The Reconstruction Of Wildfire Since The Last Glacial Period Of The Huangtu Section Of Daijie Village,shangdan Basin

The Shangdan Basin is located in the south of the Qinling Mountains and belongs to the subtropical-wet monsoon sensitive area.It is the dividing line between wet and sub-humid areas.The region is also one of the birthplaces of Chinese culture.At the same time,research on climate and vegetation changes in the geological history has achieved certain results.Therefore,this area provides an ideal place for us to study the temporal and spatial evolution of biomass burning and wildfire frequency.Based on the field history of the wildfire in the Shangdan Basin,this paper selects the typical loess-paleosol profile of Daijie Village(DJ)in Shangdan Basin as the research object.The environmental proxy indexes such as magnetic susceptibility(MS),black carbon(BC),total organic carbon(TOC),particle size and heavy metal elements of DJ profile samples were measured and analyzed.Trying to reconstruct the historical sequence of wildfire since the last glacial period in the region;exploring the relationship between wildfire-climate change-human activities since the last glacial period in the Shangdan Basin;revealing the temporal and spatial evolution of wildfire frequency in the Shangdan Basin since the last glacial period and its climate aridification response.The main conclusions of this paper are as follows:(1)The black carbon stored in the loess-paleosol profile records the history of fire evolution and its environmental changes since the last glacial period.The analysis and statistics of environmental substitution indicators such as black carbon,magnetic susceptibility,total organic carbon analysis and archaeological data were recorded in the loess-paleosol section of Daijie(DJ)in Shangdan Basin.The results show that since the last ice age(~18000aB.P.),the climate in this area is cold and dry,showing desert grassland landscape,wildfire activity rarely occurs;with the chilling temperature and precipitation and vegetation biomass gradually increase,wildfire occurs In the middle of the Holocene,the climate was warm and humid,and deciduous broad-leaved forests dominated by Pinus,Amaranthus,and Amaranthus were gradually formed,and the incidence of wildfires showed a downward trend.However,in the 6000 aB.P.~5000aB.P.and 4200 aB.P.~4000aB.P.stages,the biomass burning increased significantly,which may be related to the ancient humans burning wood deforestation and reclamation farmland during the Yangshao culture and the Longshan culture period.Related activities;in the late Holocene,the climate was further cold and dry,with a relatively obvious peak at 2000 aB.P.~1500aB.P.,which may be related to the strengthening of farming activities in the area since the Western Han Dynasty.It can be seen that since the last glacial period,the wildfire occurrence mechanism in the Shangdan Basin has been transformed from climate-based to human-oriented.(2)This paper compares the DJ profiles of the Shangdan Basin in the upper reaches of the Yangtze River Basin with the YGZ profiles in the southern part of the Weihe River Basin.The results show that the wildfires and natural evolution processes in the north and south of the Qinling Mountains are significantly different.During the accumulation period of the Malan loess,the low carbon black values of the DJ and YGZ profiles indicate that wildfire activity has rarely occurred since the last ice age,from the southern part of the Weihe River basin to the entire desert grassland environment in the upper reaches of the Yangtze River.During the mid-Holocene warm period,the natural wildfires in the upper reaches of the Yangtze River Basin and the vast areas in the southern part of the Weihe River Basin showed a downward trend.However,the DJ profile and the YGZ profile showed a significant peak in black carbon concentration during the 6000 aB.P.~5000aB.P period.The Shangdan Basin may be related to the human slash-and-burn and the burning of the pottery industry in the Yangshao culture period.In the southern part of the Weihe River Basin,the urban settlements began to take shape and the biomass burning reached the highest level.The climate in the late Holocene became more arid,and the wildfires in the Shangdan Basin were different from those in the Guanzhong Basin.However,during the period of 4200 aB.P.~4000aB.P.,the black carbon in the Shangdan Basin was significantly higher,which may be directly related to the human social production activities in the Longshan period of the study area;the black carbon and charcoal in the Guanzhong Basin during this period High values occur,when urban settlements are declining,and dry and cold climates lead to increased biomass burning.In the Yangguanzhai area of the Guanzhong Basin,the concentration of charcoal has declined in the past 3,000 years,indicating that the regional environment has deteriorated and the agricultural landscape has been basically established.The black carbon concentration in the Shangdan Basin increased significantly,indicating that the biomass burned to the highest level.This is because of the promotion of agricultural technology during the Western Han Dynasty(2000aB.P.~1500aB.P.),a large number of burned forests and deforestation,and the development of agricultural operations.(3)Through the analysis of the environmental proxy indicators and archaeological records of the DJ section of the Shangdan Basin,the preliminary judgment of the DJ profile wildfire event is provided,which provides an important basis for further understanding the wildfire history in the region.Through the identification of macroscopic stratigraphic features in the field,analysis of magnetic susceptibility,black carbon,total organic carbon,geochemical elements,archaeological ages,global climate events,and cultural heritage,a preliminary judgment of the DJ profile recorded 4 last ice glazes.Typical regional wildfire events since the beginning of the period are ~18000aB.P.,6000 aB.P.~5000aB.P.,4200 aB.P.~4000aB.P.,2000 aB.P.~1500aB.P.,both periods Climate change and human activities that have occurred since the last ice age are closely related.(4)The change of black carbon concentration in the sediments of the profile has an important indication for the evolution of production mode and the intensity of human activity.In this paper,black carbon is used as the entry point,combined with magnetic susceptibility,total organic carbon and archaeological records,to explore the relationship between wildfire events and human activities and natural environment changes since the Holocene.Studies have shown that since the Holocene,the probability of wildfire occurrence has a significant nonlinear relationship with the intensity of human activity.Since the early Holocene,a series of ancient cultural and social successions and changes in human land development and utilization in the Shangdan Basin have been closely related to wildfire activities and changes in climate and environment.In the mid-Holocene environment,the warm and humid climate,superior hydrothermal conditions and high-quality soil development,the traditional agricultural operations are in a period of prosperous development.At the same time,human-induced wildfire spreads frequently occur.In the late Holocene,the climate was further aridized,and activities such as deforestation and land reclamation were intensified,resulting in a further deterioration of the environment in the region.In the face of increased population and degraded land resources,the development of agricultural operations has been extensively opened since the Shang Dynasty.Material combustion reaches a higher value.There are 28 pictures,9 tables,129 references.