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[1] We simulate methane hydrate formation with multiphase flow and free gas within the regional hydrate stability zone (RHSZ). We find that hydrate distribution and fracture behavior are largely determined by the phase of the methane supply. We allow free gas to enter the RHSZ when porewater salinity increases to the value required for three-phase equilibrium. Fractures nucleate when the excess pore pressure exceeds the vertical hydrostatic effective stress. At Hydrate Ridge, where methane supply is dominantly free gas, hydrate saturation increases upwards and fractures nucleate high within the RHSZ, eventually allowing gas to vent to the seafloor. At Blake Ridge, where methane supply is dominantly in the dissolved phase, hydrate saturation is greatest at the base of the RHSZ; fractures nucleate here and in some cases could propagate through the RHSZ, allowing methane-charged water to vent to the seafloor.

INTRODUCTION Methane is an important link within the global carbon cycle and has become a major focus for scientific investigations over the last decades, especially since the discovery of large deposits of methane hydrates in continental margins. The majority of recent methane production is biogenic, i.e. produced either by thermogenic transformation of organic material or by methanogenesis as the final step in fermentation of organic matter carried out by methanogenic archaea in anoxic habitats (Reeburgh, 1996). There are also abiotic sources of methane, e.g. at mid-oceanic ridges, where serpentinization takes place. In marine environments, the bulk of the methane is produced in shelf and upper continental-margin sediments, which receive large amounts of organic matter from deposition (Reeburgh, 1996). As methane builds up, it migrates upwards and may reach the sediment surface. Here, its ebullition and oxidation can lead to the formation of complex geostructures, such as pockmarks or carbonate chimneys and platforms, as well as large-scale topographies, such as mud volcanoes and carbonate mounds (Ivanov et al ., 1991; Milkov, 2000). In most of the deeper continental margin and the abyssal plain sediments, methane production is low, as only 1-5 % of the surface primary production reaches the bathyal and abyssal seabed, due to degradation processes in the water column (Gage & Tyler, 1996).

The discovery of huge deep sea methane reservoirs in the form of ice-like solid crystals of methane clathrate hydrates (methane hydrates or gas hydrate for short) and carbonate deposits at the Hydrate Ridge in the Cascadia subduction zone (Suess et al. 1985) has stimulated global exploration and extensive studies of cold methane seeps and vents associated with the hydrate deposits (Paull and Dillon 2001; Trehu et al. 2004). Many deep sea methane hydrate deposits have been discovered around the world, for instance, in the Gulf of Mexico (Pohlman et al. 2008; Sassen et al. 1998, 2004), Monterey Bay of California (Gieskes et al. 2005; Lorenson et al. 2002; Stakes et al. 1999), Black Sea (Peckmann et al. 2001), Sea of Okhotsk, Eastern Siberia (Greinert and Derkachev 2004), the Gulf of Cadiz (Stadnitskaia et al. 2008), the Kuroshima Knoll of southern part of the Ryukyu Arc (Takeuchi et al. 2007), and South China Sea (Han et al. 2008; Lu 2007). It has been estimated that there are about 1,000 ∼24,000 Gt of carbon in global methane hydrate zones (Dickens et al. 1997; Harvey and Huang 1995; Kvenvolden 1988; MacDonald 1990; Makogon and Makogon 1997). Methane hydrates have the potential to be a future energy source however methane is also a greenhouse gas. The release of methane from the methane hydrate can have a profound effect on the global climate (Kvenvolden 1998). In geological record, the rapid global temperature change at the Paleocene-Eocene boundary at ∼55 millions of years ago, the Paleocene–Eocene Thermal Maximum (PETM), may be related to catastrophic methane release from sea sediments (Dickens et al. 1995; Katz et al. 2001; Zachos et al. 2005). At the interface between uprising methane from dissociation of methane hydrate and sulfate from sea water, a distinct microbial consortium mediates anaerobic oxidation of methane (AOM) through a net reaction of $$ {\text{CH}}_{4}+{\text{SO}}_{4}{}^{2-}\to {\text{HS}}^{-}+{\text{HCO}}_{3}{}^{-}+{\text{H}}_{2}\text{O}.$$ (15.1)

We observed anisotropy in the acoustic reflection amplitudes from the seafloor at Hydrate Ridge on the Oregon Margin. We analyzed acoustic data from several OBS receivers and several sound-source lines with one line being perpendicular to the others. We used time windows to isolate the acoustic data that contained the most information regarding the seafloor reflectivity. The bottom loss is directly related to these observed reflection strengths with some restrictions. The observed higher reflection strengths for the east-west line (perpendicular to bathymetrie contours) than to reflection strengths on the north-south lines is unexpected and so far unexplained. We expect that the primary causes of variations in the reflectivity are due to scattering and from heterogeneity.

[eng] Gas hydrates have been recently recognized as a key factor affecting a number of global processes such as the climatic change, sea floor stability, etc. In this thesis we present the multidisciplinary study of gas hydrate rich sediments recovered during ODP Leg 204. The main objective of this thesis is to study how the textural characteristics of marine sediments can affect the main pathways and intensity of fluid flow and how fluid flow determines the distribution of gas hydrates in the continental margins, as well as the main geochemical processes that occur during early diagenesis.To reach these objectives, a complete sedimentary and geochemical study of 581 sediment samples from southern Hydrate Ridge was carried out. The methods and techniques that were applied include: complete textural analyses, mineralogy, physical properties and geochemistry.The southern Hydrate Ridge sediments are mainly made up of four lithofacies defined as: hemipelagites, turbidites, ash layers and debrites. Mass‐transport deposits such as turbidites and debrites are more abundant in Lithostratigraphic Unit III and II, as well as in Lithostratigraphic Unit IA in the slope basin of southern Hydrate Ridge. Some increasing trends with depth can be observed in the smectite content in the clay mineral assemblages. These features suggest that the transport in suspension of fine sediments through the California Current was more effective during the Pliocene and early Pleistocene period. Bedload transport of coarse material from local and distal areas was more effective during the middle Pleistocene and Holocene due to the tectonic reactivation of the southern Hydrate Ridge uplift. During the Pleistocene and owing to the pervasive fluctuation of sealevel, gas hydrate dissociation together with the seismic movements in the Oregon margin seems a plausible triggering mechanism for mass‐movements.The results presented here confirm that the sedimentation patterns in the Hydrate Ridge region are controlled by climate and tectonic parameters such as the regional intensity of the California Current or the local tectonic movements that lead to the uplift of the Ridge. These parameters mainly control the clay mineral distribution as well as the sedimentary facies that were produced.The sedimentary fabric of gas hydrate‐rich intervals is disturbed during core recovery due to gas hydrate dissociation. The two main disturbance fabrics generated through this process are mousselike and soupy. The gas hydrate‐rich sediments analyzed for this thesis are coarser grained in respect to the hemipelagite sediments. The coarse‐grained layers such as turbidites and ash layers could act as conduits for fluids in the southern Hydrate Ridge region because of their higher porosity and permeability. In this context, methane‐rich fluids migrate through these layers from deep in the sedimentary sequence and into the gas hydrate stability zone. A number of barium fronts have been identified in southern Hydrate Ridge sediments and interstitial waters. Barite fronts were formed as a result of the barite recycling process during early diagenesis, which is controlled by the availability of methane‐rich fluids, in situ decomposition of organic matter and the sulphate gradient. Modelling of these data shows that these processes were active at southern Hydrate Ridge for a period of up to one thousand years.A number of geochemical and sedimentological processes are proposed in this thesis as plausible mechanisms to allow the survival of the barite fronts during diagenesis. The sedimentary texture plays an important role in controlling the major fluid flow pathways in the continental margins. The temporal evolution of the fluid flow can be studied in a given area through the distribution of the mineral phases that form during early diagenesis, as well as the interstitial water composition., [cat] Aquesta tesi integra els resultats de l'anàlisi sedimentológica i geoquímica de sediment marins rics en hidrats de gas, recuperats durant la campanya "Ocean Drilling Program" Leg 204 en el marge d'Oregon (USA). L'objectiu principal d'aquest estudi és conèixer les característiques sedimentàries que afecten el fluxe de fluids i gasos a través del sediment i com els fluids afecten la distribució d'hidrats de gas en aquesta àrea, així com alguns processos geoquímics que operen durant la diagènesi inicial. Els mètodes i tècniques aplicats inclouen l'anàlisi de sedimentològia, mineralogia, susceptibilitat magnètica i geoquímica.Els sediments de southern Hydrate Ridge estan formats per 4 litofàcies: hemipelagita, turbidita, cendra volcànica i debrita. La sedimentació està controlada per factors climàtics i tectònics com ara la intensitat del corrent oceànic Californià o moviments tectònics locals. Aquests paràmetres exerceixen un control fonamental en la distribució dels minerals d'argila i de les fàcies sedimentàries en el marge continental. La seva evolució des del Pliocè és discutida en aquesta tesi.Els sediments analitzats rics en hidrats de gas són més grollers que els sediments hemipelàgics. Els sediments més grollers actuen com a conductes preferents per a la circulació de fluids degut a la seva porositat i permeabilitat. En aquest context, fluids rics en metà migren des dels sediments profunds cap a la zona d'estabilitat dels hidrats de gas, on possibiliten la seva formació.Diversos fronts de barita han estat identificats en els sediments de southern Hydrate Ridge. Es formen com a resultat del reciclatge de barita durant la diagènesi inicial, controlada per la presència de fluids rics en metà, la degradació de matèria orgànica i la presència de sulfat. La modelització de les dades obtinguda mostra que aquest procés va ser actiu durant un període de >1000 anys. En aquesta tesi, es discuteixen els possibles processos geoquímics i sedimentaris que permetrien la supervivència de la barita durant la diagènesis.La textura sedimentària juga un paper molt important en el flux de fluids als marges continentals. La seva evolució temporal en una àrea determinada pot ser deduïda estudiant les fases minerals que es formen durant la diagènesi.

