tem study of in situ organic matter on continental margins occurrence and the monolayer hypothesis abstract tem photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as 1 discrete discontinuous blebs and micro blebs of differentiated and undifferentiated protoplasm 2 bacterial cells and associated muco polysaccharide networks and 3 localized smears generally associated with clay minerals and domain junctions in clay rich flocs not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness this occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or monolayer equivalent coatings on mineral grains and inorganic bioclastic particles our results are based on tem photomicrographs of samples from the northern california continental slope that fall within the monolayer equivalent envelope defined by mayer 1994 marine snow from the overlying nepheloid layer and sediments from eckernforde bay in the baltic sea these samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents textural evidence suggests that low permeabilities caused by the interaction between the organic inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins

improving the utility of ocean circulation models through adjustment of the momentum balance we propose a new method to improve the utility of three dimensional ocean circulation models the method uses climatological temperature and salinity data to adjust the momentum balance of the model while leaving the tracer equations fully prognostic and unconstrained the adjustment is accomplished by replacing density in the hydrostatic equation by a linear combination of model computed and climatological density the procedure is equivalent to adding a forcing term to the horizontal momentum equation through a modification of the model s horizontal pressure gradient term the forcing term modifies the model computed velocity field which in turn affects the model computed temperature and salinity fields through the advection term there is no adjustment of the tracer equations carried by the model assuming the linear combination coefficient to be invariant in time and space we suggest a statistical approach to estimating its optimal value we apply this semiprognostic method to the northwest atlantic a primitive equation circulation model is initialized with january climatological temperature and salinity and is forced by monthly mean comprehensive ocean atmosphere data set surface wind stress and heat flux by restoration of the surface salinity to monthly mean climatology and by flows through the open boundaries both the model computed tracer and velocity fields produced using the semiprognostic method show significant improvement over those produced by a purely prognostic calculation drift of the tracer and velocity fields away from climatology is greatly reduced further convective mixing is explicitly represented thus improving the utility of results over those obtained from pure diagnostic calculations the velocity fields obtained with the new approach are somewhat more realistic than those obtained from pure diagnostic calculations the method reproduces many well known circulation features in the region including the labrador current the gulf stream and the north atlantic current more significantly the method reproduces reasonably well the seasonal evolution of temperature and salinity in the region despite the fact that the model s tracer fields are not constrained directly by the new method this result suggests that the semiprognostic approach will be useful for examining the evolution of tracers that are not easily determined by observations

time capsule concretions unlocking burial diagenetic processes in the mancos shale using carbonate clumped isotopes abstract septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture filling as such they have been used to track variations in 13c and 18o of pore waters during diagenesis and to define diagenetic zones in clastic rocks however the 18o value of the carbonate is dependent on precipitation temperature and the 18o value of the pore fluid from which it precipitated interpretations must assume one of these parameters both of which are highly variable through time in diagenetic settings carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation allowing the pore water 18o to be back calculated here we use this technique on carbonate concretions and fracture fills of the upper cretaceous prairie canyon member mancos shale colorado we sampled concretions from two permeable horizons separated by a 5 m shale layer with one permeable horizon containing concretions with septarian fractures we show cores precipitated at cooler temperatures 31 c 660 m burial depth than the rims 68 c 1980 m burial depth and relate that to the 13ccarbonate values to suggest the concretion core precipitated in the methanogenic zone with increasing input from thermogenically produced co2 the two concretion bearing horizons have different back calculated 18oporewater values mean 2 65 and 1 13 vsmow for cements formed at the same temperature and similar 13c values suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing additionally the 18ocarbonate of the septarian fractures 13 8 vpbd are due to precipitation at high temperatures 102 to 115 c from a fluid with a mean 18oporewater of 0 32 vsmow therefore we can tie in the cementation history of the formation to temporal and spatial variations in 18oporewater

