Philippe Blanchemanche

Holocene Variations of Radiocarbon Reservoir Ages in a Mediterranean Lagoonal System

To obtain a precise radiocarbon Holocene chronology in coastal areas, it is necessary to estimate the modern 14C reservoir age R(t) and its possible variations with time in relation to paleoenvironmental changes. The modern reservoir 14C age was estimated by comparing AMS 14C ages of 2 recent mollusk shells found in sediment cores sampled in the Pala- vasian lagoonal system (south of France) with ages derived from 210Pb and 137Cs data and historical accounts of identifiable storm events. The calculated modern R(t) value of 943 ± 25 14C yr is about 600 yr higher than the global mean sea surface res- ervoir 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). 14C 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 14C 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 14C 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.