Vertical advection diffusion and redox potentials as controls on the distribution of manganese and other trace metals Dissolved in waters of the Black Sea

Abstract

Profiles of dissolved manganese, copper, iron, and zinc show that the distributions of these elements are markedly affected by redox reactions at the boundary between oxygenated surface waters and the sulfide-containing deep waters. Copper and zinc are depleted in the deep water by precipitation as insoluble sulfides. The concentrations of manganese and iron in the deep water greatly exceed those of the surface water principally because of the greater solubility of the sulfides and hydroxides of the reduced species as compared with the solubility of hydroxides and oxides of the oxidized species. The distribution of dissolved nickel and cobalt does not appear to be greatly affected by redox reactions. The profile of dissolved manganese, which shows a pronounced mid-water maximum about 40 meters below the oxygen zero boundary, has been explained with the aid of a vertical advection-diffusion model. We suggest that the Black Sea basin is currently acting as a very efficient trap for manganese. A flux of manganese, from surface particulates, of about 200 mg m−2 year−1, which is reduced and dissolved immediately upon penetrating to the sulfide-containing waters, builds up a mid-water maximum until the concentration gradient between the maximum and the deep water is sufficient to drive an equivalent diffusive flux of manganese into the deep water. Manganese is not lost by upward diffusion and advection because the reduced species is oxidized and precipitated just above the oxygen zero boundary and hence adds to the total flux of particulate manganese into the deep water. Currently the total flux of particulate manganese that is going into solution in the deep water is about 875 mg m−2 year−1 of which 675 mg m−2 year−1 is derived from the precipitation of dissolved manganese. The latter amount will increase in the future until the concentration of dissolved manganese at the midwater maximum exceeds the solubility product of some salt. Although we have performed no calculations, the shape of the dissolved iron profile indicates that a mechanism similar to that described for manganese is controlling the distribution. In addition it is likely that sulfide precipitation limits the iron concentration in the deep water.