geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part i detailed study of one oil seeping site abstract we report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data this paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites the oil seep is a complex shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre scale seafloor mounds a strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden the associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone bghsz that is used as a fluid migration route towards the crest of the diapir the combination of local depressions and seafloor amplitude anomalies linked with vertical high amplitude pipes rooted on the bghsz suggests a focused fluid flow towards the seafloor in peripheral areas the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub bottom profiler sections the combination of restricted size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas the proximity of actively oil supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally the thermogenic seep site is affected by two consistent and sub parallel reflections with negative polarity the first is interpreted as the methane related bghsz the second could correspond to the base of a thermogenic bghsz produced by a mixture of heavier gas the seafloor roughness and double bsr appear to be specific features of oil seep sites the geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation

inferred gas hydrates and clay diapirs near the storegga slide on the southern edge of the voring plateau offshore norway this paper presents a new data set including single channel airgun seismic lines okean long range side scan sonar data and gravity cores acquired on the edge of the voring plateau and in the region of the storrega slide the data acquisition was part of the 8th training through research ttr 8 expedition with r v professor logachev the acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom simulating reflectors bsrs at about 350 ms twt subbottom depth the lower bsr zone is located on the slope of the voring plateau in the immediate vicinity of the northern headwall of the storegga slide and in some places below the slide deposits this zone runs parallel to the general trend of the continental slope the spatial distribution of the upper bsr zone located upslope in an area where diapir like structures are found does not demonstrate any topographic control interpretation of high backscatter patches on the okean sonographs associates the observed structures with fluid escape features on the seabed most of them are pockmarks but in a few places diapirs are cropping out and form dome like elevations after analysing the behaviour of the bsr sometimes crosscutting and its acoustic characteristics reversed polarity and after applying seismic inversion processing to estimate the acoustic velocity change across the bsr this reflector is interpreted to represent the base of the local gas hydrate stability field ghsf this information was used to derive the regional geothermal gradient spatial variations of the inferred geothermal field appeared to be negligible the observation of two separated bsr zones suggests different environmental controls for the growth of the hydrates enhanced reflectors observed in the intermediate zone can be explained by the presence of strata bound free gas accumulations and migration combined with overlying permeability barriers therefore a model for gas hydrate formation in relation to directional fluid migration processes fluid escape features and the presence of diapiric structures is proposed in this paper our model is supported by sedimentological analyses of seafloor samples which demonstrate the presence of stiff clays with evidence of gas migration and by paleontological studies of the cores retrieved from the pockmarks the presence of a bsr in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event this observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm year finally the effect of hydrate dissociation on slope instability is considered assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re distributed through the whole sedimentary section above the paleo bsr with consuming heat and increasing pore pressure dissociation tends to shift the pt conditions back to equilibrium values it appears that the dissociation of hydrates due to sliding would cause only 0 2 increase of the pore pressure which would hardly contribute to further slope instability

vertical evolution of fluid venting structures in relation to gas flux in the neogene quaternary of the lower congo basin offshore angola abstract the processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970 a high resolution 3 d seismic survey from the continental slope of the lower congo basin allows us to study in detail the structures that developed before the formation of pockmarks as well as their subsequent geometric evolution this paper aims at reporting new type of fluid related structures that relate to pockmarks the objective of this research is to understand 1 the evolution of these structures and their genetic relationship with pockmarks and 2 how their morphologies may be influenced by changes in venting intensity over time pockmark and chimney structures in this survey are hosted within late miocene to quaternary hemipelagites above clastic reservoirs in a petroliferous basin they are all associated with positive high amplitude anomalies phaas on seismic data phaas are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units and by their association with seismic pull up effects from their planform and cross sectional geometry phaas are primarily interpreted as being due to the development of methane related carbonates but they may be associated with gas hydrates in one or two examples methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation amo once upwardly migrating methane enters the sulfate methane transition zone smtz typically within the first few tens of meters below the seabed therefore vertical accumulations of phaas are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo seabed or smtz timing of local scale fluid leakage can therefore be diagnosed by analyzing phaas at seep locations that occur in association with pockmark chimney structures a conceptual model for the evolution of different fluid venting structures is proposed here and is based on the analysis of venting structures that we expressed by 3 d seismic reflections in this model the main features are linked into a process based sequence culminating in the infill and termination of the pockmarks as fluid venting sites our model summarizes different rhythms of venting which are expressed by different types of phaas along with venting structures linear phaas and conduits inferred slow seeps they can be succeeded by sub circular phaas and shallow depressions which inferred slow to moderate seeps then the seep can become either a fast vent and terminated into a pockmark crater or it can become less active and be sealed afterwards subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations

gas venting that bypasses the feather edge of marine hydrate offshore mauritania methane can be released from the vast marine hydrate reservoirs that surround continents into oceans and perhaps the atmosphere but how these pathways work within the global carbon cycle now and during a warmer world is only partially understood here we use 3 d seismic data to identify what we interpret to be a gas venting system that bypasses the hydrate stability zone hsz offshore of mauritania this venting is manifested by the presence of the acoustic wipe out awo across a densely faulted succession above a salt diapir and a set of morphological features including a substantial 260 m wide and 32 m deep pockmark at the seabed the base of the hsz is marked by a bottom simulating reflector bsr which deflects upwards above the diapir rather than mimicking the seabed we use a numerical modelling to show that this deflection is caused by the underlying salt diapir it creates a trapping geometry for gas sealed by hydrate clogged sediment after entering the hsz some methane accumulated as hydrate in the levees of a buried canyon venting in this locality probably reduces the flux of gas to the landward limit of feather edge of hydrate reducing the volume of gas that would be susceptible for release during a warmer world

