The carbon isotopes of DIC and methane gas from gas hydrate potential area offshore SW Taiwan
Date Issued
2009
Date
2009
Author(s)
Chen, Nai-Chen
Abstract
In the previous studies, high methane flux in offshore SW Taiwan was considered as methane from gas hydrate dissociation. However, dynamic microbial activities are observed in the shallow sediments indicating mixture of methane from these shallow reactions. In this study, piston core samples from gas hydrate potential area offshore SW Taiwan during several surveys since 2007 ( r-v ORI, Leg 828, 835, 860) are collected. The 13C of dissolved inorganic carbon ( DIC ) and d13C and dD of methane in interstitial water are measured in order to identify the source of methane and biogeochemistry processes. Our results indicate the followings: 1) samples collected from lower slope of active continental margin (sites GT1, 26, 27, 28) exhibit methane which mainly generating by microbial activities (d13C = -72.8 ~ -112 ‰）; on the other hand, methane from samples collected from upper slope of active continental margin (site GT39B) is from thermogenic source (d13C = -45 ~ -63.8 ‰). 2) The lightest d13C value of methane and DIC observed at sulfate methane transition (SMT) of depth profile shows that carbon cycling within this transition indeed exists—d13C-depleted methane is generated at the top of methanogenic zone when d13C-depleted CO2 produced by microbially-mediated anaerobic oxidation of methane (AOM), and it is recycled back to methane pool by CO2 reduction. 3) d13C profile of DIC in many sites become heavier with increasing depth; this indicates that CO2 reduction become more and more important. All these evidences indicate that significant microbial activities in the study area. Therefore, methane from microbial activities should not be overlooked.
Subjects
methane
dissolved inorganic carbonate
gas hydrate
carbon isotopes
methanogenesis
Type
thesis
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