Environmental Change In JiangHe Yuan Region Since The Last Glaciation Recorded By Ximen Cuo Core

In recent decades, the ecological environment in Jiang Heyuan region is more and more deteriorating. Finding the reasons of the environmental change in this area will be undoubtedly of great theoretical significance for the scientific decision-making. Nian Bao Yu Ze Mountains is located in Jiu Zhi County in Qinghai Province, with a maximum altitude of 5369m and the growth of modern glaciers in its peak. Rivers in the south of the Mountains run to the Yangtze River, the north into the Yellow River. Ximen Cuo at the north of the Mountains was formed by the erosion of glaciers in ice age, with average water depth of 40m, small population density and extremely weak impact of human activities. The Nigequ River from Ximen Cuo flows into the Yellow River. A 12.8m-long continuous core was drilled at the water depth of 50m in the center of the lake in March 2004.The environmental change in this area is reconstructed in the paper through the lake sediment.Three of the four AMS ages are nearly in a straight line, and the bottom one because of its serious inversion, can not have its eye. Assuming that the age of the surface sediment is zero and the deposition rate of the core is constant, an average deposition rate for the whole of the core is 0.324mm/a through a linear regression of three AMS ages. The age of the bottom of the core is 39.5kaBP, in MIS3 stage. On this basis, the environmental change in this area since 40kaBP is reconstructed through organic content, magnetic susceptibility and grain size.12.80~7.30m (39.5~22.5 ka) glaciers near the lake is largest because of the higher precipitation. 7.30~6.25m (22.5-19.1ka) the climate is coldest and the lake surface is frozen at most of the time. 6.25~5.00m (19.1~15.6ka) the climate is dry and the water level is low. 5.00~3.50m (15.6~11.2ka) the climate become warmer and the water level also increase gradually. Ximen Cuo drilling revealed the climate in the ice age is very unstable.The upper 3.50m of Ximen Cuo core represents the Holocene environment. 3.50~3.10m (11.7~8.1ka) the climate is warmer and wetter. The climate becomes coldand dry suddenly at 8.4kaBP, water level decreases. 3.10~2.25m (8.1~5.9ka) the climate is at its optimum with highest temperature and largest precipitation. Temperature startes to decline at 6kaBP, marking the end of the Holocene optimum. 2.25~0.80m (5.9-2.lka) the temperature decreases dramatically and the precipitation is also lower, with some abrupt climate changes. 0.8~0m (2.1~0 ka) precipitation continued to fall, the temperature experiences several fluctuations. The Medieval Warm period and the Little Ice Age in the area are obvious.Several dry climate events are revealed in Ximen Cuo core, with low lake level. The time is 12.8ka, 11.2 ka, 9.4 ka, 8.4 ka, 6.0 ka, 4.2 ka, 3.7 ka, 2.6 ka, 2. lka, 0.7 ka.