Durham E-Theses
The Osmium Isotopic Composition of Seawater: Past
and Present
SPROSON, ADAM,DAVID
How to cite:
SPROSON, ADAM,DAVID (2017) The Osmium Isotopic Composition of Seawater: Past and Present,Durham theses, Durham University. Available at Durham E-Theses Online:http://etheses.dur.ac.uk/12262/
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2
1
TheOsmiumIsotopicCompositionofSeawater:PastandPresent
AdamD.Sproson
CollegeofSt.Hild&St.Bede
AthesissubmittedinpartialfulfilmentoftherequirementsforthedegreeofDoctorofPhilosophyatDurhamUniversity
DepartmentofEarthSciences,DurhamUniversity
June,2017
2
AbstractTheosmiumisotopiccompositionofseawater(187Os/188Os)reflectsabalancebetween
radiogeniccontinentalsourcesandunradiogenicmantleandextraterrestrialderived
sources.Reconstructionofthisvaluehasallowedustounlockvitalinformationabouta
seriesofEarthsystemprocesses,bothtodayandinEarth’sgeologicalpast.Thisbodyof
worklookstoreconstructthe187Os/188Osofseawaterforpastandpresentoceansusingthe
187Os/188Oscompositionofshalesandmacroalgae(seaweed)respectively.
The187Os/188OscompositionofIcelandic(0.16to0.99)andJapanese(0.16to1.09)
macroalgaearehighlyvariable,andreflectthemixingbetweenmultiplesources.The
187Os/188OsofIcelandiccoastalwatersisdominatedbyseawaterandlocalrivercatchments,
andhasbeenutilisedtotracetheinfluenceofbasalticweatheringontheglobalOscycle.
The187Os/188OsofJapanesecoastalwatersisdominatedbyseawaterandrivercatchments
drainingMiocene-Holocenecontinentalrocksoranthropogenicsources,andhasbeen
utilisedtotracemankind’simpactontheglobalOscycle.The187Os/188Osprofilesofshales
fromtheSilurianIreviken,Mulde,LauandKlonkbioventsaresimilartothosepreviously
recordedfortheHirnantianglaciation.ThisdatasuggeststheSilurianhasbeenpunctuated
byseveralglaciationsassociatedwithfluctuationsinglobaltemperatures,sea-levelandthe
carboncycle.WhencombinedwiththeLiisotopic(δ7Li)compositionofcarbonates,this
studysuggestsglacialprocessescausedlargechangesinoxidativeandsilicateweathering.
Thisstudyhassuccessfullyutilisedmacroalgaeasaproxyforthe187Os/188Osof
seawaterandprovenitcanbecomeapowerfultracerofEarthsystemprocessesand
humanactivity.ThisstudyhasalsoredefinedtheSilurianasanicehouse,andsuggeststhe
longtermdeclineinatmosphericCO2,duetoorogeny,land-plantdiversification,volcanic-
arcdegassingand/orpaleogeography,wasreversedbyperiodicglaciationswhichactedto
enhanceoxidativeweatheringwhilstsuppressingsilicateweathering.
3
Declaration
Ideclarethatthisthesis,whichIsubmitforthedegreeofDoctorofPhilosophyatDurham
University,ismyownworkandnotsubstantiallythesameasanywhichhaspreviously
beensubmittedatthisoranyotheruniversity.
AdamD.Sproson
CollegeofSt.Hild&St.Bede,DurhamUniversity
June2017
©Thecopyrightofthisthesisrestswiththeauthor.Noquotationfromitshould
bepublishedwithoutpriorwrittenconsentandinformationderivedfromit
shouldbeacknowledged.
4
Acknowledgements
IwouldliketodedicatethisPhDthesistomyparents,JulieandRichardSproson,for
withoutthem,noneofthiscouldbepossible.Theyhavesupportedmefinancially,
emotionallyandacademicallythroughoutmyentirelife.Theyhavealwayskeptanopen
mindandhaveneverpressuredmetotakemylifeinanyparticulardirection.Thishasled
metotheenvironmentalandpaleoclimaticsciencesIsomuchloveandtreasureandthe
startofacareerinsomethingIfeelintenselypassionateabout.ForthatIameternally
indebted.
IwouldliketothankmyPhDsupervisors,Prof.DavidSelby,Prof.KevinBurtonand
Prof.DavidHarperfortheirconstantadviceandsupportinthisendeavour.Inparticular,I
wouldliketothankProf.DavidSelbyforhisfinantialandacademicsupport.Withouthim
noneofthisresearchcouldhavebeenpossible.Healwaysallowedmetodevelopnew
ideasandcarryfurtherresearchifthemoodstruckme,withnosignoffinancialconstrait.
Thishasledtothedevelopmentofsomeuniqueresearchthathasgoneaboveandbeyond
theoriginalremitoftheproject.Inmyopinionheisanexceptionalsupervisorandthebest
supervisorIhavehad(sofar).
IwouldliketothankmyfriendsandcolleaguesinDurham.InparticularIwouldlike
tothankFienkeNanne,ThomasUnderwood,ElizabethAtar,EdwardInglisandBen
Maunder.Theyhavemadethelastfouryearssomeofthebestinmylife.Theyhavefilled
thedurationofmyPhDwithtrulyfunandenjoyableexperiencesandsupportedme
throughthehardtimes(ofwhichthereweremany).
IwouldliketothankJoannaHesselinkforhelpingmeinthelabduringthestartof
myPhDandteachingmethetricksofthetrade.IwouldalsoliketothankAntoniaHoffman
forherhelptowardstheendofmyPhDandmakingthingsrunsmoothly.ChrisOttleyand
GeoffNowellreceivemygratitudeforkeepingthemassspectrometersinoperation
5
thoughoutmyPhDandforhelpingwithanalyses.ThankyoutotherestoftheSelbyGroup
i.e.YangLi,JunjieLiuandZeyangLiuformakingthelabasaneplacetoworkandhelping
interpretmydata.
IwouldliketothankKatsuhikoSuzuki,RyokoSenda,MariaLuisaTejeda,Marc-
AlbanMilletandDieterKornfortheirassistancewithfieldworkduringmyPhD.Inparticular
IwouldliketothankDirector.KatzSuzukiforsupervisingmeduringmyresearchinJapan.I
wouldliketothankJindirchHladil,EmiliaJarochowska,JiriFryda,LadislavSlavikandDavid
Loydellfortheirtirelesseffortsinprovidingmewithenoughsamplestomakemystudyof
theSilurianpossible.TothatextentIwouldalsoliketothankPhilipPoggevonStrandmann
forhishelpandexpertiseinbothanalysingandinterpretinglithiumisotopedata.
Finally,IwouldliketothankTheJapaneseSocietyforthePromotionofScience,the
GeologicalSocietyofLondon,theCollegeofSt.Hild&St.Bede(JohnSimpsonGreenwell
memorialfund)andProf.DavidSelbyforhelpingtofundmyresearch.Withouttheir
money,noneofthisresearchwouldhavebeenpossible.
6
Contents
Abstract...................................................................................................................................2
Declaration..............................................................................................................................3
Acknowledgements.................................................................................................................4
Chapter1...............................................................................................................................10
1.1The187Os/188Osofcontemporaryseawater.................................................................13
1.1.1Osmiumisotopesinmacroalgae..........................................................................14
1.1.2OsmiumisotopesinIceland..................................................................................15
1.1.3OsmiumisotopesinJapan....................................................................................16
1.2The187Os/188OsofseawaterduringtheSilurian..........................................................18
1.2.1ClimaticchangeduringtheSilurian......................................................................18
1.2.2ApplicationoftheRe-OssystemtotheSilurian....................................................20
1.2.3ApplicationoftheLithiumisotopesystemtotheSilurian....................................21
1.3References...................................................................................................................22
Chapter2...............................................................................................................................28
2.1Introduction.................................................................................................................30
2.2Fieldandanalyticaltechniques...................................................................................33
2.2.1Samplingandstorage...........................................................................................34
2.2.2Macroalgaespeciesandhabitats.........................................................................35
2.2.3Re-Osanalysis.......................................................................................................36
2.2.3.1Macroalgae.......................................................................................................37
2.2.3.2Bedload.............................................................................................................37
2.2.3.3Seawater...........................................................................................................38
2.2.3.4MassSpectrometry............................................................................................38
2.2.4Statisticaltests.....................................................................................................39
2.3Results.........................................................................................................................40
2.3.1Macroalgae..........................................................................................................40
2.3.2Bedload................................................................................................................42
2.3.3Dissolvedload.......................................................................................................44
2.3.4Partitioncoefficient..............................................................................................46
2.4Discussion....................................................................................................................47
2.4.1BiologicalandenvironmentalcontrolsonReandOsuptakeinmacroalgae.......47
2.4.2Environmentalcontrolsonthe187Os/188Osofmacroalgae...................................52
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2.4.2.1Influenceofestuarineconditionsonthe187Os/188Osofmacroalgae.................52
2.4.2.2Influenceofbasalticweatheringonthe187Os/188Osofmacroalgae..................54
2.4.3Biologicalandenvironmentalcontrolson187Re/188Osofmacroalgae..................57
2.5Implicationsandfutureoutlook..................................................................................60
2.6References...................................................................................................................62
Chapter3...............................................................................................................................66
3.1Introduction.................................................................................................................67
3.2Fieldandanalyticaltechniques...................................................................................71
3.2.1Samplingandstorage...........................................................................................71
3.2.2Macroalgaespeciesandhabitats.........................................................................73
3.2.3Re-Osanalysis.......................................................................................................73
3.2.3.1Macroalgae.......................................................................................................73
3.2.3.2MassSpectrometry............................................................................................74
3.3Results.........................................................................................................................74
3.3.1HokkaidoandNorthernHonshu...........................................................................76
3.3.2TokyoBay.............................................................................................................78
3.3.3OsakaBay.............................................................................................................79
3.3.4IseandMikawaBay.............................................................................................80
3.3.5IzuPeninsula.........................................................................................................82
3.3.6NotoPeninsula.....................................................................................................83
3.4Discussion....................................................................................................................85
3.4.1Biologicalandenvironmentalcontrolsonthe187Re/188Osofmacroalgae...........86
3.4.2Naturalsourcesofosmiumtomacroalgae..........................................................88
3.4.3Anthropogenicsourcesofosmiumtomacroalgae...............................................92
3.4.4Anthropogenicinfluenceontheglobalosmiumcycle..........................................99
3.4.4.1AnthropogenicimpactonJapanesecoastalwaters..........................................99
3.4.4.2AnthropogeniccontributionsofosmiumfromJapan......................................100
3.4.4.3Impactofanthropogenicosmiumonsurfacewaters......................................101
3.5Implicationsandfutureoutlook................................................................................102
3.6References.................................................................................................................104
Chapter4.............................................................................................................................111
4.1Introduction...............................................................................................................112
4.2Materialsandmethods.............................................................................................116
4.2.1Geologicalsetting...............................................................................................116
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4.2.1.1Aizputecore.....................................................................................................117
4.2.1.2Lusklint&Lickershamn....................................................................................117
4.2.1.3Bartoszyce.......................................................................................................118
4.2.1.4Hunningecore.................................................................................................118
4.2.1.5Kosov...............................................................................................................118
4.2.1.6Klonkcore........................................................................................................119
4.2.2Samplepreparation............................................................................................119
4.2.3Osmiumisotopeanalysisofshales.....................................................................120
4.2.4Lithiumisotopeanalysisofbulkcarbonates.......................................................121
4.2.5Isotopemodeling................................................................................................123
4.2.5.1Osmiumisotopemodeling...............................................................................123
4.2.5.2Lithiumisotopemodeling................................................................................124
4.3Results.......................................................................................................................126
4.3.1Rhenium-osmiumisotopedata..........................................................................126
4.3.1.1Aizpute-41Core...............................................................................................129
4.3.1.2BartoszyceCore...............................................................................................130
4.3.1.3Kosovsection...................................................................................................131
4.3.1.4KlonkCore........................................................................................................133
4.3.2Lithiumisotopeandtracemetaldata................................................................134
4.3.2.1Lusklintsection................................................................................................135
4.3.2.2Lickershamnsection........................................................................................136
4.3.2.3Hunninge-1drillcore........................................................................................138
4.3.2.4Kosovsection...................................................................................................139
4.4Discussion..................................................................................................................139
4.4.1IsotopicconstraintsonSilurianseawaterchemistry..........................................141
4.4.1.1Climaticallyinducedchangesinoceancirculation..........................................141
4.4.1.2FloodBasaltVolcanism....................................................................................145
4.4.1.3Temperature-weatheringfeedbacks...............................................................146
4.4.1.4Hydrothermalactivity......................................................................................147
4.4.1.5Glaciation........................................................................................................149
4.4.2ScenariosfortriggeringSilurianGlaciations......................................................156
4.4.2.1Orogeny...........................................................................................................157
4.4.2.2Landplantdiversification................................................................................157
4.4.2.3Volcanicarcdegassing....................................................................................158
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4.4.2.4Paleogeography..............................................................................................159
4.4.2.5Orbitalforcing.................................................................................................159
4.4.2.6Whydidwenotsee‘SnowballEarth’conditionsduringtheSilurian?.............160
4.4.3WeatheringfeedbackshelpregulateatmosphericCO2......................................161
4.5Implicationsandfutureoutlook................................................................................164
4.6References.................................................................................................................166
Chapter5.............................................................................................................................178
5.1Re-Osisotopeuptakeanddistributioninmacroalgae..............................................179
5.2Re-OsisotopesinmacroalgaeasatracerofEarthsystemprocesses.......................180
5.3Re-Osisotopesinmacroalgaeasatracerofanthropogenicprocesses....................181
5.4TheOsandLiisotopiccompositionofSilurianseawater..........................................182
5.6Futureoutlook...........................................................................................................183
5.7References.................................................................................................................185
11
Temporalandspatialvariationsintheisotopiccompositionofseawaterreflecttheeffects
offluctuationinEarthsystemprocessesonoceanchemistry.Manyradiogenicisotope
systemsinseawateraresensitivetovariationsincontinentalweatheringanderosion,
makingthemapowerfularchiveforreconstructingresponsestoclimaticortectonic
change,withsilicateweatheringreceivingspecialattentiondueitsperceivedcontrolon
atmosphericCO2overgeologicaltimescales(Berneretal.,1983;Walkeretal.,1981).Of
theseradiogenicsystems,therubidium-strontium(87Rb-86Sr)radiogenicisotopesystemhas
beenthemostwidelyused,withvariationsinthemarine87Sr/86Srrecordreflecting
fluctuationsincontinentalinputscausedbyorogenesis(Raymoetal.,1988)andglaciations
(Armstrong,1971).However,asaconsequenceofthelongresidencetimeofSrinthe
oceans(2-4Myr)short-termfluctuationsininputsarehardtodetect(Richterand
Turekian,1993).
Theosmiumisotopiccompositionofseawater(187Os/188Os)reflectsabalance
betweenradiogeniccontinentalsourcesandunradiogenicmantleandextraterrestrial
derivedsources(Peucker-EhrenbrinkandRavizza,2000).Therefore,muchlikeSrisotopes,
Osisotopeshavebeenutilisedtoinferinformationaboutpastchangesincontinental
weathering(SeePeucker-EhrenbrinkandRavizza,2012).However,unlikeSr,theresidence
timeofOsintheocean(1-50kyr)issufficientlyshorttorespondtoshort-periodic
fluctuationsininput,whilststillbeinglongenoughtoattainaglobalsignal(Levasseuretal.,
1999;Oxburgh,2001;Rooneyetal.,2016).Therefore,the187Os/188Oscompositionof
seawaterofferstheabilitytodistinguishbetweenhigh-frequencyclimaticandlow
frequencytectonicforcing(Peucker-EhrenbrinkandRavizza,2000).Thishasallowedusto
unlockvitalinformationaboutaseriesofEarthsystemprocessesintheEarth’sgeological
pastsuchas:floodbasaltvolcanism(CohenandCoe,2002;DuVivieretal.,2014;Ravizza
andPeucker-Ehrenbrink,2003;TurgeonandCreaser,2008);paleoweathering(Finlayetal.,
2010;Ravizzaetal.,2001;Schmitzetal.,2004)basinconnectivity(PoirierandHillaire-
12
Marcel,2009);and,bolideimpacts(Paquayetal.,2008).However,despitethreedecades
ofwork,therestillremainsagreatdearthofdata187Os/188Osforpre-Cenozoictime(See
Peucker-EhrenbrinkandRavizza,2012).Inpart,thisstudylookstocorrectthisby
determiningthe187Os/188OsofseawaterfortheSilurian.
Inthemodernocean,the187Os/188Oscompositionofseawaterhasbeen
reasonablywellconstrainedthroughdirectanalysisusingultra-lowblanktechniques
capableofoxidisingallosmiumtoacommonoxidationstate(ChenandSharma,2009;
GannounandBurton,2014;Levasseuretal.,1998;Pauletal.,2009).Nevertheless,direct
analysisofseawaterremainsanalyticallychallengingduetothelowconcentrations
(Peucker-Ehrenbrinketal.,2013),andmeasurementsofrivers,estuariesandcoastal
watersarethereforesparse(Gannounetal.,2006;Huhetal.,2004;Sharmaetal.,2007;
SharmaandWasserburg,1997;Turekianetal.,2007).Thecompositionoftheglobal
riverineinputsthereforeremainspoorlyconstrained,raisingthepossibilitythatthe
osmiuminputintotheoceancouldbeunderestimatedbyafactorof~3(Oxburgh,2001).
Thismayhaveledtoadiscrepancybetweenoceanicosmiumresidencetimesestimated
frommassbalancecalculations(35-50kyr)andthoseinferredfromtheevolution(1-4
kyr)oftheosmiumisotoperecord(Levasseuretal.,1999;Oxburgh,2001;Rooneyetal.,
2016;Sharmaetal.,1999).
RecentworksuggestsmacroalgaeconcentratesOs(withabundancesthatvary
from12.6to78.5ppt),whilstmaintainingthe187Os/188Oscompositionoftheseawaterit
inhabits(Racionero-Gómezetal.,2016;Racionero-Gómezetal.,2017).Thissuggeststhat
macroalgaecouldactasaproxyforthe187Os/188Oscompositionoflocalwaterswhilst
removingsomeoftheanalyticalchallengesassociatedwithdirectanalysisofseawateri.e.
ultra-lowconcentrationsandmultipleoxidationstates.Macroalgaeexistingincoastal
13
waters,therefore,shouldrecordan187Os/188Ossignaturethatreflectsabalanceoflocal
inputs,includingriverineinput,localbedrock,anthropogenicactivityandseawater.
Inthisbodyofworkwewillapplythemacroalgae-Os-seawaterproxytoreal
worldsettingstotestitsabilitytorecordEarthsystemprocessesduringthepresent.In
Chapter2wewillutilisemacroalgaecollectedfromIcelandiccoastalwaterstotrace
fluctuationsinthe187Os/188OsoffreshwaterandseawateraroundIceland,anddetermine
theinfluenceofbasalticweatheringontheglobalosmiumbudget.InChapter3wewill
utilisemacroalgaefromJapanesecoastalwaterstohelpconstraintheanthropogenic
influenceontheglobalOscycle.Finally,inChapter4wewillutilisetheRe-Osisotope
systematicsofshalestodeterminefluctuationsinthe187Os/188Osofseawaterduringthe
Silurian,andshedlightonthemechanismsbehindabruptclimaticchangeduringthistime.
1.1The187Os/188Osofcontemporaryseawater
Osmiumisamongtheleastabundantelementsinseawater,thereforeearlyattemptsto
analysetheconcentrationandisotopiccompositionofseawaterdirectlywereplaguedwith
difficulties.Koideetal.(1996)useda25Lsampleofseawaterspikedwitha190Ostracer,
separatingOswithanion-exchangechromatographyandthempurifyingitusingdistillation
techniques.Sharmaetal.(1997)reducedseawaterandtracerOsbybubblingSO2(g)and
thenco-precipitatingOswithironoxyhydroxidein4-10Lsamples.Theproblemwiththese
techniquesisthattheyrequirehandlingalargevolumeofsample.Threesubsequent
techniquesattemptedtousesmallervolumesofsample(50gto1.5kg),andtriedto
equilibratetracerandwaterOsbyoxidisingittoacommonoxidationstate(OsO4).
However,ChenandSharma(2009)discoveredthateachofthesemethodsdidnotyield
identicalconcentrationstothoseofWoodhouseetal.(1999).Theyfoundhigher
temperatures(300°C)wererequiredtooxidiseallspeciesofOspresentinseawater.Witha
14
reliablechemicalseparationtechniqueinhandithasbecomepossibletomeasureOs,
althoughstillanalyticalchallenging,withaslittleas20gofseawater(Sharmaetal.,2012).
Despitethiscapability,problemsstillarisefromthenatureofOsinseawater.
ExtremelylowconcentrationsofOsinseawateri.e.90fgin100mlofseawater,meansyou
getsignificantinterferencefromproceduralblanksof~3.6fg(ChenandSharma,2009).This
iscompoundedbycontaminationfromtheuseofpolyethylenebottlesforseawater
storage(Sharmaetal,2012).WeproposetodevelopanewproxyfortheOsisotopic
compositionofseawaterbasedonOsmeasurementsofmacroalgae,whichdoesnotsuffer
fromtheseproblemsi.e.Osconcentrationsinseaweedare>50pg/gandthereforefar
higherthantheproceduralblank(~50fg),whilstthelongtermstorageofseaweeddoes
notsufferfromthestoragetechniquesused.
1.1.1Osmiumisotopesinmacroalgae
WorkconductedbyB.Racionero-GómezandmyselfattheUniversityofDurhamlooked
intothebiologicaluptakeofRe(Racionero-Gómezetal.,2016)andOs(RacioneroGomez
etal.,2017)intomacroalgae.Thesestudiesutilisedasinglemacroalgaespecies,Fucus
vesiculosus,andanalyseditsReandOsabundanceanduptake,aswellasassessingifit
couldrecordtheOsisotopecompositionoftheseawaterinwhichitlived.Itwas
demonstratedthatOsandRearenotlocatedinonespecificbiologicalstructure,butfound
throughouttheorganism.OsmiumuptakewasdeterminedbyculturingF.vesiculosuswith
differentconcentrationsofOswithaknown187Os/188Oscomposition(~0.16).Thecultured
samplestookontheisotopiccompositionofthecultureinwhichtheylived,whichis
significantlydifferenttothebackgroundcompositionofun-dopedseawater(~0.94)(Fig.1).
Thissuggestsmacroalgaecanattaintheisotopiccompositionofseawater,andtherefore
couldpotentiallyactasaproxyforunderstandingavarietyofEarthsystemprocesses.
15
Fig.1.Osmium(ppt)accumulation(circles)and187Os/188Oscompositions(squares)inF.
versiculosusunderdifferentculturemediaOsabundances.SeeRacionero-Gómezetal.
(2017)formoredetails.
1.1.2OsmiumisotopesinIceland
Icelandconsistsofanessentiallymonolithologicalbasalticterrainofvaryingages(historic
to12Ma),yieldingalargerangeinthe187Os/188Os(0.15to1.04)ofriverinedissolvedloads
(Gannounetal.,2006).Unradiogenic187Os/188Osvaluescanbeexplainedbycongruent
basaltweatheringand/orhydrothermalinput,withradiogenic187Os/188Osvaluesarising
fromtwodistinctprocesses:the187Os/188Osofglacier-fedriverscanbeexplainedbythe
entrainmentofseawateraerosolsintoprecipitationandsubsequentglacialmelting;while
the187Os/188Oscompositionofdirect-runoffandspring-fedriverscanbeexplainedbythe
incongruentweatheringofcertainprimarybasalticmineralsthatpossessexceptionallyhigh
16
187Re/188Os,whichovertimeevolvestoradiogenic187Os/188Osvalues(Gannounetal.,
2004).
Thechemistryofmacroalgaefromshallowcoastalwatersisdominatedbythe
brackishconditionsinwhichitlives.Macroalgaethereforecomeincontactwithbothfresh
waterandseawatersourcesthroughasingletidalcycle,andintheory,the187Os/188Os
compositionofmacroalgaeshouldrepresentamixingbetweenthe187Os/188Oscomposition
ofthesetwosources.InIceland,aspreviouslyexplained,the187Os/188Oscompositionof
riverineinputswillbehighlyvariable,rangingfromunradiogenicvaluesnearthecentralrift
zone,throughtohighlyradiogenicvaluesintheouterpartsofIceland.Thisdiverserangeof
187Os/188OscompositionsmakesIcelandauniqueenvironmentwithwhichtotesttheability
ofmacroalgaetorecordthe187Os/188Oscompositionoftheseawateritinhabits.
Chapter2presentsRe-Osabundanceandisotopedataformacroalgaeand
dissolvedandbedloadsfromcoastalwatersandriversdrainingbasalticwatershedsof
Iceland.Thisrepresentsthefirstexaminationoftheinfluenceofbothseawaterandriver
waterosmiumonthe187Os/188Oscompositionofmacroalgaeanddemonstratestheability
ofmacroalgaetotracefluctuationsinthe187Os/188Osoffreshwaterandseawateraround
Iceland.Chapter2utilisesthistodeterminetheinfluenceofbasalticweatheringonthe
globalosmiumbudget.
1.1.3OsmiumisotopesinJapan
Thepresent-dayopenoceanseawater187Os/188Osvalueof~1.06reflectsthebalance
betweenunradiogenicmantle-derivedOsandradiogeniccontinentalOs(Peucker-
Ehrenbrink,&Ravizza,2000).Dissolutionofbackgroundextraterrestrialmattercontributes
littletotheunradiogenicsourcesofOs.Ontheotherhand,Osreleasedbyanthropogenic
activitieshasbeendetectedincoastalsediments,lakesandestuariesfromsourcessuchas
sewagesludge,catalyticconvertors,anduseasastainingreagentinbiomedicalresearch
17
(EsserandTurekian,1993;Rauchetal.,2004;RavizzaandBothner,1996;Turekianetal.,
2007;Williamsetal.,1997).AtmosphericanthropogenicOsattributedtothesmeltingof
variousoresandcatalyticconvertors,hasalsobeendetectedinrainandsnow,whichis
impactingOsinoceanicsurfacewatersandthereforetheglobalOsbudget(Chenetal.,
2009).Theseanthropogenicsourcesaregenerallycharacterizedbyunradiogenic
187Os/188Oscompositions(~0.12)relatedtotheisotopiccompositionofPGEsrefinedfor
humanuse.
Japanoffersauniqueplaceinwhichtostudytheinfluenceofanthropogenic
processesonregionalvariationsinthemarineOscycle.Large,denselypopulated,
sprawlingmetropolitanareasoftenfallincloseproximitytocoastalwaters,inletseasor
bays.WatersintheseregionsarelikelytobedominatedbyunradiogenicOsproducedby
sewagetreatmentplants,hospitals,refineriesandvehicleexhaust.Moreover,these
metropolitanareasareoftenjuxtaposedtosparselypopulatedruraland/ormountainous
regions,withlittlehumanactivityandthereforeanthropogenicinfluence.The187Os/188Os
compositionofcoastalwatersintheseregionsisthereforelikelytobedominatedby
naturalsourcesofOs.Inparticular,Japaneserivercatchmentsaredominatedbythe
weatheringofMiocene-Holocenevolcanicandsedimentaryrocksofradiogenic187Os/188Os
compositions.Aspreviouslyexplained,the187Os/188Oscompositionofmacroalgaefrom
coastalwaterslikelyrepresentsamixingbetweenthe187Os/188Oscompositionofseawater
andlocalfreshwatersinputs.MacroalgaefromdenselypopulatedregionsofJapanwill
thereforelikelyshowastronginfluencefromhighlyunradiogenicPGE187Os/188Osvalues
derivedfromlocalhumanactivity.Meanwhile,the187Os/188Oscompositionofmacroalgae
frommoresparselypopulatedregionswillshowastrongerinfluencefromnatural
freshwatersources.
18
Chapter3presentsRe-Osabundanceandisotopedataformacroalgaefrom
TokyoBay,OsakaBay,IseBay,MikawaBay,IzuPeninsula,NotoPeninsula,Hokkaidoand
northernHonshu.TokyoBay,OsakaBayandIse/MikawaBaylieincloseproximitytothe
denselypopulatedKanto(Tokyo-Yokohama),Keihanshin(Osaka-Kobe)andChukyo
(Nagoya)metropolitanareas.The187Os/188Ossignaturefromtheseregionsismuchlower
thanexpectedfornaturalriverandoceanicsystems,butsimilartotheisotopecomposition
ofPGEores.ThissuggeststhathumanactivityhasinfluencedtheOsisotopiccomposition
ofmacroalgae,andthereforeseawater,throughtheburningofmunicipaland/orhospital
waste,processingofsewageandtheextensiveuseofautomobilesintheseareas.TheIzu
Peninsula,NotoPeninsula,HokkaidoandnorthernHonshuexhibit187Os/188Osvaluessimilar
toglobalriverwaterorPacificseawatermeasurements,suggestinglittleinfluencefrom
humanactivity.Theseresultsdemonstratetheabilityofmacroalgaetotracefluctuationsin
the187Os/188OsoffreshwaterandseawateraroundJapan,andutilisethistodeterminethe
influenceofhumanactivityontheglobalosmiumbudget.
1.2The187Os/188OsofseawaterduringtheSilurian
1.2.1ClimaticchangeduringtheSilurian
IncontrasttopreviousviewsthattheSilurianwasanenvironmentallystableperiod
betweentheOrdovicianicehouseandtheDevoniangreenhouse(Munneckeetal.,2010),it
hasmorerecentlybecomeapparentthattheSilurianischaracterisedbyahighlydynamic
climateattenuatedbymultipleshort-livedevents,strongeustaticsealevelchangeand
oceanicturnoverassociatedwithextinctioneventsafterarecoveryfromtheend-
Ordovicianglaciation(Calner,2008;Melchinetal.,2012).Thecarbonisotopiccomposition
ofcarbonate(δ13Ccarb)throughtheSilurianishighlyvariable,indicatingtheclimatesystem
andcarboncyclewereprobablymoreunstablethananyotherPhanerozoicperiodandcan
19
beregardedasoneofthemostvolatileperiodswhenconsideringtheocean-atmosphere
system(CramerandSaltzman,2005).
TheSilurianthroughtotheearlyDevonianismarkedbyfourlarge-amplitude
positivecarbonandoxygenisotopeexcursions,withtheδ13Ccarbexceeding+5‰duringthe
Ireviken,Mulde,LauandKlonkbioevents.TheLaucarbonisotopeexcursion(>+8‰)is
probablythelargestpostCambrianδ13CcarbexcursionoftheentirePhanerozoic.Such
excursionsarefarlargerthananythingintheMesozoicorCenozoic,andthereforeany
classicalinterpretationsconcerningproductivitychangesarenotviable(Bickertetal.,
1997).Theδ13Ccarbisotopeexcursionsareassociatedwithsignificantpositiveoxygen
isotope(δ18O)excursionsofapproximately1-3.5‰magnitude(Lehnertetal.,2010;
Munneckeetal.,2010;Žigaitėetal.,2010),whicharetoolargetobeexplainedbyeither
temperature,icevolumeorsalinityalone(Bickertetal.,1997).However,despitethese
discoveriesandovertwodecadesofresearch,thecauseoftheseclimateperturbationsis
stillnotunderstood.
Traditionalexplanationsfortheseeventshaveinvokedashiftbetweentwo
stableoceanic-climatestates,drivenbychangesinthelocationofdeep-waterformation
fromhightolowlatitudes(Jeppsson,1990),orglobalprecipitationratesandcontinental
runoff(Bickertetal.,1997).However,theseearlyattemptstoexplainSilurianclimatic
eventshavereceivedmuchcriticism(Johnson,2006;Kaljoetal.,2003;Loydell,1998).More
recentlyithasbeenpostulatedthatSilurianclimaticchangecouldhavebeendrivenby
glacialexpansionoverGondwana,inferredinpartfrompositiveoxygenisotopeshifts
(Trotteretal.,2016)coupledtosignificanteustaticsea-levelchange(Díaz-Martínezand
Grahn,2007;Lehnertetal.,2010),muchliketheLateOrdovicianthatprecededit(Algeoet
al.,2016;Harperetal.,2014).However,thelackofglacialsedimentsinthestratigraphic
20
recordformuchoftheSilurian(post-Wenlock)hashamperedthisnotion(Caputo,1998;
Díaz-MartínezandGrahn,2007;GrahnandCaputo,1992).
1.2.2ApplicationoftheRe-OssystemtotheSilurian
Toshedlightonthepossiblemechanismsbehindtheseabruptclimateperturbations,we
haveappliedtheRe-Osisotopesystemtoorganicrichshalesfromgeologicalformations
thatspantheIreviken,Mulde,LauandKlonkbioevents.The187Os/188Os
oforganicrichshalesmimicsthatofseawateratthetimeofdeposition,andreflectsa
balancebetweenradiogeniccontinentalsourcesandunradiogenicmantleand
extraterrestrialderivedsources(Peucker-EhrenbrinkandRavizza,2000).Therefore,much
likeSrisotopes,Osisotopeshavebeenutilisedtoinferinformationaboutpastchangesin
continentalweathering.However,unlikeSr,theresidencetimeofOsintheoceanis
sufficientlyshorttorespondtoshort-periodicfluctuationsininput,whilststillbeinglong
enoughtoattainaglobalsignal(Levasseuretal,1998;Oxburgh,2001).Therefore,theOs
isotopesystemofferstheabilitytodistinguishbetweenhigh-frequencyclimaticandlow
frequencytectonicforcing(Peucker-EhrenbrinkandRavizza,2000).
InChapter4osmiumisotopeprofilesfortheIreviken,Mulde,LauandKlonk
bioeventsarepresented.TheseprofilesaresimilartotheHirnantianglaciation(Finlayet
al.,2010),whichoccurredsome10MyrpriortotheIrevikenEvent.The187Os/188Osprofiles
arerelatedtotheweatheringoforganic-richsedimentaryrocksduringtheexpansionof
continentaliceoverGondwana,whichdeliversradiogenicOstotheocean.Thiswork
suggestsclimaticperturbationsduringtheSilurianarerelatedtoglaciationstriggeredby
decliningatmosphericCO2andtemperatures.Oncetheglaciationwastriggered,enhanced
oxidativeweatheringoforganicandsulphiderichsedimentaryrocksledtoareleaseinCO2
totheatmospherewhichhelpedreverseglobalcooling.
21
1.2.3ApplicationoftheLithiumisotopesystemtotheSilurian
Aspreviouslyexplained,variationsinOsisotopescanalsobecontrolledbyfactorsother
thancontinentalweathering,suchasfluctuationsinhydrothermalalterationofjuvenile
basalticcrustandextra-terrestrialinputs(Peucker-EhrenbrinkandRavizza,2000).Totest
thevalidityofourOsisotopecurvesastracersofcontinentalweathering,wealsoutilised
othercontinentalweatheringproxies.TheLiisotopiccompositionofseawater(δ7Li)
reflectsabalancebetweentheinputsofLifromrivers(weatheringofcontinentalsilicate
rocks)andmid-oceanridgespreadingcentres(weatheringofoceanicsilicaterocks)andthe
outputsofLifromtheincorporationintomarinesedimentsandalteredoceaniccrust
(MisraandFroelich,2012).Theδ7Liofcarbonatesisseentoreflecttheδ7Liofseawater
throughtime,andhasbeenusedtoreconstructcontinentalsilicateweatheringratesinthe
geologicalpast(MisraandFroelich,2012).Recently,PoggevonStrandmannetal.(in
review)utilisedLiisotopemeasurementsincarbonatesandshalesthatspantheHirnantian
glaciation.Apeakinδ7LioccursduringthetroughinOsisotopes(Finlayetal.,2010)
associatedwithglacialmaximum.Thisshowsastrongdeclineinsilicateweathering,
reducingtheeffectoftheEarth’sprimaryCO2removalmechanism,preventingfurther
coolingandallowingtemperaturestobuild-up,leadingtodeglaciation.
Totestthehypothesisinsection1.2.2wegeneratedLiisotoperecordsin
carbonatesfromsectionsspanningtheIreviken,Mulde,LauandKlonkbioevents(See
Chapter4).LithiumisotoperecordsshowasimilarprofiletotheHirnantianglaciation
suggestingsuppressionincontinentalsilicateweatheringunderenhancedcontinentalice
sheets.ThisreducesoneoftheEarth’smajorCO2withdrawalmechanisms,andwhen
combinedwithenhancedoxidativeweathering,leadstogreaterlevelsofatmosphericCO2.
ThisstudysuggeststhattheEarthhasastabilisingnegativefeedbackmechanisminwhicha
reductioninatmosphericCO2andglobaltemperatureswhichleadstoenhanced
22
continentaliceisultimatelyreversedbyweatheringprocesseswhichacttoincrease
atmosphericCO2andtemperatures,preventingarunawayicehouse.
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28
Chapter2
Tracingtheinfluenceofweatheringprocessesoncoastalwaterssurroundinga
basalticterrainusingosmiumisotopesinmacroalgae*
*AversionofthischapterwillbesubmittedtoGlobalBiogeochemicalCycles,co-authored
withDavidSelby,AbdelmouchineGannoun,KevinW.BurtonandJeremyM.Lloyd.
29
Thisstudypresentsrhenium(Re)andosmium(Os)abundanceandisotopedatafor
macroalgae,dissolvedloadandbedloadfromtheIcelandiccoastalwaters,anenvironment
adjacenttopredominantlybasalticterrains,ranginginagefromhistorictoca.12Ma.Both
theReandOsabundanceinmacroalgaeareshowntobeprimarilycontrolledbyuptake
fromthedissolvedloadoflocalseawater.Inahabitatwithvaryingsalinitythroughthetidal
cycle,macroalgaeReandOsabundancesvary,dependingontherelativeinfluenceoflocal
freshwaterinputs.IncorporationofReandOsintomacroalgaeiscomplicatedbyadditional
Reuptakefromsuspendedparticulatesand/orbedload,whichisnotobservedforOs,
suggestingdifferentuptakepathwaysforbothReandOs.The187Os/188Os(0.16to0.99)and
187Re/188Os(~65to40,320)compositionsofmacroalgaearehighlyvariable,andcanbe
explainedintermsofanunradiogenic187Os/188Oscontributionwithlow187Re/188Osfrom
riversdrainingyoungercatchmentsthathaveundergonecongruentbasaltweathering
(and/orhydrothermalinput),andaradiogenic187Os/188Oscontributionfromtwodistinct
sources:riversdrainingoldercatchmentsthathaveundergoneincongruentweatheringof
primarybasalticmineralsthatpossessexceptionallyhigh187Re/188Osratiosthathave
evolvedtoradiogenic187Os/188Osratios;and,NorthAtlanticseawater.Macroalgaecan
attaina187Re/188OsfarhigherthanthatrecordedfortheIcelandicgeochemicalreservoirs
duetothepreferentialuptakeofReoverOsathighReconcentrationsinthedissolved
load,thereforemacroalgaecannotbeusedtodeterminethe187Re/188Osofseawater.This
studyconfirmstheutilityofmacroalgaeasaproxyfortheOsisotopiccompositionof
seawater,whichholdsthepotentialtoelucidatearangeofEarthsystemprocesses.
