The Distribution Of Organic Carbon And Clay Minerals In A Typical Loess-paleosol Profile And Their Paleoclimatic Significance

The loess-paleosol sequence has been considered as an important information carrier recording changes in the global environment and climate of the Quaternary peroid.In this paper,a typical loess-paleosol sequence in the south of the Loess Plateau was selected as research object,and the distribution characteristics of clay minerals and organic carbon in the loess-paleosol profile have been investigated.Besides,the correlations between clay mineral chricteristics and magnetic susceptibility,and between organic carbon chricteristics and paleoclimate indexes,such as soil grain size and CaCO₃,were also analyzed to explore the significance of clay mineral and organic carbon charicterisitcs in indicating the paleoclimate changes of the Loess Plateau in the Quaternary,based on previous studies investigating paleoclimatic and paleoenvironmental evolution in the Quaternary and changes of organic carbon charicteristics in modern soil under different climatic conditions.The results can provide new evidence to reconstructing the evolution of climate and environment in the southern part of the Loess Plateau since the Pleistocene.The main results were as follows:?1?All the loess and paleosol layers of Chunhua profile had the same clay minerals,but relative contents of the clay minerals varied significantly from layer to layer.During the period from the early Pleistocene to the mid-middle Pleistocene?WL-3^S5?,the clay minerals were composed mainly of illite-montmorillonite-chlorite-kaolinite-vermiculite,and this period could be divided into two stages according to the changes in relative contents of the clay minerals.From the early Pleistocene to the early middle Pleistocene?WL-3^L11?,the relative content of illite was comparatively low while the relative content of chlorite was quite high,which indicates that a cool temperate climate prevailed in this period.However,in the mid-middle Pleistocene?S10^S5?,the relative content of illite increased,while the relative content of montmorillonite and chlorite decreased,which indicates that the climate was relatively warmer and more humid in this period.In the late middle Pleistocene?L5^S1?,the composition of clay minerals turned to be of illite-chlorite-vermiculite-kaolinite-montmorillonite,and the relative contents of illite and chlorite increased,implying a dry-cold climate dominated this period.Therefore,the changes in relative contents of the clay minerals in the Chunhua loess-paleosol profile indicate that from the early Pleistocene to the late middle Pleistocene,the climate generally became dry and cold,and experienced changes from cool temperate to warm wet and to cold dry.The ratio of illite to chlorite?I/C value?and illite crystallinity?IC value?also exhibited phased variations from the bottom to the top in the Chunhua profile,which increased first and then decreased.The distribution of I/C ratio and illite crystallinity indicates that the climate in the Pleistocene underwent an evolutionary process similar to that of the clay minerals in relative content.?2?In the Chunhua loess-paleosol profile,three paleoclimate proxies,grain size and CaCO₃,show good indication of paleoclimate changes in the Quaternary.Soil clay content and median grain size???were higher in paleosol than in the underlying loess,and psotive correlation exsited bettwen the two proxies,while CaCO₃ content showed an opposite tendency.The higher clay content and median grain size???in paleosol reflect warm and humid climate,but the higher CaCO₃ content in loess reflects cold and dry climate.All of the three paleoclimate proxies reflect that in Chunhua area,the climate generally became dry and cold since the early Pleistocene,and experienced cold dry-warm wet paleoclimatic cycle.?3?The total content and density of soil organic carbon,as well as the ratio of mineral-associated organic carbon in total organic carbon?MOC/TOC ratio?,were generally higher in paleosol layers than in the underlying loess layers of the Chunhua loess-paleosol profile.But in the soil layers below S8,the contents of total organic carbon?TOC?and mineral-associated organic carbon?MOC?tended to be stable,while MOC/TOC ratios changed more obviously.?4?Regarding to the distribution of organic carbon functional groups in Chunhua loess-paleosol profile,alkyl carbon was observed in all loess and paleosol layers,while alkoxy carbon exsited in most soil layers,and aromatic carbon and carboxyl carbon only appeared since the last interglacial period.?5?Based on the comparative analysis of organic carbon charictersitics and paleoclimatic proxies,we find that compared to total content and density of organic carbon,MOC/TOC ratios showed more significant correlations with the changes of clay content,Md???and CaCO₃ content in the Chunhua profile,which were positively correlated with the former two proxies and negatively correlated with the last one,and the correlation coefficients in the whole loess-paleosol profile were 0.54,0.59 and-0.71,respectively,while in strata below S8,the correlation coefficients were 0.63,0.68 and-0.75,respectively.Our results indicated that MOC/TOC ratios in the Chunhua loess-paleosol profile correlated with the cold dry-warm wet paleoclimatic cycle in the Quaternary.The higher MOC/TOC ratios in the loess-paleosol profile may reflect warm and humid climate,while lower ratios indicate relatively cold and dry climate.Compared to TOC,MOC/TOC ratios had greater significance in indicating paleoenvironmental evolution in the Quaternary on the Loess Plateau.?6?The results of this study indicate that clay mineral charicteristics?relative content,I/C value and IC value?and MOC/TOC in loess-paleosol profile can offer new evidence to reconstructing paleoenvironmental changes in the Quaternary,and also provide a basis for predicting responses of soil organic carbon pools to vegetation and climate changes in the future.Among these indexes,clay mineral charicteristics can better reflect the phased paleoclimate changes,while MOC/TOC ratio can better reconstruct paleoclimate evolution at a long time scale.Therefore,through investigating the charicteristics of both clay minerals and organic carbon,we find that the paleoclimate changes in Chunhua area changed from cool temperate to warm wet and then to cold dry,and generally became dry and cold from the early Pleistocene.