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

evidence for cenozoic tectonic deformation in se ireland and near offshore 1 an integrated study of topography bathymetry high resolution aeromagnetic data and structural observations demonstrates significant cenozoic fault activity in se ireland tectonically generated knickpoints and reddened fault breccias along topographic escarpments that are underlain by greywacke bedrock and trend oblique to the regional caledonian strike provide evidence for fault displacement near offshore faults with similar geometry produce present day bathymetric scarps and localized tectonic topography in the inverted kish bank basin the integration of offshore high resolution aeromagnetic data and structural interpretation of the kish bank basin provides evidence for dextral transtension on nnw trending faults and sinistral transpression on ene trending faults bounding a lower paleozoic to carboniferous basement block these faults correlate onshore with previously recognized caledonian faults producing topographic offsets and surface uplift to the north offshore structures can be traced onshore and cut exposures of the caledonian leinster granite structural analysis of these outcrops indicates post variscan deformation a major fault on the nw margin of the batholith cuts a major erosion surface developed on carboniferous carbonate rocks and nonmarine miocene deposits are preserved above this surface fault kinematics provide evidence of two paleostress systems 1 nw se 1 subvertical 2 and ne sw 3 followed by 2 a clockwise swing of 1 to nnw sse timing of deformation in both stress systems is probably post oligocene age the mechanism driving this deformation is likely ridge push much is already known about cenozoic tectonics and exhumation from offshore basins this study shows that onshore ireland has also been affected by significant tectonic activity and exhumation during the cenozoic

repeated pulses of vertical methane flux recorded in glacial sediments from the southeast bering sea 1 there is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period in this study of two sediment cores from the southeast bering sea 700 m and 1467 m water depth we identify multiple episodes during the last glacial period of intense methane flux reaching the seafloor within the uncertainty of the radiocarbon age model the episodes are contemporaneous in the two cores and have similar timing and duration as dansgaard oeschger events the episodes are marked by horizons of sediment containing 13c depleted authigenic carbonate minerals 13c depleted archaeal and bacterial lipids which resemble those found in anme 1 type anaerobic methane oxidizing microbial consortia and changes in the abundance and species distribution of benthic foraminifera the similar timing and isotopic composition of the authigenic carbonates in the two cores is consistent with a region wide increase in the upward flux of methane bearing fluids this study is the first observation outside santa barbara basin of pervasive repeated methane flux in glacial sediments however contrary to the clathrate gun hypothesis kennett et al 2003 these coring sites are too deep for methane hydrate destabilization to be the cause implying that a much larger part of the ocean s sedimentary methane may participate in climate or carbon cycle feedback at millennial timescales we speculate that pulses of methane in these opal rich sediments could be caused by the sudden release of overpressure in pore fluids that builds up gradually with silica diagenesis the release could be triggered by seismic shaking on the aleutian subduction zone caused by hydrostatic pressure increase associated with sea level rise at the start of interstadials

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

simulation of long term feedbacks from authigenic carbonate crust formation at cold vent sites fluid flow at cold vent sites is usually driven by the ascent of overpressured fluids from subsurface reservoirs porosity changes in surface sediments due to precipitation and dissolution of authigenic carbonates affects fluid flow and biogeochemical turnover from observations it is known that carbonate precipitates often occur in distinct layers in high concentrations surrounded by layers with low carbonate content using a non steady state model we simulate aragonite and calcite precipitation and dissolution in a 2 m long sediment column located under a bacterial mat at hydrate ridge cascadia accretionary margin assuming a constant pressure gradient over 7000 years fluid flow anaerobic oxidation of methane aom rates and carbonate precipitation and dissolution rates show strong oscillations evoked by changes in permeability and fluid flow over time the porosity reaches values below 0 35 in the carbonate layers that reduce the fluid flow velocity from an initial value of 30 cm a 1 to a minimum value of about 2 cm a 1 these significant changes in the fluid flow system displace the depth of sulfate penetration the simulation predicts cycles of carbonate crust formation and dissolution with a duration of 2000 2700 years resulting in several distinct carbonate layers during periods of high fluid flow aom reaches rates over 1000 mol cm 1 a 1 and methane fluxes out of the sediment reach 200 mol cm 1 a 1 during periods of low fluid flow aom is about 450 mol cm 1 a 1 and the methane flux into the bottom water vanishes completely the oscillations are dampened so that fluid flow and biogeochemical turnover slowly approach steady state after about 7000 years towards the end of the simulation period showing a 1 m thick area at the surface with carbonate concentrations of about 25 wt flow oscillations may also impact the colonization of chemoautotrophic larva and bacteria in and on the sediment the frequency of precipitation dissolution cycles of 2000 2700 years is long enough for vent biota to react to changes in the sediments for example explaining the occurrence of buried calyptogena and acharax mussel shells at former vent sites that are not presently active the evaluation of side scan sonar data reveals large areas of cemented sediments and carbonate pavements north of the investigated site the situation found in the investigated core may be characterized as intermediate stage of carbonate cementation this ongoing process will also form a solid carbonate pavement at this site in the future

