A Study On Magnetostratigraphic Date And Paleoclimate Environment Of The Cultural Layer Of Xihoudu Site

Xihoudu site is the first paleolithic cultural site belonging to the early pleistocene period which was discovered in China,the study of its cultural layer's chronology and paleoclimate environment plays an important role in discussing the evolution of early human in east Asia,but the cultural layer of Xihoudu site is relate to fluvial sand and gravel sedimentary environment,the sand and gravel layer producing the culture layer shows complex hydrodynamic conditions,at the same time,many scholars have different understandings of loess-paleosol deposits above the cultural layer and river-lacustrine deposits below the cultural layer.In addition,different dating methods are adopted for the cultural layer,and the date obtained by different methods are different,and the date obtained by the same method are not verified repeatedly.All these make scholars fail to reach a unified chronological framework for the cultural layer.However,for a long time,the paleoclimate environment corresponding to the cultural layer of the Xihoudu site has been reconstructed mainly with the help of the unearthed vertebrate fauna in the cultural layer,and there is a lack of systematic and comprehensive understanding.In order to better clarify the date and corresponding paleoclimate environment of the cultural layer of Xihoudu site,in this paper,firstly,through field research,we found a set of sand-gravel layer in the Anli section is as same as the sand-gravel layer which produced the cultural layer.With the help of this discovery,the characteristics of the sediments environment over or under the cultural layer were further explored,then the magnetostratigraphic date and paleoclimate environment of the cultural layer of the Xihoudu site were studied by means of paleomagnetic polarity dating and stratigraphic pollen analysis.Thermal demagnetization data of field samples shows that,a complete Jaramillo normal polarity subzone appears above the sand-gravel layer which produced cultural layer,which is located at a depth of 31.2-36.5 meters in section and 5.3 meters in thickness,covering the paleosoils of S10,S11,S12 and their interloess.The sand-gravel layer which produced cultural layer is located 8.1 meters below the bottom boundary of the Jaramillo normal polarity subzone.According to the international standard geomagnetic polarity time scale(GPTS),the date range of the Jaramillo positive polarity subzone is 0.99 Ma-1.07 Ma,in this paper,the deposition rate of the loess-paleosoil is considered to be stable,so the average loess-paleosoil deposition rate during the Jaramillo positive polarity subzone can be calculated as(5.3×100)/(0.8×1000)=6.625cm/ka,so the deposition date of the sand and gravel layer is:(8.1×100)/(6.625×1000)+1.07?1.21 Ma,Therefore,the magnetic date of the cultural layer of Xihoudu site is about 1.21 Ma.On the other hand,there are two types of pollen which are woody pollen and herbaceous pollen were identified from the stratigraphic palynology samples corresponding to the cultural layer of Xihoudu site,among them,woody pollens are mainly Pinus and Quercus,and herbaceous pollens include Artemisia,Aster and Poaceae.On the whole,the percentage of herbaceous pollen is 92%,and the percentage of woody pollen is 8%,the content of Artemisia and Poaceae in the pollen of herbaceous plants is dominant.In terms of pollen concentration,the pollen concentration of Pinus is 1 grain per gram,and that of Quercus is 2 grains per gram,while that of herbaceous pollens is better than that of woody pollens,among them,the pollen concentration of Artemisia is 15 grains per gram,that of Poaceae is 12 grains per gram,and that of Aster is 3 grains per gram.Secondly,the values of trees/non-trees(AP/NAP)calculated by pollen-climate index is less than 0.3,and that of Pine/Artemisia(P/A)is less than 0.05.And biomisation analysis shows the biota that matched the pollen groups in the samples are mainly distributed in TEDE,TEMX,TEGR,TEXE and POGR,on this basis,according to the similarity index formula,the biota with the highest score which matched the pollen groups in the sample is TEGR.Finally,based on the results of biomisation analysis,pollen percentage,pollen concentration and pollen-climate index,the paleoclimate environment corresponding to the cultural layer of Xihoudu site is the temperate sparse forest grassland.