戸丸 仁

Abstract The properties of host sediments and pore water considerably affect both the occurrence and formation processes of methane hydrate. In coarse‐grained layers, hydrates are generally concentrated preferentially in the pore space, and their formation is influenced by pore water salinity. To understand how geophysical and geochemical factors control the distribution of methane hydrates, we conducted numerical simulations using a one‐dimensional flow model under different reservoir and fluid conditions in the Kumano Forearc Basin, Nankai Trough, Japan. Assuming an estimated range of methane flux between 0.002 and 1.9 kg m⁻² year⁻¹, three flow scenarios were considered. When the methane flux was relatively small, the results coincided with the observed hydrate distribution. In general, a low‐methane flux decreases the hydrate saturation upward from the bottom of the methane hydrate stability, whereas a high‐methane flux increases the saturation downward. These results also suggest that the sediment structure, such as the fracture distribution, influences the sediment stress conditions and constrains the flow regime. We further examined the effects of permeability changes in the heterogeneous lithological units on the simulation results using typical permeabilities of 10⁻¹³ m² for sand and 10⁻¹⁵ m² for mud. The results showed that hydrate saturation sharply increased and decreased in adjacent high‐ and low‐permeability units, respectively. The consideration of complex stratigraphic conditions and variable fluid configurations provides an understanding of the environmental factors controlling hydrate generation and distribution, which is important for hydrate resource extraction and geohazard prevention.

This investigation presents methane, noble gas isotopes, CTD, and stable isotopic data for water samples collected in Niskin bottles at Tatar Strait during the spring seasons of 2015 and 2019 onboard the Russian R/V Akademik M.A. Lavrentyev. The results are compared to previous research carried out in 1999 in a nearby portion of the Strait and demonstrate that salinity and temperature can change appreciably. The CTD data from 1999 shows warm surface waters underlain by cold subsurface waters. In contrast, the 2015 data show the CTD data that show warm temperatures and high salinity extending down from the surface well into intermediate waters, while the 2019 data show cold surface waters underlain by even colder subsurface waters. CTD data collected above active gas plume sites along Sakhalin Island’s western shore show no substantial difference in temperature or salinity from the non-plume sites, and the methane concentrations at all of the measured sites are significantly above saturation, even in the shallow waters. Hydroacoustic data also suggest the presence of free gas and gas hydrate–coated methane bubbles from the seafloor at least to the base of upper intermediate waters. All of the intermediate and deep Japan Sea Proper waters in Tatar Strait still retain tritiogenic ³He, similar to that observed throughout much of the Japan Sea, indicating limited vertical exchange between these layers and surface waters. An analysis of the δ¹³C of dissolved inorganic carbon in the seawater shows that positive values are limited to surface waters and that the waters become progressively more depleted in ¹³C with depth. The results are consistent with research over the past several decades which has shown that ventilation of intermediate and deep Japan Sea Proper water is somewhat limited, while both the temperature and salinity of surface and subsurface water layers within the strait are sensitive to the balance between cold, less saline waters contributed by the Amur River/Primorye Current from the north and warm, saline waters contributed by the Tsushima Current from the south.

Abstract Over the past 15 years, massive gas hydrate deposits have been studied extensively in Joetsu Basin, Japan Sea, where they are associated primarily with active gas chimney structures. Our research documents the discovery of spheroidal microdolomite aggregates found in association with other impurities inside of these massive gas hydrates. The microdolomites are often conjoined and show dark internal cores occasionally hosting saline fluid inclusions. Bacteroidetes sp. are concentrated on the inner rims of microdolomite grains, where they degrade complex petroleum-macromolecules present as an impurity within yellow methane hydrate. These oils show increasing biodegradation with depth which is consistent with the microbial activity of Bacteroidetes. Further investigation of these microdolomites and their contents can potentially yield insight into the dynamics and microbial ecology of other hydrate localities. If microdolomites are indeed found to be ubiquitous in both present and fossil hydrate settings, the materials preserved within may provide valuable insights into an unusual microhabitat which could have once fostered ancient life.

