Uranium isotope compositions of mid-Proterozoic black shales: Evidence for an episode of increased ocean oxygenation at 1.36Ga and evaluation of the effect of post-depositional hydrothermal fluid flow

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.precamres.2017.06.016 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/We report U isotope data for marine black shales of the early Mesoproterozoic Velkerri Formation (Roper Group) and late Paleoproterozoic Wollogorang Formation (Tawallah Group) from the McArthur Basin, Northern Australia. An average authigenic δ238U of 0.13±0.04‰ (1SD; relative to standard CRM145) was obtained for six U- and Mo-rich shales from a ∼1m interval that was deposited at 1361±21Ma (based on previous Re-Os geochronology). After correcting for a local U isotope fractionation of ∼0.60–0.85‰ associated with U removal to anoxic sediments, we infer that global seawater at 1.36Ga had a δ238U of ∼−0.47‰ to −0.72‰, which is ∼0.1−0.3‰ lighter than modern seawater (−0.39±0.01‰). Uranium isotope mass-balance modelling suggests that