ATMOSPHERIC LEAD DEPOSITION FROM 12,400 TO CA. 2,000 YRS BP IN A PEATBOG PROFILE, JURA MOUNTAINS, SWITZERLAND

The bog at Etang de la Gruere (Jura Mountains, Switzerland) consists o f 420 cm of Sphagnum-dominated bog peat overlying 230 cm of Carer-domi nated fen pear. One hundred cm below the bog surface, there is a prono unced peak in lead (Pb) concentration (approx. 10 mu g/g) which has be en dated at 2110 +/- 30 BP and can be attributed to Roman Pb mining an d smelting. Lead concentrations in peats from deeper, much older layer s were measured using ICP-MS and found to be low and relatively consta nt (0.28 +/- 0.04 mu g/g, n = 17) from 405 cm to 235 cm which correspo nds to the period from approx. 8,000 and 5,500 years before present (B P). In this same interval, scandium (Sc) concentrations (measured usin g INAA) were 0.07 +/- 0.02 mu g/g, yielding an average Pb/Sc ratio of 4.1 +/- 1.2. These values are assumed to represent the true ''backgrou nd'' Pb and Sc concentrations and Pb/Sc ratios of pre-anthropogenic ae rosols. At 205 cm the Pb concentrations began to increase by 2 to 3 ti mes, but these are proportional to the increases in aluminum (Al), tit anium (Ti), silicon (Si), and Sc, and reflect an increase in Pb deposi tion supplied by silicate-derived soil dust. This depth, dated at 5,23 0 BP, coincides with the development of agriculture and indicates the impact of soil cultivation on metal fluxes to the air. At 115 cm, howe ver, the Pb concentrations increase out of proportion with Sc; this po int was dated at 3,000 BP and reflects the beginning of Pb contaminati on by mining and metallurgy in Europe and the Middle East. There are t wo pronounced peaks in Pb concentrations centred at 435 cm and 555 cm, corresponding with local maxima in ash and ash-forming major elements at the same depths. These samples have been dated at 8,230 BP and 10, 590 BP, respectively, indicating the Vasset/Killian volcanic events (M assif Central, France) and Younger-Dryas cold phase as the most likely explanations.