a kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at hydrate ridge cascadia margin oregon we develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at hydrate ridge on the cascadia margin of oregon that predicts how pore water chlorinity temperature and crystallized hydrate evolve after the onset of steady venting predictions are compared to observations at ocean drilling program site 1249 in the preferred model calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20m below seafloor bsf under the southern summit of hydrate ridge in similar to 650years and the vertical water flux must be less than 50kg m 2 yr hydrate accumulates more slowly between 20m bsf and the base of the hydrate stability zone where there is no free gas accumulating to observed levels of a few volume percent of hydrate in 10 5 to 10 6 years depending on the water flux that is assumed through this zone this dichotomy means that the presently observed gas venting must have been diverted to this area similar to 650years ago or be episodic and infrequent if the gas venting for the last 650years has been as observed today the latent heat of hydrate precipitation in the upper 20m of sediments would have caused the temperature to increase similar to 0 8 degrees c at similar to 20m bsf and similar to 0 2 degrees c at similar to 100m bsf this would have caused a similar to 5m rise in the elevation of the base of hydrate stability zone and decreased the rate of hydrate crystallization from 1 5kg ch4 m 2 yr 650years ago to 0 7kg ch4 m 2 yr today

lagrangian exploration of the california undercurrent 1992 95 abstract during the period 1992 95 nineteen isobaric rafos floats placed in the california undercurrent at intermediate depths 150 600 m off monterey and san francisco california reveal a region of varying width of subsurface poleward flow adjacent to the continental margin the float trajectories exhibit three patterns poleward flow in the undercurrent reversing but predominately alongshore flow adjacent to the continental margin and farther offshore anticyclonic motion accompanied by slow westward drift flow continuity of the undercurrent exists between pt reyes and at least cape mendocino with an average speed dependent on the float depth speeds were variable but common features were acceleration occurring to the south of pt arena and deceleration to the north of cape mendocino an important mechanism for floats and water to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

methane hydrate in the global organic carbon cycle the global occurrence of methane hydrate in outer continental margins and in polar regions and the magnitude of the amount of methane sequestered in methane hydrate suggest that methane hydrate is an important component in the global organic carbon cycle various versions of this cycle have emphasized the importance of methane hydrate and in the latest version the role of methane hydrate is considered to be analogous to the workings of an electrical circuit in this circuit the methane hydrate is a condenser and the consequences of methane hydrate dissociation are depicted as a resistor and inductor reflecting temperature change and changes in earth surface history these consequences may have implications for global change including global climate change

distribution and height of methane bubble plumes on the cascadia margin characterized by acoustic imaging 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

chemistry isotopic composition and origin of a methane hydrogen sulfide hydrate at the cascadia subduction zone abstract although the presence of extensive gas hydrate on the cascadia margin offshore from the western u s and canada has been inferred from marine seismic records and pore water chemistry solid gas hydrate has only been found at one location at ocean drilling program odp site 892 offshore from central oregon gas hydrate was recovered close to the sediment water interface at 2 19 m below the seafloor mbsf at 670 m water depth the gas hydrate occurs as elongated platy crystals or crystal aggregates mostly disseminated irregularly with higher concentrations occurring in discrete zones thin layers and or veinlets parallel or oblique to the bedding a 2 to 3 cm thick massive gas hydrate layer parallel to bedding was recovered at 17 mbsf gas from a sample of this layer was composed of both ch4 and h2s this sample is the first mixed gas hydrate of ch4 h2s documented in odp it also contains ethane and minor amounts of co2 measured temperatures of the recovered core ranged from 2 to 1 8 c and are 6 to 8 degrees lower than in situ temperatures these temperature anomalies were caused by the partial dissociation of the ch4 h2s hydrate during recovery without a pressure core sampler during this dissociation toxic levels of h2s 34s 27 4 were released the 13c values of the ch4 in the gas hydrate 64 5 to 67 5 pdb together with d values of 197 to 199 smow indicate a primarily microbial source for the ch4 the 18o value of the hydrate h2o is 2 9 smow comparable with the experimental fractionation factor for sea ice the unusual composition ch4 h2s and depth distribution 2 19 mbsf of this gas hydrate indicate mixing between a methane rich fluid with a pore fluid enriched in sulfide at this site the former is advecting along an inclined fault into the active sulfate reduction zone the facts that the ch4 h2s hydrate is primarily confined to the present day active sulfate reduction zone 2 19 mbsf and that from here down to the bsr depth 19 68 mbsf the gas hydrate inferred to exist is a 99 ch4 hydrate suggest that the mixing of ch4 and h2s is a geologically young process because the existence of a mixed ch4 h2s hydrate is indicative of moderate to intense advection of a methane rich fluid into a near surface active sulfate reduction zone tectonically active faulted margins with organic rich sediments and moderate to high sedimentation rates are the most likely regions of occurrence the extension of such a mixed hydrate below the sulfate reduction zone should reflect the time span of methane advection into the sulfate reduction zone

