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 pres ent anthropogenic impact on a coastal lagoon (Thau, S. France) using m ussel shells as indicators. The concentrations and isotopic compositio ns 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 determin ed to compare metal levels in different environments. Recent shells fr om 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 b asis of several natural and anthropogenic endmembers: rocks, road traf fic, Sete harbour and water treatment plants. Recent lagoon shells are clearly influenced by anthropogenic input from the harbour and road t raffic. Anthropogenic contributions vary with location. The Pb isotopi c composition was also determined on ancient shells dating from the Ro man Empire whose Pb concentrations are similar to the recent shells fr om the lagoon center. Their isotopic compositions are compared to diff erent Pb sources: local castings, English and Spanish ingots and regio nal mining districts. It is suggested that local anthropogenic impact existed since the Roman Empire and is linked with population developme nt in the area. (C) 1998 Elsevier Science Ltd. All rights reserved.