Idntifying Pedogenic Magnetic Components In Loess Using Isothermal Remanent Magnetization Acquisition Curves

Loess sediments have the characteristics of good continuity,high deposition rate,and fine grain size,which contain rich paleoclimate information.In the past 30 years,environmental magnetism has been one of the important methods to study the paleoclimate information of loess,and much great progress has been made in using magnetic susceptibility as a proxy climate indicator for the study of summer monsoon.However,the magnetic susceptibility is a comprehensive parameter that includes the contribution of the detrital component in the source region and the pedogenic component of the sedimentary region.Therefore,extracting and distinguishing the magnetic signals of the detrital and pedogenic components is related to the accurate interpretation of paleoclimate information of loess.In the mid-latitudes of the northern hemisphere,there are thick layer of loess deposits,such as Xinjiang and the Chinese Loess Plateau in China,and southern Siberia,Russia,which are good carriers for studying paleoclimate of loess.In this paper,the loess sections of Bole in Xinjiang,Xifeng in Gansu and Kurtak in Siberia are selected as the research objects.These loess sections are distributed successively in arid,semi-arid and humid regions.Their soil development degree has obvious change trend from weak to strong,which can help to extract and distinguish the pedogenic and detrital components in the loess.The research methods are mainly magnetic un-mixing,combined with room temperature magnetic parameters,CBD?citrate-bicarbonate-dithionite?method,and diffuse recflection spectrum?DRS?to identify the magnetic signals of different magnetic components in the loess,and these methods are applied to study the separation of magnetic components and its environmental significance in the Jiuzhoutai section of Gansu province.In the arid region of Xinjiang,Northwest China,the Bole section,with negligible soil development has loess with only component 2 centered at⁸0 mT.In semi-arid central China,the Xifeng section has both loess and paleosol with three coercivity components centered at²6 mT,⁸2 mT,and⁹60 mT.Component 1 has a pedogenic origin,and the remanence contribution(M_ri)increases in a positive linear relationship with the intensity of pedogenesis.In the humid region of Siberia,the Kurtak section has three coercivity components are centered at²3 mT,⁷8 mT,and¹014 mT.The M_ri of component 1 shows a low correlation with the intensity of pedogenesis.Component 3 is characterized by high-coercivity minerals and its M_ri is related to the intensity of pedogenesis.The pedogenesis in the semi-arid Xifeng section tends to produce massive fine-grained ferromagnetic minerals,so the M_ri of component 1 is positively correlated with the intensity of pedogenesis.The humid pedogenic environment of the Kurtak section is prone to form high-coercivity minerals and destroys fine-grained ferromagnetic minerals,so the M_ri of component 3 increases with the degree of pedogenesis.In the western Loess Plateau and desert staggered Jiuzhoutai section,the soil development is weak,and the loess-paleosol is dominated by detrital components,but the paleosol has pedogenic components that reflect the intensity of pedogenesis.Since the L6layer,the relative content of the detrital component and the background value of the magnetic susceptibility of the section show an upward trend,the result show that the increase of relative content of detrital component dominates the enhancement of the background value of the magnetic susceptibility of the section.The detrital component of the loess is derived from the dust source region,which indicates that the long-term trend of the background value of the magnetic susceptibility of the section reflects the strengthening of winter monsoon.The magnetic component in loess indicates different environment significance under different climatic condition.The increase of magnetic susceptibility of Xifeng section is caused by the increase of pedogenic component 1,so its M_ri value is positively correlated with pedogenic strength.The humid pedogenic environment of Kurtak section will destroy the pedogenic component 1 and transform it into component 3,resulting in the decrease of magnetic susceptibility.The increase of magnetic susceptibility of the paleosol layer in the Jiuzhoutai section is caused by the pedogenic component 1,but the rising trend of the background value of the magnetic susceptibility of the section is due to the increase of the relative content of the detrital component 2,indicating the continuous enhancement of the winter monsoon.