reefs of the deep the biology and geology of cold water coral ecosystems coral reefs are generally associated with shallow tropical seas however recent deep ocean exploration using advanced acoustics and submersibles has revealed unexpectedly widespread and diverse coral ecosystems in deep waters on continental shelves slopes seamounts and ridge systems around the world advances reviewed here include the use of corals as paleoclimatic archives and their biogeological functioning biodiversity and biogeography threats to these fragile long lived and rich ecosystems are mounting the impacts of deep water trawling are already widespread and effects of ocean acidification are potentially devastating

elongated depressions associated with pockmarks in the western slope of the norwegian trench abstract terraces or elongated depressions have been surveyed in the western slope of the norwegian trench in origin the depressions seem to be closely related to the formation of pockmarks in the norwegian trench a study area d has been selected in the western slope for the description of the elongated depressions and their associated features an area of similar size e has also been selected in the norwegian trench in order to compare the recent geology of the two areas it is found that the elongated depressions probably are caused by a combination of shallow gas seepage the geometry of subsurface bedding and the action of bottom currents furthermore examples of faulting within soft silty unconsolidated clay is documented by use of deep towed boomer data the existence of such faults could explain how gas migrates through the upper layers

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

catastrophic fluid escape venting tunnels and related features associated with large submarine slides on the continental rise off vesteralen troms north norway abstract video inspection of the seabed on the continental rise ca 2200 m water depth off vesteralen troms north norway has revealed several remarkable features including large tunnels small tunnels chimney tunnels carbonate crusts and bacterial mats the structures occur in landslide areas with slide blocks up to 2 km long 1 km wide and 100 m high we hypothesize that the large seabed tunnels in and between slide blocks and with opening diameters up to 1 m and apparent inner diameters of 10 20 cm are formed by dewatering of fine grained sediments buried by the slide masses and possibly venting of underground gas reservoirs related to sliding and loading this process probably occurred instantaneously after sliding numerous small tunnels with diameters up to a few centimetres in slide blocks of compacted sediments may have formed by dewatering following sudden pressure release or by decomposition of gas hydrates following off loading of sediments due to sliding bacterial mats and carbonate crusts indicate active leakage of subsurface fluids and methane through the seabed and the gas hydrate stability zone either from thermogenic sources or from shallow reservoirs cemented chimney tunnels up to 15 cm tall and with inner diameters up to a few centimetres occur both on slide blocks and on flat and even seabed the presence of apparently both inactive and active chimney tunnels indicates that gas and fluid release is a process that has been periodically active possibly from before sliding up to the present

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

sea bed pockmarks related to fluid migration from mesozoic bedrock strata in the skagerrak offshore norway abstract in the deepest part of the skagerrak the sea floor morphology and the subsurface geology have been mapped from multibeam echo sounder data seismic data and bedrock cores an area of high pockmark density occurs along the shallowest part of a gentle ridge in the central norwegian trench where a middle jurassic sandstone unit subcrops below 30 40 m of late weichselian sediments in contrast few pockmarks occur at the seabed where lower and upper jurassic shales and clayey siltstones subcrop the concentration of pockmarks above the middle jurassic sandstone unit is thus attributed to seepage of gas or liquids migrating up dip from deeper parts of the sedimentary basin in the skagerrak more than 90 of the observed pockmarks occur above mesozoic sedimentary rocks while less than 10 occur in areas of palaeozoic metasediments or crystalline rocks a linear concentration of pockmarks is observed above a fault that terminates at the base of the quaternary succession and this is therefore attributed to seepage of gas or liquids along the fault plane the density of pockmarks is low in areas with thick successions of quaternary sediments indicating that the pockmarks are not related to seepage of biogenic gas from these deposits pockmarks in the skagerrak are thus caused by seepage of thermogenic gas and or other pore fluids from deeper sources and their distribution may help to improve our understanding of the fluid system and migration regime of this part of the north sea

evaluation of deepwater gas hydrate systems the world s offshore continental margins contain vast reserves of gas hydrate a frozen form of natural gas that is embedded in cold near seafloor strata published estimates suggest that the energy represented by gas hydrate may exceed the energy available from conventional fossil fuel by a factor of 2 or more understanding marine hydrate systems has become critical for long term worldwide energy planning groups in several nations are attempting to evaluate the resource and to define seafloor stability problems across hydrate accumulations

an integrated view of the methane system in the pockmarks at vestnesa ridge 79 n accepted manuscript version published version available at https doi org 10 1016 j margeo 2017 06 006 accepted manuscript version licensed cc by nc nd 4 0

acoustic and biological characteristics of seafloor depressions in the north yellow sea basin of china active fluid seepage in shallow water seafloor abstract high resolution multibeam bathymetry revealed the occurrence of numerous craterlike depressions so called pockmarks in the sea floor of the north yellow sea basin nysb to investigate whether these pockmarks are related to any type of fluid flow or gas seepage from subsurface stratum acoustic survey and box samplings were conducted in a selected area containing several typical pockmarks this allowed for an integrated analysis of the morphology distribution and subsurface strata as well as a direct comparison of seafloor characteristics among different areas inside and outside of a pockmark additionally surface sediments were analyzed for particle structure and grain size based on the combination of multibeam bathymetry and backscatter intensity data 282 seafloor depressions were accurately identified moreover the formation of pockmark can be attributed to existence of gas chimneys and other shallow gases revealed by shallow seismic profile records in the subsurface strata sediments inside and outside the pockmark showed remarkable discrepancies in terms of grain size and benthic fauna additionally strong internal reflection area was observed inside the depressions where benthic habitats are abundant and sediments contain strawberry pyrite and carbonate cementation side scan sonar also recorded sediment plumes in the water column which indicates that the pockmarks are still active several pearl string like accumulations of pockmark were found in the study area which well agree with the distribution of buried channels and lakes these results thus provide new insights in interpreting shallow water fluid seepage with implications for our current knowledge of overall sedimentation process in offshore continental shelves