Abstract. The Chikyu Shallow Core Program (SCORE) has been started to provide more opportunities for the scientific ocean drilling of shallow boreholes (up to 100 m) during a short-term expedition. The proposal flow is a simplified version of that of the International Ocean Discovery Program (IODP). Although there are several limitations for a SCORE project, the opportunity to retrieve 100 m of continuous core samples will be of great interest for the scientific ocean drilling community in multiple disciplines. The first expedition of the SCORE program was implemented off Cape Erimo, Hokkaido, northern Japan. The target of the drilling was to investigate the impact of submarine mass transport on the subseafloor sedimentary biosphere. In the preliminary observation of the core samples, including X-ray computed tomography (CT) scan image analysis, chaotic and inclined beds were found and interpreted as mass transport deposit (MTD) units.

Microbial life inhabiting subseafloor sediments plays an important role in Earth's carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (102 to 103 cells cm-3) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated.

日本海東縁に発達する表層型ガスハイドレートの腑存域では，しばしば海底から海水中にガスバブルを放出するガスプルームが確認される．ガスプルームを形成するほどガスに富んだ流体が堆積物中を上方へ移動することで海底地盤を乱し，地盤強度が低下することが表層型ガスハイドレートを開発する際の障害の1つになると指摘されている．本研究では2つの海域（上越沖，最上トラフ）で海底下数mの柱状堆積物を採取し，物理·力学試験および地化学分析を行った．両海域を比較すると，物理試験でほとんど違いがみられなかったが，ガスプルームがある海域で堆積物強度が小さく，溶存CH₄ 濃度も高かった．ガスプルームが観測されなかった海域では堆積速度が大きいと，堆積物強度が小さくなる傾向を示した．どちらの海域においても，溶存CH₄ 濃度と堆積物強度に関係はみられなかった．これらの結果はガスプルームがある海域では原位置において，堆積物強度が小さいことを示唆する．

Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (delta(OH)-O-18 and delta(OM)-O-18, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2 equilibrated with H2O of the sub-samples of a small size (typically 0.3 cm(3)) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (delta O-18(PW)) squeezed from a certain length of core sediment (typically 20 cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, delta(OH)-O-18 > delta O-18(PW) > delta(OM)-O-18, as expected from isotopic fractionation that enriches O-18 in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less O-18-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between delta(OH)-O-18 and delta(OM)-O-18 as well as high CH2/CO2 ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.

Marine sediments contain eukaryotic DNA deposited from overlying water columns. However, a large proportion of deposited eukaryotic DNA is aerobically biodegraded in shallow marine sediments. Cold seep sediments are often anaerobic near the sediment-water interface, so eukaryotic DNA in such sediments is expected to be preserved. We investigated deeply buried marine sediments in the Japan Sea, where a methane hydrate deposit is associated with cold seeps. Quantitative PCR analysis revealed the reproducible recovery of eukaryotic DNA in marine sediments at depths up to 31.0m in the vicinity of the methane hydrate deposit. In contrast, the reproducible recovery of eukaryotic DNA was limited to a shallow depth (8.3m) in marine sediments not adjacent to the methane hydrate deposit in the same area. Pyrosequencing of an 18S rRNA gene variable region generated 1,276-3,307 reads per sample, which was sufficient to cover the biodiversity based on rarefaction curves. Phylogenetic analysis revealed that most of the eukaryotic DNA originated from radiolarian genera of the class Chaunacanthida, which have SrSO4 skeletons, the sea grass genus Zostera, and the seaweed genus Sargassum. Eukaryotic DNA originating from other planktonic fauna and land plants was also detected. Diatom sequences closely related to Thalassiosira spp., indicative of cold climates, were obtained from sediments deposited during the last glacial period (MIS-2). Plant sequences of the genera Alnus, Micromonas, and Ulmus were found in sediments deposited during the warm interstadial period (MIS-3). These results suggest the long-term persistence of eukaryotic DNA from terrestrial and aquatic sources in marine sediments associated with cold seeps, and that the genetic information from eukaryotic DNA from deeply buried marine sediments associated with cold seeps can be used to reconstruct environments and ecosystems from the past.

