Lakes Deposition And Environmental Changes In The Yangtze River In The Middle Reaches Of The Yangtze River

Catchment environmental changes, as one of the major and disputive subject internationally, is tackled in the paper, with Chiba Oxbow lake as its study object,which is located in the areas along the Middle and lower reaches of the Yangtze River,and talking the methods of field investigation, comparison among the sedimentary piles, drilling lacustrine deposits core in the centre of lake, laboratory analysis of sediments. By the help of water daturn and comparing the information from the sediments with historical data, the influences of human activities in the past centenary on the lake sediment are revealed. Through these analyzing and researching some conclusions were reached and acquired:(1) Based on the sample dating results of 210 Pb from Core CB-1 in central Chiba Oxbow lake, segmented deposition dating, combined with Yangtze River flood event and sedimentary facies auxiliary dating, established high-resolution chronology scale. Analysis shows that its average deposit rate is 1.49 cm/a, the whole core CB-1covers approximately 73 years since 1940 year.(2) Sedimentary size composition of core CB-1 in Chiba lake is mainly fine silt,more than half of the total content, the second is clay, then is middle silt, is little sand.Higher content of magnetic minerals in sediments, which mainly is ferrimagnetic mineral-based, and contains a small amount of in complete antiferromagnetic minerals,magnetic mineral particles to coarse-based SSD. There is a certain regularity between the composition and size of the magnetic particles: the smaller size of small magnetic domains, larger particle size large domains.According to an analysis of the age data, granularity and magnetics of deposits in Chiba lake, sample core CB-1 could be divided into 5 sedimentary chronologicalsections, to wit: ① 1940 a A.D. to 1954 a A.D. is proven to be a wet period; ② 1954 a A.D. to 1969 a A.D. is proven to be a dry period; ③ 1969 a A.D. to 1980 a A.D. is proven to be a wet/dry alternative action period; ④ 1980 a A.D. to 2000 a A.D. is proven to be a dry period and insert more than once flood; ⑤ 2000 a A.D. to 2012 a A.D. is proven to be a wet period.(3) After taking normalization of the chemical and mineral nutrients content,the enrichment coefficient of TOC, TN and TP in the deposition’s 1980 sustainability than 1.0, more than the unnatural level threshold value, indicating that about by human activities since the 1980 s, the influence, Chiba Lake gradually entered the eutrophication stage. TOC and TN unnatural level enrichment, during the same period of population growth, the expansion of arable land, as well as extensive use of fertilizers are directly linked.(4) After taking normalization of the chemical and mineral nutrients content,there is only Pb, Mn, Zn elements in 16 elements in the deposition’s obvious enrichment factor of 1986 more than 1.0, a non-natural levels of enrichment,influenced by human activities, Pb element content from 1986 to 2000 the enrichment level rise, Pb enrichment levels high and volatile, presumably with the rapid development of China’s economy at the time, car ownership increases, increasing the use of leaded petrol related decline after 2000 began to fluctuate, 2000 may be a total ban on the use of leaded petrol relevant. Mn, and Zn enrichment unnatural levels should be subject to the 1980 s after China’s rapid economic development, industry,enterprise rapid development impact.(5) Chiba Lake watershed areas are more frequent floods and droughts over the past century, by the frequency and intensity of floods and droughts analysis, found that in the 1940 s the worst flooding; the 1950 s, the most serious drought; the formation of floods and droughts and dry climate wet change "rhythm" are basically the same, natural environmental factors affect the evolution of the last centuryshakuhachi lake.(6) Annals of literature by sorting out the demographic changes since the founding of the Chiba Lake area, changes in arable land, fertilizer application rate changes and human activities such as industrial development, and analyze its coupling with the nutrients found with the enhancement of human activities, ruler eutrophic lake gradually into eight stages, more and more human activity affects the evolution of the environment.