Glacial advances in southeastern Tibet during late Quaternary and their implications for climatic changes
Introduction
Southeastern Tibet, which covers a major portion of the southeastern Qinghai–Tibet Plateau, is situated between the Nyainqentanglha Shan in the north, the Hengduan Shan in the east, and the Himalaya in the west. As the marginal mountains of the Qinghai–Tibet Plateau, these ranges are being rapidly denuded due to intense mass wasting and fluvial erosion during the Quaternary. They consist of active tectonic landforms with high mountain peaks and deep valleys. For example, the Namjagbarwa summit (Namcha Barwa) has an elevation of 7756 m a.s.l., although it is circled by the “Great Bend” of the Yarlung Zangbo River with an elevation of less than 2000 m a.s.l. This area has some of the highest values of mean annual precipitation recorded on the Qinghai–Tibet Plateau. This is because of the influence of the “Grand Canyon” of the Yarlung Zangbo River (the Brahmaputra in India), through which large amounts of precipitation from the Bay of Bengal is delivered through the South Asian Monsoon to southeastern Tibet. These topographic and orographic effects result in mean annual precipitation reaching 3000 mm and equilibrium line altitudes (ELAs) approximately 1400–1600 m lower than on the northern slopes of the western Himalaya and in the central Plateau. ELA values in southeastern Tibet are as low as they are in the Qilian Shan some 1000 km northward. This abundant precipitation also makes southeastern Tibet the largest area of original forest cover in the Qinghai–Tibet Plateau.
Southeastern Tibet is the typical region of monsoonal maritime glaciers in China. There are 4007 mountain glaciers with a total area of 8012.7 km2 in the tributary headwaters of the Yarlung Zangbo River around its “Great Bend”, according to a recent inventory by Shi et al. (2006b). The glaciers here were identified as temperate glaciers based on their internal ice temperatures of −1 °C to 0 °C, summer air temperatures of 1–5 °C near the snowline, and rapid flow velocities of 80–440 m/year. Accordingly, these glaciers have all the characteristics of temperate glaciers including enhanced subglacial erosion, and high transportation and deposition rates. This is in stark contrast to the polar continental glaciers that exist in much of the central, western, and northern Qinghai–Tibet Plateau. Well-preserved glacial landforms and sediments indicate that the glaciers in southeastern Tibet advanced and retreated repeatedly during late Quaternary, implying sensitive response of glaciers' climatic changes.
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Evidence of Quaternary glaciations in southeastern Tibet
The timing of Quaternary glaciations in southeastern Tibet has been studied since the 1970s. Li Jijun and his colleagues made field expeditions to all of the major mountain ranges and valleys in this region during 1975–1977, and suggested that two glaciations occurred based on large moraines preserved in the Boduizangbo River Valley (Li et al., 1986). Jiao and Iwata obtained radiocarbon ages for several late glacial moraines in the Boduizangbo River Valley (Iwata and Jiao, 1993, Jiao and Iwata,
Chronology of glaciations
The vestiges of Late Quaternary glaciations are plentiful in southeastern Tibet. This paper describes only those in the three areas mentioned above. In summary, at least three Quaternary glacier advances shaped many valleys in southeastern Tibet. They have been named the Guxiang, Baiyu, and Yuren Glaciations. Although they appear to be widely distributed, they are best exposed in the Boduizangbo Valley. Accordingly, this valley was selected to sample for CRN exposure ages, ESR, and radiocarbon
ELA changes associated with the glacial advances of MIS-6 and MIS-2
The well-preserved end and lateral moraines in the Boduizangbo Basin were investigated, and glacier extents for the Guxiang and Baiyu Glaciations were reconstructed based on the distribution of glacial sediments and landforms (Fig. 10, Fig. 11). This allowed reconstruction of the ELAs using the accumulation area ratio (AAR) method. This method assumes the accumulation area of the glacier represents a fixed proportion of the total glacier's area.
Firstly, the modern ELAs of glaciers were
Discussion of the climatic implications
The ELA depression during the Baiyu Glaciation is less in southeastern Tibet compared to estimates from the mountains along the eastern edge of the Qinghai–Tibet Plateau, but more than in the central plateau region. For example, the Last Glacial Maximum (LGM) ELAs along the eastern slope of Hengduan Mountains and the south slope of Himalayas were probably 800–1000 m lower than at present based on the elevation of cirques formed during LGM (Shi et al., 2006b). In the central plateau regions ELA
Conclusions
Southeastern Tibet is a region dominated by modern monsoonal maritime glaciers fed by high mean annual precipitation. It is also the type region for the Guxiang, Baiyu, and Yuren Glaciations. Complex lateral and latero-frontal moraines in the Boduizangbo River Valley were probably deposited during MIS-6, MIS-2, and MIS-1. Moraines of intermediate ages between those of MIS-6 and MIS-2 are also present, but have not been dated yet.
Reconstruction of glacier surfaces and ELAs using the AAR method
Acknowledgments
This work was supported by NSFC project No. 40771049, 40801031 and 40371013. Funding for sample collection and field work was also provided by grants from the University of Wisconsin-Madison. CRN analyses were funded by Prime Lab Seed Grant to P. Colgan. Funding, lab equipment, and travel expenses for preparation of cosmogenic samples was also provided by the Department of Geology at Grand Valley State University. Thanks to Prof. Norm Catto and two anonymous reviewers for helpful comments.
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