Authigenic minerals at site GMGS2-16 of northeastern South China Sea and its implications for gas hydrate evolution

The sulfate-methane transition zone (SMTZ) is a pivotal biogeochemical zone within an anoxic sediment column, in which anaerobic oxidation of methane (AOM) may affect the formation of authigenic minerals, such as carbonate, pyrite, barite and gypsum. In this paper, authigenic minerals in 58 marine sediment samples from site GMGS2-16 at the gas hydrate drilling area in the northeastern South China Sea were analyzed. We investigated the type, content, distribution, morphology and isotopic composition of the authigenic minerals to explore the formation mechanism of them and their implications for methane events. Carbonate, pyrite and gypsum are the three dominant authigenic minerals at site GMGS2-16, and also found is the presence of elemental sulfur particles. The distribution of the authigenic mineral contents varies greatly and shows several high concentration peaks. The authigenic carbonates are formed in irregular clumps. These carbonates have extremely negative carbon isotope (-37.3‰ VPDB~-51.7‰ VPDB) and heavy oxygen isotope compositions (3.13‰ VPDB~4.95‰ VPDB), which are typical characteristics of methane-derived carbonates, suggesting that AOM is the trigger for carbonate precipitation. Authigenic pyrites are mostly in forms of irregular masses, rod-tube and organism-filling aggregates. The sulfur isotopic values (δ³⁴S) of the authigenic pyrites range from -41.7‰ VCDT to 27.1‰ VCDT. Anomalously positive excursions of sulfur isotopic values probably stem from enhanced-AOM in the SMTZ because of high methane flux. The AOM-derived authigenic carbonates are basically consistent with the δ³⁴S-anomaly authigenic pyrite production, indicating the occurrence of episodic methane events, which may be related to natural gas hydrate dissociation at site GMGS2-16. Authigenic gypsums occur as hyaline prisms more or less elongated and as dull prismatic or lenticular crystals, and some of them are attached to the authigenic pyrites. We tentatively propose that the precipitation of the authigenic gypsums results from ion exclusion during the formation of gas hydrates and/or pyrite oxidation caused by the change in environmental redox conditions. Hence, carbonate- pyrite-gypsum authigenic mineral assemblage in marine sediments may have the possibility to be used as a proxy for development of natural gas hydrate.