Gas hydrates have been recently recognized as a key factor affecting a number of global processes such as the climatic change, sea floor stability, etc. In this thesis we present the multidisciplinary study of gas hydrate rich sediments recovered during ODP Leg 204. The main objective of this thesis is to study how the textural characteristics of marine sediments can affect the main pathways and intensity of fluid flow and how fluid flow determines the distribution of gas hydrates in the continental margins, as well as the main geochemical processes that occur during early diagenesis.To reach these objectives, a complete sedimentary and geochemical study of 581 sediment samples from southern Hydrate Ridge was carried out. The methods and techniques that were applied include: complete textural analyses, mineralogy, physical properties and geochemistry.The southern Hydrate Ridge sediments are mainly made up of four lithofacies defined as: hemipelagites, turbidites, ash layers and debrites. Mass‐transport deposits such as turbidites and debrites are more abundant in Lithostratigraphic Unit III and II, as well as in Lithostratigraphic Unit IA in the slope basin of southern Hydrate Ridge. Some increasing trends with depth can be observed in the smectite content in the clay mineral assemblages. These features suggest that the transport in suspension of fine sediments through the California Current was more effective during the Pliocene and early Pleistocene period. Bedload transport of coarse material from local and distal areas was more effective during the middle Pleistocene and Holocene due to the tectonic reactivation of the southern Hydrate Ridge uplift. During the Pleistocene and owing to the pervasive fluctuation of sealevel, gas hydrate dissociation together with the seismic movements in the Oregon margin seems a plausible triggering mechanism for mass‐movements.The results presented here confirm that the sedimentation patterns in the Hydrate Ridge region are controlled by climate and tectonic parameters such as the regional intensity of the California Current or the local tectonic movements that lead to the uplift of the Ridge. These parameters mainly control the clay mineral distribution as well as the sedimentary facies that were produced.The sedimentary fabric of gas hydrate‐rich intervals is disturbed during core recovery due to gas hydrate dissociation. The two main disturbance fabrics generated through this process are mousselike and soupy. The gas hydrate‐rich sediments analyzed for this thesis are coarser grained in respect to the hemipelagite sediments. The coarse‐grained layers such as turbidites and ash layers could act as conduits for fluids in the southern Hydrate Ridge region because of their higher porosity and permeability. In this context, methane‐rich fluids migrate through these layers from deep in the sedimentary sequence and into the gas hydrate stability zone. A number of barium fronts have been identified in southern Hydrate Ridge sediments and interstitial waters. Barite fronts were formed as a result of the barite recycling process during early diagenesis, which is controlled by the availability of methane‐rich fluids, in situ decomposition of organic matter and the sulphate gradient. Modelling of these data shows that these processes were active at southern Hydrate Ridge for a period of up to one thousand years.A number of geochemical and sedimentological processes are proposed in this thesis as plausible mechanisms to allow the survival of the barite fronts during diagenesis. The sedimentary texture plays an important role in controlling the major fluid flow pathways in the continental margins. The temporal evolution of the fluid flow can be studied in a given area through the distribution of the mineral phases that form during early diagenesis, as well as the interstitial water composition., EXTRACTE DE TESI:Aquesta tesi integra els resultats de l'anàlisi sedimentológica i geoquímica de sediment marins rics en hidrats de gas, recuperats durant la campanya "Ocean Drilling Program" Leg 204 en el marge d'Oregon (USA). L'objectiu principal d'aquest estudi és conèixer les característiques sedimentàries que afecten el fluxe de fluids i gasos a través del sediment i com els fluids afecten la distribució d'hidrats de gas en aquesta àrea, així com alguns processos geoquímics que operen durant la diagènesi inicial. Els mètodes i tècniques aplicats inclouen l'anàlisi de sedimentològia, mineralogia, susceptibilitat magnètica i geoquímica.Els sediments de southern Hydrate Ridge estan formats per 4 litofàcies: hemipelagita, turbidita, cendra volcànica i debrita. La sedimentació està controlada per factors climàtics i tectònics com ara la intensitat del corrent oceànic Californià o moviments tectònics locals. Aquests paràmetres exerceixen un control fonamental en la distribució dels minerals d'argila i de les fàcies sedimentàries en el marge continental. La seva evolució des del Pliocè és discutida en aquesta tesi.Els sediments analitzats rics en hidrats de gas són més grollers que els sediments hemipelàgics. Els sediments més grollers actuen com a conductes preferents per a la circulació de fluids degut a la seva porositat i permeabilitat. En aquest context, fluids rics en metà migren des dels sediments profunds cap a la zona d'estabilitat dels hidrats de gas, on possibiliten la seva formació.Diversos fronts de barita han estat identificats en els sediments de southern Hydrate Ridge. Es formen com a resultat del reciclatge de barita durant la diagènesi inicial, controlada per la presència de fluids rics en metà, la degradació de matèria orgànica i la presència de sulfat. La modelització de les dades obtinguda mostra que aquest procés va ser actiu durant un període de >1000 anys. En aquesta tesi, es discuteixen els possibles processos geoquímics i sedimentaris que permetrien la supervivència de la barita durant la diagènesis.La textura sedimentària juga un paper molt important en el flux de fluids als marges continentals. La seva evolució temporal en una àrea determinada pot ser deduïda estudiant les fases minerals que es formen durant la diagènesi.