<p>表層型ガスハイドレートが胚胎する日本海では複数の海域で海底から湧出するガスバブルが確認されている．潜水調査艇による海底観察から，これらの強度や位置が数日スケールで変化していることが明らかになり，表層型ガスハイドレート近傍の化学環境もガス湧出と同様，短期的に変動している可能性が示唆される．本研究は浸透圧式長期連続採水器(オスモサンプラー)を用いて，ガス湧出の強い上越沖の鳥が首海脚と，ガス湧出の弱い海鷹海脚，湧出が見られない山形沖の鳥海礁(とりみぐり)の3海域で海底下30cmの間隙水を一年間連続的に採水し，一日の解像度で間隙水に溶存する主要イオン(SO42-, Cl-, Na+, Ca2+, Mg2+, K+)とガス(CH4, C2H6)濃度を分析した．</p>

© 2016 Author(s). The Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser-drilling technology with the drilling vessel Chikyu. The drilling Site C0020 is located in a forearc basin formed by the subduction of the Pacific Plate off the Shimokita Peninsula, Japan, at a water depth of 1180 m. Primary scientific objectives during Expedition 337 were to study the relationship between the deep microbial biosphere and a series of ~2 km deep subseafloor coalbeds and to explore the limits of life in the deepest horizons ever probed by scientific ocean drilling. To address these scientific objectives, we penetrated a 2.466 km deep sedimentary sequence with a series of lignite layers buried around 2 km below the seafloor. The cored sediments, as well as cuttings and logging data, showed a record of dynamically changing depositional environments in the former forearc basin off the Shimokita Peninsula during the late Oligocene and Miocene, ranging from warm-temperate coastal backswamps to a cool water continental shelf. The occurrence of small microbial populations and their methanogenic activity were confirmed down to the bottom of the hole by microbiological and biogeochemical analyses. The factors controlling the size and viability of ultra-deep microbial communities in those warm sedimentary habitats could be the increase in demand of energy and water expended on the enzymatic repair of biomolecules as a function of the burial depth. Expedition 337 provided a test ground for the use of riser-drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling.

Subseafloor pelagic sediments with high concentrations of organic matter form habitats for diverse microorganisms. Here, we determined depth profiles of genes for SSU rRNA, mcrA, dsrA and amoA from just beneath the seafloor to 363.3 m below the seafloor (mbsf) using core samples obtained from the forearc basin off the Shimokita Peninsula. The molecular profiles were combined with data on lithostratigraphy, depositional age, sedimentation rate and pore-water chemistry. The SSU rRNA gene tag structure and diversity changed at around the sulfate-methane transition zone (SMTZ), whereas the profiles varied further with depth below the SMTZ, probably in connection with the variation in pore-water chemistry. The depth profiles of diversity and abundance of dsrA, a key gene for sulfate reduction, suggested the possible niche separations of sulfate-reducing populations, even below the SMTZ. The diversity and abundance patterns of mcrA, a key gene for methanogenesis/anaerobic methanotrophy, suggested a stratified distribution and separation of anaerobic methanotrophy and hydrogenotrophic or methylotrophic methanogensis below the SMTZ. This study provides novel insights into the relationships between the composition and function of microbial communities and the chemical environment in the nutrient-rich continental margin subseafloor sediments, which may result in niche separation and variability in subseafloor microbial populations.

Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ∼40° to 60°C sediment associated with lignite coal beds at ∼1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ∼10⁴ cells cm^-3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.