seepage in isfjorden and its tributary fjords west spitsbergen abstract this study analyses pockmark morphology and their spatial distribution in isfjorden relative to seabed morphology bedrock geology fault systems glacial landforms and processes using multibeam bathymetric data it provides insight into the possible mechanisms of pockmark formation high density pockmark field evolution and fluid migration pathways a total of 1304 pockmarks occur in the isfjorden at water depths of 40 to 320 m varying from circular to elongate in plan view their diameter ranges from 14 to 265 m and their depths from 1 to 11 m elongate pockmarks are dominant in the isfjordbanken and outer isfjorden where the seafloor is influenced by the west spitsbergen current the west spitsbergen fold and thrust belt and other fault systems sub cropping at the isfjorden seafloor correlate spatially with the high density pockmark zones the pockmarks are preferentially located on the marine sediments draping the bedrock of isfjorden fjord system they are most abundant in the areas underlain by jurassic cretaceous and the triassic lower jurassic bedrock pockmarks found ahead of submarine slope failures may have formed due to dewatering of soft sediments as a result of rapid increase of overpressure caused by deposition of glacigenic debris lobes fault conduits potential source rock and thin postglacial sediment layers are found to be crucial for the formation of pockmarks modeling results of the near shore subsea permafrost and potential gas hydrate stability zone imply thawing permafrost and gas hydrate dissociation as additional possible mechanisms for pockmark formation in isfjorden

massive vein filling gas hydrate relation to ongoing gas migration from the deep subsurface in the gulf of mexico abstract a gas hydrate mound that contains massive vein filling structure ii gas hydrate occurs on the upper continental slope 540 m water depth of the gulf of mexico southwest of the mississippi delta the mound is located in the green canyon gc block 185 adjacent to jolliet field in gc 184 jolliet field contains oil and gas that filled fault traps caused by salt deformation during late pleistocene holocene time in contrast to reservoir oil in jolliet field which shows bacterial oxidation effects the c 1 c 5 reservoir gas is unaltered by bacterial oxidation disassociated gas is assumed to have recently entered from the subsurface hydrocarbon system vertical migration of gas along faults is ongoing manifested on the sea floor by gas vents gas hydrate complex chemosynthetic communities and by a large gas plume that extends from the vents to the sea surface the isotopic properties of c 1 c 5 hydrocarbons from reservoirs gas vents and gas hydrate correlate closely although outcropping gas hydrate is transiently stable because of variations in seawater temperature the bulk of buried gas hydrate at gc 185 is stable and perhaps increasing in volume because of the copious gas flux the massive accumulation of gas hydrate at the gc 185 site is attributed to the gas that has recently entered the vents largely from jolliet field and to the synchronous activation of fault conduits allowing gas migration to the sea floor synchronous late gas charge and faulting could also explain the wide distribution of gas hydrate across the upper gulf slope

assessing the impact of volcanic activity on mid holocene climate in ireland the need for replicate data analyses of pollen tephra mineral input and degree of peat humification from three neighbouring raised peat profiles at corlea central ireland covering the period of the deposition of a tephra

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

geochemical signatures of intense episodic anaerobic oxidation of methane in near surface sediments of a recently discovered cold seep kveithola trough nw barents sea abstract at cold seep environments the detection of large 34s enrichments within bulk sulphide minerals has been frequently used as a proxy to infer the vertical migrations of the sulphate methane transition zone smtz processes related to the oxidative part of the sulphur cycle can alter the original stable sulphur isotope composition 34s of the solid sulphur phases in particular at the sediment water interface identification of paleo and present positions of methane fronts in the sedimentary records requires the use of multiple geochemical proxies authigenic enrichment of mo has been demonstrated to represent a valid and durable tracer of the past smtz depth fluctuations and of the intensities of the related methane seepages this study is based on two closely spaced short cores collected from the main drift of the kveithola trough a glacially carved depression located in northwestern barents sea active fluid seepage has been recently reported in this area and a gas flare was specifically observed in proximity of the sampling sites where the cores object of our study were retrieved in order to collect evidences of the possible occurrence of anaerobic oxidation of methane aom at the sediment water interface of the investigated area and infer the entity of the associated methane flux we combined the analyses of reduced sulphur species 34s total organic carbon and redox sensitive elements the negative 34s values within the extracted solid sulphur phases up to 49 1 for pyritic sulphur show that organoclastic sulphate reduction osr coupled with disproportionation of sulphur intermediates are the only active processes in the near surface sediments however moderate to strong enrichments of mo detected in the relatively organic carbon poor intervals of both cores and the lack of concurrent enrichments of co cu ni v and zn usually associated to osr dominated environment suggested that the sulphidic conditions favouring the mo enrichments were produced by aom therefore we infer that high methane flux events characterized the drift area of the kveithola occasionally moving upward the smtz and thus inducing intense aom in proximity of the sediment water interface of this part of the trough our results confirmed the validity of sedimentary mo as an indicator of the occurrence and intensity of past methane seepages