However,itisnotyetpossibletodirectlyrelatethemacroalgaeOsconcentrationtothatof
thewaterinwhichtheylive.Finally,theseresultssuggestthatmacroalgaeisnota
substantialsinkforeitherReorOs.Therefore,globalmacroalgaebiomass,todayorduring
theEarth’sgeologicalpast,doesnotplayasignificantroleinthemarineOsandRecycles.
30
2.1Introduction
Temperatureandatmosphericcarbondioxide(CO2)havefluctuatedwidelythroughoutthe
Phanerozoic,fromwarmgreenhousetocoldicehouseconditions.Nevertheless,
throughoutthistime,temperatureandCO2havealwaysremainedwithinthenarrowlimits
thatallowlifetoexistandevolvethroughtheinteractionbetweentheatmosphere,
hydrosphere,biosphereandlithosphere(Berneretal.,1983).Overgeologicaltimescales
(Myr)temperaturehaspartlybeencontrolledbyinteractionsbetweenatmosphericCO2
andcontinentalweathering.Risingtemperaturesstimulateincreasedchemicalweathering
ofsilicaterocksdrawingdownCO2fromtheatmosphere,leadingtoadeclinein
temperature(Berneretal.,1983;Walkeretal.,1981).Manyradiogenicisotopesystemsin
seawateraresensitivetovariationsincontinentalweatheringanderosion,makingocean
chemistryapowerfularchiveforreconstructingresponsestoclimaticortectonicchange.
Ofthese,therubidium-strontium(87Rb-86Sr)radiogenicisotopesystemhasbeen
themostwidelyused,withvariationsinthemarine87Sr/86Srrecordreflectingfluctuations
incontinentalinputscausedbyorogenesis(Raymoetal.,1988)andglaciations(Armstrong,
1971).However,asaconsequenceofthelongresidencetimeofSrintheoceans(2-4Myr)
short-termfluctuationsininputsarehardtodetect(RichterandTurekian,1993).The
osmiumisotopiccompositionofseawater(187Os/188Os)reflectsabalancebetween
radiogeniccontinentalsourcesandunradiogenicmantleandextraterrestrialderived
sources(Peucker-EhrenbrinkandRavizza,2000).Therefore,muchlikeSrisotopes,Os
isotopeshavebeenutilisedtoinferinformationaboutpastchangesincontinental
weathering(SeePeucker-EhrenbrinkandRavizza,2012andreferencestherein).However,
unlikeSr,theresidencetimeofOsintheocean(1-50kyr)issufficientlyshorttorespondto
short-periodicfluctuationsininput,whilststillbeinglongenoughtoattainaglobalsignal
(Levasseuretal.,1998;Oxburgh,2001;Rooneyetal.,2016).Therefore,the187Os/188Os
31
compositionofferstheabilitytodistinguishbetweenhigh-frequencyclimaticandlow
frequencytectonicforcing(Peucker-EhrenbrinkandRavizza,2000).
Inthemodernocean,the187Os/188Oscompositionofseawaterhasbeen
reasonablywellconstrainedthroughdirectanalysisusingultra-lowblanktechniques
capableofoxidisingallosmiumtoacommonoxidationstate(ChenandSharma,2009;
GannounandBurton,2014;Levasseuretal.,1998;Pauletal.,2009).Nevertheless,direct
analysisofseawaterremainsanalyticallychallengingbecauseofthelowconcentrations
andmultipleoxidationstates(Peucker-Ehrenbrinketal.,2013),andmeasurementsof
rivers,estuariesandcoastalwatersarethereforesparse(Gannounetal.,2006;Huhetal.,
2004;Sharmaetal.,2007;SharmaandWasserburg,1997;Turekianetal.,2007).Therefore
thecompositionoftheglobalriverineinputremainspoorlyconstrained,raisingthe
possibilitythattheosmiuminputtotheoceanisunderestimated,therebyaccountingfor
thediscrepancybetweenoceanicosmiumresidencetimesestimatedfrommassbalance
calculations(35-50kyr)andthoseinferredfromtheevolutionoftheosmiumisotope
record(1-4kyr)(Levasseuretal.,1999;Oxburgh,2001;Rooneyetal.,2016;Sharmaetal.,
1999).Althoughitisalsoproposedthatthedifferenceinresidencetimeestimatesrelates
tothelocalremovalofOs(Sharmaetal.,2007).
Recentworkindicatesthatmacroalgae(seaweed)concentratesOs(with
abundancesthatvaryfrom12.6to78.5ppt),whilstmaintainingthe187Os/188Os
compositionoftheseawateritinhabits(Racionero-Gómezetal.,2017;Rooneyetal.,
2016).Thissuggeststhatmacroalgaecouldactasaproxyforthe187Os/188Oscompositionof
localwaterswhilstremovingsomeoftheanalyticalchallengesassociatedwithdirect
analysisofseawateri.e.ultra-lowconcentrationsandmultipleoxidationstates.Macroalgae
existingincoastalwaters,therefore,shouldrecordan187Os/188Ossignaturethatreflectsa
balanceoflocalinputs,includingriverineinput,localbedrockandseawater.
32
Icelandconsistsofanessentiallymonolithologicalbasalticterrainofvaryingages
(historicto12Ma),yieldingalargerangeinthe187Os/188Os(0.15to1.04)ofriverine
dissolvedloads(Gannounetal.,2006).Unradiogenic187Os/188Osvaluescanbeexplainedby
congruentbasaltweatheringand/orhydrothermalinput,withradiogenic187Os/188Osvalues
arisingfromtwodistinctprocesses.The187Os/188Osofglacier-fedriverscanbeexplainedby
theentrainmentofseawateraerosolsintoprecipitationandsubsequentglacialmelting.
The187Os/188Oscompositionofdirect-runoffandspring-fedriverscanbeexplainedbythe
incongruentweatheringofcertainprimarybasalticmineralsthatpossessexceptionallyhigh
187Re/188Os,whichovertimeevolvestoradiogenic187Os/188Osvalues(Gannounetal.,2004).
Icelandthereforeprovidesauniqueenvironmentwithrespectto187Os/188Osinwhichto
testtheabilityofmacroalgaetorecordthe187Os/188Oscompositionoftheseawaterit
inhabits.
ThisstudypresentsRe-Osabundanceandisotopedataformacroalgaeand
dissolvedandbedloadsfromcoastalwatersandriversdrainingbasalticwatershedsof
Iceland.GiventhatwatershedsofIcelandareessentiallymonolithological,their187Os/188Os
compositionisdeterminedbytheageofbasalt,thepreferentialweatheringofconstituent
basaltmineralsandtheentrainmentofrain,seaandhydrothermalwaters(Gannounetal.,
2006).Thisstudyrepresentsthefirstexaminationoftheinfluenceofbothseawaterand
riverwaterosmiumonthe187Os/188Oscompositionofmacroalgae.Theseresults
demonstratetheabilityofmacroalgaetotracefluctuationsinthe187Os/188Osoffreshwater
andseawateraroundIceland,andutilisethistodeterminetheinfluenceofbasaltic
weatheringontheglobalosmiumbudget.
34
Fig.1.GeologicalmapofIcelandmodifiedafterJóhannesson(2014):1=Holocene
sediments;2=basicandintermediatelavas(postglacial,historic,youngerthanAD871);3=
basicandintermediatelavas(postglacial,prehistoric,olderthanAD871);4=acidlavas
(postglacial,historic,youngerthanAD871);5=acidlavas(postglacial,prehistoric,older
thanAD871);6=acidextrusives(Miocene,PlioceneandPleistocene,olderthan11,000
yrs);7=basicandintermediatehyaloclastite,pillowlavaandassociatedsediments(Upper
Pleistocene,youngerthan0.8myr);8=basicandintermediateinterglacialandsupraglacial
lavaswithintercalatedsediments(UpperPleistocene,youngerthan0.8myr);9=basicand
intermediateextrusiverockswithintercalatedsediments(UpperPlioceneandLower
Pleistocene,0.8-3.3myr);10=basicandintermediateextrusiverockswithintercalated
sediments(MioceneandLowerPliocene,olderthan3.3myr);11=basicandintermediate
intrusions,gabbro,doleriteanddiorite;12=acidintrusions,rhyolite,granophyreand
granite.Sampletypeandlocalitiesareindicatedinthelegend.Dashedlinesrepresentan
outlineofareasshowninFigures2aand2b.
2.2.1Samplingandstorage
Macroalgae,bedloadsandsandwatersfromtheIcelandiccoastlineweresampledat
eighteenlocationsduringlateAugustof2014(Fig.1;samples7-23and27).Afurthernine
locationsweresampledbetweenlateJulyandearlyAugustof2015(Fig.1;samples1-6and
24-26).Intotaltwenty-sevenmacroalgae,elevenbedloadandeightwatersampleswere
collected(Fig.1;Table1-2).Macroalgaeandbedloadwerewashedusingdeionised(Milli-
Q™)watertoremoveanyattachedsedimentandsalt.Theywerethendriedfor12hat60
°Candstoredinplasticzip-lockbags.Macroalgaeandbedloadwerelatercrushedusingan
agatepestleandmortarpriortoanalysis.Watersampleswerefilteredthrough0.2μm
celluloseacetatefiltersusingapressurizedSartorius®Teflonunit.Filtratealiquotswere
storedinpre-cleanedSavillex®TeflonbottlestopreventOscontamination(Sharmaetal.,
2012).SalinitywasmeasuredusingaHanna®HI98192conductivitymeter.
35
2.2.2Macroalgaespeciesandhabitats
Specificmacroalgaespecieswerenottargetedduringthisstudy,andsampleswere
selectedbasedontheiravailabilityatsamplesites.Therewashoweverapreferenceto
brownmacroalgaeovergreenandredmacroalgaeduetotheirrelativelyhighabundancein
Re(Masetal.,2005;Proutyetal.,2014;Racionero-Gómezetal.,2016;Yang,1991)andOs
(Racionero-Gómezetal.,2017;Rooneyetal.,2016).Fourspeciesofbrownmacroalgae
(Fucusvesiculosus,Fucusspiralis,FucusdistichusandAscophyllumnodosum)were
analysed.
Fig.2.GeologicalmapsoftheReykjanesPeninsula(a)andEasternFjords(b).Keyisthe
sameasinFigure1.
a
36
2.2.3Re-Osanalysis
TheRe-OsanalysisformacroalgaeandbedloadwerecarriedoutintheDurham
GeochemistryCentre(LaboratoryforSulfideandSourceRockGeochronologyand
Geochemistry).TheseawaterOsanalyseswereconductedatLaboratoireMagmaset
37
VolcansattheCampusUniversitairedesCézeaux,withtheRefractionprocessedatthe
DurhamGeochemistryCentre.
2.2.3.1Macroalgae
ThetechniqueforchemicalseparationofReandOsfrommacroalgaeisreportedby
Racionero-Gómezetal.(2017).Inbrief,approximately200mgofpowderedmacroalgae
wasintroducedintoaCariustubetogetherwith11NHCl(3mL),15.5NHNO3(6mL)anda
knownamountof185Re+190Ostracersolutionandheatedto220°Cinanovenfor24h.The
OswasisolatedfromtheacidmediumusingCHCl3solventextractionandthenback
extractedintoHBr.TheOswasfurtherpurifiedusingaCrO3-H2SO4–HBrmicro-distillation
(Bircketal.,1997;CohenandWaters,1996).TheremainingRe-bearingacidmediumwas
evaporatedtodrynessat80°C,withtheReisolatedandpurifiedusingbothNaOH-acetone
solventextractionandHNO3-HClanionchromatography(Cummingetal.,2013).
2.2.3.2Bedload
ThedetailedanalyticalprocedureforsilicateshasbeenadaptedfromIshikawaetal.(2014).
Approximately1gofbedloadwascrushedusinganagatemortar.Thepowderisdissolved
withHCl+HF(4mL:2mL)ina22ml-savillex®vialat100°C.Theacid-samplemediumwas
evaporatedtodrynessat80°Cbefore11NHCl(1mL)wasaddedandsubsequently
evaporatedtwicetoremoveremainingHF.Theresultingacidmediumwasintroducedinto
aCariustubetogetherwith11NHCl(3mL),15.5NHNO3(6mL)andaknownamountof
the185Re+190Ostracersolutionandheatedat220°Cfor48h.Theextractionand
purificationoftheOsandRefractionsfollowsthesameanalyticalprotocoloutlinedin
section2.3.1.
38
2.2.3.3Seawater
ThetechniqueforchemicalseparationofReandOsfromseawaterisreportedby
Racionero-Gómezetal.(2017).Briefly,~60gofwatersample,plusaknownamountof
mixed(190Os+185Re)tracersolution,togetherwith2mLofBr2,2mLofCrO3-H2SO4solution
and1.5mLof98%H2SO4weresealedintoa120mLsavillexvialandheatedto100˚Cinan
ovenfor72htoequilibratesampleandspike(GannounandBurton,2014).TheOswas
extractedfromthesampleintoliquidBr2followedbyasecondextractionofOsusing1mL
ofBr2.The1mLofliquidBr2wasaddedtothesamplesolution,reactedfor1h,andthen
removed.TheextractedBr2ismixedwith1mLof9NHBrandevaporatedtodryness,and
furtherpurifiedusingaCrO3-H2SO4–HBrmicro-distillation.TheRewaspurifiedasoutlined
forthemacroalgaesamples(Cummingetal.,2013).
2.2.3.4MassSpectrometry
ThepurifiedReandOsfractionswereloadedontoNiandPtfilamentsrespectively,and
measuredusingNTIMS(Creaseretal.,1991;Völkeningetal.,1991)onaThermoScientific
TRITONmassspectrometerusingFaradaycollectorsinstaticmode,andanelectron
multiplierindynamicmoderespectively.TheReandOsabundancesandisotope
compositionsarepresentedwith2s.e.(standarderror)absoluteuncertaintieswhich
includefullerrorpropagationofuncertaintiesinthemassspectrometermeasurements,
blank,spikecalibrations,andsampleandspikeweights.Fullanalyticalblankvaluesforthe
macroalgaeanalysisare10.9±5.9pgforRe,0.13±0.13pgforOs,witha187Os/188Os
compositionof0.61±0.34(1SD,n=4).Forthebedloadanalysisthefullanalyticalblank
valuesare15.9±0.23pgforRe,2.12±0.01pgforOs,witha187Os/188Oscompositionof
0.27±0.001(2s.e.,n=1).Fortheseawateranalysisthefullanalyticalblankvaluesare10±
39
1.3pgforRe,0.043±0.002pgforOs,witha187Os/188Oscompositionof0.72±0.02(1SD,n
=4).
Tomonitorthelong-termreproducibilityofmassspectrometermeasurements
ReandOs(DROsS,DTM)referencesolutionswereanalysed.The125pgResolutionyields
anaverage185Re/187Reratioof0.5987±0.0023(2SD.,n=8),whichisinagreementwith
publishedvalues(e.g.,Cummingetal.,2013andreferencestherein).A50pgDROsS
solutiongavean187Os/188Osratioof0.16111±0.0008(2SD.,n=8),whichisinagreement
withreportedvaluefortheDROsSreferencesolution(Nowelletal.,2008).Forthe
seawaterOsanalysisattheLaboratoireMagmasetVolcansmonitorinstrument
reproducibilityusinga1pgDTMOssolution,whichyieldsa187Os/188Osvalueof0.1740±
0.0002(2SD,n=4),whichisinagreementwithpublishedvalues(ChenandSharma,2009;
GannounandBurton,2014).
2.2.4Statisticaltests
StatisticaltestswerecarriedoutusingMATLAB®R2017a(MATLAB9.2,TheMathWorks
Inc.,Natick,MA,2017).Aone-wayanalysisofvariance(ANOVA)wasutilisedtodetermine
whetherseveralgroupsofafactorhaveacommonmean.ANOVAtestsfordifferences
betweengroupmeansbypartitioningvariationinthedataintotwocomponents:variation
ofindividualobservationsfromtheirgroupmean;and,variationofthegroupmeansfrom
theoverallmean(WuandHamada,2000;Neteretal,1996).IfduringtheANOVAtest,the
p-valueoftheF-statistic(ratioofthemeansquares)issmallerthanthesignificancelevel
(0.05),thetestrejectsthenullhypothesis,thatallgroupmeansareequal,andoneofthe
groupmeansisthereforedifferentfromtheothers.WheretheANOVAtestisusedthe
meansofeachgroup(avg),theF-statistic(F)andthep-valuewillbecited.Intheeventthe
nullhypothesisisrejectedamultiplecomparisontestiscarriedout.Thiswilldetermine
whichpairsofmeansaresignificantlydifferentfromeachother.
40
2.3Results
2.3.1Macroalgae
Table1
RheniumandosmiumabundanceandisotopedataforIcelandicmacroalgaesamples
TheReandOsabundanceandisotopedataformacroalgaearegiveninTable1.Rhenium
andOsabundancesshowalargerangefrom0.1to88.4ppbandfrom3.3to254.5ppt
respectively.Individualspecies,suchasFucusvesiculosus,Fucusspiralis,Fucusdistichus
andAscophyllumnodosumshowvariableReabundancesfrom3.6to71.9ppb,14.5to28.8
ppb,0.10to1.6ppband3.2to88.4ppb,respectively(Fig.3a).Individualspecies,suchas
Fucusvesiculosus,Fucusspiralis,FucusdistichusandAscophyllumnodosumalsoshowa
largerangeinOsabundancesfrom5.0to254.5ppt,3.3to14.5ppt,4.7to7.6pptand9.5
to53.0ppt,respectively(Fig.3a).The187Os/188Oscompositionsofthemacroalgaerange
1 Fucusvesiculosus 44.32 2.53 27.85 0.10 7879.7 452.6 0.337 0.0032 Fucusspiralis 14.52 0.57 8.81 0.07 8534.2 342.4 0.700 0.0203 Fucusvesiculosus 38.04 1.50 5.03 0.04 39405.6 1646.4 0.750 0.0304 Fucusvesiculosus 53.37 2.10 23.42 0.10 11864.8 471.8 0.750 0.0105 Fucusvesiculosus 40.52 2.32 44.92 0.16 4483.7 257.4 0.368 0.0036 Fucusvesiculosus 23.56 0.93 106.42 0.42 1087.5 43.5 0.270 0.0107 Ascophyllumnodosum 3.17 0.01 53.04 0.41 307.1 4.3 0.634 0.0128 Fucusvesiculosus 71.92 0.23 10.14 0.04 37751.4 211.2 0.926 0.0069 Fucusvesiculosus 15.39 0.05 6.93 0.06 11692.8 180.0 0.835 0.01810 Fucusvesiculosus 61.68 0.20 13.71 0.05 23395.3 120.6 0.736 0.00411 Fucusvesiculosus 25.66 0.54 7.87 0.05 17023.4 381.1 0.765 0.00912 Fucusvesiculosus 3.58 0.01 13.73 0.36 1323.9 77.5 0.535 0.04413 Fucusvesiculosus 8.86 0.19 10.59 0.12 4358.6 130.2 0.750 0.02214 Fucusdistichus 0.11 0.03 4.69 0.10 123.5 34.6 0.970 0.05815 Fucusvesiculosus 14.65 0.05 9.21 0.09 8533.3 130.4 0.995 0.02116 Fucusvesiculosus 50.36 0.13 8.34 0.04 32096.9 237.1 0.913 0.00917 Fucusvesiculosus 43.93 0.11 7.72 0.04 30419.2 226.2 0.967 0.00918 Fucusdistichus 1.55 0.01 7.41 0.05 1107.6 10.8 0.879 0.01119 Fucusspiralis 28.83 0.08 14.45 0.08 10523.5 80.0 0.854 0.00920 Fucusdistichus 0.10 0.03 7.56 0.15 64.6 20.5 0.612 0.03621 Fucusspiralis 24.96 0.08 3.30 0.03 40320.4 683.7 0.938 0.02322 Ascophyllumnodosum 4.23 0.01 21.98 0.44 1022.7 41.8 0.925 0.05323 Fucusvesiculosus 60.27 0.15 8.24 0.06 38953.6 311.5 0.930 0.01424 Ascophyllumnodosum 88.43 0.29 13.83 0.28 34155.5 1422.1 0.962 0.05625 Fucusspiralis 18.15 0.06 6.45 0.04 14725.3 135.5 0.787 0.01026 Ascophyllumnodosum 63.96 3.65 9.45 0.04 35167.1 2017.9 0.727 0.00627 Fucusvesiculosus 9.58 0.55 254.47 1.69 182.3 10.7 0.161 0.003
2s.e. 2s.e. 2s.e.187Re/188Os 187Os/188Os2s.e.[Os](ppt)
[Re](ppb)MacroalgaeSpecies
SampleLocation
41
from0.16to0.99(Fig.3b).Thereason(s)forthevariabilityisdiscussedbelow.The
187Re/188Osratiosofthemacroalgaearehighlyvariable(~65to40,320;Fig.3b).
Fig.3.Rhenium(opendiamonds)andosmium(filledsquares)abundance(a)andisotopic
composition(b)forF.vesiculosus(green),F.spiralis(orange),F.distichus(purple)andA.
nodosum(blue).
Aone-wayANOVAtestwasconductedtodetermineiftherewasanystatistically
significantvariationinReabundance,Osabundance,187Re/188Osand187Os/188Osbetween
eachmacroalgaespecies.WhencomparingFucusvesiculosus,Fucusspiralis,Fucusdistichus
andAscophyllumnodosum,theANOVAtestsforReabundance(avg=35.4,21.6,0.6,39.9;
42
F=2.29;p-value=0.1054),Osabundance(avg=34.9,8.3,6.5,24.6;F=0.45;p-value=
0.7178),187Re/188Os(avg=16904,18525,431,17663;F=1.18;p-value=0.3385)and
187Os/188Os(avg=0.68,0.82,0.82,0.82;F=0.68;p-value=0.5744)allhaveap-values
whichfallabovethesignificancelevel(p-value=0.05).Wecanthereforeacceptthenull
hypothesisthateachspecieshasacommonmeanforeachparameterstudied.
2.3.2Bedload
Table2
RheniumandosmiumabundanceandisotopedataforIcelandicbedloads
SampleLoacation
[Re](ppb) 2s.e.
[Os](ppt) 2s.e. 187Re/188Os 2s.e. 187Os/188Os 2s.e.
7 0.73 0.02 41.21 0.18 85.6 2.1 0.157 0.002
8 0.91 0.03 7.23 0.04 646.3 23.0 0.591 0.007
9 0.89 0.02 15.60 0.07 277.8 6.6 0.231 0.003
10 1.11 0.04 9.13 0.04 593.03 20.98 0.228 0.003
11 0.89 0.03 8.81 0.04 494.3 17.6 0.250 0.003
12 0.55 0.02 24.99 0.12 108.02 3.97 0.337 0.004
15 0.69 0.02 23.71 0.11 143.2 5.2 0.292 0.004
18 0.51 0.02 10.42 0.05 238.6 8.8 0.224 0.003
19 1.03 0.04 12.52 0.06 405.5 14.4 0.271 0.003
20 0.61 0.02 11.41 0.05 259.96 9.45 0.220 0.003
27 0.63 0.02 69.05 0.31 44.2 1.6 0.204 0.003
TheReandOsabundanceandisotopicdataofbedloadsarepresentedinTable2.Rhenium
andOsabundancesrangefrom0.5to1.1ppband7.3to69.1pptrespectively.This
compareswelltoReandOsabundancespreviouslyrecordedinIcelandicbasalts,which
rangefrom0.05to1.8ppband3.7to1954.9pptrespectively(Debailleetal.,2009;
Gannounetal.,2006).MacroalgaeandbedloadretainsimilarlevelsofOswiththe
exceptionofthemacroalgaesamplefromlocation27(Fig.1),whichisheavilyenrichedin
Os(~254ppt;Fig.4a).MacroalgaeisenrichedinRebyseveralordersofmagnitudewhen
comparedtobedload.Asignificantcorrelation(R2=0.6061)inReabundanceisobserved
43
betweenmacroalgaeandthecorrespondingbedload(Fig.4b).The187Os/188Oscomposition
ofthebedloadrangesfrom0.16to0.59,withanaverageof0.27,whichisclosertothe
unradiogenicend-member.Thebedload187Os/188Osvaluesaremoreradiogenicthanthose
previouslyrecordedforIcelandicbasalts(Gannounetal.,2006),althoughdatainthisstudy
isfromtheolderextremitiesofIceland(Figs.1-2),wheremoreradiogenicingrowthof
187Oswillhaveoccurred.Inalmostallcases,the187Os/188Osratiosofmacroalgaeare
significantlymoreradiogenicthanthecorrespondingbedload(Fig.5a).The187Re/188Os
ratioofthebedloadrangesfrom44.2to646.3,whichfallclosetothelowerendofthe
rangereportedforIcelandicbasalts(45-1698;Gannounetal.,2006).Althoughthe
macroalgaepossessmuchgreater187Re/188Osthanthecorrespondingbedload,astrong
correlation(R2=0.7962)betweenthemacroalgaeandbedload187Re/188Osisobserved(Fig.
5b).
Fig.4.Bedloadosmium(a)andrhenium(b)abundanceanddissolvedloadosmium(c)and
rhenium(d)abundanceversusthecorrespondingabundanceinmacroalgae.Seetextfor
discussion.
44
2.3.3Dissolvedload
Table3
RheniumandosmiumabundanceandisotopedataforIcelandicdissolvedloads
SampleLoacation
[Re](ppt) 2s.e.
[Os](ppq) 2s.e. 187Re/188Os 2s.e. 187Os/188Os 2s.e.
Salinity(ppt) ±
7 1.9 0.2 69.2 0.3 130.99 11.07 0.160 0.002 0.140 0.001
8 10.0 0.7 9.7 0.1 5467.3 375.4 0.876 0.023 33.580 0.336
9 8.3 0.6 18.9 0.2 2229.6 152.8 0.569 0.017 29.680 0.297
11 4.5 0.3 11.2 0.1 2097.98 147.97 0.693 0.014 30.080 0.301
15 2.1 0.2 8.5 0.1 1184.03 105.84 0.230 0.009 0.220 0.002
19 19.4 1.3 750.7 3.0 125.4 8.4 0.168 0.002 23.780 0.238
20 0.6 0.1 8.1 0.1 365.7 60.8 0.198 0.007 0.000 0.001
27 3.8 0.3 30.1 0.2 613.3 43.9 0.237 0.006 9.170 0.092
TheReandOsabundanceandisotopicdataoffilteredwatersamplesarereportedinTable
3.RheniumandOsabundancesrangefrom0.6to19.4pptand8.1to750.7ppq
respectively.ThesevaluesarehighlyvariablewhencomparedtooceanicRe(~8.2ppt)
(Anbaretal.,1992;Colodneretal.,1995;Colodneretal.,1993b)andOs(~10ppq)
(GannounandBurton,2014;Levasseuretal.,1998;Sharmaetal.,1997;Woodhouseetal.,
1999)concentration.However,theycomparewellwithReandOsconcentrationsfoundin
globalriverestimateswhichrangefrom24ppqto2.3ppband4.6to52.1ppqrespectively
(Colodneretal.,1993a;Levasseuretal.,1999;Milleretal.,2011;SharmaandWasserburg,
1997),withtheexceptionofsample19(750.7ppq),whichhasanOsconcentrationseveral
ordersofmagnitudehigherthanforanywatereverrecorded.Withtheexceptionof
sample19,OsconcentrationsaresimilartopreviouslyrecordedIcelandicrivers(1.0-20.5
ppq;Gannounetal.,2006).MacroalgaearehighlyenrichedinReandOswhencompared
tothedissolvedloadoftheriversmeasuredhere(Figs.4c,d),inkeepingwithprevious
observationswherebymacroalgaeisseentotakeupandconcentratetheseelementsfrom
seawater(Racionero-Gómezetal.,2016;Racionero-Gómezetal.,2017).
45
Fig.5.Bedload187Os/188Os(a)and187Re/188Os(b)anddissolvedload187Os/188Os(c)and187Re/188Os(d)versusthecorrespondingratioinmacroalgae.Seetextfordiscussion.
The187Os/188Osratioofthedissolvedloadrangesfrom0.16to0.88.Thesevalues
arelessradiogenicthanopenocean187Os/188Osvalues(~1.06)(GannounandBurton,2014;
Levasseuretal.,1998;Sharmaetal.,1997;Woodhouseetal.,1999)fallingclosetothe
rangeofIcelandicrivers(0.15-1.04)(Gannounetal.,2006).Inalmostallcasesthe
187Os/188Osratiosofmacroalgaearesignificantlymoreradiogenicthanthecorresponding
dissolvedload(Fig.5c).Themostradiogenicvalueswereobtainedformacroalgaecloseto
theoldestbasaltintheeasternandnorth-westernpartsofIceland,whilethemost
unradiogenicisotoperatioswerefoundeitherclosetothecentralactivezoneortherivers
drainingit(Table1;Fig.1).The187Re/188Osratioofthedissolvedloadrangesfrom125.4to
5467.3.Theloweranduppervaluesofthisrangearesimilartoaverageglobalriverine(227)
andseawater(4270)187Re/188Osratios,respectively(Peucker-EhrenbrinkandRavizza,2000)
suggestingtheinfluenceofbothriverwaterandseawateratthesesamplinglocations.This
46
isconfirmedbysalinitydata,whichshowsaweakcorrelation(R2=0.469)with187Re/188Os
andgenerallyhigherratiosathighersalinityandviceversa(Table3).Themacroalgae
187Re/188Osaregenerallyseveraltimesgreaterthanthecorrespondingdissolvedload(Fig.
5d).Astrongcorrelation(R2=0.9088)in187Re/188Osvaluesisobservedbetween
macroalgaeandthedissolvedload(Fig.5d).
2.3.4Partitioncoefficient
Thepartitioncoefficient(Kd)forOsandReis:
Kd=X s
[X]l
where[X]sisthetotalelementalabundanceformacroalgae,[X]listheelemental
abundanceinthedissolvedloadofseawater,andXreferstotheelementinquestion.The
Kdvaluesforsamplelocationswherebothmacroalgaeandseawatermeasurementsare
recordedintable4.
Table4
PartitioncoefficientsbetweenmacroalgaeandseawaterforReandOsabundance.
SampleLocation ReKd OsKd
7 1693 7678 7200 10499 1860 36611 5644 70215 7113 108419 1484 1920 156 93227 2536 8448
47
2.4Discussion
2.4.1BiologicalandenvironmentalcontrolsonReandOsuptakeinmacroalgae
RheniumandOsabundanceinmacroalgaeareconsistentwithpreviousstudies,showing
variationsbothbetweenspeciesandwithineachspeciesitself(Fig.3).Rhenium
abundancesinFucusvesiculosusfromsamplelocation3,8,10and24(Fig.1)comparewell
withvaluesof51.0to103.4ppbintheUK(Racionero-Gómezetal.,2016)and60.8to84.9
ppbinNorway(Masetal.,2005).However,mostFucusvesiculosusinthisstudyhavelower
concentrationsthanthosefromtheliterature,rangingfrom3.6to50.4ppb.Likewise,
FucusdisichusandAscophyllumnodosumshowalowerabundanceofRethanrecorded
valuesforCalifornia(Yang,1991)andGreenland(Rooneyetal.,2016),withtheexception
ofsample21.PreviousOsabundancedeterminationsforFucusvesiculosusfromtheUK
(33.8ppt;Racionero-Gómezetal.,2017)andAscophyllumnodosumfromGreenland(12.6
ppt;Rooneyetal.,2016)fallwithintherangesfoundinthisstudy.However,Osabundance
forFucusdistichusrecordedinGreenland(14.0ppt;Rooneyetal.,2016)isfarhigherthan
therangeforIcelandicFucusdistichusofthisstudy.
Ithaspreviouslybeensuggestedthattheageofthemacroalgae-duetoitsrapid
accumulationrates(Yang,1991)-andgeographicaldistribution-duetorelatively
ubiquitousoceanicReconcentrations(Masetal.,2005)-donotplayapartincontrolling
theReconcentrationinmacroalgae.However,seasonalvariations,thechemicalspeciesof
theReperrhenatecompound,fertilityandgrowth-mediaReconcentrationhaveallbeen
suggestedaspossiblecausesofRevariationinmacroalgae(Masetal.,2005;Racionero-
Gómezetal.,2016).Likewise,duetofastOsuptake(Racionero-Gómezetal.,2017)and
relativelyubiquitousoceanicOsconcentrations(Peucker-EhrenbrinkandRavizza,2000),it
isconceivablethatseasonalvariationsandgrowth-mediaOsconcentrationcouldcausethe
variationsinOsseeninmacroalgae(Racionero-Gómezetal.,2017).
48
Allsampleswerecollectedduringthesameseason(July-August)suggesting
seasonalitydoesplayanimportantroleinthevariationsinReandOsabundanceobserved
inthisstudy.AthighconcentrationsithasbeenfoundthatthespeciesofResaltdoesnot
greatlyaffectthelevelsofReincorporatedintomacroalgaeandbyhomogenisingthe
sample,aswasdoneinthisstudy,theinfluenceofhighRenon-fertiletipscanbemitigated
(Racionero-Gómezetal.,2016).However,culturingofmacroalgaeunderincreasing
seawaterReandOsconcentrationshasbeenshowntocauseanincreaseinuptakeinRe
(Racionero-Gómezetal.,2016)andOs(Racionero-Gómezetal.,2017)inmacroalgae.
AlthoughoceanicRe(8.2ppt)andOs(~10ppq)concentrationisseentobefairly
ubiquitous(Anbaretal.,1992;Colodneretal.,1993b;Levasseuretal.,1998;Sharmaetal.,
1997;Woodhouseetal.,1999),riverineRe(3.1ppt)andOs(9.1ppq)abundanceis
generallymuchlower,butalsohighlyvariable(Colodneretal.,1993a;Levasseuretal.,
1999;Milleretal.,2011;SharmaandWasserburg,1997).Therefore,thegeographical
distributioncouldplayanimportantroleifmacroalgaelivingonthecontinentalshelfinan
estuarinehabitatareincloseproximitytoafreshwatersource.Underestuarineconditions,
amixtureoffreshwaterandseawatercouldcausehighlyvariableReandOsabundances
throughatidalcycle,leadingtovariableabundancesinmacroalgae.
MacroalgaeinthisstudywerecollectedfromcoastalwaterssurroundingIceland,
generallyclosetothemouthsofmajorrivers(Figs.1-2).Previously,ithasbeenshownthat
ReconcentrationactsconservativelyintheAmazonestuarywithlowconcentrations(0.21
ppt)atlowsalinityandhighconcentrations(8.6ppt)athighsalinity(Colodneretal.,
1993a).Osmiumontheotherhand,hasbeenshowntobehavenon-conservativelyin
estuaries,experiencingremovalfromthewatercolumnatlowsalinitiesintemperateand
arcticestuaries(Levasseuretal.,2000;Turekianetal.,2007)andathighsalinitiesin
tropicalestuaries(Martinetal.,2001;Sharmaetal.,2007),suggestingpotentialOsremoval
atlowsalinitiesinIceland.Giventhatthemacroalgaestudiedheretendstoliveinbrackish
49
water,itwillinteractwithseawaterofvaryingsalinitythroughatidalcycle.Samples
locatedclosertoafreshwatersourcecouldthereforereceivelessReandOsthanadeeper
watersample,whichcanbeseeninsamples16,17and18(Table1;Fig.2b),whichshow
progressivelyhigherReandOsabundancesasyoumoveseaward.Thiscouldexplainsome
oftheRevariabilityinF.vesiculosus,whereassamples(7,9,11,12,13,14,15,18,20and
27;Table1;Fig.1)closetoriverineinputsgenerallyhavelowerReabundances(8.1ppb)
thanthosethatarenotspatiallynearriverineinputs(43ppb).However,Osdoesnotfollow
thistrendsuggestingthatsomeothermechanismscontrolmacroalgaeOsuptakeand
concentration.
TheOsabundanceofthedissolvedloadofwatermeasuredinthisstudy(8.1to
750.7ppq)compareswellwithglobalriverestimates(4.6-52.1ppq)(Levasseuretal.,
1999;SharmaandWasserburg,1997)withtheexceptionofsample19(750.7ppq),which
hasanOsabundanceseveralordersofmagnitudehigherthananyrecordedwater.This
highvariationinOsabundancecanbeattributedtotheinfluenceofIcelandicfreshwater
sources(1.0-20.5ppq),withlowerconcentrationsfoundinriversdrainingolder,more
radiogenic187Os/188Osbearingbasalticcatchments(1.9ppq)whencomparedtoyounger,
lessradiogenic187Os/188Osbasalticcatchments(10.5ppq)(Gannounetal.,2006).Thiscould
alsoexplainsomeofthevariationseeninthemacroalgaeOsabundance,wherebysamples
(13,14,15,16,17,18and20;Table1;Fig.1)foundclosetothemouthsofriversdraining
oldercatchmentshavelowerconcentrations(8.0ppt)thanthosedrainingyounger
catchments(153.8ppt;Sample:7and27;Table1;Fig.1).However,thisisfurther
complicatedbythepotentialinfluenceofgeothermalwatersthatpossessrelativelyhighOs
concentrations(19.7ppq;Gannounetal.,2006),whichcouldexplainthehigherOs
concentration(36.1ppt)foundinmacroalgaearoundtheReykjanespeninsula(Samples1-
6;Table1;Fig.2a).Thisissupportedbyexperimentalstudies,whichshownincreasingRe
50
andOsabundancesinculturedF.vesiculosuswithincreasingReandOsabundancesinthe
culturesmedia(Racionero-Gómezetal.,2016;Racionero-Gómezetal.,2017).
