gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

linking basin scale and pore scale gas hydrate distribution patterns in diffusion dominated marine hydrate systems the goal of this study is to computationally determine the potential distribution patterns of diffusion driven methane hydrate accumulations in coarse grained marine sediments diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse grained sediments over geologic time using one dimensional compositional reservoir simulations we examine hydrate distribution patterns at the scale of individual sand layers 1 20 m thick that are deposited between microbially active fine grained material buried through the gas hydrate stability zone ghsz we then extrapolate to two dimensional and basin scale three dimensional simulations where we model dipping sands and multilayered systems we find that properties of a sand layer including pore size distribution layer thickness dip and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand which in turn impact the distribution of hydrate throughout a sand unit in all of these simulations we incorporate data on physical properties and sand layer geometries from the terrebonne basin gas hydrate system in the gulf of mexico we demonstrate that diffusion can generate high hydrate saturations upward of 90 at the edges of thin sands at shallow depths within the ghsz but that it is ineffective at producing high hydrate saturations throughout thick greater than 10 m sands buried deep within the ghsz furthermore we find that hydrate in fine grained material can preserve high hydrate saturations in nearby thin sands with burial

short migration of methane into a gas hydrate bearing sand layer at walker ridge gulf of mexico 1 the mechanisms governing distribution of natural gas hydrates in marine sediment systems are not well understood we focus on a hole in the gulf of mexico walker ridge block 313 hole h where a 2 5 m thick gas hydrate bearing sand occurs within a 152 m thick fine grained mud interval containing gas hydrate in fractures the gas hydrate bearing sand is surrounded by two distinct hydrate free zones 10 m thick above and 3 m thick below we hypothesize that microbial methane generated within the hydrate free zones diffused into the sand and formed gas hydrate we show that the amount of methane produced in the hydrate free zones is likely enough to explain the gas hydrate content of the sand layer additionally we show that there is enough time for dissolved methane to diffuse from the hydrate free zones into the sand we conclude that methane transport over significant distances via fluid flow is not required but that microbial methane could migrate short distances to form gas hydrate in the sand layer

gas origin and migration in the ulleung basin east sea results from the second ulleung basin gas hydrate drilling expedition ubgh2 abstract we have investigated the gas chemistry of marine sediments up to 250 m below seafloor m bsf recovered during the second ulleung basin gas hydrate drilling expedition ubgh2 to identify and compare the gas origin and its migration at seismically non chimney truncated chimney and chimney sites in this basin the molecular and isotopic data c1 c2 ratios 300 13 c ch 4 d ch 4 kim et al 2012 and thought to reflect two phase methane transport processes in chimney sites deep sourced methane migrates independently of the carbon dioxide which remains dissolved in the aqueous phase leading to the extremely low c values observed in the upper 30 m bsf upward methane diffusion at non chimney sites may also add a transport induced fractionation as evidenced in mixing diagrams but this process does not significantly affect the c values 13 c co 2 shows usually minimum values at the sulfate methane transition zone smtz at all sites however in chimney sites these values have a much more depleted signature indicating faster rates of methane consumption by anaerobic oxidation of methane aom supported by the higher methane flux moreover here the massive hydrate that forms in shallow sediments incorporates gases with a lower c1 c2 ratio higher c2 c1 than the surrounding sediment as documented in hydrate bound gas bg and void gas vg samples gas hydrate dissociation during the core recovery is released enriched ethane c2h6 trapped in gas cage which leads to have high c2h6 in vg samples

marine silicate weathering in the anoxic sediment of the ulleung basin evidence and consequences marine silicate weathering msiw in anoxic sediments has been recently shown to be a significant sink for co2 generated by methanogenesis independently the roles of clay dehydration illitization in producing water and driving upward fluid advection have been well established in deep marine sediments but to date the k source required for the reaction has not been established here we present chemical and strontium isotope properties of pore fluids from seven cores in the ulleung basin which show radiogenic 87sr 86sr values up to 0 71045 very high alkalinity values maximum 130 mm and enrichment in h4sio4 na k and mg2 consistent with msiw this reaction consumes co2 generates alkalinity and acts as a k source for illitization water released from msiw supported illitization drives upward fluid flow our results highlight the importance of msiw along continental margins and its underappreciated role in carbon cycling silicate diagenesis and hydrogeology of marine systems