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

cenozoic erosion and sedimentation in the western barents sea abstract seismic reflection data and well logs are used to interpret cenozoic sedimentary environments in the south western barents sea using volumetric calculations and compaction trend analysis the cenozoic erosion and sedimentation rates have been quantified marine environments ranging from neritic to bathyal water depths dominated the southern barents sea during the late palaeocene at the palaeocene eocene boundary a southwards progradation commenced along the uplifted and eroded loppa and stappen highs during the early eocene to mid miocene sedimentation continued in the intracratonic basins as well as along the newly established continent ocean boundary in the embryonic northern norwegian greenland sea substantial erosion occurred on the westernmost margin particularly on the stappen high during this time interval in the mid miocene during a period of relative lowstand of sea level a system of lowstand fans was deposited west of the stappen high later a pronounced progradation of the shelf edge commenced during the mid miocene to late pliocene the shelf edge prograded between 20 and 40 km during the mid pliocene to early pleistocene the barents sea and adjacent land areas were uplifted a fluvial glaciofluvial drainage system developed on the continental shelf resulting in pronounced erosion there and very high sedimentation rates on the continental margin during the last 0 8 ma several large glaciations on the barents sea shelf proper have resulted in overdeepened troughs and high sedimentation rates at the continental shelfbreak and on the continental slope the overall cenozoic erosion varies spatially as well as temporally in the svalbard area close to 3 km of rocks have been eroded since the oligocene the same amount or more was eroded from the stappen high during the palaeocene and early neogene in the south western areas during the eocene mid miocene erosion occurred along the margin in addition the loppa high and adjacent areas served as sediment source areas during the late pliocene and pleistoncene the average erosion in the south western barents sea amounted to about 600 m erosion was greater in the south east than to the west and north and was more extensive where the former glacial ice streams had flowed

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

acoustic imaging of gas hydrate and free gas at the storegga slide 1 analysis of p wave velocities of an ocean bottom cable obc data set demonstrates the existence of gas hydrates and free gas on the northern flank of the storegga slide the distribution and concentration of gas hydrates and free gas show significant variation over the length of the obc average gas hydrate saturation of pore space is 5 when modeled by effective medium theory with hydrates as a sediment frame component average gas saturation is 0 45 assuming homogeneous distribution the bottom simulating reflector bsr is generally identified as termination of enhanced reflection but in some places appears as a reflection proper amplitude anomalies related to the bsr are primarily caused by the gas and not the hydrates the gas hydrates at the storegga slide develop from gas rich fluids migrating into the gas hydrate stability zone ghsz from the sediments below development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area the physical and geological properties of the sediments control the distribution and concentration of gas which migrates predominantly along strata rather than along the base of the ghsz this migration mechanism controls the flux of gas rich fluids into the ghsz and hence the distribution of gas hydrates and the variation in gas hydrate concentration

characteristics of pockmarks in the norwegian trench a 3 km wide corridor across the norwegian trench was surveyed in detail with high resolution side scan sonar and subbottom profiler boomer in order to study pockmarks the pockmarks in the norwegian trench are dish shaped depressions found in the soft silty clays they range from 2 m to 300 m in diameter and appear with a maximum density of 40 50 per km2 lineated depressions and strings of small pockmarks were discovered the strings are generally composed of individual pockmarks 10 15 m in diameter each string can be up to 400 m long some medium sized pockmarks 40 50 m in diameter were found in groups often associated with a large parental pockmark the deep towed subbottom profiler records indicated columnar disturbances fountains in the sediments below some of the pockmarks these fountains are interpreted as vents or fissures through which gas or liquids migrate buried pockmarks and anomalies interpreted as boulders possibly gas bubbles trapped in the substrata were also found the recordings bear evidence of gas or liquid migration originating from layers lying beneath the soft clay sequence a large hydrocarbon gas reservoir has recently been found during exploration drilling in the immediate vicinity of the studied pockmark area pockmarks may be indicators of deep lying gas or oil reservoirs and therefore prove interesting features in relation to hydrocarbon prospecting

pockmark like depressions near the goliat hydrocarbon field barents sea morphology and genesis abstract pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion in the present study we have analysed multibeam bathymetry and 2d 3d seismic data from the south western barents sea in relation to gas hydrate stability field and sediment type to examine pockmark genesis seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation the seabed depths in the study area are just outside the methane hydrate stability field but the presence of higher order hydrocarbon gases such as ethane and or propane in the expelled fluids may cause localised gas hydrate formation the selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults the presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder coarser sediments likely linked to removal of soft material the pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids high seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath which is blocked by overlying less permeable formations in areas of consolidated sediments the upward flow is limited to open fault locations while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region through removal of fine grained material

postglacial carbonate production by cold water corals on the norwegian shelf and their role in the global carbonate budget cold water coral reefs have been neglected in calculations of global carbonate production we present the first calculations of the amount of caco 3 produced by cold water corals on the norwegian shelf in postglacial time showing that the reef building lophelia pertusa in norwegian waters contributes 54 188 g m 2 yr locally or 0 03 0 38 g m 2 yr averaged over the entire shelf these estimates indicate that cold water reef caco 3 flux is 4 12 of the tropical reefs a first tentative global estimate indicates that cold water coral caco 3 production could add 1 to the total marine caco 3 production the spatial and temporal evolution of cold water reefs and thus their influence on atmospheric co 2 is difficult to reconstruct but factors that dominantly influence the rate of coral growth include sea level and oceanographic changes submarine slides and anthropogenic activity the results show that other shelf and slope environments should be assessed in a similar manner to further constrain and evaluate the global caco 3 production by cold water corals and their possible contribution to the caco 3 budget

