Paleoecology and paleoceanography of the Athel silicilyte,
Ediacaran–Cambrian boundary, Sultanate of Oman
D. A. Stolper1 | G. D. Love2 | S. Bates2 | T. W. Lyons2 | E. Young3,4 |
A. L. Sessions1 | J. P. Grotzinger1
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA2DepartmentofEarthSciences,UniversityofCalifornia, Riverside, CA, USA3DepartmentofEarthandSpaceSciences,University of California, Los Angeles, CA, USA4InstituteofGeophysicsandPlanetaryPhysics, University of California, Los Angeles, CA, USA
Correspondence
D.A.Stolper,DivisionofGeologicalandPlanetary Sciences, California Institute of Technology, Pasadena, CA, USA.Email:[email protected]
Present address
D.A.Stolper,DepartmentofEarthandPlanetary Science, University of California, Berkeley,CA,USA
Funding information
Division of Geological and Planetary Sciences atCaltech;NSFGRFP
Abstract
TheAthelsilicilyteisanenigmatic,hundredsofmetersthick,finelylaminatedquartzdeposit,inwhichsilicaprecipitatedindeepwater(>~100–200m)attheEdiacaran–CambrianboundaryintheSouthOmanSaltBasin.Incontrast,Meso-Neoproterozoicsinksformarinesilicaweredominantlyrestrictedtoperitidalsettings.Thesilicilyteisknowntocontainsteranebiomarkersfordemosponges,whichtodayarebenthic,ob-ligatelyaerobicorganisms.However,thebasinhaspreviouslybeendescribedasper-manentlysulfidicandtime-equivalentshallow-watercarbonateplatformandevaporiticfacieslacksilica.TheAthelsilicilytethusrepresentsauniqueandpoorlyunderstooddepositionalsystemwithimplicationsforlateEdiacaranmarinechemistryandpaleo-ecology.Toaddresstheseissues,wemadepetrographicobservations,analyzedbio-markersinthesolvent-extractablebitumen,andmeasuredwhole-rockironspeciationandoxygenandsiliconisotopes.ThesedataindicatethatthesilicilyteisadistinctrocktypebothinitssedimentologyandgeochemistryandintheoriginalbiologypresentascomparedtootherfaciesfromthesametimeperiodinOman.Thedepositionalenvi-ronmentofthesilicilyte,ascomparedtotheboundingshales,appearstohavebeenmorereducingatdepthinsedimentsandpossiblybottomwaterswithasignificantlydifferentbiologicalcommunitycontributingtothepreservedbiomarkers.Weproposeaconceptualmodelforthissysteminwhichdeeper,nutrient-richwatersmixedwithsurfaceseawaterviaepisodicmixing,whichstimulatedprimaryproduction.Thesilicanucleatedonthisorganicmatterandthensanktotheseafloor,formingthesilicilyteina sediment-starved system.Wepropose that the silicilytemay represent a typeofenvironmentthatexistedelsewhereduringtheNeoproterozoic.Theseenvironmentsmayhaverepresentedanimportantlocusforsilicaremovalfromtheoceans.
1 | INTRODUCTION
ThemakeupanddistributionofmulticellularlifeaspreservedintherockrecordchangedfundamentallyacrosstheEdiacaran–Cambrianbound-ary(~541Ma).Ourunderstandingandrecognitionofthischangeareinextricablylinkedtotheriseinthenumberofmineralizedmetazoanfossils intheCambrian(Valentine,2002).However,thecauseofthistransition and the timing and rate of this radiation remain contentious. Some explanations favor environmental drivers for these biological
changes includinga large-scale change to theoxidation stateof theatmosphereandoceans(asreviewedbyMills&Canfield,2014),global-scaleglaciations (Hoffman,Kaufman,Halverson,&Schrag,1998),ortruepolarwander(Kirschvink&Raub,2003).Alternatively,otherex-planationsfocusonchangesbetweenorwithinorganismsthemselves.Thisincludestheemergenceofnewecologicalinteractions(Marshall,2006)andnewgeneticinnovations(Davidson&Erwin,2009).
primitivemetazoancladesexistedinsomestatepriortothisradiation,although how far back is argued (Antcliffe, Callow, & Brasier, 2014;Droser & Gehling, 2015). Knowledge of the environments in whichearlyMetazoaormetazoanprecursors lived is necessary to evaluatethe link between environmental changes and the Cambrian “explo-sion” of animals (Butterfield, 2009;Mills &Canfield, 2014).MultiplelinesofevidenceexistforNeoproterozoicMetazoa.Forexample,somePrecambrianfossilshavebeeninterpretedasfossilizedancientanimalembryos (Cohen, Knoll, &Kodner, 2009; Xiao,Yuan,&Knoll, 2000),or,alternatively,asgiantsulfurbacteria(Bailey,Joye,Kalanetra,Flood,& Corsetti, 2007). Additionally, both molecular (Gold, Grabenstatter,etal.,2016;Love&Summons,2015;Loveetal.,2009)andbodyfossils(Brasier,Green,&Shields,1997;Li,Chen,&Hua,1998;Maloofetal.,2010;Yinetal.,2015)havebeeninterpretedtoindicatethepresenceofsponge-gradeorganismsintheNeoproterozoic.However,theinter-pretationofsomeofthesefossilshasbeenquestioned(Antcliffeetal.,2014).Finally,someEdiacaranbiotaarethoughttorepresentearlyan-imals(e.g.,Kimberella;Droser&Gehling,2015;Fedonkin&Waggoner,1997).ManyoftheseEdiacaranorganismsarethoughttohavelivedatornearthesediment–waterinterface(Xiao&Laflamme,2009).Thus,thegeochemistry(e.g.,oxidationstate)ofthebenthicenvironmentsinwhichtheseorganismslived(Canfieldetal.,2008)isfundamentaltoourunderstandingoftheevolutionandexpansionofcrown-groupMetazoa.
Weinvestigatedthechemicalconditionsandmolecularfossilsofor-ganismsrecordedinrocksdepositedatorneartheEdiacaran–Cambrianboundary in Oman from the Athel Basin of the South Oman Salt Basin. Weusetheseconstraintstobetterunderstandtherelationshipsbetweentheenvironmentalgeochemistryandpaleobiologyinbenthicandpelagicwaters and sediments in the basin at this time. This succession of strata wasselectedbecauseitcorrelateswithunitsthatstraddletheEdiacaran–Cambrianboundary,containshighconcentrations(>1weight%)oftotalorganicmatter,and isbelievedtohavebeendeposited in “deep”water(>100–200m deep; Amthor, Ramseyer, Faulkner, & Lucas, 2005). TheAthelBasinthuspotentiallyprovidesconstraintsonenvironmentsdistinctfromthosewithshallow,well-mixedwaters in thephoticzone thataremorecommonlyencounteredfromthistimeperiod.Critically,bitumenandkerogenorganicphasesfromtheseAthelBasinrockshaveyieldedsteranesinterpretedbysometooriginatefromdemosponges(Gold,Grabenstatter,etal.,2016;Love&Summons,2015;Loveetal.,2009),whichrequiredis-solved O2(i.e.,theyareobligateaerobes).Inparticular,wefocusedontheAthel Formation (termed theAthel “silicilyte”), a silica-rich sedimentarydepositandeconomically importantpetroleumsourcerock.Wepresentpetrographicexaminationsofthinsectionsanddataonextractable lipidbiomarkers preserved in sedimentary organicmatter, ironmineral geo-chemistry,andstable-isotoperatiosofoxygenandsilicon.Thesemeasure-mentsareusedtoinfertherelationshipsbetweentheoriginaldepositionalenvironment of the Athel Basin and the organisms it hosted.
2 | GEOLOGIC BACKGROUND
TheSouthOmanSaltBasin(Figure1)ispartofasubsurfacenetworkof evaporite basins studied extensively because of its hydrocarbon
reserves. Allen (2007), Bowring etal. (2007), and Grotzinger andRawahi (2014)providedetailedgeological reviewsofthebasin.Wefocusedon theAraGroupdepositedduring lateEdiacaran toearli-estCambriantime(Figure2).TheAraGroupconsistsdominantlyofshallow-water carbonate units bounded by evaporites (the A0–A6units).U-Pbdatesonzirconsextractedfromintra-Araashbedsrangeinagefrom547to541Ma(Amthoretal.,2003;Bowringetal.,2007).TheA4unitcontainsanashbeddatedat541±0.13Ma(Bowringetal.,2007)coincidentwithaglobalcarbon-isotopeexcursionthatmarkstheEdiacaran–Cambrianboundary inOman.Additionally, aswillbeemphasizedlater,thesecarbonateslackauthigenicchert(Grotzinger&Rawahi,2014;Ramseyeretal.,2013;Schröder,Grotzinger,Amthor,&Matter,2005;Schröder,Schreiber,Amthor,&Matter,2003).TheAraalso includes theclay-richU-ShaleandThuleilat formations (shales)and the silica-rich Athel Formation (“silicilyte”; Figure2), which areinterpreted to have formed in a deeper-water setting compared tothe carbonates. These formations as well as the Ara carbonates are encasedinevaporitesalts.TheU-ShaleiscorrelatedintimetotheA4carbonatebasedonasharedhighgamma-raylogcharacter(Amthoretal.,2005).Thesefaciesarelocalizedwithinapartlyfault-boundedbasinor“trough”(Amthoretal.,2005)referredtoastheAthelBasin.Consequently,theArashalesandsilicilytewerelikelydepositedatorwithinafewmillionyearsoftheEdiacaran–Cambrianboundary.
Amthoretal.(2005)estimatedthewaterdepthoftheAthelsedi-mentsatthetimeofdepositiontohavebeengreaterthan100–200m.Depositionoccurredinafault-boundedbasinrelativetoaco-evalplat-formcarbonates.ThisinterpretationisindependentlysupportedbytheexistenceofshalesandlaminatedquartzintheAthelBasin—suchsed-imentsaregenerallyformedinlow-energydepositionalenvironments(e.g.,belowwavebase).Additionally,theAthelsilicilytefaciesisupto400mthickand is thusmosteasily interpretedtohave formed inadeeptrough(Amthoretal.,2005).
Thesilicilyterepresentsapotentiallyuniquefaciesinthegeolog-ical record.Althoughclassifiedasa chert (Amthoretal.,2005), it isunlikemostmid-andlateProterozoiccherts.Forexample,unliketheseothercherts,thesilicilytesilicadidnotreplacepre-existingminerals.Additionally,andagainunlikemostmid-andlateProterozoiccherts,inhandsamplethesilicilyteislaminatedandfriable.Itisdominatedbyquartz,whichis,onaverage,85%oftherockbyweight(Amthoretal.,2005)—other minor phases include organic carbon, smectite/illiteclays,apatite,pyrite,anddolomite(Amthoretal.,2005).Claycontentsofthesilicilytearetypically~4–6%byweightoftherockandorganiccarbon2–5%(AlRajaibi,Hollis,&Macquaker,2015).Additionally,andmost importantly, the silicilytedid not form in a shallow, evaporiticsetting as is the case for nearly all authigenic cherts from this time period(Maliva,Knoll,&Simonson,2005).
Multiple hypotheses have been proposed to explain the originof the silicilyte. For example, the possibility of silica precipitationbya silica-secretingorganismshasbeenexploredbutultimately re-jectedbasedonsilicon isotopicmeasurements (Amthoretal.,2005;Ramseyeretal.,2013).Alternatively, thepassivenucleationof silicaonfloatingmicrobialmatsatthechemoclineinthewatercolumn(AlRajaibietal.,2015;Amthoretal.,2005)hasbeenproposed.Stillother
All previous studies have argued that the water column in theAthel Basinwas permanently reducing and likely sulfidic (Al Rajaibiet al., 2015; Amthor et al., 2005; Ramseyer et al., 2013; Schröder
& Grotzinger, 2007;Wille, Nägler, Lehmann, Schröder, & Kramers,2008). Indeed, the presence of a chemocline and pycnocline in thewater column were necessary conditions in the models of silica nu-cleation discussed above. Yet, biomarkers interpreted by some toindicate the presence of obligately aerobic, benthic demosponges,albeitwithdissolvedoxygenrequirementsaslowas~1%ofmodern
F IGURE 1 Geologicalmapofthestudyarea.Bottomrightpanelisageographicmapoftheregion.ThecentralmapshowstheplacementoftheSouthOmanSaltBasinwithinOman.Theupperleftpanelshows in detail the South Oman Salt Basin including the location of the main well studied here, Marmul Northwest 7. Based onAmthoretal.(2005)andSchröderandGrotzinger(2007)
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air-saturated seawatervalues (Mills etal., 2014), are found in rocksfromtheAthelBasin(Grosjean,Love,Stalvies,Fike,&Summons,2009;Loveetal.,2009).Any interpretationabout thepaleoecologyof thesilicilyterequiresanintegrated,self-consistentmodelforthegenera-tionofthesedimentsandoriginofthebiomarkerspreservedinthosesediments.Suchamodelshouldalsoconsiderthelackofsilicainthetime-equivalentAracarbonates.
3 | METHODS
3.1 | Sample preparation and extraction
SamplesusedforbiomarkeranalysesarelistedinTables1–3.Biomarkermeasurementsweretakenoncuttings(pulverizedrockextracteddur-ing drilling from known depths) obtained from the Marmul NW-7core (MMNW-7; Figure1). Samples for biomarkerswereprocessedaccording to the followingprocedure:Cuttingswerepre-washed inwater and then methanol for 1 min, vacuum- filtered, and finally rinsed brieflywithdichloromethane(DCM)toremoveanysurfacecontami-nation.Sampleswerethendriedandsubsequentlycrushedbyhandinasolvent-cleanedceramicmortarandpestle(pre-solventwashed).Followingthis,~2gofeachsamplewereextractedtwicein95:5v/vDCM:methanol at 100°C in a CEMMars 5microwave extractor inpre-solvent-washedTeflon tubes. Insoluble sediment residueswere
separated fromextracts via vacuum filtration. Elemental sulfurwasremovedfromsamplesbypassingextractsthroughcoppercolumnsactivatedwith1MHCl.Asphalteneswereprecipitatedfrommalteneswith an excess ofn-hexane solvent. Saturated, aromatic, and polarcompoundswere separated on activated silica gel (40–60μm, pre-heatedovernightat~220°C)firstwithpurehexane,followedby9:1v/vhexane:DCM,andfinally95:5v/vDCM:methanol.
