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Two accretionary sediment sequences from Sites 1245 and 1252 recovered during Ocean Drilling Program (ODP) Leg 204 at southern Hydrate Ridge were investigated to explore the response of geochemical partitioning of iron and sulfur isotopic composition of authigenic pyrite to non-steady-state depositional and diagenetic scenarios. Five iron species were characterized by a modified sequential extraction procedure that covers almost all iron-bearing minerals in sediment cores, including: (1) iron-bearing carbonates, mainly siderite; (2) ferric (hydr)oxides, probably ferrihydrite and/or lepidocrocite; (3) magnetite; (4) iron-bearing silicates; and (5) pyrite. Highly reactive iron has been accumulated for a long-term steady-state history and its pyritization, to varying degrees, is limited by availability of dissolved sulfide. This causes pyrite and siderite occurred in the same sedimentary layer and shows an inverse relationship between their concentrations. From this, their proportions to highly reactive iron can be chosen for evaluating the degree of sulfidization. A significant change in sulfur isotopic composition of pyrite (-42.4 to +16.8‰ VCDT) indicates that the steady-state conditions are dramatically limited, where the d34S values higher than -20‰ may result from an upward shift of SMT zone close to the seafloor or a sudden, massive depositional event. To explain the downcore sulfidization effects and pyrite d34S values, we developed two categories of conceptual scenarios based on variations in sedimentation rate and methane flux. The geochemical features similar to those derived from each scenario were searched in the sediment columns and the non-steady-state events behind the scenarios were proved to be consistent with the real observations. Thus, iron species and pyrite d34S values can be regarded as a proxy to differentiate different non-steady-state depositional and diagenetic controls on the sedimentary record. [ABSTRACT FROM AUTHOR]

An improved understanding of the effects of gas hydrate presence on seismic attenuation is important for accurate hydrate characterization and quantification. Based on a rock‐physics model recently presented for gas hydrate‐bearing fine‐grained clay‐dominated sediments, here we establish an integrated workflow for surface seismic data from extracting seismic attenuation to estimating gas hydrate concentration (Ch) in the sediment. We apply this workflow to the high‐resolution seismic data acquired at southern Hydrate Ridge, offshore Oregon, to reveal the hydrate distribution and clarify the controlling factor of hydrate formation. We first present an adaptive‐bandwidth spectral ratio method to robustly measure attenuation. The attenuation measurements show that the presence of hydrate suppresses the attenuation of the host sediment. We then calculate Ch by applying the rock‐physics model to the attenuation measurements. The estimated Ch are mostly low (<5%) in Hydrate Ridge and agrees well with the in‐situ Ch measured from core‐ or well log‐based data. Our result also suggests that the lithology and stratigraphic structures together control the distribution of gas hydrate at Hydrate Ridge, where relatively high Ch is found in the region where a gas‐charged conduit exists and in an anticlinal structure overlying a strong bottom simulating reflection. Adjacent to the anticline, however, a low amount of gas hydrate appears present, possibly due to the gas migration blocked by the anticlinal structure or the lack of gas conduits. Our study offers an effective strategy for detecting and quantifying gas hydrate in fine‐grained clayey sediments through surface seismic data. Plain Language Summary: Seismic attenuation represents the decay of seismic energy during the travel of seismic waves. Seismic attenuation is a crucial tool in detecting and evaluating gas hydrates. Our knowledge of the relation between seismic attenuation and how hydrate forms in sediment has been improved in these years. Here we present a workflow for seismic data in which we measure seismic attenuation using a novel method and calculate the hydrate content in clay‐rich sediment based on a rock physics model. We apply this workflow to the seismic reflection data in Hydrate Ridge, Oregon. The attenuation measurements show that gas hydrate suppresses the attenuation of host sediments. The gas hydrate amount estimated in this study agrees well with (a) seismic characteristics on the seismic profile and (b) the in‐situ measurements of core‐ or well log‐based techniques. Our results show that the gas hydrate distribution at Hydrate Ridge is mainly controlled by lithology and stratigraphic structures. This good agreement suggests that our method is able to not only extract attenuation but also obtain reliable estimation of hydrate amount using seismic data. Key Points: We present a workflow from extracting attenuation to estimating gas hydrate concentration for seismic reflection dataGas hydrate presence in fine‐grained clayey sediments suppresses the seismic attenuation of host sediments at Hydrate RidgeGas hydrate distribution at Hydrate Ridge is controlled by both lithology and stratigraphic structures [ABSTRACT FROM AUTHOR]

