Triple oxygen isotope analyses reveal the origin and diagenetic history of enigmatic Ediacaran carbonates

• Applicant: Dr. David Bajnai
• Subject Area: Geology ; Mineralogy, Petrology and Geochemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548933705

The Ediacaran Shuram Excursion (ca. 574–563 million years ago) is the most pronounced negative carbonate carbon isotope anomaly observed in the whole geological record. Specifically, carbonate sediments of this age are more depleted in the heavy 13C isotope than Earth’s mantle, revealing an apparent major carbon cycle perturbation coinciding with the emergence of complex life. However, the origin of the Shuram Excursion remains incompletely understood. The most widely accepted hypotheses include the oxidation of marine organic matter and subsequent authigenic carbonate precipitation, a secondary origin resulting from post-depositional diagenetic alteration, and the contribution of carbonatites. Considering the timing and scale of the Shuram Excursion, it is crucial to distinguish between these hypotheses and reconstruct the environmental conditions under which complex life evolved. The aim of this proposal is to test these hypotheses using triple oxygen isotope analyses on carbonates (that is, the analyses of the 17O/16O ratio besides the more commonly investigated 18O/16O ratio) collected from well-studied Ediacaran deposits. The oxygen isotope composition of carbonates depends on the processes and reactants involved in their formation and the formation temperature. Explicitly, carbonates of different origins (i.e., authigenic, diagenetic, pristine) are expected to show characteristic signatures in triple oxygen isotope space, which are clearly distinguishable with the current analytical precision. The proposed analyses may also provide evidence of ancient ocean temperatures and its oxygen isotope composition. Besides investigating the Shuram Excursion, the current proposal includes fundamental methodological work to investigate acid fractionation factors between carbonate and CO2.

• DFG Programme: Research Grants