distributions of gas hydrate and free gas accumulations associated with upward fluid flow in the sanriku oki forearc basin northeast japan abstract we applied automated velocity analysis to a 3d seismic data volume to obtain a high resolution seismic velocity model that we used to investigate the influence of fluid behavior on the subsurface distribution of gas in the sanriku oki forearc basin northeast japan we identified free gas accumulations as zones of low p wave velocity separated from overlying high velocity gas hydrates by the clear seismic boundary of the bottom simulating reflection bsr between them we then used conductive modeling to map upward heat flow in our study area from the depth of the bsr derived from the velocity model the estimated heat flow demonstrates that upward fluid flux has considerable influences on the distributions of both gas hydrate and free gas the areas of high heat flow representing high fluid flux correspond to underlying permeable geological features such as gas chimneys faults and the edges of porous slumps suggesting that these features provide fluid migration pathways from eocene to oligocene source rocks to free gas and hydrate accumulations in overlying sedimentary rocks the techniques we have employed can contribute to the high resolution mapping of gas hydrate and free gas accumulations in similar deep water reservoirs off the eastern continental margin of japan and at tectonically similar plate subduction margins in other regions

opening of the fram strait gateway a review of plate tectonic constraints abstract we have revised the regional crustal structure oceanic age distribution and conjugate margin segmentation in and around the lena trough the oceanic part of the fram strait between the norwegian greenland sea and the eurasia basin arctic ocean the lena trough started to open after eurasia greenland relative plate motions changed from right lateral shear to oblique divergence at chron 13 times 33 3 ma earliest oligocene a new bouguer gravity map supported by existing seismic data and aeromagnetic profiles has been applied to interpret the continent ocean transition and the influence of eocene shear structures on the timing of breakup and initial seafloor spreading assuming that the onset of deep water exchange depended on the formation of a narrow oceanic corridor the gateway formed during early miocene times 20 15 ma however if the initial lena trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep water circulation the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around chron 5 times 9 8 ma late miocene paleoceanographic changes at odp site 909 northern hovgard ridge are consistent with both hypotheses of gateway formation we cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the lena trough the gravity seismic and magnetic observations question the prevailing hypotheses on the yermak plateau and the morris jesup rise as eocene oceanic plateaus and the hovgard ridge as a microcontinent

gas hydrate distribution and hydrocarbon maturation north of the knipovich ridge western svalbard margin a bottom simulating reflector bsr occurs west of svalbard in water depths exceeding 600 m indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern north atlantic margin regional bsr mapping shows the presence of hydrate and free gas in several areas with the largest area located north of the knipovich ridge a slow spreading ridge segment of the mid atlantic ridge system here heat flow is high up to 330 mw m 2 increasing towards the ridge axis the coinciding maxima in across margin bsr width and heat flow suggest that the knipovich ridge influenced methane generation in this area this is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination to evaluate the source rock potential on the western svalbard margin we applied 1d petroleum system modeling at three sites the modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons the bulk petroleum mass produced since the eocene is at least 5 kt and could be as high as 0 2 mt most likely source rocks are miocene organic rich sediments and a potential eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the knipovich ridge the presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the svalbard margin varies spatially

gas hydrate reservoir and active methane venting province in sediments on 20 ma young oceanic crust in the fram strait offshore nw svalbard seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas hydrate stability zone ghsz the present day flux through these degassing systems is presumably at a low level on most rifted continental margins a pockmark field on the nw svalbard passive margin is located on young ocean crust 20 ma and shows evidence of ongoing episodic degassing new geophysical data from the vestnesa ridge 79 n a mounded and elongated sediment drift in the eastern fram strait reveal a gas hydrate free gas and venting system that is exceptionally more dynamic than documented elsewhere along the northeastern north atlantic margin the prominent bottom simulating reflection bsr about 200 mbsf separates anomalously high p wave velocities in the ghsz from a remarkable underlying low velocity zone indicating the presence of gas hydrate and gas in the pore space inversion of p wave velocity data using the differential effective medium theory yields a two dimensional concentration model of methane hydrate and free gas the model predicts saturations of up to 11 in the hydrate reservoir which due to the seafloor topography forms a large anticlinal permeability barrier below in the low velocity zone i e 1350 1500 m s up to 3 of free gas is predicted across the apex of the vestnesa ridge and in the immediate vicinity of extensional faults a conservative estimate indicates that 225 kg m2 of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column an elongated pockmark field consisting of 100 individual pockmarks up to 600 m wide systematically aligns the apex of the vestnesa ridge active vigorous degassing from the topography controlled pressure valve system was evident from a 750 m high and 150 m wide gas flare observed in the water column during a cruise with r v jan mayen in october 2008 the gas flare documents dynamic degassing through the corresponding chimney which penetrates the entire ghsz and into the underlying free gas zone cruises in 2006 and 2007 did not detect active gas venting above the pockmark field accordingly vigorous degassing may operate in an episodic mode where hydrothermal circulation systems through young ocean crust may play a significant role

seismic evidence for shallow gas escape features associated with a retreating gas hydrate zone offshore west svalbard active gas venting occurs on the uppermost continental slope off west svalbard close to and upslope from the present day intersection of the base of methane hydrate stability bmhs with the seabed in about 400 m water depth in the inter fan region between the kongsfjorden and isfjorden cross shelf troughs from an integrated analysis of high resolution two dimensional pre stack migrated seismic reflection profiles and multibeam bathymetric data we map out a bottom simulating reflector bsr in the inter fan region and analyze the subsurface gas migration and accumulation gas seeps mostly occur in the zone from which the bmhs at the seabed has retreated over the recent past 1975 2008 as a consequence of a bottom water temperature rise of 1 c the overall margin parallel alignment of the gas seeps is not related to fault controlled gas migration as seismic evidence of faults is absent there is no evidence for a bsr close to the gas flare region in the upper slope but numerous gas pockets exist directly below the predicted bmhs while the contour following trend of the gas seeps could be a consequence of retreat of the landward limit of the bmhs and gas hydrate dissociation the scattered distribution of seeps within the probable hydrate dissociation corridor and the occurrence of a cluster of seeps outside the predicted bmhs limit and near the shelf break indicate the role of lithological heterogeneity in focusing gas migration

high resolution p cable 3d seismic imaging of gas chimney structures in gas hydrated sediments of an arctic sediment drift the newly developed p cable 3d seismic system allows for high resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions gas chimneys shallow gas and gas hydrate reservoirs seismic imaging of a geofluid system of an arctic sediment drift at the vestnesa ridge offshore western svalbard provides significantly improved details of internal chimney structures from the seafloor to not vert similar500 m bsf below seafloor the chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments the pockmarks are not buried and align at the ridge crest pointing to recent topography controlled fluid discharge chimneys are fuelled by sources beneath the base of gas hydrate stability zone ghsz that is evident at not vert similar160 170 m bsf as indicated by a bottom simulating reflector bsr conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments episodically active pressure driven focused fluid flow could explain the hydro fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the vestnesa ridge high amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near surface gas hydrates and or authigenic carbonate precipitation acoustic anomalies expressed as high amplitudes and amplitude blanking are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and or carbonate formation in space and time