Current estimations of seabed methane release into the ocean (0.4–48 Tg yr−1) are based on short‐term observations and implicitly assume that fluxes are constant over time. However, the intensity of gas seepage varies significantly throughout a seep lifetime. We used instruments operated by the Ocean Observatories Initiative's Regional Cabled Array to monitor variations of gas emissions over the entire Southern Hydrate Ridge summit. We show that bubble plumes emanate from distinct and persistent vents. Multiple plumes can occur within each vent and the location of their outlets may shift progressively. Active bubble plumes vary temporally in number and intensity, even within single vents. Gas emission fluctuations are partly periodic and linked to the local tide. However, short‐term variability and high ebullition events unrelated to tidal cycles are also commonly observed. Our data indicate that small‐scale processes beneath or at the sediment surface are responsible for the short‐term variability of the venting activity that is otherwise modulated by tides. Furthermore, a decrease of venting at one vent may coincide with an increase in plume activity at other vents. Our results depict a spatially and temporally dynamic seep environment, the variability of which cannot be fully characterized without systematic and comprehensive monitoring of the entire area. These results indicate that flux estimations may be largely overestimated or underestimated depending on the time, duration, and place of observation. Although sudden ebullition bursts are hardly predictable, we argue that tidal cycles must be taken into consideration when estimating gas fluxes. Plain Language Summary: Methane emission from the seabed into the ocean occurs naturally along continental margins. Methane release in the form of bubbles commonly escapes the seabed and rises through the water column forming bubble plumes. Since methane is a potent greenhouse gas, understanding which factors influence the methane release rate from submarine sources is important. This study focuses on one submarine source, Southern Hydrate Ridge, located in the Northeast Pacific 85 km offshore Oregon at a 780 m water depth. We used instruments installed at the seafloor and operated through an underwater cabled observatory to monitor bubble plumes and to study why their intensity varies over time. We confirmed that pressure variations caused by tides affect methane release rates and that bubble plumes are more intense during decreasing tides than rising tides. However, we found that not all fluctuations could be accounted for by tides and that distinct bubble plumes could have decoupled behaviors. The data suggest local and temporary permeability changes near the sediment‐water interface as the most likely cause of the short‐term gas emission variability. These findings are significant because they show that methane flux estimations from submarine sources may be largely inaccurate if based on short‐term or small‐scale measurements. Key Points: Fluctuations of methane emissions from the Southern Hydrate Ridge summit are modulated by the barotropic tidePermeability changes in shallow hydrate‐bearing sediments cause local pressure buildups that produce strong short‐term venting variabilityDistinct vents have different ebullition behaviors and monitoring a single vent would give an incomplete picture of the venting dynamics [ABSTRACT FROM AUTHOR]

Sulfuric acid–water nucleation is thought to be a key atmospheric mechanism for forming new condensation nuclei. In earlier literature, measurements of sulfuric acid activity were interpreted as the total (monomer plus hydrate) concentration above solution. Due to recent reinterpretations, most literature values for H2SO4 activity are thought to represent the number density of monomers. Based on this reinterpretation, the current work uses the most recent models of H2O–H2SO4 binary nucleation along with perturbation analyses to predict a decrease in critical cluster mole fraction, increase in critical cluster diameter, and orders of magnitude decrease in nucleation rate. Nucleation rate parameterizations available in the literature, however, give opposite trends. To resolve these discrepancies, nucleation rates were calculated for both interpretations of H2SO4 activity and directly compared to the available parameterizations as well as the perturbation analysis. Results were in excellent agreement with older parameterizations that assumed H2SO4 activity represents the total concentration and duplicated the predicted trends from the perturbation analysis, but differed by orders of magnitude from more recent parameterizations that assume H2SO4 activity represents only the monomer. Comparison with experimental measurements available in the literature revealed that the calculations of the current work assuming aa represents the total concentration are most frequently in agreement with observations. [ABSTRACT FROM AUTHOR]

Abstract We present new field data from three outcrops of Miocene methane-derived authigenic carbonates in the foredeep of the northern Apennines that contain chemosynthetic fauna and record a long history (∼1 Ma) of shallow fluid seepage linked to seafloor anaerobic oxidation of methane. The studied outcrops show similar features in terms of carbonate morphology, facies, spatial distribution and lateral and vertical contacts with the enclosing sediments. Methane-derived carbonates occur in two structural positions: 1) on the slope of the accretionary wedge in hemipelagites draping buried thrust-related anticlines, and 2) at the leading edge of the deformation front in the inner foredeep, within fault-related anticlines standing above the adjacent deep seafloor as intrabasinal ridges. We compare fossil seeps with two extensively investigated modern analogues: the Hikurangi Margin, offshore New Zealand and Hydrate Ridge, on the Cascadia margin, offshore the U.S.A. These analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane-derived carbonate bodies and chemosynthetic fauna on the present-day seafloor. The comparison allows us to propose a model for the evolution of fluid seeps on thrust-related ridges. At the deformation front, uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults, mostly blind, connected to the basal detachment, favoring the migration of fluids toward the incipient anticline. Fold development generates extensional stresses in the hinge zone of the anticline, promoting the development of normal faults; fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve. In the slope setting, far from the deformation front, thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor. After reaching a mature stage within the wedge, the structure is less active and buried in the slope environment of the evolved prism. Highlights • We provide summary of recurring features at seep outcrops in the northern Apennines. • Fossil seeps are compared with analogues from modern compressive settings. • The onset and evolution of seeps are linked to the development of anticlinal ridges. • We propose a new evolutionary model for seeps on accretionary ridges. [ABSTRACT FROM AUTHOR]

