Juichiro Ashi

Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough

Abstract Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide us with the regional heat flow variations using pressure–temperature conditions for methane hydrate stability. Estimated heat flows generally show constant values about 50 mW/m2 shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concentration of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissociated hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estimated from BSR positions offer information about active processes occurring in accretionary prisms.

Microbial Diversity in Deep-sea Methane Seep Sediments Presented by SSU rRNA Gene Tag Sequencing

Microbial community structures in methane seep sediments in the Nankai Trough were analyzed by tag-sequencing analysis for the small subunit (SSU) rRNA gene using a newly developed primer set. The dominant members of Archaea were Deep-sea Hydrothermal Vent Euryarchaeotic Group 6 (DHVEG 6), Marine Group I (MGI) and Deep Sea Archaeal Group (DSAG), and those in Bacteria were Alpha-, Gamma-, Delta- and Epsilonproteobacteria, Chloroflexi, Bacteroidetes, Planctomycetes and Acidobacteria. Diversity and richness were examined by 8,709 and 7,690 tag-sequences from sediments at 5 and 25 cm below the seafloor (cmbsf), respectively. The estimated diversity and richness in the methane seep sediment are as high as those in soil and deep-sea hydrothermal environments, although the tag-sequences obtained in this study were not sufficient to show whole microbial diversity in this analysis. We also compared the diversity and richness of each taxon/division between the sediments from the two depths, and found that the diversity and richness of some taxa/divisions varied significantly along with the depth.

Molecular Characterization of Potential Nitrogen Fixation by Anaerobic Methane-Oxidizing Archaea in the Methane Seep Sediments at the Number 8 Kumano Knoll in the Kumano Basin, Offshore of Japan

ABSTRACT The potential for microbial nitrogen fixation in the anoxic methane seep sediments in a mud volcano, the number 8 Kumano Knoll, was characterized by molecular phylogenetic analyses. A total of 111 of the nifH (a gene coding a nitrogen fixation enzyme, Fe protein) clones were obtained from different depths of the core sediments, and the phylogenetic analysis of the clones indicated the genetic diversity of nifH genes. The predominant group detected (methane seep group 2), representing 74% of clonal abundance, was phylogenetically related to the nifH sequences obtained from the Methanosarcina species but was most closely related to the nifH sequences potentially derived from the anoxic methanotrophic archaea (ANME-2 archaea). The recovery of the nif gene clusters including the nifH sequences of the methane seep group 2 and the subsequent reverse transcription-PCR detection of the nifD and nifH genes strongly suggested that the genetic components of the gene clusters would be operative for the in situ assimilation of molecular nitrogen (N2) by the host microorganisms. DNA-based quantitative PCR of the archaeal 16S rRNA gene, the group-specific mcrA (a gene encoding the methyl-coenzyme M reductase α subunit) gene, and the nifD and nifH genes demonstrated the similar distribution patterns of the archaeal 16S rRNA gene, the mcrA groups c-d and e, and the nifD and nifH genes through the core sediments. These results supported the idea that the anoxic methanotrophic archaea ANME-2c could be the microorganisms hosting the nif gene clusters and could play an important role in not only the in situ carbon (methane) cycle but also the nitrogen cycle in subseafloor sediments.

Magnetic fabric development in the Tertiary Accretionary Complex in the Boso and Miura Peninsulas of central Japan

It has been demonstrated that the anisotropy of magnetic susceptibility (AMS) is a sensitive indicator for distinguishing various processes that had acted during and after the deposition of sediments. Recently, much attention has been focused on proving its usefulness in distinguishing early deformation signatures in sediments which show no visible grain-scale deformation. Here, we present the AMS results of the Neogene Miura Group of Central Japan, which was formed by arc-arc collision tectonics with subdivisions of two contrasting tectonic provinces: the southern province, a highly deformed accretionary prism and slope basin, and the northern province, a gently deformed forearc basin. Samples were collected from 21 sites in both tectonic settings. We obtained from axes orientations three typical fabric domains considered to represent sedimentary, tectonically deformed, and their intermediate types. The shape parameters of AMS also revealed distinctions clearly. Sedimentary fabrics are more foliated and anisotropic. Deformed fabrics show a more prolate shape and are closer to being isotropic. The sedimentary fabrics predominate in the northern forearc basin sequence whereas the intermediate and deformed fabrics occur predominantly in the southern province. The general trend of the shape of the magnetic fabric in studied sediments reveals a clear evolution in the pattern of the isotropic tectonic fabric constructed from foliated sedimentary fabric.

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.

Deep-tow survey in the KAIKO-Nankai cold seepage areas

Deep-sea biological communities associated with cold venting were located within the eastern portion of the Nankai Trough using a deep-towed TV system. Five continuous lines (approximately 200 km long) were observed. Bivalve communities were found both in the Tenryu Canyon outlet region and at the outcrop of the frontal thrusts of the accretionary wedge off Cape Omaezaki. The former is a site at which biological communities had been found during the KAIKO dives in 1985. However, the second site appears to be associated with larger scale venting and was chosen for the 1989 KAIKO-Nankai submersible cruise. Three small mud volcanoes with diameters of several hundred meters and heights of several tens of meters are found in the Seabeam topography map, but the present survey showed neither evidence of recent venting nor biological communities.

