Last glacial pollen record from Lanzhou (Northwestern China) and possible forcing mechanisms for the MIS 3 climate change in Middle to East Asia |
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| Authors: | Hanchao Jiang Xue Mao Hongyan Xu Jessica Thompson Ping Wang Xiaolin Ma |
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| Institution: | 1. CAS Center for Excellence in Tibetan Plateau Earth Sciences & Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. School of Earth Sciences & Key Laboratory of Western China''s Mineral Resources of Gansu Province, Lanzhou University, Lanzhou 730000, China;4. School of Geosciences, China University of Petroleum (East), Qingdao 266555, China;1. Department of Geography, Korea University, Seoul 136-701, South Korea;2. Division of Earth and Environmental Science, Korea Basic Science Institute, Chungbuk 363-883, South Korea;3. Kyrgyz Institute of Seismology, Kyrgyz Academy of Sciences, Bishkek 720060, Kyrgyzstan |
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| Abstract: | The vegetation on the northeastern margin of the Tibetan Plateau is highly sensitive to climatic changes and thus represents a potentially interesting environmental archive. Pollen samples from the Fanjiaping Loess section in Lanzhou on the western Chinese Loess Plateau (CLP) were analyzed in conjunction with OSL dating. The results indicate that pollen zone B (60.6–46.0 ka, correlative to the early MIS 3) had the greatest abundances of Cupressaceae, Tsuga, Gramineae and Cyperaceae of the entire section, suggesting a warm phase during the last glacial period. These pollen taxa decreased significantly in abundance in the zones C (46.0–39.0 ka) and D (39.0–27.0 ka), reflecting a substantial climate cooling from the middle MIS 3 to MIS 2. These results correlate with climate records from the South China Sea, the CLP, Baikal Lake, North America, North Atlantic Ocean and other regions, and probably correspond with the decline of northern high-latitude insolation and the increase of global ice volume from 50 to 20 ka. In particular, arboreal pollen, fern spore and algae abundances declined sharply since ～40 ka, while shrub and herb pollen reached the highest abundances. Conifer pollen Picea and Abies abundance also rose markedly and increased up the section. This implies significant climate deterioration and likely corresponded with substantial growth of the polar ice sheets since ～40 ka. The decreasing temperature caused by an insolation decline during the last glacial period probably reinforced the cooling effect in a ‘snow/ice/albedo’ feedback, which would result in less climate sensitivity to radiative forcing. Meanwhile, vegetation decline in the Northern Hemisphere during the last glacial period and tundra development at high latitudes possibly caused additional cooling, enhancing the growth of polar ice sheets since 40 ka. The development of polar ice sheets increased the polar-to-equator temperature and pressure gradients, strengthening the westerlies and supplying plenty of moisture to Northwest China during 40–30 ka. Lake sediments developed widely on the Tibetan Plateau during 40–30 ka, probably related to an increase in the seasonality of middle-to-low latitude insolation which caused an enhancement of glacier melting on the Plateau. |
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