Charcoal Characteristics And Paleoenvironmental Significance In Xiashu Loess In Yangzhou Area

Fire is a unique and important ecological factor in the earth’s ecosystem,which plays an important role in the global ecosystem in the past and now,and is of great significance to understanding climate change and vegetation evolution.China’s Yangtze River Delta is the key zone of the eastern monsoon region,and it is very sensitive to the evolution of the East Asian monsoon environment.Although different scholars have done a lot of research on chronology and environmental evolution in loess strata in this area,the research on ancient fire history and vegetation evolution and the relationship between ancient fire,vegetation and climate in this area is weak,which needs further study.According to the stratigraphic age of Xiashu loess in Yangzhou area,this study selected Xiashu loess in Yangzhou area of Yangtze River Delta as the research object,collected loess samples from Shishiwa profile and Tumenjin profile in this area,and established the chronological framework of Xiashu loess in Yangzhou area by means of photoluminescence dating and electron spin resonance dating.In this paper,the characteristics of charcoal in two profiles are studied,and the relationship between the changes of charcoal characteristics and paleoclimate proxy indexes such as organic carbon isotope,grain size,environmental magnetism and major elements is explored,which reveals the evolution history of paleofire in this area,and further discusses the relationship between paleofire activity,vegetation succession and climate change.Through multi-index comprehensive analysis,the characteristics of paleoenvironment evolution in the study area since the Middle Pleistocene are reconstructed,and compared with other geological records in the world,the relationship between paleoenvironment evolution in the study area and global climate change is explored.This study mainly obtains the following understandings:（1）The charcoal records of Xiashu loess in Yangzhou area can reflect the evolution history of ancient fire in this area since the Middle Pleistocene,and its evolution characteristics are as follows:In the first stage（460～352ka B.P.）,the concentration of charcoal in loess was at the lowest level,with an average of only 1577 grains/g,which indicated that fire events were rare and the intensity was small.In the second stage（352～117ka B.P.）,the concentration of charcoal was the highest,with an average of3,465 grains/g,and the concentration of charcoal in Shishiwa profile reached a maximum of 6,768 grains/g,indicating that fire incidents occurred most frequently and had the strongest intensity at this stage.In the third stage（117～46ka B.P.）,the concentration of charcoal was at a low level,with an average of 1878 grains/g,and showed a gradual increase trend,reflecting the gradual increase of fire incidents.Compared with the magnetic susceptibility curve,it is found that there is a negative correlation between the intensity of fire events and the degree of climate wetness.The low level of fire events indicates that the climate is humid,whereas the high level of fire events indicates that the climate is dry.（2）In Xiashu loess in Yangzhou area,the content of charcoal with particle size>50μm is less,and the proportion of charcoal with particle size<50μm is higher.The aspect ratio（Lo/Ro）of charcoal ranges from 0.46 to 1.88,with an average value of 0.97,and the components with particle size<50μm are much lower than those with particle size>50μm,which indicates that the charcoal in the profile mainly come from regional fire events.In different periods of ancient fire history,the average value of Lo/Ro of charcoal>50μm in the first stage was 2.5,which showed an upward trend,indicating that herbaceous vegetation had expanded to some extent,indicating that herbaceous vegetation was dominant.In the second stage,the average value of Lo/Ro of charcoal>50μm was 5.9,indicating the continuous expansion of herbaceous vegetation.In the third stage,the average value of Lo/Ro of charcoal>50μm was 2.0,indicating that herbaceous vegetation dominated and changed into herbaceous-woody mixed vegetation.The results of organic carbon isotope composition characteristics show that the average relative abundance of C₄ vegetation in different ancient fire historical periods is close,about 11%,indicating that C₃ vegetation is the main vegetation type.（3）It is found that in the second stage of ancient fire history,the frequent occurrence and high intensity of fire events revealed by the change of charcoal concentration have a good corresponding relationship with the indicated cold and dry climate characteristics of the profile paleoclimate proxy indicators,such as median particle size,volume percentage of clay particles<4μm,total organic carbon and chemical weathering index.However,in the first and third stages of ancient fire history,the above-mentioned good corresponding relationship was not observed.Comprehensive analysis of the environmental evolution characteristics indicated by various proxy indicators,and preliminary induction of the environmental characteristics in the historical evolution of ancient fires in Yangzhou area are as follows:In the first stage,there was more precipitation,and the loess in Xiashu was strongly weathered into soil,with lush vegetation;In the second stage,the climate changed rapidly from wet to cold and dry,the herbaceous vegetation expanded continuously,and the weathering and pedogenesis weakened;In the third stage,the climate changed from cold dry to warm and humid,the precipitation increased gradually,the weathering and soil formation increased continuously,and the woody vegetation increased gradually.