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 pyritebearing 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 delta S-34 values in pyrites across the carbonate pipe display an extremely wide range on a centimetre scale. The majority of pyrites have delta S-34 values ranging from 19.4 parts per thousand to 45.8 parts per thousand, 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 delta S-34 values than the majority of pyrites, respectively. Pyrites in the abnormal area I have delta S-34 values ranging from 62.2 parts per thousand to 130.3 parts per thousand, suggesting an extremely S-34-enriched microenvironment produced via Rayleigh fractionation of limited sulfate in a closed system. The model calculation indicates that the maximum positive delta S-34 value could be produced when similar to 93% sulfate is consumed. Pyrites in the abnormal area II have delta S-34 values ranging from 32.7 parts per thousand to 18.0 parts per thousand, 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 delta S-34 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 delta S-34 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.