Carbon and sulfur cycling across the Silurian-Devonian boundary in the Qujing Basin, South China

During the late Silurian - Early Devonian there were significant changes in the biosphere and environment. However, there are few integrated studies of carbon and sulfur cycling in the Qujing Basin, in South China, which has extremely well-preserved sarcopterygian fossils. Here we report C-isotopic data of organic carbon (delta 13Corg) and multiple S-isotopes of pyrite (delta 34S and Delta 33S) from two sections in the Qujing Basin. Our delta 13Corg data show an increase of -4 parts per thousand near the Silurian-Devonian boundary which can be correlated globally. We suggest that this rise in delta 13Corg near the Silurian-Devonian boundary may have resulted from the weathering of the 13C-enriched carbonate platform, rather than from oceanic anoxia. This interpretation may explain why there is little evidence of a mass extinction near the Silurian-Devonian boundary in the Qujing Basin. Our S-isotopic data show positive delta 34S values with either positive or negative Delta 33S values, and most of the delta 34S and Delta 33S data can be explained by microbial sulfate reduction and Rayleigh distillation processes. The S-isotopic data from both sections suggest that seawater sulfate concentrations were very low in the Qujing Basin during the late Silurian - Early Devonian.