Post-depositional origin of highly 13C-depleted carbonate in the Doushantuo cap dolostone in South China: Insights from petrography and stable carbon isotopes

Extremely negative carbon isotope values from the Doushantuo cap dolostone succession in the Yangtze Gorges, South China were interpreted as direct evidence of the hypothesis of methane hydrate destabilization during the Marinoan deglaciation. However, this suggestion remains uncertain due to the obscurity of the paragenetic sequence of carbonate minerals with diverse carbon isotopic compositions. Here, we conducted macroscopic and microscopic petrographic and carbon isotopic investigations of the cap dolostone succession at the Jiulongwan section in the Yangtze Gorges. Our results show that extreme delta C-13 values down to -44 parts per thousand exclusively occur in calcite, whereas delta C-13 values for the host dolomite range mostly between -4 parts per thousand and -2 parts per thousand, and clearly reveal that the host dolomite were locally dissolved and/or replaced by the calcite bearing highly C-13-depleted signals. This finding suggests that methane-oxidation activity as indicated by the strongly depleted delta C-13 signals did not occur until after the cap dolostone deposition. Thus, the presence of the extremely negative delta C-13 values could not be taken as evidence for the proposed methane hypothesis. Nevertheless, this conclusion is not contradictory to the scenario that massive releases of methane from permafrost hydrates during deglaciation, much prior to methane oxidation documented here, could drive substantial negative delta C-13 shifts well exhibited by the Doushantuo cap dolostone and others around the world. (C) 2011 Elsevier B.V. All rights reserved.