Lead fall-out isotopic signal over French northern Alps: Timing and sources constraints from distant lake sediment records

We present time-series of lead concentration and isotopic ratio in sediments from two French alpine lakes: Lac du Bourget, a suburban lake, and Lac d'Anterne, a mountain take nearby the Mont-Blanc Range. This approach aims at differentiating "local" from "regional" inputs by comparing distant sedimentary records. Pb-210 and Cs-137 dating methods were validated by historical events recognition [1-2] providing confident age-depth models spanning the last 250 years. In both lakes the sediment is lead-enriched compared to upper continental crust over the whole records. The human impact reached its maximum around 1970 AD, and then presents a rapid decline. Lead concentration and isotopic composition are correlated. When compared to data from a Mont Blanc ice core [3], Lac d'Anterne time-series yields excellent correlation over the last 250 years, confirming the accuracy of both dating process and record of atmospheric lead fall-out. Record from Lac du Bourget presents a similar pattern until 1985 AD when the signal from the high elevation sites ones, both in concentrations and isotopic ratio. Our data confirm the date of 1970 as the maximum of atmospheric lead fall-out over Western Europe. Today, Lac d'Anterne still records a weak airborne contamination, while Lac du Bourget is mostly influenced by local sources.