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

Abstract

The 3.5-3.2 Ga sedimentary record shows evidence of surface temperatures of 70 ′ 15 °C, nahcolite (NaHCO 3 ) 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 CH 4 and CO 2 atmospheric greenhouse in which CH 4 /CO 2 ratios were «1 and pCO 2 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 CO 2 through weathering. By 2.9-2.7 Ga, declining pCO 2 was associated with climatic cooling and siderite-free soils. Transitory CH 4 /CO 2 ratios of ∼1 may have resulted in the sporadic formation of organic haze from atmospheric CH 4 , reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 C-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 CO 2 due to weathering, and continued long-term tectonic recycling of CO 2 resulted in rising pCO 2 and decreasing CH 4 /CO 2 ratios in the later Archean and eventual reestablishment of a mainly CO 2 greenhouse. Similar events may have been repeated in the latest Archean and earliest Proterozoic, but gradually rising production of O 2 effectively kept CH 4 /CO 2 ratios to «1 at this time.