Trace metal distributions in shelf waters of the northwestern Black Sea

Measurements of dissolved and leachable particulate trace metals (Mn, Fe, C o, Pb, Cd, Zn, Cu and Ni) and total particulate Mn and Fe were made on seaw ater samples collected from the northwestern Black Sea during the EROS 2000 expedition conducted in July-August 1995. The investigation concentrated o n waters of the shelf and shelf edge, but included one deeper water (1440 m ) station. In the oxic layer of the deep station, the suspended particulate fractions of Mn and Fe were a major part of the total metal mass, consiste nt with the presence of the "Fine Particle Layer" which forms on the shelf and spreads all over the Black Sea with intensities decreasing from the coa st. Dissolved and total particulate concentrations were, respectively, Mn, 0.69-9.6, 1.2-29; Fe, 0.79-3.03, 2.3-7.4 nM. Dissolved Cu and Ni concentrat ions were relatively high (1-8 and 8-12 nM, respectively), and did not show any depletion in surface oxic waters, possibly as a result of strong organ ic complexation. Dissolved Pb concentrations (100-200 pM) were higher than were generally found on the shelf. This was attributed to atmospheric input s combined with less efficient scavenging of metals in these low SPM waters . The distribution of dissolved Co closely resembled that of dissolved Mn r eflecting coupling through oxidation of Mn. Concentrations of dissolved Cd and Zn were low in surface water (0.07-0.09 and 0.9-2.0 nM, respectively), and increases in concentrations with depth were sharply reversed around the top of the redoxcline. For most metals (Mn, Fe, Co, Pb, Cu, Cd, Zn) dissol ved concentrations were low in the anoxic layers as a result of solubility by formation of, or association with, solid sulphide phases. Dissolved Ni w as not affected by sulphide precipitation. At most of the shelf stations th ere were clear enhancements of dissolved Mn and Fe in the deepest waters, c onsistent with other evidence that significant benthic fluxes of these meta ls arise through the redox conditions in the region of the sediment-water i nterface. In the shelf water column, dissolved Mn and Fe concentrations ran ged between 1.2 and 1350 and between 0.4 and 181 nM, respectively; the high est concentrations were found near the bottom. Particulate concentrations o f Mn and Fe were high, implying high oxidation rates of Mn(II) and Fe(II) a nd/or high supply rates from rivers. Total particulate concentrations of Mn and Fe were 0.7-1050 and 2.3-2650 nM, respectively; the highest concentrat ions were found ill surface and bottom waters. The distributions of particu late Mn and Fe were consistent with the isopyenal transport or Mn and Fe ox yhydroxides from the shelf by the coastal circulation. Distributions of oth er trace metals (Co, Pb, Cu, Ni, Cd, Zn) were considerably influenced by ri verine inputs. Relatively high dissolved and available particulate metal co ncentrations were generally found in surface waters at stations directly in fluenced by the Danube River. Some trace metals (Co, Ni, Cd and Zn) were in fluenced by Mn and Fe cycling and increases in their dissolved concentratio ns occurred at a number of stations near the sediment-water interface. Diss olved and available particulate metal concentrations (nM) at stations oil t he shelf were, respectively: 0.171-1.80, 0.003-0.437 (Co); 0.014-0.614, 0.0 10-1.48 (Pb); 7.6-28.8, 0.048-3.75 (Cu); 11.0-17.5, 0.018-2.10 (Ni); 0.033- 0.161, 0.003-0.063 (Cd); 1.01 8.33, 0.135 7.58 (Zn). (C) 2001 Elsevier Scie nce Ltd. All rights reserved.