Milkov-2005-Gas hydrate systems at Hydrate Rid(5)

1016A.V.Milkovetal.

Fig.6.Variationofmolecularandisotopicpropertiesofvoidgases(opencircles)andhydrate-boundgases(solidsquares)fromSites1244(a),1245(b),1246(c),1247(d),1248(e),1249(f),1250(g),1250(h).ThedepthsofBSR,BSR2,HorizonsAandY,andthecalculatedbaseoftheGHSZ(Table1)areindicated.

GashydratesystemsatHydrateRidgeoffshoreOregon1017

Fig.6.(Continued)

gashydratesimplythatgashydratesinthedeepesthorizonsneartheBSR,evenatSiteswithsea?oorseepage(e.g.,Site1250),crystallizefromrelativelypuremicrobialgases(C1andC2)whilegashydratesatshallowerdepthscrystallizefromgasthatisatleastpartiallythermogenic.Theserepresentthetwoend-membergashydratesystems.Italsoappearsthatsomegashydratescrystallizefromamixtureofmicrobialandthermo-genicgases.Forexample,the?13CofC2inhydratesatSite1245decreaseswithdepth(Fig.6b)suggestingthatshallowhydrateshaveslightlyhigherportionofthermogenichydrocar-bonsthanthedeeperhydrateseventhoughthereisnosea?oorseepage.Similarly,thesamplefrom65mbsfatSite1244has?13CofC2around?43‰,which,inthecontextofothersamplesfromdeepsediments,impliesthatgashydratecontainsatleastsomethermogenicC2.Apparently,HydrateRidgeis

characterizedbyacomplex?uidmigrationsystem,whichincludesbothmigrationofthermogenicgasesfromdeepsedi-mentsorreservoirsandlocalgenerationofmicrobialgases.Onlyonegashydratefrom7.4mbsfatSite1248Bcontainedenoughi-C4andn-C4tomeasure?13C(?26.2‰and?23.0‰respectively,Table3)consistentwiththethermogenicoriginofthesegases(Clayton,1991).

5.2.3.CarbonIsotopicCompositionofCO2

Carbondioxideinmostgashydrateshas?13Cvalues(Figs.5fand8e)similartothoseintheadjacentgasvoids(Tables3andEA-1,Fig.6).TheCO2inmostgashydrateshassourcessimilartothatingasvoidsanditsisotopicpropertiesdependonthefactorsthatcontroltheisotopiccompositionof

dissolved

1018A.V.Milkovetal.Fig.7.ComparisonofcarbonisotopiccompositionofCO2andthe

C1/CO2ratioingasvoidsandgashydratesfromSite1248.Notethat

gashydratesingeneralarerelativelydepletedinCO2

.

CO2inshallowsediments.Athigh?uxSites1248to1250,the

hydrate-boundCO2maymigratefromgreatdepthstogether

withpartiallythermogenichydrocarbons.Nevertheless,some

samples(15of42)areenrichedin12Crelativetovoidgases

(Fig.6).Fourofthesesamplesarenearshallowdepthswhere

methaneisoxidizedviasulfatereduction(Boetiusetal.,2000).

ThesesamplescontainCO13

30‰(Site2with?Cvaluesasnegativeas

?121248).However,mostgashydratesamplesthatcontainC-enrichedCO2arefromdepthswhereanaerobic

oxidationofC1shouldnotbeoccurring.

Sassenetal.(1998)observedsigni?cantenrichmentofhy-

drate-boundCO2in12C(with?13Cvaluesasnegativeas

?27.8‰)ingashydrateoutcropsintheGulfofMexico.These

authorssuggestedthattheenrichmentisaresultofmicrobial

oxidationofhydrate-boundmethane.Wespeculatethatthis

explanationmaybevalidforourobservationsatHydrateRidge

whereanextensivemicrobialcommunityoxidizingmethaneis

knowntooccurnearthesea?oor(Boetiusetal.,2000;Knittel

etal.,2003).The12C-enrichedCO2istheonlygasgeochemical

evidencepointingtothemicrobialoxidationofmethane.The

hydratesamplesthatmayhaveexperiencedmicrobialoxidation

basedoncarbonisotopesofCO2donotshowsigni?cantly

higherCO2concentration(Fig.9e)ormorepositive?13CofC1

(Fig.9f)relativetonon-oxidizedsamplesasmaybeexpected

(Kastneretal.,1998;Sassenetal.,1998).Thismaybeex-

plainedbylessextensivemicrobialoxidationordifferentstart-

ingmaterialthanpurelythermogenicgashydratesitesstudied

intheGulfofMexico.Alternatively,the12C-enrichedhydrate-

boundCO2couldbearesultofcontaminationduringdecom-

positionofbicarbonateintrappedandsurroundingporewaters.

