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 phase s were measured during monitoring cruises in Gotland Basin between 1992, an d 1996. The situation during the investigation period was influenced by a m ajor inflow of dense and oxygen-rich water into the Baltic Sea at the begin ning 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 condition s as a direct effect on the geochemical behaviour of trace metals in the wa ter column the scavenging of Cd and Pb by Mn-precipitates is discussed unde r oxic conditions. In 1992 dissolved metal concentrations decreased by fact ors of 3 (Cd), 2.5 (Cu). 2 (Pb) in the anoxic deep waters, compared with su rface 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. A fter the saltwater inflow had brought oxic conditions, the elimination of d issolved Cd and Pb was influenced by high contents of Mn (IV) precipitates, while for Cu the dissolved species (hydroxo-, chloro-, and organic complex es) predominated. (C) 1999 Elsevier Science B.V. All rights reserved.