Résumé
Lead concentrations and isotopic ratios were measured along two well-dated sediment cores from two distant lakes: Anterne (2100 m a.s.l.) and Le Bourget (270 m a.s.l.), submitted to low and high direct human impact and covering the last 250 and 600 years, respectively. The measurement of lead in old sediment samples (>3000 BP) permits, in using mixing-models, the determination of lead concentration, flux and isotopic composition of purely anthropogenic origin. We thus show that since ca. 1800 AD the regional increase in lead contamination was mostly driven by coal consumption (206Pb/207Pb ∼ 1.17–1.19; 206Pb/204Pb ∼ 18.3–18.6), which peaks around 1915 AD. The increasing usage of leaded gasoline, introduced in the 1920s, was recorded in both lakes by increasing Pb concentrations and decreasing Pb isotope ratios. A peak around 1970 (206Pb/207Pb ∼ 1.13–1.16; 206Pb/204Pb ∼ 17.6–18.0) corresponds to the worldwide recorded leaded gasoline maximum of consumption. The 1973 oil crisis is characterised by a drastic drop of lead fluxes in both lakes (from ∼35 to <20 mg cm−2 yr−1). In the late 1980s, environmental policies made the Lake Anterne flux drop to pre-1900 values (<10 mg cm−2 yr−1) while Lake Le Bourget is always submitted to an important flux (∼25 mg cm−2 yr−1). The good match of our distant records, together and with a previously established series in an ice core from Mont Blanc,1 provides confidence in the use of sediments as archives of lead contamination. The integration of the Mont Blanc ice core results from Rosman et al.1 with our data highlights, from 1990 onward, a decoupling in lead sources between the high elevation sites (Lake Anterne and Mont Blanc ice core), submitted to a mixture of long-distance and regional contamination and the low elevation site (Lake Le Bourget), where regional contamination is predominant.