gas origin and migration in the ulleung basin east sea results from the second ulleung basin gas hydrate drilling expedition ubgh2 abstract we have investigated the gas chemistry of marine sediments up to 250 m below seafloor m bsf recovered during the second ulleung basin gas hydrate drilling expedition ubgh2 to identify and compare the gas origin and its migration at seismically non chimney truncated chimney and chimney sites in this basin the molecular and isotopic data c1 c2 ratios 300 13 c ch 4 d ch 4 kim et al 2012 and thought to reflect two phase methane transport processes in chimney sites deep sourced methane migrates independently of the carbon dioxide which remains dissolved in the aqueous phase leading to the extremely low c values observed in the upper 30 m bsf upward methane diffusion at non chimney sites may also add a transport induced fractionation as evidenced in mixing diagrams but this process does not significantly affect the c values 13 c co 2 shows usually minimum values at the sulfate methane transition zone smtz at all sites however in chimney sites these values have a much more depleted signature indicating faster rates of methane consumption by anaerobic oxidation of methane aom supported by the higher methane flux moreover here the massive hydrate that forms in shallow sediments incorporates gases with a lower c1 c2 ratio higher c2 c1 than the surrounding sediment as documented in hydrate bound gas bg and void gas vg samples gas hydrate dissociation during the core recovery is released enriched ethane c2h6 trapped in gas cage which leads to have high c2h6 in vg samples

abiotic methane from ultraslow spreading ridges can charge arctic gas hydrates biotic gas generation from the degradation of organic carbon in marine sediments supplies and maintains gas hydrates throughout the world s oceans in nascent ultraslow spreading ocean basins methane generation can also be abiotic occurring during the high temperature 200 c serpentinization of ultramafic rocks here we report on the evolution of a growing arctic gas and gas hydrate charged sediment drift on oceanic crust in eastern fram strait a tectonically controlled deep water gateway between the subpolar north atlantic and arctic oceans ultraslow spreading ridges between northwest svalbard and northeast greenland permit the sustained interaction of a mid ocean ridge transform fault and developing sediment drift on both young 10 ma oceanic crust since the late miocene geophysical data image the gas charged drift and crustal structure and constrain the timing of a major 30 km lateral displacement of the drift across the molloy transform fault we describe the buildup of a 2 m y long lived gas hydrate and free gas charged drift system on young oceanic crust that may be fed and maintained by a dominantly abiotic methane source ultraslow spreading sedimented ridge flanks represent a previously unrecognized carbon reservoir for abiotic methane that could supply and maintain deep water methane hydrate systems throughout the arctic

heat flow variations correlated with buried basement topography on the juan de fuca ridge flank systematic heat flow variations are commonly observed on sedimented flanks of mid ocean ridges these variations have been attributed to the convective circulation of pore fluids within the oceanic crust and have been used to estimate the scale and pattern of this circulation here we report similar systematic variations observed in a detailed heat flow and seismic reflection survey over sediment covered 3 myr old sea floor on the eastern flank of the juan de fuca ridge we find that heat flow peaks are located above buried basement ridges and adjacent to two minor basement outcrops heat flow lows are found where thick sediments fill basement valleys the simple inverse relationship between heat flow and sediment thickness defined by the data indicates that hydrothermal circulation sealed within the permeable portion of the oceanic crust is vigorous enough to maintain approximately isothermal 10 k conditions at the sediment crust interface as no systematic variations in estimated basement temperatures are observed it seems that at this location the heat flow variations result only from sediment thickness variations and do not reflect a particular convective pattern

leveling arrays as multicomponent tiltmeters slow deformation in the new hebrides island arc two benchmark arrays located on the islands of santo and efate in the new hebrides island arc have been releveled 2 to 4 times per year during the past five years 1975 1980 each array is thereby used as a multicomponent tiltmeter the arrays are located within 35 50 km of the interplate thrust zone of this convergent plate boundary each array has an aperture of about 1 km and in addition contains small subarrays with apertures of about 70 m this configuration leads to a resolving power for tilt of about one microradian the most important result of this study is that at the devils point array on efate island there has been a consistent tilting down of about five microradians in a direction subparallel to the strike of the arc this tilting is clearly resolved by both the large array and by the small aperture sub arrays the observations are accounted for by a combination of planar tilting and random error consistent with the observed misclosures although tilting is clearly established at the devils point array it is not possible at this stage to establish a unique cause for the observed tilt hypotheses consistent with the tectonic setting include interseismic strain accumulation associated with the earthquake cycle of loading and interplate boundary rupture deformations near a transition between a creeping and a locked segment of the interplate boundary and near surface deformations associated with magmatic activity and or block movements also reflected in the pattern of faulting of the uplifted holocene coral terraces at the second array at ratard santo island no tilt trend clearly emerges from the noise level however two marginally significant tile excursions precede in each case large earthquakes located several hundred km north of the array at the ratard array the ground noise small wavelength elevation disturbances and benchmark instability contributes to residuals almost as much as the measurement error thereby degrading the threshold level for detection of tilt signals the difference between the two arrays is not clearly established but seems to reflect an inherently larger level of short wavelength ground noise associated with geological properties of the ratard site

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

complex gas hydrate system in a gas chimney south china sea abstract gas hydrates and deep sea cold seeps are of interest to industry and academia for several reasons including their implications for offshore geohazards and unconventional resources and for global climate change and carbon cycling guangzhou marine geological survey gmgs implemented its fifth gas hydrate drilling expedition in the qiongdongnan sea area qdn sa of the northern south china sea scs from june to september 2018 multidisciplinary investigations including logging while drilling lwd pressure and non pressure coring gas composition analyses in situ temperature and permeability measurements pore water geochemical analyses and a regional seismic survey reveal a complex distribution of gas hydrate within a vertical chimney structure lwd profiles record high resistivity anomalies and high p wave velocity variations at 9 174 mbsf suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney hydrate bound and void gas is an admixture of methane 70 1 98 3 and heavier hydrocarbons c2h6 0 16 17 59 c3h8 0 05 9 88 i c4h10

