LAUSR

Abstract Multiple observations have shown that widespread oceanic anoxia may have played an important role in the end-Permian mass extinction and delayed biotic recovery in the Early Triassic. However, it has not been reached a consensus on the temporal and spatial changes in oceanic redox conditions at that time. Chromium (Cr) isotopes have been widely used to trace redox processes in surface environments. Here, we present the Cr isotope compositions ( δ 53 Cr) of the Upper Permian–Lower Triassic carbonate ramp from the Xiakou section in South China (eastern Paleotethys). We observed largely fractionated δ 53 Cr values (1.15‰ to 2.16‰) in the latest Permian, which are higher than most values among Phanerozoic sedimentary carbonates. Such high values may result from Cr reduction facilitated by the development of a reducing environment during the latest Permian. The high Cr isotope ratios were followed by an abrupt negative excursion of δ 53 Cr values (from average 1.78‰ to average −0.13‰) across the extinction horizon, which may be attributed to a rapid expansion of oceanic anoxia in the eastern Paleotethys. Persistent negative δ 53 Cr values covering the Early Triassic indicate that the ocean may not have recovered from anoxia for an extended period. Our Cr isotope results support a stepwise deterioration of oceanic redox environments, which has been previously proposed as a possible cause of the most severe biotic catastrophe in the Phanerozoic.