Thermoluminescence dating of the orkutsay loess section in Tashkent region, Uzbekistan, Central Asia

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The chronology of loess in Central Asia was investigated by means of thermoluminescence dating (TL) in the 1970s, when age estimates ranging from 20 ka to over 800 ka were taken as providing an absolute chronology. This has caused major misinterpretation of Central Asian loess stratigraphy and palaeoclimatic reconstruction. We present a new TL study of a loess section at Orkutsay in the Tashkent Region, Uzbekistan. Nine samples spreading over a large stratigraphic interval yield apparent TL ages ranging from 20 ka to 134 ka. We interpret our TL results with reference to recent TL dating studies of loess from other regions. We consider most of our TL age estimates obtained by the regeneration method to be underestimated. A single TL date of 92.8 ka obtained by the total bleach method for loess just below the first pedocomplex (PC 1) of the last interglacial is also significantly younger than expected. We attribute the underestimation to sensitivity change induced by the laboratory optical bleach and instability of the TL signal. We conclude that the current TL techniques as employed in this study cannot provide a reliable absolute chronology for the sequence below PC 1 and cannot provide even a relative chronology for the loess below PC2. For the Orkutsay section the dating limit is around 130 ka. The results of this study suggest that the previous Central Asian loess chronology based on early TL studies is not valid.