LAUSR

Abstract The ca. 810 Ma Bitter Springs carbon isotope anomaly is an abrupt and long-lived (5–10 Myr.) departure from the positive carbon isotope values (δ 13 C carb ) that otherwise characterize early-middle Neoproterozoic carbonate rocks. The onset of this isotope anomaly is manifested by a globally expressed ~8‰ negative shift in δ 13 C values in shallow marine carbonate strata. Given increasing evidence that metazoan diversification began well before the Ediacaran Period, the Bitter Springs anomaly is a logical interval to explore the potential relationship between biological innovation, perturbations to the carbon cycle, and oxygenation. The iodine-to-calcium+magnesium ratio (I/[Ca + Mg]) in marine carbonates is a sensitive proxy for seawater redox conditions that is increasingly being applied to reconstruct the oxygenation of the global surface ocean through Earth's history. We report I/[Ca + Mg] ratios, along with carbon and oxygen isotope ratios and major and minor element concentrations from carbonate sections spanning the Bitter Springs anomaly in Svalbard, East Greenland, and the Mackenzie Mountains (northwestern Canada). The results from Svalbard and Greenland collectively show two prominent features. The early stage of the Bitter Springs anomaly is characterized by a negative δ 13 C shift coupled to low I/[Ca + Mg] ratios compared to carbonates pre- and post-dating the Bitter Springs anomaly. The last stage of the Bitter Springs anomaly displays a positive excursion in I/[Ca + Mg], with the highest values at ~7 μmol/mol yet documented in rocks older than 580 Ma. In contrast, carbonates from the Mackenzie Mountains are uniformly low and display no variation in I/[Ca + Mg] across the Bitter Springs anomaly, which we interpret to be the consequence of alteration of primary signatures during diagenesis. The observed geochemical variations in our Svalbard and Greenland datasets are interpreted to be the result of a shift from pre-Bitter Springs ocean conditions, defined by well oxygenated surface waters and anoxic-ferruginous bottom waters, to syn -Bitter Springs ocean conditions, characterized by expanded euxinia.