九江红土的磁学特征及其环境意义

Magnetic characteristics and environmental significance of Jiujiang red earth

为了解南方红土磁性变化的机理,采用磁化率仪、扫描电子显微镜及X荧光光谱分析对九江红土剖面的磁性特征、磁性矿物的特征及风化程度进行了深入研究。研究结果表明,九江剖面上部砂土经历的风化改造强度弱于下部的网纹红土,而磁性强度高于下部网纹红土,磁性强度的增加与超顺磁颗粒及多畴磁铁矿的含量密切相关;风化强度的增加导致磁铁矿的晶形遭到破坏,同时导致红土中强磁性矿物向弱磁性矿物转化,这是造成网纹红土磁性强度低于砂土的主要原因;红土中的多畴磁铁矿并不是形成于风化成土作用过程中,而是来自红土原始母质的风化残留。从中更新世到晚更新世气候由湿热向冷干转变,风化作用强度逐渐减弱,导致上部砂土相对于网纹红土保留了更多的强磁性矿物。因此,红土中主要的载磁矿物并不是风化成土过程中的产物,其磁性特征不能反映气候环境的变化。

To understand the mechanism of magnetic variation of red earth in southern China,the authors investigated magnetic characteristics,characteristics of magnetic minerals and degree of weathering of Jiujiang red earth by using magnetic susceptibility instrument,scanning electron microscope and X-ray fluorescence spectrograph.The results indicate that the upper sandy clay has experienced weaker weathering than the lower net-like red earth,but it has a higher magnetic field strength than the lower net-like red earth.The increase in magnetic strength is closely related to the content of superparamagnetic particles and multidomain magnetite.Increasing weathering results in the destruction of the magnetite crystals and the transformation of ferromagnetic minerals to weakly magnetic minerals,which is the main reason causing a lower magnetic strength of net-like red earth than magnetic strength of the sandy clay.The results also indicate that multidomain magnetite in red earth is not formed in weathering and pedogenesis but in the weathering residue from the parent material of the red earth.From middle Pleistocene to late Pleistocene,the climate changed from hot and humid to cold and dry and weathering intensity gradually decreased,resulting in more ferromagnetic minerals in the upper sandy clay than in the lower net-like red earth.Therefore,the main magnetic-bearing minerals in red earth are not the products formed in weathering and pedogenesis,and magnetic characteristics of red earth can’t reveal the climate change.