Causes and consequences of mid-Proterozoic anoxia

Evidence for low pO2 and a ferruginous ocean characterize the mid-Proterozoic (1.8–0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export production must therefore be low to limit Corg burial and O2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (Pi) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater Pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid-Proterozoic, and low pO2 limits formation of sedimentary iron deposits (BIF). We propose that low pO2 was maintained by P limitation resulting from ferrous phosphate buffering. The near-absence of BIF and phosphorite deposition is direct and indirect consequences of the low pO2, respectively.