| Since late nineteenth century, the trend of global warming become more and more obvious. During 1880～2012, the global surface temperature increased significantly, which have risen about 0.85℃. As the global warming, the extreme warm events will increase while extreme cold events will decrease in most land area, which will cause the pressure of the living environment increase.Karst area has a wide distribution in China, the distribution area of bare karst is about 620,000 k2m in seven provinces and one city in the southwest area (Yunnan, Guizhou, Guangxi, Sichuan, Hubei, Guangdong, Hunan, Chongqing). Karst area is sensitive to climate change due to its fragile ecosystem. Such as the moderate to severe drought at Guizhou, Yunnan, Sichuan and Chongqing in Spring,2010, which has cause very serious problems on people’ daily life. In a few words, it is very important to reconstruct the paleoclimate and environment change in karst area for having a better understanding of modern climate and forecasting climate change in the future.Guilin karst area belongs to subtropical monsoon climate, impacted by East Asia Monsoon and Southwest Monsoon at the same time. As is sensitive to climate change, Guilin karst area is an ideal area for paleoclimate and environment change study. People mainly use stalagmite to reconstruct paleoclimate and environment change since the last glacial period, while seldom use the Holocene lacustrine and wetland sediments to reconstruct the vegetation, climate and environment change. Therefore we lack geological data which can constract with stalagmite. Hence we select two karst area wetland sediments in Xinancun and Baxianyan, Guilin to study to make up for the deficiency.Changes to the vegetation, climate and the depositional environment of the karst area of Guilin, Guangxi were documented from 8000 years ago to the present, using spores, pollen, freshwater algae, geochemical index analysis and AMS14C dating. Then we discuss the regular pattern of vegetation succession, evolutionary character of climate and the depositional environment, along with the influence of human activity on natural vegetation.According to the spores, pollen, freshwater algae, geochemical index data in Baxianyan, Guilin, it is found that:ca.8435-4405 cal yr BP, subtropical evergreen broadleaved forest, dominated this area, and the climate was warm and wet; ca. 6285-4405 cal yr BP, more tropical and subtropical vegetation appeared, accompany with higher TOC, TN values and lower 513C values, indicating that this period may be the peak period of the Holocene Mega thermal. Since ca.4405 cal yr BP, the vegetation was replaced by subtropical evergreen broadleaved and coniferous mixed forest, indicating that the climate was relatively cold and dry. ca.4405-3590 cal yr BP, subtropical evergreen broadleaved forest almost disappeared, and a layer of peat developed under wet and cold condition, ca.3590-580 cal yr BP, subtropical evergreen broadleaved forest expanded to some extent, indicating that the temperature had risen, while the climate was cold and dry on the whole, ca.580 cal yr BP to the present, subtropical evergreen broadleaved forest shrunk further.ca.8435-4405 cal yr BP, the climate of the area which Baxianyan profile belongs to was warm and wet, while relatively low content of Cyperaceae pollen indicated that the depression was swamp at that time. ca.4405～3590 cal yr BP, the climate was cold and dry, an increase of Cyperaceae pollen indicated that the swamp expanded, ca.3590～580 cal yr BP, the climate was cold and dry on the whole, the swamp contracted with a decrease of Cyperaceae pollen.ca.580～0 cal yr BP, a continue decrease of Cyperaceae pollen indicated that the swamp dry up.According to the spores, pollen, freshwater algae, geochemical index data in Xinancun, Guilin, it is found that:ca.5390～1990 cal yr BP, subtropical evergreen broadleaved forest with some coniferous forest, dominated this area, reflecting warm and humid climate conditions. Conversely, ca.1990～380 cal yr BP the vegetation was replaced by subtropical evergreen broadleaved and coniferous mixed forest, indicating that the climate was relatively cold and dry. ca.1990-1540 cal yr BP, ca.1360-750 cal yr BP, subtropical evergreen broadleaved forest expanded to some extent, indicating that the climate was relatively warm and wet. Two episodes, ca.1540～1360 cal yr BP and ca. 750～380 cal yr BP, subtropical evergreen broadleaved forest shrunk with a decrease of Quercus pollen and an increase of Pinus pollen, indicating that the climate was relatively cold and dry.ca.5390-580 cal yr BP, the area which Xinancun profile belongs to was swamp which the hydrodynamic force was weak. ca.5390-3160 cal yr BP, the climate was warm and wet, the swamp expanded with relatively high content of Concentricystes; ca. 3160-1990 cal yr BP, a layer of peat developed. Two episodes of swamp expansion were recorded during ca.1990-1540 cal yr BP and ca.1360-750 cal yr BP, based on the high abundance of Cyperaceae pollen and Concentricystes. Two episodes, ca. 1540-1360 cal yr BP and ca.750-380 cal yr BP, indicating a contraction of the swamp with a decrease of Cyperaceae pollen and Concentricystes.The vegetation and climate change indicated by the spores, pollen, freshwater algae, geochemical index data which come from Baxianyan and Xinancun are consistent: Before ca.4405 cal yr BP, subtropical evergreen broadleaved forest dominated these two areas, and the climate was warm and wet. ca.4405-3590 cal yr BP, subtropical evergreen broadleaved forest distributed in these two areas shrunk quickly, accompany with relatively higher 813C values, indicating that the temperature dropped quickly. During this time, an increase of Cyperaceae pollen in Baxianyan and relatively bigger grain size in Xinancun indicated that the precipitation may not changed or even increased. This period is an important climate transition period in Guilin. ca.3590-580 cal yr BP, subtropical evergreen broadleaved forest expanded to some extent in Baxianyan; ca.3160-1990 cal yr BP, subtropical evergreen broadleaved forest also expanded in Xinancun, indicating that the temperature had risen, while the climate was cold and dry on the whole, ca.580cal yr BP to the present, subtropical evergreen broadleaved forest shrunk further, accompany with the increasing trend of 8l3C values, indicating that the climate was cold and dry.About ca.750～580 cal yr BP, an increase of Poaceae pollen both in these two areas indicating that human activity have more and more impact on natural vegetation. |
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