The anaerobic oxidation of methane (AOM) by archaeal methanotrophs(ANME) functions as a major sink in oceanic methane geochemistry, and is a key biogeochemial process in the anoxic seabed. Unfortunately, demonstration of the biochemical pathway of AOM has not been possible because of the lack of pure cultures of ANME and their partner sulfate-reducing bacteria. The main reason for failing to isolate these microorganisms by cultivation is their slow growth, which is most likely a consequence of the low energy yield of the AOM reaction. This chapter discusses how in situ biogeochemical and microbiological observations of natural seabed communities and in vitro enrichments contribute to understanding of the ecology and physiology of these “uncultivables”. Successful in vitro enrichment strategies include selecting seabed inoculates with abundant ANME populations, increasing the availability of dissolved methane and sulfate by flow through reactors and hydrostatic pressure, and maintaining the apparent temperature, pH, and salinity optima of AOM, the energy delivering process.

We measured halogen concentrations and I-129/I ratios in five drilling sites of Integrated Ocean Drilling Program Expedition 311 (offshore Vancouver Island, Canada) in order to identify potential sources of fluids and methane in gas hydrate fields. Iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope I-129 (T-1/2 = 15.7 Ma) allows the age determination of organic sources for iodine. Here we report halogen concentrations in 135 pore water samples, I concentrations in 48 sediment samples, and I-129/I ratios measured in a subset of 20 pore water samples. Most I-129/I ratios fall into a range around 500 x 10(-15), corresponding to a minimum age of 25 Ma and the lowest ratio of 188 x 10(-15) (T-min = 47 Ma) was observed at 208 m below sea floor (mbsf) in Site 1326. These ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the Juan de Fuca plate, indicating that old, accreted sediments in the accretionary wedge contribute a significant amount of iodide and, by association, of methane to the gas hydrate occurrences. A geochemical transport-reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide, bromide, and ammonia in the host sediments due to organic matter degradation. The model was first tested with data from two well studied areas, Ocean Drilling Program Site 1230 (Peru margin) and Site 1245 (Hydrate Ridge). The model results for the Expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids, carrying large amounts of old iodine from deep sources. The comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry, especially at the Cascadia margin. The iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields.

Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ methanogenesis. We estimated the in situ methanogenesis rates in Hydrate Ridge (HR) sediments by coupling experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete locations in the sediment column. When starved in a biomass recycle reactor, Methanoculleus submarinus produced ca. 0.017 fmol methane/cell/day. Quantitative PCR (QPCR) directed at the methyl coenzyme M reductase subunit A gene ( mcrA ) indicated that 75% of the HR sediments analyzed contained

[1] To elucidate the geological processes associated with hydrate formation and dissociation in the marine environment under a wide range of conditions, we have developed a one-dimensional numerical computer model (simulator). The numerical model can be used to simulate the following aspects of hydrate formation, decomposition, reformation, and distribution: (1) burial history of deep marine sediments and associated phenomena (e.g., sediment compaction and consequent reduction in sediment porosity and permeability, fluid expulsion, time evolution of temperature and pressure, heat flux), (2) in situ generation of biogenic methane from buried organic carbon and methane solubility in formation brine, (3) methane hydrate formation, decomposition, reformation, and distribution in response to changes in gas concentration, pressure, temperature and fluid salinity (the hydrate formation and decomposition are treated as equilibrium processes), (4) influence of sulfate reduction zone under the seafloor on hydrate formation, (5) possible presence of a free gas zone beneath the gas hydrate stability zone, and (6) multiphase (i.e., liquid brine with dissolved gas, free gas, gas hydrate) flow through a deformable porous matrix. The model provides for a reduction/ increase in permeability due to the formation/decomposition of the gas hydrate. Initial applications of the model to study hydrate distribution at the Blake Ridge (site 997) and Hydrate Ridge (site 1249) are described. Model results are compared with chlorinity, sulfate, and hydrate distribution data.

The relation between acoustic sea-floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side-scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual sea-floor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea. More than 600 active methane seeps were hydroacoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from − 72 m to − 156 m. Multibeam and side-scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium- to high-backscatter areas which often coincide with pockmarks. High-resolution seismic data reveals the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the sea floor at locations where gas seeps and medium- to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium- to high-backscatter locations. This is confirmed by visual sea-floor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites. Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self)-sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations.

[1] 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.

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 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.

[1] Due to experimental difficulties grain size distributions of gas hydrate crystallites are largely unknown in natural samples. For the first time, we were able to determine grain size distributions of six natural gas hydrates for samples retrieved from the Gulf of Mexico and from Hydrate Ridge offshore Oregon from varying depths. High-energy synchrotron radiation provides high photon fluxes as well as high penetration depth and thus allows for investigation of bulk sediment samples. The gas hydrate crystallites appear to be (log-) normally distributed in the natural samples and to be of roughly globular shape. The mean grain sizes are in the range from 300–600 μm with a tendency for bigger grains to occur in greater depth, possibly indicating a difference in the formation age. Laboratory produced methane hydrate, starting from ice and aged for 3 weeks, shows half a log-normal curve with a mean value of ∼40 μm. This one order-of-magnitude smaller grain sizes suggests that care must be taken when transposing grain-size sensitive (petro-)physical data from laboratory-made gas hydrates to natural settings.

[1] 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.”

Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was the first drilling expedition dedicated to understanding gas hydrate processes in accretionary complexes and provided a testbed for a number of different techniques for estimating the gas hydrate content of sediments. It was also the first time that (1) digital infrared scans of core temperature were systematically recorded for all cores from within or near the gas hydrate stability zone, facilitating identification of gas hydrate samples for further study, (2) hydrate-bearing cores were recovered and logged at in situ pressure, and (3) ODP allowed acquisition of logging-while-drilling data prior to coring at a given site, providing an initial comprehensive estimate of gas hydrate distribution that was used to design the subsequent coring program. Gas hydrate estimates based on a variety of geophysical and geochemical techniques indicate a heterogeneous distribution of gas hydrate, which results in part because of two distinct regimes for delivery of gas to the gas hydrate stability zone. In the “reaction regime,” which is pervasive throughout the study region, the average gas hydrate content of the sediments is relatively low (2%–8% of the pore space), no gas hydrate is present in the upper ~30 meters below seafloor (mbsf) because the methane content of the pore water is below saturation, and the fine-scale distribution of gas hydrate depends strongly on lithology. Superimposed on the reaction regime is a “transport-dominated re1Trehu, A.M., Torres, M.E., Bohrmann, G., and Colwell, F.S., 2006. Leg 204 synthesis: gas hydrate distribution and dynamics in the central Cascadia accretionary complex. In Trehu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Drilling Program), 1–40. doi:10.2973/odp.proc.sr.204.101.2006 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis OR 97331, USA. Correspondence author: trehu@coas.oregonstate.edu 3Research Center Ocean Margins, University of Bremen, D-28334 Bremen, Germany. 4Idaho National Laboratory, Idaho Falls ID 83415, USA. Present address: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis OR 97331, USA. Initial receipt: 22 March 2006 Acceptance: 4 September 2006 Web publication: 1 November 2006 Ms 204SR-101 A.M. TREHU ET AL. LEG 204 SYNTHESIS: GAS HYDRATE DISTRIBUTION AND DYNAMICS 2 gime” in which gas is focused into a stratigraphically controlled conduit and is transported as free gas to the structural summit. At the summit, high gas pressure drives free gas into and through the gas hydrate stability zone, resulting in a shallow deposit in which gas hydrate comprises ~25% of the total sediment volume to a depth of ~25 mbsf. Geochemical data indicate that most of the gas that forms the summit deposit has migrated from greater depth and has either a thermogenic or altered biogenic character, and modeling suggests that abundant free gas is needed to form gas hydrate in these conditions. Although this deposit contains only ~2% of the estimated total volume of methane trapped in gas hydrate within the study region, it may be particularly susceptible to destabilization in response to oceanographic change. INTRODUCTION Ocean Drilling Program (ODP) Leg 204, which focused on a topographic high in the Cascadia accretionary complex ~80 km west of Newport, Oregon (USA), and 15 km east of the current deformation front (Fig. F1), was the first ODP expedition dedicated to understanding gas hydrate in an accretionary complex. Leg 204 builds on the results of Deep Sea Drilling Project (DSDP) Leg 18 (Sites 174–176) (Kulm and von Huene, 1973) and ODP Leg 146 (Sites 888–892) (Westbrook, Carson, Musgrave, et al., 1994), which studied the process of sediment subduction and accretion on the Cascadia margin, and Leg 164, drilled on the Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996), which was the first ODP leg dedicated to understanding gas hydrate processes. Leg 204 is complemented by Integrated Ocean Drilling Program (IODP) Expedition 311, which targeted a segment of the northern Cascadia margin that is characterized by coarse-grained trench sediments (Expedition 311 Scientists, 2005), in contrast to the finer-grained slope sediments that dominate the lithologies sampled during Leg 204 (Trehu, Bohrmann, Rack, Torres, et al., 2003; Su et al., this volume, Gracia et al., this volume). Nine sites were occupied during Leg 204 (Table T1; Fig. F2A). The sites were located in water depths of 780–1210 meters below sea level (mbsl) near the southern part of Hydrate Ridge (SHR). All sites were located within a 4 km × 11 km region that was imaged to a subseafloor depth of ~1 km by three-dimensional (3-D) seismic data. The 3-D seismic data provide tectonic and stratigraphic links among the sites (Fig. F2B–F2F) and enable reconstruction of the geologic history (Chevallier et al., this volume; Bangs et al., 2005). Logging-while-drilling (LWD) data acquired at the beginning of the leg provided initial estimates of the hydrate distribution as inferred from electrical resistivity anomalies. These estimates were used to plan the subsequent coring program. Immediately after recovery, all cores from within the gas hydrate stability zone (GHSZ) were scanned with a track-mounted infrared camera to detect “cold spots” indicative of recent or ongoing dissociation of gas hydrate. The sections of core showing the strongest cold spots were sampled for shipboard experiments to calibrate geophysical proxies for gas hydrate presence or to preserve the gas hydrate for shore-based structural analyses. Gas hydrate presence was also determined based on a variety of geochemical and geophysical measurements (see “Intercalibration of Different Gas Hydrate Proxies,” p. 10). All shipboard data, with limited interpretation, were presented in the Leg 204 Initial Reports volume (Trehu, Bohrmann, Rack, Torres, et al., 2003). -000 -000 -000 0 0 000 000

Hydrate Ridge is an accretionary thrust ridge located on the lower slope of the central Cascadia convergent margin. Structural mapping based on two-dimensional and three-dimensional multichannel seismic reflection profiles and gridded bathymetry coupled with deep-towed sidescan sonar data and Ocean Drilling Program (ODP) biostratigraphy suggests that seafloor fluid venting patterns are likely controlled by the seaward-vergent (SV) structural style at northern Hydrate Ridge (NHR) and by the dominantly landward-vergent (LV) structural style at southern Hydrate Ridge (SHR). North-south structural variability across Hydrate Ridge is coincident with the seafloor authigenic carbonate distribution, which varies from aerially extensive authigenic carbonate crusts at NHR to a minor focused occurrence of authigenic carbonate at SHR. The older stratigraphy exposed at the seafloor at NHR (>1.6–1.7 Ma) has likely been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR (1.7 Ma to recent), suggesting the extent of carbonates at NHR may result from a longer history of fluid flow and/or more intense venting through a more uplifted, lithified, and fractured NHR sequence. Furthermore, recent work at SHR shows that the major seafloor fluid venting site there is fed by fluid flow through a volcanic ash–bearing turbidite sequence, suggesting stratigraphic conduits for fluid flow may be important in less uplifted, LV-dominated portions of Hydrate 1Johnson, J.E., Goldfinger, C., Trehu, A.M., Bangs, N.L.B., Torres, M.E., and Chevallier, J., 2006. North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin. In Trehu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Drilling Program), 1–16. doi:10.2973/odp.proc.sr.204.125.2006 2Present address: University of New Hampshire, Department of Earth Sciences, 56 College Road, James Hall, Durham NH 03824, USA. joel.johnson@unh.edu 3College of Oceanic and Atmospheric Science, Oregon State University, Corvallis OR 97331, USA. 4University of Texas, Institute for Geophysics, 4412 Spicewood Springs Road, Austin TX 78759, USA. Initial receipt: 12 May 2005 Acceptance: 5 February 2006 Web publication: 20 October 2006 Ms 204SR-125 J.E. JOHNSON ET AL. HISTORY OF DEFORMATION AND FLUID VENTING 2 Ridge. In addition, the variability in structural style observed at Hydrate Ridge may have implications for the distributions and concentrations of fluids and gas hydrates in other accretionary settings and play a role in the susceptibility of accretionary ridges to slope failure. INTRODUCTION On the Cascadia continental margin offshore central Oregon, Hydrate Ridge (Figs. F1, F2) has been the focus of numerous geologic and geophysical investigations for nearly two decades. During the mid1980s, its location within the lower slope of the accretionary wedge initially prompted investigations of seafloor fluid flow and the dewatering processes associated with accretionary wedge deformation and development and resulted in one of the first discoveries of chemosynthetic biological communities (Suess et al., 1985; Kulm et al., 1986; Ritger et al., 1987). By the early 1990s this early work was supplemented by Ocean Drilling Program (ODP) drilling (Fig. F2; Sites 891 and 892), during which gas hydrates were first recovered (Westbrook, Carson, Musgrave, et al., 1994), and detailed structural investigation (MacKay et al., 1992; Goldfinger et al., 1992, 1996). Subsequent work, including numerous sea-floor observation and sampling expeditions since the late 1990s (e.g., Suess et al., 1999, 2001) and more recently a gas hydrate–dedicated ODP leg (Trehu, Bohrmann, Rack, Torres, et al., 2003; and Trehu et al., this volume) (Fig. F2; Sites 1244–1252), focused on the surface and shallow subsurface gas hydrate system, seeking to characterize the distribution, concentration, and behavior of gas hydrates in an active margin setting. In this paper we integrate our recent work on 1. The history of deformation, constrained by structures interpreted from seismic reflection data and core biostratigraphy; 2. The record of fluid venting, as imaged on sidescan sonar data; 3. The surface and subsurface gas hydrate distribution, constrained by seafloor observations and ODP coring; and 4. Records of slope failure, interpreted from piston cores and ODP cores in the Hydrate Ridge region. Together, these data suggest that variability in structural style may strongly influence the distribution and concentration of fluids and gas hydrates in the subsurface and the susceptibility of accretionary ridges to slope failure. GEOLOGIC SETTING The Cascadia accretionary wedge evolved in response to the oblique subduction of the Juan de Fuca–Gorda plate system (Fig. F1) and is composed of folded and faulted abyssal plain turbidites and hemipelagic clays as well as the recycled products of these uplifted sediments as slope basin fills (Kulm and Fowler, 1974; Westbrook, Carson, Musgrave, et al., 1994; Trehu, Bohrmann, Rack, Torres et al., 2003). The Quaternary portion of the accretionary wedge is widest off the Washington and northern Oregon margins, coincident with accretion of the thick Pleistocene Astoria and Nitinat Fans (Carlson and Nelson, 1987), and narrows to the south. The active accretionary thrust faults and folds of 125° 123° 127° 129° 131°W 0 lanco Tnsform ault Juan de Fuca plate Ju an d e Fu ca R id ge

Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate–bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep-seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl− and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (

The Simple Cabled Instrument for Measuring Properties In Situ (SCIMPI) is a proposed new long-term seafloor observatory system for sub-seafloor studies. The SCIMPI sensors, initially proposed, include temperature sensors, resistivity sensors, and pore pressure transducers. The design easily accommodates integration of other sensors (e.g., geophones, X-ray fluorescence, spectrophotometry). SCIMPI can be deployed autonomously with a battery and data module for 1-2 year deployments or connected to a seafloor fiber optic cable for real time data streaming. Applications include the study of sub-seafloor hydrological and deep biosphere studies; pore pressure measurements for hazards assessments; long-term temperature measurements; and earthquake monitoring. Potential near-term SCIMPI sites include Monterey Bay, Cascadia Margin and the Gulf of Mexico. In Monterey Bay, SCIMPI is proposed to be part of the Monterey Accelerated Research System (MARS) cabled observatory. On Cascadia Margin, SCIMPIs are proposed to be part of a cabled observatory, and are specifically targeted for a location called Hydrate Ridge. Hydrate Ridge studies are focused on understanding the mechanisms, rates, and extent of methane hydrate formation in this area. SCIMPIs at Hydrate Ridge will measure temperature, pore pressure, electrical resistivity, and micro-seismicity. The proposed plan is to install SCIMPIs by the IODP and then link the SCIMPI observatories into the Regional Cabled Observatory in the ORION program. Potential Gulf of Mexico sites are in the deep water environments where overpressures have been observed in shallowly buried, loose sand deposits.

[1] In marine sedimentary environments, microbial methanotrophy represents an important sink for methane before it leaves the seafloor and enters the water column. Using benthic observatories in conjunction with numerical modeling of pore water gradients, we investigated seabed methane emission rates at cold seep sites with underlying gas hydrates at Hydrate Ridge, Cascadia margin. Measurements were conducted at three characteristic sites which have variable fluid flow and sulfide flux and sustain distinct chemosynthetic communities. In sediments covered with microbial mats of Beggiatoa, seabed methane efflux ranges from 1.9 to 11.5 mmol m−2 d−1. At these sites of relatively high advective flow, total oxygen uptake was very fast, yielding rates of up to 53.4 mmol m−2 d−1. In sediments populated by colonies with clams of the genus Calyptogena and characterized by low advective flow, seabed methane emission was 0.6 mmol m−2 d−1, whereas average total oxygen uptake amounted to only 3.7 mmol m−2 d−1. The efficiency of methane consumption at microbial mat and clam field sites was 66 and 83%, respectively. Our measurements indicate a high potential capacity of aerobic methane oxidation in the benthic boundary layer. This layer potentially restrains seabed methane emission when anaerobic methane oxidation in the sediment becomes saturated or when methane is bypassing the sediment matrix along fractures and channels.

The Deep Ocean Raman In Situ Spectrometer (DORISS) instrument was deployed at the Sea Cliff Hydrothermal Field and Hydrate Ridge in July 2004. The first in situ Raman spectra of hydrothermal minerals, fluids, and bacterial mats were obtained. These spectra were analyzed and compared to laboratory Raman measurements of standards and samples collected from the site. Spectra of vent fluid (∼294°C at the orifice) at ∼2700 m depth were collected with noncontact and immersion sampling optics. Compared to spectra of ambient (∼2°C) seawater, the vent fluid spectra show changes in the intensity and positions of the water O-H stretch bands due to the elevated temperature. The sulfate band observed in seawater spectra is reduced in vent fluid spectra as sulfate is removed from vent fluid in the subseafloor. Additional components of hydrothermal fluid are present in concentrations too low to be detected with the current Raman system. A precision underwater positioner (PUP) was used to focus the laser spot on opaque samples such as minerals and bacterial mats. Spectra were obtained of anhydrite from actively venting chimneys, and of barite deposits in hydrothermal crusts. Laboratory analysis of rock samples collected in the vent field also detected the presence of gypsum. Spectra of bacterial mats revealed the presence of elemental sulfur (S8) and the carotenoid beta-carotene. Challenges encountered include strong fluorescence from minerals and organics and insufficient sensitivity of the instrument. The next generation DORISS instrument addresses some of these challenges and holds great potential for use in deep-sea vent environments.

We present grain size, granulometric statistical parameters, and calcium carbonate content of sediment samples from the summit and east and west flanks of southern Hydrate Ridge (Sites 1244–1250). These data are compared with magnetic susceptibility measurements from the same intervals. Bulk and clay mineralogy from Sites 1244 (east flank), 1247 (west flank), and 1250 (summit) are also presented. The integration of these data allows us to characterize the main sedimentary facies and composition of the Quaternary age sediments from southern Hydrate Ridge. INTRODUCTION Hydrate Ridge is a structural high located in the accretionary complex of the Cascadia subduction zone, offshore Oregon (USA) (Fig. F1A), where gas hydrates were recovered from the seafloor (e.g., Suess et al., 2001) and a ubiquitous bottom-simulating reflector (BSR) suggests widespread distribution of gas hydrates (Trehu et al., 1999). During Ocean Drilling Program (ODP) Leg 204, nine sites were drilled on southern Hydrate Ridge (Trehu, Bohrmann, Rack, Torres, et al., 2003; Trehu et al., 2004b) (Fig. F1B). High-resolution three-dimensional mulSite 1252 Site 1251 Site 1244 Site 1245 Site 1249 Site 1250 Site 1247

