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Serpentinization of ultramafic rocks provides molecular hydrogen (H2) that can support lithotrophic metabolism of microorganisms, but also poses extremely challenging conditions, including hyperalkalinity and limited electron acceptor availability. Investigation of two serpentinization-active systems reveals that conventional H2-/CO2-dependent homoacetogenesis is thermodynamically unfavorable in situ due to picomolar CO2 levels. Through metagenomics and thermodynamics, we discover unique taxa capable of metabolism adapted to the habitat. This included a novel deep-branching phylum, “Ca. Lithacetigenota”, that exclusively inhabits serpentinite-hosted systems and harbors genes encoding alternative modes of H2-utilizing lithotrophy. Rather than CO2, these putative metabolisms utilize reduced carbon compounds detected in situ presumably serpentinization-derived: formate and glycine. The former employs a partial homoacetogenesis pathway and the latter a distinct pathway mediated by a rare selenoprotein—the glycine reductase. A survey of microbiomes shows that glycine reductases are diverse and nearly ubiquitous in serpentinite-hosted environments. “Ca. Lithacetigenota” glycine reductases represent a basal lineage, suggesting that catabolic glycine reduction is an ancient bacterial innovation by Terrabacteria for gaining energy from geogenic H2 even under hyperalkaline, CO2-poor conditions. Unique non-CO2-reducing metabolisms presented here shed light on potential strategies that extremophiles may employ for overcoming a crucial obstacle in serpentinization-associated environments, features potentially relevant to primordial lithotrophy in early Earth.

We estimate the rate of upward pore fluid flow and chlorinity (Cl−) concentrations at depth through a joint analysis of Cl− concentration and temperature versus depth profiles obtained from gas hydrate related test sites drilled and cored in the Krishna-Godavari Basin off the eastern coast of India. Cl−, measured on conventional core samples, decreases with depth at all sites but some of the Cl−profiles show a prominent convex shape, whereas in situ temperature profiles, obtained by the Advanced Piston Coring Temperature probe, are mostly linear at all sites established during the National Gas Hydrate Program 02 Expedition (NGHP-02). Assuming a one-dimensional, time-dependent model for the advection of pore fluid including the sedimentation effect and depth-dependent diffusivity, we estimate darcy velocity and the Cl− concentrations at depth. The best-fit darcy velocity of 1.2–1.6 × 10−11 m/s was estimated for the sites along the crest of the regional anticlinal structure in the NGHP-02 Area B, which was significantly faster than those on the flanks of the anticline. Because the thermal diffusion coefficient is much larger than the chloride ion diffusion coefficient, estimated darcy velocities are not great enough to generate nonlinear temperature profiles with depth, which is consistent with observed linear thermal profiles.

National Gas Hydrate Program Expedition 02 was conducted in 2015 by D/V Chikyu in the western Bay of Bengal, India. Analyses of interstitial water in sediment from four drilling sites along the eastern and western transect in the Krishna–Godavari Basin show that dissolved chloride (Cl−) concentrations decreased with depth from seawater values (>550 mM) near the sediment surface to 300–400 mM at 350–400 m below seafloor (mbsf), suggesting the upward advection of low-Cl– fluid. Excursions of Cl− concentrations toward low values in gas hydrate zones were attributed to the presence of fresh water released by dissociation of gas hydrate during core recovery. At Sites in the western transect, Cl− concentrations below the gas hydrate reservoir (around 280 mbsf) were ca. 60 mM and ca. 45 mM lower than those from above the reservoir, and δ18O and δD values were 1–2‰ and 0.7–1.3‰ higher than those from above the reservoir, respectively. These results suggest that a fraction of migrating low-Cl– fluid is trapped in the gas hydrate zone, causing decreases of δ18O and δD in the residual water due to isotopic fractionation during gas hydrate formation. From mass-balance calculations, we estimated that 10–20% of the advected fluid is trapped at the gas hydrate zone. At all sites, the δ18O and δD values below the gas hydrate zone, representing relatively unaltered low-Cl– fluid, were 0.5–1‰ higher and ca. 13–15‰ lower than those of seawater, respectively. This trend is consistent with water derived from the dehydration of clay minerals, which generally occurs in deeply buried sediments. The contribution of clay mineral dehydration is relatively large at the western transect compared to that in the eastern transect. Our results imply that fluid advection contributes to the accumulation of gas hydrate in the study area.

