Grain size characteristics of blowing sediments over zibar surface: the primary results of a 1-yr filed measurement using a segmented, eight-directional sand trap (SEDST) in the Kumtagh Desert of NW China
Guangqiang Qian, Zhuanling Yang, Zhibao Dong, Wanyin Luo, Zhengcai Zhang, Junfeng Lu
Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu, China
Grain size analysis plays an important role in aeolian research because the characteristics of the blowing sediments can be recognized as the direct indicators of local wind regime and sediment source. Numerous works of grains size analysis have been done but most of the findings concerning the characteristics of surface sediments and the features of blowing particles during short-term measurement. To investigate the long-term, in-situ changes of aeolian transport, field measurements over a coarse-grained zibar surface were conducted in the Kumtagh Desert from May 2014 to May 2015 by means of a segmented, eight-directional sand trap (SEDST) designed by the authors. The SEDST can collect blowing sediments at eight directions and four heights of each direction (0 – 0.1 m, 0.1 – 0.2 m, 0.2 - 0.4 m and 0.4 - 1.0 m, respectively). During the 1-yr field measurement, the blowing sediments (total about 314.76 kg) were collected for six times with sampling intervals from 1 to 3 months depending on the wind strength and the samples were analyzed with a Vibratory Sieve Shaker (AS-200, Retsch). For the 192 samples, the mean grain size ranges between 0.21mm (2.25 phi) and 0.66 mm (0.60 phi). The sorting coefficients change between 0.56 phi and 1.21 phi which indicate that the sediments are moderately well sorted to poorly sorted. The skewness and the kurtosis vary from -0.24 to 0.04 and 0.72 phi to 1.26 phi, respectively, thus the sediments are characterized by coarse skewed to symmetrical, platykurtic to leptokurtic distribution. The mean grain size increase with height and the coarsest sediments were found at 0.4 m above the ground, this phenomenon may result from the collision and rebound of particles on the coarse-grained zibar surface. The grain size also vary with directions, in which the sediments blown from the S and NW directions are coarser than that from others, meanwhile, the finest mean grain size was found on the north. According to the geomorphologic settings of the study site, the lacustrine deposits on the east, the linear dune sands on the northwest, the alluvial materials on the north, as well the local coarse grains on zibar surface, can be recognized as the possible sediment sources. Therefore, the combination of sediment supply, wind strength and direction, as well as surface condition accounts for the variations of grain size. Our findings could provide some new information for the less studied zibars.