Research On The Changes Of The Pediment Environment And Human-land Relationship Recorded In The Loess Profile In The Upper Reaches Of The Weihe River In The Holocene

Loess-paleosol sequence is considered as a good information earrier to record environmental changes in the past,which reflects the characteristics of pedogenic transformation and paleoclimate change information and is of great significance to the research of hot issues such as global change.In recent years,with the deepening of global change research,the research on the impact of Holocene climate and environment changes on the origin and survival of human civilization has also become one of the scientific issues that have attracted much attention.The upper reaches of Weihe River are located in the transition zone of three natural regions,namely,the humid region in eastern China,the arid region in northwest China and the alpine region of Qinghai-Tibet Plateau.They can respond sensitively to East Asian monsoon and global climate change.At the same time,the upper reaches of Weihe River is one of the important birthplaces of ancient civilization in our country,and cultural sites are widely distributed.These cultural sites provide favorable conditions for studying the trajectory of human activities during Holocene.This paper mainly focuses on the characteristics of pedogenic environmental changes since Holocene,and on this basis,discusses the relationship between Holocene environmental changes and human activities.Taking Fanjiacheng(FJC)loess profile in the upper reaches of Weihe River as the research object,the physical and chemical properties such as magnetic susceptibility,grain size and chromaticity were systematically tested indoors after detailed investigation and continuous systematic sampling in the field.Combined with optical emission light(OSL)dating data,stratigraphic correlation and analysis of cultural relics,the following main conclusions were obtained:(1)The complete stratigraphic sequence of Fanjiacheng loess section and its chronological framework have been established.According to the field macroscopic characteristics and indoor physical and chemical indexes of the loess profile,and in combination with OSL dating data,the Fanjiacheng loess profile 1s divided into:topsoil MS(1 500?0 a BP)-Holocene loess L0(3 100?1 500 a BP)-paleosol S0?(5000?3 100 a BP)-loess interlayer Lx(6 000?5 000 a BP)-paleosol S0?(8 500?6 000 a BP)-transition loess Lt(11 500?8 500 a BP)-Malan loess L1(??11 500 a BP)?(2)The physical and chemical properties of loess-paleosol in the upper reaches of Weihe River were analyzed and summarized.Physical and chemical indexes of different stratigraphic units in Fanjiacheng section are obviously different.The change trends of magnetic susceptibility,clay(<5 ?m),fine sand(5?10?m),clay-powder ratio,a*,loss on ignition,hygroscopic water and other indexes are consistent,while the change trends of coarse sand(10?50 ?m),sand(>50?m),L*and other indexes are opposite to those of the previous physical and chemical indexes.Specifically,the magnetic susceptibility of the loess layer ranges from 86.63?110.20×10-8m3·kg-1,with an average value of 96.53×10-8m3·kg-1.The magnetic susceptibility of the paleosol layer ranges from 90.57?116.27×10-8 m3·kg-1,with an average value(108.59×10-8m3·kg-1)significantly higher than that of the loess layer.The grain size is mainly composed of silty sand(5?50 ?m),with the content of the whole section ranging from 62.4%?77.10%,with an average of 68.60%,which is in line with the main part of eolian loess deposition,followed by clay(<5 ?m)content ranging from 11.40%?26.40%,and sand(>50 ?m)content with the smallest proportion ranging from 8.60%?19.90%.The L*and b*(56.21,11.99)of loess L1 are higher than that of paleosol S0(55.65,11.83).a*is opposite to the change trend of L*,b*,and paleosol(4.31)is higher than that of loess layer(4.22).The burning loss and hygroscopic water content of the profile are characterized by low loess layer(13.21,0.79)and high paleosol layer(16.64,0.98).The CaCO3 content varies between 11.61%?15.83%,with the mean CaCO3 content in paleosol layer being 12.70%and that in loess layer being 14.04%.It is worth noting that loess interlayer Lx exists in the middle of ancient soil,with magnetic susceptibility appearing trough,fine sand content significantly reduced,sand content significantly increased,L*value increased,and loss on ignition content significantly reduced,indicating that secondary climate fluctuations occurred during the formation of ancient soil,during which there was a climate deterioration stage characterized by drying and cooling,which was strongly affected by winter wind and weakened by weathering and soil formation.(3)The weathering intensity of loess in the upper reaches of Weihe River is discussed and revealed.The study of physical and chemical indicators shows that the intensity of weathering soil formation between different strata of FJC profile is obviously different.The physical and chemical indexes of Malan loess L1 all reflect the basic characteristics of eolian loess,indicating weak soil formation and transformation.During the formation period of transitional loess Lt,soil formation increased.Paleosol S0 has the strongest effect on soil formation and transformation.Comparative analysis shows that the physical and chemical indexes in the middle of paleosol layer have obviously fluctuated,forming loess interlayer Lx,which indicates that the pedogenic effect was weakened during this period.The loamy strength of modern loess LO is higher than that of Malan loess L1 but obviously weaker than that of paleosol S0.Therefore,the weathering intensity of soil in FJC section shows a law of gradual increase from Malan loess layer L1 to paleosol S0 and decrease from paleosol S0 to Holocene loess L0.(4)The climate change track since the late Pleistocene in the upper reaches of Weihe River has been reconstructed.According to the characteristics of soil formation,the climate change in the upper reaches of Weihe River since Late Pleistocene can be divided into the following stages:Late Pleistocene(?— 1150abp),the climate is cold and dry.In the early Holocene(11 500?8 500 a BP),the climate began to change to warm and wet,but it was still relatively dry and cold on the whole.In the early middle Holocene(8 500?6 000 a BP),the climate was warm and humid.In the middle of the middle Holocene(6 000?5 000 a BP),there was a climate deterioration period in which temperature decreased and precipitation decreased.In the late middle Holocene(5 000?3 100 a BP),summer monsoon dominated and the climate turned warm and humid again.In the late Holocene(since 3 100 a BP),due to the influence of global climate change,the pattern of east Asian monsoon changed and the climate changed to cold and dry.(5)The evolution of human-land relationship in the upper reaches of Weihe River during Holocene was preliminarily discussed.Climate environment is a prerequisite for the development of ancient human beings and culture.Especially in the Neolithic Age,when the productivity level was low,the development and decline of human beings and their culture were closely related to the changes of natural environment.It showed that the climate was dry and cold before 8 500 a BP and the intensity of human activities was very weak.During the period of 8 500?6 000 a BP,the climate was warm and humid,the animal and plant resources were abundant,the intensity of human activities was enhanced,the primitive settlement agriculture was developed,and the first-stage culture of Dadiwan and the early Yangshao culture were developed in the area.Against the background of the deterioration of the natural environment during the period of 6000?5 000 a BP,at the beginning of this stage,it stimulated the ancestors to continue the development and prosperity of Yangshao culture by improving production technology and tools.At the end of this period,the continuous deterioration of the climate environment exceeded the ability of human beings to resist natural changes,which eventually led to the decline of Yangshao culture.The 5 000?3 100 a BP climate has improved and the intensity of human activities has continued to increase.A prosperous Majiayao culture and Qijia culture have emerged and developed.After 3 100 a BP,the climate turned dry and cold,the primitive settlement agriculture gradually declined,and the region changed from primitive settlement agriculture culture to nomadic culture.