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 18o 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

vertical tectonics and the origins of bsrs along the peru margin abstract the distribution of bottom simulating reflections bsrs which mark the base of the gas hydrate zone on marine seismic records across the peru margin is examined with respect to tectonic uplift sedimentation and carbon content of sediment where accretion of only deep ocean pelagic sediment containing less than 1 carbon is seismically imaged no bsr is observed no bsr is apparent on seismic records of carbon rich 4 8 sediment of rapidly subsiding basins bsrs are abundant on records of carbon rich sediment affected by tectonic uplift and or rapid sedimentation in areas of uplift the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate during uplift free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level the strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of bsrs off peru

strain partitioning in the zone of transition from nankai subduction to izu bonin collision central japan implications for an extensional tear within the subducting slab abstract the izu bonin ridge collision with central japan is a type example of collision of two island arcs in a subduction context based on the analysis of active faults paleomagnetic data and recent seismicity in the northern tip of the izu bonin arc and the adjacent oceanic plate we propose a tectonic schema of the izu collision system and its transition to the nankai subduction to the west the dense gps network deployed in central japan allows us to constrain the kinematics of the different blocks identified this new kinematic model demonstrates the essential role of strain partitioning there as the northward component of the izu bonin arc motion is locked in the collision zone 30 of the convergence between the philippine sea plate and central japan is accommodated within the philippine sea slab along the zenisu east izu deformation system we show that this pattern of deformation is related to an extensional tear of the philippine sea slab propagating southward along the volcanic arc we discuss the implications of this kinematic model on the seismogenic potential of the area and the dynamics of the central japan convergence zone

composition of shelf methane seeps on the cascadia continental margin 1 methane reservoirs and seeps are an active component of the continental margin carbon budget and represent a poorly characterized pathway for reduced carbon cycling and methane input to the atmosphere active gas seeps from three shelf settings on the cascadia continental margin off oregon and northern california contain nearly pure methane with a heavy carbon isotope composition 29 to 35 an extensive study of the gas seep at coquille bank oregon revealed a warm buoyant pore fluid associated with the pockmark as methane enters the water column above these seeps in a steady gas stream a fraction escapes directly to the atmosphere while the balance dissolves into local seawater measured oxidation rates are too slow for significant local oxidation within the water column near the seep large mats of pink and white bacteria including beggiatoa spp are found around the vent demonstrating the activity of sulfide oxidizers in this ecosystem

a new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere a new estimate of global methane emission into the atmosphere from mud volcanoes mvs on land and shallow seafloor is presented the estimate considered a lower limit is based on 1 new direct measurements of flux including both venting of methane and diffuse microseepage around craters and vents and 2 a classification of mv sizes in terms of area km2 based on a compilation of data from 120 mvs the methane flux to the atmosphere is conservatively estimated between 6 and 9 mt y 1 this emission from mvs is 3 6 of the natural methane sources and is comparable with ocean and hydrate sources officially considered in the atmospheric methane budget the total geologic source including mvs seepage from seafloor microseepage in hydrocarbon prone areas and geothermal sources would amount to 35 45 mt y 1 the authors believe it is time to add this parameter in the intergovernmental panel on climate change official tables of atmospheric methane sources

ocean bottom seismometer investigations in the ormen lange area offshore mid norway provide evidence for shallow gas layers in subsurface sediments a multi component ocean bottom seismometer obs survey in the ormen lange area of the storegga slide constrained the existence of free gas and possibly gas hydrates in the shallow subsurface the three investigated areas a b c lie in close vicinity to the slide scar above one of the largest deep water gas reservoirs on the mid norwegian margin generally p wave and s wave velocities are high compared to average velocities at a given burial depth due to an exposure of deeper sediments as a consequence of sediment mass movement by the storegga slide indications for the presence of gas within the sediments exist for two of the three investigated areas the gas accumulates beneath less permeable layers of glacigenic debris flow deposits average gas concentration of pore space in both areas is 0 9 area a and 0 15 area b the geophysical data do not allow a conclusive answer about the occurrence of gas hydrates their presence might be masked by high velocity debris flow deposits which occur in the subsurface nevertheless gas hydrate concentrations of pore space have been estimated to about 9 in area a and 7 in area b

