| The grain size end member(EM)model analysis can extract the sensitive grain size components of different material sources and sedimentary dynamic processes in the sediments,and further reveal the changes of the regional sedimentary environment.As the southern barrier of the Loess Plateau,the Qinling Mountains is the boundary of the humid and semi-humid regions,subtropical and warm temperate zones,subtropical monsoon climates and temperate monsoon climates in China.The climate,vegetation,soil and other environmental characteristics of the north and south sides are significantly different.The river valleys in the north and south sides of the Qinling Mountains are special areas of land-water-air interaction.The alternating influence of the Asian winter and summer monsoons provide favorable conditions for the development of aeolian loess-paleosol profiles in landforms such as river terraces and gentle slopes.At the same time,Slackwater deposits(SWD)of prehistoric flood events can occur to these aeolian loess-paleosol profiles due to the deposition of river floods.Grain sizes end member analysis of the sediments in the valley are helpful to understand the response to the valley landform development,river flood hydrological characteristics,and sedimentary weathering process to the Asian monsoon changes and human activities on both sides of the Qinling Mountains.In this study,we carried out extensive and detailed field surveys in the valley zones on the north and south sides with the Qinling Mountains,and found aeolian loes-paleosol profiles since the Late Pleistocene with paleoflow SWD at Luojiatan(LJT)in the upper reaches of the Hanjiang River,Yangguanzhai(YGZ)in the lower reaches of the Jinghe River,and Caihezhan(CHZ)in the middle reaches of the Beiluohe River.On the basis of detailed observation and systematic sampling of the above profiles,grain size analysis of sedimentological samples was carried out in the laboratory,and then combined with the comprehensive analysis results of relevant geochemical elements and other environmental substitution indicators,to identify the different sedimentary dynamic environment and material source indicated by each end member of the profile.The responses to river valley geomorphology development,river flood hydrological characteristics and sediment weathering to monsoon climate change and human activities were studied.The main conclusions are:(1)The parameterized and non-parametric endmember models were calculated for the grain size data of the LJT profile in the upper Hanjiang River,the YGZ profile in the lower Jinghe River,and the CHZ profile in the middle reaches of the Beiluohe River,respectively,and the number of end members,linearity,and linearity of the two decomposition methods were compared.Correlation,angular deviation,endmember correlation,and end member standard deviation mean,it can be clearly observed that the parameterization method has better fitting results.In this paper,the decomposition results of the parametric method are selected for analysis,in which 4 end members are obtained by decomposing the LJT profile in the upper reaches of the Hanjiang River,and 3 end members are obtained by decomposing the YGZ profile in the lower reaches of the Jinghe River and the CHZ profile in the middle reaches of the Beiluohe River.(2)The upper reaches of the Hanjiang River,the lower reaches of the Jinghe River and the middle reaches of the Beiluohe River are located on the north and south sides with the Qinling Mountains,and their environmental characteristics such as climate,vegetation and soil are significantly different,which makes the particle size endmember components separated from different research profiles.The differences are obvious and have different environmental indications.In each loess layer(Malan loess layer,transitional loess layer and modern loess layer)and soil layers(paleosol layer and modern soil layer)of LJT profile in the upper reaches of the Hanjiang River,EM1 is mainly composed of clay and fine silt with an average particle size of 3.81 μm,which represents the secondary clay mineral components formed by strong weathering and pedogenesis under the influence of the Asian summer monsoon after sediment deposition;EM2 is mainly composed of fine powder It is composed of sand with an average particle size of 8.60 μm,representing the far-source fine silt components transported by the high-altitude westerly wind and East Asian winter monsoon over the Qinling Mountains;EM3is mainly composed of coarse silt with an average particle size of 27.16 μm,representing the valley wind.Dust material transported in the form of low-altitude short-distance from river sediments and slope debris distributed in river valleys,and affected by leaching under the control of late climate change;EM4 is mainly composed of fine sand,with