NUTRIENT DYNAMICS DURING THE CENOMANIAN-TURONIAN OCEANIC ANOXIC EVENT IN THE NORTH ATLANTIC REGION

Geochemical data for sediments from the proto-Atlantic, the Tethys and the Western Interior Seaway deposited during the Cretaceous Oceanic Anoxic Event (OAE) II display spatial variability that may aid in understanding the paleoceanographic conditions during the OAE. In general, the “black shales” of OAE II are enriched in organic matter of marine origin. Elemental C/P and C/N ratios are high indicating selective removal of nitrogen and regeneration of organic and metal oxide associated phosphorus under anoxic conditions. Nitrogen isotope values are typically below 0 permil and indicate a nutrient nitrogen source most likely produced by nitrogen fixing cyanobacteria. The nitrogen isotope values also indicate that removal of nitrate during denitrification was complete as the low values also suggest little influence from incomplete denitrification which would leave nitrogen isotope values enriched. Organic matter accumulation rates, which vary considerably, are higher by up to two orders of magnitude on Northeastern coast of Africa and along the paleo-equator and drop significantly in the Central and West Atlantic, Tethys, and South Atlantic. This distribution of organic matter accumulation rates is consistent with the presence of paleo-upwelling zones that are similar to upwelling patterns that support elevated productivity in the modern Central Atlantic. Some localities also display fine, mm-sale laminations and contain organic biomarkers indicating photic-zone euxinia. Despite the spatial diversity, the expansion of organic matter-rich deposition over such a large area can be explained ultimately by one event. Opening of the gateway that separated the North and South Atlantic could have allowed incursion of more dense waters from the South Atlantic that either delivered large amounts of phosphorus or displaced phosphorus-rich deep water closer to the surface driving enhanced productivity and carbon burial.