Ocean redox changes from the latest Permian to Early Triassic recorded by chromium isotopes

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 (delta Cr-53) of the Upper Permian-Lower Triassic carbonate ramp from the Xiakou section in South China (eastern Paleotethys). We observed largely fractionated delta Cr-53 values (1.15 parts per thousand to 2.16 parts per thousand) 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 delta Cr-53 values (from average 1.78 parts per thousand to average -0.13 parts per thousand) across the extinction horizon, which may be attributed to a rapid expansion of oceanic anoxia in the eastern Paleotethys. Persistent negative delta Cr-53 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Research analyzing Cr isotopes in marine rocks in southern China revealed changes in Cr isotope compositions during the late Permian, possibly related to oceanic anoxia. The negative excursion values following the extinction horizon may indicate the expansion of oceanic anoxia, while persistent negative values in the Early Triassic suggest a prolonged recovery period for the ocean.