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

Abstract

<jats:p>Abstract. The organic speciation of dissolved cobalt (DCo) was investigated in the subtropical region of the southeastern Atlantic, and in the Southern Ocean in the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre, between 34°25´ S and 57°33´ S along the Greenwich Meridian during the austral summer of 2008. The organic speciation of dissolved cobalt was determined by competing ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using nioxime as a competing ligand. The concentrations of the organic ligands (L) ranged between 26 and 73 pM, and the conditional stability constants (log K'CoL) of the organic complexes of Co between 17.9 and 20.1. Most dissolved cobalt was organically complexed in the water-column (60 to &gt;99.9%). There were clear vertical and meridional patterns in the distribution of L and the organic speciation of DCo along the section. These patterns suggest a biological source of the organic ligands in the surface waters of the subtropical domain and northern subantarctic region, potentially driven by the cyanobacteria, and a removal of the organic Co by direct or indirect biological uptake. The highest L:DCo ratio (5.81 ± 1.07 pM pM−1) observed in these surface waters reflected the combined effects of ligand production and DCo consumption. As a result of these combined effects, the calculated concentrations of inorganic Co ([Co']) were very low in the subtropical and subantarctic surface waters, generally between 10−19 and 10−17 M. In intermediate and deep waters, the South African margins can be a source of organic ligands, as it was suggested to be for DCo (Bown et al., 2011), although a significant portion of DCo (up to 15%) can be stabilized and transported as inorganic species in those DCo-enriched water-masses. Contrastingly, the distribution of L does not suggest an intense biological production of L around the Antarctic Polar Front where a diatom bloom had recently occurred. Here [Co'] can be several orders of magnitude higher than those reported in the subtropical domain, suggesting that cobalt limitation was unlikely in the ACC domain. The almost invariant L:DCo ratio of ~1 recorded in these surface waters also reflected the conservative behaviours of both L and DCo. In deeper waters higher ligand concentrations were observed in waters previously identified as DCo sources (Bown et al., 2011). At those depths the eastward increase of DCo from the Drake Passage to the Greenwich Meridian could be associated with a large scale transport and remineralisation of DCo as organic complexes; here, the fraction stabilized as inorganic Co was also significant (up to 25%) in the low oxygenated Upper Circumpolar Deep Waters. Organic speciation may thus 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.</jats:p>

Imprint of a dissolved cobalt basaltic source on the Kerguelen Plateau

“…This would indicate that basalt dissolution rates are elementspecific and that Co dissolution could be slower than Fe dissolution, which can partially explain why DCo concentrations do not display their highest values close to C01, suspected to be the most impacted station by the lithogenic inputs from Heard Island for Fe, REE and Ra (Blain et al, 2008b;van Beek et al, 2008;Zhang et al, 2008;Chever et al, 2010). In addition, the organic complexation of DCo may enhance both the basaltic dissolution (Hausrath et al, 2009) and the stabilization of Co in the dissolved phase as previously observed in the Southern Ocean (Bown et al, 2012). Furthermore, the biological removal term could be much higher for Fe than for Co above the plateau, as suggested by the absence of a nutrient-like vertical distribution of DCo (Figs.…”

“…The latter study also showed that the organic complexation of DCo would probably promote Co stabilization and its transport from continental margins to the open ocean (Bown et al, 2012). According to the budget at station A08, vertical diffusion from the sediment (4.41-31.1 nmol m −2 d −1 , Fig.…”

“…Their chemistry could show similarities in seawater, since both elements are redox sensitive and they can occur in two different oxidation states (+II or +III) depending on seawater oxygenation (Boye et al, 2006;Ellwood and van den Berg, 2001;Saito and Moffett, 2001). Moreover Co and Fe have been shown to be strongly complexed by organic ligands, which can impact their respective bioavailability and residence time in the water column (Boye et al, 2001;Saito and Moffett, 2001;Ellwood and van den Berg, 2001;Bown et al, 2012). Despite Co and Fe having similar residence times in the whole ocean (Bruland et al, 1994;Saito and Moffett, 2002), Co can exhibit contrasted vertical distributions (e.g.…”

Toward a Regional Classification to Provide a More Inclusive Examination of the Ocean Biogeochemistry of Iron-Binding Ligands

“…Copper and zinc complexation with HStype ligands in seawater is strong (Yang and van den Berg, 2009), and copper and iron compete for L 2 type ligands in estuarine and coastal waters (Abualhaija et al, 2015). The conditional stability constants for marine organic ligands with nickel and cobalt are high (Martino et al, 2004;Ellwood et al, 2005;Bown et al, 2012;Baars and Croot, 2015) suggesting a possible competition with iron providing they bind with the same ligands. Indeed, Cu and Ni competition for binding to the same organic ligand has been recently observed (Boiteau et al, 2016b; this issue).…”

The cobalt cycle in the Tropical Pacific Ocean

“…This strong dependence of cellular cobalt quotas on Co' indicates that cobalt transport is not easily saturated at Co' concentrations below 16 pM, which are higher than that found in most oceanic settings (Bown et al, 2012b; Prochlorococcus iron quotas exhibited a more complex response to iron limitation. As with cobalt, lower dissolved Fe decreased both cellular Fe quotas and growth rates (Fig.…”