origin of gas in gas hydrates as interpreted from geochemistry data obtained during the national gas hydrate program expedition 02 krishna godavari basin offshore india abstract this report presents the result of geochemical studies carried out on cores recovered from krishna godavari basin k g basin during the national gas hydrate program expedition 02 nghp 02 nghp 02 targeted gas hydrate accumulations in sand facies in the k g basin off the east coast of the indian peninsula three areas area b area c and area e in the k g basin were investigated during this expedition 10 research sites in k g basin were cored and samples for geochemistry analysis were collected high toc tn ratios indicate an input from terrestrial organic matter analysis of gas samples from pressure cores also acquired during nghp 02 were composed of mostly methane carbon isotopic studies show that methane gas sampled from recovered gas hydrate samples were derived from biogenic microbial sources this list of factors appears to indicate that the gas associated with the recovered hydrate bearing core samples were derived from a microbial source however the biogeochemical processes responsible for generating the microbial gases found with the recovered gas hydrates may not have been limited to current limits off the gas hydrate stability zone it is important to note here that microbial process likely extend to more than 1 km deep into the basin so gas coming from below the gh stability zone can still be from a microbial source in addition the microbial gas within the stability zone was likely in part recycled from hydrate accumulations that has passed through the base of the gas hydrate stability zone as a product of sedimentation with the dissociated microbial gas migration again vertically in the section entering the gas hydrate stability zone and reforming gas hydrates although the organic carbon required for in situ methanogenesis and gas hydrate formation is abundant in the shallow sedimentary section of the kg basin the faulted fractured and deformed stratigraphy observed in the kg basin coupled with high pore filling gas hydrate saturations at many sites suggests that advective processes may be contributing to the supply of methane to the gas hydrates in the kg basin the possible migration of gas from greater depths is also supported by seismic evidence of through going migration pathways that can connect the gas hydrate system with deeper petroleum systems it is important to note here that the deep conventional gas accumulations in the basin also contain microbially sourced gas

tectonic sediment thickening fluid expulsion and the thermal regime of subduction zone accretionary prisms the cascadia margin off vancouver island tectonic sediment thickening fluid expulsion and the resulting thermal regime of the northern cascadia subduction zone accretionary prism have been described by numerical models both the sediment thickening and the fluid expulsion are found to have important thermal effects constraints on the models are provided by 1 detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom simulating reflector bsr and 2 porosity changes across the accretion zone inferred from the landward variation in the p wave velocity depth profiles the heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20 below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes the porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea cascadia basin the model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically generating a high porosity underconsolidated section reconsolidation and fluid expulsion in response to the tectonic thickening is slow occurring mainly 20 30 km landward of the deformation front the maximum predicted fluid expulsion rate is about 4 10 11 m s 1 1 3 mm a 1 the pore pressure distribution across the deformation front region has been estimated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure pore pressures are high reaching 80 of lithostatic 15 to 20 km landward of the deformation front an estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10 16 m2 near the top to about 10 18 m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data the prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism

