NOTES Chinese Science Bulletin Vol. 47 No. 20 October 2002 1739 Extension of drylands in northern China around 250 kaBP linked with the uplift of the southeast margin of Tibetan Plateau WU Haibin 1 , GUO Zhengtang 3 , FANG Xiaomin 2,3 , ZHANG Jiawu 2 & CHEN Fahu 2 1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2. Department of Geography, Lanzhou University, Lanzhou 730000, China; 3. Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Correspondence should be addressed to Guo Zhengtang (e-mail: ztguo@mail.igcas.ac.cn or ztguo@95777.com) Abstract Study on two loess sections, one located at Wu- wei near the Tengger Desert in northwestern China, another located near Ganzi at the southeast margin of the Tibetan Plateau in southwest China, reveals a coeval drying step oc- curred at ~250 kaBP. It is expressed by the increase in eolian grain-size at Wuwei, and by a drastic extension of C4 plants and a decrease of loess chemical weathering intensity at Ganzi. Examination of the available eolian data indicates that the event has also been clearly documented in the loess sections near the deserts in northern China, and in the eolian records from the North Pacific. On the contrary, the signal is rather weak for the central and southern Loess Plateau re- gions as well as for Central Asia, where the climates are in- fluenced by the southeast Asian monsoon and the westerlies, respectively. Since the climate at Ganzi is under strong con- trol of the southwest Asian monsoon, we interpret this drying step as a result of decreased influence of the southwest sum- mer monsoon. This decrease in monsoon moisture is attrib- utable to the uplift of the Hengduan Mountains, the south- east margin of the Tibetan Plateau at ~ 250 ka ago. Keywords: aridification, loess-paleosol sequences, organic carbon isotope, chemical weathering intensity, Tibetan Plateau. Since the late 1980s, tectonic events in East Asia, especially the uplift of the Tibetan Plateau, have been in- voked as driving forces behind the global and Asian envi- ronmental changes at over-orbital timescales [1, 2] . Aridifi- cation in the interior of Asia is regarded as one of the markers of global environment deterioration in the Ceno- zoic era [2] . Uplift influences climates through several mechanisms [3ü5] , including the barrier effect to moisture and the reorganizations of atmospheric and ocean circula- tions, such that the consequent climate expression varies in different regions. Understanding of the tectonic impacts on climate changes, therefore, requires a good temporal and spatial coverage of climate records. Loess is a geological record that bears information on climate changes of both dust source and depositional regions and is also a tracer of the atmospheric circulations. Earlier studies [6ü10] on the eolian deposits have provided a fare picture about the long-term evolution of the Asian aridification history. However, most of these sites are from the Loess Plateau region in northern China. In recent years, loess-soil sequences [11ü13] back to the middle Pleis- tocene have been reported from the surrounding areas of the Tibetan Plateau, which provide the possibilities to examine the impacts of uplift on climate changes in more detail. In this study, two loess-soil sequences from the mar- ginal areas of the Tibetan Plateau are comparatively stud- ied. The objective is to address the long-term environ- mental histories in the two areas over the past 600 ka, and to interpret the relationship between the changes of at- mosphere circulations and uplift of the Tibetan Plateau. 1 Materials and methods The Huangyangzhen section (37°34h N, 102°49h E) is located in the Wuwei region, Gansu Province, between the northeastern margin of the Tibetan Plateau and south- western margin of the Tengger Desert (fig. 1), with an altitude of 2100 m a.s.l. The location is well suited for addressing the relationships between the evolution of the Tengger Desert and the Tibetan uplift. The Ganzi section (31°38h N, 99°59h E, 3480 m a.s.l) situates on the sixth terrace (T6) of the Yalong Rive in Sichuan Province (fig. 1), at the southeastern margin of Tibetan Plateau. This region is within the pathway of the southwest monsoon moisture into China mainland, and is therefore a good record of the monsoon influence. The modern mean an- nual temperature (MAT) and precipitation (MAP) are 7.9k and 105 mm at Wuwei, 5.7kand 637 mm at Ganzi, respectively. Both sections were dated by geomagnetic methods and were described in detail [11, 13] . Grain-size parameters and chemical weathering in- tensity of the loess-soil sequences are widely used as proxies of paleoclimate [14,15] . In this study, the grain-size of 424 samples from Huangyangzhen was analyzed using the precipitation method following the Stokes Law. Sam- ples were taken at 10 cm intervals from the upper 32 m depth, and at 25 cm intervals for the lower 26 m depth. The contents of free Fe 2 O 3 (FeT) and total Fe 2 O 3 (FeD) of 120 samples at 10 cm intervals from Ganzi were analyzed to address the chemical weathering intensity [15] of loess. FeD was extracted using the CBD method and FeT was analyzed on completely acid-dissolved samples, both were measured on a WFD-Y2 atomic absorption unit. To ad- dress the vegetation evolution in the Ganzi region, the organic carbon isotope composition of 19 loess-paleosol samples at 0.5ü1 m intervals were analyzed. Powdered samples were acidified using 1 mol/L HCl, dried at 60k and combusted in the presence of excess cupric oxide. The