Fire is a key earth system process affecting ecosystems,land-surface properties,the carbon cycle,atmospheric chemistry,aerosols and human activities.Local fire history has been expensively reconstructed from charcoal records from lake sediments and enables us to infer the impacts of climate changes and human activities.However,long-term fire history at glacial-to-interglacial scale can be available from loess-paleosol sequences.Black carbon(BC) is a kind of material originating from biomass and fossil fuel combustion and can be used as the best indicator of paleo-biomass burning.BC is not one chemical compound or a group with well-defined characteristics,but a suite of compounds occurring along a “combustion continuum” that can be differentiated into two parts of carbon contents:combustion residues from pyrolysis and combustion emissions formed via gas-to-particle conversion,corresponding to char and soot.Char is composed mainly of micrometer particles and generally retains the morphology of its source material,and in which the countable part identified under the microscope(generally >5 μm) is called charcoal.Soot is composed mainly of submicron particles formed from the condensation of hydrocarbon radicals at high temperature(>600℃).Due to differences in the chemical and physical properties of char and soot and the resulting differences in lightabsorbing properties,the differentiation between char and soot in the environment would help us better understand their environmental and climatic impacts.Loess covers an area of about 440000 km~2 in the Loess Plateau of north-central China.Loess and paleosol consist of the sequences of the Loess Plateau and represent the different climate periods.Generally loess deposited in glacial periods,while paleosols deposited in interglacial periods.BC,char and soot contents in Lingtai and Luochuan loess sections are analyzed using the thermal optical reflectance(TOR) method to investigate the spatial-temporal changes in natural fires and its relations with the paleoclimate over the last glacial cycles.The average concentrations of BC,char and soot are 0.475,0.314,0.161 mg·g~(-1) in Lingtai sections,which is about 2 times higher than those in Luochuan section.This may be associated with the different biomass coverage in the two areas.The average char/soot ratios in the two sections are very close,varying from 2.42 to2.58.Vertical profiles of BC and char contents have similar trends in the two sections and show similar variations.High BC and char concentrations occurred in warmer and humid periods,while low BC and char concentrations occurred in colder and dry periods.This suggests that in such semiarid area in the Loess Plateau the main factor on biomass fires is from the availability of biomass fuels.In humid period with the increase of the biomass coverage the occurrence of biofires increases.Soot vertical profiles shows very different trends in the two sections during the last glacial.Soot can be used to indicate even broader-scale trends in fire activity because it is dispersed primarily through the atmosphere.Since fire occurrence is generally accompanied with dry conditions,the overall increase in soot concentrations in Lingtai section might be an indicator of gradual intensification of the drought in West China.The correlations of the magnetic susceptibility with BC and char in Lingtai section are better than that in Luochuan sections.The correlations of the magnetic susceptibility with char are better than those with BC in the two sections.This may suggest that BC and char may be the main sources of the loess magnetic susceptibility in the Loess Plateau since fires can produce high magnetic materials,while local fires appears to be the main contributor. Fire is a key earth system process affecting ecosystems, land-surface properties, the carbon cycle, atmospheric chemistry, aerosols and human activities. Local fire history has been expensively reconstructed from charcoal records from lake sediments and enables us to infer the impacts of climate changes and human activities. How long-term fire history at glacial-to-interglacial scale can be available from loess-paleosol sequences. Black carbon (BC) is a kind of material originating from biomass and fossil fuel combustion and can be used as the best indicator of paleo-biomass burning. BC is not one chemical compound or a group with well-defined characteristics, but a suite of compounds occurring occurring along a “combustion continuum” that can be differentiated into two parts of carbon contents: combustion residues from pyrolysis and combustion emissions formed via gas-to-particle conversion, corresponding to char and soot. Char is composed mainly of micrometer particles and generally retains the morph ology of its source material, and in which the countable part identifier under the microscope (generally> 5 μm) is called charcoal .oot is composed mainly of submicron particles formed from the condensation of hydrocarbon radicals at high temperature (> 600 ° C) .Due to differences in the chemical and physical properties of char and soot and the resulting differences in lightabsorbing properties, the differentiation between char and soot in the environment would help us better understand their environmental and climatic impacts. Loss covers an area of ​​about 440000 km ~ 2 in the Loess Plateau of north-central China. Loess and paleosol consist of the sequences of the Loess Plateau and the different climate period. Generally loess deposited in glacial periods, while paleosols deposited in interglacial periods.BC, char and soot contents in Lingtai and Luochuan loess sections are analyzed using the thermal optical reflectance (TOR) method to investigate the spatial-temporal changes in natural fires and its relations with the paleoclimate over the last glacial cycles. The average concentrations of BC, char and soot are 0.475, 0.314 and 0.161 mg · g -1 in Lingtai sections, which is about 2 times higher than those in Luochuan section. This may be associated with the different biomass coverage in the two areas. The average char / soot ratios in the two sections are very close, varying from 2.42 to 2.58.Vertical profiles of BC and char contents have similar trends in the two sections and show similar variations. High BC and cacophodisation in warmer and humid periods, while low BC and at high concentrations in colony and dry periods. This suggests that in such semiarid area in the Loess Plateau the main factor on biomass fires is from the availability of biomass fuels. In humid period with the increase of the biomass coverage the occurrence of biofires increases.Soot vertical profiles shows very different trends in the two sections during the last glacial.Soot can be used to ind icate even broader-scale trends in fire activity because it isially primarily through the atmosphere .ince fire is likely accompanied by dry conditions, the overall increase in soot concentrations in Lingtai section might be an indicator of gradual intensification of the drought in West China .The correlations of the magnetic susceptibility with BC and char in Lingtai section are better than that in Luochuan sections.The correlations of the magnetic susceptibility with char are better than those with BC in the two sections.This may suggest that BC and char may be the main sources of the loess magnetic susceptibility in the Loess Plateau since fires can produce high magnetic materials, while local fires appears to be the main contributor.