Chemosynthetic Biological Communities Research Articles

Pockmarks Offshore Big Sur, California Provide Evidence for Recurrent, Regional, and Unconfined Sediment Gravity Flows

AbstractRecent surface ship multibeam surveys of the Sur Pockmark Field, offshore Central California, reveal >5,000 pockmarks in an area that is slated to host a wind farm, between 500‐ and 1,500‐m water depth. Extensive fieldwork was conducted to characterize the seafloor environment and its recent geologic history, including visual observations with remotely operated vehicles, sediment core sampling, and high‐resolution, near‐bottom Chirp and multibeam surveys collected with autonomous underwater vehicles to capture the morphology and stratigraphy of the pockmarks. No evidence of high methane concentrations in sediments, chemosynthetic biological communities, or methane‐derived diagenetic byproducts was found. Chirp data and sediment cores showed alternating layers of slowly accumulating hemipelagic drapes interrupted by more reflective turbidite horizons that extend throughout the pockmark field and beyond. Chirp data showed multiple episodes of lateral migration over time in some of the pockmarks in association with erosion and infilling events. Laterally continuous turbidite horizons that overlay erosional surfaces indicated that pockmark migration occurred synchronously in multiple pockmarks separated by tens of kilometers. These shifts are presumed to be the result of asymmetrical erosion of the pockmark flanks caused by passing sediment gravity flows. While some pockmarks occur in chains, most are not clustered or randomly spaced but are regularly dispersed within the pockmark field. We hypothesize that intermittent, unconfined sediment gravity flows occurring over at least the last 280,000 years are the source of the regionally continuous turbidite deposits and the mechanism that maintained the regularly dispersed pockmarks.

Deciphering the Origin of Abiotic Organic Compounds on Earth: Review and Future Prospects

AbstractThe geologic production of abiotic organic compounds has been the subject of increasing scientific attention due to their use in the global carbon flux balance, by chemosynthetic biological communities, and for energy resources. Extensive analysis of methane (CH4) and other organics in diverse geologic settings, combined with thermodynamic modelings and laboratory simulations, have yielded insights into the distribution of specific abiotic organic molecules on Earth and the favorable conditions and pathways under which they form. This updated and comprehensive review summarizes published results of petrological, thermodynamic, and experimental investigations of possible pathways for the formation of particular species of abiotic simple hydrocarbon molecules such as CH4, and of complex hydrocarbon systems, e.g., long‐chain hydrocarbons and even solid carbonaceous matters, in various geologic processes, distinguished into three classes: (1) pre‐ to early planetary processes; (2) mantle and magmatic processes; and (3) the gas/water‐rock reaction processes in low‐pressure ultramafic rock and high‐pressure subduction zone systems. We not only emphasize how organics are abiotically synthesized but also explore the role or changes of organics in evolutionary geological environments after synthesis, such as phase transitions or organic‐mineral interactions. Correspondingly, there is an urgent need to explore the diversity of abiotic organic compounds prevailing on Earth.

Gas Migration Episodes Observed During Peridotite Alteration in the Samail Ophiolite, Oman.

Serpentinization and carbonation of mantle rocks (peridotite alteration) are fundamentally important processes for a spectrum of geoscience topics, including arc volcanism, earthquake processes, chemosynthetic biological communities, and carbon sequestration. Data from a hydrophone array deployed in the Multi-Borehole Observatory (MBO) of the Oman Drilling Project demonstrates that free gas generated by peridotite alteration and/or microbial activity migrates through the formation in discrete bursts of activity. We detected several, minutes-long, swarms of gas discharge into Hole BA1B of the MBO over the course of a 9month observation interval. The episodic nature of the migration events indicates that free gas accumulates in the permeable flow network, is pressurized, and discharges rapidly into the borehole when a critical pressure, likely associated with a capillary barrier at a flow constriction, is reached. Our observations reveal a dynamic mode of fluid migration during serpentinization, and highlight the important role that free gas can play in modulating pore pressure, fluid flow, and alteration kinetics during peridotite weathering.

