关键词: 青藏高原, 他念他翁山, 全新世早中期, 宇生核素

Abstract:

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 reflect 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 complete formations. Moraine QM3 was distributed within an altitude range of 4 600 to 5 250 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 5 290 and 5 175 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 5 360~5 270 m， and massive granite debris were scattered on the surface. Here， we investigated the QM2 moraine history of the Qinggulong Valley using cosmogenic ¹⁰Be 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 glacial event during the early-mid Holocene. In order to make regional comparisons we recalculated the ¹⁰Be ages for surrounding mountains using data compiled from previously published literature. Combined with other dating results （OSL and ¹⁴C）， 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 6 000 m. Thus， the precipitation reaching the Taniantaweng Mountains is significantly reduced， resulting in much smaller glaciers compared with those in glaciated areas with abundant precipitation， such as the southeastern QTP. Pollen from cores of Lake Rencuo and Hidden in southeast Tibet demonstrated 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 eastern 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 particularities 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 advance in the QTP. This study provides a new basis for investigating the dynamic relationships amongst glaciations， changes in the South Asian monsoon， and global climate change.

Key words: Qinghai-Tibet Plateau, Taniantaweng Mountains, early-mid Holocene, cosmogenic nuclides