Paleoglacier Change And Its Impact On Valley Landform Evolution In The Bodui Zangbo River Valley,Southeastern Tibet

During the Quaternary,glaciers responded to climatic fluctuations by repeatedly advancing and retreating,forming diverse glacial landforms in their action areas that are valuable for paleoclimate research.Additionally,glaciers are not only a major force in shaping earth surface processes at high altitudes,but they also greatly influence the geomorphological evolution of valleys at lower altitudes through their interactions with rivers and lakes.Sediments found in dammed lake and river terraces provide reliable records of the geological history of river valleys,and studying these terraces and ancient lake deposits can reconstruct the evolutionary processes of river valleys and help us understand paleo-disasters.Southeastern Tibet is a highly glaciated region on the Tibetan Plateau,with a rich record of past glaciation from the Quaternary period.This makes it an ideal location for research on Quaternary glaciation and past climates.Additionally,the tendency of the Quaternary glaciers to extend into river valleys and form glacial/moraine dams makes southeastern Tibet an excellent area for investigating the impact of glacial activity on river valley landforms and reconstructing paleo-disasters.The Bodui Zangbo River Valley in southeastern Tibet is particularly noteworthy,as it contains numerous glacial deposits,dammed lake deposits,and river terraces.These features provide a unique opportunity to study the effects of glacial activity and reconstruct the evolutionary history of the river valley.This study is based on detailed field investigations,where we used multiple dating methods to establish the ages of glacial sediments and determine the timing of glacial evolution in the basin.Specifically,we employed electron spin resonance（ESR）,optically stimulated luminescence（OSL）,and terrestrial in situ cosmogenic nuclide（TCN）¹⁰Be exposure dating.By integrating the history of regional tectonic and climate changes,as well as glacier model results,we examined the driving forces behind glacial expansion.In addition to dating glacial sediments,we used the OSL method to constrained the ages of dammed lake deposits and river terraces.Based on the ages of glacial sediments,we reconstructed the evolutionary history of paleo-dammed lake and river valleys,and analyzed the climatic factors contributing to terrace development.From our analysis,we have drawn several preliminary conclusions.These are summarized below:（1）During our recent field investigations in the Bodui Zangbo River Valley,we identified glacial sediments from an earlier glaciation,which are preserved on valley shoulders in a discontinuous manner.Using the ESR technique,we determined the age of the moraine to be 506.3±60.4 ka.Based on the age error and climatic conditions,we believe that this glaciation likely occurred during MIS12,although it might have occurred earlier.As the moraine is best preserved at Nitong Village,we have named the glacier advance that deposited the moraine the"Nitong Glaciation."The extensive glacier advance during the Nitong Glaciation was likely caused by the coupling of Tibetan Plateau surface uplift and global cooling.（2）This study employed OSL dating techniques to date the moraines,which were suspected to have formed during the early stage of the Last Glaciation,based on previous research and detailed field investigations.The resulting OSL ages range from58.7±3.6 to 64.6±3.9 ka,corresponding to MIS4.By comparing these ages with summer insolation at 30°N and paleo-temperature and precipitation proxy records from regions dominated by the Indian summer monsoon,we suggest that the decreased temperatures caused by the reduced summer insolation at 30°N were the likely causes of glacier expansion during MIS4 in southeastern Tibet.（3）Using OSL dating,we determined the age of a moraine preserved in the Linqiong Village area of the Bodui Zangbo River Valley,which ranges from～14-15 ka,corresponding to the Late Glacial（LG）.Additionally,we identified two sets of moraines at the Zhuxi and Songlong valley mouths,and dated them using ¹⁰Be exposure and OSL dating,revealing two glaciations at～20-21 ka and～14-16 ka,respectively,coinciding with the Last Glacial Maximum（LGM）and LG.Applying a coupled mass balance-glacial dynamic model to simulate the paleo-glacier of the Songlong Valley during the LGM-LG,we found that the modeled glaciers had a mean coverage of 72.97 km²,a mean volume of 6.22 km³,ELA depressions ranging from 900 to 1100 m asl,and temperature drops estimated at 6.3 to 7.8°C,with precipitation being 40-60%of present-day levels.Comparing climate proxies and synthesizing glacial chronologies across southeastern Tibet suggests that glacial events occurred during cold periods,but changes in precipitation were not directly correlated with these glacial events.Instead,lower temperatures were likely the primary driver of glacial advances in regions influenced by the Indian Summer Monsoon.（4）This study conducted field investigations on lake sediments and river terraces at various locations in the Bodui Zangbo River Valley,Yalong Zangbo River Valley,and Songlong Valley.Through geomorphology mapping and OSL dating,we analyzed the spatial relationship and developmental characteristics of lacustrine deposits and fluvial terraces,and integrated the research results of glacial landforms in the Bodui Zangbo River Valley.Our findings revealed that there were moraine-damming events in the Baiyu-Linqiong reaches,Dizhong reaches,and the Songlong Valley.During the LG,the Baiyu glacier blocked the Bodui Zangbo River,forming a paleo-lake in the Baiyu-Linqiong area with a highest lake level of～2940 m asl,a maximum area and volume of～18.8 km² and～0.13 km³,respectively.The moraine-dammed lake ceased to exist～6 ka ago.The Dingzhong paleo-lake likely formed due to the Songlong glacier advancing to the Yalong Zangbo River Valley and blocking the upper reaches around～28-30 ka.The presence of the highest terrace（T4）indicates a dam elevation of～2910 m,resulting in a maximum lake area and volume of～7.5 km² and～0.58 km³,respectively.The lake’s infill reached full capacity quickly,and then the river began to erode the dam.The Songlong paleo-lake formed during the post-LG when the Songlong glacier retreated from the valley mouth,with the highest lake level reaching～2970 m asl and causing a maximum area and volume of～1.2 km² and～0.05 km³,respectively.The lake likely failed after～7 ka.Based on the geomorphic-stratigraphic relationship of river terraces and their ages,we infer that the river first eroded and carried away some of the lacustrine deposits after the collapse of the paleo-moraine dammed lakes,and then began to cut down and form terraces.The OSL ages of river sediments indicate that the T2 and T1 terraces in the Baiyu-Xumu reaches were formed at～3-5 ka and～2.6 ka,respectively;the T3,T2,and T1 terraces in the Dingzhong reaches were formed at～20.7 ka,～16-20 ka,and～12-16ka,respectively;and the Songlonggou terrace was likely formed after～7 ka.Comparing the timing of terrace formation with paleo-climatic records,we infer that terraces were formed during periods of climatic fluctuations or transitions.