enhanced marine ch4 emissions to the atmosphere off oregon caused by coastal upwelling methane in surface waters and marine air off oregon 44 24 n 44 54 n 124 36 w 125 24 w was continuously surveyed in july 1999 during a high resolution survey after a period of steady winds from the north ch4 concentrations were high in the northeastern region near the shelf edge the highest ch4 concentrations were 2 5 times higher than equilibrium with the atmospheric partial pressure in contrast concentrations were near equilibrium in the western part of the survey area the hydrate ridge the increase in ch4 from southwest to northeast correlates with a drop in sea surface temperature sst from 16 5 c to 13 5 c toward the shelf edge the observed sst pattern was caused by summer upwelling off oregon the results suggest that ch4 derived from bottom sources near the shelf slope break and methane found in connection with shallow 100 300 m turbidity layers is transported to the surface by coastal upwelling which causes an enhanced net flux of ch4 to the atmosphere vertical profiles of the methane distribution on the shelf in october demonstrate the accumulation of methane introduced by shelf sources surface concentrations at these stations in october during nonupwelling conditions were lower than in july during upwelling and were only slightly oversaturated with respect to the atmosphere an acoustic doppler current profiler survey indicates that the observed trend cannot be attributed to a surface flow reversal in the area the low salinity waters in the core of the columbia river plume s 31 showed no enhanced ch4 concentrations the trend of higher ch4 concentrations at lower temperatures existed over the whole 17 day survey but large spatial and temporal variations existed the presence of methane sources in regions of coastal upwelling worldwide such as shallow seeps gas hydrates and intermediate nepheloid layers suggests that the enhancement of ch4 fluxes to the atmosphere by coastal upwelling occurs on a global scale

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

variability of internal frontal bore breaking above opouawe bank methane seep area new zealand large internal wave breaking is observed exceeding a vertical array of 61 high resolution temperature sensors at 1 m intervals between 7 and 67 m above the bottom the array was moored for 5 days at 969 m of opouawe bank new zealand a known methane seep area as breaking internal waves dominate sediment resuspension above sloping topography in other ocean areas they are expected to also influence methane transport despite being visible in single beam echosounder data indications for turbulence due to rising gas bubbles are not found in the present 1 hz sampled temperature records likely the mooring was too far away from the very localized bubble release spot instead the temperature sensors show detailed internal wave turbulence transitions every tidal cycle a solibore a frontal turbulent bore with a train of trailing solitary waves changes shape and intensity these solibores are highly turbulent and they restratify the bottom boundary layer thereby maintaining efficient mixing details of different turbulent bore developments are discussed averaged over a few tidal cycles and over the sensors range mean vertical eddy diffusivity amounts to 3 1 10 3 m2 s 1 and mean turbulent kinetic energy dissipation to 1 6 0 7 10 7 w kg 1 with variations over 4 orders of magnitude such turbulence will affect the distribution of dissolved methane and other geochemical species in the lower 100 150 m above the bottom and their release from the bottom the above mean values are remarkably similar to those found at various other sites in the ne atlantic ocean

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

temporal and spatial evolution of a gas hydrate bearing accretionary ridge on the oregon continental margin a seismic reflection survey on the oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin the seismic signature of gas hydrate a bottom simulating reflector bsr with negative polarity that locally cuts across stratigraphic horizons is especially well developed beneath hydrate ridge this anomalously shallow accretionary ridge was drilled during ocean drilling program leg 146 to study fluid venting in this paper we focus on the seismic data from the southern part of hydrate ridge where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system apparent disruptions of the bsr beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping a double bsr beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding on the basis of these and other observations we propose a qualitative model for the evolution of a hydrate bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor permitting construction of large ridges that are then eaten away by slumps along their margins the north to south variation in sea floor venting and subsurface seismic structure along hydrate ridge may reflect different stages in the temporal evolution of one of these ridges

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

estimates of methane output from mud extrusions at the erosive convergent margin off costa rica four mud extrusions were investigated along the erosive subduction zone off costa rica active fluid seepage from these structures is indicated by chemosynthetic communities authigenic carbonates and methane plumes in the water column we estimate the methane output from the individual mud extrusions using two independent approaches the first is based on the amount of ch4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities which ranges from 104 to 105 mol yr 1 the remaining portion of ch4 which is released into the ocean has been estimated to be 102 104 mol yr 1 per mud extrusion the second approach estimates the amount of ch4 discharging into the water column based on measurements of the near bottom methane distribution and current velocities this approach yields estimates between 104 105 mol yr 1 the discrepancy of the amount of ch4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth e g focused fluid emission through channels in sediments and fractures in carbonates extrapolated over the 48 mud extrusions discovered off costa rica we estimate a ch4 output of 20 106 mol yr 1 from mud extrusions along this 350 km long section of the continental margin these estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins almost half of the continental margins are described as non accretionary assuming that the moderate emission of methane at the mud extrusions off costa rica are typical for this kind of setting then global estimates of methane emissions from submarine mud extrusions which are based on data of mud extrusions located at accretionary and passive continental margins appear to be significantly too high

