Multiple sulfur isotope chemostratigraphy across the Permian-Triassic boundary at Chaotian, China: Implications for a shoaling model of toxic deep-waters

Previous studies on multiple sulfur isotopes (S-32, S-33, and S-34) in sedimentary pyrite at the end-Permian suggested a shoaling of anoxic/sulfidic deep-water contributing to the extinction. This scenario is based on an assumption that the sedimentary sulfur cycle was largely controlled by benthos activity, though a stratigraphic correlation between the sulfur records and ichnofabrics of the sediments at the end-Permian has not yet been examined. We report the multiple sulfur isotopic composition of pyrite in the Permian-Triassic boundary interval at Chaotian, South China. Our data can be generally explained by a mixing of sulfur in sulfide from two different sources: one produced via sulfate reduction in an open system with respect to sulfate and the other produced in a closed system. In particular, the former with the substantially low delta S-34 (<-40 parts per thousand) and high increment S-33 (up to +0.100 parts per thousand) values was likely produced via water-mass sulfate reduction or via sulfate reduction in oxic sediments with common burrows. The frequent occurrence of small pyrite framboids (mostly <5 mu m in diameter) in the Lopingian (Late Permian) Dalong Formation of deep-water facies supports the enhanced water-mass sulfate reduction in an anoxic deep-water mass. The negative increment S-33 values are observed only in the oxic limestones, and no substantial increment S-33 change is observed across the extinction horizon despite of the disappearance of bioturbation. Our results are apparently inconsistent with the previous shoaling model. We expand the model and infer that, when the deep-water was sulfidic and its shoaling rate was high, a substantial amount of hydrogen sulfide (H2S) was supplied onto the shelf via the shoaling; that resulted in the positive increment S-33 value of the bulk sediments. The observed increment S-33 variation on a global scale suggests a substantial variation in H2S concentration and/or in upwelling rate of shoaling deep-waters during the Permian-Triassic transition.

Automated summary

Previous research on multiple sulfur isotopes in sedimentary pyrite at the end-Permian suggests a mixing of sulfur from two different sources, with noticeable sulfate reduction in deep waters. This challenges the traditional shoaling model and infers substantial variations in hydrogen sulfide concentration and upwelling rate during the Permian-Triassic transition.