dynamics of gas hydrate case of the congo continental slope a numerical model is developed to study the temperature effect on stability and phase transformation of gas hydrate the model uses a mathematical formulation based on the enthalpy form of the conservation law of energy the use of the enthalpy form instead of the temperature form as often done in the literature has made the problem numerically simpler the model is then applied to describe the effect of sea bottom temperature variations on the stability of gas hydrate occurrences and on the seafloor reflectivity in sediments of the congo continental slope indeed a migrating seafloor reflectivity front is observed on 3d seimic images from two surveys performed 6 months apart and interpreted as a migrating gas hydrate stability zone seawater temperature variations were recorded over a 230 day period at the seafloor the amplitude of these variations could explain a 75 m shift in water depth of the upper limit of the gas hydrate stability zone however the response of the sediment to a perturbation applied at the seafloor is not immediate because of the effects of thermal diffusion and of latent heat of hydrate dissociation thermal modelling shows that the depth of penetration of these perturbations is around 2 m for the saturated sediment and around 6 m for hydrate bearing sediments hydrate fraction of 0 1 the switch between gas and gas hydrate generated by these temperature fluctuations concern only the first meter of sediment these appear too small to explain the migration of the seafloor reflectivity considering that the wavelength of the seismic signal is significantly larger around 10 m however for temperature fluctuations with larger wavelength the switch between gas and gas hydrate will be more important and consequently could explain the reflectivity contrast as a possible migrating gas or gas hydrate front

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

methane sources distributions and fluxes from cold vent sites at hydrate ridge cascadia margin 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 13c 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 13c 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

slope instabilities and gravity processes in fluid migration and tectonically active environment in the eastern nankai accretionary wedge kaiko tokai 96 cruise abstract one main feature imaged on the nankai slope by using the sar pasisar system deep tow single channel seismic streamer during the franco japanese kaiko tokai cruise in april 1996 is the gravitational displacement of large volumes of the sediment this slope failure is assumed to be mainly related to an increase of the slope angle beyond its threshold value tentatively linked to the uplift effect of the subduction of a paleo zenisu ridge and to the existence of a basal shear plane possibly controlled by altered properties of the sediments at the base of the gas hydrate stability field the role of pore fluid or free gas is discussed but remains unknown without any in situ data this role may be major if considering the gassy environment in the sedimentary cover where earthquake loading can cause excess pore pressure and act as a major triggering mechanism for sliding

fluid sediment interactions at eastern mediterranean mud volcanoes a stable isotope study from odp leg 160 abstract pore fluids from two odp sites at eastern mediterranean mud volcanoes have been analyzed for their cl concentration and their 18o and d isotopic composition the cl data span a wide range of concentrations from extremely depleted with respect to seawater as low as 60 mm at the crest of milano dome site 970 to strongly enriched up to 5 4 m at napoli dome site 971 chloride enrichment is known to be due to dissolving messinian evaporites whereas the source of the low cl fluid is deduced from stable isotope data presented here the isotopic composition of the endmember fluid is found to be 10 for 18o and 32 for d for low as well as for high cl waters from this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water several other processes altering the 18o d composition of pore waters are discussed and considered to be of only negligible influence the process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration mainly smectite illite transformation corresponding to a depth range of 3 5 7 km and an elevated temperature of about 120 165 c a quantitative estimate shows that this reaction is capable of producing the observed extreme cl depletion

a study of the chemistry of pore fluids and authigenic carbonates in methane seep environments kodiak trench hydrate ridge monterey bay and eel river basin analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the kodiak trench hydrate ridge monterey bay clam flats and the eel river basin coupled with the studies of the chemistry of the pore fluids have shown that these carbonates have grown within the sediment column geochemical profiles of pore fluids show that in deep water seeps kodiak trench 4450 m monterey bay 1000 m hydrate ridge 650 m 13c dic values are low isotopically light whereas in the eel river area 350 500 m 13c dic values are much higher isotopically heavier in all cases the 13c values indicate that processes of methane oxidation associated with sulfate reduction are dominant in the shallow sediments data on the isotopic composition of authigenic carbonates found at sites in kodiak trench eel river basin south and eel river basin north indicate a variable composition and origin in different geochemical environments some of the authigenic carbonates from the study sites show a trend in their 13c values similar to those of the pore fluids obtained in their vicinity suggesting formation at relatively shallow depths but others indicate formation at greater sediment depths the latter usually consist of high magnesium calcite or dolomite which from their high values of 13c up to 23 and 18o up to 7 5 suggest formation in the deeper horizons of the sediments in the zone of methanogenesis these observations are in agreement with observations by other workers at hydrate ridge in monterey bay and in the eel river basin