Serpentinization of ultramafic rocks provides molecular hydrogen (H

Deep, hot, and more alive than we thought Marine sediments represent a massive microbial ecosystem, but we still do not fully understand what factors shape and limit life underneath the seafloor. Analyzing samples from a subduction zone off the coast of Japan, Heuer et al. found that microbial life, in particular bacterial vegetative cells, decreases as depth and temperature increases down to ∼600 meters below the seafloor, corresponding to temperatures of ∼70°C. Below this limit, endospores are common—a remnant, and a potential reservoir, of bacterial life. Deeper still is a sterile zone, and below 1000 meters is a scalding realm populated by vegetative cells. At such great depths, high concentrations of acetate and sulfate coexist, and there are also signs of hyperthermophilic methanogenesis. These data provide a fascinating window into an extreme and inhospitable environment that nonetheless supports microbial life. Science , this issue p. 1230

Recent explorations of scientific ocean drilling have revealed the presence of microbial communities persisting in sediments down to ~2.5 km below the ocean floor. However, our knowledge of these microbial populations in the deep subseafloor sedimentary biosphere remains limited. Here, we present a cultivation experiment of 2-km-deep subseafloor microbial communities in 20-million-year-old lignite coalbeds using a continuous-flow bioreactor operating at 40 °C for 1029 days with lignite particles as the major energy source. Chemical monitoring of effluent samples via fluorescence emission-excitation matrices spectroscopy and stable isotope analyses traced the transformation of coalbed-derived organic matter in the dissolved phase. Hereby, the production of acetate and 13C-depleted methane together with the increase and transformation of high molecular weight humics point to an active lignite-degrading methanogenic community present within the bioreactor. Electron microscopy revealed abundant microbial cells growing on the surface of lignite particles. Small subunit rRNA gene sequence analysis revealed that diverse microorganisms grew in the bioreactor (e.g., phyla Proteobacteria, Firmicutes, Chloroflexi, Actinobacteria, Bacteroidetes, Spirochaetes, Tenericutes, Ignavibacteriae, and SBR1093). These results indicate that activation and adaptive growth of 2-km-deep microbes was successfully accomplished using a continuous-flow bioreactor, which lays the groundwork to explore networks of microbial communities of the deep biosphere and their physiologies.

金沢大学理工研究域地球社会基盤学系, Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. © 2018. American Geophysical Union. All Rights Reserved.

We examined the carbon isotopic and molecular compositions of residual gases within Miocene methane-derived carbonates collected in Japan. Methane, ethane, and propane were extracted by acid digestion of powdered carbonates. The isotopic and molecular compositions of the extracted hydrocarbons are inconsistent with conventional thermogenic and microbial gases. Despite a range of δ13C values from −67‰ to −38‰ (relative to Vienna Pee Dee Belemnite (VPDB)), the liberated hydrocarbons yielded consistently low methane to ethane + propane ratios (2–30). The extracted ethane and propane yielded anomalous δ13C values as low as −84‰, lower than those of the coexisting methane. The ethane and propane were most likely produced through thermal cracking of organic compounds preserved within the seep carbonates during burial. The observed unusual isotopic trends may be explained by the mixing of two thermogenic gas components with different carbon isotopic and molecular compositions. Nevertheless, a positive correlation between δ13C values of methane and relatively immature carbonates at one study site (Nakanomata) indicates that while methane was oxidized to bicarbonate from which carbonates precipitated, it was preserved within the host carbonate cements. Such a scenario indicates that the residual methane at least partly originates from the Miocene seep fluid. Smaller amounts of methane were also released during heating and crushing of chipped samples. The results suggest that methane was entrapped mainly within intracrystal inclusions, which is supported by the observation of abundant nanometer-scale voids in an individual crystal.

