Tracing the Three Atlantic Branches Entering the Arctic Ocean With ¹²⁹I and ²³⁶U

Abstract

This study presents the data on 129 I and 236 U concentrations in seawater samples and sea ice cores obtained during two expeditions to the Arctic Ocean that took place onboard R/V Polarstern (PS94) and R/V Lance (N-ICE2015) in summer 2015. Carbon-14 was also measured in the deep water samples from the Nansen, Amundsen, and Makarov Basins. The main goal was to investigate the distribution of 129 I and 236 U in a transect from the Norwegian Coast to the Makarov Basin to fully exploit the potential of combining 129 I and 236 U as a dual tracer to track Atlantic waters throughout the Arctic Ocean. The use of the 129 I/ 236 U and 236 U/ 238 U atom ratios allowed identifying a third Atlantic branch that enters the Arctic Ocean (the Arctic Shelf Break Branch) following the Norwegian Coastal Current that carries a larger proportion of the European reprocessing plants signal compared to Fram Strait Branch Water and Barents Sea Branch Water. The combination of 129 I and 236 U also allowed quantifying the different proportions of the La Hague stream, the Scottish stream, and Atlantic waters forming the three Atlantic branches of the Arctic Ocean Boundary Current. The results show that the 129 I/ 236 U atom ratio can now be used to identify the different Atlantic branches entering the Arctic Ocean. New input functions for 129 I, 236 U, and 129 I/ 236 U have also been described for each branch, which can be further used for calculation of transit time distributions of Atlantic waters. Plain Language SummaryIn this work we present results of artificial radionuclides that were measured in seawater samples collected in the Arctic Ocean. We measured the long-lived artificial radionuclides 129 I and 236 U to track the different water masses. These two radionuclides are present in the marine environment after the nuclear weapon tests (1950s-1960s) and from two European nuclear reprocessing plants (from 1960s until today) located in France and United Kingdom. In particular, the input of 129 I from these two reprocessing plants changed over time and can therefore also be used to estimate travel times of water masses. In this study, we collected about 150 seawater samples from 20 different stations in the Arctic Ocean in summer 2015, onboard the R/V Polarstern. Our results reveal that the mixing ratios of the two reprocessing plant effluents are different for the Fram Strait Branch and the Barents Sea Branch, contrasting with previous studies. The 129 I/ 236 U was used as a new tool to identify the characteristics of these two Atlantic branches entering the Arctic Ocean and even prove the existance of a more surface branch (Arctic Shelf Break Branch) carrying a significant proportion of reprocessing plant-derived radionuclides.

Shelf‐Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

“…The residence time of deep water in the Makarov Basin was determined to be 400–650 years based on the 226 Ra profile at station 30 (Figure S10) and the range in the 226 Ra flux from sediments in the Makarov Basin. This value is similar to previous estimates of ~300–600 years (Jones et al, ; Schlosser et al, ; Tanhua et al, ; Casacuberta et al, ; Rutgers van der Loeff et al, ). The deep waters at stations 48, 52, and 57 in the deep Canada Basin (Figure S10) were determined to have residence times of 130–270 years based on the range of 226 Ra fluxes estimated for sediments in both the Makarov and Canada Basins.…”

“…Intermediate waters move cyclonically around the Arctic; one pathway follows the Eurasian side of the Lomonosov Ridge and another crosses the Lomonosov to enter the Makarov and Canada Basins (Rudels et al, ; Schauer et al, ; Smethie et al, ). Anthropogenically sourced tracers such as 3 H, SF 6 , 129 I, and 236 U have been used to denote recent ventilation of intermediate water in the Amundsen, Canada, and Makarov Basins and indicate that the residence time of this water mass is on the order of decades (Casacuberta et al, ; Ekwurzel et al, ; Rutgers van der Loeff et al, ; Schlosser et al, ; Smethie et al, ; Smith et al, ; Smethie & Swift, ; Tanhua et al, ).…”

Tracing Atlantic Waters Using 129I and 236U in the Fram Strait in 2016

“…Yet a recent study (Casacuberta et al, ) found that the 129 I and 236 U signals from LH and SF do not completely mix in the North Sea, suggesting three different input functions for Atlantic waters entering the Arctic Ocean (two branches entering via Barents Sea, green and purple in Figure a and one via Fram Strait, orange in Figure a). The high 129 I/ 236 U ratio released from LH largely remains within the NCC, which enters the Barents Sea very close to the Norwegian coast.…”

“…Apart from the temporal variation of 129 I/ 236 U from RP, the combination of 129 I and 236 U allows distinguishing between the two main anthropogenic sources (i.e., RP and GF), as they are characterized by very different 129 I and 236 U concentrations as well as 129 I/ 236 U atom ratios (Casacuberta et al, , ). To this purpose, the dual‐tracer approach of using the 236 U/ 238 U ratio together with the 129 I/ 236 U ratio has been suggested and used to constrain sources of water masses (Christl, Casacuberta, Vockenhuber, et al, ; Casacuberta et al, ).…”

“…129 I and 236 U concentrations obtained for the outflowing waters in the Fram Strait indicate a large influence of NCC waters in the surface. Finally, we make use of the split input functions for 129 I defined recently (Casacuberta et al, ) to calculate transit times of waters transported from the Barents Sea through the Arctic Ocean to the Fram Strait by the different Atlantic branches.…”