NUMERICAL STUDY OF MECHANISMS AND TIMESCALES OF OXYGENATION AND INTERPRETATION OF GEOLOGICAL RECORDS IN THE SNOWBALL EARTH AFTERMATH

Manganese and iron formation directly above the Paleoproterozoic low-latitude glacial sediments (the Makganyene Diamictite, Transvaal supergroup, South Africa) indicate that a rise in atmospheric oxygen occurred just after the Paleoproterozoic snowball Earth. The mechanism of this oxygenation might be hypothesized as the consequence of cyanobacterial blooms induced by a large nutrient input to the ocean via enhanced weathering on land, under high pCO₂ conditions in the snowball aftermath. Paleoproterozoic snowball Earth is also marked by carbonate deposition above the Fe and Mn formation, which may correspond to cap-carbonates in Neoproterozoic snowball Earth. The occurrence of the carbonates may support the idea that extensive weathering occurred in the Paleoproterozoic post-snowball greenhouse world.

In this study, we aim to assess the hypothesized oxygenation scenario, as well as to understand mechanisms and timescales of the formation of these characteristic sediments. We developed an atmosphere-ocean biogeochemical cycle model coupled with a redox balance model, and investigated changes in climate, biogeochemical cycles (C, O, P, Ca, Mn, and Fe), and a redox state in the Snowball Earth aftermath.

The results show that, after the Paleoproterozoic Snowball Earth, long-term enhancement of productivity (~5-30 times as high as that of present level) for ~10⁵ years causes a rise of oxygen up to ~0.01PAL in ~10²-10³ years, and even up to 1PAL in ~10⁶ years. In the surface ocean, Fe and Mn delivered by upwelling of deep water are oxidized and deposit on the timescale of ocean circulation, usually on the order of thousands of years. Fe and Mn oxides deposition precede the carbonate precipitation, because high atmospheric pCO₂ and low pH in the ocean prevent the carbonate precipitation just after the deglaciation. We estimate that the carbonates start to precipitate ~10⁵ years after the deglaciation, and a rapid precipitation lasts for ~2-5 x 10⁶ years. Under the Neoproterozoic conditions, i.e., a larger solar constant and lower pCO₂ required to escape from the snowball Earth, carbonate precipitation starts relatively earlier (on the order of 10⁴ years). These results may explain the delay in the timing of carbonate deposition in Paleoproterozoic snowball Earth compared to that of cap-carbonates in Neoproterozoic.