3.2 | Lipid biomarker and Rock- Eval analyses
Sampleswereinitiallyexaminedonagas-chromatographquadrupolemassspectrometer(GC-MS;ThermoTrace—DSQ)atCaltech.1μl of samplewasinjectedfromaprogrammabletemperaturevaporizationinjector insplitlessmodeandtransferredat350°CtoaDB-5msorZB-5mscapillarycolumn (30m×0.25mm i.d.×0.25μmfilm thick-ness)withHeasthecarriergas,heatedat20°C/minfrom80to130°C,then 5°C/min to 320°C, and held for 20 min. n-alkaneandmid-chainmonomethylalkane relative abundances were calculated by manualintegrationofpeaksinthechromatogram.
SampleswerethenmeasuredatUCRiversideonaWatersAutoSpecPremierequippedwithaHP6890GCusingaDB-1MScapillarycolumn(60m×0.25mmi.d.,0.25μmfilmthickness)forbiomarkeridentifica-tionandquantification,withHeasthecarriergas.Metastablereactionmonitoring(MRM)GC-MSmeasurementsweretakenwithatempera-tureprogramof60°Cfor2min,heatingto320°Cat4°C/minwithafinalholdat315°Cfor34min.Aliquotsofthesaturatedhydrocarbonfractionswerespikedwith50ngofadeuteratedC29 sterane internal standard (d4- ααα−24-ethylcholestane) and analyzed using published
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406 | STOLPER ET aL.
MRMGC-MSmethods(Grosjeanetal.,2009;Loveetal.,2009).TheAustralian Geological SurveyOrganization (AGSO) standard oilwasusedasa referencepoint for the retention timesofcommonmole-cules.Peaksinthechromatogramswereintegratedmanually.Typicaluncertaintiesinhopanetosteraneratiosare±8%orless,ascalculatedfrommultipleanalysesofasaturatedhydrocarbonfractionfromtheAGSOstandardoil.Reproducibilityofmeasurementswasadditionallytestedbyrunningtwosaturatedhydrocarbonfractions(samples2626and2688)multipletimes.Ingeneral,mostbiomarkerratiosreportedinthispaperexhibituncertaintiesofabout1–2%(1standarddevia-tion, σ).Thehopanesandsteranesdetectedinoursamplesareatleastthree orders of magnitude higher in abundance than those found in procedurallaboratoryblankswithcombustedsand.
Rock-Evalparameters(hydrogenindex,HI;oxygenindex,OI;andTmax) and total organic carbon (TOC)measurementsweremade byWeatherfordLaboratoriesonaVinciRock-Eval6instrumentfollowingLafargue,Marquis,andPillot(1998)forRock-EvalparametersandonaLECO600CarbonAnalyzerforTOC.
3.3 | Iron speciation
Speciation among the iron minerals and total iron and aluminumcontents, were determined following the methodology de-scribed in Planavsky etal. (2011) and Poulton andCanfield (2005).Concentrations of extracted iron and aluminum were measuredvia inductively coupled plasmamass spectrometry atUCRiverside.“Highlyreactive”ironintheformofcarbonatephases(principallysi-derite),ferricoxides(hematiteinPrecambriansamples),andmagnetitewasextracted sequentially throughacid leaching.A sodiumacetatesolutionwasused initially for thecarbonate-boundFe, followedbysodiumdithioniteforhematite,andthenammoniumoxalateformag-netite(Poulton&Canfield,2005).Pyritecontentwasmeasuredfromaseparatesamplesplitviaiodometrictitrationofsulfurreleasedfromahotchromouschloridedistillation.Pyriteiron(FePy)contentwasthencalculatedfromthemeasuredpyrite-derivedsulfurassumingaFeS2 stoichiometry.Theseextractedphasesarecollectivelyreferredtoas“highlyreactive”iron(FeHR)becausetheyhaveorcouldhavereactedwithhydrogensulfidetoformpyriteduringsedimentarydiagenesis.Concentrationsoftotaliron(FeT)andaluminumweredeterminedonaseparatealiquotofsampleviaatotalaciddigest (Planavskyetal.,2011).Reproducibilityfortheironcontentofeachsequentialextrac-tiontechniqueistypically~5%(Planavskyetal.,2011).
In most samples, the measured ratio of FeHR/FeT was found to exceedunity,withvalues typically between1.04 and1.28. Such“excesses” of highly reactive Fe are not uncommon (e.g., Clarkson,Poulton, Guilbaud, &Wood, 2014) and occur due to errors associ-ated with the different methods used to measure the total iron vs. the sequential ironextractions.Whentheweightpercentironmeasuredviathespeciationextractionexceedsthatinthetotaldigest,wetreatthesummedFeHRdataasirontotal,thusensuringallFeHR/FeT ratios are≤1.Thetotal ironcontentofsample2,737wasfoundtobesig-nificantlylower(50%)thanthatofironspeciationfractionandisnotdiscussed.
3.4 | Petrography and stable- isotope measurements
Petrographic thin sections were analyzed using both reflected andtransmitted light. Inaddition,secondaryelectronmicroscopy (SEM),electronbackscatterdiffraction(EBSD),andelectron-dispersiveX-rayspectroscopy(EDS)mapsweremadeatCaltechusingaZeiss1550VPfield-emissionSEMequippedwithanOxfordX-MaxSDDEDSsystemandHKLEBSDsystem.
OxygenisotoperatiosweremeasuredatCaltechonaCameca7fsecondaryionmassspectrometer(SIMS).GrainsoftheCaltechrosequartzstandards (δ18O=8.45‰1 )wereembedded in thecenterofsampleswithepoxy,cut into thinsections,polished,and thengold-coated.Measurementswere takenwith a 133Cs+ primary ion beamwithaspotsizeof20–30μm.Secondaryoxygenions(16O− and 18O−)werecollectedviamagnethoppingonaFaradaycupatamassresolv-ingpowerof~3600.Analysesconsistedof20cycleswhere16O was counted for 1 s and 18Ofor5spercycle.Averagecountrateswere~1×109 countsper second (cps) for 16Oand~2×106 cps for 18O. Externalprecisionofstandardswas~0.8‰(1σ).
Siliconisotoperatios(expressedasδ30Si)weremeasuredvialaserablationmulticollectorinductivelycoupledplasmamassspectrometryatUCLAonaThermoFinniganNeptune.Allmethodsandstandardsfollow Shahar andYoung (2007) and Ziegler, Young, Schauble, andWasson (2010). Siliconwas extracted from the samplewith a 193-nmexcimerlaser(PhotonMachinesAnalyte)usinga52μmspotsize.Extractedsiliconwas introduced to themass spectrometerwithHeasthecarriergas,mixedwithdryargon,andthenionized.Themassspectrometerwas set to medium resolution (mass resolving powerof 7,000). Instrumental mass bias (~5%) was corrected by sample-standard bracketing using a previouslymeasured SanCarlos olivinestandard.Matrixeffectsbetweenquartzandolivinehavebeenshownpreviouslytobe<0.1‰usingthismethodology(Ziegleretal.,2010).Accuracywasconfirmedthroughcomparisontoapreviouslymeasuredisotopically spiked glass. External precisionwas ±0.2‰ (1σ),whichwasdeterminedbymaking14replicatemeasurementsofastandardoverthecourseofmeasuringsamples.As inRamseyeretal. (2013),thesevaluesarenotcorrectedfortheminorweightpercentofclaysinthesamples.Thisdoesnotinfluencethemeasurementsbeyondtheirstatedprecision(Ramseyeretal.,2013).
4 | RESULTS AND DISCUSSION
4.1 | Petrographic constraints on the formation of the silicilyte
Petrographicthinsectionsweremadeandexaminedforallsamplesused for geochemical analyses. Representative thin sections fromcore samplesof theAthel silicilyte are shownunderboth reflectedandtransmittedlight(Figure3).Laminae,usually20–30μmthick,are
definedbydifferencesincolor(e.g.Figure3a).Darkercolorsindicatelarger concentrations of organic matter and/or clay. Laminae are slightly wavy and can be both continuous and discontinuous on the scaleofathinsection.Additionally,pinchandswelloflaminaeoccur.
F IGURE 3 Petrographicimagesofsamplesfromcore.Panelsontheleft(a,c,ande)aretransmittedlightimages,whilethoseontheright(b,d,andf)arereflected.(a)and(b)arefromadepthof2,687m,(c)and(d),2,688m,and(e)and(f),2,690m.Laminae are distinguished by differences incoloration,whichreflectcomposition;darkregionsaremoreorganic/clayrichcomparedtolighter,moresilica-richlayers. Laminae may be slightly wavy and discontinuous on the scale of centimeters. Additionally,laminaearevariablyexpressedindifferentthinsectionswith,forexample,(a)and(b)showinglaminaemoreclearlythan(e)and(f)[Colourfigurecanbeviewedatwileyonlinelibrary.com]
Tobetterunderstandtherelationshipsbetweenthesilica,clay,andorganicmatter,sampleswereexaminedviaelectron-dispersiveX-rayspectroscopy (EDS) and secondary electron microscopy (Figure4).Although samples were polished before measurement, secondaryelectron images still show significant topographywith clay/organic-rich areas being slightly recessed relative to quartz-rich areas. Thisdifference likely occurs because clay and organic matter are softerthanquartzandthusmoreeasilyremovedduringpolishing.Silica-poorareasareeither truevoidspaces (i.e.,notanartifactof thinsectionpreparation)orfilledwithorganiccarbonorclay (Figure4).Theclayand carbon, on the micron scale, are generally not found in the same voids.Additionally, theclayandorganicmatterare intimatelymixedwiththesilica—thatis,therearenodistinguishableclaylayersoror-ganic layers at themicron scale.Wenote that samples are carbon-coated—assuch,onlyconcentratedaccumulationsoforganiccarbon(e.g.,discretekerogenparticles)arediscernible.
Afewkeyobservationsandconclusionscanbemadefromthesepetrographicexaminations.First,thelaminaeinthesilicilytearemor-phologicallysimple. Incontrast,microbialmatsoftencreate laminaewithpositivereliefmanifestedby“crinkly”irregularitiesatthetopofalamina(e.g.,asisobservedintime-equivalentcarbonatesoftheAra;Grotzinger&Rawahi,2014;Schröderetal.,2005)orgrowthfeatures(e.g.,stromatolites;Grotzinger&Knoll,1999).Ramseyeretal.(2013)interpretedthelaminaeasfossilizedmicrobialmats.Specifically,theyproposedthatbenthicmicrobialmatslivingatthesediment–waterin-terfacecapturedandwereencasedbysedimentingsilica.Wearguethat the silicilyte laminae lack the diagnostic features generated bymicrobialmatsandaremoreconsistentwithsimplepelagicsedimen-tation; we return to this in Section 5.1.
Second, laminae are defined by alternating organic carbon/clay- richvs.carbon/clay-poorlayers.Alternatinglaminaeofdifferingphys-icalproperties,suchasthosefoundinvarves(e.g.,Thunell,Tappa,&Anderson,1995),areoftenattributedtophysicalorchemicalchangesinthedepositionalenvironmentduetoepisodicforcingsuchas,butnotlimitedto,changesinseasonality,oxygenationofbottomswaters,orsedimentfluxes.Previously,thelayersinthesilicilytewerearguedtorepresentweeklytobiweeklyevents(~32laminaeformedperyear)byRamseyeretal.(2013).
Third, the clay and organic content that give contrast to the lami-naeinthesilicilytedonotformseparatehorizons,butinsteadareinti-matelymixedwithquartzatthemicronscale.Thecouplingoflaminaeto organic matter content could be the result of heterogeneous nucle-ationofsilicaonorganicmatter(AlRajaibietal.,2015;Amthoretal.,2005;Ramseyeretal.,2013),whichwediscussfurtherinSection5.
Fourth,handsamplesfromcoreandthinsectionsofthesilicilytelack any discernible sedimentological hallmarks of a shallow-waterdepositionalsetting.Nodesiccationcracks,mudchips,orcurl-upmatstructuresarepresentinanysamples,norhavetheybeendescribedinanypreviousstudy.Thisindicatesthatthesilicilyteformedatsig-nificantwaterdepths (>100–200m)asopposedto inaquiet-water,shallowlagoon.PossiblerippleshaverecentlybeendescribedintheU-Shale,intermixedwithplane-parallellaminations,whichcouldindicateshallowerdepositionaldepthsfortheU-Shale(AlRajaibietal.,2015).
Thesephysicalobservations set critical, first-order requirements foranymodelofthesilicilyte.Thatis,suchamodelmustexplainthepres-enceof laminae, their simplemorphology, sometimesdiscontinuousnature,andintimateassociationofthequartzwiththeorganiccarbonand clay.
4.2 | Biomarkers and organic parameters
The term “biomarkers,” as used here, refers to fossil hydrocar-bon lipids derived from ancient organisms (Brocks & Summons,2005).Biomarker-basedandRock-Evalparametersarepresented inTables1–3andaselectionofchromatogramsinFigures5–7.Yieldsofbiomarkers(ngbiomarker/mgsaturate)aregiveninTableS1.
Thebiomarkers28,30bisnorhopane(BNH)and25,28,30trisnor-hopane (TNH)areusedasproxiesforreducing,sulfidicdepositionalenvironments.BNHishypothesizedtooriginatefromsulfide-oxidizingmicrobeslivingatthetransitionbetweensulfidicandoxicconditions(Schoell,McCaffrey, Fago,&Moldowan, 1992).TNHhas been sug-gested to originate from a diagenetic demethylation of BNH or via direct synthesis by microbes (Peters, Walters, & Moldowan, 2005;Schoelletal.,1992).NoextantsourceorganismsforBNHorTNHareknown.TheratioofBNHtoC30 αβhopane,knownasthebisnorho-paneindex,iscommonlyusedasaproxyforreducingsedimentsandpossiblewatercolumnanoxia.Anindexvalueof>0.5(Caoetal.,2009)hasbeenusedpreviouslytoestablishthepresenceofreducingcon-ditions.WeincludethesumofBNHandTNHinthenumeratorofthebisnorhopane index toaccount for thepossiblediagenetic transfor-mationofBNHtoTNH.Maximumvaluesoftheindex(~2.75)occurin the center of the silicilyte, while values in the U- Shale and Thuleilat arelower,averaging~0.5(Figure8a).Thesevaluesareconsistentwiththedepositionofallsamplesinanoxicsedimentsandperhapsbottomwaters.WenotethatBNH/C30Hratios(~0.3–0.65)shownoobvioustrendthroughthesection. Instead, theelevated (BNH+TNH)/C30H valuesseeninthesilicilyte(Figure8a)areassociatedwithincreasesinsilicilyte TNH abundances. This may indicate increased demethylation of BNH to TNH in high silica conditions or a distinctive source of TNH to the silicilyte.Regardless, thehigher (BNH+TNH)/C30H values in thesilicilytesamplesrelativetotheboundingshalesindicatethatei-therthedepositionalenvironmentofthesilicilytewasmorereducingthan the bounding shales or had distinctive contribution of biomass fromorganismsthatsynthesizeTNHprecursors.ElevatedBNH/C30H
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and TNH/C30Hvalueshavebeenobservedinallotherpreviouslystud-iedsilicilytesamples(Grosjeanetal.,2009).
