TheEarth’sfirstatmosphere
AstheEarthformed,inadditiontorockysilicatesandmetalliciron,impactingbodiesemplacedvolatilematerialssuchascarbondioxide,hydrogenandhydrogencompounds(methaneandammonia),andnitrogeninthemantle.TheEarth'sfirstatmospherewasformedbyoutgasing;thevolatilegaseswerereleasedfromthemantleviavolcanicactivity
Theearlyatmospherewasprobablyrichincarbondioxide,nitrogen,methane,ammonia,hydrogen,andwatervapor;thisreducingatmospherewasdevoidoffreeoxygen(O2)Theoceanscondensedfromthewatervaporthatoutgasedfromthemantle-thecarbondioxiderichoceanswereveryacidic(carbondioxide+water=carbonicacid)
Thephotodissociationofwatervapormayhaveproducedsomefreeoxygen.Hydrogenescapedintospace;carbondioxidewasusedbyphotosyntheticorganisms;oxygenwasaddedtotheatmospherebyphotosynthesis;nitrogengraduallyincreasedbysteadyoutgasingtoeventuallybecomethemostabundantgasintheatmosphere
Photosynthesisproducedlargequantitiesofoxygenoncelifeevolvedby3.5b.y.;thiseventuallyledtotheoxidizingatmospherewithfreeoxygenthatwehavetoday
Photosynthesisproducedlargequantitiesofoxygenandremovedmuchcarbondioxidefromtheatmosphereoncelifeevolvedby3.5b.y.About3b.y.bandedironformations(BIFs)showthatfreeoxygenwasavailableperiodically,perhapsseasonally,tooxidizeandprecipitatetheabundantironcationsthatweredissolvedinoceanwater(remembertheseironcationsweretheproductofcarbonicacidweatheringsilicaterocks)
About2-1.7b.y.redbeds(hematite-cementedsandstonesandshales)startedtoform,indicatingacontinuoussupplyoffreeoxygen
About2-1.7b.y.carbonaterocks/limestonesandothercarbonaterocksbecamecommonindicatingthatoxygenhadincreasedandcarbondioxidereducedthroughphotosynthesissothatoceanacidityhaddecreasedenoughtoallowcalciumcarbonatetoprecipitate.
BandedIronFormationsaresedimentaryrocksthathavealternatinglayersofiron-richmineralsandafine-grainedsilicarockcalledchert.
Redbeds
Carbonaterocks
http://www.dailygalaxy.com/my_weblog/2008/06/ion-microprobe.html
IonMicroprobeTechnologyRevealsEarthwasHabitable4.3billionyearsago
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