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