bottom simulating reflections and geothermal gradients across the western svalbard margin seismic reflection data reveal prominent bottom simulating reflections bsrs within the relatively young 0 78 ma sediments along the west svalbard continental margin the potential hydrate occurrence zone covers an area of c 1600 km2 the hydrate accumulation zone is bound by structural tectonic features knipovich ridge molloy transform fault vestnesa ridge and the presence of glacigenic debris lobes inhibiting hydrate formation upslope the thickness of the gas zone underneath the bsr varies laterally and reaches a maximum of c 150 ms using the bsr as an in situ temperature proxy geothermal gradients increase gradually from 70 to 115 c km 1 towards the molloy transform fault anomalies only occur in the immediate vicinity of normal faults where the bsr shoals indicating near vertical heat fluid flow within the fault zones amplitude analyses suggest that sub horizontal fluid migration also takes place along the stratigraphy as the faults are related to the northwards propagation of the knipovich ridge long term disturbance of hydrate stability appears related to incipient rifting processes

effects of bottom water warming and sea level rise on holocene hydrate dissociation and mass wasting along the norwegian barents continental margin 1 gas hydrate gh stability modeling results explain why some major holocene submarine landslides along the norwegian barents margin could have been triggered by gh dissociation during the early to middle holocene not during the lowest sea levels of the last glacial maximum lgm our model results show that subbottom depths of 170 260 m below the pre slide continental slope ca 350 475 m present water depth must have passed out of gas hydrate stability zone ghsz by 8 15 ka as the effect of warm bottom water inflow at 11 ka penetrated into the subbottom overcoming the effects of pressure increase due to sea level rise slr the component of local slr due to the isostatic response to fennoscandian deglaciation is shown to be relatively insignificant particularly for the part of the upper continental slope where the slide probably began the stability relations show that gh could have formed under the ice sheet before deglaciation and below deeper shelf areas after sea levels began to rise but before significant warming near the ghsz base to the extent water deeper than 800 m has remained cold 1 to 0 c since lgm times the ghsz continued to thicken in deep water and gh dissociation could not have triggered holocene failure in that regime the present distribution of gh stability is limited to water depths greater than about 400 m in the storegga slide area and the thickness of the ghsz increases with water depth

estimation of gas hydrate concentration and free gas saturation at the norwegian svalbard continental margin we estimate the concentration of gas hydrate and free gas at an area located to the north of the knipovich ridge western svalbard margin the method is based on p wave velocities computed by reflection tomography applied to multicomponent ocean bottom seismometer data the tomographic velocity field is fitted to theoretical velocities obtained from a poro elastic model based on a biot type approach the interaction between the rock frame gas hydrate and fluid is modelled from first physical principles we obtain average hydrate concentrations of 7 and maximum free gas saturations of 0 4 and 9 depending on the saturation model

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

thermodynamic and pore water halogen constraints on gas hydrate distribution at odp site 997 blake ridge abstract marine sediment sequences with ch4 hydrate are characterized by an atypical depth profile in dissolved cl squeezed from pore space a shallow subsurface cl maximum overlies a lengthy and pronounced cl minimum this pore water cl profile represents a combination of multiple processes including glacial interglacial variations in ocean salinity advection and diffusion of ions that are excluded during gas hydrate formation at depth and release of fresh water from dissociation of hydrate during core recovery in situ quantities of gas hydrate can be determined from a measured pore water cl profile provided the in situ pore water signature prior to core recovery can be separated ocean drilling program odp site 997 was drilled into a large ch4 hydrate reservoir on the blake ridge in the western atlantic ocean previously we have constructed a high resolution pore water cl profile at this location here we present a coupled chloride hydrate numerical model to explain basic trends in the cl profile and to isolate in situ cl concentrations the model is based on thermodynamic and ecological considerations and uses established equations for describing chemical behavior in marine sediment pore water systems the model incorporates four key concepts 1 most gas hydrate is formed immediately below the so42 reduction zone 2 fluid dissolved ions and gas advect upward through the sediment column 3 ch4 hydrate dissociates at the base of hydrate stability conditions and 4 seawater salinity fluctuates during glacial interglacial cycles of the late pliocene and quaternary rates of upward advection in the model are sufficient to account for measured br and i concentrations as well as ch4 oxidation at the base of the so42 reduction zone in situ pore water cl inferred from the model is similar to that determined by limited direct sampling in situ ch4 hydrate amounts inferred from the model an average of about 4 of porosity are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques the model also predicts production of substantial quantities of free ch4 gas bubbles 2 5 of porosity at a depth immediately below the lowest accumulation of ch4 hydrate in the sediment column our explanation for the pore water cl profile at site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water cl gas hydrate and free gas in a marine sediment column

