Study On Characteristics Of Weathering And Soil Formation Of Loess-paleosol Section In Longquan Village Of The Middle Reaches Of Danjiang River

As the important information carrier which recorded the past environmental changes,the loess-palaeosol sequences has been used in the study of the evolution of palaeoclimate in the global change.The mid-reaches of Danjiang River Valley is located in the south of Qinling Mountain,which is a transition area between the North Temperate Zone and northern subtropical region,belong to sensitive climate change zone.Although covered a thick layer of eolian loess in the Danjiang River Basin,it is limited about the research for the loess deposition.For example,what are the loess stratigraphic sequences in this region?How does the weathering and pedogenic intensity under the north subtroptical monsoon climates?And so on.On the basis of field investigation,we choose the Longquan Village profile as the object of the study.Based on above the field and laboratory analysis,we study the stratigraphic sequences,physical and chemical properties including chromaticity,magnetic susceptibility,loess-on-ignition and grain size,element content as well as pedogenic process and their climate implications.Here,we present the results:(1)The stratigraphic sequence of the loess are mordem soil(MS)? palaeosol layer(S0)? transitional loess layer(Lt)? Malan loess(L1)?palaeosol layer(S1)?loess layer(L2)from top to bottom.Detailed stratigraphy characteristics are presented as follows:Modern soil(MS,0-40 cm),dull brown(7.5YR5/3),clay silt,granular structure and plant roots develop;Palaeosol layer(So,40?220 cm),dull brown(7.5YR5/4),clay silt,columnar-blocky structure,relative firm,abundant dull brown clay film develop widely;Transitional loess layer(Lt,220-320 cm),dull yellow orange(10YR6/4),silt,block-massive structure;Malan loess layer(L1,320-664 cm),dull yellow orange(10YR6/3),silt,block-massive structure,loose;Palaeosol layer(S1,664?1000 cm),dull brown(7.5YR5/6),clay silt,columnar-blocky structure,firm and more red than So;Loess layer(L2,1000??),yellow orange(10YR5/6),silt,block-massive structure and looser.(2)Physical and chemical properties in the loess-palaeosol sequences were different obviously at the LQC site:?The values of L*and b*in the L1/L2(52.54/51.47,17.44/17.06)are higher than that are in S1/S0(49.18/48.39,17.04/16.15).a*mean value is lower in the loess layer than the palaeosol layer,which is contrary with L*and b*.The values of a*in the L2 and L1 are 8.92,8.81 and 9.07,9.49 in the S0 and S1.?The range of magnetic susceptibility is 42.4?181.5 ×10-8 m3 ·kg-1 and the average value is 138.5 ×10-8m3·kg-1 in loess layer.Magnetic susceptibility have more lower values in the loess units than palaeosol units.The value is 153.0 × 10-8 m3 kg-1 in Li and 127.0 ×10-8 m3 kg-1 in L2.However,The range is 38.3-331.8 X 10-8m3 kg-1 in palaeosol layer and the average value is 183.3 ×10-8m3·kg-1,Which is 184.2 × 10-8m3·kg-1 in S0 and 237.7×10-8 m3·kg-1 in Si.?Loess units have lower average content of loess-on-ignition relative to palaeosol units.The values are respecitively:S1(45.97 g·kg-1)>S0(42.69 g.kg-1)>L2(40.54 g·kg-1)>Li(37.25 g.kg-1).?This loess sequence is mainly composed of silt(2-63 ?m)with an average proportion of 80.93%and the clay(<2 ?m)accounts for 10.30%in total composition.The average particle size(Mz)and median diameter(Md)all indicates the grain size was increased by the sequence of L1>L2>S0>S1.The sequence's grain-size composition varies from bottom to top stratigraphic unit,and the palaeosol is smaller in grain size than that of their loess stratum.These properties reflect that the weathering intensity in palaeosol is stronger than in loess.(3)The characteristics of element distribution in Longquan Village profile:the average content of Al in the constant element is higher in Sl(154.9 g·kg-1),S0(154.0 g·kg-1)than Li(150.0 g·kg-1)and L2(147.8 g·kg-1).The average content of Fe is consistent with that of A1 in the profile.The average content of Si element in S1(649.6 g kg-1)and S1(651.6 g kg-1)is lower than the Li(656.1 g kg-1)and L2(663.6 g kg-1).The average content of K has a small difference in whole profile.The average content of Ca and Na in Li is 10.7 g kg-1,reaching the maximum value of profile.The content of Ba(688.0 mg·kg-1)in trace elements is the highest in S1 unit.Ni and Cu elements have same distribution characteristics in the profile,and are higher in Si than L1/L2.The average content of Sr element(83.3 mg·kg-1)is the lowest in S0 unit,while is the highest in L1(108.5 mg kg-1).