Methane-derived authigenic carbonate from the lower Doushantuo Formation of South China: Implications for seawater sulfate concentration and global carbon cycle in the early Ediacaran ocean

Authigenic carbonate associated with anaerobic oxidation of methane (AOM), usually via microbial sulfate reduction (MSR) or ferric iron reduction, is generally characterized by extremely low delta C-13 values (<-30 parts per thousand, VPDB). This has been used as one of the major diagnostic features for the recognition of hydrocarbon seep carbonate in the geological past. Previous reports on Precambrian authigenic carbonates are rare, limiting our understanding of the effects of their deposition on the Earth's carbon isotopic mass balance. In this study, mainly based on petrographic features and pronounced negative delta C-13 values as low as -38.1 parts per thousand, we discovered authigenic calcite cement immediately above the cap dolostone in the basal Ediacaran Doushantuo Formation in the Jiulongwan section, Yangtze Gorges area, South China. Our observations not only provide direct evidence for the involvement of AOM during carbonate precipitation in the early Ediacaran (similar to 635 Ma), but also suggest that the seawater sulfate concentrations in the early Ediacaran may have been higher than previously thought. (C) 2016 Elsevier B.V. All rights reserved.