relationships between seep carbonates mud volcanism and basin geometry in the late miocene of the northern apennines of italy the montardone melange the montardone melange mm is a chaotic block in matrix unit outcropping in the montebaranzone syncline in the northern apennines the mm occurs in the uppermost part of the termina fm the middle late miocene interval of a succession deposited in a wedge top slope basin epiligurian succession the mm is closely associated with bodies of authigenic carbonates characterized by negative values of 13c from 18 22 to 39 05 pdb and chemosynthetic benthic fauna lucinid and vesicomyid bivalves in this paper we propose that the mm is a mud volcano originated by the post depositional reactivation and rising of a stratigraphically lower mud rich mass transport body canossa val tiepido sedimentary melange or olistostrome triggered by fluid overpressure we base our conclusion on 1 the mm pierces the entire termina fm and older epiligurian units and represents the direct continuation of the underlying canossa val tiepido melange 2 the geometry and facies distribution of the montebaranzone sandstone body which are compatible with a confined basin controlled by the rising of the mm 3 the systematic presence of large scale lateral extension 300 400 m seep carbonates associated with the melange suggesting a persistent gas enriched fluid vent from the ascending overpressured mud 4 blocks and clasts sourced from the mm hosted by the authigenic carbonates conveyed by ascending mud and gas enriched fluids the mm represents one of the few fossil examples of reactivation of a basin scale sedimentary melange olistostrome a three stage model showing mechanisms of mm raising is proposed

seep carbonates in a thrust related anticline at the leading edge of an orogenic wedge the case of the middle late miocene salsomaggiore ridge northern apennines italy abstract seep carbonates are important indicators of submarine fluid expulsion and dewatering of sediments along convergent margins and accretionary prisms in the northern apennine compressional wedge seep carbonates usually occur in pelitic successions from different settings of the middle late miocene foreland basin system in serravallian foredeep deposits of the salsomaggiore area seep carbonates cemented and or encrusted coarse grained sandstones and conglomerates fluid expulsion strictly related to variable intensities in tectonic stresses generated seep carbonate facies and sediment instabilities depending on the position of the fluid pathway feeders within the salsomaggiore ridge a middle late miocene intra basinal high at the leading edge of the apenninic orogenic wedge this site provides a unique opportunity for analyzing depositional characters and the three dimensional distribution and evolution of a seepage system in a thrust related anticline the resulting evolutionary model shares common features with recent ridges in other accretionary wedges in this model a strict relationship between tectonics seepage and sediment instability during a major tectonic phase is outlined

review of fossil chemosynthetic assemblages in japan abstract seventy five fossil chemosynthetic assemblages of cretaceous to holocene age have been discovered in japan many are interpreted to have been cold seep dependent and one is associated with a whale fall fossil hydrothermal vent assemblages have not been found in japan non seep assemblages of solemyids lucinids and or thyasirids are not included in this study various combinations of five diagnostic features characterize cold seep assemblages 1 the abundant and often exclusive occurrence of large invertebrate taxa suspected of harboring chemosynthetic bacteria 2 associated authigenic carbonates greatly depleted in 13c 3 a three dimensional distribution of the assemblage that is laterally restricted 4 associated pockmark or diapir related sedimentary structures and 5 the presence of conduit related tectonic or sedimentological structures such as faults slumps or debris flows excluding the whale fall assemblage we can divide cenozoic fossil chemosynthetic assemblages in japan into three types regardless of geochronologic age type i is dominated by vesicomyid bivalves and or tube worms characterized by its allochthonous or parautochthonous mode of occurrence in deep water greater than 1000 m settings along with tectonically or sedimentologically induced structures such as faults slumps diapirs or debris flows likely dependent on seepage controlled by subduction related tectonism and or on methane coming from the drastic decomposition of gas hydrate type ii is dominated by vesicomyid bivalves in most cases the species differ from those in type i and is characterized by autochthonous occurrences in muddy sediments from less than 1000 m water depth type iii is dominated by lucinoma and or conchocele and characterised by autochthonous occurrences in muddy sediments from depths of less than 300 m a living community similar to the type ii and iii assemblages has not yet been observed in the seas around japan it is noteworthy that no tectonically or sedimentologically induced structures have been observed in and around the type ii and iii fossil localities although the muddy sediments sometimes show brecciated facies that suggest an explosive effusion of methane

miocene chemoherms of the northern apennines italy in the miocene of the northern apennines methane derived limestones from various geologic settings represent important examples of fossil chemoherms and provide a basis for more accurate interpretations of seepage systems the systematic study of fossil chemoherms has allowed the definition of new field and compositional criteria for the recognition of these deposits which were previously based almost exclusively on negative carbon isotope composition and peculiar chemosynthetic communities this has constrained the relations between seepage and sedimentary and tectonic instability processes we suggest that the peculiar brecciated structures associated with fossil chemoherms are related to diapirism of overpressured pelitic sediments due to fluid venting fluidization of sediments increases the instability of mudstones in which chemoherms are contained thus favoring gravity slumping processes and the reworking of many chemoherms

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

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

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

the cenozoic ch4 derived carbonates of monferrato nw italy a solid evidence of fluid circulation in the sedimentary column abstract two types of methane derived carbonates each with different lithological and paleontological signatures have been distinguished in the oligocene miocene sediments of monferrato nw italy 1 irregularly shaped metre sized rock bodies characterized by the widespread occurrence of remains of large bivalves lucina sp these bodies include two subtypes differing in the lithological characteristics of the cemented sediments a lucina rich marly limestones encased in langhian outer shelf marls they document a slow seepage of ch4 rich fluids at the paleo sea floor b lucina bearing cemented mud breccias found either as cemented masses in the original stratigraphic position or as redeposited blocks within upper messinian chaotic block in matrix sediments these carbonate bodies document the activity of submarine mud volcanoes during the late messinian in particular they document intermittent fluid emission ranging from violent eruptive phases responsible for the extrusion of soft mud breccias at the sea floor to quiet degassing phases that promoted colonization by chemosymbiotic communities and localized cementation 2 very irregularly shaped strongly cemented mainly coarse grained clastic sediments reaching the lateral extent of few hundred meters and the thickness of few tens of meters they occur within oligocene to messinian sediments lack remains of chemosymbiotic taxa and are highly fractured with fractures filled with sediments and cements these carbonate cemented masses here referred to as macroconcretions result from the precipitation of carbonate cements within buried sediments flushed by ascending ch4 rich fluids they document a complex fluid circulation pattern in the sedimentary column in which so4 rich waters were drawn from surrounding sediments by the fast and prolonged flow of ch4 rich fluids and promoted anaerobic oxidation of methane and the precipitation of carbonate cements the polyphase network of clastic dykes and veins that characterize macroconcretions is related to an intermittent fluid flow regime dykes formed during fast upward flows of sediment rich fluids whereas carbonate filled veins formed during slower degassing phases the local lithostratigraphic and structural framework allow to correlate genetically and chronologically the macroconcretions to lucina bearing mud breccias consequently macroconcretions are here regarded as the result of cementation along fluid conduits feeding messinian mud volcanoes at the sea floor the massive release of fluids could have played a role in triggering large scale mass wasting events responsible for the emplacement of sedimentary melanges including ch4 derived rocks in the messinian