seismic characterization and continuity analysis of gas hydrate horizons near mallik research wells mackenzie delta canada gas hydrate accumulations located onshore in arctic permafrost regions are seen as a potential source of natural gas surprisingly most of the gas hydrate found in the mackenzie delta and beaufort sea areas was indirectly discovered or inferred from conventional hydrocarbon exploration programs one of these occurrences the mallik gas hydrate field figure 1 has received particular attention over the last 10 years two internationally partnered research well programs have intersected three intervals of gas hydrates and have allowed successful extraction of subpermafrost core samples with significant gas hydrates the gas hydrate intervals are up to 40 m thick and have high gas hydrate saturation sometimes exceeding 80 of pore volume of unconsolidated clastic sediments with average porosities from 25 40 at mallik the gas hydrate intervals are located at depths of 900 1100 m and are localized on the crest of an anticline

description and numerical model of pleistocene iceberg scours and ice keel turbated facies at toronto canada near toronto in southern ontario canada glaciolacustrine muds and shoreface sands of late pleistocene age early wisconsin glaciation 45 ka were deformed by icebergs ploughing the floor of an ancestral ice dammed lake ontario outcrop mapping and a ground penetrating radar survey identify ice scour structures up to 4 m deep up to 10 m wide and at least several hundred metres long with high standing lateral margins berms ploughing of lake floor sediment is also recorded by ice keel turbated facies in which storm deposited shoreface sands have been mechanically mixed with underlying muds palaeoenvironmental information for one prominent scour such as water depths estimates of the mass and draft of the ice mass and scouring forces can be determined using a simple finite element numerical model a mass of 0 04 million tonnes a keel draft of between 10 and 30 m and a scouring force of 5 103 kn is consistent with scouring by a small iceberg bergy bit or pressure ridged lake ice the former is preferred because of the considerable depth of the scour and the overall glacial depositional setting estimates of ice mass dimensions agree with independent reconstructions of palaeobathymetry no greater than 20 m derived from storm deposited hummocky and swaley cross stratified facies that fill the scour data presented here aid the identification of ice scour structures and turbated sediment in the ancient record despite being a common process on modern high latitude shelves descriptions of ancient structures and facies are few

quaternary geology and surficial sediment processes browns bank scotian shelf based on multibeam bathymetry abstract browns bank is located on the glaciated continental shelf off southern nova scotia geological mapping of browns bank is based on interpretation of multibeam bathymetric and backscatter data in conjunction with 220 line km of seismic reflection profiles and sidescan sonograms sea floor sediment samples and bottom photographs the fundian moraine part of the previously identified end moraine system on the continental shelf off nova scotia is a prominent west east flat topped multi lobate ridge identifiable in the multibeam bathymetric data and in geophysical records the fundian moraine was subjected to erosion in the surf zone during sea level rise in depths less than 100 m a series of north south roughly parallel till ridges continuous in the subsurface with the fundian moraine is interpreted as interlobate moraines suggesting formation by a tidewater glacier the browns bank moraine connected to the fundian moraine is evidence of a newly identified grounded ice position farther seaward than the latter bedforms including obstacle marks comet marks sand waves and megaripples are evidence of a vigorous anticyclonic current pattern on browns bank the combination of multibeam bathymetric imagery with high resolution geoscientific information represents a powerful technique for sea floor geological investigations

miocene shallow marine cold seep carbonate in fold and thrust western foothills sw taiwan a mound shaped authigenic carbonate buildup 50 m wide and 5 m high occurred in the middle part of the late miocene shallow marine succession the hunghuatzu formation exposed along the nantzuhsien river sw taiwan the carbonate concretions are classified into four types based on morphology which appeared to develop in upward sequence with vague boundaries 1 flat pipe shaped nodules highly dolomitic small pipes or nodules subparallel to the host strata 2 bulb shaped nodules discrete or combined small mesoclots with subspherical to irregular shapes 3 mushroom shaped concretions large mushroom like or irregular shaped blocks with chimney or vent structures and 4 carbonated layers with sedimentary structures these concretions are mainly composed of dolomite micrites quartz feldspar muscovite and clay minerals ubiquitously interweaved with blackish brown colored low magnesium calcite lmc veins low delta c 13 vpdb values 51 8 parts per thousand to 29 8 parts per thousand of all types of the carbonates reveal that the carbonates were mainly derived from anaerobic oxidation of hydrocarbons many samples are also o 18 depleted delta o 18 vpdb 11 5 parts per thousand to 1 00 parts per thousand due to the mixture with isotopically light lmc veins likely precipitated from meteoric waters there are numerous chemosymbiotic bivalve fossils genus monitilora diameter 3 4 cm in growth position just above the carbonate buildup according to the geological and isotopic signatures the hunghuatzu carbonates are related to a cold seep pseudobioherm which formed in a very shallow marine environment like tidal flat in a passive margin setting exposures of the hunghuatzu pseudobioherm may provide clues for tracking and comparing the evolution history of gas reservoir systems and hints of active tectonics of the sw taiwan region c 2012 elsevier ltd all rights reserved