controls on gas hydrate system evolution in a region of active fluid flow in the sw barents sea abstract the location and stability of gas hydrates in the sw barents sea is poorly constrained due to complex geological geochemical and geophysical conditions including poor controls on regional heat flow and gas chemistry understanding the stability of gas hydrates in this region is important as recent studies suggest destabilizing hydrates may lead to methane discharge into the ocean and possibly in to the atmosphere here we use high resolution 3d p cable seismic data combined with 3d heat flow and fluid flow models to place new constraints on gas hydrate stability in this region the 3d p cable seismic data acquired in 2009 west of loppa high show cross cutting reverse polarity high amplitude reflectors interpreted as the base of gas hydrate stability to constrain heat flow fluid flow and gas hydrate stability within the 3d seismic volume we use a 3d steady state finite difference diffusive thermal model that incorporates regional bottom water temperature from ctd casts expected geothermal gradients and gas composition derived from well data in general modelled bottom simulating reflectors are deeper than observed bsrs our analysis weighs multiple factors that might explain the discrepancy between observed and modelled bottom simulating reflector depths from this analysis we propose that the most significant discrepancies in bsr depth are likely related to changes in regional fluid heat flow and fluid geochemistry the anomalously shallow bottom simulating reflectors can be explained via vertical fluid flow that might include ensuing potential effects on gas composition pore water salinity and temperature our estimate suggest that a maximum vertical fluid flux of approximately 12 mm y is necessary to explain the most significant anomalies our study provides new insight into regional heat flow geochemistry and end member vertical fluid flux rates in the barents sea moreover it documents that the fluid flow system is active and most likely very dynamic

methane generated pockmarks on young thickly sedimented oceanic crust in the arctic vestnesa ridge fram strait acoustic backscatter imagery in the fram strait between greenland and spitzbergen reveals a 1 3 km wide 50 km long belt of 50 pointlike backscatter objects decorating the 1300 m deep crest of vestnesa ridge a 1 2 km thick sediment drift possibly underlain by a transform parallel oceanic basement ridge crustal ages 3 14 ma a 3 5 khz seismic reflection profile indicates that at least some objects are pockmarks 100 200 m in diameter and 10 20 m deep the pockmarks possibly also mud diapirs may have been formed by evolution of methane generated by the decomposition of marine organic matter in the vestnesa ridge sediment drift the ridge may be underlain by an anticlinal carapace of methane hydrate calculated to be 200 300 m thick comparable to the hydrate thickness measured just to the south the rising methane would collect in the ridge crest trap intermittently escaping to the sea floor this hypothesis is supported by multichannel evidence for bright spots and bottom simulating reflectors in the area the pockmark belt may also be located above a transcurrent fault sediment slumps on the flanks of vestnesa ridge and northeast of molloy ridge may have been triggered by plate boundary earthquakes and facilitated by methane hydrates

detecting hydrate and fluid flow from bottom simulating reflector depth anomalies methane hydrates ice like compounds that consist of water and methane represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification however it remains unclear how much hydrate exists here using a newly developed three dimensional 3 d thermal technique we reveal a novel method for detecting and quantifying methane hydrate the analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter scale horizontal resolution our study located at hydrate ridge offshore oregon united states suggests that heat flow and hydrate concentrations are coupled and that 3 d thermal analysis can be used to constrain hydrate and fluid flow in 3 d seismic data hydrate estimates using this technique are consistent with 1 d drilling results but reveal large previously unrecognized swaths of hydrate rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies the 3 d analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three

detection of gas hydrates in faults using azimuthal seismic velocity analysis vestnesa ridge w svalbard margin accepted for publication in journal of geophysical research solid earth copyright 2020 american geophysical union further reproduction or electronic distribution is not permitted

rising arctic ocean temperatures cause gas hydrate destabilization and ocean acidification vast amounts of methane hydrates are potentially stored in sediments along the continental margins owing their stability to low temperature high pressure conditions global warming could destabilize these hydrates and cause a release of methane ch 4 into the water column and possibly the atmosphere since the arctic has and will be warmed considerably arctic bottom water temperatures and their future evolution projected by a climate model were analyzed the resulting warming is spatially inhomogeneous with the strongest impact on shallow regions affected by atlantic inflow within the next 100 years the warming affects 25 of shallow and mid depth regions containing methane hydrates release of methane from melting hydrates in these areas could enhance ocean acidification and oxygen depletion in the water column the impact of methane release on global warming however would not be significant within the considered time span

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

the impact of refraction correction on leveling interpretations in southern california recent investigations show that the refraction corrections in leveling are large but can be adequately modeled the corrections are proportional to the temperature differences and the square of the sight lengths changes in sight length with time have resulted in apparent crustal movements when non refraction corrected data were used applying refraction corrections to leveling data in southern california significantly alters the size and configuration of the southern california uplift uplift at palmdale and gorman lebec relative to los angeles bm v32 during the 1953 1978 time period is no greater than 10 cm the bulk of the motion about 6 cm occurs as localized motion along the san gabriel fault during the 1953 1964 time period this motion along the san gabriel fault and uplift of 8 cm directly north of the san fernando fault between 1955 and 1968 may represent precursors to the 1971 san fernando earthquake

controls on the formation and stability of gas hydrate related bottom simulating reflectors bsrs a case study from the west svalbard continental slope 1 the growth and stability of the free gas zone fgz beneath gas hydrate related bottom simulating seismic reflectors bsrs is investigated using analytical and numerical analyses to understand the factors controlling the formation and depletion of free gas for a model based on the continental slope west of svalbard a continental margin of north atlantic type we find that the fgz is inherently unstable under a wide range of conditions because upward flow of under saturated liquid depletes free gas faster than it is produced by hydrate recycling in these scenarios the 150 m thick fgz that presently exists there would deplete within 105 106 years we suggest the fgz is in a stable state however that is formed by a diffusion dominated mechanism that produces low concentrations of gas in a fgz of steady state thickness gas forms across a thick zone because the upward fluid flux is relatively low and because the gas water solubility decreases to a minimum several hundred meters below the seabed this newly understood solubility curvature effect is complementary to hydrate recycling but becomes the most important factor controlling the presence and properties of the bsr in environments where the rate of upward fluid flow and the rate of hydrate recycling are both relatively low i e rifted continental margins if the present day fgz is in steady state we estimate that the upward fluid flux in the west svalbard site must be less than 0 15 mm a 1

modelling persistent methane seepage offshore western svalbard since early pleistocene abstract recent observations of extensive methane release into the oceans and atmosphere have raised concern as to whether rising temperatures across the arctic could drive rapid destabilization of gas hydrate reservoirs here we report modelling results from hydrate modulated methane seepage from vestnesa ridge offshore western svalbard suggesting that continuous leakage has occurred from the seafloor since the early pleistocene up until today sustained by modelled deep subsurface thermogenic sources of miocene age large scale hydrocarbon fluid migration started 6 million years ago and reached the seafloor some 4 million years later the modelling results indicate that widespread methane seepage offshore western svalbard commenced in earnest during early pleistocene significantly older than late holocene as previously reported we propose that the onset of vertical hydrocarbon migration is the response of rapid burial of potential hydrocarbon sources induced by increased sediment deposition following the onset of northern hemisphere glaciations 2 7 million years ago from the modelling results we propose that source rock intervals capable of generating hydrocarbons and hydrocarbon reservoirs buried kilometers deep have continuously fueled the gas hydrate system off western svalbard for the past 2 million years it is this hydrocarbon system that primarily controls the thermogenic methane fluxes and seepage variability at the seabed over geological times