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

fluid methane and energy flux in an active margin gas hydrate province offshore costa rica joint analysis of seafloor temperature data borehole temperature measurements and observations that constrain the position of the top and base of the gas hydrate zone ghz at sites on the non accretionary costa rican margin provide quantitative constraints on the flux of energy fluid and methane through the sediments on the overriding plate a comparison of borehole temperature data obtained in the trench at the toe of the deformed sedimentary wedge and at a midslope location indicates that the thermal regimes are perturbed by appreciable advective flux at 1 km oceanward 19 mm yr 1 and 1 4 km landward 5 7 mm yr 1 of the deformation front this observation is loosely consistent with models that predict the most rapid dewatering and highest rates of fluid expulsion within a few kilometers of the deformation front using data that constrain the thickness of the ghz at three sites on the forearc we estimate methane and fluid flux rates through application of a one dimensional analytical model that constrains the nature of the plumbing system for the gas hydrate reservoir our results indicate a two fold decrease in methane flux between a site located 400 m from the toe of the wedge and a site 12 km from the toe an intermediate site located 1 4 km from the toe of the wedge has a thick ghz extends to at least 340 365 m below seafloor that requires a very high methane supply rate 345 mol m 2 kyr 1 for a mass flux of 5 mm yr 1 we conclude that this site has the most two dimensional fluid flux regime with fluids being fed into the zone laterally by conduits and fractures and vertically by expulsion during compaction of underthrusting sediments

the nature distribution and origin of gas hydrate in the chile triple junction region abstract a bottom simulating reflector bsr is regionally distributed throughout much of the chile triple junction ctj region downhole temperature and logging data collected during ocean drilling program odp leg 141 suggest that the seismic bsr is generated by low seismic velocities associated with the presence of a few percent free gas in a 10 m thick zone just beneath the hydrate bearing zone the data also indicate that the temperature and pressure at the bsr best corresponds to the seawater methane hydrate stability field the origin of the large amounts of methane required to generate the hydrates is however problematic low total organic carbon contents and low alkalinities argue against significant in situ biogenic methanogenesis but additional input from thermogenic sources also appears to be precluded increasing thermal gradients associated with the approach of the spreading ridge system may have caused the base of the hydrate stability field to migrate 300 m upwards in the sediments we propose that the upward migration of the base of the stability field has concentrated originally widely dispersed hydrate patches into the more continuous hydrate body we see today the methane can be concentrated if the gas hydrates can form from dissolved methane transported into the hydrate zone via diffusion or fluid advection a strong gradient may exist in dissolved methane concentration across the bsr leading to the steady reabsorbtion of the free gas zone during the upward migration of the bsr even in the absence of fluid advection

methane rich plumes on the carolina continental rise associations with gas hydrates seafloor venting of microbial gases occurs at 2167 m water depth over the blake ridge diapir gas rich plumes were identified acoustically in the water column up to 320 m above a pockmarked sea floor associated with active chemosynthetic biological communities plumes and venting fluids emanate from near a small fault that extends downward toward a dome in the bottom simulating reflector indicating that fluid and or gas migration is associated with gas hydrate bearing sediment below these plumes might be caused by gas bubbles or buoyant dumps of gas hydrate that float upward from the seafloor 18 refs 3 figs

an upper jurassic methane seep limestone from the fossil bluff group forearc basin of alexander island antarctica an anomalous upper jurassic limestone bed 3 m thick is present within the volcaniclastic infill of a forearc basin exposed on alexander island antarctica the limestone formally designated the gateway pass limestone bed contains laminated crusts and blocks including hardgrounds showing radial fibrous and botryoidal calcite cements carbon isotope values are very negative delta 13 c of 33 5 per thousand to 44 6 per thousand pdb indicating a methane oxidation origin for the carbonate the limestone bed formed around a submarine cold seep in the antarctic peninsula forearc basin adjacent to the accretionary prism in contrast to adjacent nearly barren sea floor the limestone also yields a benthic island assemblage containing abundant gastropods and bivalves the seep is directly comparable to certain jurassic seeps at the convergent margin of the northeast pacific and both are related to subduction at the eastern proto pacific margin

why did ancient chemosynthetic seep and vent assemblages occur in shallower water than they do today cold seep communities have relatively low diversity are dominated by one or two taxa present in high density and high biomass in comparison with the surrounding fauna and are restricted to aphotic habitats their associated heterotrophic fauna are usually distinctive from the fauna of their surroundings in contrast a more commonplace chemoautotrophically based community occurs in shallow photic habitats the associated heterotrophic fauna includes many of the species typical of the surrounding communities and typically dominates abundance whereas the species with chemoautotrophic symbionts dominate biomass all modern seep assemblages are deeper than 550 m many fossil seep assemblages occurred in water as shallow as the mid shelf 200 m in contrast communities where species with chemoautotrophic symbionts are biomass dominants but not numerical dominants are common in shallow waters at present but rarely reported in the geological record we suggest that the absence of cold seep communities on the continental shelf presently is due to a combination of predation and competitive exclusion by primary consumers limiting the presence of species dependent on chemoautotrophic symbionts we suggest that cold seep assemblages are more common at shelf depths in the fossil record for two reasons a the biases of preservation have accentuated their distribution by transforming communities where species with chemoautotrophic symbionts dominate by biomass but not numerically into cold seep appearing assemblages b the importance of predation pressure and oligotrophy has varied with decreased predation pressure accompanying increased oligotrophy favoring cold seep communities we suggest that the paucity of shallow water assemblages with species harboring chemoautotrophic symbionts as biomass dominants in the fossil record is based on the reliance of paleoecological analysis on numerical abundance data when energy flow analyses are required to identify these assemblages the distinctiveness of the fossil seep assemblage is intensified by taphonomic processes that bias the assemblage against small individuals and epifaunal species so that diversity declines the small heterotrophic component of the assemblage is significantly reduced and the epifaunal component is minimized the final assemblage is usually dominated by the better preserved large infaunal clams which perchance are also the species with chemoautotrophic symbionts in contrast preservation does not enhance the distinctiveness of these chemoautotroph harboring species in shallow water

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

a blast of gas in the latest paleocene simulating first order effects of massive dissociation of oceanic methane hydrate carbonate and organic matter deposited during the latest paleocene thermal maximum is characterized by a remarkable 2 5 excursion in delta 13c that occurred over approximately 10 4 yr and returned to near initial values in an exponential pattern over approximately 2 x 10 5 yr it has been hypothesized that this excursion signifies transfer of 1 4 to 2 8 x 10 18 g of ch4 from oceanic hydrates to the combined ocean atmosphere inorganic carbon reservoir a scenario with 1 12 x 10 18 g of ch4 is numerically simulated here within the framework of the present day global carbon cycle to test the plausibility of the hypothesis we find that 1 the delta 13c of the deep ocean shallow ocean and atmosphere decreases by 2 3 over 10 4 yr and returns to initial values in an exponential pattern over approximately 2 x 10 5 yr 2 the depth of the lysocline shoals by up to 400 m over 10 4 yr and this rise is most pronounced in one ocean region and 3 global surface temperature increases by approximately 2 degrees c over 10 4 yr and returns to initial values over approximately 2 x 10 6 yr the first effect is quantitatively consistent with the geologic record the latter two effects are qualitatively consistent with observations thus significant ch4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum this conclusion is of broad interest because the flux of ch4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present day anthropogenic ch4 sources

temporal and spatial evolution of a gas hydrate bearing accretionary ridge on the oregon continental margin a seismic reflection survey on the oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin the seismic signature of gas hydrate a bottom simulating reflector bsr with negative polarity that locally cuts across stratigraphic horizons is especially well developed beneath hydrate ridge this anomalously shallow accretionary ridge was drilled during ocean drilling program leg 146 to study fluid venting in this paper we focus on the seismic data from the southern part of hydrate ridge where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system apparent disruptions of the bsr beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping a double bsr beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding on the basis of these and other observations we propose a qualitative model for the evolution of a hydrate bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor permitting construction of large ridges that are then eaten away by slumps along their margins the north to south variation in sea floor venting and subsurface seismic structure along hydrate ridge may reflect different stages in the temporal evolution of one of these ridges

significance of aragonite cements around cretaceous marine methane seeps abstract detailed petrography and geochemistry of carbonate precipitates in cretaceous cold seep mounds from the canadian arctic show spectacular early diagenetic products some still preserved splays and isopachous layers of fine acicular aragonite and large botryoids and crusts of low magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite the latter mineralogy is suggested by clear flat terminated cathodoluminescence patterns interpreted as ancient crystal growth steps and the former by rhombohedral terminations the early cement phases very likely precipitated in cold arctic water dominated by bicarbonates derived from bacterially oxidized methane these cements have d13c values around 44 and d18o values of 1 8 to 0 1 pdb coexistence of calcite and aragonite early cements in the cretaceous seep mounds is unusual because precipitation occurred in high latitude cold water settings and during a so called calcite sea mode as in modern marine hydrocarbon seeps the chemistry of the cretaceous system was apparently controlled by chemosynthetic bacterial activity resulting in high ahco3 that promoted precipitation of carbonates we suggest that locally fluctuations in ahco3 aso42 resulted in oscillating aragonite or calcite supersaturation and hence controlled the mineralogy of the early precipitates

heat flow and gas hydrates of the baikal rift zone multi channel seismic studies mcs performed during a russian expedition in 1989 and a joint russian american expedition in 1992 have for the first time revealed a bottom simulating reflector bsr in lake baikal these data have shown that gas hydrates occur in the southern and central basins of lake baikal in those places where the water depth exceeds 500 700 m four types of tectonic influence on the distribution of the gas hydrate were revealed a modern faults displace the bsr as they do with normal seismic boundaries b older faults displace normal reflectors whereas the bsr is not displaced c modern faults form zones where the bsr has been totally destroyed 4 processes that occur within older fault zones situated close to the base of the hydrated sediment layer lead to undulations of the bsr the thickness of the hydrate stability field inferred from seismic data ranges between 35 and 450 m heat flow values determined from bsr data range from 48 to 119 mw m2 a comparison between heat flow values from bsr data and values measured directly on the lake bottom shows an overall coincidence changes in water level and bottom water temperature that occurred in the past have had no noticeable influence on the present bsr depths or heat flow values determination of deep heat flow from bsr data is in this case more reliable than by direct measurements

stable isotopic ratios and origins of the carbonates associated with cold seepage at the eastern nankai trough two types of cold seeps were located at the eastern nankai trough 1 seeps on sediment covered seafloor of 3800 3600 m in depth at the toe of the accretionary prism and 2 those between 1900 and 2100 m situated on the backthrust of the accretionary prism covered with rough outcrops of calcareous sandstones fragments of sandstone pipes or chimneys cemented with calcite were recovered from type 1 seeps whereas in type 2 seeps massive carbonate cemented sandstones and silty sandstones were obtained many of which contain cemented fragments of different lithologies and textures including dense aggregates of fossilcalyptogena type clam shells the carbon isotopic ratios of the carbonates in these sediments range from 36 to 56 pdb the carbonates formed by mixing between isotopically light carbonate ions derived from biogenic methane carried from depth in advecting fluids and heavy marine sedimentary carbonates sharp isotopic gradients in carbon and sometimes also in oxygen from the mixing processes are still retained across the walls of the sandstone pipes from type 1 seeps whereas the isotopic ratios of the samples from type 2 seeps exhibit variation patterns related to lithologies and textures and confirm the presence of several generations of carbonate cement within a single specimen these facts and the oxygen isotopic ratios of live and fossil clam shells recovered from the survey areas suggest that the cold seeps started at least during the last glaciation and have experienced repeated destruction and reformation due to episodic supply of methane rich fluid of different isotopic ratios and or temperature

