Southern Hydrate Ridge Research Articles

Iron species and sulfur isotopic compositions of authigenic pyrite in deep-sea sediments at southern Hydrate Ridge, Cascadia margin (ODP Leg 204): implications for non-steady-state depositional and diagenetic processes

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 δ34S 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 δ34S 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 δ34S values can be regarded as a proxy to differentiate different non-steady-state depositional and diagenetic controls on the sedimentary record.

Estimation of Seismic Attenuation and Gas Hydrate Concentration From Surface Seismic Data at Hydrate Ridge, Cascadia Margin

AbstractAn 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.

Variability of Natural Methane Bubble Release at Southern Hydrate Ridge

AbstractCurrent 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.

Simulating gas hydrate formation in the southern hydrate ridge, Cascadia Margin

The Cascadia Margin in the Pacific Ocean is a widely investigated region for oceanic gas hydrates. In this study, numerical simulation models are developed to simulate the flow of methane gas up fault zones in the Southern Hydrate Ridge of the Cascadia Margin. Three different hydrate formation scenarios, common to hydrate formation mechanisms in similar settings, are simulated. The tested scenarios are based on both field observations and previous numerical simulation assessments in similar subsea hydrate reserves. Results predict that gas hydrates in this region can be formed from gas migration up faults from deeper sources. The hydrate distribution follows the slope of the southern hydrate ridge but can be disjointed at places due to the presence of low salinity fluids, which results in higher hydrate saturations. The dominant mechanisms of gas migration into the region are advection and buoyancy driven. Capillary pressure forces, density differences, and hydraulic pressure differentials define the migration pathways along with high permeability pathways through faults. The timing of gas hydrate formation ranges between 2000 years and 1650 years for the modeling scenarios tested with the likelihood that the formation time is closer to 1650 years due to the likely presence of multiple faults acting as migration pathways for the gas. For the modeling scenarios tested, the gas hydrate mass generated ranges from 6.46 × 105 kg to 6.837 × 105 kg for the unit vertical cross-section used in the simulations. Predicted results of hydrate saturation are calibrated to field measured well log data for validation and verification.

The Ocean Observatories Initiative

The Ocean Observatories Initiative (OOI) is an integrated network that enables scientific investigation of interlinked physical, chemical, biological and geological processes throughout the global ocean. With near real-time data delivery via a common Cyberinfrastructure, the OOI instruments two contrasting ocean systems at three scales. The Regional Cabled Array instruments a tectonic plate and overlying ocean in the northeast Pacific, providing a permanent electro-optical cable connecting multiple seafloor nodes that provide high power and bandwidth to seafloor sensors and moorings with instrumented wire crawlers, all with speed-of-light interactive capabilities. Coastal arrays include the Pioneer Array, a relocatable system currently quantifying the New England shelf-break front, and the Endurance Array, a fixed system off Washington and Oregon with connections to the Regional Cabled Array. The Global Arrays host deep-ocean moorings and gliders to provide interdisciplinary measurements of the water column, mesoscale variability, and air-sea fluxes at critical high latitude locations. The OOI has unique aspects relevant to the international ocean observing community. The OOI uses common sensor types, verification protocols, and data formats across multiple platform types in diverse oceanographic regimes. OOI observing is sustained, with initial deployment in 2013 and 25 years of operation planned. The OOI is distributed among sites selected for scientific relevance based on community input and linked by important oceanographic processes. Scientific highlights include real-time observations of a submarine volcanic eruption, time-series observations of methane bubble plumes from Southern Hydrate Ridge off Oregon, observations of anomalous low-salinity pulses off Oregon, discovery of new mechanisms for intrusions of the Gulf Stream onto the shelf in the Middle Atlantic Bight, documentation of deep winter convection in the Irminger Sea, and observations of extreme surface forcing at the most southerly surface mooring in the world ocean.

Time‐series measurements of bubble plume variability and water column methane distribution above Southern Hydrate Ridge, Oregon

AbstractAn estimated 500–2500 gigatons of methane carbon is sequestered in gas hydrate at continental margins and some of these deposits are associated with overlying methane seeps. To constrain the impact that seeps have on methane concentrations in overlying ocean waters and to characterize the bubble plumes that transport methane vertically into the ocean, water samples and time‐series acoustic images were collected above Southern Hydrate Ridge (SHR), a well‐studied hydrate‐bearing seep site ∼90 km west of Newport, Oregon. These data were coregistered with robotic vehicle observations to determine the origin of the seeps, the plume rise heights above the seafloor, and the temporal variability in bubble emissions. Results show that the locations of seep activity and bubble release remained unchanged over the 3 year time‐series investigation, however, the magnitude of gas release was highly variable on hourly time scales. Bubble plumes were detected to depths of 320–620 m below sea level (mbsl), in several cases exceeding the upper limit of hydrate stability by ∼190 m. For the first time, sustained gas release was imaged at the Pinnacle site and in‐between the Pinnacle and the Summit area of venting, indicating that the subseafloor transport of fluid and gas is not restricted to the Summit at SHR, requiring a revision of fluid‐flow models. Dissolved methane concentrations above background levels from 100 to 300 mbsl are consistent with long‐term seep gas transport into the upper water column, which may lead to the build‐up of seep‐derived carbon in regional subsurface waters and to increases in associated biological activity.

