Proterozoic coevolution of phytoplankton and the surface Earth redox landscape

The surface Earth oxygenation is the key process in the evolution of a planet habitable for complex life forms. As the landmark of biosphere evolution, the emergence of animal implies the atmospheric oxygen level reaching a threshold of 10% present atmospheric level. However, before this critical point, neither the evolution nor the key control of the redox landscape of the ocean-atmosphere system has been resolved. In this study, we hypothesize that the biological pump structure, placed the key control on the redox landscape of surface Earth. Our numerical model results indicate that with the Proterozoic evolution of eukaryotic phytoplanktons, the redox gradient between the ocean and atmosphere was reduced, and the ocean and atmosphere oxygenation evolved from decoupled to coupled. The Proterozoic evolution of biological pump structure significantly drove redox landscape of the surface Earth system toward more oxic, manifested as the reduction in the oceanic dissolved organic carbon and then H2S reservoir. In the Proterozoic, ocean-atmosphere redox conditions were likely influenced mainly by the biosphere, especially phytoplankton and the biological pump, according to analysis of biomarker data, isotopic and redox proxy records.