gas hydrate distribution and volume in canada gas hydrate a solid form of natural gas and water is inferred to widely occur in canadian polar and continental shelf regions and in sediment of outer continental margins although direct indications of hydrate are few and widely separated conditions potentially favorable for gas hydrate formation and stability especially low to moderate temperatures under permafrost or the deep sea combined with favorable geological conditions for gas generation and storage cover vast areas and indicate an immense potential for natural hydrocarbon gas in the upper 2 km of many canadian sedimentary basins we have analyzed the potential of gas hydrates for the vast continental shelves and arctic permafrost regions of canada mackenzie delta beaufort sea and arctic archipelago in the north and davis strait the labrador shelf scotian shelf and grand banks of newfoundland along the canadian atlantic margin and canadian pacific margin our conservative calculation suggests 1010 1012 m3 of gas hydrates in these regions has an associated methane gas potential estimated to be in the range of 1012 1014 m3 the volume of methane in hydrates in canada are geographically distributed in the following regions 0 24 8 7 x 1013 m3 in the mackenzie delta beaufort sea 0 19 6 2 x 1014 m3 in the arctic archipelago 1 9 7 8 x 1013 m3 on the atlantic margin and 0 32 2 4 x 1013 m3 on the pacific margin the total in situ amount of methane in hydrates of canada is estimated to be 0 44 8 1 x 1014 m3 as compared to a conventional canadian in situ hydrocarbon gas potential of approximately 0 27 x 1014 m3 this comparison suggests that gas hydrates represent a possible future assurance of north american energy supply if the gas can be recovered and separated from the hydrate form

foraminiferal record of environmental changes messinian of the los yesos area sorbas basin se spain the late messinian of the sorbas basin betic cordillera se spain has been sampled to estimate 1 the paleoenvironmental changes recorded by the benthic foraminifers in pre evaporitic and evaporitic units and 2 the impact of the messinian salinity crisis on the foraminiferal biodiversity in the pre evaporitic unit foraminifers indicate a circalittoral to bathyal environments and show significant changes in bottom water oxygenation simultaneously the occurrences of epiphytic damaged foraminifers and the blocks of porites point out the presence of plant substrates and coral reefs on the infralittoral zone the overlying evaporitic unit is characterized by four subaqeous selenitic gypsum layers and three marine fossiliferous intercalations each episode of gypsum deposition ends with a desiccation event each fossiliferous intercalations traduce a rapid reflooding of the basin by marine waters corresponding to infralittoral to upper bathyal environments with normal marine conditions during the evaporitic period the ecological conditions in the water column were intermittently favorable to the development of marine organisms and moreover the marginal basins of the betic cordillera were not continuously desiccated during the messinian the low percentage of extinct species 5 at the messinian pliocene boundary and the occurrences of benthic foraminifers in the evaporitic unit indicate that the infralittoral to upper bathyal environments have not been completely modified and or barren during the messinian salinity crisis i e the messinian salinity crisis is not associated with a biological crisis

late miocene seep carbonates and fluid migration on top of the montepetra intrabasinal high northern apennines italy relations with synsedimentary folding abstract during the miocene hydrocarbon seep carbonates located atop intrabasinal highs and associated with sediment instability formed commonly at the deformation front of the northern apennine collisional orogen the parallelism between the structural trend and the distribution of seep carbonates suggests a close relationship between tectonics and gas fluid emission the montepetra intrabasinal high was formed during the closure stage of the foredeep being related to the synsedimentary growth of an anticline field geometry suggests that detachment folding was the leading mechanism of anticline growth and synsedimentary instability along the anticline flanks ten different bodies of seep carbonates occur in the tortonian early messinian sediments nine in the hinge zone and one in the southern backlimb of the anticline foraminiferal study geochemistry facies investigation and the three dimensional geometry of carbonate bodies with respect to the encasing terrigenous sediments indicate a protracted late tortonian early messinian activity of fluid migration with re mobilization and ascent of sediments from the core of the anticline stabilization of chemosynthesis related communities and in situ brecciation seepage atop the intrabasinal high was fed by different circuits one related to the compaction dewatering of shallow tortonian early messinian sediments and a deeper one related to the deformation of the anticline core and to the activity of detachment surfaces and of faults propagating through the sedimentary cover

increased continental margin slumping frequency during sea level lowstands above gas hydrate bearing sediments we present sup 14 c data on sediment samples from cores of the upper 7 m of the sediment column overlying a major continental rise gas hydrate field on the southern carolina rise and inner blake ridge offshore the southeastern united states the data show that glacial age sediments are underrepresented in the cores the observation is consistent with a previously predicted association between sea level lowstands and increased frequency of sea floor slumping on continental margins containing gas hydrates 26 refs 3 figs

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

hydrocarbon seep carbonates of a miocene forearc east coast basin north island new zealand an ancient hydrocarbon seep province of 14 isolated authigenic carbonate deposits has been identified in fine grained deep marine siliciclastic strata of the miocene east coast basin north island new zealand these forearc sediments have been uplifted and complexly deformed into accretionary ridges adjacent to the still active hikurangi convergent margin older active and passive margin strata mid cretaceous to oligocene in age underlie the neogene sequence and contain oil and gas prone source rocks older mesozoic meta sedimentary rocks constitute the backstop against which the current phase of subduction related sedimentation has accumulated 24 ma present the seep carbonates up to 10 m thick 200 m across archive methane signatures in their depleted carbon isotopes to 13c 51 7 pdb and contain chemosynthesis based paleocommunities e g worm tubes bathymodioline mussels and vesicomyid lucinid and thyasirid bivalves typical of other cenozoic and modern seeps northern and southern sites are geographically separated and exhibit distinct lithological and faunal differences structural settings are variable seep associated lithologies also are varied and suggest carbonate development in sub seafloor seafloor and physically reworked diapiric expansion gas explosion gravity slide or debris flow settings similar to italian apennine seep deposits of overlapping ages peculiar attributes of the new zealand miocene seep deposits are several including digitate thrombolites of clotted microbial micrite encased in thick isopachous horizons and botryoids of aragonite seep plumbing features are also well exposed at some sites displaying probable gas explosion breccias filled with aragonite tubular concretions fluid conduits and carbonate cemented thin sandstone beds and burrows within otherwise impermeable mudstones a few seeps were large enough to develop talus debris piles on their flanks which were populated by lucinid bivalves and terebratulid brachiopods firmgrounds and hardgrounds were common as evidenced by trace fossil associations or caryophyllid coral thickets atop some seep carbonate deposits thus the new zealand examples show strong evidence of formation in sediments at or just beneath the seafloor but some were clearly exhumed by erosion to sustain later non seep epifaunal and boring paleocommunities