coupled fluid flow and sediment deformation in margin scale salt tectonic systems 2 layered sediment models and application to the northwestern gulf of mexico 1 in paper 1 we described a methodology to model coupled fluid flow and deformation in composite salt and siliciclastic tectonic systems and investigated their compaction and overpressuring behavior prior to and during continental margin scale gravitational spreading compaction driven darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional plastic yield and mechanical deformation viscous flow of the underlying salt is independent of fluid pressure paper 1 presented prototype models that are limited to single uniform sediment lithologies either sandstone type or shale type that undergo mechanical and volumetric viscous compaction in this paper we present models with layered sandstone type and shale type lithologies designed to better approximate the more complex stratigraphy of the gulf of mexico our natural example a first set of models demonstrates that layered lithologies can produce fluid pressure regimes similar to those observed in sedimentary basins we then introduce an improved formulation of viscous compaction that includes a stronger dependence on porosity and depth used as proxy for temperature thereby more effectively self limiting viscous compaction a second set of models with the improved viscous compaction formulation demonstrates that the onset of gravity spreading is mainly controlled by overpressuring in the landward end of the salt basin and that resulting shortening in the distal part is partly accommodated by horizontal compaction models with moderately high fluid pressure best reproduce conditions considered to have been necessary for large scale gravitational spreading in the northwestern gulf of mexico which led to the formation of the perdido fold belt

in situ temperature measurements at the svalbard continental margin implications for gas hydrate dynamics during expedition maria s merian msm57 2 to the svalbard margin offshore prins karls forland the seafloor drill rig marum mebo70 was used to assess the landward termination of the gas hydrate system in water depths between 340 and 446 m the study region shows abundant seafloor gas vents clustered at a water depth of 400 m the sedimentary environment within the upper 100 meters below seafloor mbsf is dominated by ice berg scours and glacial unconformities sediments cored included glacial diamictons and sheet sands interbedded with mud seismic data show a bottom simulating reflector terminating 30 km seaward in 760 m water depth before it reaches the theoretical limit of the gas hydrate stability zone ghsz at the drilling transect we present results of the first in situ temperature measurements conducted with mebo70 down to 28 mbsf the data yield temperature gradients between 38 c km 1 at the deepest site 446 m and 41 c km 1 at a shallower drill site 390 m these data constrain combined with in situ pore fluid data sediment porosities and thermal conductivities the dynamic evolution of the ghsz during the past 70 years for which bottom water temperature records exist gas hydrate is not stable in the sediments at sites shallower than 390 m water depth at the time of acquisition august 2016 only at the drill site in 446 m water depth favorable gas hydrate stability conditions are met maximum vertical extent of 60 mbsf however coring did not encounter any gas hydrates

palaeo earthquake and palaeo mudflow events at the machangyuan ruins in the huangshui river valley northeastern margin of the tibetan plateau palaeo earthquake event recorded by loess rapture fissures n30 40 w and n40 50 e trending and palaeo mudflow event recorded by red clay deposits were identified at the machangyuan ruins in the huangshui river valley at the foot of the northeastern tibetan plateau comparative analysis of grain size distributions and geochemical elements of local outcrops indicates that the red mudflows were sourced from the gullies on the valley side where unconsolidated neogene red clay formation was exposed the palaeo earthquake was associated with regional tectonic structures including a nnw trending left lateral strike slip fault lajishan fault and a ne trending local fault bazhougou fault analysis combining optically stimulated luminescence ages and radiocarbon ages of archaeological record dates the palaeo earthquake and palaeo mudflow events to ca 4 80 ka bp during that stage many neolithic settlements distributed around the machangyuan ruins enhanced human activities of the majiayao culture disturbed

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

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

pockmarks in the witch ground basin central north sea marine sediments host large amounts of methane ch4 which is a potent greenhouse gas quantitative estimates for methane release from marine sediments are scarce and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios here we use 2d and 3d seismic reflection multibeam bathymetric geochemical and sedimentological data to i map and describe pockmarks in the witch ground basin central north sea ii characterize associated sedimentological and fluid migration structures and iii analyze the related methane release more than 1500 pockmarks of two distinct morphological classes spread over an area of 225 km2 the two classes form independently from another and are corresponding to at least two different sources of fluids class 1 pockmarks are large in size 6 m deep 250 m long and 75 m wide show active venting and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources class 2 pockmarks which comprise 99 5 of all pockmarks are smaller 0 9 3 1 m deep 26 140 m long and 14 57 m wide and are limited to the soft fine grained sediments of the witch ground formation and possibly sourced by compaction related dewatering buried pockmarks within the witch ground formation document distinct phases of pockmark formation likely triggered by external forces related to environmental changes after deglaciation thus greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present day fluxes but require an analysis of the pockmark forming processes through geological time

expansion of the aftershock zone following the vanuatu new hebrides earthquake on 15 july 1981 following a large ms 7 0 interplate earthquake on 15 july 1981 in the vanuatu new hebrides islands the area defined by the aftershocks dramatically increased over a two week period of time the aftershocks were located using the cornell orstom seismograph network that has been monitoring earthquakes with magnitudes larger than mb 2 9 in the region since mid 1978 in the first few hours following the mainshock the area defined by the aftershocks was consistent with the rupture zone of a typical interplate earthquake with the observed seismic moment of 5 8 1026 dyne cm however during the 16 days following the mainshock the area of the aftershock zone increased to approximately 10 times the area of expected coseismic rupture the final aftershock zone included earthquakes that were located within the overriding and descending plates as well as along the interplate fault zone the migration of aftershocks away from the rupture zone of the 1981 mainshock indicates that at least in some cases the aftershock zone can overestimate the extent of coseismic rupture

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

erosion and sedimentation as processes controlling the bsr derived heat flow on the eastern nankai margin in 2000 a 3d mcs survey was carried out on the eastern nankai trough a strong and widespread bottom simulating reflector bsr was observed and mapped after a preserved amplitude 3d prestack migration of the seismic data we use this bsr to calculate a heat flow map over the 3d survey area this map presents some high and low heat flow areas that cannot be correlated to known active wedge faults and previously observed fluid venting sites since the heat flow anomalies are not correlated to known major faults or fluid venting sites large fluid circulation is not likely to explain the varying bsr derived heat flow distribution in order to explain this heat flow distribution quantitatively we calculate the rates of erosion and sedimentation that are necessary to create these anomalies these rates are in good agreement with observations on seismic profiles and are locally consistent with a sedimentation rate calculated from slope basin depth and reflect the distribution of erosion landsliding and deposition on the margin slope we therefore propose that the observed bsr derived heat flow anomalies are mainly due to active erosion and sedimentation and that heat flow distribution on the margin section of the 3d survey is controlled by surface processes fluid migration could occur where a bsr is absent but is probably episodic or limited where a bsr is present and does not affect the bsr derived heat flow in this part of the margin these surface processes are the consequence of the tectonic and seismic activity of the wedge and are influenced by the subduction of a wide oceanic ridge below the eastern nankai margin

heat flow estimated from bsr and iodp borehole data implication of recent uplift and erosion of the imbricate thrust zone in the nankai trough off kumano heat flow values were estimated in the nankai trough fore arc slope region off kii peninsula from the bottom simulating reflector bsr and other data obtained during iodp expeditions 315 and 316 the heat flow has an uncertainty of 20 mainly due to ambiguities in the temperature estimate at the bsr and uncertainties on the thermal conductivity bsrs occur intermittently in the imbricate thrust zone itz they are significantly shallower below anticlines designated anticlinal high zone ahz than in the adjacent synclines basal low zone blz the heat flow in the blz 55 65 mw m2 is consistent with the regional heat flow trend the shallow bsr in the ahz produces an apparent high heat flow anomaly of 70 90 mw m2 and discontinuities are observed across thrust faults the most likely cause is the transient effect of thrust faulting followed by erosion on the hanging wall side a one dimensional time dependent numerical model with latent heat for hydrate and gas transition considered indicates that the relaxation time for the bsr to reequilibrate after such a disturbance is 10 kyr although the possibility that the shallow bsr is a remnant base of hydrate stability zone cannot be ruled out if the shortening has occurred in the recent 10 kyr the convergent rate at the itz would be 30 m kyr which is much larger than the recent horizontal shortening across the megasplay fault and could even exceed the portion of the plate convergence than that accommodated at the frontal thrust

gas hydrate of lake baikal discovery and varieties abstract this paper summarizes the results of recent gas hydrate studies in lake baikal the only fresh water lake in the world containing gas hydrates in its sedimentary infill we provide a historical overview of the different investigations and discoveries and highlight some recent breakthroughs in our understanding of the baikal hydrate system so far 21 sites of gas hydrate occurrence have been discovered gas hydrates are of structures i and ii which are of thermogenic microbial and mixed origin at the 15 sites gas hydrates were found in mud volcanoes and the rest six near gas discharges additionally depending on type of discharge and gas hydrate structure they were visually different investigations using mir submersibles allowed finding of gas hydrates at the bottom surface of lake baikal at the three sites

the thermal evolution of sedimentary basins and its effect on the maturation of hydrocarbons summary the maturation of hydrocarbon source rocks depends on a range of factors including the primary rock type and its original content of organic matter kerogens the history of sedimentation and burial depth the local geothermal gradient temperature and duration of sedimentation time the standard approach to modelling this process assumes an evolving burial history but in a basin with a steady state geotherm both in the sediment column and in the underlying basement rocks this first order approach neglects any evects of changes in the geothermal profile resulting from the timing and history of the particular mechanism that formed the sedimentary basin and hence may lead to systematic overestimation or underestimation of hydrocarbon maturation in such rocks this systematic evect may be small when sedimentation rates are low but can be significant in basins with sedimentation rates that are rapid when compared with the rate of heat transport and re equilibration in the system here we describe two analytical 1 d mathematical models for divusive heat transport during sediment deposition which is rapid compared with the thermal relaxation rate the two models describe sedimentary events that can be considered either evectively instantaneous sudden or continuous in both cases we calculate a time dependent geothermal profile and the resulting maturation index of particular source rocks given the combined evects of the thermal and burial histories the models take account both of the transient cooling evect of the cold sediment blanket and of the steady state warming from the underlying basement after calibration with borehole data the method allows a reconstruction of the initial geothermal gradient at the start of the sedimentation event the model has been applied to data derived from two wells drilled in two diverent tectonic settings the pannonian basin and the central north sea in both cases there is a good agreement between the model predictions of the maturation index and empirical observations based on the vitrinite reflectance technique applied to borehole samples of actual source rocks in contrast the steady state geotherm models give a systematic overestimation of both the maximum temperature and the degree of maturation reached by the sediments by the end of the sedimentary event this overestimation is particularly evident for the case of the pannonian basin such systematic evects could have significant implications both for the interpretation of past thermotectonic events and for oilfield appraisal

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

thickness of the gas hydrate stability zone gulf of mexico continental slope the maximum thickness of the gas hydrate stability zone ghsz at key gas hydrate study sites was estimated and a generalized ghsz profile across part of the central gulf of mexico slope was constructed maximum thickness of the ghsz increases with water depth at the shallowest site 540 m water depth from about 450 m to about 1150 m at the deepest site 1930 m water depth the occurrence of gas hydrate in the subsurface is largely controlled by structural focussing of hydrocarbons with gas hydrate most abundant at the rims of salt withdrawal basins and less abundant within intrasalt basins the postulated geometry of large subsurface gas hydrate accumulations shows strong structural control bottom simulating reflectors bsrs are absent or infrequently observed in the gulf of mexico because of structural focusing of the free hydrocarbon gases that form gas hydrate new insight to the geometry of the ghsz in the gulf of mexico continental slope will contribute to improved application of geophysics to map the distribution of gas hydrates improved understanding of three dimensional geometry of gas hydrate accumulations will contribute to better assessment of gas hydrate volumes as a future energy resource as a geohazard and will constrain theories relating the role of gas hydrate in past climate change

crustal fingering facilitates free gas methane migration through the hydrate stability zone widespread seafloor methane venting has been reported in many regions of the world oceans in the past decade identifying and quantifying where and how much methane is being released into the ocean remains a major challenge and a critical gap in assessing the global carbon budget and predicting future climate c ruppel j d kessler rev geophys 55 126 168 2017 methane hydrate ch 4 5 75 h 2 o is an ice like solid that forms from methane water mixture under elevated pressure and low temperature conditions typical of the deep marine settings 600 m depth often referred to as the hydrate stability zone hsz wide ranging field evidence indicates that methane seepage often coexists with hydrate bearing sediments within the hsz suggesting that hydrate formation may play an important role during the gas migration process at a depth that is too shallow for hydrate formation existing theories suggest that gas migration occurs via capillary invasion and or initiation and propagation of fractures fig 1 within the hsz however a theoretical mechanism that addresses the way in which hydrate formation participates in the gas percolation process is missing here we study experimentally and computationally the mechanics of gas percolation under hydrate forming conditions we uncover a phenomenon crustal fingering and demonstrate how it may control methane gas migration in ocean sediments within the hsz

