沉积过程对自生黄铁矿硫同位素的约束

Constraint of Sedimentary Processes on the Sulfur Isotope of Authigenic Pyrite

自生黄铁矿是海洋沉积物中还原态硫的主要赋存形式,其形成过程与有机质矿化相关,影响全球的C⁃S⁃Fe生物地球化学循环。自生黄铁矿硫同位素分馏主要受微生物硫酸盐还原的控制,但近期的研究成果表明局部沉积环境的改变也可以影响黄铁矿硫同位素的组成,特别是在浅海环境。在浅海非稳态沉积环境内,物理再改造和生物扰动作用,导致硫酸盐还原带内生成的硫化物被再氧化,进而影响黄铁矿的硫同位素值。浅海沉积过程容易受到古气候和海平面变化的影响,引起沉积速率的剧烈波动,导致有机质和活性铁输入的不稳定,进而影响成岩系统的开放性和硫酸盐还原速率,最终影响黄铁矿的硫同位素值。另外,沉积速率的改变还影响硫酸盐—甲烷转换带的迁移,造成有机质和甲烷厌氧氧化硫酸盐还原的相互转化,产生不同的硫同位素信号。东海内陆架泥质区为研究沉积过程对自生黄铁矿的形成及其硫同位素组成的约束机制提供了很好的研究材料。该区域有很好的沉积学研究基础,自生黄铁矿丰富、并且个别层位有生物气(甲烷为主)存在,是研究边缘海C⁃S⁃Fe循环的理想场所。

Authigenic pyrite is the main mineral specie of reduced sulfur in marine sediments.Its formation process is related to organic mineralization and affects the global C⁃S⁃Fe biogeochemical cycle.Sulfur isotope fractionation of authigenic pyrite is mainly controlled by microbial sulfate reduction,but recent studies have indicated that the local depositional environment also affects the composition of pyrite sulfur isotopes,especially in shallow depositional envi⁃ronments.In an unsteady shallow environment,physical reworking and bioturbation lead to reoxidation of sulfides formed in the sulfate reduction zone,which in turn affects the sulfur isotopes of pyrite.The sedimentation process in a shallow depositional environment is readily affected by paleoclimate and sea⁃level changes,which cause drastic fluc⁃tuations in sedimentation rate as well as instable input of,for instance,organic matter and active iron.This in turn af⁃fects the openness of the diagenetic system and ultimately affects the isotopic value of pyritic sulfur.In addition,any change in sedimentation rate also affects the movement of the sulfate⁃methane transition zone,resulting in the conver⁃sion of organic matter and anaerobic oxidation methane sulfate reduction,producing different sulfur isotope signals.The study of the sulfur isotopes of authigenic pyrite in the mud area of the inner shelf of the East China Sea provides a good example for depositional control on the formation of authigenic pyrite and its sulfur isotope composition.The sed⁃imentary process of this area has been well studied,and its sediments have been shown to be enriched in authigenic pyrite and biogas(CH4).Therefore,it is an ideal site for studying the sulfur cycle in a marginal sea,and is expected to provide a new perspective on the global C⁃S⁃Fe biogeochemical cycle.