Uranium isotope systematics of a low-productivity ferruginous ocean analog: Implications for the uranium isotope record of early Earth

The uranium isotope (δ²³⁸U) paleo-redox proxy has emerged as a premier tool for understanding ocean oxygenation through Earth's history. The fidelity of this important proxy depends, however, on our mechanistic understanding of the pathways that induce uranium isotope fractionation during U(VI) reduction to U(IV). Uranium reduction has been extensively studied in euxinic (anoxic + sulfidic) environments, yet relatively few constraints currently exist on δ²³⁸U fractionation in ferruginous (anoxic + iron-rich) environments, even though ferruginous conditions may have been a dominant feature of Earth's oceans for much of the geologic past. Here, we present a comprehensive uranium isotope study of modern, meromictic, oligotrophic to mesotrophic, ferruginous Canyon Lake, Upper Peninsula, Michigan (USA), including investigation of a high-resolution profile of lake waters, lake inlet and outlet waters, and groundwater, as well as shallow cores through both oxic and ferruginous sediments. The key observation of this study is that the entire water column, oxic sediments, and ferruginous sediments have indistinguishable δ²³⁸U values near the composition of the upper continental crust. This implies a lack of δ²³⁸U fractionation in the low-productivity, ferruginous environments of Canyon Lake. We suggest that uranium cycling in Canyon Lake is dominated by adsorption and co-precipitation with iron oxides, with only a limited role for U(VI) reduction. These processes result partly from aqueous uranium speciation in the lake, with the dominance of UO₂-CO₃ complexes in the upper water column leading to a high partition coefficient of uranium during sorption to iron oxides. In addition, the dominance of CaUO₂(CO₃)₃^2– and Ca₂UO₂(CO₃)_3(aq) in bottom waters kinetically inhibits U(VI) reduction by Fe(II)_(aq). The lack of U(VI) reduction and hence δ²³⁸U fractionation in Canyon Lake, despite Fe(II)_(aq) concentrations >1.5 mM, is potentially analogous to the lack of δ²³⁸U fractionation that occurred in the Archean and Proterozoic oceans, as indicated by the carbonate δ²³⁸U record. In contrast with predictions that U(VI) should be rapidly reduced and scavenged from the water column in the presence of Fe(II)_(aq), our data suggest a limited role for U(VI) reduction and δ²³⁸U fractionation under the low-nutrient, low-productivity, ferruginous conditions that characterized the oceans on the early Earth.