孟加拉湾大陆边缘沉积物中自生黄铁矿及其古环境指示意义

Pyrite is one of the common authigenic minerals precipitated in continental margin sediments during the early diagenesis, and usually was occurred and preserved in form of solid crystals. Pyrite is believed as a result of microbial sulfate reduction coupled with the anaerobic oxidation of clasticoragnic matter and/or methane during the sedimentation and could have a potential to record the key processes between the downward seawater and the upperward pore water as well as the alternation of sediment components during the early diagenesis. The Bay of Bengal is located on the course of China proposed“Marine Silk Road”and was drilled by IODP 353 Expedition with a huge cored samples recovered from both passive and active continental margin, which makes a best opportunity for comprehensive research of authigenic pyrite in continental margin. Based on the collected 2360 samples from IODP 353 the application wants to establish firstly the various characteristics of pyrite including concentration, morphology, distribution and stable sulfur isotope etc. to recognize the position of paleo-SMTZ in sediment column from Miocene and to decipher the potential relationship between authigenic pyrites and regional sea level changes, glacial-interglacial cycles and tectonic / magmatic events. So, there will be great significance from the proposal not only theoretically in the pyrite formation mechanism but also practically in service on the gas hydrate exploration in South China Sea, as well as politically in service on the China national strategy of “One Belt, One Road”.

黄铁矿是大陆边缘海洋沉积物中早期成岩阶段最常见自生矿物之一，以固相形式保留在沉积物或地层中，是硫酸盐还原作用下沉积物中有机质和/或甲烷厌氧氧化的产物，有效记录海水下渗、流体上涌和沉积组份变化等信息。孟加拉湾国际大洋发现计划（IODP）353航次提供了长岩芯样品，为系统开展大陆边缘沉积物中黄铁矿研究创造了难得机遇。研究IODP353航次中新世以来约2360件沉积物等样品中黄铁矿等自生矿物相对含量、形貌、分布、稳定同位素组成等，利用自生黄铁矿的综合特征指示古海洋硫酸盐-甲烷交换带（paleo-SMTZ）的变迁规律，探讨它们与海平面升降、冰期-间冰期旋回和构造/热事件的成因联系。理论上创新性利用黄铁矿识别古海洋硫酸盐-甲烷转换带位置的新途径，为“将今论古”探讨地史深时古海洋甲烷事件及其古环境意义等提供现实案例，实践上有助于我国海域水合物成藏演化和勘探开发，政治上服务于 “一带一路”中国战略。

项目的研究背景是基于申请者在印度洋孟加拉湾和安德曼海参加IODP353航次期间和航次后研究工作中获得的长岩芯钻探沉积物样品，结合申请者在南海北部陆坡多年积累的岩芯沉积物样品和与国外合作者共同采集的波罗的海沉积物样品等，聚焦海洋沉积物中黄铁矿等自生矿物的成因机理及其所指示的沉积环境变迁意义等，重点研究了上述海域海洋沉积物中黄铁矿等自生矿物的含量、形貌、分布、多硫等稳定同位素组成和元素地球化学特征，分析了海洋沉积物中黄铁矿等自生矿物发育特征与海洋天然气水合物的成藏演化之间关系等，结果表明海洋沉积物中黄铁矿的形成主要受控于硫酸盐还原背景下有机质厌氧氧化过程（OSR）、甲烷厌氧氧化过程（AOM）和硫酸盐－甲烷转换带（SMTZ）位置的变迁等因素，并伴随有自生碳酸盐类矿物、石膏、蓝铁矿等自生矿物的形成，基本上厘清了大陆边缘海洋沉积物中黄铁矿等自生矿物的形成机制及其地质、地球化学识别标志，实现了项目申请书上预定的二个科学目标：即厘清大陆边缘沉积物中自生黄铁矿的形成机制和厘清水合物赋存海区沉积物中黄铁矿等自生矿物对水合物成藏演化的指示意义。同时，本项目“古今结合”分析了古海洋硫酸盐-甲烷转换带的识别标志，探讨了地质历史上重大转折期古海洋甲烷异常渗漏事件所引起的自生矿物发育特征和地质-地球化学识别标志及其古环境、古气候效应等。项目在研期间在国际著名地学期刊EPSL, GCA, Marine Chemistry, Precambrian Research, PPP等发表学术文章共10篇，其中SCI文章8篇（2篇Nature-Index）。取得的研究成果有助于在理论上开拓如何利用黄铁矿等自生矿物来识别古海洋重大甲烷事件的新研究手段，在实践上指导我国海域天然气水合物的成藏演化和勘探。