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

oxygen and carbon isotopic composition of marine carbonate concretions an overview discussion and reply in their review of isotopic data from marine carbonate concretions mozley and burns 1993 p 8 i incorrectly associate our work dix and mullins 1987 with their interpretation of the effects of recrystallization on isotopic compositions in concretions mozley and burns 1993 p 81 state late stage calcite and siderite concretions are characterized by a combination of normal marine 6 3c values and depleted 6 so values such isotopic values have been interpreted by some authors as indicating recrystallization i e that the concretion initially had normal marine carbon and oxygen isotopic compositions and that the oxygen isotopic compositions were reset at higher temperatures dix and mullins 1987 the association of our work with this statement is incorrect because we illustrated that 1 the bulk of the concretions in the middle devonian hamilton group appear to have formed during very early stages of burial within the upper 10 m p 147 2 the very negative 5 3c values of most concretion samples and systematic shifts with concretion growth illustrate the role of organic diagenesis within the zone of sulfate reduction and transitional into the upper zone of methanogenesis our fig ii and 3 the only late stage concretions containing ferroan calcite have 6 3c values ranging between 0 and 2 and this due to a subsurface shift in porewater 6 3c related to organic diagenesis our fig 11 not recrystallization p 150 in their discussion of causes for consistently depleted 5 80 values in shallow burial calcite concretions mozley and bums 1993 p 77 appear to dismiss the effect of recrystallization partly through their assumption that the original composition of concretions must be stable low magnesium calcite they cite however only three studies of concretions that unequivocally remove any possibility for recrystallization are these the exceptions or the rule combined petrographic and isotopic observations still lead us to the interpretation that recrystallization of hamilton group concretions excluding late stage concretions was very important 6180 data fall within the middle of an isotopic group defined by late stage septarian calcites and our samples of septarian calcites also fall within this group with lao values that are not too dissimilar to the bulk of concretion calcite our fig 12 we presented petrographic evidence illustrating that two stages of septafian calcite have been replaced by subsequent calcite including ferroan calcite our fig 7 and that replacement calcite is associated with silica diagenesis in the concretion bodies and migration of petroleum bearing fluids if septarian calcite can be replaced why not the concretion body calcite too with so many calcite concretions appearing to form under near surface burial conditions could the mineral water interaction that mozley and burns 1993 call upon to account for net shifts in ao include very shallow recrystallization ofa metastable magnesium beating carbonate given the diverse mechanisms for causing a shift in 6 80 mozley and burns 1993 recrystallization seems to be a plausible factor see desrochers and ai aasm 1993 that should not be discarded so readily

near surface hydrocarbon anomalies in shelf sediments off spitsbergen evidences for past seepages 1 as global warming occurs the dissociation of bound methane on arctic shelves due to ocean current temperature changes may become a major contributor to the global methane budget and thus contribute to strong positive climate feedback mechanisms however little is known about the magnitude and fate of methane emissions from shallow submarine sediments to the atmosphere in the peculiar area in this paper we present one of the first direct evidences for seepage on the northwestern barents sea shelf by studying the molecular and isotopic signatures of low molecular weight hydrocarbons in seawater near surface sediment pore space and the sediment matrix at 26 locations we provide a detailed view on the partitioning of gaseous hydrocarbons in the sediment water interface off spitsbergen in the free gas phase low concentration of methane 28 ng g wet sediment paired with constantly high isotopic values 65 is consistent with high impact of methane oxidation on the isotopic composition in contrast high concentrations of adsorbed ch4 up to 5292 ng g wet 63 m and c2 ethane through pentane up to 1724 ng g wet 63 m in the sediment matrix suggest that the adsorbed gas is reasonably well protected against microbial degradation moreover the isotopic and molecular signatures of adsorbed ch4 38 60 100 5300 ng g wet 63 m and c2h6 20 36 30 1700 ng g wet 63 m indicate an integrated but strongly varying signal of historic thermally derived hydrocarbon plumes plus in situ adsorbed gas of biological origin this suggests a minimal exchange and unlike histories of free and adsorbed gas the majority of gaseous hydrocarbons in the adsorbed pool may be the result of past seepages past upward flow of thermogenic gas and impregnation with mature hydrocarbons was particularly strong along tectonic lineaments active petroleum source rocks along the continental margin and migration of gaseous hydrocarbons at re activated fault systems might explain the anomalies along these conduits

