� Monterey Bay Aquarium Research Institute

Theoriginofsubmarinecanyonsisoneoftheoldestques-tionsinmarinegeology.FrancisShepard,thefirstgeologisttodevotehislife’sworktomarinegeology,concluded in his 1963 textbook on sub-marine canyons that no one hypothesiscould explain their origin, and that theobservations were too sparse to test thecompeting hypotheses. He pointed outthattheir featuresareverysuggestiveoferosionbyriver-likeaction,buttheirdepthistoogreattobeconsistentwithcarvingthesecanyonsduringperiodsoflowersealevel. Even though many canyon headsareclosetothemouthsofterrestrialriversystems,freshwaterfromtheriverreach-ingtheoceanislighterthansaltwater,andtherefore unlikely to scour the seafloor.DebatecontinuedfromShepard’stimetothepresentonwhethersubmarinecanyonsareactiveorwhethertheyinheritedtheirdistinctivefeaturesfromsomecatastrophiceventsinthedistantgeologicpast.

Forexample,onetheoryfortheformationofMontereyCanyon,particularlygiventhecurrentlyintermittentandminorflowoftheSalinasRiver, isthattheSacramentoRiverpreviouslydrainedintoMontereyBayandcarvedthecanyoninfrequentturbidityflows.TheupliftoftheCoastRangesseveralmillionyearsagowouldhavedivertedthatflow,makingMontereyaninactivecanyontoday.Therefore,whenMBARIScientistCharliePaullinitiatedaresearchprogramincanyondynamicsseveralyearsago,theper-ceivedriskatthetimewasthatnothingwouldhappen.InstrumentsplacedinMontereyCanyonwouldneedtowaityearsbeforeanythingsignificantwouldberecorded.

Nevertheless,thetimewasrighttoreconsidertheques-tionsfirstraisedbyFrancisShepard,asanimpressivearrayofnewtechnologyandanalyticalmeasurementscouldbebroughttobearontheinquiry.Paullandhiscollaboratorshavegatheredthenecessaryhigh-resolutionmultibeambathymetry,remotelyoperatedvehicle(ROV)-guidedobser-

A History Lesson From Monterey Canyon

Figure 1: Images showing the morphology of the Grand Canyon of the American southwest (top) and Monterey Canyon (bottom) at the same scale demonstrate that these are analogous features in terms of their size and shape. While water flowing in the Colorado River carved the Grand Canyon, a directly analogous process is not known to occur within Monterey Canyon.

Submarinecanyons,withtheirsteepsides,meanders,andtribu-taries,dominatethegeomor-

phologyofcontinentalmarginsinthesamewaythatrivervalleysdominatesubaeriallandscapesofplateausandhighplains.MontereyCanyon,whoseheadliesjustoffshoreofMBARI,isoneofthelargestsubmarinecanyonsonthePacificCoast.Innearlyeverymeasure,itisasdramaticastheGrandCanyon(Figure1).

�005 Annual Report 5

A History Lesson From Monterey Canyon

vationsandsampling,anddatafromcanyonmonitoringactivities(measurementsofnear-seafloorcurrents,turbidity,pressure,temperature,andsalinity)to“readthehistory”ofMontereyCanyon.SomeofthemoreintriguingscientifictechniquesemployedbyPaullandhisteamtoprovideaprecisechronologyofrecentsedimentaryeventsincludeobservingtheoccurrencesofpollenfromnon-nativespeciesintroducedbyEuropeansettlers,thepesticideDDTappliedtoagriculturalfieldsbetween1945and1969,andelevatedlevels of carbon-14(14C) from the detonation of nuclearweapons.Ratiosofstableisotopesofcarbonandnitrogendistinguishmarineversusterrestrialoriginofsedimentinsidethecanyonandonitsflanks.