migration and venting of deep gases into the ocean through hydrate choked chimneys offshore korea it has recently been recognized that in addition to low concentrations of widespread natural gas hydrate associated with bottomsimulating seismic refl ectors highly concentrated hydrate can occur in local seafl oor fl uid venting structures such structures extend upward into the regional gas hydrate stability fi eld and sometimes allow gases to escape into the overlying ocean these hydrate choked chimneys are especially prospective as an energy resource because they contain high hydrate concentrations furthermore they may be one of the most important conduits into the ocean atmosphere system for deep methane we present an analysis of two dimensional seismic refl ection data from offshore korea that give a complete picture of gas migrating from a deep source zone to feed hydrate choked vent structures at the seafl oor the gases migrate upward through networks of fractures imaged as steep amplitude striations in both diffuse and concentrated distributions we present an example of a high fl ux gas vent fed through fracture swarms emanating from a 10 15 km wide catchment zone of source gases residing 3 4 km below the seabed the geological context and the inferred distribution of hydrate within this feature are consistent with recent models in which seabed gas venting is a consequence of elevated pore fl uid salinities that are produced by a high fl ux of gases migrating independently of the pore water in contrast a nearby vent that is not plumbed into the same high fl ux system appears to be dominated more by gas rich liquids we present these two vent structures as type cases for high and lowfl ux fl uid escape systems in the seabed furthermore we suggest that since the amount of gas trapped as hydrate within the vents is small compared with the amount in the underlying reservoir the greatest risk for increased methane input to the atmosphere associated with climate driven oceanic warming is not the melting of hydrate but an increase in the number of deep reservoirs able to vent gases through the seabed that occurs as the regional gas hydrate stability zone thins

occurrence and seismic characteristics of gas hydrate in the ulleung basin east sea abstract multi channel seismic reflection and well log data from the ulleung basin east sea reveal several seismic signatures indicative of gas hydrate occurrence in the ulleung basin that are associated with vertically and or laterally stacked mass transport complexes the seismic indicators include a a bottom simulating reflector bsr b a seismic chimney c high amplitude reflections within the gas hydrate stability zone ghsz d acoustic blanking e enhanced reflections below the bsr and f seafloor gas escape features the bsr associated with enhanced reflections below is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas seismic chimneys are characterized by velocity pull up and reduced reflectivity on the seismic sections which appear to be caused by active migration of fluid gas vertically into the ghsz the logging data retrieved from the seismic chimneys showed elevated electrical resistivity 80 ohm m and p wave velocity 2000 m s indicating the presence of gas hydrate another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections defined by the relatively high reflectivity within the ghsz acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers where the upward migrating gas seeps into the deep water column seafloor pockmarks and mud mounds may be formed gas hydrate was recovered from the ulleung basin east sea in 2010 during the second ulleung basin gas hydrate drilling expedition ubgh2 under the korean national gas hydrate program based on the results gas fluids migrate into the ghsz through two distinct pathways 1 structural conduits which include fault and fracture systems associated with seismic chimneys and 2 stratigraphic conduits associated with inclined turbidite hemipelagic layers two types of gas hydrate occurrence were identified in the basin 1 a stratally bound type pore filling within turbidite sand layers and 2 a locally concentrated type massive nodule or fracture filling within upward growing chimneys associated with near vertical faults relatively high concentrations of gas hydrate however tend to occur in localized seismic chimneys rather than in the strata related features the successful recovery indicates that the ulleung basin provides favorable conditions for gas hydrate formation

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

evidence for sediment eruption on deep sea floor gulf of mexico a large crater has been discovered on the sea floor gulf of mexico in a water depth of 2176 meters deep tow high resolution imagery shows that the crater is cut into a low hill surrounded by near surface concentric faults approximately 2 million cubic meters of ejected sediment forms a peripheral debris field the low hill and faults may be related to mud diapirism or intrusion of gas hydrates into near surface sediments a recent eruption evacuated sediments from the crater apparently because of release of overpressured petrogenic gas

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

seafloor instabilities and sediment deformation processes the need for integrated multi disciplinary investigations abstract in this paper we present the current practice of investigations of seafloor instabilities and deformation processes based on extensive research conducted over the last years which sets the scene for future research activities in this field the mapping of the continental margins and coastal areas with ever increasing resolution systematically reveals evidence of instabilities and deformation processes both active and palaeo features in order to properly assess the hazards and risks related to these features an integrated and multi disciplinary approach is essential but challenging such an approach consists of combining field data geophysics geology sedimentology geochemistry and geotechnical data with numerical simulations constrained by results from laboratory data as such it is of paramount importance to build a common knowledge base and understanding that unify these disciplines into more complete and conceptual models constrained by all the data we review the status of this integrated approach adapted to palaeo landslides e g storegga ana vesteralen and recent deformations finneidfjord nice gulf of guinea allowing to identify gaps in our knowledge at these sites by reviewing these case studies one can conclude that each case remains highly site specific in which both the regional and local geological tectonic settings have a distinct effect of the type of instability or deformation taking place or that can take place our knowledge on the actual triggers remains poorly constrained and there is even ambiguity for historic landslides e g finneidfjord also our knowledge of the preconditioning factors is incomplete there is a general lack of geotechnical data both in situ and from laboratory and therefore modelling the dynamics e g rheology of the instabilities relies on a number of assumptions rather than facts in addition excess pore pressure and its evolution is one of the key parameters driving instabilities despite this fact in situ excess pore pressure is rarely measured or monitored much work remains to be done to relate and integrate geotechnical data with geophysics e g through inversion and rock physical models in order to obtain additional quantitative information from the sub surface but also with respect to partial saturation free gas hydrate and pore pressure behaviour or lithologies it is of critical importance to be able to identify the different processes which can lead to hazardous situations which include establishing recurrence intervals timing and frequencies through event recognition and age control and magnitudes so that proper mitigation measures can be developed in this perspective the smaller scale instabilities deserve much attention as there are many instances where such features had far reaching consequences for society e g nice finneidfjord in that perspective human interferences e g exploitation drilling blasting loading must be one of the factors that should be taken into consideration

