Regulation of atmospheric oxygen during the Proterozoic

- We model ocean/atmosphere biogeochemical cycles in the Phanerozoic and Proterozoic.
- Stable low oxygen is inconsistent with feedbacks needed to model the Phanerozoic.
- A model of Proterozoic pO2 requires a ten-fold reduction in P flux to the ocean.
- Low-oxygen rivers may limit P through efficient co-precipitation with mobile Fe2+.
- A pO2-sensitive P flux allows for a multiple steady state model of atmospheric pO2.

Many studies suggest that oxygen has remained near modern levels throughout the Phanerozoic, but was much less abundant from the “Great Oxygenation Event” around 2.4 Ga until the late Neoproterozoic around 600 Ma (Kump, 2008). Using a simple model, we show that the maintenance of atmospheric pO2 at ∼1% of present atmospheric levels (PAL) is inconsistent with modern biogeochemical cycling of carbon, sulfur and iron unless new feedbacks are included. Low oxygen conditions are stable in our model if the flux of phosphorus to the oceans was greatly reduced during the Proterozoic. We propose a mechanism to reduce this flux through the scavenging of phosphate ions with an “iron trap” driven by greater surface mobility of ferrous iron in a low pO2 world. Incorporating this feedback leads to two stable equilibria for atmospheric oxygen, the first quantitative hypothesis to explain both Proterozoic and Phanerozoic O2 concentrations.