| With Chenghu Lake as research object, which is a small shallow lake in Taihu plain, under the Holocene climate background, systematic researches have been done on the paleo-geographic environment evolution recorded by the paleo-limnological sediment from Chenghu Lake and the process of ancient climate change. According to the analysis on the lithological characters, susceptibility, granularity, clay mineral, organic matter, humification of the paleo-limnological sediment, comparing with the other cores'researches of this region, and the chronology of AMS14C dates, following mainly conclusions:1. Dated from mid-Holocene, paleo-geographic environmental evolution of Chenghu Lake region can be divided approximately into four phases,â… :About 7.2-6.8kaBP, the late of early Holocene, the location of profile sample of SC4 core is low-lying and easy to be seeped, climate is to chang from relatively cool and dry to warm and humid.â…¡:About 6.8-6.3kaBP, climate is high temperature and humidity, plants grow, and spore-pollen shows that woody plants increase during mid-Holocene period. The region of SC4 core changes to shallow swamps environment gradually and become a tiny shallow lake with the rising of water level.â…¢:About 6.3-6.1kaBP, the late of mid Holocene, each results of proxy changed strongly. Climate changing is a process from warm and humid into relatively warm and dry. The fluctuation well on the value of humification and organic matter content of climate, and have a good sensitivity. the water level down in the location of SC4 core, and it is used for developing peat. The sedimentary environment is peat swamp facies.â…£:About 6.1kaBP-5.8kaBP, analysises of grain size datas show relatively stable sedimentary environment, and the hydrodynamic condition is comparatively weak. Bule grey clay overlying on sedimental lithology is Lake Facies sediment, gray clay to oxidation increased and water level decreasing. As the low-lying land of SC4 core location, the water level rises frist but falls back late in this period. Compared with the lithological characters of SC4, SC1and SC7, when the lake face expands, the paleo-channel may be submerged, when it shrinks, the paleo-channel may be exposed to the land surface and become the watercourse. Overall, it is Lake Facies sedimentary environment. Fine particulate matter of sediments in SC1, SC4 and SC7 core keep increasing, this shows that the same sedimentary environment may happened. Magnetic and coarse silt content increased sharply, indicate that the changes connect with the influences of the human activity. Climate becomes relative warm and dry during the late of mid-Holocene. Sedimentary environment is very easily influenced by rivers around the lake.2. As a useful proxy - absorbance of peat humification, can be a good application for the analysis on paleo-limnological sediment from Chenghu Lake, well connection with Organic Matter was found. Results showed that higher humification degree indicates relatively warm and humid of climate, corresponding to high value of organic matter, and lower to cool and dry, related to lower organic matter.3. Based on analisizing proxies from SC4 core,the results shows that since 7.2kaBP, climatic change can be divided approximately into three phases in this region, changing climate from relatively cold and dry to warm and humid (ca 7.2-6.8kaBP), hot and humid (ca 6.8-6.3kaBP), and relatively warm and dry (ca 6.3-5.8 kaBP). The hot and wet mid-Holocene climate (ca 6.8-6.3kaB.P.) was Holocene Megathermal Maximum period in this area, simultaneously, there existed climate undulation on decadal-to centennial scale. Low-lying land has been characterized by developing peat and peaty sediments. The proxies of organic matter and humification displayed good sensitivity, and they provided evidences for recognizing the climate variation of Holocene Megathermal Maximum period in the downstream of Yangtze River.
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