| Temporal and spatial variations of former glaciers were documented by glacial erosional and dispositive landforms.The glacial sediments and the landforms they constitute of,provide valuable information on palaeo-climate and palaeo-envirnment reconstruction.Chronology is the basis and premise for determining the nature and magnitude of glaciation,understanding landscape evolution and paleoclimate change,and reconstructing paleo-environment.Luminescence dating is one of the few dating techniques that can be applied to glacial deposits.Incomplete/heterogeneous bleaching is the greatest challenge for glacial sediment luminescence dating,which hinders the application of traditional luminescence methods using sand and silt-size grains in glacial environments.The latest developed rock luminescence burial dating technique is able to judge whether the cobble has been fully exposed to daylight according to the luminescence-depth profile,which is one of the solutions to the problem of heterogeneous bleaching of glacial sediment luminescence dating.Testing young,including modern,glacial gravel samples is the most intuitive way to test the reliability of the new rock luminescence burial dating technique.In front of the modern glacier in Yingpu Valley,eastern Qinghai-Tibetan Plateau,130 young glacial cobbles were collected,including 37 late Holocene samples and 93 modern samples,among which 20 of the modern samples were collected from the ground surface.Firstly,the surface≤2 mm/saturated signal method was used to distinguish the degree of bleaching of cobble samples.Then,the relationship between the characteristics of cobble samples（roundness,sphericity,color,grain size,relative grain size,the average diameter of cobbles,etc.）and the degree of bleaching was explored,to seek well-bleached cobbles and their characteristics.The results indicate that there are cobbles that show characteristics of being bleached at most sampling sites.On average,20.6%of cores of the buried cobbles were well-bleached.Samples from different types of sedimentary environments show different degrees of bleaching.The cobble samples from the top of the lateral moraine have the best degree of bleaching,followed by outwash terrace and modern glaciofluvial cobbles.The glaciofluvial cobbles collected from small depression between modern recessional moraines in front of modern glacier show the worst degree of bleaching.Granite cobbles with poorer roundness,lower sphericity and lighter color were better bleached.Secondly,post-IR IRSL SAR protocols were used to measure the equivalent dose of rock slices,and single grain measurements were conducted on quartz and potassium feldspar particles.These ages were cross-compared,and compared with previous chronological works and stratigraphic and geomorphic relationships.The results show that the quartz samples in the study area are dim and not suitable for single grain dating.The feldspar single grain ages are overestimated for most samples.The ages of rock slices of modern cobbles are close to 0.The ages of the rock slices of modern glaciofluvial cobbles and cobbles from outwash terrace are consistent with the large aliquot quartz OSL ages.Ages of rock slices of cobbles from the top of lateral moraine are younger than the large aliquot quartz OSL age,but consistent with expected age.Both ages of rock slices of cobbles and large aliquot quartz grains from recessional moraine are overestimated.In comparison,cobbles show better bleaching and potential for luminescence dating.To sum up,the degree of bleaching of glacial cobbles is related to sedimentary environment,deposit process,transport distance and characteristics of cobbles.Rock luminescence burial dating of glacial sedimentary cobbles shows better potential than traditional luminescence dating using grains.This study provides a reference for the application of rock luminescence burial dating to the study of Quaternary glaciation. |
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