Delayed recovery from the end-Triassic extinction due to an increase in the extent of ocean anoxia

Abstract:

The end-Triassic mass extinction was likely triggered by a rapid rise in pCO₂ associated with the emplacement of the Central Atlantic Magmatic Province (CAMP) ca. 201 Ma. Shallow-marine anoxia has long been hypothesized to have caused the extinction and/or delayed the recovery of marine life. However, due to a lack of proxy data, the effects of CAMP emplacement on seawater chemistry remain poorly constrained. Local proxies for anoxia may not reflect widespread ocean redox conditions. However, coupled records of U concentration and isotopic composition (δ²³⁸U) in CaCO₃ sediments precipitated beneath well-oxygenated bottom waters can potentially serve as a proxy for the global extent of anoxia due to fractionation of U during reduction and associated imbalances in the marine U cycle due to redox changes. We measured δ²³⁸U and Th/U values in shallow marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m, to quantify the change in the extent of ocean anoxia during the end-Triassic extinction. We observe a ca. 0.6‰ negative excursion in δ²³⁸U beginning in the lowermost Jurassic, coeval with the onset of the negative δ¹³C excursion and persisting for the duration of subsequent high δ¹³C values in the lower–middle Hettangian (earliest Jurassic). Th/U values are generally low at the T/J boundary, peak near the nadir of the δ²³⁸U excursion, and steadily return to pre-event values by the end of the measured section. Using a numerical model of the U cycle, we demonstrate that this excursion corresponds to a thirty-fold increase in the extent of anoxia worldwide and a simultaneous increase in the riverine U flux, consistent with increased weathering and eutrophication following massive CO₂ injection from CAMP volcanism. Scenarios involving an increase in marine anoxia would also predict higher rates of organic C burial, explaining the large protracted positive δ¹³C excursion in the lower-mid Hettangian. Recovery of marine fauna, particularly reef builders, was delayed until the late Hettangian, suggesting that environmental conditions were unfavorable for >1 My after the extinction. Uranium isotope data provide strong support for the hypothesis that persistent ocean anoxia inhibited biotic recovery throughout this interval.