surface geochemical studies in the norwegian barents sea comparison with drilling results during the last 4 years a number of surface geochemical studies have been undertaken in the barents sea acreage in all of the areas has subsequently been licensed for exploration sampling for surface geochemistry has been undertaken using a gravity corer where samples were taken 2 m below the seafloor the sampling grids were either regular or irregular based on known geological features in the areas or a combination of the two the samples were analyzed for interstitial gas adsorbed gas grain size distribution organic and inorganic carbon content fluorescence of the hexane extract and heavy hydrocarbons in the hexane extract the results from the different areas ranged from barren i e no indication of petrogenic hydrocarbons at the surface anomalies of petrogenic hydrocarbons i e indicating the generation of gas condensate or oil to extremely strong anomalies of oil interpreted as indicators of direct leakage from source rocks or a defective caprock

diagenetic relationships of methanogenesis nutrients acoustic turbidity pockmarks and freshwater seepages in eckernforde bay abstract high organic loading 4 5 wt and sedimentation rates 1 4 mm yr 1 in eckernforde bay sediments lead to anaerobic conditions within the uppermost 15 cm intense bacterial sulphate reduction 0 011 0 15 mm so 4 red 2 yr 1 exhausts dissolved sulphate around 150 cm sediment depth resulting in methanogenesis at greater sediment depth by carbonate reduction 1 8 8 5 m ch 4 yr 1 extensive regions of eckernforde bay are characterized by acoustically turbid sediments 300 cm sediment depth resulting from deeper sediments supersaturated in methane forming free gas accumulation zones methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3 10 times greater than methanogenesis i e 46 95 m ch 4 ox yr 1 the pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence pockmarks i e shallow surface depressions in some eckernforde bay sediments are not associated with gas ebullition rather these physical features result from the expulsion of freshwater these episodic springs or seepages of groundwater from the underlying holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations the geochemical evidence indicates that the commercial accumulation of petroleum in the lower cretaceous dogger beta hauptsandstein schwedeneck field in eckernforde bay 1500 mbsf has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmarks

concretion morphology classification and genesis abstract a discussion of the most relevant morphological features of concretionary bodies and the different classifications and the criteria involved in these classifications is presented together with suggestions for improvements to the various classification schemes the meaning of syngenetic diagenetic and epigenetic related to the relative timing and environment of growth of concretionary bodies is also reviewed and discussed the replacement of mixed morphogenetic classifications which lead to conflicting results by classification categories based on textural features is proposed because the identification of the genetic environment of a concretionary body tells little about its history i recommend defining growth patterns in which the successive steps associated with changes in composition and or texture and the development of new structures are recorded a new path is presented that accounts for the contradiction between textural and isotope features which suggests respectively syngenetic and diagenetic or even epigenetic signatures this new path is characterized by the preservation of large porosities over very long time spans and down to depths that are somewhat greater than expected due to the inhibition of normal compaction caused by early overpressuring this overpressuring is the result of the development of hydraulic seals since approximately the syngenetic phase times the concepts of force of crystallization and displacive growth are also reviewed it is being suggested to discard some controversial interpretations of their actual importance for true concretionary displacive growth under epigenetic conditions in accordance with other authors the conclusion is reached that displacive growth is only possible if the shear strength of the host is very low and the stress field within the host is almost hydrostatic a new model the symcompactional concretionary growth model is proposed which explains how a concretionary body generally a nodule can grow while its host thins down by compaction this overpressured undercompacted model will be useful for the interpretation of an assemblage of features such as injection dikes hydraulic breccias cone in cone structures among others that are thought to be representative of the former presence of overpressured horizons these overpressured horizons could serve as detachment planes in paleotectonically active basins seals that could have controlled the movement of brines and hydrocarbons during the diagenetic evolution of the basin can also be assessed this is relevant for the identification of maturation conditions the potential development of secondary fracture permeability due to hydraulic fracturing in buried areas of the basin can also be evaluated based on the identification of formerly overpressured horizons in outcrops the origin of concretions is generally a geological puzzle clifton 1957

