The global marine phosphorus cycle: Response to climate change and feedbacks on ocean biogeochemistry. Geologica Ultraiectina (329)

This thesis focuses on the marine phosphorus (P) cycle and its response to changing environmental conditions, particularly those associated with glacial-interglacial cycles of the late Pleistocene and Ocean Anoxic Events in the Cretaceous. From a box model of the ocean phosphorus, organic carbon and oxygen cycles, climate change scenarios are applied representing these events. The effects of continental supply of reactive P, oceanic mixing, and sea level on the marine P cycle are examined on glacial-interglacial timescales. Results show that mixing is a dominant forcing during early glaciation and leads to an initial lowering of oceanic primary production, while sea level fall is the dominant forcing during late glaciation, enhancing nutrient supply to the open ocean. Post glacial periods are times of peak productivity, and primary production is generally lower during glaciations relative to post glacial periods, arguing against the biological pump hypothesis for CO2 drawdown. When shelf erosion during sea level low stands and particulate matter re-routing to the open ocean via submarine river canyons are implemented as part of the glacial-interglacial transition scenario, the results indicate that deep sea oxygen levels may lower significantly, mainly due to the supply of new material from the shelves and particulate organic matter bypassing the coastal zone. Deep-sea oxygen demand is decoupled from primary productivity in the open ocean and the phosphorus burial does not reflect ocean fertilization. When the box model is adapted to Cretaceous ocean conditions, Oceanic Anoxic Events (OAEs) can be triggered by enhanced P supply from land, for an ocean with wide continental shelves, slow circulation and high sea surface temperatures. The system is most sensitive to oceanic mixing. Model results imply OAEs can be sustained by P recycling from sediments due to low oxygen in the water column. These results are corroborated by P burial data from the geological record for OAE2. A sensitivity analysis demonstrates that low mixing of ocean waters (poor ventilation) and enhanced sedimentary P recycling are necessary to achieve ocean anoxia. Phosphorus burial in deep sea sediments is investigated as a function of bottom water oxygen and organic matter flux to the sediment water interface, using a mechanistic reactive transport model for sediment diagenesis. Hypoxic, oligotrophic conditions are thus identified as optimum for P recycling from the sediment. The mechanistically derived redox P burial is implemented in the box model of the marine P, oxygen and organic carbon cycles. Results show that, although deep-sea burial of reactive P phases changes, the biogeochemical cycling in the ocean is not impacted