Authigenic carbonate rocks at methane seeps are recognized as hosting diverse and abundant invertebrate assemblages, with potential forcing from fluid seepage and hydrography. Mensurative studies of carbonate macrofauna (>0.3 mm) at Hydrate Ridge, OR revealed little effect of water depth and overlying oxygenation (at 600 m and 800 m) but a large influence of seepage activity on density, taxonomic composition, diversity, and biological traits (feeding, lifestyle, motility, size and calcification). Rocks exposed to active seepage had 3–4× higher total macrofaunal densities than under inactive conditions. Assemblages exhibited higher species richness and reduced evenness (greater dominance) under active seepage than inactive conditions, but no difference in H ′ or rarefaction diversity. Actively seeping sites were characterized by errant (motile), bacterial grazing, small- and medium-sized, heavily calcified species, whereas inactive sites exhibited a greater diversity of feeding modes and more burrowers, sessile, large and lightly calcified species. Active rocks supported more exogonid (Syllidae), ampharetid, and cirratulid polychaetes, provannid snails, pyropeltid limpets, nemerteans, and sponges; whereas inactive rocks supported higher densities of ophiuroids, isopods, gammarid amphipods, hydroids, Typosyllis (Syllidae) and tanaids. Transplant experiments, in which rocks were transferred between active and inactive sites at Hydrate Ridge North (600 m), revealed that assemblages respond within 13 months to increase or cessation of seepage, taking on the feeding, size and calcification characteristics of the background fauna at the new site. Lifestyles and motility patterns shifted more slowly as the sessile, attached species did not track seepage as quickly. Provannid snails and pyropeltid limpets rapidly colonized rocks transplanted to active sites and disappeared when transplanted to inactive sites. Given the known variability of fluid fluxes and rapid community response, a mosaic of communities changing in space and time is hypothesized to generate the relatively high species diversity at methane seeps. [ABSTRACT FROM AUTHOR]

After ~1 yr on the seafloor at water depths of ~700 m on Hydrate Ridge in the Pacific, eight colonization experiments composed primarily of a plastic mesh cube (from here on refered to as SEA 3 , for Seafloor Epibenthic Attachment Cubes) were colonized by 1076 Cibicidoides wuellerstorfi on ~1841 cm 2 of experimental substrate. This species is typically considered an indicator of well-oxygenated conditions, and recruitment of such large numbers in bottom waters with low dissolved oxygen availability (0.24–0.37 mL/L) indicate that this taxon may not be as limited by oxygen as previously thought. Clues about substrate preferences were evident from the distribution, or lack thereof, of individuals among plastic mesh, coated steel frame, wooden dowels and reflective tape. Abundance, individual size distributions within cage populations and isotopic biogeochemistry of living foraminifera colonizing experimental substrates were compared between active seep and adjacent off-seep experiment locations, revealing potential differences between these environments. Few studies have examined foraminiferal colonization of hard substrates in the deep-sea and to our knowledge no previous study has compared foraminiferal colonization of active seep and off-seep substrates from the same region. This study provides initial results of recruitment, colonization, geochemical and morphological aspects of the paleoceanographically significant species, C. wuellerstorfi , from dynamic deep-sea environments. Further experimental deployments of SEA 3 s will provide a means to assess relatively unknown ecologic dynamics of important foraminiferal deep-sea species. [ABSTRACT FROM AUTHOR]

Although chemosynthetic ecosystems are known to support diverse assemblages of microorganisms, the ecological and environmental factors that structure microbial eukaryotes (heterotrophic protists and fungi) are poorly characterized. In this study, we examined the geographic, geochemical and ecological factors that influence microbial eukaryotic composition and distribution patterns within Hydrate Ridge, a methane seep ecosystem off the coast of Oregon using a combination of high-throughput 18S rRNA tag sequencing, terminal restriction fragment length polymorphism fingerprinting, and cloning and sequencing of full-length 18S rRNA genes. Microbial eukaryotic composition and diversity varied as a function of substrate (carbonate versus sediment), activity (low activity versus active seep sites), sulfide concentration, and region ( North versus South Hydrate Ridge). Sulfide concentration was correlated with changes in microbial eukaryotic composition and richness. This work also revealed the influence of oxygen content in the overlying water column and water depth on microbial eukaryotic composition and diversity, and identified distinct patterns from those previously observed for bacteria, archaea and macrofauna in methane seep ecosystems. Characterizing the structure of microbial eukaryotic communities in response to environmental variability is a key step towards understanding if and how microbial eukaryotes influence seep ecosystem structure and function. [ABSTRACT FROM AUTHOR]

Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by sequestering inorganic carbon via AOM-induced carbonate precipitation. These precipitates commonly take the form of carbonate nodules that form within methane seep sediments. The timing and sequence of nodule formation within methane seep sediments are not well understood. Further, the microbial diversity associated with sediment-hosted nodules has not been well characterized and the degree to which nodules reflect the microbial assemblage in surrounding sediments is unknown. Here, we conducted a comparative study of microbial assemblages in methane-derived authigenic carbonate nodules and their host sediments using molecular, mineralogical, and geochemical methods. Analysis of 16S rRNA gene diversity from paired carbonate nodules and sediments revealed that both sample types contained methanotrophic archaea (ANME-1 and ANME-2) and syntrophic sulfate-reducing bacteria ( Desulfobacteraceae and Desulfobulbaceae), as well as other microbial community members. The combination of geochemical and molecular data from Eel River Basin and Hydrate Ridge suggested that some nodules formed in situ and captured the local sediment-hosted microbial community, while other nodules may have been translocated or may represent a record of conditions prior to the contemporary environment. Taken together, this comparative analysis offers clues to the formation regimes and mechanisms of sediment-hosted carbonate nodules. [ABSTRACT FROM AUTHOR]

Abstract: Authigenic carbonate precipitation occurs within marine sediments where sulfate-dependent anaerobic methane oxidation occurs. Geochemical and isotopic analyses of authigenic carbonates are commonly used as indicators of carbon sources and environmental conditions present during carbonate formation, but burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography allows for detailed characterization of carbonate phases and their subsequent alteration. Petrographic, isotopic, and geochemical characteristics of modern offshore authigenic carbonates from central and northern Cascadia are compared with Oligocene–Pliocene fossil seep carbonates uplifted on the Olympic Peninsula. Coupled analyses show the value and complexity of separating primary vs. secondary signals with relevance to understanding fluid-burial history in methane seep provinces on tectonically active convergent margins. The modern, offshore, near-seafloor diagenetic environment (S. Hydrate Ridge and Barkley Canyon) is dominated by acicular and microcrystalline aragonite and high-Mg calcite (HMC, >12mol % Mg). PPL and CL data illustrate that aragonite and HMC phases recrystallize to intermediate-Mg calcite (IMC, 5–12mol% Mg) during burial and diagenesis and eventually to low-Mg calcite (LMC, <5mol% Mg). This diagenetic progression is accompanied by a decrease in Mg/Ca and Sr/Ca ratios. Typically an increase in Ba/Ca is consistent with a high-barium content of the methane-bearing pore fluids that drive recrystallization. CL images also discern primary carbonates with high Mn/Ca ratios, including biogenic peloids, from secondary phases related to deep fluid migration through high permeability conduits. In the secondary phases, the Mn/Ca reflects Mn-enrichment that characterizes deep sourced fluids venting at Barkley Canyon. Mn-enrichment is accompanied by depletion in 18O attributable to elevated fluid temperatures during recrystallization. [Copyright &y& Elsevier]

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reduction as dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE (∑REE) concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the re-mineralization of particulate organic matter. The highest pore water ∑REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale-normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shale-normalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in ∑REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions. [ABSTRACT FROM AUTHOR]

Methane seepage at south Hydrate Ridge (offshore Oregon, United States), one of the best-studied examples of gas venting through gas hydrates, is the seafloor expression of a vigorous fluid flow system at depth. The seeps host chemosynthetic ecosystems and release significant amounts of carbon into the ocean. With new three-dimensional seismic data, we image strata and structures beneath the ridge in unprecedented detail to determine the geological processes controlling the style of focused fluid flow. Numerical fluid flow simulations reveal the influence of free gas within a stratigraphic unit known as Horizon A, beneath the base of gas hydrate stability (BGHS). Free gas within Horizon A increases the total mobility of the composite water-gas fluid, resulting in high fluid flux that accumulates at the intersection between Horizon A and the BGHS. This intersection controls the development of fluid overpressure at the BGHS, and together with a well-defined network of faults, reveals the link between the gas hydrate system at depth and methane seepage at the surface. [ABSTRACT FROM AUTHOR]