Fig.6.Partitionscoefficients(Kd)forOs(opensquares)andRe(closedcircles)forvarying
salinities.
RheniumandOspartitioncoefficientsbetweenmacroalgaeandlocalseawater
(Table4)showalargevariationwithsalinity(Fig.6).OsmiumKdremainsrelativelyhighat
bothhighandlowsalinities,butdecreasestowardsintermediatesalinities(Fig.6).Rhenium
showsvariableKdatlowsalinitiesandhighKdathighsalinity,butdecreasestowards
intermediatesalinities(Fig.6).ThissuggestssalinitycontrolsonReandOsuptakeinto
macroalgaewithgreateruptakeatfreshwater(low)andseawater(high)salinity.Uptakeof
bothelementsisreducedatintertidalzone(intermediate)salinities.Thiscouldarisefor
severalreasonsinmacroalgaesuchas:growthrates;salinitystressorthe
conservative/unconservativebehaviourofReandOsinanestuarinehabitat.Growthrates
areknowntoincreaseatintermediatesalinities(15to20psu)insomemacroalgaespecies
(Martinsetal.,1999).However,ifReandOsuptakeislinkedtometabolicactivity,we
wouldexpecthigherabundancesinmacroalgaeatintermediatesalinitiesasmoreReand
Osareconfusedforelementsofasimilarionicradius(Racionero-Gómezetal.,2016;
0
1000
2000
3000
4000
5000
6000
7000
8000
0
200
400
600
800
1000
1200
0 5 10 15 20 25 30 35
ReK
d
OsK
d
Salinity(ppt)
51
Racionero-Gómezetal.,2017).Salinitystresscanbecausedbyfluctuatingsalinityinthe
fluidmediumwherebythemacroalgaetransportssalts,andthereforepotentiallyReand
Os,tomaintaintheircellularosmolality.Inmarineandfreshwatersystems,salinitywill
remainrelativelyconstantandthereforemacroalgaewillnotundergosalinitystress.
However,intheintertidalzone,salinitywillvarythroughatidalcycleandmacroalgaewill
thereforebeaffectedbysalinitystress.Thiscouldpotentiallypreventmacroalgaefrom
maintainingconstantReandOsuptakeandthereforeReandOsabundance(Fig.6).Aswill
bediscussedfurtherinsection2.4.2.1,Rehasbeenseentobehaveconservativelyinan
estuarinehabitatwhileOshasbeenobservedtobehavebothconservativelyor
unconservativelyinestuarinehabitats(Levasseuretal.,2000;Martinetal.,2001;Sharma
etal.,2007;Turekianetal.,2007;Milleretal.,2011).ThiscouldleadtohigherReuptake
highRe,highsalinity,seawater(Fig.6).Rheniumuptakedecreasesasyoumovelandwards
asmixingwithfreshwaterdrivestheconcentrationofRedowntowardsfreshwatervalues
(Fig.6).Osmiumontheotherhandhasbeenseentoactunconservativelyinestuarieswith
Osremovalatintermediatesalinitiesassedimentdispersalduringmixingcauses
scavengingontoparticulatematerial(Martinetal.,2001).ThiswouldacttolowerOs
abundanceinintertidalwatersandthereforelowerOsuptakeinmacroalgae(Fig.6).
However,Osabundanceinseawatermeasuredinthisstudyisnotseentovarywithsalinity
(Table3).
PreviousstudieshaveidentifiedthatReaccumulationinF.vesiculosusisvariable
acrossstructuralcomponents,indicatingsomecellsaremorespecialisedforReuptake
(Racionero-Gómezetal.,2016),whichisnotthecaseforOs(Racionero-Gómezetal.,
2017).AlthoughthebiologicalmechanismbywhichmacroalgaeextractReandOsfrom
theirmediaisnotknown,culturingstudieshaveshownthatmacroalgaedirectlytakeup
bothReandOsfromthedissolvedloadofseawaterthroughsyn-life
bioadsorption/bioaccumulation(Racionero-Gómezetal.,2016;Racionero-Gómezetal.,
52
2017).However,thesestudiesdidnotlookintoReandOsuptakeinmacroalgaefromthe
particulateorbedloadofseawater.Astrongcorrelationbetweenmacroalgaeand
dissolvedloadOsabundanceisclearfrompreviousstudies(Racionero-Gómezetal.,2017),
buttheabsenceofanycorrelationbetweenmacroalgaeandbedloadOsabundancefound
inthisstudy(Fig.4a)suggeststhatOsuptakeispurelyfromthedissolvedloadwithlittle
uptakedirectlyfromthebedload.However,macroalgaeReshowsacorrelationwithboth
thedissolved(Racionero-Gómezetal.,2016)andbedload(Fig4b).Thissuggestsa
potentialadditionalsourceofRetomacroalgaefromthebedload.However,ithasbeen
shownthattheReabundanceoftheholdfastinF.vesiculosusisrelativelylowwhen
comparedtotheotherbiologicalstructuresandthereforeReisprobablynottakenupfrom
thebedloadviathispathway(Racionero-Gómezetal.,2016).
Althoughnotmeasuredinthisstudy,ithasbeenshownthatReabundanceinthe
particulateloadofIcelandicriversiswellcorrelatedwiththerespectivebedload(Gannoun
etal.,2006).ThesimilarityofthebedloadReabundancemeasuredinthisstudy(0.5-1.1
ppb)toIcelandicrivers(0.3-1.8ppb;Gannounetal.,2006)couldsuggestapossible
correlationwithReintheparticulateloadatthesampledlocations.MacroalgaeRe
abundancecouldpotentiallybecorrelatedwiththeparticulateloadsuggestingdifferent
biologicalpathwaysfortheuptakeofOsandReinmacroalgae.
2.4.2Environmentalcontrolsonthe187Os/188Osofmacroalgae
2.4.2.1Influenceofestuarineconditionsonthe187Os/188Osofmacroalgae
Ithasbeenshownthatthe187Os/188Osoffloatingmacroalgae(SargassumfluitansandS.
natans)fromtheGulfofMexico(Rooneyetal.,2016)areindistinguishablefromthatofthe
presentdayoceanic187Os/188Osvalueof1.06(Peucker-EhrenbrinkandRavizza,2000).In
contrast,macroalgaefromwatersoffthewestcoastofGreenlanddeviatefromthisvalue
53
(0.9-1.9),insteadrecordinglocalcontinentalOsfluxintothecoastalregion(Rooneyetal.,
2016).Moreover,FucusvesiculosusfromanestuaryontheeastcoastoftheUKrecordsthe
187Os/188Osoftheseawaterinwhichitlives(0.94)andwhenculturedinseawaterdoped
withOsofaknown187Os/188Oscomposition(0.16),takesonthecompositionofthenew
source(Racionero-Gómezetal.,2017).Ithasthereforebeenpostulatedthatthe
187Os/188Osofmacroalgaecanactasaproxyforthe187Os/188Osoftheseawaterinwhichit
lives.
The187Os/188Osofmacroalgaefromthisstudy(0.16-1.0;Fig.3b)showsasimilar
rangetothe187Os/188OsofdissolvedOsinIcelandicrivers(0.15-1.04;Gannounetal.,
2006).However,whenwecomparethe187Os/188Osofmacroalgaewiththatofthe
dissolvedloadfromthesamelocationweseenorelationship(Fig.5c).Thisisprobablydue
totheentrainmentofseawaterinanestuarinehabitatfromwhichthesemacroalgaeand
watersamplesweretaken.Underestuarineconditionsmacroalgaewillcomeintocontact
withvaryingamountsoffreshwaterandseawaterthroughatidalcycle.Itscompositionwill
thereforerepresentamixingbetweenthe187Os/188Osofalocalfreshwatersourceandthe
187Os/188OsofNorthAtlanticseawater(1.02;GannounandBurton,2014).Macroalgaefrom
deeperwaters,withlittlefreshwaterinfluence,aremorelikelytotakeonan187Os/188Os
compositionsimilartoseawater(Rooneyetal.,2016;thisstudy),unliketheirshallowwater
counterparts(Racionero-Gómezetal.,2017;thisstudy).
Thisisapparentwhenthe187Os/188Osofthedissolvedloadfromthisstudy(0.16-
0.88)isplottedagainstsalinity(Fig.7).Atlowsalinity,the187Os/188Osofthefreshwater
sourceisdominantandvaluesareclosertothatexpectedforIcelandicrivers(Avg.=0.4;
Gannounetal,2006),whileathighsalinitythe187Os/188OsisclosertoaNorthAtlantic
source(Avg.=1.02;GannounandBurton,2014;GreensquareinFig.7).Foreachlocation,
macroalgaeinhabitwaterswithanintermediatesalinitybetweenthesetwoend-members
54
(blacksquaresinFig.7),andthereforethe187Os/188Osrepresentsamixtureofboth
freshwaterandseawatersources.Thisissupportedbyarctic,temperateandtropical
estuaries,whichshowanincreasingseawater187Os/188Osinfluenceasyoumoveoceanward
tohighersalinities(Levasseuretal.,2000;Martinetal.,2001;Sharmaetal.,2007;Turekian
etal.,2007).
Fig.7.Osmiumisotopic(187Os/188Os)compositionversussalinity.Blueandgreenopen
squaresrepresentvaluesforthedissolvedload(thisstudy)andNorthAtlanticseawater
(GannounandBurton,2014)respectively.Salinityhasbeeninterpolatedbasedon187Os/188Osmeasurementsformacroalgae(openblacksquares).Macroalgaegenerallyplots
atintermediatesalinitiesalongamixinglinebetweenthecorrespondingdissolvedloadfor
eachlocationandestimatesfortheNorthAtlantic.Seetextfordiscussion.
2.4.2.2Influenceofbasalticweatheringonthe187Os/188Osofmacroalgae
Duringmantlemeltingandbasaltgenesis,bothReandOsbecomefractionated.Osmium
behavesasacompatibleelementduringmeltingandisretainedinthemantle,unlikeRe,
whichismoderatelyincompatibleandentersthemelt.Mantlederivedbasaltsthushave
veryhighRe/OsvaluesandtheirprimarymineralphasescrystalliseinahighRe/Os
environment(Burtonetal.,2002;Gannounetal.,2004).Primarymineralssuchasolivine,
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40
187 O
s/18
8 Os
Salinity(ppt)
55
pyroxeneandplagioclasepossesshigher187Re/188Osthanbulk-rockorglass,andovershort-
timescales(<106yrs)canproducemeasurableshiftsinradiogenicosmium(187Osr)(SeeFig.
9inGannounetal.,2006).
InIceland,riversdrainingoldercatchments(>106yrs)areundersaturatedwith
respecttothesehigh187Re/188Osbearingminerals(olivine,pyroxeneandplagioclase)and
preferentialweatheringofthesephasesfromthehostbasalt,combinedwiththeirage,
whichallowsforradiogenicingrowthof187Osfromthedecayof187Re,causeselevated
radiogenic187Os/188Osinthedissolvedload(Gannounetal.,2006).Incontrast,rivers
drainingyoungercatchments(<106yrs)areapproachingsaturationwithrespecttothese
sameminerals,whichinthisinstancehavenothadtimetoattainsignificantradiogenic
ingrowthof187Os,andweatheringtendstowardscongruencycausingthedissolvedloadto
approachthatofbulk-rock(Gannounetal.,2006).
Macroalgae187Os/188Osvaluesinthisstudy(0.16–0.99)aregenerallymuch
lowerthanthe187Os/188Osratiosof0.81to1.89previouslyrecorded(Rooneyetal.,2016;
Racionero-Gómezetal.,2017).However,macroalgaeinthisstudyreachsimilarvaluesto
culturesdopedwiththeOsstandardsolution,DROsS(Racionero-Gómezetal.,2017),
suggestingthatanunradiogenic187Os/188OssourceoftheOstakenupbythemacroalgae.
Whenthe187Os/188Osofmacroalgaeisplottedagainstthereciprocaloftheconcentration,
allthedatafallwithinafielddelimitedbythreepotentialend-members(Fig.8a):Seawater
(radiogenic187Os/188Os,intermediate[Os]);riverwaterdraininganoldbasalticcatchment
(lessradiogenic187Os/188Os,low[Os]);and,geothermalwaterandriverwaterdraininga
youngbasalticcatchment(unradiogenic187Os/188Os,high[Os]).Variationsinthe187Os/188Os
ofmacroalgaecanthereforebeexplainedbythemixingbetweenradiogenicseawaterand:
relativelyless-radiogenic187Os/188Osbearingrivers(BluesquaresinFig.8a)drainingan
oldercatchmentwhichhasundergoneincongruentweatheringofhigh187Re/188Os,and
56
thereforeradiogenic187Os/188Os,minerals;orunradiogenic187Os/188Osbearingrivers(Pink
circlesinFig.8a)drainingyoungercatchmentswhichhaveundergonecongruent
weatheringofmineralswithunradiogenic187Os/188Os.Samplesclosetoglaciallyfedrivers
(Samples9and10;Fig.1)arelikelytoattainaradiogenic187Os/188Ossignal(Orange
diamondsinFig.8a)duetotheentrainmentofseawaterintoglacialice(Gannounetal.,
2006).
Fig.8.Osmiumisotopic(187Os/188Os)compositionagainstthereciprocaloftheOs
abundancefor(a)macroalgaesampledcloseto:riversdrainingayoungbasalticterrainor
geothermalwaters(pinkcircles);riversdraininganoldbasalticterrain(bluesquares);and,
Holocenesediments(orangediamonds)and(b)F.vesiculosuswheretheOsabundanceis
convertedtothatofseawater(blacksquares)usingtherelationshipfoundinRacionero-
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
187 O
s/18
8 Os
1/[Os](ppt)
Seawater
River(old)
River(young)
b
57
Gómezetal.(2017).Athree-pointend-membermixingdiagrambasedonextremeNorth
Atlanticseawater(Green;GannounandBurton,2014)andriversdrainingayoungbasaltic
catchment(pink;Gannounetal,2006)andoldbasalticcatchments(blue,Gannounetal,
2006).Seetextfordiscussion.
ThisrelationshipbecomesmoreapparentwhentheOsabundanceinF.
vesiculosusisconvertedtotheOsabundanceinseawaterusingtherelationshipfoundin
Racionero-Gómezetal.(2017;y=0.0004x1.6607,R²=0.9898),withitsreciprocalplotted
againstthe187Os/188OsofF.vesiculosus,andthencomparedtoanidealised3-pointend-
membermixingmodel(Fig.8b).Datafalluponeither:amixinglinebetweenseawater
(187Os/188Os=1.02,[Os]=10ppq)anddirect-runoffriversdrainingoldcatchments
(187Os/188Os=0.6,[Os]=3.65ppq);amixinglinebetweenseawateranddirect-runoffrivers
drainingyoungriversorgeothermalwaters(187Os/188Os=0.14,[Os]=22.7ppq);or,closeto
thevalueforseawateritself(Fig.8b).However,thedataisoffsetinconcentrationspace
fromanidealisedmixingmodel.Thisismostlikelycausedbytheconversionrateused,
whichwasdevelopedbydopingmacroalgaeunderexceptionallyhighseawaterOs
concentrations(Racionero-Gómezetal.,2017).Moreworkisneededtoascertainhow
macroalgaeculturesbehaveatlowerOsconcentrations,moreakintothosefoundin
nature,forF.vesiculosusandothercommonmacroalgaespecies.
2.4.3Biologicalandenvironmentalcontrolson187Re/188Osofmacroalgae
The187Re/188Osratiosofmacroalgaeshowawiderangefrom~64to40,320(Fig.3b)similar
topreviousstudies(6.8to31,983;Rooneyetal.,2016,Racionero-Gómezetal.,2017),but
farexceedingthatfoundinglobalseawater(avg.187Re/188Os=4270;Peucker-Ehrenbrink
andRavizza,2000),globalriverwater(avg.187Re/188Os=227;Peucker-Ehrenbrinkand
Ravizza,2000)andthedissolvedloadofwatersmeasuredinthisstudy(125-5467).As
58
previouslydiscussedthismaybeduetothedifferentsourcesofReandOsinmacroalgae.
Sampleslocatedclosetoayoungbasalticterrain(PinkcirclesinFig.9a)arecharacterised
byrelativelyunradiogenic187Os/188Osandlow187Re/188Oscompositions,astheirisotopic
signatureiscontrolledbygeothermalandriverwatersdominatedbycongruentweathering
ofbasalticbed-rockwithrelativelyunradiogenic187Os/188Os(0.12-0.18)andlow
187Re/188Os(23.8-1310)compositions.Sampleslocatedclosetotheolderbasalticterrains
(BluesquaresinFig.8a)exhibitradiogenic187Os/188Osandhigh187Re/188Osratios,astheir
isotopicsignatureisdominatedbyriverwatersthathavebeeninfluencedbythe
preferentialweatheringofoldprimarybasalticmineralssuchasolivine,clinopyroxeneand
plagioclase,whichhaverelativelyradiogenic187Os/188Os(0.13-0.25whichhasundergone
radiogenicingrowth)andhigh187Re/188Os(288-7164)ratios(Burtonetal.,2002;Gannoun
etal.,2004;Gannounetal.,2006).Theinfluenceofseawater,witharelativelyradiogenic
187Os/188Os(1.02)andintermediate187Re/188Os(4270;Peucker-EhrenbrinkandRavizza,
2000),willpullthemacroalgaevaluestowardsaseawaterend-member(SeeFig.9).
Macroalgaethereforeholdthepotentialtonotonlyrecordthe187Os/188Osoftheseawater
inwhichtheylive,butalsotherelative187Re/188Osratiooftheseawatersources.Thisis
confirmedbythestrongrelationshipbetweenthe187Re/188Osofmacroalgaeandthe
dissolvedloadmeasuredinthisstudy(Fig5d).
Despitethisrelationship,macroalgaecanstillexhibit187Re/188Osratiosgreater
thanexpectedforIcelandicgeochemicalreservoirs.Ithasbeenproposedthatmacroalgae
uptakeReandOsthroughthesamemechanismleadingtocompetitionbetweenthese
elementsandthereforelowerReconcentrationinmacroalgaeunderhigherOsseawater
concentrationandviceversa(Racionero-Gómezetal.,2017).Thisisillustratedwhenthe
187Re/188OsratioofmacroalgaeisplottedagainstthereciprocaloftheReconcentration
(Fig.9b).AtlowmacroalgaeReconcentration(<10ppb),thereisnocompetitionbetween
ReandOsforuptakeintomacroalgae,and187Re/188Osratiosremainconsistentlylow
59
(<5,000).AstheReconcentrationrisesabove~10ppb,ReisfavouredoverOsasthetwo
elementsbegintocompeteforuptakeintomacroalgaeleadingtoanincreasein187Re/188Os
ratios(5,000-15,000).AtexceptionallyhighReconcentration(>25ppb),Recontinuestobe
takenupandthemacroalgaebecomesenrichedinRe,leadingtoexceptionallyhigh
187Re/188Osratios(15,000-40,000).
Fig.9.(a)187Re/188Osversus187Os/188Osand(b)187Re/188OsagainstthereciprocalofRe
abundanceformacroalgaesampledcloseto:riversdrainingayoungbasalticterrainor
geothermalwaters(pinkcircles);riversdraininganoldbasalticterrain(bluesquares);and,
Holocenesediments(orangediamonds).Seetextfordiscussion.
R²=0.74086
051015202530354045
0 2 4 6 8 10 12
187 Re/
188 O
sx103
1/[Re](ppb)
b
05
1015202530354045
0.0 0.2 0.4 0.6 0.8 1.0 1.2
187 Re/
188 O
sx103
187Os/188Os
a
60
ThiscouldbefurtherexacerbatedbyadditionaluptakeofRefromthebedload
and/orparticulateloadleadingtoevenhigherReconcentrationrelativetoOsin
macroalgae(Fig.4b),ortheadditionofanother187Re/188Ossignalimpartedfromthe
bedload(Fig.5b).ThesebiologicalcontrolsonReandOsuptakecouldpossiblybethe
causeofthe187Re/188Osratiosinmacroalgaeclosetotheoutflowofolderrivercatchments
beingfarhigherthanthoseobservedforrecordedprimarybasalticminerals(Fig.9a).
2.5Implicationsandfutureoutlook
TheRe-Osdataformacroalgaepresentedherehavebeensuccessfullyusedtotracethe
influenceofbasalticweatheringonthe187Os/188Osand187Re/188OsofIcelandiccoastal
watersandsubsequentmixingwithmoreradiogenicseawater.Geothermalwaterand
riversdrainingyoungbasalticcatchmentsimpartanunradiogenic187Os/188Oscomposition
andalsoarelativelylow187Re/188Ossignaltocoastalwaters.Riversdrainingolderbasaltic
catchments,wherepreferentialweatheringofprimarybasalticmineralshighinRe/Os
ratios,andwhichhavehadsufficienttimeforthedecayof187Retoform187Osr,impartsa
moreradiogenic187Os/188Oscompositionandrelativelyhigh187Re/188Ossignaltocoastal
waters.Thisprovidesfurtherevidencetosupporttheuseofthe187Os/188Osofmacroalgae
asaproxyforthe187Os/188Osofseawater,overcomingthedifficultiesassociatedwithdirect
seawateranalysisandpotentiallybecomingausefultoolfortracingavarietyofEarth
systemprocesses.
Ifsamplesobtainedatthemouthofariverrepresentstheentiredrainagearea
(Milleretal.,2011),thenitispossibletocalculatetheentirequantityandisotopic
compositionofdissolvedOsthatissuppliedtotheNorthAtlanticfromIcelandicrivers
usingmacroalgae.TheriverscontainingF.vesiculosusstudiedhaveanannualdischargeof
2.3km3/yr,accountingfor1.3%ofthetotaldischargeofIcelandicrivers(175km3/yr).Given
61
theOsabundanceofF.vesiculosus,whichhasbeenconvertedtothatofseawaterandthen
offsettobemorerepresentativeofIcelandicgeochemicalreservoirs(SeeFig.8b),we
estimateadissolvedOsfluxof2.35kg/yr.Thesevaluesarefarhigherthanpreviously
recordedestimatesforIcelandicriversof0.98kg/yr(Gannounetal.,2006).Someofthe
discrepancybetweenthesetwovaluesislikelyduetotheunderrepresentationofIcelandic
riversinthisstudy(1.3%)whencomparedtopreviousstudies(21%)(Gannounetal.,2006),
butalsosuggeststhatalthoughthisnewlydevelopedproxyoffersthepotentialtobetter
constrainthe187Os/188Oscompositionofglobalriverineinputs,moreworkisneededto
understandtheuptakerateofOsbymacroalgaeatnaturallevelsinordertoabetter
estimatetheglobalriverineabundance.Ifthiscanbedone,macroalgaecouldholdthe
potentialtoyieldabetterunderstandingoftheglobalOscycleandoceanicresidence
times.
ItisalsopossibletoestimatetheentirequantityofReandOstakenupby
macroalgaefromIceland.Thefourmacroalgaespeciesstudiedhereaccountfor46%ofthe
totalbiomassofcommonmacroalgaespeciesfoundaroundIceland(Munda,1987).Given
theaverageReandOsabundanceforeachspecies,wecalculate4.3to14.1kgofReand
3.3to11.8gofOsareabsorbedbythesemacroalgaeovertheirlifetime.Thisisrather
insignificantwhencomparedtotheIcelandicriverinputofOstotheoceanasdescribed
previously.However,ithasbeenshownthatupondeathmacroalgaedoesnotappreciably
looseRe(Racionero-Gómezetal.,2016).IfthisisthesameforOs,and6to14%ofglobal
macroalgae(6084Tg/yr)(Krause-JensenandDuarte,2016)survivesdecompositionandis
sequesteredtosediment,thenmacroalgaecouldactasasinkof0.03to89kg/yrofReand
0.001to0.2kg/yrforOs,whenbasedontherangeofcompositionsofallmacroalgae
speciesstudiedtodate(Masetal.,2005;Racionero-Gómezetal.,2016;Racionero-Gómez
etal.,2017;Rooneyetal.,2016;Yang,1991).Thisisinsignificantwhencomparedto
estimatesforglobalsedimentsinksof18to29x103kg/yrand260to1350kg/yrforReand
62
Os,respectively(MorfordandEmerson,1999;Oxburgh,2001),suggestingmacroalgae
alonedoesnothaveasignificantcontrolintheglobalmarineReandOscycle,bothtoday
andinEarth’sgeologicalpast.
2.6References
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Armstrong,R.L.,1971.Glacialerosionandthevariableisotopiccompositionofstrontiuminseawater.Nature230,132-133.
Berner,R.A.,Lasaga,A.C.,Garrels,R.M.,1983.Thecarbonate-silicategeochemicalcycleanditseffectonatmosphericcarbondioxideoverthepast100millionyears.AmJSci283,641-683.
Birck,J.L.,Barman,M.R.,Capmas,F.,1997.Re-Osisotopicmeasurementsatthefemtomolelevelinnaturalsamples.Geostandardsnewsletter21,19-27.
Burton,K.W.,Gannoun,A.,Birck,J.-L.,Allègre,C.J.,Schiano,P.,Clocchiatti,R.,Alard,O.,2002.Thecompatibilityofrheniumandosmiuminnaturalolivineandtheirbehaviourduringmantlemeltingandbasaltgenesis.EarthandPlanetaryScienceLetters198,63-76.
Chen,C.,Sharma,M.,2009.Highprecisionandhighsensitivitymeasurementsofosmiuminseawater.Analyticalchemistry81,5400-5406.
Cohen,A.S.,Waters,F.G.,1996.Separationofosmiumfromgeologicalmaterialsbysolventextractionforanalysisbythermalionisationmassspectrometry.AnalyticaChimicaActa332,269-275.
Colodner,D.,Edmond,J.,Boyle,E.,1995.RheniumintheBlackSea:comparisonwithmolybdenumanduranium.EarthandPlanetaryScienceLetters131,1-15.
Colodner,D.,Sachs,J.,Ravizza,G.,Turekian,K.,Edmond,J.,Boyle,E.,1993a.Thegeochemicalcycleofrhenium:areconnaissance.EarthandPlanetaryScienceLetters117,205-221.
Colodner,D.C.,Boyle,E.A.,Edmond,J.M.,1993b.Determinationofrheniumandplatinuminnaturalwatersandsediments,andiridiuminsedimentsbyflowinjectionisotopedilutioninductivelycoupledplasmamassspectrometry.AnalyticalChemistry65,1419-1425.
Creaser,R.A.,Papanastassiou,D.A.,Wasserburg,G.J.,1991.Negativethermalionmassspectrometryofosmium,rheniumandiridium.GeochimicaetCosmochimicaActa55,397-401.
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Cumming,V.M.,Poulton,S.W.,Rooney,A.D.,Selby,D.,2013.AnoxiaintheterrestrialenvironmentduringthelateMesoproterozoic.Geology41,583-586.
Debaille,V.,Trønnes,R.G.,Brandon,A.D.,Waight,T.E.,Graham,D.W.,Lee,C.-T.A.,2009.Primitiveoff-riftbasaltsfromIcelandandJanMayen:Os-isotopicevidenceforamantlesourcecontainingenrichedsubcontinentallithosphere.GeochimicaetCosmochimicaActa73,3423-3449.
Gannoun,A.,Burton,K.,2014.HighprecisionosmiumelementalandisotopemeasurementsofNorthAtlanticseawater.JAAS.
Gannoun,A.,Burton,K.W.,Thomas,L.E.,Parkinson,I.J.,vanCalsteren,P.,Schiano,P.,2004.Osmiumisotopeheterogeneityintheconstituentphasesofmid-oceanridgebasalts.Science303,70-72.
Gannoun,A.,Burton,K.W.,Vigier,N.,Gíslason,S.R.,Rogers,N.,Mokadem,F.,Sigfússon,B.,2006.Theinfluenceofweatheringprocessonriverineosmiumisotopesinabasalticterrain.EarthandPlanetaryScienceLetters243,732-748.
Huh,Y.,Birck,J.-L.,Allègre,C.J.,2004.OsmiumisotopegeochemistryintheMackenzieRiverbasin.EarthandPlanetaryScienceLetters222,115-129.
Ishikawa,A.,Senda,R.,Suzuki,K.,Dale,C.W.,Meisel,T.,2014.Re-evaluatingdigestionmethodsforhighlysiderophileelementand187Osisotopeanalysis:Evidencefromgeologicalreferencematerials.ChemicalGeology384,27-46.
Jóhannesson,H.,2014.GeologicalMapofIceland.1:600000.BedrockGeology.IcelandicInstituteofNaturalHistory,Reykjavík(2ndedition).
Krause-Jensen,D.,Duarte,C.M.,2016.Substantialroleofmacroalgaeinmarinecarbonsequestration.NatureGeosci9,737-742.
Levasseur,S.,Birck,J.-L.,Allegre,C.,1999.Theosmiumriverinefluxandtheoceanicmassbalanceofosmium.EarthandPlanetaryScienceLetters174,7-23.
Levasseur,S.,Birck,J.-L.,Allègre,C.J.,1998.Directmeasurementoffemtomolesofosmiumandthe187Os/186Osratioinseawater.Science282,272-274.
Levasseur,S.,Rachold,V.,Birck,J.-L.,Allegre,C.,2000.Osmiumbehaviorinestuaries:theLenaRiverexample.EarthandPlanetaryScienceLetters177,227-235.
Martin,C.E.,Peucker-Ehrenbrink,B.,Brunskill,G.,Szymczak,R.,2001.Osmiumisotopegeochemistryofatropicalestuary.GeochimicaetCosmochimicaActa65,3193-3200.
Mas,J.,Tagami,K.,Uchida,S.,2005.RheniummeasurementsonNorthAtlanticseaweedsamplesbyID-ICP-MS:anobservationontheReconcentrationfactors.JournalofRadioanalyticalandNuclearChemistry265,361-365.
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Miller,C.A.,Peucker-Ehrenbrink,B.,Walker,B.D.,Marcantonio,F.,2011.Re-assessingthesurfacecyclingofmolybdenumandrhenium.GeochimicaetCosmochimicaActa75,7146-7179.
Morford,J.L.,Emerson,S.,1999.Thegeochemistryofredoxsensitivetracemetalsinsediments.GeochimicaetCosmochimicaActa63,1735-1750.
Munda,I.M.,1987.DistributionanduseofsomeeconomicallyimportantseaweedsinIceland,TwelfthInternationalSeaweedSymposium.Springer,pp.257-260.
Nowell,G.,Luguet,A.,Pearson,D.,Horstwood,M.,2008.Preciseandaccurate186Os/188Osand187Os/188Osmeasurementsbymulti-collectorplasmaionisationmassspectrometry(MC-ICP-MS)partI:Solutionanalyses.ChemicalGeology248,363-393.
Oxburgh,R.,2001.Residencetimeofosmiumintheoceans.Geochemistry,Geophysics,Geosystems2,1018.
Paul,M.,Reisberg,L.,Vigier,N.,2009.Anewmethodforanalysisofosmiumisotopesandconcentrationsinsurfaceandsubsurfacewatersamples.ChemicalGeology258,136-144.
Peucker-Ehrenbrink,B.,Ravizza,G.,2000.Themarineosmiumisotoperecord.TerraNova12,205-219.
Peucker-Ehrenbrink,B.,Ravizza,G.,2012.Osmiumisotopestratigraphy.TheGeologicTimeScale2012,145-166.
Peucker-Ehrenbrink,B.,Sharma,M.,Reisberg,L.,2013.Recommendationsforanalysisofdissolvedosmiuminseawater.Eos,TransactionsAmericanGeophysicalUnion94,73-73.
Prouty,N.G.,Roark,E.B.,Koenig,A.E.,Demopoulos,A.W.,Batista,F.C.,Kocar,B.D.,Selby,D.,McCarthy,M.D.,Mienis,F.,Ross,S.W.,2014.Deep-seacoralrecordofhumanimpactonwatershedqualityintheMississippiRiverBasin.GlobalBiogeochemicalCycles28,29-43.
Racionero-Gómez,B.,Sproson,A.,Selby,D.,Gröcke,D.,Redden,H.,Greenwell,H.,2016.Rheniumuptakeanddistributioninphaeophyceaemacroalgae,Fucusvesiculosus.RoyalSocietyOpenScience3,160161.
Racionero-Gómez,B.,Sproson,A.D.,Selby,D.,Gannoun,A.,Gröcke,D.R.,Greenwell,H.C.,Burton,K.W.,2017.Osmiumuptake,distribution,and187Os/188Osand187Re/188OscompositionsinPhaeophyceaemacroalgae,Fucusvesiculosus:Implicationsfordeterminingthe187Os/188Oscompositionofseawater.GeochimicaetCosmochimicaActa199,48-57.
Raymo,M.E.,Ruddiman,W.F.,Froelich,P.N.,1988.InfluenceoflateCenozoicmountainbuildingonoceangeochemicalcycles.Geology16,649-653.
Richter,F.M.,Turekian,K.K.,1993.Simplemodelsforthegeochemicalresponseoftheoceantoclimaticandtectonicforcing.EarthandPlanetaryScienceLetters119,121-131.
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Rooney,A.D.,Selby,D.,Lloyd,J.M.,Roberts,D.H.,Lückge,A.,Sageman,B.B.,Prouty,N.G.,2016.Trackingmillennial-scaleHoloceneglacialadvanceandretreatusingosmiumisotopes:InsightsfromtheGreenlandicesheet.QuaternaryScienceReviews138,49-61.
Sharma,M.,Balakrishna,K.,Hofmann,A.W.,Shankar,R.,2007.ThetransportofOsmiumandStrontiumisotopesthroughatropicalestuary.GeochimicaetCosmochimicaActa71,4856-4867.
Sharma,M.,Chen,C.,Blazina,T.,2012.Osmiumcontaminationofseawatersamplesstoredinpolyethylenebottles.LimnologyandOceanography:Methods10,618-630.
Sharma,M.,Papanastassiou,D.,Wasserburg,G.,1997.Theconcentrationandisotopiccompositionofosmiumintheoceans.GeochimicaetCosmochimicaActa61,3287-3299.
Sharma,M.,Wasserburg,G.,1997.Osmiumintherivers.Geochimicaetcosmochimicaacta61,5411-5416.
Sharma,M.,Wasserburg,G.,Hofmann,A.,Chakrapani,G.,1999.Himalayanupliftandosmiumisotopesinoceansandrivers.GeochimicaetCosmochimicaActa63,4005-4012.
Turekian,K.K.,Sharma,M.,Gordon,G.W.,2007.ThebehaviorofnaturalandanthropogenicosmiumintheHudsonRiver–LongIslandSoundestuarinesystem.GeochimicaetCosmochimicaActa71,4135-4140.
Völkening,J.,Walczyk,T.,Heumann,K.G.,1991.Osmiumisotoperatiodeterminationsbynegativethermalionizationmassspectrometry.InternationalJournalofMassSpectrometryandIonProcesses105,147-159.
Walker,J.C.G.,Hays,P.B.,Kasting,J.F.,1981.Anegativefeedbackmechanismforthelong-termstabilizationofEarth'ssurfacetemperature.JournalofGeophysicalResearch86,9776.
Woodhouse,O.,Ravizza,G.,Falkner,K.K.,Statham,P.,Peucker-Ehrenbrink,B.,1999.Osmiuminseawater:verticalprofilesofconcentrationandisotopiccompositionintheeasternPacificOcean.EarthandPlanetaryScienceLetters173,223-233.
Yang,J.S.,1991.Highrheniumenrichmentinbrownalgae:abiologicalsinkofrheniuminthesea?Hydrobiologia211,165-170.
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Chapter3
TracinganthropogenicosmiumaroundJapanusingtheosmiumisotopic
compositionofmacroalgae*
*AversionofthischapterwillbesubmittedtoEnvironmentalScienceandTechnology,co-
authoredwithDavidSelbyandKatsuhikoSuzuki.
67
Thisstudypresentsrhenium(Re)andosmium(Os)abundanceandisotopedatafor
macroalgaefromJapanesecoastalwatersadjacenttodenselypopulatedmegacities
(Tokyo,OsakaandNagoya)andsparselypopulatedregionsofJapan(TheIzupeninsula,The
Notopeninsula,northernHonshuandHokkaido).The187Os/188Oscompositionof
macroalgaeishighlyvariable(0.16to1.09)andcanbeexplainedintermsofmixing
betweenseawaterand:riversdrainingMiocene-Holocenecontinentalrockswitha
radiogenic187Os/188Oscomposition;and/or,anthropogenicsourcesofOswithan
unradiogenic187Os/188Oscomposition.TheOsemittedfromcatalyticconvertorusein
vehiclespredominantlycontrolstheOsisotopebudgetofcoastalwatersindensely
populatedregionsofJapan.However,Osderivedfrommedicalresearch,municipalsolid
wasteincineratorsandsewageoutflowcangeneratelocalisedpointsourcesof
anthropogenicOs.AnthropogenicOsattributedtocoastalcities,whichistransferredvia
surfacewaterstoworld’soceans,representsasubstantialsourceofunradiogenicOswhich
isdrivingsurfaceseawatertolower187Os/188Osratios(~0.95)thandeeperwaters(~1.05).
ThissuggeststhedemandinPGEs,theirrefiningandeventualuseincatalyticconvertorsis
impactingtheglobalOscycle.Finally,thisstudysuggeststhatosmiumisotopesin
macroalgaecouldbecomeapowerfulenvironmentalindicatorandtracerofcontinental
inputstothemarineOscycle.
3.1Introduction
Theosmiumisotopiccomposition(187Os/188Os)ofseawaterreflectsabalancebetween
radiogeniccontinentalsourcesandunradiogenicmantleandextraterrestrialderived
sources,andhastraditionallybeenusedtotraceawiderangeofEarthsystemprocesses,
bothtodayandintheEarth’sgeologicalpast(Peucker-EhrenbrinkandRavizza,2000,2012).
However,humanactivityisperturbingthemarineOscycle(Chenetal.,2009)and
anthropogenicOscontaminationhasbeendetectedinestuaries(Turekianetal.,2007;
68
Williamsetal.,1997),coastalsediments(EsserandTurekian,1993;RavizzaandBothner,
1996),lakes(Rauchetal.,2004)andprecipitation(Chenetal.,2009)(SeeTable1).
