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U-Th vs. AMS ¹⁴C dating of shells from the Achenheim loess (Rhine Graben)

¹ BRGM, Service Analyses/Isotopes, 3 Avenue Claude Guillemin, BP 6009, F-45060 Orléans cedex 02.
² Institut de Radioprotection et de Sureté Nucleaire, Département Protection de l’Environnement, BP 17, F-92262 Fontenay-aux-Roses cedex.

Abstract

Shells extracted from loess deposits at Achenheim (Rhine Graben) have been investigated for AMS 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 stratigraphic 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.

Key Words: U-Th dating • ¹⁴C dating • Loess deposits • Rhine Graben • Quaternary