Grain Size Characteristics Of Palaeo-aeolian Sand In The South Tibetan River Valley During Holocene Typical Periods

In the study of global environmental change,aeolian activities are an important part of its research.The characteristics of palaeo-aeolian sand are an important part of aeolian activities.Aeolian sediments have developed extensively in the Tibetan Plateau.The complex evolutionary process of erosion and accumulation of aeolian sediments sensitively records the climate change of the plateau,which provides an important basis for studying the changes of the Tibetan Plateau and its surrounding palaeo-environment.Grain size analysis method is widely used in the study of aeolian sediments and is one of the commonly methods to study the evolution of the Quaternary environment.The grain size characteristics of aeolian sediments can reflect the aeolian activity,which is of great significance for judging sedimentary environment and aeolian landforms.In this paper,six profiles in Dinggye of the Pengqu area and the middle reaches of the Yarlung Zangbo River are selected.Based on the chronological data,the study analyzed the grain size composition and parameters,which provides a theoretical basis to reveal the grain size characteristics of Holocene palaeo-aeolian sand in this area.The results shows as follows:(1)The grain size characteristics of the Holocene palaeo-aeolian sand in the southern Tibet valley are as follows: The main component were silver sand,fine sand and medium sand.During the Holocene,the grain size tended to be coarser,and the sorting tended to be worse.The skewness tended to be positive,the kurtosis tended to be moderately sharp.In Dinggye of the Pengqu area,the main components of palaeo-aeolian sand were silver sand,medium sand and fine sand during the early Holocene(12.83~6.56 ka B.P.).The average grain size was relatively coarse,the sorting was good,the skewness tended to be positive,and the kurtosis was sharp.During the mid-Holocene(6.56~3.68 ka B.P.),the main components of palaeo-aeolian sand were silver sand,fine sand and very fine sand,which gradually became thinner.The sorting was worse,the skewness tended to be extremely positive and the kurtosis was sharpest.During the late Holocene(since3.68 ka B.P.),the main components of palaeo-aeolian sand were silver sand,medium sand and very fine sand.The grain size was coarsest,the sorting was best,the skewness tended to be positive.The kurtosis was the medium sharp.In the middle reaches of theYarlung Zangbo River,the main components of palaeo-aeolian sand were silver sand and medium sand during the early Holocene(8.56~6.56 ka B.P.).The grain size was coarse,the sorting was good,the skewness tended to be positive,and kurtosis was sharp.During the mid-Holocene(6.56~3.58 ka B.P.),the main components of palaeo-aeolian sand were silver sand,fine sand and very fine sand,which gradually became thinner.The sorting was worse,the skewness tended to be positive,and the kurtosis was the medium sharp.During the late Holocene(since 3.58 ka B.P.),the main components of palaeo-aeolian sand were silver sand and medium sand,which gradually became coarsest.The sorting was worst.The skewness tended to be positive,and the kurtosis was the medium sharp.(2)The internal differences the palaeo-aeolian sand between Dinggye area and the middle reaches of the Yarlung Zangbo River are as follows.The average grain size became finer then coarser during the Holocene.On the whole,the average grain size of holocene palaeo-aeolian sand in Dinggye area was coarse and the sorting was poor.In the middle reaches of the Yarlung Zangbo River,the average grain size of paleoaeolian sand was relatively thinner and the sorting was better.Compared with the sand in the middle reaches of the Yarlung Zangbo River,the grain size of the sand in Dinggye area were coarser during the early and late Holocene,which were affected by the provenance and climate.(3)The grain size composition of Holocene palaeo-aeolian sand in the south Tibetan river valley reflected the aeolian activity in different periods.During the early Holocene(12.83~6.56 ka B.P.),the aeolian activity in the southern Tibetan valley continued to weaken as a whole,with a short-term enhancement period(8.60~6.80 ka B.P.).During the middle of the Holocene(6.56~3.50 ka B.P.),the aeolian activity continued to be weak.During the late Holocene(since 3.50 ka B.P.),aeolian activity increased.The change trend of aeolian activity in this area was consistent with the trend of aeolian activity in the foothills of the Gonghe Basin in the northeast of the Tibetan Plateau.