Dissolved Molybdenum, Tungsten, and Vanadium at the Oxic–Anoxic Interface in the Black Sea

The 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) in the studied area was about 165 m (at a potential density of ∼16.2 kg m^–3). Water samples with 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 zone, molybdenum decreased when the hydrogen sulfide concentration exceeded ∼8 μM and reached 3.3 nmol/kg at 320 m. The tungsten concentration decreased from 160 pmol/kg at the surface to 113 pmol/kg at the redox interface (in the suboxic layer at depth 150 m) in the presence of particulate manganese. As Mn oxyhydroxides dissolved in the hydrogen sulfide zone, W concentrations increased to 221 pmol/kg at a depth of 180 m, along with an increase in dissolved Mn. The distribution of W at the redox interface is controlled by the sorption properties of Mn 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 of 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 t of Mo and 1200 t of V are annually buried in the sediments. Tungsten is thought to be supplied in significant amounts to the Black Sea in the form of suspended and colloidal matter in riverine waters, and this matter passes then into seawater in the process of suboxic diagenesis in sediments.