Study On Environmental Indication Significance Of Sediment Grain Size And Magnetic Susceptibility In Qilian Mountains And Surrounding Areas

Sediments in the Qilian Mountains and surrounding areas are sensitive to climate change,and have become a hot spot in the study of past environmental changes in my country.Grain size and magnetic susceptibility indexes record important information about past environmental changes and become necessary means for environmental change research.However,the comprehensive study of grain size and magnetic susceptibility in the Qilian Mountains and the surrounding areas is slightly insufficient,and the relationship and difference of the two environmental indicators are still unclear.Therefore,it is particularly important to establish a basic framework for sediment grain size and magnetic susceptibility.Fully explore the environmental information contained in sediment particle size and magnetic susceptibility,explore the modern and past climate evolution laws in Qilian Mountains and surrounding areas,and enrich the environmental change research database in western my country.certain practical significance.In this study,three field investigations were conducted in the Qilian Mountains and surrounding areas,and a total of 545 surface sediment samples and 213 Xiaosugan Lake(SGH)profile samples were collected.The collected samples were measured for indicators,and the spatial distribution characteristics of particle size and magnetic susceptibility of surface sediments were analyzed;then preliminary clustering was carried out according to the proximity propagation clustering algorithm(AP clustering method),and then the Qilian Mountains and surrounding areas were divided into the The surface sediments are classified according to the depositional environment.The characteristics of grain size and magnetic susceptibility in different depositional environments are analyzed respectively,and their environmental indication significance is discussed.On this basis,the research results of the grain size and magnetic susceptibility of stratigraphic sediments in the Qilian Mountains and surrounding areas are collected,and the difference and connection between the two indicators of surface layer and stratigraphic sediments are compared and analyzed,and they are applied to the study of paleoenvironmental changes in the SGH profile..The main conclusions of this paper are as follows:1.The samples with larger particle size in the Qilian Mountains and surrounding areas are concentrated in the eastern region and low-altitude regions,and there are more samples with silt components.According to the characteristics of particle size frequency and probability curve,the sedimentary environment and sedimentary dynamics can be identified,and the surface sediments can be divided into five groups according to the sedimentary environment and modern environmental characteristics:aeolian sediments,lacustrine-like sediments,and lakeside floodplain sediments.sediments,fluvial sediments,flood sediments.Aeolian sediments are near-source sediments and are affected by the winter monsoon;the rest of the sediments are mainly affected by rivers or lakes,the particles increase in turn,and the hydrodynamic force gradually increases.Grain size is closely related to the degree of climate humidity,but the correlation is different in different depositional environments.From the lakeside to the center of the lake,the particle size of the sediment gradually decreases,and the hydrodynamic force is weakened,which is affected by the runoff into the lake.2.The magnetic susceptibility of surface sediments is highly correlated with longitude.The magnetic susceptibility of lacustrine-like facies,lakeside floodplain facies and fluvial facies is bounded by 97?98°E(near the 200 mm isoprecipitation line).The magnetic susceptibility changes synchronously with precipitation in space,which is mainly controlled by the Asian summer monsoon;the magnetic susceptibility west of 97-98°E is inversely correlated with the degree of climate humidity,and is affected by the transport of wind and dust.The magnetic susceptibility of fluvial sediments is also affected by fluvial erosion and bedrock properties.The correlation between the magnetic susceptibility and grain size of the surface sediments on the east and west sides of 97?98°E is opposite.3.The similarity of grain size and magnetic susceptibility of stratigraphic sediments is mainly reflected in that both can be used to indicate the degree of climate humidity.The difference is that sediment particle size is closely related to kinetics,but has different meanings in different depositional environments.The particle size in the aeolian profile reflects the change of the intensity of the winter monsoon,and the lake profile mostly indicates the dry and wet climate change;the magnetic susceptibility is in different circulation systems.There is a difference in the significance of sediment magnetic susceptibility in the Asian summer monsoon control area and climate humidity degree,while the core area of the westerly mode is on the contrary,and the response of sediment magnetic susceptibility to climate in the transition area is more complicated.4.The grain size characteristics indicate that the SGH profile is dominated by lakeside facies and shallow lake facies,which are less affected by precipitation.Xiaosugan Lake is a lake type controlled by a typical arid climate,and the climate in this area has a trend of aridification.Below the depth of about 3 m(that is,the R line),lacustrine facies are mainly deposited,with small particle size,low magnetic susceptibility,and relatively humid climate;below the R line,lacustrine and shallow lake facies are mainly deposited,and The particle size is larger,the magnetic susceptibility is higher,and the environment is arid.