Combined acoustical and seismic studies of Axial Seamount

The NSF Ocean Observatories Initiatives Regional Cabled Array at Axial Seamount provides a remarkable facility for sustained studies of an active submarine volcano. A local seismic network supports a real-time earthquake catalog and hydrophones monitor sounds in the water column. In April 2015, Axial Seamount erupted shortly after the cabled observatory came on line. The seismic record of brittle deformation in the summit caldera during the eruption is complemented by acoustic observations that tracked a dike propagating along the north rift using T phases and recorded both impulsive and continuous sound sources on the seafloor at the sites of erupting lava. Seafloor pressure gauges are presently documenting the reinflation of the volcano towards its next eruption and these geodetic studies would be enhanced by deploying a transponder network to support horizontal ranging. The availability of real-time data to detect and localize future eruptions will enable effective autonomous and ship-based responses to study how hydrothermal systems and their chemosynthetic biological communities respond to eruptions. The seismic and acoustic networks could also support studies of fin and blue whales to understand whether their distribution is modulated by the enhanced productivity associated with the presence of a hydrothermally active seamount.

Isolating Detrital and Diagenetic Signals in Magnetic Susceptibility Records From Methane‐Bearing Marine Sediments

AbstractVolume‐dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post‐depositional alteration by H2S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane‐bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X‐ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high‐resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ13C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo‐positions of the sulfate‐methane transition zone, past enhancements of OSR or paleo‐methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps.

Hydrogen and Abiotic Hydrocarbons: Molecules that Change the World

Molecular hydrogen (H2), methane, and hydrocarbons with an apparent abiotic origin have been observed in a variety of geologic settings, including serpentinized ultramafic rocks, hydrothermal fluids, and deep fractures within ancient cratons. Molecular hydrogen is also observed in vapor plumes emanating from the icy crust of Saturn’s moon Enceladus, and methane has been detected in the atmosphere of Mars. Geologic production of these compounds has been the subject of increasing scientific attention due to their use by chemosynthetic biological communities. These compounds are also of interest as possible energy resources. This issue summarizes the geological sources of abiotic H2 and hydrocarbons on Earth and elsewhere and examines their impact on microbial life and energy resources.

Active mud volcanoes on the continental slope of the Canadian Beaufort Sea

AbstractMorphologic features, 600–1100 m across and elevated up to 30 m above the surrounding seafloor, interpreted to be mud volcanoes were investigated on the continental slope in the Beaufort Sea in the Canadian Arctic. Sediment cores, detailed mapping with an autonomous underwater vehicle, and exploration with a remotely operated vehicle show that these are young and actively forming features experiencing ongoing eruptions. Biogenic methane and low‐chloride, sodium‐bicarbonate‐rich waters are extruded with warm sediment that accumulates to form cones and low‐relief circular plateaus. The chemical and isotopic compositions of the ascending water indicate that a mixture of meteoric water, seawater, and water from clay dehydration has played a significant role in the evolution of these fluids. The venting methane supports extensive siboglinid tubeworms communities and forms some gas hydrates within the near seafloor. We believe that these are the first documented living chemosynthetic biological communities in the continental slope of the western Arctic Ocean.

Spatial distribution of sister species of vesicomyid bivalves Calyptogena okutanii and Calyptogena soyoae along an environmental gradient in chemosynthetic biological communities in Japan

Vesicomyid bivalves have a substantial biomass in deep-sea chemosynthetic biological communities in the Pacific. Using a novel multiplex-PCR (mPCR) method to identify the co-occurring vesicomyids in Sagami Bay, we analyzed the distribution of Calyptogena okutanii and Calyptogena soyoae along environmental gradients. All the known distributions of C. okutanii indicated the different preferences in salinity and temperature to those of C. soyoae, and in Sagami Bay, depth seemed to be an important environmental factor, too. Although the concentration of hydrogen sulfide in sediment was not examined, our results showed that the distributions of these two Calyptogena clams were affected by salinity and temperature.

Active submarine eruption of boninite in the northeastern Lau Basin

Subduction of oceanic crust and the formation of volcanic arcs above the subduction zone are important components in Earth’s geological and geochemical cycles. Subduction consumes and recycles material from the oceanic plates, releasing fluids and gases that enhance magmatic activity, feed hydrothermal systems, generate ore deposits and nurture chemosynthetic biological communities. Among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. These lavas contain information about the early stages of subduction, yet because most subduction systems on Earth are old and well-established, boninite lavas have previously only been observed in the ancient geological record. Here we observe and sample an active boninite eruption occurring at 1,200 m depth at the West Mata submarine volcano in the northeast Lau Basin, southwest Pacific Ocean. We find that large volumes of H2O, CO2 and sulphur are emitted, which we suggest are derived from the subducting slab. These volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. The eruption has been ongoing for at least 2.5 years and we conclude that this boninite eruption is a multi-year, low-mass-transfer-rate eruption. Thus the Lau Basin may provide an important site for the long-term study of submarine volcanic eruptions related to the early stages of subduction. Boninite lavas are erupted during the early stages of subduction, however they have previously been found only in the ancient geological record. Discovery of an active boninite eruption shows that abundant volatile gases derived from the subducting slab drive this violent eruptive activity, even in the deep sea.

