Study On Red Clay Sediments In Qinghai-Tibet And Yunnan-Guizhou Plateau Of China And Their Enviromental Significance

The evolution of the Quaternary climate and environment is the focus of global change researces,especially loess and red clay are the most widely distributed sediments of Quaternary in China.The loess research has achieved remarkable improvements,however,the red-clay research is relatively leg-behind,especially in the plateau region of the Qinghai-Tibet and upper reaches of the Yangtze River.In order to explore the internal relations between the genesis of red clay and regional climate change,this thesis selects three typical red clay profiles located in Yushu of Qinghai Province,Yunlong and Dongchuan of Yunnan Province as research objects.The analytical methods include the X-ray powder diffraction(XRD),scanning electron microscope(SEM),trace and REE geochemical analysis,particle size analysis,magnetic susceptibility and soil color analysis,etc.The analytical results show that,NASC and CI normalized REE patterns between those three typical red clay profiles and semi-weathering purple sand shale are similar,as well,the ?Eu(NASC)values of the three profiles are 1.26(Dongchuan),1.14(Yunlong)and 1.08(Yushu)respectively,which are consistent with the ?Eu(NASC)range of red clay(0.83-1.29)developed on the parent material of purple sand shales.The above evidence shows that the main sources of red clay materials in the study area are mainly purple sand shales.The normal probability cumulative curves in three profiles have in consistence,two cut points near 3~5? and 11? separately,frequency curve of the numerical values close to the 7?~8?,mean that it is mainly the hydrodynamic depositional environment.The first mode of Dongchuan profile is 10~11?,which indicates that it experienced extensive weathering pedogenesis after deposition.Mineral analysis shows that all these three profiles of chromogenic minerals are hematite.Mineral composition of the Yushu profile is illite,irregular illit/smectite interstratified clay mineral,a small amount of palygorskite and chlorite.The Yunlong profile clay minerals are illite,kaolinite,a small amount of chlorite.Compositions of the Dongchuan profile are illite,chlorite,and it contains lots of gibbsite and goethite.The Mineral analysis results indicate that the arid and semi-arid climate was the main trait of Yushu area,relatively tendency to warm climate in Yunlong,very warm and wet climate in Dongchuan.Under the observation of SEM,the clay minerals in three profiles appear as fragmentary and irregular edges,indicate the origin of terrigenous debris and had undergone varying degrees of chemical weathering.The chemical weathering index CIA values of three profiles of Dongchuan,Yunlong and Yushu are 0.96,0.83 and 0.71,and the ratios of light to heavy rare earths are 10.71,9.03 and 7.88,respectively,that point out weathering pedogenesis intensity reduced in order.The general characteristics of three red clay profiles reflect the space zonation of climate,it may be mainly controlled by the influence of the plateau winter monsoon.The Yunlong and Dongchuan red clay sediments have similar elevation level,but with different sedimentary environments.They also have been influenced by the summer wind-southwest monsoon and the local terrain.The various characteristics of each profile in the longitudinal reflect the change of regional environment.Middle section of the sedimentary environment in the Yushu area is most stable,which reflects the relatively humid climate in arid-semi-arid setting.At least two obvious climate and environment changes occurred in the sedimentary process of the Yunlong profile.The leaching effect in the Dongchuan area was strongest,the red clay reached the stage of strong Al-rich stage,and the sedimentary environment was complex.