Hydrate-bearing sediment cores were retrieved from the Joetsu Basin (off Joetsu city, Niigata Prefecture) at the eastern margin of the Japan Sea during the MD179 gas hydrates cruise onboard R/V Marion Dufresne in June 2010. We measured molecular and stable isotope compositions of volatiles bound in the gas hydrates and headspace gases obtained from sediments to clarify how the minor components of hydrocarbons affects to gas hydrate crystals. The hydrate-bound hydrocarbons at Umitaka Spur (southwestern Joetsu Basin) primarily consisted of thermogenic methane, whereas those at Joetsu Knoll (northwestern Joetsu Basin, about 15 km from Umitaka Spur) contained both thermogenic methane and a mixture of thermogenic and microbial methane. The depth concentration profiles of methane, ethane, propane, CO2, and H2S in the sediments from the Joetsu Basin area showed shallow sulfate-methane interface (SMI) and high microbial methane production beneath the SMI depth. Relatively high concentrations of propane and neopentane (2,2-dimethylpropane) were detected in the headspace gases of the hydrate-bearing sediment cores obtained at Umitaka Spur and Joetsu Knoll. Propane and neopentane cannot be encaged in the structure I hydrate; therefore, they were probably excluded from the hydrate crystals during the structure I formation process and thus remained in the sediment and/or released from the small amounts of structure II hydrate that can host such large gas molecules. The lower concentrations of ethane and propane in the sediment, high delta C-13 of propane and isobutane, and below-detection normal butane and normal pentane at Umitaka Spur and Joetsu Knoll suggest biodegradation in the sediment layers.

Halogen concentrations and I-129/I ratios were determined in pore waters from the Nankai Trough subduction system, collected during IODP Expeditions 315, 316, 322, and 333 along the NanTroSEIZE transect. The transect allowed the first direct comparison of iodine results across an active subduction system, from subducting oceanic sediments to the accretionary prism, and the overlying forearc basin. In contrast to the other halogens (Cl and Br) iodine concentrations show large variations within and among the cores at all sites landward of the trough, I concentrations increase rapidly with depth and reach values several orders of magnitude higher than those in seawater, but are only slightly higher than seawater values at the seaward sites. Methane concentrations follow a similar pattern. Host sediments of the fluids are younger than 7 Ma in all the cores, but the ages of iodine in pore waters at the landward sites reach values beyond 30 Ma. In contrast, iodine seaward of the trough is in isotopic equilibrium with the host sediments, resulting in very similar iodine and sediment ages. The distribution of iodine concentrations and ages indicates that iodine at the landward sites has been transported there in aqueous fluids, probably together with methane, from old formations in the upper plate. The specific fluid pathways potentially were influenced by features such as the megasplay fault in the prism or the decollement. The results demonstrate large-scale transport of fluids carrying iodine and other compounds such as methane from old layers in the upper plate to surface locations landward of the Nankai Trough, while separate, but only local hydrologic processes occur in the marine sediments moving toward the trough. (C) 2014 Elsevier Ltd. All rights reserved.

Marine gas hydrate and cold-seep systems, which maintain a large amount of methane in the seabed, may critically impact the geochemical and ecological characteristics of the deep-sea sedimentary environment. However, it remains unclear whether marine sediments associated with gas hydrate harbor novel microbial communities that are distinct from those from typical marine sediments. In this study, microbial community structures thriving in sediments associated with and without gas hydrate in the eastern Japan Sea were characterized by 16S rRNA gene-based phylogenetic analyses. Uncultivated bacterial lineages of candidate division JS1 and a novel group NT-B2 were dominant in the sediments from gas hydrate-associated sites. Whereas, microbial populations from sites not associated with gas hydrate were mainly composed of Bacteroidetes, Nitrospirales, Chlamydiales, Chlorobiales, and yet-uncultured bacterial lineages of OD1 and TM06. The good correlation between the dominance of JS1 and NT-B2 and the association of gas hydrate could be attributed to the supply of more energetically favorable energy sources in gas-rich fluids from the deep subsurface than refractory organic matter of terrigenous and diatomaceous origin. (C) 2013 Elsevier Ltd. All rights reserved.

