Research ArticleGEOCHEMISTRY

Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

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Science Advances  13 Jun 2018:
Vol. 4, no. 6, eaao4631
DOI: 10.1126/sciadv.aao4631
  • Fig. 1 Location of the study sites (KMV#5) and seismic profiles of the Kumano forearc basin.

    (A) Bathymetric map showing the location of KMV#5 in the Kumano forearc basin and IODP Sites C0002 and C0009. (B) Transect seismic cross-section in the middle part of the Kumano Basin based on the data from Morita et al. (17). (C) P-wave velocity predicted by three-dimensional (3D) tomography inversion during 3D prestack depth migration based on the data from Tsuji et al. (18). The overpressure zone (fluid or gas accumulation zone) is identified as a low-amplitude and low-velocity zone at 400 to 700 mbsf in the forearc basin sequence. The low-velocity zone, which is located above the ridge because of the megasplay fault displacement, suggests that the overpressured fluids are moving upward along the interpreted ancient megasplay faults. The mud volcanoes are located along the northern extension of the megasplay faults.

  • Fig. 2 Chloride concentration and stable isotopic compositions of pore water in sediments of KMV#5.

    Vertical profile of Cl (A), δ18O (B), and δD (C) of pore water in sediments. Circle and square plots represent pore water sample and water from dissociated gas hydrate fragment, respectively. Dashed line represents the averaged isotopic value of methane hydrates, and blue-filled range denotes original δ18O and δD values of pore water before the formation of methane hydrates.

  • Fig. 3 Depth profiles of biogeochemical parameters in sediments of KMV#5.

    (A) Vertical profiles of CH4 concentrations. (B) δ13C and δD of methane. (C) Concentrations of SO42−, acetate, and dissolved inorganic carbon (DIC). (D) δ13C of acetate, DIC, and total organic carbon (TOC). (E) H2 concentrations. Δ13CH3D temperatures are shown in (B).

  • Fig. 4 Graphs of stable isotopic gas classifications in sediments of KMV#5.

    (A) Relationship between C1/C2 and δ13CCH4 with respect to gas source (12) at KMV#5. Black solid line represents the most plausible two–end-member mixing scenario between biogenic and thermogenic hydrocarbons, and the black dashed curves show the possible range based on varying assumptions. Blue dashed curve shows the scenario between the biogenic hydrocarbon with normal δ13CCH4 (−65‰) and thermogenic hydrocarbons. Percentage labels represent contribution of biogenic methane to the total methane. (B) Relationships between δ13CCH4 and δ13CCO2 with isotope fractionation lines (14). (C) Relationships between δ13CCH4 and δDCH4 with respect to gas source (14). In (A) to (C), the data from the other mud volcanoes in the Kumano Basin (19), Nankai Trough area (see Supplementary Methods), and Cascadia margin (27) are plotted. (D) Clumped isotopologues of methane from KMV#5 and other locations (3, 47, 48). Solid green curve represents isotopic equilibrium, with εmethane/water calibration [εmethane/water = (D/H)methane/(D/H)water − 1] given by Horibe and Craig (60). Green shading represents ranges of εmethane/water calibrations from other published studies (47).

  • Fig. 5 Depth profiles of cell abundance, taxonomic composition of microbial communities, potential activities, and in situ energy yields in sediments of KMV#5.

    (A) Cell abundance in the mud volcano samples (black dots) and the Kumano Basin sediment at Site C0002 (white dots), approximately 30 km south of KMV#5. The dashed line indicates the minimum quantification limit of sedimentary microbial cells, representing the upper limit of 95% confidence intervals of negative controls. (B and C) Taxonomic compositions of bacterial (B) and archaeal (C) communities in mud volcano sediments based on 16S ribosomal RNA (rRNA) gene sequences. Number in parentheses indicates the sample depth. nd, not detected. (D) Potential activities of homoacetogenesis, hydrogenotrophic methanogenesis, acetoclastic methanogenesis, and hydrogenase assessed by radiotracer incubation experiments. (E) Gibbs free energy yields of homoacetogenesis and hydrogenotrophic methanogenesis under in situ conditions (H2, 28.1 mM) and headspace H2 concentrations.

