The largest negative carbon isotope excursions in Neoproterozoic carbonates caused by recycled carbonatite volcanic ash

The late Ediacaran Shuram Excursion (SE) records the most prominent negative delta C-13 excursions (delta(61)3C = -12 parts per thousand) during Earth's history. It has been hypothesized to have resulted from oxidation of dissolved organic matter, diagenetic or authigenic precipitates. However, the origin of the SE remains enigmatic; current models face challenges regarding the significant amount of atmospheric oxygen needed to balance such extensive oxidation and sustained inputs of light carbon with extremely negative C isotope compositions. Here, we show that the Doushantuo Formation at the Jiulongwan section in South China, a key stratum recording the SE event, contains mineralogical and geochemical signatures related to igneous processes. Both the occurrence of ankerite, feldspar, moissanite and euhedral quartz in the SE samples and the relatively consistent Ce anomalies of carbonate and O isotopes of quartz indicate a contribution from an igneous source. In particular, the SE samples have trace element and C isotope compositions similar to those of recycled carbonatites formed by decarbonation and melting of sedimentary carbonate rocks. These observations suggest that the deep cycle of ancient carbonate rocks, which were subjected to decarbonation during subduction, melting and eruption related to the breakup of the Rodinia supercontinent, contributed to the SE. This igneous model for the SE may provide a connection between the deep and shallow carbon cycles of the Earth. (C) 2021 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Automated summary

Studies suggest that the Shuram Excursion event in the late Ediacaran period is geologically and geochemically related to igneous processes in the Doushantuo Formation in South China. These findings imply that ancient carbonate rocks, subjected to decarbonation during subduction, melting, and eruption related to the breakup of the Rodinia supercontinent, contributed to the Shuram Excursion event. This igneous model may provide a connection between the deep and shallow carbon cycles of the Earth.