large gas hydrate accumulations on the eastern nankai trough inferred from new high resolution 2 d seismic data previous studies have revealed the presence of a widespread bottom simulating reflector bsr on the eastern nankai slope as well as the occurrence of enigmatic high amplitude reflections that extend well above the bsr new high resolution 2 d seismic data were collected on the eastern nankai slope during the french japanese sfj cruise in year 2000 and ava analyses of the enigmatic reflectors are conducted at the studied location these analyses suggest that high amplitude anomalies above the bsr delineate the top of gas hydrate rich sediments several tens of the sediment porosity would be filled with gas hydrates between the bsr and a sharp boundary 30 to 60 m above to account for these observations we propose that an invasion by free gas of the present day gas hydrate stability zone occurred in the past several mechanisms for this intrusion are discussed

bottom currents interpreted from iceberg ploughmarks revealed by multibeam data at tromsoflaket barents sea abstract multibeam data have been acquired on eastern tromsoflaket a bank in the southwestern barents sea comprising a plateau detailed examination of the multibeam backscatter data however reveals a high variability of sediments especially in and around iceberg ploughmarks we observe coarser sediments on the levees either side of the ploughmarks than inside the furrows moreover we frequently note asymmetries in the distribution of fine sediments across these ploughmark features drawing inspiration from previous studies of bottom currents which use the distribution of sediments across bedforms e g sand waves we have interpreted the direction of bottom currents on tromsoflaket by examining the local distribution of fine sediments across iceberg ploughmarks which are the most prominent bedform in this area further integration of our results with existing grain size current speed models have allowed estimates of the bottom current speeds to be obtained such interpretations are particularly valuable in this area where availability of bottom current measurements sparse our approach makes valuable and novel use of multibeam data and results show good agreement with the limited set of available current data and models we can identify near bottom pathways for the norwegian atlantic current and norwegian coastal current the two major current systems in this region and the patterns of circulation help to explain the sedimentation regime and identify recent erosion and deposition areas

grounding pleistocene icebergs shape recent deep water coral reefs the widespread view that scleractinian corals in cold and deep waters of high latitudes are slow growing organisms that do not form reefs is challenged by the discovery of a huge coral reef over 13 km in length 10 to 35 m in height and up to 300 m in width formed by the coral lophelia pertusa in water depths of 270 to 310 m at 64 n on the sula ridge mid norwegian shelf cruises in 1994 1995 and manned submersible operations in may 1997 provide data and observations from which the structure and development of the sula ridge coral reef have been determined the fennoscandian icesheet retreated from the mid norwegian shelf prior to 12 000 years before present and modern oceanographic conditions were established at 8000 years before present coral growth since that time has resulted in a large deep water shelf reef for which recent stable isotopic studies have demonstrated high growth rates for these azooxanthellate cold water corals information on the geometry of deep water coral reefs and their environmental controls is still fragmentary controversial and raises issues of conservation in this area of active fishing and oil exploration this paper reports on the discovery of what is probably one of the largest deep water coral reefs existing in the northeast atlantic and indicates that its siting is due to post glacial structures iceberg plough marks events the second storegga slide and local conditions on the seafloor surprisingly reef accumulation rates on the sula ridge are comparable with those measured on tropical coral reefs

neogene and quaternary depositional environments on the norwegian continental margin 62 n 68 n abstract 2d multichannel seismic and mini sleeve gun profiles reveal that an up to 1 5 s twt thick sedimentary succession has been deposited on the norwegian continental margin 62 n 68 n during neogene and quaternary well defined miocene depocentres have evolved both in the north sea fan region and along the flanks of structural highs in the voring basin miocene sediments are on the other hand mainly absent within the storegga slide scar seismic facies analyses show that these deposits locally are characterised by a mounded and or migration aggradation pattern which we relate to a current influenced depositional environment hence the early neogene sediments on the norwegian continental margin are classified as contourites the contourites have most likely been deposited in connection with the establishment of a deep water exchange in the norwegian greenland sea due to the opening of the fram strait and subsidence of the greenland scotland ridge at about 2 5 ma a significant change in this depositional environment took place down slope sedimentary processes became now more important and throughout the late plio pleistocene sediments were mainly sourced from the norwegian mainland and the adjacent continental shelf causing depocentres to evolve along the shelf edge

petroleum system analysis of the hammerfest basin southwestern barents sea comparison of basin modelling and geochemical data abstract exploration in the hammerfest basin southwestern barents sea has proven several petroleum systems and plays with the presence of multiple source rocks of mainly jurassic and triassic age to date several fields and discoveries have been found and are described to mainly contain gaseous hydrocarbons with the presence in some cases of an oil leg our 3d hammerfest basin model shows that the jurassic hekkingen formation and the triassic snadd and kobbe formations reached high maturity levels gas window in the western and the northwestern margin at the same time this model reproduces the main hydrocarbon accumulations that have been found in the basin an analysis of the volumetrics and the proportion of oil and gas contributions to each field and discovery suggests that the gas contribution stems mainly from triassic source rocks while the oil phases contain variable proportions from the jurassic hekkingen formation and triassic source rocks gas isotope and maturity related biomarker ratios confirm the maturity trends derived from the basin modelling results light hydrocarbons indicate the influence of secondary processes biodegradation and long distance migration in the petroleum from the goliat field and the tornerose discovery age related biomarker ratios such as the etr extended tricyclic terpane ratio and the c28 c29 steranes ratio did not provide a clear separation when evaluating a contribution from jurassic vs triassic source rocks