Abstract: While bubble plumes have been acoustically imaged in the water column above marine gas hydrate deposits in many studies, little is known about the temporal variation in plume intensity. In July 2008, we conducted surveys using 3.5 and 12kHz echosounders and a 75kHz acoustic Doppler current profiler (ADCP) over the northern and southern summits of Hydrate Ridge, on the Cascadia continental margin. Our study included multiple surveys at both sites, including a survey of the northern summit that was repeated 16 times in 19h. Seafloor depth at the northern summit is ∼600m, well within the hydrate stability zone (HSZ), which was below ∼510m during our survey based on CTD data. Three distinct flares (a term used to denote the acoustic signature of bubble plumes) were detected at Northern Hydrate Ridge (NHR) and one was detected at Southern Hydrate Ridge (SHR), coincident with where flares were observed a decade ago, indicating that the supply of gas is stable on this time scale. High-resolution bathymetric surveys of NHR and SHR acquired with an Autonomous Underwater Vehicle (AUV) flown ∼50m above the seafloor indicate that flare locations are correlated with a distinctive pattern of short-wavelength seafloor roughness, supporting the inference of long-term stability in the location of bubble expulsion sites. As in previous studies at Hydrate Ridge, flares were not detected with the 3.5kHz echosounder but were clearly imaged at 12kHz. By reprocessing routine shipboard ADCP data, we show that they are also observed in at 75kHz, indicating that a wide range of bubble sizes is present. The intensity of the flares varied strongly with time. Two primary sources for flares were observed. One, located on the regional topographic high, showed continuous activity, with two times periods of particularly strong flares that are not correlated with tidal height. The other, located on a local topographic high, shows a pulse of increased backscatter that occurred on a falling tide. While the time period of observation is not enough to constrain the effect of tidal changes in seafloor pressure on venting, the data suggest that tides are not the dominant factor controlling release of bubbles from the seafloor. The data support previously suggested models in which temporary sealing of vents by gas hydrate formation and breaking of these barriers as gas pressure builds up is responsible for “burp-like” pulses of gas expulsion. We also report the first observations of flares originating within the HSZ that extend well above the HSZ with little loss of backscatter intensity. These represent a new source of methane injection into the upper ocean and possibly the atmosphere. Flare extension above the HSZ may be due to coating of some bubbles by oil or biofilms or to inclusion of particulate matter (possibly including floating pieces of hydrate) in the plumes. Although this study provides tantalizing new information on both short-term variability in gas expulsion rate and long-term stability of vent sites, longer, well-calibrated observations that integrate bubble flux over entire vent fields as a function of time are needed to develop accurate models for the flux of methane into the ocean and atmosphere from seafloor methane vents. [Copyright &y& Elsevier]

Abstract: Hydrate Ridge (HR), located on the northeastern Pacific margin off Oregon, is characterized by the presence of outcropping hydrates and active methane seepage. Additionally, permanent low oxygen conditions overlay the benthic realm. This study evaluated the relative influence of both seepage and oxygen minima as sources of habitat heterogeneity and potential stress-inducing features on the bathyal metazoan benthos (primarily nematodes) at three different seep and non-seep HR locations, exposed to decreasing bottom-water oxygen concentrations with increasing water depth. The nematode seep communities at HR exhibited low diversity with dominance of only one or two genera (Daptonema and Metadesmolaimus), elevated average individual biomass and δ13C evidence for strong dependance on chemosynthesis-derived carbon, resembling deep-sea seeps worldwide. Although the HR seep habitats harbored a distinct nematode community like in other known seep communities, they differed from deep-sea seeps in well-oxygenated waters based on that they shared the dominant genera with the surrounding non-seep sediments overlain by oxygen-deficient bottom water. The homogenizing effect of the oxygen minimum zone on the seep nematode assemblages and surrounding sediments was constant with increasing water depth and concomitant greater oxygen-deficiency, resulting in a loss of habitat heterogeneity. [Copyright &y& Elsevier]

Methane hydrates, ice-like compounds that consist of water and methane, represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification; however, it remains unclear how much hydrate exists. Here, using a newly developed three-dimensional (3-D) thermal technique, we reveal a novel method for detecting and quantifying methane hydrate. The analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter-scale horizontal resolution. Our study, located at Hydrate Ridge, offshore Oregon (United States), suggests that heat flow and hydrate concentrations are coupled and that 3-D thermal analysis can be used to constrain hydrate and fluid flow in 3-D seismic data. Hydrate estimates using this technique are consistent with 1-D drilling results, but reveal large, previously unrecognized swaths of hydrate-rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies. The 3-D analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three. [ABSTRACT FROM AUTHOR]

Abstract: 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. 500–4000years; ) 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 (). Vents are inherently dynamic and flux varies greatly in magnitude and even flow direction over short time periods (hours-to-days), often tidally-driven (). 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 ) 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. [Copyright &y& Elsevier]

SUMMARY Gas hydrates are a potential energy resource and hazard for drilling and infrastructure, yet estimates of global volume vary by over three orders of magnitude. Hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations. A marine electromagnetic survey was carried out at Hydrate Ridge, Oregon, USA, with the aim of testing the use of controlled source electromagnetic (CSEM) and magnetotelluric (MT) methods to map gas hydrate and free gas below the gas hydrate stability zone. A 2-D CSEM inversion supports the scenario deduced from previous seismic and drilling results, which indicate two mechanisms of hydrate emplacement: a transport-dominated and reaction-dominated regime. A prominent resistive region of 2.5-4 Ωm at a depth of about 130 mbsf, near the seismic bottom simulating reflector (BSR), suggests that 27 to 46 per cent of the bulk volume is filled with hydrate, depending on whether Archie's Law or the Hashin-Strikman bounds are used. This is representative of a reaction-dominated regime for hydrate emplacement, and where a significant low velocity zone exists based on a seismic tomography inversion, suggests large quantities of free gas below the BSR. Electrical resistivity logging while drilling (LWD) data show general agreement with the CSEM inversion model except for a CSEM-derived resistive region at seismic horizon A, known to transport free gas into the gas hydrate stability zone. Inversion of MT data collected simultaneously during the CSEM survey provides a complimentary low-resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model. [ABSTRACT FROM AUTHOR]