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

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

gas hydrates at the storegga slide constraints from an analysis of multicomponent wide angle seismic data geophysical evidence for gas hydrates is widespread along the northern flank of the storegga slide on the mid norwegian margin bottom simulating reflectors bsr at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 outside but also inside the storegga slide scar area traveltime inversion and forward modeling of multicomponent wide angle seismic data result in detailed p and s wave velocities of hydrate and gas bearing sediment layers the relationship between the velocities constrains the background velocity model for a hydrate free gas free case the seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3 and 6 in a zone that is as much as 50 m thick overlying the bsr hydrates are most likely disseminated neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably average free gas concentrations beneath the hydrate stability zone are approximately 0 4 to 0 8 of the pore volume assuming a homogeneous gas distribution the free gas zone underneath the bsr is about 80 m thick amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone bghs this gives rise to enhanced reflections that terminate at the bghs the stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations gas bearing layers can be as thin as 2 m

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

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

bunger hills east antarctica ice free at the last glacial maximum optically stimulated luminescence dating of glaciofluvial and glacial lake shoreline sediments indicates that the bunger hills area in coastal east antarctica was largely ice free by the last glacial maximum lgm deglaciation commenced as early as 30 ka and the southern hills were completely exposed by 20 ka the sediments do not record evidence of an lgm readvance previous reconstructions of lgm ice limits for the area are incompatible with this new evidence

holocene sedimentary record from lake tutira a template for upland watershed erosion proximal to the waipaoa sedimentary system northeastern new zealand abstract a holocene lake record from northeastern new zealand provides a detailed record of environmental controls on upper watershed sedimentation and is proximal to the waipaoa margins source to sink focus site in that context lake tutira in hawke s bay was cored in 2003 to recover a complete sedimentary record since the lake s formation ca 7 2 ka the 27 14 m long core contains alternating lithotypes that are sedimentary responses to lacustrine organic accumulation normal to severe rainfalls earthquakes and volcanism a diatom allochthonous ranking scheme pollen counts and c and n percentages were used to identify intra lake and watershed derived storm deposits and modes of lithotype deposition the lithotypes and depositional modes are tephras volcanic airfall organic rich mud algal rich lake sedimentation massive to weakly graded brown silty clay beds homogenites and redeposited lake sediments grey graded sandy mud beds intense storm delivered sediment and thin yellow clay layers run off from small storms using 12 tephras and 3 radiocarbon ages to provide a chronology the long term sedimentation rate is ca 3 3 mm year which increases to 10 mm year following european colonisation storm beds occur in response to rainfall events with no obvious correlation to el nino southern oscillation polarity or strength moreover no single climate index appears to correlate strongly with the historic rainfall event record having characterised and identified storm beds over the lake s history a hindcast relationship implies that around 53 pre historic storms occurred with a magnitude similar to the severe cyclone bola event of 1988 plus 7 potentially larger storm events despite the prominence of storm beds a summation of the total percent thickness as an indication of the relative modes of emplacement for each lithotype shows that proportionally the balance of intra lake versus storm sources preserved in the lake bed is 69 and 26 respectively as well as storms lake sedimentation is strongly influenced by earthquakes that destabilize the terrigenous sediment laden lake margins to generate homogenites represented by the brown silty clay beds these deposits tend to be thicker after a hiatus in seismic activity and after sustained periods of lake margin loading as inferred from the occurrence of thick graded storm beds comparison with marine records on the adjacent continental margin suggests that more terrestrial events are captured in the lake record due to i close hillslope lake connectivity with little intervening storage of sediment compared with the waipaoa sedimentary system and ii the preservation of event stratigraphy at tutira compared to its reduced preservation in the dynamic marine environment only major storms such as cyclone bola leave an imprint traceable to the ocean whereas identifiable sedimentary responses to individual earthquakes are localized although through landscape preconditioning and sediment production they contribute to the overall high terrigenous input to the ocean in contrast low frequency high magnitude perturbations volcanic eruptions european deforestation are preserved through the source to sink sedimentary system consistent with earlier hypotheses