the geochemical characteristics of the hydrate bound gases from the nyegga pockmark field norwegian sea the nyegga pockmark field located south east of the voring plateau norwegian sea was investigated on the uob cruises gs07 148 2007 and gs08 155 2008 remotely operated vehicle rov monitoring and successful recovery of gas hydrate give insights to the origin of the hydrocarbon gases in the complex pockmark feature named g11 isotope values of methane 13c ranging from 72 4 to 66 2 and d from 202 0 to 198 0 in the hydrate bound gas suggest that the methane is microbially produced and originates from co2 reduction it is proposed that the ethane from the gas hydrate has mixed thermogenic and microbial contributions which is supported by the fact that the gas hydrate also contains propane and isobutane the composition of the hydrate bound gas thus leads to small inclusions of structure ii sii in the hydrate while the major part is structure i si the isotope data and gas composition together with the geological setting suggest that the hydrate bound gas originated predominantly from a free gas system located beneath the gas hydrate stability zone and accumulated under glaciated debris formations the free gas system is mainly microbial methane from in situ production however a contribution of thermogenic gas from deeper polygonal faults to the free gas system seems evident from the composition of the gas the rov monitoring strengthens previous suggestions that the fauna in the complex pockmark g11 are supported by ongoing micro seepage

assessing marine gas emission activity and contribution to the atmospheric methane inventory a multidisciplinary approach from the dutch dogger bank seep area north sea abstract we present a comprehensive study showing new results from a shallow gas seep area in 40 m water depth located in the north sea netherlands sector b13 that we call dutch dogger bank seep area it has been postulated that methane presumably originating from a gas reservoir in 600 m depth below the seafloor is naturally leaking to the seafloor our ship based subbottom echosounder data indicate that the migrating gas is trapped in numerous gas pockets in the shallow sediments the gas pockets are located at the boundary between the top of the late pliocene section and overlying fine grained sediments which were deposited during the early holocene marine transgression after the last glaciation we mapped gas emissions during three r v heincke cruises in 2014 2015 and 2016 and repeatedly observed up to 850 flares in the study area most of them 80 were concentrated at five flare clusters our repeated analysis revealed spatial similarities of seep clusters but also heterogeneities in emission intensities a first calculation of the methane released from these clusters into the water column revealed a flow rate of 277 l min sd5140 with two clusters emitting 132 and 142 l min representing the most significant seepage sites above these two flare clusters elevated methane concentrations were recorded in atmospheric measurements our results illustrate the effective transport of methane via gas bubbles through a 40 m water column and furthermore provide an estimate of the emission rate needed to allow for a contribution to the atmospheric methane concentration

authigenic carbonates in upper miocene sediments of the tertiary piedmont basin nw italy vestiges of an ancient gas hydrate stability zone a wide array of carbonate rich rocks has been recognized in the tertiary piedmont basin nw italy hosted in lower messinian slope deposits carbonate cements show negative 13 c values and positive 18 o values suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabilization two groups of rocks have been distinguished 1 lucina bearing mud breccias representing the seafloor product of an ancient seepage site and 2 lucina free concretions originating below the sediment water interface within this group two subtypes have been further distinguished stratiform concretions and cylindrical concretions stratiform concretions result from precipitation of dolomite in the pores of muddy sediments some of them display a brecciated structure others show a network of septarian like cracks that are empty filled with sediments or zoned carbonate cements their internal features are related to the formation of gas hydrates within the sediments and to their destabilization thus these rocks mark a portion of the sedimentary column located within a paleo gas hydrate stability zone cylindrical concretions represent ancient fluid conduits related to the upward migration of ch 4 rich fluids subsequent to gas hydrate destabilization the carbonate rich rocks of the tertiary piedmont basin stand as one of the first examples of methane derived rocks that record successive episodes of dissociation and re formation of gas hydrates and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone

geological controls on fluid flow and gas hydrate pingo development on the barents sea margin vista a basic research program in collaboration between the norwegian academy of science and letters and equinor

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

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

comparison and implications from strikingly different authigenic carbonates in a nyegga complex pockmark g11 norwegian sea authigenic carbonates sampled in a complex pockmark g11 from the nyegga area south western voring basin reveal strikingly different morphologies a comparative study of macro and micro features of these neighbouring samples was approached in order to attempt a reconstruction of the pockmark dynamics the carbonate morphologies include thin 1 cm platy carbonate crusts blocky and massive carbonate ridges up to several metres in size and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity our observations indicate that three distinct mechanisms of fluid seepage and carbonate precipitation occurred within the pockmark petrography studies reveal that the samples contain three generations of predominantly aragonitic cements in addition zoned crystals with coexisting mg calcite and dolomite are abundant in one sample suggesting a trend of gradual increase of the sulphate concentrations in the pore fluids during carbonate precipitation isotope measurements on bulk carbonate cements show consistent values with 13 c as low as 52 1 18 o as high as 6 4 and 87 86 sr of 0 7092 anaerobic oxidation of methane close to seafloor conditions can explain these isotope data the carbonates have a common carbon source despite the differences in morphology methane probably originated from the dissolution of gas hydrates as supported from calculated 18 o enrichment in the pore fluids and the methanogenic carbon we propose a model where the differences in bulk carbonate morphology are related to local sediment properties depth of carbonate precipitation and the flow rate of the seeping fluids

geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part ii a regional validation abstract the lower congo basin is known to discharge a substantial volume of oil towards the sea surface from more than one hundred seafloor seep sites distributed throughout the deep province of the lower congo basin a large geochemical coring survey confirmed the presence of oil on the seabed the combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets this study aims to test the characteristics previously identified at regional scale the active discharge zone is limited to the distal province of the basin it is characterised by strong compression shortening due to the sliding of the post salt super sequence resulting in numerous salt diapirs that control the location of seafloor oil seep sites the paper describes the bathymetric reflectivity amplitude and sub bottom profiler characteristics of a group of thermogenic seep sites they correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone the sub bottom profiler data shows that the hummocky mounds are connected by a network of faults to high amplitude bodies buried under a consistent sediment thickness based on the definition of specific geophysical features seafloor mounds complex shape and irregular pockmarks positive anomalies of seafloor amplitude high amplitude vertical pipes we identified a series of potential oil seep sites at basin scale the mounds are particularly recognizable using the seismic curvature attribute we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35 240 per km2 they are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile unusual double bsrs occur over the study area these are also a specific feature of thermogenic seep sites barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface the proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite based monitoring or gas dominated seep sites