ThesenewobservationsrevealthatMontereyCanyonisanextremelyactivefeature.Gravelandcoarsesand,withacompositionsimilartothesandfoundonlocalbeaches,arecapturedinthecanyonheadastheyaretransportedalongshorevialittoraldrift.Periodicallytheaccumulationsofsedimentinthecanyonheaddepositsfail,producingsubmarinelandslidesorturbidityeventsthatareinmanywayssimilartoavalanchesonland.InstrumentsdeployedintheaxisofupperMontereyCanyon(atdepthsoflessthantwokilometers)from2001to2005recordedevidenceofanunexpectednumber(aboutten)ofdistinctturbidityevents,rangingfrommildincreasesinnear-bottomturbiditywithnodetectablechangeinbottomcurrents,tomajor,energetic,down-canyonflowswithhighcurrentspeedsandturbidity,largevolumesof sedimenttransport,anddisplacementofmooredsystems.Thelargeeventsoftendestroyedorburieddeployedinstruments(Figure2).Conditionswere

documented by intact instrument systems during fourlargeturbidityeventsfromlate2002tomid2005,leadingtheCanyonDynamicsteamtoreconsiderwhatconstitutesa“50-yearevent”.Currentspeedsduringeventsreachedmorethan200centimeterspersecondneartheseabedanddecreasedinspeedwithheightabovethebottom.Sedimenttransporteventsofthismagnitudeoccurmorethanonceayearintheuppercanyon.Basedontheknowntimingoftheseeventsandcomparisonwithlocalearthquake,storm,andriverdischargedata,themostcommontriggersfortheobservedeventsareintensestormwavedisturbanceand/orrandomcanyonwallfailures.EvenduringthelongdryseasonwhentheSalinasandPajaroRiversarejustbarelyornotflowing,materialmovesdownthecanyon,liningtheaxisofthecanyonwithsandandgraveldeposits.

Measurementsofsedimenttransportintheuppercanyonnear1,300metersdepthhighlightboththehighvolumeoftransportinthecanyonaxisandthedominanceoftrans-portduringturbidityflows.Mooringswithsedimenttraps,currentmeters,andopticalbackscattersensorsdeployedinthecanyonaxisbyMBARIScientistJimBarry’sgrouprevealed that sedimentation in the canyon is normallyabout15to20timesgreaterthanmeasuredatthesamedepthonSmoothRidge,onthecontinentalslopeoutsidethecanyon.Moreimportantly,ratesofcanyonsedimenttransportanddepositionduringeventswere300to500timesthoseobservedduringnon-eventperiods.Thus,asinglesignificanteventperyearcanresultinmorethan85 percent of the annual inorganic and organic carbontransportbudget.

Figure �: Right: A damaged remote instrument node (RIN) that was moved during a sediment transport event in upper Monterey Canyon. This heavy steel structure was found, using ROV Ventana, about 50 meters from where it was deployed. Its mast was broken off and tipped up against a ledge on the side of the canyon. Left: The yellow cable, which became buried in the sediment, connects with a cluster of instruments that Ventana excavated from the sediment.

� Monterey Bay Aquarium Research Institute

A History Lesson From Monterey Canyon

TheaxialchannelassociatedwithMontereyCanyonextendsasadistinctcontinuouschanneltodepthsofmorethanfourkilometers,wherethewinnowedmaterialisultimatelydepositedontotheMontereyFan.Paull’steamhasexten-sivelysampledthesedimentinthesubmarinechannelandthe fan using a vibracoring system (Figure 3) mountedonMBARI’sremotelyoperatedvehicleTiburon.Thewide-spread occurrence of sediment with DDTr (the sum ofDDTanditsdecayproducts)inthetopfivecentimetersofsedimentthroughoutthecanyonandfansystemindi-catethatsignificantvolumesoffinegrainsedimentshavebeentransportedacrossthemarginandontothefansince1945(Figure4).Thecanyonisapparentlyamajorconduitfortransportingasubstantialamountoforganicmaterialfromlandandthecoastalzonetothedeepseainmoderntimes.Ithaslongbeensuspectedthatverticaltransportofcarbonfromtheupperoceantothedeepseaawayfromthecoastalzoneisinsufficienttomaintainthesizeofthebenthiccommunity.Thus,Paull’suseofDDTasatracerforcontemporarycarbontransporttothedeepseasug-geststhatcanyonsmightbeimportantinsupplyingthatmissingcarbontonourishthedeepsea.

Totestthishypothesis,in2004monitoringequipmentwasdeployedin3,450-meterwaterdepthswithintheaxisofthedeep-seachannel—calledShepardMeanderafterFrancisShepard—atthedown-slopeendofMontereyCanyon.TheShepardMeanderexperimentcontinuedthroughout2005withtheregularservicingoftheseinstruments(Figure5).Sedimenttrapsandcurrentmetersdeployedwithinthecanyonaxisandoutsidethecanyononitsflanksareprovid-

ingaseveral-yearrecordofbothverticalandlateralflowandcarbontransportgradientsinthevicinityofthecanyon.