  • Fig. 6 Schematic figure illustrating methanogenesis in the deep mud volcano sediments associated with fluid migration via the megasplay fault.

    The cross-section is based on seismic profiles (see Fig. 1 and fig. S1).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaao4631/DC1

    Supplementary Text

    Supplementary Methods

    fig. S1. Photographs and Raman spectra of methane hydrate and a seismic profile of KMV#5.

    fig. S2. Thermal gradient based on the temperatures measured in situ at KMV#5 and IODP site C0009.

    fig. S3. Chloride concentrations versus stable isotopic compositions of water.

    fig. S4. Depth profile of the estimated methane hydrate saturation in pore space based on δ18O and δD.

    fig. S5. Average linkage clustering analysis based on the Bray-Curtis dissimilarity distance of 16S rRNA genes.

    fig. S6. Community network analysis based on Spearman’s correlation coefficient.

    fig. S7. Characteristics of the isolated methanogenic archaeon strain 1H1.

    fig. S8. Chemical and stable isotopic compositions of hydrocarbon gases in the Nankai Trough area.

    fig. S9. Mixing curves of Δ13CH3D between biogenic and thermogenic methane end-members consistent with a final clumped isotopologue temperature of 30°C for the resultant mixture.

    table S1. Geochemical data from KMV#5 analyzed in this study.

    table S2. δ13C-CH4, δD-CH4, and Δ13CH3D temperature of Hybrid-PCS sediment core samples.

    table S3. Production test of gasses from Hybrid-PCS sediment core samples.

    table S4. Cell concentration in sediment core samples from KMV#5.

    table S5. Diversity indices of microbial communities in sediment core samples from KMV#5 based on 16S rRNA gene sequence analysis.

    table S6. Activity of methanogenesis, acetogenesis, and hydrogenase based on radiotracer incubation analyses.

    table S7. Concentration of archaeal core and IPLs.

    table S8. Thermogenic and biogenic end-member values for mixing calculation.

    References (6179)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Supplementary Methods
    • fig. S1. Photographs and Raman spectra of methane hydrate and a seismic profile of KMV#5.
    • fig. S2. Thermal gradient based on the temperatures measured in situ at KMV#5 and IODP site C0009.
    • fig. S3. Chloride concentrations versus stable isotopic compositions of water.
    • fig. S4. Depth profile of the estimated methane hydrate saturation in pore space based on δ18O and δD.
    • fig. S5. Average linkage clustering analysis based on the Bray-Curtis dissimilarity distance of 16S rRNA genes.
    • fig. S6. Community network analysis based on Spearman’s correlation coefficient.
    • fig. S7. Characteristics of the isolated methanogenic archaeon strain 1H1.
    • fig. S8. Chemical and stable isotopic compositions of hydrocarbon gases in the Nankai Trough area.
    • fig. S9. Mixing curves of Δ13CH3D between biogenic and thermogenic methane end-members consistent with a final clumped isotopologue temperature of 30°C for the resultant mixture.
    • References (61–79)

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    Other Supplementary Material for this manuscript includes the following:

    • table S1 (Microsoft Excel format). Geochemical data from KMV#5 analyzed in this study.
    • table S2 (Microsoft Excel format). δ13C-CH4, δD-CH4, and Δ13CH3D temperature of Hybrid-PCS sediment core samples.
    • table S3 (Microsoft Excel format). Production test of gasses from Hybrid-PCS sediment core samples.
    • table S4 (Microsoft Excel format). Cell concentration in sediment core samples from KMV#5.
    • table S5 (Microsoft Excel format). Diversity indices of microbial communities in sediment core samples from KMV#5 based on 16S rRNA gene sequence analysis.
    • table S6 (Microsoft Excel format). Activity of methanogenesis, acetogenesis, and hydrogenase based on radiotracer incubation analyses.
    • table S7 (Microsoft Excel format). Concentration of archaeal core and IPLs.
    • table S8 (Microsoft Excel format). Thermogenic and biogenic end-member values for mixing calculation.

    Download Tables S1 to S8

    Files in this Data Supplement:

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