Discussion Of The Relationship Between The Topsoil Grain Size On The South And The Modern Southern Border Of The Tengger Desert

In northern China, high spatial and temporal instability exists in the southern desert border, and the region is very sensitive to the global climate change, meanwhile the migration of the southern desert border threatens the survival of surrounding inhabitants, so it is very important to strengthen the research about the migration of the southern desert border of northern China. To investigate the relationship between modern topsoil grain size of northern China and the modern southern desert border, we collected56eolian topsoil samples on the southern Tengger Desert, then selected two near north-south transects and used the grain size class-standard deviation method fou the research. We finally obtained three sensitive grain size components (ranking by degree of sensitivity from high to low):61.8-185.4μm,18.2-61.8μm and2.3-18.2μm.The total19grain-size indicators which include three sensitive grain-size components from the56eolian topsoil samples, grain-size parameters (standard deviation, skewness and kurtosis), mode grain size, average grain size, median grain size,>185.4μm(%) and various grain-size indicators proposed by other researchers:<0.5μm(%)、<5μm(%)、2-8μm(%)、>16μm(%)、>20μm(%)、>30μm(%)、>40μm(%)、>63μm(%) and>74μm(%),have been converted to the grain-size contour figures. In the19grain-size contour figures, the contour of the61.8-185.4μm(%) component has the highest identical degree with the southern border of the Tengger Desert, followed by the>63μm(%) component and>74μm(%), and then the components of the median grain size, the mean grain size,>40μm(%),>30μm(%),>20μm(%),18.2-61.8μm(%),>16μm(%),2.3-18.2μm(%),2-8μm(%),<5μm(%),<0.5μm(%),the mode grain size and>185.4μm(%) in turn, finally the grain-size parameters(standard deviation, skewness and kurtosis) with the lowest identical degree.The changing law of the sensitivity of the grain-size indicators is:the larger the grain size of the particle group,the greater the sensitivity of the grain-size indicator to the migration of the southern border of the Tengger Desert.But there is a upper limit (185.4μm) of the grain size.showing a fact that the coarse particles are from near-source desert. Compared to the other18indicators, the indicator of61.8-185.4μm%is not only the most sensitive one in theory, but also has the highest identical degree between its contour and the southern desert border in practical terms. Furthermore, it defines the upper limit of the coarse grain as well.This not only verified the feasibility of firstly applying grain size class-standard deviation into the extraction of sensitive grain size indicators of desert border,but also identified the sensitive degree of the different grain-size indicators to the desert border migration.Through observing the grain size-frequency distribution curves of the Minqin sandstorm, we found that the sensitive grain group of61.8-185.4um is exactly the mode grain-size range of the sandstorm. It shows that in the downwind direction of the southern Tengger Desert, the most sensitive grain-size group to the desert border migration mainly comes from local sandstorm, or in other words, that the sandstorm is the main power for this group’s transportation.In the range of180km from the southern Tengger Desert border,on the east of Qinwangchuan basin,there is a linear correlation function existed between modern southern border of the Tengger Desert and the modern topsoil sensitive grain size, which is:y=-0.145x+35.379(R2=0.9344).In the southern desert of northern China,the coarse grains in the eolian deposits are mainly from the near-source desert, the component of61.8-185.4um within loess of the southern Tengger Desert could be regarded as the indicator of the migration of the southern desert border, which could be used to quantitatively reconstruct the desert palaeo-border in the future.