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

analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope cascadia margin sediments contain a rich reservoir of carbon derived both from terrestrial input and sea surface productivity a portion of this carbon exists as solid gas hydrate within sediment pore spaces which previous studies have shown to be a methane reservoir of substantial size on both the vancouver island and oregon portions of the cascadia margin multichannel seismic reflection profiles on the cascadia margin show the widespread presence of bottom simulating reflectors bsrs within the sediment column indicating the gas hydrate reservoir extends from the deformation front at 3000 m depth to the upper limit of gas hydrate stability near 500 m water depth in this study we compile an inventory of methane bubble plume sites on the cascadia margin identified in investigations carried out for a range of interdisciplinary goals that also include sites volunteered by commercial fishermen high plume density anomalies are associated with both the continental shelf 180 m and the depth of the upper limit of methane hydrate stability depth mhsd that occurs near 500 m in the ne pacific the observed anomalies on the cascadia slope may be due to the warming of seawater at intermediate depths suggesting that modern climate change has begun to destabilize the climate sensitive hydrate reservoir within the cascadia margin sediments reanalysis of similar plume images on the north american atlantic slope suggests a lack of correlation between observed plume depths and the mhsd for much of the latitudinal range

methane in the northern atlantic controlled by microbial oxidation and atmospheric history during may august 1997 the distributions of dissolved methane and ccl3f cfc11 were measured in the atlantic between 50 and 60 n in surface waters throughout the region methane was observed to be close to equilibrium with the atmospheric mixing ratio implying that surface ocean methane is tracking its atmospheric history in regions of north atlantic deep water formation despite the different atmospheric history and ocean chemistry of ch4 and cfc11 their spatial distribution patterns in the water column are remarkably similar one dimensional distributions have been simulated with an advection diffusion model forced by the atmospheric histories the results suggest that the similar patterns result from the increasing input of ch4 and cfc11 to newly formed deep waters over time combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale

oceanic methane layers the hydrocarbon seep bubble deposition hypothesis bubble plumes from hydrocarbon seeps drive upwelling flows in the water column that can disappear if the bubbles dissolve this may lead to formation of a layer enriched in gases and substances transported by the bubbles a process we term bubble deposition a review of observed dissolved methane layers in the north sea showed their existence in an area of active seeping pockmarks at a height of 20 30 m above the sea bed well below the thermocline to test the bubble deposition hypothesis rising seep bubbles were simulated numerically the model predicted a dissolution depth consistent with the observed methane layer for 2700 m radius bubbles the model also predicted that bubbles smaller than 3400 m dissolved subsurface decreasing to 2000 m for a 10 cm s 1 upwelling flow we speculate that this layer may be attractive to marine organisms although north sea seeps are not oily this mechanism also applies to oily bubbles from hydrocarbon seeps or a leaking undersea gas oil pipeline thus bubble deposition can create a subsurface oil layer which rises far slower than either the bubble stream or droplets entrained in the stream

spatial variation in fluid flow and geochemical fluxes across the sediment seawater interface at the carlos ribeiro mud volcano gulf of cadiz submarine mud volcanism is an important pathway for transfer of deep sourced fluids enriched in hydrocarbons and other elements into the ocean numerous mud volcanoes mvs have been discovered along oceanic plate margins and integrated elemental fluxes are potentially significant for oceanic chemical budgets here we present the first detailed study of the spatial variation in fluid and chemical fluxes at the carlos ribeiro mv in the gulf of cadiz to this end we combine analyses of the chemical composition of pore fluids with a 1 d transport reaction model to quantify fluid fluxes and fluxes of boron lithium and methane across the sediment seawater interface the pore fluids are significantly depleted in chloride but enriched in lithium boron and hydrocarbons relative to seawater pore water profiles of sulphate hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34 180 cm depth below seafloor clay mineral dehydration and in particular the transformation of smectite to illite produces pore fluids that are depleted in chloride and potassium profiles of boron lithium and potassium are closely related which suggests that lithium and boron are released from the sediments during this transformation pore fluids are expelled into the water column by advection fluid flow velocities are 4 cm yr 1 at the apex of the mv but they rapidly decrease to 0 4 cm yr 1 at the periphery the associated fluxes of boron lithium and methane vary between 7 301 0 5 6 and 0 806 mmol m 2 yr 1 respectively we demonstrate that fluxes of li and b due to mud volcanism may be important on a global scale however release of methane into the overlying water column is suppressed by microbial methanotrophy

