In the Badain Jaran Desert, 223 surface sediment samples from different dune types and geomorphological positions were collected and analyzed by means of sieving. The shifting dunes have a mean grain size between 2.104-2.706  Φ and are mainly composed of fine sands whose content may reach 49.5%-66.1%. It is coarser than sands in the Taklamakan Desert and the Tengger Desert in China, and has a moderate grain size on global scale. The sorting parameters show that the dune sands are well to very well sorted with a distribution of positively skewness and mesokurtic. The finest and the coarsest grains can be found on the surface of the shrub canopy dunes and the stoss base of transverse dunes, respectively. The sediments from inter-dunes have a mean grain size of 1.409 Φ, which are coarser and worse sorted than dune sands and with a positively skewness and leptokurtic distribution. Generally, the frequency curve is unimodal for dune sands while it is bimodal or multimodal for inter-dune sediments.

On the surface of typical transverse mega-dunes, the grains become finer and better sorted from the windward slope bases to the dune crests, and the coarsest sands occur near the toe of the stoss while the finest and the best sorted sands are found at the crests and the nearest slip faces. The similarity of grain size parameters shows that the transverse mega-dune and barchan dune or barchanoid chains are both belong to the transverse dune system and endure some similar dynamic processes despite of the scale difference. From a regional perspective, in the NW-SE cross sections which are parallel to the dominant wind direction, the mean grain size has a slight fining trend downwind in the north central of desert where the dune heights are relative lower than the neighboring mega-dunes. However, the mean grain size has no distinct downwind change in the sections occupied by mega\|dunes. In the SW-NE cross sections which are parallel to the Yabrai Mountain, although it's difficult to find the variation in grain size parameters along the sections, the sands become coarser with the section closer to the mountain. Generally, the Badain Jaran Desert is considered to be one of the important source region of dust, but there is nearly no silt and clay portions in the north central region according to the grain size analyses. Meanwhile there is relatively high contents of silt and clay at the southeast of the desert, the transverse mega-dune region. A further research is needed to determine the dust release region and processes in the Badain Jaran Desert.

 Results indicate that the microscopic factors which can affect the airflow structure and thereby the saltating processes, such as topography change and vegetation cover, would play an important role in the sorting process except the macroscopic factors (e.g. source region and dominant wind). The regularity of grain size parameters on a dune scale is better than on a regional scale which is resulted from the coactions of the macro and micro factors. The paper can provide some useful information for analyzing the material sources and formation processes of the mega-dunes and even also for estimating the amount of the release of dust in the Badain Jaran Desert.

 

This research is supported by National Natural Science Foundation of China(4801007) and West Light Foundation of The Chinese Academy of Sciences;