Milkov-2005-Gas hydrate systems at Hydrate Rid(6)

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BiS24451222111Fig.10.Comparisonofmolecular(a)andcarbonisotopic(b)prop-ertiesofgashydratesfromSite1248andvoidgasessamplednearHorizonAatSites1245and1248.Thecarbonisotopicpropertiesofgasesarepresentedintheformofa“naturalgasplot”ofChungetal.(1988),wherenisthenumberofcarbonatomsofindividualgasmolecules.

ofthethermogenicmethanecomponent(?13CofC1is?45‰)indicatethatthegasinHorizonAwasdilutedbyasigni?cantportion(?80%)ofthemicrobialC1duringmigrationfromgreatdepths.

ThecomparisonofgascompositionfromHorizonAdrilledatSites1248and1245andshallowgashydratesfromSite1248suggeststhatmostgashydratesaredepletedinC2?gasesrelativetoHorizonA(Fig.10a).Thegashydratesamplefrom7.4mbsfatSite1248Bhasvalues?13CofC1-C4thatarealmostidenticaltothevaluesinthegasfromHorizonA(Fig.10b),suggestingsimilarsources.The?13CofC3inothergashy-dratesfromSites1248to1250issimilarto?13CofC3fromHorizonA(Figs.10band11)andsupportsthatshallowgashydratesatSites1248to1250alsocrystallizefromgasmi-gratedfromHorizonAtothesea?oor.MostshallowgashydratesfromSites1248to1250haveC1andC2enrichedinlightisotope12CrelativetoC1andC2inHorizonA,butthe

Table4.MolecularandisotopicpropertiesofvoidgasesfromsedimentsnearHorizonA.

GashydratesystemsatHydrateRidgeoffshoreOregon1021

carbonisotopiccompositionofC3issimilar(Fig.11).This

suggeststhatwhengasesfromHorizonAmigratetothesea-

?oor,theyaremixedwithshallowmicrobialC1andC2,but

thereisverylittle(ifany)microbialC3inshallowsediments.

6.SYNTHESIS

6.1.GasHydrateSystems

Ourresultsshowthatmolecularandisotopicpropertiesof

gashydratesvaryintheHydrateRidgearea.Allavailablegas

geochemicaldataareconsistentwiththecrystallizationofgas

hydratefromtwodifferentgassourcesandtheirmixingin

variousproportions(Figs.9a–9d).Oneend-membergassource

isadeeppetroleumsystemsupplyingrelativelymaturether-

mogenicC1-C5andCO2gasesintotheGHSZ,andtheother

sourceismicrobialgenerationofC1,C2,andCO2.Gas?uxinto

theGHSZ,?uidmigrationpathwaysandtheratesofmicrobial

processesultimatelycontrolthedistributionandconcentration

ofgashydratesinthestudiedareaofHydrateRidge.

MostrecoveredgashydratesarefromSites1248to1250in

theareaofhighsea?oorre?ectivity,ventingofhydrocarbon

gases,abundantsea?oorcarbonatesandchemosyntheticcom-

munities(Suessetal.,1999;Sahlingetal.,2002;Torresetal.,

2002).Highconcentrationsofgashydrate(upto43%of

porespace)werefoundtoco-existwithhypersaline(salinity

?105gkg?1)porewaterandfreegasinthisareaat14mbsf

atSite1249(Milkovetal.,2004b).Theshallowgashydrates

fromtheseSitescontainmixedmicrobialandthermogenicC1

andC2andmostlythermogenicC3?hydrocarbongases.

ThisFig.11.VariationofcarbonisotopicpropertiesofC2andC3fromgashydrates(Sites1248–1249)andgasvoidsnearHorizonA(Sites1245,1247,1248,and1250).Thevariationof?13CingasesfromHorizonAisaresultofvariousmaturityandperhapsmoderatebio-degradation,whichaffectsmostlyC3.NotethatallbutonegashydratesampleshaveC2signi?cantly(by?5‰)enrichedin12C.ThisisinterpretedasaresultofadditionalcontributionofmicrobialC2tohydrate-forminggasesduringmigrationfromHorizonAtothesea-

?oor.

Fig.12.Gashydrateformationtemperature(a),pressure(b),andC1concentration(c)(solidlines)incomparisontoobservedconditions(brokenlines)andtheconceptualmodelofgashydrateandfreegasoccurrence(d)atHydrateRidge.Thetemperaturepro?le(a)isconstructedbasedondatafromHeeschenetal.(2003)(watercolumn)andthegeothermalgradient55°C/km(subbottom).Thepressurepro?le(b)assumesgenerally,hydrostaticpressureinporespace,andsomeoverpressureinthegascolumnwithinHorizonA(TréhuandFlemings,2003).Equilibriumtemperature(a)andpressure(b)arefromSloan(1998)andassumethathydrate-forminggasis100%C1andporewatersalinityis3.5wt.%.Theoreticalmethanesolubilitypro?le(c)isconstructedbasedontheapproachofXu(2002)andtheschematicobservedpro?lesaregenerallyconsistentwithmeasurementsofMilkovetal.(2003).Themodelofgashydrateandfreegasdistributioninsediments(d)isbasedonavailableinformationfromLeg204andprevioussea?oorcoringandseismicstudies,butshouldberegardedasconceptual.Distributionoffreegasandhydrate-coatedgasbubblesinthewatercolumnisinferredfromthestudyofHeeschenetal.(2003)andtheoccurrenceoffreegaswithintheGHSZisbasedontheevidencepresentedbyMilkovetal.(2004b).Arrowsindicatemigration(advectionand/ordiffusion)ofallochthonousgasesinsubsurface.TherelativelocationofSites1244,1246,1249and1250isapproximate(seeFig.1forexactlocation)andthereisnohorizontalscale.

