Milkov-2005-Gas hydrate systems at Hydrate Rid(7)

Incontrast,Sites1246and1247arelocatedintheareaslessaffectedbyfaultingandlackingdirectsupplyofdeephydro-carbonsintotheGHSZ.HorizonAislocated?30mbelowtheGHSZatsite1247andadvectionand/ordiffusionofhydrocar-bonsfromthathorizonintotheGHSZisapparentonC1/C2?and?13CofC2pro?les(Fig.6d).However,theonlygashydratesamplerecoveredfrom93mbsfatSite1247apparentlydoesnotincludemuchC1orC2fromHorizonA.

6.2.FormationoftheShallowGasHydrateAccumulation

attheSouthernSummitSuessetal.(1999)suggestedthatsea?oorgashydratesatHydrateRidgerepresenta“secondary”phaseastheyformfromgashydratedecomposedindeepsedimentsneartheBSR.Incontrast,theextensivegasgeochemicaldatasetfromLeg204providescompellingevidencethatshallowgashydratescrystallizemostlyfromgasthatisamixtureofmicrobialgasgeneratedinrelativelyshallowsediments(predominantlywellbelowtheBSR)andthermogenicgasgeneratedatdepthsontheorderof2to2.5km(probablyfromtheaccretionarycomplex).ThemajorityofthemixedgasissuppliedalongHorizonA.Gashydratesareformingasaconsequenceofgassaturationofporewaterinsedimentsinandadjacenttothemigrationpathways.Althoughsomeinsitumicrobialgasisneededtoexplaindifferencesinsomemolecularandisotopicpropertiesofshal-lowgashydratesandthegasinHorizonA,thecontributionofthatmicrobialgasisminor.Thisinterpretationsupportsrecentinferencesthatthebulkofgashydrateinmodernsedimentsisstable(Sassenetal.,2001b)andisnotthecauseofsea?oorseepageashasbeenargued(Suessetal.,1999;VanRensber-genetal.,2002).

Itappearsfrom3Dseismicdata(Figs.2band2c)thatthegashydrateaccumulationattheSouthernSummitmaybecon-nectedtotheintersectionofHorizonAwiththebaseoftheGHSZwhereaseismic“brightspot”isobserved(Tréhuetal.,2002).Ithasbeenshownthatvolcanicglass-richsedimentswithinHorizonAhavehighgassaturation(Collettetal.,2003;Tréhuetal.,2003;TréhuandFlemings,2003).Inthisstudy,wedemonstratedthatshallowgashydratesatSites1248to1250havemolecularandisotopicpropertiessimilartogaswithinHorizonA.Integrationofavailablegeological,geophysical,andgeochemicaldataallowustoproposethefollowingsce-nariofortheformationofthegashydrateaccumulationatSouthernSummit.Itappearsthatmixedthermogenicandmi-crobialfreeanddissolvedgasmigratesfromdeepaccretionarycomplexalongHorizonA.Whengasreachedthebaseofthe

GashydratesystemsatHydrateRidgeoffshoreOregon1023

GHSZandstartedtoprecipitateasgashydrateintheporespace,therelativepermeabilityofhydrate-bearingsedimentsdecreased(NimblettandRuppel,2003).Asgashydratescon-tinuedtoaccumulate,afreegascolumnwasstratigraphicallytrappedbyupdipgashydrateaccumulationandlesspermeablemudlayersoverlayingHorizonA.Whenthegascolumnhasreachedacertainmaximumheightthatcouldbeholdbackbythesealingcapacityofthemudlayersandgashydrates,con-tinuinggassupplycriticallyincreasedoverpressureandde-creasedtheeffectivestressofsealingmuds.GasstartedtoleakverticallyintotheGHSZthroughmudporesandhydraulicfractures,asitdoesinoverchargedhydrocarbontrapswellbeneaththeGHSZelsewhere(e.g.,Finkbeineretal.,2001).Gasinexcessofsolubilityinshallowsedimentsprecipitatedasgashydrates.Thesuggestedmechanismofoverpressurecaus-ingverticalleakageissimilartothatoutlinedbyTréhuandFlemings(2003)whosuggest,basedonseismicdata,thatthecurrentgascolumnwithintheHorizonAbelowtheGHSZmaybe110to120mthick.

Fromsonar(Johnsonetal.,2003)andseismic(Tréhuetal.,2002,2003)data,submersibleobservations(Torresetal.,2002)anddeeppenetrationsoftheGHSZatSites1250and1249,itappearsthatthemaingashydrateaccumulationattheSouthernSummithasa3Dshapegenerallyresemblingamush-room.Althoughtheexactpositionofthemainmigrationcon-duitisnotknown,itislikelylocatednearthesea?oorcarbonatepinnacle(Figs.2cand12d).Atthesamedistancefromthemigrationconduit,gashydratesappeartobemoreabundantandincludemoreallochthonousgasesintheshallowsedimentsthanindeepsediments.Theobserveddistributionofgashy-dratesmaybearesultofmorefavorablethermodynamic(Figs.12aand12b)andkineticconditionsforgashydrateprecipita-tionintherelativelyshallowsediments(SolovievandGins-burg,1997),obstructionoffreegasbelowthesea?oorgashydrateandcarbonatecap,andlateralrefocusingofgasintostructurallyhighsettings.WespeculatethatthesmallergashydrateaccumulationatSite1248islessdevelopedbecausegasmigratedfromHorizonAtothesea?oorrelativelyrecently.MuchlessC1isneededtosaturateporewaterandprecipitategashydratenearthesea?oorthanatthebaseoftheGHSZ(Fig.12c).Thismayfurtherleadtotheformationofaconically-shaped“diffusionhalo”(SolovievandGinsburg,1997)inwhichgasdiffusesawayfromthemainmigrationconduitsforlongerdistanceinshallowsedimentsthanindeepsediments.Therefore,itappearsthatthegeneral3DshapeofthegashydrateaccumulationatSouthernSummitofHydrateRidgemaybegovernedbytheexistenceofpointsourcesofgasbelowtheGHSZandvariationofthermodynamicconditionsandgassolubilityinsediments.

The3Dshapeofthegashydrateaccumulationprobablyisfurthercontrolledbyheterogeneityofrelativepermeabilityofsedimentsandbylocalstructurethatfavorsupdip(NE)migra-tionoffreegastothebathymetricsummitoftheridgeatSite1249(Figs.2cand12d).Theexistenceoffreegasbelowthesea?oorisevidencedbyobservationsofMilkovetal.(2004b)inaPCScorefrom?14mbsfatSite1249inwhichfreegasenrichedinC3?hydrocarbongasco-occurwithhighgashy-drateconcentrationsandbrines.Apparently,thegashydratecapexistsattheSouthernSummitandlimitsverticalmigrationof?uidsintothewatercolumn.

7.CONCLUSIONS

Alargenumberofmolecularandisotopicanalysesonvoidandhydrate-boundgasesfromHydrateRidge(ODPLeg204)allowustoconstraintheoriginofnaturalgasesandtheirmixingduringthemigrationthroughthesedimentsectionalongspeci?cpathways.Themain?ndingsofthisstudyaresumma-rizedbelow:

1.ShallowhighlyconcentratedgashydratesattheSouthernSummitcharacterizedbysigni?cantgas?uxatthesea?oor(Sites1248–1250)arerelativelyenrichedinC2?gases.Theycrystallizepredominantlyfromhydrocarbongaseslikelygeneratedasdeepas2to2.5kmbelowthesea?ooralongpermeableconduitssuchasHorizonA.Althoughthehydrate-boundgasislargelymicrobialC1,includedC2?gasesaremainlythermogenicinorigin.TheenrichmentofC3insomeofthegashydratesamplesmaysuggestthatsmallamountsofsIIgashydratesaremixedascrystalintergrowthswithlargelysIgashydratesintheshallowsediments.

2.DeepdispersedgashydratesatSouthernSummit(Sites1248–1250)andinthesurroundinglowergas?uxarea(Sites1244–1247)containonlyC1,CO2andminorC2andthusmaybesIhydrates.Theycrystallizemainlyfrominsitugeneratedmicrobialgas,althoughsomegashydratesneartheSouthernSummitandinfaultedareasofSites1244and1245maycontainaminorfractionofthermogenicgasmigratedfromgreaterdepth.

3.H2Singashydratesispresentonlyinlowconcentrations(0.007–1.2%)inafewsamplesrecoverednearthesea?oor.4.The12C-enrichedCO2maybeaddedtotheexistinghydrate-boundCO2asaresultofanaerobicoxidationofhydrate-boundCAlthoughSO2?

1.4

mayfacilitatethisprocessnearthesea?oor,wespeculatethatotherelectronacceptors(e.g.,Fe3?,S0)maybeinvolvedinthereactionindeepersedi-ments.

5.ItappearsthatgasissuppliedfromHorizonAthroughself-createdpermeablepathwaysasoverpressuredgasde-stroysthesealingcapacityofoverlayingmuds.Gashydratedistributionandconcentrationwithinthesedimentsiscon-trolledby(1)thermodynamicconditionsandlocalsolubilityofgasfavoringnucleationandgrowthofgashydrateandwiderlateraldiffusionofgasinrelativelyshallowsedi-ments;(2)trappingoffreegasbelowthesea?oorgashydratecapandrefocusingitalongrelativelypermeablehorizons;(3)localstructuralsettingthatfavorslateralmi-grationofhydrocarbonstothecrestofthesummit(anticline).Severalissuesremainunresolvedandshouldbeaddressedinfutureresearch.Forexample,itisnotclearwhylessthermo-genicC2?gasispresentatSite1250locatedclosertotheassumedmainmigrationconduitrelativetothemoredistantSite1249.Heterogeneityofrelativepermeabilityinsedimentsin3Dspacemayexplainthisobservation,andin-depthsedi-mentologicalandstructuralstudiesareneededtobetterunder-stand?uidmigrationwithintheSouthernSummit.Thepro-posedgasmigrationmechanismfromHorizonAintotheshallowsedimentsshouldbevalidated.Timinganddynamicsofgasmigrationandgashydrateformationwithrespectto

1024A.V.Milkovetal.

structuraldevelopmentofHydrateRidgeandtheunderlyingaccretionarycomplexarenotaddressedhereandareanimpor-tantdirectionoffuture3Dbasinmodelingefforts.

Acknowledgments—ThisresearchuseddataprovidedbytheOceanDrillingProgram(ODP).TheODPissponsoredbytheU.S.NationalScienceFoundation(NSF)andparticipatingcountriesundermanage-mentofJointOceanographicInstitutions(JOI),Inc.FundingforthisresearchwasprovidedbytheU.S.ScienceSupportProgram.WeacknowledgecommentsbyothermembersoftheLeg204scienti?cparty.WethankT.Lorenson,J.Greinert,ananonymousreviewerandtheassociateeditorR.Burrussforusefulsuggestions.Associateeditor:R.C.Burruss