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

on the calculation of total heat salt and tracer fluxes from ocean hydrothermal events abstract total heat salt and other tracer masses released during a hydrothermal event are shown to be proportional to but not necessarily equal to volume integrals of resulting water column anomalies proportionality coefficients depend on anomaly definition on background hydrographic and tracer profiles on expansion coefficients of the equation of state at an appropriate pressure and on tracer to heat anomaly ratios at the venting source for gorda ridge event plumes which are described in other papers of this issue volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2 4 if the discharge is not anomalously saline under certain combinations of hydrographic and source anomaly conditions not unlike those found on the east pacific rise at 10 n the apparent total heat released during an event can be deceivingly zero this analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source one consequence is that the ratio of anomalous 3 he to heat in gorda ridge event plumes is approximately 2 4 larger in the water column than it is at the source results are independent of the entrainment process involved in event plume formation and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth

continuity of the poleward undercurrent along the eastern boundary of the mid latitude north pacific abstract several recent data sets improve our view of the poleward undercurrent of the california current system as part of a triennial national marine fisheries service nmfs survey of pacific whiting a series of 105 shipboard acoustic doppler current profiler adcp velocity sections across the shelf break from 33 to 51 n at about 18 km meridional spacing were collected from july to august 1995 significant 0 05 m s 1 subsurface poleward flow occurred in 91 of the sections a mean cross shelf section using the entire data set has statistical significance revealing an undercurrent core 0 1 m s 1 from 200 275 m depth 20 25 km off the shelf break the mean poleward volume transport in a 125 325 m layer is 0 8 0 2 10 6 m 3 s 1 we focus particular attention on the cape blanco to cape mendocino region and the nmfs results are compared with shipboard adcp three weeks later from a study of coastal upwelling processes near cape blanco adcp streamfunction maps are derived and strongly suggest that one portion of flow is continuous over the 440 km meridional extent of the analysis region other portions of the flow show evidence of offshore turning separation and the formation of anti cyclonic eddies we also note that isopycnic potential vorticity from alongslope ctd stations during the nmfs survey appears to be a tracer for the poleward flow

methane seepage along the hikurangi margin of new zealand geochemical and physical data from the water column sea surface and atmosphere the concentration and carbon isotope values of dissolved methane were measured in the water column at rock garden omakere ridge and wairarapa areas in the first dedicated cold seep investigation along the hikurangi margin of new zealand these measurements provide a high resolution impression of the methane distribution in the water column and show that these seep sites are actively venting methane with varying intensity the highest concentrations up to 3500 nm measured in water samples obtained from conductivity temperature depth ctd operations were at faure site of rock garden here seafloor bubble release was observed by rov the omakere ridge area is actively venting over almost its entire length not vert similar 25 km in particular at bear s paw a newly discovered seep site in the wairarapa area another new seep site called tui was discovered where methane measurements often exceeded 500 nm no evidence was obtained from water column or sea surface measurements along the hikurangi margin to indicate that methane from seeps is reaching the sea surface in fact a consistent upper boundary was observed at a density of 26 85 kg m3 which occurs at about 500 m below sea surface above which methane decreased to background concentrations no obvious oceanographic feature is associated with this 500 m ch4 boundary bubble dissolution calculations show that about 500 m was also the model derived maximum bubble rise height a wide range of 13cch4 values from 71 to 19 vpdb were measured with the highest ch4 concentrations having the lowest 13cch4 values of about 71 to 68 simple mixing and isotope fractionation calculations show that changes of 13cch4 values are predominantly caused by the dilution of seep fluids with the seawater with some anaerobic oxidation also occurring

estimating the thickness of the free gas zone beneath hydrate ridge oregon continental margin from seismic velocities and attenuation recent ocean drilling program odp results in the oregon accretionary prism and on the blake ridge indicate that the zone containing free gas beneath the hydrate bearing near surface sediments is considerably thicker than previously thought in this paper we present results from travel time inversion of refracted seismic waves that show very low 100 in the normal accretionary complex sediments west of hydrate ridge these results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change

quantification of gas bubble emissions from submarine hydrocarbon seeps at the makran continental margin offshore pakistan 1 evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 khz echosounder records flares was obtained at the convergent makran continental margin the hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column dives with the remotely operated vehicle quest 4000 m revealed that several individual bubble vents contributed to one hydroacoustic anomaly analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume we found that a low volume flux flare 1 at 575 mbsl 90 ml min caused a weak hydroacoustic signal in echograms whereas high volume fluxes flare 2 at 1027 mbsl 1590 ml min flare 5 c at 2870 mbsl 760 ml min caused strong anomalies the total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies an order of magnitude estimate further considers the temporal variability of some of the flares assuming a constant flux over time and allows a large range of uncertainty inherent to the method our results on the fate of bubbles and the order of magnitude estimate suggest that all of the 40 32 106 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean

upper ocean water mass characteristics of the california current summer 1993 abstract two large high resolution upper ocean surveys of the california current region were conducted in the summer of 1993 temperature and salinity were measured from a seasoar vehicle and velocity was measured by a ship borne acoustic doppler current profiler both surveys extended from the continental margin to 128 w and consisted of zonal sections 28 km apart the first survey 7 28 june extended from 39 5 n to 36 25 n and the second 16 august to 1 september extended from 39 0 n to 37 0 n the current field was fairly simple in june showing an equatorward surface jet with only one gentle meander a nearly continuous poleward california undercurrent near the continental margin and two anticyclonic subsurface eddies offshore of the jet by august meanders in the surface jet had strengthened and eddies dominated the flow field large scale trends in the water mass characteristics spiciness along three isopycnal surfaces 26 4 26 0 and 25 6 kg m 3 show that temperature and salinity tend to increase towards the equator and towards shore consistent with poleward advection along the continental margin local minima in spiciness tend to coincide with the equatorward surface jet local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m and seem to originate in the california undercurrent over the continental slope at local latitudes we identified several different anticyclonic eddies each with different water mass characteristics all of the eddies we observed had diameters larger than the baroclinic radius of deformation water mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6 8 months

a multioffset vertical seismic profiling experiment for anisotropy analysis and depth imaging a multioffset vertical seismic profile vsp was carried out in the rocky mountain foothills of southern alberta canada the purpose of this experiment was to investigate whether the dipping shale strata exhibit p wave velocity anisotropy and if so to calculate the thomsen anisotropy parameters for use in anisotropic depth migration traveltime inversion of first arrival data from the multioffset vsp revealed that the dipping mesozoic clastics in the area exhibit seismic velocity anisotropy of about 10 the anisotropy parameters derived from this experiment were then used in anisotropic prestack depth migration of data from a surface seismic line close to the vsp well comparison of the anisotropic migration with the corresponding isotropic prestack depth migration showed that the target was imaged incorrectly in the isotropic case a lateral shift of 180 m in the updip direction of the overlying beds was observed the image obtained with an anisotropic velocity model was also better focused than that obtained assuming isotropic velocities

massive dissociation of gas hydrate during a jurassic oceanic anoxic event in the jurassic period the early toarcian oceanic anoxic event about 183 million years ago is associated with exceptionally high rates of organic carbon burial high palaeotemperatures and significant mass extinction heavy carbon isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon which is isotopically light in contrast examples of light carbon isotope values from marine organic matter of early toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12c versus 13c refs 1 2 5 6 here however we report carbon isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic biospheric and atmospheric carbon reservoirs and that this occurred despite the enhanced burial of organic carbon we propose that as has been suggested for the late palaeocene thermal maximum some 55 million years ago the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental margin sediments

coastal upwelling in the california current system abstract coastal upwelling in the california current system has been the subject of large scale studies off california and baja california and of small scale studies off oregon recent studies of the winds along the entire coast from 25 n to 50 n indicate that there are significant along shore variations in the strength of coastal upwelling which are reflected in the observed temperature distribution active upwelling appears to be restricted to a narrow coastal band about 10 25 km wide along the entire coast but the region influenced by coastal upwelling may be much wider intensive observations of the upwelling zone during summer off oregon show the presence of a southward coastal jet at the surface a mean vertical shear a poleward undercurrent along the bottom and persistently sloping isopycnals over the continental shelf most of the upwelling there occurs during relatively short periods several days long of upwelling favorable winds during the upwelling season off oregon the offshore ekman transport is carried by the surface ekman layer and the onshore return flow occurs through a quasi geostrophic interior it is not known whether the structure and dynamics observed off oregon are typical of the upwelling zone along the entire coast though some of the same features have been observed off baja california current and future research will eventually show whether the oregon results are also applicable in the region of persistently strong upwelling favorable winds off northern california and in the region of complex bathymetry off central and southern california

a poleward jet and an equatorward undercurrent observed off oregon and northern california during the 1997 98 el nino abstract the timing and intensity of the effects of the 1997 98 el nino on sea surface temperature sst and coastal sea level along the us west coast are examined using in situ time series measurements and the effects on upper ocean currents on the continental shelf and slope off oregon and northern california are examined using repeated shipborne adcp transects a mid shelf mooring off newport oregon and an hf surface current radar an initial transient positive anomaly was observed in both adjusted sea level and sst during may june 1997 followed by anomalously high coastal sea levels generally strongest during september 1997 through february 1998 and abruptly returned to normal in late february 1998 and by positive temperatures anomalies over the mid shelf that persisted longer into april 1998 low frequency coastal sea level anomalies propagated poleward at 2 1 m s poleward flow over the shelf and slope was enhanced at most depths during the el nino compared with following years northward currents in the upper 12 m over the continental shelf off newport oregon averaged 13 7 cm s stronger during august 1997 through february 1998 than during the same period the following year enhanced poleward flow was present at all latitudes sampled during november 1997 and february 1998 particularly over the continental slope these transects also provided clear views of a fall winter equatorward undercurrent which was both strongest and had the most alongshore similarity of form during the enso finally subsurface intensified anticyclonic eddies originating in the poleward undercurrent appear to be a recurrent feature of the circulation off newport late in the upwelling season

