EVALUATING THE ORIGIN OF HIGH-AMPLITUDE CARBON ISOTOPE EXCURSIONS IN TONIAN TO CRYOGENIAN CARBONATES OF AUSTRALIA

Carbon isotope records from Neoproterozoic carbonates are characterized by two features that are largely unique to the era: (1) prolonged periods with high δ¹³C_carb values and (2) high amplitude departures to negative δ¹³C_carb values. In studies that interpret these features as recording global perturbations, they form the backbone of stratigraphic correlation. Recent challenges to the validity of Neoproterozoic carbon-isotope records as a proxy for the global inorganic carbon pool have focused on feature (2) being a result of local or diagenetic processes while taking feature (1) to be a robust portion of the record. Two of these negative shifts, the Bitter Springs Stage and the Trezona Anomaly, are currently of critical importance to stratigraphic correlations for the first 365 million years of the Neoproterozoic. One argument for attributing these anomalies to global perturbations focuses on their occurrence on multiple continents. While this argument is potentially powerful, the current paucity of radiometric age constraints from many sections renders it circular. Pairing chemostratigraphic records with physical stratigraphy from many parallel sections in Australian basins, we use geologically-focused arguments, without the ambiguity of global correlation, to evaluate the primary nature of the excursions. In the Bitter Springs Formation of central Australia, the shape of δ¹³C_carb variability across the Bitter Springs Stage is tightly consistent across more than 400 km--both in uplifted sediments and those that remain at depth. Similar basin-scale reproducibility is found for the Trezona Anomaly in South Australia. The replicability of δ¹³C_carb trends on such scales is difficult to reconcile with the negative values being wholly attributable to diagenesis. Across the Bitter Springs Stage organic carbon isotope values shift sympathetically, suggesting significant changes to the composition of dissolved inorganic carbon. While organic carbon isotope values do not vary in the Trezona Formation, erosional truncation of the anomaly by Marinoan ice sheets suggests that the δ¹³C_carb excursion is a primary low-temperature feature. In general, primary porosity due to varying lithofacies does not exert a control on observed δ¹³C_carb values, but can be correlated to more variable values for δ¹⁸O_carb.