evidence for a thick free gas layer beneath the bottom simulating reflector in the makran accretionary prism abstract seismic reflection data from the makran continental margin indicate the presence of a strong and widespread bottom simulating reflector bsr we apply a nonlinear full waveform inversion technique to multichannel reflection data from this area to investigate the detailed velocity structure and hence the origin of the bsr our result shows an abrupt decrease in the compressional wave velocity from 2 2 to 1 3 km s at a depth of 500 m below the sea bed the low velocity zone is unusually thick 200 350 m and may contain large quantities of free gas similar to some of the recently drilled blake ridge sites of odp leg 164 the voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas hydrate stability field relative to the sediment column due to uplift and sedimentation in the accretionary wedge

composition and spatial organization of a cold seep community on the south barbados accretionary prism tectonic geochemical and sedimentary context abstract deep faunal assemblages were observed on cold seeps between 1000 and 2000m depth along the southern termination of the barbados accretionary prism during the caracolante ii and diapicar cruises faunal composition and microdistribution of a cold seep community were analyzed at one site using deep sea photographs and seismic 3 5 kz profiles the community is dominated by large mussels up to 20cm long possibly related to the genus bathymodiolus tiny vesicomyid clams and vestimentiferan clusters in which animals are up to 2m long there is a high density of sponges three species bryozoans and gorgonians other taxa occasionally associated with this assemblage include galatheid crabs anemones holothurians and possibly pennatulaceans which are numerous at boundaries of the site a typical deep sea fauna of fish asteroids and ophiuroids is also scattered about the vicinity this seep community is distributed as a band of discontinuous colonies 40 50m in width and more than 10km in length living colonies alternate with accumulations shells and bare areas the substratum is hard mud covered by a ferrugineous crust and cemented by carbonates the early diagenetic character of the bottom and the occurrence of biological assemblages indicate that seepages of sulphur and or methane which have been revealed along the decollement zone of the barbados prism must diffuse through the sea floor these biological and geological features are located on the crests of ne sw anticlinal ridges and mud volcanoes which have been built by diaprism

role of naturally occurring gas hydrates in sediment transport naturally occurring gas hydrates have the potential to store enormous volumes of both gas and water in semi solid form in ocean bottom sediments and then to release that gas and water when the hydrate s equilibrium conditions are disturbed therefore hydrates provide a potential mechanism for transporting large volumes of sediments under the combined low bottom water temperatures and moderate hydrostatic pressures that exist over most of the continental slopes and all of the continental rises and abyssal plains hydrocarbon gases at or near saturation in the interstitial waters of the near bottom sediments will form hydrates the gas can either be autochthonous microbially produced gas or allochthonous catagenic gas from deeper sediments equilibrium conditions that stabilize hydrated sediments may be disturbed for example by continued sedimentation or by lowering of sea level in either case some of the solid gas water matrix decomposes released gas and water volume exceeds the volume occupied by the hydrate so the internal pressure rises drastically if large volumes of hydrate are decomposed part of the once rigid sediment is converted to a gas and water rich relatively low density mud when the internal pressure due to the presence of the compressed gas or to buoyancy is sufficiently high the overlying sediment may be lifted and or breached and the less dense gas cut mud may break through such hydrate related phenomena can cause mud diapirs mud volcanos mud slides or turbidite flows depending on s diment configuration and bottom topography

evidence of climatic control on hydrocarbon seepage in the miocene of the northern apennines the case study of the vicchio marls abstract the vicchio outcrop in the tuscan apennines contains excellent exposures of a miocene methane derived carbonate system made up of more than 80 carbonate bodies enclosed in marly sediments facies analysis composition and a detailed biostratigraphic study of the carbonates and enclosing vicchio marls allowed us to document the role of climatic changes and eustasy on seepage in these ancient deposits results of our study indicate that the stratigraphic horizon bearing seep carbonates is constrained by two planktonic foraminiferal events the acme end ae of turborotalita cf t quinqueloba 13 75 ma and the acme 1 beginning a 1 b of paragloborotalia siakensis 13 32 ma encompassing about 400 000 years the ae of t cf t quinqueloba approximates the mid miocene global cooling event mi3b as defined by the 18 o maxima 13 78 ma the glacio eustatic sea level drop associated with this cooling event is estimated to be approximately 60 m assuming that the highest rates of glacio eustatic sea level fall coincide with 18 o maxima the timing of the seepage onset at about 13 75 ma is concomitant with the phase of sea level lowering the ascent and emission of methane rich fluids may have been triggered by pressure drop due to the eustatic fall associated with the mi3b event the moderate intensity of fluid expulsion is suggested by the lack of brecciation by the scarcity of detrital particles in the carbonates and by the pervasive occurrence of carbonate in the enclosing marls this type of seepage differs from focused fault confined fluid flows typical of other tectonically controlled apenninic seep carbonates

sediment instability related to fluid venting in miocene authigenic carbonate deposits of the northern apennines italy chemosynthetic carbonates identified by isotopic palaeoecological and sedimentological features are concentrated in middle late miocene satellite and foredeep deposits of the northern apennines chemoherms in the foredeep are hosted in thick pelitic intervals probably deposited in intrabasinal structural highs which are entirely or partly involved in large slumps in many cases associated with extrabasinal slides sediment textures in carbonates and in the enclosing foredeep pelitic sediments indicate a link between hydrocarbon fluid venting sediment deformation and mobilisation and tectonics the intensity and style of fluid release phases directly influenced chemoherm typology and also determined overpressure conditions in low shear strength pelitic sediments favouring sediment mobilisation and influencing slope instability which widely affected the apennine foredeep chemosynthetic carbonates are associated with sites of tectonically fractured and compressed sediments in the apennine foredeep thrust belt system thus indicating a relation with the tectonic loading of the apennine thrust sheets which favoured fluid expulsion along forerunner faults possible gas hydrate contributions to fluid expulsion processes are discussed based on sediment textures compared with modern vent areas finally sediment instability may have facilitated a large amount of fluid escape thus stopping carbonate precipitation

