21 Influence of Epeiric Sea Dynamics on the Interpretation of Isotope Secular Curves (Posters)

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

Paper number: 21-1
Program: THEME SESSIONS
Time: 4:30 PM – 6:00 PM

Authors

Abstract

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.