中文摘要：

始新世-渐新世之交（EOT）,全球气候急剧变冷,由＂温室期＂进入了＂冰室期＂,是新生代以来重大气候事件之一.为了更好揭示该时期陆地气候环境变化过程及其对全球变化的响应机制,本文选取位于青藏高原东北缘西宁盆地始新世/渐新世过渡期的地层,通过距离西宁市区约7km的塔山村一带（36.5＆#176;N,101.8＆#176;E）的塔山剖面马哈拉沟组中的孢粉样品分析,建立了两个孢粉组合,自下而上为：Ⅰ Nitrariadites-Meliaceoidites-Quercoidites （35.4～33.9Ma）,Ⅱ Quercoidites-Labitricolpites-Qinghaipollis（33.3 ～ 33.2Ma）,表明西宁盆地植被由晚始新世的暖温带半干旱的疏林灌丛、疏林,在早渐新世向干旱的温带疏林草原的转化,指示该区域在33.9～33.3Ma期间气温明显下降并伴随干旱化持续现象.西宁地区该植被变化可能是对EOT时期全球变冷事件的响应.

英文摘要：

During the Eocene-Oligocene transition （EOT） the global climate cooled from ＂greenhouse＂ to ＂icehouse＂ at ca. 33.5Ma, which is one of the most prominent events during Cenozoic climate deterioration. However, few records of continental climate changes during this period are available, especially in Asia. To determine the behavior of Eocene-Oligocene continental climate changes, the Late Eocene to Early Oligocene sedimentary succession from the Xining Basin in the northeastern margin of the Tibetan Plateau was selected for pollen analysis. A total of 24 pollen samples were collected from the Mahalagou Formation of the Tashan section （36.5~N, 101.8~E） , which is located ca. 7km south of Xining City. The section is 103.3m thick （ca.35.5Ma to 33.1Ma） and composed of red mudstones with 24 muddy gypsum or gypsiferous mud layers. Ten samples contained relatively rich pollen fossils. There were 109 palynomorphs belonging to 70 genus and 43 families. According to pollen variations from the lower to upper parts of the section, two spore pollen assemblages were established： I Nitrariadites-Meliaceoidites - Quercoidites （ 35.4 ~ 33.9Ma ） , I1 Quercoidites - Labitricolpites - Qinghaipollis （ 33.3 ~ 33.2Ma）. In Assemblage I, high abundances of shrub taxa such as Ephedripites and Nitrariadites are found. In addition, thermophilic broadleaf taxa including Meliaceoidits and Rutaceoipollenites are also found. Others pollens such as the Quercoidites and QinghaipoUis are minor consituents. In Assemblage 11, the proportion Ephedripites, Nitrariadites, Meliaceoidits is less than in Assemblage I. The abundance of herb taxa such as Labitricolpites and Qinghaipollis, however, is greater. Pollen analysis demonstrates that the vegetation of Xining Basin changed from Late Eocene semiarid warm sparse forest shrub to Early Oligocene arid temperate sparse forest grassland. These results indicate that the cooling occurred at ca. 33.9-33.3Ma and is coincident with aridification in Xining Basin, which may correspond t