PotentialsourcesofanthropogenicOstotheatmosphereandthemarineenvironment
include:(i)thecombustionoffossilfuels;(ii)smeltingofplatinumgroupelement(PGE:Os,
Ir,Pt,Pd,Ru,Rh)sulphideores;(iii)smeltingofbase-metal(Pb,Ni,Cu,andZn)sulphide
ores;(iv)smeltingofaluminiumore(v)smeltingofchromiumore;(vi)exhaustfrom
automobilecatalyticconvertors;(vii)emissionsfromincinerators;and,(viii)sewage
outflow(SeeTable1).
Table1
ComparisonofnaturalandanthropogenicsourcesofOsandRe.Datareferencesare:
seawater(Peucker-EhrenbrinkandRavizza,2000);riverwater(Peucker-Ehrenbrinkand
Ravizza,2000);precipitation(Chenetal.,2009);loess(Peucker-EhrenbrinkandJahn,2001);
cosmicdust(Peucker-EhrenbrinkandRavizza,2000);HThydrothermal(Peucker-Ehrenbrink
andRavizza,2000);LThydrothermal(Peucker-EhrenbrinkandRavizza,2000);fossilfuels
(SelbyandCreaser,2005;Selbyetal.,2007);base-metalsulphideores(Lambertetal.,
1998;Morganetal.,2002;Walkeretal.,1994);PGEores(McCandlessandRuiz,1991);
chromites(Walkeretal.,2002);aluminiumsmelter(Gogotetal.,2015);airborneparticles
(Rauchetal.,2005);urbansediments(Rauchetal.,2004);carcatalysts(Poirierand
Gariépy,2005);sewagesludge(EsserandTurekian,1993);MSWIashes(Funarietal.,2016).
[Os](fg/g) 187Os/188Os [Re](pg/g) 187Re/188Os
Natural
Seawater 10 1.06 8.2 4270
RiverWater 9.1 1.4 428 227
Precipitation 0.3-16.9 0.16-0.98
Loess 3.1x104 1.05 3x105 50
CosmicDust 5x108 0.13 3.7x107 0.4
HThydrothermal 2.8-38 0.13-0.39 LThydrothermal 98 0.11 Anthropogenic
FossilFuels 0.01-295x103 0.9-6 0.3-40.7x103 400-1300
Base-metalSulphideOre 0.02-89x106 0.13-24 0.6-286x103 0.8-5232
PGEOre
0.15-0.19
69
Chromites 2-1320x106 0.12 0.01-9.3x103 0.01-2.2
AluminiumSmelter 30x103 2.5
AirborneParticles 0.001-0.82pg/m3 0.3-2.8
UrbanSediments 56-395x103 0.36-2.2
CarCatalysts 6-228x103 0.16-0.19
SewageSludge 10-100x103 0.6-1.7
MSWIAshes 26-1650x103 0.24-0.7
Highlyradiogenicanthropogenic187Os/188Ossourcesincludethesmeltingof
bauxiteore(~2.5)andfossilfuels(1.1to13.7)(Finlayetal.,2011;Gogotetal.,2015;Selby
andCreaser,2005;Selbyetal.,2007).Incontrast,the187Os/188OsvaluesofPGEsulphide
oresfromtheBushveldComplex,SouthAfrica(0.15to0.2),chromites(0.12to0.14),
municipalsolidwasteincinerator(MSWI)ash(0.24to0.7)andsewagesludgearegenerally
unradiogenic(EsserandTurekian,1993;Funarietal.,2016;McCandlessandRuiz,1991;
RavizzaandBothner,1996;Turekianetal.,2007;Walkeretal.,1994;Williamsetal.,1997).
The187Os/188Osofbase-metalsulphideoresarehighlyvariable,andinsomecasescanbe
relativelyradiogenic(e.g.Sudbury,Canada)duetocontaminationfromcontinental
sources,whilstinothercasestheycanrecordunradiogenic187Os/188Osvaluessimilarto
mantlederivedsources(e.g.Noril’sk,RussiaandYilgarncraton,Australia)(Lambertetal.,
1998;Morganetal.,2002;Walkeretal.,2002).Roughly95%(Jones,1999)ofallPGEsare
derivedfromunradiogenicsources(0.15-0.2),whichisreflectedinthe187Os/188Os
measurementsofthecatalyticconvertorsfromwhichtheyaremade(0.1-0.2)(Poirierand
Gariépy,2005),suggestingthe187Os/188Osofautomobileexhaustwillbeofsimilarvalues
(Chenetal.,2009).
Smelters(Gogotetal.,2015;Rodushkinetal.,2007b),incinerators(Funarietal.,
2016;Turekianetal.,2007;WilliamsandTurekian,2002)andsewageoutflow(Esserand
Turekian,1993;RavizzaandBothner,1996;Turekianetal.,2007;Williamsetal.,1997)
representpointsourcesinanthropogenicOs,whereascatalyticconvertorsrepresenta
70
regionalinfluenceindenselypopulatedareas(PoirierandGariépy,2005).Onaglobalscale,
therefiningofPGEores,andtoalesserextentautomobileexhaust,hasdrivenglobal
precipitationtowardstheisotopiccompositionofores(~0.2)(Chenetal.,2009).Moreover,
present-daysurfacewatershavelower187Os/188Osvalues(~0.95)thandeepwaters(~1.05),
suggestinghumanshaveimpactedtheglobalOscycleandtheworld’soceans(Chenetal.,
2009).Osmiumisotopesthereforeholdthepotentialtobecomeapowerfultracerof
hydrologic,oceanographicandanthropogenicprocesses‘similartoPbfromleadedgasoline
usagebefore1978ortritiumfromatmosphericatomicbombtestingintheearly1960s’
(Chenetal.,2009).
Despitethispotential,andtheestablishmentofultra-lowblankanalytical
techniquescapableofoxidisingallosmiumtoacommonoxidationstate(ChenandSharma,
2009;GannounandBurton,2014;Levasseuretal.,1998;Pauletal.,2009),thedirect
analysisofOsinseawaterstillremainsanalyticallychallengingduetoultra-lowOs
concentrations(Peucker-Ehrenbrinketal.,2013).Therefore,themeasurementsof
anthropogenicOsinrivers,estuariesandcoastalandoceanicwatersremainsparse.Recent
workindicatesthatmacroalgae(seaweed)concentratesOs(withabundancesthatvary
from3.3to254.5ppt),whilstmaintainingthe187Os/188Oscompositionoftheseawaterit
inhabits(Racionero-Gómezetal.,2017;Rooneyetal.,2016;Chapter2).Thissuggeststhat
macroalgaecouldactasaproxyforthe187Os/188Oscompositionoflocalwaterswhilst
removingsomeoftheanalyticalchallengesassociatedwithdirectanalysisofseawateri.e.
ultra-lowconcentrationsandmultipleoxidationstates.Macroalgaeexistingincoastal
waters,therefore,shouldrecordan187Os/188Ossignaturethatreflectsabalanceoflocal
inputs,includingriverineinput,localbedrockandseawaterandhasbeenutilisedto
understandnaturalprocessesinGreenland(Rooneyetal.,2016)andIceland(Chapter2).
Japanoffersauniqueplaceinwhichtostudytheinfluenceofanthropogenic
processesonregionalvariationsinthemarineOscycle.Large,denselypopulated,
71
sprawlingmetropolitanareasoftenfallincloseproximitytocoastalwaters,inletseasor
bays.Thesemetropolitanareasareoftenjuxtaposedtosparselypopulatedruraland/or
mountainousregions,withlittlehumanactivityandthereforeanthropogenicinfluence.
ThisstudypresentsRe-OsabundanceandisotopedataformacroalgaefromTokyoBay,
OsakaBay,IseBay,MikawaBay,IzuPeninsula,NotoPeninsula,Hokkaidoandnorthern
Honshu.
TokyoBay,OsakaBayandIse/MikawaBaylieincloseproximitytotheKanto
(Tokyo-Yokohama),Keihanshin(Osaka-Kobe)andChukyo(Nagoya)metropolitanareas.The
187Os/188Ossignaturefromtheseregionsismuchlowerthanexpectedfornaturalriverand
oceanicsystems,butsimilartotheisotopecompositionofPGEores.Thissuggeststhat
humanactivityhasinfluencedthe187Os/188Osofmacroalgae,andthereforeseawater,
throughtheburningofmunicipaland/orhospitalwaste,processingofsewageandthe
extensiveuseofautomobilesintheseareas.TheIzuPeninsula,theNotoPeninsula,
HokkaidoandnorthernHonshuexhibit187Os/188Osvaluessimilartoglobalriverwateror
Pacificseawatermeasurements,suggestinglittleinfluencefromhumanactivity.These
resultsdemonstratetheabilityofmacroalgaetotracefluctuationsinthe187Os/188Os
compositionsoffreshwaterandseawateraroundJapan,andutilisethistodeterminethe
influenceofhumanactivityontheglobalosmiumbudget.
3.2Fieldandanalyticaltechniques
3.2.1Samplingandstorage
MacroalgaefromOsakaBay(SampleLocation16-22)weresampledat7locationsduring
September2014.MacroalgaefromTokyoBay(SampleLocation1-15),theIzupeninsula
(SampleLocation31-36)andtheNotopeninsula(SamplesLocation37-43)weresampledat
28locationsbetweenJulyandSeptember2015.Afurther8locationsintheIse(Sample
72
Location23-24)andMikawa(SampleLocation25-30)BaysweresampledduringDecember
2015.MacroalgaefromnorthernHonshu(SampleLocation44-48)weresampledat5
locationsduringSeptember2016.SamplesfromHokkaidowerepurchasedattsukijifish
marketinTokyo.Intotalsixty-fourmacroalgaesampleswerecollected(Fig.1;Table1).
Macroalgaewerewashedusingdeionised(Milli-Q™)watertoremoveanyattached
sedimentandsalt.Theywerethendriedfor12hat60°Candstoredinplasticzip-lock
bags.Macroalgaewerelatercrushedusinganagatepestleandmortarpriortoanalysis.
Fig.1.MapofJapan.MacroalgaesamplelocationsfornorthernHonshuandHokkaidoare
displayed(Circles;Table2).DashedlinesrepresentanoutlineofareasshowninFigures2
(TokyoBay),3(OsakaBay),4(IseandMikawaBay),5(TheIzupeninsula)and6(Kanazawa
andtheNotopeninsula).
73
3.2.2Macroalgaespeciesandhabitats
Specificmacroalgaespecieswerenottargetedduringthisstudy,andsampleswere
selectedbasedontheiravailabilityatsamplesites.Therewashoweverapreferenceto
brownmacroalgaeovergreenandredmacroalgaeduetotheirrelativelyhighabundancein
Re(Masetal.,2005;Proutyetal.,2014;Racionero-Gómezetal.,2016;Yang,1991)andOs
(Racionero-Gómezetal.,2017;Chapter2;Rooneyetal.,2016).Twentyspeciesof
macroalgaewereanalysed:Monostromanitidum;Urosporapenicilliformis;Hizikia
fusiformis;Undariapinnatifidia;DictyopterisundulataHolmes;Gloiopeltiscomplanata;
Spatoglossumcrissum;Gloiopeltisfurcate;Sargassumfusiforme;Sargassummuticum;
GelidumelegansKutzing;Gracilariavermiculophylla(Ohmi)Papenfuss;Sargassumhorneri;
Grateloupiacarnosa;Schizymeniadubyi;Grateloupialanceolate;PorphyrateneraKjellman;
Pachydictyoncoriaceum(Holmes)Okamura;Gracilariabursa-pastoris;and,Laminaria
japonica.
3.2.3Re-Osanalysis
TheRe-OsanalysisofmacroalgaewascarriedoutintheDurhamGeochemistryCentre
(LaboratoryforSulfideandSourceRockGeochronologyandGeochemistry).
3.2.3.1Macroalgae
ThetechniqueforchemicalseparationofReandOsfrommacroalgaeisreportedby
Racionero-Gómezetal.(2017).Inbrief,approximately200mgofpowderedmacroalgae
wasintroducedintoaCariustubetogetherwith11NHCl(3mL),15.5NHNO3(6mL)anda
knownamountof185Re+190Ostracersolutionandheatedto220°Cinanovenfor24h.The
OswasisolatedfromtheacidmediumusingCHCl3solventextractionandthenback
extractedintoHBr.TheOswasfurtherpurifiedusingaCrO3-H2SO4–HBrmicro-distillation
74
(Bircketal.,1997;CohenandWaters,1996).TheremainingRe-bearingacidmediumwas
evaporatedtodrynessat80°C,withtheReisolatedandpurifiedusingbothNaOH-acetone
solventextractionandHNO3-HClanionchromatography(Cummingetal.,2013).
3.2.3.2MassSpectrometry
ThepurifiedReandOsfractionswereloadedontoNiandPtfilamentsrespectively,and
measuredusingNTIMS(Creaseretal.,1991;Völkeningetal.,1991)onaThermoScientific
TRITONmassspectrometerusingFaradaycollectorsinstaticmode,andanelectron
multiplierindynamicmoderespectively.TheReandOsabundancesandisotope
compositionsarepresentedwith2s.e.(standarderror)absoluteuncertaintieswhich
includefullerrorpropagationofuncertaintiesinthemassspectrometermeasurements,
blank,spikecalibrations,andsampleandspikeweights.Fullanalyticalblankvaluesforthe
macroalgaeanalysisare10.9±5.9pgforRe,0.13±0.13pgforOs,witha187Os/188Os
compositionof0.61±0.34(1SD,n=4).
Tomonitorthelong-termreproducibilityofmassspectrometermeasurements
ReandOs(DROsS,DTM)referencesolutionswereanalysed.The125pgResolutionyields
anaverage185Re/187Reratioof0.5987±0.0023(2SD.,n=8),whichisinagreementwith
publishedvalues(e.g.,Cummingetal.,2013andreferencestherein).A50pgDROsS
solutiongavean187Os/188Osratioof0.16111±0.0008(2SD.,n=8),whichisinagreement
withreportedvaluefortheDROsSreferencesolution(Nowelletal.,2008).
3.3Results
TheReandOsabundanceandisotopedataformacroalgaearepresentedinTable2.The
ReandOsabundanceshowalargerangefrom0.02to21.43ppband1.96to122.76ppt
respectively.The187Os/188Osand187Re/188Oscompositionsarehighlyvariableandrange
75
from0.16to1.09and8.7to11234.5respectively.Thereason(s)forthevariationis
discussedissection4.Insection3.3.2to3.3.6,thepositionofsamplelocationsare
comparedtoseveralindicatorsofhumanactivity.Populationdensitydata(ESRI,2014),Os
pollutionsources(GoogleEarth,2016)andCO2emissiondatafromEAGrid2010(Kannariet
al.,2007)canbefoundinpanelb,candd(Figs.2to6)respectively.
Table2
RheniumandosmiumabundanceandisotopedataforJapanesemacroalgaesamples
SampleLocation MacroalgaeSpecies
[Re](ppb) 2s.e.
[Os](ppt) 2s.e. 187Re/188Os 2s.e. 187Os/188Os 2s.e.
TokyoBay1 Monostromanitidum 0.02 0.03 3.90 0.08 28.7 38.9 0.519 0.0322 Monostromanitidum 3.56 0.20 11.27 0.22 1603.9 113.4 0.541 0.0313 Monostromanitidum 2.88 0.05 3.33 0.07 4462.4 215.1 0.674 0.0414 Monostromanitidum 0.07 0.03 7.67 0.08 42.2 19.4 0.363 0.011
5Urospora
penicilliformis 3.72 0.04 24.03 0.11 789.5 9.3 0.582 0.0065 Monostromanitidum 0.305 0.001 12.18 0.05 124.7 1.0 0.397 0.0046 Monostromanitidum 3.65 0.04 29.19 0.56 639.5 26.7 0.592 0.034
7Urospora
penicilliformis 0.52 0.03 29.86 0.31 87.9 5.4 0.463 0.014
7Urospora
penicilliformis 0.938 0.009 30.06 0.31 156.3 3.6 0.428 0.012
8Urospora
penicilliformis 0.233 0.003 26.57 0.50 43.9 1.9 0.436 0.025
rptUrospora
penicilliformis 0.37 0.03 18.14 0.34 101.6 9.3 0.455 0.0269 Monostromanitidum 0.127 0.001 17.21 0.13 37.0 0.6 0.418 0.009 9 Monostromanitidum 0.269 0.001 17.60 0.08 76.7 0.6 0.450 0.005 10 Monostromanitidum 0.05 0.03 3.77 0.08 71.0 40.5 0.551 0.03411 Monostromanitidum 0.06 0.03 3.44 0.07 85.7 45.2 0.669 0.04112 Hizikiafusiformis 1.63 0.09 10.00 0.07 861.6 50.3 0.867 0.01213 Undariapinnatifidia 2.14 0.02 5.85 0.04 1948.2 27.2 0.950 0.015
14Dictyopterisundulata
Holmes 0.10 0.03 7.40 0.15 68.5 21.7 0.910 0.053
15Gloiopeltiscomplanata 7.65 0.14 17.10 0.20 2362.2 66.3 0.857 0.026
OsakaBay
16Spatoglossum
crassum 1.33 0.07 13.25 0.08 502.6 26.7 0.445 0.00716 Gloiopeltisfurcata 0.58 0.03 21.31 0.22 137.3 8.5 0.497 0.01417 Sargassumfusiforme 3.64 0.17 122.76 0.54 143.6 6.9 0.165 0.00217 Undariapinnatifidia 4.21 0.15 23.91 0.45 876.0 47.7 0.380 0.022rpt Undariapinnatifidia 3.49 0.17 24.87 0.39 697.4 40.6 0.364 0.01718 Undariapinnatifidia 7.05 0.25 22.78 0.25 1589.2 65.5 0.628 0.01818 Undariapinnatifidia 4.72 0.22 14.34 0.20 1683.0 91.7 0.597 0.02319 Undariapinnatifidia 4.21 0.15 18.81 0.36 1144.2 62.5 0.591 0.03419 Gloiopeltisfurcata 0.55 0.04 11.36 0.12 246.6 18.5 0.521 0.01520 Sargassumfusiforme 2.58 0.10 25.89 0.16 518.8 20.0 0.758 0.01021 Sargassummuticum 0.38 0.03 16.59 0.18 114.5 10.3 0.508 0.01522 Undariapinnatifidia 3.18 0.12 19.34 0.37 830.8 45.6 0.497 0.029
IseBay23 Gelidumelegans 0.16 0.03 88.80 0.84 8.7 1.7 0.158 0.005
76
Kutzing
23
Gracilariavermiculophylla(Ohmi)Papenfuss 0.08 0.03 27.06 0.49 15.0 5.4 0.185 0.011
24 Undariapinnatifida 1.37 0.07 22.01 0.13 318.5 16.7 0.602 0.008MikawaBay
25 Sargassumhorneri 0.36 0.03 10.62 0.21 176.3 18.5 0.742 0.04326 Sargassummuticum 0.24 0.03 7.31 0.15 167.3 23.8 0.689 0.04127 Grateloupiacarnosa 3.82 0.18 5.30 0.09 3661.5 217.3 0.544 0.02728 Sargassummuticum 21.43 1.00 14.78 0.08 7523.2 354.7 0.716 0.00829 Schizymeniadubyi 0.19 0.03 3.00 0.06 327.6 55.1 0.513 0.032
30Grateloupialanceolata 0.27 0.03 3.51 0.04 390.0 48.1 0.537 0.017
IzuPeninsula31 Hizikiafusiformis 4.83 0.17 16.92 0.19 1504.3 62.5 0.842 0.025
32Porphyratenera
Kjellman 0.61 0.04 7.78 0.09 407.4 26.4 0.704 0.022
33
Pachydictyoncoriaceum(Holmes)
Okamura 4.22 0.15 10.25 0.14 2129.3 97.2 0.692 0.027
34Dictyopterisundulata
Holmes 4.79 0.17 15.21 0.14 1628.8 64.8 0.697 0.01735 Hizikiafusiformis 0.65 0.04 28.10 0.32 122.0 7.6 0.905 0.02636 Undariapinnatifidia 0.89 0.04 8.69 0.06 537.4 26.9 0.776 0.010
KanazawaandtheNotoPeninsula37 Monostromanitidum 0.16 0.03 5.18 0.10 158.6 30.7 0.459 0.028
38Gracilariabursa-
pastoris 0.12 0.03 6.13 0.12 100.8 25.5 0.476 0.02838 Monostromanitidum 0.03 0.03 2.29 0.05 70.2 66.1 0.483 0.031
39Gracilariabursa-
pastoris 0.61 0.04 19.24 0.38 166.0 13.2 0.848 0.049
40Dictyopterisundulata
Holmes 3.18 0.15 27.16 0.54 623.1 39.0 0.937 0.05441 Sargassummuticum 1.73 0.09 32.80 0.21 279.1 14.1 0.888 0.011
42Dictyopterisundulata
Holmes 2.56 0.11 12.20 0.07 1101.0 49.0 0.822 0.00943 Sargassummuticum 1.60 0.07 25.26 0.17 341.0 16.1 1.017 0.013
NorthernHonshu44 Laminariajaponica 0.82 0.04 3.98 0.05 1078.3 54.7 0.763 0.02445 Laminariajaponica 2.12 0.06 6.40 0.04 1727.7 54.1 0.765 0.00846 Laminariajaponica 1.44 0.05 16.16 0.14 470.1 17.3 0.884 0.01847 Laminariajaponica 0.60 0.03 12.10 0.14 263.2 16.0 0.921 0.02748 Laminariajaponica 3.92 0.11 11.10 0.19 1877.8 83.3 0.923 0.045
Hokkaido49 Laminariajaponica 7.71 0.02 3.69 0.05 11234.5 257.2 1.016 0.03150 Laminariajaponica 2.637 0.009 4.02 0.08 3517.9 154.1 0.984 0.05951 Laminariajaponica 1.27 0.07 1.96 0.05 3485.2 269.9 1.015 0.06852 Laminariajaponica 2.878 0.009 5.06 0.11 3078.9 132.4 1.075 0.06453 Laminariajaponica 11.87 0.04 11.32 0.10 5689.1 82.2 1.093 0.022
3.3.1HokkaidoandNorthernHonshu
TheReandOsabundanceinmacroalgaefromNorthernHonshuandHokkaido(Fig.1)
variesfrom0.6to11.87ppband1.96to16.16pptrespectively.The187Os/188Osand
187Re/188Oscompositionsrangefrom0.76to1.09and263.2to11234.52respectively.No
discerniblepatterncanbefoundbetweenReandOsabundanceandgeographicalposition.
78
Fig.2.Samplelocationsand187Os/188Osmacroalgaevalues(a),populationdensity(b),potentialOspointsourcelocations(c)andannualvehicleCO2emissionsint-CO2km-2yr-1(d)fortheTokyoBayarea.
The187Os/188Osand187Re/188Oscompositionofmacroalgaeisgenerallyhigherin
macroalgaefromHokkaido(avg.187Os/188Os=~1.03;avg.187Re/188Os=~5401)than
northernHonshu(avg.187Os/188Os=~0.85;avg.187Re/188Os=~1083).
3.3.2TokyoBay
TheReandOsabundanceinmacroalgaefromTokyoBay(Fig.2a)variesfrom0.02to7.65
ppband3.33to30.06pptrespectively.The187Os/188Osand187Re/188Oscompositionsrange
from0.36to0.95and28.7to4462.4respectively.Nodiscerniblepatterncanbefound
betweenReabundanceand187Re/188Oscompositionandgeographicalposition.Osmium
abundanceisgenerallyhigherinmacroalgaewithproximitytocentralTokyo(Samples5to
9).The187Os/188OscompositionofmacroalgaefromTokyoBayarehighlyvariable.
Relativelyunradiogenicvalues(~0.36to0.67)arefoundinmacroalgaefromthenorthern
andnorth-easternpartsofthebay,withconsistentlylow(~0.4)187Os/188Osvaluesnear
centralTokyo(Samples7to9).MacroalgaemoreproximaltothePacificOcean,possess
187Os/188Osvaluesthatbecomeprogressivelymoreradiogenic(~0.7to0.95).
ThedenselypopulatedcitiesofYokohama,TokyoandChibaoccupywestern,
northernandnorth-easternpartsofTokyoBayrespectively(Fig.2b).Alargenumberof
hospitalsoccupythewesternandnorthernpartsofthebay,whilemunicipalsolidwaste
incinerators(MSWIs)andsewagetreatmentplantsdominatepartsofthebaycloseto
centralTokyoandChiba(Fig.2c).MajorhighwaysconnectingthecitiesofYokohama,Tokyo
andChiba,generatehighannualvehicleCO2emissionsalongtheedgeofthenorthernpart
oftheBay(Fig.2d).Majorhighwaysandtrafficcongestionleadtoexceptionallyhigh
vehicleemissionsincentralTokyo(Fig.2d).
79
3.3.3OsakaBay
TheReandOsabundanceinmacroalgaefromOsakaBay(Fig.3a)variesfrom0.38to7.05
ppband11.36to122.76pptrespectively.The187Os/188Osand187Re/188Oscompositions
rangefrom0.17to0.76and114.5to1683respectively.Nodiscerniblepatterncanbe
foundbetweenReabundance,Osabundanceand187Re/188Oscompositionsand
geographicalposition.The187Os/188OscompositionofmacroalgaefromOsakaBayishighly
variable.SamplesalongtheeasterncoastofAwajiIslandvaryfromrelativelyunradiogenic
187Os/188Osvalues(0.16to0.45)towardsthesouth(Samplesite18)andthecentre(Sample
site17)torelativelyradiogenic187Os/188Osvalues(0.52to0.63)inthenorth(Samplesite18
and19).Themostradiogenic187Os/188Osvalues(~0.76)canbefoundclosetocentralOsaka
(Samplesite20),withrelativelyhomogenous187Os/188Osvaluesof~0.5occurringsouthof
Osaka(Samplesite21and22).
c
b
d
a
80
Fig.3.Samplelocationsand187Os/188Osmacroalgaevalues(a),populationdensity(b),
potentialOspointsourcelocations(c)andannualvehicleCO2emissionsint-CO2km-2yr-1
(d)fortheOsakaBayarea.SeeFig.2forpopulationdensityinformation.
ThedenselypopulatedmegacityofOsakaliestothenorth-eastofthebayand
thelargecityofKobeoccupiesthenortherncoast(Fig3b).Thesecitiesgenerallyrepresent
morethan4000peoplepersquarekm,whileAwajiIslandtothesouth-westhasa
populationlowerthan400peoplepersquarekm(Fig.3b).Alargenumberofhospitals,
MSWI,sewagetreatmentplantsandsteelmillsoccupytheeasterncoastlineofOsakaBay
(Fig.3c).Asmallsectionoftheeasternbayisoccupiedbyseverallargeoilrefineries(Black
circlesinFig.3c).ThreehospitalsareclusterednearcentralAwajiIslandwithafurthertwo
atthenortherntipoftheisland(Fig.3c).MajorhighwaysrunningtoOsakaandKobe
generatehighannualvehicleCO2emissionsalongthenorthernedgeoftheBay(Fig.3d).
ExceptionallyhighvehicleemissionscanbefoundincentralOsaka(Fig.3d).
3.3.4IseandMikawaBay
TheReandOsabundanceinmacroalgaefromIseandMikawaBay(Fig.4a)varyfrom0.16
to21.43ppband3to88.8pptrespectively.The187Os/188Osand187Re/188Oscompositions
rangefrom0.16to0.74and8.7to7521.2respectively.Nodiscerniblepatterncanbefound
betweenReabundance,Osabundance,and187Re/188Osandgeographicalposition.The
187Os/188OscompositionofmacroalgaefromIseandMikawaBayarehighlyvariable.
Moderatelyradiogenic187Os/188Osvalues(~0.51to0.54)arefoundalongthenorthern
coastlineofMikawaBayclosetothecitiesofGamagori(Sample29)andNishio(Sample
30).The187Os/188OsvaluesalongthesoutherncoastlineofMikawaBayaregenerallymore
radiogenic(~0.69to0.74)withtheexceptionofsample27(~0.54).IntheIseBay,
187Os/188Osvaluesarerelativelyunradiogenic(~0.16)closetothecityofMatsusaka(Sample
81
23),withmoreradiogenic187Os/188Osvalues(~0.6)closetothemouthofthebayandthe
PhilippineSea(Sample24).
b
cd
a
82
Fig.4.Samplelocationsand187Os/188Osmacroalgaevalues(a),populationdensity(b),
potentialOspointsourcelocations(c)andannualvehicleCO2emissionsint-CO2km-2yr-1
(d)fortheIseandMikawaBayareas.SeeFig.2forpopulationdensityandpointsource
information.
ThedenselypopulatedcitiesofGamagoriandNishiooccupythenortherncoast
ofMikawaBay,withToyohashitotheeast(Fig.4b).Thedenselypopulatedcityof
MatsuakaoccupiesthewesternsectionofIseBay(Fig.4b),withthemegacity,Nagoya,to
thenorthofthemap.SeveralhospitalsarespreadacrossthewesterncoastlineofIseBay
withamoredenselygroupinginToyohashi(Fig.4c).Severalsewagetreatmentplants
occupytheeasternandnorthwestregionsofMikawaBay(Fig.4c).VehicleCO2emissions
arerelativelyhighinToyohashiandalongthewesterncoastlineoftheIseBay(Fig.4d).
3.3.5IzuPeninsula
TheReandOsabundanceinmacroalgaefromtheIzuPeninsula(Fig.5a)varyfrom0.61to
4.83ppband7.78to28.1pptrespectively.The187Os/188Osand187Re/188Oscompositions
rangefrom0.69to0.91and122to2129.3respectively.Nodiscerniblepatternisobserved
betweenReabundance,Osabundance,and187Re/188Osandgeographicalposition.The
187Os/188OscompositionofmacroalgaefromtheIzuPeninsulaarevariable,butallrelatively
radiogenic(~0.7to0.91).Alongthesoutherntipofthepeninsula(SampleLocation32to
34)the187Os/188Osvaluesare~0.7,withslightlymoreradiogenicvaluesbeingfoundfurther
northonthewesterncoast(187Os/188Os=~0.84)andeasterncoast(187Os/188Os=0.78to
0.91).TheIzuPeninsulaisgenerallysparselypopulated,withthelargestcity(Numazu)to
thenorthwestofthepeninsula(Fig.5b).SeveralhospitalsandMSWIslinetheeasterncoast
ofthepeninsula(Fig.5c).VehicleCO2emissionsarelowacrosstheentirepeninsula,with
theexceptionofthecityofNumazutothenorth(Fig.5d).
83
Fig.5.Samplelocationsand187Os/188Osmacroalgaevalues(a),populationdensity(b),potentialOspointsourcelocations(c)andannualvehicleCO2emissionsint-CO2km-2yr-1(d)fortheIzuPeninsula.SeeFig.2forpopulationdensityinformation.
3.3.6NotoPeninsula
TheReandOsabundanceinmacroalgaefromtheNotoPeninsula(Fig.6a)varyfrom0.03
to3.18ppband2.29to32.8pptrespectively.The187Os/188Osand187Re/188Oscompositions
rangefrom0.46to1.02and70.3to1101respectively.TheReabundanceofmacroalgae
fromKanazawaandthewesterncoastoftheNotoPeninsula(SampleLocation37to39)is
b
c d
a
84
Fig.6.Samplelocationsand187Os/188Osmacroalgaevalues(a),populationdensity(b),
potentialOspointsourcelocations(c)andannualvehicleCO2emissionsint-CO2km-2yr-1
(d)fortheNotoPeninsula.SeeFig.2forpopulationdensityinformation
lower(0.03to0.61ppb)thanformacroalgaeofthenorthernandeasterncoastoftheNoto
Peninsula(1.6to3.18ppb).TheOsabundanceinmacroalgaeislower(2.29to6.13ppt)
ba
c d
85
nearthecityofKanazawa(SampleLocation37and38)thanformacroalgaeoftheNoto
Peninsula(12.2to32.8ppt).The187Re/188Oscompositionofmacroalgaefromthewestern
coastoftheNotoPeninsula(SampleLocation37to39)arelower(70.2to166)thanforthe
northernandeasterncoastoftheNotoPeninsula(279.1to1101).The187Os/188Os
compositionofmacroalgaefromtheIzuPeninsulaishighlyvariable.Relatively
unradiogenicvalues(0.46to0.48)arefoundclosetothemajorcityofKanazawa(Sample
Location37and38),whereassamplessurroundingtheNotoPeninsulaarerelatively
radiogenic(0.82to1.02).
TheNotoPeninsulaisgenerallysparselypopulated,withthelargestcity
(Kanazawa)tothesouthwest,andthecitiesofTakaoka,ImizuandToyamatothesouth
eastofthepeninsula(Fig.6b).Hospitals,MSWIsandsewagetreatmentplantsaregenerally
clusteredinmajorcities,withseveralhospitalsspreadaroundtheNotoPeninsula(Fig.6c).
VehicleCO2emissionsarelowacrosstheentirepeninsula,withtheexceptionofKanazawa
tothesouthwestandTakaokaandToyamatothesoutheast(Fig.6d).
3.4Discussion
Thelargerangeinthe187Os/188Oscompositionofmacroalgae(0.16to1.09)impliesthat
bothhighlyradiogenicandhighlyunradiogenicsourcesofOscontributetothebulkOsin
macroalgae.NaturalsourcesofradiogenicOsincludeseawater,chemicalweatheringof
radiogeniccontinentalcrustandthedepositionofAeoliandust,whileanthropogenic
sourcesincludethesmeltingofradiogenicbase-metalsulphideoresandcontamination
fromfossilfuels.NaturalsourcesofunradiogenicOsincludehydrothermalalterationof
oceaniccrust,volcanism,cosmicdustandthechemicalweatheringofjuvenilebasaltic
crust,whereasanthropogenicsourcesinvolveexhaustfromautomobilesandthe
processingofPGEores,chromitesandunradiogenicbase-metalsulphideores.Inthe
followingsectionswewillfirstdiscusstheenvironmentalandbiologicalinfluenceonReand
86
Osuptakeinmacroalgaebeforediscussingeachofthesourcespresentedabovetoinfer
thattheOsproducedbyhumanactivitydominatestheisotopiccompositionofcoastal
watersinproximitytomajorJapanesecities.
3.4.1Biologicalandenvironmentalcontrolsonthe187Re/188Osofmacroalgae
The187Re/188Osofmacroalgaeshowawiderange8.7to11234.5(Table2)similarto
previousstudies(6.8to40,320)(Racionero-Gómezetal.,2017;Chapter2;Rooneyetal.,
2016).ThismaybeduetothedifferentsourcesofReandOstomacroalgae.Whenthe
187Re/188Osofmacroalgaeisplottedagainstthe187Os/188Osofmacroalgae(Fig.7a),most
Japaneseseaweedsshowalargevariationin187Os/188Osforasmallrangein187Re/188Os
whencomparedtoIcelandicmacroalgae(greycirclesinFig.7a).Thismaysuggestthat
JapanlacksasourcehighinReandthereforehigh187Re/188Osvalues,suchasoldprimary
basalticminerals(SeeChapter2;Gannounetal.,2006).Insteadthemacroalgaeappearto
bedominatedbysourceslowinResuchasriverwater(avg.187Re/188Os=227;Peucker-
EhrenbrinkandRavizza,2000).The187Re/188Osratiosincreaseasriverinesourcesmixwith
seawaterwithhigher187Re/188Os(avg.187Re/188Os=4270;Peucker-EhrenbrinkandRavizza,
2000).ThisisparticularlydominantinHokkaido(blacksquaresinFig.7a)wheremacroalgae
fromdeeperwatershaveastrongerinfluencefromPacificOceanseawater,andthus
higher187Re/188Osvalues(Fig.7a).
Whenthe187Re/188OsofmacroalgaeisplottedagainstthereciprocaloftheRe
concentration(Fig.7b)weseeasimilarrelationshiptoIcelandicmacroalgaefromChapter2
(greycirclesinFig.7b).ThissuggestssimilarmechanismscouldcontrolReandOsuptakein
Japanesemacroalgae.IfReandOsaretakenupviathesamepathwayitwillleadto
competitionbetweentheseelementsandthereforelowerReconcentrationinmacroalgae
underhigherOsseawaterconcentrationandviceversa(Chapter2;Racionero-Gómezetal.,
2017).AtlowReconcentration,thereisnocompetitionbetweenReandOsforuptakeinto
87
macroalgae,andthe187Re/188Osratiosremainconsistentlylow.AstheReconcentration
rises,ReisfavouredoverOsasthetwoelementsbegintocompeteforuptakeinto
macroalgae,leadingtoanincreaseinthe187Re/188Osratios.AtexceptionallyhighRe
concentrations,RecontinuestobetakenupandthemacroalgaebecomesenrichedinRe,
leadingtoexceptionallyhigh187Re/188Osratiosinmacroalgae(Fig.7b).
y=445.66x-0.917R²=0.82191
0.00
0.01
0.10
1.00
10.00
100.00
0.01 0.10 1.00 10.00 100.00
187 Re/
188 Os
x103
1/Re(ppb)
0
5
10
15
20
25
30
35
40
45
0.0 0.2 0.4 0.6 0.8 1.0 1.2
187 Re/
188 Os
x103
187Os/188Os
TokyoBayBosoPeninsulaOsakaBayIseBayMikawaBayIzuPeninsulaKanazawaNotoPeninsulaKanazawaHokkaidoIceland
a
b
88
Fig.7.a)The187Re/188Osagainstthe187Os/188OsofmacroalgaefromIceland(Chapter2)andJapan(Chapter3).b)187Re/188OsratiosagainstthereciprocaloftheReconcentrationforJapanese(blacksquares)andIcelandic(greycircles)macroalgae.
3.4.2Naturalsourcesofosmiumtomacroalgae
Alargenumberofmacroalgaeinhabitbrackishwaters,andtheir187Re/188Osand187Os/188Os
compositionwillthereforerepresentamixingbetweenfreshwaterriverineinputsand
seawater(SeeChapter2).The187Os/188OscompositionandOsabundanceofadepthprofile
fromtheEastPacificOceanhasbeenconstrainedat~1.04and10ppqrespectively(Chen
andSharma,2009;GannounandBurton,2014;Woodhouseetal.,1999).Further,the
hydrogenous187Os/188Oscompositionofpresentdaymarinesedimentssuggestthatthe
SeaofJapanhasasimilar187Os/188Oscompositionof~1.03(Dalaietal.,2005).
