ISOTOPIC VARIABILITY OF THE LATE NEOPROTEROZOIC DOUSHANTUO CAP CARBONATE IN SOUTH CHINA: PROCESSES AND IMPLICATIONS

Millimeter- to centimeter-scale isotope analysis of the thin (< 5m) and regionally persistent Doushantuo cap carbonate (~635 Ma) overlying the glaciogenic Nantuo Formation in south China reveals strong variability in both absolute d¹³C values and d¹³C trends in two platform to basin transects. These include (1) d¹³C values as low as -41‰ in well-preserved clotted limestone crusts; (2) up to 5‰ variation in d¹³C within single specimens; (3)d¹³C values 2–5‰ more depleted in isopachous cements and internal sediments than in coexisting dolomicritic matrix; (4) positive d¹³C values in the basal cap carbonate in some sections; (5) strong isotopic variation between limestone and dolomite, (6) clear positive and negative d¹³C–d¹⁸O covariation in some sections; (7) a negative to positive d¹³C shift within the cap carbonate in platform sections coinciding with a mineralogical change from dolomite to limestone and an up to +6‰ shift in d¹⁸O; and (8) a comparable d¹³C shift at a stratigraphically higher level in slope to basinal sections where d¹⁸O values are more consistent and mineralogic phases changes are absent. Such isotopic variability suggests that d¹³C values from cap carbonates record a variety of local processes including, but not limited to, methane oxidation and carbonate precipitation near seawater/sediment contact and in pore space, carbonate dissolution and reprecipitation, dolomitization, and burial diagenesis. The data challenge the widely held view that absolute d¹³C values from cap carbonates represent a more or less unmodified surface ocean seawater signature, and they raise questions about the time-stratigraphic significance of the d¹³C values.