The Grain Size Distribution Characteristics of Surface Sediments from Plateau Lakes in Yunnan Province and Their Environmental Significances

Key words: 
• Yunnan Province /
• plateau lakes /
• surface sediments /
• grain size distribution characteristics /
• source analysis /
• sedimentary dynamics

Abstract: To investigate the distribution pattern and significance of grain size and provide accurate analogues for grain size records from lake cores, we analyzed surface sediments from 15 lakes of Yunnan Province, China for grain size. Meanwhile, the grain size components of the surface sediments of the 15 lakes were partitioned using the log-normal distribution function method. The results showed that frequency curves of samples indicated polymodal and consisted of two to five grain-size components with the modal size varying within ranges of 0.6~423.0 μm. These components are specified from fine to coarse modes as clay (C1), fine silt (C2), silt (C3), fine to medium sand (C4) and coarse sand (C5). The grain size analysis reveals that the C1 components were the long-term superfine dust and aerosol in the atmosphere which entered the lake water body through the process of gravity and precipitation, and may have represented the background level of atmospheric dust and aerosols in southwest China; C2 and C3 were the main components of surface sediments, the modal size of which reflected the precipitation intensity of the watershed; C4 and C5 components only appeared in minority surface sediments of lakes with the contents were extremely low, and were significantly related to the lake hydrodynamic force. For the large lakes, such as Dianchi and Fuxian Lakes, it requires two processes for the sand components to reach the central part of lakes: process of from drain basin to lakes and from lakeshore to the middle of lakes, and the second one plays a main role, affected by lake current, water depth, gradient of lake basin and so on. Under the background of the drought in southwest China, some small lakes in Yunnan shrinked and even disappeared. However, because of the decrease of precipitation and weakening of general circulation, surface runoff and hydrodynamics of lakes recede, thus the material which transmit to the center of the lakes were primarily fine grain. Small lakes located in the mountainous region, with the limited catchment area and conspicuous terrain slope, can be affected by the surface runoff directly and strongly. Therefore, the increase of coarse grain in the sediments of this kind of lakes reflected the enhancement of precipitation or the precipitation intensity.