The mechanics of intermittent methane venting at South Hydrate Ridge inferred from 4D seismic surveying

Sea floor methane vents and seeps direct methane generated by microbial and thermal decompositions of organic matter in sediment into the oceans and atmosphere. Methane vents contribute to ocean acidification, global warming, and providing a long-term (e.g. 500–4000 years; Powell et al., 1998) life-sustaining role for unique chemosynthetic biological communities. However, the role methane vents play in both climate change and chemosynthetic life remains controversial primarily because we do not understand long-term methane flux and the mechanisms that control it ( Milkov et al., 2004; Shakhova et al., 2010; Van Dover, 2000). Vents are inherently dynamic and flux varies greatly in magnitude and even flow direction over short time periods (hours-to-days), often tidally-driven ( Boles et al., 2001; Tryon et al., 1999). But, it remains unclear if flux changes at vents occur on the order of the life-cycle of various species within chemosynthetic communities (months, years, to decades Leifer et al., 2004; Torres et al., 2001) and thus impacts their sustainability. Here, using repeat high-resolution 3D seismic surveys acquired in 2000 and 2008, we demonstrate in 4D that Hydrate Ridge, a vent off the Oregon coast has undergone significant reduction of methane flow and complete interruption in just the past few years. In the subsurface, below a frozen methane hydrate layer, free gas appears to be migrating toward the vent, but currently there is accumulating gas that is unable to reach the seafloor through the gas hydrate layer. At the same time, abundant authigenic carbonates show that the system has been active for several thousands of years. Thus, it is likely that activity has been intermittent because gas hydrates clog the vertical flow pathways feeding the seafloor vent. Back pressure building in the subsurface will ultimately trigger hydrofracturing that will revive fluid-flow to the seafloor. The nature of this mechanism implies regular recurring flow interruptions and methane flux changes that threaten the viability of chemosynthetic life, but simultaneously and enigmatically sustains it.

Seismic identification of along-axis hydrothermal flow on the East Pacific Rise

Hydrothermal circulation at the axis of mid-ocean ridges affects the chemistry of the lithosphere and overlying ocean, supports chemosynthetic biological communities and is responsible for significant heat transfer from the lithosphere to the ocean. It is commonly thought that flow in these systems is oriented across the ridge axis, with recharge occurring along off-axis faults, but the structure and scale of hydrothermal systems are usually inferred from thermal and geochemical models constrained by the geophysical setting, rather than direct observations. The presence of microearthquakes may shed light on hydrothermal pathways by revealing zones of thermal cracking where cold sea water extracts heat from hot crustal rocks, as well as regions where magmatic and tectonic stresses create fractures that increase porosity and permeability. Here we show that hypocentres beneath a well-studied hydrothermal vent field on the East Pacific Rise cluster in a vertical pipe-like zone near a small axial discontinuity, and in a band that lies directly above the axial magma chamber. The location of the shallow pipe-like cluster relative to the distribution and temperature of hydrothermal vents along this section of the ridge suggests that hydrothermal recharge may be concentrated there as a consequence of the permeability generated by tectonic fracturing. Furthermore, we interpret the band of seismicity above the magma chamber as a zone of hydrothermal cracking, which suggests that hydrothermal circulation may be strongly aligned along the ridge axis. We conclude that models that suggest that hydrothermal cells are oriented across-axis, with diffuse off-axis recharge zones, may not apply to the fast-spreading East Pacific Rise.