crustal structure beneath the central oregon convergent margin from potential field modeling evidence for a buried basement ridge in local contact with a seaward dipping backstop models of magnetic and gravity anomalies along two e w transects offshore central oregon one of which is coincident with a detailed velocity model provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop the models indicate that the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america 50 million years ago has a seaward dip of less than 60o3 seismic magnetic and gravity data are compatible with no more than 2 km of subducted sediments between the siletz terrane and the underlying crystalline crust of the juan de fuca plate the data also suggest the presence of a n s trending 200 km long basaltic ridge buried beneath the accretionary com plex from about 43on to 45on although the height and width of this ridge probably vary along strike it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the siletz terrane beneath heceta bank several models for the origin of this ridge are discussed these include a sliver of siletz terrane detached from the main siletz terrane dur ing a late eocene episode of strike slip faulting imbrication and thickening of subducted oceanic crust in place an aseismic ridge rafted in on the subducting oceanic crust during the past 1 2 mil lion years and a series of ridges and or seamounts rafted in over a longer period of time and trans ferred from the subducting plate to the overlying plate the last model is the most consistent with the complicated history of local uplift subsidence and slope instability recorded in the ridges basins and banks of this part of the margin we speculate that the massive seaward dip ping western edge of the siletz terrane in this region inhibits subduction of seamounts and sedi ments resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction this process may also be responsible for thickening of lower accretionary complex material over steepening of slopes leading to massive slumping and north south extension through strike slip faulting in the accretionary complex to the west of the buried ridge regardless of its origin the ridge may currently be acting as an asperity inhibiting subduction

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

contrasting hydrogeologic regimes along strike slip and thrust faults in the oregon convergent margin evidence from the chemistry of syntectonic carbonate cements and veins samples of carbonate cemented sedimentary rocks were collected during 11 alvin dives in two regions along the cascadia margin a northern strike slip fault zone and a southern thrust fault zone second ridge we characterized 35 samples petrographically and chemically northern area samples are dominantly sandstones second ridge samples are all mudstones vugs and high percentages of carbonate cement in the mudstones suggest that these rocks were never deeply buried the abundance of veins and brecciation in samples from both regions attests to their proximity to faults northern area cements are dominantly calcite or magnesian calcite whereas second ridge cements generally are more dolomitic oxygen and sr isotopic values of carbonate cements indicate the involvement of two very different fluid reservoirs during cementation one reservoir generally has low 18 o very low 13 c and high 87 sr 86 sr ratios but the other has high 18 o moderately negative 13 c and 87 sr 86 sr ratios lower than modern seawater values in the northern area diagenetic carbonates are subdivided into two populations of coupled oxygen and carbon isotopic values population i has 18 o pdb values of 3 to 5 and 13 c pdb values of 55 to 45 population ii has 18 o values of 13 to 4 and 13 c values of 25 to 1 such common low 18 o values in carbonate cements have not been observed at any other accretionary wedge 87 sr 86 sr ratios of northern area samples range from 0 7128 to 0 7088 oxygen and sr isotopic values show a correlation between increasing 87 sr 86 sr ratios and decreasing 18 o values oxygen and carbon isotopic values of most diagenetic carbonates from the second ridge range from 3 to 10 and 55 to 38 respectively enrichments of 18 o probably result from a combination of dolomitic mineralogy cold bottom water temperatures and the presence of decomposing gas hydrate in the subsurface 87 sr 86 sr ratios of carbonates from the second ridge range from 0 7086 to 0 7091 the geochemical signatures of diagenetic cements from the northern area suggest that fluids were derived from greater than 2 km depth perhaps from the decollement prominent gullies throughout the northern area are probably underlain by strike slip faults that provide conduits for upward fluid migration cements at the second ridge precipitated from shallowly derived fluids the difference in fluid source depth is related to proximity to the fluid conduits and different orientations of minimum principal stress and resultant hydrofractures in the two fault regimes

relationship of pore water freshening to accretionary processes in the cascadia margin fluid sources and gas hydrate abundance 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