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

thermal evolution of the new zealand hikurangi subduction margin impact on natural gas generation and methane hydrate formation a model study abstract we present an integrated 2d model of thermal and microbial generation of methane migration into the gas hydrate stability zone hsz and formation of methane hydrates the model reconstructs the shallow 0 20 km thermal structure of the subduction interface between the australian plate and the subducting pacific plate and the trench basin pegasus basin modelled temperatures of less than 110 c within pegasus basin constrain the generation of oil and gas whilst a cool thermal regime is predicted to limit thermogenic generation of gas to a burial depth of 10 km it extends the interval where prolific microbial gas generation occurs the modelled rate of microbial generation of methane increases beneath the hsz and peaks at 1600 m below seafloor diffusive upward migration of microbially generated methane is interpreted to lead to widespread methane hydrate formation and the presence of a semi continuous bottom simulating reflector bsr predicted average hydrate saturation within the hsz is 0 9 for a modelled sedimentary organic matter content of 0 5 and 1 6 for 1 organic matter in fine grained pegasus basin sediments considerably higher concentrations of methane hydrate of up to 20 70 are predicted to occur where gas migration is focussed within the frontal anticline and proto thrust zone southeast of the modern accretionary wedge and in channel and basin floor sandstones related to the hikurangi channel the hikurangi channel sedimentary system transported coarse clastic sediments eroded from the rising southern alps along the eastern margin of the pegasus basin since the miocene it provides carrier beds specifically for transport of thermogenic gas generated close to the subduction interface a buried mesozoic accretionary wedge originating from subduction of the pacific plate beneath gondwana further focusses the migration of gas focussed migration of thermogenic gas leads to the highest predicted hydrate concentrations in potential channel sand reservoirs

chemical and isotopic signature of bulk organic matter and hydrocarbon biomarkers within mid slope accretionary sediments of the northern cascadia margin gas hydrate system abstract the chemical and isotopic compositions of sedimentary organic matter som from two mid slope sites of the northern cascadia margin were investigated during integrated ocean drilling program iodp expedition 311 to elucidate the organic matter origins and identify potential microbial contributions to som gas hydrate is present at both locations iodp sites u1327 and u1328 with distinct patterns of near seafloor structural accumulations at the cold seep site u1328 and deeper stratigraphic accumulations at the slope basin site u1327 source characterization and evidence that some components of the organic matter have been diagenetically altered are determined from the concentrations and isotopic compositions of hydrocarbon biomarkers total organic carbon toc total nitrogen tn and total sulfur ts the carbon isotopic compositions of toc 13ctoc 26 to 22 and long chain n alkanes c27 c29 and c31 13c 34 to 29 suggest the organic matter at both sites is a mixture of 1 terrestrial plants that employ the c3 photosynthetic pathway and 2 marine algae in contrast the 15ntn values of the bulk sediment 4 to 8 are consistent with a predominantly marine source but these values most likely have been modified during microbial organic matter degradation the 13c values of archaeal biomarker pentamethylicosane pmi 46 4 and bacterial sourced hopenes diploptene and hop 21 ene 40 9 to 34 7 indicate a partial contribution from methane carbon or a chemoautotrophic pathway our multi isotope and biomarker based conclusions are consistent with previous studies based only on the elemental composition of bulk sediments that suggested a mixed marine terrestrial organic matter origin for these mid slope sites of the northern cascadia margin

the stable carbon isotope biogeochemistry of acetate and other dissolved carbon species in deep subseafloor sediments at the northern cascadia margin abstract ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor but to date little is known about their metabolic activities to better understand the biogeochemical processes in the deep biosphere we investigate the stable carbon isotope chemistry of acetate and other carbon bearing metabolites in sediment pore waters acetate is a key metabolite in the cycling of carbon in anoxic sediments its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ this study reports our findings for a methane rich site at the northern cascadia margin ne pacific where expedition 311 of the integrated ocean drilling program iodp sampled the upper 190 m of sediment at site u1329 13c values of acetate span a wide range from 46 0 to 11 0 vs vpdb and change systematically with sediment depth in contrast 13c values of both the bulk dissolved organic carbon doc 21 6 1 3 vs vpdb and the low molecular weight compound lactate 20 9 1 8 vs vpdb show little variability these species are interpreted to represent the carbon isotopic composition of fermentation products relative to doc acetate is up to 23 1 depleted and up to 9 1 enriched in 13c broadly 13c depletions of acetate relative to doc indicate flux of carbon from acetogenesis into the acetate pool while 13c enrichments of pore water acetate relative to doc suggest consumption of acetate by acetoclastic methanogenesis isotopic relationships between acetate and lactate or doc provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment in particular they suggest that acetogenic co2 reduction can coexist with methanogenic co2 reduction a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron accepting process further the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments

sources of fluids and gases expelled at cold seeps offshore georgia eastern black sea four seep sites located within an 20 km2 area offshore georgia batumi seep area pechori mound iberia mound and colkheti seep show characteristic differences with respect to element concentrations and oxygen hydrogen strontium and chlorine isotope signatures in pore waters as well as impregnation of sediments with petroleum and hydrocarbon potential all seep sites have active gas seepage near surface authigenic carbonates and gas hydrates cokheti seep iberia mound and pechori mound are characterized by oil stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release pechori mound is further characterized by deep fluid advection of lower salinity pore fluids the pechori mound pore fluids are altered by mineral water reactions at elevated temperatures between 60 and 110 c indicated by heavier oxygen and lighter chlorine isotope values distinct li and b enrichment and k depletion strontium isotope ratios indicate that fluids originate from late oligocene strata this finding is supported by the occurrence of hydrocarbon impregnations within the sediments furthermore light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at pechori mound iberia mound and colkheti seep c15 hydrocarbons at the oil seeps are allochtonous whereas those at the batumi seep area are autochthonous the presence of oleanane an angiosperm biomarker suggests that the hydrocarbon source rocks belong to the maikopian formation in summary all investigated seep sites show a high hydrocarbon potential and hydrocarbons of iberia mound colkheti seep and pechori mound are predominantly of thermogenic origin however only at the latter seep site advection of deep pore fluids is indicated

microbial vs thermogenic gas hydrates in the south falkland basin bsr distribution and fluid origin abstract the south falkland basin hosts a working petroleum system as well as one of the most recently discovered gas hydrate provinces of the south atlantic ocean using three dimensional reflection seismic data a series of bottom simulating reflections bsrs are interpreted within two contrasting settings 1 the thrust cored anticlines developed by the oblique convergence of the scotia and the south american plates and 2 the foreland basin formed to the north of this plate boundary these bsrs are interpreted as the base of the gas hydrate stability zone and are associated with seismic indicators of underlying free gas accumulations and overlying hydrate bearing sediments in the foreland basin the bsr is laterally continuous for tens of kilometres whereas in the fold belt bsr occurrences are restricted to limited portions of the thrust cored anticline crests these observations calibrated with sedimentological analyses and gas geochemistry argue that the gas source for the gas hydrates within the thrust cored anticlines is unrelated to in situ microbial generation of methane but instead is associated with the vertical seepage of thermogenic fluids from deeper cores of the anticlines in contrast the nature of the sediments in the foreland basin appears more favourable for the generation of shallow microbial methane this study highlights that in specific tectonic and depositional environments the character of the bsr observed on reflection seismic data with the limited support of in situ data can be used to predict the most likely source of natural gas hydrate systems

distribution and isotopic signatures of archaeal lipid biomarkers associated with gas hydrate occurrences on the northern cascadia margin abstract we have investigated the distributions and carbon isotopic compositions of archaeal membrane lipids in gas hydrate bearing sediments collected from the northern cascadia margin offshore from vancouver island sites u1327 and u1328 by the r v joides resolution during iodp expedition 311 archaeal lipid biomarkers including glycerol dialkyl glycerol tetraethers gdgts tend to become abundant below 100 mbsf meters below sea floor tricyclic biphytane bp 3 which is a robust biomarker derived from gdgt crenarchaeol and other bps exhibit 13c values of ca 20 and become abundant between 130 and 230 mbsf at site u1328 in this depth range concentrations of ammonium and phosphate in interstitial waters also increase suggesting that a larger population and higher activity of heterotrophic community consisting of crenarchaeota and other archaea decompose the sedimentary organic matter thereby liberating ammonium and phosphate such crenarchaeotic activity can produce other metabolic products such as molecular hydrogen by fermentation of organic matter during diagenesis furthermore near the organic matter decomposition zone 130 to 230 mbsf a probable methanogen biomarker 13c depleted bp 1 with 13c values as low as 48 8 becomes abundant indicating that methanogens utilize these diagenetic products the molecular and isotopic distributions of archaeal lipid biomarkers indicate that the archaeal community plays an important role in the biogeochemical cycles of deep sea sediments including both methanogenesis and nutrient recycling

