HIGH-RESOLUTION PB-210 DATING OF LAKE CONSTANCE SEDIMENTS - STABLE LEAD IN LAKE CONSTANCE

Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated by gamma-spectrometry for (210)pb, (CS)-C-134, (CS)-C-137, Am-241, Th-234, and other members of the U-238 decay chain. The sediments were dated using the constant-flux model for (210)pb, and accumulation rates were determined. These range from 0.04 to 0.65 g/cm(2)/yr (BO90/13b) and 0.04 to 0.8 g/cm(2)/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16 g/cm(2)/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for (210)pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and (210)pb datings. With Am-241 a further radioactive isotope is available, which can, due to the half-life of Pu-241 (t(1/2) = 14.4 yr), be detected now by gamma-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.