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

Abstract

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 δ 13C data on carbonates ( δ 13C carb) and their associated organic matter ( δ 13C org) from a shelf-margin section (Yangjiaping) of the Doushantuo Formation. In this section, the Doushantuo Formation starts with cap dolostones presenting δ 13C carb values around − 5‰ and δ 13C org values between − 30.3 and − 27.6‰. Up-section, phased variations in δ 13C carb and δ 13C org describe positive and negative excursions, while their difference (Δ 13C dol–org = δ 13C carb − δ 13C org) remains around + 29.2‰. These new data allow the first reconstruction of lateral variations of δ 13C carb, δ 13C org and Δ 13C 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 δ 13C carb and Δ 13C dol–org in the inner-shelf section, phased variations in the shelf-margin section with positive δ 13C carb and Δ 13C dol–org close to 29‰, and dominantly negative δ 13C carb with δ 13C org as low as − 35‰ in the basin. These variations are incompatible with the idea that the δ 13C carb can systematically be used as a proxy of ocean surface waters. Assuming that δ 13C carb are acquired in bottom waters and/or upper sediments, we show that the heterogeneous δ 13C carb and Δ 13C 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 δ 13C 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 δ 13C 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.