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. By evaluating the high contents of CRS and 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 CRS and TOC and δ13CTOC indicated that organoclastic sulfate reduction 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 both CRS and hand-picked pyrite indicated that the two 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 CRS and 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与现代甲烷渗漏的记录载体。此外，甲烷渗漏环境中的铁硫循环长期以来备受关注。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-甲烷转换带（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）。本次研究针对南海神狐海域采集的14米长岩芯，采用扫描电子显微镜（SEM）、能谱分析（EDS）等多种方法开展实验。通过评估铬还原硫（CRS）的高含量与多种矿物的赋存特征，我们识别出两处古SMTZ，表明该区域曾发生两期甲烷渗漏事件。针对浮游有孔虫的加速器质谱14C（AMS14C）测年以及碳、氧同位素测试结果显示，沉积物沉积序列自海洋同位素阶段3（MIS3）延续至海洋同位素阶段1（MIS1）。铬还原硫与总有机碳（TOC）、总有机碳碳同位素（δ¹³C_TOC）之间的低相关性表明，有机碎屑硫酸盐还原并非该岩芯中的主导生物地球化学反应。在多个深度层位中发现的黄铁矿含量升高、草莓状与立方体黄铁矿的平均粒径及标准差增大，以及铬还原硫和手选黄铁矿的硫同位素（δ³⁴S）极端负值，均指示两处古SMTZ位于海底表面或其附近。全岩芯广泛分布的单质硫（尤其在SMTZ层位）暗示甲烷渗漏活动已趋于衰退。此外，在多个层位中发现的黄铁矿与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，我们进一步得出结论：该独特岩芯中的铁硫循环直接受SMTZ位置变化的影响。当甲烷通量增强时，铬还原硫与黄铁矿含量升高，草莓状黄铁矿粒径变大。在甲烷渗漏活动减弱、SMTZ向沉积物深部迁移后，先期形成的黄铁矿被含氧海水氧化，生成中间产物并最终形成铁（氢）氧化物（尤以草莓状针铁矿为代表）。因此，本研究结果为通过各类含硫和/或含铁矿物重建相对完整的铁硫循环提供了独特参考依据。