QUANTITATIVE CONSTRAINTS ON THE EXTENT OF MIDDLE PROTEROZOIC OCEAN ANOXIA FROM MODELING OF THE OCEANIC MASS BALANCE OF RHENIUM
To interpret ancient ocean redox conditions, we constructed an oceanic mass balance model for Re based on modern source and sink fluxes estimated from a compilation of Re burial rates observed in modern marine basins. In addition, we have compiled a large database of Re concentrations in anoxic organic-rich mudrocks. The latter compilation reveals an expansion of marine anoxia following the relatively oxygenated conditions of the Great Oxidation Event. This is quantitatively confirmed by modeling Re authigenic enrichments in mudrocks by using the recent, advanced mass-balance approach developed for the Cr and Mo records (Reinhard et al., 2013; PNAS 110, 5357-5362). Using our model, we find that widespread anoxia (a minimum of 40-50% of the seafloor area) is required to achieve the observed average Re concentration in Middle Proterozoic mudrocks. Given that the Mo record is consistent with 1-10% of euxinic seafloor, our Re model suggests that ferruginous conditions may have been more pervasive in the Middle Proterozoic ocean than previously estimated.