An algorithm is presented for the inversion of marine controlled-source electromagnetic (CSEM) data that uses a 2D finite difference (FD) forward driver. This code is demonstrated by inverting a CSEM data set collected at Hydrate Ridge, Oregon, consisting of 25 seafloor sites recording a 5-Hz transmission frequency. The sites are located across a bathymetric high, with variations in water depth of ≈ 300 m along the 16-km profile. To model this complex seafloor bathymetry accurately, the FD grid was designed by careful benchmarking using a different 2D finite element (FE) forward code. A comparison of the FE and FD forward model solutions verities that no features in the inversion are due to inaccuracies of the FD grid. The inversion includes the local seawater conductivity-depth profile as recorded by the transmitter's conductivity-temperature-depth gauge, because seawater conductivity is known to have a significant effect on the CSEM responses. An apparent resistivity pseudosection of the CSEM data resembles the 2D inversion in general appearance. However, the inversion provides depth and geometric control of features that cannot be provided by the pseudosection and eliminates artifacts generated from the pseudosection projection. [ABSTRACT FROM AUTHOR]

Abstract: Marine sediments harbor an enormous quantity of microorganisms, including a multitude of novel species. The habitable zone of the marine sediment column begins at the sediment–water interface and probably extends to depths of several thousands of meters. Studies of the microbial diversity in this ecosystem have mostly relied on molecular biological techniques. We used a complementary method – analysis of intact polar membrane lipids – to characterize the in-situ microbial community in sediments covering a wide range of environmental conditions from Peru Margin, Equatorial Pacific, Hydrate Ridge, and Juan de Fuca Ridge. Bacterial and eukaryotic phospholipids were only detected in surface sediments from the Peru Margin. In contrast, deeply buried sediments, independent of their geographic location, were dominated by archaeal diether and tetraether lipids with various polar head groups and core lipids. We compared ring distributions of archaeal tetraether lipids derived from polar glycosidic precursors with those that are present as core lipids. The distributions of these related compound pools were distinct, suggestive of different archaeal sources, i.e., the polar compounds derive from sedimentary communities and the core lipids are fossil remnants from planktonic communities with possible admixtures of decayed sedimentary archaea. This in-situ production of distinct archaeal lipid populations potentially affects applications of the TEX86 paleotemperature proxy as demonstrated by offsets in reconstructed temperatures between both pools. We evaluated how varying cell and lipid stabilities will influence the sedimentary pool by using a box-model. The results are consistent with (i) a requirement of continuous inputs of freshly synthesized lipids in subsurface sediments for explaining the observed distribution of intact polar lipids, and (ii) decreasing lipid inputs with increasing burial depth. [Copyright &y& Elsevier]

Summary The anaerobic oxidation of methane (AOM) by methanotrophic archaea and sulfate-reducing bacteria is the major sink of methane formed in marine sediments. The study of AOM as well as of methanogenesis in different habitats is essentially connected with the in situ analysis of stable isotope (13C/12C, D/H) signatures ( δ-values). For their kinetic interpretation, experimental (cultivation-based) isotope fractionation factors ( α-values) are richly available in the case of methanogenesis, but are scarce in the case of AOM. Here we used batch enrichment cultures with high AOM activity and without background methanogenesis from detrital remnants to determine 13C/12C and D/H fractionation factors. The enrichment cultures which originated from three marine habitats (Hydrate Ridge, NE Pacific; Amon Mud Volcano, Mediterranean Sea; NW shelf, Black Sea) were dominated by archaeal phylotypes of anaerobic methanotrophs (ANME-2 clade). Isotope fractionation factors calculated from the isotope signatures as a function of the residual proportion of methane were 1.012-1.039 for 13CH4/12CH4 and 1.109-1.315 for CDH3/CH4. The present values from in vitro experiments were significantly higher than values previously estimated from isotope signature distributions in marine sediment porewater, in agreement with the overlap of other processes with AOM in the natural habitat. [ABSTRACT FROM AUTHOR]

