Tracing Atlantic water transit times in the Arctic Ocean: coupling reprocessing-derived ²³⁶U and colored dissolved organic matter to distinguish different pathways

The Fram Strait is a key region for investigating the exchange of Atlantic water with the Arctic Ocean. Uranium-236 (²³⁶U) from the two European nuclear reprocessing plants (NRPs) at La Hague and Sellafield provides a unique fingerprint in Atlantic water which can be used for studying its circulation patterns in the Arctic Ocean. And NRPs-derived ²³⁶U (²³⁶U_NRPs ) can be identified by its ²³³U/²³⁶U signature. In this study, we use colored dissolved organic matter (CDOM) absorption to constrain the selection of three Atlantic branch waters that carried different inputs of ²³⁶U_NRPs in Fram Strait. This can potentially provide better estimates of transit times of Atlantic waters in the Arctic Ocean. High CDOM levels (a₃₅₀ ≥ 0.35 m⁻¹) in Fram Strait reflect the passage of Atlantic water transported to the Arctic by the Norwegian Coastal Current (NCC) and its extension and subsequently along the Siberian continental slope and shelf where the Ob, Yenisei and Lena rivers supply terrestrial organic matter with significantly high CDOM content. Conversely, low CDOM water represents Atlantic water that has remained off the shelf. Based on CDOM absorption, potential temperature (), potential density (σ) and ²³⁶U concentration, the path of a given body of Atlantic water could be inferred and an appropriate NRPs input function constrained so that transit times could be estimated. Our results indicate that Arctic High CDOM Water (a₃₅₀ ≥ 0.35 m⁻¹ ) sourced from the NCC and Barents Sea Branch Water (BSBW) in the Barents Sea Opening has an average of 7–27 yrs transit time in the upper ∼200 m of the western shelf of the Fram Strait. Atlantic Low CDOM Water (a350 < 0.35 m⁻¹ , > 2 ◦C) sourced from the Fram Strait Branch Water (FSBW) has a short pathway from the eastern Fram Strait. Arctic Low CDOM / High ²³⁶U Water (a₃₅₀ < 0.35 m⁻¹ , ≤ 2 ^◦C, σ ≤ 27.97 or ²³⁶U concentration ≥15 × 10⁶ atom/L) sourced from the BSBW and the FSBW has an average of 22–28 yrs transit time. These findings demonstrate how combining measurements of CDOM with ²³⁶U_NRPs can improve the robustness in estimation of transit times of different Atlantic water pathways in the Arctic Ocean. There are limited ways to empirically derive estimates and the values provided offer unique data for comparison with estimates from regional circulation models.