14C and for U-Th dating by the isochron method on single shells. The two chronometers provide results that are not in agreement: the obtained 14C date is 38 ka B.P. In contrast, U-Th results provide much older ages. The 238U/232Th - 230Th/ 232Th isochron diagram gives an age of 69 kyr, whereas a slightly younger age of 64 kyr is derived from the other, 234U/232Th - 230Th/232Th, diagram. 234U/238U ratios have not been found to vary from a shell to another: the average value is 1.199, slightly higher than radioactive equilibrium. For both isochrons, the Y-axis intercept is slightly negative, which indicates that there is no evidence for a significant detrital component. Thorium 232 that is present in the shells is probably incorporated in the material. The negative intercepts suggest that the two ages have been probably "aged" resulting from a slight mobilisation of uranium. By constraining the Y-axis intercepts at 0, ages that are in good agreement from one to another, can be calculated at 60.9 kyr and 60.4 kyr, respectively. In both cases, the statistical quality of the alignment is only slightly deteriorated. Since U-Th ages are in agreement with strat igraphic data, in contrast to the 14C date, one may believe that they date the end of early diagenesis, with uranium trapping. In contrast, the AMS 14C date is either thought to reflect analytical limitations of AMS 14C dating for such type of material, or to date a younger geologic event. In this latter case, a possible hypothesis is that this event has triggered the slight opening of the 238U - 234U system. In any case, this study demonstrates once again that surface formations often tell such complex stories that only one single dating method cannot usually be sufficient to describe precisely these histories. Instead, informations derived from different independent chronometers have to be compared.
a BRGM, Service Analyses/Isotopes, 3 Avenue Claude Guillemin, F-45060 Orléans cedex 02, France
b Institut de Radioprotection et de Sureté Nucleaire, Département Protection de l'Environnement, BP 17, F-92262 Fontenay-aux-Roses cedex, France