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1、Geobiology(2005),3,53–76Acoupledatmosphere–ecosystemmodeloftheearlyModelingtheArcheanbiosphereBlackwellPublishing,Ltd.ORIGINALARTICLEArcheanEarthP.KHARECHA,1,3J.KASTING1ANDJ.SIEFERT21DepartmentofGeosciencesandAstrobiologyResearchCenter,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA2Departm
2、entofStatistics,RiceUniversity,Houston,TX77251,USA3NASAGoddardInstituteforSpaceStudies(ColumbiaUniversity),NewYork,NY10025,USAABSTRACTAcoupledphotochemical-ecosystemmodelhasbeendevelopedtosimulatetheearlyArcheanbiosphere.Themodelincorporateskineticandnutrientlimitationsonbiologicalproductivity,alo
3、ngwithconstraintsimposedbymetabolicthermodynamics.WehaveusedthismodeltopredictthebiogenicCH4fluxandnetprimarypro-ductivity(NPP)ofthemarinebiospherepriortotheadventofoxygenicphotosynthesis.Organismsconsideredincludechemotrophicandorganotrophicmethanogens,H2-,H2S-,andFe-usinganoxygenicphototrophs,S-r
4、educingbacteria,CO-usingacetogens,andfermentativebacteria.CH4productionandNPPinourmodelarelimitedbythedownwardfluxofH2,CO,S8,andH2Sthroughtheatmosphere–oceaninterfaceandbytheupwellingrateofFe2+fromthedeepoceans.Forreasonableestimatesofthesupplyratesofthesecompounds,wefindthatthebiogenicCH4fluxshouldh
5、averangedfromapprox-imately1/3to2.5timesthemodernCH4flux.IntheanoxicArcheanatmosphere,thiswouldhaveproducedCH4concentrationsof100ppmvtoasmuchas35000ppmv(3.5%),dependingontherateatwhichhydrogenescapedtospace.RecentcalculationsindicatingthathydrogenescapewasslowfavourthehigherCH4concen-trations.Calcu
6、latedNPPislowerthaninthemodernoceansbyafactorofatleast40.Inourmodel,H2-basedmetabolismismoderatelymoreproductivethanFe2+-basedmetabolism,withS-basedmetabolismbeingconsiderablylessproductive.Internalrecyclingofsulphurwithinthesurfaceoceancouldconceivablyraiseratesofsulphurmetabolismbyafactorof10hig
7、herthanthevaluespredictedbyourmodel.Althoughexplicitclimatecalculationshavenotbeenperformedhere,ourresultsareconsistentwiththeideathattheArcheanclimatewaswarm,andpossiblyveryhot.Someormostofourecosystemscenariosa