Abstract: Free or hydrate-bound gas in the seafloor has been of scientific, ecologic and economic interest for many years because it predominantly contains high concentrations of low-molecular-weight hydrocarbons. A prerequisite of accurate quantifications of gases in sediments is to preserve pressure and temperature close to the in situ conditions during recovery. Here we introduce two new sediment coring devices that allow for the recovery of near-surface gas- and gas-hydrate-bearing sediments and subsequent investigations using several different techniques such as visualisation by computerized tomography, quantitative degassing, and sediment and porewater analyses. The first coring tool, the Multiple Autoclave Corer (MAC), resembles a standard multiple corer in terms of applications, size and core length of about 55cm. The second tool, the Dynamic Autoclave Piston Corer (DAPC), is similar to a piston corer in application and size and enables one to take cores of up to 2.5m length. Both focus on the investigation of near-surface sediments, which are most strongly affected by changes in bottom-water temperature and hydrostatic pressure, which in turn influence continental slope stability. Some results from recent offshore applications show the potential of these tools. [Copyright &y& Elsevier]

Methane-related carbonates from Hydrate Ridge typically show several macroscopically distinguishable mineral phases, namely whitish aragonite, lucent aragonite, and gray micrite. The relationship of these phases to particular microorganisms or biogeochemical processes is as yet unclear. We used a miniaturized biomarker technique on mg samples, combined with factor analysis and subsequent electron microprobe analysis, to study lipid biomarkers and chemical compositions of the individual phases. This allows us to identify particular mechanisms involved in the formation of the different carbonate precipitates. Our combined analysis of biomarkers and petrographic traits shows that most of the lipids related to the anaerobic oxidation of methane (>90% by weight) are concentrated within only a minor compartment (~20% by volume) of the Hydrate Ridge carbonates, the whitish aragonite. The patterns indicate that the whitish aragonite represents fossilized biofilms of methanotrophic consortia containing mainly archaea of the ANME-2 group and sulfate reducing bacteria, whereas the precipitation of the lucent aragonite may have lacked the immediate proximity of microorganisms during formation. By contrast, the gray micrite formed by incorporation of allochthonous organic and inorganic matter during carbonate precipitation induced by the anaerobic oxidation of methane involving ANME-1 archaea. [ABSTRACT FROM AUTHOR]

Abstract: Hydrate Ridge off the coast of Oregon, USA, is a prime example for gas hydrate occurrences in active margin settings. It is part of the Cascadia Margin and was the focus of Ocean Drilling Program (ODP) Leg 204, which successfully recovered fluids from nine sites from the southern part of the ridge. Iodide concentrations in pore fluids associated with gas hydrates are strongly enhanced, by factors up to 5000 compared to seawater, which allows the use of this biophilic element as tracer for organic source regions. We applied the cosmogenic isotope 129I (T 1/2 =15.7 Ma) system to determine the age of the organic source formation responsible for the iodide enrichment. In all sites at ODP Leg 204, 129I/I ratios were found to decrease with depth to values around 250×10−15, corresponding to minimum ages of 40 Ma, but in several sites, maxima in the 129I/I ratios point to the local addition of young iodide. The results indicate that a large amount of iodide was derived from deep accreted sediments of Eocene age, and that additional source regions provide iodide of Late Miocene age. The presence of old iodide in the pore waters suggests that fluid pathways are open to allow transport over large distances into the gas hydrate fields. The strong correlation between iodide and methane in hydrate fields coupled with the similarity in transport parameters in aqueous solutions suggests that a large fraction of methane in gas hydrates also has old sources and is transported into the present locations from source regions of Eocene age. [Copyright &y& Elsevier]

Methane-related carbonates from Hydrate Ridge typically show several macroscopically distinguishable phases, namely whitish aragonite, lucent aragonite, and gray micrite. The relationship of these phases to particular microorganisms or biogeochemical processes 5 is as yet unclear. We used a miniaturized biomarker technique on mg samples, combined with factor analysis and subsequent electron microprobe analysis, to study lipid biomarkers and chemical compositions of the individual phases. This allows us to identify particular mechanisms involved in the formation of the different carbonate precipitates. Our combined analysis of biomarkers and petrographic traits shows that most of the lipids related to the anaerobic oxidation of methane (>90% by weight) are concentrated within only a minor compartment (∼20% by volume) of the Hydrate Ridge carbonates (whitish aragonite). The patterns indicate that the whitish aragonite represents fossilized biofilms of methanotrophic consortia, whereas the precipitation of the lucent aragonite does not seem to be directly controlled by microorganisms. The gray micrite shows a partly Mg-calcitic mineralogy, higher pyrite contents, and a much higher proportion of allochthonous biomarkers. The formation of these precipitates is interpreted to reflect periodic methane-rich fluid pulses that disrupted the sediments and promoted the growth of the respective methanotrophic consortia along fluid pathways. [ABSTRACT FROM AUTHOR]