Curiously,whileseverallargesedimenttransporteventshaveoccurredintheupperreachesofMontereyCanyoninthelast18months,nosignificantsedimentologicaleventshavebeenrecordedatthedeepShepardMeandernodeinthelowercanyonduringthesametimeperiod.However,thesedatahavealreadyprovidedauniqueconstraintonhowfrequentlythedeep-seabenthosisfedfromland.Andthoughtheuppercanyonisanactiveconduitforcarbontransport,thehighfrequencyofgravityfloweventscandisturb benthic communities. Barry’s group has docu-mentedthelowabundanceofsessilefaunaintheuppercanyonandtheirincreaseinabundancewithdepth,pre-sumablyduetothelowerfrequencyofdisturbanceeventsinthelowercanyon.Clearly,thereappeartobeecologicaltrade-offsforthefaunainMontereyCanyon.Areasintheuppercanyonwiththehighestfoodavailabilitymayalsobethemosthazardoustoinhabit.Conversely,thelowercanyonisrelativelyfood-poor,butsafer.Thus,thecanyon’s

“carbonhighway”maybeanimportantconduitfeedingthe

Figure �: ROV Tiburon is launched with the vibracoring system mounted on the front.

20

15

10

5

00 100 200 300 400

Distance along thalweg (km)

DDTr

(ppb

)

Figure �: Plot showing the mean concentrations of the pesticide DDT and its associ-ated residues found in seafloor sediment versus the distance along the thalweg of the channel that extends through Monterey Canyon and continues across the Monterey Fan. Detectable levels of DDTr can be found out to nearly �00 kilometers along this channel and provide a tracer for modern sediment transport.

�005 Annual Report �

A History Lesson From Monterey Canyon

deepsea,butitstrafficofsedimentflowsmaybedangerousforcanyoninhabitants.

Thelargeroleofshort-lived,episodiceventsinboththetransportofmaterialsacrossthecontinentalmarginandasapotentiallydominantinfluenceonthestructureofbenthicfaunalcommunitiesincreasesthecomplexityoftheirstudy.Ratherthantheregularbeatandtempoofoceanographicprocessesoperatingondiurnal, tidal,orseasonalperiods,episodicandrareeventsrequiresensorstoremaindeployedandactiveover longperiods,oftensampling under higher frequencies to fully capture aneventofinterest.Infact,largeeventsintheuppercanyonhaveoccasionallycapturedthesensorsdeployedtomeasurethem,sendingthemdown-canyonwiththeavalanche-liketurbidityflows.

Todate,theexperimentalapparatusatShepardMeanderhasbeenpoweredbyseafloorbatteries,andreplenishingthebatteriesandretrievingthedatabothrequireavisitfromanROV.Inthenexttwoyears,theexperimentwillbeupgradedtouseMBARI’snewMontereyOceanObservingSystem(MOOS),whichusesafiber-opticcabletobringpowerandcommunicationstothedeepseafromsolarpanels,windturbines,andsatellitecommunicationsonamooringattheseasurface.Theexperiencessofarwithdeployingandservicinginstrumentshaveprovidedinsightintohowto

designsubmarinecable-connectedseafloornodesthatwillconnecttotheMOOSmooringinthefuture.

Theobservationthatlittlesedimentologicalactivityhap-pened in the lower canyon during the first 18 monthsoftheShepardMeanderexperimentisatoddswiththelargenumberofenergeticeventsrecordedfromtheuppercanyon.Vibracorestratigraphies(seecoverimage)showthattheseafloorwithintheaxialchannelisunderlainbyhigh-energydeposits(gravel,coarsesand,andrip-upclasts)thatareatleastameterthick.Effortstodatematerialsinthesecoresindicatethatthelastmajorsedimenttransporteventtoreachthelowercanyonoccurredabout60to150yearsago,andmight,speculatively,havebeenassociatedwithamajordisturbancesuchasthe1906earthquake.Theobservationthatcoarse-sediment-carrying,high-energyeventsaremuchmorecommonintheuppercanyonthanatShepardMeandersuggeststhattheuppercanyonactsasacapacitor,accumulatingsandandothercoarsesedimentsthatultimatelymovedownthelowercanyontothefaninmuchlessfrequentbutcatastrophicevents.

