Effect of urbanization on metal deposition in the bay of Töölönlahti, Southern Finland

A sediment core from the Bay of Töölönlahti in the centre of Helsinki, Finland was analysed by a selective extraction method in order to examine the geochemical impacts of urbanization. Concentrations of Al, Ca, Cu, Fe, Mn and Zn in the extracts and total P and S were measured at intervals of 1 cm to a depth of 100 cm. The data were subjected to principal components analysis. Sedimentation rate was determined using 137Cs- and 14C-measurements and SCP-analysis and it varied from 0.4 to 0.6 mm•yr−1. Most of the background metal deposition in the basin is acid soluble. The intense eutrophication began around 1900 and is reflected in the sediment by a rise in the concentrations of S, P, organically bound Cu, Fe, Mn, Zn and hydroxide Zn, Total Al, Ca and Mn decrease towards the surface, probably because of dilution by organic inputs or by biogenic silica. The total Cu and Zn were high in the surface with a large proportion of labile forms.