evidence for a thick free gas layer beneath the bottom simulating reflector in the makran accretionary prism abstract seismic reflection data from the makran continental margin indicate the presence of a strong and widespread bottom simulating reflector bsr we apply a nonlinear full waveform inversion technique to multichannel reflection data from this area to investigate the detailed velocity structure and hence the origin of the bsr our result shows an abrupt decrease in the compressional wave velocity from 2 2 to 1 3 km s at a depth of 500 m below the sea bed the low velocity zone is unusually thick 200 350 m and may contain large quantities of free gas similar to some of the recently drilled blake ridge sites of odp leg 164 the voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas hydrate stability field relative to the sediment column due to uplift and sedimentation in the accretionary wedge

carbonate cements indicate channeled fluid flow along a zone of vertical faults at the deformation front of the cascadia accretionary wedge northwest u s coast to understand the relation between fluid seeps and structures sedimentary rocks were collected with the dsrv alvin from a vertical fault zone that transects the deformation front of the cascadia accretionary wedge the rocks contained diagenetic carbonate cement that was precipitated from fluids expelled during accretion carbon oxygen and strontium isotope data are consistent with a fluid source at 2 km depth most carbon isotopes range from 1 to 25 pdb peedee belemnitel standard consistent with a thermogenic methane source oxygen isotopes show extreme 18 o depletions 4 to 13 pdb that are consistent with precipitation from fluids with temperatures as high as 100 c 87 sr 86 sr values of 0 70975 to 0 71279 may be due to strontium in fluids derived from clay rich parts of the stratigraphic section the ubiquity of carbonate precipitates and the isotope data indicate that the vertical fault zone is an efficient conduit for fluid dewatering from deep levels of the accretionary wedge

oxygen isotope fractionation in marine aragonite of coralline sponges oxygen isotope values of the extant caribbean coralline sponge ceratoporella nicholsoni are compared with published temperatures and 18o of water calculated from salinities the measured values from aragonitic sponge skeletons have a mean offset of 1 0 0 1 from calculated calcite equilibrium values aragonite calcite 1 0010 this is in good agreement with published values from synthetic aragonite they further agree with published near equilibrium oxygen isotope values of temperate and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges these results and the mode of skeleton formation of ceratoporella nicholsoni suggest that these sponges precipitate aragonite close to isotopic equilibrium the temperature dependence of oxygen isotopic fractionation between the aragonite of ceratoporella nicholsoni and water is only roughly constrained by the available data due to the narrow temperature range of the caribbean reef sites however as the data suggest oxygen isotopic equilibrium we can calculate a well constrained temperature equation combining temperate and cold water equilibrium values from molluscs and foraminifera with our sponge data full size image 1 k and full size image 1 k

hydrate ridge a natural laboratory for the study of microbial life fueled by methane from near surface gas hydrates hydrate ridge cascadia convergent margin is characterized by abundant methane hydrates at and below the seafloor active venting of fluids and gases chemosynthetic communities composed of giant sulfide oxidizing bacteria and clams authigenic carbonates and some of the highest methane oxidation rates ever found in the marine environment fluid flow rates vary over six orders of magnitude among closely spaced settings at the crest of hydrate ridge the distribution of benthic communities is mainly related to the sulfide flux from the subsurface sediments produced by anaerobic oxidation of methane aom even at high flux rates aom removes most of the methane seeping from the subsurface less than 50 of the methane escapes from bacterial mats from calyptogena fields 15 and from acharax beds no methane is emitted to the water column the precipitation of carbonate derived from aom is a significant and permanent carbon sink hence compared to the amount of gas hydrates and methane derived carbonates stored at the summit of hydrate ridge the emission of methane is relatively low due to the efficient filtering capacity of methanotrophic microbial communities this present review updates current knowledge of the geology of hydrate ridge summarizes recently published data on geomicrobiology and biogeochemistry and derives a local methane budget