distribution of subsurface hydrocarbon seepage in near surface marine sediments hydrocarbon seeps in surficial marine sediments are of two types active and passive active seeps occur where gas bubbles pockmarks or bright spots are visible on seismic profiles and where chemosynthetic communities are present in conjunction with large concentrations of migrated hydrocarbons macroseeps these generally occur where generation and migration of hydrocarbons from source rocks are ongoing today at maximum burial or where significant migration pathways have developed from recent tectonic activity passive seeps occur where concentrations of migrated hydrocarbons are usually low microseeps with few or no geophysical anomalies these occur typically in areas where generation and expulsion is relict no longer at maximum burial or regional seals prevent significant vertical migration the type of seepage controls the distribution of migrated hydrocarbons in the near surface sediments and should dictate the sampling equipment and approach used to detect seeps active seeps are usually detected near the water sediment interface in the water column or at the sea surface and at relatively large distances from major leak points most conventional sediment and water samplers can capture active seeps the gulf of mexico santa barbara channel and parts of the north sea have active hydrocarbon seeps passive seeps can only be detected relatively far below the water sediment interface and require samples to be collected near leak points sampling equipment must penetrate the zone of maximum disturbance or any shallow migration barriers in areas where surficial sediments are coarse grained or compacted conventional gravity corers will not work other options for subsurface sampling include vibracores jet cores and rotary cores precise location of samples site specific using seismic profiles to locate leak points is critical to detect passive hydrocarbon seeps the beaufort and bering seas offshore alaska and parts of the north sea contain passive seeps

depositional environment and source rock potential of miocene strata from the central fram strait introduction of a new computing tool for simulating organic facies variations abstract organic rich sediments were recognized in early miocene strata from the norwegian greenland sea during odp leg 151 three organic geochemical subunits were distinguished in hole 909c using detailed organic geochemical and microscopic analyses toc up to 3 wt hi up to 200 mg hc g toc and 13corg values 24 26 indicate the predominance of terrestrial type iii organic matter in subunit 3 18 16 2 ma biomarker and vitrinite reflectance values r0 0 5 point to rather fresh immature terrestrial organic matter supplied by river discharge from adjacent vegetated coastal areas although the sediments of subunit 3 have a fair to good source rock potential they are insufficiently mature to generate significant amounts of oil or gas to test if the moderate generation potential and source rock quality of subunit 3 are applicable to the entire area the computer software of mod was applied several modelling runs testing the most probable depositional scenarios result even under conservative assumptions in the formation of good to excellent source rocks towards the basin margins

carbon isotopic distribution of methane in deep sea hydrothermal plume myojin knoll caldera izu bonin arc implications for microbial methane oxidation in the oceans and applications to heat flux estimation the concentration and stable carbon isotopic composition 13c of methane have been measured for both seafloor hydrothermal venting fluids and effluent plume waters supplied from the vents at the myojin knoll caldera izu bonin arc in the western north pacific the hydrothermal end member concentrations and 13c of methane show near homogeneity among vents 41 2 mol kg and 16 3 0 8 pdb respectively while those in the effluent plume are stratified in the caldera and vertically exhibit a large variation 2 1 11 nmol kg and 29 0 to 11 3 pdb respectively comparison of concentration and 13c data between vent fluids and plume waters suggest that the methane is not conserved but microbially oxidized along with the plume gradual decrease of diffusive methane flux in proportion to distance from the vent fields supports the occurrence of significant microbial oxidation within the plume assuming steady state emission of methane from the vents 1 kinetic isotope effect due to the microbial oxidation k12 k13 2 methane flux from the vents fvents 3 heat flux from the vents q and 4 average turnover time of hydrothermal methane tall are estimated to be k12 k13 1 005 0 001 fvents 90 340 mol d q 30 110 mw and tall 60 240 d respectively the estimated turnover times however are not uniform within the water column around the vents a turnover time of less than 50 d for methane is one of the shortest values in the pelagic ocean while those at the distant points correspond to values more typical for deep ocean water

