MODELING DYNAMICS OF THE RISE OF OXYGEN DURING THE PALEOPROTEROZOIC: DEEP-WATER OXYGENATION AND SULFATE ACCUMULATION IN THE POST-SNOWBALL OCEAN

Atmospheric oxygen level has shifted markedly from anoxic (<10^-5 Present Atmospheric Level, PAL) to oxic (≥10^-2 PAL) during the early Paleoproterozoic (2.45-2.2 billion years ago). The rise of oxygen has been recognized as a dynamic transition from low to high oxygen steady states, associated with the extensive overshoot lasting for ~10⁸years. Geochemical studies suggest that deep-water oxygenation and accumulation of sulfate ions in the oceans occurred during this period, possibly in response to the overshoot of the atmospheric oxygen levels. Understanding mechanism and dynamics of such drastic changes in the atmosphere-ocean chemistry would be critical to reveal co-evolution of the environment and life on Earth. However, there have been no quantitative studies that account for the oxygen transition with an overshoot and associated changes in the marine chemistry.

In this study, we suggest that the Paleoproterozoic snowball glaciation was the trigger for the oxygen transition with an overshoot. Biogeochemical cycle modeling shows that super greenhouse conditions after the snowball glaciation caused high nutrient input from continents to oceans, which led to elevated organic carbon burial in the ocean. This caused a rapid jump from low to high oxygen steady states within 10⁴ years and an oxygen overshoot by up to ~1 PAL that persisted for 10⁶-10⁸ years after the glaciation. This oxygen overshoot caused long-term oxygenation of the deep ocean, which is consistent with the geochemical evidence reported from the Francevillian Group, the Republic of Gabon. In addition, the results show that the high atmospheric oxygen levels enhanced oxidative weathering of continental sulfides, leading to accumulation of sulfate ions in the oceans by up to 1-10 mM for 10⁷-10⁸ years. This may account for the global sulfate depositions at 2.2-2.1 Ga, including sulfate evaporates found in the post-glacial sedimentary sequence in the Transvaal Supergroup, South Africa. We conclude that an accumulation of CO₂ during the Paleoproterozoic snowball glaciation resulted in disequilibrium in carbon cycles in the atmosphere-ocean system after the deglaciation, which would have been a sufficient driving force for the oxygen transition with a long-term overshoot.