Fig.8.Osmiumisotopecomposition(187Os/188Os)ofmacroalgaefromHokkaidosamplelocations(SeeFig.1).BlacklineshowsdatafromdirectPacificOceanseawatermeasurements(SeeGannounandBurton,2014).
These187Os/188Osvaluesarereflectedinthe187Os/188Oscompositions(Table2)of
macroalgaefarmedfromdeeperpristinewaters>2milesofftheeastcoastofHokkaido
89
(~1.02),whichisindistinguishable,withinuncertainty,fromOsisotopemeasurements
obtaineddirectlyfromseawater(Fig.8).The187Os/188Oscompositionofmacroalgaefrom
shallowercoastalwatersaroundJapanwillrepresentthemixingbetweenPacificseawater
values(~1.04)andlocalcontinentalriverineinputswithadiverserangeof187Os/188Os
compositionsandOsabundancesdependentontheunderlyinglithologybeingweathered.
ThelongchainofJapaneseislandsalongthenorthwestmarginofthePacific
Oceanconsistsoftwotrench-arcsystems:theEastJapanIslandArcwhichmarksthe
boundarybetweentheEurasian,PacificandPhilippineSeaplates;andtheWestJapan
IslandArcwhichrepresentstheboundarybetweentheEurasianandPhilippineSeaplates
(Hashimoto,1991).Thesefeaturesgenerateacomplexgeologythatcanbebrokendown
intofivemajorelements:poorlylithifiedNeogene-QuaternarysedimentsandPaleogene
sedimentaryrocks(~40%);accretionarycomplexesconsistingmainlyofmelange,
mudstoneandsandstone(~17%);non-alkalineNeogene-Quaternaryvolcanicrocks(23%);
graniticrocksintrudedduringtheCretaceous(~10%);and,metamorphicrocks(~4%).The
underlyingingeologyofJapanthereforerepresentsalargerangeofagesandsourcesi.e.
mantletouppercrust.
Parent-daughterfractionationoftheRe-Ossystemduringmantlemeltingleads
toRebecomingpreferentiallypartitionedintothemeltoverOs.Asaproductofmantle
differentiation,continentalcrustischaracterisedbyhigherRe/Osratiosrelativetothe
mantle.Withanaveragecontinentalcrustageof~2.2Gyr,insitudecayof187Rehasledto
theaccumulationofappreciable187Osandaradiogenic187Os/188Oscompositionof~1.4
(Peucker-EhrenbrinkandJahn,2001).Theerosionandsubsequentweatheringofcomplex
underlyinglithologieshasledtohighlyvariable187Os/188Osvalues(0.64to2.94)andOs
abundances(4.6to52.1ppq)inglobalrivers(Levasseuretal.,1999a).Themajorcities
studiedhere,andmorethan70%oftheJapanesepopulation,resideontheJapanese
‘plains’whicharelargelyunderlinedbyQuaternarysediments(Hashimoto,1991).Itis
90
thereforeexpectedthatthe187Os/188Oscompositionofriversdrainingtheseregionstobe
similartotheglobalriverineaverageof~1.54(Levasseuretal.,1999a).
Pleistocene-Holocenesedimentsandsedimentaryrocksoccupytheentire
drainagebasinoftheKantoplains(Ohtaetal.,2011),Osakaplains(Ohtaetal.,2005;Ohta
etal.,2007),Kanazawaplains(Ohtaetal.,2004)andIseplains(Ohtaetal.,2005;Ohtaet
al.,2007),andthereforerepresentthedominantcontrolonthechemicalcompositionof
riversdrainingtherelevantwatershed(Ohtaetal.,2005).Wewouldthereforeexpectthe
187Os/188OsofwatersinTokyoBay(Fig.2)torepresentamixturebetweenradiogenicrivers
(~1.54)drainingthecoastalplainsandPacificseawater(~1.04)entrainedintothebayfrom
thePacificOceanviaestuarinegravitationalcirculation.However,the187Os/188Osof
macroalgaefromTokyoBay(Fig.2a),OsakaBay(Fig.3a),Kanazawa(Fig.6a)andIseBay
(Fig.4a)arerelativelyunradiogenicandrangefrom0.36to0.95,0.17to0.76,0.16to0.6
and0.46to0.48(Table2).ThissuggeststhatOsfromnaturalsourceshasverylittle
influenceontheisotopiccompositionofTokyoBay.
Althoughmostregionsstudiedareunderlainbysedimentaryplains,exceptions
tothisincludetheRyokebelt,IzupeninsulaandNotoPeninula.TheRyokebeltis
characterisedbylowP/Ttypemetamorphismandextensivefelsicigneousactivitythat
coversa~800kmstretchofinnerSouthwestJapan(Nakajima,1994).Graniticrocksare
dominantovermetamorphicrocksthroughouttheRyokebelt,andwerederivedfrom
magmasproducedinthelowercrustand/oruppermantle;withpotentialassimilationof
metamorphicrocksorPrecambriancrust(Yuharaetal.,2000).Weatheringofgranitic
materialdominatescentralAwajiIsland,KobecityalongthenorthcoastofOsakaBayand
theYahagiRiver,whichflowsintonorthernMikawaBay(Ohtaetal.,2005;Ohtaetal.,
2007).TheweatheringofhighpressuremetamorphicrocksdominatestheToyoRiver
whichflowsintotheeasternMikawaBay(Ohtaetal.,2005;Ohtaetal.,2007).Moststudies
suggesttheweatheringofold(Precambrian)graniticterraindelivershighlyradiogenicOs
91
(187Os/188Os=≥2.5)toriversdrainingthesecatchments(Chenetal.,2006;Ehrenbrinkand
Ravizza,1996;Huhetal.,2004).However,theRyokeBeltismadeupofmorerecent
(Cretaceous)I-typegraniteswithsomesedimentarycomponentsoftheMinoTerrain
(IshiharaandWu,2001).Althoughhardtoestimatewithoutdirectmeasurements,thelow
OsconcentrationcombinedwithhighRe/Osratiosingranites(Johnsonetal.,1996),
combinedwithpossiblecrustalcontaminationofradiogenicOswouldleadtohigh
187Os/188OsvaluesinsilicicmaterialsincetheCretaceous(Alvesetal.,1999;Alvesetal.,
2002;Hartetal.,2003;Hartetal.,2002).Wethereforesuggestthatrelativelyunradiogenic
187Os/188OsvaluesofmacroalgaefromOsakaBay(0.17to0.76)andMikawaBay(0.51to
0.74)arenotrelatedtotheweatheringoflocalgranites.
The187Os/188Oscompositionofmacroalgaefromthesouthandeastcoastofthe
IzuPeninsula(SampleLocation31to36;Fig.5a)andthecoastoftheNotoPeninsula
(SampleLocation39to43;Fig.6a)rangefrom0.69to0.91andfrom0.82to1.02
respectively.Thesevaluesareconsistentlylowerthanthe187Os/188Oscompositionof
seawater(~1.04),suggestingtheinfluenceofanunradiogenicendmember.Lowhuman
activity(Fig.5bandFig.6b)andtheabsenceofsubstantialanthropogenicsourcesofOsin
theregionsstudied(Fig.5c,dandFig.6c,d)suggeststhatthedominantsourceof
unradiogenicOsislikelytobenatural.FormationsexposedontheIzuPeninsulaarealmost
entirelycomposedofsubmarineandterrestrialvolcanicseruptedsincetheearlyMiocene,
andtheirreworkeddeposits(KoyamaandUmino,1991).Thenorth-easttipofthe
peninsulaisdominatedbyrecent(0-0.6Ma)HigashiizuandShiofukibasaltictoandesitic
lavasandpyroclastics.ThiscontrastswiththesouthernIzuPeninsula,whichisdominated
byolder(MiddleMiocenetoEarlyPleistocene)basalticandandesiticvolcanicsfromthe
YugashimaandShirahamagroup.Mioceneandesiticlavaandsedimentaryrocksfromthe
IwaineandKurosendaidominatethelithologyoftheNotopeninsula(Japan,1992).
92
Weatheringofthesevolcanicsystemsdominatesstreamandcoastalsediments
emanatingfromthem(Ohtaetal.,2005;Ohtaetal.,2004;Ohtaetal.,2007).Wewould
thereforeexpectthe187Os/188Oscompositionofriversintheseregionstohaveamore
mantlesignature(~0.12)duetotheweatheringofbasalticmaterialdominatinglocal
catchmentareas(Gannounetal.,2006).However,thedecayofabundant187Reintheolder
YugashimaandShirahamagroupsandtheNotoPeninsulamayleadtoradiogenicingrowth
of187Osandthereforehigher187Os/188Oscompositionsintheseregions(Gannounetal.,
2004;Gannounetal.,2006).The187Os/188OscompositionsofmacroalgaefromtheIzu
coastalwatersthereforeconstitutesamixingbetweenradiogenicPacificseawater(~1.04)
andunradiogenicriversdrainingabasalticterrain(>0.12).Thisissupportedbythe
187Os/188OscompositionofmarinesedimentsfromtheYasakaestuaryinnorthernKyushu,
whichrangefrom0.67to0.73,andrepresentamixtureoflocallyweatheredMiocene
volcanicsandseawater(Zhengetal.,2014).
3.4.3Anthropogenicsourcesofosmiumtomacroalgae
AnthropogenicOshasbeendetectedinestuaries(Turekianetal.,2007;Williamsetal.,
1997),lakes(Rauchetal.,2004),coastalsediments(EsserandTurekian,1993;Ravizzaand
Bothner,1996),biologicalorganisms(Rodushkinetal.,2007a,b),airborneparticles(Rauch
etal.,2005)andprecipitation(Chenetal.,2009).SourcesofanthropogenicOsinclude
hospitals(EsserandTurekian,1993;Turekianetal.,2007;Williamsetal.,1997),MSWIs
(Funarietal.,2016),vehicleexhaust(PoirierandGariépy,2005)andsmelters(Chenetal.,
2009;Rodushkinetal.,2007b),andcanbeeitherdirectlyintroducedintoriversandcoastal
watersviasewageoutflow(EsserandTurekian,1993;RavizzaandBothner,1996;Turekian
etal.,2007;Williamsetal.,1997)ordirectlytotheatmosphereasvolatileOsO4during
high-temperatureprocesses(Smith,1974).The187Os/188OscompositionofOsfromthese
sourcesdependsontheoriginalmaterialusedintheircreation.The187Os/188Os
93
compositionoffossilfuelsarehighlyradiogenic,rangingfrom1to13.7(Finlayetal.,2011;
SelbyandCreaser,2005).ThePGEsulphideoresandchromiteshaveunradiogenic
187Os/188Osvaluesof0.15to0.2(McCandlessandRuiz,1991)and0.12to0.14(Walkeret
al.,2002)respectively.The187Os/188Oscompositionofbase-metalsulphidedepositsare
highlyvariable:rangingfromvaluessimilartoPGEores(Lambertetal.,1998;Walkeretal.,
1994),throughtohighlyradiogenicvaluescausedbycontaminationfromcontinentalcrust
duringtheircreation(Morganetal.,2002).
Osmiumhasmajoruses–asatissuestainforelectronmicroscopyandasa
catalystinsteroidsynthesis–inmedicalresearch(Smith,1974).AnthropogenicOsfrom
biologicalandmedicalresearchlaboratorieshasbeendetectedintheHudsonRiver-Long
IslandSoundestuarinesystemandChesapeakeBaywithanestimated187Os/188Os
compositionof~0.13(EsserandTurekian,1993;Helzetal.,2000;Turekianetal.,2007;
Williamsetal.,1997).Thedominantmodeoftransportisbelievedtobeseweroutflow
fromnearbyhospitalsandatmospherictransportfromhospitalincinerators,theinfluence
ofwhichcanbedetectedupto70kmfromthesource(EsserandTurekian,1993;Ravizza
andBothner,1996;Williamsetal.,1997).Althoughnotmeasuredhere,sewagefrom
outflowsinNewHaven(EsserandTurekian,1993),NewYorkCity(Williamsetal.,1997)
andBoston(RavizzaandBothner,1996)have187Os/188OscompositionsandOsabundances
ofbetween0.15to0.3and0.57to4.01ppbrespectively.
DenselypopulatedregionsofJapan(SeepanelbinFigs.2to6)aregenerally
servedbyalargenumberofmajorhospitals(SeeredcirclesinpanelcofFigs.2to6).We
wouldthereforeexpectasignificantinfluxofunradiogenicOsfromhospitalssituatedin
centralTokyoandYokohama(Fig.2c),centralOsaka(Fig.3c),westernIseBayandeastern
MikawaBay(Fig.4c)andKanazawa(Fig.6c).The187Os/188Oscompositionofmacroalgae
fromtheseregionsaregenerallyhighlyunradiogenicrangingfrom0.39to0.55nearcentral
TokyoandYokohama,0.5to0.76nearOsaka,0.16to0.19inwesternIseBay,0.51to0.54
94
innorthernMikawaBayand0.46to4.48nearKanazawa(Table2).Additionally,
macroalgaesamplesfromcentralAwajiIsland(SampleLocation17)showhighly
unradiogenic187Os/188Osvaluesof0.17to0.38,closetothepreviouslyrecorded187Os/188Os
ofsewage(0.15to0.3).Thisismostlikelyduethehighnumberofhospitalsinthenearby
cityofSumototothesouthorthesewagetreatmentplanttothenorth(Fig.3c).This
suggeststhatthemacroalgaehaveincorporatedOsrelatedtomedicalresearchfrompoint
sourcessuchashospitalsandsewagetreatmentplantseitherviasewageoutflowor
incinerationofmedicalwaste(Fig.9).However,despitetheubiquitousnatureofhospitals
inmajorcitiese.g.TokyoandOsaka,the187Os/188Oscompositionofmacroalgaefromthese
regionsarenotuniformlylow.Forexample,macroalgaeinTokyoBayshowanincreaseto
moreradiogenicvalueswestwardsfromcentralTokyo,despitetheubiquityofhospitalsin
thisregion.Meanwhile,themacroalgaeclosesttocentralOsaka(SampleLocation20)has
thehighest187Os/188Osvalues(0.76)forOsakaBay.Thissuggeststhatalthoughmedical
facilitiescanofferapointsourceforanthropogenicOse.g.AwajiIsland,other
anthropogenicsourcescandominateinotherregions.
Fig.9.MacroalgaeOsconcentration(filledcircles)andisotopiccomposition(opencircles)
withdistancefromhospitalsincentralAwajiIsland(redcircles)oranoilrefineryincentral
Osaka(green).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40
187 Os/
188 Os
[Os](p
pt)
Distancefromsource(km)
95
MSWIsareconsideredoneofthebestoptionsformunicipalsolidwaste
managementinindustrialisedcountries(Chandleretal.,1997)andarewidelyusedin
majorJapanesecities.AnalysisofbottomandflyashfromItalianMSWIsshowawiderange
of187Os/188Oscompositions(0.24to0.7)andconcentrations(0.026to1.65ppb),andithas
beenpredictedthattheOsreleasedfromMSWIsmokestackswillrangefrom16to38ng
Os/m2/yr(Funarietal.,2016).ThisisthereforeanimportantsourceofOsnearcentral
TokyoandOsaka(SeelightgreencirclesinFig.2cand3c).Theseadditionalinputsof
potentiallyunradiogenicOscouldcausetherelativelyunradiogenic187Os/188Osvaluesin
regionswithrelativelylowvehicleemissionsi.e.Yokosuka(SampleLocation10),Chiba
(SampleLocation1,2and5),Kanazawa(SampleLocation38)andthesouthcoastofOsaka
Bay(SampleLocation21and22).
Automobilecatalyticconvertorscouldpotentiallyprovidealarger,regionally
dispersed,sourceofanthropogenicOs.Directmeasurementsofcatalyticconvertorsyield
unradiogenic187Os/188OsvaluesandOsabundancesof0.1to0.2and6to228ppt
respectively(PoirierandGariépy,2005).Moreover,newcatalyticconvertorscouldbe
responsibleforasmuchas120pgOs/m2inthefirstyearoftheirlife(PoirierandGariépy,
2005),andanthropogenicOssourcedfromvehicleexhausthasbeendetectedinairborne
particles(Rauchetal.,2005)andglobalprecipitation(Chenetal.,2009).Asthemajorityof
PGEsaresourcedfromtheBushveldComplex(SouthAfrica)andNoril’sk(Russia),we
expectcatalyticconvertorsinJapantohaveunradiogenicvaluesof0.15to0.2(Jones,
1999).HerewehaveutilisedtotalvehicleCO2emissionsfor2015fromtheEastAsianAir
PollutantEmissionGrid(EAGrid2011)database(Kannarietal.,2007)totryanddetermine
theinfluenceofcatalyticconvertorsonregionalvariationsinanthropogenicOs(Seepanel
dinFigs.2to6).
Wecangenerallyseethatinregionsofhighvehicleemissions,wegetthemost
unradiogenic187Os/188Osvaluesinmacroalgae.The187Os/188Oscompositionofmacroalgae
96
incentralTokyorangefrom0.42to0.46.Thesevaluesareindistinguishablefromthe
hydrogenousloadofpreviouslyrecordedmarinesedimentsfromtheTamaRiver(~0.3to
0.4)(Zhengetal.,2014),coincidenttowherethehighestvehicleemissionsinJapanare
found(Fig.2d).The187Os/188Oscompositionofmacroalgaebecomesmoreradiogenic(0.55
to0.67)inregionsofintermediatevehicleemissionssuchasYokosukaandChiba,withthe
exceptionofSampleLocation4whichishighlyunradiogenic(0.36).The187Os/188Os
compositionofmacroalgaebeginstoreachmoreradiogenic187Os/188Osvalues(0.85to
0.95),closetoPacificOceanestimates(~1.04),inregionsoflowvehicleemissionsonthe
BosoPeninsula(Fig.2d).InOsakaBay,macroalgae187Os/188Osvaluesarerelatively
unradiogenic(0.5to0.59)inregionsofintermediatevehicleemissions(Fig.3d).However,
macroalgaefromSampleLocation20and17wherevehicleemissionsarehighestand
lowestrespectively,recordthemostradiogenic187Os/188Osvalues(0.76)andunradiogenic
values(0.17to0.38)respectively.Inthecaseofsamplelocation17,aspreviously
explained,localinfluencefromhospitalandsewageoutflowwillcontributeOswithan
unradiogenic187Os/188Oscompositiontothisregion.Meanwhile,samplelocation20isclose
toanoilrefinery(Fig.3c),whichcouldactasapointsourceofanthropogenicOs.Osmium
inoilisgenerallyhighlyradiogenic(187Os/188Os=1to13.7)(Finlayetal.,2011;Selbyand
Creaser,2005;Selbyetal.,2007)andanyoilspillsoratmosphericemissionwilltherefore
carrysimilarcompositions.Thisisthereforethemostlikelycauseofradiogenic187Os/188Os
values(0.76)inthisarea(Fig.9).MikawaandIseBayshowsimilartrendswithrelatively
unradiogenicvaluesclosetotheregionsofhighestvehicleemissions(Fig.4d).
Finally,otherpotentialpointsourcesofanthropogenicOsincludetheprocessing
ofchromites,PGEoresandbase-metalsulphideores.AlthoughPGEoreandchromite
smeltersarenotknowntoexistintheregionsstudiedhere,awidenumberofsteelmills
arelocatedintheJapaneseindustrialcentressuchasTokyoandOsaka(Fig.2cand3c).The
187Os/188Oscompositionofbase-metalsulphidedepositsarehighlyvariable:rangingfrom
97
unradiogenicmantlevalueslessthan0.3(Lambertetal.,1998;Walkeretal.,1994),
throughtohighlyradiogenicvaluesgreaterthan0.9(Morganetal.,2002).Notknowingthe
sourceoforeusedatJapanesesteelmills,andduetothelackofstudiesofemissions,itis
hardtoestimatetheinfluenceofthesesourcesonregionalfluctuationsatmosphericOs.
However,itshouldbenotedthatsteelmillscouldpotentiallyactasasignificantsourceof
anthropogenicOswithpotentiallyhighlyvariableisotopiccomposition.
00.10.20.30.40.50.60.70.80.91
0 2000 4000 6000 8000 10000
187 O
s/18
8 Os
PopulationDensity(people/km2)
y=-0.097ln(x)+1.3351R²=0.84271
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2000 4000 6000 8000 10000
187 O
s/18
8 Os
PopulationDensity(people/km2)
a
b
98
Fig.10.187Os/188Osagainstpopulationdensityfora400km2areasurroundingeach
samplefor(a)populatedbaysstudieshere(Mikawa,Ise,OsakaandTokyoBay)and(b)
TokyoBay.Blacksymbolsrepresentmacroalgaeaffectedbypointsourcessuchashospitals
andoilrefineries(SeeFig.9).
Inconclusion,duringthisstudywehavefoundawiderangeofnaturaland
anthropogenicsourcestoJapanesewaters.InregionssuchasHokkaido,northernHonshu,
theIzuPeninsula,andNotoPeninsulawherehumanactivityislow,theosmiumisotope
budgetofcoastalwatersisdominatedbynaturalOsfromPacificOceanseawaterand
riverinesourcesdominatedbytheweatheringoflocalrocktypessuchasvolcanic,granitic
andsedimentaryrocks.IndenselypopulatedregionssuchasTokyo,Yokohama,Chiba,
Osaka,Kanazawa,MatsuakaandToyohashi,anthropogenicsourcesofOsdominate.
OsmiumfromcatalyticconvertorsdominatestheOsisotopebudgetincoastalwaterin
theseregions.However,pointsourcescanbecomemoreinfluentialinsampleslocated
nearhospitals,sewagetreatmentplantsandMSWIs.Thisisbestshownwhenthe
187Os/188Oscompositionofmacroalgaeisplottedagainstpopulationdensity(Fig.10).In
areasoflowpopulationdensity,the187Os/188Osofmacroalgaerangesfrom0.6to1(Fig.
10a).However,aspopulationdensityincreases,theisotopiccompositionofmacroalgae
becomesmoreunradiogenic(0.6to0.4).Pointsourcessuchashospitalsandoilrefineries
pulltheisotopiccompositionofmacroalgaetowardstheirrespectivesourcecompositions
awayfromthistrend(BlackcirclesinFig.10a).Thisrelationshipbecomesmoreapparent
whenthe187Os/188OsofmacroalgaeisplottedagainstpopulationdensityforTokyoBay
(Fig.10b).MacroalgaeinthelesspopulatedregionoftheBosoPeninsulahaveanisotopic
compositionof~0.9.However,asyoumoveintothebay,andtowardsthemorepopulated
regionstotheNorthoftheBay,theisotopiccompositionbecomesmoreunradiogenic
(0.4).
99
3.4.4Anthropogenicinfluenceontheglobalosmiumcycle
3.4.4.1AnthropogenicimpactonJapanesecoastalwaters
Thisstudyhasshownthatthe187Os/188Oscompositionofmacroalgaecansuccessfullytrace
thefluctuationsinnaturalandanthropogenicprocessesaroundJapan.Whenthe
187Os/188Oscompositionofmacroalgaeisplottedagainstthereciprocalofthe
concentration,allofthedatafallwithinfielddelimitedbyfourpotentialend-members(Fig.
9):seawater(radiogenic187Os/188Os,intermediate[Os]);riverwaterdrainingQuaternary
sedimentarymaterial(radiogenic187Os/188Os,low[Os]);riverwaterdrainingMiocene
volcanicrocks(intermediate187Os/188Os,low[Os]);and,anthropogenicOswithaPGE
source(unradiogenic187Os/188Os,high[Os]).Variationsinthe187Os/188Osofmacroalgaecan
thereforebeexplainedbythemixingbetweenthesedistinctsources.
Fig.11.187Os/188OsratiosagainstthereciprocaloftheOsconcentrationforJapanese(Chapter3)andIcelandic(Chapter2)macroalgae.SeeFig.7bforkey.
HighlypopulatedregionssuchasTokyoBay(redsquares),OsakaBay(dark
orangesquares),IseBay(lightorangesquares)andKanazawa(yellowsquares)fallona
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6
187 Os/
188 Os
1/[Os](ppt)
100
mixinglinebetweenradiogenicseawaterand/orriverwaterandanthropogenicsources
(Fig.11).Meanwhile,sparselypopulatedregionssuchasHokkaido(blacksquares),the
BosoPeninsula(lightbluesquares),theIzuPeninsula(darkgreensquares)andnorthern
Honshu(lightbluesquares)fallonamixinglinebetweenradiogenicseawaterand
radiogenicorrelativelyunradiogenicriverwater(Fig.11).Thissuggeststhathumanactivity
hasaddedanadditionalsourceofOstotheJapanesemarineOsbudgetandisdrivingthe
systemtowardsunradiogenicvalues.Thishascausedthe187Os/188Osprofileofarelatively
oldislandarclikeJapan,totakeonthe187Os/188Osprofileofayoungmid-oceanriftsystem
likeIceland(greycirclesinFig.11).
3.4.4.2AnthropogeniccontributionsofosmiumfromJapan
Osmiumoxidebecomesvolatileatacatalyticconvertorsoperatingtemperature(>400°C),
releasing75-95%ofitsOstotheatmospherewithinthefirstyearofuse(Poirierand
Gariépy,2005).Ithasbeenestimatedthata1kgmonolithiccatalystcontainsbetween6
and228pptOswhichwillbereleasedduringitslifetime(PoirierandGariépy,2005).Given
apopulationof127millionpeopleinJapan,androughly5.5millionnewvehiclesonthe
roadinJapanin2015,weestimateroughly23peoplepercar.Givenapopulationof~15.5
M,~4.2Mand~9.3MpeoplefortheTokyo-Kawasaki-Yokohama-chiba,Osaka-Kobeand
Mie-Aichiimetropolitanregionsrespectively,thisamountsto~675,000,~182,000and
~400,000vehiclesrespectively.Ifweassumeeachcarhasa1kgmonolithiccatalyst,this
amountsto3to105,1.5to54and0.2to8.4pgOs/m2/yrrespectively.
ThisiscomparabletopreviousOsemissionestimatesforNewYorkCityof3to
126pgOs/m2/yr(PoirierandGariépy,2005).ThissuggeststhatOsemissionfromcatalytic
convertorsaresignificantinurbanareaswhencomparedtonaturalOsinputsof~810pg
Os/m2/yr(PoirierandGariépy,2005)fromcontinentalerosionandatmosphericdustinputs
of1pgOs/m2/yr(WilliamsandTurekian,2002).However,pointsourceOsemissions,such
101
asMSWIs(16to38ngOs/m2/yr),canbecomefarmoresignificantthenvehicleemissionsin
denselypopulatedregions(Funarietal.,2016).
3.4.4.3Impactofanthropogenicosmiumonsurfacewaters
Contemporarysurfaceseawaterhasalower187Os/188Oscomposition(~0.95)thandeep
waters,whichhasbeenattributedtohumanactivities(Chenetal.,2009;Levasseuretal.,
1999b).Ifweassumethatsurfacewatersatthemouthofabay(M)representsthemixing
betweenananthropogenicsourceofOs(A)comingfromthebayandanaturalsourceofOs
comingfromseawater(SW),wecandescribetheisotopiccompositionandconcentration
atthemouthusingthefollowingequation:
187Os/188OsM Os SW+ Os A = 187Os/188OsSW[Os]SW+187Os/188OsA[Os]A
Wecanthenrearrangetheaboveequationtodeterminetheconcentrationof
anthropogenicosmium:
Os A =Os SW(187Os/188OsSW-187Os/188OsM)
187Os/188OsM − 187Os/188OsA
Ifweassumethattheisotopiccompositionofmacroalgaefromthemouthofthe
bayrepresents187Os/188OsM(187Os/188OsTokyoBay=0.86;187Os/188OsOsakaBay=0.47;
187Os/188OsIseBay=0.6),andgivena187Os/188OsSWand[Os]SWforthePacificOcean
(187Os/188OsPacific=1.04;[Os]Pacific=8ppq)andan187Os/188OsArepresentativeofPGEores
(0.15)weestimatetheconcentrationofanthropogenicOsinsurfacewatersatthemouth
ofTokyo,OsakaandIseBaystobe2ppq,14ppqand8ppqrespectively.Givenaflowrate
of8x109m3/yr,8x109m3/yrand37x109m3/yrforsurfacewatersleavingTokyo,Osaka
102
andIseBayrespectively(SmithandYanagi,1997),weestimateatotalofapproximately4.2
kgOs/yrofanthropogenicOsisdeliveredtothePacificOceanfromthesethreebays.
Usinganaveragevalueof1.4kgOs/yrdeliveredbyeachJapanesemegacityand
consideringtherearecurrently16megacitiesgloballysituatedincoastalregions,this
equatesto22.6kgOs/yrdeliveredfromglobalcoastalcities.However,coastalmegacities
onlyrepresent242millionpeopleor~8.2%ofpeoplelivingwithinthecoastalregion
(PellingandBlackburn,2014).Therefore,apotential276kgofanthropogenicOscouldbe
deliveredviasurfacewaterstotheworld’soceanseveryyearifweextrapolatetotheentire
globalcoastalpopulation.Chenetal.(2009)calculated2,391kg/yrarerequiredtodrive
theOsisotopiccompositionofocean’ssurfacewaters(depth=200m)from1.05to0.95.
TheysuggestedthatthemajorityofthisanthropogenicOsisdeliveredtotheatmosphere
duringtherefiningofPGEores.However,thisstudysuggests~12%ofanthropogenicOsin
surfacewaterscanbeattributedmainlytotheuseofPGEsincatalyticconvertors,and
confirmshumanactivityisimpactingtheglobalOsbudget.
3.5Implicationsandfutureoutlook
TheRe-Osdatapresentedheresuggestsmacroalgaecansuccessfullytracetheimpactof
environmentalandhumanactivityontheglobalosmiumbudget.Itparticular,the
187Os/188OscompositionofmacroalgaehastracedfluctuationsinnaturalOsfromthe
weatheringofvolcanicsandsedimentarymaterial,anditsmixingwithseawaterfromthe
PacificOceanandtheSeaofJapaninbrackishwatersaroundHokkaido,northernHonshu,
theIzuPeninsulaandNotoPeninsula.Moreover,the187Os/188Oscompositionof
macroalgaehasalsosuccessfullytracedtheimpactofanthropogenicOsemissionsfromthe
useofcatalyticconvertors,MSWIs,hospitalsandrefineriesonwatersinTokyoBay,Osaka
BayandIse/MikawaBayneardenselypopulatedregionsofJapan.Thissupportsprevious
workthatindicatesOsisotopescanactasapowerfultracerofearthsurfaceprocessessuch
103
asbasalticweatheringinIceland(Chapter2)andcontinentalweatheringinGreenland
(Rooneyetal.,2016).
Thewidespreaduseofcatalyticconvertorshasledtotheremovalofarangeof
pollutantsfromtheenvironment.Ironically,thishasledtoglobalOspollutionattheEarth’s
surfacefromtheproductionofplatinumforuseincatalyticconvertors.Unlikeprevious
studies,wesuggestthattherefineryofPGEsdoesnothaveasignificantinfluenceonthe
JapanesemarineOsbudget.Instead,thewidespreaduseofOsinmedicalresearchandthe
lackoftreatmentinsewageandmunicipalwastehasledtosignificantcontaminationin
regionsdirectlyadjacenttohospitals,MSWIsandsewagetreatmentplants.Indensely
populatedregions,OsemissionsfromcatalyticconvertorsdominateregionalOsbudgets,
andrepresentasignificantsourceofOstotheatmosphere.TransferofanthropogenicOs
fromthesesources,viasurfacewaters,totheworld’soceansrepresents~12%of
anthropogenicOsinputtothesurfaceocean,whichactstodrivethe187Os/188Os
compositionofseawaterfrom1.05to0.95.ThisstudyechoesChenetal.(2009)who
suggestedthat‘Osisotopescouldbeavaluabletracerforthehydrologicalcycle,similarto
Pbfromleadedgasolineusagebefore1978ortritiumfromatmosphericatomicbomb
testingintheearly1960s.’
FurtherworkisneededtounderstandthespecificuptakerateofOsfrom
seawaterbymacroalgaeinasimilarmannertoRacionero-Gómezetal.(2017).Suchdata
forJapanesemacroalgaespecieswillprovideabetterestimateoftheOsabundancein
Japanesecoastalwaters.ThiswillhelpyieldabetterunderstandingoftheglobalOscycle
andoceanicresidencetimes.Itwouldbewisetoutiliseotherisotopesystemsin
conjunctionwiththeOsisotopesystemtodistinguishbetweenthesourcesof
anthropogenicOsfrompopulatedregions.Forinstance,AlandorganicChaveusedin
conjunctionwithosmiumisotopestotracetheflowofanthropogenicOsrelatedtosewage
104
outflow.ThiswillallowthepossibilitytodistinguishOsattributedtocatalyticconvertors
fromothersourcesincomplexregionssuchasTokyo.
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Chapter4
Osmiumandlithiumisotopeevidenceforfluctuatingoxidativeandsilicate
weatheringduringperiodicSilurianglaciations*
*AversionofthischapterwillbesubmittedtoNature,co-authoredwithPhilipPoggevon
Strandmann,TimothyM.Lenton,DavidSelby,EmiliaJarochowska,JiriFryda,JindrichHladil,
DavidLoydell,LadislavSlavik,MikaelCalnerandAxelMunnecke
112
TheSilurian(419.2to443.8Ma)wasoneofthemostclimaticallyunstableperiodsofthe
Phanerozoic,punctuatedbyfourlargepositivecarbonisotope(δ13C)excursions,associated
withrapidturnoverinmarinebiota.Despiteovertwodecadesofresearch,thecauseof
theseclimaticfluctuationsisstillunclear.Hereweuseosmium(187Os/188Os)andlithium
(δ7Li)isotopemeasurementsofshalesandcarbonatesspanningfourofthemost
prominentSilurianδ13Cexcursions,toassesstheroleofcontinentalweatheringduringthis
time.Wefindtwopeaksofradiogenic187Os/188Oscompositionseithersideofthelightest
δ7Livaluesduringeachδ13Cexcursion.Geochemicalmodellingattributesthistoperiodic
continentalglaciations,whichacttoenhancetheoxidativeweatheringoforganic-and
sulphide-richlithologies,whilstsuppressingglobalsilicateweathering.Theproductionof
atmosphericCO2fromoxidativeweathering,coupledtoareductioninsilicateweathering–
andthereforeatmosphericCO2removal–actedtoreversethelong-termdeclinein
atmosphericCO2andglobaltemperaturesdrivenbyorogenesis,landplantdiversification,
reducedvolcanicarcdegassingand/orchangesinpaleogeographyduringtheSilurian.
4.1Introduction
OverthepasttwodecadesithasbecomeapparentthattheSilurianisthemostclimatically
unstableperiodofthePhanerozoic,punctuatedbyfourlargeamplitude(>5‰)positive
carbonisotope(δ13C)excursions(Fig.1)associatedwithfluctuationsinthecarboncycle,
seawatertemperaturesandfaunalextinctionrates(Calner,2008;Lehnertetal.,2010;
Melchinetal.,2012;Munneckeetal.,2010;Nobleetal.,2005).Traditionalexplanationsfor
theseeventshaveinvokedashiftbetweentwostableoceanic-climatestates,drivenby
changesinthelocationofdeep-waterformationfromhightolowlatitudes(Jeppsson,
1990),orglobalprecipitationratesandcontinentalrunoff(Bickertetal.,1997).However,
113
theseearlyattemptstoexplainSilurianclimaticeventshavereceivedmuchcriticism
(Johnson,2006;Kaljoetal.,2003;Loydell,1998).
Figure1.Osmium(187Os/188Os,greensquares),lithium(δ7Li,reddiamonds),oxygen(δ18O,
bluecircles)andcarbon(δ13C,blackcircles)isotoperatiosforshaleandcarbonatesections
measured.a,Klonk,b,Kosov(Os),c,BartoszyceIG-1,d,Aizpute-41,e,Kosov(Li),f,
Hunningeandg,Lusklint&Lickershamn.Datafromthisstudyiscomparedtotheglobal
conodont(C)andgraptolite(G)bio-events(Melchinetal.,2012),δ13C(Saltzmanand
Thomas,2012;Crameretal.,2011a),δ18O(SeeTrotteretal.,2016),glacialtillites(thick
dashedblackline)(Caputo,1998;Díaz-MartínezandGrahn,2007;GrahnandCaputo,1992)
andsea-levelreconstructions(HaqandSchutter,2008;Johnson,2006,2010;Loydell,
1998).
114
MorerecentlyithasbeenpostulatedthatSilurianclimaticchangecouldhave
beendrivenbyglacialexpansionoverGondwana,inferredinpartfrompositiveoxygen
isotope(δ18O)shifts(Fig.1)(Azmyetal.,1998;Brandetal.,2006;ErikssonandCalner,
2007;Kaljoetal.,2003;Lehnertetal.,2010;Trotteretal.,2016;Žigaitėetal.,2010)
coupledtosignificanteustaticsea-levelchange(HaqandSchutter,2008;Johnson,2006,
2010;Loydell,1998),muchliketheLateOrdovicianthatprecededit(Algeoetal.,2016;
Harperetal.,2014).However,thelackofglacialsedimentsinthestratigraphicrecordfor
muchoftheSilurian(post-Wenlock)hashamperedthisnotion(Caputo,1998;Díaz-
MartínezandGrahn,2007;GrahnandCaputo,1992).
Intheabsenceofstratigraphicevidenceforglacialsediments,seawater
chemistrycanbecomeapowerfularchiveforreconstructingearthsystemresponsesto
climaticortectonicchange,andseveralisotopesystemshavebeenutilisedtoreconstruct
continentalweatheringanderosion.Traditionally,therubidium-strontium(87Rb-86Sr)
radiogenicisotopesystemhasbecomethemostwidelyused,andvariationsinmarine
87Sr/86Srareseentoreflectfluctuationsincontinentalinputscausedbyorogenesis(Raymo
etal.,1988)andglaciation(Armstrong,1971).However,thelongresidencetimeof
strontiumintheoceans(2-4Myr)andmultiplecontinentalsourcesfrombothcarbonate
andsilicateweathering(Jacobsonetal.,2002;PalmerandEdmond,1992)meansthat
short-periodicfluctuationsinunambiguousinputsarehardtodetect(Hodelletal.,1990;
RichterandTurekian,1993).Unlikestrontium,OsandLiisotopesystemscanovercome
someofthesedifficulties.
