Study On The Sequence Of The Upper Loess Layer And The Characteristics Of Weathering And Soil Formation In The First Terrace Of Hanzhong Basin

Since the late Pleistocene,climate change research has been the focus of global change research.The loess-palaeosol sequence is the good information carrier of the ancient climate change.The Hanzhong basin in the upper reaches of Hanjiang River area belongs to the transition region between the north subtropical and warm temperate zone,and,the climate is very responsive.It is significant to improve the research on region integrity of ancient climate in Hanjiang River and global climate change by studying the pale climate evolution.Through field trips many times,we found that there covered a large number of thick layer loess in river terrace of Hanzhong region,but,for a very few researches on loess,there are still many questions to be studied.Our study took the section in Junwangcun profile of Hanzhong basin as the research object,and based on the detailed field trip,we have done the systematic sampling,and,later,have analyzed the physical properties of loess including magnetic susceptibility,granularity,chromaticity,ignition loss and the composition of geochemistry.Depending on the related analysis of experimental data,we studied the stratigraphic sequence of loess-palaeosol,Physical and chemical properties of the loess,the chemical weathering of loess characteristics and ancient climate evolution.Following is the main conclusions:(1)The macroscopic characteristics and stratigraphic sequence of loess-palaeosol was presented:The loess-palaeosol stratigraphic sequences from the bottom to top are AD-Li-Lt-S0-L0-MS.Based on the field observations to describe were presented as follow:Gravel layers(AD)had the gray(5YR 8/2),pure sand,a typical dual structure;Malan loess(Li)had the dull yellow orange(10YR6/4)with the silt texture,uniform massive texture and porous.Transitional loess(Lt)had the dull yellow orange(10YR7/4)with the silt,block-massive structure.Plalaesol(S0)had the dull brown(5YR3/4)with typical clay texture,edge block structure,relative tight,abundant dull brown clay film;Modern loess(L0)had the dull yellow orange(10YR 6/4)with silt texture,mass-block structure and relatively loose.Modern soil(MS)had the dull brown(5YR8/2)with the clay silt texture,aggregate structure and plant roots.(2)Physical and chemical properties of the loess:The physical and chemical properties of the loess were different in the junwangcun profile.The variability of magnetic susceptibility,clay,clay/silt,loess-on-ignition,redness(a*)and redness/yellowness(a*/b*)had the similar properties in the junwangcun profile.There is the maximum present in the palaeosol layers S0,but the lower values present in the loess(Li,L0,Lt),and they have the variation characteristics of the sequence S0>L0>Lt>L1.However,The sand and lightness(L*)had the lowest values present in the palaeosol layers S0 and the higher values present in the loess(L1,L0,Lt).It is performanced specificlyas follows:The magnetic susceptibility in the palaeosol layers So(212.09×10-8 m3·kg-1)is significantly higher than in the loess layers Li(61.31×10-8 m3·kg-1);The grain-size in junwangcun profile are mainly composed of silt content(62.56%)and the second is clay content(19.97%),and the clay in the palaeosol layers S0(28.35%)is more fine than loess layers(20.34%),the a*and a*/b*in the palaeosol layers are highly than loess,but the b*in the palaeosol layers(15.53)is smaller than the loess(16.70).Which reflected that the climate was warm and humid during the palaeosol layers S0 developed and the period affected by the East Asian summer monsoon strongly,the rainfall was rich,the temperature was high,the biological was activity and the chemical weathering of loess was strong.However,the climate was cold and dry during the loess layers(L1,L0,Lt),the rainfall was lower.There were some sandstorms,dust loess accumulation rate was fast and the chemical weathering of loess was weak.