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Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
Evolution of the atmosphere between 3.9 and 1.8 Ga can be related to changes in physical, chemical and biological surface processes. Changes in surface processes, in turn, took place in response to growth of continents. Growth of stable continents favoured expansion of epeiric seas and resultant increase in oxygen productivity. Prior to 2.6 Ga, continents probably comprised less than 10% of the Earth’s crust and, as a consequence, physical sedimentation and volcanism, and chemical and biological sedimentation took place mainly in oceanic and island-arc settings. Following major crustal growth at 2.6 Ga, surface processes became dominated by widespread stromatolite growth and iron and manganese precipitation in vast epicontinental seas.
Prior to 2.6 Ga, oxygen was consumed mainly in ocean-crust alteration. Following growth of large continents, the ocean crust oxygen reservoir decreased in scale freeing oxygen for the second major reservoir, namely reduced iron and manganese. Only after oceanic sinks were utilized did oxygen escape to the atmosphere. This was taking place on a large scale by 1.8 Ga at which time the atmosphere had an oxidation state necessary for formation of continental red beds and palaeosols.
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