Holocene Environmental Change Of Pedogenensis In The Middle Reaches Of The Jinghe River Basin

Some Holocene Loess profiles with a single palaeosol layer have been found in the middle reaches of Jinghe River Basin A layer Heilu soil was formed in these profiles. The profiles in Ertang Site, Co.Changwu and Duanjing Site, Co.binxian of the middle reaches of Jinghe River Basin are studied in this paper.Field investigation, grain-size analysis, magnetic susceptibility, Total Fe, total organic carbon(TOC), CaCO3 and Rb, Sr, Ba ,Rb/Sr measurement of Holocene in this region, have revealed the characteristics of the environmental change of Pedogenensis during late 11500 years:(一) Three pedological phrases have been found on Holocene environmental change of Pedogenensis in the middle reaches of Jinghe River Basin :(1) 11500-8500a B P., summer monsoon was stronger and winter monsoon weaker Therefore, Temperature went higher graduately and precipitation got more. Dust-storms weakened, pedological process was becoming strong slowly.(2) 8500-3100a B.P., pedological process was very stronger, summer monsoon reached its maximum, Precipitation increased remarkably, so was temperature. Annual precipitation and temperature have 50-150mm and 1-3 ℃ higher than its present respectively, A layer of Heilu soil was formed on this region.(3) <3100a B.P., Climate deteriorated again. Summer monsoon was weaker and winter stronger. A layer of Loess has been accumulated since 3100a BP., which has the heilu soil buried. This means the last 3100 years is a dry period with intensified dust accumulation. The soil have been deteriorated.(二) During the late glacial age, short-term climatic events of the Younger Dryas and Boiling and Allerod were well recorded in these profiles. The time was same as 12800-11500a BP and 14700-12800a B.P. recorded in Greenland ice cores.(三) During the Holocene Climate Optimum(8500-3100a B.P.), A single layer of soil (S0) was formed in this region, but this is not an indication that pedological process was characterized by continuous high temperature, but with certain periodic changes. The falls of many index curves conclusively demonstrate that there weremany sudden climatic changes during the Holocene Optimum, and that a further research is needed.(四) The late term of Holocene Climate Optimum (3400-3100 a B.P.) was corresponding with the cultivating layer or cultural layer of the Predynastic Zhou. During this term, the soil resource deteriorated significantly, which indicated that human activities had great influence on the pedogenensis. The top boundary of the cultivating layer or cultural layer was 3100 a B.P., which represents a climate change from the period dominated by southeastern monsoon to the period dominated by northwestern monsoon Therefore, land-use was shifted from arable farming to nomadism. Due to the southern migration of the nomadism, the people of the Predynastic Zhou of arable farming community was forced to move the south to the Guanzhong Basin.(五) It is of great importance for the study Holocene Environmental Change of Pedogenensis in the middle reaches of the Jinghe if magnetic susceptibility is applied to the recovery of temperature and precipitation. Then, a further and profound research is required in future.