Study On Soil Erosion Characteristics And Climate Response Of Fengshi Loess In Yunxian Basin, Upper Reaches Of Hanjiang River

Hanjiang River Valley is located at the south side of Qinling Mountains. It is also the climate transition zone of the South and North China and can be extremely sensitive to one slight change of the environment. It plays an irreplaceable role in environment change study. However, the studies of climate records for the area mainly recorded by peat and stalagmite, there have not found the climate researches recorded by loess-paleosol. Therefore, we should pay more attention to carry out the climate change research with different ancient induction magnet. Extensive field investigation were carried out between in upper reaches of Hanjiang River valley, there has three terrace developed, the II and III level terrace were incomplete caused by corrode, but the loess-paleosol sequence laid on the first level terrace was little human disturbance and little affected by the evolution of the natural environment and soil erosion. Therefore the loess-paleosol sequence laid on the first level terrace could record the climate changed process since last glaciation very well. This research choose the HPC profiles at Yunxian Basin as research objects and sampling the sediment samples. OSL dating and correlation of strata dating were used to established the Chronological sequence of loess-paleosol lied on the first terrace, and combining with the composition characteristics of loess-paleosol (particle size, elements, morphology characteristics (particle shape, pore, coarse particle, formation), comprehensive analysis the loess-paleosol sequence characteristics and its response to climate change, we draw the main conclusions as following:(1) According to the color, grain size, magnetic susceptibility distribution in the profiles, the loess-paleosol sequence were divided into 5 layers, which from top to bottom were MaLan loess (L1), transition layer (Lt), paleosol layer (So), modern loess(Lo) and top soil (TS). The methods of luminescence dating (OSL), cultural dating and correlation of strata dating were applied to determine the age of loess-paleosol sequence. The top and bottom boundary age of So were 3.1 ka B.P. And 8.5 ka B.P., the top and bottom boundary age of L1 were 11.5 ka B.P and 50 ka B.P..(2) The basic constituent of HPC profile in Yunxian Basin are as follows:①The redness value, magnetic susceptibility, loss on ignition, hygroscopic water indicates different weathering intensity order as:S0>Lt>L0>L1.②The grain size composition were consistent with the whole profile, the average particle size (Mz) and median diameter (Md) all indicates the grain size was increased by the sequence of S0�'Lt�'TS�'L0�'L1.③The main component of HPC profile are Si, Al, Fe. Compare the concentration of mobile element (Na、Ca、Sr、Ba) and immobile element (Fe、Al、Rb、Cu、K) concentration among the different layers, the mobile element concentration of So is lower than the layers of loess, the immobile elements concentration changed opposite with mobile element.④The concentration variability of crystalline Fe, amorphous Fe, free Fe were well correspondence with the total Fe, which is S0<Lt<L0<L1.⑤The pore morphology of loess-paleosol was mainly composed by complex stacked pore, the single pore mainly were bubble and cystic shape. The shape parameters (length, diameter, perimeter, area, circularity, C/F10μm of coarse increased by the order of S0, Lt, L0, L1. Soil formation mainly was secondary clay, iron and manganese content condensate (Fe Mn disseminated forms), biomass and so on. The clay film on the pore cell was the thickest in different layers.The chemical weathering index (CIA) of loess-paleosol indicates that the loess on the first terrace was experienced the medium weathering intensity. By the means of multiple geochemical parameters, the A-CN-K diagram, the characteristics of element migration, which all indicates that the chemical weathering intensity of different layers was S0> Lt> L0> L1.Based on the relationship between the weathering intensity of Loess-paleosol sequence and climate change, the process of climate change in the upper reaches of Hanjiang River area since the last glacial maximum can be divided into 4 stages, which were last glacial maximum (50～11.5 ka B.P.), the early Holocene (11.5～8.5 ka B.P.), the mid Holocene (8.5～3.1 ka B.P.), the late Holocene (3.1～0 ka B.P.), The climate characteristics were cold and dry, cool and dry, warm and humid, cool and dry.