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

holocene paleoceanography and glacial history of the west spitsbergen area euro arctic margin two sediment cores from the west spitsbergen area euro arctic margin md99 2304 and md99 2305 have been investigated for paleoceanographic proxies including benthic and planktonic foraminifera benthic foraminiferal stable isotopes and ice rafted debris core md99 2304 is located on the upper continental margin reflecting variations in the influx of atlantic water in the west spitsbergen current core md99 2305 is located in van mijenfjord picturing variations in tidewater glacier activity as well as fjordocean circulation changes surface water warmer than today was present on the margin as soon as the van mijenfjord was deglaciated by 11 200 cal years bp relatively warm water invaded the fjord bottom almost immediately after the deglaciation a relatively warm early holocene was followedby an abrupt cooling at 8800 cal years bp on the continental margin another cooling in the fjord record 8000 4000 cal years bp is documented by an increase in ice rafted debris and an increase in benthic foraminiferal d 18 o the ird record indicates that central spitsbergen never was completely deglaciated during the holocene relatively cool andstable cond itions similar to the present were establishedabout 4000 cal years bp

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

experimental measurements of the diffusion parameters of light hydrocarbons in water saturated sedimentary rocks i a new experimental procedure an automatic experimental set up has been developed for the determination of diffusion parameters of hydrocarbon gases through water saturated rock samples diffusive flow of hydrocarbons through rock slices is monitored by gas chromatography experiments are carried out according to the time lag method yielding diffusion coefficients solubility coefficients and diffusion permeabilities diffusion coefficients down to 10 12 m2 s 1 10 8 cm 2 s 1 may be determined routinely on rock samples of 2 10 mm thickness maximum errors in diffusion coefficients are estimated around 20 and reproducibility was found to range between 10 and 20 specific features of this set up are automatic sampling and data acquistion high sampling frequency and maintenance of water saturation of the rock samples throughout the experiment

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

sources of methane for marine gas hydrate inferences from a comparison of observations and numerical models the supply of methane for marine gas hydrate is commonly attributed to local conversion of organic material within the zone of stability or to migration of methane bearing fluids from a deeper source area however the relative importance of these methane sources is not well known we use numerical models for the formation of gas hydrate to assess the influence of methane supply on observable features of hydrate occurrences model predictions for the pore fluid chlorinity and the volumes of hydrate and free gas are compared with observations from the blake ridge and cascadia margin we find that the observations from the blake ridge can be explained with a deep methane source if a saturated fluid rises with a velocity of 0 26 mm yr 1 an equally good fit to the observations is obtained by combining in situ production of methane with an upward fluid velocity of 0 08 mm yr 1 which is compatible with estimates of fluid velocity at site 997 observations from the cascadia margin can be explained with a deep methane source if the upward fluid velocity is 0 42 mm yr 1 the predicted volume of hydrate 2 of the pore space is much smaller than some previous estimates and a layer of free gas 1 of the pore space develops below the stability zone allowing for in situ production at the cascadia margin lowers the upward fluid velocity 0 35 mm yr 1 needed to explain the pore water chlorinity

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

ocean warming and gas hydrate stability on the mid norwegian margin at the storegga slide abstract the sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment in the storegga slide area on the mid norwegian margin evidence for the existence of gas hydrates both inside and outside the storegga slide complex comes from reflection seismic profiles where a bottom simulating reflection marks today s base of the gas hydrate stability zone paleo bottom water temperatures show a relatively fast increase at approximately 12 5 10 ka calendar years following the younger dryas while stable warm water conditions have prevailed since then despite sea level rise this warm water inflow caused a major reduction in the thickness of the gas hydrate stability zone along the upper slope of the mid norwegian margin modelling results indicate that critical hydrate stability conditions and consequently the maximal potential pore pressure build up occur at the location of the storegga slide headwall although the major phase of hydrate melting predates the storegga slide event dated at 8 2 ka calendar years reduced hydrate stability conditions could have facilitated or contributed to sub marine slope failure additionally the bottom simulating reflections within the slide complex seem to have nearly adjusted to new equilibrium conditions highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes climate change geohazards

