Global control of riverine U isotopes by source and weathering processes

The use of marine sedimentary uranium isotopes (δ238U) to investigate past variability in the redox state of the ocean-atmosphere system rests on a robust quantification of the riverine U isotope flux, the dominant U input to the ocean. However, there is no consensus on the relative importance of lithology versus fractionation during weathering in controlling the riverine U isotope flux to the ocean. Here we report the riverine dissolved and solid uranium isotope composition from fourteen of the world’s largest rivers, integrating lithologies and processes relevant at the global scale, which we compare with data from the literature. The riverine dissolved 238U ranges from -0.47 to 0.08 ‰, while the solids range from -0.65 to -0.19‰. Collectively, our results indicate a dominant control by weathering processes on the global riverine U isotope flux, with the notable exception of the Mackenzie Basin. No relationship emerges between riverine dissolved U isotope and lithological tracers. Whilst river solid U isotope variability reflects different relative contributions from erosion of silicate versus organic-rich sedimentary rocks, most large rivers lie close to the silicate erosion end member, and mass balance calculations indicate that silicate rock weathering can explain the total riverine U isotope flux. In this view, elemental and isotope partitioning of U between the riverine dissolved and solid loads suggests a dominant control by weathering processes that fractionate U isotopes. This interpretation is supported by the relationship between the fraction of U remaining in solution following secondary mineral formation during weathering and riverine U isotope signatures. Finally, a complex pattern emerges between climate and U isotope fractionation and weathering intensity. However, based on data collected so far, the U isotope system appears analogous to lithium isotopes, whereby moderate weathering intensity leads to the largest shift in river dissolved load δ238U values under optimal conditions for U incorporation into secondary mineral formation.