Causes and consequences of mid‐Proterozoic anoxia

Evidence for low p O 2 and a ferruginous ocean characterize the mid‐Proterozoic (1.8–0.8 Ga). Considerations of redox sources and sinks imply that generation of O 2 via organic carbon (C org ) burial must be low to maintain a low p O 2 atmosphere for geologically long intervals, yet low oxygen should result in increased C org preservation. Marine export production must therefore be low to limit C org burial and O 2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (P i ) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater P i is consistent with the hiatus in sedimentary phosphorite deposits during the mid‐Proterozoic, and low p O 2 limits formation of sedimentary iron deposits (BIF). We propose that low p O 2 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 p O 2 , respectively.