measurements of transience and downward fluid flow near episodic methane gas vents hydrate ridge cascadia aqueous flux measurements within cold seep regions on northern hydrate ridge cascadia indicate a high degree of variability with extended periods of downflow and reversals of flow direction over periods of weeks to months local episodic venting of free methane gas was also observed the instruments recorded similar changes in hydrologic flow patterns both on and off clam fields the magnitude of the flow rates decreasing away from the clam field the coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled we propose that these observations are consistent with the action of a gas driven pump that operates somewhat like a geyser the hypothesis of gas driven pumping of seawater through northern hydrate ridge has important ramifications for the mass fluxes through this region

widespread venting of methane rich fluids in late cretaceous campanian submarine springs tepee buttes western interior seaway u s a from new mexico to montana campanian shales contain abundant submarine spring and seep deposits aligned along early laramide basement faults from spring conduit to sea floor characteristic lithofacies are a a vuggy pelletoid micrite core to 30 m height 5 m width b lucinoid bivalve coquina in pelletoid micrite or grainstone c carbonate slump breccias in shale d concretion bearing shale and e organic rich shale stable isotope analyses of early zoned marine core rock cements suggest a an oxidized methane source for the carbonate carbon b venting of methane charged fluids along active fracture zones and c methanogenesis in older and coeval organic carbon rich muds six consistently zoned foraminifer and macrofaunal communities suggest a strong environmental stress gradient over a few metres from spring vents to the adjacent sea floor these methane springs and their biotas were dynamic and episodically active over a 1 25 m y time span

climatically driven impacts on sedimentation processes in the bay of quiberon south brittany france over the last 10 000 years the bay of quiberon nw france which is protected by the quiberon peninsula provides well preserved sedimentary archives for high resolution reconstruction of holocene climate variability very high resolution seismic profiles were obtained from two seistec boomer seismic campaigns conducted in the bay the seismic surveys reveal the deposition of five distinct sedimentary units this succession is correlated with radiocarbon dated sediments sampled in a long casq core md08 3204cq the analysis of the sediments shows significant changes in grain size indicating a complex series of palaeo climate events dated at 8800 8600 4270 3860 2470 and 1060 cal yr bp these may be correlated with millennial timescale climate variability corresponding to the phases of holocene rapid climate changes rccs associated with negative north atlantic oscillation nao index and usually characterized by weaker winter storms on the contrary periods between rccs are characterized by the predominance of westerly winds and stormy conditions for brittany i e positive nao however only storm events occurring during the rccs have been preserved in the bay of quiberon this paper aims to reconstruct the holocene sedimentary infill of the bay highlighting the role of episodic acceleration phases of the sea level rise on the preservation of sedimentary archives thus the bay of quiberon provides substantially a complete sedimentation record of the last 10 000 years characterized by a series of complex palaeo environmental changes

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

seafloor manifestations of fluid seepage at the top of a 2000 metre deep ridge in the eastern nankai accretionary wedge long lived venting and tectonic implications abstract the results of the 1989 kaiko nankai diving cruise to the eastern nankai accretionary wedge show that a narrow corridor on the top of a 2000 m deep ridge is characterized by concentrated surface manifestations of fluid seepage and chaotic outcrop orientations suggesting active faulting along a right lateral shear zone modern biological indicators of fluid venting include bivalve vesicomyid species and vestimentiferan tube worms ancient manifestations of fluid venting consist of similar vesicomyid species found in 20 and 150 ka concretions this suggests relatively constant biotypes carbonate deposits including fossil cemented shells were observed near active sites either as paving stones diffuse seepage and cementation cut by channels or as pinnacles local seepage and cementation despite its proximity less than 15 km a nearby 3800 m deep site where substantial fluid discharge occurs seems to be completely separate from the 2000 m deep ridge site the 3800 m deep site differs from its shallower counterpart in terms of its seafloor manifestations and the composition of its fluids the sediment at the top of the ridge ranges in age around 500 300 ka which attests that the transition from a depositional to an erosional stage was younger than 800 ka but older than 200 ka this transition can be due to either the incorporation of this unit from the trench fill into the accretionary wedge front or the uplift of a slope basin the first hypothesis leads to a 2 5 cm yr minimum progradation rate associated with a 2 5 mm yr minimum uplift rate from the paleotrench to the present ridge the second hypothesis implies a recent uplift of a piggy back basin at a minimum rate of 1 mm yr

cold seep deposits of beauvoisin oxfordian southeastern france and marmorito miocene northern italy microbially induced authigenic carbonates the relation of two well known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study archaea are found to be associated with the formation of oxfordian seep carbonates from beauvoisin and with a miocene limestone from marmorito tube worm limestone carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative 13ccarbonate values respectively highly positive values 15 reflect the use of 13c enriched co2 produced by methanogenesis low 13c values of the marmorito carbonates 30 indicate the oxidation of seepage derived hydrocarbons likewise the 13c content of specific tail to tail linked isoprenoids biomarkers for archaea was found to be strikingly depleted in these samples as low as 115 the isotopic signatures corroborate that archaea were involved in the cycling of seepage derived organic carbon at the ancient localities another miocene limestone marmorito limestone shows a strong imprint of methanotrophic bacteria as indicated by 13c values of carbonate as low as 40 and biomarker evidence epifluorescence microscopy and field emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation in addition to lucinid bivalves previously reported from both localities we infer that sponges from beauvoisin and tube worms from marmorito depended on chemosynthesis as well low 13c values of nodules related to sponge taphonomy 27 indicate that sponges might have been linked to an enhanced hydrocarbon oxidation tube worm fossils from marmorito closely resemble chemosynthetic pogonophoran tube worms from recent cold seeps and are embedded in isotopically light carbonate 13c 30

