676 Science in China Ser. D Earth Sciences 2005 Vol.48 No.5 676—688 Copyright by Science in China Press 2005 The vegetation and climate change during Neocene and Early Quaternary in Jiuxi Basin, China MA Yuzhen 1 , FANG Xiaomin 1,2 , LI Jijun 1 , WU Fuli 1 & ZHANG Jun 1 1. Key Laboratory of Western China’s Environmental Systems and College of Resources and Environment, Lanzhou University, Lanzhou 730000, China; 2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Correspondence should be addressed to Ma Yuzhen (email: yzhma@lzu.edu.cn) Received April 4, 2003 Abstract Sporopollen record in the Laojunmiao Section at Yumen in the Hexi Corridor foreland depression at the northern margin of the Tibetan Plateau revealed that during the period of 13.0―11.15 Ma the ecological environment of the Jiuxi Basin is characterized by steppe vegetation and a semi-moist climate. During 11.16―8.60 Ma prevailed forests of cypress and a still warmer, moister climate; steppe vegetation and dry climate began probably at about 8.6 Ma. Although aridification had been relaxed time and again during 8.40―6.93 Ma (forest-steppe, warm-semi-moist), 6.64―5.67 Ma (open-forest and steppe, warmer-semi-moist) and 5.42―4.96 Ma (steppe, semi-arid), the climate in the region became drier and drier in response to the fre- quent occurrence of aridity during 6.93 ―6.64 Ma (steppe, semi-arid), 5.67―5.42 Ma (de- sert-steppe, arid), 3.66―3.30 Ma (desert-steppe, arid) and 2.56―2.21 Ma (desert, arid). Perhaps the important findings of our study are the notable expansion of drought-enduring plants during 3.66―3.30 Ma and about 2.56 Ma and the replacement of vegetation by vast arid desert. Keywords: Tibetan Plateau, Hexi Corridor, sporopollen record, ecological environment, vegetation evolution, aridific- ation. DOI: 10.1360/03yd0110 Desertification and aridification in the inland of Asia are the important scientific issues pertaining to the existing environment of mankind and the sustain- able development of society in western China. The onset and evolution sequence, development and proc- ess of history and the mechanism of this transition are thought to be one of the most attractive hot objects of research on climatic changes in the Northern Hemi- sphere during the Cenozoic, which may have involved glaciation in the Northern Hemisphere and the uplift of the Tibetan Plateau. Studies of large-scale eolian dust accumulation in the Xifeng and Lingtai loess sec- tions on the Loess Plateau showed that desertification must have occurred in the inland of Asia during Late Miocene (8 Ma) and must have been associated with the uplift of the Tibetan Plateau during this period of time [1―5] . Development of aridification in Asia and South America reflected the enhancement of dust ac- cumulation rates in South and North Pacific during 8―7.7 Ma. Particularly at 3.6 Ma there occurred a drastic increase in dust accumulation rate, which is also explained to be closely related to the uplift of the