Incision rate of the Manas River, northern Tian Shan: Insight from luminescence dating of terrace cobbles

Fluvial deposits in piedmont environments are usually enriched by cobbles, which can be used for luminescence dating, as an alternative to the conventionally employed sand-sized minerals. In this study, a modified multielevated-temperature post-infrared infrared stimulated luminescence (MET-pIRIR) procedure was employed to establish luminescence-depth profiles of upper and lower sides of fluvial cobbles and obtain the burial ages of cobbles from high terrace of the Manas River in northern Chinese Tian Shan. The age-stimulation temperature (A-T) plateau was combined with the conventional age-depth (A-D) plateau in luminescence-depth profile to evaluate the reliability of burial ages of cobble luminescence dating from the aspects of fading and bleaching of signals. When the signal was only bleached to a limited depth given rise to no A-D plateau, the A-T plateau could serve as an internal-check to evaluate the degree of bleaching of the luminescence signal just through a single rock slice. Our results showed that self-evidenced and reliable buried ages of fluvial cobbles could be determined by only using first sub-surface rock slice in the presence of A-T plateau. Thereafter, the deposition age of cobble from top of T6 terrace and bottom of T5 terrace were 23.6-24.0 ka and 22.7 ka, respectively, inferring a very fast incision rate of 20 mm/a. The observed variability in bleaching depth between upper and lower sides across different cobble samples suggested complex transport dynamics.