regional uplift gas hydrate dissociation and the origins of the paleocene eocene thermal maximum much of the north atlantic region experienced hundreds of metres of transient uplift during the latest paleocene regional uplift should cause dissociation of gas hydrates stored in marine sedimentary strata and the subsequent release of carbon to the atmosphere should enhance warming through the greenhouse effect we have developed models of the spatial distribution of uplift to match observational constraints from sedimentary basins fringing the north atlantic in order to provide quantitative estimates of the carbon released upon uplift these models are coupled to a simple parameterisation of carbon storage in sub seafloor gas hydrates results of modelling indicate that large quantities of carbon can be released by uplift with some models generating over 2000 gt of carbon release the possibility that latest paleocene regional uplift occurred over the same time interval as onset of the paleocene eocene thermal maximum petm around 30 kyr cannot be ruled out using current stratigraphical observations or mantle convection models in this case the uplift mechanism alone can account for the magnitude and timing of the carbon isotope excursion associated with the petm alternatively if the duration of latest paleocene uplift was closer to the upper bound of around 1 myr or if other paleocene uplift events were equally significant then gas hydrate dissociation upon uplift could have contributed to longer term changes in ocean temperature and carbon isotope composition through the late paleocene these temperature changes in combination with development of the north atlantic land bridge during the latest paleocene culmination of uplift may be able to force rapid changes in ocean circulation and cause phenomena such as the petm

gas hydrate destabilization enhanced dewatering benthic material turnover and large methane plumes at the cascadia convergent margin mixed methane sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the cascadia convergent margin vent fields from which methane charged low salinity fluids containing sulfide ammonia 4he and isotopically light co2 escape are associated with these exposures they characterize a newly recognized mechanism of dewatering at convergent margins where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward this process augments the convergence generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths the enhanced turnover results from the oxidation of methane hydrogen sulfide and ammonia by vent biota the injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide a large fraction of the methane is oxidized within the water column and generates 13c anomalies of the dissolved inorganic carbon pool

geophysical constraints on the surface distribution of authigenic carbonates across the hydrate ridge region cascadia margin on active tectonic margins methane rich pore fluids are expelled during the sediment compaction and dewatering that accompany accretionary wedge development once these fluids reach the shallow subsurface they become oxidized and precipitate cold seep authigenic carbonates faults or high porosity stratigraphic horizons can serve as conduits for fluid flow which can be derived from deep within the wedge and or if at seafloor depths greater than 300 m from the shallow source of methane and water contained in subsurface and surface gas hydrates the distribution of fluid expulsion sites can be mapped regionally using sidescan sonar systems which record the locations of surface and slightly buried authigenic carbonates due to their impedence contrast with the surrounding hemipelagic sediment hydrate ridge lies within the gas hydrate stability field offshore central oregon and during the last 15 years several studies have documented gas hydrate and cold seep carbonate occurrence in the region in 1999 we collected deep towed seamarc 30 sidescan sonar imagery across the hydrate ridge region to determine the spatial distribution of cold seep carbonates and their relationship to subsurface structure and the underlying gas hydrate system high backscatter on the imagery is divided into three categories i circular to blotchy with apparent surface roughness ii circular to blotchy with no apparent surface roughness and iii streaky to continuous with variable surface roughness we interpret the distribution of high backscatter as well as the locations of mud volcanoes and pockmarks to indicate variations in the intensity and activity of fluid flow across the hydrate ridge region seafloor observations and sampling verify the acoustic signals across the survey area and aid in this interpretation subsurface structural mapping and swath bathymetry suggest the fluid venting is focused at the crests of anticlinal structures like hydrate ridge and the uplifts along the daisy bank fault zone geochemical parameters link authigenic carbonates on hydrate ridge to the underlying gas hydrate system and suggest that some of the carbonates have formed in equilibrium with fluids derived directly from the destabilization of gas hydrate this suggests carbonates are formed not only from the methane in ascending fluids from depth but also from the shallow source of methane released during the dissociation of gas hydrate the decreased occurrence of high backscatter patches and the dramatic reduction in pockmark fields imaged on the eastern part of the survey suggest gas hydrate near its upper stability limit may be easily destabilized and thus responsible for these seafloor features high backscatter along the left lateral daisy bank fault suggests a long history of deep seated fluid venting probably unrelated to destabilized gas hydrate in the subsurface