Drilling on Hydrate Ridge, offshore Oregon, during ODP Leg 204 enabled us to investigate fabrics of gas hydrate samples in a wide depth range of the gas hydrate stability zone (GHSZ). X-ray computerized tomographic imaging on whole-round samples, frozen in liquid nitrogen, revealed that layered gas hydrate structures are related to variable processes occurring at different sediment depths. Shallow gas hydrates often form layers parallel or sub-parallel to bedding and also crosscut sedimentary strata and other gas hydrate layers, destroying the original depositional fabric. The dynamic processes interacting with this complicated plumbing system in this shallow environment are responsible for such highly variable gas hydrate fabrics. Gas hydrate layers deeper in the sediments are most often dipping with various angles, and are interpreted as gas hydrate precipitates filling tectonic fractures. These originally open fractures are potential candidates for free gas transportation, and might explain why free gas can rapidly emanate from below the bottom-simulating reflector through the GHSZ to the seafloor. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge in the accretionary complex of the Cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment. We present a detailed local analysis of four component (4C) ocean bottom seismometer (OBS) data and show the importance of multi-frequency and shear wave data for determining hydrate reservoir properties. A detailed model of the elastic parameters at the bottom simulating reflector (BSR) is developed by using synthetic seismogram modelling. The main focus in this study is an amplitude-versus-offset (AVO) analysis of shear waves, which originate by PS-conversion at the BSR in 73 m below the seafloor (bsf). The AVO analysis enables the determination of the shear wave velocity above the BSR. A velocity of 400 m/s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the BSR. Information about the attenuation and the shape of the BSR transition zone is obtained from the frequency-dependent reflection amplitudes of the BSR. The BSR is shown to be a gradual type transition zone of 1.5–2.5 m thickness. Average Q factors of Q p =150 for P-waves and of Q s =35 for S-waves are determined within the gas hydrate stability zone (GHSZ). The low Q s factor points to a pronounced attenuation of S-waves in the uppermost sediments. From rock physics modelling, the hydrate concentration is estimated to vary locally between 3–12% of the pore space. Below the BSR, free gas concentrations of 0.5% and 8% are determined for homogeneous and patchy distributed gas, respectively. [Copyright &y& Elsevier]

Abstract: As part of Ocean Drilling Program Leg 204 at southern Hydrate Ridge off Oregon we have monitored changes in sediment electrical resistivity during controlled gas hydrate dissociation experiments. Two cores were used, each filled with gas hydrate bearing sediments (predominantly mud/silty mud). One core was from Site 1249 (1249F-9H3), 42.1 m below seafloor (mbsf) and the other from Site 1248 (1248C-4X1), 28.8 mbsf. At Site 1247, a third experiment was conducted on a core without gas hydrate (1247B-2H1, 3.6 mbsf). First, the cores were imaged using an infra-red (IR) camera upon recovery to map the gas hydrate occurrence through dissociation cooling. Over a period of several hours, successive runs on the multi-sensor track (includes sensors for P-wave velocity, resistivity, magnetic susceptibility and gamma-ray density) were carried out complemented by X-ray imaging on core 1249F-9H3. After complete equilibration to room temperature (17–18 °C) and complete gas hydrate dissociation, the final measurement of electrical resistivity was used to calculate pore-water resistivity and salinities. The calculated pore-water freshening after dissociation is equivalent to a gas hydrate concentration in situ of 35–70% along core 1249F-9H3 and 20–35% for core 1248C-4X1 assuming seawater salinity of in situ pore fluid. Detailed analysis of the IR scan, X-ray images and split-core photographs showed the hydrate mainly occurred disseminated throughout the core. Additionally, in core 1249F-9H3, a single hydrate filled vein, approximately 10 cm long and dipping at about 65°, was identified. Analyses of the logging-while-drilling (LWD) resistivity data revealed a structural dip of 40–80° in the interval between 40 and 44 mbsf. We further analyzed all resistivity data measured on the recovered core during Leg 204. Generally poor data quality due to gas cracks allowed analyses to be carried out only at selected intervals at Sites 1244, 1245, 1246, 1247, 1248, 1249, and 1252. With a few exceptions, data from these intervals yield low to no gas hydrate concentration, which corresponds to estimates from downhole resistivity logs. However, since the gas cracking may be the result of gas hydrate dissociation, this is a biased sampling. Cores that had contained some gas hydrate may have been excluded. [Copyright &y& Elsevier]

Abstract: We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions. [Copyright &y& Elsevier]

Abstract: ODP Leg 204, which drilled at Hydrate Ridge, provides unique insights into the fluid regime of an accretionary complex and delineates specific sub-seafloor pathways for fluid transport. Compaction and dewatering due to smectite–illite transition increase with distance from the toe of the accretionary prism and bring up fluids from deep within the accretionary complex to sampled depths (≤600 mbsf). These fluids have a distinctly non-radiogenic strontium isotope signature indicating reaction with the oceanic basement. Boron isotopes are also consistent with a deep fluid source that has been modified by desorption of heavy boron as clay minerals change from smectite to illite. One of three major horizons serves as conduit for the transport of mainly fluid. Our results enable us to evaluate fluid migration pathways that play important roles on massive gas hydrate accumulations and seepage of methane-rich fluids on southern Hydrate Ridge. [Copyright &y& Elsevier]

Abstract: Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as −48.1 ‰ PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The δ18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 ‰ PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 ‰ PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place. [Copyright &y& Elsevier]