1022A.V.Milkovetal.

gashydratesystemisrelatedtothe?uxofallochthonousmixedmicrobialandthermogenicgasesfromthedeepaccretionarycomplex(Fig.12d).Isotopicdatasuggestthatthethermogenicgasesweregeneratedasdeepas2to2.5kmandmixedwithmicrobialgasduringmigrationintoandalongHorizonA.Inmostgashydratesamples,additionalmicrobialC1andC2wereaddedwithintheGHSZ.ThetotalportionofmicrobialC1inshallowgashydratesmaybe80to85%,butmostofthisC1wasprobablyexsolvedfromaccretedsedimentsandonlyaminoroverprintistheinsituC1currentlybeinggeneratednearthegashydrates.Incontrast,mostoftheC2(?80%)andC3(almost100%)inthegashydratesattheSouthernSummitismigratedthermogenicgas.Weconcludethatthehighgashydratecon-centrationsattheSouthernSummitofHydrateRidgearere-latedtorelativelyrapidmigrationofgasfromgreaterdepthsintotheGHSZ.IncontrasttosomeotherstructuralgashydrateaccumulationsintheGulfofMexico(Sassenetal.,1998)ortheCaspianSea(GinsburgandSoloviev,1998),gashydratesattheSouthernSummitofHydrateRidgecontainonlyminoramountsofthermogenicgases.

Nogashydratewasrecoveredatdepth?40mbsfatSite1249,anddeepsedimentsatSites1248and1250hostrelativelylowamountsofgashydrate(0.5–2%ofporespace,Milkovetal.,2003).AtSite1248,deepgashydratesappeartocontainamixtureofinsitugeneratedmicrobialC1andC2andalloch-thonousgaseswiththermogeniccontribution.AtSite1250,deepgashydratescontainmostlymicrobialgasthatwaslikelygeneratedinsitu.Itappearsthatthebulkofdeepgasesmigrat-ingalongHorizonAbypassdeepsedimentsatSite1250butsaturatetheshallowsediments(Figs.2cand12d).Thisobser-vationimpliesthattheverticalmigrationconduitsarerelativelynarrowandperhapsarerestrictedtoacousticallyincoherent(wipeout)zonesandhigh-re?ectivityzonesobservedonseis-micpro?les(Tréhuetal.,2003).Sites1249and1250weredrilledintheareaofashallowhigh-re?ectivityzone,whileSite1248sampledthesedimentwithinandbeneathaverticalwipe-outzone.(Tréhuetal.,2003,Figs.2band2c).ThepatternofC2?hydrocarbonsinvoidgasesandgashydratesindicatesthat,inadditiontovertical?ow,lateralmigrationofgasoccursandresultsinamoreextensivearealdistributionofgashydratesinshallowsedimentsthanindeepsediments(Fig.12d)asdis-cussedbelow.

AtSites1244to1247,gashydratesoccuratdepths?40to50mbsf.DirectmeasurementsofmethaneconcentrationattheseSitessuggestthatgashydrateconcentrationsarerela-tivelylow(average?2%ofporespacewithintheGHOZ)(Milkovetal.,2003)andsomeintervalswithintheGHOZdonotcontaingashydrates(Milkovetal.,2004c).Theseobser-vationstogetherwiththelackofsea?oorhydrocarbonseepageandrelativelydeep(8–12mbsf)sulfate-methaneinterfaceattheseSites(Tréhuetal.,2003)indicatethatgas?uxissignif-icantlylowerrelativetotheSouthernSummit.Gasgeochemi-caldatasuggestthatthesegashydratescrystallizefrommostlymicrobialC1andC2.Thesourcesofthesemicrobialgasesmaybedifferent.AtSites1244and1245,theshallowestgashy-drateshave?13CofC2around?45‰,whereasdeepergashydratesatSites1246and1247have?13CofC2around?52‰(Fig.5b).Thisobservationmayindicatethatgashydratesys-tematSites1246and1247isrelatedmainlytotheinsitugenerationofmicrobialgases(Fig.12d).GashydratesatSites

1244and1245maycontainsomeallochthonousgasmigratedfromthedeepersediments(Fig.12d).Differentstructuralset-tingsmayexplaindifferentgassourcesattheseSites.Site1244islocatedinthefaultedandfracturedareaoftheupliftedaccretionarycomplex(Fig.2a),whichissuitableformigrationofhydrocarbonsfromthedeepsediments.Site1245penetratesHorizonsYandY=(withintheGHSZ)whichappeartobelesspermeablethanHorizonAbutstillmayfacilitatethemigrationofgasfromtheaccretionarycomplex(Figs.2aand2b).Stress-inducedboreholebreakoutsfoundatSites1244and1245(Goldbergetal.,2003)alsoindicatethattheseSitesarelocatedinatectonicallyactiveareamorefavorablefor?uidmigrationfromdeepconduits.

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