Carbon isotope variability across the Ediacaran Yangtze platform in South China: Implications for a large surface-to-deep ocean δ13C gradient

Abstract

New isotope data obtained from relatively conformable, carbonate-rich strata of the Ediacaran Yangtze platform in South China reveal substantial δ 13C variability. In platform sections, four negative δ 13C anomalies with a nadir down to ≤− 8‰ (PDB) are present in the interval between the cap carbonate level (∼ 635 Ma) and the Precambrian/Cambrian boundary (∼ 542 Ma), while in slope and basinal sections, δ 13C values are negative through the entire Doushantuo Formation (∼ 635–551 Ma). If these δ 13C values are close to their primary seawater signature, they imply a strong (≥ 10‰) surface-to-deep ocean δ 13C 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 δ 13C 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 13C-depleted carbon from the oceanic DOC reservoir to the surface ocean during regression. Inconsistencies in Ediacaran δ 13C profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in δ 13C from an inferred average seawater signature.