an average of The particle size is 103.87 μm,which represents the coarse-grained matter carried by the valley wind from the river sediments and slope debris near the source.In each paleoflood SWD layer,the contents of EM1,EM2 and EM3 all showed high values and were between the loess layers and soil layers,indicating that they were mainly derived from the surface soil sediments distributed over both side with the river valley transported by heavy rain and flood erosion of the upper reaches of the Hanjiang River;and EM4 clearly represents the coarse-grained suspended sediment transported and deposited under the flood environment in the upper reaches of the Hanjiang River.In the loess layers(Malan loess layer,transitional loess layer and modern loess layer)and soil layers(paleosol layer and modern soil layer)in the YGZ profile of the lower Jinghe River,EM1 is mainly composed of clay and fine silt,with an average particle size of 6.87 μm,which mainly represents the secondary clay mineral components that have undergone strong weathering and pedogenesis under the influence of the East Asian summer monsoon after sediment deposition;EM2 is mainly composed of coarse It is composed of silt,with an average particle size of 25.38 μm,which represents the dust material carried by the East Asian winter monsoon in the form of low-altitude and short-distance transportation and has been severely affected by later human activities;EM3 is mainly composed of coarse silt,with an average particle size of 48.09 μm,representing the coarse particulate matter carried by the East Asian winter monsoon in the form of low altitude and short distance.In each paleoflood SWD layer,the contents of end member components EM1,EM2 and EM3 are between the loess layers and soil layers,which indicates that they are mainly derived from the surface soil deposits distributed over both side with the river valley transported by heavy rain and flood erosion of the upper Jinghe River.In each loess layer(transitional loess layer and modern loess layer)and soil layer(paleosol layer and modern soil layer)of CHZ profile in the middle reaches of Beiluohe River,EM1 is mainly composed of clay and fine silt with an average particle size of 8.64 μm,which mainly represents the secondary clay mineral components formed by strong weathering and pedogenesis under the influence of the East Asian summer monsoon after sediment deposition;EM2 is mainly composed of coarse It is composed of silt,with an average particle size of 34.53μm,which represents the dust material carried by the East Asian winter monsoon in the form of low altitude and short distance;EM3 is mainly composed of coarse silt and fine sand,with an average particle size of 79.33 μm,which represents Coarse particulate matter transported by valley winds from near-source fluvial sediments and slope debris.In each paleoflood SWD layer,the contents of endmember components EM1 and EM3 are between the loess layers and soil layers,and the content of EM2 is slightly higher than that of each loess layer and soil layer,which indicates that EM1,EM2,EM3 are mainly It is derived from the surface soil sediments distributed on both sides of the valley carried by the rainstorm and flood erosion of the Beiluohe River.(3)The difference between the grain size end member components of the sediments since the Middle and Late Pleistocene in the research profile of the river valleys on the north and south sides of the Qinling Mountains is large.First,compared with the YGZ profile in the lower reaches of the Jinghe River and the CHZ profile in the middle reaches of the Beiluohe River,the LJT profile in the upper reaches of the Hanjiang River shows that the secondary clay mineral component(EM1)formed by strong weathering and pedogenesis has a finer average particle size and a higher content.higher(between 34.21% and 50.91%).This is because the LJT profile in the upper reaches of the Hanjiang River is located in the subtropical monsoon humid climate zone.In a warmer and humid environment,the weathering pedogenesis is strong,which will make the sedimentary strata of this profile form more abundant secondary clay minerals.Secondly,the average particle size of endmember components(EM2 and/or EM3)representing the transport power of the winter monsoon is smaller in the LJT profile of the upper Hanjiang River than in the YGZ profile of the lower reaches of the Jinghe River and the CHZ profile of the middle reaches of the Beiluohe River.This is because the upper reaches of the Hanjiang River are located in the south of the Qinling Mountains,and dust particles larger than 20 μm are mainly transported in the form of low altitude and short distances,which are difficult to be transported over the Qinling Mountains by the East Asian winter monsoon and deposited in the valleys of the upper Hanjiang River. |
