A New Negative Carbon Isotope Interval Caused by Manganese Redox Cycling After the Shuram Excursion

Several negative C isotope excursions (CIEs) occurred at the end of the Neoproterozoic era which have been generally attributed to the oxidation of organic carbon using sulfate as the terminal electron acceptor and the subsequent release of 13C-depleted dissolved inorganic C (DIC). Based on new analyses from the Doushantuo Formation in South China, we observe a negative C isotope excursion right after the well-known Shuram excursion. This excursion is equivalent to the similar to 550 Ma negative CIE which is globally expressed within several continental margins. However, the origins of this CIE in the termination of Ediacaran remain unresolved. Here, we hypothesize that this post-Shuram negative CIE was caused by a localized manganese cycling that began with the oxidation of hydrothermal Mn(II) in a water column to insoluble Mn(IV)-oxide, followed by accumulation of Mn(IV)-oxide to the seafloor and its subsequent dissolution via Mn(IV) reduction leading to the release of dissolved Mn(II) and 13C-depleted DIC into ambient seawaters. This ultimately led to the precipitation of particulate Mn(II)-carbonate characterized by low delta 13Ccarb values ranging from -11.1 parts per thousand to -2.8 parts per thousand. The presence of microbial fabrics in association with the Mn(II)-carbonate further suggests that Mn(II)-carbonate precipitation took place at the seafloor in shallow sun-lit waters rather than in the deeper sediment pile, which archived ambient seawater C isotopic signal. Although most Ediacaran negative CIEs were generally attributed to sulfate reduction, our findings suggest that at a local level, Mn cycling can also lead to negative CIE in the Neoproterozoic, and potentially at other times in Earth's history. The late Neoproterozoic era witnessed several carbon isotopic excursions (CIEs), among them the Shuram excursion, considered the largest negative CIE in Earth's history. In this study, we identify a new negative CIE, comparable to the 550-Ma negative CIE observed in multiple continental margins immediately following the well-documented Shuram excursion. This occurrence exists within Member IV of the Doushantuo Formation, located at northern margin of the Yangtze Platform in South China. Our findings reveal that this post-Shuram negative CIE primarily resulted from localized manganese (Mn) cycling. This process originated with the oxidation of hydrothermal Mn(II) in oxic water column, leading to the formation of insoluble Mn(IV)-oxide which then served as the primary electron acceptor for the mineralization of organic carbon in O2-depleted zones. This reduction released dissolved Mn(II) and 13C-depleted dissolved inorganic carbon into the surrounding waters. Our data suggest that, at a local scale, Mn cycling can contribute to negative CIEs during the Neoproterozoic, and potentially during other times in Earth's history. This challenges the prevailing notion that most Ediacaran negative CIEs were attributed to sulfate reduction. A new negative carbon isotope interval exists immediately after the Shuram excursion at the end of Ediacaran Period Mn(II) oxidation formed Mn(IV)-oxide, triggering organic C oxidation and precipitating Mn(II)-carbonate with negative C isotope values Pulsed input of hydrothermal Mn redox cycling overwhelms sulfate reduction as the trigger for the newly discovered negative CIE