A multilayered water column in the Ediacaran Yangtze platform? Insights from carbonate and organic matter paired δ13C

Large carbon isotope fluctuations have been systematically reported for Ediacaran carbonate rocks, the meaning of which remains controversial. In order to better understand the mechanisms behind such variations, we present new paired delta C-13 data on carbonates (delta C-13(carb)) and their associated organic matter (delta C-13(org)) from a shelf-margin section (Yangjiaping) of the Doushantuo Formation. In this section, the Doushantuo Formation starts with cap dolostones presenting delta C-13(carb) values around - 5 parts per thousand and (delta C-13(org) values between -30.3 and -27.6 parts per thousand. Up-section, phased variations in delta C-13(carb) and delta C-13(org) describe positive and negative excursions, while their difference (Delta C-13(dol-org) = delta C-13(carb) - delta C-13(org)) remains around + 29.2 parts per thousand. These new data allow the first reconstruction of lateral variations of delta C-13(carb), delta C-13(org) and Delta C-13(carb-org) for a shelf-to-basin cross-section of the Yangtze platform (South China) after integration with results reported previously for two other sections. Across the Yangtze platform, the isotope signals reveal strong lateral heterogeneities, with complex variations of delta C-13(carb) and Delta C-13(dol-org) in the inner-shelf section, phased variations in the shelf-margin section with positive delta C-13(carb) and Delta C-13(dol-org) close to 29 parts per thousand, and dominantly negative delta C-13(carb) with delta C-13(org) as low as - 35 parts per thousand in the basin. These variations are incompatible with the idea that the delta C-13(carb) can systematically be used as a proxy of ocean surface waters. Assuming that delta C-13(carb) are acquired in bottom waters and/or upper sediments, we show that the heterogeneous delta C-13(carb) and Delta C-13(dol-org) are compatible with a stratified water column composed of up to three layers: (i) an oxic surface layer, where dissolved inorganic carbon (DIC) is probably in isotope equilibrium with the atmosphere; (ii) an intermediate euxinic layer with a lower delta C-13(DIC) due to organic matter oxidation by ongoing sulphate reduction; (iii) a bottom euxinic layer that seems to be restricted to the inner-shelf lagoonal facies, lacking sulphate. containing methane and with a higher delta C-13(DIC) due to DIC production by methanogenesis. If our model holds true, it suggests that not only negative but also positive Ediacaran carbon isotope excursions may reflect ocean stratification, the positive excursion possibly recording a sulphate-free methanogenic layer at the bottom of restricted basins. (C) 2009 Elsevier B.V. All rights reserved.