10.5281/ZENODO.4027082
Dubicka Zofia
0000-0003-1105-4111
Faculty of Geology, University of Warsaw
Danuta Peryt
Institute of Paleobiology, Polish Academy of Science
Marcin Barski
Faculty of Geology, University of Warsaw
Maciej Bojanowski
Institute of Paleobiology, Polish Academy of Science
Anomalies in the marine nutrient cycle triggering biotic crises on continental shelves during oceanic anoxic events
Zenodo
2020
2020-02-04
Journal article
https://zenodo.org/record/4027082
10.5281/zenodo.3636060
Creative Commons Attribution 4.0 International
Open Access
<p>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 δ<sup>13</sup>C<sub>org</sub> and δ<sup>15</sup>N<sub>org</sub> data indicate well oxygenated oligotrophic environment and a collapse of primary productivity in an epicontinental basin that have apparently left their mark on the biota. Because the event was concurrent with the development of extensive and extreme oceanic bottom water anoxia reaching the photic zone in oceanic settings, we infer that the biotic crisis in the shelve seas during 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 water on the photic zone resulted in primary productivity based mainly on ammonium assimilation, as recorded by strongly <sup>15</sup>N-depleted organic matter deposited in the oceans at the CTBE. We propose that unlike the oceanic settings, productivity in the well-oxygenated, oligotrophic epicontinental seas was nitrate-based, as evidenced by strongly <sup>15</sup>N-enriched organic matter deposited in the epicontinental sea at the same time. These very high δ<sup>15</sup>N<sub>org</sub> values (> +5‰) were related to spreading of shallow oceanic waters carrying <sup>15</sup>N-enriched nitrate onto the epicontinental settings.</p>