Sediment records of the stable isotopic composition of N (d15N) show light d15N 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 d15N during the event is generally attributed to an increase in N2-fixation and incomplete uptake of ammonium 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 d15N from pre-OAE 2 to OAE 2 occurs in the open ocean, but with d15N never lower than -3 ppm. 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 d15N signal, especially in the open ocean. Incomplete uptake of ammonium for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of ammonium acting as a major source. In the southern proto-North Atlantic, where bottom waters were euxinic, the light d15N signature is largely explained by upwelling of ammonium . Our study provides an overview of regional differences in d15N in the proto-North Atlantic and highlights the role of lateral exchange of water and nutrients, in addition to local biogeochemical processes, in determining d15N values of OAE 2 sediments.

New nitrogen isotope measurements of bulk sediments for 5 sites in the North Atlantic (Wunstorf, Bass River, DSDP 381, ODP 641 and ODP 1276). Additionally, the sample number, the depth, the carbon isotope, the total nitrogen and organic carbon content, the phosphorus and the organic carbon to phosphorus ratios are given. All sites include the Oceanic Anoxic Event 2 (OAE2) interval.

Supplement to: Ruvalcaba Baroni, Itzel; van Helmond, Niels A G M; Tsandev, I; Middelburg, Jack J; Slomp, Caroline P (2015): The nitrogen isotope composition of sediments from the proto-North Atlantic during Oceanic Anoxic Event 2. Paleoceanography, 30(7), 923-937