Nitrogen cycle dynamics during the Cretaceous Oceanic Anoxic Event 2 in the Western Interior Seaway.

The Cretaceous Ocean Anoxic Event 2 (OAE2, 94 Ma) was a time of high sea-level and warm greenhouse climate, and was characterized by enhanced marine productivity and carbon burial. These conditions resulted in the widespread deposition of black shales and a global perturbation of the carbon cycle. While decades of research have disentangled some of the complex biogeochemical dynamics during OAE2, a detailed understanding of the nitrogen cycle during this event (i.e., nitrogen transformations through redox-controlled microbial processes), remains elusive. We present a high-resolution (centennial-to-millennial time scales) bulk 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 ecology and biogeochemistry from a marginal setting in the Western Interior Seaway (WIS). By comparing our record to data from planktonic ecology (e.g., foraminifera, nanoplankton, algal biomarkers) from the same site, as well as other nitrogen isotope records during this event, we propose a schematic model for nitrogen cycling that invokes changes in marine productivity, ecological successions, water circulation, and redox conditions across OAE2. A new astronomical timescale is used to estimate rates of change and temporal relationships between nitrogen cycling and other biogeochemical and climatic feedbacks. We will discuss potential implications of nitrogen cycling during OAE2 for the study of global change in the Anthropocene.