| There are large areas of Gobi,desert and sandy land in northern China,with a total area of 1.308 million km~2,accounting for 13.6%of the total land area.In these areas,wind and sand activities are frequent,land desertification is serious,and the ecological environment is extremely fragile.Under the influence of climate environment,the flow of desert sand,the intensity of wind-blown sand activities and the vegetation coverage will change significantly,thus affecting the regional radiation balance and regional climate.In addition,a large amount of dust released from these areas is an important participant in the land-sea system,and also has an important impact on the global climate system.Mu Us Sandy Land is one of the four major sandy lands in China,located in the northwest edge of East Asian summer monsoon,and in the desert/loess transitional zone and farming-pastoral ecotone.It is a potential source area and transfer station of loess,with fragile ecological environment and sensitive response to monsoon activities.There are many types of sedimentary strata in the sandy land,which provide geological archives for reconstructing paleoenvironment and paleoclimate changes.In this paper,the ancient limnetic profile of the southeast margin of Mu Us Sandy Land was selected,and on the basis of full field investigation,the grain size and magnetic susceptibility characteristics of different types of sediments in this stratum were analyzed,and the grain size components of different types of sediments were extracted by end-member analysis.Finally,the environmental significance of different grain size components and magnetic susceptibility indications of sediments was discussed.The main conclusions are:(1)Different types of sediments show different particle size composition and distribution characteristics,among which sandy sediments have coarse particles,with an average particle size of 3.4?,and the particle size of each component is very fine sand>fine sand>silt>clay>medium sand.Peat layer sediments have the finest particles,with an average particle size of 4.3?,and the clay content increases obviously,with silt>very fine sand>fine sand>clay>medium sand in the particle size components.The average particle size of lacustrine sediments is 4.18?,and the particle size of each component is silt>very fine sand>fine sand>clay>medium sand.Different types of sediments also show obvious differences in sorting.Sandy sediments are better than peat sediments and lacustrine sediments,with a sorting coefficient of 0.99,while peat sediments and lacustrine sediments are worse,with a sorting coefficient of 1.74?1.68.The skewness of grain size distribution of different types of sediments is positive or extremely positive,and the kurtosis is peak distribution.The grain size distribution of sandy sediments includes two types,one is a single-peak distribution with high and narrow peaks,which is a typical aeolian sand deposit,and the other is a double-peak distribution,with the main peak's modal particle size getting smaller and the peak value getting lower.The second peak's distribution is 8?20?m.The particle size distribution curve of peat layer presents asymmetric bimodal distribution,with the main peak mode particle size of 100?140?m and the secondary peak mode particle size of 8?20?m.The particle size distribution of lake sediments is bimodal,with the main peak mode particle size of 110?130?m and the secondary peak mode particle size of 10?30?m.(2)According to the end-member analysis of different types of sediments by using Gen.Weibull distribution function.Four end-members can be separated from sandy sediments,EM1(33.01?m)?EM2(92.10?m)?EM3(121.84?m)?EM4(176.94?m).Peat sediments can be separated into three end members,EM1(15.65?m)?EM2(101.10?m)?EM3(161.18?m).Three end members can be separated from lake sediments,EM1(9.82?m)?EM2(24.95?m)?EM3(121.84?m).(3)The low frequency magnetic susceptibility of XHZ section is 3.53?27.83×10-8m~3/kg,with an average value of 13.12×10-8m~3/kg.The high frequency magnetic susceptibility ranges from 4.27 to 27.03×10-8m~3/kg,with an average value of 13.07×10-8m~3/kg.The changes of low frequency magnetic susceptibility and high frequency magnetic susceptibility values of different types of sediments are different.The low frequency magnetic susceptibility of sandy sediments is 20.16×10-8m~3/kg,the high frequency magnetic susceptibility is 20.04×10-8m~3/kg.The low frequency magnetic susceptibility of peat sediments is 8.82×10-8m~3/kg,the high frequency magnetic susceptibility is 8.91×10-8m~3/kg.The low frequency magnetic susceptibility of lake sediments is 5.26×10-8m~3/kg,the high frequency magnetic susceptibility is 5.41×10-8m~3/kg.The magnetic susceptibility of sandy sediments is higher than that of peat and lacustrine sediments,and shows a clear boundary with peat and lacustrine sediments.(4)There are obvious differences in grain size characteristics of sandy sediments at different depths,indicating that the climate is unstable at this stage and there are many wet and dry fluctuations.The bimodal distribution of peat and lacustrine sediment grain size indicates the influence of wind and water on sediment grain size.The particle size end-member(10?16?m)of peat and lacustrine sediment indicates typical hydrodynamic deposition,but due to the difference of hydrodynamic conditions between them,the separated particle size end-member 1 components are slightly different.Sandy sediments,peat and lacustrine sediment separate 2?3 end-member with grain size>90?m.This grain size component is a near-surface jump component,which is a close deposit,indicating the strength of wind-sand activity.There is a positive correlation between the low-frequency magnetic susceptibility of the profile and the grain size components of>63?m,which indicates that the magnetic minerals of the profile sediments mainly exist in the coarse-grained sediments of>63?m.High magnetic susceptibility indicates that more coarse-grained materials and more fine-grained materials in the sediment,which shows a humid climate environment and weak wind-sand activity. |
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