Nitrogen dynamics during the Cenomanian-Turonian oceanic Anoxic Event 2: A model study for the proto-North Atlantic

Evidence from sediment core records and model studies suggest that increased nutrient supply played a key role in the initiation of the Cenomanian-Turonian oceanic anoxic event 2 (OAE2; ~94 Ma). However, the relative roles of nitrogen (N) and phosphorus (P) availability in controlling primary productivity during the event are not fully understood. Here, we expand an existing multi-box model of the coupled cycles of P, carbon and oxygen in the proto-North Atlantic ocean with the marine N cycle. With the updated version of the model, we test the hypothesis that enhanced availability of P can fuel N₂-fixation, increase primary productivity and drive large parts of the proto-North Atlantic to anoxia during OAE2. In a sensitivity analysis, we demonstrate that N dynamics in the proto-North Atlantic respond strongly to variations in oxygen and P supply from the Pacific Ocean and to changes in circulation. The implemented N cycle weakly modifies the carbon cycle, implying that P was the major nutrient controlling primary productivity during OAE2. Our model suggests that both N₂-fixation and upwelling of recycled ammonium (NH₄⁺) were enhanced during OAE2 and that N₂-fixation was the major source of N in the proto-North Atlantic. Denitrification was more important in the water column than in sediments, with high rates in the open ocean and in the Western Interior. High P inputs in the proto-North Atlantic led to widespread N₂-fixation, which more than compensated for the loss of N through denitrification. As a consequence, rates of primary productivity and organic carbon burial were high.