Tracking the spatial extent of redox variability in the mid-Proterozoic ocean

Emerging geochemical evidence suggests highly heterogeneous ocean redox conditions in the mid-Proterozoic. Quantitative estimates of the extent of different modes of anoxia, however, remain poorly constrained. Considering the complementary redox-related behaviour, uranium and molybdenum isotopes can be combined to reconstruct ancient marine redox landscapes, which has not been applied to the mid-Proterozoic. In this study, we present new δ²³⁸U and δ⁹⁸Mo data for shales from the ~1.4 Ga Xiamaling Formation, North China Craton, together with independent redox proxies, including Fe speciation and redox-sensitive trace metals. We find that most oxic and dysoxic samples retain low U and Mo concentrations, with δ²³⁸U and δ⁹⁸Mo values indistinguishable from continental crust. While euxinic samples record the highest authigenic δ²³⁸U and δ⁹⁸Mo, consistent with efficient reduction of U and Mo. Samples deposited under ferruginous conditions exhibit a wider range of δ²³⁸U and δ⁹⁸Mo values that generally fall between the (dys)oxic and euxinic end-members. Using a coupled U-Mo isotope mass balance model, we infer limited euxinia but extensive low productivity, ferruginous conditions in mid-Proterozoic oceans.