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