Since July 2021 are based on historical data on I-129, U-236 and new data collected in more than 20 expeditions that took place in the Arctic, North Atlantic and Arctic gates, from 2021 until today (see figure in attach). The main results obtained that aim addressing the previous research qüestions are:
(i): We have constrained the pan-Arctic circulation and transport timescales of Atlantic waters in the Arctic Ocean in 2015. Results from expeditions that took place in 2020/2021 are almost concluded, and new data from years 2022 - 2024 are under analysis.
The combination of I-129 with another radionuclide tracer of anthropogenic origin, namely U-236, allow for the estimation of Transit Time Distribution (TTD) of Atlantic waters in the Arctic Ocean and estimated circulation timescales from 10-15 years (from the entrance to the Arctic Ocean) to the North Pole, longer than 20 years to the Arctic Canada Basin and about 30 years to finally exit the Arctic Ocean through the Fram Strait (Casacuberta&Smith, 2023). Further sampling in 2020 (and afterwards) has allowed to extend the only result that existed for the Arctic Canada to be between 29 and 34 years, indicating a stable circulation over the past 15 years (Payne et al, 2024).
A recent work by Raimondi et al. (2024) has compared for the first time the results obtained by the transient tracers U-236 and I-129 with the gas tracers CFC-12 and SF6. Although these tracers have all different origins and historical inputs to the oceans, they all proved to be very complementary to understand circulation regimes in the Arctic Ocean and to estimate the storage of anthropogenic carbon in the Arctic Ocean.
(ii): We assessed the presence and variability of Pacific Water the central Arctic and in their outflow through the Fram Strait.
In the central Arctic, a yearly sampling since 2020 has taken place in the Canada Basin, and significant efforts are devoted to understanding the presence of these transient tracers in the Pacific summer and winter waters. First results by Payne et al. (2024) show that while Pacific summer waters are mostly dominated by waters of Pacific origin, the Pacific winter waters seem to contain between 25 - 40% of Atlantic-derived waters.
In the Fram Strait, both I-129 and U-236 proved to be a novel tool of water mass composition. From the tracer data collected between 2016 and 2019, Wefing et al. (2022) suggested an increase in Pacific-origin waters of about 20% for the uppermost layer of the EGC. This time-series has now been extended to year 2023 with the work of Scheiwiller et al. (manuscript in preparation) and preliminary results show that while more Pacific waters seemed to ouflow the Arctic in years 2016 and 2017, a shift back towards more Atlantic derived waters happened during 2021.
(iii): Quantifying the tracer-based fluxes and changes of deep-water fomation in the subpolar North Atlantic and further soutwards spreading through the Deep Western Boundary Current.
Results by Castrillejo et al. (2022) of samples collected in the subpolar North Atlantic (SPNA) showed that I-129 was present at deep waters and spreading with Iceland-Scotland Overflow Waters and Labrador Sea Waters, intruding the Atlantic Ocean at unprecedented rate and labelling much larger extensions and water masses than in the recent past. Results from Dale et al. (2024) and Leist et al. (submitted to Frontiers in Marine Sciences) extend the results from Castrillejo, by adding data on U-236 and show the potential of combining both tracers to constrain the composition of overflow waters in the SPNA.
(iv): Develop a new method that combines the cosmogenic long-lived radionuclides Ar-39 and C-14 to estimate ventilation timescales in the deep Arctic Ocean. This method builds up on previous methods developed by Wally Broecker and colleagues in the 1980s and establishes a new way of calculating timescales of ventilation using the Transit Time Distribution approach. This study by Raimondi et al. includes the new results on C-14 and Ar-39 collected in the Canada Arctic Basin in 2022 and it will be soon submitted.
