Study On The Geochemical Characteristics Of Elements And Grain-size In Surface Sediments From Lakes In Qinghai-Tibetan Plateau

In the process of rebuilding paleoclimate, the understanding of environment substitute indexes was one of the important research contents. Nowadays the earth chemistry, ecology, geophysics were main paleoenvironmental indexes which could record the environmental information of oceans, lakes, glaciers, loess. In those indexes, compositions of major and trace elements were one of indexes that was used widely. But there was many problems to be solved in that element acted as the geochemical indicators to rebuild paleoenvironment. As the unique geographical location and geological structure, Qinghai-Tibetan Plateau had an special effect on the formation of the east Asian monsoon and the changes of global climate and environment. Qinghai-Tibetan Plateau had many kinds of lakes whose sediment recorded rich information of climate and environment, which had very important significance to evolution of global paleoenvironmental system. So the objectives of this study were some lakes(puddles) in Hoh Xil. Based on analysing the the discrepancy of water salinity, the study divided those lakes into four categories as freshwater lakes, brackish lakes, saline lakes and salt lakes. By analysing the frequency distribution curve and probability cumulative curve of grain size, geochemical characteristics of elements in surface sediments, mineral composition, the paper studied the major transportation action and the enrichment regularity of elements of surface sediments under the different environmental conditions in those lakes. The main conclusions were as follows:1 The fresh water lakes is mainly composed of the kkxl-5,13 two lakes in the study area:Grit had grain size greater than 63μm consititued about 70-90% of total deposit in those lakes. Grit migrated by way of jumping load and traction load that was the dominant way. About 20-10% of the total deposit was silt whose way to migrate was mainly by jump load, only part of the smaller particle size of clay showed the migration by suspended way. Clay component held ratio less than 5% in total deposit that migrate to lake by suspension load. Characteristics of grain size reflected sediment in Qinghai-Tibetan Plateau could indicated the temporary water with high water power or characteristics of multisource as a whole.2 Brackish lakes have a large proportion in study lakes. Those lakes divided into three categoried based on the characteristics of grain size parameters in those lake sediment.The first kind lake composed by three lakes included the sampling points of kkxl-4,7,25: Particles had size greater than 63 μm occupied about 70% of total sediment, and the mode of traction load was their main migration way, and about 10% of the component in this kind of sediment migrated by jump load; The silt occupied about 25% of total sediment and migrated into the lakes by the mode of jump load; Clay had less content in those lakes and the migrated way to the lake was mainly by the suspension load mode.The second category was composed by eight lakes which included the sampling points of kkxl-2,9,10,12,14,22,24,26. The compenent that had grain size larger than 350 μm consitituted about 10% of total sediment in those lakes and they migrated into the lakes mainly by mode of traction load. The sediments that had grain size between 63 um and 350 μm accounted for about 40-70% of total sediment in those lakes, and the jump load mode was their main way to move into the lakes. The component of clay was less than 10% in those lake sediments and had the way of suspend load to move into the lakes.The third type lake mainly included six lakes that kkxl-3,11,15,21,23,27 were sampled there. Among them, the particle size larger than 250 um was mainly migrated into lakes by the way of traction load, and had a proportion about 10% in their total sediment. The particle size between 63 μm and 250 μm occupied a large proportion about 20-30% in the lake sediments and migrated way were traction load and jump load. Silt have a percent of 10 in those lakes sediment and migrated into the lake mainly by the form of suspended and jumping. The components of clay occupied about 20% of their sediments and had a suspensed form to move into those lakes.3 Saline lakes contained two lakes in sampling points of kkxl-18,19. Among those lakes sediment, about 5% of the total sediment moved into the lakes by the way of traction load and had a grain size larger than 250 um. The components migrated by jump load occupied about 85% of the total sediment in lake of kkxl-18, but in the lake of kkxl-19, there were just about 15% of sediment. With the migration form of suspended solids in the lake, there were only 10% of sediments in the lake of kkxl-18. There were about 80% of sediments migrated by the way of suspended load in the lake of kkxl-19.In the study sediment of those lakes, distibutions of Ca, Mg and Sr were extremely influenced by the distribution of carbonate in the lakes while the Si were affected by the quartz particulate distribution.. In a word the hydrodynamic force was the key factor on elements distribution in the lakes. In addition, distributions of major elements and trace elements were affected by sediment grain size significantly.Standardized the content of the major elements and trace elements in the lake sediments by UCC, we found the As were enriched in the lakes abnormally, followed by Ca, Mg, Cr, Pb, V, while the K and Na were depleted obviously in all kinds of lakes. In addition, Al and Rb were depleted in fresh water but were kept ballanced in brackish water and salt water. Results indicated the Qinghai Tibet Plateau Lakes were charcterized by both the fast surface runoff and cold water, and weak chemical weathering. The K and Na were exist resulted by the strong dissolution in the river and the devlopment of lake were affected strongly by the high salinity of elements.