Abstract
The Cenomanian–Turonian Greenhouse-Hothouse interval, encompassing Oceanic Anoxic Event 2 (OAE-2), represents one of the most intensively studied episodes of global environmental change in Earth History. However, despite extensive research, nitrogen isotope records from this interval remain geographically limited with a recent compilation highlighting that some continental margins were under-represented (e.g. one location and/or limited stratigraphic coverage per margin). This prevents a comprehensive understanding of the local and regional variability in nitrogen cycling in these Greenhouse to Hothouse conditions. This study provides new nitrogen isotope data, integrated with an extensive multidisciplinary dataset from multiple cores and outcrops across a single margin to provide new insights into the spatial and temporal variability in marine nitrogen cycling and marine primary producer community response. This study demonstrates that nitrogen isotope depletion in the Cretaceous Western Interior Seaway began in the Early Cenomanian (approx 97-98 Ma) , significantly earlier than previously recognized. The occurrence of δ15N-depleted organic matter, coupled with biomarker and palynological evidence for abundant (cyano-) bacteria, indicates enhanced diazotrophy in oxygen-deficient surface waters resulting in ammonium as the dominant form of fixed nitrogen in the photic zone. These conditions favoured low diversity eukaryote communities dominated by blooms in Prasinophyceae and Peridiniales (Dinophaceae), likely due to their ability to more efficiently assimilate ammonium through alternative nitrogenases enzymes. However, the observed δ15N values, even under broadly similar environmental conditions impacted by periodic volcanic events, can vary significantly over relatively short distances suggesting that, despite large-scale controls, local physiographic conditions significantly influenced the nitrogen cycle and related biological responses. This highlights the need to integrate data from multiple stratigraphic sections across a basin to fully capture the complexity of nitrogen cycle dynamics and associated ecological feedback during extreme climate events.