Carbon isotope variability across the Ediacaran Yangtze platform in South China: Implications for a large surface-to-deep ocean delta C-13 gradient

New isotope data obtained from relatively conformable, carbonate-rich strata of the Ediacaran Yangtze platform in South China reveal substantial delta C-13 variability. In platform sections, four negative delta C-13 anomalies with a nadir down to <= -8 parts per thousand (PDB) are present in the interval between the cap carbonate level (similar to 635 Ma) and the Precambrian/Cambrian boundary (similar to 542 Ma), while in slope and basinal sections, delta C-13 values are negative through the entire Doushantuo Formation (similar to 635-551 Ma). If these 613C values are close to their primary seawater signature, they imply a strong ( >= 10 parts per thousand) surface-to-deep ocean delta C-13 gradient that is consistent with long-term deep ocean anoxia and the presence of a large dissolved organic carbon (DOC) reservoir. The two prominent negative delta C-13 excursions within the Doushantuo Formation above the cap carbonate level are associated with shoaling and local exposure of the platform. The anomalies may thus record remineralization of a large oceanic DOC pool via sulfate reduction that transferred C-13-depleted carbon from the oceanic DOC reservoir to the surface ocean during regression. Inconsistencies in Ediacaran delta C-13 profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in delta C-13 from an inferred average seawater signature. (C) 2007 Elsevier B.V. All rights reserved.