南海北部天然气水合物成藏区自生黄铁矿原位铁同位素组成对甲烷渗漏的响应

Methane emissions from sediments are widely observed along continental margins worldwide, which have been considered as an important source for gas hydrate formation. In areas of diffusive seepage, methane is mostly oxidized in the sediments at the expense of sulfate by a biogeochemical process known as anaerobic oxidation of methane (AOM). This process enhances the formation of authigenic pyrite, which is a crucial recorder of variations in local environmental conditions during AOM. The geochemical cycles of iron and sulfur are intrinsically tied to and eventually recorded in pyrite. Many studies have focused on the sulfur isotopic composition of sedimentary pyrite, however, its associated fractionation process is complicatedly controlled by multiple factors. Hence, studies on the iron isotopic composition of pyrite would provide a new insight into the dynamics of biogeochemical process in methane-rich sediments. Some recent applications of iron isotopic compositions of pyrite have shed light on tracing AOM process. However, it is challenging to unequivocally unravel the intrinsic characteristics of AOM-derived pyrite solely based on the bulk iron isotope analysis, because of the common coexistence of pyrite resulting from organiclastic sulfate reduction. To better understand how AOM affects the iron isotope pattern of authigenic pyrite and its implication for seepage activities, we will apply LA-MC-ICP-MS to analyze the in-situ iron isotopic composition of pyrite from three gas hydrate-bearing sedimentary environments of the Shenhu, Taixinan and Qiongdongnan areas, South China Sea. In addition, we will try to find out the differences of the iron isotope patterns of pyrite from these areas and further explore the mechanism of how early diagenetic processes affect the iron isotope patterns of pyrite in different seepage areas. The obtained results will help to trace the diagenetic history of the methane-rich environments and provide new insight into the exploration of gas hydrate.

海底甲烷渗漏是发生在全球大陆边缘沉积物中的普遍现象，对天然气水合物的成矿具有重要影响。大部分甲烷在甲烷厌氧氧化反应（AOM）中被耗尽并促进自生黄铁矿的形成，它们有效记录了甲烷活动的信息。黄铁矿是铁、硫循环的共同载体，前人对其硫同位素开展了大量研究，然而鉴于硫同位素分馏受到多重因素影响，借助铁同位素可为了解黄铁矿化过程提供新思路。目前黄铁矿铁同位素在示踪AOM方面已彰显出良好前景，但由于早期硫酸盐还原作用的影响，黄铁矿常具有多期次的结构特征，故采用全岩分析难以还原AOM相关黄铁矿的内在信息。本项目拟采用LA-MC-ICP-MS对南海神狐海域、台西南盆地和琼东南盆地水合物成藏区自生黄铁矿进行原位铁同位素分析，系统研究AOM对黄铁矿铁同位素组成的影响，并揭示铁同位素对甲烷渗漏的响应，同时探明影响不同区域黄铁矿铁同位素组成之间存在差异的因素，为追溯南海北部甲烷渗漏演化过程及水合物远景预测提供依据。

自生黄铁矿作为冷泉/天然气水合物区沉积物中甲烷厌氧氧化作用（AOM）的重要产物，是我们了解冷泉环境中生物地球化学过程和元素循环的重要研究对象。本项目采用原位分析手段对南海北部多个站位自生黄铁矿进行了（LA-MC-ICP-MS）原位Fe同位素和（LA-ICP-MS）原位微量元素分析，发现在硫酸盐-甲烷转换带（SMTZ）中的自生黄铁矿均表现出较高的Fe同位素值（HS148站位:−0.03‰至+1.81‰; DH-CL11站位:−1.18‰至+1.21‰），并且不同类型黄铁矿（草莓状黄铁矿、增生型黄铁矿和自形黄铁矿）的铁同位素组成差异较大，其中增生型和自形黄铁矿具有更高的Fe同位素值，该特征与黄铁矿S同位素较为一致。同时SMTZ处黄铁矿的微量元素如Mn、Pb、Zn、Cu、V也非SMTZ处的黄铁矿具有更高的特征，而Mo、Sb、As的含量受到浅部有机质参与的硫酸盐还原过程的影响。研究认为SMTZ处黄铁矿Fe同位素以及Mn等元素含量的一致升高主要受到AOM过程的影响。该过程形成大量的硫化氢，促使沉积物中大量活性铁锰矿物发生溶解，一方面使得黄铁矿的Fe同位素具有较高的特征，另一方面导致铁锰矿物溶解时释放出的微量元素得以进入黄铁矿。因此，结合黄铁矿Fe、S同位素和微量元素能更准确地识别甲烷厌氧氧化作用，有利于我们深入了解甲烷渗漏过程及其相关的元素循环过程。我们进一步探讨了AOM对冷泉碳酸盐岩结核Mo同位素的影响，发现大部分碳酸盐结核表现出明显的Mo含量富集以及较高的Mo同位素组成特征（δ98Mo值均约为+1.9‰）（标样 NIST3134定义为+0.25‰)。这些特征指示了甲烷强烈喷溢过程中AOM释放出大量的硫化氢，促使孔隙水中的溶解钼酸盐几乎定量被黄铁矿捕获，并最终保存在碳酸盐结核中，从而说明冷泉系统的Mo同位素体系是我们了解冷泉环境的铁、硫和微量元素循环的一个新的工具。