Significance Microbial cells are widespread in diverse deep subseafloor environments; however, the viability, growth, and ecophysiology of these low-abundance organisms are poorly understood. Using single-cell–targeted stable isotope probing incubations combined with nanometer-scale secondary ion mass spectrometry, we measured the metabolic activity and generation times of thermally adapted microorganisms within Miocene-aged coal and shale bed samples collected from 2 km below the seafloor during Integrated Ocean Drilling Program Expedition 337. Microorganisms from the shale and coal were capable of metabolizing methylated substrates, including methylamine and methanol, when incubated at their in situ temperature of 45 °C, but had exceedingly slow growth, with biomass generation times ranging from less than a year to hundreds of years as measured by the passive tracer deuterated water., The past decade of scientific ocean drilling has revealed seemingly ubiquitous, slow-growing microbial life within a range of deep biosphere habitats. Integrated Ocean Drilling Program Expedition 337 expanded these studies by successfully coring Miocene-aged coal beds 2 km below the seafloor hypothesized to be “hot spots” for microbial life. To characterize the activity of coal-associated microorganisms from this site, a series of stable isotope probing (SIP) experiments were conducted using intact pieces of coal and overlying shale incubated at in situ temperatures (45 °C). The 30-month SIP incubations were amended with deuterated water as a passive tracer for growth and different combinations of 13C- or 15N-labeled methanol, methylamine, and ammonium added at low (micromolar) concentrations to investigate methylotrophy in the deep subseafloor biosphere. Although the cell densities were low (50–2,000 cells per cubic centimeter), bulk geochemical measurements and single-cell–targeted nanometer-scale secondary ion mass spectrometry demonstrated active metabolism of methylated substrates by the thermally adapted microbial assemblage, with differing substrate utilization profiles between coal and shale incubations. The conversion of labeled methylamine and methanol was predominantly through heterotrophic processes, with only minor stimulation of methanogenesis. These findings were consistent with in situ and incubation 16S rRNA gene surveys. Microbial growth estimates in the incubations ranged from several months to over 100 y, representing some of the slowest direct measurements of environmental microbial biosynthesis rates. Collectively, these data highlight a small, but viable, deep coal bed biosphere characterized by extremely slow-growing heterotrophs that can utilize a diverse range of carbon and nitrogen substrates.

金沢大学理工研究域地球社会基盤学系, We have discovered deep-sea mud that is extremely enriched in rare-earth elements and yttrium (together called REY) in the Japanese Exclusive Economic Zone around Minamitorishima Island, in the western North Pacific Ocean. The maximum total REY concentration reaches approximately 7000 ppm, which is much higher than that reported for conventional REY deposits on land and other known potential REY resources in the ocean. The extremely REY-rich mud is characterized by abundant phillipsite and biogenic calcium phosphate. In addition, the stratigraphic layer with the highest REY concentration occurs just -3 m beneath the seafloor. The shallow burial of these strata together with the high REY content, especially those of heavy rare-earth elements, suggest that the newly discovered extremely REY-rich mud may be a promising REY resource. © 2016 by The Geochemical Society of Japan.

金沢大学理工研究域地球社会基盤学系, We report detailed lithological and chemical characteristics of deep-sea sediments, including rare-earth elements and yttrium-rich mud (REY-rich mud), in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island. Three research cruises obtained fourteen sediment cores collected by piston coring. Based on the visual descriptions and geochemical analysis of the sediment cores, we confirm the presence of REY-rich mud containing more than 400 ppm total REY (ΣREY) in the southern and northwestern areas of the Minamitorishima EEZ. The REY-rich mud layers are characterized by abundant grains of phillipsite, biogenic calcium phosphate, and manganese oxides, and are widely distributed in relatively shallow depths beneath the seafloor. In contrast, relatively thick, non-REY-rich mud lies near the seafloor in the northern areas of the EEZ. In the three cores from the southern part of the EEZ, we also confirm the presence of highly/extremely REY-rich mud layers. Further accumulation of geochemical data from the sediments will be required to constrain the extent of the highly/extremely REY-rich mud layers. © 2016 by The Geochemical Society of Japan.