The homohopane index (HHI) is the percent abundance of C35 hopanesrelativetothesummedC31 to C35hopaneabundances.TheHHIproxyisbasedontheobservationthatthesidechainsofbacte-riohopanepolyols(theprecursorstoC35hopanoids)arepreferentiallycleavedunder oxic conditions, creatingC31–34 hopanes. In contrast,undersulfidicconditionsthesidechainsarethoughttobestabilizedvia enhanced cross-linkingwith other organic molecules (Bishop &Farrimond, 1995; Sinninghe Damsté, Van Duin, Hollander, Kohnen,&DeLeeuw,1995).Thus,C35hopanepreservationisenhancedoverotherhopanes insulfidicenvironments.Generally,HHIvalues inex-cessof5%areinterpretedtoindicatereducingconditions(Caoetal.,2009).MaximumHHIvaluesbetween20and25%occurinthesilici-lyte,whiletheboundingshaleshavevaluesnear10%(Figure8b).Thisproxyindicatesthattheoriginalsedimentsofallsamplesandpossiblytheoverlyingwaterswereataminimumepisodicallyanoxicandsul-fidicduringthetimeofdeposition.Thehighervaluesofthisratiointhesilicilyteindicatestrongerreducingconditionsintheoriginaldeposi-tionalenvironmentofsilicilyteascomparedtotheshales.
Thefinalredox-sensitivebiomarkerproxywepresentisthegam-macerane index.This proxy employs the ratio of the abundance ofgammacerane to the abundance of C30 αβhopane.Itsutilityrestsontheobservationthatgammacerane’sprecursor,tetrahymanol(Haven,Rohmer,Rullkötter,&Bisseret,1989),isdominantlyproducedbycili-ateslivinginanaerobicormicroaerophilicenvironmentsatthechemo-cline(Havenetal.,1989).Onthisbasis,enhancedgammaceranelevelsareusedtoinferwatercolumnstratificationduetobothredox-andsalinity-driven gradients (Schoell, Hwang, Carlson, &Welton, 1994;Sinninghe Damsté etal., 1995). Some Proteobacteria also producegammacerane precursors (Kleemann etal., 1990), includingmethyl-ated and dimethylated forms. Thus, gammacerane, in and of itself, is notauniquebiomarkerforredox-orsalinity-stratifiedwaters.
Regardless, the gammacerane index reachesmaximumvalues inthe silicilyte, ranging from 0.1 to 0.3, while the bounding shales range
from~0.05to0.20(Figure8c).Theincreaseinthegammaceraneindexislinkedtoincreasesingammaceranecontent(asopposedtoadropinC30 αβhopanecontent).Specifically,averagevaluesforgammaceraneconcentrations(ngbiomarker/mgsaturatedextract)are4.7×higherinthe silicilyte relative to the shales, while for C30 αβ abundances they are2.7×higher(TableS1).Themeasuredgammaceraneindexvaluesarenotparticularlyelevatedwhencomparedtorestricted,hypersalinedepositionalenvironments(e.g.,>2;Jiamoetal.,1986).Forcompari-son,Precambrianbitumensfromarangeof faciessampled inSouth
F IGURE 4 ElectronmicroscopeimagesfromathinsectionfromtheMMNW-7wellat2,686.7metersdepth.(a)Asecondaryelectronmapofarepresentativeportionofthethinsection.Thethinsectionwaspolishedandcarbon-coatedbeforemeasurement.Thetopography,despitepolishing,wasgeneratedbydifferentialpolishingofquartzcomparedtovoidspacesandclayintherock.(b)Abackscatteredelectronmapofthesamearea—thelargebrightspotsarepyrite.(c)Anelementalmapgeneratedbyenergy-dispersiveelectrons.Weshowmapsofoxygen(blue;representingsilicagroups),carbon(green;representingorganiccarbon),aluminum(red;representingclays),andsulfur(yellow;representingpyrite).Aluminumalsotrackspotassiumconcentrationsindicatingthepresenceofclaysasopposedtoacontaminantfrompolishing.Carbonandaluminumarefoundinvoidspacesbetweensilicaaccumulations.Organic carbon and aluminum show no obvious association, although both are found in the large central void. Otherwise they arefoundseparatelyinthesmallerspaces.Organiccarbonisfoundconcentratedinsmallspacesthroughoutthesample[Colourfigurecanbeviewedatwileyonlinelibrary.com]
Omanshowgammaceraneindexvaluesfrom0to0.35(Grosjeanetal.,2009). Additionally, our measured values are consistent with pre-viousmeasurementsofgammacerane indexvalues fromsamplesofthesilicyliteandtheboundingshales(Table3;Grosjeanetal.,2009).Consequently,gammaceraneindexvaluesareelevatedbutnotexcep-tionalintheAthelBasinrelativetootherNeoproterozoicformationsfromOman.Assuch,theincreaseingammaceraneindexvaluesfromthe shales to the silicilyte, although consistent with increased restric-tionandpossiblymorereducing/salineconditions,doesnotsignaladrasticchangeinthewatercolumnproperties.
4.2.2 | Biomarkers indicative of their source organisms
Biomarkers are also used to infer the presence and relative abun-dance of organisms present in the original depositional environ-ment(Petersetal.,2005).Wepresentdataforthe2-methylhopaneindex (2-MHI), %C29 sterane values, the sterane/hopane ratio, the24- iso/n-propylcholestane (24-ipc/npc) ratio,andtheC24 mid- chain monomethylalkane/n-alkane(C24MMA/n)ratio(Figure9).
The 2- MHI is a ratio of the abundance of 2α-methylhopane(a C31 hopanoid) to the combined abundances of C30 and 2α-methylhopanes. The biologically produced precursors of 2α- methylhopanes,the2α-methylhopanoids,aresometimesattributedtoacyanobacterialsource(Summons,Jahnke,Hope,&Logan,1999).However,phylogeneticanalysesbyRiccietal. (2014)demonstratethatthegenesrequiredtosynthesize2α-methylhopanoidsarewide-spread within the Alphaproteobacteria. Furthermore, RhizobialesProteobacteria produce both 2α-methylhopanoids and the gam-macerane precursor tetrahymanol (Rashby, Sessions, Summons, &Newman,2007).
Thehighest2-MHIvalues for samples from this studyoccur inthe silicilyte and form a distinctmaximum (Figure9a). Specifically,silicilyte2-MHIvaluesrangefrom10to18%. Incontrast, lower2-MHI values occur in the overlying and underlying shales, ranging between6and10%.Thehighvalues inthesilicilyteareconsistentwithelevatedvalues (15%) found inothersilicilyte rocks (Grosjeanetal., 2009). For comparison, Proterozoic samples often show 2-MHIvaluesof~5–19%(Summonsetal.,1999)andvaluesof2–16%were measured from a range of Neoproterozoic–Cambrian rock
F IGURE 5 Totalion-currentchromatogramsofsaturatedhydrocarbonfractionsfor(a)MMNW-72,737(silicilyte)and(b)MMNW-72,537(ThuleilatShale).n-alkanesaregivenbythetotalnumberofcarbonatomsinthemolecule,n.Forthemid-chainmonomethylalkaneseries,thetotalcarbonnumberisgivenbythe#inMMA-#.NotethatMMAsarenotsinglecompoundsbutaco-elutionofmultiplemethylalkaneswithdifferentmid-chainmethylationpositions.Thesearerepresentativechromatogramsoftheshaleandsilicilytesamples.Pr,pristane;Ph,phytane
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bitumensfromSouthOman(Grosjeanetal.,2009).Thus,the2-MHIratio in the silicilyte is high both locally in Oman and globally for the Neoproterozoic.This indicates a distinct shift in the population ofbacteriageneratinghopanoidsduringsilicilytedepositionrelativetothe shales.
The%C29steranevaluecomparestheabundanceofC29 steranes relative to the total abundance of C27, C28, and C29 steranes. It is used inNeoproterozoic-andCambrian-agedrockstoquantifythecontribu-tionofgreenalgaetothepreservedorganicmatterrelativetoothereukaryotes(Kodner,Summons,Pearson,King,&Knoll,2008;Volkman,Barrett,Dunstan,&Jeffrey,1994).Forthisbiomarkerratio,higherval-ues indicatea largercontributionfromgreenalgae.Values for%C29 rangefrom60to65%intheshalesandfrom65to71%inthesilici-lyte(Figure9b).Forcomparison,valuesfromdifferentNeoproterozoicfaciesfromOmanrangefrom60to85%(Grosjeanetal.,2009),andthus also show a strong C29 sterane predominance.These relation-ships suggest that the contribution of green algae relative to othereukaryoteswashigherduringthedepositionofthesilicilytecomparedto the bounding shales.
Therelativecontributionofeukaryoticvs.bacterialbiomasstothepreserved sedimentaryorganicmatter is generally inferredbyusinga ratio of the total abundance of C27–C29 steranes, sourced from eu-karyotes,tothatofC27–C35hopanes,sourcedfrombacteria(sterane/hopaneratio;Moldowan,Seifert,&Gallegos,1985).Sterane/hopaneratiosrangefrom~0.8to1.4 inAthelsamples,withaminimumob-served in the silicilyte (Figure9c). For comparison, typical sterane/
hopaneratiosfromNeoproterozoicrocksinOmanrangefrom0.2to2.1 (Loveetal.,2009).Thesevalues implythatbacteria increased inabundancerelativetoeukaryotesduringthedepositionofthesilicilytevs. the bounding shales.
Thepresenceandabundanceof24-isopropylcholestane(24-ipc)inpreservedorganiccarbonhavebeenusedpreviously to infer thepresenceofadultdemosponges inancientdepositionalsettings, in-cluding in Neoproterozoic samples (Love & Summons, 2015; Loveetal.,2009;McCaffreyetal.,1994).However,alternativeoriginshavealsobeenhypothesized(Antcliffe,2013;Brocks&Butterfield,2009;Leeetal.,2013),whichwediscussfurtherbelow(seeSection6).Theabundanceof24-isopropylcholestaneisgenerallynormalizedagainst24- n-propylcholestane (24-npc) abundances.This is the 24-ipc/npcratio. 24-npc is used as a biomarker for marine pelagophyte algae(Loveetal.,2009;Moldowanetal.,1990).Valuesforthe24-ipc/npcratiorangefrom0.4to0.8intheshalesand0.6to1.9inthesilicilyte(Figure9d).Importantly,nearlyallsamplesexceedthepreviouslyes-tablished24-ipc/npcratiobaselinevalueof0.5proposedforthepos-itive identificationofdemospongeproductionof24-ipc (Loveetal.,2009).ThisvalueissettobesignificantlyaboveaveragePhanerozoicvalues(0.0–0.3).
A distinct maximum in 24-ipc/npc ratios occurs in the silici-lyte (Figure9d). For comparison, 24-ipc/npc ratios from diverseNeoproterozoic bitumens from South Oman range from 0.5 to 16,with an average value of 1.5 in the Huqf Supergroup (Love etal.,2009).Basedonmeasuredabundances, thehigher24-ipc/npc ratio
F IGURE 6 MRMGC-MSchromatogramsforaselectionofsteranesstudied.Parenttodaughtertransitionsaregivenintheupperleft-handcorner of the chromatograms
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inthesilicilytesamplescouldnotbeattributedsimplytoadropintheabsolute abundance of 24- n-propylcholestane(TableS1).Specifically,theabundanceof24-ipc(ng24-ipc/mgsaturates)is3.5timeshigheron average in the silicilyte than in the Thuleilat shale and U- Shale.
Alesswell-constrained,butdistinctive(andunusual)groupofal-kanebiomarkers found in the silicilyte,Thuleilat shale, andU-Shaleis a series of mid-chain monomethylalkanes (MMAs) with carbonnumbers ranging from C14 through C30. These MMAs are sometimes colloquiallyreferredtoas“x-peaks”andareabundantonlyfromthe
lateNeoproterozoicthroughearlyOrdovician (Bazhenova&Arefiev,1996; Fowler & Douglas, 1987; Grantham, 1988; Grosjean etal.,2009;Kelly,Love,Zumberge,&Summons,2012;Klomp,1986;Peters,Clark,Gupta,McCaffrey,&Lee,1995).Theyareparticularlycommonin Ediacaran–Early Cambrian rocks. Although their origins are un-known,theseMMAshavebeenproposedtobebiologicallysourced(Summons, Powell, & Boreham, 1988).We focus on the C24 MMA homologand report its abundance relative to theC24 n-alkane (C24 MMA/nratio).
F IGURE 7 ThetopthreepanelsareaselectionofMRMGC-MSchromatogramsforhopanes.Parenttodaughtertransitionsaregivenintheupperright-handcornerofeachchromatogram.ThebottompanelisthecombinationofmultipleMRMGC-MSchromatogramsforavarietyofmajorhopanoids:CnHdesignatesahopanoidwithncarbonatoms.Tm, 17α(H)-22,29,30-trisnorhopane;Ts,18α(H)-22,29,30trisnorhopane;g,Gammacerane,BNH,Bisnorhopane;TNH,trisnorhopane;2-Me,2methylhopane;3-Me,3methylhopane
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ThedistinctiveMMAseriesarepresentinallrockbitumensana-lyzedinthisstudy.ThelowestC24 MMA/n ratios occur in the shales, which range in C24 MMA/nvaluesfrom0.2to0.3(Figure9e).Thesilic-ilyte, incontrast,hasexceptionallyelevatedvaluesrangingfrom0.5to1.5(Figure9e).Forcomparison,C24 MMA/n ratios in Precambrian bitumens from Oman range from 0.1 to 1.7, with the highest val-uesconsistentlyoccurring inothersilicilytesamples (Grosjeanetal.,2009).MMAscanmakeupsignificantportions(tensofpercent)ofthealkanesinthesesamples(Figure5a).This impliesthattheorganismsproducingMMAprecursorscontributedasignificantportion(oforder10%)ofbiomasstothepreservedsedimentaryorganicmatter.