evidence from three dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid escape chimney in the nyegga pockmark field offshore norway in recent years it has become evident that features commonly called gas chimneys provide major routes for methane to pass through the methane hydrate stability zone in continental margins and escape to the ocean one of many such chimneys lying beneath pockmarks in the southeastern voring plateau off norway was investigated with a high resolution seismic experiment employing a 2 d array of sixteen 4 component ocean bottom seismic recorders at approximately 100 m separation and a dense network of shots to define the 3 d variation of the chimney s structure and seismic properties the tomographic model derived from p wave travel times shows that p wave velocity inside the chimney is up to 300 m s higher than in the surrounding strata within the methane hydrate stability zone the zone of anomalously high velocity about 500 m wide near its base narrowing to about 200 m near the seabed extends to a depth of 250 m below the seafloor the depth extent of this zone and absence of high velocity beneath the base of the methane hydrate stability field make it more likely that it contains hydrate rather than carbonate if a predominantly fracture filling model is appropriate for the formation of hydrate in low permeability sediment the maximum hydrate concentration in the chimney is estimated to be 14 27 by total volume depending on how host sediment properties are affected by hydrate formation doming of the strata penetrated by the chimney appears to be associated with the emplacement of hydrate accompanying the invasion of the gas hydrate stability zone by free gas

stress path testing of an anisotropic sandstone the berea sandstone used in this study is transversely isotropic with respect to elastic response with p wave velocities of 2 160 m s normal to bedding and 2 290 m s parallel to bedding a variation of only 6 triaxial compression and extension tests involving failure by loading and unloading were performed along the two directions of symmetry with axial stress applied parallel to bedding the internal friction angle was approximately 55 for compression and extension indicating no intermediate stress effect for the linear mohr coulomb criterion however for axial stress normal to bedding the friction angle in compression was 50 whereas in extension it was 44 this anomalous behavior was attributed to strength anisotropy of the sandstone

pore fluid chemistry of the north anatolian fault zone in the sea of marmara a diversity of sources and processes as part of the 2007 marnaut cruise in the sea of marmara an investigation of the pore fluid chemistry of sites along the main marmara fault zone was conducted the goal was to define the spatial relationship between active faults and fluid outlets and to determine the sources and evolution of the fluids sites included basin bounding transtensional faults and strike slip faults cutting through the topographic highs the basin pore fluids are dominated by simple mixing of bottom water with a brackish low density pleistocene lake marmara end member that is advecting buoyantly and or diffusing from a relatively shallow depth this mix is overprinted by shallow redox reactions and carbonate precipitation the ridge sites are more complex with evidence for deep sourced fluids including thermogenic gas and evidence for both silicate and carbonate diagenetic processes one site on the western high displayed two mound structures that appear to be chemoherms atop a deep seated fluid conduit the fluids being expelled are brines of up to twice seawater salinity with an exotic fluid chemistry extremely high in li sr and ba oil globules were observed both at the surface and in cores and type ii gas hydrates of thermogenic origin were recovered hydrate formation near the seafloor contributes to increase brine concentration but cannot explain their chemical composition which appears to be influenced by diagenetic reactions at temperatures of 75 c 150 c hence a potential source for fluids at this site is the water associated with the reservoir from which the gas and oil is seeping which has been shown to be related to the thrace basin hydrocarbon system our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes

the spatial and temporal distribution of pipe formation offshore namibia a group of nearly 400 pipe structures from the continental slope of northern namibia are analysed for their spatial and temporal distribution the pipes most likely formed as a result of highly focused fluid venting and understanding the factors controlling their distribution in space and time is key to their genesis we analysed their spatio temporal distribution using an arbitrary chronostratigraphic timescale from which it is concluded that the pipes did not form at the same time pipe formation is shown to be intermittent and persistent with 2 29 pipes forming in each of the 20 arbitrary time intervals that are considered to span the neogene period the spatial distribution of these pipes is clustered to dispersed spatial statistics conducted on the distribution of pipe formation timings have shown that two statistically significant groups of pipes exist within the population 1 in the north and west and 2 in the south with the former occurring prior to the latter locally pipe formation is sporadic with clusters and outliers occurring during the same time period a conceptual model is proposed whereby pipe formation in specific locations is the result of localised breaching of the seals for isolated pressure cells which are locally independent yet broadly controlled an inferred basinal fluid source is thought to determine the broader patterns of pipe formation and the focus of this fluid source shifts from north to south with time at a local scale multiple local factors interact producing a sporadic pipe formation distribution through a prolonged period of highly focused fluid migration once formed the pipes continued to focus fluids intermittently leading in some cases to later pockmark formation

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick and are common where gas is present in near seafloor sediments this paper investigates how gas might lead to pockmark formation the process is envisioned as follows a capillary seal traps gas beneath a fine grained sediment layer or layers perhaps layers whose pores have been reduced in size by hydrate crystallization gas accumulates until its pressure is sufficient for gas to invade the seal the seal then fails completely a unique aspect of capillary seals releasing a large fraction of the accumulated gas into an upward propagating gas chimney which displaces water like a piston as it rises near the seafloor the water flow causes the sediments to become quick i e liquefied in the sense that grain to grain contact is lost and the grains are suspended dynamically by the upward flow the quickened sediment is removed by ocean bottom currents and a pockmark is formed equations that approximately describe this gas piston water drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor for uniform near surface sediment permeability this is a buoyancy control not a permeability control the rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure plausible estimates of these parameters suggest gas chimneys at blake ridge could reach the seafloor in less than a decade or more than a century depending mainly on the permeability of the deforming near surface sediments since these become quick before gas is expelled gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation however detecting gas chimney growth might be a useful risk predictor any area underlain by a gas chimney that extends half way or more to the surface should be avoided c 2009 elsevier ltd all rights reserved