Subseafloor microbes beneath active hydrothermal vents are thought to live near the upper temperature limit for life on Earth. We drilled and cored the Iheya North hydrothermal field in the Mid-Okinawa Trough, and examined the phylogenetic compositions and the products of metabolic functions of sub-vent microbial communities. We detected microbial cells, metabolic activities and molecular signatures only in the shallow sediments down to 15.8 m below the seafloor at a moderately distant drilling site from the active hydrothermal vents (450 m). At the drilling site, the profiles of methane and sulfate concentrations and the δ13C and δD isotopic compositions of methane suggested the laterally flowing hydrothermal fluids and the in situ microbial anaerobic methane oxidation. In situ measurements during the drilling constrain the current bottom temperature of the microbially habitable zone to ~45 °C. However, in the past, higher temperatures of 106–198 °C were possible at the depth, as estimated from geochemical thermometry on hydrothermally altered clay minerals. The 16S rRNA gene phylotypes found in the deepest habitable zone are related to those of thermophiles, although sequences typical of known hyperthermophilic microbes were absent from the entire core. Overall our results shed new light on the distribution and composition of the boundary microbial community close to the high-temperature limit for habitability in the subseafloor environment of a hydrothermal field.

Elmira is a medium-to-large gastropod of uncertain systematic affinity, which has so far been reported only from a presumably Eocene methane-seep deposit in Cuba. This study reports a mass occurrence of Elmira shimantoensis Kiel and Nobuhara sp. nov. from a Late Cretaceous hydrocarbon-seep deposit in Shikoku, Japan, called the Sada Limestone. Its paleoecology is reconstructed based on its mode of occurrence, carbonate petrology, and stable carbon isotope analyses. The fauna of the Sada Limestone in general is characterized by an abundance of large chemosymbiotic bivalves of the family Thyasiridae and of “Serpula” tubes. The mass occurrence of Elmira shimantoensis was found in a lens-shaped carbonate body with a flat top and a concave base, 6.5 m in length and less than 2 m in thickness, consisting of multiple layers of shell accumulations, which were formed by shell-winnowing and the filling of a depression in slope mud. The scarcity of Elmira shimantoensis elsewhere in the Sada Limestone suggests that it formed locally from a gregarious population in the vicinity of the depression, possibly on hard ground. The matrix of the mass occurrence is rich in dolomite and ankerite, and is less depleted in 13C (δ13C values of calcite: −5.3 to −2.4 ‰; of dolomite: −8.3 ‰) than the matrix of the enclosing thyasirid-rich and tube-rich limestones. This suggests that the gastropod mass occurrence was cemented below the sulfate reduction zone and has thus undergone little anaerobic methane oxidation. Therefore, Elmira shimantoensis is reconstructed here as a bacteria grazer on a hard substrate such as exposed carbonate mounds rather than as a species that relied on chemosynthetic symbionts for nutrition.

Hydraulic fracturing is a prominent method of natural gas production that uses injected, high-pressure fluids to fracture low permeability, hydrocarbon rich strata such as shale. Upon completion of a well, the fluid returns to the surface (produced water) and contains natural gas, subsurface constituents, and microorganisms (Barbot et al., 2013; Daly et al., 2016). While the microbial community of the produced fluids has been studied in multiple gas wells, the activity of these microorganisms and their relation to biogeochemical activity is not well understood. In this experiment, we supplemented produced fluid with 13C-labeled carbon sources (glucose, acetate, bicarbonate, methanol, or methane), and 15N-labeled ammonium chloride in order to isotopically trace microbial activity over multiple day in anoxic incubations. Nanoscale secondary ion mass spectrometry (NanoSIMS) was used to generate isotopic images of 13C and 15N incorporation in individual cells, while isotope ratio monitoring-gas chromatography-mass spectrometry (IRM-GC-MS) was used to measure 13CO2, and 13CH4 as metabolic byproducts. Glucose, acetate, and methanol were all assimilated by microorganisms under anoxic conditions. 13CO2 production was only observed with glucose as a substrate indicating that catabolic activity was limited to this condition. The microbial communities observed at 0, 19, and 32 days of incubation did not vary between different carbon sources, were low in diversity, and composed primarily of the class Clostridia. The primary genera detected in the incubations, Halanaerobium and Fusibacter, are known to be adapted to harsh physical and chemical conditions consistent with those that occur in the hydrofracturing environment. This study provides evidence that microorganisms in produced fluid are revivable in laboratory incubations and retained the ability to metabolize added carbon and nitrogen substrates.

