Development of Optical Dating Techniques for Chinese Loess within the Loess Plateau and Beyond

Loess, which is widely distributed across many parts of China, provides valuable records of changes in wind pattern and aridity of East Asia throughout the Quaternary. Chronologic framework and paleoclimatic significance of the Chinese loess deposits on the Loess Plateau have been extensively studied. However, loess sequences outside the Loess Plateau have received much less attention and a lack of accurate chronology poses major uncertainty in the correlation of paleoclimate events over wide areas. Optically stimulated luminescence (OSL) dating has been widely applied to the Late Pleistocene loess deposits within the Loess Plateau during the last decade and most of the studies have used quartz extracted from the loess samples. Recently, underestimation of OSL ages has been reported for quartz when samples with expected age older than about 70 ka are dated. Infrared stimulated luminescence (IRSL) signal of feldspars saturates at much higher dose than quartz and therefore holds a potential for dating loess deposited before the Late Pleistocene. However, feldspars are not commonly used for dating due to the observed instability of the IRSL signals which could lead to severe age underestimation. In this study, loess samples from sites within the Loess Plateau and in Xinjiang of northwest China and Nanjing of southern China have been examined. Experiments were designed to investigate the the accuracy and limitations of the dating protocols for quartz and feldspars. Significant difference in luminescence behavior has been observed for samples from different regions. For the last glacial loess samples from northwestern part of the Loess Plateau, quartz OSL ages obtained by both single aliquot regenerative (SAR) dose and sensitivity corrected multiple-aliquot regenerative (sc-MAR) protocols are severely underestimated, in contrast to the last glacial loess of central Loess Plateau. A recently proposed technique using post-IR IRSL signal of feldspars was tested with samples from different loess fields. A thermally transferred signal which interferes the post-IR IRSL signal was documented and a modified protocol was developed to overcome this effect. Together with the correction for anomalous fading by dose rate correction method, the refined protocol allows us to differentiate the ages of loess deposited during last three glacial-interglacial cycles at Luochuan in the central Loess Plateau and to extend the age range of the loess sequences in Xinjiang and Nanjing where the reported optical ages have never been older than 100 ka.