Ediacaran carbon isotope records contain significant and long-lasting negative anomalies for which convincing explanations have not been given. We recognized a diagenetic overprint contributing to the 120-m-thick negative anomaly interval in the upper part of the Ediacaran Doushantuo Formation at Yangjiaping, Hunan province in South China. Coarse-grained carbonate facies deposited in an outer shelf setting of the Yangtze platform provided an opportunity to examine diagenetic components by measuring micro-drilled sub-samples. Scattered δ13C values of bulk samples (+5 to -14‰) in this anomaly interval are here attributed to the secondary precipitation of highly 13C-depleted (up to -33‰) calcite cement on dolomite sedimentary grains of less negative δ13C. This diagenetic overprint was not observed in the intervals above or below the anomaly. In these intervals the δ13C difference between the cements and the grains is insignificant. Strontium isotopic variation in the anomaly interval is identical to that of the interval with negative δ13C anomaly in the middle Doushantuo Formation reported from the Three Gorges area, which is presumably correlated with the Gaskiers glaciation. This correlation scheme indicates a long-term hiatus (~35 myrs) between the anomaly horizon and the overlying Dengying Formation at Yangjiaping. We suggest that the anaerobic oxidization of reduced carbon (organic carbon or methane) by sulfate reduction was the most likely process responsible for 13C-depleted secondary calcite, which progressively filled intergranular porosity. An increase in atmospheric oxygen during Ediacaran and the Gaskiers glaciation enhanced oxidative weathering and elevated seawater sulfate concentrations to a level sufficient for diagenetic alteration. This co-occurred with an increase in 87Sr/86Sr from 0.7080 to 0.7085 in middle Ediacaran. We also show that the δ13C profile of the Doushantuo Formation at Yangjiaping is similar to that at Weng'an in Guizhou province, another section recording an outer shelf depositional environment. Although further study is needed, correlation between the Yangjiaping Unit 3 with the δ13C-scattered interval in Weng'an suggests that the embryo-bearing phosphorites at Weng'an were formed likely before the Gaskiers glaciation.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology