柴达木盆地西部千米湖泊沉积物中的盐类矿物及环境意义

Minerals of lacustrine sediments are one of important environmental proxies. Salt minerals are originated chemically from evaporation of lake water,and therefore have much closer relationship with environmental and climatic changes than other minerals. Chahansilatu sub-basin, located in the Western Qaidam basin, has developed numerous lacustrine sediments since Late Mid- Pleistocene. These lacustrine sediments consisted of numerous salt minerals. However,there were not enough information on its process of evolution. A 938.5m-long core was drilled in the sub-basin in 2008 by one cooperative group between China and German. In terms of primary investigation and examination on the salt minerals and lithology, the borehole could record the whole evolution process of the sub-basin from fresh, alkaline, salt to dry lake. At present, some work on the upper 400m sediments has been done, such as the type of salt minerals, the geochemistry of main and minor elements, grain size and paleo-magnetism of sediments etc. Some results have been published on Journal of Quaternary International. Based on the above and previous research work, the project aims to use some instruments (X-Ray Diffraction, Polarizing Microscope, ICS and ICP-MS ) to get the following information: (a) types, distribution and semi-quantities of salt minerals; (b)shape and chemical composition of halite; (c) shape and chemical composition of gypsum; (d) sedimentary characteristic of salt minerals; (e) relationships between environmental changes and salt minerals; and (f) the sequence of salt minerals and environmental information during the evolution process of the sub-basin since 2.77Ma.

湖泊沉积物中的矿物是研究环境变化的重要指标之一，特别是湖水浓缩形成的盐类矿物，与气候环境变化的关系密切。察汗斯拉图是柴达木盆地西部的次级盆地，形成于中更新世，集聚了大量含盐矿物的湖泊沉积物，且研究程度相对薄弱。2008年，中德合作在该次级盆地钻取了一个938.5m的深钻，据初步的岩性和盐矿物观察，沉积物完整地记录了古湖盆从淡水、咸水至干涸的全过程。目前已获得该钻孔的古地磁年代（底部为2.77Ma）、并对上部400m进行了初步的盐矿物、常量和微量元素、粒度等环境指标的分析，部分成果已发表。本项目拟在已有研究基础上，用X射线衍射仪、偏光显微镜、离子色谱仪、质谱仪等手段，高分辨率地研究整个钻孔的盐矿物种类、分布、含量及环境意义，并分析石盐和石膏晶体的形貌、晶体化学成分及环境意义，恢复2.77Ma以来该次级盆地的盐矿物沉积序列、环境意义和古湖泊演化过程中的环境变化。

本项目高分辨率地研究了柴达木盆地西部钻孔SG-1(938.5m, 年代2.77~0.1Ma)的盐矿物种类、分布和含量，恢复了2.77Ma 以来该次级盆地的盐矿物沉积序列和环境意义，特别是详细研究了石膏的晶体形貌变化和因素分析。钻孔岩性表现为碳酸盐粘土层与盐层的交替，盐类矿物种类复杂多样，分布也不均匀。主要是石盐和石膏，其它盐矿物有芒硝，无水芒硝，钙芒硝，杂卤石和白钠镁矾，少量或微量尤钠钙矾、重晶石、碳酸钠钙石、钠钙矾、苏打石、斜碳钠钙石、一水碳酸钠和变水方硼石、无水石膏、碳酸钙镁盐。偶尔出现含水碘化镁(仅出现在700.47m)。700m至底部，盐类矿物种类少。.钻孔中石膏形态主要有柱状、透镜状、扁平状、双晶、玫瑰状等。柱状是优先生长的形态。石膏形态特征与沉积环境的关系比较复杂，盐类离子，如K+, Na+，Mg2+和Sr2+对石膏晶面生长具有选择性抑制作用，是造成石膏形态变化的主要因素。石膏晶体的晶胞参数变化范围为a=5.927-12.57，b= 9.654-16.231，c=5.163-14.02，=113.54-119.88，晶胞体积V=601.55-712.65（Ǻ3），晶胞参数与晶格的替代离子含量有关。.2.77Ma以来，该次级盆地经历了两个大的干旱阶段2.2-2.0Ma和1.2-0.1Ma，这两个阶段与柴达木盆地更新世的成盐期一致。1.2Ma,0.9Ma和0.6Ma盐类矿物种类和含量出现明显变化，钾镁盐几乎全部出现在0.6Ma-0.1Ma阶段，干旱化趋势明显。根据大于45cm纯盐层的分布，又可以分成24个小的干旱阶段：119-133ka, 141-147ka, 163-171ka, 186-203ka, 235-243ka, 284-314ka, 330-340ka, 353-358ka, 371-391ka, 412-452ka, 540-544ka, 568-585ka, 613-628ka, 659-660ka, 702-707ka, 737-755ka, 1096-1097ka, 1213-1222ka, 2052-2055ka, 2088-2092ka, 2147-2150ka, 2169-2175ka, 2189-2197ka, 和 2219-2221ka。盐层分布说明气候干湿波动频繁。干旱化的重要驱动因素是全球变变冷，其次是高原隆升。