Uranium isotope evidence for an expansion of marine anoxia during the end‐Triassic extinction. (18th August 2017)

• Record Type: Journal Article
• Authors: Jost, Adam B.; Bachan, Aviv; van de Schootbrugge, Bas; Lau, Kimberly V.; Weaver, Karrie L.; Maher, Kate; Payne, Jonathan L.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 18:Number 8(2017)
• Page Start: 3093
• Page End: 3108
• Publication Date: 2017-08-18
• Subjects: Jurassic -- Hettangian -- redox -- carbonates; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2017GC006941
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The end‐Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes ( δ 238 U) in CaCO3 sediments deposited under locally well‐oxygenated bottom waters can passively track seawater δ 238 U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238 U(VI) relative to 235 U(VI) in anoxic marine sediments. We measured δ 238 U in shallow‐marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ 238 U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ 13 C excursion and persisting for the duration of subsequent high δ 13 C values in the lower‐middle Hettangian stage. The δ 238 U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100‐fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering, which is consistent with weathering‐induced eutrophication following the rapid increase in p CO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ 238 U implies that anoxia could have delayed biotic recovery well into the Hettangian stage. Key Points: We observe a −0.7‰ excursion in the uranium isotopic composition of lowermost Jurassic limestone The excursion is consistent with a 100‐fold increase in the areal extent of anoxic sea floor Widespread anoxic conditions likely delayed reef recovery until the mid‐late Hettangian