capillary effects on hydrate stability in marine sediments 1 we study the three phase liquid gas hydrate stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases the capillary pressure a measure of the pressure difference across a curved phase interface exerts a key control on the methane solubility in liquid hydrate l h and liquid gas l g systems by calculating the l h and l g solubilities as a function of water depth pressure and pore size interface curvature we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three phase zone the top of the three phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases the thickness of the three phase zone increases as the distribution of pore sizes widens the top of the three phase zone can overlie or underlie the bulk three phase equilibrium depth at blake ridge we predict that the three phase zone is 27 7 m thick and that the top of the three phase zone lies 13 m above the predicted bulk equilibrium depth this reconciles the observation of the bottom simulating reflector bsr at blake ridge that is shallower than the predicted bulk equilibrium depth in contrast at hydrate ridge where water depth is shallower we predict that the three phase zone is 20 4 m thick and that the top of the three phase zone lies 0 7 m below the predicted bulk equilibrium depth our model which predicts an upward shift in the top of free gas occurrence with increasing water depth pressure is compatible with worldwide observations that the bsr is systematically shifted upward relative to the bulk equilibrium depth as water depth pressure is increased

erosion of the seafloor at the top of the gas hydrate stability zone on the hikurangi margin new zealand 1 the dissociation of gas hydrates in sediment pores is thought to decrease seafloor strength potentially facilitating submarine slides because of the generation of overpressured gas and the melting of load bearing or cementing solid hydrate here we present findings that suggest gas hydrates may lead to a previously unknown mechanism of seafloor erosion gas hydrate bearing sub sea ridges on the hikurangi margin are eroded close to the top of the hydrate stability field in the ocean we hypothesize that gas hydrate instability may lead to ridge erosion by a combination of two processes hydrate destabilization caused by depressurization during ridge uplift and repeated pore volume expansion and contraction from hydrate dissociation and formation triggered by fluctuating water temperatures

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

distribution of gas hydrates on continental margins by means of a mathematical envelope a method applied to the interpretation of 3d seismic data 1 a 3d seismic volume from the nankai trough accretionary wedge se japan is used to evaluate the subsurface distribution of gas hydrates as a function of structural and stratigraphic complexity variable heat flow patterns and the presence of subsurface fluid conduits eleven equations were modified for depth pressure and temperature modeled in 3d and compared with the distribution of bottom simulating reflections bsrs offshore nankai the results show that the equations produce overlapping and thus potentially consistent predictions for the distribution of bsrs leading us to propose the concept of a bsr stability envelope as a method to quantify the subsurface distribution of gas hydrates on continental margins in addition we show that the ratio r between shallow and deep bsrs of seven subenvelopes which are defined by bsr stability equations indicates local gas hydrate equilibrium conditions values of r 1 the majority of bsrs are located in warmer structural traps the method in this paper can be used to recognize any divergence between observed and theoretical depths of occurrence of bsrs on 3d or 4d time lapse seismic volumes in the nankai trough our results point out for equilibrium conditions in bsrs located away from the megasplay fault zone and major thrust faults this latter observation demonstrates the applicability of the method to a the recognition of subsurface fluid conduits and b the prediction of maximum and minimum depths of occurrence of gas hydrates on continental margins under distinct thermal and hydrologic conditions

contourites in the fram strait abstract multichannel seismic reflection profiles from the fram strait show many features that can be attributed to bottom current influence on the sediments by comparison with well known contourite sequences elsewhere a greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of svalbard the currents here have probably been controlled by local topography we interpret sequences with abundant diffractions in the broader basins to be current related probably deposited during periods of intensified circulation the continental slopes on the svalbard side have fairly continuous sequences whereas the younger sequences on the greenland lower slope onlaps the lowermost one this can be attributed to differences in current regime sediment supply or margin subsidence the contourites of the lower slope in the northeastern part of the strait about 1000 m water depth can be explained by long term influence of the west spitsbergen current but current influenced patterns down to 3000 m water depth are not compatible with the weak deep water circulation observed at present the oldest contourites west of svalbard are estimated to be of late miocene to pliocene age between 10 and 3 ma based on correlation to the dsdp site 344 and inferred basement ages near the present plate boundary hiatuses may be present beneath the contourites the onset of currents may be related to opening of the gateway between greenland and svalbard to climatic cooling and to palaeogeographic changes outside the fram strait contourite deposition has continued through the late neogene and quaternary but vertical changes in the pattern suggest that current intensities have decreased through time

molecular and stable isotope compositions of natural gas hydrates a revised global dataset and basic interpretations in the context of geological settings abstract a global dataset of molecular and stable isotope compositions of gases released from 209 different specimens of natural gas hydrate is presented and discussed the 26 hydrate bearing areas from 21 geographic regions are grouped into high gas flux hgf settings low gas flux lgf settings and hydrated gas accumulations hga methane ch4 is the most abundant hydrate bound gas while co2 and c2 hydrocarbon gases are frequently present in small amounts non hydrocarbon gases such as h2s are uncommon the stable isotope composition of hydrate bound gases varies significantly e g 13c of ch4 from 74 7 to 39 6 suggesting that gases of both microbial and thermogenic origin form gas hydrates hydrate bound gases are derived from autochthonous located predominantly within the gas hydrate stability zone ghsz and allochthonous located in deep sediments sources the occurrence and concentration of gas hydrates in sediments are controlled not by the origin of gases but rather by their sources which in turn strongly depend on geological setting allochthonous gases microbial and or thermogenic dominate in hgf and hga settings where they are focused in the shallow ghsz along faults within mud volcanoes in permeable carrier beds and other geological features from underlying petroleum systems relatively high concentrations of gas hydrate average 5 15 of pores and locally up to 100 of volume occurring over small areas are typical of hgf and hga settings in contrast autochthonous and diffuse allochthonous gases mostly microbial occur in stratigraphically and structurally simple lgf settings and result in relatively low concentrations of gas hydrate average 2 of pores in the ghsz spread over large areas the major implication of this finding is that successful prediction of resource and geohazard potential of gas hydrates is possible only if regional petroleum systems extending well below the ghsz are properly evaluated in addition to the diagnosis of origins and sources of hydrate bound gases molecular and isotopic data help to better identify hydrate bearing intervals and provide valuable insights into the dynamics of hydrate bearing sites

the challenges of quantifying the carbon stored in arctic marine gas hydrate the quantification of the carbon stored in gas hydrate gh bearing marine sediments still remains a challenge despite recent efforts to develop approaches to better estimate the gh inventory globally these estimates are still highly unconstrained due to insufficient field data and poor understanding of the mechanisms fuelling the gh stability zone ghsz here we use geophysically derived gh saturations to constraint estimates of model derived arctic marine gh inventory at present we also estimate the potential carbon released from gh dissociation under a seabed warming of 2 c over 100 yr we estimate an inventory ranging between 0 28 and 541 gt of carbon which upper bound results in average gh saturations of 0 25 our upper bound is mainly controlled by our imposed upwards carbon rich fluid flow of 0 01 cm yr 1 and it is five times greater than the most recent estimate that only considers in situ degradation of particulate organic carbon poc to obtain the seismically inferred gh saturations of 5 10 offshore west of svalbard and in the beaufort sea an upwards advection of carbon rich fluids equivalent to 0 02 0 04 cm yr 1 is required this mechanism may be the most important source of carbon reaching the ghsz in arctic marine sediments a 2 c seabed temperature increase over 100 yr may reduce the gh inventory by about 88 44 0 7 gt c if poc is the only source and by about 5 4 29 7 gt c if the main source of carbon is the upwards advection of carbon rich fluids

