Impact of high CO2 levels on ocean biogeochemistry
Ancient oceanic anoxic events (OAEs) were a feature of the Earth's distant past when the sea became completely depleted of oxygen below its surface, resulting in profound changes to the global climate cycle and oxygenation of the world's oceans. This period also saw the burial of oil and gas reserves in carbon-rich black shale layers that required specific environmental conditions for their formation. The EU-funded 'Biogeochemistry in a high CO2 world: Lessons from the ocean anoxic events' (BIGCOW) project studied these ancient OAEs. Work involved an investigation into the role of oxygen supply (comprising oxygen solubility and ocean circulation) and marine productivity in creating these extreme conditions. Researchers set up an Earth system model to account for the main ocean dynamics and biogeochemistry of the Cretaceous climate 145–66 million years before present. The model was used to determine the impact of higher temperatures and marine productivity against a set of observations for seafloor anoxia. The model revealed that temperature alone was not capable of reproducing observations; the patterns of recorded ocean anoxia were instead the result of greater marine productivity due to increased nutrient content. The aim of BIGCOW was to investigate the interconnected response of nutrient and carbon cycles in a world with high CO2 levels. The project's conclusions have major implications for understanding how the Earth system responds to long-term CO2 release and warming, and how and when conditions for oil source rock formation are created. The discovery that warming alone exerts only a minor influence in promoting seafloor anoxia reduces concerns that any such similar occurrence could be a possibility in the near future.