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

isotopic and trace element arguments for the lower crustal origin of hercynian granitoids and pre hercynian orthogneisses massif central france abstract the cadomian hercynian basement of the massif central france contains both pre metamorphic orthogneisses 600 410 ma and post metamorphic undeformed granitoids 360 275 ma major trace element and sr nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid a granodiorite monzogranites and leucogranites orthogneisses have moderately enriched ree patterns la yb 5 3 13 3 variable hree contents ybn 4 2 25 and strong negative eu anomalies eu eu 0 23 0 36 their tnd values are low 5 8 to 1 7 indicating a strong crustal component in the source but tsr values are also generally low 25 to 100 and overlap the field of age corrected isotopic values for lower crustal granulite xenoliths from the massif central the source of the orthogneisses is therefore considered to be dominated by a lower crustal meta igneous component with additional input from a lower crustal metasedimentary source granodiorites monzogranites tend to show strong lree enrichment la yb 11 26 with small eu anomalies eu eu 0 54 0 75 and ybn values between 6 and 8 5 these granitoids form the least evolved end members of a series trending to highly evolved leucogranites with similar la yb ratios 11 36 but with generally larger eu anomalies eu eu 0 24 0 62 and lower ybn values 3 8 initial isotopic ratios of the leucogranites tnd 7 9 to 4 5 and tsr usually 100 mostly fall within the field of age corrected lower crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower crustal metasediments granodiorites and monzogranites have tnd between 6 5 and 3 4 and tsr values

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

submarine pingoes indicators of shallow gas hydrates in a pockmark at nyegga norwegian sea complex pockmarks up to 300 m wide and 12 m deep are located in the nyegga area in the norwegian sea bathymetric data and direct visual documentation and sampling with rovs remotely operated vehicles have shown that these pockmarks contain abundant methane derived authigenic carbonate rocks furthermore geochemical results and the finding of seep associated organisms including tubeworms and bacteria shows that the pockmarks are still active fluid flow locations hovland m svensen h forsberg c f johansen h fichler c fossa j h jonsson r rueslatten h 2005 complex pockmarks with carbonateridges off mid norway products of sediment degassing marine geology 218 191 206 here we report the discovery of localized pingo like sediment mounds up to 1 m high and 4 m wide they occur inside one of the nyegga complex pockmarks g11 all of the seven structures we investigated have four characteristics in common 1 they have a positive topography rounded mounds and cones 2 they are partly covered in bacterial mats indicating ongoing fluid flow 3 they are partly covered in a carpet of small living tubeworms polychaetes which utilize methane 4 they have distinct corrosion pits on their surfaces indicating fluidization and point source corrosion of the covering sediments probably caused by localized sub surface hydrate dissociation we interpret the features as true submarine pingoes formed by the local accumulation of hydrate ice below the sediment surface it is inferred that the pingoes are formed as documented hydrocarbon gases methane ethane propane and butane migrate upwards through distinctive sub surface channels or conduits inside the pockmark we suggest that these submarine hydrate pingoes manifest the exact locations where fluid flow through the seafloor is currently active and that they can therefore be used as small scale indicators of active seepage

repeated fluid expulsion through sub seabed chimneys offshore norway in response to glacial cycles abstract focused fluid flow through sub seabed sediments is a common phenomenon on continental margins worldwide however the governing controls and timing of this fluid release have been difficult to understand in particular for fluid flow features buried beneath sub surface sediments a link between fluid flow activity ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated norwegian margin new high resolution p cable 3d seismic data from the nyegga area on the mid norwegian margin reveal at least two more periods of fluid expulsion from sub seabed sediments the 3d seismic data show depositional patterns within chimney features expressed as truncations of seismic horizons against the flanks of the chimneys the truncations are interpreted by analogy with present day observations as evidence for buried carbonate mounds and or sediment wash out during formation of pockmarks in the past the truncations are hence an indicator and a chronological marker for fluid expulsion in the past the classification of chimneys results in three major groups 1 chimneys that have been formed and consecutively reactivated one or two times during the last 200 kyr and that have a fluid flow expression at the present seafloor 2 chimneys that are approx 125 160 ka bp old without any associated fluid flow expression at the present day seafloor 3 chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum 18 25 ka bp the observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region the emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the naust formation at nyegga

methane cold seeps as biological oases in the high arctic deep sea cold seeps can support unique faunal communities via chemosynthetic interactions fueled by seabed emissions of hydrocarbons additionally cold seeps can enhance habitat complexity at the deep seafloor through the accretion of methane derived authigenic carbonates mdac we examined infaunal and megafaunal community structure at high arctic cold seeps through analyses of benthic samples and seafloor photographs from pockmarks exhibiting highly elevated methane concentrations in sediments and the water column at vestnesa ridge vr svalbard 79 n infaunal biomass and abundance were five times higher species richness was 2 5 times higher and diversity was 1 5 times higher at methane rich vestnesa compared to a nearby control region seabed photos reveal different faunal associations inside at the edge and outside vestnesa pockmarks brittle stars were the most common megafauna occurring on the soft bottom plains outside pockmarks microbial mats chemosymbiotic siboglinid worms and carbonate outcrops were prominent features inside the pockmarks and high trophic level predators aggregated around these features our faunal data visual observations and measurements of sediment characteristics indicate that methane is a key environmental driver of the biological system at vr we suggest that chemoautotrophic production enhances infaunal diversity abundance and biomass at the seep while mdac create a heterogeneous deep sea habitat leading to aggregation of heterotrophic conventional megafauna through this combination of rich infaunal and megafaunal associations the cold seeps of vr are benthic oases compared to the surrounding high arctic deep sea