Two types of plane beds under upper-flow-regime in flume experiments: evidence from grain fabric

Abstract Laboratory experiments that aimed to produce plane beds under upper-flow-regime were conducted using a flume of 15×700 cm with quartz sands of mean size of 0.72 mm (0.47 φ ). Two types of plane beds were formed: (1) laminae formed and buried by the migration of low-relief asymmetric ripples near- or subcritical flow (Froude Number, Fr =0.94, Run 5), and (2) laminae formed without producing any apparent low-relief bedform but under supercritical flow ( Fr =1.98, Run 12). The mean flow velocity (83 cm/s) and the mean depth (8 cm) in Run 5 are larger than those in Run 12 (62 cm/s and 1 cm, respectively). Upcurrent grain imbrications in Run 12, however, are better developed than those in Run 5. Such an imbrication pattern may be useful to recognize the formational processes of plane beds under upper-flow-regime.

Atribacteria from the Subseafloor Sedimentary Biosphere Disperse to the Hydrosphere through Submarine Mud Volcanoes

Submarine mud volcanoes (SMVs) are formed by muddy sediments and breccias extruded to the seafloor from a source in the deep subseafloor and are characterized by the discharge of methane and other hydrocarbon gasses and deep-sourced fluids into the overlying seawater. Although SMVs act as a natural pipeline connecting the Earth’s surface and subsurface biospheres, the dispersal of deep-biosphere microorganisms and their ecological roles remain largely unknown. In this study, we investigated the microbial communities in sediment and overlying seawater at two SMVs located on the Ryukyu Trench off Tanegashima Island, southern Japan. The microbial communities in mud volcano sediments were generally distinct from those in the overlying seawaters and in the well-stratified Pacific margin sediments collected at the Peru Margin, the Juan de Fuca Ridge flank off Oregon, and offshore of Shimokita Peninsula, northeastern Japan. Nevertheless, in-depth analysis of different taxonomic groups at the sub-species level revealed that the taxon affiliated with Atribacteria, heterotrophic anaerobic bacteria that typically occur in organic-rich anoxic subseafloor sediments, were commonly found not only in SMV sediments but also in the overlying seawater. We designed a new oligonucleotide probe for detecting Atribacteria using the catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). CARD-FISH, digital PCR and sequencing analysis of 16S rRNA genes consistently showed that Atribacteria are abundant in the methane plumes of the two SMVs (0.58 and 1.5 × 104 cells/mL, respectively) but not in surrounding waters, suggesting that microbial cells in subseafloor sediments are dispersed as “deep-biosphere seeds” into the ocean. These findings may have important implications for the microbial transmigration between the deep subseafloor biosphere and the hydrosphere.

Friction properties of the plate boundary megathrust beneath the frontal wedge near the Japan Trench: an inference from topographic variation

The 2011 Tohoku-Oki earthquake (Mw 9.0) produced a fault rupture that extended to the toe of the Japan Trench. The deformation and frictional properties beneath the forearc are keys that can help to elucidate this unusual event. In the present study, to investigate the frictional properties of the shallow part of the plate boundary, we applied the critically tapered Coulomb wedge theory to the Japan Trench and obtained the effective coefficient of basal friction μb′ and Hubbert-Rubey pore fluid pressure ratio (λ) of the wedge beneath the lower slope. We extracted the surface slope angle and décollement dip angle (which are the necessary topographic parameters for applying the critical taper theory) from seismic reflection and refraction survey data at 12 sites in the frontal wedges of the Japan Trench. We found that the angle between the décollement and back-stop interface generally decreases toward the north. The measured taper angle and inferred effective friction coefficient were remarkably high at three locations. The southernmost area, which had the highest coefficient of basal friction, coincides with the area where the seamount is colliding offshore of Fukushima. The second area with a high effective coefficient of basal friction coincides with the maximum slip location during the 2011 Tohoku-Oki earthquake. The area of the 2011 earthquake rupture was topographically unique from other forearc regions in the Japan Trench. The strain energy accumulation near the trench axis may have proceeded because of the relatively high friction, and later this caused a large slip and collapse of the wedge. The location off Sanriku, where there are neither seamount collisions nor rupture propagation, also has a high coefficient of basal friction. The characteristics of the taper angle, effective coefficient of basal friction, and pore fluid pressure ratio along the Japan Trench presented herein may contribute to the understanding of the relationship between the geometry of the prism and the potential for generating seismo-tsunamigenic slips.

Settling of Earthquake-Induced Turbidity on the Accretionary Prism Slope of the Central Nankai Subduction Zone

Studied of turbidite deposits can provide valuable paleoseismological information, such as earthquake recurrence intervals. However, there are few studies based on in situ observations of gravity flows during or soon after earthquakes. We used a remotely operated vehicle in dive surveys near the epicenters of the 2004 off the Kii Peninsula earthquakes (maximum magnitude 7.4) soon after and 6 years after the events. Video observations 20 days after the main shock revealed two turbid layers exhibiting different degrees of turbidity: a dilute suspension layer (DSL) and a bottom turbid layer (BTL). Pervasive distributions of DSL on the prism slope suggest that clouds of suspended sediments were induced by sliding, slumping, seafloor shaking, or sediment gravity flow at multiple locations during the earthquakes. BTL is a highly turbid layer of around 2.4 m thickness, the upper surface of which is sharply defined. That BTL was observed only in a slope basin implies that it was formed by a sediment gravity flow dumping muddy deposits in a depression. Our dive observations revealed that seafloor disturbance by seismic shaking plays a significant role in sediment redistribution in subduction zones.