The iron (III) imprint on the Ediacaran Shuram carbon isotope excursion

The Ediacaran Shuram Excursion marks the largest negative carbonate carbon isotope (δ13Ccarb) anomaly in Earth’s history and has been attributed to sulfate-driven oxidation of a large marine organic carbon reservoir. However, growing evidence suggests that multiple electron acceptors—beyond O2 and SO42−—were likely operative in the sulfate-limited Precambrian oceans. Here, we combined carbonate-associated Fe(II) and Mn(II) contents with Mo isotopes from a stratigraphically representative section of the Ediacaran Doushantuo Formation in South China. Geochemical and mineralogical evidence indicates a shift from manganous to ferruginous conditions, with sulfate reduction confined to pore waters. Redox reconstructions further suggest that the δ13Ccarb anomaly was partly overprinted by syndepositional diagenetic processes involving Fe(III) reduction. These findings highlight the role of authigenic carbonate formation in shaping ancient δ13Ccarb records without requiring the extensive global oxidation proposed by previous models. Hence, we hypothesize that enhanced Fe(III) oxide input might have buffered CH4 and H2S toxicity in benthic environments, favoring Ediacaran biological innovations.