Modeling phosphorus cycling and carbon burial during Cretaceous Oceanic Anoxic Events

Abstract

In the greenhouse world of the Cretaceous, there were episodes of oxygen depletion in the deep ocean associated with enhanced organic carbon burial in sediments (black shale formation). In this study, we use a box model of the oceanic phosphorus (P), organic carbon (orgC) and oxygen cycles to explore the hypothesis that variations in marine P availability control deep ocean oxygen depletion (anoxia) and the formation of black shales under Cretaceous ocean conditions. We find that, for the Cretaceous ocean, with large continental shelves, slow oceanic overturning and high sea surface temperatures, Oceanic Anoxic Events (OAEs) can be triggered by enhanced P supply from land, and that the system is particularly sensitive to oceanic mixing. In our baseline scenario, the deep sea becomes completely anoxic, while the shelves attain only partial anoxia. Sedimentary burial differs between the shelves and open ocean: while organic carbon burial is enhanced in both regions, deep sea reactive P burial decreases dramatically under anoxia, but not on the shelves, where oxygen depletion is not complete. Furthermore, our model results imply that OAEs can be sustained by P recycling from sediments under low-oxygen conditions. Ultimately, however, the feedbacks which result in the accumulation of atmospheric oxygen terminate the anoxic event. Atmospheric oxygen is modulated by land processes such as forest fires and oxidative weathering, which limit the rise of atmospheric O 2. Our model findings are corroborated by P burial data from the geological record for OAE2 (approximately 94 Myr BP). Through a sensitivity analysis we identify two necessary criteria for OAEs: low mixing of surface and deep waters (poor ocean ventilation) and enhanced sedimentary P recycling under low oxygen conditions. When these criteria are met, ocean anoxia is a robust result to a mild increase of continental supply of phosphorus, under a wide range of environmental conditions.