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Geologic evidence for Archean atmospheric and climatic evolution: Fluctuating levels of CO₂, CH₄, and O₂ with an overriding tectonic control

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA

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.

Key Words: Archean • carbon dioxide • methane • atmosphere • greenhouse

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