The Response of Nitrogen and Sulfur Cycles to Ocean Deoxygenation Across the Cenomanian-Turonian Boundary

The Cretaceous Oceanic Anoxic Event 2 (OAE2) is a greenhouse episode of severe marine anoxia at the Cenomanian-Turonian boundary. This time interval is characterized by rising sea surface temperature, enhanced marine biological productivity, and widespread occurrence of organic-rich black shales. With an export of biological production to the deep ocean, organisms consume vast amounts of oxygen and subsequently utilize nitrate and sulfate as electron acceptors in organic matter degradation, thereby affecting biogeochemical cycles of nitrogen and sulfur. However, due to a lack of comprehensive compilation of global records, the temporal and spatial distribution of oceanic oxygen loss is yet to elucidate. To bridge this gap, this review summarizes the published marine nitrogen and sulfur isotope data sets from worldwide locations across the Cenomanian-Turonian boundary. The compiled records suggest that anoxic and euxinic waters have initially developed on the southern edge of the proto-North Atlantic before the OAE2 and spread to open oceans of the proto-North Atlantic, epicontinental seas of the Western Tethys and Western Interior Seaway at the onset of the OAE2. Meanwhile, sites above the storm base and in deep seas of the Tethys remain relatively oxic throughout the time. We suggest that distinct nutrient levels and circulation structures primarily control the regional difference in marine redox states. Our findings depict the evolutionary history of ocean deoxygenation and biogeochemical cycles during the past hyperthermal event, with implications for how future global warming may impact seawater chemistry and marine ecosystems.