Uranium isotope evidence for an expansion of marine anoxia during the end-Triassic extinction

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 (δ²³⁸U) in CaCO₃ sediments deposited under locally well-oxygenated bottom waters can passively track seawater δ²³⁸U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of ²³⁸U(VI) relative to ²³⁵U(VI) in anoxic marine sediments. We measured δ²³⁸U 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 δ²³⁸U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ¹³C excursion and persisting for the duration of subsequent high δ¹³C values in the lower-middle Hettangian stage. The δ²³⁸U 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 pCO₂ during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ²³⁸U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.