The Optical Stimulated Luminescence Dating Of Bottom Glacial Sediment Below 216.6m-Depth Chongce Ice Core In West Kunlun Mountains

Ice core record plays one of the most important role in reconstructing the paleoclimate and paleoenvironment information because of its unique propertites such as reserving a large amount of information,high resolution and long time scale.A precise chronology is the essential first step for a sound interpretation this paleoclimatic information.However,dating of ice cores drilled in the high mountain glaciers is difficult because seasonal variations cannot be traced at depth due to rapid thinning of the ice layers.Here we provide the first luminescence dating of the basal sediment of the Chongce ice cap in the northwest Tibetan Plateau.It is also problem whether the OSL method is reliable in basal glacial deposit of ice core.We attempt to explore the reliability of OSL of coarse and fine grain quartz in basal glacial deposits underlying depth ice cores.Finally,using this dating method,we constrain the oldest age of the 216.6 meters Chongce ice core.The details are as follows:1.Assessing the measurement reliability of equivalent dose in glacial deposits at the bottom of the depth ice core:The nine fine grain(4-11 ?m)quartz aliquots(total 12)and half of coarse grain(90-150?m)quartz(n = 6)successfully corrected the sensitivity changes because recycling ratios range between 0.9 and 1.1.The recuperation values of all the fine grain quartz aliquots are below 5%.Only the aliquots with recycling ratios and recuperation values within the acceptable range were used for finally dating.2.Analyzing the bleaching degree of coarse and fine grain quartz.Fast component in fine grain quartz accounts for around 87%of the natural signal,indicating that the signal is fast component dominant based on decay curves and components separation.But for coarse grain quartz,it includes not only the fast components but also medium and slow components.The De distribution shows a narrow and nearly symmetric shape for the fine quartz grains.Above indicating sufficient bleaching of the fine quartz grains than coarse grain quartz.3.Estimation of annual dose rate.The dose rate is determined by environmental dose and water content,and the former was controlled external dose rate of sample,which is calculated by the radioactive elements(e.g.U,Th and K)content of sample.For water content,it is difficult to measure because of few previous studies in glacial deposits covered by ice and/or snow.Then,we assume that the water content is zero(not only solid ice and no liquid water)and the maximum saturation moisture content(30%).Combining the water content and environmental dose,we calculated the annual dose rate is 5.81 ±0.46Gy/ka and 4.24±0.37Gy/ka when the water content is 0 and 30%,respectively.4.Constraining the age of 216.6 m Chongce ice core.The OSL age of fine grain quartz was determined by both equivalent dose and annual dose rate.De value of fine grain quartz exhibited Gaussian distribution and is average of 178 ± 9Gy.Then,the OSL age of fine grain quartz is 31 ± 3ka(0%water content)and 42 ± 4 ka(30%water content).Namely,the OSL age of base glacial sediment gives an upper limit age of 42± 4 ka for 216.6 m Chongce ice core.The successful application of OSL dating method in glacial deposits covered by ice has important implication in ice core dating of Tibetan Plateau and great potential to constrain age of ice core in other regions.