中文摘要：

揭示西安地区第5层古土壤中还原层的类型和水分平衡，对认识西安地区S5古土壤发育时的气候和植被以及地下水补给来源具有重要科学意义。通过对西安地区第5层古土壤（S5）的调查和多个剖面的观察，在西安东郊任家坡剖面、南郊双竹村剖面和蓝田田家坡剖面首次发现该层古土壤黏化层中含有1~5mm大小的低价氧化铁的灰绿色斑点。低价铁的灰绿色斑点是在地势较高和地下水位较深的还原作用下形成的，属于高位还原层，不同于以往认识的潴育化类型。灰绿色斑点指示S5古土壤发育时土壤中上部出现了雨季积水并处于还原环境，当时雨季还原层含水量达到了饱和状态，土壤中上部含水量为42%左右，土壤水分非常充足。该层古土壤的红色铁质胶膜迁移到了上部古土壤黏化层顶界之下6.3 m深的黄土中，表明当时高含量重力水分布达到了6.3 m左右深度，至少出现过较长时期适于茂盛森林植被发育的气候。在西安地区S5古土壤发育时，土壤水分的收入量大于支出量，土壤水分为显著正平衡。当时大气降水在经过蒸发、蒸腾与地表径流损失之后，还有较多剩余的水分通过入渗补给深部土层水。S5古土壤发育时降水较丰富是导致该层古土壤出现高位还原层的主要因素。

英文摘要：

【Objective】To reveal the water loggogenic type of the fifth palaeosol in Xi’an area, and identify the water content, moisture balance and possible vegetation types when the S5 developed.【Method】Field investigation, electron microscope observation and energy spectrum analysis were used to research relevant problem.【Result】The fifth palaeosol was composed of three layers, which was the best ancient soil in the loess. Based on the field investigation of the fifth palaeosol in Xi’an area, celadon spots were first discovered in the argillic horizon of this palaeosol, proving the accumulated water in the middle and upper paleosol and reducing environment during the development of S5. The red ferruginous adhesive film developed very well, which has two kinds of micro structure under the electron microscope, one was crystal structure composed of newborn clay minerals, the other was amorphous colloid structure. According to the energy spectrum analysis, the content of Fe2O3、Al2O3 and SiO2 in the red ferruginous adhesive film and celadon spots was between 12.85%～17.15%、20.45%～24.84% and 47.19%～54.17%, with average of 15.28%、23.07% and 52.13%, separately. Resulting from the deposition of green iron film on the surface of the red adhesive film, the difference of Fe2O3content between the red ferruginous adhesive film and celadon spots was quite small. The migration depth of red ferruginous adhesive film is 1.8m under the bottom of argillic horizon, and that was 6.3m from the top of the palaeosol.【Conclusion】These celadon spots in the fifth palaeosol were formed under the condition of deep ground water level. The major factor was abundant precipitation for reducing layer appearance above deep ground water level. The distribution depth of gravity water is up to 6.3m when the S5developed and the climate would have been favorable to sustain forest development for a long period. At that time moisture content of reducing layer reached saturation state and that in the upper part of the soil was about