glacial interglacial deep water changes in the nw pacific inferred from single foraminiferal δ18o and δ13c abstract oxygen and carbon isotope values of single benthic foraminiferal tests in a core from the shatsky rise nw pacific ocean show greater intra horizon variance during the holocene than during the last glacial maximum lgm this greater variance is caused by the introduction of glacial specimens some 20 cm upward from their original deposition layer due to bioturbation in contrast foraminiferal populations belonging to glacial layers do not include holocene specimens the difference in direction of bioturbation greatly modifies climate information in horizons formed during and after deglacial events after omitting glacial specimens from holocene sediments the glacial interglacial difference in 18o suggests that pacific deep water temperature changed by 2 4 3 8 c at the most the 13c values suggest that nutrient concentration was higher during the lgm than the holocene the glacial deep north pacific ocean apparently was influenced by cold deep waters of southern origin

deciphering late cenozoic development of the western svalbard margin from odp site 986 results abstract the present work is based on sedimentological data from odp site 986 drilled off western svalbard the data include grain size distribution bulk mineralogy clay mineralogy total carbon total organic carbon and carbonate determinations from over 500 samples the results show interesting trends across the seismic sequence boundaries defined earlier and can be related to changes in climate and depositional styles the interpretation of these results together with the seismic observations and interpretations made earlier have shed new light on the timing of the onset of major glaciations and sedimentary processes on the svalbard margin during plio pleistocene the estimate for the timing of the first major glacial advance is revised to 1 6 ma from the earlier estimate of 2 3 ma barents sea was the likely source for the sediments in the 2 3 1 6 ma interval followed by svalbard as the main sediment source in the post 1 6 ma interval it is also suggested that in view of their morphology and proposed sedimentary processes the sedimentary wedges off svalbard barents sea margin are better described as slope aprons than in terms of glacial fans or trough mouth fans used earlier in the literature

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

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

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

excess pore pressure resulting from methane hydrate dissociation in marine sediments a theoretical approach 1 this study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments the excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation on the other hand the magnitude of excess pore pressure in well connected sediment pores is generally smaller depending primarily on the hydrate dissociation rate and the sediment permeability volume expansion due to gas hydrate dissociation in well connected pore spaces is related via darcy s law to an increase in pore pressure and its gradient in sediment which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area the magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level the excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation tectonic uplift sea level fall heating or inhibitor injection the excess pore pressure is found to be potentially able 1 to facilitate or trigger submarine landslides in shallow water environments 2 to result in the formation of vertical columns of focused fluid flow and gas migration and 3 to cause the failure of a sediment layer confined by low permeability barriers in relatively deep water environments

upward shifts in the southern hydrate ridge gas hydrate stability zone following postglacial warming offshore oregon 1 high resolution three dimensional 3 d seismic reflection data acquired on the r v thomas g thompson in 2000 reveal a pair of bottom simulating reflections bsrs across a broad region of southern hydrate ridge offshore oregon the primary bsr bsrp is a regionally extensive reflection that lies 120 150 m below seafloor and exhibits typical characteristics of a gas hydrate bsr we also imaged a second weaker bsr bsrs 20 40 m below bsrp with similar characteristics bsrs is interpreted as a remnant of a bsr that probably formed during the last glacial maximum 18 000 years ago when the base of the gas hydrate stability zone ghsz was deeper an increase in bottom water temperatures of 1 75 2 25 and a corresponding sea level rise of 120 m could have produced the bsr shift the preservation of bsrs for at least 5000 years which is the time since subseafloor temperatures stabilized following ocean warming after the last glacial maximum implies very slow upward advective and diffusive flow of methane 1 m 1000 years in the vicinity of bsrs bsrs appears where there are no resolvable steeply dipping faults and fractures consistent with very low advective flow rates and has dispersed where vertical fractures are visible free gas released by the shift in the bsr either migrates so slowly that it remains stable beneath the ghsz or is directed upward along fractures to reform as hydrate in the ghsz there is no evidence for release of this free gas into the ocean or atmosphere

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials