The carbon isotopes of DIC and methane gas from gas hydrate potential area offshore SW Taiwan

Two possible processes could cause the sulfate reduction which is usually found in marine sediments: (1) the bacteria consume sedimentary organic matter with sulfate reduction (or organic matter degradation) oxidation of sedimentary organic matter; (2) anaerobic methane oxidation (AMO). d13C data of dissolved inorganic carbon (DIC) in the interstitial water are considered to be able to distinguish above mentioned two processes. Therefore, the carbon isotopes of methane and DIC of cored samples from three sites: GT1 (at the active margin), GT39B (at the offshore mud volcanoes), and GT44 (at the passive margin), respectively in the gas hydrate potential area offshore SW Taiwan, will be analyzed in this study to better constrain the methane sources and chemical processes occurred in the region. The d13C-methane ranges from -90 to -80 permil at site GT1, indicating a biogenic source. Nevertheless, the values of -63 to -45 permil at site GT39B show that the methane gas is mainly derived from a thermogenic source at that site. Meanwhile, the carbon isotopic characteristics of DIC at the depth of sulfate-methane interface indicate that AOM process played a major role at sites GT1 and GT39B. However, the effect of organic matter degradation cannot be ignored due to the high sedimentation rate in the area. Furthermore, the role of each process at each site will be discussed in this study.