New insights on the role of organic speciation in the biogeochemical cycle of dissolved cobalt in the southeastern Atlantic and the Southern Ocean

摘要

The organic speciation of dissolved cobalt (DCo) was investigated in thesubtropical region of the southeastern Atlantic, and in the Southern Oceanin the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre,between 34°25′ S and 57°33′ S along the Greenwich Meridian duringthe austral summer of 2008. The organic speciation of dissolved cobalt wasdetermined by competing ligand exchange adsorptive cathodic strippingvoltammetry (CLE-AdCSV) using nioxime as a competing ligand. Theconcentrations of the organic ligands (L) ranged between 26 and 73 pM, andthe conditional stability constants (log K'_CoL) of the organiccomplexes of Co between 17.9 and 20.1. Most dissolved cobalt was organicallycomplexed in the water-column (60 to >99.9%). There were clearvertical and meridional patterns in the distribution of L and the organicspeciation of DCo along the section. These patterns suggest a biologicalsource of the organic ligands in the surface waters of the subtropicaldomain and northern subantarctic region, potentially driven by thecyanobacteria, and a removal of the organic Co by direct or indirectbiological uptake. The highest L:DCo ratio (5.81 ± 1.07 pM pM?1)observed in these surface waters reflected the combined effects of ligandproduction and DCo consumption. As a result of these combined effects, thecalculated concentrations of inorganic Co ([Co']) were very low in thesubtropical and subantarctic surface waters, generally between 10−19and 10−17 M. In intermediate and deep waters, the South African marginscan be a source of organic ligands, as it was suggested to be for DCo (Bownet al., 2011), although a significant portion of DCo (up to 15%) can bestabilized and transported as inorganic species in those DCo-enrichedwater-masses. Contrastingly, the distribution of L does not suggest anintense biological production of L around the Antarctic Polar Front where adiatom bloom had recently occurred. Here [Co'] can be several orders ofmagnitude higher than those reported in the subtropical domain, suggestingthat cobalt limitation was unlikely in the ACC domain. The almost invariantL:DCo ratio of ~1 recorded in these surface waters also reflected theconservative behaviours of both L and DCo. In deeper waters higher ligandconcentrations were observed in waters previously identified as DCo sources(Bown et al., 2011). At those depths the eastward increase of DCo from theDrake Passage to the Greenwich Meridian could be associated with a largescale transport and remineralisation of DCo as organic complexes; here, thefraction stabilized as inorganic Co was also significant (up to 25%) inthe low oxygenated Upper Circumpolar Deep Waters. Organic speciation maythus be a central factor in the biogeochemical cycle of DCo in those areas,playing a major role in the bioavailability and the geochemistry of Co.