Sedimentary Characteristics And Environmental Significance Of Dangmuqu Profile In The Eastern Section Of Nianqing Tanggula Mountain During The Middle And Late Holocene

The eastern section of Nianqing Tanggula Mountain is located in the southeast of Qinghai-Tibet Plateau and is sensitive to climate change.As a good information carrier to recover the evolution of paleo-sedimentary environment,the ice-water sediments in this area recorded the process of environmental warming and humidification in the middle and late Holocene.Based on the results of OSL dating and the characteristics of section deposition,the grain size,geochemical elements and magnetic susceptibility of Taimuqu sediments have been measured and analyzed in detail,and the climatic and environmental characteristics of the middle and late Holocene in the Taimuqu area have been restored.At the same time,the section was compared with different carriers and sections in different study areas,and the characteristics of paleoclimate and environment recorded by the caudal water sediments in the middle and late Holocene were discussed.The results show that:（1）The stratified structure of DMQ section is developed,and the sedimentary layers are distributed with boulters,lenses and drop stones.The light years are 5.78±0.22ka BP and4.22±0.14ka BP.The upper limit of the age of the study area is 3.06ka BP according to the lacustrine deposition rate.（2）The sediment in DMQ section is mainly sand-grade particles with an average particle size of 2.17φ,a mean standard deviation of 1.33,poor sorting,an average skewness of 0.17,and a mean peak state of 1.13.The grain size frequency curve is mostly single-peak distribution,and the main peak is concentrated in the range of 50-1000μm.The grain size is coarse,and the probability cumulative curve is multi-stage,reflecting the basic characteristics of the ice water sediments.（3）Analysis results of chemical elements showed that:The average value of Si O₂,Al₂O₃,Fe₂O₃,Ca O,K₂O,0.39%,and Na₂O is 69.70%,13.11%,2.70%,2.05%,4.89%,0.39%,and2.14%,respectively.The average value of CIA is 59.06.The sa and saf values were 5.32 and4.42,respectively.The A-CN-K triangle showed that the sediment inclined to the A-K line,and the migration ability of major elements was Ca>Mg>Mn>Ti>P>Fe>Al>Na>K>Si,indicating that the sediment was mainly physically weathered,and chemical weathering was weak,and was in the early stage.（4）The profile magnetic susceptibility is generally low,ranging from 10.15×10^-8m³kg^-1to 27.23×10^-8m³kg^-1,with an average of 18.32×10^-8m³kg^-1and a frequency magnetic susceptibility of 2.36%.The profile fluctuates.The mean value and amplitude of magnetic susceptibility of the ice-water sediments were larger than that of the moraines in the study area.As for the influencing factors,the magnetic susceptibility in the study area is mainly controlled by the parent rock,while climate,particle size and Fe₂O₃are not the dominant factors.（5）The paleoclimate evolution sequence of the middle and late Holocene in the eastern section of Nianqing Tanggula Mountain is as follows:The first stage:the age is 5.78～4.22ka BP,the sediment of this section has more coarse material,typical distribution of gravel layer,average sand grade content is 90.08%,average Mz is 1.9φ,standard deviation is 1.24,sorting is good,and frequency curve mainly shows single peak.The contents of Ca O and Mg O were low,the contents of Al₂O₃and K₂O were high,and the CIA value was 59.33.The mass magnetic susceptibility is 18.65×10^-8m³kg^-1.It indicates that this stage is in the most suitable period of Holocene with high temperature,abundant precipitation and relatively strong weathering and eluviation.The second stage:the age of 4.22～3.06ka BP,the sediment of this section has more fine grained material,the sand and gravel layers are distributed alternately,the average sand grade content is 81.09%,the average Mz is 2.58φ,the standard deviation is 1.46,and the sorting is poor.The contents of Ca O and Mg O were higher,the contents of Al₂O₃and K₂O were lower,and the CIA value was 58.65.The mass magnetic susceptibility was 17.83×10^-8m³kg^-1.Compared with stage I,the climate of this section is relatively dry and cold,mainly dominated by physical weathering,and the hydrodynamics is weak.