Late Cenozoic Climate Change In Weinan Area Inferred From Magnetic Susceptibility And Grain Size Of Drill Cores Sediments

Guanzhong Basin(also known as Weihe Basin),surrounded by Qinling in the south and Chinese Loess Plateau in the North,is a graben faulted basin with three-side surrounded by mountains and one-side open to the east.Since the Cenozoic,more than a kilometer of lacustrine sediment and aeolian dust has deposited in the basin.Due to tectonic uplift of the Qinling Mountains,these Cenozoic strata in the northern edge of the Qinling Mountains exposed.Weinan area,one of the most typical regions for the Cenozoic strata in China,preserved particularly continuous sedimentary sequence and provided excellent archivements to study the environmental changes and paleoclimate evolution since the Cenozoic.Guanzhong agglomeration is the starting point of the Silk Road Economic Belt and plays a vital role in communicating the inland and Eurasian land bridges and the Maritime Silk Road.In the process of construction of Silk Road belt,there are many important geological and environmental problems which restrict the development of regional economy and urban agglomeration in the process of urban planning and construction,new rural land use,and it is urgent to raise the understanding of these basic geologic conditions and major geological problems.Therefore,we extract the paleoenvironmental evolution information contained in the formation sediments in the study area and explores the laws and mechanisms of environmental change in the geological period in the region,especially during the Quaternary and find the historical similarity,in order to understand the stage and position of modern environmental change in its evolution history,scientifically and rationally predict the future environment and climate change and assess climatic effect,and provide some practical guiding for human activities and production construction in the region.Based on the study of two drilled cores(GT2 and GT3)in loess tableland and the valleys respectively in the Weinan region,the east of Guanzhong Basin.This study makes use of the paleogeomagnetic to establish the chronology of borehole drilling,and studies the drilling sedimentary stratigraphy and magnetism.Combining with the existing data,the history of climate and environment change since the late Cenozoic in the region was reconstructed by using the granularity and magnetization,and the following conclusions were drawn:The chronology framework of GT2 hole and GT3 hole was established by using the paleogeomagnetic dating method.The base age of GT2 hole was 5.25 Ma,B/M,M/G,Ga/Gi was located at the 59 m,129m,247 m,The corresponding deposition rate is 7.56cm/Ka,5.0cm/Ka and 6.9cm/Ka respectively;The base age of GT3 hole is 3.85 Ma,B/M,M/G was located in 62 m,173m,197 m respectively,The corresponding deposition rate is 7.9cm/Ka?6.7cm/Ka?5.5cm/Ka respectively.;2.The significance of paleo-climatic environment of borehole sediment particle size and magnetic susceptibility is clarified.In the lake facies deposition,the particle size and the magnetic susceptibility all reflect the lake hydrodynamic condition and the sedimentary facies change,coarse grains represent low lake surface,ground erosion and hydrodynamic strengthening,sedimentary of fluvial facies or shallow lacustrine facies,indicating arid climate environment;fine grain represents higher lake,surface erosion and hydrodynamic weakening,sedimentary of deep lacustrine facies,indicating humid climate environment.But in the Eolian loess deposition,grain size reflects the wind strength,can be regarded as the winter Wind Index,the magnetic susceptibility mainly relates to the temperature and precipitation,represents the summer wind intensity;3.Based on the analysis of sedimentary strata,magnetic susceptibility and particle size characteristics of GT2 and GT3 drilling porous cores,the climatic evolution sequence since Pliocene was established.The record of magnetic susceptibility and grain-size characteristics of sediments indicate that the climate change in the region since the Pliocene has been characterized by a gradual increase in the overall trend of drought,which can be roughly divided into 4 major change stages.Namely,stage?: Pliocene(4.8-2.58Ma),Lithology is dominated by lacustrine sandy clay deposition,contains a small amount of silty sand layer and fine sand interlayer,deep lacustrine deposition,magnetic susceptibility as a whole is low.Fine components dominate and lake develop.Overall,it reflects relatively moist climate and environmental conditions and the summer monsoon is stable;Stage?: early Early-Pleistocene(2.58-2.0Ma),Lithology is mainly silty sand,and contains a lot of sand,the overall variation trend of magnetic susceptibility is first low after high,coarse particles dominates and the lake shrinkages for many times,which indicates that winter monsoon is stronger than the last stage and indicates the climate characteristics of the intensification of drought.Stage ?: early Early-Pleistocene to Middle-Pleistocene(2.0-1.2Ma),lithology is complex,magnetic susceptibility and particle size vary greatly.By analyzing synthetially the variation characteristics of magnetization,granularity and lithology of two drilled holes we found the overall climate change in Weinan region in this period is great.Overall,it gets further arid.The lake range is greatly reduced,and some areas begin to accumulate aeolian accumulation;Stage ?: Middle-Pleistocene to Holocene(1.2-0Ma),This stage experienced three stages,namely,the East Asian winter and summer monsoon enhancement,the East Asian winter wind significantly enhanced,East Asia winter and summer wind peak period.And presented cyclic drought climate strengthening process.