Carbon isotopes and origin of the Neoproterozoic cap dolostone in different sedimentary environments

A remarkably negative excursion is recognized for the carbon isotopic values of the cap dolostone from the Yongyi secton in Yinjiang, Guizhou just like those of the Neoproterozoic cap carbonates on a global scale.The cap dolostone may be originated from abundant continental weathering products entering into oceans after the deglaciation of the "snowball earth". The mean isotopic values of δ¹⁸O_PDB and δ ¹³C_PDB for the Yongyi section are lower at 6 ‰ and 6.5 ‰, respectively than those for the Abazhai section in Qingzhen, Guizhou in the clastic tidal-flat environment; higher at 2.3 ‰ and 2.5 ‰, respectively than those for the Tianping section in Dayong, Hunan in the platform foreslope environment, and consistent with those for the Longjing section in Tongren, Guizhou in the same shallow-sea and shelf environments. All these values indicate carbon isotopic gradients along the palaeoenvironmental transection through the clastic tidal-flat facies to the platform foreslope facies and to the shallow-sea and shelf facies. The previous authors contended that the C isotopic stratification may be attributed to the effect of biological pumping in a postglacial ocean. The author in this paper, however, argues that the precipitation of the carbonate rocks cited above may not be synchronous in different environments. Instead the carbon isotopic gradients may result from the combination of the constant input of the continental weathering products, variations in CCD interface and bloom of biotas.