Radiometric dating of young and old calcrete.

Abstract

To obtain a better understanding of the relationship between calcrete genesis and the results of different absolute dating methods, thermoluminescence (TL), radiocarbon ( (super 14) C) and uranium/thorium (U/Th) were applied to coeval samples taken from a very young calcrete profile in Namibia. The methodically different ages reflect the characteristics of the applied dating methods, the genetics of calcrete and different events of calcrete genesis. The conventional (super 14) C ages and the TL dates cover the last 50 ka, while the corresponding U/Th dates of coeval samples are many times larger. Uranium-series dates are not related to the deposition of the host material or to its cementation if the ages are smaller than ca. 120 ka. The TL clock is set to zero during eolian transport and the corresponding radiometric ages of the quartz and feldspar grains date the time of their deposition. The (super 14) C ages of the cement correspond, on the other hand, to a time shortly after the onset of the cementation and long before its termination. In the case of very old calcrete, the mixture of young and old cement results in ambiguous ages if they cannot be confirmed by an independent technique.