a natural hydrate dissolution experiment on complex multi component hydrates on the sea floor abstract dissolution of natural hydrate cores was measured using time lapse photography on the seafloor at barkley canyon 850 m depth and 4 17 c two types of hydrate fabrics in close contact with one another were studied a yellow hydrate stained with condensate oil and a white hydrate from thermogenic origins both fabrics contained methane as well as heavier hydrocarbons these multi component hydrates were calculated to be well within p t stability conditions 2 s while the white hydrate dissolved faster at 0 25 0 02 mmol gas m 2 s to determine the controlling mechanism for the observed changes in the hydrate cores experimental results were compared with an engineering correlation for convective mass transfer using water velocity as a fitting parameter the correlation agreed well with results from a previous dissolution experiment on well characterized synthetic hydrates even with a number of other unknowns when applied to the natural hydrate the mass transfer correlation predicted the dissolution rate within 20 this seafloor based experiment along with visual observations of seafloor hydrate dissolution over a 3 day period were used to further understand the fate of natural seafloor hydrates exposed on the seafloor by showing that mass transfer is the rate controlling mechanism for dissolution of these natural hydrate outcrops proper hydrodynamic calculations can be employed to give a refined estimate on hydrate dissolution rates

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

a silica deposit of possible tertiary age in the carboniferous limestone near birr county offaly ireland summary stony clays comprising weathered bryzoan rich chert and silica clay are found adjacent to carboniferous limestone on the west and a weathered dolomitized zone on the east near birr county offaly ireland a gravity survey together with a stratigraphy obtained from boreholes and other test borings suggests a depth of about 40 m and a surface width or diameter of approximately 60 m for the deposit indirect evidence points to a possible tertiary age the interpreted origin of the stony clays is by silification of carboniferous limestone and preservation by solution subsidence along a possible fault zone

paleo methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western svalbard we present stable isotope and geochemical data from four sediment cores from west of prins karls forland ca 340 m water depth offshore western svalbard recovered from close to sites of active methane seepage as well as from shallower water depths where methane seepage is not presently observed our analyses provide insight into the record of methane seepage in an area where ongoing ocean warming may be fueling the destabilization of shallow methane hydrate the 13c values of benthic and planktonic foraminifera at the methane seep sites show distinct intervals with negative values as low as 27 8 that do not coincide with the present day depth of the sulfate methane transition zone smtz these intervals are interpreted to record long term fluctuations in methane release at the present day landward limit of the gas hydrate stability zone ghsz shifts in the radiocarbon ages obtained from planktonic foraminifera toward older values are related to methane derived authigenic carbonate overgrowths of the foraminiferal tests and prevent us from establishing the chronology of seepage events at shallower water depths where seepage is not presently observed no record of past methane seepage is recorded in foraminifera from sediments spanning the last 14 ka cal bp 14c ams dating 13c values of foraminiferal carbonate tests appear to be much more sensitive to methane seepage than other sediment parameters by providing nucleation sites for authigenic carbonate precipitation foraminifera thus record the position of even a transiently stable smtz which is likely to be a characteristic of temporally variable methane fluxes

