Residua in Loess of China

Key words: 
• weathered deep loess /
• residua /
• rain events leached soil /
• subtropical climate /

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.