(3)The geochemical characteristics of loess:Every element had the low varble coefficients(<10%)in the junwangcun profile.And SiO2.Fe2O3.Al2O3 are the main elements(84.78%).The stable elements including major elements(Fe,Al,K)and trace elements(Ti,V,Mn,Ni,Cu,As)had the high value in the paleosol layers S0 and had the lower value in the loess layers(L0,Lt,Li).The active elements(Ca,Na,Mg,Sr,and Zn)was leached out and migrated.The migration variability of major elements which are normalized by Ti in the palaeosol layers S0 presents different trend in junwangcun profile.Major elements Fe,A1 and K were obvious enrich in the palaeosol layers S0,But major elements Na,Mg,Ca and Si were leached out.The migration trend of major elements is Na(-11.82%)>Mg(-11.42%)>Ca(-10.66%)>Si(-2.97%)>Ti(0.00%)>K(1.28%)>Al(5.35%)>Fe(5.46%).and the major elements in the L1-S1.L1-S2 and L1-S3 also have the consistent migration patterns.(4)The chemical weathering of loess characteristics:The element ratios and chemical weathering parameters indicating the chemical weathering of the loess in Hanzhong Basin was in the stage of primary transition to medium.The chemical weathering intensity of different layers in JWC profile is Li<Lt<L0<S0.It is performanced specificlies follows:the chemical weathering parameters(Rb/Sr,Fe/Mg,K/Na,Ki)and the chemical index of alteration CIA with magnetic susceptibility has the similar change trend line(CIA=66.64).It has the maximum in the layers of S0 and the minimum in the layers of L1.And the chemical weathering parameters(Sa,a,Ba)with magnetic susceptibility has the opposite change trend line.It has the maximum in the layers of Li and the chemical weathering parameters numerical of the layers(L1,Lt,L0)are higher than the paleosol layers S0.Which reflected the weathering of the paleosol layers S0 was stronger than the loess layers.The A-CN-K diagram indicates that the Ca,Na elements was leached out and the K element was relatively stable.The chemical weathering of the junwangcun profile was in the medium stage.Which experienced the chemical weathering of plagioclase and the secondary weathering products mainly were montmorillonite and illite.The loess of junwangcun profile has experienced intense eluviation in 52?200 cm depth.Which shows that the loess with sticky layer characteristics belongs to the humid luvisols.(5)The climate change since late pleistocene:Based on the stratigraphic sequence of the loess-paleosol,the chemical weathering of loess characteristics rebuild the climate change since late pleistocene,which can be divided into 4 stages.The late Pleistocene(55.0?11.5 ka B.P.),the chemical weathering of loess was weak and the climate was cold and dry.The early Holocene(11.5?8.5 ka B.P.),the chemical weathering of loess was increased and the climate was gradually warmer and humidor.The mid Holocene(8.5?3.1 ka B.P.),the chemical weathering of loess was intensity and the climate was warm and humid.The late Holocene(3.1 ka B.P.?0),the climate cool and dry.Besides during the last stage of late pleistocene(38.8-25.6 ka B.P.)it existed three climate fluctuations,the chemical weathering of loess was more strength compared with L1.It reflected that the period of late Pleistocene climate is not long-term stability in the stage of cold and dry.(6)The comparison of Qinling mountains north and south on both sides of the loess:Compared the physical and chemical index and the chemical weathering parameters between Hanzhong in the south of Qinling mountains and guanzhong in the north of Qinling mountains.It is showed specificlyas follows:The magnetic susceptibility of the paleosol layers S0 in the Hanzhong(212.09×10-8m3·kg-1)is higher the Guanzhong(146.80×10-8 m3·kg-1).The grain-size are mainly composed of silt content,the grain-size of loess in the Hanzhong is more fine than the Guanzhong.The chemical weathering parameters K/Na(3.03),Rb/Sr(1.18),Ki(2.58),CIA(68.28)in the south of Qinling mountain are higher than the north of Qinling mountain.It showed that the degree of chemical weathering was different and the chemical weathering of Hanzhong was stronger than the Guanhzong,it reflected that the climate more warm and humid in the south of Qinling mountains than the north of Qinling mountains.And the weathering of the paleosol were more stronger than the loess regardless of the Qinling mountains north and south region,it indicated that the loess has the similar sedimentary process in south and north Qinling mountains and the climate of paleosol development period more humid and warm than the loess development period.