UseofMMAs to infer past environmental conditions or the or-ganismspresentinanancientenvironmentischallengingduetotheirpoorly understood origins. Specifically, no living organismhas beenfound that produces the observed series ofMMAs.Most recently,Love, Stalvies, Grosjean, Meredith, and Snape (2008) hypothesizedthat they are derived from bacteria living at the interface between sul-fidicandoxidizingconditionswithinbenthicmicrobialmats.Ifcorrect,the presence of these biomarkerswould require sulfidic conditionsduringMMAincorporationintokerogenandasignificant(10sofper-cent)contributionofbenthicbiomasstothepreservedorganiccarbon.Suchaninterpretation,whilespeculative,isconsistentwiththefind-ingsofHöld, Schouten,Jellema, andSinningheDamsté (1999)whodemonstratedthatMMAsareboundtokerogeninsamplesfromtheHuqfgroupinOmanthroughcarbon–sulfurbonds.Additionally,othermid-chainmethylalkaneserieshavepreviouslybeenassociatedwithmicrobialmat sources (Kenigetal., 1995;Petersetal., 2005;Shiea,Brassell,&Ward,1990).Finally,higherabundancesofmonomethyl-alkanes anddimethylalkanes relative ton-alkanes in Proterozoicvs.Phanerozoicrockbitumenshavebeenusedasevidenceofextensivebenthicbacterialmatproduction inmiddleProterozoicmarineenvi-ronments(Pawlowska,Butterfield,&Brocks,2013).
historiesandspanonlya350mdepthrange,theyshouldcontainor-ganicmatteratsimilarthermalmaturities.Anyapparentdifferencesinmaturity can be used to diagnose differences in bitumen generation and preservationduetodifferingsourcerockchemistry(e.g.,claysvs.quartzlithologies)and/orcontamination.Weusedthreemolecularparametersforthisevaluationbasedonhopaneandsteraneside-chainepimeriza-tion(Figure10)inadditiontotheRock-Evalparameters(Table1).
The Ts/(Tm+Ts) C27 hopane ratio is commonly used for thermalmaturity estimations. Ts is 18α(H)-22,29,30 trisnorhopane andTm is17α(H)-22,29,30-trisnorhopane.BecauseTs is less stable than Tm at el-evatedtemperatures,thermalmaturationcausesthisratiotoincrease(Seifert&Moldowan,1978)uptoamaximumvalueof1.Measuredra-tios range from 0.35 to 0.45. There are no obvious differences in ratios betweenthesilicilyteandshalesorstratigraphicheight.Thesevaluescorrespondtoearlyoil-windowthermalmaturity(Petersetal.,2005).
The relative abundance of different diastereoisomers of ho-panesandsteranes isalsousedtoestimate thermalmaturities.Weemployed side-chain epimerization ratios for C31 αβ hopane 22S/(22S+R) ratios and C29 ααα sterane 20S/(S+R) ratios (Seifert &Moldowan,1980).Inbothcases,theRconfigurationisproducedex-clusively by biology. During thermal maturation, the R configuration converts to the S configuration.Maximumvalues of 0.6 forC31 αβ hopane22S/(22S+R)and0.55forC29 αααsterane20S/(S+R)havebeenestablished(Petersetal.,2005).Thehopaneratiosrangefrom0.54to0.58,andthesteraneratios rangefrom0.47to0.5. Innei-thercasehavetheparametersreachedthemaximumpotentialvalues.They are all consistent with early–middle oil- window thermal maturity (Petersetal.,2005).Nodependenceonstratigraphicheightorrocktypeisobservedsupportingauniformthermalmaturitythroughthestratigraphicintervalstudied.
Rock-Evalparameters (seeSection3.2)alsoreflectorganicmattersources and maturity. Tmax(i.e.,thetemperatureofmaximumgenerationofhydrocarbonsfromkerogenbreakdownduringRock-Evalpyrolysis)rangesforallsamplesaresimilar(~408–432°C;Table1)andindicatethesamplesareofathermalmaturitylowerthanpeakoil-windowthermalmaturity (Peters,1986), inagreementwiththedataofGrosjeanetal.(2009),andthusconsistentwiththeearly–middleoil-windowthermalmaturitiesderivedfromthebiomarkerdata.
F IGURE 8 Biomarkerratiosvs.stratigraphicheightforredox-influencedindices. See Section 4.2.1 for details. (a)Bisnorhopaneandtrisnorhopaneabundances relative to C30hopaneabundances.(b)Gammaceraneabundancesrelative to C30hopaneabundances.(c)Thehomohopaneindex.Thesebiomarkerratios indicate that the entire system was reducinginthesedimentsandpossiblydeeperinthewatercolumnbelowthemixedlayerwiththesilicilyteamorereducing/stratified environment than the boundingshales[Colourfigurecanbeviewedatwileyonlinelibrary.com]
Thekeyobservationfromthebiomarkerratiosandhydrocarbonpro-filesisthatthesilicilyteandboundingshalesdiffermarkedlyintheirorganicbiomarker assemblagesother than for the thermalmaturityparameters. The differences in biomarker profiles between forma-tions are stratigraphically systematic (Figures8 and 9). However,a critical question iswhether these differences result from originaldifferences in the environment and biological communities presentduringdepositionofthesedimentsorareinsteadtheresultofeitherfacies-dependentdiageneticalteration,contaminationfromdrillingorlaboratoryanalysis,orcontaminationfromflowofanorganicphasethrough the units before drilling.
Differential alteration between the silicilyte and shale units is unlikely because the biomarker-based thermalmaturity parameters,whicharealsosensitivetoalteration (e.g.,claycatalysis;Rubinstein,Sieskind,&Albrecht,1975),areidenticalwithinerrorinallsamples.
Contamination from both post-depositional organic migrationand sampling, extraction andprocessing is a serious concern for all
biomarkerstudies.Laboratorycontaminationisunlikelygiventhedis-tinctivenatureofthebiomarkersandthetypicallowproceduralblanksfound for combusted sand. For example, the presence of abundantmid-chainmonomethylalkanes found in the silicilyte, the overall el-evated%C29steranecontents,andthepresenceofelevated24ipc/npc ratios are distinctive traits Neoproterozoic to early Cambrianrocks.Suchbiomarkersandbiomarkerratiovaluesarenotobservedin routine laboratory contaminants or typical drilling fluids (Love &Summons,2015).
Furthermore,theseratiosareconsistentwithpreviousbiomarkermeasurements made on different Athel Basin rocks processed andmeasured in other laboratories. This includes measurements of both bitumenandbiomarkers releasedby the laboratory-basedhydropy-rolysisofkerogen(Table3;Grosjeanetal.,2009;Love,Snape,Carr,&Houghton,1995;Loveetal.,2008).Measurementsofbiomarkersre-leasedfromkerogenarelesssusceptibletocontaminationduringlab-oratorypreparationandanalysisorviaflowoforganicphasesthroughemplacedsections(Loveetal.,2008).Basedonthis,thesimilarityofbiomarkerratiosfrombitumenandkerogeninAthelBasinrockswas
F IGURE 9 Biomarkerratiosvs.stratigraphicheightforbiomarkersrelatedtosourceorganismalinputs.SeeSection4.2.2fordetails.(a)The2methylhopaneindex.(b)PercentabundancesofC29 steranes relative to the sum of C27–29steranes.(c)SumofC27–29 sterane abundances relative to the sum of C27–35hopaneabundances.(d)24-isopropylcholestaneabundancerelativeto24-n-propylcholestaneabundance.(e)Abundance of C24-mid-chainmonomethylalkaneseriesrelativetoC24 n-alkaneabundance[Colourfigurecanbeviewedatwileyonlinelibrary.com]
F IGURE 10 Biomarkerratiosvs.stratigraphicheightformaturityrelatedbiomarkers.SeeSection4.2.3fordetails.(a)Relative abundance of Ts[18α(H)-22,29,30trisnorhopane]relativetothesummedabundance of Ts and Tm[17α(H)-22,29,30-trisnorhopane].(b)Relativeabundanceof C3122Shopanoidsvs.thesummedabundance of 22S and 22R configurations. (c)RelativeabundanceofC29 αααS hopanoidsvs.thesummedabundanceof αααS and αααR configurations. These biomarkerratiosindicatethattheshalesand silicilyte are all the same thermal maturity, within the early to middle oil window[Colourfigurecanbeviewedatwileyonlinelibrary.com]
arguedpreviouslytodemonstratethatthebiomarkersextractedfrombitumenwere original to the sedimentary environment (Love etal.,2009). Measured values of biomarker ratios reported in this studyoverlapthosepresentedinLoveetal.(2009)forbiomarkersmeasuredfrombitumen and released through hydropyrolysis from kerogen intheThuleilatShale,Silicilyte,andU-Shale(Table3)—this isdiscussedmoreextensivelybelow.Weadditionallyscreenedsamplesforsuspi-ciousmolecularfeaturesindicativeofgeologicallyyoungerinputs(e.g.,biomarkersrelatedtoangiosperms,ofwhichtherearenone).Assuch,weconcludethebiomarkersarenotcontaminantsderivedduringdrill-ing or laboratory analysis.
An alternative means for contamination is the migration of organic phases(e.g.,oil)throughtheformationsstudied.Biomarkersfromsuchanexogenoussourcecouldbeleftbehindintheformations.Multiplelinesofevidenceindicatethatsuchaprocessisunlikelytohaveinflu-enced our measured ratios:
1. Allsectionsareencasedinimpermeablesalt(Amthoretal.,2005),and not in contact with other units. This salt barrier preventsmigration of exogenous oils at depth after the salts isolatedthe units.
2. Thepermeabilityofthesilicilyteislowandrequiresextensivehy-draulic fracturing to induce sufficient permeability for oil extrac-tion.Suchlowpermeabilitymakesmigrationofotheroilsintotheunitdifficultpost-lithification.
3. Introductionofexogenousbiomarkersfromoilmigrationthroughtheunitswouldbeexpectedtosmoothoutanyoriginallypresentgradients in biomarker ratios between units or as a function ofdepth. However, as discussed, distinctive, coherent trends andstrongdifferencesinbiomarkerratiosareseennotonlybetweenthesilicilyteandboundingshales,butalsowithinthespecificfor-mationsthemselves(e.g.,Figures8and9).Furthermore,ourmeas-ured biomarker ratio values are consistentwith thosemeasuredpreviously on samples from these formations (Table3; Grosjeanetal.,2009;Loveetal.,2008;Loveetal.,2009).
4. Biomarkerratiosintheunitsaredistinctiveforthetimeperiod(thelateNeoproterozoic),includingthepresenceofthediscussedseriesofmid-chainmethylalkanes,high%C29 sterane ratios, and high 24-ipc/npcratios(Grantham,1988;Grosjeanetal.,2009;Kellyetal.,2012;Love&Summons,2015;Loveetal.,2009;McCaffreyetal.,1994;Petersetal.,1995,2005).Phanerozoicoilswouldbeunlikelytointroducebiomarkerswiththesecharacteristics.
5. Biomarkerratiosderivedfromthekerogen-boundbiomarkerpoolthroughhydropyrolysisofkerogenfromotherAthelshaleandsilici-lytesamples—asdiscussedabove,amethodthatislesssusceptibletocontaminationbymigratedhydrocarbons—showsimilardiffer-ences between the shales and silicilyte as do the measurements of bitumens (Table3).Specifically, the rangeofvaluesweobservedareingeneralagreementwithpreviouslymeasuredvaluesinboththe solvent-extractable and kerogen phases of these formationspenetratedbyotherwells(Table3).ThisincludestheMMAs,%C29 steranes,steranes–hopaneratios,gammacerane/C30H ratio, 2-me-thylhopaneindex,andC30 ipc/npcratio(Table3;Grosjeanetal.,
2009; Love etal., 2008; Love etal., 2009). This overlap stronglysupportsthepresenceofthesebiomarkers(ortheirprecursors)inthe original sediments.
6. Finally, saturateyields are typically1–6mg/g rock in all samples(Table2).Theseyieldsareconsistentwiththerelativelylowther-mal maturity and moderate to high organic matter content of our samplesandareordersofmagnitudehigherthanArcheansampleswhere contamination is of key concern (Brocks, Logan, Buick,&Summons,1999).Contaminationthatinfluencesthemeasuredra-tios is more difficult in such organic-rich, low thermal maturity rocks.
Consequently, although contamination can never be completelyruledout,thereisnoevidencetoindicatethepresenceofcontaminationandsignificantevidencethroughtheseself-consistencychecksinsup-portof themeasuredbiomarkersbeing indigenous.Thus,weproceedbelowfollowingthisinterpretation.
4.2.5 | Implications of the biomarker results
Allbiomarkerparametersrelatedtosourceinputsandthegeochemi-cal conditionsof thedepositional environment exhibit similar, first-ordertrends:MeasuredratiosorparametersbegintochangeatthetopoftheThuleilatShale,reachmaximumorminimumvaluesinthemiddle interval of the silicilyte, and then decrease or increase in the silicilyteup to theboundarywith theU-Shale.Basedon theestab-lishedframeworkforinterpretationsofbiomarkerratios,theseresultssuggestthatthedepositionalenvironmentofthesilicilytewasmorereducinginthesedimentaryporewatersand/orinthewatercolumnthan in the shales and had a change in the organisms contributing to thepreservedorganicmatterrelativetotheshales.
Aconcentratedeffortto identifybiomarkersforphoticzoneeu-xinia in the aromatic hydrocarbon fraction (e.g., isorenieratane orotheraryl isoprenoids)wasunsuccessful.ThishasbeenthecaseforotherNeoproterozoic–CambrianrocksinSouthOman(Grosjeanetal.,2009)andsuggeststhatawatercolumnchemocline, ifpresent,wasbelowthephoticzone. Importantly,anoxicconditionsdeeper inthewatercolumndonotprecludethepresenceofcyanobacteriaorgreenalgae(asindicatedbythebiomarkermeasurements)inashallowerandoxicphoticzone.
4.3 | Iron speciation
To further characterize the redox state of the depositional envi-ronment of the silicilyte and bounding shales, we also measured chemical speciation parameters for iron minerals in samples fromMMNW-7. The distribution of iron in different mineral phases(Poulton&Canfield,2005)issensitivetodepositionalanddiageneticredoxconditionsandfullyindependentofthelipidbiomarkerproxiesusedabove.Themethodisbasedontheobservationthat“reactive”ironisburiedandpreservedaseitherironoxideslikehematiteandmagnetite;ironcarbonateminerals;or,ifsulfideispresent,aspyrite(Canfield,Raiswell,&Bottrell,1992).Systemswithhighpyrite iron
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relativetototalreactiveironareironlimitedforpyritegeneration.Insuchcases,freesulfidecanaccumulateintheporefluidsandeventhewatercolumn(Poulton&Canfield,2005;Raiswell,Buckley,Berner,&Anderson,1988;Raiswell&Canfield,1998).FePy/FeHRvalues>0.7–0.8aregenerallyassumedtoindicatepermanentlysulfidicconditionsintheporewatersorthewatercolumn(Poulton&Canfield,2011;Reinhard,Raiswell,Scott,Anbar,&Lyons,2009).Additionally,FeHR/FeTandFeT/Alweightpercentratiosarecommonlyelevatedwhenthewatercolumnisanoxicrelativetosedimentsreceivingthesameterrigenousmaterialunderoxicconditions.Therefore,FePy/FeHR val-ues>0.7combinedwithFeHR/FeT values elevated relative to source sedimentarymaterialareoftentakenasafingerprintforwatercol-umneuxinia.Incontrast,lowFePy/FeHR(<0.7)butelevatedFeHR/FeT ratios (relative to the source terrigenousmaterial) are assumed toindicateanoxicwatercolumnsdominatedbydissolvedFe(II) ratherthan sulfide.
