Modern sedimentation rate dating and environmental implication of Nushan Lake

Abstract

为探讨本世纪以来女山湖生态环境的演化历史,以¹³⁷Cs和²¹⁰pb放射性同位素为测试手段,分析该湖近期沉积物中¹³⁷Cs和²¹⁰Pb的含量分布,并根据其沉降特征和衰减规律,对沉积物做了年代定位,分段推导出平均沉积速率,建立柱状沉积年代序列。在此基础上,利用沉积记录,对沉积速率的变化进行环境解释,探讨了本世纪以来湖泊环境的演化过程。结果表明,1929~1938年沉积速率达4.8mm/a,是淮河流域洪涝使泥沙倒灌入湖的结果,1958年以后沉积速率增强则直接受控于水利工程的拦截作用。;Nushan Lake, located in the Tanlu Fault zone, Jiashan County, Anhui Province, covers an area of 100 km² with the deepest depth of 3m. According to the spot investigation, lacustrine mud in the northern part is over 8m in depth. The terrestrial deposit input is mainly from the Chihe River of the south. Occasionally, the mud-sand input is carried by the Huaihe River in the north of Nushan Lake. The short streams around the lake basin also participate in the lake sedimentation. A chronologic succession of a core taken from Nushan Lake has been buUt to reveal the evolutionary history of ecological environments based on determining sedimentation rates of the core with radinuclides ²¹⁰Pb and ¹³⁷Cs datings. Then, combining with some environmental paramenters such as χ_LF, χ_M, Gralasize, Pigments, TOC, the environmental implication of the lake in the 20's century is discussed. It is indicated that the major factors to influence the sedimentation rate of Nushan Lake axe (1) mainly ia-flow from Chihe River; (2) large amount of input mud-sand from Huaihe River; and (3) human activities in this century. Four stages can be divided for the environmental change of Nushan Lake in this century:Stage I (depth 19.5~14.5cm, 1904~1929):The sedimentation rate is 2.0nim/a. It is stable for the lake sedimental environment. Stage Ⅱ(depth 4.5~10.0cm, 1929~1938):Thesedimentation rate is 2.0mm/a. Debris deposits inflow of Huaihe drainage led to the unstable lake sedimentary environment. It is abundant for the terristrial organic matter. Stage Ⅲ (depth 10.0~5.0cm, 1938~1966):The sedimentation rate is 1.8m/a. It has experienced the lasting 4-year-arid-event in 1941~1944, which leads to less mud-sand and lower sedimentation rate. Stage Ⅳ (depth 5.0~1.5cm, 1966~1985):The sedimentation rate is 1.8m/a. A series of water-conservancy project in the Huaihe drainage play an important role, which has reduced the influence of natural factors and makes lake sedimentary environment stable. Stage Ⅴ (depth 1.5~0cm, 1985~1991):The sedimentation rate is 3.0m/a. The water-conservancy project of Nushan Lake in 1980's leads to the obvious increasing of sedimentation rate.