Pb isotopes in sediments of Lake Constance, Central Europe constrain the heavy metal pathways and the pollution history of the catchment, the lake and the regional atmosphere

Pb isotope ratios and Pb concentrations of well-dated sediments of Lake Con stance, Central Europe have been analyzed using thermal ion mass spectromet ry. Sequential extraction studies indicated isotope homogeneity of the leac hable Pb components within the investigated layers. Since the middle of the 19th century a significant anthropogenic Pb component appeared in the lake sediments, and rapidly approached concentration levels similar to that of the geogenic Pb background (20 ppm) at the beginning of the 20th century. A nthropogenic Pb was predominantly transferred to the lake sediments via the atmosphere. Pb sources were coal combustion, industrial ore processing and leaded gasoline. The flux of a fluvial Pb component to the lake sediments, additive to atmospheric Pb deposition, peaked in about 1960. This flux is attributed to (re)mobilization of Pb from polluted parts of the lake catchm ent, and indicates the change of catchment soils from a pollution sink to a heavy metal source. The strong reduction of anthropogenic Pb in the upperm ost lake sediments since the 1960's has been caused by advances of environm ental protection. The lake sediments record the changing fluxes and the isotope composition o f the deposited aeolian Pb pollution. During the 20th century aeolian Pb fl uxes to the lake sediments were in the range of 1-4 mu g/cm(2)/a. During pe ak emission periods of gasoline Pb to the atmosphere (1960-1990) the aeroso l Pb isotope composition was rather constant (Pb-206/Pb-207: 1.12-1.13) and probably a mixture of Canadian and Australian with Russian and Central Eur opean Pb types. Aeolian Pb isotope and Pb flux trends in the lake sediments as a whole agree well with the trends found in Alpine glaciers (Doring et al., 1997a,b) and in ombrotrophic peat bogs of Switzerland (Shotyk et al., 1996). However, different industrial Pb components were deposited in the ar chives of aeolian pollution during the early 20th century. Copyright (C) 19 99 Elsevier Science Ltd.