长江中下游第四纪沉积物发育土壤磁性增强的环境磁学机制

Environmental Magnetism of Magnetic Enhancement for Soils Formed on Quaternary Sediments in Yangtze River Valley

对长江中下游第四纪沉积物 (Q3 黄土 ,Q2 红土和Q3 红土 )发育土壤的磁性增强现象及其物理机制进行了讨论。结果表明 :(1)铁磁性矿物是土壤磁性的主要载体 ,它们是成土过程中形成的稳定单畴 (SSD)和超顺磁性 (SP)态的次生磁性矿物 ,没有显示重要的反铁磁性矿物的贡献 ;(2 )土壤磁化率 (χ)的高低与成土过程产生的稳定单畴和超顺磁性颗粒呈极显著正相关 ,指示了风化成土作用的强度 ,磁化率可作为反映成土环境变化的代用指标 ;(3)频率磁化率 (χfd) 5 %可作为土壤中的超顺磁性颗粒存在与否的临界值 ,土壤 χfd值的高低同样反映了风化成土作用的强度 ,可用作研究第四纪环境变化的有用工具之一。

On the basis of environmental magnetism study on soils formed on Quaternary sediments in Yangtze River valley,the soil magnetic enhancement and its mechanism were discussed.Results showed that soils formed on Quaternary sediments exhibited a distinct ferrimagnetic profile with the magnetic susceptibility enhancement of soil horizons with respect to that of parent materials. It is suggested that formation of pedogenic ferrimagnetic minerals resulted in enhancement of magnetic susceptibility in soils.In addition,the pedogenic ferrimagnetic minerals are mostly superparamagnetic(SP) and stable single domain(SSD)grains.It is found that highly significant relationships between the magnetic susceptibility( χ ) values of the samples and frequency dependent susceptibility ( χ fd ) and between χ values of samples and anhysteretic susceptibility( χ ARM ).It is suggested that χ can be considered an indicator for the environmental fluctuation recorded in soils. Results also suggested that χ fd value of 5% can be regarded as an indicator to reflect the formation of pedogenic SP and SSD ferrimagnetic minerals in the continuous pedogenic development of soils.Thus,magnetic susceptibility can be used to study the Quaternary environmental changes.