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 (10 2 to 10 3 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., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 802

Chemical and isotopic compositions of hydrothermal fluids from Hatoma Knoll in the southern Okinawa Trough were investigated. The hydrothermal fluids were derived from a single pure hydrothermal fluid source, but they underwent phase separation beneath the seafloor prior to venting. Only vapor-like fluids vent at the Hatoma system, and the most active area is around the center of the crater, based on the location of the maximum temperature and the lowest Cl– concentrations. Compared with other hydrothermal systems in the world, at Hatoma the pH and alkalinity, as well as the B, NH4+, K, Li, CO2, and CH4 concentrations, were higher, and the Fe and Al concentrations were lower, suggesting that the characteristics of the Hatoma hydrothermal fluids are comparable to those of the other Okinawa Trough hydrothermal fluids. Helium isotope ratios were lower than those of sediment-starved hydrothermal systems, suggesting that 4He derived from the sediment is supplied to the hydrothermal fluids in Hatoma Knoll. The carbon isotope ratios of CO2 in the hydrothermal fluids indicate an influence of organic carbon decomposition. The carbon isotope ratios of CH4 in the hydrothermal fluids imply that most of the CH4 originated from microbial methane produced in a recharge zone of the hydrothermal system. Although sediment influences are a typical feature of Okinawan Trough hydrothermal fluids, the Hatoma hydrothermal system has the lowest carbon isotope ratios of CH4 among them, which suggests that Hatoma is the most highly influenced by the sediments in the recharge zone. Thus, the degree of the sediment influences has a variable in each hydrothermal field in the Okinawa Trough., 論文

To investigate fluid regimes in the Nankai accretionary wedge, we estimated the temperatures of lithium (Li) reservoirs from the Li isotope ratio (7Li/6Li) in fluids recovered from Kumano mud volcano No. 5 (KMV#5) in the Nankai forearc basin. The 7Li/6Li ratios in the KMV#5 fluids are among the lightest reported from submarine mud volcanoes. The estimated Li reservoir temperatures (310 °C at maximum) were significantly higher than the smectite-to-illite conversion temperature (60–150 °C), whereas the temperature of fluid from the deeper Nankai decollement was estimated to be 150 °C at maximum. Data obtained from previous conductivity surveys across the trench suggest that there are two fluid reservoirs associated with distinct dehydration processes in the Nankai subduction zone. From these results, we inferred that the discontinuous mud diapirism in the Kumano forearc basin results from the periodic injection of deep-seated fluid accumulated in the corner of the serpentinized forearc mantle wedge that travels upward via a thrust fault. The intermittent upwelling of deep-seated fluid from the Nankai accretionary wedge due to periodic breaches of a low-permeability barrier above the fluid reservoir may be associated with seismogenic fault activity.

A sediment squeezing tool for use in interstitial water extraction at the onboard laboratory of D/V Chikyu was modified for use at pressures up to 112.3 MPa. A pair of titanium water-gathering plate originally developed by Central Research Institute of Electric Power Industry (CRIEPI) replaces a thin mesh to allow squeezing of sediment at a higher pressure without jamming of sediments. The modified tool endured a longer period of squeezing at high pressure, and yielded greater volume of interstitial water in test experiments. The acquired fluid sample composition showed no major changes even at the pressure of 112.3 MPa, although further tests must be conducted to assess the potential dehydration of clay mineral at high pressures. However, squeezing of sandstone of approximately 12% porosity yielded no interstitial water at all, showing system limitations for low-porosity samples. The improved tool and the data from these experiments are expected to be useful when low-porosity sediments from a deep hole are obtained.