natural flux of greenhouse methane from the timor sea to the atmosphere 1 methane gas bubbles from the cornea seep were sampled at the sea surface in the timor sea continental shelf area in june 2005 total bubble gas flux was 0 076 to 0 76 l m 2 h 1 during the 6 h d 1 periods of low neap tides in june 2005 this bubble gas contained an average of 26 mmol ch4 l 1 and about 0 16 and 0 006 mmol l 1 of ethane and propane we estimate the daily flux from the sea surface to the atmosphere to be 0 012 to 0 12 mol ch4 m 2 d 1 or 0 13 to 1 3 t ch4 d 1 from an area of about 0 7 km2 this methane flux came from a 500 1400 m carbonate pavement dome on the seafloor at 84 m water depth the seep hard ground was swath mapped and 3 5 khz subbottom profile data indicate that the seep dome was strongly reflective with poor penetration into the subsurface consistent with the presence of a carbonate hard ground carbon and deuterium isotope ratios 13c 41 to 42 d 157 to 158 of the seep bubble gas indicate that this methane had a thermogenic origin and was in the same isotopic range as gas within the late cretaceous cornea oil and gas field we could not detect inputs of fluids containing nutrients or short lived radium isotopes at this site commonly associated with other cold seeps tens to a hundred of kilometers seaward from the cornea seep site water column dissolved methane concentrations in this sector of the timor sea shelf and slope were 100 500 times supersaturated with respect to the atmosphere and thus the water column is expected to be degassing additional methane to the atmosphere perhaps there are thousands of other methane seeps of similar magnitude to the cornea seep on this shelf and slope to account for all the excess dissolved methane 86 000 t measured in the water column these measured and calculated fluxes provide evidence for the hypothesis that shallow sea seeps may be a significant source of atmospheric methane in contrast to deep sea vents where most of the methane is dissolved and oxidized in seawater and does not reach the atmosphere

the geomorphological and geological importance of palaeosurfaces this special publication is a collection of papers which identifies describes and interprets the occurrence of palaeosurfaces in the geological record the concept of a palaeo surface is one which is generally understood across the geomorphological and geological sciences but according to discipline and application often differs in the emphasis and detail of its interpretation in order to encompass the widest range of contributions and views upon the subject of palaeosurfaces a broad remit was deliberately adopted during the preparation of this volume cations of ancient palaeosurfaces the research areas represented are geographically widespread and include examples from australia africa south america india and europe

oceanic gas hydrate instability and dissociation under climate change scenarios 1 global oceanic deposits of methane gas hydrate clathrate have been implicated as the main culprit for a repeated remarkably rapid sequence of global warming effects that occurred during the late quaternary period however the behavior of contemporary oceanic methane hydrate deposits subjected to rapid temperature changes like those predicted under future climate change scenarios is poorly understood and existing studies focus on deep hydrate deposits under equilibrium conditions in this study we simulate the dynamic response of several types of oceanic gas hydrate accumulations to temperature changes at the seafloor and assess the potential for methane release into the ecosystem the results suggest that while many deep hydrate deposits are indeed stable under the influence of rapid seafloor temperature variations shallow deposits such as those found in arctic regions or in the gulf of mexico can undergo rapid dissociation and produce significant carbon fluxes over a period of decades

ocean oxygen depletion due to decomposition of submarine methane hydrate global warming could decompose submarine methane hydrate and cause methane release into the ocean the released methane causes oxygen depletion via oxidation however its global impact is yet to be quantitatively investigated we have projected the potential impact of oxygen depletion due to methane hydrate decomposition via numerical modeling we find that the global methane hydrate inventory decreases by approximately 70 35 under four times twice the atmospheric co2 concentration and is accompanied by significant global oxygen depletion on a timescale of thousands of years in particular we demonstrate the great expansion of suboxic and hypoxic regions having adverse impact on marine organisms and ocean biogeochemical cycles this is because hydrate decomposition primarily occurs in the pacific ocean where present day seawater has low oxygen concentration besides the decrease in oxygen solubility and reduced ventilation associated with global warming the process described in this study is also important in oxygen depletion

