Variability in the natural Uranium Isotopic composition of the Arctic Ocean

Abstract. The Arctic Ocean plays a significant role in the Global Thermohaline Circulation and is strongly sensitive to ongoing changes. Due to climate change, rising riverine freshwater inputs into the Arctic Ocean and rising instability of the Arctic Ice covering have been observed. The resulting changes in the ocean mixing and circulation can give valuable insights on future impacts. Here, we study whether over the past twenty years additional freshwater input is traceable in the isotopic composition of uranium, using the natural uranium ²³⁴U/²³⁸U isotope composition. Water samples taken at six stations across the Arctic Ocean are analyzed using high-precision MC-ICP-MS. For each station, a depth profile of ²³⁴U/²³⁸U ratios, as ‰ deviation from secular equilibrium (δ²³⁴U), is obtained and compared to a previous study by Andersen et al. (2007), which included similar stations taken 20 years prior. Surface water δ²³⁴U reveals overall 1–2 ‰ lower values as compared to 20 years ago, thus increased freshwater contributions, which are rich in ²³⁴U, are not traced within the upper 30m of the Arctic Ocean. Instead a stronger downward mixing over the past 20 years is detected. Below 300 m δ²³⁴U is homogeneous within measurement uncertainty and yields a δ²³⁴U value of 146.94 ‰ ± 0.09 ‰ (2σ mean, N=88, HU1-equilibrium scale), which is identical to the global ocean mean value of 146.75 ± 0.28 by Kipp et al. (2022) and confirms that the interior Arctic U isotope composition is in steady state and set by the long-term global ocean circulation. In addition, we determined the Arctic Ocean ²³⁸U/²³⁵U ratio obtaining a mean δ²³⁸U value of −0.348 ‰ ± 0.046 ‰ (N=68) that is also identical within uncertainty to the global Ocean mean δ²³⁸U value of −0.379 ‰ ± 0.023 ‰ (Kipp et al., 2022) indicating absence of local redox conditions on the Arctic Ocean U cycle.