福建北部闽江流域第四纪红土的磁学特征及其环境意义

Magnetic Characteristics of Quaternary Red Earth Sequence from the Minjiang Drainage Basin in Northern Fujian and Its Environmental Significance

南方第四纪红土是记录南方古环境信息的良好载体和第四纪古气候环境研究的重要内容。本文对采自福建北部闽江流域大横第四纪红土剖面的样品进行磁化率、等温剩磁、饱和等温剩磁和代表样品的κ-T曲线等多磁学参数的详细测量和分析,研究结果表明:大横剖面的磁化率范围为8.87~35×10-8m3·kg-1,饱和等温剩磁范围为51.77×10^(-5)~294.51×10-5Am^2·kg^(-1),总体偏低,说明大横剖面的磁性矿物含量较低,可能是福建北部化学风化作用强烈且温暖湿润的环境导致的;频率磁化率、非磁滞磁化率及χARM/SIRM比值说明剖面自上而下,磁性矿物颗粒大小总体上呈现变粗趋势,由SP+SD颗粒到SD+MD颗粒的转变,剖面磁性矿物以磁铁矿为主,并含有磁赤铁矿、赤铁矿和纤铁矿等。本文探讨了红土剖面的不同沉积层磁学特征变化及对环境的指示意义,对认识该地区第四纪红土的风化特征及发育环境有一定借鉴意义。

Quaternary red earth sequences in southern China have been generally considered as potential paleoenvironmental records,because they are one of the most important terrestrial sources of information for reconstructing paleoenvironment and paleoclimate at a regional scale.Lack of research on Quaternary red earth sequences in Fujian province hindered the understanding of causal mechanism and associated environment effects on Quaternary red earth in Fujian province.In this research,the environmental magnetism records from a Quaternary red earth sequence of Daheng(DH)section in the Minjiang Drainage Basin were presented.In practice,detailed measurements and analysis of multi-magnetic parameters were performed with focuses of magnetic susceptibility,isothermal remanence,saturated isothermal remanence andκ-T curves of representative samples collected at the Daheng Quaternary red earth section in the Minjiang River Basin of the northern Fujian,China.The magnetic characteristics of different sedimentary layers in the red earth sections and their implications for the local environment were interpreted for understanding of the weathering characteristics and the associated development environment of the Quaternary red earth in this area.Results showed that the magnetic susceptibility ranged between 8.87×10-8~35×10-8 m 3·kg-1,and the saturation isothermal remanence was within a range of 51.77×10-5~294.51×10-5 Am 2·kg-1.Generally,the obtained values were relatively low in extent,suggesting that there was a low magnetic mineral content at the Daheng sections,possibly caused by a strong,warm and humid environment in the northern part of Fujian.And the frequency susceptibility,non-hysteresis magnetic susceptibility andχARM/SIRM ratio indicated that the values of them increased from base to top,suggesting that the particle size of magnetic mineral may exhibited a decreasing trend from base to top.Investigations of the environmental magnetism records revealed that the particle size of magnetic mineral increased from interval SP+SD to that of SD+MD;and the mass of MD magnetic mineral particles was more than others.Moreover,κ-T curves of selected samples indicated that the magnetite was the dominant magnetic mineral in the DH section,with a secondary component of maghemite,hematite,and lepidocrocite.