The degree of gas hydrate saturation at Integrated Ocean Drilling Program (IODP) Site C0002 in the Kumano Basin, Nankai Trough, was estimated from logging-while-drilling logs and core samples obtained during IODP Expeditions 314 and 315. Sediment porosity data necessary for the calculation of saturation were obtained from both core samples and density logs. Two forms of the Archie equation (quick-look' and standard') were used to calculate gas hydrate saturation from two types of electrical resistivity log data (ring resistivity and bit resistivity), and a three-phase Biot-type equation was used to calculate gas hydrate saturation from P-wave velocity log data. The gas hydrate saturation baseline calculated from both resistivity logs ranges from 0% to 35%, and that calculated from the P-wave velocity log ranges from 0% to 30%. High levels of gas hydrate saturation (&gt;60%) are present as spikes in the ring resistivity log and correspond to the presence of gas hydrate concentrations within sandy layers. At several depths, saturation values obtained from P-wave velocity data are lower than those obtained from bit resistivity data; this discrepancy is related to the presence of free gas at these depths. Previous research has suggested that gas from deep levels in the Kumano Basin has migrated up-dip towards the southern and seaward edge of the basin near Site C0002. The high saturation values and presence of free gas at site C0002 suggest that a large gas flux is flowing to the southern and seaward edge of the basin from a deeper and/or more landward part of the Kumano Basin, with the southern edge of the Kumano Basin (the location of site C0002) being the main area of fluid accumulation.

2013年夏季に東京海洋大学の「海鷹丸」によって(UT13航海)隠岐トラフ，秋田‐山形沖から採取された堆積物における間隙水化学成分・間隙水溶存ガスの地球化学分析を行い，日本海東縁に広がるガスハイドレート胚胎状況やガスハイドレートの形成・分解と堆積物中の化学的環境の変動要因について考察した．隠岐トラフでは0.5-3cm程度大きさのフレーク状ガスハイドレート片が，秋田‐山形沖では直径8cm以上のガスハイドレートが海底下1～6mの表層堆積物中で確認された．間隙水中の硫酸イオンやメタン濃度からは，硫酸―メタン境界は浅くメタン供給量が高いことに加え，硫化水素の存在がガスハイドレートの安定領域を拡大し，浅部でもガスハイドレートが形成しやすい環境となっていることが明らかになった．SMI以深でも硫酸,カルシウム,マグネシウムプロファイルに小さなピークが複数みられ，過去のSMI変動に伴う間隙水―堆積物反応を記録している可能性が示唆された．

日本海東縁上越沖は深部ガス含有流体フラックスが強く、メタン湧水や表層型ガスハイドレートの存在が知られている。その分布や起源の理解を目的に実施されたMD179航海では、深部ガス含有流体の影響と考えられる地球化学的特徴を確認することに成功した。メタン湧水地点では、間隙水中の硫酸イオン、塩化物イオン濃度が深部に向かって減少し、一方で溶存無機炭素濃度は上昇した。さらに、炭素同位体比の高いメタンと溶存無機炭素が検出されたことから、熱分解起源メタンのみならず、微生物起源メタンも深部流体中に付加されていると解釈できる。こうした地球化学的特徴は、メタン湧水地点から離れるにつれ弱まっていくものの、約5 kmに渡る広範囲で深部流体が影響していた。また、堆積物中の微生物群集構造も、深部流体の分布に対応して特徴的であった。類似した微生物生態系は、南海トラフやカスカディアマージンなどでも報告されており、ガスハイドレート環境を表す一つの指標になると期待される。

In: Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., the Expedition 314/315/316 Scientists (Eds.)

Pore waters were taken from core sediments of Sites C0001, C0004, and C0008 on the landward slope of the Nankai Trough and Site C0002 in the forearc basin of the Nankai accretionary prism off Kumano during Integrated Ocean Drilling Program Expeditions 315 and 316. The carbon isotopic ratios of CH4 and total carbon dioxide (Sigma CO2) in dissolved gases were measured. The contribution of thermogenic CH4 was negligible at all sites, while carbon isotopic separation between CH4 and Sigma CO2 indicated that CH4 formation was mainly by microbial hydrogenotrophic methanogenesis. Evaluation of the isotopic fraction of the initial substrate Sigma CO2 pool showed larger fractionation at Site C0002 than at the other sites in the transect. In addition, the NH4+ concentration was higher at Site C0002 than at the other sites, indicating that organic matter degradation occurred more actively at Site C0002 than at the other sites. Therefore, CO2 and H-2 as well as NH4+ were actively generated by the organic matter degradation at Site C0002, which could stimulate methanogenesis utilizing CO2 and H-2 as substrates at Site C0002. The high sedimentation rate at Site C0002 in the forearc basin was due to the geomorphological setting of the site, within the outer ridge rimming the sediment-filled Kumano Basin, leading to organic matter burial without aerobic degradation on the surface of the seafloor, which preserve labile organic matter for utilization by methanogenesis. On the other hand, slope sediments were already exposed by organic matter degradation, which leaves scarce labile organic matter for supporting CH4 generation. Geomorphology was possibly an important factor controlling CH4 formation and accumulation, and the Kumano Basin sediments have greater potential as a CH4 hydrate reservoir than the landward slope sediments in the Nankai accretionary prism off Kumano.

