中国黄土中的风化壳研究

据野外调查和室内鉴定,在布容期 0.78～ 0MaBP形成的黄土剖面中确定了三个时期的风化壳,它们主要发育在黄土高原东南部。黄土中的风化壳是红褐色古土壤在特定的气候条件下转变而成的,厚度在 3～ 6m之间,风化壳剖面常由 3个层次构成,发育时间为 5～ 10万年不等。风化壳上部是发育好的红色古土壤,中部是褐黄色强风化破碎黄土,下部是棕黄色弱风化黄土。粘土矿物分析表明,黄土中的风化壳为伊利石-蒙脱石型。黄土中风化壳发育时的年均降水量在 90 0～ 10 0 0mm之间,是黄土高原降水丰富的气候事件的显示。黄土高原东南部和中部风化壳的发育证明黄土中有些古土壤确属淋溶型森林土壤,当时亚热带气候至少迁移到了黄土高原中部.

Residua in Loess of China

According to the field survey formed before 0.7 Ma, and lab oratory analyses, the residua of 3 periods are indentified in the section of loess which are composed of 1st. 4th and 5th layer r edbrown paleosols and the weathered loess layers under them. They mainly devel oped in the south-east Loess Plateau and also exist in the middle of the Loess Plateau. The thickness of residua almost varies from 3 to 6 m and normally consist of 3 to 4 layers. From top to bottom, 1st layer is red-brown pale osols,2nd layer is brown-red weathered broken loess, 3rd layer is brown-yellow weathered broken loess with ferriginous film, the 4th layer is brown-yellow weathered broken loess without ferriginous film. The apparent diffe rence between residua and paleosol is that the thickness of the former is bigger than the latter, and that there are 2 to 3 layers weathered loess more in resid ua than in paleosols. The macroscopic feature of weathered loess is the deeper color, more weathering cracks and broken soil body. 　　Particle size analysis revealed that component of clay particle in palesols v ari es from 40% to 60%, and im weathered loess varies from 40% to 45% . Magneti c sus ceptibility in paleosol generally varies from 100 to 280 (10-6SI) , and in weathered loess varies from 60 to 100 (10-6SI). The CaC O3 content in weathered loess is mainlly less than 1%, and unweathered loess is usually above 8% . This shows that weathered and unweathered loess layer are apparently different. 　　The X-ray diffraction of six clay film samples reveals that the clay mineral I n residua is mainlymontmorillonite, secondly is illite and kaolinite. So we can determine that the residua in loess is mont morillonite-illite types. The surv ey in the migrated depth of red ferriginous clay film and CaCO3 nodules indica te t hat the paleosol which converted into residuun developed strongly, CaCO3 and Fe 2O3 in them obviously migrated out of the paleosol layer. The illuvial layer of CaCO3 nodules is not at the bottom of paleosol, it lies in the weathered loess which is under the paleosol bottom 1 to 2 meters. 　　Through the contrast, the components of weathering section of the paleosols in loess and modern sub-tropical Yellow-brown Earth are primarily same, it devel oped more strongly than north sub-tropical residua. CaCO3 and Fe2O3 migrated d epth and microstructure identification indicate that CaCO3 illuvial layer in the 1st , 4th and 5th paleosols don't belong to component part of paleosol section, and acid medium occurred during middle and late period developing these 3 pa leosols which are leached forest soil. The annual mean temperature was about 17 ℃ when the 1st ,4th and 5th residuum developed in Xi'an area, and annual mean p recipitation was 900 to 1000mm, it is 4℃ higher than nowday's annual mean tem pe rature, and 300 to 400 mm more than nowday's annual mean precipitation. The deve lopment of residua in loess indicates that the climatie zone migrated greatly at that time, and the sub-tropical climate migrated at least to the middle of Loes s Plateau.