Multiple S-isotopic constraints on paleo-redox and sulfate concentrations across the Ediacaran-Cambrian transition in South China

Abstract

Changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in Earth history. During the Ediacaran–Cambrian transition, two major biological events, including the extinction of the Ediacaran biota and the Cambrian explosion, occurred approximately coincident with perturbations of the carbon and sulfur cycle. These dramatic biological changes and coeval geochemical events are commonly thought to be linked, but their causal relationships remain controversial. Here we present multiple S-isotopic compositions of pyrite (δ34Spy and Δ33Spy) from the Xiaotan section spanning the Ediacaran-Cambrian transition interval in South China. Our isotopic results show that negative Δ33Spy values, together with positive δ34Spy values, occurred across the Ediacaran-Cambrian boundary. The negative Δ33Spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters. The coincidence between the negative Δ33Spy and the approximate timing of the extinction of the Ediacaran biota implies that paleo-redox perturbations may have contributed to the disappearance of the Ediacaran biota. Negative Δ33Spy values coupled with positive δ34Spy values were also observed in the Cambrian Zhongyicun Member when the phosphorites were deposited, suggesting a strong linkage between the upwelling of nutrient-rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas. In addition, using box models and stoichiometry calculations, we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive δ13Ccarb, δ34Spy and Δ33Spy excursions observed at Xiaotan within Cambrian Stage 2. Our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mM or lower in the early Cambrian oceans. The probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early Cambrian animals.