linking continental slope failures and climate change testing the clathrate gun hypothesis it has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice core atmospheric methane record during the pleistocene because submarine sediment failures can involve as much as 5000 gt of sediment and have the capacity to release vast quantities of methane hydrates one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental slope failure and atmospheric methane correlate to test the clathrate gun hypothesis we have collated published dates for submarine sediment failures in the north atlantic sector and correlated them with climatic change for the past 45 k y more than 70 by volume of continental slope failures during the past 45 k y was displaced in two periods between 15 and 13 ka and between 11 and 8 ka both these intervals correlate with rising sea level and peaks in the methane record during the bolling allerod and preboreal periods these data support the clathrate gun hypothesis for glacial interglacial transitions the data do not however support the clathrate gun hypothesis for glacial millennial scale climate cycles because the occurrence of sediment failures correlates with heinrich events i e lows in sea level and atmospheric methane a secondary use of this data set is the insight into the possible cause of continental slope failures glacial period slope failures occur mainly in the low latitudes and are associated with lowering sea level this finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause the bolling allerod sediment failures were predominantly low latitude suggesting an early tropical response to deglaciation e g enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters in contrast sediment failures during the preboreal period and the majority of the holocene occurred in the high latitudes suggesting either isostatic rebound related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound causing destabilization of gas hydrates

the silurian of gotland sweden facies interpretation based on stable isotopes in brachiopod shells the silurian of gotland sweden consists of 440 m of carbonate deposits repeatedly uniform sequences of micritic limestones and marls are interrupted by complex structured reefs and by adjacent platform sediments generally the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated the carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than to paleotemperature carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions consequently the facies succession on gotland results from global paleoclimatic conditions changes in terrigenous input due to different rates of weathering and freshwater runoff rather than sea level fluctuations control the carbonate formation of the silurian on gotland

thermogenic hydrocarbons in unconsolidated sediment of eel river basin offshore northern california thermally produced hydrocarbons were recovered from unconsolidated sediment ponded within a bathymetric depression on the surface of a shale diapir in the offshore eel river basin of northern california evidence that the hydrocarbons are thermogenic consists of the following 1 very high concentrations of hydrocarbon gases particularly ethane through butanes c2 c4 2 methane having a carbon isotopic composition relative to the pdb standard of 43 and 44 per mil 3 presence of gasoline range c5 hydrocarbons and 4 presence of a complex mixture of heavy hydrocarbons c15 with n alkanes having a petroleumlike distribution this mixture of gaseous and liquid hydrocarbons likely originated deep within the basin an migrated to the surface through fractures and faults developed during the emplacement of the diapir the presence of thermogenic hydrocarbons in unconsolidated surface sediment indicates that conditions for petroleum generation have existed within this offshore basin

observations of gas hydrates in marine sediments offshore northern california biogenic gas hydrates were recovered in shallow cores 6 m deep from the eel river basin in offshore northern california between 40 38 and 40 56 n the gas hydrates contained primarily methane 13c 57 6 to 69 1 and occurred as dispersed crystals small 2 20 mm nodules and layered bands within the sediment these hydrates recovered in sediment at water depths between 510 and 642 m coincide nearly but not exactly with areas showing bottom simulating reflectors bsrs on seismic reflection records this study confirms indirect geophysical and geologic observations that gas hydrates are present north of the mendocino fracture zone in sediment of the eel river basin but probably are absent to the south in the point arena basin this discovery extends the confirmed sites of gas hydrates in the eastern pacific region beyond the peruvian and central american margins to the northern california margin

methane sources feeding cold seeps on the shelf and upper continental slope off central oregon usa we report on a bathymetric mapping and remotely operated vehicle surveys along the 100 600 m region offshore oregon from 43 50 n to 44 18 n we interpret our results in light of available geophysical data published geotectonic models and analogous observations of fluid venting and carbonate deposition from 44 30 n to 45 00 n the methane seepage is defined by juxtaposition of a young prism where methane is generated by bacterial activity and its release is modulated by gas hydrate dynamics against older sequences that serve as a source of thermogenic hydrocarbons that vent in the shelf we hypothesize that collision of a buried ridge with the siletz terrane results in uplift of gas hydrate bearing sediments in the oncoming plate and that the resulting decrease in pressure leads to gas hydrate dissociation and methane exolution which in turn may facilitate slope failure oxidation of the released methane results in precipitation of carbonates that are imaged as high backscatter along a 550 60 m benthic corridor

