Paleoclimate Records Of Neogene Red Clay In Altun,Western China

The Eolian deposit in the Chinese Loess plateau is the most widespread and the most continuous area in western China.Based on researching for the Loess-Paleosol sequence and the underlying red clay,a series of advances have been made in the inland aridity,the evolution of the paleo-monsoon and its constraints on the uplift of the Tibetan Plateau.The Eolian deposits found in the western part of China also provide indispensable data for inland aridity research.Basing on the previous field investigation,this paper takes the Neogene red clay in the Altun Mountain area as the research object,and based on the magnetic stratigraphy,proxy indices to paleoclimatology such as magnetic susceptibility,grain size,calcium carbonate,Rb/Sr,hematite,etc,to reconstruct the palaeoclimate evolution process of the Altun area and explore its indicative significance for the aridity of the Asian interior.The grain size analysis for the section was carried out systematically,and the End Member Model(EMM)for the grain size of sediment was used to decompose.As it turns out,It can be divided into three End Member: EM1(medium size 5.2?m),it should be a concentration size distribution in 2-6?m,grain size distribution in the westerlies of the North Pacific Ocean and grain size distribution of Chinese loess.EM2(medium size 20?m)shows a negative skewed asymmetric distribution,and the Medium Size is between 32-16?m.It is the dust material which is moved by the low-level westerly to transport short distance;EM3 is a bimodal distribution,and the grain size is 57?m and 2.5?m,it represents the variation intensity of near-source of wind-stroke dynamics during the duststorm event,which reflects the characteristics mixed deposition.By comprehensive analysis,the climatic substitution index(grain size,magnetic susceptibility,calcium carbonate,hematite content,etc.)of Altun red clay shows that,the Altun region experienced the six stages of Climate Change in the Late Miocene(13-11.2Ma),Early Miocene(11.2-10.3 Ma),Middle Miocene(10.3-8.0Ma),Late Miocene(8.0-6.0Ma),Early Pliocene(6.0-3.6Ma),Late Pliocene(3.6-2.6Ma).The late Miocene(13-11.2Ma)was warm and humid with obvious dry and wet changes.In the Early Miocene(11.2-10.3 Ma)climatic change to cold-dry;The climate of late Miocene to Pliocene(10.3-2.6Ma)was dry and rainless overall.Based on climate index of red clay in Altun mountain,Combined with other geological evidence and records,which shows that the mutation occurred before and after 11.2-10.3Ma in the Asian interior,and a aridity event was recorded in the middle and late Miocene.From the regional point of view,the 11 Ma aridity events recorded in the Altun red clay are confirmed to be a regional climatic event,which is confirmed by the oxygen isotopes from the Tarim,Qaidam Basins and Linxia basins in the western part of China.The Altun red clay records show that the intensity of the inland aridity has gone thought four stages: 10.8~10.3 Ma,8~6 Ma,5.2~4.3 Ma and 3.6~2.8 Ma.Combined with the previous studies of other geological records of the the profile and the related geological records of the adjacent areas,it is shown that the onset time of inland aridity may be 11 Ma.The westerly wind is dominant in this area,global cooling is the leading factor of the intensification of inland aridity,and the stage uplift of the Tibei Plateau is promoted.