Multi-mineral coupling reveal iron-sulfur cycles in a receding methane seep

Published: 22 February 2023| Version 1 | DOI: 10.17632/bjbnff2gts.1
文覃 张


The studies aiming to uncover diverse minerals as archive of paleo- and modern methane seep are extensive. Furthermore, Fe-S cycle in methane seep has attracted our attention for a long time. The predominated biogeochemical reaction in methane seep is sulfate reduction coupled with the anaerobic oxidation of methane and mainly occurs in sulfate–methane transition zone (SMTZ). The H2S generated from this reaction combined with active iron in the sediment and eventually formed pyrite (FeS2). Here, we studied a core with length of 14 m sampled from Shenhu Area, South China Sea by combining multiple methods such as SEM and EDS test, AMS14C dating of planktonic foraminifera. By coupling various minerals with respect to their features, we found two paleo-SMTZs which means there were twice methane seep activities. The AMS14C dating and carbon and oxygen isotope test planktonic foraminifera exhibited the successive sedimentation from MIS3 to MIS1. The increasing contents of pyrite and mean diameter as well as standard deviation of framboid, cubic pyrite emerged in several intervals and the extremely negative δ34S value of hand-picked pyrite indicated both of SMTZs situated at or near the surface of seafloor. The vast elemental sulfur emerged through the core (especially in SMTZ) implied the methane seep activity had subsided. Moreover, intermediate species of pyrite and framboid goethite (pyrite pseudomorphs) discovered in various intervals further confirmed this viewpoint. Basic on these results, we further illuminated the Fe-S cycle in this unique core directly influenced by the modification of SMTZ position. High content of pyrite and larger framboid formed during methane flux intensified. While after the methane seep activity weaken and SMTZ migrated into deep sediment, former pyrite oxidized by penetrating oxidic seawater and thus formed intermediate species and subsequently Fe (hydrogen) oxide (especially framboid goethite). Therefore, our result provided a rare reference to establish relatively complete Fe-S cycle through diverse bearing-sulfur and/or iron minerals.



Marine Geology