Pyritemakesupbetween22and62%ofthehighlyreactiveironpoolinthesilicilyteandshalesamples(Table4;Figure11),consistentwith substantial amounts of bacterial sulfate reduction. There is no obviousrelationshipbetweenfaciesorstratigraphicheightandFePy/FeHRvalues.Themaximumvalue fromtheshaleunits is0.62,while
Weaddressedthepossibilityofoxidationbycomparingtheout-side,exposedportionofadrillcorefromthesilicilyte(asopposedtocuttings)toafreshlyexposed,unoxidizedsurface0.5–1cmintothecore.FePy/FeHRvaluesare0.60forthefreshinteriorsampleand0.57fortheexteriorsample.Thus,thetwomeasurementsaresimilar,whichindicatesthatoxidationduringstorage,atleastinthecores,hasneg-ligibly alteredFePy/FeHR values. However, the lower values in other samples (as low as 0.22)may be the result of later oxidation giventhatcuttingshavehighersurfacearea-to-volumeratioscomparedtocore.Regardless,~0.6isthemaximumobservedFePy/FeHR value for boththeshaleandsilicilytesamples.FePy/FeHR ratios of 0.6 fall below the traditional0.7–0.8 thresholds routinelyused to indicateperma-nentlysulfidicdepositionalconditionsinthewatercolumn(Poulton&Canfield,2011;Reinhardetal.,2009).
ThemeasuredFeHR/FeT (0.6–1.0)andFeT/Al (0.5–1.1)ratiosareelevated relative to average Phanerozoic values for marine shales
This interpretation of the iron speciation data, that is, that thewater columnwasanoxic andcontained significant amountsofdis-solvedFe(II) is in disagreementwith previous interpretations of thechemical conditions in the Athel Basin water column. Specifically,multipleother inorganicproxieshavebeenusedpreviouslytoarguethatsulfidicconditionswerepresentinthewatercolumnoftheAthelBasinincluding:molybdenumisotopes(Willeetal.,2008),tracemetalabundances(AlRajaibietal.,2015;Schröder&Grotzinger,2007),sul-furisotopes(Schröder,Schreiber,Amthor,&Matter,2004),andpyriteframboidsizedistributions(AlRajaibietal.,2015).Additionally,redox-sensitivebiomarkerratiossuchasthebisnorhopaneandhomohopaneindices discussed above are generally thought to become elevated only under sulfidic conditions.
This disagreement may result from the comparison of FeHR/FeT and Fe/Al ratios from the Athel Basin to values for averagePhanerozoicmarineshales.Specifically,knowledgeoftheinitialFeHR/FeTandFe/Alvaluesforthesourcedetritalmaterialsisnecessarytoevaluate whether the values from the Athel Basin are actually elevated
relative to the source materials. The Athel Basin was not an openmarinesystem,butratherwasasemi-enclosedbasinwithunknownand potentially locally controlled FeHR/FeT and Fe/Alvalues for thesourcesiliciclastics.Thus,thehighFeHR/FeTandFe/Alvaluesrelativetomarine shalescould simplybedue to sourcematerials thatwereelevatedrelativetotypicaldetrital inputstomarinesystems.SuchacaseoccurstodayintheMediterranean(Lyons&Severmann,2006).Consequently,givenuncertaintiessurroundingthebackgroundFeHR/FeTandFeT/Alratiosforasemi-enclosedbasinliketheAthelBasin,theironspeciationdataonlyrequire,basedonthe~0.6maximumFePy/FeHRvalues,significantproductionofsulfideinsedimentsandpoten-tially intermittent sulfide accumulation in the water column. Such a conclusion isalsoconsistentwith thebiomarkerdataand the inter-pretationofotherinorganicredox-sensitiveproxiesdiscussedabove.This is not to say that low O2conditionsdidnotexistbelowthemixedlayerintheAthelWaterColumn.Rather,thedataindicatethatanoxic,sulfidic conditionswere not a permanent condition at depth in thewatercolumn.Wereturntothegeochemicalconditionsofthewatercolumn in Section 5.2
4.4 | Stable isotopic composition of silica in the silicilyte
Thestableisotopiccompositionofsilica(δ18O and δ30Si)canprovideinsightsintosilicaformationalanddiageneticprocesses(Blatt,1987;Douthitt,1982).Wepresentdatafromsilicilytequartzforbothiso-topicsystems.
4.4.1 | Silicon isotope ratios as a test for the spongal origin of the silicilyte
Thehighabundanceof24-isopropylcholestanecanbeinterpretedtoindicatedemospongeslivedintheAthelBasin.Assomeextantdem-ospongessecreteopalspicules,spongesareapossiblesourceofthesilicilyte silica. Sponges exhibit some of the largest silicon isotopicfractionationsknown,precipitatingopalupto5‰lowerinδ30Si than aqueous silica (Hendry & Robinson, 2012). Although the seawaterδ30Si value at the timeof theEdiacaran–Cambrianboundary is notknown, it has been estimated to be 1.3‰ (Ramseyer etal., 2013).Thus, ifspongesplayedakeyrole inprecipitatingthesilicapresentin the silicilyte, the δ30Sivalueofquartz inthesilicilyte ispredictedto be negative.
Wetested thispredictionbymeasuringδ30Si values from three different samples from the MMNW-7 core (depths of 2,687–2,690m;Table5).δ30Sivaluesrangefrom~+0.6to+1.5‰andaresimilartotherangeof0.4–1.2‰measuredbyRamseyeretal.(2013)onsilicilyterocksfromotherwells.Thesevaluesdonothavenegativeδ30Sivaluesaspredictedabove.Additionally,theyarenotdistinctintheir δ30Si values from other cherts formed during this time frame (0.5±1.5‰;Robert&Chaussidon,2006).Consequently,andinac-cordancewith the conclusionofRamseyer etal. (2013), the siliconisotopicvaluesareinconsistentwithaspongaloriginforthesilicainthe silicilyte.
F IGURE 11 Ironspeciationmeasurements,specificallytheratioofpyrite(FePy)vs.highlyreactiveiron(FeHR),whichincludestheironpoolthatcanreactwithdissolvedsulfide(ordidreactinthecaseofpyrite)duringsedimentarydiagenesis.Ratios>0.7–0.8aregenerallyconsideredtorepresentsulfidicenvironments.Asallsamplesarebelowthesethresholdvalues,weinterpretthistoindicatethatthesilicilytedidnotforminapersistentlysulfidic(euxinic)watercolumn[Colourfigurecanbeviewedatwileyonlinelibrary.com]
4.4.2 | Oxygen isotope ratios and diagenesis in the silicilyte
Oxygenisotoperatiosreflectthesourceandenvironmentalconditions(e.g.,temperature)ofsilicaprecipitationorlatermodificationsuchasconversionfromopaltoquartz(Blatt,1987;Knauth&Epstein,1976).Wemeasuredmatrix(i.e.,non-fracturefilling)quartzδ18O(δ18Oqz)val-uesviaSIMSanalyses (seeSection3.5)onsamples fromMMNW-7coreatdepthsof2,686.7(n = 55),2,687.8(n = 21),and2,690(n = 57)meters, where ndenotesthenumberofspotanalysesforeachsample.MeasurementsarereportedfollowingMarin-Carbonne,Chaussidon,andRobert(2010)bytreatingeachspotanalysisasaGaussiandistri-bution defined by the mean and 1σ uncertainty of the measurement. The δ18Ofrequencydistributionforagivensampleisfoundbysum-mingthedistributionsforallspotanalysesandthennormalizingtothetotalnumberofspotsmeasured.
The δ18Ovalues canbeused to calculate equilibrium formationtemperatures of the quartz in water of fixed isotopic composition.Using the quartz–water isotope fractionation-factor calibration ofKnauthandEpstein(1976)andaδ18Owaterof0‰,the10–12‰rangein δ18O indicates quartz formation over a ~70°C range, with mostquartzgeneratedbetween48and55°C.Althoughtheoxygenisotopiccompositionofseawater isnotwellconstrainedforthistimeperiod(thusaffectingtheabsolutetemperaturecalculation),the~70°Crangeisindependentofthisassumptionandinconsistentwiththeinitialfor-mationorsubsequent isotopic re-equilibrationof thequartzoropalprecursorinseawaterorshallowsedimentswheretemperaturerangesof ~70°C are not found. Instead, this calculation,which assumes aconstant δ18O value for the formational fluid, indicates formation or isotopic re-equilibration of silicilyte quartz over depths of multiplekilometers,whichwouldallowsamplestoformovera large (>50°C)temperaturerange.
Alternatively, ranges in δ18Oqz can be caused by variable forma-tion/pore-waterisotopiccompositions.Forexample,ifdiagenesisoc-cursathighertemperaturesthansilicaprecipitation,theprogressiverecrystallizationofopaltochertcancauseporewaterstoevolvetohigher isotopic compositions (Marin-Carbonne etal., 2010). Quartzgenerated from this evolving pore water will necessarily exhibit arange of isotopic compositions. For example, at the 20μm scale Marin-Carbonneetal. (2010)observed5–7‰rangesinδ18O in dif-ferentchertsfromtheGunflintFormation(1,800Ma).Theseauthorsexplainedthisrangeastheresultofchangesintheisotopiccomposi-tionofporewatersatburialdepths>2km(assuminga25°Cgeotherm)
F IGURE 12 δ18Odata(referencedtoVSMOW)measuredviasecondaryionmassspectrometryfromthreeseparatesilicilytecoresamplesfromMMNW-7.Measurementsaretreatedasfrequencydistributionstofullyrepresenttheerrorofthemeasurements.Individualsampleshaveawidthof~10‰withatotalrangeforallsamplesof~12‰.Thisdistributionindicateseitheralarge(~70°C)rangeinformationtemperature,orarangeintheisotopiccompositionoftheporewaters[Colourfigurecanbeviewedatwileyonlinelibrary.com]
duetoprogressivealterationofopaltoquartzoverthisdepthinterval.Consequently,largerangesofδ18Oqz values can also be attributed to evolvingporewatersbutstillrequireconversionfromopaltoquartzovermultiplekilometersinthesedimentarycolumn.Wenotethatsil-iconisotopeswillnotundergosuchisotopicchanges.Thisisbecausesilicon dominantly occurs in silicaminerals as opposed dissolved inwater in silica- rich sediments. In other words, silicon has a lower water rockratiorelativetooxygeninsilica-richsediments.
Regardless of mechanism, the range in δ18Oqz values indicates that thequartzinthesilicilyteformed(oratleastrecrystallizedfromapre-cursor)inanevolvingenvironmentalsystemovermultiplekilometersofdepthinthesediments.Thequartzisthusunlikelytobeprimary,butinsteadwaslikelyopalthatrecrystallizedtoquartzduringdiagenesis.Thishypothesispotentiallyexplainsfeaturesseeninthethinsections:Forexample,theblockier,poorlyexpressedlaminaeofsomesamples(Figure3eandf)mayresultfromdeeper, laterconversionofopaltoquartzaftercompactionanddewateringhavealreadydisruptedorigi-nallaminae.Thus,thesefeaturesneednotbeexplainedinamodelforthe initial formation of the laminae in the silicilyte.
5 | MECHANISMS FOR SILICA
PRECIPI TATION AND A PALEOCEANOGRAPHIC MODEL FOR THE ATHEL BASIN
Here,wepresentaconceptualmodelfortheformationofthesilicilytethat is consistent with the observations and measurements discussed above.Wefirstexaminethepossiblephysicalandchemicalconditionsrequiredforsilicaprecipitation.
5.1 | Potential silica precipitation mechanisms
Understanding the source of silicilyte silica is critical for any model ofitsformation.Silica(e.g.,asopal)canprecipitateduetobiologicalsecretions(e.g.,spongespiculesanddiatomfrustules)orduetosilicasupersaturationinducedbyevaporation,bypH,density,ortempera-turechanges,orthroughsorptionandprecipitationon,forexample,organicmatter(i.e.,templating).Asdiscussedabove,boththepositiveδ30Sivaluesandlackofanyobservedfossilspiculesareinconsistentwithaspongaloriginforthesilicilyte.Ahydrothermalsourceforthesilica was ruled out based on a rare earth element study of the silicilyte (Schröder&Grotzinger,2007).
Acriticalobservationisthatshallow-watertime-equivalentAracar-bonateslackdiageneticcherts(Grotzinger&Rawahi,2014;Ramseyeretal.,2013;Schröderetal.,2003,2005).IntheMeso-Neoproterozoic,themajorfluxofsilicaoutoftheoceansisthoughttohaveoccurreddue to the evaporative supersaturation of opal in shallow waters(Malivaetal.,2005).Thus,itisintriguingthattheco-evalAracarbon-ates,whichrepresentatransientlyevaporiticenvironment,lacksilica.Toexplainthis,weproposetheAthelBasinactedasafilter,removingsilicafromincomingwatersbeforetheypassedacrosstheshallowercarbonateplatforms.ThisrelationshiprequiressilicatonotonlyhaveprecipitatedintheAthelBasinbutalsotohavebeendrawndownto
sufficientlylowlevelsthatevaporationonthecarbonaterampfailedtoachievesufficientsilicaoversaturationforopalprecipitation.This,forexample,makesevaporation,andlargepH,temperature,orsalinitychangesunlikelymechanismsforsilicaremovalintheAthelBasin.ThisisbecauseneitherprocesscanlowerH4SiO4 concentrations below the solubilitylimitofopalandthuspreventsilicaprecipitationontheevap-orative carbonate shelf.