The impacts of lithologic structure and geothermal gradient on subseafloor microbial communities were investigated at a marginal site of the Iheya North hydrothermal field in the Mid-Okinawa Trough. Subsurface marine sediments composed of hemipelagic muds and volcaniclastic deposits were recovered through a depth of 151 m below the seafloor at site C0017 during Integrated Ocean Drilling Program Expedition 331. Microbial communities inferred from 16S rRNA gene clone sequencing in low-temperature hemipelagic sediments were mainly composed of members of the Chloroflexi and deep-sea archaeal group. In contrast, 16S rRNA gene sequences of marine group I Thaumarchaeota dominated the microbial phylotype communities in the coarse-grained pumiceous gravels interbedded between the hemipelagic sediments. Based on the physical properties of sediments such as temperature and permeability, the porewater chemistry, and the microbial phylotype compositions, the shift in the physical properties of the sediments is suggested to induce a potential subseafloor recharging flow of oxygenated seawater in the permeable zone, leading to the generation of variable chemical environments and microbial communities in the subseafloor habitats. In addition, the deepest section of sediments under high-temperature conditions (∼90°C) harbored the sequences of an uncultivated archaeal lineage of hot water crenarchaeotic group IV that may be associated with the high-temperature hydrothermal fluid flow. These results indicate that the subseafloor microbial community compositions and functions at the marginal site of the hydrothermal field are highly affected by the complex fluid flow structure, such as recharging seawater and underlying hydrothermal fluids, coupled with the lithologic transition of sediments.

We present a method for determining sub-milligram quantities of biogenic opal. The method employs the inductive high-temperature carbon reduction method for dehydration of opal and reduction of silica, and a continuous-flow isotope ratio mass spectrometry system for direct analysis of the oxygen isotope ratio in the evolved carbon monoxide. The accuracy and precision of the online analytical system were evaluated by isotopic analysis of various quantities of standard SiO2 in the range 40–538 µg. The time required to analyse a single sample was relatively short (50 min); hence, our method is suitable for routine analysis for paleoenvironmental studies that require large amounts of time-series data. The method was validated for samples in the sub-milligram range and can be applied to oxygen isotope analysis of various types of biogenic opal that have not been analysed because of their small amounts in natural samples. We successfully applied our method to (i) the first oxygen isotope analysis of monospecific radiolarian skeletons and (ii) high-resolution oxygen isotope analysis of Holocene diatom frustules from the Southern Ocean.

International Ocean Discovery Program (IODP) Expedition 370 aimed to explore the limits of life in the deep subseafloor biosphere at a location where temperature increases with depth at an intermediate rate and exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Drilling Site C0023 is located in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174 at the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m. ODP Leg 190 in 2000, revealed the presence of microbial cells at Site 1174 to a depth of ~600 meters below seafloor (mbsf), which corresponds to an estimated temperature of ~70°C, and reliably identified a single zone of higher cell concentrations just above the decollement at around 800 mbsf, where temperature presumably reached 90°C; no cell count data was reported for other sediment layers in the 70°–120°C range, because the limit of manual cell count for low-biomass samples was not high enough. With the establishment of Site C0023, we aimed to detect and investigate the presence or absence of life and biological processes at the biotic–abiotic transition with unprecedented analytical sensitivity and precision. Expedition 370 was the first expedition dedicated to subseafloor microbiology that achieved time-critical processing and analyses of deep biosphere samples by simultaneous shipboard and shore-based investigations. Our primary objectives during Expedition 370 were to study the relationship between the deep subseafloor biosphere and temperature. We aimed to comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor to ~120°C at the sediment/basement interface and thus likely encompasses the biotic–abiotic transition zone. We also aimed to determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex. To address these primary scientific objectives and questions, we penetrated 1180 m and recovered 112 cores across the sediment/basalt interface. More than 13,000 samples were collected, and selected samples were transferred to the Kochi Core Center by helicopter for simultaneous microbiological sampling and analysis in laboratories with a super-clean environment. Following the coring operations, a temperature observatory with 13 thermistor sensors was installed in the borehole to 863 mbsf.