temporal variability of in situ methane concentrations in gas hydrate bearing sediments near bullseye vent northern cascadia margin 1 to assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate in situ methane concentrations were measured near bullseye vent on the northern cascadia continental margin a long term sampling device collected overlying water and pore fluid samples from 25 cm above seafloor at the sediment water interface swi and 7 cmbsf centimeters below seafloor over a 9 month period august 2009 may 2010 these samples provide a record at 4 day resolution of in situ methane ethane propane sulfate and chloride concentrations as well as stable carbon isotope ratios of methane 13c ch4 and dissolved inorganic carbon 13c dic we show that pore fluids near the swi are saturated or supersaturated with respect to methane 80 mm and the methane flux from the seabed is variable over time we hypothesized that regional seismic activity controlled this variable ch4 flux in the northern cascadia continental margin setting however we found no direct correlation between earthquakes and ch4 flux we also posited alternative controls on ch4 flux variability such as storms regional oceanography and microbial activity again no direct correlation was seen this study takes first steps toward exploring which physical factors play a role in methane flux from hydrate bearing sediments

fluid migration directions inferred from gradient of time surfaces of the sub seabed the role of sub seabed topographically controlled fluid migration is assessed to improve our understanding of distributions of acoustic chimneys at the nyegga pockmark field on the mid norwegian continental margin 3d seismic data interpretations resulted in topographic gradients of seismic time surfaces and rms amplitude maps topographical gradient maps and flow tracing allowed identifying migration pathways and trapping locations for free gas within the shallow sub seabed the occurrence of acoustic chimneys pockmarks and mounds correlate with identified fluid migration pathways and gas trapping locations an important factor that controls the trapping locations and the lateral distribution of seeps on the seabed at nyegga is the variation through time of the depth of the base of the gas hydrate stability zone bghsz fluids can derive from gas hydrate systems that are suspected of being a biogenic source and or tertiary domes that are considered to show leakage of thermogenic fluids to the shallow geosphere

seismic evidence and formation mechanism of gas hydrates in the zhongjiannan basin western margin of the south china sea abstract an analysis of 3d seismic data from the zhongjiannan basin in the western margin of the south china sea scs reveals seismic evidence of gas hydrates and associated gases including pockmarks a bottom simulating reflector bsr enhanced reflection er reverse polarity reflection rpr and a dim amplitude zone daz the bsr mainly surrounds zhongjian island covering an area of 350 km2 in this 3d survey area the bsr area and pockmark area do not match each other where there is a pockmark developed there is no bsr the gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m a mature pockmark usually consists of an outside trough a middle ridge and one or more central pits with a diameter of several kilometers and a depth of several hundreds of meters the process of pockmark creation entails methane consumption dense faults in the study area efficiently transport fluid from large depths to the shallow layer supporting the formation of gas hydrate and ultimately the pockmark

seafloor depressions on the nigerian margin seabed morphology and sub seabed hydrate distribution abstract gas hydrate quantification using acoustic data requires proper knowledge of the mineralogy of their host sediment in this paper a petrophysical model allowing gh quantification at sites where mineralogy profiles are absent is proposed this approach is applied to a high gas flux pockmark system in the gulf of guinea where in situ acoustic and geotechnical measurements together with core measurements could have been correlated and tied to seismic data projections of the in situ measurements on seismic profiles have shown that the study area not only accommodates zones of shallow and dense gh but also zones where solid hydrate and free gas coexist as well as pockets of free gas further analysis of several seismic profiles has allowed illustrating the detailed gh occurrence zone within the study area estimate its volume and its occupancy ratio of the pockmark correlations between gh content and 3d bathymetry sections have allowed to draw a link between different gh contents and the morphology of the pockmark which also shares similarities with the morphology of the gh occurrence zone it accommodates

a large methane plume east of bear island barents sea implications for the marine methane cycle a pockmark field extending over 35 km2 at 74 54 n 27 3 e described by solheim and elverhoi 1993 was re surveyed and found to be covered with more than 30 steep sided craters between 300 and 700 m in diameter and up to 28 m deep the craters are thought to have been formed by an explosive gas eruption anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the barents sea shelf from the characteristic vertical decrease of methane towards the sea surface it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98 between 300 m depth and the sea surface degassing to the atmosphere is minimal based on the shape of the methane concentration gradient nevertheless the net flux of methane from this area of the barents sea is about 2 9 104 g ch4 km 2 yr 1 and thus in the upper range of the presently estimated global marine methane release this flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter the amount of methane consumed in the water column however is about 50 times greater and hence should significantly contribute to the marine carbon inventory

neodymium isotopic composition of quaternary island arc lavas from indonesia abstract 143 nd 144 nd ratios measured in quaternary lavas from java and the banda arc of indonesia range from 0 51242 to 0 51280 and exhibit an inverse correlation with 87 sr 86 sr isotopically the indonesian samples resemble andean rather than island arc lavas the samples from java plot either within or adjacent to the mantle array towards higher 87 sr 86 sr ratios samples from the banda arc and the anomalous calc alkaline volcano papandajan are characterized by relatively low 143 nd 144 nd and high 87 sr 86 sr ratios these characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas furthermore the banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust a high k trachyte from the alkaline volcano muriah java has isotopic characteristics of the mantle 143 nd 144 nd 0 51270 87 sr 86 sr 0 70424 which implies that the extreme enrichment in large ion lithophile elements in its source must have occurred only shortly before its formation the inferred 143 nd 144 nd ratio of the unmodified mantle beneath java and the banda arc is lower than that observed in mid ocean ridge basalt which may have important implications for a better understanding of the geochemical structure of the mantle

