Rise of oxygen induced by Paleoproterozoic snowball glaciation: Insights from biogeochemical cycle modeling

Earth's atmosphere is considered to have been oxidized in early Paleoproterozoic ∼2.45-2.22 billion years ago. Geological and geochemical studies suggest that the oxidation occurred immediately after the Paleoproterozoic snowball glaciation based on the global appearance of proxy indicators for high levels of atmospheric oxygen preserved in sediments deposited after the glaciation. Accordingly, it has been speculated that the global warming in the glacial aftermath would have enhanced nutrient supply to the ocean via chemical weathering, which leads to a cyanobacterial bloom. Although this proposed scenario is qualitatively convincing, there have been no study to assess the scenario quantitatively. Here we developed an atmosphere-ocean biogeochemical cycle model and assessed the perturbation caused by the Paleoproterozoic snowball glaciation, in the aim of estimating the impact of such a large-scale glaciation on the redox state of earth's surface. Biogeochemical cycle model experiments demonstrate that high atmospheric CO₂ levels and consequent high surface temperature (∼ 0.7 atm and 320 K, respectively) in the glacial aftermath enhanced the global weathering rate on the order of 10 times higher than that of today. Assuming the continental nutrient flux to the ocean is proportional to the global weathering rate, the global biological productivity increases by an order of magnitude compared to the present level. We found that the atmospheric oxygen level rises to 0.01 PAL (present atmospheric level) rapidly after the glaciation (e.g., within 10³ years), then reaches ∼1 PAL owing to high levels of biological productivity sustained by greenhouse conditions. Eventually, the oxygen level decreases to a stable level around 0.01 PAL. We also found that calcite precipitation is prevented in the ocean during the first 10⁵ years after the glaciation. Carbonate minerals precipitated from seawater may record carbon isotope ratio of 2-8‰ by the long-lasting, high levels of biological productivity. We conclude that deglaciation from the large-scale Paleoproterozoic glaciation may have caused a rapid and great shift in the redox conditions of Earth's atmosphere and ocean, rising atmospheric O₂ levels from < 10^-5 PAL to > 0.01 PAL. We suggest that the deposition of iron and manganese oxides from seawater would have occurred immediately after the glaciation, preceding cap carbonate deposition (10⁵ years after the glaciation), which has positive carbon isotope ratios. This proposed scenario in the aftermath of glaciation is consistent with the reported geological records of the Paleoproterozoic glaciations in South Africa and North America.