Nitrogen cycle dynamics during the Cretaceous Oceanic Anoxic Event 2 (OAE2; ~94 Ma) in the Western Interior Seaway

The Cretaceous Ocean Anoxic Event 2 (OAE2, ~94 Ma), a time period of high sea-level and a warm greenhouse climate, was characterized by enhanced marine productivity and carbon burial in sediments, which resulted in the widespread deposition of black shales and a global perturbation of the carbon cycle. While decades of research have disentangled complex ecological dynamics during OAE2, a detailed understanding of the nitrogen cycle during this event (i.e., nitrogen transformations through metabolic and/or water-column redox reactions), remains elusive. I present a high-resolution (~300-3,000 years) bulk organic nitrogen stable isotope record from an expanded sedimentary section of OAE2 in the Smokey Hollow #1 core (SH1, southern Utah) that provides a record of changes in marine biogeochemistry from a marginal setting in the Western Interior Seaway (WIS). The &delta;13Corg record shows the characteristic chemostratigraphic phases of OAE2 seen globally. The &delta;15Norg&nbsp;record shows a 2-phased 15N depletion of ~6&permil;, which is a larger than has been previously reported during OAE2, and may suggest a buildup of 15N depleted ammonium from efficient nitrogen remineralization. Variations in the C/N ratio from 1.8 to 30.2 reflect variable input of organic matter sources to the study area throughout the event, including terrestrial (soil) and marine organic matter. The &delta;15Norg record exhibits a moderate but statistically significant correlation with available data on terrestrial-derived biomarkers, likely reflecting the impact of transgressional sequences and/or terrestrial input on nitrogen cycle dynamics at this study location. My results indicate that nitrogen cycling in the western margin of the WIS was affected by increased recycling of ammonium in the oceans, as well as changes in nitrogen input from continental sources, across OAE2.</p