The Historical Reconstruction Of Holocene Fire In Ningshao Plain And Its Relationship With Climate Environment And Human Activities

The Holocene climate is warming rapidly,ancient human activities have increased,and climate and human activities have had a double impact on the ecological environment such as fire and vegetation evolution.However,it is difficult to distinguish between natural and man-made fires.The unique geography and historical conditions of the Ningshao Kuahuqiao area provide an ideal place for carrying out research on the relationship between vegetation change and fire activity-vegetation climate-human activity.This research combined modern processes and research methods of environmental historical reconstruction to conduct a combustion experiment on modern vegetation in the study area,extracted and observed the charcoal,and measured the black carbon isotope composition(δ¹³C_BC)at the same time to obtain end-member values for the sedimentary strata in the area.On this basis,the typical sedimentary profile of the area was selected for the charcoal analysis of the Kuahuqiao profile（KHQ14?15）and the Hemudu profile（HMD1401）.To explore the history of Holocene fire evolution in the Ningshao Plain and its coupling relationship with climate,environment,vegetation and human activities,we reached the following conclusions:Theδ¹³C_BC of modern C3 and C4 plants in the Ningshao Plain area is-29.11‰and-13.25‰,respectively.The morphological characteristics of the charcoal after simulated crushing and deposition can still retain the original characteristics.The average L/W of herbaceous charcoal is 4.73,and the average L/W of arbor leaf charcoal is 1.93.The total organic carbon content（TOC%）of KHQ14?15 profile and its carbon isotope(δ¹³C_TOC)measurement results show that there are large fluctuations in carbon isotope values around 7700 cal a BP.It can be speculated that changes in the sedimentary geomorphic environment and the impact of human activities have led to organic matter input.The results of the black carbon content（BC%）and its carbon isotopes in the profile showed that the vegetation was sparse and the fire activity was weak during the last glacial period;the Holocene fire activity was generally enhanced,and the burning vegetation was mainly C4 herbs.About the black carbon and charcoal result,there is a peak at 8650 cal a BP,theδ¹³C_BC is positively deviated,and the comprehensive spore-pollen data infer that the environment has evolved from tidal flat deposition to estuary deposition;7700 cal a BP,the Kuqhuqiao ancestors entered the agricultural development period,and the use of fire for reclamation may cause fire.The main reason for the unusually high activity.In Hemudu area,before 9500 cal a BP,the black carbon content was relatively low,and theδ¹³C_BC was slightly negative.The study believed that the climate in this period was cold,the frequency of fire was low,and the burning vegetation was mostly C3 plants.After 9500 cal a BP,the fire activity increased and the number of C4 plants burned gradually increased,which may be caused by the rising temperature.In the early stage of Hemudu culture（7010～6500 cal a BP）,fire activity was enhanced,and the dominant burning vegetation types changed alternately,which may be related to the agricultural activities of the Hemudu ancestors.Since the late Holocene,due to the problem of sample resolution,the black carbon content has only appeared the maximum value of the whole sequence at 3500 cal a BP,and the corresponding black carbon isotope value is very negative.It is preliminarily judged that this may be the result of the occurrence of high-intensity large-scale fire events caused by the sudden deterioration of climate and the burning of a large number of C3 woody vegetation.Compared with the last glacial period,the fire activity of the Holocene Ningshao Plain was significantly enhanced,but the occurrence of larger fire events was significantly different in time,which may be caused by the influence of local geomorphic environment and human activities.In addition,the types of main burning vegetation in two different profiles at the same period are different.The results of this paper can provide a basic theoretical basis for decision-making for ecological environment construction and sustainable development under the background of global warming and enhanced human activities in the future.