Age determination and pyrite framboids analysis of sediment cores from the northern continental margin of the South China Sea

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 d34S 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.