长江水下三角洲沉积物磁性特征空间差异及其冲淤指示意义

三角洲冲淤具有空间异质性，而沉积物的磁性特征可灵敏地反映物源、沉积动力特征以及早期成岩作用影响，是揭示空间异质性的有效方法。选取长江水下三角洲20~35 m水深的四个约2 m长的柱样进行磁学和碳、硫地球化学分析，结合粒度及年代资料，探讨了长江水下三角洲沉积物柱样磁性特征空间差异及其冲淤指示意义。研究结果显示，粒度表征的动力分选影响沉积物的磁性特征，这在年代较新（<350 a）的柱样中表现尤为明显，即细颗粒沉积物中富集亚铁磁性矿物，从而具有较高的磁化率。铁、碳、硫及硫同位素特征揭示了长江水下三角洲沉积物较弱的硫酸盐还原特征。沉积物年龄及沉积速率影响沉积物成岩改造强度，进而导致了磁性特征垂向变化的空间差异，整体上看，远离现代沉积中心的柱样，成岩改造特征更为明显。磁化率值的大小、成岩分带的完整性及其反映的铁还原带和硫酸盐还原带转换深度，一定程度上可以揭示三角洲的淤积和侵蚀特征。

Spatial Variation of the Magnetic Properties from the Yangtze River Subaqueous Delta Deposits and Their Implications for Erosion/Deposition Study

Delta deposits show great spatial heterogeneity in erosion and depositional patterns. Magnetic properties of sediments are sensitive to the sediment source， hydrodynamics， and early diagenesis and， therefore， can indicate spatial heterogeneity in an efficient way. In this paper， four cores （ca. 2 m in length） from the subaqueous Yangtze River Delta were subjected to environmental magnetic， organic carbon， and sulfur analyses. In combination with particle size analysis and dating results， this paper discusses the spatial variations of magnetic properties and their implications for indicating erosion/deposition patterns. Our results show that hydrodynamic sorting plays an important role in the magnetic property variations. In general， ferromagnetic minerals are enriched in fine?grained sediments in young deposits （<350 a）， resulting in higher magnetic susceptibility （χ）. The Yangtze River subaqueous delta deposits exhibit a weaker sulfate reduction， as revealed by carbon， sulfur， and sulfur isotope characteristics. Such a feature is similar to that reported from Amazon delta deposits. Sediment age and sedimentation rate affect the degree of diagenetic alteration， and therefore， the spatial and temporal variations of magnetic properties. Cores distant from the modern depo?center show a greater diagenetic alteration of magnetic properties. Magnitude of χ， diagenetic zonation of χ， and the depth of the ferruginous to sulfidic boundary can reflect the spatial pattern of erosion and deposition in deltaic deposits.