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

Abstract

Sediment records of the stable isotopic composition of N (δ15N) show light 15N 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 anincrease in N2-fixation and incomplete uptake of NH+4 for phytoplankton growth. Surprisingly, published δ15N values for OAE 2 vary widely, even for similar locations. Using analyses of δ15N for sediments from three open-ocean and two coastal sites, we suggest that this reported variation is likely related to the treatment of sediment samples with acid prior to the δ15N analysis. A compilation of all available data for unacidifed samples 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 NH+4 for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of NH+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 NH+4. Our study provides an overview of regional diferences in δ15N in the proto-North Atlantic and highlightsthe role of upwelling and high lateral exchange of water and nutrients, in addition to localbiogeochemical processes, in determining δ15N values of OAE 2 sediments.