Multiple sulfur-isotopic evidence for a shallowly stratified ocean following the Triassic-Jurassic boundary mass extinction. (15th June 2018)

• Record Type: Journal Article
• Authors: Luo, Genming; Richoz, Sylvain; van de Schootbrugge, Bas; Algeo, Thomas J.; Xie, Shucheng; Ono, Shuhei; Summons, Roger E.
• Is Part Of: Geochimica et cosmochimica acta. Volume 231(2018)
• Page Start: 73
• Page End: 87
• Publication Date: 2018-06-15
• Subjects: Oceanic anoxia -- Oceanic stratification -- Black shale -- Sulfur isotopes -- Isorenieratane -- Epeiric sea; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2018.04.015
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: 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 (δ 33 S, δ 34 S and δ 36 S) 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 34 S-enriched pyrite, low Spy /TOC ratios, and values of Δ 33 Spy (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 34 S-depleted pyrite, high Δ 33 Spy, 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 34 S-, 13 C-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 δ 13 C values, and a delay in the recovery of marine ecosystems following the Tr-J boundary crisis.