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Multiple sulfur-isotopic evidence for a shallowly stratified ocean following the Triassic-Jurassic boundary mass extinction

Author:
  • Genming Luo
  • Sylvain Richoz
  • Bas Van De Schootbrugge
  • Thomas J. Algeo
  • Shucheng Xie
  • Shuhei Ono
  • Roger E. Summons
Publishing year: 2018-06-15
Language: English
Pages: 73-87
Publication/Series: Geochimica et Cosmochimica Acta
Volume: 231
Document type: Journal article
Publisher: Elsevier

Abstract english

The cause of the Triassic-Jurassic (Tr-J) boundary biotic crisis, one of the ‘Big Five’ mass extinctions of the Phanerozoic, remains controversial. In this study, we analyzed multiple sulfur-isotope compositions (δ33S, δ34S and δ36S) of pyrite and Spy/TOC ratios in two Tr-J successions (Mariental, Mingolsheim) from the European Epicontinental Seaway (EES) in order to better document ocean-redox variations during the Tr-J transition. Our results show that upper Rhaetian strata are characterized by 34S-enriched pyrite, low Spy/TOC ratios, and values of Δ33Spy (i.e., the deviation from the mass-dependent array) lower than that estimated for contemporaneous seawater sulfate, suggesting an oxic-suboxic depositional environment punctuated by brief anoxic events. The overlying Hettangian strata exhibit relatively 34S-depleted pyrite, high Δ33Spy, and Spy/TOC values, and the presence of green sulfur bacterial biomarkers indicate a shift toward to euxinic conditions. The local development of intense marine anoxia thus postdated the Tr-J mass extinction, which does not provide support for the hypothesis that euxinia was the main killing agent at the Tr-J transition. Sulfur and organic carbon isotopic records that reveal a water-depth gradient (i.e., more 34S-, 13C-depleted with depth) in combination with Spy/TOC data suggest that the earliest Jurassic EES was strongly stratified, with a chemocline located at shallow depths just below storm wave base. Shallow oceanic stratification may have been a factor for widespread deposition of black shales, a large positive shift in carbonate δ13C values, and a delay in the recovery of marine ecosystems following the Tr-J boundary crisis.

Keywords

  • Geology

Other

Published
  • ISSN: 0016-7037
Sylvain Richoz
E-mail: sylvain [dot] richoz [at] geol [dot] lu [dot] se

Senior lecturer

Lithosphere and Biosphere Science

+46 46 222 78 89

Sölvegatan 12, Lund

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