Temporal variation of methane flares in the ocean above Hydrate Ridge, Oregon

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.

Transient hydraulic fracturing and gas release in methane hydrate settings: A case study from southern Hydrate Ridge

Episodic seafloor methane venting is associated with focused fluid flow through fracture systems at many sites worldwide. We investigate the relationship between hydraulic fracturing and transient gas pressures at southern Hydrate Ridge, offshore Oregon, USA. Two colocated seismic surveys, acquired 8 years apart, at Hydrate Ridge show seismic amplitude variations interpreted as migration of free gas in a permeable conduit, Horizon A, feeding an active methane hydrate province. The geophysical surveys also reveal transients in gas venting to the water column. We propose that episodic gas migration and pressure fluctuations in the reservoir underlying the regional hydrate stability zone (RHSZ) at southern Hydrate Ridge influence methane supply to the RHSZ and are linked with periodic fracturing and seafloor methane venting. We model the effect of pore pressure variations within the deep methane source on fracturing behavior with a 1D model that couples multiphase flow, hydrate accumulation, and pore pressure buildup. As the reservoir pressure increases, fractures open when the pore pressure exceeds the hydrostatic vertical effective stress. Gas then flows through the fractures and vents at the seafloor while hydrate precipitates in the fracture system. We show that active seafloor gas venting occurs for approximately 30 years, and that the available methane reservoir is exhausted 30 to 55 years after the onset of pressure buildup. This provides important constraints on the time scale of transient fluid flow at southern Hydrate Ridge, and illustrates how pore pressure pulses affect fluid flow and fracturing behavior in active methane hydrate provinces.

Gas hydrate concentration and characteristics within Hydrate Ridge inferred from multicomponent seismic reflection data

A seismic experiment composed of streamer and ocean bottom seismometer (OBS) surveys was conducted in the summer of 2002 at southern Hydrate Ridge, offshore Oregon, to map the gas hydrate distribution within the hydrate stability zone. Gas hydrate concentrations within the reservoir can be estimated with P wave velocity (Vp); however, we can further constrain gas hydrate concentrations using S wave velocity (Vs), and use Vs through its relationship to Vp (Vp/Vs) to reveal additional details such as gas hydrate form within the matrix (i.e., hydrate cements the grains, becomes part of the matrix frame or floats in pore space). Both Vp and Vs can be derived simultaneously by inverting multicomponent seismic data. In this study, we use OBS data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments. Once Vp and Vs are estimated, they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration. We model Vp using an equation based on a modification of Wood's equation that incorporates an appropriate rock physics model and Vs using an empirical relation. The gas hydrate concentration is estimated to be up to 7% of the rock volume, or 12% of the pore space. However, Vp and Vs do not always fit the model simultaneously. Vp can vary substantially more than Vs. Thus we conclude that a model, in which higher concentrations of hydrate do not affect shear stiffness, is more appropriate. Results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area.

Diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems

Mineral magnetic results and electron microscope observations from gas hydrate-bearing marine sediments cored at southern Hydrate Ridge during Ocean Drilling Program Leg 204 (Sites 1244 to 1252, Cascadia Margin, offshore Oregon) demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially-mediated diagenetic reactions in the sulphate, the anaerobic oxidation of methane (AOM), and the methanic/gas hydrate zones. Geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide, whether it derives from microbially-driven sulphate reduction in the sulphate zone, in the AOM zone or in deep sediments undergoing AOM, so that pyritization reactions are not driven to completion. Our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record, because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred. Formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times, which is likely to compromise paleomagnetic results from greigite- and pyrrhotite-bearing marine sediments.

Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties

Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.

Interpreting in situ pressure and hydraulic properties with borehole penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern Hydrate Ridge, offshore Oregon

Two borehole penetrometers, Fugro‐McClelland's Piezoprobe and the Ocean Drilling Program's (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg 204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon. Pore pressure is interpreted to be hydrostatic and the sediment's coefficient of consolidation is interpreted to lie between 6.92 and 7.8 × 10−7 m2/s, which is in approximate agreement with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation 14 times sooner than the DVTPP. The observed and modeled pore pressure responses illustrate how penetrometer geometry impacts our ability to interpret in situ properties and demonstrate under what conditions these tools can be effectively used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than the DVTPP does. This generates a narrower zone of pressure increase around the piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds, pressure dissipation of the Piezoprobe is retarded and forms a “bench,” or flat spot, on the dissipation curve. Owing to its distinct two‐radius geometry, it is possible to apply a consistent method to estimate in situ pressure from partial dissipation record based on the position of the “bench.”

