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The possibility of low but nontrivial atmospheric oxygen (O 2 ) levels during the mid‐Proterozoic (between 1.8 and 0.8 billion years ago, Ga) has important ramifications for understanding Earth's O 2  cycle, the evolution of complex life and evolving climate stability. However, the regulatory mechanisms and redox fluxes required to stabilize these O 2  levels in the face of continued biological oxygen production remain uncertain. Here, we develop a biogeochemical model of the C‐N‐P‐O 2 ‐S cycles and use it to constrain global redox balance in the mid‐Proterozoic ocean–atmosphere system. By employing a Monte Carlo approach bounded by observations from the geologic record, we infer that the rate of net biospheric O 2  production was    3  .   5   −  1.1    +  1.4Tmol year −1  (1σ), or ~25% of today's value, owing largely to phosphorus scarcity in the ocean interior. Pyrite burial in marine sediments would have represented a comparable or more significant O 2  source than organic carbon burial, implying a potentially important role for Earth's sulphur cycle in balancing the oxygen cycle and regulating atmospheric O 2  levels. Our statistical approach provides a uniquely comprehensive view of Earth system biogeochemistry and global O 2  cycling during mid‐Proterozoic time and implicates severe P biolimitation as the backdrop for Precambrian geochemical and biological evolution.

A sluggish mid‐Proterozoic biosphere and its effect on Earth's redox balance