Modern Sedimentary Environmental Change In Yunnan Plateau Lakes

In recent years, with the rapid growth of economy and society, the global impact of environmental change becomes increasingly prominent. In the short time-scale, anthropogenic influence on environmental change is becoming the highlighted focus of global change research. Understanding the past environmental change is the key to recognize the present and predict the future. In more geological archives, lake sediments, in which continuous, stable, high-resolution records and wide geographical coverage are preserved, have the ability to recover the history of terrestrial environmental variation, and can be useful for making predictions about the trend of future environmental changes. Because of the special geographical location of the Highland Lakes, with the intense interfere of human beings, eutrophication and environmental pollution of these Lakes become more and more serious, and harmful environmental problems have become a limit factor of regional economic development. Study on the environmental change of different historical periods by the lake sediments will help to reconstruct the past environmental information in the studying areas.In the dissertation, Lake Fuxian and Lake Dianchi, two typical lakes in the Yunnan-Guizhou Plateau of Southwest China, were chosen to discuss the modern deposition rate with the mass depth, analyze the relation on the sediment particle size and rainfall at a short time-scale. The nutrients pollution in history of Lake Fuxian and Lake Dianchi was studied and the heavy metal pollution was evaluated by the potential ecological risk index on the basis of accurate chronologies by radionuclide dating (210Pb and 137Cs). The main results were showed in following:(1) The distribution of 210PbeX and 137Cs in the fourteen cores in Lake Fuxian and Lake Dianchi was disordered. The activity of 210Pbex shows a toothed shape within the mass depth of the sediment. The vertical profile of 137Cs gives obvious 137Cs peaks in the cores corresponding to in 1963 and 1986, since the onset of 137Cs fallout in 1954. The majority of core exist the 137Cs peak in 1975 as a result of the Chinese atmospheric nuclear tests. When using the two dating methods to evaluate the modern sedimentation rates of the two study lakes, a little deviation was existed, explaining the possible reasons in different views. Human activities of historical period had played a very important role in changing the environment of the Lakes.(2) According to the particle size characteristics of Lake Fuxian, the C-M chart of all cores was located on the left or beneath the?region, and it means that the sedimentary environment of Lake Fuxian in history was mostly deposited by a still water. Because of the impact of the River Ge, the hydrodynamic force of each part in Lake Fuxian was different, and did not present more and more thinner from the shore to the center of the lake. Comparing with the local rainfall in the past 50 years with sediment grain-size in the region of Lake Fuxian, it is obvious that the sediment grain-size displayed accordant variation trend with the fluctuations of rainfall. All these findings indicating that the good correlation between grain-size and the fluctuations of rainfall shows sediment grain-size could reflect the rainfall changes trend in short time-scale and high resolution investigations, i.e. larger sediment grain size reflected more rainfall and wetter climate while smaller sediment grain size reflected less rainfall and drier climate.(3) The value of TOC and TN'showed a rising trend, but the value of TP showed a fluctuation change related to the chemical nature of phosphorus and mining around the Lake Fuxian, it is still an oligotrophic lake. The value of C/N was between 7 and 20, and a small part was more than 20 which implied that the organic matter of Lake Fuxian had began to receive the terrestrial plant residues from the catchments, except suffering to the internal pollution by sediment itself. With the impact of lower vegetation cover and land resources, reclaiming land from lakes, and lager population of Kunming, the value of TOC, TN and TP in Lake Dianchi showed an increasing trend, especially having a greater fluctuation change in the last three decades. Therefore, the eutrophication situation of Lake Dianchi was extremely serious.(4) The concentrations of Cr, Cu, Zn, As and Pb in Lake Fuxian and Lake Dianchi were analyzed on the basis of 210Pb and 137Cs dating. The ecological risk of single pollutant such as Cr, Cu and Zn in the two lakes was low level. In addition, the ecological risk of As and Pb was low level in Lake Fuxian, but in Lake Dianchi was low to moderate. The value (RI) of multiple-factor potential ecological risks in Lake Fuxian was less than 40, which showed a moderate pollution degree. To Lake Dianchi, its value (RI) focuses on between 30 and 80 which mean relatively a moderate to high pollution degree in the lake.