Historical Sedimentary Records And Environmental Changes In Chaohu Lake

Chaohu is a relatively closed lake under monsoon in eastern China and Hangbu River is Chaohu's main tributary. Samples, including surface soil and suspending clay and sand from Hangbu river and five core samples from Chaohu lake, were collected to obtain information about past environmental evolutions in the basin of Chaohu Lake by means of ~210Pb and ~137Cs and grain size and environmental magnetism. Historical evolutions recorded by words in the basin were also collected to compare with those in sediments. Through these analyzing and researching, some conclusions below about historical environmental evolutions in the Basin of Chaohu Lake were reached and acquired:Chaohu sediments basically can be named after clay-fine sand and standard deviation is relatively bad. Near the mouth of Hangbu River and shore with high sediment accumulation rates, sedimentary grain size is coarse and grain size parameters vary greatly on the depth which reflect complicated and unstable sedimentary environment, while near the center of the lake with low sediment accumulation rates, sediment grain size is relatively fine and grain size parameters vary continuously on the depth which reflect stable sedimentary environment. Fast depositing of clay and sand reflects high intensity human activities and serious soil erosion in the Basin of Chaohu Lake.Magnetite is the dominant magnetic mineral in Chaohu sediments, and soils from Hangbu River Catchment are the main source of lake sediments. Chaohu sediments with higher proportion of sands usually have higher concentration of magnetite with coarse size, while sediments with lower proportion of sands usually have lower concentration of magnetite with fine size. Content of sand fraction in cores is a. reflection of water energy which is dependent on the distance to the bank. Therefore, the distinct magnetic properties of different cores are due to sedimentary dynamics. It is found that xARM /SIRM can be used to indicate the variation of clay contents in sediments.Environmental parameters vary and change regularly on the space in Chaohu Lake. Sediment distribution is mainly controlled and affected by west shore advancing to the center and hydrodynamics and disasters of flood and drought and human activities. Being controlled by hydrodynamics of Hangbu River, environmental parameters of four core samples have a corresponding change and variety on the depth.Comparisons of grain size and accumulation rates of four cores were not same at different corresponding period, which reflects Hangbu River movement between south and north in history. The records about evolutions of Hangbu River from books and papers also can confirm the movement.Grain size parameter of Chaohu sediments record flood-years well that happened in the basin of Chaohu Lake at the historical period. Six serious flood years recorded by words including 1823,1849,1911,1915,1932 and 1954 can be reflected obviously in the sediment cores. Chaohu is a basin in which floods happened frequently and there is an especially strong flood every about 40 years.Sediment cores record five serious flood-years and important human activities which affects greatly environment of the basin of Chaohu Lake since 1950.The flood-years include 1954,1969,1974,1991 and 1998.The human activities include Longhekou reservoir built in 1960 and Chaohu brake built inl962 and soil erosion induced by destroying forest in catchment of Hangbu River from the early of 1950's to the end of 1960's and alteration of the mouth of Hangbu River in 1978.