中文摘要：

利用野外调查与观察、电镜观察和x-射线衍射法，对西安市蓝田县白鹿塬东端安村北约1km的第四纪黄土-古土壤剖面（34°17′N，109°32′E）中第4层古土壤（S4）的铁锰结核样品进行了实验分析。整个S4古土壤风化剖面厚度为5．6m，粘化层的厚度为1．5m，在S4古土壤层下部5．0～5．6m深度范围发现了具有指示当时土壤水分含量、地下水富集、水分循环等作用的针铁矿（α-FeO（OH））与铁锰结核富集层。针铁矿呈褐黄色薄膜分布于土体表面。含针铁矿的铁锰结核呈黑褐色球形，电镜下主要呈颗粒状、球形和呈菊花形等形态。针铁矿和铁锰结核呈层分布在同一层位，形成于当时的地下水位附近。在铁锰结核样品中，针铁矿含量为0．9％～3．3％，粘土矿物伊利石含量为6．O％～15．5％，高岭石含量为2％～5％，石英含量为61．1％～66．6％，斜长石含量10％～16．4％，钾长石含量3．8％～6．O％。针铁矿与铁锰结核的发育和迁移深度指示，在西安蓝田安村附近S。古土壤发育时期，年降水量至少为900mm，且重力水分布深度达到了5．6m，5．6m深度范围内的含水量大于25％，在针铁矿和铁锰结核发育层位，含水量接近饱和。当时土壤水分含量很充足，土壤水分平衡为正，有较多水分补给地下水，适于茂盛森林植被发育。研究显示，组成铁锰结核的物质成分来自S4古土壤粘化层中高价铁锰氧化物迁移，并随着地下水位的升降而发生还原和氧化聚集形成。

英文摘要：

The weathered section （34°17′N, 109°32′E） of the fourth paleosol, located at about l km far away from the north of An Village in Lantian of Xi＇an, at the eastern Bailuyuan, was investigated in this study. The ferromanganese nodules were analyzed by the means of electron microscope and X-ray diffraction. The field investigations showed that Quaternary loess strata developed well in the study area. The total thickness was about 125m and 28m from Early Pleistocene to Holocene loess and from the bottom of S5 to top of Holocene loess, respectively. Argillic horizon of S4 paleosol was 1.5m in thickness, while the whole weathered section was 5.6m. The enrichment layers of goethite （α-FeO （OH）） and ferromanganese nodules were discovered between 5.0m and 5.6m, which could indicate the soil moisture, groundwater enrichment and water circulation at that time. Goethite was distributed on the earth surface as yellow-brown film. Ferromanganese nodules were brown-black spherical, granular and looked like chrysanthemum and so on in the electron microscope. Goethite and ferromanganese nodules were distributed in the same layer and displayed layering distribution, forming near the groundwater level at that time. In the ferromanganese nodules containing goethite, the relative concentrations of goethite were from 0.9% to 3.3%, and were from 6.0% to 15.5% for the clay mineral illite. The kaolinite concentrations were from 2% to 5% and quartz ranged from 61.1% to 66.6%. The plagioclase and potash feldspar concentrations were 10% -16.4% and 3.8%- 6.0%, respectively. The depth of the development and transformations of goethite and ferromanganese nodules demonstrated that the annual rainfall was at least 900mm during the development of S4. The distribution depth of gravity water was up to 5.6m and the moisture was more than 25% within the range. In the development layer of goethite and ferromanganese nodules, the water content was closed to saturation. At that time, soil moisture was very sufficient, and water balan