light hydrocarbon gases in shallow sediments in the northern north sea abstract large amounts of data on hydrocarbons in shallow marine sediments 0 6 m below sea floor have been gathered on the norwegian sector in conjunction with surface exploration for petroleum seepage in this paper some results from a re investigation of five surveys is presented the surveys include a total of 2136 shallow cores and cover mainly the stord basin and the viking graben areas offshore western norway adsorbed gases interstitial gases headspace gases grain size and inductively coupled argon plasma elemental data have been investigated table 1 the adsorbed methane ranges from 10 to 5000 ppb it has a uni modal distribution with a median concentration of 504 ppb weight weight the interstitial methane ranges from less than 1 to 500 ppb it has a uni modal distribution with a median concentration of 13 4 ppb there are two classes of headspace methane samples a normal class comprising about 90 of the samples lies in the range 0 1 to 500 ppb with a median of 2 1 ppb the second class lies in the range 500 to more than 100 000 ppb with a median of 6990 ppb the ratio of unsaturated to saturated alkane homologues in the interstitial and headspace gasses is on average equal to one and show positive correlation this rules out thermogenic hydrocarbons as a direct source for the headspace and interstitial gasses the proportion of unsaturated hydrocarbons in the adsorbed gas is much lower the highest interstitial methane contents are found in sediments with high contents of coarse material principal component analysis show that the yields dryness and isotopic composition of adsorbed methane correlate positively with the contents of calcium and strontium the sr ca ratio in the sediments and the proportion of silt size material in the sediments we infer from this that a major component in the adsorbed gas is cogenetic with carbonates in the fine fraction probably resulting from in situ bacterial degradation of organic matter distinct and unquestionable regional headspace gas anomalies are found within the quaternary kleppe senior kse formation of soft silty marine glaciomarine clays outside of kse only background levels of headspace methane is observed some of the most prominent headspace gas anomalies correlate very well with a shallow gas accumulation in a sand body just 10 20 m below sea floor we conclude that gas found at the surface has migrated upwards from this sand body the 1 3 c 1 2 c isotopic ratio for the anomalous methane 51 to 100 ppt and its extremely dry characteristics are typical for a biogenic origin we speculate that shallow biogenic gas may be a secondary form of carbon related to leakage from petroleum deposits at depth in this very prolific area

late cenozoic seismic stratigraphy and glacial geological development of the east greenland and svalbard barents sea continental margins abstract the european program on the late cenozoic evolution of the polar north atlantic margins ponam running from 1990 to 1994 resulted in a wealth of seismic data and sediment cores particularly from the svalbard and barents sea continental margin but also from areas of the east greenland margin deep sediment coring was undertaken on both margins during odp legs 152 and 162 this paper uses the ponam generated information as well as previous and subsequent seismic and core data to review our present understanding of the glacial geological development of these regions four main depositional facies have been identified these are represented by hemipelagic glaciomarine sediments sandy silty debris flows diamictic debris flows and tills deposited directly beneath grounded ice the temporal and spatial distribution of these four facies determines the architecture and seismic character of the two continental margins the two regions show significant differences with regard to both the timing of initial glaciation and the response of the ice sheets to climatic variability glacial deposition has taken place on the east greenland margin at least since 7 ma but apparently only since 2 5 ma on the svalbard barents sea margin significant glacial expansion across the continental shelf probably occurred in the early and late pliocene in east greenland whereas the glaciers on svalbard and the barents sea did not reach the shelf edge until the early pleistocene despite a shorter glacial period average glacial sedimentation rates are 2 3 times higher on the svalbard barents sea margin than on the studied parts of the east greenland margin the svalbard barents sea ice sheet has advanced and retreated much more frequently during the plio pleistocene than the greenland ice sheet glacial regime polar on greenland and polythermal to temperate in svalbard and the barents sea and the more easily erodible preglacial bedrock of svalbard and the barents sea are thought to be main factors responsible for the differences in ice sheet behavior and consequently glacial sediment production