Theosmiumisotopiccomposition(187Os/188Os)ofseawaterreflectsabalance
betweentheweatheringofradiogenicOs-richsedimentaryrocksorsilicatemineralsand
unradiogenicmantleandextraterrestrialderivedsources(Georgetal.,2013;Peucker-
EhrenbrinkandRavizza,2000).Likewise,thelithiumisotopiccomposition(δ7Li)ofseawater
115
reflectsthebalancebetweenchemicalweatheringofcontinentalsilicateminerals,high
temperatureweatheringofoceanicsilicatemineralsatmid-oceanridgespreadingcentres
(Chanetal.,1993;ElderfieldandSchultz,1996;Huhetal.,1998),andincorporationinto
alteredoceaniccrustandauthigenicclays(Chanetal.,1992;Chanetal.,2006;Jamesetal.,
1999;MackenzieandGarrels,1966;MisraandFroelich,2012;Seyfriedetal.,1998;Verney-
Carronetal.,2011;Vigieretal.,2008).Whencombinedwithrelativelyshortresidence
timesinseawater(~1-50kyrforOsand~1-1.5myrforLi)(Huhetal.,1998;Oxburgh,
2001;Rooneyetal.,2016;StoffynegliandMackenzie,1984),theseisotopesystemshave
permittedtheabilitytounlockvitalinformationaboutaseriesofEarthsystemprocesses
suchasfloodbasaltvolcanism(CohenandCoe,2002;DuVivieretal.,2014;Ravizzaand
Peucker-Ehrenbrink,2003;TurgeonandCreaser,2008),paleoweathering(Hathorneand
James,2006;Lechleretal.,2015;MisraandFroelich,2012;PoggevonStrandmannetal.,
2013;Ravizzaetal.,2001;Schmitzetal.,2004),basinconnectivity(PoirierandHillaire-
Marcel,2009),bolideimpacts(Paquayetal.,2008)andmantleandperidotiteformation
(PoggevonStrandmannetal.,2011).Intandem,thesesystemshaverecentlybeenutilised
todeterminehowsilicateweatheringhasbeeninfluencedbycontinentalicevolume,
atmosphericCO2andglobaltemperaturesduringtheLateOrdovicianHirnantianmass
extinction(Finlayetal.,2010;PoggevonStrandmannetal.,inreview).
Here,shalesweremeasuredforosmiumisotopes(187Os/188Os),andbulk
carbonatesweremeasuredforlithiumisotopes(δ7Li)andtracemetals,instratigraphic
sectionsthatspanthelateTelychian-earlySheinwoodian,mid-Homerian,mid-Ludfordian
andlatePridoli-earlyLochkovianpositivecarbonisotopeexcursions(SeeFig.1).Thisstudy
representsthefirstapplicationofOsandLiisotopestostratigraphicsectionsfromthe
Silurian.TheresultsdemonstratetheabilityofOsandLiisotopestotraceglobal
fluctuationsinEarthsystemprocesses,andwhencombinedwithdynamicisotopemodels,
elucidatepotentialcauses,suchasfluctuationsinoceancirculation,floodbasaltvolcanism,
116
globaltemperature,hydrothermalactivityand/orcontinentalicevolume.Finally,thisstudy
looksatgeologicalandclimatictrendsinatmosphericCO2duringtheSilurian,andrelates
themtonegativefeedbackmechanismsthathelpmaintainahabitableplanet.
4.2Materialsandmethods
4.2.1Geologicalsetting
Inthisstudy,fourshalesectionswereanalysedforosmiumisotopesandfourbulk
carbonatesectionswereanalysedforlithiumisotopes(SeeFig.3to9).Combined,these
sectionscoveredfourperiodsofSiluriantime:thelateTelychiantoEarlySheinwoodian;
midHomerian;lateLudfordian;andtheSilurian-Devonianboundary.Shalesfromthe
Aizpute-41core(Latvia)andcarbonatesfromtheLusklint&Lickershamnsections(Gotland,
Sweden)coverthelatestTelychiantoearliestSilurian.ShalesfromtheBartoszycecore
(Poland)andbulkcarbonatesfromtheHunninge-1core(Sweden)coverthemidHomerian.
ShalesandbulkcarbonatesfromtheKosov(CzechRepublic)sectioncovertheLate
Ludfordian.ShalesfromtheKlonkcore(CzechRepublic)covertheSilurian-Devonian
Boundary.ThelocationofthesesectionscanbefoundinFigure2.Sectionswerechosen
becausetheyhaveallhadextensivecarbonisotopeandbiologicalstratigraphycarriedout.
Meanwhiletheyrepresentthemostdistalcoastalenvironemntsawayfromrestricted
basins,andthereforearethemostlikelysectionstorecordoceansignatures.Thefollowing
willdetailthegeology,samplingstrategyandpaleoenvironmentofeachsectionstudied.
117
Figure2.Silurian(425Ma)paleogeographicmap(adaptedfromMelchinetal.,2012).
SamplelocationsfromGotland(Hunninge-1),Aizpute-41,BartoszyceandtheBarrandian
(KosovandKlonk)arehighlighted.Darkandlightbrownrepresentthepositionof
continentsandcontinentalshelvesrespectively,duringtheSilurian.Whitelinesrepresent
theconfigurationofmoderncontinents.
4.2.1.1Aizputecore
ThelateTelychian-earlySheinwoodianAizpute-41coreislocatedinthetownofAizpute,
situatedinwesternLatvia,inthedeepershelfpartoftheEasternBaltoscandianBasin.The
latestLlandoverybedsconsistofgreenishandbrownishgreymarlstones,whiletheearliest
Wenlockconsistsofgreen,greyandbrownmarlstoneswithcalcareousmarlstones(Loydell
etal.,2003).Samplingstrategyofgraptolite-rich,relativelyhightotalorganiccarbon(TOC)
shaleswasfollowedaccordingtoLoydelletal.(2003).Theδ13Ccarbdataforthedrillcore
materialcanbefoundinCrameretal.(2010).
4.2.1.2Lusklint&Lickershamn
ThelateTelychian-earlySheinwoodianLowerVisbyformationandtheearliest
SheinwoodianUpperVisbyformationcanbefoundoutcroppingatLusklintand
Lickershamnalongthenorth-westerncoastofGotland,Sweden.TheLowerVisby
formationconsistsofupto12mofregularalternationsof2-5cmthick,wavybeddedto
118
nodularargillaceouslimestonesandapproximately10cmthickmarlswhichwere
depositedinadistalshelfenvironment,belowstormwavebaseandthephoticzone
(Maier,2010;Calneretal.,2004).However,thebeddingintheUpperVisbyformationis
notasregular,withtheratioofmarltolimestoneincreasingintheuppermostpartofthe
formationwheredetriticlimestonesbecomemoredominant(Maier,2010).Samplesfrom
theLowerVisbyformationweresampledfromLusklintwhilesamplesfromtheUpperVisby
formationweresampledfromLusklint.Theδ13Ccarbdataforthesamplescanbefoundin
Maier(2010).
4.2.1.3Bartoszyce
Themid-HomerianBartoszyceIG1boreholeislocatedintheeasternpartofthePeribaltic
SyneclizeofthePolishpartoftheEastEuropeanplatform.Thecoreconsistsofsparsely
bioturbated,light-greylaminatedandcalcareousmudstones(Porębskaetal.,2004).
SamplingstrategyofrelativelyhighTOCshales,δ13Ccarbandδ18Ocarbdatacanbefoundin
Porębskaetal.(2004).
4.2.1.4Hunningecore
Themid-Homerianhunninge-1coreislocatedintheHunningequarryofwesternGotland,
Sweden.TheGannarvememberconsistsofalternatingbedsofbrownish,argillaceous,fine
dolostonewithsiltydolomarlstoneandalumshales(Calneretal.,2006).Theoverlyingbara
oolitememberandbrickclaymemberconsistofcoatedgrainsandargillaceous,nodular
limestonealternatingwithshalerespectively(Calneretal.,2006).Samplingstrategyfor
carbonatesandδ13CcarbdatacanbefoundinCalneretal.(2006).
4.2.1.5Kosov
Themid-LudfordianKosovsectionislocatedintheBarrandianregionoftheCzechRepublic.
Thekozlowskiibiozoneconsistsofalternatingbedsofgreyfinelylaminatedshaleandlight-
greypackstonesandgrainstones,whiletheoverlyingPristiograptusdubiuspostfrequens
119
biozoneconsistsofalternatingbedsoflight-greypackstones/grainstonesandmudstonesor
greycoarselylaminatedcalcareousshales.Samplingstrategyofcarbonatesandrelatively
highTOCshalesandδ13CcarbdatacanbefoundinFrýdaandManda(2013).
4.2.1.6Klonkcore
ThePridoli-Lochkovian(Silurian-Devonian)GSSPislocatedintheCzechRepublic35km
southwestofPrague.ThelatestPridolianandearliestLochkovianbedsconsistofgrayish-
black,platy,mostlyfine-grainedbituminouslimestonesalternatingwithcalcareousshale
interbedswithoccasionalstringersofcrinoidallimestones(Beckeretal.,2012).Sampling
strategyofrelativelyhightotalorganiccarbon(TOC)drillcorematerialwasfollowed
accordingtoCricketal.(2001).δ13Ccarbandδ18Ocarbdataforthedrillcorematerialcanbe
foundinBuggischandMann(2004).
4.2.2Samplepreparation
Priortocrushing,20-80gofshalesamplewaspolishedtoeliminatecontaminationfrom
cuttinganddrillingmarksandsampleswithanysignsofveiningorweatheringwere
avoided.Theshalesampleswerethendriedat60°Cfor~12hbeforebeingbrokeninto
chipswithnometalcontact.Bulkcarbonatesandshaleswerecrushedtoafinepowder
(~30μm)inaZirconiaceramicdishusingashatterbox.Bulkcarbonateswereleachedusing
asequentialextractionmethod(PoggevonStrandmannetal.,2013;Tessieretal.,1979),
whereby~0.1gofcarbonatewasleachedfor5hatroomtemperatureusingNaacetate
bufferedtopH5byaceticacid.Leachingofinterstitialsilicateswasmonitoredusing
elementalratiossuchasAl/CaandMn/Ca.TheAl/Caratiomustbegreaterthan>0.8
mmol/molbeforesilicate-derivedLiwillperturbthed7Limeasuredincarbonates(Pogge
vonStrandmannetal.,2013).ThesamplepreparationandRe-Osisotopeandtracemetal
analysiswascarriedoutattheDurhamGeochemistryCentre(LaboratoryforSulfideand
120
SourceRockGeochronologyandGeochemistry)atDurhamUniversity.TheLiisotope
analysiswascarriedoutatthestableisotopelabtheUniversityofOxford.
4.2.3Osmiumisotopeanalysisofshales
Rheniumandosmiumabundancesandisotopiccompositionsweredeterminedusing
isotopedilutionnegativethermalionisationmassspectrometryusingCrVI–H2SO4digestion
andsolventextraction(CHCl3),micro-distillationandanionchromatographymethods
(Creaseretal.,1991;Cummingetal.,2013;SelbyandCreaser,2003).TheCrVI–H2SO4
digestionemployedhereprincipallydissolvestheorganicfractionofashale,thusliberating
thehydrogenousRe-Osloadofthesediment,andthereforeavoidingdetrital
contamination(Kendalletal.,2004;SelbyandCreaser,2003).
Forallshalesamplesbetween0.5and1gofpowderwasaddedtoacarius-tube
withaknownamountof185Re-190Ostracersolutionand8mlof0.25g/gCrVIO3-4NH2SO4at
220°Cfor48h.OsmiumwasisolatedfromtheacidmediumusingCHCl3solventextraction,
withbackextractioninHBr,andthenfurtherpurifiedusingamicro-distillationtechnique.
RheniumwasisolatedusingNaOH-C3H6Osolventextractionandpurifiedusinganion
chromatography.TheisolatedReandOsfractionswereloadedontoNiandPtfilaments
respectively,andtheirisotopiccompositionwasdeterminedusingaThermoScientific
TRITONmassspectrometerusingFaradaycollectorsandthesecondaryelectronmultiplier,
respectively.
TotalproceduralblanksforReandOsare1.1and0.1pgrespectively,withan
average187Os/188Osof1.3(n=6).RawReandOsoxidevalueswerecorrectedforoxygen
contributionandmassfractionation.Calculateduncertaintiesincludethoseassociatedwith
massspectrometermeasurements,blankabundanceandisotopiccomposition,spike
121
calibration,andsampleandspikeweights.In-housestandardsolutionsofReandOs
(DROsS)yieldanaverage185Re/187Revalueof0.59872±0.00135(1SD,n=24),and
187Os/188Osof0.16101±0.000401(1SD,n=41),respectively,whichisidentical,within
uncertaintytothepreviouslypublishedvalues(Nowelletal.,2008).
Initial187Os/188Os(187Os/188Osi)valuesinthisstudyweredeterminedfromRe-Os
dataandthe187Redecayconstant(1.666e−11a−1)(Smoliaretal.,1996)andinterpolated
graptolitebiozoneages(Melchinetal.,2012).Analyticaluncertaintyforindividual
calculatedOsiis<0.05.Thereproducibilityofcalculated187Os/188Osiwasbasedon15
analysesoftheUSGSrockreferencematerialSBC-1(BushCreekShale)andhasavalueof
~0.65±0.1(2SD).Thisuncertaintywasusedtoaccountforthemaximumuncertaintyin
thesampleset.Calculated187Os/188Osiratiosassumeclosedsystembehaviorafter
depositionwithrespecttobothrheniumandosmiumandthatthe187Os/188Osratiosreflect
theisotopecompositionofthelocalseawateratthetimeofsedimentdeposition,andare
unaffectedbymineraldetritus.
4.2.4Lithiumisotopeanalysisofbulkcarbonates
AsplitofeachsamplesolutionwasretainedforcationanalysisusinganElanQuadrupole
inductivelycoupledplasmamassspectrometer.Sampleswerematrixmatchedto10μg/g
Caandcalibratedagainstasetofsyntheticstandardsmadeupfromsingleelement
solutions.TheAl/CaratioincarbonateswasmonitoredtodetecttheinfluenceofLileached
fromsilicateclays.Previousworksuggestscarbonatesmustbe>0.8mmol/molbefore
carbonateLiisotoperatiosbecomemeasurablyperturbedbyLileachedfromclays(See
PoggevonStrandmannetal.,2013).Accuracyandprecisionwereassessedbyrepeated
analysesofseawater,theinternationalreferencematerialJLs-1and,inordertoassess
reproducibilityofbothanalysesandcarbonateleaching,repeateddissolutionsandanalysis
122
ofasampleofthePlenusMarlfromEastbournewereundertaken.Samplereproducibility
ofLi/CaandAl/Cawas~7%(2SD,n=6).
Thelargerpartofeachsample(typicallycontaining5–10ngLi)waspurifiedby
passingitthroughatwo-stagecation-exchangeprocedure(PoggevonStrandmannetal.,
2011).GiventhatLiisotopesfractionateduringcationchromatography,itiscriticaltohave
columnyieldscloseto100%(Tomascak,2004).Toassesstheefficacyofthisprocess,splits
ofthesolutionwerecollectedbeforeandafterthecollectedbracketforLi,andwere
analysedforLicontent.Resultsshowedthat<0.1%ofLiwaspresentinthesesplits.
ThetotalproceduralblankforLiisotopeanalysisis~0.02ngLi,whichis
insignificantcomparedtothemassofsampleused.AnalyseswereperformedonaNu
PlasmaHRmulti-collectorICP-MS,usingasample-standardbracketingsystemrelativeto
theLSVECstandard(Fleschetal.,1973).Eachsamplewasmeasuredthreeseparatetimes
duringananalyticalsession,repeatmeasurementsbeingseparatedbyseveralhours,but
duringthesameanalyticalsession.Eachindividualmeasurementconsistedof10ratios(10
stotalintegrationtime),givingatotalintegrationtimeof300s/sample.Atanuptakerate
of75μl/min,thesensitivityfora20ng/mlsolutionis~18pAof7Li.Background
instrumentalLiintensity,typically~0.01pA,wassubtractedfromeachmeasurement.
Accuracyandexternalreproducibility,asassessedfromseawater,is31.1±0.6‰(2SD,n=
16,chemistry=16).Precisionwasalsoassessedfromrepeatedanalyses(includingleaching
andchemistry)ofanin-housemarlstandard,whichalsogivesareproducibilityof±0.6‰
(n=7).
123
4.2.5Isotopemodeling
DynamicboxmodelsforseawaterOsandLicycleshavebeenutilisedtoexplorecausesof
thevariationsseeninthedata.ThemodelswereadaptedfromthosepresentedinPogge
vonStrandmannetal.(2013).
4.2.5.1Osmiumisotopemodeling
TheOsmodelwasconstructedfromthefollowingmassbalanceequation:
+,Os
+-= 𝐹sil + 𝐹ORLW + 𝐹lth + 𝐹hth + 𝐹dust − 𝐹out Eq.1
whereNistheseawaterOsreservoir,andFxrepresentstheinputandoutput
fluxes:sil=riverinputrelatedtosilicateweathering;ORLW=riverinputrelatedtothe
weatheringoforganic-sulphide-richlithologies;lht=low-temperaturehydrothermal;hth=
high-temperaturehydrothermal;anddust=aeoliandust.Theisotopebalanceequationis
thengivenby:
𝑁Os+1SW
+-= 𝐹sil 𝑅sil − 𝑅SW + 𝐹ORLW 𝑅ORLW − 𝑅SW + 𝐹lht 𝑅lth − 𝑅SW +
𝐹hth 𝑅hth − 𝑅SW + 𝐹dust 𝑅dust − 𝑅SW − 𝐹out(𝑅out − 𝑅SW)
Eq.2
whereRxistheisotoperatioofthevariousfluxes.Finally,thecalculationofthe
sinkofOsfromseawaterisbasedontheassumptionthatpartitioningintothesinkisdue
toaconstantpartitioncoefficientk,where:
𝐹out = 𝑘×𝑁 Eq.3
Theseequationsdifferfromtheoriginalmodel(PoggevonStrandmannetal.,
2013)astheriverineinputhasbeenpartitionedintotwocomponentsderivedfromsilicate
weatheringandorganic-sulphide-richlithologyweathering(ORLW)(SeeGeorgetal.,2013).
DuringtheCenozoic,riverineOsfluxesarelargelycontrolledbytheoxidativeweatheringof
124
ORLandrepresents70%ofriverineOsbudget,withsilicateweatheringrepresentingthe
other30%(Georgetal.,2013;Lietal.,2009).However,duringtheSilurian,theabsenceof
extensiveterrestrialvegetationwilllowertheavailabilityoforganic-richsedimentsfor
weathering,andsilicateweatheringwillthereforerepresentamoredominantcontrolon
theriverineOsbudget(60%).Cosmicdustisnotincludedinthemodelasitisunlikelyits
fluxchangedduringthistime.
4.2.5.2Lithiumisotopemodeling
TheLimodelwasconstructedfromthefollowingmassbalanceequation:
+,Li
+-= 𝐹riv + 𝐹hth − 𝐹sed Eq.4
whereNistheseawaterLireservoir,andFxrepresentstheinputandoutput
fluxes:riv=river;hth=hydrothermal;andsed=sediment(combineduptakeintomarine
sediments,andalterationoftheoceaniccrust).Theisotopebalanceequationisthengiven
by:
𝑁Li+1SW
+-= 𝐹riv 𝑅riv − 𝑅SW + 𝐹hth 𝑅hth − 𝑅SW − 𝐹sed(𝑅sed − 𝑅SW) Eq.5
whereRxistheisotoperatioofthevariousfluxeswhereSW=seawater.Rsinkis
givenby∆sink=Rsink-Rsw,where∆7Lisink=15-16‰(Chanetal.,1993;Huhetal.,1998;Misra
andFroelich,2012).Finally,thecalculationofthesinkofLifromseawaterisbasedonthe
assumptionthatpartitioningintothesinkisduetoaconstantpartitioncoefficientk,
where:
𝐹out = 𝑘×𝑁 Eq.6
ThefractionationofLiuptakeintocarbonatesisaccountedforbythemodel.
Assumingamodernoceanicsink(Hazenetal.,2013)andhydrothermalδ7Li,an
125
unfractionatedriverineδ7Li(~0‰)similartocontinentalcrust(Sauzéatetal.,2015)is
requiredtoproducetheδ7LiofseawaterrecordedfortheHirnantian(Poggevon
Strandmannetal.,inreview).Suchanisotopicallylightriverinefluxwasmostlikelydueto
thedominationofillites,whichcauselittlefractionation(MillotandGirard,2007),priorto
theadventofterrestrialplants(PoggevonStrandmannetal.,inreview).Liwasmodelled
over10kyrsteps,whileOswasmodeledover5kyrsteps.Table1liststhevaluesusedin
themodelsforeachsystem.
Table1
Modelinputparameters.StartingparametersarebasedonPoggevonStrandmannetal.,
2013andPoggevonStrandmannetal.,2017.However,Osriverinefluxhasbeenmodified
torepresentcontributionsfromtheweatheringofbothsilicatesandORL.Seetextfor
detailsonmodelperturbations.
StartingParameters Li Os
Friver(Gmol/yr) 20 Silicate(mol/yr) 600
ORLW(mol/yr) 400
Fhydro(Gmol/yr) 9.3 low-Thydro(mol/yr) 95.4
high-Thydro(mol/yr) 366
Fdust(mol/yr)
150
Rriver 0 Silicate 0.6
ORLW 1.34
Rhydro 7 low-Thydro 0.878
high-Thydro 0.115
Rdust 1.05
126
4.3Results
4.3.1Rhenium-osmiumisotopedata
RheniumandosmiumisotopecompositionsandabundancedatafortheAizpute-41
(Latvia),Bartoszyce(Poland),Kosov(CzechRepublic)andKlonk(CzechRepublic)samples
arepresentedinTable2.
Table2
RheniumandosmiumabundanceandisotopedatafortheAizpute-41,Bartoszyce,Kosov
andKlonkshalesamples.Initial187Os/188Os(187Os/188Osi)werecalculatedusinggraptolite
biozoneagesfrom(Melchinetal.(2012).
Depth Re 2s.e. Os 2s.e. 187Re/188Os 2s.e. 187Os/188Os 2s.e. 187Os/188Osi 2s.e.
(m) (ppb) (ppt)
Aizpute-41Core,Latvia
910.06 2.22 0.01 65.0 0.7 206.9 2.9 2.10 0.04 0.602 0.015
910.19 2.31 0.01 67.2 0.7 208.8 3.0 2.11 0.04 0.595 0.014
910.60 2.42 0.01 74.0 0.8 196.9 2.8 2.06 0.04 0.629 0.015
910.90 2.91 0.05 73.8 0.6 244.6 4.9 2.33 0.03 0.558 0.013
911.33 3.23 0.03 87.2 0.7 226.6 3.0 2.18 0.03 0.540 0.010
912.00 2.79 0.05 77.3 0.6 217.4 4.3 2.04 0.03 0.466 0.011
912.90 2.17 0.04 98.9 0.5 121.7 2.3 1.30 0.01 0.414 0.008
914.01 2.02 0.15 84.9 0.5 136.6 10.1 1.59 0.01 0.602 0.045
914.74 5.60 0.01 130.7 1.1 271.3 2.5 2.53 0.03 0.564 0.009
914.80 5.53 0.02 135.4 1.1 250.0 2.3 2.20 0.03 0.391 0.006
914.95 3.26 0.01 115.1 0.7 166.7 1.2 1.83 0.02 0.620 0.007
915.90 7.88 0.25 128.0 0.9 433.6 14.0 3.66 0.03 0.521 0.017
916.50 10.52 0.19 152.8 1.0 502.4 9.2 4.06 0.03 0.422 0.008
917.70 10.20 0.33 144.1 1.1 525.5 17.0 4.28 0.03 0.468 0.016
919.96 9.59 0.17 144.5 1.1 481.5 8.9 4.00 0.03 0.515 0.010
924.70 1.88 0.01 81.2 0.5 131.7 1.1 1.51 0.01 0.557 0.007
924.98 1.52 0.01 78.6 0.5 108.2 0.8 1.38 0.01 0.593 0.007
925.21 3.77 0.04 98.3 0.5 237.6 2.6 2.32 0.01 0.597 0.007
925.65 9.64 0.03 152.7 1.3 460.6 3.5 4.07 0.04 0.733 0.009
928.01 16.74 0.04 207.6 1.8 644.7 4.5 5.18 0.05 0.511 0.006
930.38 17.95 0.32 230.0 1.8 606.8 11.2 4.83 0.04 0.433 0.009
932.35 27.46 0.07 274.0 2.2 929.8 5.2 7.23 0.05 0.489 0.004
934.20 15.86 0.04 249.7 1.9 439.6 2.9 3.47 0.03 0.287 0.003
935.82 5.98 0.02 143.5 0.9 253.5 1.7 2.13 0.02 0.296 0.003
127
BartoszyceCore,Poland
1674.20 12.46 0.22 162.3 1.3 597.0 11.1 4.83 0.04 0.551 0.011
1672.90 12.55 0.40 196.3 1.5 454.8 14.8 3.78 0.03 0.517 0.017
1670.00 8.71 0.02 131.5 1.0 482.9 3.0 4.06 0.03 0.595 0.006
1666.65 9.46 0.17 147.8 1.0 461.5 8.5 3.93 0.03 0.618 0.012
1665.00 14.96 0.27 238.4 1.6 449.5 8.3 3.86 0.03 0.638 0.013
1663.75 3.01 0.05 69.8 0.6 275.5 5.5 2.65 0.03 0.670 0.016
1662.30 1.71 0.06 42.7 0.5 253.2 8.9 2.53 0.05 0.711 0.029
1661.80 1.10 0.02 27.9 0.4 244.2 6.7 2.35 0.07 0.603 0.024
1661.70 1.29 0.01 30.0 0.4 268.5 5.7 2.44 0.07 0.513 0.018
1661.40 1.17 0.02 34.6 0.5 203.6 5.6 2.04 0.06 0.580 0.023
1660.72 0.74 0.02 26.2 0.6 166.2 8.7 1.76 0.10 0.567 0.044
1660.70 0.51 0.01 34.2 0.7 81.1 3.4 1.18 0.07 0.594 0.042
1660.00 4.09 0.04 87.4 2.1 298.5 12.4 2.61 0.15 0.471 0.033
1659.75 3.80 0.07 106.3 0.9 221.5 4.4 2.32 0.03 0.727 0.017
1659.65 0.53 0.01 39.8 0.2 72.9 1.1 1.16 0.01 0.639 0.012
1657.95 3.81 0.01 126.9 1.0 177.5 1.5 1.87 0.02 0.601 0.009
1656.72 4.38 0.25 126.4 0.9 210.1 12.1 2.12 0.02 0.611 0.036
1652.36 8.61 0.49 192.2 1.2 283.8 16.3 2.54 0.02 0.500 0.029
1648.20 7.93 0.02 184.6 1.3 272.9 2.0 2.57 0.02 0.615 0.007
1647.20 9.32 0.02 204.1 1.2 290.9 1.5 2.60 0.02 0.515 0.004
Kosov,CzechRepublic
-7.60 5.60 0.39 79.7 1.0 499.9 35.7 3.78 0.07 0.235 0.017
-6.5 3.28 0.01 55.9 0.6 401.1 4.9 3.33 0.05 0.486 0.010
-6 4.26 0.01 60.9 0.7 518.7 6.1 4.25 0.07 0.570 0.011
-5.00 4.45 0.08 100.9 0.8 285.9 5.8 2.79 0.03 0.761 0.017
-3.65 9.34 0.53 134.0 1.0 511.4 29.4 4.13 0.03 0.509 0.029
-2.20 2.07 0.04 22.3 0.3 802.9 19.5 6.25 0.13 0.559 0.018
-1.00 8.35 0.16 102.7 1.1 623.6 12.8 4.66 0.06 0.243 0.006
-0.15 15.28 0.87 145.7 1.4 949.4 54.7 6.86 0.06 0.129 0.008
0.15 1.65 0.01 25.1 0.2 473.3 5.9 3.89 0.05 0.533 0.009
1.75 0.33 0.03 12.9 0.1 146.1 11.7 1.48 0.03 0.443 0.036
2.10 0.84 0.02 30.0 0.5 160.7 5.4 1.61 0.06 0.472 0.024
4.25 0.22 0.00 10.4 0.1 124.8 3.1 1.63 0.04 0.740 0.024
9.45 0.64 0.04 27.0 0.6 140.7 10.0 1.93 0.11 0.930 0.085
14.60 1.16 0.00 21.9 0.3 355.3 7.6 3.12 0.09 0.605 0.022
16.60 3.02 0.01 35.4 0.4 721.7 7.2 5.90 0.08 0.788 0.013
KlonkCore,CzechRepublic
16.37 1.93 0.07 66.3 0.5 175.0 6.4 2.02 0.03 0.789 0.031
17.53 2.77 0.01 81.6 1.1 206.7 4.3 2.13 0.06 0.682 0.024
18.67 7.30 0.26 176.3 1.5 259.9 9.5 2.44 0.03 0.619 0.024
19.82 4.45 0.16 152.2 1.2 172.9 6.3 1.87 0.02 0.662 0.026
20.73 2.60 0.01 137.6 1.4 110.7 1.6 1.78 0.03 1.006 0.024
21.22 1.98 0.01 104.8 1.1 110.6 1.6 1.76 0.03 0.984 0.024
22.25 2.37 0.01 111.1 1.1 126.8 1.8 1.93 0.04 1.044 0.025
128
23.27 1.81 0.01 64.0 0.7 176.5 2.7 2.39 0.05 1.149 0.029
23.45 3.50 0.01 76.4 1.1 304.0 5.8 3.03 0.08 0.898 0.030
23.65 6.46 0.02 148.2 1.9 282.7 4.7 2.77 0.06 0.792 0.022
23.95 2.35 0.01 59.6 0.7 248.2 4.3 2.48 0.06 0.745 0.021
24.25 4.61 0.02 142.2 1.7 198.0 3.3 2.17 0.05 0.787 0.022
24.4 6.00 0.02 158.1 1.9 234.3 4.0 2.29 0.05 0.643 0.019
24.54 8.84 0.03 182.4 1.6 321.0 3.2 3.00 0.04 0.750 0.012
24.65 3.52 0.02 124.9 1.5 167.0 3.0 1.88 0.04 0.710 0.021
24.72 3.76 0.02 125.2 1.5 181.9 3.2 2.11 0.05 0.832 0.024
24.77 4.07 0.02 161.2 1.8 146.6 2.5 1.70 0.04 0.671 0.019
24.87 5.77 0.02 150.2 1.8 235.5 4.1 2.22 0.05 0.567 0.016
24.87 6.04 0.02 159.7 1.3 232.2 2.1 2.22 0.03 0.590 0.009
25.01 3.12 0.02 84.5 1.1 229.3 4.4 2.33 0.06 0.721 0.022
25.21 5.06 0.02 123.4 1.6 258.1 4.6 2.47 0.06 0.665 0.020
25.35 6.71 0.02 136.2 1.8 325.3 5.7 2.97 0.07 0.691 0.020
25.42 2.54 0.01 111.1 1.2 137.3 2.0 2.00 0.04 1.039 0.025
25.60 6.34 0.02 116.8 1.4 370.3 5.3 3.32 0.07 0.725 0.018
25.98 3.75 0.01 118.8 1.3 190.5 2.7 2.07 0.04 0.732 0.018
26.22 2.13 0.01 99.4 1.0 123.5 1.8 1.62 0.03 0.754 0.018
26.45 5.13 0.02 152.4 1.3 215.9 2.0 2.66 0.03 1.148 0.017
26.78 10.42 0.04 175.5 2.4 411.3 6.9 3.48 0.08 0.602 0.017
27.25 6.47 0.02 208.0 2.4 186.2 3.1 1.98 0.04 0.673 0.019
27.8 17.46 0.06 200.9 1.4 729.0 3.9 5.81 0.03 0.697 0.005
28.34 4.55 0.02 105.6 0.9 274.3 2.6 2.60 0.03 0.678 0.010
29.28 6.86 0.24 185.3 1.0 229.0 8.2 2.30 0.01 0.690 0.025
30.15 9.95 0.35 224.8 1.3 283.7 10.1 2.65 0.02 0.665 0.024
31.45 6.94 0.25 206.0 1.1 204.8 7.3 2.14 0.01 0.701 0.025
129
4.3.1.1Aizpute-41Core
Figure3.Osmium(187Os/188Os,greensquares),oxygen(δ18Ocarb,bluecircles)andcarbon
(δ13Ccarb,blackcircles)isotoperatiosforshalesandcarbonatesfromtheLlandovery-
WenlockAizputecore.Biozone,lithologyandcarbonandoxygendatahavebeenadapted
fromCrameretal.2010.Seetextfordetails.
TheReandOsabundancesand187Re/188Osand187Os/188Osratiosarevariablethroughout
theAizpute-41section([Re]=1.52to27.46ppb;[Os]=65to274ppt;187Re/188Os=108to
930;187Os/188Os=1.3to7.2;Table2;Fig.3).Initial187Os/188Osvaluesrangefrom0.29to
0.73(Table2d;Fig.2).From934.82to965.28m,withinthelatestlapworthiandearliest
murchisonibiozones,187Os/188Osiincreasesfrom~0.29to~0.73.From925.65to916.5m,
187Os/188Osidecreasesfrom~0.73to~0.42.The187Os/188Osithenfluctuatesbetween~0.39
and~0.62duringthelatterpartofthemurchisonibiozone.From912.9to910.6m,spanning
130
thefirmusbiozone,the187Os/188Osiincreasesfrom0.41to0.63.Ahiatusinthe
riccartonensisbiozonepreventsfurtheranalysisintotheWenlock.
4.3.1.2BartoszyceCore
Figure4.Osmium(187Os/188Os,greensquares),oxygen(δ18Ocarb,bluecircles)andcarbon
(δ13Ccarb,blackcircles)isotoperatiosforshalesandcarbonatesfromthemid-Homerian
Bartoszycesection.Biozone,lithologyandcarbonandoxygendatahavebeenadapted
fromPorębskaetal.(2004).Seetextfordetails.
TheReandOsabundancesand187Re/188Osand187Os/188Osratiosarevariablethroughout
theBartoszyceIG-1core([Re]=0.5to15ppb;[Os]=26.2to238.4ppt;187Re/188Os=73to
131
597;187Os/188Os=1.2to4.8;Table2;Fig.4).Initial187Os/188Osvaluesrangefrom0.47to
0.73(Table2;Fig.2c).From1672.9to1662.3m,fromthelatestlundgrenitothebaseof
thenassabiozone,the187Os/188Osiincreasesfrom~0.52to~0.71.Immediatelyafterwards,
the187Os/188Osidecreasesfrom~0.71to~0.51whereitremainsrelativelylow(~0.5)
between1661.7to1660m.From1660to1659.7m,the187Os/188Osisharplyincreasesfrom
~0.47to~0.73.The187Os/188Osithenproceedstodecreasethroughouttherestofthenassa
biozone.
4.3.1.3Kosovsection
TheReandOsabundancesand187Re/188Osand187Os/188Osratiosarevariablethroughout
theKosovsection([Re]=0.2to15.3ppb;[Os]=10.4to145.7ppt;187Re/188Os=124.8to
949.4;187Os/188Os=1.5to6.9;Table2;Fig.5).Initial187Os/188Osvaluesrangefrom0.13to
0.93(Table2;Fig.2b).From7.6to5m,the187Os/188Osiincreasesfrom~0.23to~0.76.Prior
tothebaseofthedubiuspostfrequensbiozone,the187Os/188Osidecreasesfrom~0.76to
~0.13between5and0.13m.The187Os/188Osithenincreasesfrom~0.13to~0.93between
0.13and-9.45mbeforedecreasingagaintowardstheendofthedubiuspostfrequens
biozone
132
.
Figure5.Osmium(187Os/188Os,greensquares),Lithium(δ7Li,redsquares),oxygen(δ18Ocarb,
bluecircles)andcarbon(δ13Ccarb,blackcircles)isotoperatiosforcarbonatesandshales
fromtheLudfordianKosovsection.Biozone,lithologyandcarbonandoxygendatahave
beenadaptedfromFrýdaandManda(2013).Seetextfordetails.
133
4.3.1.4KlonkCore
Figure6.Osmium(187Os/188Os,greensquares),oxygen(δ18Ocarb,bluecircles)andcarbon
(δ13Ccarb,blackcircles)isotoperatiosforcarbonatesandshalesfromtheSilurian-Devonian
GSSPatKlonk.BiozoneinformationadaptedfromCricketal.(2001).Carbonandoxygen
isotopedatahavebeenadaptedfromBuggischandMann(2004).Seetextfordetails.
TheReandOsabundancesand187Re/188Osand187Os/188Osratiosarevariablethroughout
theKlonkcore([Re]=1.8to17.4ppb;[Os]=59.6to224.8ppt;187Re/188Os=110to729;
187Os/188Os=1.6to5.8;Table2;Fig.6).Initial187Os/188Osvaluesrangefrom0.57to1.15
134
(Table2;Fig.2a).From31.45to26.78m,the187Os/188Osiaremoderatelyradiogenic,
rangingfrom~0.6to~0.7.Between26.78and25.35m,the187Os/188Osifluctuatesbetween
~0.6and~1.15.AcrosstheSilurian-Devonianboundary,the187Os/188Osiisalsomoderately
radiogenic,withvaluesbetween~0.57and~0.83(25.35-23.95m).From23.95to23.27m,
the187Os/188Osiincreasesfrom0.74to1.15beforesubsequentlydecreasingfrom1.15to
0.62duringtheearliestDevonian.
4.3.2Lithiumisotopeandtracemetaldata
LithiumisotopemeasurementsandtracemetaldatafortheHunninge-1(Sweden)and
Kosov(CzechRepublic)samplesarepresentedinTable3.