measurements of transience and downward fluid flow near episodic methane gas vents hydrate ridge cascadia aqueous flux measurements within cold seep regions on northern hydrate ridge cascadia indicate a high degree of variability with extended periods of downflow and reversals of flow direction over periods of weeks to months local episodic venting of free methane gas was also observed the instruments recorded similar changes in hydrologic flow patterns both on and off clam fields the magnitude of the flow rates decreasing away from the clam field the coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled we propose that these observations are consistent with the action of a gas driven pump that operates somewhat like a geyser the hypothesis of gas driven pumping of seawater through northern hydrate ridge has important ramifications for the mass fluxes through this region

towards quantifying the reaction network around the sulfate methane transition zone in the ulleung basin east sea with a kinetic modeling approach abstract we present a kinetic model based upon pore water data collected from eight sites drilled during the second ulleung basin gas hydrate drilling expedition ubgh2 in 2010 three sites were drilled at locations where acoustic chimneys were identified in seismic data and the rest were drilled on non chimney i e background environments our model coupled a comprehensive compositional and isotopic data set is used to illustrate the different biogeochemical processes at play in those two environments in terms of reactions around the sulfate methane transition zone smtz organic matter decomposition is an important process for production of methane dissolved inorganic carbon dic and consumption of sulfate in the non chimney sites whereas anaerobic oxidation of methane aom dominates both carbon and sulfur cycles in the chimney environment different sources of methane mediate aom in the two settings internally produced methane through co 2 reduction cr and methanogenesis fuels aom in the non chimney sites whereas aom is sustained by methane from external sources in the chimney sites we also simulate the system evolution from non chimney to chimney conditions by increasing the bottom methane supply to a non chimney setting we show that the higher ch 4 flux leads to a higher microbial activity of aom and more organic matter decomposition through methanogenesis a higher methanogenesis rate and a smaller cr contribution relative to aom in the chimney sites is responsible for the isotopically light dic and heavy methane in this environment relative to the non chimney sites

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

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

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

theoretical simulation of the evolution of methane hydrates in the case of northern south china sea since the last glacial maximum the distribution and inventory of gas hydrates in a region is determined by the sediment characteristics methane supply and evolution of the reservoir in recent decades the geo environmental constraints and sources of methane have been intensively investigated and numerous experimental and numerical simulation tools have been developed to evaluate the inventory of hydrates however information regarding the evolution of hydrate reservoirs remains limited this study developed a simulator to theoretically model the evolution of specific reservoirs since the last glacial maximum lgm the lgm was a recent cold epoch that occurred approximately 18 000 years ago since the lgm the earth s climate system has experienced a continuous increase of surface temperature and rising sea level given a sufficient supply of methane and a transport system hydrates may form in marine sediments if sea level rises or melt if the temperature increases a one dimensional simulator that represents the sediment was developed and uses the current hydrate profiles as the initial conditions and reliable paleoenvironmental data obtained in sites located in the northern shelf of the south china sea scs as boundary conditions two types of hydrate profiles were reversely simulated till the lgm 1 a gaussian profile which was observed in the shenhu area and 2 a trapezoidal profile which was observed in the dongsha area scs the evolution and past quantities of local hydrate reservoirs were estimated the model results demonstrated that shallow 500 700 m below the seafloor or mbsf moderate saturation 50 pore volume or v v hydrate deposits will form if they are subjected to recent climate changes the inventory of hydrates in the nscs increased by only 0 7 over the past 18 000 years under various scenarios a sensitivity analysis was performed to identify the most pertinent parameters that control the formation and dissociation of hydrates including the grain size temperature gradient and deposition depth the distribution and depth of the reservoir were determined to be the most critical factors in the evolution of the studied hydrates

chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume the aging of the hydrothermal plume over the endeavour segment of the juan de fuca ridge was estimated by measuring the222rn3he ratio in the plume as it dispersed despite uncertainties in the source function of hydrothermal input it wa determined that the relative sequence of removal from the plume ish2 c 222rn ch4 mn where c is a measure of particle concentration and the mean life of222rn is 5 5 days h2 is removed from the plume within hours of input while mn is not removed within the two week timescale of the radon helium clock entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone this is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient like profile in the deep ocean thus we found that significant fractions of the si and226ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height