benthic macrofauna and megafauna assemblages in the arctic deep sea canada basin abstract the benthic component of an interdisciplinary expedition on icebreaker uscg healy analyzed box core samples and seafloor photographs to quantify deep sea assemblages in the canada basin and chukchi borderland area soft bottom metazoan macrofauna 250 m were collected with 33 box cores at 11 stations ranging from 817 to 3961 m water depth in the high arctic canada basin from 28 june to 25 july 2005 megabenthic epifauna was quantified with a digital camera platform suspended 2 89 2722 ind m 2 and total biomass ranged from 0 2 to 1 6 g wet weight 0 06 m 2 2 macrofauna diversity numbered between 8 and 55 taxa per station abundance biomass number of taxa and diversity indices of macrofauna declined significantly with water depth polychaetes crustaceans and bivalves dominated the macrofaunal densities biomass and species numbers the megafauna visible in the seabed photographs was assigned to a total of 67 provisional taxa the lowest taxon count per station was 11 and the highest was 51 at the pockmark station epifauna abundance was also highest in this pockmark mean 5 8 ind m 2 and generally lower at deepest stations 0 1 0 9 ind m 2 but was skewed by large numbers of the holothuroid kolga hyalina epifauna observed in the center of the pockmark was significantly different from all other station groups but did not include known chemosynthetic fauna or bacterial mats multivariate analysis indicated that station groupings chukchi slope canada basin abyss and the chukchi cap were significantly different in community composition in the macrofauna box corer samples but could not be similarly distinguished as such in the epifauna photographs

increased fluid flow activity in shallow sediments at the 3 km long hugin fracture in the central north sea the north sea hosts a wide variety of seafloor seeps that may be important for transfer of chemical species such as methane from the earth s interior to its exterior here we provide geochemical and geophysical evidence for fluid flow within shallow sediments at the recently discovered 3 km long hugin fracture in the central north sea although venting of gas bubbles was not observed concentrations of dissolved methane were significantly elevated up to six times background values in the water column at various locations above the fracture and microbial mats that form in the presence of methane were observed at the seafloor seismic amplitude anomalies revealed a bright spot at a fault bend that may be the source of the water column methane sediment porewaters recovered in close proximity to the hugin fracture indicate the presence of fluids from two different shallow 500m sources i a reduced fluid characterized by elevated methane concentrations and or high levels of dissolved sulfide up to 6 mmol l 1 and ii a low chlorinity fluid cl 305 mmol l 1 that has low levels of dissolved methane and or sulfide the area of the seafloor affected by the presence of methane enriched fluids is similar to the footprint of seepage from other morphological features in the north sea

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

high resolution 3d seismic data indicate focussed fluid migration pathways above polygonal fault systems of the mid norwegian margin seismic attribute analysis and interpretation of high resolution 3d and 2d seismic data reveal focussed fluid flow processes through the gas hydrate stability zone ghsz at the northern flank of the giant storegga slide diffusive fluid migration predominantly starts from a widespread polygonal fault system in fine grained sediments of the miocene kai formation the overlying 600 700 m thick plio pleistocene naust formation shows spatially related soft sediment deformation and overlying fluid conduits a low relief antiform structure connects to an overlying 250 m high 300 m wide and 3 km elongated columnar zone where seismic signatures suggest self enhanced permeability i e natural hydraulic fracturing push down effects create an elongated depression caused by increased gas accumulations where a cluster of vertical cylindrical acoustic pipe structures originates these pipe clusters pierce the ghsz and indicate focussed fluid flow pathways originating from potentially overpressured sediments high seismic reflection amplitudes at the seafloor above the pipe structures may indicate pockmarks with authigenic carbonates and or gas hydrates the observed objects and seismic features presented are not stand alone indicators for fluid flow but a joint perspective illustrates that they are vertically tied together providing new insights to the effects of focussed fluid flow

a predictive numerical model for potential mapping of the gas hydrate stability zone in the gulf of cadiz this paper presents a computational model for mapping the regional 3d distribution in which seafloor gas hydrates would be stable that is carried out in a geographical information system gis environment the construction of the model is comprised of three primary steps namely 1 the construction of surfaces for the various variables based on available 3d data seafloor temperature geothermal gradient and depth pressure 2 the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples and 3 the calculation of the thickness of the hydrate stability zone the solution is based on a transcendental function which is solved iteratively in a gis environment the model has been applied in the northernmost continental slope of the gulf of cadiz an area where an abundant supply for hydrate formation such as extensive hydrocarbon seeps diapirs and fault structures is combined with deep undercurrents and a complex seafloor morphology in the gulf of cadiz the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions and explains the geometry and distribution of geological structures derived from gas venting in the tasyo field gulf of cadiz and the generation of bsr levels on the upper continental slope

geophysical signatures of mud mounds at hydrocarbon seeps on the louisiana continental slope northern gulf of mexico abstract mud mounds are common features at cold seeps in many areas of the world they are abundant in the northern gulf of mexico where they occur at the numerous hydrocarbon seeps on the continental margin although the mounds are widely known their geophysical signatures and geology have not been widely studied in detail in this study we examined mounds in two areas of the upper louisiana slope using geophysical data of several types 11 12 khz side scan sonar 3 5 khz echo sounder profiles near bottom 2 12 khz chirp sonar profiles and three dimensional 3d multichannel seismic data in side scan images the mud mounds appear as ovoid high backscatter spots associated with larger irregular high backscatter zones along faults and fault complexes three different sonar responses were noted 1 a more or less uniform appearance 2 a central higher backscatter spot surrounded with a lower return halo bulls eye and 3 a more or less uniform high backscatter area with an acoustic dead spot near its center near bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics implying that the long range acoustic signature of a given mound results from the integration of seafloor characteristics in these records anomalous seafloor echo types include strong seafloor reflectors hardbottom or buried hardbottom acoustic wipe out attenuation and acoustic turbidity prolonged echoes cores from acoustically anomalous seafloor areas indicate that a variety of seep related material has been introduced into the near seafloor sediments the prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath seafloor seismic amplitude maps from these mounds are either generally uniform with a positive or negative anomaly or display a ring of higher reflectivity around the mound periphery positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging the ring feature is thought to result from the preferential concentration of seep debris in the mound periphery relative to the center some mounds which show no seismic reflection anomaly from the seafloor appear inactive altogether observations from this study are consistent with the mud mounds being formed by sedimentary volcanism the data imply they are holocene features that have a life cycle that includes hardbottom formation which progressively crusts over the fluid vent ending its activity

