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ABSTRACT.To obtain a precise radiocarbon Holocene chronology in coastal areas, it is necessary to estimate the modern 14 C reservoir age R(t) and its possible variations with time in relation to paleoenvironmental changes. The modern reservoir 14 C age was estimated by comparing AMS 14 C ages of 2 recent mollusk shells found in sediment cores sampled in the Palavasian lagoonal system (south of France) with ages derived from 210 Pb and 137 Cs data and historical accounts of identifiable storm events. The calculated modern R(t) value of 943 ± 25 14 C yr is about 600 yr higher than the global mean sea surface reservoir age. This high value, probably due to the relative isolation of the lagoon from marine inputs, is in good agreement with other R(t) estimates in Mediterranean lagoonal systems (Zoppi et al. 2001;Sabatier et al. 2008). 14 C ages were also obtained on a series of Holocene mollusk shells sampled at different depths of the ~8-m-long core PB06. Careful examination of the 14 C ages versus depth relationships suggests that R(t) in the past was lower and similar to the value presently measured in the Gulf of Lion (618 ± 30 14 C yr, Siani et al. 2000). The change in R(t) from 618 to 943 yr is thought to result from final closure of the coastal lagoon by the sandy barrier, due to the along-shore sediment transfer.