A Multi-Tracer Study of Ventilation and Anthropogenic Carbon Storage in the Arctic Ocean

The Arctic Ocean (AO) is a critical sink for anthropogenic carbon (C_ant), sequestering emissions via intermediate and deep water formation, storing it for long periods of time. Understanding the timescales of these ventilation processes is essential for calculating the current inventory of C_ant as well as predicting the AO’s capacity to store CO₂ in a warming climate. However, observational constraints remain limited; while standard transient tracers (SF₆, CFC-12) and other radionuclides successfully resolve surface and intermediate layers, they often fail to capture the older waters of the deep basins. Atom Trap Trace Analysis (ATTA) has opened a new way to measure deep ocean water residence times by using the radioactive noble gas ³⁹Ar. Here we show the first fully resolved vertical profiles of Arctic ventilation and C_ant using Transit Time Distributions (TTDs) derived from a novel combination of short-lived tracers and long-lived radioisotopes (³⁹Ar, ¹⁴C). Their vertical distribution brings key information on ocean ventilation and hence the storage of anthropogenic carbon. We apply a Bayesian Inference framework to fit the TTD parameters from the different tracer data constraints.

We find that the mixing regime across the Nansen, Amundsen, and Makarov Basins is more advection-dominated than previously assumed in the deep basins. The profiles reveal that the Arctic stores up to 33% of its total C_ant inventory below 1,500 m—vastly exceeding the global ocean average of ∼7%. While the deep Makarov Basin holds roughly half the carbon content of the Eurasian Basin, both reservoirs play a disproportionate role in deep sequestration. Conversely, we demonstrate that for the Atlantic Water Layer, which contains the bulk of the carbon, adding long-lived radioisotopes offers negligible improvement over standard tracers. These findings refine the Arctic carbon budget and highlight the necessity of adding long-lived radionuclides for constraining the deep ocean sink.