THE PATTERNS OF ENVIRONMENTAL CHANGES SINCE LATE PLEISTOCENE IN NORTHWESTERN CHINA
Li Jijun (Department of Geography, Lanzhou University)
Recent researches have indicated that there are two patterns of environmental changes since late Pleistocene in Northwestern China. Xinjiang represents the pattern of the westerly belt which follows the traditional concept that the ice age was synchronous with the pluvial age. The Gansu and Qinghai region was mainly influenced by the monsoons and was dry and cold in the ice age and warm-humid in the interglacial period. Chaiwopu lake in Xinjiang was utterly different from Qinghai lake (Koks Nor) of monsoon area in terms of the fluctuation of water level. Data collected by Wang Jingtai show that the terrace 3 of Chaiwopu lake was 25—28 m higher than the modern lake surface. The dark-grey lacustrine deposits near the surface of terrace 3 had been dated by ~(14)C method at 15 030±140 a B. P., 14 800±100 a B. P. and 12 240±120 a B. P., while the water level of Qinghai lake dropped down and almost dried up during last glacial maximum (Zhang and others, 1988). The sedimentary sequence in Loess Plateau since late Pleistocene has been well recognized through recent studies and it can be treated as a model. Take the example of the section in Beiyuan of Linxia in Gansu Province, the lower part of the section consists of three paleosols and two thin loess layers imbeded. These paleosols and loesses are evidently corresponding to the 5 sub-stages (a, b, c, d, e) of the deep sea isotope stage 5, showing that climate was mild and moist in the last interglacial period and thus the brown paleosols developed. According to TL dating, the last interglacial period persisted for about 140 000—80000 aB.P., before it came to the last glaciation, the formation of aeolian Malan Loess in a great thickness indicating an arid and cold climate. However, from 53 000 a to 27 000 aB.P. the climate was cool and moist, the Loess Plateau was covered by coniferous-broadleaf forests which contained a great number of conifers (Picea, Abics) or by woody grassland. In the valley bottom and low land area, ponds and swamps were formed and dark-green and black mud was deposited, in which the rotten wood logs were buried and the fossils of Salawusu Fauna can be found in some places. Judging by ~(14)C dating material, this cool-moist stage should be comparable with the deep sea isotope stage 3, by which Malan Loess was divided into the lower and upper parts. The end of Malan Loess seems to signify the beginning of Holocene, i.e. the present interglacial period, in which some paleosols were developed. The pentad deposition pattern mentioned above has been proved to be a typical one in late Pleistocene in Northwestern China or at least in the monsoon-affecting area. However, it will vary in sedimentary facies due to the changes of depositional environment. For example, in the inner part of Qaidam basin lakes were entirely dried up during last glacial maximum and rock saltdeposited (Chen et al., 1987). In this way, the above mentioned climatic change pattern presented an alternating deposition of lacustrine layers and rock salt.