Itiscurrentlythoughtthatsigni?cantmicrobialoxidationof

CwithabundantSO2?1islimitedtoshallowsediments4that

providestheelectronacceptorneededtofacilitatetheanaerobic

oxidationofC1(Valentine,2002).AtHydrateRidge,hydrate-

boundCO2isenrichedin12CrelativetoadjacentgasvoidsbyFig.8.Histogramsillustratingisotopicproperties(?13CofC[c],C[e],and?DofC1[a],C23[d],CO21[b])ofhydrate-boundgasescollectedonLeg204.asmuchas15‰indeepSO2?4-freesedimentsatSiteswithnosigni?cantsea?oorseepage(e.g.,samplefrom65mbsfatSite1244).LorensonandCollett(2000)alsofound12C-enrichedCO2ingashydratefromadepthof331mbsfatSite997

on

GashydratesystemsatHydrateRidgeoffshoreOregon1019

Fig.9.Relationshipsbetweenthekeymolecularandisotopicpropertiesofhydrate-boundgasesfromHydrateRidge.Arrowsshowtheincreasingcontributionofthermogenicgasesasfollowsfromincreasesinvaluesof?13CofC1(a,b,d),?13CofC2(c,d),?DofC1(a),andconcentrationofC2(c),anddecreaseinratioC1/C2?(b).Somegashydratesmayhavehydrate-boundC1partiallyoxidizedtoCO2,althoughmostgashydratesdonotshowagoodcorrelationbetween?13CofCO2andtheconcentrationofCO2(e)or?13CofC1and?13CofCO2(f).

BlakeRidge.Theseobservationsmaysuggestthathydrate-boundC1isactivelyoxidizedevenwheresulfateiscompletelydepleted.Itispossiblethatsomemicrobesuseotherelectronacceptors(Fe3?orS0)thatoxidizeC1underanoxicconditions.Thismechanismissupportedbytheobservationofthemeta-?2?2?

stablesul?demineralgreigite(Fe3S4)associatedwith2Fe

highconcentrationofdisseminatedgashydratesinpores(Mus-graveandHollamby,2003).Morespeci?cstudiesareneededtoelucidatethemechanismsandreactionsinvolvedinthepossi-bleoxidationofC1indeepmarinesediments.

5.3.CompositionandOriginofGasesinHorizonAand

TheirRelationtoShallowGasHydratesTréhuetal.(2002)suggestedthattheprominentseismicHorizonA(Fig.2)mightbeassociatedwithamigrationcon-duitthatsupplies?uidsintotheGHSZandmayfeedseepageattheSouthernSummitofHydrateRidge.ThishorizonwasdrilledonLeg204(Sites1245,1247,1248,and1250)andwasfoundtorepresentarelativelypermeablelayerofsandandashwithsigni?cantgassaturation(Tréhuetal.,2003).VoidgasescollectedfromHorizonAandadjacentsedimentscontainsig-ni?cantC1(mean96.15%)andCO2(mean2.35%)andminoramountsofC2?hydrocarbons(Table4,Fig.10a).BecausegasesmigratingalongHorizonAareapparentlyenrichedinC2?gasesrelativetosedimentssurroundingthishorizon(Figs.6b,6d,6e,and6g),itsuggeststhatgasesmigratingalongHorizonA?ndtheirwaytothesea?oorandformtheshallowgashydrateaccumulationatSites1248to1250.

ThecarbonisotopiccompositionofgasfromHorizonA(Table4andFig.10b)indicatesthatalthoughmostorallC2?gaseshaveapredominantlythermogenicorigin,C1maybemainlymicrobial.Chungetal.(1988)proposedtoextrapolatethecarbonisotopecompositionofC2,C3andC4toestimatethe?13CofpurethermogenicC1andtocalculatethecontributionofmicrobialmethanetothemixedgases.Usingthismethod,weestimatethatthermogenicC1thatiscogeneticwiththeC2?gasesfromHorizonAshouldhavea?13Cvaluearound?45‰(Fig.10b).Moreover,theprojectedcarbonsourceoftheC2?gasesmayhave?13Cvaluesbetween?15‰and?20‰,whichisconsistentwithmarineorganicmatterphotosynthe-sizedduringpost-OligoceneperiodsofloweredatmosphericCO2(Arthuretal.,1985;Deanetal.,1986).Kineticmodels(e.g.,Rooneyetal.,1995;BernerandFaber,1996;Tangetal.,2000)furthersuggestthatgaswiththeobservedisotopiccom-positionofC2-C5wasgeneratedattemperaturesintherange120to140°C(assumingthatgasisgeneratedfromTypeIIkerogeninMioceneoryoungermarineshalesatheatingrates1to5°C/Ma).IfthegeothermalgradientatHydrateRidgeis?55°C/km(Table1,Tréhuetal.,2003),thenmostoftheC2?gasesandsomeC1encounteredinHorizonAweregeneratedasdeepas?2to2.5km.ThesegaseshavemixedwithmicrobialC1duringmigrationtoshallowsediments.ThecarbonisotopiccompositionofC1migratingwithinHorizonA(?13CofC1isperhapsaround?61‰)andtheestimatedisotopic

composition

1020

A.V.Milkovetal.

)

‰(14

C91fo–D?2

O673614

......C410125–5

8

96

C...523-i222)

–––‰,C34

91770

1C.....?23926-(n22122n–––––oitisop4

874866

mC......555656-oi222222––––––ccipot403620

o3

......siC465657222221n––––––obraC194660

2

......C909001232333––––––238847

1

......C155441666666––––––2OC924166245330/11C?2C477078/90222612411CS

2ddddddHnnnnnn2

211832

O445823C337069......3213205

3Cdddd1d

-nnnnn0.n0)

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