Grain-size And Morphological Characteristics Of Micro Charcoal In Ganzi Loess And Its Environmental Significance Since The Last Interglacial

Microcharcoal refers to the black inorganic carbon compounds produced by incomplete combustion of plants,which is one of the ideal indicators to indicate natural fire events.By analyzing the frequency and intensity of fire and its relationship with climate and vegetation types,it is important to understand the occurrence and development of fire on different time scales and explore the relationship between carbon and surface vegetation.So far,however,the differences in statistical methods and environmental indicators of different regions and their influencing factors need to be further studied.Based on the Ganzi loess sequence in the Western Sichuan Plateau,this paper carried out the detailed identification and grain size statistical analysis of the microcharcoal,discussed the characteristics of the concentration,grain size distribution and morphological changes of the microcharcoal since the last interglacial,and discussed the fire evolution history and paleoenvironmental significance of the area by the microcharcoal in combination with a variety of environmental proxy indicators.The main results include the following three aspects :(1)The results show that the frequency distribution curve of each sample basically presents a single peak lognormal distribution,and the content of microcharcoal distributed at both ends is very low,mainly concentrated in 10-30 ?m;the cumulative curve is a relatively smooth "s" shape,the change rate of the first and last segments is very low,and the change rate of the curve in the middle is very large;according to the accumulation of the number of microcharcoal,the stability of various grain size parameters of microcharcoal is discussed.It is found that the average grain size,median and distribution range of microcharcoal tend to be stable when the number of statistical particles increases to 1000.(2)According to the length/width ratio(Lo / Ro)of microcharcoal,two shape types of microcharcoal can be divided: sub-long(> 2.5)and sub-round(< 2.5),which are used to indicate that microcharcoal are the products of combustion of herbaceous and woody vegetation respectively.From the experimental results,the content of sub-round microcharcoal in each sample is much higher than that of sub-long microcharcoal,which can account for about 80% of the whole sample.The results show that the ratio of local and regional natural fire is less.According to the ratio of La/Sm,the ratios of local and regional natural fires were studied.The results show that: the carbon content of less than 50 ?m is more than 90%,that is,the microcharcoal in Xinshi samples are mainly small particles.The statistical analysis of the grain size of microcharcoal in the Xinshi loess section of Ganzi shows that the average grain size of the microcharcoal varies from 18.8 to 28.2 ?m.And the average grain size is 24 ?m.The kurtosis of the frequency distribution curve of microcharcoal grain ranges from 2.6 to 4.0,the skewness ranges from-0.54 to-0.13,and the grain size of microcharcoal records is generally negative(slightly fine).The standard deviation changes between 0.61 and 0.73,which indicates that the separation of microcharcoal is good,indicating that the transport medium and moving force of microcharcoal are relatively stable.(3)From the whole profile,the microcharcoal concentration fluctuates with the depth of the profile,and the peak value appears in the paleosoil development period(interglacial period),and the content of microcharcoal of greater than 50 ?m is greater,but the concentration is lower in the loess accumulation period(glacial periode).The reasons for the difference of microcharcoal concentration in different geological periods are mainly related to climate and surface vegetation.In the period of warm and humid climate,the vegetation is well development,and the occurrence of fire events with appropriate or even low frequency will lead to high microcharcoal concentration;in the dry and cold glacial period,although it is beneficial to the occurrence of natural fire events,there is no biomass needed for combustion,which lead to the amount of microcharcoal produced after the fire event is less.Therefore,the change of the environment during the glacial-interglacial period has an important influence on the evolution history of fire in Ganzi area.The microcharcoal can effectively indicate the history of the fire events at orbital scale,and other environmental index can be used to indicate the paleoenvironmental evolution process in the study area.The study reconstructed the evolution history of ancient fire in the eastern part of Qinghai Tibet Plateau since the last interglacial period by microcharcoal record,which is of great significance to understand the environmental change process in the eastern Tibetan Plateau.