The lack of detected sand-carrying events in the lowercanyonisalsoatoddswiththevibracoringdatathatshowsubstantialpost-1945transportoffinesedimentouttotheMontereyFan.ROVdivesintheaxisofthecanyonfrequentlyencounterlow-visibilitywaterconditionsbelowtheshoul-dersofthecanyon,whicharecausedbysuspendedsediment,especiallyduringtherainyseason.MBARIScientistKenJohnsondemonstratedthattheseturbidwatermassesarecharacterizedbylowersalinityandhighertemperaturethanthenormalambientseawater,indicatingtheinfluenceofterrestrialriverflowatthefloorofthecanyon(Figure6).TheseconditionsarelikelytohappenwhentheSalinasRiver discharges a fine-sediment plume into the oceanasitbreaksthroughaseasonalbermneartheheadofthecanyon.Thedensityofthesuspendedsedimentisenoughtooffsetthebuoyancyofthefreshwater.Observationssuchasthesesuggestthatthetransportoffinesedimentdownthecanyonandontoitsflanksinvolvesdifferentprocessesandismoreorlessdecoupledfromthetransportofsandandothercoarsefractions.

Paull’sdatafromMontereyCanyontellmorethanjustthehistoryofthecanyon.Thefinesediments,largelyderivedfromerosionofthelandsurface,alsorecordthehistoryof

Figure 5: ROV Tiburon switches instrument package on a remote instrument node in the axis of Monterey Fan Channel at Shepard Meander. Photo shows multiple instru-ments including two current profilers (yellow canisters with four red transponders); the one on the left will replace the one that had been deployed for six months.

� Monterey Bay Aquarium Research Institute

A History Lesson From Monterey Canyon

land-usepracticesintheSalinasValley.Forexample,thepresenceofpollengrainsfromalien,introducedspecieswithinthefine-grainedsedimentsindicatesthatmorethanfivemetersofsedimentweredepositedwithinhistoricaltimes(post-1700)onthecanyonflanks.Theaverageannualsedimentationratefrom1700to1945ismorethandoubletherateforthetimeperiodsince1945,whichiswelldeter-minedbythepresenceofDDTresiduesinthesediment.Thisobservation suggests that thedestructive land-use

practicesassociatedwiththeearlystagesofmoderndevel-opmentintheSalinasandPajaroValleyscausedrapidlossofvirginsoilsandasubsequentincreaseintheproportionofoldersoil(asdeterminedbyits14Ccontent)asoldersoilhorizonswereexhumedbyerosion.Asharpdecreaseintheamountofsoilbeingflushedoffthelandoccurredinthemid1900sasdamsandotherimpoundmentstructureswerebuiltontherivers.

HalfacenturyafterFrancisShepardfirstposedtheques-tionsconcerningthedynamicsofsubmarinecanyons,wefinallyhavesomeanswersforMontereyCanyonthatcanbeusedasastartingmodelforcanyonselsewhereintheworld’s oceans. Canyons are indeed very active, with asubstantialportionoftheircoarsebedloadcomingfromalong-shoretransportratherthandirectinputfromariversystem.Canyonsarecreatedandmaintainedbysurgesoflarge volumesofmaterialdowntheiraxes triggered bystormsandmasswastingofthecanyonwalls.Thetrans-port isdiscontinuous intimeandspace,withsomesec-tionsactingasreservoirsthatstorethematerialformorecatastrophiceventslater.

Finally,thestudyofsubmarinecanyonsisimportanttomore than just marine geologists. These canyons arelikely important conduits for lateral transport of mate-rial, includingfood, fromthehighly-productivecoastalzonetothemoreimpoverisheddeepsea.ThesizeofthelandslideeventsinferredfromthegeologicrecordsuggeststhatlargetransporteventsinMontereyCanyonevenhavethepotentialtogeneratelocaltsunamis.

Figure �: ROV dive into the river plume at 1�00 meters shows light transmission going to zero and temperature, salinity and density signals reversing.O

3 4 5 6

20 40 60 80

27.0 27.1 27.2 27.3 27.4

34.0 34.1 34.2 34.3 34.4

100

600

%T Temp. Salin.

3/16/95

700

800

900

1000

1100

1200

% Transmittance

Depth

(m)

Salinity

Density ( )

Temperature C