(4)The migration variability of major elements and trace elements which are normalized by K in the palaeosol units presents different trend at the LQC site.Detail characteristics are as follows:the elements of Al,Fe,Si and Mg in constant elements show different extent of enrichment characteristics,while Na,Ca elements are migrated.the contents of As,Rb,Ni,Zu,Ba and Cu in the trace elements were enriched in different degrees,while Sr elements are strongly migrated.The migration trend of major elements is:Na>Ca>K>Mg>Si>Al>Fe;In addition,The migration trend of trace elements is:Sr>Cu>Ba>Zn>Ni>Rb>As.(5)The chemical weathering of the loess in the Danjiang River was in the stage of intermediate weathering,with Ca,Na loss seriously and potassium feldspar changed a little.Chemical weathering parameters CIA(72.59)indicated the chemical weathering of the loess in LQC was in the stage of primary transition to medium.And the A-CN-K diagram shows that the major elements Ca,Na rapidly leach out,while K relative stability.The orders of the two chemical weathering parameters ba and a were in L1>L2>Si>S0.The orders of Na2O/K2O was Li(0.39)>L2(0.33)>S1=S0(0.32)and Si02/Al2O3 was L2(4.49)>Li(4.38)>S0(4.24)>Si(4.20),which all indicated the weathering of the palaeosol(S0/S1)were stronger than the loess(L1/L2).(6)The characteristics of weathering and pedogenesis can reflect the evolution of palaeoclimate in the loess-palaeosol sequences.The last interglacial period(125000-75000 aB.P.),the climate envolution expericened warming ? cooling ? warming again progress.At the depth of 870?930 cm,hydro-thermal characteristics are the best,and the strongest pedogenesis process.The last glacial period(75000-11500 aB.P.),the climate is in a dry and cold state as a whole,and weak weathering and pedogenesis process.The early-Holocene period(11500-8500 aB.P.)may have experienced a climate anomaly period.The mid-Holocene period(8500-3100 aB.P.),warm and humid climate,relatively strong weathering and pedogenesis process.The late-Holocene period(since 3100 aB.P.),local climate is more affected by human activities.(7)Compared the composition of chemical elements and the degree of the weathering of Danjiang loess and the Chinese Loess Plateau,it had obtained some similarities and differences.The average contents of Al and Si are high in this two rigions.The CV(coefficient of variation)of chemical elements is extrmenly low(<17%)except Ca,which suggests that the composition of either major elements or trace elements is very consistent within the whole profile.The average content of Al,Fe,K in Danjiang loess are higher than that in the Chinese Loess Plateau,while the average content of Ca,Mg,Na are lower.It is reflected that unstable compositions of the loess in the Danjiang River had been leached more,where it experienced the stronger leaching.The magnetic susceptibility(153.0 × 10-8 m3 · kg-1)of the L1 in the Danjiang River is higher the Luochuan,which is 82.7×10-8 m3·kg-1 of loess layer.The average values of Si(237.7 × 10-8 m3 ·kg-1)and So(184.2 × 10-8 m3 ·kg-1)are both higer than the palaeosol layer in Luochuan(159.0×10-8 m3·kg-1).The chemcial index of alteration(CIA:70.00),Ki(2.20)of the L1 in the Danjiang River are higher than that in the loess of Luochuan,Where are 63.77,1.64 in loess.The value of CIA and Ki in S1(72.85,2.53)and S0(75.01,2.76)are higher than that in the palaeosol of Luochuan,where are 67.36,1.94 in palaeosol.However,Na/K(0.39),ba(0.43),a(0.23)are lower than that in the loess of Luochuan,Where are 0.57,0.64,0.33 in loess.At the same time,The value of Na/K,ba and a in So(0.32,0.33,0.16)and S1(0.32,0.37,0.18)are higher than that in the palaeosol of Luochuan,where are 0.44,0.54,0.25 in palaeosol.It shows that the weathering of the palaeosol were more stronger than the loess.It also reflected that the loess experienced similar sedimentary process in Danjiang River and the Chinese Loess Plateau.However,the weathering of the loess in Danjiang River was stronger with its higher weathering indexs,which testified the ancient climate was more warm and wet in Danjiang River.