Quartz luminescence dating and its application to sediments from Manas Lake in northwestern China

Abstract

Optically stimulated luminescence (OSL) dating has been a powerful tool in dating late Quaternary sediments deposited in a wide variety of environments. This study focuses on OSL dating of quartz grains extracted from sediments. Methodological aspects about different components of quartz OSL are studied. The dating method has been applied to lacustrine and paleoshoreline sediments from Manas Lake in northwestern China. Quartz OSL signal consists of fast, medium and slow components. In conventional dating protocol, the initial OSL signal, which is usually dominated by fast component, is used to evaluate the equivalent dose (De). However, erroneous results can be obtained if the initial OSL signal contains significant proportions of the medium and slow components. Three samples were used to demonstrate such methodological aspect. The De values of these samples were found to decrease significantly with stimulation time. This is attributed to increasing influence of thermally unstable medium and slow components in the later part of OSL signals. The De values of fast and medium components were obtained by fitting the De(t) plots (i.e. apparent De against stimulation time). The fast component De values obtained for the three samples range from 330 to 390 Gy, while the medium component gave significantly lower De values of 40 Gy. The apparent De values obtained from the initial OSL signals (~210-250 Gy) were underestimated by more than 100 Gy relative to the fast component De, which is a result of significant contribution of medium components (~20%) in the initial signals. OSL dating has been applied to sediments from the lake beaches of Manas Lake. The lake is a closed lake basin in an arid region in northern Xinjiang Province of China, with its current lake bed at 244 m a.s.l. Samples from Manas Lake gave flat De(t) plots and thus no fitting is required for corrections of other component contributions. The dating results show that lacustrine episodes dated at ~80-73 ka ago were recorded in the northwestern side of the lake at 270 m a.s.l., while paleoshoreline to near-shore environments during ~80-90 ka ago were recorded in the opposite side of the lake at 262 m a.s.l. The ~80 ka old sedimentary layers in the southeastern side are overlain by paleoshoreline sediments formed within the last ~1 ka, separated by a large age gap. Combining the results with previous studies, it is concluded that breaks in sedimentary records are common in the lake area at elevation > 260 m a.s.l. When comparing sedimentary environments at different time from different sites, it is suggested that a small amount of uplift of the northwestern side of Manas Lake relative to the southeastern side have occurred during the last 80 ka.