seismic evidence for fluid escape from mesozoic cuesta type topography in the skagerrak abstract gas and fluid dynamic processes in the mesozoic to recent strata of the southern slope and center of the norwegian trench skagerrak have been analyzed by high resolution multi channel seismic data collected with a watergun as the seismic source on board of rv heincke in spring 2003 three profiles across the southern slope and the center of the trench and four profiles cover the north eastern termination of the trench the data resolve the quaternary succession and the subcropping mesozoic strata the separating unconformity shows a typical cuesta topography caused by glacial erosion on the slope contourite drifts are present above a water depth of approximately 400 m internal reflections of the quaternary are obscured by acoustic turbidity particularly phase reversed bright reflections occurring in the entire lower jurassic to upper cretaceous succession suggest the presence of fluids therein sea floor pockmarks have been observed on the southern slope and in the center of the trench both slope and trench pockmarks are vertically correlated with the sub crop of fluid charged mesozoic rocks beneath which indicate lateral fluid escape from porous mesozoic rocks fluids that migrate towards a slope pockmark are capable to wash out gas from the quaternary leaving gas free patches behind since no thermogenic gas has ever been measured in core samples from the slope and trench freshwater seepage is a possible explanation in this case the water outflow might represent artesian springs

multiple episodes of fluid flow in the sw barents sea loppa high evidenced by gas flares pockmarks and gas hydrate accumulation abstract gas and gas hydrates have gained considerable attention in the last 20 yr due to their possible impact on global climate change from methane escalations our study area the sw barents sea is unique in its evolution due to the effect of glaciations which removed thick layers of sediment from the seabed unloading due to erosion and deglaciation resulted in opening of pre existing faults and creation of new ones facilitating fluid migration and eventually escape into the water from the subsurface expressions of hydrocarbon gas accumulation and fluid flow such as gas hydrates and pockmarks are widely distributed in the barents sea several gas flares some of them 200 m high in echograms occur along a segment of the ringvassoy loppa fault complex indicating open fractures and active fluid flow observation of gas flares along regional fault complexes outside pockmark regions indicates that present gas escape occurs along faults only the pockmark formation and gas flares belong to different episodes of fluid flow the pockmarks are absent outside soft sediment depocentres indicating that their existence is related to the presence of recording medium the thin sediment cover in pockmarks and their penetration down into glaciomarine sediments indicate that they formed after deposition of these sediments and that fluid escape was active to the very recent past methane hydrate stability zone mhsz modelling shows that after the last glacial maximum 18 000 20 000 14c years ago the mhsz has thinned from 600 m to zero in most parts of the sw barents sea the fluid expulsion probably happened after retreat of the glaciers causing release of methane from dissociation of methane hydrates through fluid escape processes which lasted until recently bottom simulating reflectors bsr observed are probably due to structure ii hydrates formed from gas containing higher order hydrocarbon components such as ethane propane etc along with methane the study area falls close to the new gigantic oil discoveries skrugard and havis and north of the snohvit hydrocarbon field

holocene environmental history of the aishihik region yukon canada a 2 7 m sediment core from wa01 a small pond in southwest yukon canada unofficial name 61 14 41 n 136 55 35 w 1000 m above sea level documents the postglacial vegetation and climate history of the aishihik basin open birch shrub tundra established at the site ca 10 900 cal years bp immediately after deglaciation picea glauca established ca 10 200 cal years bp and an open spruce forest ecosystem has since remained dominant on the landscape a gradual increase in alnus crispa starting ca 6600 cal years bp has also been noted in other studies from central yukon suggesting a significant climate transition in yukon at this time an alternating pattern in sediment loss on ignition also beginning ca 6600 cal years bp may be broadly related to trends in 18o values from the mount logan and jellybean lake oxygen isotope records and is most likely reflective of periodic changes in available moisture in the region resulting from alterations in the strength and position of the aleutian low press

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

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

acoustic evidence of shallow gas accumulations and active pockmarks in the izmir gulf aegean sea abstract based on high resolution chirp seismic multibeam bathymetry and side scan sonar data collected in the izmir gulf aegean sea in 2008 and 2010 gas related structures have been identified which can be classified into three categories 1 shallow gas accumulations and gas chimneys 2 mud diapirs and 3 active and inactive pockmarks on the chirp profiles shallow gas accumulations were observed along the northern coastline of the outer izmir gulf at 3 20 m below the seabed they appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic rich highstand fan sediments from the gediz river the diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter clockwise rotation of anatolian microplate furthermore the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs several pockmarks exist close to fault traces to the east of hekim island most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008 another chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active the surveys indicate that the gas seep is an ongoing process in the gulf based on the chirp data we proposed that the pockmark formation in the area can be explained by protracted seep model whereby sediment erosion and re distribution along pockmark walls result from ongoing or long lasting seepage of fluids over long periods of time the existence of inactive pockmarks in the vicinity however implies that gas seepage may eventually cease or that it is periodic most of the active pockmarks are located over the fault planes likely indicating that the gas seepage is controlled by active faulting

iceberg crater marks on the sea floor labrador shelf abstract sidescan sonograms of the labrador seabed show evidence of numerous subcircular iceberg generated impact craters some of these craters are found in linear associations crater chains having been formed by a single iceberg keel alternately impacting and rotating free of the seabed a three degrees of freedom iceberg model is used to generate crater chain configurations an unstable trapezoid shaped iceberg of 6 million t with a draft of 140 m in a water depth of 160 m is used in the simulations soil pressures acting on the penetrating keel introduce a vertical heave motion which depending on phase relationships with the swinging motions of the iceberg results in a variety of noncoherent crater depths and crater chain morphologies including chains with skips where craters appear to be missing on sidescan sonograms depending on the position of the final static orientation of the iceberg keel it may generate a crater chain if the keel comes to rest above the seabed or a crater chain may lead into a normal scour if the keel remains below the seabed both configurations are observed in nature