Microbial methanogenesis in subseafloor sediments is a key process in the carbon cycle on the Earth. However, the cultivation-dependent evidences have been poorly demonstrated. Here we report the cultivation of a methanogenic microbial consortium from subseafloor sediments using a continuous-flow-type bioreactor with polyurethane sponges as microbial habitats, called down-flow hanging sponge (DHS) reactor. We anaerobically incubated methane-rich core sediments collected from off Shimokita Peninsula, Japan, for 826 days in the reactor at 10 degrees C. Synthetic seawater supplemented with glucose, yeast extract, acetate and propionate as potential energy sources was provided into the reactor. After 289 days of operation, microbiological methane production became evident. Fluorescence in situ hybridization analysis revealed the presence of metabolically active microbial cells with various morphologies in the reactor. DNA-and RNA-based phylogenetic analyses targeting 16S rRNA indicated the successful growth of phylogenetically diverse microbial components during cultivation in the reactor. Most of the phylotypes in the reactor, once it made methane, were more closely related to culture sequences than to the subsurface environmental sequence. Potentially methanogenic phylotypes related to the genera Methanobacterium, Methanococcoides and Methanosarcina were predominantly detected concomitantly with methane production, while uncultured archaeal phylotypes were also detected. Using the methanogenic community enrichment as subsequent inocula, traditional batch-type cultivations led to the successful isolation of several anaerobic microbes including those methanogens. Our results substantiate that the DHS bioreactor is a useful system for the enrichment of numerous fastidious microbes from subseafloor sediments and will enable the physiological and ecological characterization of pure cultures of previously uncultivated subseafloor microbial life. The ISME Journal (2011) 5, 1913-1925; doi: 10.1038/ismej.2011.64; published online 9 June 2011

We compare here results of iodine dating in fluids collected from mud volcanoes and gas hydrate occurrences associated with active margins. This is a compilation of previously reported data for 7 subduction zones around the Pacific Rim with slab ages ranging from 6 Ma to 130 Ma, where we determined iodine concentrations and (129)I/I ratios in more than two hundred pore water samples. Iodine ages consistently are older than the host sediments and show an age distribution independent of the slab ages associated with the subduction zones. The results suggest that iodine in gas hydrates and mud volcanoes is predominantly derived from organic matter in the upper plates of subduction zones and that iodine derived from host sediments or subducting marine sediments has only a minor presence in these fluids. Because potential source sediments typically are found at lateral distances of 20 km or more, our results also suggest that fluid movement is possible over considerable distances in fractures present in the upper plate sediments. The association between iodine and methane suggests that our results can be extrapolated to the origin and transport of methane to mud volcanoes and gas hydrates. Throughout all studied sites around the Pacific margins, ages of source sediments for iodine were found to fall into the same range, which starts at the early Eocene (similar to 50 Ma) and has a broad peak around 30 Ma. Our results indicate that the occurrence of iodine and methane in gas hydrates and mud volcanoes is the result of transport in aqueous fluids and long-term remobilization of carbon in the upper plates of subduction zones.

海底堆積物中には塵や火山灰、プランクトンの死骸などが堆積している。それらの化学組成を調べることで、過去の堆積環境についての情報が得られる。また、新しいエネルギー源として注目されているメタンハイドレート産出地域における間隙水中には、ヨウ素が非常に高濃度で含まれており、ヨウ素とメタンの間には何か関係性があると考えられる。本研究では、日本海のメタンハイドレート産出地域で採取された海底堆積物及び炭酸塩ノジュールに注目し、加熱分離法を用いて、Cl、Br、Iの分析を行った。また、得られた分析結果をもとに海底面からの深度分布をみるとともに、間隙水の濃度との関係を検討した。

We report here results comparing the efficiency of three iodine extraction methods for the determination of I-129/I ratios in small samples. The direct organic extraction had the highest average yield. I-129/I ratios determined from targets prepared by direct organic extraction and silver extraction are comparable to each other within the error limits. The smallest sample from which a reliable I-129/I ratio was measured, started with an iodine input of 0.2 mg iodine and contained 0.14 mg AgI in the target. (C) 2009 Elsevier B.V. All rights reserved.

The Akita and Niigata Basins are part of the Green Tuff region of Honshu, Japan, which hosts major occurrences of oil and gas and of hydrothermal sulfide-sulfate Kuroko Deposits. The GreenTuff formation is associated with the opening of the Japan Sea, which occurred in the Early Miocene and was accompanied by large-scale volcanic activities. We measured halogen concentrations and I-129?/I ratios in oil and gas field brines and hot springs in order to determine the origin of iodine and. indirectly, of hydrocarbons in this area and their association with geological processes during the formation of the Japanese Island Arc. Based on the concentration of iodine, these samples fall into three categories: Type A (I &gt; 190 mu M), Type B (150 mu M&gt;I&gt;90 mu M), and Type C (I&lt;50 mu M). Samples in Types A and B come predominantly from oil/gas fields and have I-129/I ratios below 300 x 10(-15), corresponding to ages essentially in the Eocene epoch. In contrast, samples in Type C are generally from hot springs and have I-129/I ratios above 400 x 10(-15), corresponding to Oligocene to Miocene ages. Source formations for Type C were accumulated during the active opening of the Japan Sea, however, those for Types A and B probably are organic-rich Eocene sediments or older basement rocks which predate the volcanic formations associated with the backarc spreading of the Japanese Island. The observation that oil and gas is predominantly found in reservoir formations of middle to late Miocene age suggests mobilization of these brines in association with the tectonic and volcanic/hydrothermal activities during the opening of the Japan Sea which also led to the formation of the Kuroko Deposits in the same area. The hot spring samples of Type C reflect these processes most directly. Because of the close association of iodine with methane and other hydrocarbons, oil and gas in the Green Tuff area probably is also derived from sources considerably older than the current reservoir formations. (c) 2009 Elsevier B.V. All rights reserved.

The Umitaka Spur in the Joetsu basin on the eastern margin of the Japan Sea is characterized by a number of features indicative of high methane flux such as a BSR and gas plumes rising through the water column. These are likely linked to the presence of hydrocarbon source rocks underneath the spur. The interstitial water chemistry at the spur was measured in order to delineate the behavior and evolution of the gas hydrate system across the region. The sulfate depth profiles indicate that the SMI depths become shallower toward the crest of the spur and are shallowest (less than 200 cmbsf) at two plume sites, suggesting that the methane fluxes are highest around the plumes. Depth profiles of dissolved Cl- all fall into one of four distinguishable trends: Type-I where concentrations linearly increase with sediment depth, Type-II where concentrations linearly decrease with depth,Type-III where concentrations remain constant and Type-IV where concentrations exhibit negative spikes caused by dissociation of gas hydrate during core recovery and handling. Type-I cores commonly recovered at central plume site are characterized not only by steep Cl- gradients (+13.2 to +51.0 mM/m), but also extreme depletions in D and O-18 (-0.97 to -0.54 parts per thousand VSMOW/m for delta D-H2O and -0.16 to -0.10 parts per thousand VSMOW/m for delta O-18(H2O)) suggesting gas hydrate formation at shallow depth. The Cl- gradients of Type-II cores recovered from the southern part of the spur have little variation (-14.8 to -8.3 mM/m). Gas hydrate dissociation along base of the gas hydrate stability zone best explains both the similar pattern of freshening with depth and the widespread distributions of Type-if cores. Although gas hydrate dissociation is known to release isotopically heavy water, Type-II cores show progressive depletions of both D and 180 with downward depth. The apparent contradiction between Cl- concentrations and the isotopic compositions of Type-II cores is likely due to reequilibration by strong burial diagenesis observed all over the Japan Sea sediments. Regional dissociation of gas hydrate is likely to have been triggered by sea level drop which facilitated dissociation of subsurface gas hydrate. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface.High heat flow, similar to 100 mK/m, deposition of organic-rich strata, similar to 1.0 to 1.5% TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface(SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. delta C-13 of methane hydrate and seep gases are mostly within -30 to -50%, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with delta C-13 of -50 to -100%.Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys.The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600. 1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters,(ii) degassing from ascending fluids,(iii) bound water effect and deficiency of free-waters, and(iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.Increased accumulation of methane hydrate(specific gravity similar to 0.91 g/cm(3)) in shallow sediments should have caused a gravity imbalance of methane hydrate bearing sediments, and eventually the methane hydrate blocks lifted and floated up to the sea surface(auto-collapse). Crater-like depressions and valleys are the heritage of such an auto-collapse process.Dark colored, thinly laminated units with a very low abundance of benthic foraminifers occur in 27 to 18 kyrBP, approximately the period of the LGM, indicating low-oxygen, euxinic conditions. Furthermore, delta C-13 of benthic foraminifers from the dark laminated unit exhibits sharp negative excursion toward similar to 21 kyrBP. A sea-level fall of similar to 120 m toward the LGM released the pressure of gas hydrate-bearing sediments, and presumably triggered the dissociation of subsurface methane hydrate, which, in turn, destabilized the entire gas chimney hydrate system, collapsing the gas chimney and leaving large and deep pockmarks.

