晚新生代以来中国西北植被演化及反映的干旱化过程

亚洲内陆干旱化是全球新生代大陆环境变化中最引人瞩目的重大事件之一，一直是古气候研究的热点。但到目前为止，亚洲内陆干旱化的起源、演化及其机制仍存在争议。本文系统整理了具详细定年的30条孢粉序列，利用生物群区化方法，重建了晚新生代以来我国西北植被的时空演化历史，探讨了干旱化过程。结果显示：约36~28Ma，我国西北植被以森林为主，气候湿润；约27~24Ma，出现草原，开始呈现干旱化趋势；约23~18Ma荒漠扩张；约8Ma以来，大部分森林被荒漠、草原替代，干旱程度显著增强。上述干旱化历史与全球变冷、青藏高原隆升及副特提斯海退缩在时代上有一定的关联性。因此认为，亚洲内陆干旱化的起源可能与青藏高原隆升及副特提斯海的退缩有关，而之后干旱化过程受全球变冷影响逐步加强，约8Ma以来的急剧干旱化可能与北半球高纬变冷的关系更为密切。

Aridification in Northwestern China since the Late Cenozoic evidenced by the vegetation change

The aridification in the Asian Interior is one of the key climate events in the Cenozoic. Over the past score years, although numerous studies from separated locations within the continental interior obtain the fundamental framework for the aridification, discrepancy exists among various records from different areas, and the history of the aridification is still open to debate. To better constrain the evolution of aridification in inner Asia, a promising approach is to reconstruct a long-term, well-dated environmental history based on regional integrated analysis. In addition, Northwestern China is located in the internal arid area of Asian, with a typical continental climate, so the vegetation change in this region is more sensitive to the aridity. In order to achieve reliable vegetation change data, our collected palynological records are selected based on the following criteria: 1)the record should be well dated. The records constrained solely on lithological correlations or pollenchronologies are removed from consideration, and the remaining records are dated mainly by detailed magnetostratigraphic ages; 2)palynological records must cover detailed pollen spectrum information to ensure the accuracy of the biomization. A total of 30 palynological sequences met these criteria and have been used within this study. Then the chronologies of the palynological sequences were unified to the 2012 Geomagnetic Polarity Time Scale. Finally, we used the biomization method to reconstruct the vegetation changes in Northwestern China during the Late Cenozoic. The results show a relatively humid climatic condition with mainly forest in Northwestern China from ca.36Ma to 28Ma. Steppe started to expand during ca.27Ma and 24Ma, indicating an aridification trend, and changed to desert during the interval from ca.23Ma to 18Ma. Aridity was significantly increased since ca.8Ma, as evidenced by the replacement of forests by steppe and deserts. We attribute the early stepwise aridification to the retreat of the Paratethys Sea and to the uplift of Tibetan Plateau, and the subsequent gradual aridification trends to the ongoing Late Cenozoic global cooling. The significant aridification since ca.8Ma may be linked with the enhanced cooling in the northern high latitudes.