| Glaciers are sensitive to climate change and the legacy of past glacial advances during Quaternary,including abandoned moraines,trimlines and other glacial geomorphic features,provides direct valuable information that can be used to reconstruct the paleo-environment of the cryosphere in the past.The Qilian Mountain is located in the northeastern Tibetan Plateau and duo to the uplift of Qilian Mountain and the climate of glaciation during Quaternary,abundant and typical glacial erosion and sedimentation landforms have preserved in this region.In addition,Qilian Mountain is located in a transition region between the westerlies and the monsoonal regions.If the glacial chronology of Qilian Mountain is determined and paleoclimate is derived from the glacial record,we will not only reconstruct the chronological framework of the northeastern Tibetan Plateau,but also decipher the difference of glacial history between the westerlies and the monsoonal regions.Moreover,reconstructing past climate changes derived from the glacial record is critical for understanding regional climate-glacier interactions and the forcing paleoclimatic conditions.Based on the field investigation of Quaternary glacial erosion and sedimentation landforms in the Qiyi Glacier valley,Bailang River basin and Laolongwan valley,we determined the chronological framework of Quaternary glaciations in the Qilian Mountain with cosmogenic 10Be technique.According to the numerical age control and well preserved moraines,we used the longitude section paleoglacier model to reconstruct the paleoglacier surface geometry of Little Ice Age?LIA?in the Qiyi Glacier valley,Heinrich1?B-H1?and global last glacial maximum?LGMG?and MIS3 in the Bailang River basin,early Holocene?eH?and Younger Dryas?YD?and Heinrich1?L-H1?in the Laolongwan valley.Furthermore,the Shi's formula of the relationship between the annual atmospheric temperature and the annual precipitation at ELA?P-T model?,the Lapse-rate model?LR model?,the Energy and mass balance model?EMB model?and the degree-day model are used to reconstruct the paleoclimate based on the reconstruction of paleoglacier surface geometry.Some preliminary conclusions are as follows:?1?The 10Be exposure ages of Qilian Mountain are 0.23±0.030.40±0.05 ka,9.87±1.0210.44±1.07 ka,10.44±1.0712.09±1.05 ka,12.21±1.2014.48±1.42 ka,17.85±1.7819.71±1.91 ka,22.09±2.1530.49±3.01 ka,35.25±3.5247.07±4.71 ka,42.66±4.62 ka46.50±5.10 ka and >107.27±10.88123.82±12.59 ka.Based on the 10Be exposure ages and the principles of geomorphology and stratigraphy,8 glacial events are recorded in the Qilian Mountain at >MIS6,MIS6,MIS3,LGMG?H1?YD?eH and LIA: the two group of moraines present in Qiyi Glacier valley were form during LIA and MIS3,respectively;Laolongwan valley record H,YD and H1 glacial advance,and there are two older sets of moraines,which are present outer of the moraines deposited in H1 may be formed during MIS2?LGMG?and MIS3,respectively;Bailang River basin record five glacial ecents,corresponding to H1,LGM,MIS3,MIS6 and >MIS6.There are no 10Be exposure ages of the oldest moraines in Bailang River basin.Based on the principles of geomorphology and stratigraphy and a Electron Spin Resonance?ESR?age,we suggest its age is >MIS6,perhaps corresponding to MIS12.So far it is the oldest glacial advance found in the northeastern Tibetan Plateau.?2?Based on the observed and modeled data of ELA in the Qiyi Glacier,we estimated the modern ELA of Qiyi Glacier is 4824 m;According to the relationship between summer atmospheric temperature and the annual precipitation at ELA,the modern ELA of Bailang River basin and Laolongwan valley are 4691 m and 4642 m respectively.Based on the longitude section paleoglacier model,the length of paleoglacier during LIA,eH,YD,B-H1,L-H1,LGMG and MIS3 is 3.6 km,4.9 km,7.3 km,7.3 km,10.8 km,9.3 km and 10.6 km respectively.The ELA of LIA,H,YD,BH1,LH1,LGM and MIS3 were 4773 m,4344 m,4256 m,4335 m,4199 m,4157 m and 4100 m respectively,calculated by maximum elevation of lateral moraines?MELM??toe-to headwall altitude ratio?THAR?? accumulation area ratio?AAR?and area altitude balance ratio?AABR?methods according to the reconstructed glacial extent and surface elevation.With adjustments for the tectonic uplift?uplift rate of Qiyi Glacier valley and Bailang River basin is 3.09 mm/a,Laolongwan valley is 1 mm/a?,the true ELA was 4772 m,4332 m,4240 m,4276 m,4177 m,4026 m and 3917 m respectively.Therefore,compared to present,the depression of ELA was 52 m,310 m,402 m,423 m,465 m,638 m and 751 m.?3?Based on the P-T model,LR model,EMB model and degree-day model,considering the annual precipitation ratio?compared to present?of Qilian Mountain during LIA,eH,YD,H1,LGMG and MIS3 were 0.41,0.81.5,0.51,0.51,0.31.5 and 0.51 respectively,the drops of annunal tempreture were 0.15 ?,-0.93.8 ?,2.56.5 ?,2.27 ?,3.39.4 ? and 3.97.4 ? respectively.The glacial advance during LIA,YD,H1,LGM and MIS3 were mainly caused by the depression of temperature,and glacial advance during H was the result of the coupling of the low temperature and abundant precipitation.In addition,the local last glacial maximum?LGML?of Bailang River basin occurred during MIS3 and its glacial extent was more extensive than LGMG.Our results and other records suggested that LGML of Bailang River basin resulted from lower temperatures and slightly reduced precipitation compared with present,whereas the glacial during LGMG was more restricted because of much lower precipitation. |
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