Palaeoenvironment and carbon isotope stratigraphy of Upper Proterozoic carbonates of the Yangtze Platform

The δ¹³C values for Phanerozoic marine carbonates display secular trends, mainly in the range -1 to 2‰¹. These have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater, brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs^1,2. Carbon isotope trends are not as well established for the Precambrian, but there are indications that Upper Proterozoic sequences contain carbonates with unusually high δ¹³C values^3-9. Here we present the results of the first carbon isotopic study of the Upper Proterozoic carbonate strata from the type locality of the Sinian System in the Yangtze Gorges sections near Yichang, South China, and discuss their implications for accumulation of organic matter. We also compare the results for these Chinese strata with δ¹³C trends in other Upper Proterozoic-Cambrian sequences, and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences.