Abstract:

Central Asia, consisting of extended arid and semiarid areas, is one of the major sources of global dust and aerosol in the Northern Hemisphere. Dust emission, transportation and deposition over the region have received an increasing interest. Loess deposits, as a geologic record of dust, enable the possibility of archiving dust variabilities over the past, and give insights into the underlying mechanisms of dust activities. Here, we examined the grain size data of two late Pleistocene loess-paleosol sequences, named Nilka (NLK) and Qingshuihe (QSH), in the Ili Basin, eastern Central Asia, with focus on intervals with the highest sedimentation rates in both loess sections. Both sites are located on the south slope of the northern Tianshan Mountains, but represent different topographies and climate conditions. Parametric curve-fitting and hierarchical cluster analysis techniques were applied to partition the whole spectrum of the measured grain size distributions and provided a more reliable and more representative statistical descriptors. The end members unmixed under distant eolian dynamics were used to reveal the dust accumulation processes in the Ili Basin in detail. The results suggest that the Ili loess mainly consists of distal and proximal sediments, with the high-level westerlies and the meso-scale regional winds as the transport agents respectively. Distal clayed mineral particles transported by the high-level westerlies can contribute to the loess sediments from the eastern basin due to the wet deposition, where precipitation is high. Nevertheless, this is not true for the western part of the basin where the clay fractions are difficult to deposit. Proximal sediments dominate the Ili loess, and the grain-size composition and formation process of the Ili loess can be dependent on the availability of the local particulate materials with different sizes. Additionally, the floating dust derived from "non-dust storm processes" (after sand and dust storms) has significant impacts on the loess formation during periods of decreasing dust storm frequency. Whereas sparse vegetation is likely to create favorable conditions to the resuspension of the floating dust components when dust storms occur frequently. A conceptual model about dust accumulation in the Ili Basin is proposed based on the above results.

Key words: Ili Basin, loess sediments, end-member modeling analysis of grain-size distributions, dust accumulation, control of sand-dust weather