  The possibility of low but nontrivial atmospheric oxygen (O2) levels during
the mid-Proterozoic (between 1.8 and 0.8 billion years ago, Ga) has important
ramifications for understanding Earth's O2 cycle, the evolution of complex life
and evolving climate stability. However, the regulatory mechanisms and redox
fluxes required to stabilize these O2 levels in the face of continued
biological oxygen production remain uncertain. Here, we develop a
biogeochemical model of the C-N-P-O2-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 O2 production was 3.5 (+1.4 - 1.1) Tmol year-1
(1-sigma), 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 O2 source than organic carbon burial, implying a
potentially important role for Earth's sulphur cycle in balancing the oxygen
cycle and regulating atmospheric O2 levels. Our statistical approach provides a
uniquely comprehensive view of Earth system biogeochemistry and global O2
cycling during mid-Proterozoic time and implicates severe P biolimitation as
the backdrop for Precambrian geochemical and biological evolution.
