Iodine records from the Ediacaran Doushantuo cap carbonates of the Yangtze Block, South China

Abstract

Late Neoproterozoic cap carbonates formed following the Cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the Snowball Earth events. Large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post-Marinoan cap carbonates across the globe, but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations. In this study, we present I/(Ca + Mg) profiles of four sections from a shallow-to-deep water transect of the basal Doushantuo cap carbonates, one of the most extensively studied Ediacaran units in South China. Most samples have I/(Ca + Mg) values lower than 1 μmol/mol with a few exceptions in the shelf sections up to ~2.5 μmol/mol. All calcite samples with extremely low δ13C have iodine signal below detection limit, suggesting carbonate precipitation from anoxic seawater or diagenetic fluids. Among the four sections, the slope section has the lowest I/(Ca + Mg) values, suggesting that it may have experienced more extensive fluid-buffered diagenesis and more severe loss of IO3–. In combination with available major, trace, rare earth element and isotope data, the temporal I/(Ca + Mg) variations support freshwater mixing and deep-water upwelling in proximal sites and transient oxidation during deposition of the Doushantuo cap carbonates.