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 lowproductivity (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.