The Spatial Variation Of Loess Magnetic Characteristics And Its Mechanism On Loess Plateau Since 0.5Ma

In Chinese Loess Plateau(CLP),the S5 is the most prominent and well-developed paleosol unit,and can be distinguished by its great thickness and darkest reddish color.The climate during S5 formation was much warmer and wetter than today,and has been interpreted as representing a climatic optimum for the past 0.5 Myr.Generally,the S5 paleosol corresponds to the highest susceptibility,which is one of the summer monsoon proxies.However,in the southernmost CLP,such as Baoji,the annual rainfall is more abundant than that in the central CLP,but the magnetic susceptibility value of the S5 layer with the strongest pedogenesis is not the highest in the profile,and is even lower than that of in Xifeng and Lingtai in the central CLP,even lower than the S3 with weaker pedogenesis in the same Baoji profile.The reason for the low magnetic susceptibility of S5 paleosol on the southern edge of CLP has not yet been fully known.In this paper,S5 samples from Xifeng,Lingtai,Linyou,and Baoji sections have been studied,and comparisons of S5 and S3 samples from Baoji were also carried out in detail.This study was mainly based on rock magnetism and geochemical methods to analyse magnetic mineral types,concentration,and grain sizes during pedogenesis.The transformation of magnetic minerals under various oxidizing and reducing conditions are analysed for interpretation of the low magnetic susceptibility values at S5 palaeosol in Baoji section.The result will provide an understanding basis for the weak relation between magnetic susceptibility and the degree of pedogenesis in the southernmost CLP.The main conclusions are as follows.Their magnetic characteristics indicate that the main magnetic minerals in Xifeng,Lingtai,Linyou,Baoji loess-paleosol and the topsoil samples are similar:magnetite,maghemite,hematite and goethite.With pedogenesis enhanced,the Curie temperature rised from?580? to?600? at thermomagnetic curves,and the magnetic minerals were transformed,which may be due to the warm-wet and oxidizing pedogenic conditions resulting in surface oxidation of the magnetite grains,and forming small thermally stable maghemite grains in the outermost rim of the grain's surface.All above these could lead to a higher Curie temperature,?600?,rather than 580? and higher magnetic susceptibility.This maybe confirmed that the main ferrimagnetic minerals formed during pedogenesis were maghemite rather than magnetite.The loss of magnetization during heating and cooling process(?Ms)show a negative relationship with pedogenic development,indicating that under certain pedogenetic conditions,the small thermally unstable maghemite gradually decrease with pedogenic development.The contents of elements Al,Si,Fe,Ti,K,Mg,Cu,Mn and Zn are higher in paleosol than those in loess,but the contents of Ca,Na and Mg showed significantly lower.The contents of Al,Si,Fe,Ti,K,Cu,Mn,Zn and the ratios Fe/Mg,Rb/Sr are the highest,while the contents of Ca,Na,Mg and the ratio of Cs/Rb are the lowest in S5 paleosol.These results clearly indicate that the pedogenic development is the most significantly strong for S5,chemical weathering was the strongest during S5 paleosol development than other paleosol units.The climate for S5 period was the most warm and wet for the last 0.5 Ma.Xifeng S5,Linyou S5 and Baoji S3 paleosols were formed under oxidation evnironment.Under such pedogenic conditions,maghemite and hematite were formed,the anti-ferromagnetic minerals are mainly hematite,superparamagnetic(SP)maghemite is the main contributor to the magnetic properties.However,the contents of ferrimagnetic mineral(mainly maghemite)in Baoji S5 are lower percentage than that in Xifeng S5,Linyou S5 and Baoji S3.With pedogenic developing,the proportion of ferrimagnetic component reduces while the conten of anti-ferromagnetic minerals(mainly goethite)increases.The single domain(SD)ferrimagnetic paticles mainly contribute to the enhancement of magnetic susceptibility,anti-ferrimagnetic minerals also do some contribution to it.The ratio of ?ARM/? in Xifeng,Linyou and Baoji sections gradually decreased since the S5,especiallly obvious to the southeast,indicating that the SD ferrimagnetic grains produced during pedogenic process decreased since S5,and the degree of pedogenesis and southeast summer monsoon gradually weakened,climate gradually dried.There is a good positive correlation between frequency dependent susceptibility and the mean annual precipitation in the CLP topsoils.Climatic condition,such as temperature and precipitation,especially the latter one,is explained as a major factor effecting susceptibility enhancement in the Loess Plateau.There are amounts of dark brown Fe-Mn film on the cranny surface and voids of S5 palaeosol in Baoji section,because the soil after the deposition was under alternating oxidation and reduction conditions,soil-forming process under such oxidizing conditions generates fine-grain ferrimagnetic minerals which could increase susceptibility.Pedogenisis occurs under waterlogged conditions,fine-grained magnetite,maghemite and hematite are gradually dissolved,and those will be finally converted into goethite and limonite and other weak magnetic minerals,mainly into goethite for its redox stability.Such transformation of magnetic minerals may result in SP ferrimagnetic mineral reduction and reduced susceptibility in Baoji S5 paleosol.