567 The concentration and carbon isotopic composition (δ13C) of acetate in marine sediment are such recent additions to the pore-water geochemical analyses, following the technological advancement of a sensitive analytical tool (Heuer et al., 2006), which was developed to understand subseafloor biogeochemical processes via acetate. It has been pointed out that acetate in sediment could be sensitive to a temperature increase, resulting in increased acetate concentrations after incubation of sediment at high temperature (Wellsbury et al., 1997). In this brief note, we simulated two possible cases of shipboard sample processing; i) short-term storage of the corer with rubber stoppers in a refrigerator at near in-situ temperature (4°C) until pore water extraction to minimize air contamination and temperature change represents an ordinarily procedure on board, even though a small temperature change from the absolute in-situ temperature to 4°C and air contamination is unavoidable, ii) storage at room temperature for 19 hours represents a worst possible condition. Such a worst case occasionally occur to cut core samples which have to be warmed to room temperature for non-destructive measurement of physical properties using a multi-sensor core logger (MSCL) for 12–17 hours before pore-water extraction (Ijiri et al., 2012). Those two samples have been provided to meaIncrease in acetate concentrations during sediment sample onboard storage: a caution for pore-water geochemical analyses

Biogeochemical processes involving acetate in sub-seafloor sediments from piston core PC23B from the Bering Sea shelf break were inferred by examining the stable carbon isotopic relationships between acetate and other relevant carbon compounds: total organic carbon (TOC) in the sediment solid phase, and dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in pore water. Throughout the core, the isotopic composition of acetate (δ 13 C acetate ), from −31‰ to −29‰, was 13 C-depleted by ca. 7‰ vs. DOC (δ 13 C DOC ) and its depth profile approximately paralleled that of δ 13 C DOC , suggesting that the principal process producing acetate was fermentation of dissolved organic compounds. However, the 13 C depletion in δ 13 C acetate indicates some contribution of acetogenesis to total acetate production, because acetogenesis results in 13 C depletion of the acetate produced. The relative contribution of acetogenesis via the H 2 /CO 2 reaction, calculated by using a two source isotope mixing model, increased with depth in the sulfate reduction zone from 10% to 15% and was constant at 19% in the methanogenic zone. The acetogenic contribution to acetate production in the methanogenic zone underlying the sulfate reduction zone is consistent with reported observations, whereas the occurrence of acetogenesis in the sulfate reduction zone may be related to the contribution of terrestrial organic matter (OM) to the sedimentary OM in that depth interval, because the terrestrial component likely includes precursors that favor organoautotrophic acetogenesis. The high acetate concentration (up to 81 μM) and TOC content (up to 1.4%) at the same depth (

We measured terrestrial plant biomarker (long-chain n-alkanes) in the sediment cores taken from the Sea of Okhotsk to examine paleoclimatic utility of long-chain n-alkanes in marine sediments. This study demonstrates that sedimentary record of n-alkane in the sea has a high potential to provide important complementary paleo-climate/paleo-environmental information. Molecular distributions of long-chain n-alkanes in marine sediments show a typical signature of terrestrial plant wax derived n-alkanes with strong odd carbon number predominance from the last glacial to the present, suggesting a source of long-chain n-alkanes in the Okhotsk Sea sediments has been terrestrial higher plants throughout the time. The down core profiles of concentrations of C25–C35 n-alkanes in XP07-C9 collected from the northwestern site revealed three events of enhanced terrestrial organic matter input during the last deglaciation. The two pronounced events correspond to Melt Water Pulse (MWP) events 1A (14.5–12.5 ka) and 1B (11–6.5 ka). These events possibly linked to increases in river discharge and erosion of submerged continental shelf due to drastic rise in sea level. Down core profiles of molecular distributions of n-alkanes in the Okhotsk Sea sediments significantly vary over the last 25 kyr, and are similar to that of a peat core sequence in the East Russia and essentially consistent with pollen data from marine and peat core sequences.

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 (∑CO2) in dissolved gases were measured. The contribution of thermogenic CH4 was negligible at all sites, while carbon isotopic separation between CH4 and ∑CO2 indicated that CH4 formation was mainly by microbial hydrogenotrophic methanogenesis. Evaluation of the isotopic fraction of the initial substrate ∑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 H2 as well as NH4+ were actively generated by the organic matter degradation at Site C0002, which could stimulate methanogenesis utilizing CO2 and H2 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., 論文