青藏高原东南部他念他翁山全新世早中期冰进事件研究

Study on early-mid Holocene glacial advance events in the Taniantaweng Mountains,southeastern Qinghai-Tibet Plateau

他念他翁山位于横断山脉西部,是海洋性冰川向亚大陆性冰川的过渡区,维持冰川发育的降水补给主要由西南季风带来,该区第四纪冰川进退对西南季风波动有较为直接的反映。正是这一地理位置的特殊性,使得该区第四纪冰川研究具有重要意义。采用野外地貌调查与宇宙成因核素测年技术相结合的方法,试图查明他念他翁山青古隆槽谷全新世期间冰川地貌学特征,并确定其发生的具体时间,探讨其响应机制。结果显示:他念他翁山全新世早中期冰川波动的时限介于(6.13±0.37)~(8.83±0.50)ka,可能是当时西南季风强盛,降水增加的结果。研究可为探究冰川作用与西南季风气候变化及全球气候变化之间的动力学联系提供新依据。

Taniantaweng Mountains(30°45′~30°11′N,96°30′~97°30′E)is situated towards the west of the Hengduan Mountains and lies in the transition zone of the southeastern Qinghai-Tibet Plateau(QTP)and the Yunnan-Guizhou Plateau(YGP).During the Quaternary period,multiple glaciations occurred on the planation surface of the middle section of Taniantaweng Mountains;hence,it contains numerous typical glacial erosional and depositional landforms.The Quaternary glaciers in this area are marine glaciers,maintained by precipitation brought by the South Asian monsoon.Thus,the advances and retreats of these Quaternary glaciers directly re⁃flect the fluctuations of the South Asian monsoon and are highly important for Quaternary glacial research.The Qinggulong Valley is about 7 km long and possesses numerous groups of preserved moraines in clear and com⁃plete formations.Moraine QM3 was distributed within an altitude range of 4600 to 5250 m,and extended from upstream to downstream for approximately 7 km.ESR and OSL dating results reveal that these moraines were formed during the LGM and correspond to MIS 2.Moraine QM2 was preserved at altitudes between 5290 and 5175 m in the cirque,displayed a greyish-yellow colour and an arcuate distribution and was recorded lying across the cirque mouth.Moraine QM1 was distributed 5360~5270 m,and massive granite debris were scat⁃tered on the surface.Here,we investigated the QM2 moraine history of the Qinggulong Valley using cosmogen⁃ic ^(10)Be surface exposure dating techniques.The three samples for QM2 yielded ages of(6.13±0.37),(8.83±0.53)and(8.37±0.50)ka,and we assigned an age range of 8.83 to 6.13 ka to this moraine,indicating a gla⁃cial event during the early-mid Holocene.In order to make regional comparisons we recalculated the ^(10)Be ages for surrounding mountains using data compiled from previously published literature.Combined with other dating results(OSL and ^(14)C),we suggest that the early-mid Holocene occurred on the eastern and southeastern QTP.Air temperature and precipitation are two major factors affecting glacier change.The water vapor carried by the southwest monsoon,which influences the development of glaciers in Taniantaweng Mountains,is obstructed by high terrain barriers above 6000 m.Thus,the precipitation reaching the Taniantaweng Mountains is significant⁃ly reduced,resulting in much smaller glaciers compared with those in glaciated areas with abundant precipita⁃tion,such as the southeastern QTP.Pollen from cores of Lake Rencuo and Hidden in southeast Tibet demonstrat⁃ed that the mean air temperature in January was likely 2~3℃higher than present,and mean annual precipitation during the 8~6 ka BP was 100 mm greater than the current values.At high altitudes abundant precipitation fall as snow,which led to positive glacial mass balance and glacial advance.The glacial advance in Taniantaweng Mountains during the early-mid Holocene may have resulted from an increase in precipitation from the southwest monsoon.At present,the geochronological evidence of the early-mid Holocene glacial advance event in the east⁃ern QTP is primarily based on TCN,OSL,and radioactive 14C.The dating technology used in the current study is limited by the complex geological and geomorphic processes that have occurred in this region,and by the par⁃ticularities of glacial deposition in mountain areas.Furthermore,the resolution of the glacial geomorphic age since the Holocene is insufficiently high,leading to large uncertainties in the chronological data.There are also few geochronology data available to support the glacial advance in this period.Therefore,in this study,we aim to provide an initial chronological framework and preliminary results for the early Holocene glacial advance.More accurate glacial geochronology data are required to further understand the early-mid Holocene glacial ad⁃vance in the QTP.This study provides a new basis for investigating the dynamic relationships amongst glacia⁃tions,changes in the South Asian monsoon,and global climate change.