以东海内陆架泥质区F17柱样为对象,在放射性同位素测年、磁性测量、地球化学和粒度分析的基础上,探讨了早期成岩作用对陆架沉积物磁性特征的影响.结果表明,F17柱样磁性特征存在显著的垂向变化,表现为随深度的增加,沉积物磁性减弱、磁性颗粒变粗和不完整反铁磁性矿物比例上升.粒度以及Al,Ti,Fe,Mn的含量,有机碳和总硫的分析表明,沉积物物源相对均一,粒度虽然存在垂向变化,但不是磁性特征垂向变化的主要原因,早期成岩作用是导致上述磁性特征垂向变化的主要因素.早期成岩过程中,亚铁磁性矿物含量随深度下降的现象与其他地区报道的结果类似,但磁性矿物类型和颗粒大小随深度变化的模式,不同地区存在一定差异.基于磁性矿物对早期成岩过程的敏感性,磁性测量方法对认识铁在陆架沉积物中的迁移和转化具有积极作用.%This paper discusses the influence of early diagenesis on magnetic properties of sediments from inner shelf of the East China Sea, using combined granulometric, magnetic and geochemical analyses. Sediment core F17 (26°40'N ,120°30'E) ,271cm in length,was collected from a water depth of 41.4m using a gravity core sampler. The sediment core displays a yellowish brown color in the top 40cm layer, which gradually changes into grey color with depth. The sediments are dominated by clay silts. The core was sectioned at 1cm intervals, dried at low temperature of 40℃. Except an aliquot for particle size analysis, the remaining parts were ground with mortar and pestle before magnetic and geochemical analyses. The sediment samples were analyzed for 210Pb and 137Cs activities using HPGe γ spectrometer( GWL-120210S) for establishing core chronology. Sedimentation rate for the top 50cm layer is averaged at 0. 49cm/a,which is in accordance with reported sedimentation rates in the study area. Assuming such a sedimentation rate for the whole core, then the core is deposited in the last 550 years. Based on the result of radionuclide dating, magnetic measurements and particle size analysis were carried out at 2cm intervals, while geochemical analysis at 10cm intervals. The results indicate that the mean particle size of F17 ranges between 4μm and 12μm, which increases with depth. Magnetic properties of core F17 display marked vertical variations, which can be divided into four zones, showing a general declining concentration of ferrimagnetic mineral and coarsening grain size with depth. In contrast, the proportion of antiferromagnetic minerals increases relatively with depth. Although particle size varies with depth,it plays a minor role in the change of magnetic properties. Both Al and Ti show minor variations with depth, indicating that sediment provenance is relatively stable. Total sulfur ( TS ) content increases with depth. Organic carbon to TS ratio ( C/S ) and Mn/Fe, indicators of redox condition of sediments, display obvious variations. The C/S ratio is highest in the top 25cm layer, and then declines with depth until it reaches a relatively stable value. The Mn/Fe ratio is higher in the top 117cm layer and lower in the layer below 117cm. All these indicators suggesting increased reducing condition with depth. Scanning electron microscopy examination reveals the presence of iron sulfides. Therefore early diagenesis is the main factor behind the changes of magnetic properties. Ferrimagnetic minerals dissolve under reducing conditions and are converted into iron sulfides such as pyrite. Since antiferromagnetic minerals are more resistant during the process of iron oxide reduction, its proportion in the iron oxide assemblage increases with depth. Although the general pattern of declining ferrimagnetic mineral concentration with depth is consistent with reports from elsewhere, the vertical variations of magnetic mineral assemblage and grain size vary with place. Due to the sensitivity of magnetic minerals to early diagenesis, magnetic measurements can play a significant role in the study of iron migration and transformation in continental shelf deposits.
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