Understanding Mid-Cryogenian Biogeochemical Cycles using Biomarkers from a Mongolian Carbonate

Within the Dzabkhan carbonate platform in Mongolia is the ~1.5km thick carbonate succession Tsagaan Oloom Formation. Glacial deposits, cap carbonates and chemostratigraphy from this succession have recently allowed its integration into the global Cryogenian record (1). Aside from the organic-poor dolomites in Arctic Alaska, and the well-studied carbonate section in the Congo Craton, the Tsagaan Oloom Formation contains one of the only known thick carbonate succession to span most of the period between the Sturtian and Marinoan glaciations. A distinctive negative δ¹³C anomaly in carbonate and total organic carbon, referred to as the Tayshir anomaly, has been identified within these strata (1). The significant negative excursion in carbonate δ¹³C starts at a baseline of +8 permil and then decreases to almost -8 permil before increasing again to +8 permil within approximately 100 meters of stratigraphy. In organic carbon, this signal covaries with the carbonate signal, with organic carbon ~26 permil more negative than carbonate (2). Samples from multiple sections that span the Tayshir anomaly were analyzed for lipid biomarkers. This study focused on the identification of tricyclic terpanes, steranes and triterpenoids to identify the depositional environment, maturity and microbial communities that contributed biomass to the sediments during deposition. Aromatic compounds, including aryl isoprenoids that indicate inputs from green sulfur bacteria and the presence of sulfidic conditions, are also addressed. 1. Macdonald, F. et al., Geology 37, 123-6 (2009). 2. Johnston et al., in review.