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1、AnearlyArchaeandeep-watermicrite-BIF:Implicationsforbiogeochemicalcycling,andoriginofbanded-ironformationJelteHarnmeijer1,2,*September20091CenterforAstrobiologyandEarlyEarthEvolution,2DepartmentofEarth&SpaceSciences,UniversityofWashington,Seattle,WA,98195-1310,U.S.A.*E-mail:jel
2、te@u.washington.edu.Phone:+1-206-543-9419.Keywords:banded-ironformation,BIF,dolomite,ArchaeanAbstractDeepwatersedimentarycarbonates,andpelagicmicriticprecipitatesinparticular,arenotablyscarceintheArchaean.Here,wedescribeanewlydiscoveredbasinal~3.52Gainterlaminatedmagnetite-carb
3、onateoutcropfromthePilbara’sCoonterunahSubgroupinWesternAustralia.Fieldandtexturalfeaturespointtopelagicprecipitationofmicriticcalciumcarbonate,withtraceelementconcentrationsindicatingacalciticprecursor.Thermodynamicconsiderationsruleoutasideriteprecursortomagnetite.Theintroduc
4、tionofmixedsub-aeriallyeruptedandesitic-rhyolitictuffallowedforaverydifferentflowregimeininterstitialwatersofthisquiescentbasin,formerlydominatedbybasalticflowvolcanismandhydrothermalism,andresultedindepositionofa>35mthickstackofhighlypermeabletephra,thermallyinsulatedandadvect
5、ivelyisolatedbyathoroughlysilicifiedbasecappingtheunderlyingbasalticpile.TheinteractionofambientArchaeanseawaterwiththeseacidicvolcanicsenabledmimeticdolomitizationwhileinhibitingcharacteristicArchaeansilicificationbymafic-derivedsiliceoushydrothermalfluidsbeneaththesediment-wa
6、terinterface.MicriticlaminaesampleastratifiedArchaeansurfaceoceanbearingd13surfaceDIC=-3.0±1.0‰,whilecarbonate-hostedkerogensamplesapelagicbiotabearingd13surfaceorg=-26.1±2.4‰.Thishighdegreeofcarbonfractionation,togetherwithalkalinity-inducedpelagicprecipitation,andtheoxidation
7、ofmagnetiteoritsdiageneticprecursor,hintatthedominanceofoxygenic-overferrous-iron-photosynthesizerssurprisinglysoonafterthecurtainroseoverorganicevolutionearlyinEarth’shistory.Inthisscenario,seasonaldestratificationexposedoxygenatedalkalinesurfacewaterstounderlyingiron-saturate
8、dacidicwaters,callinganabrupthalttocarbonateirrigation
