PROVENANCE,FORMING MECHANISM AND TRANSPORT OF LOESS IN CHINA
Sun Jimin (Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029)
Although Chinese loess studies have been in the scientific literature for a long period, the origin and provenance of loess materials in different geomorphological units still remains controversial. For the most extensive and the thickest loess deposits on the well known Loess Plateau, a desert origin and a vast provenance (including the gobi and sand deserts in the nearby region and those in the three northwestern inland basins (the Junggar Basin, the Tarim Basin, and the Qaidam Basin) had been proposed. However, the isotopic, chemical and mineralogical characteristics of loess derived from the three basins are quite different from that of the Loess Plateau samples, suggesting that these basins are not the source areas of loess deposits on the plateau. It is argued that the gobi in the southern Mongolia and the adjoining gobi and sand deserts (including the Badain Jaran Desert, Tengger Desert, Ulan Buh Desert, Hobq Desert and Mu Us Desert) in China are the source areas for the Loess Plateau. However, although these gobi and sand deserts are regarded as the main source regions, they serve as dust and silt holding areas rather than dominant producers. Mountain processes (including glacial grinding, frost weathering, salt weathering, tectonic processes, and some fluvial comminution) in the Gobi Altay Mts., Hangayn Mts. and the Qilian Mts. have played an important role in producing the vast amounts of loess sized material for forming the Loess Plateau. In contrast, the alluvial sediments of the Yellow River and eolian abrasion processes in the sand deserts can be only minor sources for the plateau. The zonation of gobi, sand desert and loess from northwest to southeast, is largely the results of wind sorting of the huge alluvial deposits from the above high mountain regions. Dust entrained from different geomorphological units of China has different contributions to the proximal and distal regions. Dust derived from the Junggar and Qaidam basins is transported by near surface winds, and thus mainly accumulates on the windward slopes of the local mountains (local dust). For the Tarim Basin, dust can be transported not only by the near surface winds (accumulated in the windward slopes of the Kunlun Mts., known as local dust), but also by the westerlies whenever the dust is entrained to an elevation of 5 000m a. s. l. In the latter case, dust from the Tarim Basin (the Taklimakan Desert) can be transported out of the basin and ultimately to the remote Pacific (long distance dust). In most cases, dust entrained from the gobi in Mongolia and adjoining gobi and sand deserts in China is transported by near surface winds to the plateau region, southeastern China and the near Pacific, serving as medium distance dust. Occasionally (about 10%), the entrained dust from the above gobi and sand deserts can be transported by the westerlies to the remote Pacific Ocean and even to the United States (long distance dust).
【Fund】： 国家杰出青年科学基金项目 (批准号 :40 12 5 0 0 9)资助
【CateGory Index】： S151
【CateGory Index】： S151