| 摘要: |
| 柴达木盆地分布着的大量浅水盐湖，是重建过去全球变化的重要载体。盐湖沉积物粒度的测定及其解释对湖水动力学、地貌学和沉积学的研究具有十分重要的意义；粒度作为湖泊沉积物中最直接的物理指标，在不同的湖泊中存在着多解性，因此，系统地研究不同湖泊表层沉积物的粒度空间分布特征十分必要。本文以柴达木盆地的小柴旦湖为研究对象，对湖泊中23个表层沉积物样品进行粒度分析，并结合2个风成砂样品和1个湖泊钻孔样品（XCD3），系统研究其表层沉积物粒度特征，探讨可能影响粒度分布的外力因素。结果表明：（1）在两条采样路线上，样品粒度随水位深度增加逐渐变细，其平均粒径的变化范围自50.2 μm至5.3 μm，说明小柴旦湖这类小型浅水盐湖的沉积特征符合湖泊中湖滨至湖心沉积物颗粒逐渐变细这一规律；（2）凡是湖泊水深超过200 cm的样品，粒度频率分布曲线具有较高的一致性，表明在小柴旦湖中较细颗粒主要赋存于深水区，指示深水区沉积环境相对稳定；（3）湖滨浅水区沉积物颗粒特征复杂，粒度频率分布曲线呈多峰态，可能受洪积扇、植被和风浪的影响。选取柴旦气象站1956 — 2013年气象数据中极大风速计算风浪对水体的影响，最大影响深度为0.4 m，说明湖滨深度小于0.4 m的区域受风浪影响。结合湖泊表层沉积物及周围风成砂粒度特征，对小柴旦3（XCD3）钻孔沉积物粒度特征进行分析，发现：钻孔中68 — 70 cm和77 — 80 cm层位粒度特征指示水位快速下降，湖泊干涸并沉积风成砂；其余层位均表现为相对稳定的湖相地层，湖泊水位与现代相近。 |
| 关键词: 小柴旦湖 表层沉积物 粒度频率分布曲线 |
| DOI：10.7515/JEE201705006 |
| CSTR：32259.14.JEE201705006 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41361047）；青海省科技厅自然科学基金项目（2017-ZJ-901） |
| 英文基金项目:National Natural Science Foundation of China (41361047); Natural Science Foundation of Qinghai Provincial Science and Technology Department (2017-ZJ-901) |
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| Grain size distribution characteristics of surface sediments in Xiao Qaidam Lake |
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LÜ Shunchang, E Chongyi, SUN Yongjuan, ZHANG Jing, ZHAO Yajuan, YANG Long
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1. Key Laboratory of Physical Geography and Environmental Processes of Qinghai Province, Qinghai Normal University, Xining 810008, China
2. Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China
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| Abstract: |
| Background, aim, and scope Sediments from shallow salt lakes in arid basins are important carriers for reconstructing the past global climatic changes. The grain size of sediments in such lakes provides basic understanding of the lake hydrodynamics, geomorphology and sedimentology. However, as a simple physical indicator, the grain size could be differently interpreted in different lakes. Here we present a detailed study on the relationship between the grain size of surface sediments and hydrological conditions including water level and hydrodynamic changes within Xiao Qaidam Lake, a shallow salt lake in Qaidam Basin, NE Tibetan Plateau. Materials and methods Surface sediments (23 samples) were collected along two sampling routes across the lake from south to north of the lake. Two aeolian sand samples around the lake and one short core (XCD3, 93 cm)sediments were used for comparisons. The grain size frequency curves, average grain size, sorting, skewness and kurtosis were calculated based on the measurements of Malvern MS 2000. Results Results suggest that the grain size of the sediments becomes finer with the increase of water depth. The average grain size of the surface sediments is 5.3—31.8 μm. The sorting coefficient ranges from 1.36 to 2.59 and the kurtosis varies between 0.69 and 1.18, with skewness between −0.25 and 0.37. In deeper water area (>200 cm), the median grain size is less than 11.2 μm and the frequency curves show a similar unimodal distribution. In the shallower water (<200 cm),the median grain size is much coarser and the frequency curves show different multi-peak distributions, resulting from the combination of processes. Discussion According to the data of maximum wind speed from 1956 to 2013 at the Qaidam weather station, the maximum depth influenced by wind-waves is calculated to be 40 cm, indicating the surface sediments under water depth >40 cm cannot be affected by the wind-waves. Similar grain size characteristics are found between core and surface sediments. Two stages in the short core (68—70 cm and 77—80 cm) show a similar grain size characteristic with aeolian sand around lake, indicating rapid drop of the lake level and aeolian sand reached the core site. The remaining stages demonstrate a relatively stable lake level, which is close to that of the modern. Conclusions (1) There is a decreasing trend in lake sediment grain size with the increasing of lake depth and the distance from the shore to the center of the lake. (2) The samples’ grain size frequency distribution curves have a good consistency at the case of the lake water depth beyond 200 cm, indicating that the sedimentary environment of the Xiao Qaidam Lake is relatively stable in deep water with small external forces. (3) The characteristics of sediment grain size in the shallow lake area are complex, which is significantly affected not only by hydrological settings but also surrounding landform and vegetation in lake. Recommendations and perspectives Therefore, the characteristics of grain size in sediments from shallow lakes are complex, significantly affected not only by hydrological settings but also surrounding landform and vegetation. Detailed investigations on surface sediments are necessary before grain size of core sediments could be used for paleoenvironmental reconstructions. |
| Key words: Xiao Qaidam Lake surface sediments grain size frequency distribution curves |
