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Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and (238)U/(235)U isotopic compositions (δ(238)U) of Upper Permian-Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ(238)U across the end-Permian extinction horizon, from ∼3 ppm and -0.15‰ to ∼0.3 ppm and -0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelves-global biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.

The end-Permian mass extinction horizon is marked by an abrupt shift in style of carbonate sedimentation and a negative excursion in the carbon isotope ( δ 13 C) composition of carbonate minerals. Several extinction scenarios consistent with these observations have been put forward. Secular variation in the calcium isotope ( δ 44/40 Ca) composition of marine sediments provides a tool for distinguishing among these possibilities and thereby constraining the causes of mass extinction. Here we report δ 44/40 Ca across the Permian-Triassic boundary from marine limestone in south China. The δ 44/40 Ca exhibits a transient negative excursion of ∼0.3‰ over a few hundred thousand years or less, which we interpret to reflect a change in the global δ 44/40 Ca composition of seawater. CO 2 -driven ocean acidification best explains the coincidence of the δ 44/40 Ca excursion with negative excursions in the δ 13 C of carbonates and organic matter and the preferential extinction of heavily calcified marine animals. Calcium isotope constraints on carbon cycle calculations suggest that the average δ 13 C of CO 2 released was heavier than -28‰ and more likely near -15‰; these values indicate a source containing substantial amounts of mantle- or carbonate-derived carbon. Collectively, the results point toward Siberian Trap volcanism as the trigger of mass extinction.