LUMINESCENCE DATING OF FINE-GRAIN LACUSTRINE SEDIMENTS FROM THE LATE PLEISTOCENE UNTERANGERBERG SITE (TYROL, AUSTRIA)

For age determination purposes, the luminescence signals from the fine-grained (4-11 mu m) quartz and polymineral fractions of samples taken from a Pleistocene palaeolake site in the Eastern Alps of Austria were investigated. Optically stimulated luminescence (standard blue optical luminescence at 125 degrees C (OSL), infrared stimulated luminescence at 50 degrees C (IRSL) and post- infrared 50/225 stimulated luminescence at 225 degrees C (pIRIR(225))) were measured and analysed. In order to reveal the potential masking of anomalous 225 fading of the natural signal by incomplete bleaching prior to deposition, anomalous fading and bleaching characteristics were addressed. Anomalous fading tests showed average g-values of <2%/decade for both the IRSL and pIRIR signals. Bleachability, 50/225 225 i.e. the rate and extent of natural signal loss during direct exposure to sunlight was derived experimentally. The bleaching experiment showed rapid and thorough bleachability of the OSL and IRSL signals, and a distinctively slower bleaching of the pIRIR(225) signal. The comparison of ages calculated from the different measurements shows that both the fading of uncorrected and corrected IRSL ages are consistent within uncertainties with the uncorrected quartz OSL ages. The corrected and uncorrected pIRIR 50/225 225 ages, however, show a large overestimation when compared with the OSL ages. Considering the small fading rates and the relatively poor bleachability of the pIRIR(225) signal, this may be due to incomplete bleaching. For the IRSL ages the results suggests 225 50/225 that the samples are well bleached and do not suffer from anomalous fading. The results of this study allow to constrain the origin and duration of the Unterangerberg palaeolake to the Middle Wurmian (ca. 55 to 40 ka) and hence provide the necessary geochronological framework for environmental and climatic proxy studies of this poorly known time interval prior to the Last Glacial Maximum.