Restricted Oxygen-Deficient Basins on the Northern European Epicontinental Shelf Across the Toarcian Carbon Isotope Excursion Interval

The worldwide recognition of the Toarcian carbon isotope excursion (T-CIE) in organic-rich sedimentary rocks has been linked to an oceanic anoxic event (OAE) which implies the world's deep oceans were anoxic ∼183 Ma. The majority of independent redox observations used to build this argument were mainly obtained from T-CIE organic-rich sediments deposited on northern European epicontinental shelf. However, increasing evidence has shown that this shelf had limited connection with the open ocean, making it unsuitable for reconstructing the T-CIE ocean redox structure. To unveil such controversy, we present integrated δ¹⁵N_bulk and δ¹⁵N_ker from Dotternhausen profile, Germany, combined with literature data from other T-CIE profiles. Both δ¹⁵N_bulk and δ¹⁵N_ker values are predominantly between +0.3 and +2.5‰. These positive near-zero δ¹⁵N values imply enhanced N₂ fixation by cyanobacteria using molybdenum (Mo)-based nitrogenase to compensate bioavailable N loss following quantitative denitrification and/or anammox in a strongly redox-stratified marine setting. Such N isotope composition contradicts the typical sedimentary δ¹⁵N values (>3‰) induced by partial water-column denitrification and/or anammox in modern-ocean oxygen minimum zones. We rather propose the existence of local oxygen-deficient basins on northern European epicontinental shelf where dissolved N underwent extensive denitrification and/or anammox causing bioavailable N deficiency. Mo-based diazotrophy thus played a critical role in discriminating N isotope compositions among multiple hydrographically restricted T-CIE marginal basins. Restricted oxygen-depleted environments on the northern European epicontinental shelf unlikely represent the open-ocean redox landscape. The existence of the global OAE thus needs comprehensive redox investigations on Tethys and/or Panthalassa deep-sea T-CIE successions to validate.