geochemical surface exploration for hydrocarbons in north sea approximately 350 sediment samples were taken in the central part of the north sea and hydrocarbon composition methane c1 and ethane c2 yields and carbon isotope composition of the methane dgr13c1 adsorbed in the sediment were determined methane generation or oxidation by bacterial activity can not be deduced from the analytical data this fact is explained by the assumption of a threshold of methane concentration in sediments below which methane oxidizing bacteria are inactive and that this threshold was not reached in the area investigated background gases in the surface sediments characterized by low c1 yields and dgr13c1 37 ppt can be distinguished from thermal sediment g ses which are obviously derived from a hydrocarbon generating source rock in the subsurface source rock data obtained from several wells situated on a cross section through the investigated area were compared to the geochemical patterns of the sediment gases and used to define profile sections with good dgr13c1 approx 40 ppt yield c1 approx 217 ppb and poor source rock properties dgr13c1 approx 33 ppt yield c1 approx 24 ppb an assessment of the source rock potential of the whole area under investigation has been carried out using the geochemical data of the surface sediment gases and taking into account that the c1 and c2 yields are generally higher in clay sediments than in sand samples the number of producing wells was related to the number of all wells 271 in the areas characterized by surface geochemistry as good or poor source rock areas the comparison showed that the relative number of producing wells is higher in areas geochemically characterized as good hydrocarbons in surface sediments of good north sea areas are not generated by bacteria at shallow depth they are formed thermally from the organic carbon of deep source rocks and migrate to the surface therefore they are indicative of thermal hydrocarbon generation in the subsurface

waxing and waning of microbial laminites in the aftermath of the marinoan glaciation at the margin of the amazon craton brazil abstract at the margin of the amazon craton west central brazil the mirassol d oeste formation comprises an emblematic 16 m thick marinoan cap dolostone deposited directly upon diamictites of the puga formation laterally continuous and morphologically simple microbialites a common component of marinoan cap carbonates begin two meters above the base of the formation and continue upwards as stratiform domical and irregular wavy microbial laminate boundstones for approximately 10 m these microbialites consist of alternating thin and thick laminae of peloids micritized remains of microbial colonies and subordinate micrite with spar filled fenestrae that differ in the greater relative abundance of fenestrae in the thicker laminae the microbialites developed profusely in a calm platformal environment below storm wave base level but within the photoic zone without any contribution of siliciclastics the waxing phase of growth vertical dolomicrite filled tubestone structures up to 10 m occupy the apical portion of the broad domical microbial laminites confined percolation of fluids seems the most plausible hypothesis for their origin the wanning phase began in the final meter of the boundstone consists of irregularly wavy microbial laminites indicating prolonged oscillatory wave action followed by dolomitic peloidal grainstone packstone with megaripple and millimetric macropeloids generated by oscillatory wave action during tradewinds the hydrodynamic change and a rising sea level put an end to microbialite deposition in the mirassol d oeste cap carbonate

hydrocarbon plumbing systems above the snohvit gas field structural control and implications for thermogenic methane leakage in the hammerfest basin sw barents sea abstract based on the analysis of the high resolution 3d seismic data from the sw barents sea we study the hydrocarbon plumbing system above the snohvit and albatross gas field to investigate the geo morphological manifestation and the dynamics of leakage from the reservoir fluid and gas escape to the seafloor is manifested in this area as mega pockmarks 1 2 km wide large pockmarks

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

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

thermogenic hydrocarbons in surface sediments of the bransfield strait antarctic peninsula antarctica is the only continent from which petroleum deposits have not been encountered despite the occurrence of potential hydrocarbon basins onshore and around its margins we have discovered thermogenic hydrocarbons in unconsolidated recent sediments from the king george basin bransfield strait west antarctica these sediments possessed a marked petroliferous smell throughout the length of the 8 6 m gravity core the basin sediments are glacial marine deposits dominated by turbidites and contain abundant autochthonous organic matter as a result of high seasonal primary productivity in this most fertile part of the circumpolar ocean1 the maturation of this material into hydrocarbons may have been accelerated by the high geothermal gradient in the basin associated with back arc spreading created by the subduction of the drake plate into the south shetland trench2 sediments in this rifted basin are frequently intruded by volcanic sills and dykes3 giving rise to acoustic features shown in fig 1 the presence of these thermogenic hydrocarbons provides the first demonstration of active source rocks in antarctica

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control