PALEODOSE UNDERESTIMATES CAUSED BY EXTENDED-DURATION PREHEATS IN THE OPTICAL DATING OF QUARTZ

An optical dating study of some quartzose sediments from northern and southern Australia north Africa and north-western Europe has revealed that palaeodose (P) understimates of 10-40% are obtained when an extended-duration preheat of 160 degrees C for 16 h is used. For these samples in the 0-60 ka age range, a preheat of 220 degrees C for 5 min produces the correct P, as infected from the concordance with thermoluminescence (TL) palaeodose determinantions on the same sedimentary sample or on burnt flint. Independent support for use of the 220 degrees C preheat is given by the agreement between optical ages younger than 30 ka C-14 age determinations on associated charcoal. The deleterious effect of the 160 degrees C preheat is illustrated by growth curves of opticallky stimulated luminescence (OSL) vs added dose. These curves show that the 160 degrees C preheat induces a dose-dependent increase in sample sensitivity, which may be related to activation of the pre-dose mechanism. The 160 degrees C preheat also causes low-dose supralinearity, although the supralinearity correction is insufficient to reduce significantly the degree of P underestimation. A normalization method based on preheat-induced sensitivity changes of the 110 degrees C TL peak, albeit limited by saturation effects, warrants further investigation.