| Quaternary loess sediments are widely distributed,recorded the global and regional environmental evolution in detail.The long-sequence loess profile in the western area is less than the central area of the Loess Plateau.In the past,more basic grain-size parameters and content indexes have been used.The End-Member method of grain size has not been used in the long-sequence profile in the western part of the Loess Plateau.In this study,the grain size change of Kaole profile in Linxia area is taken as the main research object.Based on the grain size parameters,using the EndMember method analyzed the characteristics change.Based on the regional environmental changes,the palaeoenvironmental evolution characteristics recorded on the profile are preliminarily analyzed.The clastic sediments in the Kaole section can be classified by Shepard.It is considered that the sediment types are mainly silt and sandy silt,and the grain size is generally coarse.The standard deviation of profile is 2.74 for loess layer and 2.78 for paleosol layer,the separation effect is poor.The average skewness is-0.79,the loess layer is-0.87,the paleosol layer is-0.69,and the values of each layer are between-0.33 and 1.50.The grain size curves show a positive skewness distribution,and the distribution is more inclined to fine grain.The average kurtosis value is 3.34,the loess layer is 3.50,and the paleosol layer is 3.15.The value of each layer is between 2.75 and 4.50.The distribution of sediments is scattered and the curve is wide and short.The particle size of loess can be decomposed into two elements by End-Member analysis.The average particle size of EM1 is 12.95 ?m,which is widely distributed and has a low peak value.It may be transported by a normal and long-term upper air current,which may indicate the change of westerly wind.The average size of EM2 is 43.43 ?m,and its distribution is relatively concentrated.The size of EM2 is coarser,which may be mainly transported by the winter monsoon.According to the four gravel events in the profile,the climate change can be divided into four stages:(1)1.5-1.2 Ma,which roughly corresponds to the Loess Layer L14 and below.The grain size curve shows that the climate gradually turns to cold and dry in this stage.The sudden increase of sand content at the end of the period probably recorded the flood event.The climate changed from cold and dry into warm and humid rapidly.(2)1.2-0.8 Ma,corresponds roughly to the L14-L8 layer.There are secondary fluctuations in S13,S11 and S8.The End-Member curves fluctuate greatly in this stage,and the climate may be unstable.The westwind and winter monsoon vary a lot.(3)0.8-0.15 Ma,corresponds roughly to L8-L2 layer.The S5 layer in the middle is formed by three paleosol horizons,with short loess inside.The decrease of EM1 value was slowed down at this stage.The redness value increased,which may reflect the higher degree of Paleosol development.(4)0.15-Ma,corresponds roughly to the layer since S1,including the last interglacial period and the glacial period.The development degree of S1 in paleosol layer is the highest in profile,and the content of EM1 decreases rapidly.The environmental changes are mainly controlled by west wind and winter monsoon Since 1.5Ma. |
PDF Full Text Request