Acyclic hydrocarbons and ketones in cold-seep carbonates from central Hokkaido, northern Japan

Limestones associated with fossil chemosynthetic biological communities are found in Tappu Kanajirizawa and Teshionakagawa Abeshinaigawa, central Hokkaido, northern Japan. They were studied to ascertain their lipid distributions and δ13C values of lipid in order to investigate molecular records of archaea and bacteria associated with the anaerobic oxidation of methane (AOM). The limestones contain the tail-to-tail linked irregular isoprenoid hydrocarbons, 2,6,11,15-tetramethylhexadecane (crocetane), and its C25-homologue 2,6,10,15,19-pentamethylicosane (PMI). Furthermore, C30-homologue 2,6,10,15,19,23-hexamethyltetracosane (squalane) was detected. The abundance of these compounds indicates a pronounced role of particular archaea in the cold-seep. The δ13C values of PMI and crocetane were depleted, ranging from -129‰ to -116‰DB. New biomarkers found were C13 and C18 isoprenoid ketones, which could potentially be from anaerobic methanotrophic archaea. Small amounts of C14 and C19 isoprenoid ketones were also detected. The C13, C14 and C18 isoprenoid ketones also had extremely low δ13C values ranging from -115‰ to -104‰. δ13C values suggest that the origin of the isoprenoid ketones are lipids of anaerobic methanotrophioc archaea, but the diagenetic pathway leading to them is uncertain. This study shows that the limestones are cold seep carbonates and that isoprenoid ketones are potentially useful markers for anaerobic methanotrophic archaea.

Authigenic carbon entombed in methane-soaked sediments from the northeastern transform margin of the Guaymas Basin, Gulf of California

Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a ∼10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (−53.6±0.8‰ PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (−51.9±8.1‰ PDB). However, the δ13C value of the CO2 in the vent gas (+12.4±1.1‰ PDB) is very distinct from those for dissolved inorganic carbon (DIC) (−35.8‰ to −2.9‰ PDB) found elsewhere along the scarp, including underneath CBCs. The δ13C values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (−40.9‰ to +12.9‰ PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative δ13C values, while carbonate rocks that clearly formed in the subsurface have positive δ13C values (up to +23.0‰) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (∼+12‰); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate–methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO2 signature (+12.4‰) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending on how it migrated through the sediment column.

Nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the Nile deep-sea fan (eastern Mediterranean Sea)

Abstract During the NAUTINIL cruise (September–October 2003), mud volcanoes and pockmarks located in four selected areas of the Nile deep-sea fan (caldera, central, eastern, North Alex) were investigated at water depths ranging from 500 to 3019 m. Authigenic carbonate crusts were observed directly by a submersible in each of these fluid-venting areas, in close association with specific chemosynthetic biological communities. Authigenic carbonates occur typically as pavements, slabs and mounds on the seafloor, but are also present as millimeter- to centimeter-size concretions dispersed within sediments. Mineralogical analyses of carbonate crusts and concretions indicate that aragonite and high-Mg calcite represent the most dominant carbonate phases. Low-Mg calcite, dolomite and ankerite also occur as minor components. Petrographic observations of carbonate crusts and concretions show that they are composed mainly of microcrystalline carbonate cement, with minor amounts of detrital minerals, lithoclasts and bioclasts. Aragonite is present as microcrystalline cement or acicular crystals infilling bioclasts and voids. Pyrite occurs as framboids or cubic crystals, which are often associated with authigenic carbonates, thereby indicating that sulfate reduction was active during carbonate precipitation. Numerous millimeter- to centimeter-size euhedral gypsum crystals have been observed within carbonate crusts and concretions, and as isolated crystals in sediments recovered from the eastern province. In this area, precipitation of gypsum is related to the presence of rising sulfate-rich fluids, which originate from the dissolution of underlying Messinian evaporites. Millimeter-size barite concretions have also been discovered in sediment from the central province and precipitated from ascending fluids, which are enriched in barium due to the dissolution of biogenic and/or authigenic barite below the depth of sulfate depletion. The oxygen and carbon isotopic compositions of the carbonates display very large ranges, from −0.67‰ to 4.15‰ Vienna PeeDee Belemnite (V-PDB) and from −42.14‰ to 3.10‰ V-PDB, respectively. Most carbonates exhibit δ18O values around 3‰, indicating that they have precipitated in isotopic equilibrium with bottom seawater. In contrast, two carbonate concretions from the caldera and the eastern areas are characterized by lower δ18O values (−0.67‰ V-PDB and 0.98‰ V-PDB), which may reflect a contribution from 18O-poor (continental?) water or, most likely, a local high heat flow. A few carbonate crusts exhibit slightly positive δ13C values, which indicate that seawater was the main source of carbon for those carbonates. Authigenic carbonates are typically depleted in 13C, revealing that the major carbon source for those carbonates derives from anaerobic oxidation of methane driven by microbial consortia of archaea and sulfate reducing bacteria.

