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  • 1
    ISSN: 1432-0495
    Keywords: 210Pb dating ; Lead ; Sediments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated byγ-spectrometry for210Pb,134Cs,137Cs,241Am,234Th, and other members of the238U decay chain. The sediments were dated using the constant-flux model for210Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm 2/yr (BO90/13b) and 0.04 to 0.8g/cm 2/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/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 for210Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and210Pb datings. With241Am a further radioactive isotope is available, which can, due to the half-life of241Pu (t 1/2=14.4yr) be detected now byγ-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.
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  • 2
    ISSN: 1432-0495
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Lake Zürich occupies a glacially overdeepened perialpine trough in the northern Middlelands of Switzerland. A total of 154.4 m of Quaternary sediments and 47.3 m of Tertiary Molasse bedrock has been cored from the deepest part of the lake, some 10 km south of the city of Zürich. Some 16.8 m of gravels and sands directly overlying the bedrock include basal till and probably earliest subglacial fluvial and lacustrine deposits. These are overlain by 98.6 m of fine-grained, glacial-aged sediments comprising completely deformed proglacial and/or subglacial lacustrine muds, separated by four basal mud tills. The lack of interglacial sediments, fossils, and other datable material, and the presence of severe sediment deformation and unknown amounts of erosion prevent the establishment of an exact chronostratigraphy for sediments older than the upper mud till. Above it some 8.6 m of lacustrine muds were deposited, folded, faulted, and tilted during the final opening of the lake at about 17,500–17,000 years ago. Superimposed are 30.4 m of final Würm and post-glacial sediments comprising (from oldest): cyclic proglacial mud, thick-bedded and laminated mud, a complex transition zone, laminated carbonate, laminated marl, and diatom-calcite varves. These sediments reflect changing catchment and lacustrine conditions including: glacial proximity, catchment stability, lake inflow characteristics, thermal structure, chemistry, and bed stability. Average sedimentation rates ranged from 11 cm yr−1 immediately after glacier withdrawal, to as low as 0.4 mm yr−1 as the environment stabilized. The lack of coarse outwash deposits separating the fine-grained glaciolacustrine sediments from a corresponding underlying basal till suggests that deglaciation of the deep northern basin of Lake Zürich was by stagnation-zone retreat rather than by retreat of an active ice-front.
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