中国科学院南京地质古生物研究所机构知识库

The microbial sulfate reduction plays a significant role in regulating marine organic matter degradation. Thus the variation of ancient marine sulfate levels is closely coupled with the changes in global organic carbon burials and Earth's surface redox evolution in geological time. It is widely assumed that seawater sulfate concentration was deficient during the anoxic Cryogenian Marinoan glaciation (ca.650-635 Ma) which represents the most extreme icehouse condition in Earth's history. However, such a low seawater sulfate level has not been explicitly confirmed. To address this, here we carried out pyrite content and sulfur isotope (delta S-34(py)) analysis of the Cryogenian non-glacial (the Datangpo Formation) and the Marinoan glacial successions (the Nantuo Formation) in South China. The Datangpo and Nantuo Formation show relatively high values of delta S-34(py), ranging from +19.9 parts per thousand to +32.9 parts per thousand, and low values of pyrite content (mean = 0.72 wt%). These pyrites are dominated by euhedral crystals with a rare occurrence of framboids, indicating an early diagenetic origin, i.e., pyrite formed in sediments with sulfate supplied from seawater. Using and a one-dimensional diffusion-advection-reaction (DAR) model, we calculate marine sulfate levels. The modeling results show that the seawater sulfate concentration is <1 mM during the transition from the top Datangpo to the basal Nantuo Formation, suggesting a shrink of the marine sulfate reservoir at the onset of Marinoan glaciation. We propose that the low marine sulfate concentration was resulted from a decrease in continental chemical weathering. This study provides a quantitative constraint on marine sulfate levels during the Cryogenian snowball Earth event, and it may shed new light on our understanding of the marine biogeochemical changes in the Neoproterozoic.