庐山JL剖面红土磁化率特征及古环境记录

Characteristics of Magnetic Susceptibility and Its Paleoenvironmental Records from JL Red Earth Section,Lushan Area

江西九江庐山地区JL剖面红土磁化率测试结果表明:1.典型网纹红土的磁化率远小于弱网纹红土和棕黄色黄土-古土壤,数值上,棕黄色黄土-古土壤是网纹红土的2.33倍。剖面各层段频率磁化率均值变化于10.09%～13.71%,且与磁化率显著正相关,表明风化成土作用中形成的超顺磁性颗粒对红土磁性增强具有重要贡献。2.湿润气候和土壤渍水的假潜育环境下铁磁性矿物易于分解、转化、淋失,这可能是导致网纹红土磁化率异常偏小的主因。从而在剖面上磁化率与CIA变化趋势相反,两者峰谷波动又基本一致。3.结合ESR年代,JL红土磁化率沿剖面自下而上分为4个波动式增大的阶段,可与早更新世以来季风环流演变过程中的几个重要转折期对应,记录了大尺度内的古环境变化信息。早更新世以来该区环境湿润程度波动式减弱,气候自中更新世以来由湿热向干冷转变。

Magnetic susceptibility is one of the most remarkable indices in researches on Quaternary environment.920 samples were collected from JL red earth section,Lushan area,Jiangxi province,and susceptibility was measured on all these samples.The results indicate that：（1） The magnetic susceptibility of weak plinthitic red earth and loess-paleosol is rather higher than typical plinthitic red earth,loess-paleosol is 2.88 times the value of plinthitic red earth.The average values of frequency-dependent magnetic susceptibility in different layers varies from 10.09% to 13.71%.Notable positive correlation exists between magnetic susceptibility and frequency-dependent magnetic susceptibility,which shows that superparamagnetic grains formed in pedogenesis contribute significantly to the enhancement of magnetic susceptibility.（2） Pedogenic ferromagnetic minerals are prone to dissolve,transform and leach in pesudogleyization environment caused by damp and soil waterlogging,which results in the abnormality of magnetic susceptibility from plinthitic red earth,so compared with CIA,the variety of magnetic susceptibility along the section displays a reverse trend,but the fluctuation of high and low values is relatively coincident.（3） Based on ESR chronology,generally speaking,the magnetic susceptibility of JL red earth increases by four phases from bottom to top,which reflects the evolution of East Asian monsoon and large-scale paleoenvironmental changes from the early Pleistocene to the late Mid-Pleistocene in Lushan area.The degree of wetness had weakened in fluctuation since early Pleistocene and the climate in Lushan area had transferred from humid-hot to dry-cold since Mid-Pleistocene.