Distribution Of Black Carbon In Subarctic Pacific Sediments And Reconstruction Of Late Quaternary Wildfires

Black carbon(BC)is mainly produced when biomass,biofuels and fossil fuels are incompletely burned.Because of its stable chemical properties and inertness,BC produced by combustion can be transported through the atmosphere or water for a long distance and widely exists in the environment,BC is widely used as a good proxy for reconstructing historical records of wildfires due to the fact that humans use biomass as the main source of combustion during geological periods and the inertia of BC.Wildfire,as one of the key factors,has a significant impact on the global climate by influencing radiation forcing and changing surface albedo.However,there is no clear conclusion about the past fire activity patterns and driving forces in the current research.In the glacial-interglacial cycle,long-term and high-resolution wildfire records from subarctic Pacific sediments are still scarce,and the mechanisms of wildfire occurrence are still elusive.The Pacific Ocean,as the largest ocean,plays a very important role in global climate change and is one of the main sources of BC emission.In this study,we use the surface sediments from the Pacific marginal sea to study the spatial distribution characteristics of BC in the North Pacific Ocean,analyze the source apportionment of BC with the value of char/soot in the sediments,and explore the emission characteristics of BC in modern combustion sources.The study will provide the basis for further study on the biogeochemical process of BC in sediments.Then the sediment core LV76-18 in the subarctic Pacific Ocean was taken as the research object.We established its age model,analyzed BC,char and soot in the sediments,and reconstructed the wildfire records of the past 320ka.The main factors affecting the occurrence of wildfire were explored,and the main mechanism of wildfire occurrence was studied on the earth orbit scale.The main conclusions of this study are as follows:The study on the spatial distribution of BC in the surface sediments of the Pacific marginal seas shows that the overall spatial distribution characteristics are obvious,and the pollution level from the near shore to the center of the sea area gradually decreases,and the pollution near the coast is the most serious.The mean value of BC in bering Sea surface sediment samples was 0.71±0.40 mg/g,and char was 0.14±0.11 mg/g and 0.58±0.34 mg/g respectively.The soot values of BC,char and soot were 0.59±0.18 mg/g,0.15±0.10 mg/g and 0.45±0.18 mg/g,respectively.In the Sea of Japan,the pollution is relatively serious.The concentration of BC ranges from 1.10 to 1.52 mg/g,with an average of 1.22±0.17 mg/g.The average values of char and soot were 0.20±0.14 mg/g and 1.02 ± 0.15 mg/g.By analyzing its source apportionment,BC in Japan Sea mainly comes from diesel exhaust emissions.The pollution sources of Bering Sea mainly come from gas emissions.The sea of Okhotsk mainly comes from the combustion pollution of gasoline and diesel oil.The above studies show that the pollution of BC in the Marginal sea of the Pacific Ocean mainly comes from human source pollution.In LV76-18 sediment samples,the contents of BC,char and soot showed a consistent trend.The content of soot is richer than that of char,and the maximum values of mass accumulation rate(MAR)appear in MIS2 period,in which the value of BC ranges from 0.08 to 15.53 mg cm-2 ka-1,char ranges from 0.03 to 6.6 mg cm-2 ka-1,and soot ranges from 0.05 to 8.86 mg cm-2 ka-1.About 90%of BC sources in the subarctic Pacific Ocean come from wildfire burning and long-distance atmospheric transmission in Siberia.The wildfire records show that there is a significant difference in wildfire activity patterns between glacial and interglacial periods.During the warm and humid interglacial period,although there are some small fluctuations,the occurrence of wildfires is relatively stable and the incidence rate is low.However,the incidence of high-intensity wildfires in the glacial periods is high.Drought mainly increases the fire risk by reducing fuel moisture,and drought regulated by ice volume affects the occurrence of wildfires.In addition to climate factors,the available fuel load also has a crucial influence on the development of wildfire.The spectrum analysis show that wildfire has a precession period of 23ka,which proves that the fundamental driving force of fire activity is the climate change caused by the change of the earth’s orbit,and there is a close relationship between wildfire and climate.