Environmental Magnetic Characteristics And Paleoclimate Of Early Oligocene-middle Miocene Paleosols In Lanzhou Basin

The process and driving mechanism of the aridification of Asian inland and the evolution of Asian monsoon have always been hot issues in the study of global paleoclimate and paleoenvironment.Lanzhou Basin is the intersection of the northwest inland arid region,the East Asian monsoon region,and the Qinghai-Tibet Plateau alpine region,which is an ideal region for studying aridification of Asia inland and the evolution of the Asian monsoon.In this paper,we took the paleosols of the Xianshuihe Formation in Duitinggou(DTG)section of Lanzhou Basin as the research object,and used the pedology and environmental magnetism to determine the types of paleosols,and through stratigraphic comparison with the magnetostratigraphic chronological framework,the ages of paleosol layers of the Xianshuihe Formation were determined,and then the climate characteristics of Lanzhou Basin during the early Oligocene-middle Miocene were analyzed,and the climate driving mechanisms were discussed.This study is helpful to understand the impact of the uplift of the Qinghai Tibet Plateau on the climate in the northern hemisphere and the development of the monsoon climate in China.The main insights gained are as follows:(1)The paleo-Aridisol and paleo-Luvisol indicate the arid to semi-arid and semi-humid climate of the study area when the paleosols formed.The contents of antiferromagnetic minerals and SD particles in paleosols are high and are major contribution to magnetism;the degree of weathering and pedogenesis reflected by HIRM and SIRM are basically consistent with the characteristics of paleosol types.(2)According to the variation of magnetic parameters on the sections,the climate of Xianshuihe formation can be divided into three stages:Oligocene(?31-23.8 Ma):section 5-8,the values ofc,c_ARM,SIRM and HIRM increased sharply from the lowest to the higher from bottom to top,reflecting the increase in the SD grains and hard magnetic components,which indicate that the pedogenetic degree and oxidation of paleosols are enhanced.The MAP is 401-720mm and MAT is 9.5-12.2?,indicating the climate changed from cold-dry in early Oligocene(?29 Ma)to warm-dry in late Oligocene(?27-25 Ma).Early Miocene(?23.8-16 Ma):section 9-10 and 15-11,the values ofc,c_ARM,SIRM and HIRM decreased overall,reflecting the decrease in the content of SD grains and hard magnetic components,which indicate that the pedogenetic degree of paleosols weakened.The average value of MAP is 610 mm and MAT is 11.3?,indicating the climate was cold-dry in early Miocene(?23 Ma)and warm-dry in early Miocene(?20-17 Ma).Middle Miocene(?16-15 Ma):section 18-19,the values ofc,c_ARM,SIRM and HIRM increased to the highest value of the profile,indicating that the pedogenetic degree of paleosols was enhanced.In addition,MAP and MAT reached the highest values of 961 mm and 13?respectively,reflecting the warm and humid climate.(3)The climate change from Oligocene to middle Miocene in Lanzhou Basin was controlled by the expansion of the Antarctic ice sheet,the concentration of CO₂,and the evolution of seasonal patterns.The uplift of the Qinghai-Tibet Plateau blocked the transport channel of water vapor in the Lanzhou Basin,intensified the cold-dry climate in early Oligocene(?29 Ma)and early Miocene(?23 Ma),on the other hand,it strengthened the East Asian monsoon and brought water vapor to Lanzhou basin,which strengthened the wetting in middle Miocene(?16-15 Ma).The regression of the Para-Tethys Sea exacerbated the aridity process of Lanzhou Basin,resulting in a relatively dry climate in Oligocene(?29 Ma,?27-25 Ma)and early Miocene(?23 Ma,?20-17 Ma),while the transgression brought water vapor to Lanzhou Basin and strengthened the wetting of middle Miocene(?16-15 Ma).