Anothermechanismforsilicaprecipitationisthroughnucleationonorganicmatter(i.e.,templating).Ithasbeenhypothesizedthatmicro-bialcells,throughsilicasorption,cancauselocalsilicaoversaturationandprecipitationofquartzprecursors innominally silicaundersatu-ratedwaters(Fein,Scott,&Rivera,2002).Sorptiononlivingcellsdoesnotappeartoenhancesilicaprecipitationoverabioticrates(Feinetal.,2002;Konhauser,Jones,Phoenix,Ferris,&Renaut,2004;Mountain,Benning, & Boerema, 2003; Yee, Phoenix, Konhauser, Benning, &Ferris,2003).Incontrast,decayingorganicmatterdoesappeartoen-hance silica precipitation relative to abiotic systemsvia interactionsbetweenexposedorganicfunctionalgroupsanddissolvedSiO4(Ferris,Fyfe,&Beveridge,1988;Knoll,1985;Knoll&Golubic,1979;Leo&Barghoorn,1976).Thus,silicaprecipitationonorganicmatterdebrisinthewatercolumncouldactasasedimentarysinkforsilica.Thispro-cesswouldrequirethatenoughsilicasorbedontoorganicmattertopreventsilicaformationonthecarbonateshelvesduringevaporation.This scavenging could occur on floating, decaying microbial mats, as envisionedbyAlRajaibietal.(2015)andAmthoretal.(2005),onben-thicmatsubstrates,orsimplyondecayingparticulateorganicmattersinkingthroughthewatercolumn.
Nucleation on intact mats floating in the water column was favored byAlRajaibietal.(2015)andAmthoretal.(2005)becauseitprovidesareadyexplanationforthepresenceoflaminaeinthesilicilyte;eachlaminationwould represent the remnantofamicrobialmat that felltotheseafloorcoveredinsilica.Itisimportanttonote,asdiscussedabove(Section4.1),thatthepresenceoflaminaeneitherrequiresnordemonstrates thepresenceofmicrobialmats.Before the adventofsignificantbioturbationintheCambrian,laminatedrocksderivedfromchemicalsedimentswereubiquitous(Grotzinger&James,2000).Suchlaminated rocks can form throughpurely abioticmeansvia the set-tling of particles from thewater column (Grotzinger&Knoll, 1999;Grotzinger&Rawahi,2014;Grotzinger&Rothman,1996).Althoughthepresenceofmicrobialmatsisprobableinsuchenvironments,theirimportanceincreating laminaerequiresthe identificationofspecifictextures such as trapping and binding textures, crinkly laminae, oreven low- relief stromatolites. None of these are found in the silicilyte. Thus,althoughmicrobialmatsmayberesponsibleforthelaminaeasarguedbyAlRajaibietal.(2015)andAmthoretal.(2005),thereisnodirect evidence for this mechanism, nor is it necessary.
5.2 | Coupled chemical and physical model for the formation of the Athel silicilyte
Wepropose amodel for theAthelBasin inwhich biologically pro-ductive surfacewaters generated organicmatter that, via templat-ing, caused silica nucleation (Figure13). These silica-coatedorganic
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particleswouldhavebeensufficientlydensetosettletotheseafloorand thus create the silica- enriched sediment that later became the sil-icilyte.Asthesilicilyteisenrichedinsilicacomparedtoorganicmatter(~85:1byweight),themodelrequiresthatmostoftheoriginalorganicmatterwasremineralizedatdeptheitherinthesedimentsorbottomwaters.Thisremineralizationwouldreleasenutrients(e.g.,phosphate)backtothewatercolumn,which,whenmixedbackwithsurfacewa-ters,wouldallowforcontinuedprimaryproductioninthephoticzone.
This model is consistent with many of the observations made from thepetrographic,organic,andinorganicmeasurements:Tofirstorder,the model ensures silica delivery to the Athel Basin sediments and would lower silica levels in the water column such that silica could notprecipitateontheadjacentcarbonateshelf.Additionally,becauseprimaryproductionrates,whichcontrolsilicafluxestothesediment,commonlyvaryseasonally,silicafluxeswouldbeexpectedtovarytoocreating a mechanism to generate laminae.
The organic matter delivered to the sediment/water interface wouldstimulaterespirationremovingoxygen(ifpresent)orconvertingsulfate to sulfide. The loss of O2 and increased reduction of sulfate to sulfideiscapturedbytheincreaseinbiomarkersrelatedtoreducingconditionsinsilicilytebiomarkersandelevatedFePy/FeHR in the sed-iments. Importantly,wenote that the organic biomarker ratios thataresensitivetoredox/stratificationconditionsdo not differentiate be-tweensedimentaryandbottomwaterconditions.Thus,thebiomark-ers only necessitate increasingly reduced conditions in the sediments.
Mixing of deep, nutrient-richwaterswith surfacewaters in theAthelBasinwouldalsohaveinfluencedthephotosyntheticcommuni-tiespresent.Differencesinthephotosyntheticcommunitiesarecon-sistentwith thechanges in the2-MHI,%C29biomarkerparameters,andsterane:hopaneratiosinthesilicilytevs.theshales.Theincreasein2-MHIand%C29valuesinsilicilyterelativetotheshalesamplescanbeinterpretedtoindicateincreasesintherelativeimportanceofcy-anobacteriatoallbacteria(althoughalternativeexplanationsarealsopossibleasdiscussedabove).Thehigher%C29 values in the silicilyte relativetotheshalesamplesindicatesanincreaseinthecontributionofgreenalgaetopreservedalgalbiomass.Finally,thelowersterane/hopaneratiointhesilicilytevs.theboundingshalessuggestsasubtleincreaseinbacteriarelativetoeukaryoticbiomassinwatercolumn.
Ourproposedmodeloftheenvironmentalconditionsduringsilici-lytedepositioninwhichdeeper,O2-depleted,andnutrient-richwatersmixedwithsurfacewaterscanexplainthesebiomarkershifts.Inmodernsystems,upwelledlowO2watersoftenhaveexperiencedlossofnitratevia denitrification either in the water column or sediments. As a result, organisms that can fixnitrogenor are adapted tousingammonia astheir nitrogen source instead of nitrate have an advantage in such envi-ronments.Indeed,inthemodernocean,upwelleddenitrifiedwatersareareasofintensenitrogenfixation(Deutsch,Sarmiento,Sigman,Gruber,&Dunne,2007).Ascyanobacteriaarethemajornitrogenfixersintheoceanstoday(Zehr,2011),theincreaseinthe2-methylhopaneindex(possiblyindicatingincreasedcyanobacterialcontributionstobacterial
F IGURE 13 SchematicrepresentationforsilicaandorganicmatterformationintheAthelBasin.Weproposeamodelwherewaterflowsintothebasinandmixesverticallyabovethesiteofsilicadeposition.Silicanucleatesviatemplationonnewlyformedorganicmatterandthensinkstotheseafloor,formingthesilicilyte.Theorganicmatteristhenrespiredatdepth(leavingbehindasilica-enrichedresidue)andthenutrientsreturnedtothesurfaceviamixing,fuelingfurtherproductionandsilicanucleation.SilicaprecipitationonorganicmatterwouldremovesilicafromthesystembeforeshoalingontheAraplatformpreventingsilicaoversaturationduringevaporationandcarbonategrowth[Colourfigurecanbeviewedatwileyonlinelibrary.com]
biomass)andthelowersterane/hopaneratiosduringsilicilytedeposi-tionisconsistentwithourmodel.Finally,greenalgaehaveastrongerpreferenceforammoniaovernitrateascomparedtootheralgae.Thus,greenalgaecouldfarebetterinnitratelimitedenvironments(Litchman,Klausmeier,Schofield,&Falkowski,2007).This isconsistentwiththeincreased%C29 sterane values in the silicilyte vs. the bounding shales.
Finally,theMMAsindicatethatanunknownorganism,orgroupoforganismscontributedsignificantly(~10sofpercent)tothetotalmassof preserved organic matter during the deposition of the silicilyte.Asdiscussed above (Section4.2.2), no knownorganismshavebeenfoundthatproducetheprecursorsof theMMAsfound in thesilici-lyte.However,Loveetal. (2008)hypothesizedthatsulfide-oxidizingorganisms living at the chemocline in microbial mats may have been the source of these alkanes. If correct, such organisms could haveoccupiedthesediment–waterinterfaceandmayhavethrivedduringsilicilyte deposition due to lower siliciclastic depositional rates (seebelow)—generallyhighsiliciclasticinputspreventtheestablishmentofmicrobialmats.AsdiscussedinLeeetal.(2013),suchelevatedMMAvalues could indicate substantial contributions of benthic biomass to thepreservedorganicmatter.
Aquestion iswhy the shales,whichalsohave similarbiomarkerratiosandironspeciationvalues,didnotalsodevelopintosilicilytes.An explanation is that during shale deposition, terrigenous inputsoverwhelmedthelower,backgroundsilicafluxes.Thesilicilytewouldthus represent a sediment-starved, condensed geological section.Furthermore, sorptionoforganic carbononto thehigh surface-areaclayminerals(Hedges&Keil,1995;Mayer,1994)couldhaveremovedorganic carbon from the water column, thus lowering the concentra-tionsofsubstratespromotingsilicanucleation.Sorptionandprotec-tionoforganicmatteronclayminerals(Hedges&Keil,1995;Mayer,1994)couldalsoexplaintherelativeenrichmentoforganiccarboninthe shales vs. the silicilyte.
Importantly,thismodel implicitlyrequiresthatsilicilyte-likeenvi-ronmentsdidnotonlyexistduring theA4cycle (Figure2), but alsoexistedthroughouttheentiredepositionoftheAracarbonates.Thisisbecause no Ara carbonates contain significant concentrations of auth-igenicquartz.Thus,thepresenceofolderoryoungersilicilytesduringAratime,orinsystemsoutsideoftheAthelBasinwouldprovidead-ditional evidence for this model. Preliminary data from two silica- rich deposits(66and77%byweightSiO2)inOmanfromawellcorrelatedto theA3 carbonatemay indicate other silicilyte-like environments.ThesesamplescontainMMAs,whicharefoundinabundanceinthesilicilyte.However,wenotethatMMAsarenotuniquetothesilicilyteand are found elsewhere in high abundances in the South Oman Salt Basin(Leeetal.,2013).Additionalinterrogationofthiscoreisneces-sary,butopensthepossibilityforothersilicilytesinOman.
6 | 24- IPC/NPC RATIOS AND ATHEL BASIN WATER COLUMN CHEMISTRY
The biomarker ratios measured that are sensitive to redox condi-tions(thebisnorhopane,homohopane,andgammaceraneindices)all
indicatethatanoxic/stratifiedconditionsbecamemoreintenseinthesedimentsandpotentiallyatthesediment–water interfaceor inthewatercolumnduringsilicilytedepositionascomparedtoshaledeposi-tion.Theironspeciationdataareconsistentwithsulfideaccumulationin thesedimentsandpotentially intermittently in thewatercolumnduringbothshaleandsilicilytedeposition.Concurrenttothechangein the redox-sensitive biomarker ratios, 24-ipc/npc ratios rose dur-ingsilicilytedeposition relative to theshales. Interestingly, theonlyknown organisms that synthesize significant quantities of 24-ipcprecursorsareadultdemosponges (seeLove&Summons,2015foracompeltediscussion),whicharebenthic,obligateaerobes.At firstblush,increasinglyreducingconditionsatdepthwouldappearincon-sistentwithanenvironmentthatfavorsobligatelyaerobiceukaryotes.Weexploretwoexplanationsfortheconcurrentchangesinthebio-markerandironspeciationratios.
Inthefirstexplanation,weallowthatthe24-ipcprecursorswereproducedbyadultspongeslivingatthesediment–waterinterface.Thishypothesishasstrongmeritasadultspongesaretheonlyknownor-ganisms that synthesize sufficient quantities of 24-ipc precursors tocreateelevated(>0.5)24-ipc/npcratios(Love&Summons,2015;Loveetal., 2009). Furthermore, algae thatmake trace amounts of 24-ipcprecursors today appearnot tohave acquired this capacityuntil thePhanerozoic(Gold,Grabenstatter,etal.,2016).Thisexplanationaddi-tionallyfollowstheinterpretativeframeworksputforwardbypreviousstudies (e.g., Gold, Grabenstatter, etal., 2016; Grosjean etal., 2009;Love & Summons, 2015; Love etal., 2009; McCaffrey etal., 1994).Importantly, thisscenariorequiresthatspongesproliferated inan in-creasingly O2-deficient environment during silicilyte deposition. Wenote,though,thatepisodicmixingofdeepandshallowwaters,asweproposed occurred during silicilyte formation (see Section5), couldhaveallowedfortimesofincreasedoxygenationatthesediment–waterinterface.Additionally,itisimportanttostressthattheredox-sensitivebiomarkersandironspeciationdataonlyrequireshiftsinsedimentaryconditions.Thus,thepresenceoflow(andperhapsepisodic)butfiniteconcentrations of O2 indeepwatersof theAthelbasin is consistentwiththeseothermeasuresoftheredoxconditionsintheAthelBasin.
The redox-sensitive biomarkers do indicate that O2 levels were likelyloweratthesediment–waterinterface(givenanincreaseinre-ducingconditions in thesediment).Thus,akeyquestion iswhethersponges living at low dissolved O2 concentrations is reasonable to propose? Theoretical and experimental studies show that spongescansurviveataround~1%ofpresentatmosphericoxygenlevels(Millsetal.,2014;Sperling,Halverson,Knoll,Macdonald,&Johnston,2013).Additionally, sponge interiors can become anoxic (Hoffmann etal.,2005),spongesharborobligateanaerobicsymbionts(Hoffmannetal.,2005;Webster,Wilson,Blackall,&Hill,2001),andspongescansur-viveepisodesofanoxia(Bell&Barnes,2000;Millsetal.,2014).Thus,thepresenceofspongebiomarkersinasystempoisedattheboundarybetweenanoxicandoxicconditionsisperhapsnotactuallysurprising.
