Optical dating of young lacustrine sediment from Manas Lake in northwestern China

Abstract

Manas Lake is a closed and dried lake in Junggar Basin with catchment area of 11,000 square kilometers. The lake is particularly sensitive and important to the study of paleo‐climate changes, because the climate in the catchment area is dominantly influenced by the Westlies. Dating of lake paleo-shorelines and lacustrine sediments can provide chronological evidence of water level and climate changes. No evidence for Holocene high lake level was found by previous studies.

The ages of samples were determined using Optical Stimulated Luminescence methodologies. Slope method was used for the determination of equivalent dose of quartz. Series of Lx and Tx from a number of aliquots for each regenerative dose are analyzed to obtain an averaged slop value of the Lx/Tx, by separately fitting Lx-Tx plots under each regenerative dose. In this thesis, slope method was tested for a series of young lacustrine sediment. It is found that using the slope method can save 40% time of conventional SAR procedure. The errors were reduced with large number of natural aliquots. This approach can effectively improve the reliability of optical dating. It is preferred for dating of young samples, because of weak signals and poor precision.

Fifteen lacustrine sediments and shoreline deposits were dated. These samples were from a borehole, a profile section and shoreline deposits of the latest high lake level. High-resolution data were achieved by using slope method of OSL signals from quartz for the lacustrine sediments. Magnetic susceptibility of these samples was measured as a proxy for revealing paleo-climate changes. The OSL ages suggested that the lake had experienced a rapid deposition process during 200~600 years ago. Such ages agreed well with the ages of last high lake level, which was the age of little ice age. It is concluded that the paleo-climate of Lake Manas was cold and wet during the little ice age. Sedimentation gaps with age gap of 70ka were found between sediments of little ice age and sediments beneath, from both the lacustrine borehole and paleo‐shorelines. It implies that Lake Manas probably drained for a considerable time.