DECOUPLED CARBONATE AND ORGANIC CARBON ISOTOPE RECORD: AN ENVIRONMENTALLY CONTROLLED PHENOMENON

Decoupled carbonate and organic carbon isotopes of Neoproterozoic successions have been interpreted with two contrasting views. In one interpretation, invariant organic carbon isotopes across the negative carbonate carbon isotope anomalies record the buffering effects of a large dissolved organic carbon (DOC) reservoir in the ocean. In another interpretation, negative carbonate carbon isotope anomalies associated with invariant organic carbon isotopes record burial diagenetic alteration of carbonate carbon isotopes. Paired carbonate and organic carbon isotope data of the Ediacaran to early Cambrian (ca. 635 Ma to 521 Ma) strata across the different facies zones in South China show large variations in both carbonate and organic carbon isotopes. The large temporal variations in organic carbon isotopes do not support the existence of a large DOC reservoir in Ediacaran oceans. The consistency of carbonate carbon isotope trends, although at varying magnitudes, suggests that the negative carbonate carbon isotope anomalies are not completely a diagenetic artifact, although diagenetic modification on absolute isotope values has certainly happened. The observed carbonate and organic carbon isotope patterns and lateral variations are similar to variations that have been observed in modern surficial environments. In restricted environments such as shelf lagoons and lakes where a shallow chemocline develops, organic carbon isotopes would decouple from carbonate carbon isotopes. Carbonate and organic carbon isotopes would covary above the chemocline but show large differences below the chemocline. Considering the fact that pervasive anoxic conditions may have dominated the Ediacaran to early Cambrian oceans, changes in chemocline would have resulted in large carbon isotope variations across sedimentary basins. Interpretation of the Neoproterozoic carbon isotope record from individual sections of different continents needs to be scrutinized by their depositional environments.