An alternative explanation for the increase in 24-ipc/npc ratiosduringsilicilytedepositionalongwiththeapparentincreaseinreduc-ingconditionsatdepthisthat24-ipcprecursorswerenotgeneratedby adult demosponges. For example, in addition to adult sponges,
Alternativesourcesforthe24-ipcprecursorsinNeoproterozoic–CambrianOmanrocksthathavebeenconsideredincludepelagicspon-galancestors(e.g.,stem-groupspongesorstem-groupanimals;Brocks&Butterfield, 2009; Lee etal., 2013). Such scenarios are appealingastheywouldallowthebiomarkerandironspeciationdatatobein-terpretedinastraightforwardmanner;thatis,thatthewatercolumnwasgenerallyanoxicbelowthemixedlayeraspreviousstudieshaveargued (AlRajaibi etal., 2015;Amthor etal., 2005;Ramseyer etal.,2013;Schröder&Grotzinger,2007;Willeetal.,2008).Additionally,if such organisms did not generate spicules, this could explain thelackoffossilspiculesinNeoproterozoicOmanrocksdespiteelevated(>0.5)24-ipc/npcratios—itshouldbenoted,though,thatnotalladultdemosponges produce spicules. Such a scenario is independentlysupportedby thehypothesis that stem-groupspongeswerepelagic(Nielsen,2008).Althoughgenetic analysesof theenzymes involvedin the synthesis of 24-ipc precursors in sponges is consistentwithspongesbeingthesourceofthesebiomarkersintheNeoproterozoic(Gold,Grabenstatter, etal., 2016), suchanalysesarealsoconsistentwith24-ipcbeing generatedby stem-group spongesor stem-groupanimals (Gold, O’reilly, Luo, Briggs, & Summons, 2016). Finally, wenotethatelevatedabundancesof24-ipcand24-ipc/npcratios>0.5arefoundinnumerousNeoproterozoic,Cambrian,andOrdovicianoilsand bitumens (McCaffrey etal., 1994; Peters etal., 2005). If stem-group sponges or animalswere responsible for theNeoproterozoic24-ipcbiomarkers,thentheyeitherwouldhavehadtohavepersisted~100millionyearsintothePhanerozoicorbeenreplacedbyspongesasthesourceofthe24-ipcinthePhanerozoic.
Regardlessofwhether the24-ipcprecursorsweregeneratedbycrown-group, adult, benthic sponges or ancestor to sponges (andthereforeanimals),thesilicilyteappearstohavebeenasiteofprefer-ential24-ipcprecursorgeneration.Consequently,watercolumnssim-ilartotheAthelBasinduringsilicilytegeneration,thatis,deep-watersettings with high-productivity water columns, may have been thesortsofenvironmentswhereearlyanimalsproliferated.
7 | THE CONNECTION BETWEEN THE
SILICILYTE AND GLOBAL BIOGEOCHEMICAL CONDITIONS
Although no other bona fide silicilytes exist in the geologic record,Brasier,Antcliffe,andCallow(2011)notedanincreaseinsubtidalsi-licificationattheEdiacaran–Cambrianboundary.Thisexpansionisev-idencedbythelaminatedchertsintheTalFormationinIndia(Brasier
Theseotherformationsarenotexactanaloguestothesilicilyte.For example, the Tal Formation cherts are associated with phos-phorite deposits (Mazumdar & Banerjee, 1998) while some (butnotall)Yangtzeplatformchertsmayhavehada localhydrothermalsource (Wang etal., 2012). Regardless, the presence of other pro-ductive,deep-watersilicadeposits formedclose to the timeof thesilicilytecould indicatethatthesilicilyte isanexpressionofamoreglobalphenomenonoccurringattheEdiacaran–Cambrianboundary.Onepossibilityforsuchanexpansion,asproposedbyRamseyeretal.(2013),iselevatedsilicafluxestotheoceansduetoincreasedsilicateweathering rates.
Consequently,beforetheexpansionofspongesandradiolaria intheCambrian,whichfundamentallyalteredthelocationofsilicadepo-sition(Maliva,Knoll,&Siever,1989),deep-watersilicadepositioninproductivewatersmayhavebeenanimportantsinkofsilicatothesolidEarth.Thesilicilytewouldrepresentanendmemberofthesesystemswherelowsiliciclasticinputsallowedforthefullexpressionofasilica-dominateddepositionalenvironment.Thispredictsthatsilicilyte-likeformations could be present in other deep-water, sediment-starvedbasinsatthetimeoftheEdiacaran–CambrianboundaryandperhapsevenearlierintheProterozoic.
8 | SUMMARY AND CONCLUSIONS
Thesilicilyteisanenigmaticrocktype,issingularintherockrecord,hasnoknownmodernanalogues,andappearsonlyattheEdiacaran–Cambrian boundary. Previous studies proposed that its originaldepositional environmentwas a permanently anoxic, sulfidicwatercolumn. Silica precipitation occurred due to either nucleation onfloatingmicrobialmatsorduetomixingofsurfaceanddeepwaterswith significantlydifferent salinities (AlRajaibi etal., 2015;Amthoretal., 2005; Ramseyer etal., 2013; Schröder & Grotzinger, 2007;Willeetal.,2010).
Ourbiomarkerresultsindicatethatthesilicilytereceivedorganicmatterfromadistinctassemblageofbiologicalsourcesascomparedtotheboundingshales.Thisindicatesthat:(i)ThesilicilytewasmorereducingatdepthinthewatercolumnorinitssedimentporewatersthantheboundingThuleilatShaleandU-Shale.(ii)Thesilicilyterela-tive to thebounding shales receivedahigher inputoforganicmat-tersourcedfrombacteriavs.eukaryotes,fromgreenalgaevs.othereukaryotes, and from bacteria like cyanobacteria or Proteobacteriathat synthesize 2-methylhopane precursors vs. other bacteria. (iii)Highmonomethylalkaneabundancespotentiallysuggestasignificantinputoforganicmatter to theultimatelypreservedhydrocarbons inthesilicilytefrombenthicmicrobialmats.And(iv),basedonmeasure-ments of 24 iso/n-propylcholestane ratios (and total abundancesof24-isopropylcholestanevs.totalorganiccarbon),thesilicilytereceived
Ironspeciationdataindicatethatsulfidewasproducedinthesys-tem (asevidencedby thepresenceofpyrite).However, theamountof pyrite relative to other phases of iron susceptible to pyritizationis lowerthanthatcommonlyobservedinpermanentlyeuxinicwaterbodies.ThisdifferenceindicatesthattheAthelwaterswerenotper-manentlysulfidicandlikelyexperiencedepisodicventilationwithoxy-genatedseawaterandthusredoxvariabilityinbottomwaters.
Based on both the new observations presented here as wellas thosemade in previous studies,we propose that the silicilyteformed via silica precipitation on organic matter formed in theupperwatercolumnof theAthelBasinduringmixingofnutrient-richdeeperwaterswithoceanwatersenteringthebasin.Thissilicathensettledtotheseafloorandformedthesedimentaryprecursorsofsilicilyte.Organicmatterwasremineralizedatdepthcreatingthesilica enrichments observed in the silicilyte. Silicon isotopemea-surementsofthequartzdonotindicatethatspongalopalwastheprecursortoquartzsilicilyte.Theoxygenisotopesofquartzinthesilicilyteshowthatthesilicatransformedtoquartzduringdiagen-esisovermultiple kilometersofdepth in the sedimentary columnunderevolvingtemperaturesandor/pore-watercompositions.
Thesilicilyte,althoughuniqueintherecord,appearstohaveana-logsinotherbasinsofEdiacaran–Cambrianboundaryage.Theseothersystemsaccumulatedsilica indeeperwaterandarealso thought tohavebeenproductiveenvironments.Thissimilaritymayindicatethatthe silicilyte is a sediment- starved end member of a form of silica depositionthatoccurredatthistime.Itmayrepresentanunderappre-ciatedformofsilicaremovalfromtheoceansintheProterozoic.
ACKNOWLEDGMENTS
We acknowledge funding from the Eaton Fellowship administeredby the Division of Geological and Planetary Sciences at Caltech and theNSFGRFP.PetroleumDevelopmentOmanisthankedforprojectplanningassistance,sampleaccess,andstimulatingscientificdiscus-sions.WeacknowledgetheMinistryofOilandGasoftheSultanateofOmanforpermissiontoaccesssamplesandtopublishtheresults.Wethankthreeanonymousreviewersandourhandlingeditor,JochenBrocks,forhelpfulcommentsthatgreatlyimprovedthemanuscript.
CONFLICT OF INTEREST
All authors declare no conflict of interest.
REFERENCES
AlRajaibi,I.M.,Hollis,C.,&Macquaker,J.H.(2015).OriginandvariabilityofaterminalProterozoicprimarysilicaprecipitate,AthelSilicilyte,SouthOman Salt Basin, Sultanate of Oman. Sedimentology, 62,793–825.
Allen, P. A. (2007). The Huqf supergroup of Oman: Basin developmentandcontext forNeoproterozoicglaciation.Earth- Science Reviews, 84, 139–185.
Amthor, J. E., Grotzinger, J. P., Schröder, S., Bowring, S. A., Ramezani, J., Martin, M. W., & Matter, A. (2003). Extinction of cloudina and
namacalathus at the precambrian-cambrian boundary in oman.Geology, 31, 431–434.
Amthor,J. E.,Ramseyer,K., Faulkner,T.,&Lucas,P. (2005). Stratigraphyand sedimentology of a chert reservoir at the Precambrian- cambrian boundary: The Al Shomou Silicilyte, South Oman Salt Basin. GeoArabia, 10,89–122.
Antcliffe, J. B. (2013). Questioning the evidence of organic compoundscalledspongebiomarkers.Palaeontology, 56,917–925.
Bailey,J.V.,Joye,S.B.,Kalanetra,K.M.,Flood,B.E.,&Corsetti,F.A.(2007).Evidence of giant sulphur bacteria in Neoproterozoic phosphorites.Nature, 445,198–201.
Bazhenova, O. K., &Arefiev, O. A. (1996). Geochemical peculiarities ofPre-Cambrian source rocks in the East European Platform. Organic Geochemistry, 25, 341–351.
Bishop,A. N., & Farrimond, P. (1995).A newmethod of comparing ex-tendedhopanedistributions.Organic Geochemistry, 23,987–990.
Blatt, H. (1987). Oxygen isotopes and the origin of quartz. Journal of Sedimentary Research, 57, 373–377.
Bowring,S.A.,Grotzinger,J.P.,Condon,D.J.,Ramezani,J.,Newall,M.J.,&Allen,P.A.(2007).Geochronologicconstraintsonthechronostrati-graphicframeworkoftheNeoproterozoicHuqfSupergroup,Sultanateof Oman. American Journal of Science, 307,1097–1145.
Brasier,M.D.,Antcliffe,J.B.,&Callow,R.H.T.(2011).EvolutionarytrendsinremarkablefossilpreservationacrosstheEdiacaran–Cambriantran-sitionandtheimpactofmetazoanmixing.InD.J.Bottjer&P.A.Allison(Eds.),Taphonomy: Process and bias through time(519–567).Dordrecht:Springer.
Brasier,M.D.,Green,O.,&Shields,G. (1997).Ediacarianspongespiculeclusters from southwestern Mongolia and the origins of the Cambrian fauna. Geology, 25, 303–306.
Brocks,J.J.,&Butterfield,N.J.(2009).Biogeochemistry:Earlyanimalsoutin the cold. Nature, 457, 672–673.
Brocks, J. J., Logan, G.A., Buick, R., & Summons, R. E. (1999).Archeanmolecular fossils and the early rise of eukaryotes. Science, 285, 1033–1036.
Canfield, D. E., Poulton, S.W., Knoll, A. H., Narbonne, G.M., Ross, G.,Goldberg,T.,&Strauss,H. (2008).FerruginousconditionsdominatedlaterNeoproterozoicdeep-waterchemistry.Science, 321,949–952.
Canfield,D. E., Raiswell, R., &Bottrell, S. (1992).The reactivity of sedi-mentary iron minerals toward sulfide. American Journal of Science, 292, 659–683.
Cao, C., Love,G.D.,Hays, L. E.,Wang,W., Shen, S., & Summons, R. E.(2009).Biogeochemicalevidenceforeuxinicoceansandecologicaldis-turbancepresagingtheend-Permianmassextinctionevent.Earth and Planetary Science Letters, 281,188–201.
Clarkson,M.,Poulton,S.,Guilbaud,R.,&Wood,R. (2014).AssessingtheutilityofFe/AlandFe-speciationtorecordwatercolumnredoxcon-ditions in carbonate- rich sediments. Chemical Geology, 382, 111–122.
Davidson,E.H.,&Erwin,D.H.(2009).AnintegratedviewofPrecambrianeumetazoan evolution. Cold Spring Harbor Symposia on Quantitative Biology, 74,65–80.
Deutsch, C., Sarmiento, J. L., Sigman, D.M., Gruber, N., & Dunne, J. P.(2007). Spatial coupling of nitrogen inputs and losses in the ocean.Nature, 445, 163–167.
424 | STOLPER ET aL.
Douthitt, C. (1982). The geochemistry of the stable isotopes of silicon.Geochimica et Cosmochimica Acta, 46,1449–1458.
Fein,J.B.,Scott,S.,&Rivera,N.(2002).TheeffectofFeonSiadsorptionbyBacillus subtiliscellwalls:Insightsintonon-metabolicbacterialprecipi-tation of silicate minerals. Chemical Geology, 182, 265–273.
Ferris,F.G.,Fyfe,W.S.,&Beveridge,T.J.(1988).MetallicionbindingbyBacillus subtilis: Implications for the fossilization of microorganisms.Geology, 16,149–152.
Fowler,M.,&Douglas,A.(1987).Saturatedhydrocarbonbiomarkersinoilsof LatePrecambrian age fromEasternSiberia.Organic Geochemistry, 11, 201–213.
Gold, D.A., Grabenstatter, J., DeMendoza,A., Riesgo,A., Ruiz-Trillo, I.,& Summons, R. E. (2016). Sterol and genomic analyses validate thespongebiomarkerhypothesis.PNAS, 113,2684–2689.
Gold,D.A.,O’reilly,S.S.,Luo,G.,Briggs,D.E.,&Summons,R.E.(2016).Prospects for sterane preservation in sponge fossils from museumcollectionsandtheutilityofspongebiomarkersformolecularclocks.Bulletin of the Peabody Museum of Natural History, 57,181–189.
Grantham, P. (1988). Origin of crude oils in Oman. Journal of Petroleum Geology, 11,61–80.
Grotzinger,J.P.,&James,N.P.(2000).Precambriancarbonates:evolutionofunderstandingInJ.P.Grotzinger&N.P.James(Eds.),Carbonate sed-imentation and diagenesis in the evolving precambrian world (pp.3–20).Tulsa,OK:SEPMSpecialPublication.
Grotzinger,J.P.,&Knoll,A.H.(1999).StromatolitesinPrecambriancarbon-ates:Evolutionarymilepostsorenvironmentaldipsticks?Annual Review of Earth and Planetary Sciences, 27,313–358.
Grotzinger, J., & Rawahi, Z. (2014). Depositional facies and plat-form architecture of microbialite- dominated carbonate reservoirs, Ediacaran-CambrianAraGroup,SultanateofOman.AAPG Bulletin, 98, 1453–1494.
Hoffmann,F.,Larsen,O.,Thiel,V.,Rapp,H.T.,Pape,T.,Michaelis,W.,&Reitner, J. (2005). An anaerobic world in sponges. Geomicrobiology Journal, 22, 1–10.