natural oil seepage at kobuleti ridge eastern black sea abstract analysis of advanced synthetic aperture radar satellite images in combination with water column and seafloor investigations documented natural oil seepage from pechori mound and colkheti seep in 1000 1200 m water depth in the eastern black sea offshore georgia hydroacoustic imaging of the water column using multibeam echosounder evidenced numerous gas emissions from both structures gas bubbles rose as high as 45 m below sea surface it is proposed that oil coatings around gas bubbles hamper their dissolution allowing them to reach the sea surface where widespread oil slicks are formed slow rise velocities 14 1 cm s 1 of bubbles with radii of 2 6 0 3 mm n 101 derived from video data obtained with the remotely operated vehicle marum quest4000 corroborate the assumption that bubbles are oil coated high resolution seafloor mapping with the autonomous underwater vehicle marum seal5000 at colkheti seep revealed a crater strewn morphology whose formation is explained by frequent rafting of shallow gas hydrate deposits satellite imaging of oil slicks on the sea surface above both sites indicates that oil seepage is rather persistent since 2003 an order of magnitude estimation of minimum oil seepage rates suggests discharge rates of 40 liters per hour from both sites the data presented are the first comprehensive description of oil seepage in the black sea

sources of methane inferred from pore water δ13c of dissolved inorganic carbon in pockmark g11 offshore mid norway abstract pockmark g11 is the most spectacular one among the pockmarks located at the southern border of voring plateau and 1 2 km away from the northern flank of the storegga slide mid norwegian continental margin for the first time detailed pore water geochemical studies were conducted to address methane hydrate occurrence methane seepage and associated geochemical processes and methane characteristics in the pockmark pore waters collected from five sediments cores inside and one sediment core outside the pockmark were analyzed for dissolved cl sulfate so42 total hydrogen sulfide h2s ca2 mg2 dissolved inorganic carbon dic 13cdic and 18o methane hydrates were recovered in all sediments below 0 75 m in a core inside pockmark g11 which is in good accordance with heavy oxygen isotope 1 9 to 2 3 smow and low cl concentrations 84 9 to 16 1 mm in pore waters pore water profiles indicate that upward migrating methane fluids are spatially variable in the pockmark with methane fluxes ranging from below detection in the center and outside to 0 30 0 54 mol m 2 a 1 inside in the cores with active methane fluxes maximum dic concentrations 19 4 to 21 5 mm and corresponding minimum 13cdic values 52 3 to 54 6 pdb occur within sulfate methane transition smt zones from 0 40 to 0 50 m below seafloor mbsf close to the seafloor simple mass balance modeling and 13cdic measurements within the smt zones suggest that methane in shallow sediments within pockmark g11 is microbial in origin pore water geochemistry and seabed observations suggest that methane fluxes inside pockmark g11 are episodic and derived mostly from the recycling of methane hydrate at depth during sediment burial

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

the quartz diorites of limousin elemental and isotopic evidence for devono carboniferous subduction in the hercynian belt of the french massif central a chain of late devonian early carboniferous intrusions on the western margin of the french massif central the limousin tonalite belt consists of quartz diorites which are unusual among french hercynian granitoids in their low silica contents 53 68 wt sio2 and abundant hornblende they represent the earliest and most primitive plutonism post dating the main phase of hercynian metamorphism and have chemical characteristics strongly resembling andesitic lavas of calc alkaline affinity they differ from other massif central hercynian granitoids in their low 87sr86sr360 0 7047 0 7059 and high nd360 0 7 to 1 8 their initial pb composition was rather homogeneous 206pb204pb360 18 15 18 38 207pb204pb360 15 57 15 62 208pb204pb360 38 06 38 26 and their 207pb204pbi values are significantly lower than those of other hercynian granitoids the limousin quartz diorites are considered to have originated from an enriched mantle with characteristics modified by subduction of oceanic crust and terrigenous sediments the primitive quartz dioritic magmas were then contaminated in the lower continental crust by an afc process involving acid metaigneous or metasedimentary lithologies the occurrence of such plutons in the earliest phase of hercynian plutonism in the massif central is probably related to minor subduction at an active continental margin

gas hydrate pingoes deep seafloor evidence of focused fluid flow on continental margins gas hydrates in the shallow subsurface form one of the largest reservoirs of methane in the global organic carbon cycle seafloor seeps and associated features represent the venting points of methane released from the shallow lithosphere to the hydrosphere and atmosphere here we document the discovery of seep related seafloor mounds in the kwanza basin offshore angola and employ high resolution three dimensional seismic analysis to unravel the subsurface plumbing system and the origin of mounds mounds with distinct morphologies and geophysical signatures illustrate different development stages associated with the formation and dissociation of shallow gas hydrate linked to thermogenic fluid migration along salt diapir flanks draining deeply buried salt minibasins the mounds are more than an order of magnitude larger than previously described submarine hydrate pingoes and comparable to hydraulic pingoes commonly found in terrestrial periglacial environments suggesting hydrate volumes of individual mounds up to 1 1 106 m3 equivalent to 2 0 108 m3 of methane gas the interpretation of seismically well defined seep related seafloor mounds brings new insight to the occurrence and development of concentrated near surface gas hydrate accumulations and their relationship with thermogenic fluid migration and host sediment properties along continental margins

a model for water transport and ice lensing in freezing soils a one dimensional model for water transport and ice lensing in incompressible saturated and solute free soil specimens is proposed for the simulation of small scale frost heave tests in the laboratory the model considers 1 open system freezing in which the variables t and pw are independent 2 an ice lens that continues to grow as long as enough energy is available in the frozen fringe to produce the work required for mass transfer to the ice lens and 3 a new ice lens that forms when the vertical strain in the frozen soil reaches the instantaneous tensile failure strain the proposed frost heave model is amenable to computer simulation procedures which provide predictions of rate of frost heave and rate of pore freezing front penetration as functions of applied load thermal and water flow regimes and soil properties position time of initiation and ultimate thickness of individual ice lenses emerge also as a part of the solution a quantitative comparison of observed and simulated responses for various boundary conditions is shown for a reference soil known as devon silt the model was found to exhibit many of the characteristics observed in frost heave tests on devon silt and predicted the effects of overburden pressure on frost heave rate