Dissolved Molybdenum, Tungsten, and Vanadium at the Oxygen–hydrogen Sulfide Zone Boundary in the Black Sea

Distribution of dissolved molybdenum, tungsten, and vanadium was investigated in the northeastern part of the Black Sea down to a depth of 320 m. The depth of hydrogen sulfide appearance (the onset of the anaerobic zone) was about 165 m (at a potential density ~16.2 kg m–3) in the studied region. Water samples representing dissolved (< 0.45 μm) species and dissolved plus labile particulate species of the elements were collected in July 2016 and 2017. The concentration of dissolved Mo increased with depth in the oxic zone, from 36 to 39 nmol/kg, and showed no difference from the sum of dissolved and particulate forms. In the anoxic, molybdenum decreased with the appearance of more than ~8 μM hydrogen sulfide reaching 3.3 nmol/kg at 320 m. The concentration of tungsten decreased from 160 pmol/kg at the surface to 113 pmol/kg at the redox interface (in the suboxic layer at depth 150m) in the presence of particulate manganese. As Mn oxyhydroxides dissolved in the hydrogen sulfide zone, W concentrations increased to 221 pmol/kg at the depth 180m, along with an increase in dissolved Mn. The distribution of W at the redox interface is controlled by the sorption properties of manganese oxide. Dissolved vanadium was depleted at a depth of 5 m and increased with depth in the oxic zone to 13 nmol/kg, with a decrease in the suboxic zone (down to 7.1 nmol/kg). In the anoxic zone, a maximum V concentration (up to 15.2 nmol/kg) was observed, coinciding with the maximum of dissolved Mn. The calculated balance of Mo and V in the Black Sea showed that about 1200 of Mo and 1200 of V are buried in sediments annually. As for tungsten, it is assumed its significant supply to the Black Sea in the form of suspended and colloidal matter from rivers, transformed in the process of suboxic diagenesis in sediments.