Uranium reduction and isotopic fractionation in reducing sediments: Insights from reactive transport modeling

Uranium concentrations and isotopic ratios (U-238/U-235, denoted as delta U-238) have been used to provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The potential to constrain changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. Because uranium reduction primarily occurs in sediments underlying anoxic water columns rather than in the anoxic water column itself, the removal of uranium in organic-rich shales is the largest lever on seawater delta(238) U. Accordingly, accumulation and isotopic fractionation are modulated by local variations in productivity, basin connectivity, sedimentation rate, and bottom-water redox conditions. To isolate the processes at the sediment-water interface that control delta U-238 and uranium accumulation in reducing sediments, we constructed a reactive transport model that couples biogeochemical reactions to diffusive transport and the burial of solutes and minerals. Using the model framework, we test the sensitivity of authigenic uranium isotopic fractionation and accumulation to oxygenation, permeability, sedimentation rate, organic carbon delivery, and basin restriction. Our results demonstrate that these external forcings produce diagnostic patterns in isotopic fractionation. Specifically, the model predicts that authigenic delta(238) U is sensitive to productivity because of the associated organic carbon burial rate. Moreover, our results suggest that the isotopic offset does not vary significantly due to changing bottom-water O-2 concentrations, but the amount of accumulation does-a result that differs from previous estimates. Water column uranium reduction adds additional complexity to the ultimate delta U-238 value. The predictive patterns derived from model results can offer insight into local depositional conditions, such as sedimentation patterns. Collectively, these effects-including bottom-water redox conditions and related reducing sediments-alter the isotopic signature of the overlying water column according to the authigenic delta U-238 value and the diffusive fluxes arising from porewater concentration gradients. More broadly, this work provides important new constraints on the major controls on the delta U-238 of sediments while also supporting its use as a proxy for global marine redox conditions. (C) 2020 Elsevier Ltd. All rights reserved.