fluid migration and fluid seepage in the connemara field porcupine basin interpreted from industrial 3d seismic and well data combined with high resolution site survey data this study documents the suite of processes associated with source to seafloor fluid migration in the connemara field area on the basis of 3d seismic data well logs 2d high resolution seismic profiles subbottom profiles short cores and sidescan sonar data the combination of datasets yields details about fluid migration pathways in the deep subsurface in the unlithified shallow subsurface and about the distribution of fluid and gas seeps pockmarks at the sea floor the connemara field area is characterized by vertical fluid migration pathways seismic chimneys or gas chimneys that extend from the top of the jurassic sequence cross cutting the entire cretaceous sequence to the upper tertiary deposits over a vertical distance of up to 1 5 km their localization is mainly structurally controlled to the crest of tilted fault blocks along the main hydrocarbon migration pathways these chimneys are important conduits for focused vertical fluid gas flow from the deep to the shallow subsurface however gas seeps pockmarks at the sea floor are almost randomly distributed which indicates a change from focused to diffuse fluid gas migration in shallow unconsolidated sediment where the vertical chimneys reach up to unlithified eocene to miocene sands widespread deformation interpreted as fluidization occurs around the main conduit this deformation affects about 32 of the entire unconsolidated tertiary section late eocene miocene a plio pleistocene glaciomarine drift with up to five horizons with iceberg ploughmarks seals the tertiary sands in the near surface sediments it is observed that gas accumulation occurs preferentially at iceberg ploughmarks it is inferred that lateral migration at five levels of randomly oriented ploughmarks dispersed gas over larger areas and caused random pockmark distribution at the sea floor independent from the underlying focused migration pathways this study demonstrates that fluid flow migration changes from structurally controlled focused flow in the deep consolidated subsurface to diffuse flow controlled by sediment variability in the shallow subsurface this result is relevant to a better understanding of the distribution of seepage induced features at the seafloor related to focused hydrocarbon migration pathways known from industry data and fluid flow modeling

structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the harstad basin southwest barents sea the harstad basin is a structural block on the continental shelf of sw barents sea where gas hydrates likely occurred below the grounded ice sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites we present an integrated study of fluid flow systems in the harstad basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane derived carbonate crusts more than 190 acoustic gas flares were registered in water column many of them in association with pockmarks and carbonate crust fields however weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation in the western harstad basin seeps of microbial methane occur mainly above tertiary formations that are pinching out below the glacial sediments high amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within tertiary deposits in contrast gas seeping in the eastern harstad basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted mesozoic rocks occurring below glacial sediments this spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts with 13 c values typically between 55 and 42 and 40 and 20 vpdb for carbonate crusts associated with microbial and thermogenic fluids respectively u th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor

types of fluid related features controlled by sedimentary cycles and fault network in deepwater nigeria abstract the seismic characteristics of focused fluid related features such as pockmarks and carbonate build ups on the nigerian continental slope have been investigated using complementary seismic data sets reprocessed 3d exploration data and 2d very high resolution hull mounted and near bottom seismic data coupled with the results of previous studies sedimentological geotechnical and geochemical analyses the results show different types of fluid related features within the hemipelagic phase of long duration 1 myr turbidite hemipelagic cycles they are the product of the disturbance of the hosting sediments following two main distinct processes 1 no or low sediment deposition caused by fluid escape associated with seafloor settlement benthic fauna carbonate precipitation gas hydrate formation and 2 post depositional erosion caused either by fluid release dewatering from underlying features such as channels and upward migration from a deep source or by collapse destabilisation of gas hydrates the temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation the spatial distribution of fluid related features seems to be controlled by the type of fault small fault or structural lineament

elongate depressions on the southern slope of the norwegian trench skagerrak morphology and evolution abstract multibeam echosounder surveys conducted on the southern slope of the norwegian trench between norway and denmark show that elongate depressions interpreted as current modified pockmarks are widespread in water depths of 100 500 m in the holocene recent succession the long axes of the depressions are oriented at an angle of about 15 downslope relative to the depth contours the long axes appear to almost exactly parallel the effective bottom currents in this part of the skagerrak the present current directions were probably established about 4000 years ago following a major shift in the oceanographic circulation pattern the most characteristic feature of the depressions is their large size with maximum depths of 45 m widths of 400 m and lengths of 2 km strong bottom currents continuously transport particles thrown into suspension by the seepage process away from the pockmarks and prevent further deposition continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high sedimentation rates are thought to be responsible for the exceptional size of many depressions the shallow gas probably has a mixed biogenic thermogenic origin however the thermogenic component is considered to be most important for the formation of pockmarks seepage from permeable layers in the jurassic cretaceous bedrock is evidenced by linear rows of depressions above these horizons while pockmarks above sub surface faults indicate gas seeps also from there in shallow water areas the shape and size of pockmarks are locally dependent on their location with respect to giant sand waves which trend sub parallel to the depth contours and are interpreted to be of late weichselian to early holocene age in deeper water areas the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls

venting and seepage systems associated with mud volcanoes and mud diapirs in the southern tyrrhenian sea abstract high resolution swath bathymetry and backscatter data seismic chirp profiles multibeam water column acoustic measurements and sediment samples were collected on a cold seep province in the southeastern tyrrhenian sea at a water depth of 500 1000 m the mud volcanoes characterized by a high backscatter signature are the site of gas venting at the seafloor that formed a 630 m high plume in the water column the mud volcanoes feature a precipitation of iron oxy hydroxide crusts and pyritized and sulfur burrows in the sub surface and authigenic siderites also cementing burrows further down showing a sharp transition from the oxic zone toward the sulfate methanogenic zone the mud flows are characterized by an intermediate backscatter seafloor and by the presence of gas in the sediment only 2 m below the seafloor the mud flows consist of 1 m thick drapes of water rich mud extending downslope from the mud volcanoes they act as sealing layers that prevent large fluxes of gas venting at the seafloor low venting and favor oxic conditions close to the sediment water interface and the abundant precipitation of post oxic siderites a few meters below the seafloor the mud diapirs are characterized by a low backscatter seafloor and large fields of pockmarks in coincidence with the normal faults organogenic carbonate crusts form at or very close to the seafloor and are associated with chemosymbiontic bivalves lucinids the youngest shells are ams radiocarbon dated 640 440 bp suggesting that the seepage activity may have been clogged by the carbonates only very recently similarities between the normal faults in the study area and the tectonic setting of the inland calabrian arc show that normal faults can control the location of fluid pathways and probably also the rising of the mud diapirs

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

focused hydrocarbon migration in shallow sediments of a pockmark cluster in the niger delta off nigeria the niger delta is one of the largest hydrocarbon basin offshore africa and it is well known for the presence of active pockmarks on the seabed during the guineco mebo cruise in 2011 long cores were taken from a pockmark cluster in order to investigate the state of its current activity gas hydrates oil and pore water were sampled for geochemical studies the resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks there is a clear tendency for gas migration within the hydrate bearing pockmarks and oil migration within the carbonate rich one this trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks we also demonstrate that anaerobic oxidation of methane aom is the main process responsible for the depletion of pore water sulfate with depths of the sulfate methane transition zone smtz ranging between 1 8 and 33 4 m in addition a numerical transport reaction model was used to estimate the age of hydrate layer formation from the present day sulfate profiles the results show that the sampled hydrate layers were formed between 21 and 3750 years before present overall this work shows the importance of fluid flow on the dynamics of pockmarks and the investigated cluster offers new opportunities for future cross site comparison studies our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades this article is protected by copyright all rights reserved

do norwegian deep water coral reefs rely on seeping fluids hundreds of large coral reefs up to 45 m high exist a on the continental shelf and b on morainic threshold ridges in fjords of northern mid and southwest norway they occur in water depths between 400 and 40 m and contain a large variety of megafauna the most common frame building coral is lophelia pertusa l the oldest reef found to date is 8600 calendar years old determined by radiocarbon dating of buried lophelia skeleton even though many of these reefs have been known to science for over 200 years there is as yet no viable and unifying hypothesis to explain their existence in deep cool waters other than perhaps the hydraulic theory presented by the author in 1990 it states that primary producers mainly bacteria are locally formed and concentrated at reef locations due to seepage of light hydrocarbons mainly methane on the continental shelf and nutrient rich groundwater in the fjords the hydraulic theory is supported by the following topographic reflection seismic and geochemical indicators for the continental shelf reefs a seaward dipping sedimentary permeable strata enhanced acoustic seismic reflectors adjacent pockmark craters locally elevated light hydrocarbon sediment content locally elevated seawater methane content for the fjord reefs b threshold morainic substratum rather than adjacent hardrock substratum sub surface dipping sedimentary layers h2s smell of near surface sediment samples these indicators or reef associated observations have been documented by german british and norwegian researchers over the last 10 15 years however all the listed indicators do not necessarily occur at each of the respective shelf and fjord reef locations simultaneously

structural and stratigraphic controls on subsurface fluid flow at the veslemoy high sw barents sea abstract subsurface and seafloor fluid flow anomalies are gaining large interest after the finding of five new hydrocarbon discoveries and observation of large gas flares in the sw barents sea in the present study we have analysed structural and stratigraphic controls on fluid flow towards the seafloor using gravity cores selected based on subsurface gas anomalies observed on seismic data from the veslemoy high sw barents sea the subsurface fluid flow at the veslemoy high is observed to be controlled by 1 the morphology and orientation of regional faults structural highs and sedimentary basins 2 the presence of paleocene silica ooze deposits that changes microstructure with temperature thereby controlling fluid flow and 3 the location of regional and local open faults formed by glacial loading and unloading analysis of extractable organic matter in subsurface holocene sediments corroborates the active migration pathways inferred from seismic data micropalaeontological studies on benthic foraminifera reveal methane seep associated assemblages that confirm the interpretation of subsurface gas anomalies in seismic data we ultimately link these new results to the geological evolution history of the region to give a comprehensive model for the fluid flow system within the study area

the effect of hydrate content on seismic attenuation a case study for mallik 2l 38 well data mackenzie delta canada 1 observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation however sonic log measurements in the mallik 2l 38 well and cross hole tomography measurements in the mallik field have shown that attenuation increases with hydrate saturation we studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation we found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect we modelled attenuation in terms of biot and squirt flow mechanisms in composite media we have used our model to predict observed attenuations in the mallik 2l 38 well mackenzie delta canada

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge