Comparison Of The Weathering Characteristics Of Loess On The East And West Sides Of The Southern Section Of Liupan Mountain

Loess is an important information carrier for studying the environmental evolution in the past.Liupan Mountains is a transitional zone between semi-arid and semi-humid climate in China,and it is also the intersection of East Asian monsoon and westerly wind in China.It is sensitive to environmental changes.There are a large number of flat platforms with different areas to the west of the southern section of Liupan Mountains.Since Quaternary,there have been relatively thick loess deposits,but there are few studies on the chemical weathering characteristics and weathering intensity of loess in this area.After careful field investigation in Tianshui area,Gansu province,west of the southern part of Liupan Mountains,fanjiacheng(FJC)section is selected as the research section,and combined with laboratory experiments,the geochemical and weathering characteristics of loess west of the southern part of Liupan Mountains are analyzed in detail,and compared with Baoji liangcun(LC)section in Shaanxi province east of the southern part of Liupan Mountains,so as to explore the weathering characteristics difference of loess on the east and west sides of-the southern part of Liupan Mountains and the reasons for the difference.This paper draws the following conclusions:(1)The stratigraphic sequence of FJC Holocene loess section west of the southern section of Liupan Mountains has been established and stratigraphic dating has been carried out:topsoil(MS,1,500 a BP to date)modern loess layer(L0,3,100?1,500 a BP)? paleosol layer(S01,5,000?3,100 a BP)loess interlayer(Lx,6,000?5,000 a BP)?paleosol layer(S02,8,500?6,000 a BP)?transitional loess layer(Lt,11,500?8,500 a BP)? malan loess layer(L1,>11,500 a BP).(2)Geochemical characteristics of FJC section west of south Liupan Mountains:CV of each element is small,ranging from 1.42%to 8.24%,with SiO2,Al2O3,CaO and Fe2O3 as main elements.The contents of Ca(78.1 g·kg-1)and Sr(226.5 mg·kg-1)in ancient soil S0(S01,S02)are lower than those in loess layer(L0,Lx,Lt,Li)(83.4 g g·kg-1,249.0 mg·kg-1),indicating that Ca and Sr are leached,while the contents of other elements(Si,Al,Fe,K,Mg,Na,Ti,Mn,Cu,Zn,Ni,Rb,Ba)in ancient soil layer are higher than those in loess layer.Compared with the upper continental crust,the FJC profile is significantly enriched in Ca,Mn and Ni,while significantly depleted in Na and Sr.(3)The geochemical characteristics of loess on the east and west sides of the southern Liupan Mountains are obviously different:CV of each element in FJC section is less than that in LC section,which indicates that the loess element differentiation in the east of the southern Liupan Mountains is more significant.The content of Ca(81.1 g·kg-1)in FJC section is significantly higher than that in LC section(44.4 g·kg-1).The variation of Na in stratum is differentiated,specifically,the content of Na in FJC section(14.8 g·kg-1)is higher than that in loess layer(14.1 g·kg-1).The standardization of element UCC shows that the loess elements on the east and west sides of the southern Liupan Mountains have the same composition and uniform material sources,the loess elements on the east of the southern Liupan Mountains are subjected to greater chemical weathering in the later period,and Ca leaching is more intense.(4)Weathering characteristics of FJC section west of south Liupan Mountains:the mobility of elements relative to Ti in ancient soil S0 indicates that Ca,Mg,Si,Fe,Mn,Cu,Zn,Ni,Rb,Sr,Ba are relatively leached,while Na and K are relatively enriched.The migration sequences of major elements and trace elements are respectively:Ca>Mg>Si>Fe>Al>K>Na,Sr>Ba>Rb>Ni>Zn>Mn>Cu.Rb/Sr(0.43),Ba/Sr(2.02),ba(0.73),a(0.48)in paleosol S0 are higher than those in loess layer(0.38,1.83,0.71,0.46),while Ca/Mg(3.25),K/Na(1.59),CIA(57.95),Ki(1.178)are lower than those in loess layer(3.48,1.63,58.46,1.183),the chemical parameters together reflect the strongest weathering effect in the middle Holocene in the west of south Liupan Mountains.CIA(56.92?59.64)and A-CN-K triangle show that the FJC section is still in a low weathering stage with weak weathering intensity.(5)Comparison of loess weathering characteristics on the east and west sides of the southern Liupan Mountains:CDThe elements in the area east of the southern Liupan Mountains are more active and more weathered.The leaching degree of soluble elements Ca,Na,Mg and Sr in ancient soil S0 in LC section is higher than that in FJC section,while the enrichment degree of Al,Fe,Si,K,Mn,Cu,Zn,Ni,Rb and Ba is higher than that in FJC section.In paleosol S0,Rb/Sr(0.43),Ba/Sr(2.02)of FJC profile are smaller than LC profile(1.37,6.13),Ca/Mg(3.35)is larger than LC(0.58).For the whole section,K/Na,CIA,Ki(1.61,58.27,1.182)of FJC section are lower than LC section(2.51,69.65,1.86),ba,a(0.72,0.47)are higher than LC section(0.44,0.25).The FJC section is located further from the A-K line than the LC section in the A-CN-K triangle.?The weathering intensity between layers in FJC section and LC section indicated by K/Na,CIA,Ki,ba,a and A-CN-K triangle diagram is different.In LC section,higher K/Na,CIA,Ki and lower ba and a represent the formation period of paleosol S0 with high weathering degree,while in FJC section,lower K/Na,CIA,Ki and higher ba and a represent the formation period of paleosol S0 with high weathering degree.(6)Reasons for the difference in loess weathering characteristics between the east and west sides of the southern Liupan Mountains:?Climate factors are the reasons for the difference in loess weathering intensity between the east and west sides of the southern Liupan Mountains.Based on the analysis of the climate in the east and west sides of the southern Liupan Mountains from 1955 to 2003,it is found that the climate on the east side is warmer and wetter with less evaporation,which indicates that the soil forming environment on the east side is better,considering the present and the past,it is inferred that the climate in the east of the southern Liupan Mountains is also superior to the west side in geological history.The warm and humid climate environment makes the chemical weathering process of loess on the east of the southern Liupan Mountains faster,the biological activity stronger,the migration ability of elements higher,and the weathering degree higher.?The differentiation of chemical elements in loess is the differentiation rule of Na in FJC section west of south Liupan Mountains and LC section east of south Liupan Mountains.K/Na,CIA,Ki,ba,a,A-CN-K triangle maps are the cause of differentiation when indicating weathering intensity between different strata.Specifically,the higher content and the most active Ca in FJC section cause the change of Na relative content,thus causing the above-mentioned climate substitution indexes to differentiate when indicating weathering degree of various strata inside the two sections.