| The most significant changes of global environment are the stepwise decline in temperature and the formation and evolution of polar glaciation during the Cenozoic era. Particularly, the characteristics of gological and environmental condition in Asia have experienced changes thoroughly. The uplift of Tibetan Plateau, the evolution of Asian monsoon and the history of aridification of Asia’s interior are the most intriguing and challenging topics in geosciences. Although many progresses have been achieved, the important scientific problems, such as the age, cause, and mechanism of these major environmental events are unsolved and need further investigation. The Tianshui basin sits at the junction of the monsoon region, the northwest arid area, and the Tibetan Plateau, just it the peak of’monsoon triangle’, a region very sensitive to climatic change. Moreover, the widely-distributed, continuous mudflat/distal fan and shallow lake deposits in this basin provide an ideal setting for the study of paleoclimate history during Neogene. In previous works, the age of Red Clay at the eastern Loess Plateau was dated around 8 Ma. However, all geological and paleontologic evidence from the Linxia Basin indicates the climate during the early-middle Miocene was warm and humid until the later Late Miocene (7-8 Ma). Recently, Guo et al. (2002) suggested that the onset of Asian desertification was by the early Miocene epoch (22 Ma) based on the field observation, mineral assemblage, grain size and morphology and susceptibility of the Tertiary red clay from Tianshui Basin which is similar to that of Quaternary loess. Information from paleovegetation buried in the strata of basins surrounding the Tibetan Plateau provides more direct evidence for the aridification of the Asian interior. Thus, we carry out a palynulogical research at Yawan section which chronology had been well constrained by the high-resolution magnetostratigraphy and biostratigraphy, in the Tianshui Basin where sporopollen recod is scarce. Based on the sporopollen record of the Yawan section, main acquaintance and conclusions can be drawn as follows:1. The Middle-Late Miocene sporopollen record of Tianshui Basin reveals that three paleovegetation and palaeoclimatic shifts occurred at 14.7 Ma,11.7 Ma,8.5 Ma, respectively. During the period 17.1～14.7 Ma, the vegetation of Tianshui area was characterized by temperate to warm-temperate and north subtropical forest, composed mainly of Betula, Quercus, and Ulmus, indicating a warm and humid climate. The forest vegetation changed to forest or forest-steppe during the period between 14.7 and 11.7 Ma, showing that the climate became drier. During the period between 11.7 and 8.5 Ma, temperate to warm-temperate forest occupied the study area again, indicating the climate became more humid than the interval of 14.7～11.7 Ma, but still less humid than the earlier period between 17.1 and 14.7 Ma. During the period from 8.5 to 6.1 Ma, the zonal vegetation changed from forest to forest-steppe vegetation, suggesting that the climate was warm and semi-arid or semi-humid.2. The climate parameters were obtained by applying the Coexistence Approach based on the sporopollen zones.During the period 17.1～14.7 Ma, the mean annual temperature (MAT):20.9～13.3℃, the warmest month temperature (WMT):25～22.5℃, the coldest month temperature (CMT):5.9～1.9℃, the difference of temperature between warmest month temperature and coldest month temperature (DT):25.8～12.3℃, the mean annual precipitation (MAP):1389.4～797.5 mm, the maximum monthly precipitation (MMaP):268.1～161.4 mm, the minimum monthly precipitation (MMiP):14.1～6.9mm.During the period between 14.7 and 11.7 Ma, MAT:14.6～11.3℃, WMT:27.4～19.8℃, CMT: 5.5～0.3℃, DT:27.6-12.8℃, MAP:1254.7～601.1 mm, MMaP:218.8～141.5 mm, MMiP: 14.1～6.9 mm.During the period from 11.7 to 8.5 Ma, MAT:14.6～10.2℃, WMT:27.4～19.8℃, CMT: 5.5～0.2℃, DT:25.8～12.8℃, MAP:1254.7～797.5 mm, MMaP:237.5～137.3 mm, MMiP: 14.1～5.8 mm.During the interval of 8.5～6.1 Ma, MAT:14.6～9℃, WMT:27.4～18.7℃, CMT:5.5～4.4℃, DT:26.9～15.5℃, MAP:1254.7～573.9 mm, MMaP:218.8～129.4 mm, MMiP:14.1～5.8 mm.3. Based on analysis of the spoeropollen record of the Tianshui Basin and Miocene climatic condition in central Asia, the aridification of Asia’s interior probably started at about later Late Miocene, coincidentally with the onset of Hipparion red clay deposition in North China. The formation and development of Arctic ice-sheet and/or El Nino may be responsible for this drying climate event; in addition, the uplift of Tibetan Plateau may also contribute.4. The evolution of East Asian summer monsoon can be divided into four stages during the period 17.1～6.1 Ma based on the sporopollen data and climate parameters obtained by applying Coexistence Approach. The East Asian summer monsoon was strong during the period 17.1～14.7 Ma; the interval of 14.7～11.7 Ma was marked by an obviously decline of the East Asian summer monsoon; during the period 11.7～8.5 Ma the East Asian summer monsoon was strengthened again; during the period from 8.5 to 6.1 Ma, the East Asian summer monsoon was generally weak. This general trend of the East Asian summer monsoon appears to fit well with the Middle Miocene Climatic Optimum (MMCO), Middle Miocene Cooling and a northward-shift of the ITCZ (caused by East Antarctic ice-sheet growth). This correspondence appears to indicate that the East Asian summer monsoon was driven by global climate changes. In addition, because the synchronism of water and heat is the distinctive characteristics of modern East Asian monsoon, the cool and humid climatic condition during the period 11.7～8.5 Ma may indiate that the modern East Asian monsoon had not occurred at the latest by 8.5 Ma. |
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