Environmental Magnetism Of Loess In West Sichuan

The western Sichuan Plateau,located in the southeastern margin of the Tibetan Plateau,is deeply controlled by Indian Ocean monsoon,westerly monsoon and plateau monsoon,where aeolian loess is widely distributed.So the study of this area can reveal the eastern atmospheric circulation pattern of Tibetan Plateau and the climate change of its surrounding areas and the environmental effects with the uplift of the Tibetan Plateau,and even the environmental evolution in the middle Pleistocene and the global regional major geological events.Therefore,the typical loess profiles and surface soils in the western Sichuan Plateau are analyzed by systematic environmental magnetism combined with non-magnetic indicators.The results show that:（1）The results of environmental magnetism show that the magnetic minerals in Ganzi section,Nianlong section,Luhuo section and surface soil are mainly controlled by the ferromagnetic low coercive magnetite,with a number of hematite and weak magnetic hematite.The main magnetic particles are superparamagnetic（SP）and single domain（SD）particles,which contain a large number of stable single domain particles（SSD）,with a small number of multi-domain（MD）and pseudo-single domain（PSD）particles apart from the Luhuo profile.The magnetic susceptibility enhancement mechanism of loess in Western Sichuan is similar to the Loess Plateau,which is mainly controlled by ferromagnetic minerals of fine SP and SD particles produced by the soil pedogenesis.Among the three profiles,Luhuo section has the highest magnetic mineral content,Nianlong section has the lowest magnetic mineral content,and temperature may be one of the important influencing factors.There is a phenomenon that strong magnetic minerals are transformed into weak one in surface soil and paleosoil of this area.Compared with the index of magnetic mineral content,the magnetic particle size index has a clearer indication significance for the changes of climate.（2）The analysis of diffuse reflectance spectra and chromaticity indices of loess in Western Sichuan and the magnetic parameters show that the content of magnetic minerals,magnetic particle size and chromaticity indices,diffuse reflectance indices are more complex.Compared with the index of magnetic mineral content,the magnetic particle indices of Ganzi section are better correlated with chromaticity indices and diffuse reflectance indices,which indicates the weak magnetic minerals,hematite and goethite,is closely related to magnetic mineral content and magnetic particle size in process of soil formation.（3）Geochemical element analysis of loess in Western Sichuan shows that SiO₂,Al₂O₃ and Fe₂O₃ are the main oxides of major elements in the sections of Nianlong,Ganzi and Luhuo.The chemical weathering degree is low or medium,and the chemical weathering process of soil is similar in this area.Combined with environmental magnetism,it is shown that the content of magnetic minerals and particle size are related to the leaching and enrichment of geochemical elements,the degree of chemical weathering and the degree of plagioclase weathering.It is indicated that the change of magnetic mineral content and particle size can indicate the soil intensity in a certain extent,but the correlation between magnetic particles and weather is better.In addition,the content of rare earth elements in the Ganzi section is higher than that in the Loess Plateau and the distribution patterns of rare earth elements is similar which indicate the provenances are relatively stable.（4）The results of grain size analysis show that the grain size components of Ganzi,Fuhuo and Nianlong profiles in Western Sichuan are mainly of silt grade,which belong to aeolian dust deposits.The sedimentary environment of Ganzi profile is the most stable,but the Fuhuo profile is the most complex,which may be affected by flowing water erosion,related substances and human activities.The content of magnetic minerals and the change of magnetic particle size are related to the change of particle size components.In loess period,the content of fine silt decreases and the content of coarse silt increases,while the content of magnetic minerals decreases and the magnetic particle becomes coarser;In paleosol period,the content of clay and fine silt increases and the content of coarse silt decreases,while the content of magnetic minerals increases and the magnetic particle becomes finer.（5）The magnetic and non-magnetic indices of the Ganzi section indicate that the climate of the S3^S5 stage of the Ganzi section is warmer and wetter,the soil undergoes moderate chemical weathering,the sedimentary environment is more stable,the climate changes in cold and warm fluctuations,the content of magnetic minerals is less,and the content of magnetic particles is more and finer.The reason may be that more rainfall in this stage leads to the dissolution of strong magnetic minerals in soil.So magnetic susceptibility can not well indicate climate change,but magnetic particle size can well indicate the degree of soil development.During the period of L3-S0 stage,the climate changed obviously:the range of climate change increased and the fluctuation pattern tended to be dry and cold.In this stage,the content of magnetic minerals decreases gradually,the magnetic particles become coarser,the chemical weathering intensity decreases obviously,and the chemical weathering intensity changes from medium to low.The L3 stage may be an important period of the uplift stage of the Tibetan Plateau as a barrier to water vapor in the southwest.