a multidisciplinary study of middle miocene seep carbonates from the northern apennine foredeep italy several pelitic intervals are intercalated at various levels within the marly arenaceous turbiditic successions of the middle miocene northern apennine foredeep they range in thickness from 30 to 200 m and represent sedimentation on top of ephemeral structural highs related to blind faults sediments are made up of hemipelagites and fine grained turbidites and include 13c depleted carbonates related to the rising of methane rich fluids hydrocarbon seep carbonates large portions of pelitic intervals are involved in chaotic masses by soft sediment deformation slumps slides intraformational breccias revealing an intense sediment instability during middle miocene a stratigraphic mineralogic and geochemical study was conducted on two of these pelitic intervals castagno d andrea mt citerna in order to reconstruct carbonate development the composition of fluids and to document the connections between fluid seepage and syndepositional tectonics this multidisciplinary approach has allowed us to discriminate between the two examined pelitic intervals in terms of age depositional rates and conditions organic carbon and post depositional processes seep carbonates are characterized by chemosymbiotic fossil communities autoclastic fractures and brecciation carbonate bodies show complex facies relationships as indicative of different stages in chemoherm growth the compositional study evidences the peculiar chemistry of chemoherm carbonates calcite low in mg and sr compared with carbonates in associated enclosing pelites and with modern chemoherms in general the non carbonate components within the chemoherms are enriched in detrital minerals and depleted in phyllosilicates with respect to the enclosing pelites the mineralogical changes in the clay component within the brecciated unit of the castagno d andrea chemoherms suggest authigenic precipitation of the mg rich phases isotopic analyses show the distinct carbon signature in the chemoherms from the two examined intervals castagno d andrea chemoherms more depleted from 15 8 up to 41 3 pdb than mt citerna from 5 2 up to 16 7 and the transitional 13c depletion trend observed moving from chemoherms to the enclosing pelites moderately depleted and te bouma sequence turbidites in the range of marine carbonates a slight but significant enrichment in 18o up to 1 4 pdb is observed for all chemoherms when compared to values of carbonate phases present in enclosing pelites geochemical data indicate that the brecciated facies of seep carbonates are related to an explosive release of gaseous fluids probably associated with the rise of deep hypersaline fluids

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

deep tow survey in the kaiko nankai cold seepage areas deep sea biological communities associated with cold venting were located within the eastern portion of the nankai trough using a deep towed tv system five continuous lines approximately 200 km long were observed bivalve communities were found both in the tenryu canyon outlet region and at the outcrop of the frontal thrusts of the accretionary wedge off cape omaezaki the former is a site at which biological communities had been found during the kaiko dives in 1985 however the second site appears to be associated with larger scale venting and was chosen for the 1989 kaiko nankai submersible cruise three small mud volcanoes with diameters of several hundred meters and heights of several tens of meters are found in the seabeam topography map but the present survey showed neither evidence of recent venting nor biological communities

eocene deep sea communities in localized limestones formed by subduction related methane seeps southwestern washington densely populated communities of soft bottom dwelling taxa similar to those found today along subduction zones off the coasts of japan and oregon have been discovered in very localized deep water limestones of late middle to late eocene age along the southwestern margin of washington subduction was prevalent in this area during this time and compressive forces squeezed subsurface methanerich waters onto the ocean floor where opportunistic bivalves especially modiolus calyptogena and thyasira vestimentiferan tube worms serpufid tube worms siliceous sponges very small limpets trochid and turbinid archaeogastropods and other macrobenthos colonized these assemblages are the earliest recorded biologic communities formed in response to methane seeps in subduction zones

trapping and migration of methane associated with the gas hydrate stability zone at the blake ridge diapir new insights from seismic data abstract the blake ridge diapir is the southernmost of a line of salt diapirs along the carolina trough diapirs cause faulting of the superjacent sediments creating pathways for migration of fluids and gas to the seafloor we analyzed reflection seismic data from the blake ridge diapir which is located in a region with known abundant gas hydrate occurrence a striking feature in these data is a significant shallowing of the base of gas hydrate stability bghs over the center of the diapir the seafloor is warped up by about 100 m above the diapir from about 2300 m to about 2200 m the bghs as indicated by a bottom simulating reflection bsr is about 4 5 s off the flanks of the diapir rising to about 4 15 s at the center above the diapir a fault system appears to rise vertically from the bghs to about 0 05 s below the seafloor 40 50 m it then diverges into several steeply dipping faults that breach the seafloor and cover an area 700 m in diameter other secondary faults diverge from the main fault or emerge directly from the bghs near the crest of the diapir gas and other fluids may migrate upward through the faults we performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections a low frequency anomaly over the center of the diapir indicates high seismic attenuation this is interpreted to be caused by migration of fluids probably methane along fault pathways the migration of gas i e probably mainly methane through the gas hydrate stability zone is not yet understood we speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate even at depths where gas hydrate is stable away from the diapir alternatively gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates the shallowing of the bsr may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids high pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in bsr shallowing we therefore investigated the possible effect of pore water salinity on shallowing of the bsr we found that bsr shallowing may theoretically be entirely caused by increased salinity over the diapir although geologically this would not be reasonable this observation demonstrates the potential importance of pore water salinity for lateral variations of bsr depths in particular above salt structures

anatomy of seep carbonates ancient examples from the miocene of the northern apennines italy ancient hydrocarbon seep carbonates of miocene age have been reported from marine sedimentary successions of the northern apennines in the form of irregularly shaped micritic masses containing abundant fossil remains of oligotypic assemblages they are recognized by their peculiar biological sedimentological and geochemical features and represent an excellent on land example of carbonate bodies interpreted as the remains of ancient cold seeps in the apenninic chain the abundance and the extent of the outcrops provide a rare opportunity to study the geometry facies distribution and internal structures of fossil methane derived carbonates the present study reports an integrated view of the sedimentology mineral paragenesis and geochemistry of these seep carbonates in two different tectono sedimentary settings foredeep and satellite basins the main result of this study is the recognition of 17 facies in the investigated seepcarbonates the detailed facies and microfacies analysis led to the identification of different structures which occur in welldefined parts of the carbonate masses the recurring and broadly similar stratigraphy and facies association from the two considered geological environments is diagnostic of the different stages of seep carbonate formation and evolution d 2005 elsevier b v all rights reserved

timing constraints on building an intermediate plutonic arc crustal section u pb zircon geochronology of the sierra valle fertil la huerta famatinian arc argentina 1 the sierra valle fertil range in northwestern argentina exposes a tilted crustal section through the ordovician famatinian arc from 25 km to shallow crustal paleodepths fourteen new u pb zircon crystallization ages of magmatic rocks from sierra valle fertil area show that this section of the arc was built over a short time interval during the ordovician between 485 and 465 ma zircon rim ages demonstrate that high grade metamorphism and migmatization were synchronous with magmatic emplacement inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto south american continent in the ordovician which represents a part of gondwana docking of the precordilleran terrane outboard of proto south america led to the cessation of arc magmatism in the valle fertil area and preservation of the arc in its early stages after 60 myr of subduction during the pampean and famatinian magmatic stages our age data from the sierra valle fertil indicate that wet mafic magmas were emplaced into the section at a rate typical for modern island arcs 30 km3 km 1 myr 1 the production of intermediate to felsic magmas in the upper plate was a result of partial melting of the metasedimentary framework and hybridization with mantle derived melts and resulted in the generation of tonalites granodiorites and granites in the upper crust at a rate of 100 km3 km 1 myr 1 similar to major magmatic flare ups in the mature arcs of north american cordillera the sierra valle fertil arc section is a type example of an immature cordilleran arc before major crustal thickening and crustal overturn takes place in the upper plate the famatinian arc section may also be an equivalent to the crustal architecture of more mature modern island arcs such as parts of the aleutians caribbean and japan

