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

Emerging geochemical evidence suggests considerable redox heterogeneity in the mid-Proterozoic ocean. However, quantitative estimates of the extent of different modes of anoxia remain poorly constrained. Due to their complementary redox-related behavior, uranium and molybdenum isotope data can be combined to reconstruct ancient marine redox landscapes, but this approach has not been applied to the mid-Proterozoic. We present new δ²³⁸U and δ⁹⁸Mo data for marine rocks from the ca. 1.4 Ga Xiamaling Formation, North China craton, together with independent redox indicators (Fe speciation and redox-sensitive trace metals). We find that most samples deposited under oxic or dysoxic conditions retain low U and Mo contents, with δ²³⁸U and δ⁹⁸Mo values indistinguishable from continental crust, demonstrating a dominant detrital signal. By contrast, euxinic samples with authigenic enrichments in U and Mo record the highest authigenic δ²³⁸U and δ⁹⁸Mo values, consistent with efficient reduction of U and Mo. Samples deposited under ferruginous conditions exhibit a wider range of intermediate 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 (<0.5% of the global seafloor area) but extensive low-productivity (dys)oxic and ferruginous settings in ca. 1.4 Ga oceans. This redox landscape would have provided potentially habitable conditions for eukaryotic evolution in the mid-Proterozoic.