Fire History Of Late Holocene Recorded By Gonghai Lake Sediment And Its Relations With Climate Change And Human Activity

Fire is one of the factors that promotes ecosystem revolution;There are close and direct relations between fire and both climate change and human activity.Therefore,fire history reconstructing can help us to know that how spatial and temporal distributions of fire were influenced by climate change and human activity.Both two factors above have a key significance to understand the relation between human being and earth environment of the past.The researches about fire history have been processed on global scale.The proxies for fire history reconstructing contain charcoal,tree-ring fire scar,black carbon,polycyclic aromatic hydrocarbons?PAHs?and levoglucosan,etc.Different proxies were used to reconstruct fire history at a variable time scale ranging from hundreds of years to hundreds million of years,indicating how fire was influenced by climate change and some periods of longtime scale human activity.However,there are certain problems among existing fire-history researches,including that resolution is not high enough,how fire is influenced by climate change is not sufficiently clear,and the factor of human activity is not deeply researched.To explore and try to resolve these problems,we tried to reconstruct fire history in late Holocene?2014AD-800 BC,past 2800 a from now?by sediment of Gonghai Lake,taking PAHs as the proxy.The top 4.7 m sediment of Gonghai Lake was analyzed,PAHs were extracted by acetonitrile,and their concentrations in the extracted solution were measured by High Performance Liquid Chromatography.The concentrations of total 16 kinds of common PAHs in the environment?Take abbreviation as 16PAHs in this dissertation?were calculated as well as total PAHs fluxes.The former was judged to be better to mirror the fire history of Gonghai Lake area,by comparing these two curves with sediment rate curve,and according to 16PAHs'transportation patterns during depositing into Gonghai Lake.The total concentration of 16PAHs of all samples range from 92.03 ng·g^-1 to 1017.94 ng·g^-1.The lowest concentration appears in the time of Mid-Tang Dynasty,whereas the highest concentration was obtained from the toppest sample,which represents the nearest contemporary era.Certain relations between fire history with both climate change and human activity can be revealed by comparing concentration of 16PAHs with summer monsoon strength,paleotemperature,the population of north part of China,and the economic index.In the scale of hundreds and thousands of years,climate change and human activity were synergetic factors causing influences on fire history.In general,when climate was warm and the summer monsoon was strong,the population was synchronously large and the level of economic could be high at the same era.All of the situations above could increase the quantity of 16PAHs in sediments,indicating relatively high fire intensity.The average concentration of 16PAHs after the Reform and Opening-up Policy in China?1978 AD up to now?is 499.15 ng·g^-1,of modern times?1850¹978 AD?is 304.72 ng·g^-1,of Little Ice Age?1220¹850 AD?is 177.30ng·g^-1,of Medieval Climate Abnormal?910¹220 AD?is 246.85 ng·g^-1,of Sui and Tang Dynasty?581-910 AD?is 181.21 ng·g^-1,and before Sui Dynasty?580 AD⁷80BC?is 228.61 ng·g^-1.On decade scale,about the strength of summer monsoon,it appears that fire occurred more in dry events during wet periods and wet events during dry periods.Referring to temperature,high fire intensity often corresponded to high temperature in summer,while warm winter-half-year could also be accompanied with relative strong fire activity.Thereinto,the influence to fire by summer monsoon varies among different time scales,whereas the influence to fire by temperature shows to be consistent among different time scales.Turning to the aspect about the relation between fire and human activity,it appears that small population led to small influence on fire by human activity.With the population growing,the influence to fire by human activites became more and more significant.Despite the fire generated by arms and ammunitions in wars could discharge 16PAHs to the environment,it is not reliable to extract war events from fire history.The concentration of high mocular weight PAHs has strong positive cerrelation with population,and it can indicate social productive forces of different ages.When the population was large,the“natural fires”resulted from careless fire use by human beings may occupy high proportion among paleofires.