Research On The Formation Age Of The Hanjiang First Terrace And Its Overlying Loess Response To Climate Change Since MIS-3

Loess deposits provide a detailed archive for understanding the pedogenic dynamics controlled by paleomonsoon climate change,and hence hold an important position in the Quaternary Science.The upper Hanjiang River is located in a transition zone between temperate and subtropical monsoon climate,Qinling and Dabashan mountainous in China,which is a special geographic location and is more sensitive to East Asian monsoon climate change.The upper Hanjiang river valley consists of a series of intermontane basins,and four-level river terraces were found in these basins.The terraces are generally covered by aeolian loess with a thickness of about 5?20 m.Loess deposits in the high terraces are incomplete after long-term slope flow erosion.Whereas,loess in the first terrace is widely distributed,with a continous and complete sedimentary stratum,and is considered to be an ideal research material to reconstruct the pedogenic intensity as well as monsoon climate change since uplift of the first terrace of Hanjiang river.However,study on the formation age of the low terrace of the Hanjiang river,complete stratigraphy and chronology sequences of loess covered on the river terrace,weathering and pedogenic characteristics of loess deposits and its implication for longer time-scale climate change have seldom been comprehensively and systematically reported.In the paper,several loess profiles with clear and complete stratigraphic sequences in the first terrace in different reaches of the upper Hanjiang River,including Junwangcun(JWC)and Zhanggou(ZG)profiles in the Hanzhong basin,Luojiatan(LJT)profile in the Ankang basin,Tuojiawan(TJW)and Huangpingcun(HPC)profiles in the Yunxian basin,were selected for continous sampling and high-resolution analyzing after detailed field investigation.On the basis of observation of loess macroscopic features in field,measurements of magnetic susceptibility,color variation(L*,a*,b*),grain-size and chemical elements,micromorphology analysis,and optically stimulated luminescence(OSL)dating were carried out.Based on these systematical researches,we can draw a series of new understandings.The conclusions are as following:1)Firstly,the OSL dating of samples from the top part of sand layer of fossil floodplain and samples from the bottom boundary of overlying loess deposits in these study profiles in the first terrace of Hanjiang river are carried out.Sample from the bottom boundary of overlying loess deposits were OSL dated to 46.1 ±4.1?57.2 ± 2.5 ka,implying the upper bound of formation age of river terrace.Sample from the top part of sand layer of fossil floodplain were OSL dated to 54.4 ±2.8?55.1 ±5.2 ka,implying the lower bound of formation age of river terrace.These results indicate that the uplift age of the first river terrace is approximately 55.0 ka B.P.,and indicate that the modern landform pattern of the first terrace of Hanjiang river were gradually formed in this period.2)Comprehensive analysis of field characteristics(colour,texture and structure),various physicochemical indexes and OSL dating enabled a precise identification of stratigraphic boundaries and chronology sequences,that is Holocene loess L0(3.0?0.0 ka B.P.)—Paleosol S0(8.5?3.0 ka B.P.)—transitional loess Lt(11.5?8.5 ka B.P.)—Malan loess L1-L1(21.0?11.5 ka B.P.)—weaker paleosol L1-S1(23.0?21.0 ka B.P.)—Malan loess L1-L2(24.5?23.0 ka B.P.)—weaker paleosol L1-S2(31.0?24.5 ka B.P.)—Malan loess L1-L3(33.0?31.0 ka B.P.)—weaker paleosol L1-S3(34.0?33.0 ka B.P.)—Malan loess L1-L4(36.0?34.0 ka B.P.)—weaker paleosol L1-S4(44.5?36.0 ka B.P.)—Malan loess L1-L5(55.0?44.5 ka B.P.).The chronology frame laid an important chronological foundation for the study of the pedogenic intensity and monsoon climate change.3)The loess and paleosols in the upper Hanjiang river are mainly composed of silt fraction(5?50 ?m)that varies between 56.60%?67.62%and the total content of major element SiO2,Al2O3 and Fe2O3 is between 780.64?851.43 g·kg-1,generally presenting characteristics of dust accumulation.Micromorphological features show that Malan loess L1 containslittle number of clay coatings that is mainly characterized by residual clay with a yellow brown color;it is loose and porous,voids are dominated by a large number of small irregular packing voids with very rough and zigzag void walls;and the soil particles are coarser,irregular,sub-angular or angular,and mineral composition are dominated by quartz,feldspar and mica.The paleosol S0 is enriched by abundant clay coatings that is dominated by illuvial clay and has a pale red brown color;it is dense and hard with the lowest porosity and voids are dominated by channels or circular voids with very smooth void walls;soil particles are finer and rounder,and mineral composition of coarse particles mainly dominated by quartz.The loess L0 and Lt hold medium number of clay coatings that are mainly dominated by residual clay with a yellow brown color,voids are dominated by packing voids with some long-thin type channels with rough void walls.And there