| Revealing the mechanism and causes of past global changes is the basic task of the research of global environment changes,the climate variability of last glaciation and the Holocene is the focus part of the project:Past Global changes(PAGES).The upper reaches of Hanjiang River Valley is located in the north subtropical zone,which is a transition area between the North Temperate Zone and northern subtropical region,belong to sensitive climate change zone.It plays an irreplaceable role in environment change study in epipleistocene.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 2011 and 2013 in upper reaches of Hanjiang River valley,there has three terrace developed,the ? and ? 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 MTS,QFC,GXHK profiles at YunXian and YunXi sections in upper reaches of Hanjiang river valley as research objects and sampling the sediment samples.OSL dating,cultural 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,minerals,of state),morphology and structural characteristics(quartz particle morphology,particle shape,pore,coarse particle,formation),weathering characteristics(Morphological weathering degree,chemical weathering degree,weathering degree indicated by other physical and chemical properties)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 bottom to top were Ma Lan loess(L1),transition layer(Lt),paleosol layer(S0),modern loess(L0)and top soil(Ts),respectively.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 S0 were 3100 a B.P and 8500 a B.P.,the top and bottom boundary age of L1 were 11500 a B.P and 20000 a B.P..(2)The basic constitue of loess-paleosol sequence in Yunxian-Yunxi section in Hanjiang river are as follows:? According to grain size distribution,grain size parameters and its structure diagram,Grain size genetic discrimination formula,all of them indicates the loess-paleosol sequence was wind deposit products,distinct from the aqueous material;? The 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 So ? Lt ? Ts ? L0 ? L1.? The composition of loess-paleosol in Hanjiang River and Weihe River valley all quite similar to the UCC(upper continental crust)average composition,which indicates both the loess of two regions all come from multiple sources and mixed well,all were aeolian loess.The main component are Si02,Al2O3,Fe2O3,their mean sum up to about 80%.Compare the concentration of mobile element(MgO,Na2O,CaO)and immobile element(Fe2O3?Al2O3?K2O)concentration among the different layers,the mobile element concentration of S0 is lower than the layers of loess,the immobile elements concentration changed opposite with mobile element.?The concentration variability of crystalline Fe,amorphous Fe,free Fe were well correspondence with the total Fe,which is S0>Lt>L0>L1;?The loess-paleosol was mainly composed by feldspar and quartz feldspar content,but the feldspar content was decreased and clay minerals(illite smectite mixed layer,such as kaolinite,vermiculite)increased in the layer of S0.(3)The characteristics of quartz particle morphology,loess-paleosol micro structure,pore,soil formation was observed by means of scanning electron microscope,optical microscope and analyzed by the software of Q-win,we draw the conclusions as following:?The Micro morphology of quartz mainly were prismatic and subangular,the surface structure were dominated by V pits,Saucer pit,V pits,conchoidal fracture.But the frequency of round and sub-rounded quartz were decreased in order of S0?Lt?L0?L1,and the frequency of surface chemical structure were increased in the order of S0?Lt?L0?L1.? The Micro structure type is changed downward the profile,the Micro structure type of the layers of Ts,L0,S0,Lt,L1 were flocculent gel structure,Inlaid semi cemented structure,Clot bonding structure,Inlaid semi cemented structure,Support micro cement structure.?The pore morphology of loess-palesol was mainly composed by complex stacked pore,the single pore mainly were bubble and cystic shape.The porosity of pore in different layers of loess-paleosol sequence was S0>Lt>L0>L1;?The shape parameters(length,diameter,perimeter,area,circularity,C/F(10?m)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,whose thickness were between 100?1000 ?m.(4)By the means of geochemical parameters,shape weathering index(MWI),physicochemical properties,which all indicates the weathering intensity order of loess-paleosol sequence on the first terrace in the Yunxian-Yunxi section of Hanjiang river were S0>Lt>L0>L1,the characteristics as following:?The chemical weathering index(CIA)of loess-paleosol indcates that the loess-paleosol 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.?Micro morphology weathering index(MWI)were established and were applied to analyzed the morphology weathering intensity,the order is S0>Lt>L0>L1;?the magnetic susceptibility,Kd,LOI,hygroscopic water also indicates different weathering intensity order as:S0>Lt>L0>L1.(5)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(20000?11500a B.P.),the early Holocene(11500?8500a B.P.),the mid Holocene(8500?3100a B.P.),the late Holocene(3100?0 a B.P.),The climate characteristics were cold and dry,cool and dry,warm and humid,cool and dry.The Mid Holocene climate stage were not always warm and humid,there were a cold climate event occurred about 6000?5000a B.P.,which was responsed in many parts of the world.In addition,the loess-paleosol compared with peat,stalagmite in the same region recorded the climate change process and compared with the loess-paleosol recorded the climate change between Hanjiang valley and Weihe valley,we found that the climate change process since last glaciation was not completely consistent by different area and different medium.The consistency was reflected the climate change all can be divided into 4 stages and the temperature were cold,cool,warm,cold correspondingly.The difference was reflected as the start and end time of the 4 stage,which caused by the different of topography,medium,sampling density,dating methods. |
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