中国黄土中的风化壳研究

Residua in Loess of China

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 。

According to the field survey formed before 0.7 Ma, and laboratory analyses, the residua of 3 periods are indentified in the section of loess which are composed of 1st. 4th and 5th layer redbrown paleosols and the weathered loess layers under them. They mainly developed 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 paleosols,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 difference 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 residua 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 varies from 40% to 60%, and im weathered loess varies from 40% to 45% . Magnetic susceptibility in paleosol generally varies from 100 to 280 (10 -6 SI), and in weathered loess varies from 60 to 100 (10 -6 SI). The CaCO 3 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 in residua is mainlymontmorillonite, secondly is illite and kaolinite. So we can determine that the residua in loess is mont morillonite illite types. The survey in the migrated depth of red ferriginous clay film and CaCO 3 nodules indicate that the paleosol which converted into residuun developed strongly, CaCO 3 and Fe 2O 3 in them obviously migrated out of the paleosol layer. The illuvial layer of CaCO 3 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 developed more strongly than north sub tropical residua. CaCO 3 and Fe 2O 3 migrated depth and microstructure identification indicate that CaCO 3 illuvial layer in the 1st , 4th and 5th paleosols don't belong to component part of paleosol section, and acid medium occured during middle and late period developing these 3 paleosols 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 precipitation was 900 to 1 000mm, it is 4℃ higher than nowday's annual mean temperature, and 300 to 400 mm more than nowday's annual mean precipitation. The development 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 Loess Plateau.