5. Conclusions
The Pb content was measured in ten hinges of modern Ostrea edulis, collected in three sites along the French Atlantic coast, which are characterized by different levels of Pb content in the surface sediments. LA-ICP-MS tracks perpendicular to growth increments enabled to monitor intra-annual Pb/Ca ratio fluctuations recorded by oysters over several years. No ontogenetic trend was observed. Within-a-shell reproducibility tests performed in the foliated calcite of hinges show reproducible Pb/Ca profiles in each O. edulis. This result illustrates a homogeneous distribution of Pb/Ca within each growth line. Secondly, inter-shell reproducibility was tested by comparing adjusted Pb/Ca profiles recorded in the specimens of each site. Results show that all oysters within a given site record the same Pb/Ca variations within error of the method. Second order differences in peaks intensity up to 50% can be observed between specimens, which we consider can be explained either by the smoothing effect, or averaging method we used, which can become important when growth lines are narrow. A second source of variability comes from potential micro-environmental heterogeneities in Pb distribution at the water-sediment interface. A synthesis of Pb mean values measured in marine bivalves sampled in multiple environmental contexts shows a range of values from 0.01 ppm up to 50 ppm, and a general distribution of values independent of the considered species, but highly dependent from the environmental context (pre-industrial shells and modern shells from Iceland < 0.08 ppm < shells from environments influenced by distant anthropogenic sources and atmospheric inputs < 2 ppm < shells from Pb-contaminated sites – vicinity of smelters, spillage of mine waters – < 5 ppm < shells from exceptionally contaminated sites or specific layers < 50 ppm). Mean values measured in O. edulis shells in this study range from 0.15 ppm to 0.84 ppm, and are in a good agreement with the environmental context of the sampling sites. Furthermore, the confrontation of mean values measured in the surface sediments of each site and mean values of Pb/Ca recorded in shells show a linear relationship. O. edulis presents a wide geographical distribution along European coasts and is abundant in shell middens since the Mesolithic. Thus, the existence of such a relationship suggests that O. edulis can potentially be an accurate bio-monitoring tool of both modern and ancient environmental bioavailable Pb.
