Uranium Isotope Evidence for Temporary Ocean Oxygenation Following the Sturtian Glaciation

Abstract:

The link between widespread ocean oxygenation in the Neoproterozoic and the rise of animals has long been debated, largely because the timing and nature of oxygenation of the oceans remain poorly constrained. Strata deposited during the Cryogenian non-glacial interlude (660 to 635 Ma), between the Sturtian and Marinoan Snowball Earth glaciations, contain the earliest fossil evidence of animals. To quantitatively estimate patterns of seafloor oxygenation during this critical interval, we present uranium isotope (δ²³⁸U) data from limestone of the Taishir Formation (Fm) in Mongolia in two stratigraphic sections that are separated by ~75 km across the basin. The Taishir Fm hosts two large δ¹³C excursions that co-vary in total organic and inorganic (carbonate) carbon: a basal carbonate δ¹³C excursion to -4‰ in the Sturtian cap carbonate, followed by a rise to enriched values of +8‰, a second negative δ¹³C excursion to -7‰ referred to as the Taishir excursion, followed by a second rise to +10‰. Above the Sturtian glacial deposits, in the stratigraphic interval below the Taishir excursion, δ²³⁸U compositions have a mean value that is similar to that of modern seawater. After the Taishir excursion, the δ²³⁸U record exhibits a step decrease of ~0.3‰, and δ²³⁸U remains approximately constant until the erosional unconformity at the base of the Marinoan glacial deposits. We use a box model to constrain the uranium cycle behavior that best explains our observations. In the model, the best explanation for the less negative post-Sturtian values of δ²³⁸U is extensive oxygenation of the seafloor. Moreover, the model demonstrates that the higher δ²³⁸U values of the post-Sturtian limestones are inconsistent with an increased flux of uranium to the oceans due to post-Snowball weathering as the primary driver of the excursion. Thus, we favor a scenario in which there was a rise in oxygen levels following the Sturtian glaciation followed by a decrease in seafloor oxygenation coincident with the Taishir δ¹³C excursion. The U isotopic data, combined with modeling results, challenge the notion of a unidirectional oxygenation history of Neoproterozoic oceans.