Surface Deposits In The Hexi Corridor And Its Adjacent Areas And Implications For Provenance Of Asian Dust

The study area is located in Gansu Province and western Inner Mongolia of northwester China (38°-42°N; 90°-100°E). The region is characterized by a continental desert climate. There are different types of surface deposits in the study area, e.g., the desert (the Tengger and Badain Jaran desert), the Gobi desert (mainly alluvial fans along the mountains), the river deposits (Shiyanghe River and Heihe River) and the detrital deposits at the piedmont of Qilian Mountains. The Heihe River and Shiyanghe River are derived from the Qilian Mountains, linking the diiferent deposits in the Hexi Corridor. To study the material linkage among deserts, rivers plains, gobi desert and wethering deposits from the Qilian Mountains in the region is of significance for understanding of the potential dust source areas in Northwest China.In this study, we have sampled four types of surface deposits in the Hexi Corridor and its adjacent regions, including Badain Jaran Desert and Tengger Desert, the Gobi deserts, the flood plains of Shiyanghe River and Heihe River, and the deposits at the piedmont of Qilian Mountains (including loessic deposits). Based on the results of particle size, magnetic parameters, and element compositions of the bulk samples and< 38μm fraction, the conclusions were drawn as follows:(1) The grain sizes, major elemental compositions and magnetic susceptibilities of the bulk samples in the regions exhibits an evident end-member pattern, i.e. the deposits at the piedmont of Qilian Mountains and the sandy desert. The deposits of river plains and gobi desert fall into the range of the end-members. However, there is no distict difference in the trace elemental compositions of the different deposits. Meanwhile, the positive correlation of SIRM and x and the similar trend in chemical weathering to that of the A-CN in the ACNK diagrams indicate that the parent materials of the surface deposits in the Hexi Corridor and its adjacent areas are nearly consistent, which may be resulted from the processes of detritus origin and transport related to the Qilian Mountains.(2) The magnetic susceptibilities of the bulk samples and fine-grained components of the surface deposits seem not to be affected by the ferrous mobilization of magnetic minerals because the weak weathering in the study area. The susceptibilities may be influenced by the fluvial and/or wind sortings and the inhertage of ferrous minerals in the parent materials. The greater the distance is, the more difficultly the magnetic minerals are transported. Meanwhile, the strong winds in desert regions blow light minerals leeward, resulting in the lowest magnetic susceptibility in the bulk desert samples and the highest values in the fine-grained component of desert sand.(3) The Loess Plateau is a leeward dust-deposition area of the Hexi Corridor region. The distribution characteristics of the bulk sample and fine-grained components and the end-FD distribution curve of trace element compositions suggest that the study area is the main potential dust source areas, but not the sole source. Dust in the source areas has, to a certain extent, experienced mixing processes. The fine particles, which are able to be transport to downwind regions, have overall experienced the deprivation of Ca, Na. When the detrital materials from the Qilian Mountains are transport by fluviation to the marginal desert areas in the Hexi Corridor and adjacent areas, the finewould be sorted by the water and wind agents in the regions of desert, gobi and river plains. The different climatic conditions of their sedimentary environments cause the lowest CIA value and the largest particle size in the samples of the marginal desert area.(4) Under the processes of fluviation, the debris generated by the weathering in the Qilian Mountain forms alluvial fans both at the piedmouts and near the rivers and lakes. With the increasing aridity in northwestern China since the late Cenozoic, a large number of rivers and lakes in Central Asia has been shrinking or drying out. In the background of global cooling, the loose debris of the study area has undergone intensive deflation due to strengthened surface wind regimes, the aeolian landforms appear widely, and the loess dposits accumulate continously. There exits a close linkage between the surface deposits on the Hexi Corridor and aeolian deposits in the downwind regions.