The main objectives of the project were to monitor, characterize, and quantify in situ the rates of formation and dissociation of methane hydrates at and near the seafloor in the northern Gulf of Mexico, with a focus on the Bush Hill seafloor hydrate mound; to record the linkages between physical and chemical parameters of the deposits over the course of one year, by emphasizing the response of the hydrate mound to temperature and chemical perturbations; and to document the seafloor and water column environmental impacts of hydrate formation and dissociation. For these, monitoring the dynamics of gas hydrate formation and dissociation was required. The objectives were achieved by an integrated field and laboratory scientific study, particularly by monitoring in situ formation and dissociation of the outcropping gas hydrate mound and of the associated gas-rich sediments. In addition to monitoring with the MOSQUITOs, fluid flow rates and temperature, continuously sampling in situ pore fluids for the chemistry, and imaging the hydrate mound, pore fluids from cores, peepers and gas hydrate samples from the mound were as well sampled and analyzed for chemical and isotopic compositions. In order to determine the impact of gas hydrate dissociation and/or methane venting across the seafloormore » on the ocean and atmosphere, the overlying seawater was sampled and thoroughly analyzed chemically and for methane C isotope ratios. At Bush hill the pore fluid chemistry varies significantly over short distances as well as within some of the specific sites monitored for 440 days, and gas venting is primarily focused. The pore fluid chemistry in the tub-warm and mussel shell fields clearly documented active gas hydrate and authigenic carbonate formation during the monitoring period. The advecting fluid is depleted in sulfate, Ca Mg, and Sr and is rich in methane; at the main vent sites the fluid is methane supersaturated, thus bubble plumes form. The subsurface hydrology exhibits both up-flow and down-flow of fluid at rates that range between 0.5 to 214 cm/yr and 2-162 cm/yr, respectively. The fluid flow system at the mound and background sites are coupled having opposite polarities that oscillate episodically between 14 days to {approx}4 months. Stability calculations suggest that despite bottom water temperature fluctuations, of up to {approx}3 C, the Bush Hill gas hydrate mound is presently stable, as also corroborated by the time-lapse video camera images that did not detect change in the gas hydrate mound. As long as methane (and other hydrocarbon) continues advecting at the observed rates the mound would remain stable. The {_}{sup 13}C-DIC data suggest that crude oil instead of methane serves as the primary electron-donor and metabolic substrate for anaerobic sulfate reduction. The oil-dominated environment at Bush Hill shields some of the methane bubbles from being oxidized both anaerobically in the sediment and aerobically in the water column. Consequently, the methane flux across the seafloor is higher at Bush hill than at non-oil rich seafloor gas hydrate regions, such as at Hydrate Ridge, Cascadia. The methane flux across the ocean/atmosphere interface is as well higher. Modeling the methane flux across this interface at three bubble plumes provides values that range from 180-2000 {_}mol/m{sup 2} day; extrapolating it over the Gulf of Mexico basin utilizing satellite data is in progress.« less

[1] P-wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments, making it an excellent means to investigate gas hydrate systems. However, seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy. The presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis; however anisotropy can help reveal the distribution of hydrates. Here we report on the evidence of seismic anisotropy at Hydrate Ridge along the Cascadia convergent margin. We find that the south summit is anisotropic, while the basin side (east of south summit) is isotropic. Anisotropy is likely caused by the hydrate veins. We interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates.

[1] Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled-source electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep-towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.

[1] A series of biogeochemical studies were conducted at the southern summit of Hydrate Ridge, offshore Oregon. Using the submersible DSV Alvin, sediment push cores were collected from two distinct seep environments characterized by the presence of clam fields (CF) or microbial mats (MM) at the sediment-water interface; samples were also collected from a nearby reference site characterized by a barren surface at the sediment-water interface. Sediment samples from each setting were analyzed for the depth distributions of total organic carbon (concentrations, δ13C and Δ14C), total sedimentary nitrogen, and microbial abundance. Pore fluids were extracted and analyzed for sulfate, alkalinity, sulfide, organic carbon, and volatile organic acids. These depth distributions clearly indicate the presence of three distinctive biogeochemical settings in the surface sediments of Hydrate Ridge, and provide the basis for a comparative biogeochemical analysis. Both CF and MM sites display properties indicating enhanced microbial activity in the subsurface, compared with the reference site. MM sites display evidence of net biomass production in the subsurface; however, a loss of sediment nitrogen relative to the reference site indicates that mineralization is also enhanced. Calculations based on the removal of nitrogen indicate that greater than 30% of autochthonous organic material is lost to enhanced mineralization in the top 23 cm of one MM site. An isotope mass balance of sediment-bound organic carbon indicates a mixed source, including methane and allochthonous organic carbon dissolved in the seep fluids. The concentrations of organic carbon dissolved in seep fluids reach values of 22 mM and provide a first indication that advective transport of dissolved organic carbon from great depth may supply an important source of energy and carbon to “methane seep” communities.

Seismic wave velocities vary in the presence of gas hydrate and free gas in the sediments. Seismic properties (velocities) of the gas-hydrate bearing sediments allow us to identify the presence of gas hydrates, to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. Accuracy in the estimation of distribution and saturation of gas hydrates and free gas depends on the interval velocities of Pand S-waves. We have carried out an interactive velocity analysis of Pand converted S-waves in the tau-p (intercept time – ray parameters) domain, which directly gives the interval velocities. This requires multicomponent seismic data. A two-ship seismic experiment was carried out (to record multicomponent seismic data) in summer 2002 at the Hydrate Ridge to map the gas hydrate. Our approach to multicomponent velocity analysis comprises three steps: 1) P-wave velocity analysis, 2) PP to PS event correlation, and 3) Swave velocity analysis. PP to PS correlation is performed using synthetic seismograms. Observed velocities are matched with modeled velocities to estimate gas hydrate saturation. Pand S-wave velocities are modeled with a “Modified Wood equation” which is a modification of Wood equation with a rock physics model and an empirical relation, respectively. We present results from the multicomponent ocean bottom seismometer data recorded at the Hydrate Ridge, offshore Oregon. The Pwave velocity is found to be more sensitive to the saturation of gas hydrates and free gas than S-wave velocity. Gas hydrate is estimated to be upto 7% of rock volume (12% of pore space). The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus we conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region. Introduction Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). In marine environments, methane hydrates are usually stable at temperatures in the range of 0C to 150C, water depth greater than 500 m, and sediment depths up to 300 meters below sea floor. Formation of gas hydrate is favorable in the deeper ocean (methane gas, water and thermodynamic conditions are available), which means huge amount of gas (about 15000 giga tones) may be present in the offshore gas hydrates (Kvenvolden, 1999). Free gas is usually present below the gas hydrates. Hydrates and free-gas make a strong acoustic interface, which is evident in seismic section as a Bottom Simulating Reflector (BSR). Offshore gas hydrates system has got attention from the geoscientist because of its potential to be 1) a drilling hazard, 2) a future energy source, 3) continental slope stability, and 4) a factor affecting global carbon cycle. Many of these relations are not well established, which can be better constrained if distribution and saturation of gas hydrates and free gas are known. Gas hydrates and free-gas are generally detectable with seismic methods (Yuan et al., 1996) since the seismic velocity, in general, increases in the presence of gas hydrates and decreases in the presence of free gas. The velocity also increases if the hydrate content in the pore spaces increases (Hyndman and Spence, 1992). Therefore, seismic velocity has been widely used to estimate the saturation of gas hydrates and free gas (e.g., Lee et al., 1996; Helgerud et al., 1999; Lu and McMechan, 2002). Other methods gives local estimation of gas hydrates using resistivity data from well logs, chloride measurement from core data, infra-red image of cores, and pressure core sampler (Dickens et al., 1997; Trehu et al., 2004). Hydrate Ridge (HR) is a 25-km long and 15-km wide accretionary ridge in the Cascadia convergent margin (MacKay, 1995) formed as the Juan de Fuca plate subducts obliquely beneath the North American plate (Figure 1). Hydrates and its seismic proxies (BSR, and amplitude blanking) appear to be well developed beneath HR (Trehu et al., 1999). A two-ship seismic experiment was conducted at the HR during summer 2002 with the seismic ship Maurice Ewing and the drilling ship JOIDES Resolution. The cruise was designed to acquire surface seismic (streamer recording, MCS) and subsurface seismic [Vertical seismic profiles (VSP) and Ocean bottom seismometer (OBS)] data to map the distribution and saturation of gas hydrates which is possible with accurate estimation of elastic properties (Pand S-waves velocities) of gas hydrateand free gasbearing sediments. Seismic velocity analysis is normally performed in the offset-time ( t x − ) domain, which gives root mean square (RMS) velocity. The resultant RMS velocity is converted to interval velocity using the Dix equation (Dix, 1955). Interval velocity can be directly estimated with data analysis in the tau-p (intercept time – ray parameters) domain (Stoffa et al., 1981). We have carried out an interactive velocity analysis for Pand S-waves in the taup domain. S-wave in marine setting is a converted shear wave, and can be recorded with multicomponent receivers as OBS, and VSP geometry. We use the OBS data recorded on the HR. Presence of weak anisotropy due to the hydrate veins has been reported by Kumar et al. (2004), however we will consider isotropy model in this study. Once interval velocities are estimated, they are

To constrain the fluxes of methane (CH4) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (−63 to −66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L−1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (−56 to −28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (−25 to −6‰ PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of “downcurrent” plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h−1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × 104 mol h−1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally.