gas hydrate stability zone modelling in areas of salt tectonics and pockmarks of the barents sea suggests an active hydrocarbon venting system the barents sea seabed exhibits an area of major glacial erosion exposing parts of the old hydrocarbon basins in this region we modelled the gas hydrate stability field in a 3d perspective including the effects of higher order hydrocarbon gases we used 3d seismic data to analyse the linkage between fluid flow expressions and hydrate occurrences above old sedimentary basin systems and vertical faults pockmarks showed a relation to fault systems where some of them are directly connected to hydrocarbon bearing sedimentary formations the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is critically analysed in relation to both geological formations and salt tectonics our analysis suggests a highly variable ghsz in the barents sea region controlled by local variations in the parameters of stability conditions recovery of gas hydrate sample from the region and presence of gas enhanced reflections below estimated bsr depths may indicate a prevalent gas hydrate stable condition

3d thermobaric modelling of the gas hydrate stability zone onshore central spitsbergen arctic norway abstract dissociation of onshore natural gas hydrates nghs could lead to the release of methane directly to the atmosphere especially in arctic regions such as svalbard where enhanced climate warming has the capacity to promote rapid methane evasion to the atmosphere following the decay of permafrost and glacier ice here we present the first assessment of the ngh stability zone ghsz in central spitsbergen a climate sensitive part of svalbard where thermobaric conditions appear favourable for onshore ngh formation we developed an approach incorporating regionally constrained 3 dimensional parameterisation of temperature pressure and phase boundary 93 methane 7 ethane 35 ppt salinity to define the ghsz this resulted in an up to 650 m thick mean 308 m ghsz covering 74 8 of the study area thickening significantly in the east where the climate is colder perturbation of the base case parameters was undertaken to quantify the sensitivity of the ghsz to the variation in environmental conditions across the study area we present 26 examples of these deterministic scenarios and show that the largest changes in the ghsz were observed when either the ethane content to 20 or the regional pore water pressure to 125 hydrostatic were increased the ghsz also increased markedly when the geothermal gradient was reduced from 33 to 26 c km 1 but was almost completely inhibited by a dry gas 100 methane greater salinity 50 ppt or exposure to an increase in surface temperatures relative to the mean annual air temperature e g by 2 c most parameters affected both the upper and the lower stability boundary of the ghsz with the exception of the geothermal gradient which impacted primarily upon the latter given that svalbard is host to a proven petroleum system we conclude that nghs almost certainly exist onshore svalbard

potential serpentinization degassing and gas hydrate formation at a young 20 ma sedimented ocean crust of the arctic ocean ridge system 1 global assessment of methane must consider the role of mid ocean ridges fluids from serpentinized mantle and gabbro material are noteworthy on ocean ridges although they are not very well understood only a few sedimented ocean ridges exist worldwide and they may document past and ongoing serpentinization driven migration of gas rich fluids this study is based on two multichannel reflection seismic profiles acquired across a sedimented segment of the ultraslow spreading knipovich ridge offshore nw svalbard seismic data allow suggesting a potential link between inferred areas of serpentinization transfer of carbon from the deep seated host rocks through the sediments above by diapirism and methane capture within the gas hydrate stability zone at the eastern flank of the knipovich ridge the origin of sediment remobilization features can be related to intrusions and the degassing process from mantle serpentinization these disturbances in sediments overlying the oceanic crust can be observed in seismic data and are interpreted as diapirs in shallower sediments at the predicted base of the gas hydrate stability zone the seismic data show a bright spot with all the characteristics of a gas hydrate related bottom simulating reflector bsr such as enhanced reflection amplitude phase reversal relative to the seabed reflection and crosscutting of sedimentary strata the bsr occurs at about 200 ms two way time within a sequence of marine sediments two dimensional concentration models of methane hydrate using the differential effective medium theory predict saturations of up to 26 of methane hydrate in the pore space of sediments in the gas hydrate reservoir

active gas venting through hydrate bearing sediments on the vestnesa ridge offshore w svalbard abstract gas hydrate systems offshore western svalbard are extensive and include the whole vestnesa ridge an elongated sediment drift north of the molloy transform and just east of the molloy ridge one of the shortest segments of the slow spreading north atlantic ridge system the crest of the vestnesa ridge at water depth between 1200 1300 m is pierced with fluid flow features seafloor pockmarks vary in size up to 700 m in diameter pockmarks are generally larger at the eastern start of the vestnesa ridge than at its western termination a recent cruise with r v jan mayen discovered methane flares in the water column above the pockmark field at the onset of the vestnesa ridge over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m it is still unclear what has triggered the increase in gas expulsion from seafloor sediments high resolution 3d seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems at the western end of the vestnesa ridge sub seafloor fluid flow features resemble well described chimney structures however the seismic expression of the active fluid flow features is much broader much more chaotic dome shaped and is not very similar to a typical chimney structure the vestnesa ridge gas hydrate and free gas system occurs within few kilometers of a mid oceanic ridge and transform fault which makes this gas hydrate system unique on earth the close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas hydrate and free gas system the high heat flow together with the high tectonic activity of this region a thick sedimentary cover a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free gas system leading to high variability in gas supply gas migration and gas hydrate build up and dissociation

thermogenic methane injection via bubble transport into the upper arctic ocean from the hydrate charged vestnesa ridge svalbard we use new gas hydrate geochemistry analyses echosounder data and three dimensional p cable seismic data to study a gas hydrate and free gas system in 1200 m water depth at the vestnesa ridge offshore nw svalbard geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source the presence of thermogenic gas and temperatures of similar to 3 3 degrees c result in a shallow top of the hydrate stability zone thsz at similar to 340 m below sea level mbsl therefore hydrate skinned gas bubbles which inhibit gas dissolution processes are thermodynamically stable to this shallow water depth this was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl at the seafloor bubbles are released from acoustically transparent zones in the seismic data which we interpret as regions where free gas is migrating through the hydrate stability zone hsz these intrusions result in vertical variations in the base of the hsz bhsz of up to similar to 150 m possibly making the shallow hydrate reservoir more susceptible to warming such arctic gas hydrate and free gas systems are important because of their potential role in climate change and in fueling marine life but remain largely understudied due to limited data coverage in seasonally ice covered arctic environments