impact of the arctic ocean atlantic water layer on siberian shelf hydrography this paper examines the role of the arctic ocean atlantic water aw in modifying the laptev sea shelf bottom hydrography on the basis of historical records from 1932 to 2008 field observations carried out in april may 2008 and 2002 2009 cross slope measurements a climatology of bottom hydrography demonstrates warming that extends offshore from the 30 50 m depth contour bottom layer temperature time series constructed from historical records links the laptev sea outer shelf to the aw boundary current transporting warm and saline water from the north atlantic the aw warming of the mid 1990s and the mid 2000s is consistent with outer shelf bottom temperature variability for april may 2008 we observed on shelf near bottom warm and saline water intrusions up to the 20 m isobath these intrusions are typically about 0 2 c warmer and 1 1 5 practical salinity units saltier than ambient water the 2002 2009 cross slope observations are suggestive for the continental slope upward heat flux from the aw to the overlying low halocline water lhw the lateral on shelf wind driven transport of the lhw then results in the bottom layer thermohaline anomalies recorded over the laptev sea shelf we also found that polynya induced vertical mixing may act as a drainage of the bottom layer permitting a relatively small portion of the aw heat to be directly released to the atmosphere finally we see no significant warming up until now over the laptev sea shelf deeper than 10 15 m in the historical record future climate change however may bring more intrusions of atlantic modified waters with potentially warmer temperature onto the shelf which could have a critical impact on the stability of offshore submarine permafrost

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

dynamic response of oceanic hydrate deposits to ocean temperature change the dynamic response of oceanic hydrate deposits to ocean temperature change matthew t reagan and george j moridis earth sciences division lawrence berkeley national laboratory 1 cyclotron rd berkeley ca 94720 usa

time dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing 1 buffett and archer 2004 showed a model of the steady state global ocean clathrate reservoir that reproduces drill data and predicts a global methane inventory of about 5000 gton c the model inventory is very sensitive to ocean temperature with factor of two changes in methane inventory in response to temperature changes of 1 5 c here we explore the potential time dependent behavior of the clathrate reservoir assuming simple first order rate constants for methane accumulation and meltdown in response to changes in the predicted steady state reservoir for melting we divide the reservoir into two parts one melting catastrophically and the other slowly evolution of clathrates through geologic time is very sensitive to the choice of these rate constants a positive feedback exists during a transient meltdown event stronger if the catastrophic melting fraction is high and its melting timescale is fast a clathrate reservoir governed by such kinetics would melt down periodically on a multimillion year timescale determined by the rate constant for clathrate accumulation ideally we would like a model that predicts runaway melting in the paleocene and stability in the pleistocene however the model is most unstable when climate is cold rather than warm opting for stability in the pleistocene constrains the melting kinetic parameters leaving one degree of freedom the grow in timescale must be 5 myr or longer to get the deglacial 13c right but shorter than 10 myr in order for clathrates to have built up through geologically recent cooling the model thus constrained predicts methane fluxes of 200 gton c or less on deglaciations but eventual releases of 2000 4000 gton c in response to a 2000 gton c anthropogenic carbon release anthropogenic climate change differs from a deglaciation in that it warms the ocean to temperatures not seen in millions of years

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

thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic communities gulf of mexico continental slope abstract where abundant at the sea floor thermogenic gas hydrates impact bacterially mediated processes in chemosynthetic communities dependent on methane and h2s our main gas hydrate sites are at 540 m water depth and relatively low temperature 7 c gas hydrates outcrop as vein fillings in hemipelagic muds near gas vents within chemosynthetic communities molecular and isotopic properties of hydrate forming c1 c5 hydrocarbons and co2 provide insight to bacterially mediated processes hydrate bound methane is altered by bacterial oxidation as indicated by enrichment of 13c and deuterium d and by co2 depleted in 13c the degree of gas hydrate alteration appears related to duration of exposure at the sea floor in hydrate associated sediments bacterial oxidation of a mixed pool of hydrocarbons yields a net production of co2 depleted in 13c bacterial oxidation of hydrate bound methane and free hydrocarbon gases in adjacent sediments could contribute to gas hydrate decomposition some thermogenic carbon in sediments could be recycled via methanogenesis to yield a net production of bacterial methane depleted in 13c our results strengthen the hypothesis that gas hydrates could favor life in other extreme environments at low temperatures

methane under saturated fluids in deep sea sediments implications for gas hydrate stability and rates of dissolution abstract deep sea sediments contain earth s largest reservoir of methane ch 4 3000 gtons c trapped within ice like crystals known as gas hydrates understanding the controls on gas hydrate stability is critical because methane released from hydrate destabilization is hypothesized to be a powerful agent of past and potentially future climate change hydrates are stable under high pressure low temperature moderate salinity and saturated gas conditions yet the degree of gas saturation is rarely known in nature because in situ dissolved pore water ch 4 concentrations are rarely measured here we report measurements of these concentrations in sediments immediately surrounding deep sea gas hydrate deposits and show that pore fluids are greatly under saturated with respect to expected values for equilibrium with methane gas hydrate this indicates that the hydrates are dissolving even though they are found within the appropriate pressure and temperature stability field however dissolution rates calculated from the in situ ch 4 data are significantly less than dissolution rates predicted for methane under saturated pore water in direct contact with pure methane gas hydrate if equilibrium ch 4 concentrations exist immediately adjacent to the hydrate surface diffusion retarding factors found naturally in ocean sediments such as oil coatings or biofilms appear to enhance stability in outcropping hydrate deposits the in situ seafloor evidence provided herein leads us to hypothesize that the stability of the worldwide hydrate deposits may be much greater than predicted from diffusion kinetics because biological microbial excretion of slime or surfactants and or physical processes oil coatings effectively armor and stabilize exposed hydrate surfaces substantially retarding their dissolution

bivalve shell horizons in seafloor pockmarks of the last glacial interglacial transition a thousand years of methane emissions in the arctic ocean we studied discrete bivalve shell horizons in two gravity cores from seafloor pockmarks on the vestnesa ridge 1200 m water depth and western svalbard 79 00 n 06 55 w to provide insight into the temporal and spatial dynamics of seabed methane seeps the shell beds dominated by two genera of the family vesicomyidae phreagena s l and isorropodon sp were 20 30 cm thick and centered at 250 400 cm deep in the cores the carbon isotope composition of inorganic 13c from 13 02 to 2 36 and organic 13c from 29 28 to 21 33 shell material and a two end member mixing model indicate that these taxa derived between 8 and 43 of their nutrition from chemosynthetic bacteria in addition negative 13c values for planktonic foraminifera 6 7 to 3 1 concretions identified as methane derived authigenic carbonates and pyrite encrusted fossil worm tubes at the shell horizons indicate a sustained paleo methane seep environment combining sedimentation rates with 14c ages for bivalve material from the shell horizons we estimate the horizons persisted for about 1000 years between approximately 17 707 and 16 680 years b p corrected the seepage event over a 1000 year time interval was most likely associated with regional stress related faulting and the subsequent release of overpressurized fluids

atmospheric methane from organic carbon mobilization in sedimentary basins the sleeping giant abstract the mass of organic carbon in sedimentary basins amounts to a staggering 1016 t dwarfing the mass contained in coal oil gas and all living systems by ten thousand fold the evolution of this giant mass during subsidence and uplift via chemical physical and biological processes not only controls fossil energy resource occurrence worldwide but also has the capacity for driving global climate only a tiny change in the degree of leakage particularly if focused through the hydrate cycle can result in globally significant greenhouse gas emissions to date neither climate models nor atmospheric co2 budget estimates have quantitatively included methane from thermal or microbial cracking of sedimentary organic matter deep in sedimentary basins recent estimates of average low latitude eocene surface temperatures beyond 30 c require extreme levels of atmospheric co2 methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric co2 to values as much as 20 times higher than pre industrial values increased natural gas emission could have been set in motion either by global tectonic processes such as pulses of activity in the global alpine fold belt leading to increased basin subsidence and maturation rates in the prolific jurassic and cretaceous organic rich sediments or by increased magmatic activity such as observed in the northern atlantic around the paleocene eocene boundary increased natural gas emission would have led to global warming that was accentuated by long lasting positive feedback effects through temperature transfer from the surface into sedimentary basins massive gas hydrate dissociation may have been an additional positive feedback factor during hyperthermals superimposed on long term warming such as the paleocene eocene thermal maximum petm as geologic sources may have contributed over one third of global atmospheric methane in pre industrial time variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years but also over geologically short periods of time earth system models linking atmospheric ocean and earth surface processes at different timescales with the sedimentary organic carbon cycle are the tools that need to be developed in order to investigate the role of methane from sedimentary basins in earth s climate

paleozoic and mesozoic high pressure metamorphism at the margin of ancestral north america in central yukon eclogite and blueschist in the yukon tanana and slide mountain terranes in the yukon and alaska preserve a partial record of middle paleozoic late paleozoic and early mesozoic subduction and subsequent unroofing near faro eclogites are commonly mylonitized and partially retrogressed the analysis of fe mg exchange between garnet and clinopyroxene suggests temperatures t in the 400 500 c range for eclogite and jadeite 40 mol quartz suggest minimum pressures p of 11 13 kilobar at that temperature range clinopyroxene garnet epidote quartz suggest pressure near 15 kilobar p h 2 o p s eclogite near last peak which contains brown hornblende late stage oligoclase and biotite yielded p t estimates up to 625 c at a minimum pressure of 14 kilobar eclogite near ross river yielded temperatures near 525 c and a minimum pressure near 13 kilobar the previously proposed permian triassic age of high pressure tectonism for the entire yukon tanana terrane in the yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities in this study isotopic 40 ar 39 ar analysis of white mica from eclogite in the simpson range yielded a date of 344 1 ma eclogite and blueschist from ross river yielded white mica 40 ar 39 ar dates of 267 3 ma and 273 3 ma respectively white mica in eclogite from faro was dated at 260 3 ma white mica in two eclogites from stewart lake about 2 km apart yielded 40 ar 39 ar dates of 228 1 ma and 346 3 ma white mica in last peak eclogite for which a concordant u pb zircon date is 269 ma yielded an integrated date of 236 1 ma overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others the cooling ages indicate significant plate convergence as early as carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone when this is considered together with the range of ages of other high pressure occurrences the north american cordillera is seen to be characterized by multiepisodic and diachronous high pressure metamorphism

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

recent changes in shelf hydrography in the siberian arctic potential for subsea permafrost instability summer hydrographic data 1920 2009 show a dramatic warming of the bottom water layer over the eastern siberian shelf coastal zone 10 m depth since the mid 1980s by 2 1 c we attribute this warming to changes in the arctic atmosphere the enhanced summer cyclonicity results in warmer air temperatures and a reduction in ice extent mainly through thermodynamic melting this leads to a lengthening of the summer open water season and to more solar heating of the water column the permafrost modeling indicates however that a significant change in the permafrost depth lags behind the imposed changes in surface temperature and after 25 years of summer seafloor warming as observed from 1985 to 2009 the upper boundary of permafrost deepens only by 1 m thus the observed increase in temperature does not lead to a destabilization of methane bearing subsea permafrost or to an increase in methane emission the ch4 supersaturation recently reported from the eastern siberian shelf is believed to be the result of the degradation of subsea permafrost that is due to the long lasting warming initiated by permafrost submergence about 8000 years ago rather than from those triggered by recent arctic climate changes a significant degradation of subsea permafrost is expected to be detectable at the beginning of the next millennium until that time the simulated permafrost table shows a deepening down to 70 m below the seafloor that is considered to be important for the stability of the subsea permafrost and the permafrost related gas hydrate stability zone

the sensitivity of gas hydrate reservoirs to climate change perspectives from a new combined model for permafrost related and marine settings abstract gas hydrate reservoirs store large quantities of gas in sediments on continental margins in deep lakes and in continental and relic sub shelf permafrost the gas hydrate structure is only stable at sufficiently low temperature and high pressure and may therefore collapse under changing climatic conditions if a temperature rise or pressure drop e g through falling sea level is effective enough to dissociate hydrate deposits methane the most common gas component in hydrates and a potent greenhouse gas is released from the hydrate structure and may eventually enter into the atmosphere this may generate a positive feedback effect as resulting enhanced greenhouse gas levels would additionally warm the atmosphere and hence maintain or reinforce hydrate dissociation the significance of this mechanism has been debated over the past decades often within the framework of geologically rapid quaternary climatic oscillations and present day climate warming an extensive set of studies has addressed the climate sensitivity of gas hydrate reservoirs in various study areas and geological settings and by means of various approaches no real consensus has yet been reached on the matter in this study we seek to evaluate the sensitivity of gas hydrate reservoirs to changes in global climate from a more general perspective by firstly reviewing the available literature and secondly developing a new numerical model to quantify gas hydrate destabilization under changing environmental conditions qualities of the model include the wide applicability to both marine and permafrost related hydrate reservoirs and the integrative approach combining existing hydrate formation models with a dissocation model that accounts for the consumption of latent heat during hydrate dissociation to determine which settings are most vulnerable and to acquire insight into the extent fashion and rates of hydrate dissociation we apply the model to four distinct types of hydrate reservoirs across a hypothetic high latitude continental margin under two specific cases of climate change the last deglaciation following the last glacial maximum and present day climate warming the simulations indicate that hydrates on the upper continental slope and in association with thin sub shelf permafrost are most sensitive to the imposed climatic variations whereas deepwater and onshore permafrost related reservoirs react in a more stable way however the deep i e at several tens to hundreds of meters subsurface depth stratigraphic type hydrates considered in this study constitute by far the largest fraction of the global gas hydrate volume but dissociate on slow timescales of thousands to hundreds of thousands of years even in the most sensitive environments in contrast shallow i e at or a few meters below the surface or seafloor structural type hydrates are able to respond to climatic variations on sub millennial timescales but the volumes of gas they may release are probably insignificant to the global carbon cycle and climate quaternary and present day climate change do affect the stability of gas hydrate reservoirs but at long timescales where hydrate volumes are large and on short timescales where hydrate volumes are small consequently gas hydrates dissociate to an extent that is too small or at a pace that is too slow to create a strong positive feedback effect while the release of methane from the disintegration of gas hydrates is observed on different margins today it is not likely to have played a leading role in quaternary climatic variations or to become a significant process in the coming centuries as a result of present day rising temperatures

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

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western svalbard offshore western svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone llghsz 400 m water depth it is hypothesized that this methane may in part come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters to evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments and the role of anaerobic oxidation in regulating methane fluxes across the sediment seawater interface we have characterised the chemical and isotopic compositions of the gases and sediment pore waters the molecular and isotopic signatures of gas in the bubble plumes c1 c2 1 104 13c ch4 55 to 51 d ch4 187 to 184 are similar to gas hydrate recovered from within sediments 30 km away from the llghsz modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the llghsz providing no evidence for any recent change in methane supply due to gas hydrate dissociation however at greater water depths within the ghsz there is some evidence that the supply of methane to the shallow sediments has recently increased which is consistent with downslope retreat of the ghsz due to bottom water warming although other explanations are possible we estimate that the upward diffusive methane flux into shallow subsurface sediments close to the llghsz is 30 550 mmol m 2 yr 1 but it is 20 mmol m 2 yr 1 in sediments further away from the seafloor bubble plumes while anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10 of the total methane flux dissolved gas into the shallow subsurface sediments most of which escapes aom as it is transported in the gas phase

water permeability in hydrate bearing sediments a pore scale study permeability is a critical parameter governing methane flux and fluid flow in hydrate bearing sediments however limited valid data are available due to experimental challenges here we investigate the relationship between apparent water permeability k and hydrate saturation sh accounting for hydrate pore scale growth habit and meso scale heterogeneity results from capillary tube models rely on cross sectional tube shapes and hydrate pore habits thus are appropriate only for sediments with uniform hydrate distribution and known hydrate pore character given our pore network modeling results showing that accumulating hydrate in sediments decreases sediment porosity and increases hydraulic tortuosity we propose a modified kozeny carman model to characterize water permeability in hydrate bearing sediments this model agrees well with experimental results and can be easily implemented in reservoir simulators with no empirical variables other than sh results are also relevant to flow through other natural sediments that undergo diagenesis salt precipitation or bio clogging