The effect of redox processes on the partitioning of Cd, Pb, Cu, and Mn between dissolved and particulate phases in the Baltic Sea

The concentrations of Cd, Pb, Cu, Mn in the dissolved and particulate phases were measured during monitoring cruises in Gotland Basin between 1992 and 1996. The situation during the investigation period was influenced by a major inflow of dense and oxygen-rich water into the Baltic Sea at the beginning of 1993. This terminated a stagnation period of nearly seventeen years and caused oxygen concentrations to increase, mainly in the deep waters of the Bornholm- and Gotland basins. Beside the change of the redox conditions as a direct effect on the geochemical behaviour of trace metals in the water column the scavenging of Cd and Pb by Mn-precipitates is discussed under oxic conditions. In 1992 dissolved metal concentrations decreased by factors of 3 (Cd), 2.5 (Cu), 2 (Pb) in the anoxic deep waters, compared with surface concentrations of 0.16 nM/kg (Cd), 0.29 nM/kg (Pb) and 10.1 nM/kg (Cu), due to the formation of sulfidic metal species scavenged by particles. After the saltwater inflow had brought oxic conditions, the elimination of dissolved Cd and Pb was influenced by high contents of Mn (IV) precipitates, while for Cu the dissolved species (hydroxo-, chloro-, and organic complexes) predominated.