[1] 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

The anaerobic oxidation of methane (AOM) is one of the major sinks for methane on earth and is known to be mediated by at least two phylogenetically different groups of anaerobic methanotrophic Archaea (ANME-I and ANME-II). We present the first comparative in vitro study of the environmental regulation and physiology of these two methane-oxidizing communities, which occur naturally enriched in the anoxic Black Sea (ANME-I) and at Hydrate Ridge (ANME-II). Both types of methanotrophic communities are associated with sulfate-reducing-bacteria (SRB) and oxidize methane anaerobically in a 1:1 ratio to sulfate reduction (SR). They responded sensitively to elevated methane partial pressures with increased substrate turnover. The ANME-II-dominated community showed significantly higher cell-specific AOM rates. Besides sulfate, no other electron acceptor was used for AOM. The processes of AOM and SR could not be uncoupled by feeding the SRB with electron donors such as acetate, formate or molecular hydrogen. AOM was completely inhibited by the addition of bromoethanesulfonate in both communities, indicating the participation of methanogenic enzymes in the process. Temperature influenced the intensity of AOM, with ANME-II being more adapted to cold temperatures than ANME-I. The variation of other environmental parameters, such as sulfate concentration, pH and salinity, did not influence the activity of both communities. In conclusion, the ecological niches of methanotrophic Archaea seem to be mainly defined by the availability of methane and sulfate, but it remains open which additional factors lead to the dominance of ANME-I or -II in the environment.

[1] 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 (

In this study we investigated by using 16S rRNA-based methods the distribution and biomass of archaea in samples from (i) sediments above outcropping methane hydrate at Hydrate Ridge (Cascadia margin off Oregon) and (ii) massive microbial mats enclosing carbonate reefs (Crimea area, Black Sea). The archaeal diversity was low in both locations; there were only four (Hydrate Ridge) and five (Black Sea) different phylogenetic clusters of sequences, most of which belonged to the methanotrophic archaea (ANME). ANME group 2 (ANME-2) sequences were the most abundant and diverse sequences at Hydrate Ridge, whereas ANME-1 sequences dominated the Black Sea mats. Other seep-specific sequences belonged to the newly defined group ANME-3 (related to Methanococcoides spp.) and to the Crenarchaeota of marine benthic group B. Quantitative analysis of the samples by fluorescence in situ hybridization (FISH) showed that ANME-1 and ANME-2 co-occurred at the cold seep sites investigated. At Hydrate Ridge the surface sediments were dominated by aggregates consisting of ANME-2 and members of the Desulfosarcina-Desulfococcus branch (DSS) (ANME-2/DSS aggregates), which accounted for >90% of the total cell biomass. The numbers of ANME-1 cells increased strongly with depth; these cells accounted 1% of all single cells at the surface and more than 30% of all single cells (5% of the total cells) in 7- to 10-cm sediment horizons that were directly above layers of gas hydrate. In the Black Sea microbial mats ANME-1 accounted for about 50% of all cells. ANME-2/DSS aggregates occurred in microenvironments within the mat but accounted for only 1% of the total cells. FISH probes for the ANME-2a and ANME-2c subclusters were designed based on a comparative 16S rRNA analysis. In Hydrate Ridge sediments ANME-2a/DSS and ANME-2c/DSS aggregates differed significantly in morphology and abundance. The relative abundance values for these subgroups were remarkably different at Beggiatoa sites (80% ANME-2a, 20% ANME-2c) and Calyptogena sites (20% ANME-2a, 80% ANME-2c), indicating that there was preferential selection of the groups in the two habitats. These variations in the distribution, diversity, and morphology of methanotrophic consortia are discussed with respect to the presence of microbial ecotypes, niche formation, and biogeography.

Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.

[1] The results of a deployment of aqueous flux meters at the Bush Hill hydrate mound show that persistent hydrologic instability is a primary feature of the globally abundant gas- and hydrate-rich cold seep environment. Seven flux meters were deployed for 14 weeks in the area of the Bush Hill hydrate mound. Instruments were deployed on microbial mats, bivalves, adjacent to the surface hydrate mound, and a site without visible fauna. Flow rates were observed to range from downflow of 0.01 mm/day to upflow of >15 mm/day. Temporal variability and a major hydrological event were observed to occur on all instruments. There is evidence that this event was related to a gas expulsion episode. The two instruments which exhibited downflow during the event were near surface hydrates and bubbling vents. Nearby instruments recorded a rapid increase in flow rates at the time of the event with a subsequent decrease in rates to the previous background values. Two instruments showed significant output of seawater-like fluids, while the others output typical methane seep-type pore fluids. These results at a passive margin site, along with our previously published convergent margin results at gas-rich northeast Pacific cold seeps (Hydrate Ridge, Eel River margin), illustrate the hydrologic complexity of these environments. We now propose that these and our earlier results are characteristic of seafloor environments which have abundant free gas and hydrates. These mechanisms have major consequences for the near-surface geochemical and microbial environment and for the way we interpret measurements made in these areas.

[1] Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend

Summary We have observed P-wave seismic anisotropy at the south summit of Hydrate Ridge (HR), Cascadia convergent margin, offshore Oregon, on walkaway vertical seismic profiles and ocean bottom seismometer data. Models based on the assumption of vertical transverse isotropy (VTI), require the vertical velocity to be higher than the horizontal velocity that is uncommon in the conventional sand-shale formations. Since HR is characterized by high velocity hydrate veins, emulating vertical layering, an HTI model (TI with a horizontal axis of symmetry) seems more appropriate. Our anisotropy analysis consists of two-steps, 1) building of 2D anisotropic model comprising homogeneous layers based on the traveltime-error-contour analysis assuming VTI and HTI medium, and 2) derivation of continuously varying heterogeneous model using a rigorous traveltime inversion with very-fast simulated annealing. Based on our result we established relationships between P-wave anisotropic parameters and distribution of gas hydrate and free gas at HR. Observed P-wave anisotropy are, 1) 11% (N-S) to 15% (E-W) in the layer A (from sea floor to 70 meter below sea floor) containing heterogeneous hydrate veins, 2) 7% (N-S) to 10% (E-W) in the layer B (from layer A to bottom simulating reflector) containing relatively homogeneous hydrate and 3) 3% in the layer C (gas-bearing sediments below layer B). Anisotropy values are proportional to the hydrate concentration at the HR. Distribution and concentration of gas-hydrates can be further used for studying possibilities of gas-hydrate as a geohazard, an economic energy source, and a cause for world margin stability and climate change.

