Closing in on the marine 238U/235U budget

• Dissolved riverine 238U/235U is close to the average continental crust.
• Authigenic U in reducing sediments has elevated 238U/235U compared to seawater.
• Available uranium isotope data are consistent with steady-state for U in the modern ocean.

Significant redox-driven variability in the ratio between the two long-lived U parent isotopes, 238U and 235U, has recently been discovered. Thus, the 238U/235U system provides a promising new tool to evaluate redox changes in the past using the geological record. For such reconstruction to be successful, however, the modern marine U isotope cycle needs to be quantified. Here we compile U isotope ratios for the global dissolved riverine U flux. A total of 30 river samples covering a range of catchment sizes, latitudes, climates and continents were measured. Although variability is observed, the mean 238U/235U in the dissolved riverine U flux entering the ocean is close to the average for the continental crust, suggesting only minor 238U/235U fractionation during U weathering and transport. This riverine data is complemented by U isotope data in reducing sediments, the main oceanic U sink. Reducing sediments from an oxygen minimum zone off the coast of Washington State USA, show authigenic U accumulation with elevated 238U/235U compared to seawater, consistent with observations in other reducing marine sediments. However, these data also highlight that isotope partitioning between sediment and pore-water can cause variability in the 238U/235U of the accumulated authigenic U. The new data from this study, placed in the context of the growing body of data on modern marine U isotope cycling, suggest that U could be at steady-state in the modern ocean.