Ammonium upwelling and alternative nitrogenase enzymes as drivers of isotopically depleted δ15N signatures during OAE 3 in the Western Interior Seaway

The Cretaceous period is characterized by intermittent oceanic anoxic events (OAEs), which are typically denoted by increased organic carbon (OC) burial and a positive excursion in carbonate carbon isotope values (δ 13 C carb ). OAE 3 occurred in the Western Interior Seaway (WIS) from the late Coniacian to the early Santonian over a period of 3 Ma. Two hypotheses for enhanced OC burial have been proposed: (1) increased primary production export or (2) increased anoxic or euxinic conditions that made burial processes more efficient. To examine this question we analyzed the USGS Portland core from the WIS for bulk stable nitrogen isotopes (δ 15 N bulk ). The δ 15 N bulk value of sediments can track changes in sources of nitrogen (N), and how much of the lighter N isotope is taken up through biologically mediated processes that occur under specific redox/environmental conditions. The core yielded unusually isotopically depleted δ 15 N bulk values (median = − 2.45‰), which ranged from − 5.2‰ to 0.02‰. Previously published OAE 2 N isotope data are significantly more positive than our OAE 3 data, suggesting that N sources or N cycling processes may differ between OAEs in the Cretaceous. The δ 15 N bulk values are more variable and more negative before OAE 3, and less variable during and after OAE 3. Previously collected trace metal data supports the use of alternative nitrogenases in producing such negative δ 15 N bulk values. Based on redox proxies for OAE 3, anoxic conditions likely sustained an ammonium pool in excess of what was needed by phytoplankton, further contributing to a depleted δ 15 N bulk signal.