Extremely variable sulfur isotopic compositions of pyrites in carbonate pipes from the northern South China Sea: Implications for a non-steady microenvironment

To further constrain the microenvironment during the growth of carbonate pipes at cold seeps, we present the whole-rock carbon and oxygen, and in situ pyrite sulfur isotopic compositions along the cross-sections in pyrite-bearing carbonate pipes from the northern South China Sea. The whole-rock geochemical data suggest that these carbonate pipes were formed in an anoxic and methane-rich environment. Furthermore, in situ δ34S values in pyrites across the carbonate pipe display an extremely wide range on a centimetre scale. The majority of pyrites have δ34S values ranging from 19.4‰ to 45.8‰, reflecting the dominance of sulfate-driven anaerobic oxidation of methane (AOM) at and below the sulfate methane transition zone (SMTZ). Two abnormal areas show heavier or lighter δ34S values than the majority of pyrites, respectively. Pyrites in the abnormal area I have δ34S values ranging from 62.2‰ to 130.3‰, suggesting an extremely 34S-enriched microenvironment produced via Rayleigh fractionation of limited sulfate in a closed system. The model calculation indicates that the maximum positive δ34S value could be produced when ∼93% sulfate is consumed. Pyrites in the abnormal area II have δ34S values ranging from −32.7‰ to −18.0‰, and are interpreted as a result of early organiclastic sulfate reduction with a low proportion of later sulfate-driven AOM above the SMTZ. Collectively, the extreme variations in δ34S values point to locally migrating SMTZ in a non-steady microenvironment during the growth of carbonate pipes. This study reveals that the in situ pyrite δ34S value is a diagnostic tool for tracing the change in the local microenvironment, and could be a step towards a more comprehensive picture of biogeochemical cycles at continental margins. •The carbonate pipes in the northern SCS were formed in an anoxic and methane-rich environment.•The pyrites within carbonate pipes yield the extremely variable δ34S values indicating a non-steady microenvironment.•The migration of the sulfate methane transition zone is identified during the growth of carbonate pipes.