Thallium isotopic evidence for the Tonian rise and Cryogenian fall of Neoproterozoic oxygen levels

Estimating dissolved oxygen (O2) concentrations in seawater during the Neoproterozoic is central to testing hypotheses about the role of O2 in animal evolution. Here we apply the thallium (Tl) isotope redox proxy to samples stratigraphically below the ca. 810-million-year-old (Ma) Bitter Springs Carbon Isotope Excursion and spanning the interval between the two Snowball Earth glaciations (ca. 662–650 Ma) to constrain the evolution of Neoproterozoic bottom water redox conditions. Thallium isotopes can be used to reconstruct the global extent of oxygenated oceanic bottom waters because the primary control on seawater Tl isotope compositions (ε205Tl) over million-year time scales is changes in the amount of 205Tl removal by Mn oxides on the seafloor. Samples spanning an ~20-m.y. period preceding the Bitter Springs excursion from the Tonian Reefal Assemblage (n = 18/30) yield ε205Tlauth values lower than global oceanic inputs (ε205Tl ~–2±), with some samples approaching the modern seawater ε205Tl value of –6±. These sustained low ε205Tlauth values require enhanced burial of Mn oxides elsewhere on the seafloor, which we interpret as evidence for the oxygenation of the deep ocean in the Tonian. In contrast, the majority of samples from the Cryogenian Hay Creek Group (n = 13/16) yield ε205Tlauth values similar to global oceanic inputs, suggesting that the deep ocean was not ventilated at this time. This indicates that Earth’s deep ocean was not gradually oxygenated throughout the Neoproterozoic, but rather experienced intervals of increased and decreased O2 concentrations.