Late Eocene pollen records and palaeoenvironmental changes in northern Tibetan Plateau

The Eocene palaeovegetation landscape and palaeoclimate reconstructed from the pollen records in the Jiuquan Basin, northwest China provide some important information on the early uplift of the Tibetan Plateau and the origin and evolution of the aridification in northwest China. The records show the arid-semiarid scrubs with open forest palynofloras controlled by the subtropical high existed in northwest China during the 40.2–33.4 Ma. Four pollen assemblages are found: Nitrariadites-Cheno-podipollis-Pinaceae assemblage (40.2–37.9 Ma) is followed by Chenopodipollis-Nitrariadites assemblage (37.9–34.6 Ma), Pinuspollenites & Abietineaepollenites-Chenopodipollis assemblage (34.6–33.9 Ma), and Chenopodipollis-Nitrariadites assemblage (33.9–33.4 Ma). The percentage of thermophilic types is in anti-correlation with that of the dry types, which means the palaeoclimate is relatively warm-wet or cold-dry during most of that time. Such aridity may be related to the water vapor reduction and the planetary wind system movement northward in response to the cooling caused by small-ephemeral ice-sheets. Supported by the National Key Program for Developing Basic Sciences (Grant No. 2005CB422001), the National Science Foundation of China (Grant Nos. 40334038, 40421101) and the President Fund of Chinese Academy of Sciences