Magnetic Characteristics And Environmental Significance Of The Duanjiapo Red Clay At The Southern Edge Of The Loess Plateau

The loess-red clay sequence of the Loess Plateau China is rich in palaeoclimatic and paleoenvironmental information.It is one of the best terrestrial sedimentary materials for investigating the formation and evolution of the East Asian Monsoon,the uplift of the Qinghai-Tibet Plateau,and the aridity of Asia’s interior.With the establishment of a good comparison between the magnetic susceptibility of the loess-paleosol sequences and the deep-sea oxygen isotope records,the study of the environmental magnetism of the loess-paleosol sequences in China has entered a new stage,and a relatively complete system of magnetic indicators has since been developed.However,the applicability of the existing climate proxies in the red clay sequence is still controversial.Therefore,it is of great theoretical importance to apply environmental magnetism to correctly interpret the magnetic properties of the Late Miocene-Pliocene red clay sequences and their relationship with the climatic environment and then select reliable paleoclimate proxies to invert the paleoclimate changes.This paper studied the Duanjiapo red clay sequence at the southern edge of the Loess Plateau,and systematic magnetic parameters,geochemical elements,and diffuse reflectance spectroscopy measurements were carried out.The magnetic properties of the red clay sequence were analyzed by magnetic means,including the type,magnetic domain,and content of the magnetic minerals.The characteristics of the pedogenic environment of the red clay sequences during the deposition period were examined using non-magnetic methods.At the same time,an effort was made to investigate the environmental significance of the magnetic parameters of the Duanjiapo red clay sequences by combining the magnetic features and pedogenic environmental characteristics.Through the study of the above,the following revelations and conclusions are obtained:（1）The magnetic minerals of the Duanjiapo red clay sequence were consistent with the overlying loess-paleosol sequence.They were primarily ferrimagnetic minerals with low coercivity,such as magnetite and maghemite,while a certain amount of incomplete antiferromagnetic minerals,such as hematite and goethite,were also present.The magnetic domains of the magnetic particles in the Duanjiapo red clay sequences were primarily below 0.2μm.The ferrimagnetic minerals were dominated by single-domain（SD）and superparamagnetic（SP）particles.From the bottom to the top of the profile,the content of soft magnetic minerals in the red clay sequence showed an increasing trend.In contrast,the absolute content of hard magnetic minerals remained almost constant.Within the adjacent stratigraphic layer,the strongly developed paleosol layer contained more soft magnetic minerals with finer magnetic particles than the weakly developed paleosol and loess layers.（2）SiO₂,A1₂O_3,CaO,and Fe₂O₃ dominated the oxides of major elements in the Duanjiapo red clay sequence.The high CaO content in the profile might be related to the precipitation-induced calcareous leaching and redeposition during the development of the red clay and the intense surface evaporation during the dry heat period.Overall,the Duanjiapo red clay sequence was at a primary to moderate level of weathering and pedogenesis.In contrast,the red clay sequence underwent intense chemical weathering from 3.6 to 2.6 Ma,resulting in increased migratory of elements like Ca and Na and increased relative enrichment of Al and Fe,reflecting the wetter climatic environment during this period.（3）a^*and b^*,which represent the contents of hematite and goethite in the Duanjiapo red clay sequence,respectively,could be used as indicators of the strength of the East Asian Summer Monsoon.Ca^*b^*,which denoted the degree of chemical weathering,had the same environmental significance as the CIA index.The composite hematite and goethite indicator a^*/b^*,which could both indicate changes in mean annual temperature,had the same environmental significance as Hm/Gt.The variability of both suggested that the annual mean temperature during the formation of the Duanjiapo red clay tended to rise in stages from the bottom to the top of the profile.（4）In the Duanjiapo red clay sequence,fine-grained（SP,SD）ferromagnetic minerals produced by pedogenesis were the leading cause of its high magnetic susceptibility,consistent with the enhanced mechanism of the magnetic susceptibility of the loess-paleosol sequence.The magnetic parametersχandχ_fdwere indicative and could be used as indicators for the East Asian Summer Monsoon when inverting the paleoclimate.The magnetic indicators(χandχ_fd)and the non-magnetic indicators（CIA and a^*）suggested that the East Asian Summer Monsoon strengthened in phase since 6.25 Ma.