Surface and subsurface manifestations of gas movement through a N–S transect of the Gulf of Mexico

Large volumes of gas have vented through a north–south transect of the offshore northern Gulf of Mexico. An overview of surface and subsurface manifestations of this gas venting is presented. This gas movement has caused extensive alteration of reservoir oils to the north of the transect which are estimated to have equilibrated with, or been gas washed by, as much as 30 volumes of gas for every volume of oil. This gas washing entrains and carries upward the most volatile oil components depositing them in either shallower reservoirs or venting them to the overlying sediments and the water column. A significant amount of this gas bypasses the reservoirs and vents upward into the overlying sediments and waters. In spite of the significant amounts of the gas involved, the venting at the seafloor appears to occur primarily through highly localized faults and fractures. This gas discharge is spatially and temporally heterogeneous, making it difficult to estimate the actual hydrocarbon fluxes involved. This upward gas movement leaves characteristic signatures at the sediment water interface including carbonate pavements in older seep areas, and chemosynthetic biological communities, methane hydrates, and gas seeps in more recent long-term seep areas. In some cases where gas venting is very recent, massive disruption of surface and subsurface sediments is observed to be occasionally accompanied by mud volcanoes. Venting can be vigorous enough to produce methane gas bubbles, which appear to be injected rapidly into surface waters and which may constitute a significant source of methane, a greenhouse gas, to the atmosphere. In the northern Gulf of Mexico, gas venting is sometimes accompanied by natural oil slicks at the sea surface, which can be tracked for many miles in non-productive areas. These gas-venting signatures are not unique to the Gulf of Mexico; similar seep features are observed in sediments worldwide. The widespread occurrence of these seep features, which may or may not be related to subsurface oil and gas deposits, may explain why use of surface seeps has often proved to be so controversial in oil exploration. Indeed, most seeps are probably not linked with economic subsurface petroleum reservoirs. The relationships between surface seep features and productive subsurface reservoirs along a N–S transect of the northern Gulf of Mexico are presented as an example of how all surface and subsurface geochemical, geological, geophysical data might be used together to better constrain interpretations regarding the nature and dynamics of subsurface oil and gas deposits and their plumbing in frontier areas.

Distribution of chemosynthetic biological communities in Monterey Bay, California

We report the first quantitative evaluation of the distribution of seafloor chemosynthetic biological communities on a regional scale. The results are based on the analysis of video images and navigation from 792 benthic remotely operated vehicle dives conducted on the continental margin in Monterey Bay, California. These communities are common, occurring within 5% of the 25-m-square grid cells within which there have been bottom observations within 45 km of the bay9s head and within 9% of the visited cells that are below 550 m water depth. Although it has been previously assumed that these communities are associated with fluid seepage from faults, they are not more common within known fault zones. Surprisingly, the communities in Monterey Bay occur preferentially on steep slopes, which are commonly sites of recent erosion.

Bioerosion by chemosynthetic biological communities on Holocene submarine slide scars

Geomorphic, stratigraphic, and faunal observations of submarine slide scars that occur along the flanks of Monterey Canyon in 2.0–2.5 km water depths were made to identify the processes that continue to alter the surface of a submarine landslide scar after the initial slope failure. Deep-sea chemosynthetic biological communities and small caves are common on the sediment-free surfaces of the slide scars, especially along the headwall. The chemosynthetic organisms observed on slide scars in Monterey Canyon undergo a faunal succession based in part on their ability to maintain their access to the redox boundaries in the sediment on which they depend on as an energy source. By burrowing into the seafloor, these organisms are able to follow the retreating redox boundaries as geochemical re-equilibration occurs on the sole of the slide. As these organisms dig into the seafloor on the footwall, they often generate small caves and weaken the remaining seafloor. While chemosynthetic biological communities are typically used as indicators of fluid flow, these communities may be supported by methane and hydrogen sulfide that are diffusing out of the fresh seafloor exposed at the sole of the slide by the slope failure event. If so, these chemosynthetic biological communities may simply mark sites of recent seafloor exhumation, and are not reliable fluid seepage indicators.

