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 ( δ 15 N) show light δ 15 N 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 δ 15 N during the event is generally attributed to an increase in N 2 fixation and incomplete uptake of4 +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 δ 15 N from pre‐OAE 2 to OAE 2 occurs in the open ocean but with δ 15 N never lower than −3‰. Using a box model of N cycling for the proto‐North Atlantic during OAE 2, we show that N 2 fixation is a major contributor to the δ 15 N signal, especially in the open ocean. Incomplete uptake of4 +for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of4 +acting as a major source. In the southern proto‐North Atlantic, where bottom waters were euxinic, the light δ 15 N signature is largely explained by upwelling of4 + . Our study provides an overview of regional differences in δ 15 N in the proto‐North Atlantic and highlights the role of lateral exchange of water and nutrients, in addition to local biogeochemical processes, in determining δ 15 N values of OAE 2 sediments.