Timing of past glaciations in Kanchenjunga Himal, Nepal by optically stimulated luminescence dating of tills

Abstract

Optically stimulated luminescence (OSL) was used to date glacial sediments to elucidate the ages of past glacial advances in Kanchenjunga Himal, eastern Nepal. Seven glacial sediments samples were collected from four different moraines, and quartz grains were extracted from samples. However, the luminescence from feldspars existing as minor contamination and/or microinclusion in the quartz grains was so bright it masked the quartz signal. After etching the fraction 20 min extra with HF, the natural infrared stimulated luminescence (IRSL) signal, which indicates the existence of feldspar, almost disappeared. Equivalent dose ( D E) values were estimated by the single aliquot regenerative-dose protocol, using two different quartz fractions: fraction A, which was etched 20 min extra, and fraction B, which was prepared by the standard procedure still had a strong feldspar signal. Although the natural IRSL intensity for fraction A was so weak, the regenerated IRSL was relatively strong and not negligible for most of the aliquots. Such aliquots were omitted because the sensitivity of the feldspar is much higher than quartz and the difference in the sensitivity can cause D E underestimation. Out of 42 aliquots from four samples for fraction A, only six aliquots from two samples were remained. The D E values for single aliquots were uniformly distributed for fraction B. Moreover, the calculated OSL ages for fractions A and B from the two samples agreed with each other. It is suggested that both quartz and feldspars grains or microinclusions in quartz grains were likely to have been bleached, while glacial debris has been transported on the glacier. The resulting OSL ages suggest that the glaciers expanded at 5–6, 8–10, and 20–21 ka in the Kanchenjunga Himal.