Biotic and isotopic vestiges of oligotrophy on continental shelves during Oceanic Anoxic Event 2

The widespread expansion of the oxygen minimum zone onto shelves has been commonly regarded as a primary cause of benthos extinction in epicratonic sea ecosystems during the Cenomanian–Turonian boundary event (CTBE). However, neither lithology, geochemical proxies, nor micropaleontological data support this hypothesis. Instead, our integrated foraminiferal and dinoflagellate cyst study, corroborated by δ13Corg and δ15Norg data, indicate that the biota were impacted by an abrupt shift to well oxygenated oligotrophic conditions and a collapse of primary productivity in the epicontinental Central European Basin. Because the event was concurrent with the development of extensive and extreme oceanic bottom water anoxia that reached the photic zone in oceanic settings, we infer that the biotic crisis in the shelf seas during OAE2, and possibly during other OAEs, was triggered by this anomalous nutrient cycling in Earth’s oceans. This phenomenon was presumably associated with intensive denitrification combined with anammox activity in the deep ”ammonium oceans”, which caused a significant loss of biologically reactive nitrogen from the ocean system. Impingement of ammonium-rich anoxic waters on the photic zone resulted in primary productivity based primarily on ammonium assimilation, as recorded by strongly 15N-depleted organic matter deposited in the oceans during the CTBE. We propose that, unlike in the oceanic settings, productivity in the well-oxygenated, oligotrophic epicontinental seas was nitrate-based, as evidenced by strongly 15N-enriched organic matter deposited in the contemporaneous epicontinental sea. These very high δ15Norg values (> +5‰) were related to the spreading of shallow oceanic waters carrying 15N-enriched nitrate onto epicontinental settings