stromatolitic fabric of authigenic carbonate crusts result of anaerobic methane oxidation at cold seeps in 4 850 m water depth methane seepage leads to mg calcite and aragonite precipitation at a depth of 4 850 m on the aleutian accretionary margin stromatolitic and oncoid growth structures imply encrustation of microorganisms microbial mats in the host sediment with a unique growth direction downward into the sediment forming crust shaped lithologies biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol which contain extremely low 13c values this indicates the presence of methane consuming archaea and 13c values of 42 to 51 pdb indicate that methane is the carbon source for the carbonate crusts thus it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate reducing bacteria and that carbonate precipitation proceeds downward into the sediment where ascending cold fluids provide a methane source strontium and oxygen isotope analyses as well as 14c ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3 000 years ago

the origin of thermogenic gas hydrates on the northern cascadia margin as inferred from isotopic 13c 12c and d h and molecular composition of hydrate and vent gas the isotopic 13c and d and hydrocarbon compositions of hydrate bound and vent gas collected from the seafloor of barkley canyon northern cascadia margin offshore vancouver island canada were evaluated to characterize the gas and infer the type and maturity of the source rock kerogen the hydrate gas contained methane having 13c values from 43 4 to 42 6 d values from 143 to 138 and a large percentage 14 9 to 31 9 of c2 to c5 hydrocarbons these data are consistent with a thermogenic gas source the data from barkley canyon are interpreted within the context of similar data from the gulf of mexico and caspian sea thermogenic hydrates which occur in regions where the petroleum systems supporting gas and oil generation are better understood a stable carbon isotope based natural gas plot model and d h data from the hydrate gas indicate that the source rock for the barkley canyon hydrate and vent gas had primarily type iii kerogen mixed with a small fraction of type ii kerogen oil from a seep having a pristane phytane ratio of 3 2 was identified as a gas condensate which supports the conclusions drawn from the gas data a mechanism for explaining how fluids are conducted from the deep petroleum reservoir to the seafloor of barkley canyon is proposed

methane gas release from the storegga submarine landslide linked to earlyholocene climate change a speculative hypothesis the 3000 km3 storegga submarine landslide occurred c 8150 years ago from an area rich in gas hydrates off the western coast of norway a synchronous increase of 80 100 ppb in atmospheric methane c

seismic investigations of a vent field associated with gas hydrates offshore vancouver island 1 a three dimensional seismic reflection survey was carried out over a gas hydrate field offshore vancouver island in the area of ocean drilling program leg 146 site 889 890 the area of investigation is located on the accretionary prism of the cascadia margin in water depths of 1200 1500 m several seismic blank zones were observed in the seismic data over a frequency range from 30 hz to 3 5 khz where the degree of blanking increases with seismic frequency time slices and selected horizon slices were used to define the spatial distribution of the blank zones the zones range from 80 m to several hundred meters in width one blank zone almost circular with a diameter of 400 m has a distinct seafloor expression it shows the characteristics of a mud carbonate mound and is probably associated with free gas expulsion hydrate was found at four sites with a piston corer within this blank zone at depths of 3 8 m below the seafloor we present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults the blank zones represent conduits for fluids and gas migrating upward blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults the blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor

tectonic and geological framework for gas hydrates and cold seeps on the hikurangi subduction margin new zealand the imbricated frontal wedge of the central hikurangi subduction margin is characteristic of wide ca 150 km poorly drained and over pressured low taper not vert similar 4 thrust systems associated with a relatively smooth subducting plate a thick trench sedimentary sequence not vert similar 3 4 km weak basal decollement and moderate convergence rate not vert similar 40 mm yr new seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures and to establish the geological framework for gas hydrates and fluid seeps we discuss the stratigraphy of the subducting and accreting sequences characterize stratigraphically the location of the interplate decollement and describe the deformation of the upper plate thrust wedge together with its cover sequence of miocene to recent shelf and slope basin sediments we identify approximately the contact between an inner foundation of deforming late cretaceous and paleogene rocks in which widespread out of sequence thrusting occurs and a 65 70 km wide outer wedge of late cenozoic accreted turbidites although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1 2 myr this differs from the offshore hawkes bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin resulting in uplift and complex deformation of the lower slope and a narrow 10 20 km active frontal wedge five areas with multiple fluid seep sites referred to informally as wairarapa uruti ridge omakere ridge rock garden and builders pencil typically lie in 700 1200 m water depth on the crests of thrust faulted anticlinal ridges along the mid slope uruti ridge sites also lie in close proximity to the eastern end of a major strike slip fault rock garden sites lie directly above a subducting seamount structural permeability is inferred to be important at all levels of the thrust system there is a clear relationship between the seeps and major seaward vergent thrust faults near the outer edge of the deforming cretaceous and paleogene inner foundation rocks this indicates that thrust faults are primary fluid conduits and that poor permeability of the cretaceous and paleogene inner foundation focuses fluid flow to its outer edge the sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the decollement within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector bsr and commonly a seismically resolvable shallow fault network through which fluids and gas percolate to the seafloor no active fluid venting has yet been recognized over the frontal accretionary wedge but the presence of a widespread bsr an extensive protothrust zone 200 km by 20 km in the hikurangi trough and two unconfirmed sites of possible previous fluid expulsion suggest that the frontal wedge could be actively dewatering there are presently no constraints on the relative fluid flux between the frontal wedge and the active mid slope fluid seeps article outline