temporal variation of methane flares in the ocean above hydrate ridge oregon abstract while bubble plumes have been acoustically imaged in the water column above marine gas hydrate deposits in many studies little is known about the temporal variation in plume intensity in july 2008 we conducted surveys using 3 5 and 12 khz echosounders and a 75 khz acoustic doppler current profiler adcp over the northern and southern summits of hydrate ridge on the cascadia continental margin our study included multiple surveys at both sites including a survey of the northern summit that was repeated 16 times in 19 h seafloor depth at the northern summit is 600 m well within the hydrate stability zone hsz which was below 510 m during our survey based on ctd data three distinct flares a term used to denote the acoustic signature of bubble plumes were detected at northern hydrate ridge nhr and one was detected at southern hydrate ridge shr coincident with where flares were observed a decade ago indicating that the supply of gas is stable on this time scale high resolution bathymetric surveys of nhr and shr acquired with an autonomous underwater vehicle auv flown 50 m above the seafloor indicate that flare locations are correlated with a distinctive pattern of short wavelength seafloor roughness supporting the inference of long term stability in the location of bubble expulsion sites as in previous studies at hydrate ridge flares were not detected with the 3 5 khz echosounder but were clearly imaged at 12 khz by reprocessing routine shipboard adcp data we show that they are also observed in at 75 khz indicating that a wide range of bubble sizes is present the intensity of the flares varied strongly with time two primary sources for flares were observed one located on the regional topographic high showed continuous activity with two times periods of particularly strong flares that are not correlated with tidal height the other located on a local topographic high shows a pulse of increased backscatter that occurred on a falling tide while the time period of observation is not enough to constrain the effect of tidal changes in seafloor pressure on venting the data suggest that tides are not the dominant factor controlling release of bubbles from the seafloor the data support previously suggested models in which temporary sealing of vents by gas hydrate formation and breaking of these barriers as gas pressure builds up is responsible for burp like pulses of gas expulsion we also report the first observations of flares originating within the hsz that extend well above the hsz with little loss of backscatter intensity these represent a new source of methane injection into the upper ocean and possibly the atmosphere flare extension above the hsz may be due to coating of some bubbles by oil or biofilms or to inclusion of particulate matter possibly including floating pieces of hydrate in the plumes although this study provides tantalizing new information on both short term variability in gas expulsion rate and long term stability of vent sites longer well calibrated observations that integrate bubble flux over entire vent fields as a function of time are needed to develop accurate models for the flux of methane into the ocean and atmosphere from seafloor methane vents

archaea mediating anaerobic methane oxidation in deep sea sediments at cold seeps of the eastern aleutian subduction zone cold seeps in the aleutian deep sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation this process is active in the anaerobic sediments at the sulfate reduction methane production boundary and is probably performed by archaea working in syntrophic co operation with sulfate reducing bacteria diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2 6 11 15 tetramethylhexadecane crocetane and 2 6 10 15 19 pentamethylicosane pmi as well as the glycerol ether lipid archaeol 2 3 di o phytanyl sn glycerol these biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates carbon isotopic compositions of the biomarkers are strongly depleted in 13c with values of 13c as low as 130 3 pdb the process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with 13c values of 39 2 and 41 8 and of the carbonate precipitates with values of 45 4 and 48 7 this suggests that methane oxidizing archaea have accumulated within the microbial community which is active at the cold seep sites the dominance of crocetane in sediments at one station indicates that probably due to decreased methane venting archaea might no longer be growing whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane the assemblages are apparently dependent on specific conditions found at each cold seep environment selective conditions probably include water depth temperature degree of anoxia and supply of free methane

observations of boundary mixing over the continental slope observations of mixing over the continental slope using a towed body reveal a great lateral extent several kilometers of continuously turbulent fluid within a few hundred meters of the boundary at depth 1600 m the largest turbulent dissipation rates were observed ove ra5k mhorizontal region near a slope critical to the m2 internal tide over a submarine landslide perpendicular to the continental slope enhanced mixing extended at least 600 m above the boundary increasing toward the bottom the resulting vertical divergence of the heat flux near the bottom implies that fluid there must be replenished intermediate nepheloid layers detected optically contained fluid with u s properties distinct from their surroundings it is suggested that intermediate nepheloid layers are interior signitures of the boundary layer detachment required by the near bottom flux divergance

lagrangian measurement of subsurface poleward flow between 38 n and 43 n along the west coast of the united states during summer 1993 subsurface lagrangian measurements at about 140 m showed that the path of the california undercurrent lay next to the continental slope between san francisco 37 8 n and st george reef 41 8 n during mid summer 1993 the mean speed along this 500 km path was 8 cms 1 the flow at this depth was not disturbed by upwelling centers at point reyes or cape mendocino results also demonstrate the ability to acoustically track floats located well above the sound channel axis along the california coast

a survey of methane isotope abundance 14c 13c 2h from five nearshore marine basins that reveals unusual radiocarbon levels in subsurface waters methane ch4 in the subsurface ocean is often supersaturated compared to equilibrium with the modern atmosphere in order to investigate sources of ch4 to the subsurface ocean isotope surveys 14c ch4 13 c ch4 2 h ch4 were conducted at five locations skan bay sb santa barbara basin sbb santa monica basin smb cariaco basin cb and the guaymas basin gb depth distributions of ch4 concentration and isotopic abundance were determined for both the sediment and water column at the sb sbb smb and cb sites ch4 emitted from seeps on the continental shelf adjacent to the sbb as well as seeps and decomposing clathrate hydrates in the gb was also collected purified and analyzed methane isotope distributions in the sediments were consistent with known methanogenic and methanotrophic activity seep and clathrate hydrate derived ch4 was found to be depleted in radiocarbon however surprising results were obtained in the water column at all sites investigated in sb the radiocarbon content of the subsurface ch4 concentration maximum was on average 41 less than its suspected sediment ch4 source suggesting ch4 seepage in the bay in the sbb smb and cb the 14c ch4 contents in the subsurface ocean were 1 2 to 3 6 times greater than modem carbon quantities suggesting a source of 14c from atmospheric nuclear weapons testing nuclear power plant effluents or cosmogenic isotope production copyright 2008 by the american geophysical union

