The lateral distributions of Mn concentrations in the sediments of two Swiss lakes under varying oxygen conditions have been determined. The comparison of Mn distribution patterns with oxygen in the deep-water provides strong evidence for a geochemical-focusing effect, which is driven by the redox cycle of manganese. Conditions essential for this process to occur are anoxic sediments in contact with oxic deep-water. Average sedimentary manganese concentrations determined for different water-depth ranges are directly proportional to the area of shallower sediments. This result indicates that geochemical-focusing of manganese in lake sediments is a promising proxy indicator for the reconstruction of oxygen conditions during deposition.

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Distinct layers with accumulated iron and manganese oxyhydroxides are found in the recent sediments of Lake Baikal (Siberia). In the South and Central Basins, these concretions accumulate close to the sediment-water interface. In northern Lake Baikal and the area of Academician Ridge, however, massive Fe/Mn crusts are formed within several thousand years at redox fronts 10 to 15 cm below the sediment surface. In some places, precipitated iron and manganese oxyhydroxides are spatially separated. The patterns are a result of secondary iron and manganese oxide precipitation. This natural long-term experiment allows the analysis of competitive adsorption and coprecipitation of trace elements with iron and manganese oxides in sediments. Background concentrations in the sediment of oxoanions (P, As, Sb, Mo); of trace metals (Cr, V, Cu, Zn, Cd, Pb); and of Mg, Ca, Sr, La, Ce, Pr, Nd, and Sm were analyzed by inductively coupled plasma mass spectrometry. Despite the differences in catchment geology of the many tributaries, they are remarkably uniform in sediment cores from different basins of Lake Baikal. Enrichment factors of P and As within Fe crusts revealed concentrations up to 14 and 58 times higher than the background, respectively. No enrichment of P and As was found in the Mn layers. By contrast, Mo accumulated exclusively in the Mn layer with up to 35-fold enrichment. Sb was only slightly enriched in both the Fe and the Mn layers. Among the trace metals studied, only Cd was found at elevated concentrations with a preference for the Mn layer. Ca and Sr were correlated with both Fe and Mn accumulations. The study quantifies the well-known specific adsorption and coprecipitation of P and As at authigenic iron oxides and of Mo on manganese oxides. In addition, the enrichment of Cd at manganese oxides in contrast to the conservative behavior of Zn and Pb reveals highly selective accumulation processes.

In Switzerland, unleaded gasoline was introduced in 1985. In order to study the effects of decreasing lead emissions from gasoline on a lake sediment, profiles of lead concentrations and 206Pb/207Pb ratios have been determined in cores from Lake Zug. Sediment samples were digested and analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Accurate dating of sediment cores was achieved using four independent methods showing the results within a range of <10%. Pb emissions in Switzerland and Pb concentrations in the sediment correlated well. Background Pb concentrations in pre-industrial sedi ments were 16 μg/g in Lake Zug and 13 μg/g in Lake Baldegg (for comparison); background 206Pb/207Pb ratios were 1.20 in both lakes. In Lake Zug, lead concentrations reached a maximum of 110 μg/g in a depth corresponding to ca. 1970, the time of highest lead emissions. The 206Pb/207Pb ratio profile provided additional information because this ratio of Pb used for gasoline additives in Europe ranges from 1.04 to 1.10, and the ratio of background Pb in central Europe is about 1.20. The ratios in the sediment had a minimum of 1.13 ca. 1980 and an increase up to 1.16 since ca. 1990. The isotope ratios reflected the frac tion of Pb emissions from gasoline.