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June 14, 2018

Methane Hydrates of Microbial Origin Found in Submarine Mud Volcano


An international research team led by Drs. Akira Ijiri and Fumio Inagaki at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: president, Asahiko Taira) has revealed that microbially produced methane hydrates exist 590 m beneath the summit of an active submarine mud volcano in the Kumano forearc basin of the Nankai Trough, off Japan’s Kii Peninsula.

Submarine mud volcanoes are formed by the upward intrusion of deformable lower-density materials from several kilometers below the seabed. This process transports deep-sourced fluids, elements, and hydrocarbons to the seafloor, supporting chemosynthetic benthic life including microbial communities that mediate the anaerobic oxidation of methane with sulfate reduction. However, the biogeochemical and microbiological characteristics of deep submarine mud volcanoes have remained largely unknown.

During research cruises by the deep-sea drilling vessel D/V Chikyu in 2009 and 2012, scientists drilled 200 m into the summit of a highly active submarine mud volcano in the Kumano forearc basin and obtained core samples. Their analyses of deep biosphere and carbon cycling in this setting, using geochemical and geophysical data with microbiological methods, estimated that methane hydrate could exist as deep as 590 m below the summit (112–160 m above the seafloor). The amount of methane was estimated as 3.2 billion m3, ten times larger than expected in a single mud volcano. In addition, more than 90% of the methane appears to be microbial in origin and was produced in sediments 400–700 m below the seafloor, the mud volcano’s source layer. In this environment, low-salinity water derived from clay mineral dehydration is supplied from an old accretionary prism through a mega-splay fault; this fluid seems to promote microbial activities such as hydrogenotrophic methanogenesis.

These results indicate that the production and migration of fluid in an oceanic plate subduction zone is strongly related to the production of natural gas by microorganisms living in the deep subseafloor. These findings are highly important for a better understanding of biogenic gas resource generation under the seafloor as well as the relationship between earth dynamics and the deep biosphere.

This study was supported by JSPS KAKENHI Grant Number JP23681007, JP26287128、JP17H01871 and JP26251041, the JSPS Funding Program for Next Generation World-Leading Researchers (GR102), and the East Asia and Pacific Summer Institutes program through the National Science Foundation and JSPS (no. 1308171).

The above results were published in Science Advances on June 14, 2018 (JST).

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

Authors: Akira Ijiri1,2*, Fumio Inagaki1,2,3*, Yusuke Kubo4, Rishi R. Adhikari5, Shohei Hattori6, Tatsuhiko Hoshino1,2, Hiroyuki Imachi2,7, Shinsuke Kawagucci2,7, Yuki Morono1,2, Yoko Ohtomo1,2, Shuhei Ono8, Sanae Sakai7, Ken Takai2,7,9, Tomohiro Toki10, David T. Wang8, Marcos Y. Yoshinaga11, Gail L.Arnold12, Juichiro Ashi13, David H. Case14, Tomas Feseker11, Kai-Uwe Hinrichs11, Yojiro Ikegawa15, Minoru Ikehara16, Jens Kallmeyer5, Hidenori Kumagai2, Mark A. Lever12, Sumito Morita17, Ko-ichi Nakamura18, Yuki Nakamura13, Manabu Nishizawa7, Victoria J. Orphan14, Hans Røy12, Frauke Schmidt11, Atsushi Tani19, Wataru Tanikawa1,2, Takeshi Terada20, Hitoshi Tomaru21, Takeshi Tsuji22, Urumu Tsunogai23, Yasuhiko T. Yamaguchi13,24, and Naohiro Yoshida6,9