isotopically heavy oxygen containing siderite derived from the decomposition of methane hydrate authigenic siderite from the miocene and pliocene sediments of the blake outer ridge formed in a methane fermentation zone in equilibrium with anomalously heavy oxygen water as evidenced by its heavy carbon 1 6 to 12 5 13c and heavy oxygen 0 1 to 7 9 18o values blake outer ridge sediments are impregnated with methane hydrate which is generally enriched in 18o relative to ambient water this suggests that decomposition of methane hydrate and subsequent release of large amounts of methane and heavy oxygen water affected the formation of the 18o enriched siderite gas hydrate reaction in sediments is an important process for causing a dramatic variation of oxygen isotopic composition of the interstitial water and of authigenic carbonates formed during burial diagenesis

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

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

geological geochemical and microbiological heterogeneity of the seafloor around methane vents in the eel river basin offshore california marine methane vents and cold seeps are common features along continental margins worldwide serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities the eel river basin erb located on the continental slope off northern california contains active methane vents and seep associated chemosynthetic biological communities cbc on the crests of anticlines in 520 m water depth seep related features on the seafloor have a patchy distribution and include active bubbling vents chemosynthetic clam beds and sulfide oxidizing bacterial mats methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane aom to develop a comprehensive understanding of the complex biological geochemical and physical processes associated with and influencing seafloor methane seepage a multidisciplinary approach is required here we present an integrative multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the eel river basin and the complex interactions defining the geochemistry mineralogy and microbiology within this environment

holocene mass wasting on upper non polar continental slopes due to post glacial ocean warming and hydrate dissociation 1 incorporating both late post glacial bottom water warming and eustatic sealevel rise into a mh methane hydrate stability model explains why at least some major submarine landslides e g the storegga slide norwegian margin may have been triggered by mh dissociation during the early holocene rising sealevels not during the lowest sealevels of the lgm last glacial maximum 18 20 ka in the case of storegga failure may have been initiated either below the upper slope or under the shelf for fresh pore water especially with admixed ethane at water depths below ca 800m persistent low bottom water temperatures allowed the sealevel rise to thicken the mh stability zone over time ruling out holocene initiation of failure mh release by holocene mass wasting cannot have initiated deglaciation

chemoherms on hydrate ridge unique microbially mediated carbonate build ups growing into the water column two active chemoherm build ups growing freely up into the oceanic water column the pinnacle and the south east knoll chemoherms have been discovered at hydrate ridge on the cascadia continental margin these microbially mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle shaped morphology with steep flanks the recovered rocks are pure carbonates dominated by aragonite based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates the fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates the strongly 13c depleted carbon isotope values of the samples as low as 48 1 pdb are characteristic of methane as the major carbon source for the carbonate formation the methane rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom simulating reflector bsr and rise through fault systems the 18o values of the aragonitic chemoherm carbonates are substantially higher as high as 5 0 pdb than the expected equilibrium value for an aragonite forming from ambient seawater 3 5 pdb as a first approximation this indicates formation from glacial ocean water but other factors are considered as well a conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates this model explains the function of the consortium of archaea and sulfate reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane aom and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate dependent anaerobic oxidation of methane within an oxic environment the contact between beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro environment where carbonate formation takes place