南海北部白云凹陷08CF7岩心沉积物的磁化率特征及其意义

海洋沉积物的磁性异常与甲烷渗漏活动及其产物密切相关。为揭示南海北部天然气水合物区沉积物磁化率的变化特征及其意义,对白云凹陷08CF7岩心331个沉积物样品的磁化率进行测定和分析。08CF7岩心沉积物磁化率变化范围为5.54×10-6~26.56×10-6SI,平均值为11.41×10-6SI。磁化率随深度变化可分为0~188 cm、188~240 cm及240cm以下等3段,分别与硫酸盐富集带、硫酸盐-甲烷转换界面(SMT)的上部过渡带及SMT相对应。研究表明,海底天然气渗漏是08CF7岩心沉积物磁化率异常变化的可能原因,其机理是:在强烈的还原环境中,甲烷厌氧氧化反应的产物HS-与沉积物中的Fe3+发生反应,形成顺磁性的黄铁矿。沉积物磁化率变化能快速了解SMT状况及其深度,提供了一种识别南海北部天然气水合物的间接方法。

Characteristics and Implications of Magnetic Susceptibility of Sediments in Core 08CF7 from the Baiyun Sag in Northern South China Sea

Magnetic anomalies in marine sediments are related to methane venting and its products.In order to understand the characteristics of magnetic variability in sedimentary prism from the gas hydrate areas and their possible implication,magnetic susceptibility(MS) of 331 sediment samples in core 08CF7 from Baiyun sag in the northern South China Sea was investigated.MS values range from 5.54×10-6SI to 26.56×10-6SI with a mean value of 11.41×10-6SI.According to the colors and MS values of sediments,the susceptibility profile of core 08CF7 was divided into three different sedimentary layers,0 to 188 cm,188 to 240 cm and below 240 cm,which were corresponded to the sulfidic zone,the transition zone between the sulfidic zone and the upper sulfate methane transition(SMT) and SMT,respectively.Our results indicate that sub-sea gas venting is responsible for the magnetic anomalies of sediments in core 08CF7.A possible mechanism is proposed:under the strongly reducing environments,HS-of AOM product reacts with Fe3+ in sediments,resulting in precipitation of paramagnetic pyrite.MS variability of marine sediments is feasible to estimate the occurrence and depth of SMT,which provides an indirect method to identification of gas hydrate below the seafloor in the northern slope of South China Sea.