兰州西津黄土-古土壤序列中黏土矿物特征

Preliminary clay minerals on the loess-paleosol sequence in the Xijin core,Lanzhou

采用扫描电镜和X射线衍射实验方法,分析兰州西津黄土层和古土壤层中的黏土矿物质量分数,计算其化学风化指数和结晶度,结合粒度和磁化率指标探讨黏土矿物质量分数比值的古环境意义.研究表明:黄土-古土壤序列的黏土矿物组合类型比较单一,黄土层中主要有伊利石、绿泥石和高岭石;古土壤层中主要为伊利石、绿泥石、高岭石、蒙脱石和伊/蒙混层矿物.西津钻孔中的黏土矿物对化学和物理风化较为敏感,古土壤发育时的水解程度强于黄土.与西津黄土的磁化率和粒度指标一样,黏土矿物质量分数比值在黄土和古土壤层中的差异能进行古环境重建.

The clay mineral types and their relative content in the loess and paleosol layers were identified by using the scanning electron microscopy and X-ray diffraction in the Xijin loess in Lanzhou. The chemical index and the degree of crystallinity had been calculated based on the clay mineral content. The pa- leoenvironmental significance of the clay mineral content ratios was determined after comparing them with the grain size and magnetic susceptibility of the Xijin loess. The results indicated that the type of the clay mineral assemblage in the loess-paleosol sequence was relatively simple. Illite, chlorite and kaolinite were the main minerals in the loess layer, while illite, chlorite, kaolinite and montmorillonite dominated the clay minerals in the paleosol samples. It was also demonstrated that the clay minerals in the Xijin core were sensitive to the chemical and physical weathering, the hydrolytic degree of the paleosol layer was stronger than the loess layer. The clay mineral content ratios of the loess layers were obviously distin- guished from the paleosol layers. This characteristic showed a similar variation of the magnetic suscepti- bility and grain size proxies, which supported that the clay minerals of the Xijin loess has preserved abundant environmental information.