Ocean Temperature-Regulated Dichotomy of Carbon Isotopes in Post-Marinoan Badami Carbonates, India

The post-Marinoan Snowball Earth witnessed rapid carbonate deposition on marine platforms worldwide. Evidence of organic matter degradation, alkalinity development and carbonate precipitation is present in the sedimentary record. A diverging trend in the carbon isotopic value of inorganic and organic carbon is proposed here as a proxy for organic alkalinity. The carbon isotopic signatures in the organic matter and carbonate precipitates, along with temperature estimates, indicate that organic matter degradation is regulated by rising temperatures. The clumped-isotopic composition of post-Marinoan Badami carbonates strongly suggests that low-Mg carbonates are pristine, providing a unique window into the carbon cycle processes. These carbonates record a lower temperature end-member in the range of 2 to 26°C with abundant presence of total organic carbon (TOC). In contrast, samples with higher temperatures are dolomitic and contain low TOC. The partitioning of carbon isotopes between organic and inorganic phases enables tracing the source of dissolved carbon species derived from organic matter degradation in the shallow, equatorial, oxygenated marine environment, and is modelled using the Rayleigh fractionation approach. The magnitude of the isotopic signature present in the Badami carbonates exhibits a clear depositional control, similar to the neighbouring Neoproterozoic Doushantuo Formation in South China.