| The chemical evolution of Earth's early atmosphere is a subject of continuing debate. I have used chemical and mineralogical data from fossil weathering profiles, paleosols, to constrain atmospheric PO2 and atmospheric PCO2 during the late Archean and Paleoproterozoic. Every definitely identified pre-2.44 Ga paleosol as well as the 2.245 to 2.203 Ga Hekpoort paleosol suffered significant Fe loss from the top portion of the soil during weathering. This loss indicates that atmospheric PO2 was less than 5 x 10-4 atm at all times prior to 2.2 Ga. Iron loss was negligible during formation of the 2.2 to 2.0 Ga Wolhaarkop paleosol and during formation of all paleosols that formed as a result of subaerial weathering after 2.0 Ga. Thus, atmospheric PO2 probably has been ≥ 0.03 atm since sometime between 2.2 and 2.0 Ga.; Much of the Fe released from the top portions of pre-2.2 Ga weathering profiles was reprecipitated in their lower portions during weathering. The distribution of Fe, Mg, Mn, Co, and Ni within the Hekpoort paleosol indicates that these elements were incorporated into pedogenic smectites as this soil formed. The composition of chlorites in other paleosols also indicates that Fe reprecipitated as a constituent of smectite. The formation of smectite rather than siderite as these paleosols developed indicates that PCO2 was less than 2 x 10-2 atm prior to 2.2 Ga. One-dimensional cloud-free climate models suggest that other atmospheric greenhouse gases contributed to a net greenhouse effect equivalent to PCO2 > 10-1 atm during the late Archean in order to prevent massive glaciation.; Isotopic and chemical analyses indicate that the metamorphosed remains of methanotrophic organisms are preserved in sericitic material near and at the top of the 2.765 +/- 0.01 Ga Mt. Roe #2 paleosol in Western Australia. The presence of methanotrophs at the atmosphere-lithosphere interface during the late Archean is consistent with atmospheric PCH4 greater than 100 m atm. Such high levels of methane would have compensated for the difference between the maximum greenhouse forcing due to the presence of CO2 at 2.765 Ga and the apparent minimum greenhouse forcing required to maintain a glacier-free Earth at that time. |
