The nitrogen isotope composition of sediments from the proto-North Atlantic during Oceanic Anoxic Event 2

Sediment records of the stable isotopic composition of N (δ¹⁵N) show light δ¹⁵N values at several sites in the proto-North Atlantic during Oceanic Anoxic Event 2 (OAE 2) at the Cenomanian-Turonian transition (∼94 Ma). The low δ¹⁵N during the event is generally attributed to an increase in N₂ fixation and incomplete uptake of 4+ for phytoplankton growth. A compilation of all reliable data for the proto-North Atlantic during OAE 2 demonstrates that the most pronounced negative shift in δ¹⁵N from pre-OAE 2 to OAE 2 occurs in the open ocean but with δ¹⁵N never lower than -3‰. Using a box model of N cycling for the proto-North Atlantic during OAE 2, we show that N₂ fixation is a major contributor to the δ¹⁵N signal, especially in the open ocean. Incomplete uptake of 4+ for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of 4+ acting as a major source. In the southern proto-North Atlantic, where bottom waters were euxinic, the light δ¹⁵N signature is largely explained by upwelling of 4+. Our study provides an overview of regional differences in δ¹⁵N in the proto-North Atlantic and highlights the role of lateral exchange of water and nutrients, in addition to local biogeochemical processes, in determining δ¹⁵N values of OAE 2 sediments.