Recent and past anthropogenic impact on a Mediterranean lagoon: Lead isotope constraints from mussel shells

The aim of this study was to determine the variations of past and present anthropogenic impact on a coastal lagoon (Thau, S. France) using mussel shells as indicators. The concentrations and isotopic compositions of Pb in recent shells from the edge and the center of a lagoon and in ancient shells (bronze age and 1st–6th century) have been determined to compare metal levels in different environments. Recent shells from the center of the lagoon show somewhat lower Pb concentrations than shells from the edge of the lagoon. Using Pb isotopes, it is possible to constrain Pb sources and their contribution to the shells on the basis of several natural and anthropogenic endmembers: rocks, road traffic, Sète harbour and water treatment plants. Recent lagoon shells are clearly influenced by anthropogenic input from the harbour and road traffic. Anthropogenic contributions vary with location. The Pb isotopic composition was also determined on ancient shells dating from the Roman Empire whose Pb concentrations are similar to the recent shells from the lagoon center. Their isotopic compositions are compared to different Pb sources: local castings, English and Spanish ingots and regional mining districts. It is suggested that local anthropogenic impact existed since the Roman Empire and is linked with population development in the area.