云南高原湖泊表层沉积物粒度特征及环境指示意义

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

选取云南15个高原湖泊,利用瑞典产HTH重力采样器提取沉积物短钻,取顶部0~0.5 cm作为湖泊现代沉积物进行粒度分析并利用分析软件对粒度频率曲线进行拟合,探讨各组分的沉积特征和环境指示意义。结果显示,滇池等15个云南湖泊表层沉积物频率曲线呈多峰态,含有2至5个组分,从细到粗分别为黏土组分(C1)、细粉砂组分(C2)、粉砂组分(C3)、细—中砂组分(C4)、粗砂组分(C5),拟合众数粒径范围为0.6~423.0μm。C1组分来自大气中长期悬浮的超细粉尘和气溶胶,通过自然沉降和降水进入湖泊水体,反映西南地区大气粉尘背景值;C2和C3是湖泊沉积物中的主要组分,反映流域内降水和水动力大小;C4和C5组分只出现在洱海、滇池、差黑海等部分湖泊湖心沉积物中,且含量非常低,该组分与湖泊水动力密切相关。对于滇池、抚仙湖等大型湖泊而言,粗颗粒进入湖心需两个过程:从流域地表进入湖泊水体的输入过程和从湖滨到达湖心的传输过程。湖心沉积物中粗粒含量主要受第二个过程影响,而该过程的强弱与湖流、水深、湖盆坡度等因素有关。西南地区干旱背景下湖泊快速收缩直至消失时,降水量的大幅减少使得地表径流动力减弱,加之大气环流偏弱以及水生植物覆盖面积比例增大,湖水动力减弱,由湖滨向湖心传输的物质以细粒碎屑为主。位于山地地区的小型湖泊,由于汇水面积有限、地形坡度较大,坡面径流可将地表风化碎屑物带入湖盆甚至湖心位置,这类湖泊沉积物中粗粒物质的增加反映流域内降水量或降水强度增大。

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.