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 δ13C variability. In platform sections, four negative δ13C 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, δ13C values are negative through the entire Doushantuo Formation (~ 635-551 Ma). If these δ13C values are close to their primary seawater signature, they imply a strong (≥ 10‰) surface-to-deep ocean δ13C 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 δ13C 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 δ13C profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in δ13C from an inferred average seawater signature. © 2007 Elsevier B.V. All rights reserved.