Research On Quaternary Palaeoclimate In Yibin Area

Yibin area is located in the upper reaches of the Yangtze River,where the Minjiang River and the Jinsha River merge into the Yangtze River.So,the geographical location of Yibin area is significant.The paleoclimate of Yibin area has great significance for studying the Quaternary paleoclimatic evolution of the Yangtze River system.Based on the analysis of the macroscopic characteristics of sediments of the terraces and modern floodplain in Yibin area,it is found that the color of gravel layers and silty and muddy sediments of the upper floodplain in the fifth,fourth and third terraces are mainly brownish red and brownish yellow,and the color of gravel layers and silty and muddy sediments of the upper floodplain in the second and first terraces and sediments in the modern floodplain are gray and grayish yellow,indicating the paleoclimate of the Middle Pleistocene to the early stage of late Pleistocene(0.73-0.09Ma)in this area in was mainly warm and humid,and it began to change to dry and cool in the late Pleistocene(0.05Ma).Based on the analysis of the contents of geochemical elements and their related ratios,the contents of clay minerals,the chemical index and crystallinity of illite,and the characteristics of sporopollen assemblage in the sediments of the terraces and modern floodplain in Yibin area,it is found that the CIA value of the fifth,fourth and third terraces is between 81.58-91.60,the Rb/Sr ratio is large,the main clay mineral is kaolinite,the chemical index of illite is greater than 0.5,and the sporopollen combination is mainly woody plants pollen.However,in sediments of the second terrace and the modern floodplain,the CIA value is between 49.60-64.21,the ratio of Rb/Sr is small,the main clay minerals are illite and chlorite,the chemical index of illite is less than 0.5.Many indexes indicate that the paleoclimate in Yibin area was warm and humid when the fifth,fourth and third terraces formed from middle Pleistocene to the early stage of Late Pleistocene After the formation of the second terrace in the late Pleistocene,the paleoclimate changed to dry and cold.Based on the previous research data of Quaternary paleoclimate in the Three Gorges and Yichang area,it is found that the paleoclimate in the Three Gorges and Yichang area was mainly warm and humid when the fifth and fourth terraces formed and was dry and cold when the third,second and first terraces formed,which is different from the climate transformation time in Yibin area.Based on the analysis of the Quaternary paleoclimate background,the uplift of the Qinghai Tibet Plateau and the evolution process of the Southwest Monsoon,it is found that the paleoclimate characteristics of the Middle Pleistocene in Yibin area were consistent with the background of the paleolimate background in China,and the later drying and cooling were related to the environmental effects caused by the uplift of the Qinghai Tibet Plateau.The difference of the Quaternary paleoclimate evolution between the Three Gorges and Yichang areas and study area shows that the evolution of paleoclimate was also related to the evolution of the Southwest Monsoon at the same time.