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| Titel |
Methane flux in potential hydrate-bearing sediments offshore southwestern
Taiwan |
| VerfasserIn |
Nai-Chen Chen, Tsanyao Frank Yang, Pei-Chuan Chuang, Wei-Li Hong, Hsuan-Wen Chen, Saulwood Lin, Li-Hung Lin, Ryo Mastumoto, Akihiro Hiruta, Chih-Hsien Sun, Pei-Ling Wang, Tau Yang, Shao-Yong Jiang, Yun-Shuen Wang, San-Hsiung Chung, Cheng-Hong Chen |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250124722
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| Publikation (Nr.) |
 EGU/EGU2016-4200.pdf |
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| Zusammenfassung |
| Methane in interstitial water of hydrate-bearing marine sediments ascends with buoyant fluids
and is discharged into seawater, exerting profound impacts on ocean biogeochemistry and
greenhouse effects. Quantifying the exact magnitude of methane transport across different
geochemical transitions in different geological settings would provide bases to better
constrain global methane discharge to seawater and to assess physio-chemical contexts
imposed on microbial methane production and consumption and carbon sequestration in
marine environments. Using sediments collected from different geological settings
offshore southwestern Taiwan through decadal exploration on gas hydrates, this study
analyzed gas and aqueous geochemistry and calculated methane fluxes across different
compartments. Three geochemical transitions, including sulfate-methane transition zone
(SMTZ), shallow sediments, and sediment-seawater interface were specifically
focused for the flux calculation. The results combined with previous published
data showed that methane fluxes at three interfaces of 2.71×10−3 to 3.52×10−1,
5.28×10−7 to 1.08×100, and 1.34×10−6 to 3.17×100 mmol m−2 d−1, respectively. The
ranges of fluxes suggest that more than 90 % of methane originating from depth was
consumed by anaerobic methanotrophy at the SMTZ, and further >90% of the
remnant methane was removed by aerobic methanotrophy prior to reaching the
sediment-seawater interface. Exceptions are sites at cold seeps where the percentage of
methane released into seawater can reach more than 80% of methane at depth. Most
sites with such high methane fluxes are located at active margin where thrusts and
diapirism are well developed. Carbon mass balance method was applied for the
calculation of anaerobic oxidation of methane (AOM) and organotrophic sulfate
reduction rates at SMTZ. Results indicated that AOM rates were comparable with
fluxes deduced from concentration gradients for most sites. At least 60% of sulfate
infiltrating from seawater was consumed by AOM. Gas compositions and methane
carbon isotopes show microbial gas dominated at passive margin and lower slope of
active margin; by contrast, thermogenic gas source was prevalent at upper slope
of active margin. In summary, transport of deeply sourced methane in potential
hydrate-bearing sediments is strongly controlled by geological structures and microbial
processes. For most of sites, anaerobic and aerobic methanotrophy in sediments
act as efficient biofiltration for the removal of methane. For sites with strong fluid
advection, a great fraction of deeply-sourced methane could escape from anaerobic and
aerobic methanotrophy and be discharged into seawater column. The changing
mechanisms for gas generation from passive to active margin highlights the interplay
between in situ methanogenesis, sediment loading, and connectivity of fluid conduits. |
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