Multimineral coupling reveals the iron–sulfur cycle in a receding methane seep

Many studies have aimed to establish various minerals as archives of paleo- and modern methane seeps. Furthermore, the Fe-S cycle in methane seeps has attracted attention for a long time. The predominant biogeochemical reaction in methane seeps is sulfate reduction coupled with the anaerobic oxidation of methane, which mainly occurs in the sulfate–methane transition zone (SMTZ). The H2S generated from this reaction combines with active iron in the sediments and eventually forms pyrite (FeS2). Here, we studied a core with a length of 14 m sampled from the Shenhu area, South China Sea, via multiple methods, such as SEM and EDS tests and AMS 14C dating of planktonic foraminifera. By evaluating the presence of various minerals, we found two paleo-SMTZs, which means that there were two methane seepage events. AMS14C dating and the carbon and oxygen isotopic test for planktonic foraminifera indicated successive sedimentation from MIS3 to MIS1. The low correlations between pyrite and TOC and δ13CTOC indicated that OSR was not the dominant biogeochemical reaction in this core. The increasing contents of pyrite and the mean diameter as well as the standard deviation of framboid and cubic pyrite found in several depth intervals and the extremely negative δ34S value of hand-picked pyrite indicated that both SMTZs were situated at or near the surface of the seafloor. The vast elemental sulfur that was distributed throughout the core (especially in the SMTZ) implied that the methane seep activity had subsided. Moreover, the intermediate species formed during pyrite and framboid goethite formation (pyrite pseudomorphs) that were discovered in various intervals further confirmed this viewpoint. Based on these results, we further concluded that the Fe-S cycle in this unique core was directly influenced by changes in the SMTZ position. High pyrite contents and larger framboids formed when methane flux intensified. After the methane seep activity weakened and the SMTZ migrated to deeper sediments, previously formed pyrite was oxidized by oxygen-containing seawater and thus formed intermediate species and ultimately Fe (hydrogen) oxide (especially framboid goethite). Therefore, our results provide a unique reference to establish a relatively complete Fe-S cycle through diverse sulfur- and/or iron-bearing minerals.诸多研究致力于将各类矿物确立为古、现代甲烷渗漏的沉积记录载体。此外，甲烷渗漏环境中的铁硫循环长期以来备受学界关注。甲烷渗漏环境中占主导的生物地球化学反应为硫酸盐还原耦合甲烷厌氧氧化，该反应主要发生于硫酸盐-甲烷过渡带（sulfate–methane transition zone, SMTZ）。此反应生成的硫化氢与沉积物中的活性铁结合，最终形成黄铁矿（FeS₂）。本研究针对南海神狐海域采集的一根长14 m的沉积物岩芯，采用扫描电子显微镜（Scanning Electron Microscope, SEM）、能谱仪（Energy Dispersive X-ray Spectroscopy, EDS）测试以及浮游有孔虫加速器质谱（AMS）¹⁴C测年等多种方法开展分析。通过评估各类矿物的赋存状态，研究识别出两处古SMTZ，表明研究区域曾发生两期甲烷渗漏事件。浮游有孔虫的AMS ¹⁴C测年与碳、氧同位素测试结果显示，沉积物沉积序列自深海氧同位素3阶段（MIS3）持续至深海氧同位素1阶段（MIS1）。黄铁矿与总有机碳（Total Organic Carbon, TOC）以及δ¹³C_TOC之间的低相关性表明，有机质硫酸盐还原（Organic Sulfate Reduction, OSR）并非本岩芯中的主导生物地球化学反应。在多个深度层位中观测到黄铁矿含量升高，同时草莓状黄铁矿与立方黄铁矿的平均粒径及粒径标准偏差均有所增大，且手工分选得到的黄铁矿的δ³⁴S值呈极端负值，上述特征表明两处古SMTZ均位于海底表面或近表层。全岩芯（尤其在SMTZ层位内）广泛分布的单质硫暗示甲烷渗漏活动已衰退。此外，在多个层位中发现的黄铁矿及草莓状针铁矿形成过程中的中间产物（黄铁矿假象）进一步佐证了这一观点。基于上述结果，本研究进一步推断，该独特岩芯中的铁硫循环直接受SMTZ位置变化的调控。当甲烷通量增强时，会形成高含量黄铁矿与较大粒径的草莓状黄铁矿；当甲烷渗漏活动减弱、SMTZ向沉积物深部迁移时，先前形成的黄铁矿会被含氧海水氧化，生成中间产物并最终转化为铁（氢）氧化物（尤以草莓状针铁矿为典型代表）。因此，本研究结果为通过各类含硫和/或含铁矿物构建相对完整的铁硫循环记录提供了独特的参考依据。