Enhanced framboidal pyrite formation related to anaerobic oxidation of methane in the sulfate-methane transition zone of the northern South China Sea

Many studies have confirmed that size distributions of framboidal pyrite can be an effective indicator of bottom-water redox conditions in modern as well as ancient sedimentary environments. However, one environment in which production of framboidal pyrite has not been sufficiently studied to date is the sulfate-methane transition zone (SMTZ), in which anaerobic oxidation of methane (AOM) is coupled with microbial sulfate reduction (MSR) to enhance iron sulfide mineral precipitation (e.g., FeS2, FeS, and Fe3S4). Here, we document for the first time size distributions for pyrite framboids from the SMTZ, based on data from two sites in the methane hydrate-bearing region of the northern South China Sea. On the basis of framboid size, pyrite concentration, and sulfur isotope data, we propose new insights into the formation process of authigenic pyrite framboids within the SMTZ. We conclude that (1) Enhanced anaerobic oxidation of methane (AOM) not only plays a dominant role in the accumulation of 34S-enriched pyrite but also is responsible for formation of highly variable and sometimes exceptionally large pyrite framboids in the SMTZ; (2) Most framboids occur in 'framboid clusters' which commonly exhibit rod-like shapes, secondary overgrowths, heavier δ34S values, and unusual size distributions (e.g., mean size > 20 μm and standard deviation > 3.0 μm) in the SMTZ; (3) Pyrite framboids formed in the SMTZ, which have characteristics different from those formed in the sulfate reduction zone (SRZ), do not comment on redox conditions of the overlying water column; and (4) Framboid occurrences with similar characteristics in ancient marine deposits may be considered indicators of enhanced AOM and mark the former position of the SMTZ in the paleo-marine system. In addition, significant quantities of elemental sulfur were observed in the SMTZ, possibly related to anaerobic oxidation of hydrogen sulfide and fluctuations of the SMTZ.