Nitrogen isotopes of bulk sediments for OAE2 samples

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.

Palynology and sea surface temperature reconstruction for the mid-Cretaceous Oceanic Anoxic Event 2 at ODP Hole 210-1276A

The mid-Cretaceous is thought to be a greenhouse world with significantly higher atmospheric pCO2 and sea-surface temperatures as well as a much flatter latitudinal thermal gradient compared to the present. This time interval was punctuated by the Cenomanian/Turonian Oceanic Anoxic Event (OAE-2, ~ 93.5 Myr ago), an episode of global, massive organic carbon burial that likely resulted in a large and abrupt pCO2 decline. However, the climatic consequences of this pCO2 drop are yet poorly constrained. We determined the first, high-resolution sea-surface temperature (SST) record across OAE-2 from a deep-marine sedimentary sequence at Ocean Drilling Program (ODP) Site 1276 in the mid-latitudinal Newfoundland Basin, NW Atlantic. By employing the organic palaeothermometer TEX86, we found that SSTs across the OAE-2 interval were extremely high, but were punctuated by a remarkably large cooling (5-11 °C), which is synchronous with the 2.5-5.5 °C cooling in SST records from equatorial Atlantic sites, and the "Plenus Cold Event". Because this global cooling event is concurrent with increased organic carbon burial, it likely acted in response to the associated pCO2 drop. Our findings imply a substantial increase in the latitudinal SST gradient in the proto-North Atlantic during this period of global cooling and reduced atmospheric pCO2, suggesting a strong coupling between pCO2 and latitudinal thermal gradients under greenhouse climate conditions.