Geological, geochemical, and microbiological heterogeneity of the seafloor around methane vents in the Eel River Basin, offshore California

Marine methane vents and cold seeps are common features along continental margins worldwide, serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities. The Eel River Basin (ERB), located on the continental slope off Northern California, contains active methane vents and seep-associated chemosynthetic biological communities (CBC) on the crests of anticlines in ∼520-m water depth. Seep-related features on the seafloor have a patchy distribution and include active bubbling vents, chemosynthetic clam beds, and sulfide-oxidizing bacterial mats. Methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane (AOM). To develop a comprehensive understanding of the complex biological, geochemical and physical processes associated with, and influencing seafloor methane seepage, a multidisciplinary approach is required. Here we present an integrative, multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the Eel River Basin and the complex interactions defining the geochemistry, mineralogy and microbiology within this environment.

Numerical modeling of carbonate crust formation at cold vent sites: significance for fluid and methane budgets and chemosynthetic biological communities

At many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane. Carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments. The range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear. Here, we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate. Starting with the conditions prevailing at a previously investigated reference site located on Hydrate Ridge, off Oregon, several parameters are systematically varied in a number of numerical experiments. These parameters include coefficients of bioturbation and bioirrigation, sedimentation rate, fluid flow velocity, methane concentration in the ascending vent fluids, and pH and saturation state at the sediment–water interface. The simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane (>50 mM) and if bioturbation coefficients are low (<0.05 cm 2 a −1). Moreover, high sedimentation rates (>50 cm ka −1) inhibit crust formation. Bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust. Crusts only form over a rather narrow range of upward fluid flow velocities (20–60 cm a −1), which is somewhat enlarged (up to 90 cm a −1) if the overlying bottom waters are supersaturated with respect to calcite. At higher flow rates, methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment. The formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years (100–500 a). Crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport. Therefore, rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation. Due to the moderate flow rates and the slow diffusive transport, only very little methane escapes into the bottom water overlying carbonate-encrusted vent areas.

地学における先端テクノロジー 深海調査技術 有人潜水調査船と無人探査機

The development of deep sea research systems-manned and unmanned submersibles-made it possible to carry out visual surveys of the deep seafloor at any depth on the earth. These surveys have revealed a great number of phenomena that occur on the seafloor, such as hydrothermal vents and chemosynthetic biological communities at mid-oceanic ridges.Unmanned submersibles include ROV (Remotely Operated Vehicle), UROV (Untethered Remotely Operated Vehicle), which have thrusters and are operated by mother ship through a tether cable or a thin optical fiber respectively, AUV (Autonomous Underwater Vehicle) which moves under pre-programmed commands and does not have a cable, and deep-tow system which is also operated by mother ship through. a cable but does not have a thruster.In order to see objects under very low illumination in deep seas, a Super-HARP (High-gain Avalanche Rushing Amorphous Photoconductor) video camera which is far more sensitive than CCD cameras, was developed. Recently, the Super HARP Hi-vision video camera, which provides high-resolution video images, has been developed.The location of the submersible in water is obtained by three acoustic positioning systems. These are called LBL (Long Base Line), SBL (Short Base Line), and SSBL (Super Short Base Line) positioning systems.In order to achieve effective dive surveys by manned submersibles or ROVs, it is necessary to select dive points by considering the results of pre-surveys carried out over a wide area, including bathymetric mapping using multi-narrow beam echo sounder, side-scan sonar survey, and deep-towed camera surveys.Submersibles enable not only visual observation, but also sampling and geophysical measurement at aimed points on the deep seafloor. In order to construct multi-disciplinary real-time and long-term deep seafloor observatories, such as off Hatsushima Island Observatory and VENUS off Okinawa Island Observatory, or to carry out borehole measurements, submersibles have been playing important roles in installing instruments, extending cables, and connecting connectors of instruments to underwater telemetry systems, or in recovering data. Submersibles are also necessary when searching for the causes of events observed by the observatory.The ROV and the Super HARP Hi-vision video camera made it possible for many scientists on board to see high-resolution video images of the seafloor simultaneously as if they were viewing it with the unaided eye. The AUV will enable surveys over wide areas in the near future. However, observations by submersibles are limited by survey time. On the other hand, long-term observatories enable observations only at fixed points. Complimentary observations using both long-term observatories and submersibles are expected in order to extend survey area in terms of both temporality and spatiality to provide an understanding of phenomena that occur on the seafloor.