Li同位体ツールを用いて，ガスハイドレートに関与する流体の生成温度（深度）を推定する事を試みた。試料は日本海直江津沖で採取されたものを用いた。その結果，分析試料のLi同位体比は海水と深部流体の2成分混合で説明できる事が明らかとなった。その際に，推定される深部流体端成分のδ7Li値は約+7‰であった。ここでガスハイドレート試料に含まれる泥のLi同位体比が本地域の堆積物相の値を反映していると仮定した場合，δ7Li値が+7‰の深部流体を作り出すためには約150℃程度が必要となる。この温度は日本海の地温勾配では，海底下1400~1900mに相当する。

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface.<br> High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ¹³C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ¹³C of -50 to -100‰.<br> Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys.<br> The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.<br>(View PDF for the rest of the abstract.)

Iodine concentration and radioisotopic composition (129I/I) were measured in the pore waters from the gas hydrate occurrence in the forearc basin offshore Shimokita Peninsula, north-eastern Japan, to determine the source formation of I and accompanying hydrocarbons. Iodine concentrations correlate well with the alkalinity and SO4 patterns, reflecting degradation stages of I-rich buried organic matter, rapidly increasing in the sulfate reduction interval, and becoming constant below 250 meters below the seafloor with an upwelling flux of 1.5 x 10-11 mu mol cm-2 year-1. The 129I/I ratios of 300 x 10-15-400 x 10-15 in deep pore waters suggest ages for iodine and hydrocarbon sources as old as 40 Ma. These ages correlate well with the coaly source formations of the Eocene age thought to be responsible for the conventional natural gas deposits underlying the gas hydrate stability zone. Similar profiles are observed in 129I/I ratios of pore waters in the gas hydrate stability zone from the forearc basin in the eastern Nankai Trough, offshore central Japan, where pore waters are enriched in I and reach ages as old as similar to 50 Ma through the sediment column. At the outer ridge site along the trough, on the other hand, relatively younger I are more frequently delivered probably through thrusts/faults associated with subduction. The nature of source formations of I and hydrocarbons in the offshore Shimokita Peninsula has a more terrestrial contribution compared with those in the Nankai Trough, but these formations are also considerably older than the host sediments, suggesting long-term transport of I and hydrocarbons for the accumulation of gas hydrates in both locations.

In Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists<br /> Contributed to the section of “Inorganic Geochemistry”

In Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists<br /> Contributed to the section of “Inorganic Geochemistry”

In Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists<br /> Contributed to the section of “Inorganic Geochemistry”

In Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists<br /> Contributed to the section of “Inorganic Geochemistry”