Multiple sulfur isotopic evidence for the origin of elemental sulfur in an iron-dominated gas hydrate-bearing sedimentary environment.

Elemental sulfur is commonly regarded as the product of oxidative sulfur cycling in the sediment. However, reports on the occurrence of elemental sulfur in seepage areas are few and thus its origin and mechanisms controlling its distribution are insufficiently understood. Here, we analyzed the multiple sulfur isotopic compositions for elemental sulfur and pyrite from an iron-dominated gas hydrate-bearing sedimentary environment of the South China Sea to unravel the impact of sulfate-driven anaerobic oxidation of methane (SO 4 -AOM) on the formation of elemental sulfur. The multiple sulfur isotopes reveal variable ranges for both elemental sulfur and pyrite (δ 34 S: between −15.7 and +23.3‰ for elemental sulfur and between −35.3 and +34.4‰ for pyrite; Δ 33 S: between −0.08 and +0.06‰ for elemental sulfur and between −0.03 and +0.15‰ for pyrite). The enrichment of 34 S in pyrite throughout the sediment core suggests pronounced SO 4 -AOM in paleo-sulfate-methane transition zones (SMTZ). In addition, the occurrence of seep carbonates with very negative δ 13 C values (as low as −57‰, V-PDB) coincides with the inferred paleo-SMTZs and agrees with formerly locally pronounced SO 4 -AOM. Interestingly, the multiple sulfur isotopic composition of elemental sulfur reveals a different pattern from that of pyrite derived from organoclastic sulfate reduction (i.e., with low δ 34 S and high Δ 33 S values for the latter). In comparison to coexisting pyrite, most of the elemental sulfur reveals higher δ 34 S values (as much as +28.9‰), which is best explained by an enrichment of 34 S in the residual pool of dissolved sulfide generated by SO 4 -AOM. As an intermediate sulfur phase, elemental sulfur can form via sulfide oxidation coupled to iron reduction, but it can only persist in the absence of free sulfide. Therefore, the occurrence of 34 S enriched elemental sulfur is likely to represent an oxidative product after hydrogen sulfide had vanished due to vertical displacement of the SMTZ. Our observations suggest that elemental sulfur may serve as a useful recorder for reconstructing the dynamics of sulfur cycling in modern and possibly ancient seepage areas. [ABSTRACT FROM AUTHOR]