| The uplift of the Tibetan Plateau resulting from the collision between the Indian and Eurasian plates and their continued convergence is one of the greatest tectonic events during the Cenozoic. Its presence not only change the Asian geomorphic patterns, but also have enormous influence on the global and regional climate. Thus the formation, uplift of the Tibetan Plateau and its impact on the environmental has been present-day important subject in international geological research.Numbers of Tertiary sedimentary basins were formed in the internal and surrounding regions of the Tibetan Plateau, as a result of the tectonic uplift of the Tibetan Plateau. The Xunhua basin, which is an important part of the basin-mountain coupling system in the northeastern Tibetan Plateau, holds continuous fine fluvial-lacustrine sediments from the Oligocene to the late Miocene (between～30.0 Ma and 5.8 Ma), preserves a relatively complete record of the deformation and uplift of the northeastern of the Plateau and the environment and climate change, and is ideal to examine the course of interaction and mechanism between the uplift of Plateau and climate changes.Based on the high resolution magnetostratigraphy, a detailed investigation of the sedimentary facies, provenance and sedimentary accumulation rate conducted on the Oligocene-Miocene sediments within the Xunhua basin. We divided the sedimentary evolution responding to the tectonic uplift into three phases:(1) the initial uplift to stable development period from 30.0 Ma to 22 Ma; (2) the middle uplift to stable development period from 22 Ma to 7.3 Ma; and 3) the later strong uplift period between 7.3 Ma and 5.8 Ma.The global climate has undergone tremendous changes, one of the most significant change is the global cooling process marked by the origin and expansion of polar ice-sheet. Contemporaneously, The prominent environmental change in Asian is the formation and development of East Asian monsoon and aridification environments. For the variety natural proxies of climate changes, plants are particularly sensitive to the ecological environment changes, palynological flora becomes the most direct and sensitive records of the environment and climate change in continental history. This paper is based on the analysis of Palynology samples from the Yangquangou and Xigou section of the Xunhua basin, and obtained a relatively detailed pollen record of our continent, as indicators of the long-term terrestrial paleoecosystem and paleoclimate changes in the Asian interior of NW China.On the basis of the variations in abundance of the dominant taxa, the palynological diagram is subdivided into six palynological Assemblage from the bottom up. Assemblage I (30～24.8 Ma) Pinaceae-Chenopodipollis-Nitrariadites-Ephedripites. Assemblage Ⅱ(24.8～23.0 Ma) Pinaceae-Chenopodipollis-Nitrariadites-Ephedripites-Quercoidites. Assemblage III (23.0-17.4 Ma) Pinaceae-Chenopodipollis-Betulaepollenites-Juglanspollenites. Assemblage IV (17.4～13.6 Ma) Pinaceae-Betulaepollenites-Juglanspollenites. Assemblage V (13.6-8.0 Ma) Pinaceae-Betulaepollenites-Graminidites-Ephedripites-Chenopodipollis. Assemblage VI (8.2-5.8 Ma) Ephedripites-Nitrariadites-ChenopodipollisThe palynological record shows that shrub-steppe dominated the Xunhua region with dry and cold climate from 30.0 Ma to 24.8 Ma; During 24.8～23.0 Ma the vegetation change gradually from shrub-steppe to forest-steppe with warmer climate. Then the pollen diversity increase and the forest-steppe is replaced by forest and continuously develop until～13.6 Ma. Forest taxa has obviously decreased and proportion of xerophilous herbs have enhanced since～13.6 Ma with continuous dryer and colder. In a word, the vegetation evolution can be subdivided into three stage:shrub (30.0-24.8 Ma), shrub-steppe (24.8～23.0 Ma), forest (23～13.6 Ma) and desert steppe (13.6-5.8 Ma).The elevation history of Tibetan Plateau has been studied by many researchers. Several quantitative evidence indicates that the Tibetan Plateau have been uplifted significantly before Late Oligocene. In order to quantitative reconstruct paleoclimate and paleoelevation in northeastern Tibetan Plateau, we conduct the Coexistence Approach on the advantage taxa of the palynological zones and obtain the Oligocene-Miocene paleoelevation and paleoclimatic parameters of the Xunhua basin and surroundings as follows:Late Oligocene-Early Miocene (30～23.0 Ma):Paleoelevation 2800-3200 m, MAT 7.8～14.9℃, MAP 446～1031 mm; Early Miocene (23.0～17.4 Ma):Paleoelevation 2400～2800 m, MAT 11.3～20.9℃, MAP 798～1540 mm; Middle Miocene (17.4～13.6 Ma):Paleoelevation 2400～2600 m, MAT 13.3～20.9℃, MAP 798～1540 mm; Middle-Late Miocene (13.6～8.0 