Large-scale metal distribution in Baltic Sea sediments

The large-scale distribution pattern of metals in surface sediments shows elevated levels of Cd, Zn and Pb in the Baltic proper. The distribution of As is the opposite, with highest levels in the Bothnian Bay. Mercury shows elevated levels both in the Bothnian Bay and in coastal areas of the Bothnian Sea as well as in the archipelago of Stockholm. The vertical distribution of metals in sediment cores indicates mean enrichment factors of 10 for Cd, and 2–3 for Hg, Zn, Pb and As in the surface sediments of the Baltic proper. The enrichment factors for As, Hg and Pb are highest in the Bothnian Bay (18, 20 and 15 respectively as mean values, calculated from data expressed as ppm dry weight). Besides the anthropogenic load, the redox potential also has a marked influence on the distribution of Cd, Zn, Pb and Cu in the sediments of the Baltic proper. Laminated sediments at anoxic sites show elevated concentrations and drastic increases towards the sediment surface, of Cd and Zn especially, indicating an excess deposition to the sediments as metal sulphides. The vertical concentration profiles and mass balance calculations indicate that the reduced anthropogenic emissions are not yet reflected in the sequestering of As and Pb in the sediments of the Gulf of Bothnia, and that about half the loads of Cd, Zn and Hg are exported to the Baltic proper. The major part (about 90%) of the input of Cd to the Baltic proper is deposited in the sediments.