Iodine proxy evidence for increased ocean oxygenation during the Bitter Springs Anomaly

The Neoproterozoic Bitter Springs Anomaly (BSA; 810-800 Ma) is characterised by an 8 ‰ negative δ¹³C excursion and is coeval with multiple indicators of increasing oxygenation of the ocean and atmosphere. Here, we use carbonate iodine contents to provide the first constraints on the evolution of local upper ocean redox conditions spanning the BSA. Iodine speciation in seawater is strongly redox sensitive, and carbonates precipitated proximal to O₂-depleted water record low I/(Ca + Mg). Data from the Akademikerbreen Group of Svalbard show a major rise of I/(Ca + Mg) during the recovery phase of the BSA. Other relatively high I/(Ca + Mg) values are also associated with rising δ¹³C throughout the section. Combined with existing palaeoredox proxies (e.g., Cr and S isotopes), our new iodine data most likely reflect an oxygenation event.