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

Yunfeng Wang,Jorge E Spangenberg,Frantz Ossa Ossa,Martin Wille,Ronny Schoenberg

doi:10.1029/2020pa004207

Abstract

AbstractThe 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 δ15Nbulk and δ15Nker from Dotternhausen profile, Germany, combined with literature data from other T‐CIE profiles. Both δ15Nbulk and δ15Nker 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.

Highlights

During the Mesozoic era, several particular periods of widespread deposition of organic-rich black shales occurred during extreme perturbations to global climate and ocean redox structure, known as Oceanic Anoxic Events (OAEs)
The worldwide recognition of the Toarcian carbon isotope excursion (T-CIE) in organicrich sedimentary rocks has been linked to an oceanic anoxic event (OAE) which implies the world's deep oceans were anoxic ∼183 Ma
The hypothesis of the suggested global deep ocean anoxia is, challenged by the findings that most of the Toarcian fine-grained organic-rich sedimentary successions were deposited in hydrographically restricted anoxic basins especially in the northern European epicontinental shelf region (e.g., Fantasia et al, 2018; Frimmel et al, 2004; Sælen et al, 2000), which were appreciably isolated from the open ocean (Dickson et al, 2017; McArthur, 2019; McArthur et al, 2008; Remírez & Algeo, 2020)

Summary

Introduction

During the Mesozoic era, several particular periods of widespread deposition of organic-rich black shales occurred during extreme perturbations to global climate and ocean redox structure, known as Oceanic Anoxic Events (OAEs). Of these events, the early Jurassic (Toarcian) OAE at ∼183 Ma is marked by a specific interval of rapid global warming (Jenkyns, 1988), second-order marine mass extinction (e.g., Harries & Little, 1999; Little & Benton, 1995) and widespread oxygen deficiency (Pearce et al, 2008), manifested in deposits of anoxic marine sediments around the globe. Contrasting redox states have been proposed for the European epicontinental shelf seas during the T-CIE, in that the development of anoxia in the bottom waters was largely limited to the northern part of the European epicontinental shelf, while the southern part remained mainly oxygenated (Ruvalcaba Baroni et al, 2018)

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