Geologic evidence for Archean atmospheric and climatic evolution: Fluctuating levels of CO2, CH4, and O2 with an overriding tectonic control
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abstract
The 3.5-3.2 Ga sedimentary record shows evidence of surface temperatures of 70 15 C, nahcolite (NaHCO3) as a primary evaporitic mineral, and an aggressive weathering regime even in the absence of land vegetation. These features are best explained by a mixed CH4 and CO2 atmospheric greenhouse in which CH4/CO2 ratios were 1 and pCO2 was at least 100-1000 times the present value and perhaps as high as several bars. The formation of large areas of continental crust at 3.2-3.0 Ga, including the Kaapvaal and Pilbara cratons, resulted in the gradual depletion of atmospheric CO2 through weathering. By 2.9-2.7 Ga, declining pCO2 was associated with climatic cooling and siderite-free soils. Transitory CH4/CO2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric CH4, reflected in one or more isotopic excursions involving global deposition of abnormally 13C-depleted organic C. Surface temperatures of <60 C after 2.9 Ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes. Eventual lowering of new continental blocks by erosion, reduced loss of atmospheric CO2 due to weathering, and continued long-term tectonic recycling of CO2 resulted in rising pCO2 and decreasing CH4/CO2 ratios in the later Archean and eventual reestablishment of a mainly CO2 greenhouse. Similar events may have been repeated in the latest Archean and earliest Proterozoic, but gradually rising production of O2 effectively kept CH4/CO2 ratios to 1 at this time. 2004 Geological Society of America.