is almost no secondary carbonate in the loess and paleosol in these study profiles.4)Compared with loess layer,magnetic susceptibility,redness(a*),Fe2O3,loss-on-ignition,CIA and clay content are higher,luminosity(L*)is lower and the soluble element Ca,Na,Mg are leaching strongly in the paleosol.These characteristics imply that the loess deposits experienced a series of pedogenesis,including formation of strong magnetic iron minerals and rubification,decomposition of silicate minerals and formation of clay minerals,formation of organic matter,strong eluviation in the weathering and pedogenic process in the north subtropical monsoon climate environment.Secondly,based on deep analysis of various chemical weathering parameters(CIA,PIA,CIW,CPA,K2O/Na2O,and so on),results indicate that the chemical weathering strength of loess and paleosol in the upper Hanjiang river does not show obvious regional difference,and are in the intermediate weathering stage.And the A-CN-K diagram show thatmajor elements Ca,Na are rapidly leaching,K relative stability.The clay minerals produced in the weathering process are dominated by Smectite and Illite,and do not reached the strong weathering stage that is dominated by Kaolinite and Gibbsite.To better reveal weathering characteristics in the monsoon transition zone,comparative study with Luochuan loess in the north of Qinling mountains and Xiashu loess in the south of Qinling mountains were carried out.Results show that the major elemental compositions of loess in the upper Hanjiang river show a high similar with Luochuan and Xiashu loess,implying a similar sedimentary background of aeolian deposit.But the chemical weathering intensity of these loess deposits present a significant regional variation that is Luochuan loess<loess in the upper Hanjiang River<Xiashu loess.The changing trend is well consistent with the spatial variation of modem summer monsoon climate change in China,suggesting that the chemical weathering differences are mainly controlled by impact of summer monsoon climate.This provides crucial evidence for understanding response of loess weathering to the East Asian monsoon.5)The soil developed from loess in the upper Hanjiang river have a A-AB-Bt-BC-C profile structure.The Bt layer in different profiles are 50?170 cm in thick,and exhibit redder color of dark red brown(5YR),red brown(5YR)or pale brown(7.5YR).Its clay content increased obviously compared with overlying eluvial horizon(A?AB)and underlaying parent material horizon(C),and absolute increase in the amount of ?3%.Microscopic observious shows that Bt horizon is rich in residual and illuvial clay coating.These features coincide with the diagnostic characteristics of Argic horizon,suggesting that the soil developed in the subtropical environment belongs to a kind of Argosols.The climatic environment of the Hanjiang river meets the requirements of soil temperature and moisture of Udic Argosol.The Argic horizon exhibits dense and hard,prismatic-bloky,abundant clay film with the thickness of0.5 mm,and often accompanied by Fe-Mn film and nodules,macthing the diagnostic characteristics of Clay pan.Hence,the soil can be sorted as Clayplan-Udic Argosol.These results exactly reveal soil property and can provide a solid basis for soil improvement using in the future.6)Four weaker paleosols were first identified in the Malan loess L1,revealing that the famous warm-wet periods during MIS-3 in the last glacialalsoexsit in the Hanjiang River valley.The weathering and pedogenic intensity was very weak in the formation of Manlan loess L1,reflecting that the climate is overall in a relative cold during 55?11.5 ka B.P..The four weaker paleosols were developed during the period of 23.0?21.0 ka B.P.?31.0?24.5 ka B.P.?34.5?33.0 ka B.P.and 44.5?36.0 ka B.P.,implying climate fluctuation during the last glacial.The pedogenic intensity(taking clay content of TJW profile as an example)of four weaker paleosols L1-S1?L1-S2?L1-S3 and L1-S4(18.44%?20.60%?15.91%and 14.48%)are significantly higher than the adjacent upper and lower loess layers,higher than the loess L1(13.32%),and lower than thepaleosol S0(25.94%),and present such variation characteristics of L1-S2>L1-Si>L1-S3>L1-S4.Among them,the pedogenic intensity of weaker paleosols L1-S3 and L1-S4 arerelative weak,indicating that the climate was mainly clod-dry in 55.0?31.0 ka B.P.,but during which the climate became weak warm-wet in 44.5?36.0 ka B.P.and 34.5?33.0 ka B.P.;Weaker paleosols L1-S1 and L1-S2 experienced higher pedogenic intensity and weaker paleosols L1-S2 was highest,indicating that the climate presents a obvious prolonged period of about 10.0 ka,warm-wet period in 31.0?21.0 ka B.P.,during which the climate was at the most warm and wet stage of last glacial in 31.0?24.5 ka B.P.,and became relative cold-dry in 24.5?23.0 ka B.P..The climate shows a persistent 9.5 ka,dry-cold stage in 21.0?11.5 ka B.P.,corresponding to the last glacial maximum.These results first reveal the evidence of climate change during MIS-3 recorded by loess deposits in the Hanjiang river,and reveal regional climate change response to global change during this period.