Fracture networks and hydrate distribution at Hydrate Ridge, Oregon

Analysis of resistivity-at-the-bit (RAB) images acquired during Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, Oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled. Fractures on the ridge crest at Sites 1247, 1248, and 1250, are typically high-angle (> 60°) with isotropically distributed strikes, while those on the ridge flanks at Sites 1244–1246 have lower dips (20–60°) and strike parallel to the trend of the ridge. The eastern slope basin, Site 1251, lacks significant fracturing. These differences indicate that the minimum principal stress ( σ 3) direction rotates to horizontal at the ridge crest as a result of topographically-driven extension and gravitational collapse of the ridge. Chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration, which is confined to high permeability sediments and fracture zones by capillary pressure constraints. In contrast, on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping σ 3 and the predominance of aqueous, rather than gaseous, flow.

Estimates of in situ gas hydrate concentration from resistivity monitoring of gas hydrate bearing sediments during temperature equilibration

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.

Passing gas through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon

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.

Fluid sources, fluid pathways and diagenetic reactions across an accretionary prism revealed by Sr and B geochemistry

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.

Upward shifts in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon

High‐resolution three‐dimensional (3‐D) seismic reflection data acquired on the R/V Thomas G. Thompson in 2000 reveal a pair of bottom simulating reflections (BSRs) across a broad region of southern Hydrate Ridge, offshore Oregon. The primary BSR (BSRp) is a regionally extensive reflection that lies 120–150 m below seafloor and exhibits typical characteristics of a gas hydrate BSR. We also imaged a second weaker BSR (BSRs), 20–40 m below BSRp, with similar characteristics. BSRs is interpreted as a remnant of a BSR that probably formed during the Last Glacial Maximum 18,000 years ago, when the base of the gas hydrate stability zone (GHSZ) was deeper. An increase in bottom water temperatures of 1.75°–2.25° and a corresponding sea level rise of 120 m could have produced the BSR shift. The preservation of BSRs for at least 5000 years, which is the time since subseafloor temperatures stabilized following ocean warming after the Last Glacial Maximum, implies very slow upward advective and diffusive flow of methane (<1 m/1000 years in the vicinity of BSRs). BSRs appears where there are no resolvable steeply dipping faults and fractures, consistent with very low advective flow rates, and has dispersed where vertical fractures are visible. Free gas released by the shift in the BSR either migrates so slowly that it remains stable beneath the GHSZ or is directed upward along fractures to reform as hydrate in the GHSZ. There is no evidence for release of this free gas into the ocean or atmosphere.

Painting a picture of gas hydrate distribution with thermal images

A study of cold thermal anomalies identified in continuous down‐core records of thermal structure acquired by an infrared (IR) camera during Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, is combined with core‐based observations of structure and lithology, to present new insights into the influence of lithology on hydrate distribution in an accretionary margin. Hydrate distribution is spatially associated with <0.5 cm–thick, laterally discontinuous sand horizons that compose <1% of the sedimentary section at all sites. Proximity to the fractured ridge crest determines the extent to which individual sand horizons host hydrate, with 70% of the horizons hosting hydrate at the crest and only 29% in the basin. This trend is consistent with focused up‐dip migration of fluids along permeable lithologic conduits, and shows that lateral flow may be as important as vertical diffusion and fracture flow in determining gas hydrate distribution at Hydrate Ridge.

Living benthic foraminifera in sediments influenced by gas hydrates at the Cascadia convergent margin, NE Pacific

Living (Rose Bengal stained) foraminifera in gas-hydrate-influenced sediments at the Cascadia convergent margin were investigated. Foraminiferal assemblages from the southern Hydrate Ridge and neighboring basins were compared in terms of abundances, vertical distribution, diversity, and species composition. At Hydrate Ridge, the presence of shallow gas hydrates and increased porewater sulfide concentrations was indicated by extensive bacterial mats of Beggiatoa sp. and clam beds of the bivalve mollusk Calyptogena sp., generating different biological zones. Living foraminifera were found in all biological zones, in sediment layers down to 5 cm. They showed highly variable densities within all zones. The average abundance of benthic foraminifera at Hydrate Ridge differs from neighboring basins. Average species diversities are comparable between biological zones, while the average number of species increases from bacterial mats to clam fields and surrounding sediments. Foraminifera can be characterized by 5 principal component communities which explain 97.3% of the variance of the live assemblages at the southern Hydrate Ridge and neighboring basins. At Hydrate Ridge, 2 foraminiferal zones can be distinguished: (1) an Uvigerina peregrina community which characterizes sediments covered with bacterial mats and clam fields; (2) a ?Spiroplectammina biformis community in the surrounding non-seep sediments. Foraminiferal assemblages in the neighboring Western and Eastern Basin differ from the Hydrate Ridge stations.

Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments

Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of 18O into the hydrate structure and hence the enrichment of 16O in the surrounding brine. We measured δ18O values as high as 2.27‰ relative to Peedee belemnite that correspond to a fluid composition of −1.18‰ relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the 44Ca in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems.