Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate

Abstract

We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is . 100 times higher in nearsurface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep-water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end-member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace-nutrient-limited Southern Ocean.

Diagnosing oceanic nutrient deficiency

“…Data on the distributions of the macronutrient elements N, P, Si [44,[62][63][64][65][66][67], the micronutrient elements Fe, Mn, Ni, Zn and Cd [64,[68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84] and the non-nutrient element Al [67,85,86] were downloaded from the IDP2014 collection [20] and imported into a Matlab™ environment for analysis. For each sampling location and/or seawater volume/water mass, a ranking of the relative deficiencies of multiple nutrient elements was calculated by dividing observed dissolved concentrations by an assumed 'typical' stoichiometric ratio within newly formed organic (biological) material (figure 1) [11,18].…”

Determination of the Side-Reaction Coefficient of Desferrioxamine B in Trace-Metal-Free Seawater

“…Organic ligands dominate the solution speciation in seawater of many trace metals, notably Fe (Rue and Bruland, 1995), Co (Baars and Croot, 2015), Ni (van den Berg and Nimmo, 1987), Cu (Jacquot et al, 2013), Zn (Jakuba et al, 2012), Cd (Baars et al, 2014), and Pb (Capodaglio et al, 1998). Ligand concentrations and conditional stability constants can be measured for specific metals using electrochemical techniques (Pižeta et al, 2015), but the identity of these molecules remains largely unknown.…”

“…Schijf et al (2015) recently measured stability constants of metal-DFOB complexes in 0.7 M NaClO 4 and found them to be similar to published conditional stability constants of complexes with marine organic ligands for Cu, Zn, and Pb, yet orders of magnitude smaller for Ni and Cd. It has indeed been suggested that natural Cd-specific ligands contain sulfurbearing groups (Bruland, 1992;Baars et al, 2014) and do not resemble siderophores. Such comparisons are only meaningful if stability constants measured in non-coordinating media can be adjusted to seawater conditions by correction with a suitable side-reaction coefficient (SRC).…”

Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade

“…Studying both elements in the Southern Ocean is also of interest because of their use as paleo-tracers for past ocean circulation and nutrient availability. Speciation measurements south of the Polar Front reveal high concentrations of dissolved bioavailable Zn and Cd (Baars et al 2014) that help to explain high apparent Zn/P and Cd/P uptake ratios in the Southern Ocean Baars et al 2014). Local changes in potential Cd uptake rates, inferred from Cd* (defined as the deviation of total dissolved Cd from a linear deep water Cd vs. P relation), suggest that the Cd/P uptake ratios are also dependent on the availability of Zn, Mn, and Fe, which leads to a peculiar band of negative Cd* in sub-Antarctic mode water and Antarctic Intermediate Water (Baars et al 2014;Fig.…”