Table3
LithiumisotopeandtracemetaldatafortheHunninge-1andKosovcarbonatesamples.
Depth δ7Li 2s.d. Mg/Ca Al/Ca Mn/Ca Sr/Ca(m) (‰) (μmol/mol) (μmol/mol) (μmol/mol) (μmol/mol)
Lusklint&Lickerdhamn,Sweden38 11.0 0.2 6.50 0.27 0.51 0.1730 12.3 0.2 4.81 0.17 0.49 0.0921 11.1 0.3 5.11 0.15 0.45 0.1014 11.6 0.6 4.39 0.12 0.38 0.124 11.6 0.5 5.53 0.18 0.50 0.181 11.1 0.8 4.45 0.17 0.47 0.09
-0.19 13.5 0.5 4.00 0.15 0.41 0.09-0.66 12.2 0.1 6.63 0.22 0.38 0.20-2.25 16.1 0.2 5.36 0.14 0.29 0.22-3.27 17.6 0.5 4.18 0.21 0.22 0.15-4.2 16.1 0.5 4.15 0.08 0.26 0.12-5.8 14.3 0.1 5.15 0.20 0.32 0.17
Hunninge-1Drillcore,Sweden -4.4 11.2 0.7 39.32 0.536 2.74 0.75
-2.8 11.4 0.3 28.51 0.290 3.64 0.74-1.5 11.4 0.5 40.49 1.006 2.46 0.66-1.1 10.6 0.5 32.11 0.521 1.76 0.29-0.7 9.9 0.1 20.51 0.123 1.97 0.39-0.4 12.2 0.2 41.29 0.827 1.89 0.60-0.2 12.5 0.2 40.12 0.536 1.47 0.680.2 12.7 0.3 48.25 1.002 1.07 0.96
135
0.7 12.6 0.6 24.21 0.108 0.67 0.831 13.4 0.3 15.61 0.007 0.50 0.701.5 13.8 0.2 19.00 0.007 0.53 0.631.8 14.7 0.6 24.94 0.114 0.49 1.274.8 13.0 0.7 22.07 -0.005 0.61 1.35
Kosov,CzechRepublic -4.77 9.6 0.6 13.64 -0.018 1.23 0.56-2.65 6.2 0.5 12.69 0.066 0.78 0.55-2.2 7.8 0.4 14.60 0.014 0.74 0.51-1.25 5.8 0.6 14.48 0.108 0.85 0.44-1 4.6 0.1 12.34 0.200 0.85 0.79-0.7 7.8 0.6
-0.45 6.2 0.4 -0.05 15.4 0.2 10.93 0.052 0.47 0.97
0.5 6.2 0.7 0.9 9.0 0.3 13.24 0.129 0.35 0.95
1.15 5.3 0.5 1.5 6.3 0.6 2.7 6.2 0.2 3.3 5.6 0.2 4.05 6.1 0.1 13.67 0.072 0.41 0.99
5.28 12.8 0.6 13.29 0.065 0.31 0.98
4.3.2.1Lusklintsection
TheMg/Ca,Al/Ca,Mn/CaandSr/CaratiosarevariablethroughouttheHunninge-1drillcore
(Mg/Ca=4.38to6.5μmol/mol;Al/Ca=0.12to0.27μmol/mol;Mn/Ca=0.38to0.51
μmol/mol;Sr/Ca=0.09to0.18μmol/mol;Table3;Fig.7).TheAl/Caratiosremainbelow
the~0.8mmol/molthreshold,suggestinglittleinfluencefromLileachedfromclays.The
δ7Livaluesrangefrom11to12.3‰(Table3;Fig.7).Theδ7Livaluesremainrelatively
constantthroughoutthebicornisandprocerusbiozones.
136
Figure7.Lithium(δ7Li,redsquares)andcarbon(δ13Ccarb,blackcircles)isotoperatiosfor
carbonatesfromtheLlandovery-WenlockLusklintsection.Biozone,lithologyandcarbon
datahavebeenadaptedfromMaier(2010).Seetextfordetails.
4.3.2.2Lickershamnsection
TheMg/Ca,Al/Ca,Mn/CaandSr/CaratiosarevariablethroughouttheHunninge-1drillcore
(Mg/Ca=4to6.63μmol/mol;Al/Ca=0.08to0.22μmol/mol;Mn/Ca=0.22to0.41
μmol/mol;Sr/Ca=0.09to0.22μmol/mol;Table3;Fig.7).TheAl/Caratiosremainbelow
the~0.8mmol/molthreshold,suggestinglittleinfluencefromLileachedfromclays.The
δ7Livaluesrangefrom12.2to17.6‰(Table3;Fig.8).Theδ7Livaluesrelativelylow(~13
‰)atthebaseoftheupperprocerusbiozonesbeforerisingto17.6‰bytheendofthe
upperbiozone,decreasingagainto~14.3‰duringthelowerranuliformiszone.
137
Figure8.Lithium(δ7Li,redsquares)andcarbon(δ13Ccarb,blackcircles)isotoperatiosfor
carbonatesfromtheLlandovery-WenlockLickershamnsection.Biozone,lithologyand
carbondatahavebeenadaptedfromMaier(2010).Seetextfordetails.
138
4.3.2.3Hunninge-1drillcore
Figure9.Lithium(δ7Li,redsquares),oxygen(δ18Ocarb,bluecircles)andcarbon(δ13Ccarb,
blackcircles)isotoperatiosforcarbonatesfromthemid-HomerianHunningecore.Biozone,
lithologyandcarbonandoxygendatahavebeenadaptedfromCalneretal.(2006).Seetext
fordetails.
139
TheMg/Ca,Al/Ca,Mn/CaandSr/CaratiosarevariablethroughouttheHunninge-1drillcore
(Mg/Ca=15.6to48.3μmol/mol;Al/Ca=-0.005to1.006μmol/mol;Mn/Ca=0.49to3.64
μmol/mol;Sr/Ca=0.29to1.35μmol/mol;Table3).ThemajorityofAl/Caratiosremain
belowthe~0.8mmol/molthreshold,suggestinglittleinfluencefromLileachedfromclays.
However,samplesfromadepthof-1.5and0.2mhaveanAl/Caof~1suggestingpossible
influencefromLileachedfromclays.Theδ7Livaluesrangefrom9.9to14.7‰(Table3;Fig.
9).From4.4to1.5m,justpriortothebaseofthenassabiozone,δ7Liremainsrelatively
constant(~11.3‰).Justafterthebaseofthenassabiozone,δ7Lidropsto9.9‰before
rapidlyincreasingto14.7‰between0.7and-1.8m.Theδ7Lithendecreasesto13‰.
4.3.2.4Kosovsection
TheMg/Ca,Al/Ca,Mn/CaandSr/CaratiosarevariablethroughouttheKosovsection
(Mg/Ca=10.93to14.6μmol/mol;Al/Ca=-0.02to0.2μmol/mol;Mn/Ca=0.31to1.23
μmol/mol;Sr/Ca=0.44to0.99μmol/mol;Table3).Al/Caremainsbelowthe0.8mmol/mol
threshold,suggestinglittleinfluencefromLileachedfromclays.Theδ7Livaluesrangefrom
4.6to15.4‰(Table3;Fig.2e).From4.77to1m,priortothebaseofthedubius
postfrequensbiozone,δ7Lidecreasesfrom9.6to4.6‰.Between1and0.05m,theδ7Li
increasesfrom4.6to15.4‰beforedecreasingagainto5.3‰by-1.15m.Theδ7Livalues
remainrelativelyconstantat~6‰beforesubsequentlyincreasingto12.8‰between-
4.05and-5.28m.
4.4Discussion
TheOsandLiisotoperecords(Fig.1andFigs.3to9)showsimilarprofilesforeachtime
periodstudied,butwithdifferingmagnitudesofchange.Priortotheδ13Cexcursionthereis
140
changeof0.19to0.56inthe187Os/188Oscompositiontomoreradiogenicvalues.This
changeinthe187Os/188Oscompositionisoftenassociatedwithanincreaseinδ18Oof
between0.55and1.74‰(Fig.1andFigs.3to9).Thisisfollowedbyadeclineinthe
187Os/188Oscompositiontopreviousvalues.Thisintervalcontinuestobecharacterizedby
highδ18Ovalues.Duringtheδ13Cexcursionofbetween0.9and8.29‰,the187Os/188Os
compositiongenerallyremainslow(unradiogenic).Incontrast,theδ7Liincreasesby4.8to
9.2‰.Duringtherelativelyplateauedδ13Cinterval,theδ18Oandδ7Livaluesbeginto
returntopre-excursionvalues.Duringthisdecline,the187Os/188Osvaluesincreaseby0.26
to0.8beforereturningtothemoreunradiogenicpre-excursionvalueseitherintimewith
thedescendinglimboftheδ13Crecordorpriortoit,withexceptionoftheTelychian-
Sheinwoodianboundary,whichhasahiatusaftertheascendinglimboftheδ13Cexcursion.
Processesthatcouldcausethesevariationsincludecontaminationduringsample
processing,diagenesis,oraprimaryseawatersignaldrivenbylocalorglobalchangesin
Earthsystemprocesses.ContaminationofReandOsfromthedetritalfractionofanalysed
shaleswasavoidedusingbytheCrO3–H2SO4digestionmethod,whilecationexchangeor
leachingofclays,whichcouldimpartanisotopicallylightδ7Lisignal,wasmonitoredby
analysingcation/Caratiosofthecarbonatesamples(Seesection4.2).Diagenesiscanbe
discountedbecausesimilartrendsandabsolutevaluesarerepeatedinsectionsfrom
temporallyseparatedsections(Thisstudy;Finlayetal.,2010;PoggevonStrandmannetal.,
inreview).Furthermore,carbonandoxygenisotopesinstudiedprofilesshowsimilarvalues
toothercorrelatedsectionsthatspanthesameintervals(Fig.1;SaltzmanandThomas,
2012;Trotteretal.,2016,Crameretal.,2011).Itisthereforesuggestedthattheisotopic
shiftsinthecarbonateandshalesectionsmustrepresentprimaryseawatersignatures.
InthefollowingsectionwewilldiscusshowchangesinavarietyofEarthsystem
processescaninfluenceboththe187Os/188Osandδ7Liofseawater,anddetermineifthese
141
processescouldhavecausedthevariationsinisotopedataseeninthisstudyusingdynamic
OsandLicyclemodels.Initiallywewillexploretraditionaloceancirculationmodels(Bickert
etal.,1997;Jeppsson,1990)usedtoexplainlithologicalandcarbonandoxygenisotope
variationsduringtheSilurian.Wewillthendiscussthepotentialofrapidclimaticcooling
(e.g.Lechleretal.,2015;PoggevonStrandmannetal.,2013;Ravizzaetal.,2001),flood
basaltvolcanism(e.g.DuVivieretal.,2014;Lechleretal.,2015;PoggevonStrandmannet
al.,2013)andhydrothermalactivityaspossibleexplanations.FinallywewilldiscussSilurian
glacialexpansionsoverGondwanaasadriverofglobalenvironmentalchange(Azmyetal.,
1998;Brandetal.,2006;Kaljoetal.,2003;Trotteretal.,2016).
4.4.1IsotopicconstraintsonSilurianseawaterchemistry
4.4.1.1Climaticallyinducedchangesinoceancirculation
Severalocean-climatemodelsweredevelopedduringthe1990stotryandexplainthe
observedchangesinfaunalturnover,lithologyandcarbonandoxygenisotopes.Thefirst
modelwasestablishedbyJeppsson(1990)toexplainobservedchangesinthelithologyand
conodontfaunasofGotland.Themodelproposesthatmajorbio-eventsoccurredduring
thetransitionbetweentwostableoceanicstates,knownasPrimoandSecundoEpisodes,
drivenbyachangeinthelatitudinalpositionofdeep-waterformation.Primoepisodes
werecharacterisedbylowatmosphericCO2concentration,coolerclimatesandrelatively
lowglobalsealevel.Downwellingofcold-densehighlatitudesurfacewatersventilatedthe
deepocean,causingupwellingofoxic,nutrient-richwatersatlowlatitudes,whichdrovean
increaseinprimaryproductivityandshelffaunadiversity.Lowlatitudeclimatewashumid,
andhighrainfallwouldhaveintensifiedcontinentalweathering,deliveringabundantclay
andnutrientstoshelfseas.TheSecundoepisodeswerecharacterisedbyhigher
atmosphericCO2,warmerclimatesandthereforethermalexpansionoftheocean.Cold-
142
densewatersnolongerformedathighlatitudes,andsalinitydrivendeep-waterformation
occurredatintermediatelatitudesinstead.Thiscreatedastratifiedoceanwithlower
primaryproductivityandshelffaunadiversity.Thelowlatitudeclimatewasdry,leadingto
lowerrun-offandadeclineincontinentalweatheringintensity.TheIreviken,Muldeand
Laueventsallsupposedlyculminatedfromaswitchbetweenthesetwooceanicstates
(Jeppsson,1998).
Aspreviouslyexplained,theswitchbetweenPrimoandSecundoepisodeswould
havehadaprofoundinfluenceonlowlatitudecontinentalweatheringintensityand
thereforetheriverinefluxandisotopiccompositionofosmiumandlithiumdeliverytothe
ocean.DuringthePrimoepisodes,warmertemperatures,ahumidclimateandhigher
precipitationrateswouldbeassociatedwithenhancedchemicalweatheringratesof
radiogeniccontinentalcrustresultinginanincreaseinthe187Os/188Oscompositionof
seawater(Peucker-EhrenbrinkandRavizza,2000;Peucker-EhrenbrinkandRavizza,2012;
Ravizzaetal.,2001).Highintensityweatheringandextensiveclayformation,asproposed
bytheJeppsson(1990)model,wouldlead7Li-depletedclaysandothersecondaryminerals
toremainintheweatheringzoneforalongtime,leadingtodissolutionofthesephasesand
relativelylowδ7Liinriverwaterandthereforeadecreaseintheδ7Liofseawater(Bouchez
etal.,2013;Dellingeretal.,2015).DuringtheSecundoepisodes,lowertemperatures,a
morearidclimateandlowerprecipitationrateswouldbeassociatedwithreducedchemical
weatheringratesandthereforeresultinaloweringthe187Os/188Oscompositionof
seawater(Peucker-EhrenbrinkandRavizza,2000;Peucker-EhrenbrinkandRavizza,2012).A
lowerweatheringintensityandreducedclayformationwouldcause6Litoberetainedin
precipitatedsecondaryminerals,leadingtoisotopefractionationandrelativelyhighδ7Liin
thedissolvedloadofriversandanincreaseintheδ7Liofseawater(Bouchezetal.,2013;
Dellingeretal.,2015).
143
Accordingtotheabovescenarios,aswitchbetweenthesetwo(Primo-Secundo)
statesduringtheIreviken,MuldeandLaueventswouldbeassociatedwithpermanent
changefromrelativelyradiogenic187Os/188Osandlowδ7Litorelativelyunradiogenic
187Os/188Osandhighδ7Li.However,theobservedvariationsin187Os/188Osandδ7Li,
associatedwithcontinentalinputs,fromthisstudy(Fig.1)donotfollowthistrend.
Therefore,ourdatadoesnotsupportaswitchbetweenPrimoandSecundoepisodesasa
driverofSilurianclimateperturbations,atleastintermsofcontinentalprecipitation
changes.
ThemodelbyJeppsson(1990)waslaterfurtherdevelopedbyBickertetal.
(1997)usinggeochemicaldata.TheynoticedthattheIreviken,MuldeandLaueventswere
allassociatedwithanomaliesintheδ13Candδ18OrecordfromGotland.Bickertetal.(1997)
arguedthatthepositiveshiftsinδ18Oduringtheseeventsweredrivenbysalinity
fluctuationsassociatedwithchangesinglobalevaporation-precipitationratesand
continentalrun-off.AsaresulttheoceancirculationmodelofJeppsson(1990)wasadapted
accordinglytothestableisotopefluctuationsintoshiftsbetweenhumidperiods(H-
periods)withhighcontinentalinputandupwellingincoastalwaters,andaridperiods(A-
periods)withlittlecontinentalrunoffanddownwellingincoastalwaters.
Generally,themodelsuggeststhattheSilurianclimateisinahumidstate
punctuatedbyshortaridepisodes.Afullhumid-aridcycle(A-Hmodes)duringtheIreviken,
MuldeandLaueventswouldbeassociatedwithadeclineinlow-latitudecontinental
weatheringintensityduringA-periods.TotesttheA-Hmodesaseriesofdynamicmodels
wereruntoconstraintheinfluenceofagradualdecreaseinglobalcontinentalweathering
ratesby~50%,over250kyr,ontheOsandLiisotopesystems(Fig.10).Duringaswitch
fromanH-toA-period,adeclineinriverinefluxesrelatedtosilicateand/ororganic-and
sulphide-richlithologyweathering(Fig.10b)causesashifttolessradiogenic187Os/188Os
144
values(Fig.10a),duetoareductioninthefluxofradiogenicOsintotheocean.The
187Os/188Osvaluesproceedtoplateauatrelativelyunradiogenicvaluesfor~750kyr(Fig.
10a),beforerisingtomoreradiogenicvaluesastheriverineinputofmoreradiogenicOs
resumesastheclimatereturnstoahumidmode(Fig.10b).ForLi,theδ7Ligradually
increasestomorepositivevalues,peakingtowardstheendofthearidperiod(Fig.10c).
DuringtheswitchbacktoanH-period,thefluxofcontinentalderivedLiincreases(Fig.
10d),drivingtheδ7Libacktopreviousvaluesoverthenext5Myr(Fig.10d).
Figure10.DynamicmodelofOs(a)andLi(c)isotopesforchangesincontinental
precipitationratesaccordingtoBickertetal.(1997).Themodelshownwasgeneratedby
assuminga50%dropincontinentalweatheringandthereforeriverineflux(b,d)duringa
switchfromahumid-period(H)toanarid-period(A).InthecaseofLi,theweathering
congruencyismodelledbyvaryingtheisotoperatiooftheriverineendmember(d).See
textfordetails.
Thisissupportedbythedatafromthisstudy(Fig.1),whichshowslessradiogenic
187Os/188Oscompositionsatthesametimeasthepeakinδ7Livaluesofsimilarmagnitudes
tothemodel(Fig.10)duringtheA-periodsassociatedwiththeMuldeandLauevents.We
145
alsoseelessradiogenic187Os/188OscompositionsduringtheIrevikenandKlonkevents(Fig.
1).However,wewouldexpectthecorrespondingH-periodstobeassociatedwithrelatively
radiogenic187Os/188Oscompositions(Fig.10a),whichwouldpersistformuchoftheSilurian.
Incontrast,short-livedpeakscharacterizedbyradiogenic187Os/188Oscompositionsare
observed,withbackgroundlevelsduringpredictedH-periodssimilartothedurationofthe
A-period(Fig.1).Inaddition,theascendinglimbtowardsradiogenic187Os/188Os
compositionsisassociatedwithanincreaseinδ18Oinmostinstances(Fig.1).Accordingto
theBickertetal.(1997)model,thisshouldbetheconverse,withadeclinetounradiogenic
187Os/188Oscompositionsduringanincreaseinδ18Oasalowercontinentalfluxisassociated
withmoresalinecoastalwaters.Thisiscompoundedbypreviouscriticismsthatarguethe
conceptofgloballyincreasingaridityorhumidityisnotsupportedbymodellingorexisting
data,andisthereforedifficulttoargueasacauseforglobalevents(Johnson,2006;Kaljoet
al.,2003;Loydell,1998;Munneckeetal.,2010).
4.4.1.2FloodBasaltVolcanism
Periodsofintensesubmarineorcontinentalvolcanismandtheemplacementoflarge
igneousprovinces(LIPs)haveaprofoundinfluenceonglobalclimateandare
penecontemporaneouswiththeonsetofoceananoxicevents(OAE).Additionally,newly
formedbasalticterrainscanhavealargeimpactonsecularvariationsintheosmium
(Bottinietal.,2012;CohenandCoe,2002;DuVivieretal.,2014;RavizzaandPeucker-
Ehrenbrink,2003;TurgeonandCreaser,2008)andlithium(Lechleretal.,2015;Poggevon
Strandmannetal.,2013)isotoperecords.Submarinefumaroleandhydrothermalalteration
/weatheringofjuvenilebasaltsdeliversafluxofunradiogenicOstotheoceans,drivingthe
187Os/188Oscompositionofseawatertowardsmantlevalues.Further,duringOAEs,abrupt
globalwarmingassociatedwithrisingatmosphericCO2causedenhancedweatheringof
146
maficsilicatematerialwhichdeliveredlowδ7Li(high[Li])inputstotheocean,drivingthe
δ7Liofseawatertolowervalues.Therefore,anyigneousactivityduringtheSilurian
intervalsstudiedherewouldbemetbyarapiddecreaseinboththe187Os/188Osandδ7Liof
seawater,whichisnotobserved(Fig.1).Furthermore,thisiscompoundedbythelackof
evidenceforLIPsduringtheSilurian.
4.4.1.3Temperature-weatheringfeedbacks
Overgeologicaltimescales(Myr)temperaturehaspartlybeencontrolledbyinteractions
betweenatmosphericCO2andcontinentalweathering(Berneretal.,1983;Walkeretal.,
1981).Risingtemperaturesstimulateincreasedchemicalweatheringofsilicaterocks
drawingdownCO2fromtheatmosphere,leadingtoadeclineintemperatureandviceversa
(Berneretal.,1983;Walkeretal.,1981).Rapidtemperaturefluctuationsinthegeological
pastcouldinfluencecontinentalweatheringandthereforesecularvariationsinbothOsand
Liisotoperecords.ThisissupportedbyOsandLiisotopedatafromthePaleocene-Eocene
ThermalMaximum(PETM)andOAEsrespectively,whichareinterpretedtoreflectabrupt
releaseofgreenhousegaseswhichresultedinanincreaseinglobaltemperatures,
stimulatingcontinentalsilicateweatheringanddeliveringradiogenicOsandisotopically
heavyLitotheocean(Lechleretal.,2015;PoggevonStrandmannetal.,2013;Ravizzaet
al.,2001).
Silurianoxygenisotoperecordsofphosphates(Fig.1)(Trotteretal.,2016)show
globallyrecognisedpositiveexcursions,indicativeofcoolingduringtheIreviken,Muldeand
Lauevents.Likewise,oxygenisotopedatafromsectionsstudiedhereshowanincreasein
δ18Ointimewith,orslightlypreceding,theOsandLiisotopeexcursions(Fig.1).According
totheabovetheory,arapidcoolingwouldbeassociatedwithadeclineinglobal
continentalsilicateweatheringandthereforedecreasethefluxoftheradiogenicOsand
147
isotopicallyheavyLitotheocean,drivingthe187Os/188Osandδ7Licompositionofseawater
tolowerandhighervaluesrespectively.However,thisstudyshowsanincreaseinthe
187Os/188Oscompositionofthehydrogenouscomponentoftheorganic-richsedimentary
units,andthusbyinferencethatofthecontemporaneousseawaterduring,orslightly
precedingtheincreaseintheδ18Ovalues;withtheexcursiontomorepositiveδ7Livalues
occuringsometimeaftertheinitialriseintheδ18Ovalues(Fig.1).Thissuggeststhat
temperaturechangealoneisnotthedriverofOsandLiisotopevariationsseeninthis
study.
4.4.1.4Hydrothermalactivity
Inthemodernoceans,hydrothermalactivityatmid-oceanridgesaccountsforasignificant
inputofOsandLitotheocean,witha187Os/188Oscompositionandδ7Livalueof~0.12and
~8‰respectively(HathorneandJames,2006;MisraandFroelich,2012;Peucker-
EhrenbrinkandRavizza,2000).Theisotopiccompositionofthesefluxesisseentobe
relativelyconsistentthroughtime,howeverachangeintheirfluxcouldcausevariationsin
theisotopiccompositionofseawater.Althoughhardtoconstrain,areductioninpluton
emplacement(Hardie,1996)andanincreaseintheMg/Caratioofseawater(Stanleyand
Hardie,1998)suggestsareductioninsea-floorspreadingduringthelate-Wenlocktoearly-
Ludlow(Crameretal.,2011b).However,thelowresolutionofavailabledatapreventsan
estimationofvariationsofhydrothermalactivityovertherelativelyshorttimescales
studiedhere.
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Severaldynamicmodelswereruntoconstraintheinfluenceofshortperiodic
fluctuationinthefluxofhightemperaturehydrothermalactivityontheOsandLiisotope
systems(Fig.11).Twoscenariosweremodelled:1.Apulseddecreaseinthehydrothermal
fluxby~50%(Fig.11b);and,2.Asingle,extendedperiodofreduced(~50%)hydrothermal
flux(Fig.11d).Duringthepulsedscenarioweseeasimultaneousincreasetomore
radiogenic187Os/188Oscompositionsandmorepositiveδ7Livalueswitheachdecreasein
hydrothermalflux(Fig.11a).Althoughthe187Os/188Oscompositionsbecomemore
unradiogenicintimewithanincreaseinhydrothermalflux,theδ7Livaluesshowadelayed
response,decliningovertheproceeding250kyr(Fig.11a).Thiscompareswellwiththe
187Os/188Osdatafromthisstudy(Fig.1),whichshowtwouniformpeaksinradiogenic
187Os/188Oscompositions.However,themagnitudeofchangepredictedbythemodel(0.07)
ismuchsmallerthanmeasuredisotoperatios.Likewise,theδ7Livaluesmeasuredhere(Fig.
1)showonesingularpeakinδ7Litowardsthesecondpeakin187Os/188Oscompositions,
whichisnotseeninthemodel(Fig.11a).
Figure11.DynamicmodelofOsandLiisotopesforapulsedreductioninhydrothermalflux
(a)andanextendedperiodofreducedhydrothermalflux(c).Themodelshownwas
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generatedbyassuminga50%dropinhydrothermalfluxovertworelativelyshort250kyr
periods(b)oroverasingle,moreextended750kyrperiod(d).Seetextfordetails.
Duringanextendedperiodofreducedhydrothermalflux(Fig.11d),the
187Os/188Oscompositionsandδ7Livaluesincreaseoverasimilarperiodoftime(Fig.11c).
ThiscompareswellwithLiisotopedatafromthisstudy(Fig.1),whichshowsasingular
peakinδ7Li.However,themagnitudeofchangepredictedbythemodel(<0.5‰)isfartoo
smallwhencomparedtotheexcursionobserved(>4‰).Likewise,the187Os/188Os
compositiondeterminedhere(Fig.1)showtwopeaksinradiogenic187Os/188Osvalues,
whichisnotseeninthemodel(Fig.11c).Inbothmodeloutputs(Fig.11aandc)theδ7Li
decreasesby>0.3‰belowbackgroundlevelsaftertheexcursionduetoadependenceof
thepartitioncoefficientonseawaterconcentration,whichremainslowevenaftertheinput
hasincreased.Thesemodellingstudies(Fig.11)suggestthatfluctuationsinthe
hydrothermalfluxcannotcausethevariationsinOsandLiisotopedataseenhere(Fig.1).
4.4.1.5Glaciation
OsmiumandLiisotopevariationsfoundinthisstudybearastrikingresemblancetothose
measuredfortheHirnantianglaciation(Finlayetal.,2010;PoggevonStrandmannetal.,in
review),some12MyrsearlierthantheTelychian-Sheinwoodianboundary(Fig.1).Authors
postulatedthesevariationsweredrivenbyfluctuationsinchemicalweatheringrates
relatedtoenhancedcontinentalicevolumeoverGondwana.DuetothesimilaritiesinOs
andLiisotopetrendsdeterminedhere,combinedwithreoccurringglobaltrendsinoxygen
isotopes(Azmyetal.,1998;Calner,2008;Lehnertetal.,2010;Munneckeetal.,2010;
Nobleetal.,2005;Trotteretal.,2016),carbonisotopes(Crameretal.,2011a;Crameret
al.,2010;Munneckeetal.,2010;SaltzmanandThomas,2012),sea-levelreconstructions
(HaqandSchutter,2008;Johnson,2006,2010;Loydell,1998),marinefaunalturnover
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(Calner,2008;Cooperetal.,2013;Melchinetal.,2012),andinthecaseoftheearly
Sheinwoodian,thepresenceofglacialsediments(Díaz-MartínezandGrahn,2007),we
suggestchangesincontinentalweatheringcausedbyglaciationeventscouldhaveoccurred
duringeachofthefourSiluriantimeintervalsstudied.
Duringglacialadvance,physicalerosionoftheunderlyingbedrockprovides
abundantfreshlycomminutedrockflourtoice-sheetmarginsandtheproglacialzone
(Tranter,1982).Subglacialandproglacialconditionspromotesulphideoxidationandthe
chemicalweatheringofcarbonatesinthesenewlyformedreactivemineralsurfaces
regardlessofunderlyinglithology(Andersonetal.,2000;Cooperetal.,2002;Fairchildet
al.,1999;Tranteretal.,2002)whilststimulatingthemicrobiallymediatedoxidationof
ancientorganicmattersuppliedbycomminutedshales(Petschetal.,2001a;Wadhamet
al.,2004).SedimentarysulphidesandorganicmatterareoftenassociatedwithhighOs
concentrationsandhighlyradiogenic187Os/188Oscompositions(Jaffeetal.,2002;Peucker-
EhrenbrinkandHannigan,2000;Peucker-EhrenbrinkandRavizza,2000;Pierson-Wickmann
etal.,2002;RavizzaandTurekian,1989),andtheirerosionandsubsequentoxidationby
advancingice-sheetswouldimpactontheglobalriverineOsend-member,drivingseawater
tomoreradiogenicvalues.Theinitialpeakin187Os/188Osvaluesseeninthisstudy(Fig.1)
andduringtheHirnantianglaciation(Finlayetal.,2010)canthereforebeattributedto
glacialadvance.
However,lowtemperaturesinnewlyglaciatedregionswouldacttosuppress
chemicalsilicateweatheringtolowerratesthanpreviouslynon-glaciatedregions
(Anderson,2005,2007;Andersonetal.,2000;Andersonetal.,1997;Gislasonetal.,2009;
MaherandChamberlain,2014).Ifphysicalerosionratesincrease,whilesilicatechemical
weatheringremainsconstantordecreases,weatheringintensitywilldeclineandtherefore
increasethedissolvedδ7Livaluesofrivers(Bouchezetal.,2013;Dellingeretal.,2015;Li
151
andWest,2014;PoggevonStrandmannetal.,2017).Likewise,regionsoflowchemical
weatheringintensitywilldrivetheδ7Liofglacialriverstomorepositivevalues,as6Liis
retainedbysecondaryminerals(PoggevonStrandmannetal.,2006),andtheformationof
Fe-oxyhydroxidesduringsulphideoxidationunderice-sheetswillalsopreferentiallyuptake
6Liontomineralsurfaces,contributingtotheriseintheδ7Liofglacialwaters(Wimpennyet
al.,2010).Anincreaseintheδ7Livaluesoftheriverinefluxcombinedwithadecreaseinthe
overallglobalriverinefluxofLi,duetothegradualcoveringofGondwanabycontinental
ice-sheetspreventingweatheringoftheunderlyingsilicateminerals(KumpandAlley,1994;
PoggevonStrandmannetal.,inreview),willdrivetheδ7Livalueofseawatertomore
positivevalues(Fig.1).
Asglacialexpansionbegantoslowandglacialmaximumwasestablished,the
previouslyhighdenudationrateswouldhavediminishedalongwiththeoxidationof
proglacialandsubglacialsulphidesandorganicmatter,reducingtheinputofOswitha
radiogenic187Os/188Oscompositiontotheoceans.Whendiminutivedenudationiscoupled
withenhancedlowlatitudecontinentalcoverbyicesheetsandgenerallycolder,drier
conditions,overallglobalchemicalweatheringrateswouldhaveremainedlowunder
glacialmaximum(KumpandAlley,1994),maintaininghighδ7Livaluesandlow187Os/188Os
compositionsinseawater(Fig.1)(Finlayetal.,2010;PoggevonStrandmannetal.,in
review).FortheHirnantianglaciation,authorshavesuggestedthatthistransientdeclinein
silicateweathering,andthereforeadeclineinoneoftheEarth’smajoratmosphericCO2
withdrawalmechanisms,wouldhavecausedanincreaseinatmosphericCO2thatultimately
terminatedtheglaciation(Kumpetal.,1999;PoggevonStrandmannetal.,inreview).The
similaritybetweenLiisotoperecordsfortheHirnantian(PoggevonStrandmannetal.,in
review)andthisstudy(Fig.1)suggestasimilarmechanismfordeglaciationcouldhave
occurredthroughouttheSilurian.
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Whateverthecauseofdeglaciation,theextensiveavailabilityoffreshmaterial,
increasedmeltwaterandgenerallywetterconditionsduringthedemiseofcontinentalice
sheetswouldenhancetheoxidationand/orweatheringofsulphides,shalesandsilicates
leftbehind(BluthandKump,1994;DreverandZobrist,1992;Gaillardetetal.,1999;Huh
andEdmond,1999;Meybeck,1987;Petschetal.,2001b;Vanceetal.,2009;Wildmanetal.,
2004).ThiswouldcauseashiftintheglobalriverineOsend-member,andtherefore
seawater,tomoreradiogenicvalues(Finlayetal.,2010;Peucker-EhrenbrinkandBlum,
1998)creatingthesecondpeakinthe187Os/188Oscompositionrecord(Fig.1).Greater
weatheringintensityofsilicateswilldecreaseLiisotopefractionationbetweenthe
dissolvedandsuspendedloadsofglacialrivers,asless6Liisretainedinsecondaryminerals,
drivingδ7Litolowervalues(Huhetal.,1998;PoggevonStrandmannetal.,2006).Asthese
newlyformedriversystemsbecomemoremature,continuedweatheringofsuspended
materialwillincreasethesaturationstateofthedissolvedloadwithrespecttosecondary
minerals,loweringδ7Livaluesfurther(Dellingeretal.,2015;PoggevonStrandmannetal.,
2006).Adecreaseintheδ7LivalueandLiconcentrationofriverineend-memberswould
leadtoadecreaseinδ7Livalueofseawater,eventuallyrestoringthesystemtopre-
excursionvalues(Fig.1).
Severaldynamicmodelswereruntoconstraintheinfluenceofglacialprocesses
ontheOsandLiisotopesystems(Fig.12).Biozoneboundaryages(Melchinetal.,2012)
wereusedtoconstrainthetimingsofglaciation(~250kyr;lightblueboxdenotedbyGin
Fig.12),glacialmaximum(~500kyr;darkblueboxdenotedbyGMinFig.12)and
deglaciation(~250kyr;lightblueboxdenotedbyDinFig.12).Duringglaciation,the
relativecontinentalfluxrelatedtosilicateweathering(Fig.12band12e)gradually
decreasesby50%,whiletheweatheringoforganic-sulphide-richlithologiesincreasesby
~80%(Fig.12c).InthecaseoftheOsisotopesystem,thiscausesaswitchfromariverine
budgetdominatedbysilicateweathering(60%oftotalFriv),toariverinebudgetdominated
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byORLW(70%oftotalFriv).ThehighlyradiogenicnatureofORL(187Os/188Os=1.34;Table1)
whencomparedtosilicateminerals(187Os/188Os=0.6;Table1)causestotalRrivtobecome
moreradiogenic,whilstFrivremainsconstant,drivingthe187Os/188Oscompositionof
seawatertomoreradiogenicvalues(Fig.12a).Meanwhile,areductioninriverineinputs
relatedtosilicateweathering(Fig.12e)drivesδ7Litomorepositivevalues(Fig.12d).
Figure12.DynamicmodelofOs(a)andLi(d)isotopesduringachangeinhigh-latitude
continentalicevolume.Themodelshownwasgeneratedbyassuminga50%decreasein
continentalsilicateweathering(b,e)duringglaciation(G),whichremainslowduringglacial
maximum(GM),beforeincreasingduringdeglaciation(D).InthecaseofOs,twoperiodsof
enhancedORLWduringglaciationanddeglaciationaregeneratedbyassuminga70%
increaseintheriverinefluxrelatedtoORLW(c).InthecaseofLi,theweathering
congruencyismodelledbyvaryingtheisotoperatiooftheriverineendmember(f).See
textfordetails.
Duringglacialmaximum,icesheetmovementstops,causingtheimmediate
cessationofenhancedORLWandtherelativefluxofFORLWdropstobelowpre-excursion
values(Fig.12c).Thiscausesadecreaseinthe187Os/188Oscompositionofseawaterasthe
inputofradiogenicOstoseawaterisremoved(Fig.12a).Theδ7Liofseawatercontinuesto
increase,reachingpeakvaluestowardstheendoftheglacialmaximum(Fig.12d).During
154
deglaciation,theriverinefluxrelatedtosilicateweatheringgraduallyincreasesbacktopre-
excursionlevels(Fig.12band12e),whiletheriverineinputrelatedtoORLWshowsa
secondinstantaneousincreasebeforegraduallydecliningtopre-excursionlevels(Fig.12c).
Thiscausesanincreaseinthe187Os/188Oscompositionofseawaterbeforethe187Os/188Os
compositiondecreasestopre-excursionvalues(Fig.12a).Theδ7Livalueofseawater
graduallydecreasestopre-excursionvaluesoverthenext~2Myr(Fig.12d).
ThiscompareswelltoOsisotopedatafortheIreviken,Mulde,LauandKlonk
events(Fig.1),whichallshowasimilartwo-peakprofiletomodeloutputs(Fig.12a).
However,themagnitudeofchangefrommodeloutputs(~0.12)islowerthandescribedin
thedata(0.19-0.8).Possiblecausesofthesediscrepanciescouldbe:amisrepresentationof
modelinputparameters;or,astrongerinfluencefromlocalcontinentalsourcesofOsin
coastalshelfsamplinglocalities,whichwilldrivelargervariationsinrecorded187Os/188Os
whencomparedtoawell-mixedocean.Lithiumisotoperecords(Fig.1)showasimilar
singularpeakinδ7Livaluestothemodeloutputs(Fig.12d).Ascenarioinwhichriverflux
alonevariedthroughouttheglaciationeventcausesaδ7Liexcursionof~3‰(Fig.12d),
whichcompareswelltorecordeddatafortheMuldeandLauevents(<4.4‰;Fig.1).Peak
δ7Livaluesoccurduringdeglaciationandthesecondpeakofradiogenic187Os/188Osvalues
(Fig.12aandd),consistentwithdatafortheMuldeevent(Fig.1).Ascenarioinwhichthe
riverfluxdecreased,whileriverineδ7Liratiosincreasedcausedavariationinδ7Li(~6‰)
greaterthanobservedinthedata,suggestingweatheringcongruencyremainedconstant
throughouttheglaciation.