Höld, I. M., Schouten, S., Jellema, J., & Sinninghe Damsté, J. S. (1999).Originof freeandboundmid-chainmethylalkanes inoils,bitumensand kerogens of themarine, InfracambrianHuqf Formation (Oman).Organic Geochemistry, 30,1411–1428.
Kelly, A. E., Love, G. D., Zumberge, J. E., & Summons, R. E. (2012).Hydrocarbon biomarkers of Neoproterozoic to Lower Cambrian oilsfrom eastern Siberia. Organic Geochemistry, 42, 640–654.
Kirschvink,J.L.,&Raub,T.D. (2003).Amethane fuse for theCambrianexplosion: Carbon cycles and true polar wander. Comptes Rendus Geoscience, 335,65–78.
Kleemann,G.,Poralla,K.,Englert,G.,Kjøsen,H.,Liaaen-Jensen,S.V.,Neunlist,S.,&Rohmer,M.(1990).TetrahymanolfromthephototrophicbacteriumRhodopseudomonaspalustris:Firstreportofagammaceranetriterpenefromaprokaryote.Journal of General Microbiology, 136, 2551–2553.
Knauth, L. P., & Epstein, S. (1976).Hydrogen and oxygen isotope ratiosin nodular and bedded cherts. Geochimica et Cosmochimica Acta, 40, 1095–1108.
Knoll,A.H.(1985).Exceptionalpreservationofphotosyntheticorganismsinsilicifiedcarbonatesandsilicifiedpeats.Philosophical Transactions of the Royal Society of London B, Biological Sciences, 311, 111–122.
Kodner, R. B., Summons, R. E., Pearson, A., King, N., & Knoll, A. H.(2008).Sterols inaunicellular relativeof themetazoans.PNAS, 105, 9897–9902.
Konhauser, K. O., Jones, B., Phoenix, V. R., Ferris, G., & Renaut, R.W.(2004).Themicrobialroleinhotspringsilicification.AMBIO: A Journal of the Human Environment, 33,552–558.
Lafargue, E.,Marquis, F., & Pillot,D. (1998). Rock-Eval 6 applications inhydrocarbonexploration,production,andsoilcontaminationstudies.Oil & Gas Science and Technology – Revue de L’Institut Français du Pétrole, 53, 421–437.
Lee, C., Fike, D., Love, G., Sessions, A., Grotzinger, J., Summons, R., &Fischer,W.(2013).Carbonisotopesandlipidbiomarkersfromorganic-rich faciesof theShuramFormation,SultanateofOman.Geobiology, 11,406–419.
Leo, R. F., & Barghoorn, E. S. (1976). Silicification of wood. Botanical Museum Leaflets, Harvard University, 25, 1–47.
Litchman,E.,Klausmeier,C.A.,Schofield,O.M.,&Falkowski,P.G.(2007).Theroleoffunctionaltraitsandtrade-offsinstructuringphytoplank-ton communities: Scaling from cellular to ecosystem level. Ecology Letters, 10,1170–1181.
Love,G.D.,Grosjean,E.,Stalvies,C.,Fike,D.A.,Grotzinger,J.P.,Bradley,A. S., … Summons, R. E. (2009). Fossil steroids record the appear-ance ofDemospongiae during theCryogenian period.Nature, 457, 718–721.
Love, G. D., Stalvies, C., Grosjean, E., Meredith, W., & Snape, C. E.(2008). Analysis of molecular biomarkers covalently bound withinNeoproterozoic sedimentary kerogen. Paleontological Society Papers, 14,67–83.
Lyons,T.W.,&Severmann,S.(2006).Acriticallookatironpaleoredoxprox-ies:New insights frommodern euxinicmarine basins.Geochimica et Cosmochimica Acta, 70,5698–5722.
Maldonado,M.(2006).Theecologyofthespongelarva.Canadian Journal of Zoology, 84,175–194.
Maldonado,M.,&Bergquist,P.R.(2002).PhylumPorifera.InC.M.Young&M.A.Sewell(Eds.),Atlas of marine invertebrate larvae(pp.21–50).SanDiego: Academic Press.
| 425STOLPER ET aL.
Maliva,R.G.,Knoll,A.H.,&Siever,R.(1989).Secularchangeinchertdis-tribution:Areflectionofevolvingbiologicalparticipation in thesilicacycle. Palaios, 4,519–532.
Maliva, R.G., Knoll,A.H.,& Simonson,B.M. (2005). Secular change inthePrecambriansilicacycle:Insightsfromchertpetrology.Geological Society of America Bulletin, 117,835–845.
Maloof,A.C.,Rose,C.V.,Beach,R.,Samuels,B.M.,Calmet,C.C.,Erwin,D.H.,…Simons,F.J.(2010).Possibleanimal-bodyfossilsinpre-Marinoanlimestones from South Australia. Nature Geoscience, 3,653–659.
Marin-Carbonne,J.,Chaussidon,M.,&Robert,F.(2010).Microscaleoxygenisotopevariationsin1.9GaGunflintcherts:Assessmentsofdiagenesiseffectsandimplicationsforoceanicpaleotemperaturereconstructions.Geochimica et Cosmochimica Acta, 74, 116–130.
Marshall, C. R. (2006). Explaining the Cambrian “explosion” of animals.Annual Review of Earth and Planetary Sciences, 34,355–384.
Mayer,L.M.(1994).Surfaceareacontroloforganiccarbonaccumulationin continental shelf sediments. Geochimica et Cosmochimica Acta, 58, 1271–1284.
McCaffrey,M.A., MichaelMoldowan, J., Lipton, P. A., Summons, R. E.,Peters,K.E.,Jeganathan,A.,&Watt,D.S.(1994).PaleoenvironmentalimplicationsofnovelC30steranesinPrecambriantoCenozoicagepe-troleum and bitumen. Geochimica et Cosmochimica Acta, 58,529–532.
Mills,D.B.,Ward, L.M.,Jones,C., Sweeten,B., Forth,M.,Treusch,A.H., & Canfield, D. E. (2014). Oxygen requirements of the earliestanimals. Proceedings of the National Academy of Sciences, 111(11),4168–4172.
Moldowan,J.M.,Fago,F.J.,Lee,C.Y.,Jacobson,S.R.,Watt,D.S.,Slougui,N.-E.,…Young,D.C.(1990).Sedimentary12-n-Propylcholestanes,mo-lecular fossils diagnostic of marine algae. Science, 247,309–312.
Peters, K. E., Walters, C., & Moldowan, J. (2005). The biomarker guide: Volume II. Biomarkers and isotopes in petroleum systems and earth history. Cambridge,UK:CambridgeUniversityPress.
Raiswell, R., Buckley, F., Berner, R.A.,&Anderson,T. (1988).Degreeofpyritizationofironasapaleoenvironmentalindicatorofbottom-wateroxygenation.Journal of Sedimentary Research, 58,812–819.
Raiswell,R.,&Canfield,D.E.(1998).Sourcesofironforpyriteformationinmarine sediments. American Journal of Science, 298,219–245.
Ramseyer,K.,Amthor,J.E.,Matter,A.,Pettke,T.,Wille,M.,&Fallick,A.E. (2013). Primary silica precipitate at the Precambrian/Cambrianboundary in the South Oman Salt Basin, Sultanate of Oman. Marine and Petroleum Geology, 39(1),187–197.
Rashby, S. E., Sessions,A. L., Summons,R. E.,&Newman,D.K. (2007).Biosynthesisof2-methylbacteriohopanepolyolsbyananoxygenicpho-totroph.PNAS, 104,15099–15104.
Ricci, J. N., Coleman, M. L.,Welander, P. V., Sessions, A. L., Summons,R. E., Spear, J. R., & Newman, D. K. (2014). Diverse capacity for2-methylhopanoid production correlates with a specific ecologicalniche. The ISME Journal, 8(3),675–684.
Rubinstein, I., Sieskind,O.,&Albrecht,P. (1975).Rearrangedsterenes ina shale: Occurrence and simulated formation. Journal of the Chemical Society, Perkin Transactions, 1,1833–1836.
Schoell,M.,Hwang,R.J.,Carlson,R.M.K.,&Welton,J.E.(1994).Carbonisotopic composition of individual biomarkers in gilsonites (Utah).Organic Geochemistry, 21,673–683.
Schoell,M.,McCaffrey,M.A.,Fago,F.J.,&Moldowan,J.M.(1992).Carbonisotopic compositions of 28, 30-bisnorhopanes and other biologicalmarkersinaMontereycrudeoil.Geochimica et Cosmochimica Acta, 56, 1391–1399.
Schröder,S.,&Grotzinger,J.P.(2007).EvidenceforanoxiaattheEdiacaran-Cambrianboundary:Therecordofredox-sensitivetraceelementsandrare earth elements in Oman. Journal of the Geological Society of London, 164,175–187.
Schröder,S.,Grotzinger,J.P.,Amthor,J.E.,&Matter,A.(2005).CarbonatedepositionandhydrocarbonreservoirdevelopmentatthePrecambrian-Cambrian boundary: The Ara Group in South Oman. Sedimentary Geology, 180,1–28.
Schröder,S.,Schreiber,B.C.,Amthor,J.E.,&Matter,A. (2003).Adepo-sitional model for the terminal Neoproterozoic-Early Cambrian AraGroupevaporitesinsouthOman.Sedimentology, 50,879–898.
Schröder,S.,Schreiber,B.C.,Amthor,J.E.,&Matter,A.(2004).Stratigraphyand environmental conditions of the terminal Neoproterozoic-CambrianPeriodinOman:Evidencefromsulphurisotopes.Journal of the Geological Society of London, 161,489–499.
Seifert,W.K.,&Moldowan,J.M.(1978).Applicationsofsteranes,terpanesand monoaromatics to the maturation, migration and source of crude oils. Geochimica et Cosmochimica Acta, 42,77–95.
Seifert,W.K.,&Moldowan,J.M.(1980).Theeffectofthermalstressonsource-rock quality asmeasuredbyhopane stereochemistry.Physics and Chemistry of the Earth, 12,229–237.
Shahar,A.,&Young,E.D.(2007).AstrophysicsofCAIformationasrevealedbysiliconisotopeLA-MC-ICPMSofanigneousCAI.Earth and Planetary Science Letters, 257,497–510.
Shen,Y.,&Schidlowski,M. (2000).NewC isotopestratigraphy fromsouth-westChina:ImplicationsfortheplacementofthePrecambrian-Cambrianboundary on theYangtze Platform and global correlations.Geology, 28, 623–626.
Shiea,J.,Brassell,S.C.,&Ward,D.M.(1990).Mid-chainbranchedmono-anddimethylalkanes inhotspringcyanobacterialmats:Adirectbio-genic source for branched alkanes in ancient sediments? Organic Geochemistry, 15, 223–231.
SinningheDamsté,J.S.,VanDuin,A.C.T.,Hollander,D.,Kohnen,M.E.L.,&DeLeeuw,J.W.(1995).Earlydiagenesisofbacteriohopanepolyolderiv-atives:Formationoffossilhomohopanoids.Geochimica et Cosmochimica Acta, 59, 5141–5157.
426 | STOLPER ET aL.
Sperling,E.A.,Halverson,G.P.,Knoll,A.H.,Macdonald,F.A.,&Johnston,D.T.(2013).Abasinredoxtransectatthedawnofanimallife.Earth and Planetary Science Letters, 371, 143–155.
Summons, R. E., Jahnke, L. L., Hope, J. M., & Logan, G. A. (1999).2-Methylhopanoidsasbiomarkersforcyanobacterialoxygenicphoto-synthesis. Nature, 400, 554–557.
Summons,R.E.,Powell,T.G.,&Boreham,C.J.(1988).PetroleumgeologyandgeochemistryoftheMiddleProterozoicMcArthurBasin,NorthernAustralia:III.Compositionofextractablehydrocarbons.Geochimica et Cosmochimica Acta, 52, 1747–1763.
Thunell,R.C.,Tappa,E.,&Anderson,D.M. (1995).Sediment fluxesandvarve formation in Santa Barbara Basin, offshore California. Geology, 23,1083–1086.
Valentine,J.W.(2002).PreludetotheCambrianexplosion.Annual Review of Earth and Planetary Sciences, 30,285–306.
Wang,J.,Chen,D.,Wang,D.,Yan,D.,Zhou,X.,&Wang,Q.(2012).PetrologyandgeochemistryofchertonthemarginalzoneofYangtzePlatform,westernHunan, SouthChina, during theEdiacaran-Cambrian transi-tion. Sedimentology, 59,809–829.
Webster,N.S.,Wilson,K.J.,Blackall,L.L.,&Hill,R.T.(2001).PhylogeneticdiversityofbacteriaassociatedwiththemarinespongeRhopaloeidesodorabile. Applied and Environmental Microbiology, 67, 434–444.
Wille,M.,Nägler,T.F.,Lehmann,B.,Schröder,S.,&Kramers,J.D.(2008).Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary. Nature, 453,767–769.
Wille,M.,Sutton,J.,Ellwood,M.J.,Sambridge,M.,Maher,W.,Eggins,S.,&Kelly,M.(2010).Siliconisotopicfractionationinmarinesponges:Anewmodel for understanding silicon isotopicvariations in sponges.Earth and Planetary Science Letters, 292,281–289.
Xiao,S.,&Laflamme,M.(2009).Ontheeveofanimalradiation:Phylogeny,ecology and evolution of the Ediacara biota. Trends in Ecology & Evolution, 24, 31–40.
Xiao, S.,Yuan, X., & Knoll,A. H. (2000). Eumetazoan fossils in terminalProterozoicphosphorites?PNAS, 97,13684–13689.
Yee, N., Phoenix,V. R., Konhauser, K.O., Benning, L. G., & Ferris, F. G.(2003).The effect of cyanobacteria on silica precipitation at neutralpH: Implications forbacterial silicification in geothermal hot springs.Chemical Geology, 199,83–90.
Zehr, J. P. (2011). Nitrogen fixation by marine cyanobacteria. Trends in Microbiology, 19, 162–173.
Ziegler,K.,Young,E.D.,Schauble,E.A.,&Wasson,J.T.(2010).Metal–silicatesilicon isotope fractionation inenstatitemeteorites andconstraintsonEarth’scoreformation.Earth and Planetary Science Letters, 295,487–496.
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How to cite this article:StolperDA,LoveGD,BatesS,etal.PaleoecologyandpaleoceanographyoftheAthelsilicilyte,Ediacaran–Cambrianboundary,SultanateofOman.Geobiology. 2017;15:401–426.https://doi.org/10.1111/gbi.12236
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