Yao Wo Village Profile Record In The East The Years Since The Environmental Evolution Of High-resolution Study

Holocene is the most closely period related to human development. The environmental characteristics and evolution in this period has always been one of the key issues of global change research and also has great significance in predicting future environmental changes. On this basis we select a typical Holocene loess-paleosol sequence in YHC Village in the East of Guanzhong as the research object. Through the comprehensive study to its physical and chemical properties (magnetic susceptibility, grain size, CaCO3, loss on ignition, water absorption, etc.) and micro-morphological features, extracting the environmental instructions, we come to the following understanding:(1) The physical and chemical properties and micro-morphological data in YHC profile jointly reflect that the stratigraphic sequence during the Holocene in the East of Guanzhong from the bottom to up is:Malan loess layer (Li), the transitional loess layer (Lt), paleosols (So), modern loess layer (Lo) and the surface soil (Ts).(2) Through the study to the typical physical and chemical properties of YHC profile, the results show that the content of magnetic susceptibility, loss-on-ignition, hygroscopic water and clay in soil are higer than those in loess. But the content of calcium carbonate, coarse fraction and sand-size particles in soil are lower than those in loess. This feature shows that characteristics of climate change during the Holocene in the of Guanzhong are:during the development period of paleosol So, summer monsoon was strong,climate was warm and humid, precipitation was significantly higher than other periods, leaching and chemical weathering reaction were strong and of the soil had a higher degree development. In the loess layer (L0, L1,Lt), The climate was dry and cold, little precipitation, chemical weathering and leaching function was weak, the soil had a lower degree development, and the loess soil had a much faster accumulation speed than development.(3) Through the study of major elements of Ca, Na, Mg, K, Al, Fe, Si and trace elements Mn, Co, Cu, Zn, Ti, Cr, Pb we found that the elements Na, Ca, Sr were obviously migrated and leached during the chemical weathering progress so that they had a high value in loess. And the elements Al, K, Si, Fe, Mn, Co, Cu, Zn, Ti, Cr had a higher value in soil layer than loess layers. And Rb/Sr, Ba/Sr ratio both had the highest value in the soil layer while had lower value in loess layers. This indicates that the migration of elements may reflect the climate and environmental changes from the Holocene.(4) Through the study of K/Na, CIA, residual coefficient and SiO2/Al2O3 coefficient, the results showed that:K/Na, CIA, residual coefficients all had the highest value in the soil layer while had lower value in loess layers. But the SiO2/Al2O3 coefficient showed a opposite result. It suggesting that the weathering characteristics during the Holocene in the East of Guanzhong are:the paleosols (So) layer had reached the level of moderate weathering under the conditions of warm and humid, which made it to be the highest intensity of the weathering in the entire profile. And the Malan loess had the weakest degree of weathering in the entire profile, which is a further evidence of the climate change characteristics-during the period of Holocene in the East of Guanzhong area, the climate during paleosols was hot and humid while dry and cold during the loess.(5) The characteristics of porosity in the profile shows that:the paleosols had the largest void ratio and the gap wall of the holes and the channels was smooth, which-as instruction of the porosity was formed by pore volume biological activities and pore water; modern loess and loess transition had a smaller void ratio and lots of were simply heaped by native matters, indicating that the small amount of biological activity and the role of weak water.And the C/F(>10μm) having a less value in the paleosols than in the soil layer also shown that during the Holocene in the East of Guanzhong the climate in paleosol was hot and humid, having abundant rainfall, conducive to leaching and biological activities, during loess was dry and cold and less leaching of soluble substances.(6) The microstructure characteristics in the profile shows:it was a high degree of the iron and manganese contamination, a small number of secondary calcite in the paleosol, and a large number of needle-like calcite surrounded the porosity. But in the loess layer was less iron and manganese contamination, and lots of calcite were originally heaped. This indicate that the microstructure characteristics also prove that during the Holocene the paleosol the climate was warm and moist, had a more leaching of soluble substances and were more conducive to a variety of chemical reactions, resulting a volume biological activity, strong pedogenesis and redox reactions. During modern loess the climate was dry and cold, dust storms was frequent, loess accumulation over into the soil, precipitation was less which was not conducive to leaching of elements and chemical reaction, a small amount of biological activity.(7) YHC profile alternative indicators reflect a common feature:the study area in the early Holocene the climate was dry and cold, and then went from dry and cold climate to hot and humid climate, till the paleosol the climate achieved the most suitable, abundant rainfall, higher temperature, all kinds of chemical reaction, leading to loess into the soil strongly, and then the climate had undergone dramatic changes, from the humid in paleosol into the dry and cold climate in modern loess. And then loess accumulation more than soil formation, and the climate in the modern period has improved, but the disturbance of human activity in soil is relatively large.