Records of terrestrial sulfur deposition from the latest Permian coals in SW China

Different sulfur fractions of low sulfur (average 0.11%) coals from the latest Permian in southwestern China were analyzed isotopically at a high vertical resolution to examine the ramifications for potential terrestrial extinction mechanisms at the Permo-Triassic boundary. Sorg (organic sulfur) accounts for 87% of the total sulfur content on average, while Spy (pyritic sulfur) is very low (13% of the total sulfur content and ca. 0.01% of the whole sample weight) and relatively constant throughout the profile except for some insignificant additions in clay-rich layers. These indicate the possible differential contribution of local diagenesis during the early stages of peat accumulation. The δ34Sorg values have a relatively narrow range, from + 1.5‰ to + 7.6‰, while the δ34Spy values have a wider range, from + 0.8‰ to + 34.1‰, suggesting that pyrite was possibly formed in different stages and that there was negligible bacterial contribution to the primary organic sulfur fraction. Thus, the organic sulfur signal in the coal is largely sourced from the original plant material and records temporal variations in the isotopic composition of the sulfur input to the coal forming environment. The stratigraphically lower coal (B3) has δ34Sorg values around + 4‰ while the stratigraphically higher coal (B2), which is closer to the Permian–Triassic boundary, has clearly higher δ34Sorg values, ranging from + 5.3 to + 7.6‰. This change is most likely due to increased marine sulfate aerosol inputs into the coal-forming peatland caused by coastline retreat during Late Permian transgression. Importantly, there is no evidence in these discrete intervals of coal formation to indicate a dominant role of atmospheric inputs of isotopically lighter sulfur from either H2S released from euxinic oceans or volcanic SO2. This militates against theories that implicate such sources of sulfur in terrestrial extinction mechanisms at the end-Permian.