Ma):Paleoelevation 2400～2800 m, MAT 7.8～14.9℃, MAP 446～1031 mm; Late Miocene (8.0～5.6 Ma):Paleoelevation 2400～4000 m, MAT 2.8～14.9℃, MAP 403～1031 mm.The East Asian monsoon and aridification in Central Asia environment system is an important component of the global environment system and it covers a large portion of Asia. Recently, several studies have shown that mechanism of Asian monsoon-arid environment system have been controlled not only by the global cooling, but have also been affected by other significant events related to their geographical locations, such as the uplift of the Tibetan Plateau and the retreat of the Paratethys Sea. Although growing attention was paid to the Central Asia Miocene environment evolution in past decades, the initial time and geodynamic mechanisms of East Asian monsoon and aridification in Central Asia environment system remain inconclusive.In this paper, based on the ecological habits of pollen fossils, we divide them into xerophytic taxa, thermophilic taxa and the relatively humid coniferous taxa as a climatic proxy and use the various of mean annual temperature (MAT) and mean annual precipitation (MAP) as an auxiliary index to explore the environment evolution of Xunhua basin. By comparing them with the North Pacific eolian flux and the global deep-sea δ18O isotope record, main conclusions are summarized as following:Around 23.0 Ma, Palynological records show that the vegetation in the Xunhua basin and its surrounding mountains changes from steppe to forest, with climate shifting form a cold-arid climate to relative warm-wet conditions. This may be related to the extent of the East Asian monsoon close to the Xunhua basin, bringing more precipitation. By using the Coexistence Approach to the fossil pollen records, a maximum paleoelevation of 2800-3200 m asl is estimated in the Xunhua basin and its surroundings in the late Oligocene-Early Miocene (30-23.0 Ma). Combined with the geological evidence of around 25 Ma, we infer that Tibetan Plateau uplift plays a key role in East Asian summer monsoon dynamics. In addition, a large number of numerical experiments illustrate that the Paratethys Sea retreats to the Turan Plate have occurred before Early Miocene. Paratethys Sea retreats can strengthen the East Asian summer monsoon and greatly increase humidity in the monsoon areas. In views of this, we suggest that the Paratethys retreat may also play important roles in the formation of East Asian monsoon environmental pattern.During 23.0～13.6 Ma, the vegetation types are dominated by mixed coniferous-broadleaved forests with higher values of mean annual temperature (MAT) and mean annual precipitation (MAP), indicating the strengthening of East Asian summer monsoon. Global climate may be the key factor for the strengthening of East Asian summer monsoon, while the influence of the uplift of Tibetan Plateau is limited.The most significant change in Palynological records is that the increase in xerophytic taxa percentages and decrease in the relatively humid coniferous taxa, indicating aridification in the Xunhua basin and its surrounding mountains since 13.6 Ma, with climate shifting form a warm-wet climate to relative cold-arid conditions. We conclude that this shift may be associated with the gradual weakening of East Asian summer monsoon and strengthening of East Asian winter monsoon. And the aridification in the Xunhua basin since 13.6 Ma is done by two main drying events occurred at around 13.6 Ma and 8.0 Ma. During the same period, both the thermophilic taxa percentages and mean annual temperature (MAT) show a decreasing trend, recording a gradually cooling process. It corresponds well with the Cenozoic global temperature curve, suggesting that aridification in the Xunhua basin and its surrounding mountains since 13.6 Ma is mainly controlled by Global cooling. Since late Miocene, a series of mountain building in northeastern Tibetan Plateau could effectively disturb the local atmospheric circulation and precipitation patterns in the mountainous regions, causing the windward of the mountains obtain high precipitation, while the lee of mountains have drier conditions. Coinsidence between tectonic uplift and drying event around ～13.6 Ma and 8.0 Ma, suggest that the Tibetan Plateau uplift is also contributed to the intensified aridification of the Xunhua basin. Furthermore, as the timing of final retreat of the Paratethys from the central Asia are still poorly constrained, we can not rule out its influence to this major drying event in the Xunhua basin. The cause of evolution of the East-Asian monsoon needs further detailed investigation. |
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