Abstract: PCFA, PFeand four kinds of active Fe components in sediment core 9734 from the continental slope of the northern South China Sea gas hydrate potential zone, near Jiulong Methane Reef, were extracted and analyzed. The results showed that the sedimental PCFA is the dominant P ingredient at this location, with its content fluctuating downwards from the seabed from 0.34% to 3.24%. PFe content was found to be 01.38%, displaying several upanddown fluctuations. Similarly, Fepy and Fecarb contents fluctuated between 0.050.72% and 0.140.64%, with averages of 0.27% and 0.33%, respectively; both contents increased at the top of SMTZ, while Fepy content decreased sharply and Fecarb content increased slowly at the bottom of SMTZ. And Feox and Femag contents varied between 0.320.73% and 0.180.34%, with averages of 0.46% and 0.25%, respectively. The variation trends of Feox and Femag were similar. Combining with TS, AVS, SO2-4 and other indicators, our analysis showed that CO2-3, produced by two types of reductive reactions in core 9734, inhibited the formation of apatite core; and furthermore, SMTZ can be identified quickly from sharp increments of Feox and Femag contents and opposite variation trends of Fepy and Fecarb. Recurrent fluctuations of PCFA and PFe under SMI, as observed in this study, shall provide us with new ideas in studying paleoSMI and the range of effects from decomposition of methane hydrate.

Key words: sulfate methane transition zone, methane hydrate, iron components, phosphorus forms, early diagenesis