Influence of anthropogenic activity on the lead isotope signature of Thau Lake sediments (southern France): origin and temporal evolution

Lead concentrations and isotopic compositions were determined on both bulk sediments deposited in the Thau lake in southern France during the last 200 years, and leachates derived from a series of sequential leachings of the sediments, making it possible to identify the sources, natural (i.e. indige nous lithologic) or anthropogenic, and to quantify the different inputs of Pb. Two distinct inputs of Ph could be distinguished. One of these corresponds to the terrigenous material entering the basin, representative of the local natural Pb 'background'. Its supply remained steady most of the time with (206)pb/(207)pb ratios of 1.200 +/- 0.003, except at the time of heavy stor ms producing voluminous and sudden depositions, such as that of September 1 875, This Pb supply is mainly hosted by the detrital silicate fraction of t he sediments. The second Pb input is a direct consequence of anthropogenic activities of various industrial and domestic emissions in the region, part icularly due to the city of Sete and, to a lesser extent, to the villages i n the watershed. The (206)pb/(207)pb ratios Of this input are of 1.142-1.16 2. The Pb added to gasoline could also be identified in the uppermost sedim ents, because of its specific Pb-206/(207)pb ratios of 1.069-1.094. The lea ching experiments also showed that the anthropogenic Pb is mainly hosted by the oxi-hydroxides of the sediments and to a lesser extent by the carbonat es. It may also be adsorbed on particle surfaces, while only limited amount s are bound to organic matter. (C) 2000 Elsevier Science Ltd. All rights re served.