摘要:
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.
Abstract:
According to the field survcv formed before 0.7 Ma, and lahoratorv analvses, the residua of 3 periods arc indcntificd in the section of loess which arc composed of 1st.4th and 5th lavcr rcdhrown palcosols and the weathered loess lavcrs under them.Thcv mainly developed in the south-cast Loess Plateau and also exist in the middle of the Loess Plateau.The thickncss of residua almost varies from 3 to 6 m and normally consist of 3 to 4 layers.From top to hottom} 1st layer is red-brown palcosols,2nd layer is brown-red weathered broken loess 3rd layer is brown-yellow weathered broken loess with fcrriginous film the 4th layer is brown-yellow weathered broken loess without fcrriginous film.The apparent difference between residua and palcosol 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 palcosols.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 palcsols varies from 40% to 60%,and im weathered loess varies from 40 % to 45 %.Magnetic susceptibility in palcosol generally varies from 100 to 280(10-6SI),and in weathered loess varies from 60 to 100(10-6SI).The CaC03 content in weathered loess is mainllv less than 1%,and unweathered loess is usually above 8 %.This shows that weathered and unwcathcrcd loess layer are apparently different. The X-ray diffraction of six clay film samples reveals that the clay mineral in residua is mainlvmontmorillonite, secondlv is illitc and kaolinitc.So we can determine that the residua in loess is mont morillonitc-illitc types.The survey in the migrated depth of red fcrriginous clay film and CaC03 nodules indicate that the palcosol which converted into rcsiduun dcvclopcd strongly CaC03 and Fe203 in them obviously migrated out of the palcosol layer. The illuvial layer of CaC03 nodules is not at the bottom of palcosol, it lies in the weathered loess which is under the palcosol bottom 1 to 2 meters. Through the contrast the components of weathering section of the palcosols in loess and modern sub-tropical Ycllowbrown Earth are primarily same it developed more strongly than north sub-tropical residua. CaC03 and Fe203 migrated depth and microstructure identification indicate that CaC03 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 OOOmm} it is 4℃ higher than nowdav's annual mean temperature, and 300 to 400 mm more than nowdav'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.