estimation of heat and chemical fluxes from a seafloor hydrothermal vent field using radon measurements the circulation of seawater through newly formed ocean crust at mid ocean ridge spreading centres is important in the oceanic heat and chemical budgets the heat transfer in submarine hydrothermal systems accounts for 25 of the total global heat loss1 the transfer of mass resulting from basaltic alteration affects the geochemistry of seawater2 and is responsible for the formation of ore deposits on and beneath the seafloor3 various geophysical techniques have been employed in efforts to determine the heat output from hydrothermal vent fields4 9 however the magnitudes of the heat and chemical fluxes through these systems remain uncertain here we introduce a geochemical approach for estimating the flux from a vent field based on radon 222rn measurements in the overlying effluent plume this method was applied successfully in september 1986 during a 23 day expedition to an active vent field on the 170 km endeavour segment of the juan de fuca ridge fig la we estimate the heat flux from this site to be l 5 l09 w

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

stable carbon isotopic evidence for methane oxidation in plumes above hydrate ridge cascadia oregon margin 1 the transport and consumption of methane in the water column in the vicinity of the cold seeps of hydrate ridge on the cascadia oregon margin were characterized using measurements of the stable carbon isotope composition of methane the 13c ch4 values measured in the water column ranged from approximately 65 to 16 pdb the combination of measured methane concentration data and the stable carbon isotope values from the same depths support the hypothesis of biogenically produced methane which enters the water column from dissolving bubbles released from cold seepages likely as a consequence of destabilized methane hydrate 13c ch4 65 pdb kinetic fractionation factors associated with aerobic bacterial methane oxidation in the water column were calculated using a rayleigh distillation equation applied to a subset of the data fractionation factors ranged from 1 002 to 1 013 mean 1 008 and were in the lower end of the range of those reported in the literature a result likely due to the influence of temperature and mixing in plume waters the fraction of methane remaining after oxidation calculated using the same rayleigh model approach suggests that the aerobic oxidation of methane in the water column over hydrate ridge is nearly quantitative

u th systematics and ages of authigenic carbonates from hydrate ridge cascadia margin recorders of fluid flow variations uranium u concentrations and activity ratios 234u of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon rich fluids cold seeps at hydrate ridge off the coast of oregon usa the low u concentrations mean 1 3 0 4 g g and high 234u values 165 317 of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor their 230th 234u ages span a time interval from 0 8 to 6 4 ka and cluster around 1 2 and 4 7 ka in contrast chemoherm carbonates precipitate from marine bottom water marked by relatively high u concentrations mean 5 2 0 8 g g and a mean 234u ratio of 166 3 their u isotopes reflect the 234u ratios of the bottom water being enriched in 234u relative to normal seawater simple mass balance calculations based on u concentrations and their corresponding 234u ratios reveal a contribution of about 11 of sedimentary pore water to the bottom water from the u pore water flux and the reconstructed u pore water concentration a mean flow rate of about 147 68 cm a can be estimated 230th 234u ages of chemoherm carbonates range from 7 3 to 267 6 ka 230th 234u ages of two chemoherms alvin and se knoll chemoherm correspond to time intervals of low sealevel stands in marine isotope stages mis 2 4 5 6 7 and 8 this observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments the 18o pdb ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times deviations of the chemoherm 18o values from the marine 18o record can be interpreted as to reflect temporally and spatially varying bottom water and or vent fluid temperatures during carbonate precipitation between 2 6 and 8 6 c

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system

episodic methane concentrations at seep sites on the upper slope opouawe bank southern hikurangi margin new zealand along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere with numerous cold seeps already known the convergent hikurangi margin east of north island new zealand was selected as a target area for further detailed multidisciplinary investigation of cold seeps within the new vents and associated projects methane and temperature sensors mets were deployed at selected seep sites on the opouawe bank off the southeastern tip of north island and near the southern end of the imbricate thrust hikurangi margin together with seismic ocean bottom stations they remained in place for about 48 h while seismic data were collected the seeps were associated with seep related seismic structures methane concentrations were differing by an order of magnitude between neighbouring stations the large differences at sites only 300 m apart demonstrate that the seeps were small scale structures and that plumes of discharged methane were very localised within the bottom water high methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow methane discharge from the seafloor was episodic which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides the strong semi diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater despite dispersal by currents fluid flow through fissures fractures and faults close to the mets positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the mets small earthquakes do not appear to be correlated with seawater methane anomalies