One of the key questions related to the prediction of the hydrate distribution and quantity is the nature of the interface at the base of the gas hydrate stability zone (GHSZ), where it is generally considered that free gas is present below. The presence of gas below the gas hydrate stability zone is a major contributor to the formation of a bottom simulating reflector (BSR) present in seismic profile records. Improved geophysical methods were used in September 2000 during cruise Sonne SO-150 to seismically investigate the nature of the BSR across Hydrate Ridge, which is part of the Cascadia accretionary complex, offshore the Oregon continental margin. The experiments were carried out by using a variety of source and receiver acquisition systems. Within the scope of Ocean Drilling Program (ODP) Leg 204, Southern Hydrate Ridge was drilled in the year 2002 providing additional information to verify the results derived from the seismic data and supporting ongoing analyses with relevant acoustic log data. The analyses, presented in this study, focus on deriving P-wave velocities from the seismic data to quantify the gas hydrate content within the GHSZ. By combining ocean bottom hydrophone (OBH) data with surface (SCS) and deep tow streamer data, an accurate velocity-depth distribution is determined, which is constrained by the acoustic logs of Ocean Drilling Program (ODP) Leg 204 and the depth of the BSR is predicted within a 5 m uncertainty. In general, the P-wave velocities are below 1600 m/s, averaged over the entire GHSZ, which leads to an estimate of about 1% of hydrate of pore space. This is in general below the average value of 2% resulting from ODP analyses, but is within an uncertainty of 1%, when considering thin hydrated layers affecting the mean velocity distribution. The analyses of the frequency dependent amplitude variations of the BSR signal, result in the presence of a thin gas bearing layer below the BSR with an average thickness of 8 ± 4 m. The existence of a thick gas zone of several 100 m beneath the GHSZ, as observed in other hydrate provinces, is ruled out by P-wave velocity considerations. The presence of a thin gas layer leads to the assumption that amplitude variation along the BSR, in addition to a change in the gas concentration, is caused by tuning effects. Amplitude and velocity anomalies are not necessarily the result of a change in gas hydrate saturation. Further, it is assumed that this thin gas layer is an indication for gas hydrate in the GHSZ being recyled due to tectonic uplift and a major contributor for the free gas below.

Biogeochemical and molecular biological studies revealed new insights into the anaerobic oxidation of methane (AOM), but physiological aspects remained mostly unclear. Here incubation-experiments on AOM were performed, which gave insights into the physiology of the organisms. With sediment samples from Hydrate Ridge the dependence of sulphate-reduction on methane was shown, as well as the 1:1 stoichiometry. Vertical activity profiles confirmed that higher methane fluxes lead to higher activity. AOM showed a dependence on temperature, Hydrate Ridge samples were psychrophilic while samples from the Black Sea were mesophilic. Elevated methane partial pressure had a strong effect and increased the activity 2 to 5fold. Since AOM is most probably a syntrophic process by a sulphate-reducer and a methane-oxidizer we tried to find the intermediate that is exchanged between them. But non of the discussed and tested ones (H2, acetate, formate, methanol) seems to be the one in question. BES however stopped the AOM activity, indicating, that one of the partners is a methanogen operating in reverse. Molybdate only stopped the activity in Hydrate Ridge samples. In Black Sea samples the activity was only reduced. Antibiotics only active against bacteria did not effect the process of AOM. It was possible to increase the activity of the samples by continuously incubating them under elevated methane partial pressure (1,37MPa) and a regular exchange of the medium to supply the organisms with sulphate and methane and to remove sulphide. In 20 month the activity increased 6,5fold and also the amount of consortia increased significantly. The assumed doubling time of a consortium is 4.7 month. The classical isolation of sulphate-reducing-bacteria and methanogens seperatly lead to a number of strains, non of which however was closely elated to the members in the consortia. Also co-culture experiments did not reveal their ability to oxidize methane.

[1] Fluxes of gas, energy, and fluid (water with dissolved gas) are known to control the distribution of gas hydrate and free gas in gas hydrate reservoirs, but theoretical studies have so far not focused on the effect of sediment properties (e.g., permeability) on these fluxes during the formation of gas hydrate deposits. Using a transient, one-dimensional, two-phase (gas hydrate and fluid) numerical model based on a coupled conservation approach, we examine permeability clogging during gas hydrate formation in end-member porous media and fractures initially lacking hydrate and determine the resulting changes in fluid and advective energy flux. Gas hydrate accumulates most rapidly at depth in both systems, a result consistent with borehole logging and seismic studies on the Blake Ridge. Consideration of the ratio of surface area to fluid volume explains model results that imply more rapid permeability reduction in the porous media system and more uniform hydrate formation as a function of depth in the fracture system. Depending on fluid flux, fracture aperture, or pore sizes, and other factors, the timescale for permeability clogging due to the accumulation of gas hydrate ranges from thousands to millions of years. Regenerating gas hydrate to a concentration of 10–15% of pore space in marine settings with fluid fluxes characteristic of Hydrate Ridge or Blake Ridge requires 103–105 years, meaning that hydrate deposits in these settings cannot be viewed as a naturally renewable resource. These results also imply that significant time is required to reestablish gas hydrate deposits after large-scale dissociation events associated with global-scale climate change.

Submersible investigations of the Cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence. Acoustic profiles of these bubble plumes in the water column in the vicinity of Hydrate Ridge offshore Oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column. The gas vent sites remained active over the span of two years, but varied dramatically on time scales of a few hours. All plumes emanated from local topographic highs near the summit of ridge structures. The acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m, independent of the seafloor depth. This coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m, suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution. The upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone.

The inversion of seismic data offers a tool for the remote extraction of physical properties of the sediments from surface-recorded data. Physical properties of the gas hydrate-bearing sediments derived from inversion of seismic help us identify the presence of gas hydrates. They can also be used to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. A 3-D multi-channel seismic (MCS) and ocean bottom seismometer (OBS) survey in the Hydrate Ridge, offshore Oregon was conducted to image structures controlling the migration of methane-rich fluid and free gas and to map the gas-hydrate distribution. We first obatined preliminary Vp and Vs profiles from OBS data by interactive analysis and then used them as a starting model to estimate Vp from the streamer data by waveform inversion. The results from our inversion and interpretation study in Hydrate Ridge reveal that: (1) both 3-D streamer and OBS data show a strong BSR indicating the presence of gas hydrate above and free gas below; (2) interactive P- and S-wave velocity analysis of OBS data allows us to identify the presence of a “conversion surface” in the gas hydrate-bearing sediments; (3) inverted velocity profiles show a lowvelocity layer existing below the sea floor and above the normal gas hydrate, suggesting a new geological model of gas hydrates; (4) two types of hydrate fabrics, massive and porous hydrates, observed by deep-towed video survey, were identified in the P-wave velocity profiles, and three main layers of gas hydrate-sediments separated by the conversion surface and BSR are distinguished; (5) the profiles reflecting the physical properties of sediments, such as the P-wave velocity, acoustic impedance and Poisson’s ratio profiles, are able to map the distribution of gas hydrates and show very similar trends of lateral variation of the main layers; (6) a series of faults in the accretionary complex under the ridge not only offer pathways for methane and fluid ascending from deeper layers but also control the distribution of the porous hydrates with low velocity below the seafloor, and (7) Hornbach et al. (2003) suggest their results using velocity analysis of seismic reflection data on the Blake Ridge is the first direct seismic detection of concentrated hydrate confirmed by velocity analysis. Our results of direct inversion of seismic data extend these results to greater resolution of the entire seismic data set. Further, our results may be the first seismic indication of visually observed porous hydrate zone.