• 责任者：Guo Gaoxuan (Beijing Hydrogeology and Engineering Geology Team ; Beijing 100195 ; China) ; Jiang Hanchao
• 刊名：Quaternary Sciences
• 出版年：2013
• 卷期：33(6)
• 关键词：更新世 ; Pleistocene ; 孢粉组合 ; palynological assemblage ; 北京 ; Beijing
• 页码：p.1160-1170
• 图表：3 illus., 61 refs.
• 分类号：1400
• issnNo：ISSN1001-7410
• isbnNo：CN11-2708/P

     To explore evolutional characteristics of vegetation in North China during the Pleistocene period, pollen analysis was conducted on the fluviolacustrine sediments of the X5 core in Beijing. The X5 core （39°57＇04.3＂N, 116°30＇55.3＂E） has stratigraphie units of Quaternary, Neogene,Palaeogene and Cretaceous age, and is 862.7m in total depth. The upper 426. 35m contains multiple conglomerate-sand-sih-elay cycles. Magnetostratigraphic results suggest that the upper 422.12m spans the past 3.58Ma and is generally continuous. Fifty-one pollen samples from the upper 422. 12m were analyzecl and 38 samples contained 100 -351 pollen and spore grains（ mean 202 grains/ sample）. A total of 52 pollen and spore types were identified. Shrub and herb taxa were dominant in most of the samples,ranging from 10.3% to 80.9% in abundance with an average of 46.7%. They were dominated by Corylus, Artemisia,Chenopodiaceae and Cyperaeeae. Also present were Gramineae, Polygonaceae, Compositae, Thalictrum, Labiatae,and Myriophyllum. Arboreal pollen taxa were less dominant, fluctuating between 5.9% and 80.6% and averaging to 31.4%. They were dominated by Pinus, Quereus and Ulmus. Also present were Pieea, Abies, Podocarpus, Tsuga, Cupressaeeae,Alnus, Betula, Juglans, Carya, and Castanea. Fern spores and algae ranged from 1.3% to 11.4% in content with an average of 5.6%. These suggest that the study area has generally been covered by sparse-wooded steppe or steppe with some conifers growing on the northern and western mountains since 2.3Ma, especially since 1.7Ma. Content variations of major pollen taxa indicate a warm and semi-arid climate in the study area during 2.3 - 2.1Ma. From 1.7Ma to 1.45Ma, temperature in winter probably decreased in Beijing, resulting in distinct increase of coniferous trees on the western and northern mountains. In the meanwhile,the summer monsoon also strengthened,leading to evident development of broadleaved trees and hygrophilous herbs. These suggest that, relative to the early period （2.3 - 2.1 Ma）, seasonality during this period （ 1.70 - 1.45 Ma） increased in the study region. During 1.45 - 1.05Ma, the climate in the study area remained previous features with small fluctuations. During 1.05 -0.94Ma, the conifer Pinus decreased and increased since 0.94Ma, probably corresponding to the mid-Pleistocene climate transition and subsequent polar ice-sheet development. Polar ice volume increased and meridional temperature gradient deepened in the Northern Hemisphere, leading to strengthening of the winter monsoon in East Asia. On the other hand, during 1.05 - 0.94Ma, both broadleaved trees and hygrophilous herbs/ferns increased to some degree and showed a declining trend upwards, implying that the summer monsoon generally strengthened during 1.05 - 0. 94Ma and weakened since 0. 94Ma. Accordingly,it is clear that both the winter and the summer monsoon circulations strengthened during the mid-Pleistocene climate transition. However, with the development of polar ice sheets and global cooling, the winter monsoon kept strengthening while the summer monsoon declined gradually. It＇s why vegetation and environments in many regions of North China recovered momently and deteriorated continuously from the mid-Pleistocene climate transition. From ca. 0.5Ma, the conifers represented by Pinus decreased significantly and the broadleaved trees also declined,revealing a drying climate. Hygrophilous herbs and trees were gradually replaced by Artemisia and Chenopodiaeeae. The evolving pattern of  palynoflora in the study area is well consistent with environmental change in other regions of East Asia, North Pacific  and even the Southern Hemisphere. Nevertheless,what is responsible for the occurrence of mid-Pleistocene climate  transition and significant climate shift at ca. 0.5Ma remains unknown and deserves further investigation.