Characteristics And Sedimentary Significance Of Magnetic Susceptibility And Grain Size Of Lake Sediments In Lianhuan Lake

Lianhuan Lake located in the Daqing region which was the ecological environment fragile area in the north of China. Four sediment cores from Lianhuan Lake were collected to obtain information about past environmental evolutions in the basin of Lianhuan Lake by means of ²¹⁰Pb and ¹³⁷Cs and grain size, as well as environmental magnetism. Historical evolutions recorded by words in the basin were also collected to compare with those in sediments. Through the analyzing and researching, some conclusions below about historical environmental evolutions in the Basin of Lianhuan Lake were reached and acquired:1. The composition of grain size in the Lianhuan Lake mainly was fine silt, and the overall grain size was coarse. Within the four cores, grain size in AMT core was the coarsest, while in HSH core was the finest. Good positive correlation was found between clay and fine silt, indicated that consistent change and the same sources in the relative fine compositions.2. There was a finer trend from the bottom to the top of grain size in sediments, and the evolutionary orderliness was fine-coarse-fine. Grain size analysis revealed the climate changes and sandstorms phenomenon. It inferred that from the late 1940s to mid-1990s the climate turned drought, intensity and frequency of sandstorms in the Basin of Lianhuan Lake which was confirmed by the precipitation information and other history information.3. On the basis of analyzing grain size proxies, the evolution history of Lianhuan Lake was discussed. The dominate composition was changed from lacustrine materials to aeolian materials during the early 1800s to the early 1900s. Then grain size distribution was characterized by the "three-peak" feature, which showed that the peaks of clay and sand were dominated, while the aeolian materials peak was weaken from the end 1940s to the mid-1990s. Since the end 1940s the aeolian materials peak was strengthened and the grain size frequency curve changing into the "single-peak"feature which is the representative character of lake evolution.4. As the low-frequency magnetic susceptibility of four cores in the Lianhuan Lake, the concentration in HSH core was the highest, while the concentration in TLH core was the lowest. The vertical contents of low-frequency magnetic susceptibility showed that relatively low contents in the lower parts of four cores, high contents in the upper parts of the cores. Magnetic susceptibility revealed the changes of human activities on land use patterns and intensity in the basin. Through the analysis the contents changes of magnetic susceptibility in the cores, it reflected the history of human activities. Since the 19th century, human activity from the initial low intensity of land use, and then experienced a long period of relative stability on the role of land-applied to the mid-20th century, from the mid-1960s to the early 1980s in the early human agricultural and industrial activities carried out significantly, has caused soil erosion on the watershed, but only remain in the surface erosion of soil. Since the mid-90s, the intensity of human activities further enhanced, have led to erosion in some areas turning over the deep soil.5. On the correlation analysis between sediments grain size compositions and magnetic susceptibility parameters got the following conclusions: In addition to AMT in the size composition and magnetic core parameters no significant correlation between the expression other than, THL, HSH and XHL emerged Xlf and Xhf and 16 64μm thick silt at the 0.01 level components significantly correlated. This paper studied the grain size and magnetic susceptibility comic Lake core characteristics are closely related, into the lake depends on the concentration of magnetic material and component size of 16 64μm which the meal sand level, magnetic susceptibility of the content can reflect basin erosion conditions, and thus a chain lake basin with the basin of Hunan components related to the degree of soil erosion. Lake sediments may reflect the evolutionary history of the environment, and can reveal the way by human activity and intensity changes.