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

paleoceanographic evolution of the sw svalbard margin 76 n since 20 000 14c yr bp abstract two cores from the southwestern shelf and slope of storfjorden svalbard taken at 389 m and 1485 m water depth have been analyzed for benthic and planktic foraminifera oxygen isotopes and ice rafted debris the results show that over the last 20 000 yr atlantic water has been continuously present on the southwestern svalbard shelf however from 15 000 to 10 000 14c yr bp comprising the heinrich event h1 interval the bolling allerod interstades and the younger dryas stade it flowed as a subsurface water mass below a layer of polar surface water in the benthic environment the shift to interglacial conditions occurred at 10 000 14c yr bp due to the presence of a thin upper layer of polar water surface conditions remained cold until ca 9000 14c yr bp when the warm atlantic water finally appeared at the surface neither extensive sea ice cover nor large inputs of meltwater stopped the inflow of atlantic water its warm core was merely submerged below the cold polar surface water

removal of methane through hydrological microbial and geochemical processes in the shallow sediments of pockmarks along eastern vestnesa ridge svalbard the recent discovery of methane seeps in the arctic region requires a better understanding of the fate of methane in marine sediments if we are to understand the contributions of methane to arctic ecosystems and climate change to this goal we analyze pore water data from five sites along eastern vestnesa ridge a sediment drift off north west svalbard to quantify the consumption of dissolved methane across the sulfate methane transition zone which are 3 5 m below seafloor from the investigated sites we use transport reaction models to quantify the hydrology as well as the carbon mass balance in the sediments pore water profiles and our model results demonstrate that hydrological microbial and geochemical processes reactions efficiently remove methane carbon from fluid over different time scales we interpret the nonsteady state behavior of the first 50 70 cm of our pore water profiles from the active sites as an annual scale downward fluid flow due to a seepage related pressure imbalance such downward flow supplies sulfate which enhances methane consumption through anaerobic oxidation of methane aom within this depth range our steady state modeling confirms the efficiency of aom in consuming dissolved methane in the upper 0 8 1 2 m of sediments based on the phosphate profiles we estimate that aom at the active pockmarks may have been operating for the last two to four centuries precipitation of authigenic carbonate removes more than a quarter of the dissolved inorganic carbon produced by aom and fixes it as authigenic carbonate in the sediments a process that sequestrates methane carbon over geological time

bottom simulating reflector dynamics at arctic thermogenic gas provinces an example from vestnesa ridge offshore west svalbard the vestnesa ridge comprises a 100 km long sediment drift located between the western continental slope of svalbard and the arctic mid ocean ridges it hosts a deep water 1000 m gas hydrate and associated seafloor seepage system near seafloor headspace gas compositions and its methane carbon isotopic signature along the ridge indicate a predominance of thermogenic gas sources feeding the system prediction of the base of the gas hydrate stability zone for theoretical pressure and temperature conditions and measured gas compositions results in an unusual underestimation of the observed bottom simulating reflector bsr depth the bsr is up to 60 m deeper than predicted for pure methane and measured gas compositions with 99 methane models for measured gas compositions with 4 higher order hydrocarbons result in a better bsr approximation however the bsr remains 20 m deeper than predicted in a region without active seepage a bsr deeper than predicted is primarily explained by unexpected spatial variations in the geothermal gradient and by larger amounts of thermogenic gas at the base of the gas hydrate stability zone hydrates containing higher order hydrocarbons form at greater depths and higher temperatures and contribute with larger amounts of carbons than pure methane hydrates in thermogenic provinces this may imply a significant upward revision up to 50 in the case of vestnesa ridge of the amount of carbon in gas hydrates

conductive heat flow variations from bottom simulating reflectors on the hikurangi margin new zealand 1 bottom simulating reflectors bsrs represent the base of a gas hydrate zone underlain by widespread free gas the extent of bsrs 40 000 km2 has been mapped on the continental slope of the hikurangi margin using industry and research seismic data a conductive model was used to calculate heat flow from the depth of the bsr to the surface a regional trend is observed in which heat flow uncorrected for sedimentation effects decreases from an average of 50 mw m 2 at the toe of the accretionary prism to 35 mw m 2 at a distance 100 km landward from the deformation front this trend reflects the processes of thickening accretionary wedge sediments and subduction of the pacific plate and is in general agreement with heat flow values obtained using bottom hole temperature data local variations of heat flow were also observed notably low and high values over the crest and flanks of anticlines numerical modeling indicates that much of this heat flow variation is due to a combination of variable seabed bathymetry and sedimentation effects the dynamic effects of thrust faulting are found to be small and fluid flow along near surface faults is not predicted to contribute significantly to the observed heat flow anomalies along the modeled section

quaternary uplift of the torres islands northern new hebrides frontal arc comparison with santo and malekula islands central new hebrides frontal arc coral reef terraces on the torres islands have recorded the quaternary uplift of part of the northern new indian plate underthrusts the arc from the west 14c and 230thp34u ages for torres fossil corals indicate that for approximately the past 100 000 years the islands uplifted at a constant rate ranging geographically from 0 7 to 0 9 yr the occurrence of similar uplift rates and eastward tilting directions and the absence of major crosscutting structures or rupture zone boundaries suggest that the islands may be part of a single tectonic block or arc segment the frontal arc morphology in the torres area is typical of frontal arcs farther south the central new hebrides frontal arc has no physiographic trench at the plate boundary west of santo and malekula islands and the axes of the maximum uplift rate are at the seaward edge of the plate the d entrecasteaux ridge dr a massive e w trending bathymetric feature on the indian plate is underthrusting santo and malekula the eastern parts of santo and malekula are physiographically equivalent to the torres islands and seem to have been on a former axis of maximum uplift rate however in response to subduction of the dr the uplift axes of santo and malekula may have shifted westward in a few hundred thousand years the new uplift axes caused the western parts of these islands to achieve their present topographic dominance over the earlier uplift axes holocene uplift rates on southern santo and northern malekula are about twice the average rates of the previous 100 000 years the accelerated holocene uplift could be explained if a particularly prominent part of the dr has recently underthrust santo and malekula d hebrides frontal arc these isles lie about midway between the volcanic chain and trench axis where the

a new 6 myr stratigraphic framework for the atlantic arctic gateway abstract a comprehensive stratigraphic framework for the atlantic arctic gateway has been established for the last 6 myr on the yermak plateau nw svalbard based on new paleomagnetic and biostratigraphic sampling and correlation of high resolution seismic data between ocean drilling program odp holes 911a 910c and 912a the new results indicate a late miocene age for the base of both holes 911a and 910c providing the first complete late neogene record for the marginal arctic ocean and extending the previously published age model by several million years during the late miocene pleistocene the southern yermak plateau was dominated by contourite deposition from the intensification of the northern hemisphere glaciation at 2 7 ma glacial fans from nw svalbard prograded onto the southernmost yermak plateau this indicates that the ice sheet on nw svalbard reached the shelf break at 2 7 ma much earlier than on the western barents sea svalbard margin 1 5 ma simultaneous with the first shelf break glaciation on the western barents sea svalbard margin at 1 5 ma we observe the first signs of extensive glacial erosion on the yermak plateau indicating a regional glacial intensification at this time for the whole barents sea svalbard region our results agree well with the widening deepening of the fram strait and the opening of the atlantic arctic gateway during the middle miocene