USING SIMS DATA TO UNDERSTAND THE ROLE OF AUTHIGENIC CARBONATE IN THE ORIGIN OF CHAOTIC STRATIGRAPHIC VARIATIONS OF CARBON ISOTOPES IN THE EARLY EDIACARAN DOUSHANTUO FORMATION

Carbonate carbon isotopes have been used widely to study Precambrian paleoclimate, paleoceanography, and chemostratigraphic correlation. This is particularly true in the study of Precambrian climatic change, oceanic and atmospheric redox conditions, and stratigraphic correlation, because few Precambrian fossils provide diagnostic information about paleoenvironments and biostratigraphic zonation. One premise of these applications of carbon isotopes is that primary or oceanic carbon isotope signatures are faithfully preserved in carbonate rocks. Several recent studies, however, suggest that carbonate carbon isotopes can be altered by the syn-sedimentary addition of authigenic carbonate minerals. In this study, we analyzed carbonate carbon isotope compositions of lower Ediacaran dolostones in Member II of the Doushantuo Formation in the Yangtze Gorges area of South China, using both conventional isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS). IRMS analysis of microdrilled powders revealed chaotic stratigraphic variations in carbonate carbon isotope compositions ranging from –5 to +7‰ over 30 m of strata. SIMS analysis of three selected samples showed comparable isotopic variations between –5 and +9‰ at millimeter-micrometer scales. Integrated with petrographic observations, the new SIMS data suggest that much of the chaotic stratigraphic δ¹³C variation is related to the presence of authigenic calcite with either highly positive or negative isotopic values. The results suggest that SIMS offers an effective tool to evaluate the origin and chemostratigraphic significance of carbon isotope excursions, particularly when such excursions are stratigraphically irregular and regionally variable.