湿润气候条件下温度对土壤磁化率影响的再认识

NEW INSIGHT INTO THE TEMPERATURE DEPENDENCE OF PEDOGENIC MAGNETIC SUSCEPTIBILITY IN HUMID CLIMATE REGION

土壤磁性对环境因子(降水量、温度)的响应机制是古气候定量反演的重要研究内容。对北半球温带和热带地区土壤的调查表明,当年均降水量超过1000~1200 mm的条件下,土壤磁化率与降水量的相关关系由正转负。但研究区内高降水量地区通常对应高温地区,因此磁化率变化的受控机制还有待进一步厘清。对浙江中部金华山地区不同海拔土壤剖面样品进行环境磁学和漫反射光谱测试分析,并与黄土高原典型剖面进行对比,以期在前人研究的基础上探讨不同水热条件下土壤磁性变化特征及其影响因素。分析结果表明:中亚热带季风气候条件下,1)土壤中次生的主要载磁矿物为磁铁矿,同时含有较多的赤铁矿和针铁矿;相比较而言,较低的气温有利于成壤成因磁铁矿的形成,而较高的温度有利于成壤成因赤铁矿的形成;2)较高的气温有利于次生磁铁矿颗粒的粒径增大;3)与黄土高原相比,浙江金华山地区的高温条件极大地限制了细粒(超顺磁和单畴)磁铁矿的生成,而有利于生成赤铁矿,导致土壤磁性随年降水量增大而降低更显著。因此,对于发育于暖湿气候条件下土壤,在使用磁学参数反演古土壤记录的湿度历史时,应充分考虑温度的影响。

The paleoclimate implication of magnetic susceptibility is an important issue of quantitative climate-recover study.The previous study of Chinese loess evidenced that the magnetic susceptibility of loessic soil usually could be used to recover the mean annual precipitation(MAP)of the area.While,more and more magnetic investigations from soils in tropical and subtropical area discovered the correlation between soil magnetic susceptibility and precipitation changed from positive to negative as the MAP exceeded 1000~1200 mm.In terms of details,the decrease magnetic susceptibility was commonly discovered from warm and humid region rather than humid and mild area.Therefore,it needs to make clear whether the humidity or temperature of soil leads the magnetic susceptibility decrease is still in mystery.In this study,we collected two soil sections from Jinhua Mountain,Zhejiang,China,in which the climate is prevalent subtropical monsoon climate.There two sections have a more than 1000 m difference in altitude,due to which the climate in the section Panqian Village section(PQ:29°13'41″N,119°39'42″E,1130 m a.s.l.)is significant by mild and humid climate and in the section Shiqiangjiao Village section(SQ:29°9'44″N,119°38'10″E;110 m a.s.l.)is significant by warm and humid climate.According to comparison the magnetic properties of these two section,we hope to insight understand the mechanism of magnetic susceptibility decrease in humid area.The thickness of the PQ and SQ section are 171 cm and 100 cm,respectively and we have taken 19 soil samples at 9 cm interval in PQ section,10 samples at 10 cm interval in SQ sections.Detailed environmental magnetic measurement including χ_(lf),χ_(fd),χ_(fd)%,χ_(ARM) and SIRM were conducted in all soil samples to reveal the magnetic characteristics.In addition,representative samples were selected to conduct the temperature dependence of magnetic susceptibility curve(κ-T)and diffuse reflectance spectrum(DRS)to determine the magnetic minerals composition.The results show that:(1)The soils both contain abundant of hematite(Hm)and goethite(Gt),and its magnetic susceptibility is dominated by magnetite(Mt).The concentration of magnetic minerals in the two soil profiles is significantly different:the average χ_(lf) of the PQ profile is 193.24×10^(-8) m^(3)/kg,while the average χ_(lf) of the SQ profile is only 18.3×10^(-8) m^(3)/kg.The Mt content of the PQ profile is significantly higher than that of the SQ profile,while the Hm content in the latter is relatively higher.(2)The new formed Mt particles are generally coarser than those in the parent material in both profiles.In addition,the newly formed Mt particles in the soil profile in the foothills area are larger than the soil profiles in the mountain top area.(3)By comparing the magnetic properties of these two sections and Chinese Loess Plateau,the results show that the relatively high temperature and humid conditions greatly limit the formation of fine-grained(superparamagnetic:SP and single-domain:SD)Mt and promote the formation of Hm,which caused a more prominent decrease in soil magnetism with the increase of annual precipitation.In addition,the pedogenic Mt is getting coarser in warm condition.Therefore,we suggest that it needs to fully consider the influence of temperature change in the paleo-precipitation recover investigations basing on the magnetic record from paleosoil sediment.