Paleowildfire Events And Its Climatic Implications In Middle Jurassic Coal Seams In Eastern Junggar Basin,Xinjiang

Wildfire is an important interference factor in the evolution process of forest and swamp system.The destruction of the original ecosystem by wildfire and the emissions of CO₂,NH₃,SO₂ and aerosols make it possible for the ecosystem to Leap or backward development,greatly changing the original evolution process of the biological community,even in a certain period change regional climate and environment.The study of ancient wildfire is of great significance to the understanding of climate change in geological history.Based on the study of coal petrology and geochemistry of Middle Jurassic Xishanyao Formation coal samples in Wucaiwan mining area,Junggar basin,this paper expounds the generation period and type of paleowildfire in the study area,and systematically analyzes the paleoenvironmental characteristics of paleowildfire and its impact on climate in combination with the evolution law of ecosystem and indoor test results.The discussion that inertinite is the product of incomplete combustion of plant body is accepted by more and more scholars.With it,the significance of indicating paleowildfire is also more prominent.In this paper,inertinite and polycyclic aromatic hydrocarbons(PAHs)were measured in 22 samples collected from the coal seam of Xishanyao Formation in Wucaiwan Mine Area,eastern Junggar Basin.The results show that the content of inertinite is 31.14%to 82.64%,the average is 55.78%,the average reflectance of the inertinite is 1.08%to 1.54%,and the high content of the inertinite is the main feature of the coal seam.The results of experiments show that the range of?PAHs is large among different layers(3881?6525 ng/g),with an average value of 4433ng/g,and the content of PAHs in different kinds of PAHs in the same layer is very different.The characteristics of inertinite,reflectance and PAHs content are analyzed comprehensively.It is concluded that the coal seam has experienced at least three frequent wildfires during the deposition period.According to the temperature interpretation of the reflectance data of inertinite,the wildfires in these three periods are all low temperature ground fires.By establishing the correlation between wildfire and other indexes(Vitrinite/Inertinite,the proportion of dry forest swamp facies,the proportion of mesophyte and the proportion of tropical-subtropical climate)recorded in the vertical direction of ultra thick coal seam,it is concluded that wildfire frequency has a great correlation with the degree of environmental dryness and wetness.At the same time,the reconstruction of atmospheric oxygen content during the Middle Jurassic Xishanyao Formation deposition period by inertinite content in coal shows that the atmospheric oxygen content at that time was as high as 28.1%.Finally,from the perspective of global carbon cycle,the possible impact of wildfire on climate and environment is analyzed.We think that under different conditions,the impact of wildfire on climate is different.When the carbon cycle between the biosphere,lithosphere and atmosphere is smooth,wildfires release CO₂ and other greenhouse gases by burning vegetation,increase the carbon transport to the atmosphere,enhance the greenhouse effect,thus expanding the surface vegetation coverage,and a large number of vegetation makes the atmospheric oxygen content rise,realizing the positive feedback mechanism between wildfires and the environment.However,a large amount of coal deposited in the Middle Jurassic strata in the world indicates that peat swamps were developed in the world during this period.A large amount of carbon stored in the lithosphere compressed the carbon storage in the biosphere and the atmosphere.The biosphere began to play a dominant role in the active fixation of carbon.The concentration of atmospheric CO₂ decreased and the greenhouse effect decreased.At this time,a large number of greenhouse gases produced by wildfire will still aggravate the greenhouse effect,and the damage to large area of vegetation will directly weaken the absorption of CO₂ by vegetation.Wildfire plays a role in maintaining greenhouse effect.