Glacial advances in southeastern Tibet during late Quaternary and their implications for climatic changes

Abstract

Southeastern Tibet is the type area for monsoonal maritime glaciers in the Qinghai–Tibet Plateau with more than 4000 mountain glaciers covering an area of approximately 8000 km². Based on moraines in the Boduizangbo River Valley, glacial advances have been recognized for the Guxiang, Baiyu, and Yuren Glaciations. Cosmogenic radionuclide surface exposure, ESR, and radiocarbon dating methods are used to estimate moraine ages. The dates suggest that glacial advances occurred during MIS-6, MIS-2 and MIS-1. Moraines of possible MIS-4 or MIS-3 age are also present, but have not yet been dated. During the Guxiang Glaciation, glaciers converged in the Boduizangbo Valley and formed a glacier approximately 100 km long, reaching the Parlongzangbo River Valley. During the Baiyu Glaciation the glacier was about 80 km long, yet did not reach the main valley of the Parlongzangbo River. During the Yuren Glaciation, glaciers advanced only within tributary valleys feeding the Boduizangbo River. Based on glacier extent, reconstructed equilibrium line altitudes during the Guxiang and Baiyu Glaciations were approximately 800 m and 600 m lower, respectively than at present. Mean annual air temperatures were depressed by approximately 6.6–7.8 °C compared to present values, estimating a 40% decrease in precipitation during glaciations.

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.