clathrate eustasy methane hydrate melting as a mechanism for geologically rapid sea level fall although submarine methane hydrates or clathrates have been highlighted as potential amplifiers of modern global climate change and associated glacio eustatic sea level rise their potential role in sea level fall has not been appreciated recent estimates of the total volume occupied by gas hydrates in marine sediments vary 20 fold from 1 2 x 10 sup 14 to 2 4 x 10 sup 15 m sup 3 using a specific volume change on melting of minus 21 dissociation of the current global inventory of hydrate would result in a decrease of submarine hydrate volume of 2 4 x 10 sup 13 to 5 0 x 10 sup 14 m sup 3 release of free gas bubbles present beneath hydrates would increase these volumes by 1 1 2 0 x 10 sup 13 m sup 3 the combined effects of hydrate melting and subhydrate gas release would result conservatively in a global sea level fall of 10 146 cm such a mechanism may offset some future sea level rise associated with thermal expansion of the oceans it could also explain anomalous sea level drops during ice free periods such as the early eocene the cretaceous and the devonian

geological evidence for mud diapirism on the mediterranean ridge accretionary complex abstract a mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the mediterranean ridge this breccia is composed of subrounded clasts supported by a clay rich matrix and ranges in age from cretaceous to early miocene sedimentological analysis allows identification of a primitive intrusive and a reworked mud breccia the latter being formed by mud extrusion and gravitational reworking of the former homogeneous mud sorted by grain size which occurs at the top of the diapirs has been interpreted as a lake deposit based on composition and carbon isotopic ratios gases present within the diapiric material suggest a partial thermogenic origin and gas escape structures have been identified in the mud breccia and host sediments carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep sea lithification in the mediterranean sea the presence of a marker bed composed of mn nodules and bacterial colonies the mud lake deposits showing lower carbonate content than primitive mud breccia and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures evidence of active mud diapirism can be found from about 300 000 yrs b p to the present

giant submarine landslides submarine landslides can generate enormous turbidity currents that carry that carry 500 km3or more of sediment down to the oceans abyssal plains the slides can cause tsunamis and may release large amounts of methane to the air

bacterial methane oxidation in sea floor gas hydrate significance to life in extreme environments samples of thermogenic hydrocarbon gases from vents and gas hydrate mounds within a sea floor chemosynthetic community on the gulf of mexico continental slope at about 540 m depth were collected by research submersible the study area is characterized by low water temperature mean 7 c high pressure about 5 400 kpa and abundant structure ii gas hydrate bacterial oxidation of hydrate bound methane ch sub 4 is indicated by three isotopic properties of gas hydrate samples relative to the vent gas from which the gas hydrate formed 1 methane bound methane is enriched in sup 13 c by as much as 3 8 pdb peedee belemnite 2 hydrate bound methane is enriched in deuterium d by as much as 37 smow standard mean ocean water and 3 hydrate bound carbon dioxide co sub 2 is depleted in sup 13 c by as much as 22 4 pdb hydrate associated authigenic carbonate rock is also depleted in sup 13 c bacterial oxidation of methane is a driving force in chemosynthetic communities and in the concomitant precipitation of authigenic carbonate rock that modifies sea floor geology bacterial oxidation of hydrate bound methane expands the potential boundaries of life in extreme environments

low sea level emplacement of a very large late pleistocene megaturbidite in the western mediterranean sea large volume turbidites termed megaturbidites or megabeds result from catastrophic slope failures and the associated downslope transport of enormous quantities of sediment from continental margins to the deep sea such large sediment failures can generate tsunamis and in terrains underlain by gas hydrates clathrates may be associated with the release of substantial amounts of the greenhouse gas methane it has been proposed that the megaturbidite events may be triggered by seismic activity or may result from gas hydrate release itself caused by a lowering of hydrostatic pressure on clathrates as a result of low sea level previous conclusions on the significance of sea level change however have been conditional because of the lack of absolute times of turbidite emplacement here we use accelerator mass spectrometry radiocarbon dating in five widely spaced cores to constrain the date of emplacement of a large volume 500 km3 bed in the balearic basin of the western mediterranean this turbidite is exceptional in its magnitude and represents the main sedimentation event in the balearic basin over the past 100 kyr our data provide an estimate of 22 000 calendar years before present for emplacement of the megabed a time when sea level stood at its lowest level during the last glacial maximum the coincidence of these dates is consistent with emplacement due to clathrate destabilization caused by low sea level although other triggering mechanisms such as seismic shock cannot be ruled out

relationship of clathrate instability to sediment deformation in the upper neogene of california widespread soft sediment deformation ssd in upper bathyal continental margin sediments has been explained by compaction related dewatering and related physical processes that can destabilize margin sediments we present the hypothesis that ssd in such facies resulted in part from instability of gas hydrates clathrates leading to overpressurization in sediments and resulting fluid flow and brecciation clathrate related deformation must be widespread in upper bathyal continental margin methanogenic sequences at depths where clathrates are destabilized by changes in bottom water temperatures and sea level a close association exists between an array of ssd features with clathrate hosting sediments in ocean drill cores similar features are conspicuous in the partly laminated upper neogene upper bathyal sediments of the sisquoc formation in southern california suggesting that clathrates may have been responsible for abundant and well developed ssd in these units the lithology methane producing organic content and depositional environment of the sisquoc formation are similar to those features in modern clathrate hosting settings these characteristics in conjunction with the upper bathyal paleobathymetry of the unit the geothermal gradient of the region and 13 c values of benthic foraminifera and dolomite concretions suggest the occurrence of clathrates in the shallow subsurface of these sediments we suggest that the analyzed parts of the sisquoc formation represent an exhumed zone of ssd resulting from clathrate instability in the shallow subsurface of the continental margin sedimentary rocks exhibiting an association of ssd features that resulted from clathrate formation and dissociation are named clathrites