LANDFORM EVOLUTION AND LATE QUATERNARY ENVIRONMENTAL CHANGES AROUND LAKE PUMOYUM CO IN THE SOUTHERN PART OF THE TIBETAN PLATEAU

There have been heated debates on the relationship between environmental changes in Tibet and the global climate change during the Glacial-Interglacial cycle. However there are still many uncertainties on the timing and extent of Quaternary glaciations for most areas on the plateau. In this study, we have investigated landform and unconsolidated materials around Lake Pumoyum Co (altitude: 5010 m, E90 ^o 30', N28 ^o 32') to clarify environmental changes in the southern part of the Tibetan Plateau during Late Quaternary. Around the lake there exist several moraines, which can be classified into three groups. The innermost terminal moraines are located about tens to hundred meters apart from the front of present glaciers. Being unweathered and non-vegetated, they seem to have been formed quite recently such as during the Little Ice Age. The middle terminal moraines are little vegetated and hardly weathered. The outermost terminal moraines are not much weathered but covered with grass, and gradually change into the highest outwash terraces. Moreover the outwash terraces gradually merge into the highest river terraces along the outlet of the lake. The relative height between the highest river terrace surface and the present river bed along the outlet is about 10 meters. In the vicinity of the northern shore there exists the lowest divide of the lake basin, the altitude of which is about 10 meters higher than that of the present lake surface. One wind-gap or paleo-channel was identified there. These Observations suggest that the water level of the lake was once about 10 meters higher than the present one. Based on the relationship between the outwash terraces and other river terraces, the period of the higher water level seems to have coincided with that of an advance of neighbouring glaciers. Indeed an older moraine from one tributary valley glacier is found to have once blocked the lower course of the outlet. Therefore the period of high lake level seems to have been mainly due to damming up of the outlet. A ¹⁴ C age (16,000+-70 yBP) from loess deposits which cover older glacial diamicton and several OSL ages from the highest river terrace deposits indicate that the age of the period was almost around the Last Glacial Maximum (LGM). It is concluded that the extent of glaciers in and around the study area was limited and Lake Pumoyum Co was present during the LGM.