Elemental composition of a deep sediment core from Lake Stocksjoen in the Forsmark area

A deep sediment core was taken from Lake Stocksjoen, situated within the Forsmark site investigation area. The 55 cm long sediment core, representing the entire history of the lake (approx 430 years) was sliced in 5 cm portions and analysed for various chemical elements, using ICP-MS technique. In total, 54 different elements - classified as main elements, heavy metals and trace elements - were analysed. In general terms, three different patterns of stratigraphy were derived from all the analysed elements. Calcium, manganese, lead and mercury occurred in highest concentrations in the upper sediments (<30 cm depth). Phosphorus, zinc, cadmium, antimony, tin and strontium occurred in more even proportions throughout the sediment core. All the other elements were substantially reduced in the upper parts (<30 cm) compared to the deeper parts of the sediment core. Metals that are considered as airborne pollutants were found in low or moderate concentrations. This is in concert with other investigations of pollutants that have been performed in the Forsmark area. The sediment of Lake Stocksjoen is highly organic, and has been so during the entire history of the lake. Much of the organic Material seems to be refractory and less susceptible for mineralisation and respiration during the prevailing environmental conditions. This corresponds well with the characteristic gelatinous cyanophycee gyttja found in the lower parts of the sediment core. Although speculative, the pronounced changes in elemental composition of the sediment at 30 cm depth may correspond to the final isolation of the lake from the Baltic Sea, which occurred approximately 230 years ago. The deeper parts (below 30 cm depth) thus may represent the time period with regular intrusions of brackish water into the lake basin. One important factor governing the environmental conditions and the resulting elemental composition of the sediment is the unusually thick 'microbial mat', which is characteristic for the surface sediments of Lake Stocksjoen and other oligotrophic hardwater lakes in the Forsmark area. Reducing conditions and low pH, which frequently are found close beneath the surface layers of many lake sediments, are in this case directed to the deeper layers. The primary producers of the microbial mat promotes precipitation of calcium carbonates and co-precipitation of phosphorus compounds, and prevents recycling of e.g. phosphorus to the lake water during large parts of the year. The data gathered in this investigation will be further used in future evaluations of transport and turnover of various elements within the catchments of the Forsmark site investigation area, as well as in research regarding ontogenetic patterns of lake ecosystems