The Guanzhong Basin In The Central Loess - Paleosol Sequence Records Of Holocene Climate Change Research

Loess-paleosol sequence, as a good carrier to save environmental information in the past,records the information of different scales of environmental change. In this paper,we select Holocene loess-paleosol sequence profile (NSC) in Xianyang Nansi Village as a research object. The profile is located on the first level of the river terraces of the lower Weihe River. The profile is composed of Late Pleistocene loess and paleosol. The stratigraphic sequence is very complete, and boundaries of stratigraphic units are clear. We study edimentary characteristics, physicochemical properties and weathering and pedogenesis, from which we extract climate change information, to provide direct evidence for researching Holocene environmental change in Guanzhong basin.In the wild, we detailed describe the macroscopic characteristics of NSC profile, and conduct stratigraphic classification and Systematic sampling. Indoor we make a systematic testing to color, structure, physical and chemical properties and composition(such as magnetic susceptibility,grain size,pH value,the content of calcium carbonate,hygroscopic water and loss on ignition, major elements, trace elements and so on). We make a comprehensive and integrated analysis of the experimental data, using which discussing pedogenic intensity and its change, climate change and other issues since the late Pleistocene in Guanzhong Basin region,to obtain the understanding of the following:(1)NSC profile sequence is continuous and is saved relatively complete,it detailed and accurate records of climate environmental evolution at this area in different period of climate. Through field research and comprehensive analysis of experimental data, We find that the stratum unit of study section from the lower to the upper is MaLan loess layer L1�'transition layer Lt�'ancient soil layer S0�'modern loess layer L0�'surface soil TS.(2)Various climate indicators at NSC profile shows different change trend in different horizons.Magnetic susceptibility,clay content,loss on ignition and moisture are higher in soil layers,their average values are140.9×10-8m/kg、26.5%、1.22%and3.62%, and they are lower in loess layers, average values are57.7×10-8m3/kg、19.0%、0.58%and2.37%. The peak of coarse silt content,CaO3content and pH value is in loess layers, their values are49.8%,16.91%and8.58.Their lower values are in ancient soil layer,and their values are42.8%、2.70%and8.15.These characteristic changes of indica-tors show that southeast monsoon climate prevails during the ancient soil developing, a-nd climate is warm and moist, weathering pedogenesis is significant;The monsoon in East Asia is becoming weakened during loess accumulation period.The northwest monsoon becomes strong and the climate is cold and dry.The weathering leaching is weak and transformation degree is low.(3) Major elements such as Fe,Al,Si and K appears high in ancient soil layer and their average values are61.8g/kg、141.9g/kg、595.9g/kg and27.1g/kg. They are lower in loess layers,and average values are47.3g/kg、112.0g/kg、56.1g/kg and20.9g/kg.The value of Ca,Mg and Na is high in loess layer and average values are89.1g/kg,32.4g/kg and108.9g/kg. They are low in ancient soil layer and average values are27.4g/kg,28.5g/kgand13.0g/kg.These elements exhibit different supergene geochemical behavior in different periods,compared with Al,Fe and K show relative enrichment characteristics, but Ca,Na and Mg produces marked eluviation.Their migration ability in the order is Na>Ca>Mg>Si>Fe>Al>K.(4) Smobility and migration of trace element are different.Cr,Pb,Co and Ga have strongest activities and largest migration ability.Nb,P,Ti and V show weak migration properties,while Cu,Mn,Ni and Zn is relative enrichment.Their migratory ability in order is Cr>Pb>Ga>Co>Nb>P>Ti>V>Ni>Mn>Zn>Cu.(5) Geochemical parameters such as potassium sodium,residual coefficient, SiO2/(CaO+MgO) ratio,chemical alteration index CIA,Rb/Sr ratio,Ba/Sr ratio and magnetic susceptibility show significant positive correlation,and their distribution curve with the formation have substantially synchronism trend with magnetic susceptib-ility. These suggests that during the soil development,climate condition is advantageous and leaching effect remarkable,and the chemical weathering degree is higher than loess accumulation.(6) NSC profile shows pedogenic environmental change process in Weihe Valley and records climate environmental change since Holecene:Late Pleistocene(11500a B.P.) ago,winter monsoon effect is obvious,and climate is dry and cold,sandstorms frequently arriving at the area,forming thick loess layer Li.The early Holocene (11500-8500a B.P.) is a transition phase from cold wet climate to warm wet climate.sandstorm activity becoming weakened,pedogenesis enhancing,and forming a transition layer Lt.During the mid Holocene(8500-3100a B.P.),summer monsoon effect in East Asia is very remarkable,and climate is very warm and humid,sandstorm arriving at this area significantly reducing,and under the strong weathering transformation,sedi-ments forming soil layer.At late Holocene(3100a B.P.),East Asian monsoon pattern changes and the northwest monsoon effect begins to apparently increase,the climate becoming drought,sandstorm frequently arriving to Weihe Valley and depositing,form-ing modern loess L0.