Magnetic Properties And Its Environmental Significance Of Loess Under Different Sedimentary Environments In Iran And Pakistan

Loess is widely distributed on the Earth’s surface,and it is a good archive to record paleoclimate.As a proxy for paleoclimate,the magnetic susceptibility curves of the Chinese Loess Plateau can be compared with those of the deep-sea sediments to record the climatic changes in geological time.Magnetic susceptibility（MS）is a physical parameter,which is related to the species,content and size of magnetic minerals.The reason why the MS of loess stratum can record the paleoclimate is that the process of soil formation after the deposition of aeolian loess is controlled by the local climatic conditions,which affects the formation,preservation or change of magnetic minerals in the sediments,causes the MS to change.There are many relationships between loess MS and paleoclimate,for example,MS is positively correlated with paleoclimate rainfall intensity in the semi-arid central part of the Chinese Loess Plateau;in the extreme arid desert loess that dominated by physical weathering,and loess deposit in high humidity and rainfall condition of Alaska and Siberia are negatively correlated with MS.Furthermore,there is no clear relationship between MS and paleoclimate/pedogenesis intensity if the degree of dry and humidity oscillates in the transition zone between the three relationships.Therefore,before taking the MS of a loess section as the paleoclimate proxy index,it is necessary to have a macroscopic comprehending of the climatic conditions in the study area.The study is based on three sections（P₁,P₂,SH）in the Tigris and Euphrates region of Iran and the Attock section in Pakistan.This paper using systematic environmental magnetism,grain-size analysis,and geochemistry experiments,in the arid and semi-arid environments,discuss the magnetic properties and environmental significance of loess under different sedimentary environments in a region of western Iran in Tigris and Euphrates and Pakistan.Research has led to the following findings:1)The three sections in western Iran（SH,P₁ and P₂,P₁ and P₂ are loess sections adjacent to the same river）all have low MS.SH profile has higher topography and slightly higher MS while P₁ and P₂ section have lower MS,which may be related to low topography and frequent dry-wet alternating.The Attock（ATT）section of Pakistan is a mountainous loess near Islamabad.The MS is slightly higher than that of western Iran,but it is still very low,far lower than that of The Chinese Loess Plateau region and central Asia.By comparing with the average value of the upper Continental Crust（UCC）chemical properties,combined with the results of chemical weathering intensity,the total content of primary magnetic minerals is low.The proportion of soft magnetic minerals in each section is relatively high,the content of hard magnetic minerals is low,and there are a great number of paramagnetic minerals.The loess layer of SH section contains magnetite and thermically unstable maghemite,goethite and hematite,while the paleosol layer contains maghemite,hematite and goethite but does not contain magnetite,the goethite content higher than loess layer.P₁ section contains hard magnetic minerals hematite and goethite,more paramagnetic minerals,but no magnetite.The P₂ section is similar to that in P₁ section,containing a large amount of paramagnetic minerals,but lacks magnetite.It also contains hard magnetic minerals goethite and hematite,and the content of goethite is higher in the gray layer.The loess layer of ATT section contains maghemite,magnetite,hematite and goethite,while the paleosol layer contains maghemite,goethite and hematite but does not contain magnetite,which may indicate that magnetite cannot be stably preserved in long-term humid environment.2)The element ratio K/Na,weathering index Wi,loss coefficient Lc,the coefficient of eluviation Ki,SiO₂/（Fe₂O₃+Al₂O₃）and Na₂O/Al₂O₃ and K₂O/Al₂O₃ models all show that:P₁ and P₂ sections are less susceptible to chemical weathering,and the difference of chemical weathering among different samples are small.The chemical weathering of SH section is slightly higher than that of P₁ and P₂ section on the whole,and the degree of chemical weathering of diverse stratum samples exist discrepancy.The majority of the ATT section samples have undergone chemical weathering with near moderate intensity,and the chemical weathering intensity among the samples is quite different.The chemical weathering intensity of some samples in the same layer has great variability,which may indicate that the strata experienced several secondary climatic fluctuations during the deposition period.3)Under the arid and semi-arid climate,rainfall and sedimentary environment have great influence on the transformation and change mechanism of magnetic minerals in loess,the long-term hydrodynamic and dry-wet alternate sedimentary environment can change the content and composition of magnetic minerals by water action,short-term and strength weak hydrodynamic and dry-wet alternate environment can make some magnetic mineral that formed and preserved in arid and semi-arid environment dissolution and transformation.The frequent dry-wet alternation during primary formation and fluvial environment can even dissolve and take away the Fe element in sediments,as gray-white layer of the P₂ section.Whereas the precipitation is more through the change of the effective humidity of the sedimentary environment,and then affect the chemical weathering and pedogenic process,which has a great relationship with temperature.