Anthropogenic Carbon in the Arctic Ocean: Perspectives From Different Transient Tracers

In this study we investigated the physical characteristics of the Atlantic layer in the Arctic Ocean (AO) and its role in the distribution and storage of anthropogenic carbon (Cant). The novelty of this work is to use the Transit Time Distribution method (TTD) with the radionuclides 129I and 236U and its comparison to the commonly applied gas tracers, CFC-12 and SF6. Overall, our examination of two distinct tracer pairs, along with the novel TTD method in comparison to a classical approach, revealed a notable agreement, highlighting the robustness of these Cant estimates. The TTD analysis based on radionuclides showed that whereas the Eurasian Basin has shorter transit times and is dominated by strong mixing conditions, the Amerasian Basin is characterized by longer transit times and a strong advective flow. Overall, the Cant concentrations obtained from radionuclides confirm that the distribution in the AO follows its circulation patterns, with higher concentrations in the Eurasian Basin (similar to 50 mu mol kg-1) compared to the Amerasian one (similar to 36-42 mu mol kg-1). An estimated partial inventory of 0.85 +/- 0.17 and 1.0 +/- 0.03 Pg C was assessed for 2015 from the novel application of TTD with radionuclides and gas tracers, respectively. Finally, we identified the saturation of gas tracers as a larger source of uncertainty for Cant estimation compared to the uncertainty associated to different radionuclides' input functions, thus remarking the importance of including non-saturation dependent tracers, such as radionuclides, as an additional tool to support Cant estimates in the AO. The oceans store large amounts of the CO2 emitted by human activities (anthropogenic carbon; Cant). In particular, the Arctic Ocean (AO) is one of the regions where Cant can be sequestered for long periods of time. This region is changing much faster than the rest of the world oceans. Nevertheless, the effect of these changes on its capacity to store Cant has been studied only by a few authors. Because Cant cannot be measured in the ocean, here we provide new estimates of Cant in the AO by using other human-produced substances (tracers) that have been deliberately released to the North Sea by the nuclear industry: the two artificial radionuclides 129I and 236U. To mimic Cant from these tracers we used a statistical method called the Transit Time Distribution (TTD). We found that results from our tracers compare well to those from other long-used tracers (the gases CFC-12 and SF6), proving that these radionuclides can reliably estimate Cant and even have certain advantages compared to gas tracers. Anthropogenic Carbon in the Arctic Ocean was estimated for the first time using artificial radionuclides 129I and 236UA multi-tracer approach is recommended to better understand the physical properties of the Arctic basinThe comparison to gas tracers shows that these radionuclides can provide robust estimates of Cant in the Arctic Ocean