Organic Carbon Isotopic Composition Of Harbin Loess Since The Mid-pleistocene And Its Paleovegetation Significance

Loess is a thick layer of yellow silt deposited by wind transport during the Quaternary.Loess is the most abundant information base for the evolution of geological environment in Quaternary because of its wide distribution,continuous deposition and long time span.Studying the organic carbon isotope composition of the loess-paleosoil sequence has great significance for the reconstruction of paleo-vegetation and paleo-climate.However,for Northeast China,the research on the organic carbon isotope composition of Harbin loess,it has not been started,which hinders our understanding of vegetation change and its dynamic relationship with paleoclimate based on the carbon isotope composition in this region Therefore,in this article,the organic carbon isotope composition,Optically Stimulated Luminescence,(OSL)and Electron Spin Resonance,(ESR)of the Harbin Huangshan core loess-paleosoil sequence are studied,revealing the paleoenvironment of the Harbin area since the Middle Pleistocene.And the evolution of ancient vegetation and its driving mechanism,mainly draw the following conclusions:(1)Based on the OSL and ESR dating data and the deep-sea oxygen isotope stack comparison model,the age frame of Huangshan core is established,and the bottom boundary age of loess is about 0.46 Ma.(2)According to the magnetic susceptibility test,the magnetic susceptibility of Harbin area is different from that of the Loess Plateau.The average magnetic susceptibility of the loess layer and paleosol layer are 33.7×10^–8m³·kg^–1and 22.9×10^–8m³·kg^–1,the magnetic susceptibility of the loess layer is higher than that of the palaeosoil layer,and presents the periodic fluctuations in the glacial-interglacial period.Therefore,the magnetic susceptibility of the Harbin area can effectively show the change of the summer monsoon.(3)The range of organic carbon isotope composition of the loess-paleosoil since the Middle Pleistocene in the Harbin huangshan core is from–26.6‰to–21.3‰,with an average value is–25.0‰.The average values of organic carbon isotopes in the loess layer and paleosol layer are–25.1‰and–24.8‰,respectively.The loess layer is 0.3‰lower than the paleosol layer,but the S0 and S2 paleosol layers are obviously lower.(4)The relative abundance of C₃/C₄ plants in the past was estimated using the binary mixture model.It is revealed that the vegetation type in Harbin area is C₃ plants with absolute predominance.(5)Comprehensive comparison of deep-sea oxygen isotope,sea level temperature and organic carbon isotope composition of Chifeng loess showed that the organic carbon isotope of glacial loess was negative and the organic carbon isotope of interglacial paleosol(such as S1,S3 and S4)was positive,indicating temperature It is the main controlling factor for changes in plant carbon isotope composition in Harbin.However,in certain periods,such as the middle of S0,S2 and S3 paleosol,the organic carbon isotopic composition was significantly more negative than that of the adjacent loess layer,indicating that precipitation had an effect on the change of organic carbon isotopic composition.The synergistic changes of temperature and precipitation during interglacial period controlled the variation of organic carbon isotopic composition of loess-paleosol in Harbin,which reflected the complexity of plant growth response to climate in Harbin.