ThefailureofSilurianoceancirculationmodels(Seesection4.1.1),floodbasalt
volcanism(Seesection4.1.2),rapidcooling(Seesection4.1.3)andvariationsin
hydrothermalactivity(Seesection4.1.4)indescribingvariationsinOsandLiisotopesfound
here(Fig.2),leadsustotheconclusionthatperiodicglaciationsarethemostlikelycauseof
155
Silurianclimateevents,basedonthecurrentunderstandingoftheOsandLiisotope
systems.Thedataandmodelsareconsistentwithglaciationeventsduringthelate
TelychiantoearlySheinwoodian,mid-Homerian,mid-LudfordianandtheSilurian-Devonian
boundary.Duringglaciation,expandingcontinentalicesheetsenhancephysicalerosion
whilststimulatingchemicalweatheringoforganicandsulphiderichrocksasevidencedby
anincreaseinthe187Os/188Oscompositiontomoreradiogenicvalues(Fig.1;Fig.12a).
Meanwhile,glacialcover,subglacialprocessesanddecliningglobaltemperaturesreduce
silicateweathering,decreasingtheriverinefluxofLitotheocean,leadingtoanincreasein
theδ7Livalueofseawater(Fig.1;Fig.12e).Decliningtemperaturescoupledtoenhanced
continentalicevolumedrivetheδ18Oofseawatertomorepositivevalues(Azmyetal.,
1998;Calner,2008;Lehnertetal.,2010;Munneckeetal.,2010;Nobleetal.,2005;Trotter
etal.,2016).Adropineustaticsea-level(HaqandSchutter,2008;Johnson,2006,2010;
Loydell,1998)exposescarbonateshelvestoweathering,drivingtheδ13Cofseawaterto
morepositivevalues(Crameretal.,2011a;Kumpetal.,1999;SaltzmanandThomas,2012).
Astheglacialmaximumisreached,icesheetexpansionabruptlyterminates,reducingthe
availabilityoffreshlycomminutedrockfortheoxidationofancientorganicmatterand
sulphiderichlithologiesandthereforereducingthefluxofOstotheoceanbearinga
radiogenic187Os/188Oscomposition(Fig.1;Fig.12a).Extensivestablecontinentalice-sheets
maintainlowlevelsofsilicateweathering(Fig.1;Fig.12e)andsea-level(Johnson2006,
2010;HaqandSchutter,2008;Loydell,1998)whilstmaintainingahigherδ18Ovalueof
seawater(Trotteretal.,2016).Duringdeglaciation,risingtemperaturesandtheincreased
availabilityofmeltwaterandfreshlycomminutedand/orscouredbedrock,enhances
chemicalsilicateweatheringandtheoxidationoforganicandsulphiderichrocks,drivingan
increaseinthe187Os/188Oscompositionofseawater,andadecreaseintheδ7Livalueof
seawater(Fig.1;Fig.12aand12e).Anincreaseinglobaltemperaturesandareductionin
continentalice-volume,drivesadecreaseintheδ18Ovalueofseawater(Trotteretal.,
156
2016)andanincreaseinglobaleustaticsea-level(HaqandSchutter,2008;Johnson,2006,
2010;Loydell,1998).
Acoupleddropineustaticsealevelandglobaltemperatureswouldhavehada
profoundinfluenceonmarinebiota.TheIreviken,Mulde,LauandKlonkbio-eventsare
definedbyglobalextinctionsofconodonts,graptolites,acritarchs,andotherbenthos
(Aldridgeetal.,1993;Calner,2008;Cooperetal.,2013;Jeppsson,1990,1997;Jeppsson,
1998;JeppssonandAldridge,2000;Loydell,2007;Melchinetal.,1998;Štorch,1995).
Cooperetal.(2013)utilisedgraptoloidevolutionaryratestotrackglobalclimaticchange.
RelativelycalmOrdovicianextinctionandoriginationratesgavewaytohighlyvolatilerates
duringthelateKatianthroughtotheEarlyDevonian,withsharpextinctionepisodes
triggeredbyenvironmentalcrisis(Cooperetal.,2013).Thissupportstheideapresentedin
thisstudy,inwhichaswitchfromrelativelystablegreenhouseconditionsduringtheEarly
andMiddleOrdovician,torelativelyunstableicehouseconditionsduringtheHirnantian
andSilurian,createdhighlyvolatileconditionsformarinebiota.Abruptglaciations
presentedherecoincidewithhighgraptloidextinctionratesduringtheearlySheinwoodian,
midHomerian,lateLudfordianandlatestPridolian,withsubsequenthighlevelsof
originationsduringrecoveryphases(Cooperetal.,2013).
4.4.2ScenariosfortriggeringSilurianGlaciations
Severalmechanisms,usuallyinvolvingadropbelowthresholdatmosphericCO2levels(~6
PAL;Pohletal.,2014),havebeenputforthastriggersfortheHirnantianglaciation,and
includechangesin:orogeny(Kumpetal.,1999);landplantdiversification(Lentonetal.,
2012;Poradaetal.,2016);volcanicarcdegassing(McKenzieetal.,2016;Poggevon
Strandmannetal.,inreview);and,paleogeography(Nardinetal.,2011).Likethe
Ordovicianthatprecededit,manyoftheseprocesseswereactiveduringtheSilurian,
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providingseveralpossiblecandidatesforthemechanismthattriggeredtheglaciations
proposedinthisstudy.
4.4.2.1Orogeny
IthasbeenarguedthatenhancedweatherabilityofsilicaterocksduringtheTaconic
orogeny,ledtoalong-termdeclineinatmosphericCO2whicheventuallydroppedbelow
thethresholdlevelsthatwouldallowcontinentalicetoexpand(Kumpetal.,1999).During
theSilurian,AvaloniaandBalticacollided,withsubsequentcollisionofthesecombined
landmasseswithLaurentiatoformtheScandianorogeny(CocksandTorsvik,2002;Torsvik
andCocks,2013).Therapidrisetomoreradiogenic87Sr/86Srisotoperatiosduringthe
Silurianhasbeenattributedtoenhancedweatheringofoldsialiccrustexposedduringthe
aforementionedformationoftheCaledonian-Appalachianorogenicbelt(Crameretal.,
2011b).Moreover,steepinflectionpointsintheSrisotoperecordoccurpriortoand/or
duringthelate-Telychian,mid-Homerian,mid-LudfordianandtheSilurian-Devonian
boundary(Crameretal.,2011b;Frýdaetal.,2002).Theserepeatingperiodsofenhanced
weatherabilityofexposedsialiccrustwouldhaveledtothelong-termdeclinein
atmosphericCO2andglobaltemperatureswhich,ifdroppedbelowthresholdlevels(Kump
etal.,1999),wouldinitiatetheformationofcontinentalicesheets.
4.4.2.2Landplantdiversification
Ithasbeenhypothesized(Lentonetal.,2012)thattheexpansionofnon-vascularland-
plantsduringtheOrdovicianacceleratedglobalchemicalweatheringbyuptothreetimes
modernweatheringfluxes(Poradaetal.,2016),leadingtothedrawdownofenough
atmosphericCO2(<6PAL)totriggerthegrowthoficesheets.Moreover,thedevelopment
158
ofvascularlandplantsduringtheDevonianisthoughttohavecausedadramatic
drawdowninatmosphericCO2thatledtoglobalcoolingandpolarglaciationsduringthe
LateDevonian,ultimatelyleadingtoCarboniferousicehouseconditions(Algeoand
Scheckler,2010;AlgeoandScheckler,1998;Berner,1997).DuringtheSilurian,non-
vascularandvascularlandplantsexpandedgeographicallytoinhabitnewcontinentsand
landmasses(Steemansetal.,2009).Meanwhile,thedevelopmentofevolutionarytraitsin
vascularland-plants,suchasmoreextensiverootingstructuresand‘theseedhabit’,
originallyassignedtotheDevonian(AlgeoandScheckler,2010;Algeoetal.,1995;Algeo
andScheckler,1998),arenowthoughttohavedevelopedearlier,duringtheSilurian
(Edwardsetal.,2014;Gensel,2008;Kenricketal.,2012).Geographicalexpansionand/or
thedevelopmentofmoreextensiverootsystemswouldacttoenhancetheweathering
ratesofsilicates,leadingtoareductioninatmosphericCO2,globalcoolingandperiodic
glaciations.
4.4.2.3Volcanicarcdegassing
Recently,variationsinvolcanicarcactivityhavebeenshowntohaveadirectrelationship
withclimaticshiftsbetweenicehouseandgreenhouseconditions(McKenzieetal.,2016).
ContinentalcollisionsduringtheassemblyofGondwanaledtoareductionincontinental
arcvolcanism,andthereforeatmosphericCO2emissions,culminatinginglobalcoolingand
theHirnantianglaciation(McKenzieetal.,2016;PoggevonStrandmannetal.,inreview).In
theMid-LateSilurian,theIapetusOceanclosedduringtheformationofLaurussia.This
newlyformedcontinentrapidlytravelledsouthward,eventuallycollidingwithGondwana,
causingtheclosureoftheRheicOceanbytheCarboniferous(Cocksandtorsvik,2002).This
wouldhaveledtoareductionincontinentalarcvolcanism,asevidencedbycumulativeage
159
distributionsofSilurianZircons(McKenzieetal.,2016),leadingtoglobalcoolingand
intermittentSilurianglaciations.
4.4.2.4Paleogeography
AnotherexplanationfortheupperOrdovicianthroughtolateLlandoveryglacialperiod
involveschangesinpaleogeographythatcausedthemovementoffreshlyexposedvolcanic
rocks(Lefebvreetal.,2010;Youngetal.,2009)throughtheintertropicalconvergencezone
(ITCZ),stimulatingcontinentalweatheringandadeclineinatmosphericCO2belowa
thresholdthatwouldallowcontinentalglaciationstooccur(Nardinetal.,2011).In
particular,theprogressiveamalgamationofBaltica,AvaloniaandLaurentiaattropical
latitudesledtohighrunoffandthereforeweathering,maintainingthelowatmosphericCO2
requiredtoinitiatetheearlySilurianglaciations(Nardinetal.,2011).Althoughthismodel
couldexplainthemechanismbywhichaglaciationjustaftertheTelychian-Sheinwoodian
boundary(Fig.1),itprecludestheexistenceoflateSilurianglaciationsduetodecreasesin
continentalrunoffascontinentallandmassesmoveoutoftheITCZ(Nardinetal.,2011).
However,thismaybeduetothelowtemporalresolutionofthemodel.
4.4.2.5Orbitalforcing
AlthoughtherehavebeenmanyPaleozoic-centricmechanismsputforthasdriversfor
Paleozoicglaciations(Seesection4.4.2.1to4.4.2.4)ithasrecentlybecomeapparentthata
more‘Cenozoic-style’scenariomayhavecausedtheend-Ordovicianglaciations(Ghienneet
al.,2014).TheHirnantianglaciationisgenerallyviewedasasingularevent,withmarine
extinctionstiedtoglaciationanddeglaciation.However,highresolutionstratigraphic
recordshaveledtothediscoveryofseveralhigh-orderglacialcycles,suggestingamulti-
orderclimatesignalsimilartotheCenozoic(Sutcliffeetal.,2000;Ghienneetal.,2014).
Bothtwo(Sutcliffeetal.,2000)andthree(Ghienneetal.,2014)glacialcyclesofice-sheet
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growthhavebeenrecordedfortheend-Ordovician,controlledbya0.1Myreccentricity
periodicity(Sutcliffeetal.,2000)or1.2Myrobliquitymodulation(Ghienneetal.,2014)
respectively.
MuchlikepreviousviewsfortheOrdovician,theSilurianclimaticeventsare
generallyseentohaveasingularpeak,lackinginhighorderglacialcycles.However,the
Silurianisseverelylackinginhighresolutionstratigraphicandisotopicrecords,precluding
thediscoveryoforbitallyforced‘Cenozoic-style’glacialcycles.Moreover,OsandLiisotopic
recordsfromthisstudy(SeeFig.1)areofsuchlow-resolutionthattheypreventrecognition
ofsuchevents.However,Silurianoxygenisotope(Trotteretal.,2016)andsea-level((Haq
andSchutter,2008;Johnson,2006,2010;Loydell,1998)recordsshowhigh-order
fluctuationspossiblerelatedtoglacialeustasyonglacial-interglacialtimescales(SeeFig.1).
Furthermore,theHirnantian,Sedgwicki,Ireviken,Mulde,LauandKlonkeventsappearto
bespacedover4to5Myrintervals(SeeFig.1)suggestingapacingtoSilurianglaciations
whichcouldbesetbytheinteractionbetweeneccentricity,obliquityandprecessioncycles,
ultimatelycontrolledthroughchangesininsolation.Moreworkwillneedtobegenerate
high-resolutionstratigraphicandisotopicrecordsforSilurianclimaticeventsinasimilar
mannertotheend-Ordovician(SeeSutcliffeetal.,2000andGhienneetal.,2014).
4.4.2.6Whydidwenotsee‘SnowballEarth’conditionsduringtheSilurian?
Largeglaciationsencompassingtheentireglobe,otherwiseknownas‘SnowballEarths’,are
believedtohaveoccurredduringtheNeoproterozoic(1000–542Ma),asevidencedfrom
theexistenceoflow-latitudeglacialdepositsatsea-level(SeeHoffman&Schrag,2002and
referencestherein).Theseeventscouldhavepotentiallybeentriggeredbyhighobliquity
causedbythelunar-formingimpact(SeeWilliams,2008)orpositiveice-albedofeedbacks
whichledtoglacialsthatlastedtensofmillionsofyearsbeforetheslowaccumulationof
atmosphericCO2duetovolcanicdegassingbroughtthemtoanend(SeeHoffman&Schrag,
161
2002).However,unliketheNeoproterozoicsnowballEarths,Paleozoicglaciationssuchas
thosefoundintheCambrian,LateOrdovician,Silurian,LateDevonianandCarboniferous
aregenerallynotmetbylow-latitudeglacialdepositsanddeglacialcap-carbonatesand
negatived13Cexcursions(>10‰).Thissuggeststherewaseithermore/lessfeedback
mechanismsactiveduringtheNeoproterozoicwhencomparedtothePaleozoic,that
allowedforrunawayicehouseconditionstooccur.
Phanerozoicpaleogeographyreconstructions(CocksandTorsvik,2002;Torsvik
andCocks,2013)oftensituatescontinentalmassesathigh-latitudesoveroneorbothof
thepoles,eitherasasupercontinente.g.Pangeaorasmallercontinente.g.Antarctica.
However,unlikethePhanerozoic,NeoproterozoicsnowballEarthsaredominatedbya
severelackofhigh-latitudecontinentalmasses.Thiswouldhavecreatedanabsenceof
continentalice-weatheringfeedbacksproposedinthisstudyfortheSilurian(Seesection
4.4.1.5).Therefore,oncepolarglaciationisinitiated,sea-iceformationwouldproceedto
lowlatitudesviapositiveice-albedofeedbackwithoutthesubsequentreductioninsilicate
weatheringandenhancementofoxidativeweatheringassociatedwithcontinentalice,
whichwouldotherwiseacttoincreaseatmosphericCO2leadingtoglobalwarming.This
suggestsweatheringfeedbackshelpregulateatmosphericCO2,preventingrunaway
icehouseconditionsandsnowballEarths(Seesection4.4.3).
4.4.3WeatheringfeedbackshelpregulateatmosphericCO2
WeatheringregulatesatmosphericCO2overmultimillionyeartimescalesandcanbe
summarisedbyseveralchemicalreactions(BernerandBerner,2012;Berner,2006;Berner
etal.,1983;Torresetal.,2014;Walkeretal.,1981).ThedissolutionofMg-andCa-bearing
silicatemineralsbycarbonicacid,andthesubsequenttransportofsolutestotheocean
whereinorganiccarbonisburiedasmarinecarbonates,sequestersatmosphericCO2:
CO2+(Ca,Mg)SiO3→(Ca,Mg)CO3+SiO2 Eq.7
162
Ontheotherhand,theoxidativeweatheringofancientsedimentaryorganicmatter
releasesCO2backtotheatmosphere:
CH2O+O2→CO2+H2O Eq.8
OthersourcesofatmosphericCO2includetheoxidativedissolutionofpyriteandthe
subsequentdissolutionofcarbonatesbysulfuricacid:
15O2+4FeS2+14H2O→4Fe(OH)3+8H2SO4 Eq.9
CaCO3+H2SO4→CO2+H2O+Ca2++SO42- Eq.10
and
2CaCO3+H2SO4→2Ca2++2HCO3-+SO4
2-↔CaCO3+CO2+SO42-+Ca2+
Eq.11
Reactions7to11showastrongdependenceonclimaticparameterssuchastemperature,
runoffand/orphysicalweathering(Georgetal.,2013;Torresetal.,2014).Duringthe
Cenozoic,concurrentincreasesinthe87Sr/86Sr,187Os/188Osandδ7Lirecords(Klemmetal.,
2005;McArthuretal.,2001;MisraandFroelich,2012)suggestenhancedphysicaland
chemicaldenudationduringextensiveupliftofmountainrangesmayhavestimulatedCO2
consumptionviasilicateweathering(Raymoetal.,1988).However,intheabsenceof
enhancedCO2productionfromothersources,thiswouldhavestrippedtheatmosphereof
allitsCO2withinafewmillionyears(BernerandCaldeira,1997).Thisdeclinein
atmosphericCO2mayhavebeenoffsetbyareleaseofCO2fromasimultaneousincreases
inoxidativeweatheringofancientorganiccarbon(Lietal.,2009)and/orcarbonate–sulfuric
acidweathering(Torresetal.,2014).
DuringQuaternaryglacial-interglacialcycles,enhancedphysicalweatheringleft
behindfine-grainedmaterialforchemicalweatheringatglacialterminals(BellandLaine,
163
1985;GoodbredandKuehl,2000;Hinderer,2001;ThomasandThorp,1995).Thiscaused
deglacialpulsesinsilicateweatheringthatcouldhavetheoreticallyloweredatmospheric
CO2by10-20ppm(Vanceetal.,2009).However,icecoresrecordariseinCO2during
deglaciations(Petitetal.,1999)duetoaconcurrentreleaseofCO2fromtheenhanced
weatheringoforganic-andsulphide-richsedimentaryrocks,whichexceededCO2
consumptionbysilicateweathering(Georgetal.,2013).
Here,weproposesimilarprocessesoccurredduringtheSilurian.Thelong-term
declineinatmosphericCO2causedbyorogeny,land-plantdiversification,volcanic
degassingand/orpaleogeographicchanges,inducedglobalcooling.Eventuallyatmospheric
CO2droppedbelowthresholdlevels(~6PAL)triggeringanexpansionofsouthern
hemisphereicesheetsoverGondwana,whichproceededtoexpandviapositiveice-albedo
feedback.Duringglaciation,enhancedphysicalweatheringandglacialprocessesactedto
increasetheoxidationofancientorganiccarbonandsedimentarysulphides,which
subsequentlydissolvedcarbonates,releasingatmosphericCO2totheatmosphere.
Meanwhile,silicateweatheringdeclined,partiallysuppressingoneoftheEarth’smajorCO2
removalmechanisms.Thecombinedinfluenceofthesetwoeffectsbegantoreversethe
globalcoolingtrend.Duringglacialmaximum,relativelylowsilicateweatheringrates
allowedCO2tobuildupintheatmosphereviaotherprocessese.g.volcanism,leadingto
rapidwarmingandeventuallydeglaciation.Thisdeglaciationexposedscouredbedrockto
theatmosphereandgeneratedfreshlycomminutedglacialtill,actingasafertilesubstrate
formeltwatertochemicallyattack.AdeclineinatmosphericCO2duetoenhancedsilicate
weatheringatglacialterminalswouldhavebeenlargelyoffsetbyenhancedoxidationof
organic-andsulphide-richsedimentaryrocks.ThisworklendsitselftotheideaoftheEarth
havingaself-regulatingclimatethatallowslifetoremainwithinhabitableconditions
(BernerandCaldeira,1997;Berneretal.,1983;Garrelsetal.,1976;Walkeretal.,1981).
164
4.5Implicationsandfutureoutlook
OsmiumandLiisotopedatapresentedhereareinterpretedtotracefluctuationsin
continentalweatheringthroughSilurianclimaticperturbations.DuringtheSilurian,
orogeny,thediversificationandglobalexpansionoflandplants,changesinpaleogeography
and/orareductioninvolcanicarcdegassing,ledtoalong-termdeclineinatmosphericCO2
andglobalcooling.DuringthelateTelychian-earlySheinwoodian,mid-Homerian,mid-
LudfordianandacrosstheSilurian-Devonianboundary,atmosphericCO2droppedbelow
thethresholdlevels(~6PAL)thatwouldhaveallowedcontinentaliceoverGondwanato
expandthroughice-albedofeedbacks.Oncetriggered,theexpansionofcontinentalice
enhancedthedenudationofunderlyingbedrockandtheproductionoffine-grained
materialforchemicalattack.Subglacialandproglacialprocessesfavourtheoxidationof
ancientorganiccarbonandsulphides,whilstsuppressingsilicateweathering,causinganet
releaseofCO2totheatmosphere.Underglacialmaximum,theproductionoffine-grained
materialceased,andoxidativeweatheringratesdiminished.Areductioninsilicate
weathering,andthereforeoneoftheEarth’smajorCO2removalmechanisms,allowedCO2
tobuildupintheatmosphereviaotherprocesses,leadingtoglobalwarmingandrapid
deglaciation.Duringdeglaciation,retreatingice-sheetswouldhaveenhancedchemical
weatheringthroughtheprovisionoffreshmaterial,increasedmeltwaterandgenerally
wetterconditions.Theoxidativeweatheringoforganicandsulphide-richlithologieswould
havelargelyoffsetthereductioninatmosphericCO2fromenhancedsilicateweathering
duringdeglaciation.
FluctuationsintheSilurianδ18Orecordweredrivenbychangesincontinentalice
volumeandglobaltemperatures,withmorepositiveδ18Ovaluesduringglacialperiods.The
associateddropineustaticsealevel,exposedcarbonateshelvestoweatheringanddrove
positiveshiftsintheδ13Crecord,attainingthehighestδ13Cvaluesduringglacialmaximum.
Acoupleddropineustaticsealevelandglobaltemperatureswouldhavehadaprofound
165
influenceonmarinebiota.TheIreviken,Mulde,LauandKlonkbio-eventsaredefinedby
globalextinctionsofconodonts,graptolites,acritarchs,andotherbenthos,withhighlevels
oforiginationsduringpost-glacialrecovery.
IncontrasttopreviousviewsoftheSilurianasagreenhouseworldpunctuated
bylargecarboncycleperturbationsassociatedwithchangesinoceancirculationand
precipitationrates,thisstudypresentsanewoutlookontheSilurian.Wesuggestthe
SilurianisanicehouseworldlikethelateOrdovicianthatprecededit,punctuatedby
glaciationsassociatedwithabruptclimaticandbiologicalchange.Thelong-termdeclinein
atmosphericCO2duringtheSilurianwasperiodicallyreversedbynegativefeedback
mechanismsassociatedwithsaidglaciations,andpreventeda‘runaway’icehouse,helping
tomaintainahabitableplanet.Futureworkwillneedtofocusongeneratingsimilar
osmiumandlithiumisotopecurvesfordifferentpaleogeographiclocationstovalidatea
globalsignalandhelpascertainthetimingsofglaciation,glacialmaximumanddeglaciation.
Moreworkisnecessarytohelpdeterminebetweenorogeny,landplantdiversification,
paleogeographicchangesorvolcanicarcdegassingasthedriverofatmosphericCO2decline
andglobalcooling.
MuchliketheCenozoic,generatingOsandLiisotopecurvesfortheentire
Siluriancouldhelptrackthetimingandextentoforogenicevents.Moredetailedsporeand
macrofossilrecordsforSilurianland-plantscouldhelptrackevolutionaryandgeographical
changesandhelpascertaintheirpotentialinfluenceonglobalweatheringandatmospheric
evolutionthroughcomputermodelling.Modellingcouldalsohelppredicttheinfluenceof
enhancedvolcanicweatheringonatmosphericCO2duringLaurussia’spassagethroughthe
ITCZ.Finally,thehuntforglacialsedimentsinSouthAmericaandAfricashouldberesumed
toprovideunequivocalevidenceforSilurianglaciationsandtheirtiming.
166
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Thisbodyofworkhasdeterminedtheosmiumisotopiccompositionseawaterforpastand
presentoceansbyapplyingRe-Osisotopemeasurementstoorganic-richshalesand
macroalgaerespectively.Thishasledtoabetterunderstandingofbasalticweatheringin
Iceland,mankind’sinfluenceontheOsisotopecycleandthediscoveryofseveralSilurian
glaciations.Thisfinalchapterbrieflydrawstogetherkeyaspectsofeachchapterandthen
suggestspossibledirectionsforfuturework.
5.1Re-Osisotopeuptakeanddistributioninmacroalgae
ResearchconductedbyB.Racionero-GómezattheUniversityofDurham,inpartunder
supervisionbymyself,lookedatReandOsuptakeanddistributioninthecommon
macroalgaespecies,Fucusvesiculosus(Racionero-Gómezetal.,2016;Racionero-Gómezet
al.,2017).ThesestudiesdemonstratedReandOsvarieswithinthemacroalgaeandthatRe
andOsarenotlocatedwithinonespecificstructure.Rhenium,andthereforemostlikely
Os,isnotheldwithinthechloroplastsorcytoplasmicproteins.RheniumandOsabundance
inculturedmacroalgaeincreasedwithincreasingculturemediaabundance.Moreover,
culturedmacroalgaedopedinseawaterwithaknownOsisotopiccomposition,tookonthe
isotopicsignatureofthefluid,withnosignsoffractionation(Racionero-Gómezetal.,
2017).Rheniumdidnotaccumulateindeadmacroalgae,suggestingsyn-life
bioadsorption/bioaccumulation(Racionero-Gómezetal.,2016).
ThisworksuggestedReandOsinmacroalgaearetakenupdirectlyfromthewater
columninwhichtheylive,wheretheyaccumulatetofarhigherconcentrationsthan
seawater.Meanwhile,theuptakeofOsbymacroalgaedoesnotfractionateOsisotopes
andmacroalgaeretainstheisotopiccompositionofseawater.Thisovercomesseveral
problemsassociatedwithdirectOsisotopeanalysisinseawater,suchasultra-low
concentrationsandmultipleoxidationstates(Peucker-Ehrenbrinketal.,2013).Itis
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thereforepossibleforOsisotopesinmacroalgaetobecomeaproxyfortheOsisotopic
compositionofseawater,apowerfultracerofEarthsystemprocesses.Moreworkwillneed
tobedonetounderstandtheuptakemechanismandstoragelocationforReandOswithin
themacroalgae.
5.2Re-OsisotopesinmacroalgaeasatracerofEarthsystemprocesses
Chapter2utilisedReandOsabundanceandisotopedatafromIcelandicmacroalgae,
dissolvedloadandbedload.ThisworkestablishedthatReandOsabundanceandisotopic
compositionreflectedthatofthebrackish,estuarinehabitatsinwhichtheylive.Inthe
instanceofIceland,estuarineconditionsrepresentthemixingbetweenriversdraining
eitheryoungerbasalticcatchmentsthathaveundergonecongruentweathering,or,older
basalticcatchmentsthathaveundergoneincongruentweatheringofprimarybasaltic
mineralswithNorthAtlanticseawater.However,althoughthe187Os/188Oscompositionof
macroalgaeiscontrolledbythatofseawater,the187Re/188Oscompositionofseawaterisfar
higherthanIcelandicgeochemicalreservoirs.ThissuggeststhepreferentialuptakeofRe
overOsathighambientseawaterReconcentrations,anotionsupportedbyfurtherdata
presentedinChapter3.
Thisstudybuiltonpreviouswork(Seesection5.1)byconfirmingtheuseofOs
isotopesinmacroalgaeasaproxyforthe187Os/188Oscompositionofseawaterinareal-
worldsetting.However,italsobecameapparentthatmacroalgaecouldnotbeusedto
tracefluctuationintheOsabundanceand187Re/188Oscompositionofseawater.Morework
willneedtobedonetounderstandOsuptakeratesinarangeofcommonmacroalgae
species.ThiswillallowformoreaccurateestimatesoftheOsabundanceofseawater,
whichinturncanbeutilisedalongside187Re/188Osratiostohelpconstraintheresidence
timeofOsintheocean.Finally,macroalgaeisnotasubstantialsinkofReandOsandglobal
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macroalgaebiomassdoesnotplayasignificantroleinthemarineOsandRecycle,both
todayandinEarth’sgeologicalpast.
5.3Re-Osisotopesinmacroalgaeasatracerofanthropogenicprocesses
Chapter3utilisedReandOsabundanceandisotopedatainJapanesemacroalgaetotrace
notonlynaturalprocesses,butalsoanthropogeniccontaminationassociatedwithhuman
activityindenselypopulatedregionsofJapan.The187Os/188Osofmacroalgaereflects
mixingbetweenseawaterand:riversdrainingMiocene-Holocenecontinentalrocks;and/or,
anthropogenicsources.Inparticular,theuseofPGEoresincatalyticconvertorshasledto
thewidespreadreleaseofappreciableunradiogenicOstotheatmosphereandmarine
environmentsurroundingmajorJapanesecities.Thisiscompoundedbyfurtherpoint
sourcereleaseofPGEderivedOsassociatedwithmedicalresearchandmunicipalsolid
wasteprocessing.
Thisstudybuiltonpreviouswork(Seesection5.1and5.2)byconfirmingthatthe
187Os/188OsofmacroalgaecouldtraceenvironmentalfluctuationsinanthropogenicOs
relatedtothewidespreaduseofPGEores.Osmiumisotopesinmacroalgaecantherefore
becomepowerfultracersofpollution.However,moreworkwillneedtobedonetofind
waystodistinguishbetweenthevariousanthropogenicsourcesofOs.Onewaycouldbeto
developtheuseofotherisotopeandelementalsystemswithspecificassociationtovehicle
use,municipalsolidwasteorrelevantmedicalresearchsources.Asanexample,Aland
organiccarbonconcentrationsinsedimentshavebeenusedinconjunctionwithOs
isotopestotraceanthropogenicOsassociatedwithsewage.Finally,Chapter3suggested
coastalcitiesareasignificantsourceofanthropogenicOstotheocean,andtheuseofPGE
oresincatalyticconvertorshasledtolower187Os/188Osratiosinglobaloceanicsurface
waters.
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5.4TheOsandLiisotopiccompositionofSilurianseawater
InChapter4,Re-OsandLiisotopesystematicsweremeasuredinorganic-richshalesand
carbonates,fromsectionsthatspannedtheSilurianIreviken,Mulde,LauandKlonk
bioevents.OsmiumandLiisotopeprofilesweresimilartothosepreviouslyrecordedforthe
Hirnantianglaciation(Finlayetal.,2010;PoggevonStrandmannetal.,inreview)andare
associatedwithfluctuationintheweatheringoforganic-sulphide-richshalesandsilicates
respectively.ThissuggeststhatmuchliketheOrdovician,theSilurianispunctuatedby
severalglaciationeventsassociatedwithenhancedcontinentalicevolumeoverGondwana,
andissupportedbycarbonandoxygenisotopedataandsea-levelreconstructions.
Ascontinentaliceexpandssubglacialandproglacialprocessesacttoenhancethe
weatheringoforganicandsulphiderichshales,deliveringanincreasedfluxofradiogenicOs
totheoceans.Meanwhile,subglacialprocessesacttosuppresssilicateweatheringcausing
areductioninthedeliveryofisotopicallylightLitotheocean.Underglacialmaximumthe
supplyoffreshlycomminutedshalesceasesalongwithweatheringandthesupplyof
radiogenicOstotheocean.However,theenhancedicecovermaintainsrelativelylow
levelsofsilicateweathering.Duringdeglaciation,subglacialandproglacialprocesses
enhancetheweatheringoforganicandsulphiderichshalesandsilicates,delivering
radiogenicOsandisotopicallylightLitotheocean.
Thisstudyhasseveralmajorimplications:
1. TheSilurianwastraditionallythoughtofasagreenhouseliketheDevonianthat
proceedsit.However,thisworksuggeststheSilurianisanicehousemuchlike
theOrdovicianthatprecededit.
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2. TheSilurianispunctuatedbyseveralglaciationsassociatedwithminormarine
extinctionsandareductioninglobaltemperaturesandsea-level.
3. OsmiumandLiisotopescanbeusedintandemtoreconstructfluctuationin
continentaloxidativeandsilicateweatheringrespectively.Ifusedproperly,
theseproxiescouldhelpusunderstandhowweatheringinfluences
atmosphericCO2throughgeologicaltime.
4. Subglacialandproglacialprocessesleadtoenhancedoxidativeweathering
whilstsuppressingsilicateweatheringduringglaciations.Thisactstoreverse
globalcoolingbyreleasingnetcarbondioxidetotheatmosphere.
5. Along-termdeclineinatmosphericCO2associatedwithorogeny,land-plant
diversification,reducedvolcanicarcdegassingand/orpaleogeography,is
ultimatelyreversedbyweatheringprocessesassociatedwithenhanced
continentalicevolume,preventingrunawayicehouseconditions.
6. ThisstudysupportstheideaoftheEarthhavingaself-regulatingclimatethat
maintainsahabitableplanet.
5.6Futureoutlook
Osmiumisotopesinmacroalgaeasaproxyforthe187Os/188Oscompositionofseawater
remainsinitsinfancydespitethegreatstridesmadeinthisbodyofwork.Onemajor
limitationistheinabilitytodeterminetheOsabundanceinseawaterusingmacroalgae.
MoreworkwillneedtobedonetodetermineOsuptakerelationshipsinasimilarmanner
topreviouswork(Racionero-Gómezetal.,2017)butatnaturalOslevels.Thiswillthen
havetoberepeatedforthemostcommongloballydistributedmacroalgaespecies.This
studyhasshownthe187Os/188Osofmacroalgaerepresentsthatoftheseawaterinwhichit
lives,andifitcouldbecombinedwithareconstructionof[Os],wouldallowthe
determinationofcontinentalinputsofOstotheocean.Thiswouldallowforbetter
constraintsonthemarineOscycleandOceanicresidencetimes.
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Thisworkhasshownthe187Os/188Osofmacroalgaecantraceanthropogenic
sourcesofOs.However,itisdifficulttodistinguishbetweenthevarioussourcesduetothe
similarnatureofisotopicsignatures.Otherisotopeorelementalsystemswithaspecific
affinityforoneofthesesourceswillneedtobedevelopedinmacroalgaetodistinguish
betweenthesesources.Thiscouldhelpmacroalgaebecomeapowerfultracerofhuman
activityandausefulenvironmentalindicator.Inparticular,itcanbeusedtotracethe
influenceofwidespreadcatalyticconvertoruse,‘similartoPbfromleadedgasolineusage
before1978ortritiumfromatmosphericatomicbombtestingintheearly1960s’(Chenet
al.,2009).
ThisbodyofworkprovidedfurthergeochemicalevidenceforperiodicSilurian
glaciationsbyusingOsandLiisotopes.However,physicalevidenceformid-lateSilurian
glaciationsstillremainselusive.Moreworkwillneedtobedonetofindglacialsediments
forthetimeperiodsstudied.ThiswillprovideunequivocalevidenceforaSilurianicehouse
world.Chapter4suggestedthatOsandLiisotopeprofilesrepresentchangesinoxidative
andsilicaterespectively.However,likemostisotopessystems,thesesignalsremain
ambiguous.Datacollectedinthisstudycouldreflectchangesinweatheringofvariable
continentalrocks(notjustoxidativeweathering)orachangeinweatheringcongruency.
Moreweatheringproxiesneedtobeutilisedtoseparateoutthesesignals.Moreover,
sampleswereonlyanalysedinoneregionallocation.Inordertodetermineaglobalsignal
samplesfromgloballydistributessitesneedtobemeasured.
Finally,thecauseofSilurianglaciationsremainselusive.Mostproposed
mechanismsinthisstudysuggestalong-termdeclineinatmosphericCO2andglobal
temperaturesresultingfromchangesinorogeny,land-plantdiversification,volcanic-arc
degassingorpaleogeography.Moreworkwillneedtobedonetodeterminebetweenthese
185
driversofclimaticchange.Inparticular,complexcarboncyclemodellingfortheSilurian
mayprovideunparalleledinformation.
5.7References
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global-scaleatmosphericcontamination.ProceedingsoftheNationalAcademyofSciences
oftheUnitedStatesofAmerica106,7724-7728.
Finlay,A.J.,Selby,D.,Gröcke,D.R.,2010.TrackingtheHirnantianglaciationusingOs
isotopes.EarthandPlanetaryScienceLetters293,339-348.
Peucker-Ehrenbrink,B.,Sharma,M.,Reisberg,L.,2013.Recommendationsforanalysisof
dissolvedosmiuminseawater.Eos,TransactionsAmericanGeophysicalUnion94,73-73.
PoggevonStrandmann,P.A.E.,Desrochers,A.,Murphy,M.J.,Finlay,A.J.,Selby,D.,Lenton,
T.M.,inreview.GlobalclimatestabilisationbychemicalweatheringduringtheHirnantian
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Racionero-Gómez,B.,Sproson,A.,Selby,D.,Gröcke,D.,Redden,H.,Greenwell,H.,2016.
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Racionero-Gómez,B.,Sproson,A.D.,Selby,D.,Gannoun,A.,Gröcke,D.R.,Greenwell,H.C.,
Burton,K.W.,2017.Osmiumuptake,distribution,and187Os/188Osand187Re/188Os
compositionsinPhaeophyceaemacroalgae,Fucusvesiculosus:Implicationsfordetermining
the187Os/188Oscompositionofseawater.GeochimicaetCosmochimicaActa199,48-57.