东海外陆架浅孔沉积物的岩性和磁性特征及成因讨论?

利用东海外陆架的10根柱状样，进行磁性、粒度、氯离子含量和粘土矿物分析。结果表明：在东海外陆架扬子浅滩前缘，浅部沉积地层自上而下可分为两层。上层为砂泥混合物，含大量贝壳碎片，磁化率与饱和等温剩磁较强，主要是磁铁矿富集所致，反映了冰后期海侵对晚更新世沉积物的强烈改造。研究还发现，现代海底表层沉积物中存在铁硫化物，它是浅海还原环境中铁磁性矿物次生变化的产物。下层为坚硬致密的黄褐色泥质粉砂和粉砂质泥（硬土层），磁化率与饱和等温剩磁较弱，但局部存在剩磁异常高，从矫顽力等磁性特征证明它是硫复铁矿的富集所致。另外，在硫复铁矿富集层中绿泥石和氯离子含量明显富集，反映该硬土层是早期海侵沉积物在末次冰期干冷气候条件下的风化产物。

Lithologic and magnetic characteristics and origin of shallow bore sediments on outer continental shelf of East China Sea

Ten vibrocores obtained from the outer continental shelf of the East China Sea were measured for their magnetic properties, grain size, Cl- concentration and clay mineral. Results show that the sub-bottom strata at the brim of Yangtze shoal consists of two layers：(1) The upper layer is the mixture of sand and mud with a large quantity of shell fragments, which is characterized by high values of magnetic capacity (x) and saturation isothermal remanent magnetization (SIRM). Magnetite(Fe3O4) is enriched in the layer of sand and mud mixture, representing the strong reworking and deposition in situ by the post-glacial transgression. Iron sulphides found at the sea bottom indicate the secondary transformation of the magnetic minerals in the reduction shallow sea. (2) The lower layer contains the hard and compact yellowish brown clayey silt and silty clay (stiff mud) with a generally faint magnetic property. But extremely high values of SIRM/χ and coercivity (Bc and Bcr) appear locally in the lower layer, evidencing the occurrence of greigite. The concentrated chlorite and Cl- of the greigite bearing sediment shows the stiff mud is the efflorescent result of the early transgression sediment in the subsequent relatively dry climate during the last ice age.