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 δ13C data on carbonates (δ13Ccarb) and their associated organic matter (δ13Corg) from a shelf-margin section (Yangjiaping) of the Doushantuo Formation. In this section, the Doushantuo Formation starts with cap dolostones presenting δ13Ccarb values around − 5‰ and δ13Corg values between − 30.3 and − 27.6‰. Up-section, phased variations in δ13Ccarb and δ13Corg describe positive and negative excursions, while their difference (Δ13Cdol–org = δ13Ccarb − δ13Corg) remains around + 29.2‰. These new data allow the first reconstruction of lateral variations of δ13Ccarb, δ13Corg and Δ13Ccarb–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 δ13Ccarb and Δ13Cdol–org in the inner-shelf section, phased variations in the shelf-margin section with positive δ13Ccarb and Δ13Cdol–org close to 29‰, and dominantly negative δ13Ccarb with δ13Corg as low as − 35‰ in the basin. These variations are incompatible with the idea that the δ13Ccarb can systematically be used as a proxy of ocean surface waters. Assuming that δ13Ccarb are acquired in bottom waters and/or upper sediments, we show that the heterogeneous δ13Ccarb and Δ13Cdol–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 δ13CDIC 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 δ13CDIC 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.