Glacial Landform Mapping And Reconstruction Of Typical Regional Paleoglacier Scale In The Yarlung Tsangpo Gorge And Its Surrounding Areas

The Tibetan Plateau contains the largest concentration of glaciers outside of the Polar Regions and has been extensively and repeatedly glaciated during the Quaternary.In addition,the glaciers in the region were controlled by various climatic systems and presented different properties,which induced the Quaternary glacial evolution and its climate driving mechanism to become more complex.The typical monsoonal temperate glaciers in the Southeastern Tibet,not only the modern glaciers are large in scale and occupy a high proportion of the regional area,but also the magnitude of glacial advance during the Quaternary glaciation is particularly dramatic,which not only strongly modified the topography of the glacier action area,but also even caused the damming of the valley at one time,which in turn affected the topographic evolution of the regional valley scale.The key to the study of the above processes and their mechanisms is to clarify the timing and extent of Quaternary glaciation.However,the current research in this area in Southeast Tibet is still incomplete.Therefore,focusing on glacier evolution characteristics on short time scales by means of glacial landform mapping,dating and paleoglacier scale simulation in and around the Yarlung Tsangpo Gorge can further improve the spatial and temporal sequence of Quaternary glaciation in southeastern Tibet and reveal information on the exact extent,location and age of glacial damming;in addition,through quantitative comparison with valley topographic parameters,it is also important to systematically understand how glacial surface processes influenced the regional valley-scale geomorphological evolution through damming.In this paper,the most extensive glacial geomorphology in Southeastern Tibet was firstly mapped through DEM and remote sensing data of the Yarlung Tsangpo Gorge and its surrounding areas,combined with several detailed scientific fieldwork;secondly,¹⁰Be exposure dating was used to date the glacial remains formed in the main valley of a typical branch valley glacial damming in the upper reachs of the Yigongzangbu River,and combined with the existing glacial evolution sequence around the study area to improve the chronological framework of the paleo-glacier evolution in the region;furthermore,the paleo-glacial scale and paleoclimate conditions of typical regions were reconstructed based on the 2-D glacier flow model;finally,the influence of glacial damming on the topographic evolution of the valley was explored by comparing the location,extent and age of glacial damming with the topographic parameters of the valley.The following main preliminary conclusions were obtained:（1）Based on remote sensing images,DEM data and scientific fieldwork,a glacial landform map（71844 km²）of the Yarlung Tsangpo Gorge and its surrounding areas was drawn,which mainly includes three kinds of glacial relics（glacial valleys,marginal moraines and hummocky moraines）and ice-dammed paleolake deposits.The results of the glacial landform mapping show that the glaciers had advanced substantially during the glacial period,such as the convergence of smaller compound valley glaciers into larger compound or even reticulated valley glaciers in the Bodui Zangbo and Palung Zangbo regions;and the upper reaches of the Yigongzangbu and Niyang rivers,the southern side of the Niyang River basin and the western side of the Sedi La Mountains even form three ice cap glaciations,with an area 7,15 and 30times larger than modern glaciers,respectively.In addition,many branch valley glaciers in the middle and lower reaches of the Yarlung Tsangpo River,the Bodui Zangbo and the Yigongzangbu have even reached down to the main valley and may have triggered large-scale damming in several places.（2）The ¹⁰Be exposure dating of the glacier deposits at the mouth of Bumdo Gully in the upper Yigongzangbu range from 11.58±1.07-13.77±1.25 ka（n=8）,with a peak age of 13.15±0.95 ka,which may correspond to the Younger Dryas（YD）event.This suggests that glaciers in the region may be sensitive to sub-orbital scale climate events;it also indicates that glacial damming in the region can occur not only during Last Glacial Maximum,but even during the millennial-scale cold events at the LGM termination.（3）Based on the DEM,meteorological data,modern glacier extent and spatial distribution of glacier relics in the typical area of the upper Yigongzangbu,the glacier scale in the YD period of Bumdo Gorge was accurately reconstructed by setting the precipitation data to 60%-100%of the modern one with the existing paleoclimatic background,the results show that the glacier area during this period could reach541.6-574.5 km²,with corresponding ELA between 4532-4658 m and cooling of up to5.9-4.2°C;meanwhile,using the existing climatic background of the Last Glacial Maximum（g LGM）as input conditions（ΔT=-8°C,ΔP=40%）,the reconstruction of its glacial area shows 1.2 times increase compared to the YD,and a corresponding decrease of 393 m in ELA.（4）A comprehensive comparison of the simulation results based on glacier extent with the cross-sectional longitudinal profile of the valley reveals that the glacier damming location within the main valley matches the area of dramatic decrease in the longitudinal profile specific drop（2900-3200 m）at either YD or g LGM,showing that glacial damming can create at least a local base-level,thus controlling the longitudinal profile morphology of the valley;in addition,differences in the scale of glacial damming within the main valley of this section can create different valley profile;these processes may be the main reason for the formation of the 2900 m and3100 m knickpoints,and suggest that the evolutionary processes of Quaternary glaciations in the region,especially the triggered glacial damming,can indeed have a significant impact on the evolution of valley-scale landscapes.