Kerogen-specific isotope variations during the end-Permian mass extinction in South China

Abstract

Sulfur isotopic variation has been widely used to study the environmental backdrop of the end-Permian mass extinction (EPME), although research to date has focused mostly on carbonate-associated sulfate (CAS) and pyrite. Study of the δ34S of kerogen, as well as of its H- and C-isotopic compositions, is relatively rare, although such data have the potential to shed new light on our understanding of the proximate driver of the EPME event. Here, we report paired δ34Spyrite and δ34Skerogen values for three widely spaced end-Permian marine sections from South China (Meishan, Shangsi, and Guanzi) that represent a transect from upper-slope (shallow-water) to basinal (deep-water) facies, as well as additional data for Meishan (i.e., δ34SCAS, δ2Hkerogen, and δ13Ckerogen). Our results show that oxidation of isotopically light H2S in upwelled anoxic watermasses led to negative shifts of δ34SCAS, δ34Skerogen and δ34Spyrite in the ocean-surface layer during the Late Permian, and an increase of Δ34SCAS-kerogen in the upper-slope Meishan section. High Δ34Skerogen-pyrite (>10‰) in the Meishan section indicates that slow pyritization occurred considerably below the sediment-water interface. In contrast, the lower-slope Shangsi and deep-water Guanzi sections show more negative δ34S values for both kerogen and pyrite, with smaller Δ34Skerogen-pyrite (mainly 5–10‰), implying open-system microbial sulfate reduction (MSR) in an anoxic deep watermass. A large negative shift in δ2Hkerogen from the uppermost Permian to the lowermost Triassic is most plausibly attributed to increased sedimentation rates and organic matter inputs due to collapse of terrestrial ecosystems. High sedimentation rates may have been contributed to observed subsequent increase of (poly)sulfide δ34S within the Lower Triassic in shallow-water facies. Our study thus demonstrates the utility of integrated analysis of S-isotopes in pyrite, kerogen, and CAS, coupled with H- and C-isotopic data of kerogen for tracking dynamic environmental changes during the Permian-Triassic transition and possibly other major events in Earth history.