Evidences Of Widespread Wildfires In Coal Seams From The Middle Jurassic Of The Northwest China And Its Impact On Paleoclimate

Wildfires are an important part of the earth,s ecosystem,and have an important effect on the circulation of many elements in the ecosystem.The occurrence of wildfires is closely related to the type of climate,vegetation and combustibles.The gas produced by combustion will have an important impact on the climate and environment,and the change of climate will affect the growth of vegetation.During the Jurassic period,the atmospheric CO2 was obviously high and the climate change was obvious.However,there are few studies on the climate change in the Jurassic.Coal samples of the middle Jurassic strata in northwest China were collected in this study.Coal samples were collected from the Shendong mining area on the northern margin of the Ordos basin,the Binchang mining area on the southern margin of the Ordos basin,the Hami mining area on the Tuha basin,and the Fukang mining area on the southern margin of the Junggar basin.The evidence of wildfires from the coal samples in Jurassic in northwest China,types of wildfires,and the effects of wildfires on paleoclimate and coal-forming environment were studied using the knowledge of coal geology,coal petrology,mineralogy,petrology,element geochemistry and organic geochemistry.At the same time,according to the relationship between the content of inertinite in coal and the content of ancient oxygen,the oxygen content in the atmosphere of middle Jurassic is estimated.We had achieved the following results and understanding:Evidences of the occurrence of wildfires from the coal petrology are found in the coal samples from the middle Jurassic in Northwest China.A large number of macro-charcoal were found in the samples of this study,which is the product of incomplete combustion.Numerous studies have shown that the fusain component is equivalent to the macro-charcoal,and the inertinite group in coal is equivalent to the micro-charcoal.In the northwest area,the content of the inertinite group in the middle Jurassic coal ranges from 45%to 72%,which is much higher than that in the late Carboniferous and early Permian coal in north China.The difference of the content of inertinite in the coal indicates that the wildfires occurred more frequently in Jurassic than in late carboniferous and early Permian.Evidences of the occurrence of wildfires from the PAHs are found in the coal samples from the middle Jurassic in Northwest China.The most representative molecular markers produced by combustion are polycyclic aromatic hydrocarbons(PAHs),especially those over the 4-rings.3-5 cyclic aromatic hydrocarbons have been detected in all the coal samples in the study area,and some samples have detected the typical high temperature combustion product-coronene.Through the calculation of the ratio of polycyclic aromatic hydrocarbons isomers,the results showed that fluoranthene,pyrene,benzo(a)anthracene,indenopyrenem and benzoperylene were formed by the combustion.Oxygen-containing aromatic compounds,the products of incomplete combustion of organic matter,such as benzo-benzofuran,dino-benzofuran,and benzo-dibenzofuran,were also detected in the coal samples.These molecular markers indicated that extensive and frequent wildfire events occurred in northwest China during the middle Jurassic.The wildfires in the middle Jurassic in northwest China were mainly surface fires with medium and low temperature.The average reflectance of the inertinite in the middle Jurassic coal in northwest China is 1.77%,with the reflectance accounting for 75.96%between 0.7 and 2%.According to the relationship between the reflectivity of inertinite and the combustion temperature,Jurassic wildfires burning temperature was calculated,and the burning temperature was about 400OC.Observed under the scanning electron microscopy(SEM),layer between plant cells were homogeneous,suggesting that the burning temperature of wildfires was at least above 325?.The relative abundance of 5-ring and 6 ring aromatic compounds was low in all samples.Therefore,the main type of wildfire was low-temperature fire in the peat formation period,and the frequency of high-temperature fire was less.The climate of the Yan,an Formation in the early Middle Jurassic in the northern margin of the Ordos Basin was obvious changed.The values of CAI,CaO/MgO,Al2O3/MgO and Sr/Cu in the floor and roof of main coal seam of Yan,an Formation in the northern margin of the Ordos Basin have a tendency to increase first and then decrease.The mineral assemblage in the floor and roof of main coal seam changed from illite and chlorite to kaolinite,illite and montmorillonite,and then to illite and chlorite assemblages,which indicates that the climate was relatively dry and cold in the first and second periods of Yan,an Formation,and relatively warm and humid in the third and fourth periods of Yan,an Formation.However,in the fifth sedimentary period the climate became relatively dry and cold.The huge amount of CO2 released by wildfires during the Jurassic period may have contributed to climate change.During the coal-forming period in the early-middle Jurassic in northwest China,forest combustion released 5.56×1011 tons of carbon into the atmosphere,with the amount of 1.73×1012 tons of CO2,1.1×1011 tons of CO,and 1.0×1010 tons of CH4.Under the condition of conservative calculation parameters,the amount of CO2 released by forest fires in the peat swamp system of the Middle Jurassic in Northwest China alone was equivalent to 57.7%of the total CO2 in the atmosphere at present(3×1012t).A large amount of data showed that the global CO2 concentration in the early Middle Jurassic was on the rise as a whole,and the CO2 concentration was four times higher than today.Extensive and frequent wildfire events in the Middle Jurassic may be one of the reasons for the rise of CO2 concentration and the change of climate from cold and dry to warm and humid.During the middle Jurassic,the oxygen level in the atmosphere of the ordos basin was 27.9%,higher than the present level(21%).The content of inertinite in the coal from ranged from northwest China ranged 45%to 72%,with an average value of 55.76%.According to Glasspool's establishment of the relationship between the content of inertinite in coal and the atmospheric oxygen level,the atmospheric concentration in the Middle Jurassic of Ordos basin was calculated to be 27.9%.Macro-charcoal distributed in the roof and floor of coal seam indicates that water erosion occured in the burned area.Debris earried by rivers may caused river siltation and diversion.Abandoned river channels and backshore ponding areas may provide favorable sites for peat formation.