Biogeochemical Modeling of the Second Rise of Atmospheric Oxygen

Abstract:

The second rise of atmospheric oxygen (~600 Ma) marked an increase of atmospheric pO₂ from a poorly constrained value of 0.1% < pO₂ < 10% of present atmospheric level (PAL) in the early and mid Proterozoic to >10%PAL¹. The event is important because it ushered in the modern era of animal life. To understand the evolution of Earth’s habitability, it is therefore key to understand the cause of this 2^nd rise.

Here, we quantitatively examine possible causes for the 2^nd rise of oxygen. We use a biogeochemical box model² originally developed to calculate the oxygen evolution before and after the 1^st rise of oxygen (~2.4 Ga). The Claire et al. (2006) model calculates the evolution of atmospheric oxygen and methane given production and loss fluxes associated with the oxygen, carbon, and iron cycles. Because the model was unable to drive pO₂ to end-Proterozoic levels, the authors suggested that another buffer, such as sulfur, is needed to explain the 2^nd rise of oxygen. The sulfur and oxygen cycles are tied through various biogeochemical interactions; therefore, once sulfur (as sulfate) began to accumulate in Proterozoic oceans, it likely began to heavily influence the oxygen cycle. We have added a sulfur biogeochemical cycle to this model, enabling exploration of mechanisms that buffer pO₂ at intermediate levels in the Proterozoic and fail to do so in the Phanerozoic.

Preliminary results show evolution of oxygen and methane that are consistent with geologic proxies. However, the model-generated 2^nd rise of oxygen is dependent upon sulfur fluxes that have uncertain magnitudes, so we will present the sensitivity of our results to model assumptions while constraining scenarios for the 2^nd rise of atmospheric O₂. In the future, we will also integrate isotopic fractionation effects, which will allow comparison with isotopic data from sedimentary sulfides, carbonates, and organic carbon.

¹Canfield, C., 2014, Treatise on Geochemistry, 197 ²Claire, M.W., et al., 2006, Geobiology, 4, 239