Description: This study evaluates the trends in anthropogenic carbon (Cant) in the different sub-basins of the Arctic Ocean between 1987 and 2015. Data were extracted from the GLODAPv2 data product as well as two GEOTRACERS cruises in the Arctic Ocean from 2015 and Cant was evaluated using the transient time distribution (TTD) approach. In the Nansen and Amundsen sub-basins, the Cant trend in the Atlantic Waters (AW, depths: 200–500 m) and dense AW (dAW, depths: 500–800 m) is about +0.7 µmol kg−1 yr−1. As we move into the Makarov and West Canadian sub-basins, the Cant trend in the AW and dAW is smaller. The upper Polar Deep Water (uPDW, depths: 800–1600 m) has a Cant trend of about +0.4–0.5 µmol kg−1 yr−1 in the Nansen, Amundsen and West Canadian sub-basins. The trend is smallest in the South Canadian sub-basin, with a value of about +0.2 µmol kg−1 yr−1. Ventilation primarily governs Cant trends while the influence of the Revelle factor plays a secondary role. The increase in the Cant column inventory is estimated to be 0.7–1.0 mol C m−2 yr−1 in the Nansen, Amundsen and Makarov sub-basins. By extrapolating the results from our defined sub-basins to the full Arctic Ocean, we estimate an Cant accumulation of 25 Tg C yr−1 in the Arctic Ocean and an inventory of about 3.6–3.9 Pg C between 2005 and 2015.

Description: We evaluate the decadal evolution of ventilation and anthropogenic carbon (C-ant) in the Nordic Seas between 1982 and the 2010s. Ventilation changes on decadal timescale are identified by evaluating decadal changes in mean ages and apparent oxygen utilization in each of the four main basins of the Nordic Seas (the Greenland and Iceland Seas, and the Norwegian and Lofoten Basins). The ages are derived from the transient time distribution approach, based on the transient tracers chlorofluorocarbon-12 (CFC-12) and sulfur hexafluoride (SF6). The different decades show different phases in ventilation, with the 2000s being overall better ventilated than the 1990s in all basins. For the Greenland Sea, we also show that the 2010s are better ventilated than the 2000s, with a clear shift in hydrographic properties. The evolution of concentrations and inventory of C-ant is linked to the ventilation state. The deep waters get progressively older over the analyzed period, which is connected to the increased fraction of deep water from the Arctic Ocean.Plain Language Summary The ocean region between Greenland, Iceland, and Norway, called the Nordic Seas, is a main site of deep-water formation. This process produces dense waters and brings surface waters to larger depths, thereby ventilating the water below. This transports, among other things, man-made CO2 (anthropogenic carbon; C-ant) and oxygen from the atmosphere into the interior ocean, thereby reducing the amount of CO2 stored in the atmosphere. This study investigates how the ventilation has changed in the Nordic Seas from 1982 to the 2010s. We find that the ventilation has changed with time, from a rather well-ventilated state in 1982, to a reduced ventilation in the 1990s, and then a restrengthened ventilation from the 2000s.