Natural copper-binding ligands in the Arctic Ocean. The influence of the Transpolar Drift (GEOTRACES GN04)

The Arctic Ocean is a unique biogeochemical environment characterized by low salinity surface waters, extensive sea-ice coverage, high riverine inputs, large shelf extension and the long residence time of deep waters. These characteristics determine the distribution of dissolved bio-essential trace metals, such as copper (Cu), and the dissolved organic-binding ligands capable of complexing it. This work reports the concentrations and conditional stability constants of dissolved Cu-binding ligands (L-Cu and log K-Cu2+L(cond)) measured in samples from the Polarstern (PS94) expedition, as part of the international GEOTRACES program (cruise GN04). Full-depth profile stations from the Barents Sea, Nansen Basin, Amundsen Basin and Makarov Basin were analysed by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV). The basins and water masses presented a wide range of L-Cu concentrations (range: 1.40 - 7.91 nM) and log K-Cu2+L(cond) values (range: 13.83 - 16.01). The highest variability of Cu-binding ligand concentrations was observed in surface waters (<= 200 m), and mean concentrations increased from the Barents Sea and Nansen Basin (2.15 +/- 0.31 nM and 1.93 +/- 0.35 nM, respectively) to the Amundsen (3.84 +/- 1.69 nM) and Makarov Basins (4.40 +/- 2.03 nM). The influence of the Transpolar Drift (TDP) flow path was observed in the Amundsen and Makarov Basins, especially on Cu-binding ligand concentrations (L-Cu range: 3.96 - 7.91 nM). In contrast, deep waters (>200 m) showed no significant differences between basins and water masses in terms of L-Cu concentrations (range: 1.45 - 2.78 nM) and log K-Cu2+L(cond) (range: 14.02 - 15.46). The presence of strong Cu-binding ligands (log K-Cu2+L(cond)>13) in surface waters stabilises the excess of dissolved copper (dCu) transported in the TPD and favours its export to the Fram Strait and Nordic Seas.