红黏土磁化率增强机制

The Magnetic Susceptibility Enhancement Mechanism of the Red Clay Sequence

对于黄土-古土壤序列,磁化率增强机制已经被大家认可,并且磁化率被广泛作为古夏季风强弱变化的代用指标。红黏土伏于第四纪黄土之下,同为风成沉积物,对红黏土磁化率增强机制的研究程度不高,且对强发育红黏土获得比较低的磁化率值的原因不清楚。本文通过对佳县红黏土进行系统岩石磁学分析得出:(1)红黏土磁性矿物种类以磁铁矿为主,同时含有一定量磁赤铁矿、赤铁矿;磁性矿物平均粒径为PSD颗粒;强发育层含有较多的软磁性矿物,弱发育层含有较多的硬磁性矿物。(2)χ、χfd和χARM两两之间表现出很好线性正相关关系,说明红黏土中含有大量SP和SD颗粒亚铁磁性矿物,这些细小磁颗粒是在成壤过程中产生,成壤作用导致古土壤层磁化率升高。(3)红黏土χfd、χfd%值较高,说明经历较强的成壤过程,成壤环境与黄土高原黄土-古土壤序列类似,与阿拉斯加、西伯利亚黄土-古土壤明显不同。

Pedogenetic enhancement mechanism of magnetic susceptibility in loess/paleosol sequences of the Chinese Loess Plateau has been widely accepted. Magnetic susceptibility is used as a proxy of summer paleomonsoon intensity. The red clay underlying the loess/paleosol sequences is also aeolian deposit, however, its magnetic susceptibility enhancement mechanism has not been studied as intensively as that of the loess/ paleosol sequences, and the reason why the strongly developed red clay gets low susceptibility is unclear. Studies were made on the magnetic mineralogy, magnetic concentration and magnetic grain size distribution of Jiaxian red clay. （1） The magnetic minerals of the red clay are magnetite, maghemite and hematite, similar with Quaternary loess/paleosol sequences; the average magnetic grain size is PSD; the strongly developed red clay contains much soft magnetic minerals （magnetite and maghemite） and the weakly developed red clay contains much hard magnetic minerals （hematite and goethite）. （2） There is a positively linear relationship between χ,χfd and ;χARM, which implies that the red clay contains a lot of ultrafine ferrimagnetic grains （SP and SD）. The grains produced during the pedogenesis process account for the susceptibility increase. （3） The values of χfd and χfd% of red clay are higher than the loess/paleosol sequences, which implies the red clay underwent a strong pedogenesis development process. The environment during the paleosol developed of red clay is similar with the loess/paleosol sequence of the Chinese Loess Plateau, different from that of the high latitude Alaska and Siberia.