Kochi Institute for Core Sample Research, JAMSTEC
Research and Development Center for Submarine Resources, JAMSTEC
Research and Development Center for Ocean Drilling Science, JAMSTEC
Center for Deep Earth Exploration, JAMSTEC
Department of Earth and Environmental Sciences, University of Potsdam
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology
Department of Subsurface Geobiology Research and Analysis, JAMSTEC
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology
Earth-Life Science Institute (ELSI), Tokyo Institute of Technology
Department of Chemistry, Biology and Marine Science, Faculty of Science, University of The Ryukyus
MARUM and Department of Geosciences, University of Bremen
Center for Geomicrobiology, Department of Biological Sciences, Aarhus University
Atmosphere and Ocean Research Institute, The University of Tokyo
Division of Geological and Planetary Sciences, California Institute of Technology
Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI)
Center for Advanced Marine Core Research, Kochi University
Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST)
Department of Earth and Space Science, Graduate School of Science, Osaka University
Marine Works Japan Ltd.
Department of Earth Sciences, Graduate School of Science, Chiba University
Department of Earth Resources Engineering, Kyushu University
Graduate School of Environmental Studies, Nagoya University
Department of Earth and Planetary Science, The University of Tokyo, Tokyo

Figure 1. Location of the mud volcano (KMV#5) drilled in this study. (A) General location of the Kumano forearc basin. (B) Specific locations of KMV#3–KMV#7. KMV#5 is among most active of the 13 or more mud volcanoes observed in the Kumano basin.


Figure 2. Methane hydrate observed in the KMV#5 sediment core. (A) Photograph of small patchy grains of methane hydrates in the core. (B) Infrared thermal imaging of the core. Hydrate dissociation creates a negative (cold) temperature anomaly that can be rapidly imaged using an IR camera. (C) Photograph of a methane hydrate chunk observed in the sediment core. (D) X-ray image of a core section at full pressure in the Pressure Core Analysis and Transfer System showing mud clasts and methane hydrate veins in a matrix of hemipelagic mud.


Figure 3. Vertical profiles of biogeochemical data from sediment cores. (A) Methane concentrations; values measured using the pressure core sample (PCS) were over 100 mM, higher than the in-situ solubility. (B) Stable carbon and hydrogen isotopic compositions of methane. (C) Stable carbon isotopic compositions of acetate, dissolved inorganic carbon (DIC), and total organic carbon (TOC). DIC is highly enriched in 13C. (D) Concentration of H2, generally several tens of ㎛, four orders of magnitude higher than those in ordinary marine sediments (several nM). (E) Potential metabolic activities. High activity of hydrogenotrophic methanogenesis was observed.


Figure 4. (A) Carbon isotopic composition of methane versus methane/ethane concentration ratio. Methane in KMV#5 was enriched in 13C compared to observations elsewhere in the Nankai Trough because the lighter 12C in DIC is preferentially consumed by microorganisms during hydrogenotrophic methanogenesis, leaving the residual DIC pool enriched in 13C. As more DIC converts to CH4, both the accumulated methane and remaining DIC pools become increasingly 13C-enriched. (B) Clumped isotopologues of methane from KMV#5 and other locations. The apparent equilibrium temperature based on the clumped isotopologue was ~30 °C.


Figure 5. P-wave velocity predicted by 3D-tomography inversion during 3D-prestack depth migration based on data from Tsuji et al. (2015). The overpressure zone (fluid- or gas-accumulation zone) is identified as a low-amplitude and low-velocity zone at 400–700 mbsf in the forearc basin sequence. The low-velocity zone, which is located above the ridge due to the mega-splay fault displacement, suggests that the overpressured fluids are moving upward along the interpreted ancient mega-splay faults. The mud volcanoes are located along the northern extension of the mega-splay faults.


Figure 6. Characteristics of the isolated methanogenic archaeon strain 1H1 from the KMV#5 core. (A-B) Photomicrographs of strain 1H1 grown on H2/CO2 medium. Shown are (A) a phase-contrast micrograph and (B) a fluorescence micrograph of the same field. Bars represent 10 μm. (C-D) Effect of (C) temperature and (D) NaCl concentration on the specific growth rate of strain 1H1 on H2/CO2 medium; this preferentially grows under very low-salinity conditions.


Figure 7. Schematic figure illustrating methanogenesis in the deep mud volcano sediments associated with fluid migration via the mega-splay fault.


(For this study)
Akira Ijiri, Senior Scientist, Geomicrobiology Group,
Kochi Institute for Core Sample Research
(For press release)
Tsuyoshi Noguchi, Manager, Press Division, Public Relations
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