Cosmogenic ¹⁰Be Constraints Glacier Behaviors Since The Last Glacial Maximum In The Haizishan Area,southeastern Tibetan Plateau

The Last Glacial Maximum（LGM）,which occurred between 26.5 ka and 19.0 ka,marked a significant event in the Earth’s history when global ice volume reached its peak.This event was followed by a period of the last glacial termination before enter to the onset of the interglacial Holocene.The transition from the LGM to the onset of the Holocene is widely regarded as the most significant climate reorganization+n on earth over the past 100 ka and has been commonly considered to be globally synchronized.However,the driving mechanism behind this synchronicity remains obscure.Although it is widely recognized that the synchronous increase of CO₂concentration in global atmosphere around 18 ka would be the primary cause of the LGM termination,recent studies have revealed that mountain glaciers from several regions in both hemispheres retreated progressively during the LGM.This suggests that other unidentified regional forcings,such as high latitude summer solar in the Northern Hemisphere and the Northern Continental Ice Sheet,may have driven the glacier changes during this period.Mountain glaciers in the mid-latitude regions,such as the Tibetan Plateau（TP）,are recognized as sensitive proxies of climate variation,and their fluctuations could reflect millennial to centennial scale climatic changes.Moreover,the TP is geographically located in a considerable distance away from both Northern Atlantic and the Northern Continental Ice Sheet,as well as the Southern Ocean,which is a primary source of CO₂ emissions.As a result,the glaciers developed on the TP are not subject to direct forcing by abovementioned factors.Therefore,the glacial landforms,left behind by glacial fluctuations in the TP,can provide the key information for clarifying this conundrum.However,the related glacial chronologies as regards the LGM and its termination remain extremely limited in the TP and surrounding mountains,hindering a systematic understanding of glacial evolution and potential climatic mechanisms forcing glacial fluctuations in the region.To address this problem,the glacial landforms left by the marine-type glaciers were studied as they are particularly sensitive to climate and possess significant erosional power,rendering them more suitable for recording the short-scale glacial fluctuations since LGM.The Haizishan（HZS）area is located in the southeastern TP.This area is distinguished by the presence of extensive and well-maintained glacial landforms,left behind by marine-type glaciers.In this study,we conducted a comprehensive analysis of the glacial geomorphology,chronology of moraines and glacial molded bedrock surfaces using ¹⁰Be dating,paleo-glacial scale,and paleoclimate reconstruction in two valleys:Lahaku valley and Yangying Co valley.Additionally,we aslo compiled ¹⁰Be chronological data during LGM and its termination from available published ¹⁰Be exposure ages across the Hengduan Mountains,to comprehend the regional glacial behaviors.We then compare the timing of glacial activities with multiple climate proxies with the aim of shedding new light on the driving mechanisms behind glacial fluctuations.This will provide reliable paleo-glacial evidence for the climate driving mechanisms of the TP and the global during the LGM and its termination.The results of detailed geomorphic mapping and cosmogenic ¹⁰Be surface exposure dating reveal that the formation ages of nine moraines（LM1-LM9）in the Lahaku valley were tentatively defined as 20.7±0.5,20.2±0.9,20.9±0.8,18.9±0.3,18.6±1.0,17.0±0.6,16.2±0.8,16.4±1.0 and 16.5±0.9 ka.Additionally,consistent¹⁰Be surface exposure ages of 14.3±0.5 were obtained for the glacial molded bedrock surfaces in the upstream of the Lahaku valley.The formation ages of two moraines（YM2 and YM1）in the Yangying Co valley were tentatively defined as 21.0±0.6 and17.3±0.6.These age results suggest that the millennial to centennial scale glacial fluctuations occurred in the HZS during the LGM to the Heinrich 1（H1）,and rapid deglaciated around～14 ka.Combined with previously published and recalculated ¹⁰Be exposure ages from the HZS area and the entire Hengduan Mountains since LGM,it is inferred that the behavior of paleo-glaciers in the HZS area mainly experienced the several millennial-centennial scale glacial fluctuations during the LGM to H1.After H1,the glaciers retreated into the valley,and then experienced a rapid deglaciation,corresponding the B?lling-Aller?d warm event（B-A）.Subsequently,most areas of HZS may no longer have glaciers（the age of Luanshibao rock avalanche at～5 ka may provide indirect support for this view）.Furthermore,the glacial fluctuations in the Hengduan Mountains since LGM mainly occurred at LGM,the end of LGM and the begin of H1,H1,B-A,Young Drays（YD）,Early-middle Holocene,and Neoglacial.The results of glacial scale and paleoclimatic reconstruction showed that during the LGM,the end of LGM and the begin of H1,and H1 in the Lahaku valley,the glacial length was 12.3 km,9.3 km,and 8.2 km,respectively,while the mean glacial thickness was 201 m,193m,and 164 m,respectively.The glacial area was 40.5 km²,21.4 km²,and 20.1 km² during these periods,and the glacial volume was 5.1 km³,2.5 km³,and2.0 km³.The Equilibrium line altitude（ELA）was reduced by 614 m（4586 m）,592 m（4608 m）,and 567 m（4633 m）compared to the modern theoretical ELA at 5200 m.In the Yangying Co valley,the glacial length during the LGM and H1 was 1.8 km and 1.6km,while the mean glacial thickness was 82 m and 51 m.The glacial area was 2.0 km²and 1.6 km²,while the glacial volume was 0.13 km³ and 0.08 km³ during these periods.The ELA were reduced by 586 m（4613 m）,564 m（4636 m）compared to modern theoretical ELA（5200 m）.Based on the available precipitation data from paleoclimatic proxies,it is assumed that the precipitation during LGM,the end of LGM and the begin of H1,and H1 is 40%,40%and 50%-60%of that in modern precipitation data.The corresponding summer temperature was estimated to be lower than moder summer temperature by 1.6-5.2℃,1.6-5.0℃,and 1.7-4.3℃,respectively.We thus infer that the retreating pattern of the glaciers in the HZS area was attributed to the increased summer temperature.The comparison between the glacial chronology and paleoclimatic proxies indicate that the main driving mechanisms of the glacial fluctuations in the HZS area were the changed regional summer temperature,which is consistent with the results of paleoclimatic reconstruction.It is further inferred that the glacial fluctuations in the HZS area since the LGM may be controlled by the variation of sea surface temperature（SST）in the Indo-Pacific Warm Pool（IPWP）,which could be linked to ocean-atmosphere interaction induced by the strength of Atlantic Meridional Overturning Circulation（AMOC）caused by changes in North Hemisphere continental ice sheets.