impact of hot fluid advection on hydrocarbon gas production and seepage in mud volcano sediments of thick cenozoic deltas hydrocarbon seeps are ubiquitous at gas prone cenozoic deltas such as the nile deep sea fan ndsf2 where seepage into the bottom water has been observed at several mud volcanoes mvs3 including north alex mv namv4 here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of namv to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments at the centre high upward flux of hot 70 c hydrocarbon rich fluids is indicated by an absence of biomarkers of anaerobic oxidation of methane aom and nearly constant methane ch4 concentration depth profile the presence of lipids of incompatible thermal maturities points to mixing between early mature petroleum and immature organic matter indicating that shallow mud has been mobilized by the influx of deep sourced hydrocarbon rich fluids methane is enriched in the heavier isotopes with values of 13c 46 6 vpdb and d 228 vsmow and is associated with high amounts of heavier homologues c2 suggesting a co genetic origin with the petroleum on the contrary at the periphery a lower but sustained ch4 flux is indicated by deeper sulphate methane transition zones and the presence of 13c depleted biomarkers of aom consistent with predominantly immature organic matter values of 13c ch4 60 vpdb and decreased concentrations of 13c enriched c2 are typical of mixed microbial ch4 and biodegraded thermogenic gas from plio pleistocene reservoirs of the region the maturity of gas condensate migrated from pre miocene sources into miocene reservoirs of the western ndsf is higher than that of the gas vented at the centre of namv supporting the hypothesis that it is rather released from the degradation of oil in neogene reservoirs combined with the finding of hot pore water and petroleum at the centre our results suggest that clay mineral dehydration of neogene sediments which takes place posterior to reservoir filling may contribute to intense gas generation at high sedimentation rate deltas

carbon dioxide and hydrogen sulfide degassing and cryptic thermal input to brimstone basin yellowstone national park wyoming abstract brimstone basin a remote area of intense hydrothermal alteration a few km east of the yellowstone caldera is rarely studied and has long been considered to be a cold remnant of an ancient hydrothermal system a field campaign in 2008 confirmed that gas emissions from the few small vents were cold and that soil temperatures in the altered area were at background levels geochemical and isotopic evidence from gas samples 3 he 4 he 3r a 13 c co 2 3 however indicate continuing magmatic gas input to the system accumulation chamber measurements revealed a surprisingly large diffuse flux of co 2 277 t d 1 and h 2 s 0 6 t d 1 the flux of co 2 reduces the 18 o content of the overlying cold groundwater and related stream waters relative to normal meteoric waters simple isotopic modeling reveals that the co 2 likely originates from geothermal water at a temperature of 93 19 c these results and the presence of thermogenic hydrocarbons c1 c2 100 and 13 c ch 4 46 4 to 42 8 in gases require some heat source at depth and refute the assumption that this is a fossil hydrothermal system

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

tracing crustal fluids applications of natural 129i and 36cl the two cosmogenic isotopes 129i and 36cl have half lives and geochemical characteristics that make their application of interest in the tracing of crustal fluids oil field brines and geothermal fluids the focus of this review is to compare 129i data from volcanic fluids with those from mud volcanoes and gas hydrate locations associated with the same subduction zone in order to demonstrate that fundamentally different hydrologic systems are present in active margins whereas 129i i ratios in volcanic fluids are site dependent and show a relation to the ages of subducting marine sediments ratios in fore arc fluids are similar in all sites investigated and are independent of the ages of the host sediments and the age of the subducting slab volcanic fluids contain iodine transported in sediments from the trench to the main volcanic zone whereas iodine in fore arc fluids is derived from organic material stored in the upper plates of subduction zones

national gas hydrate program expedition 01 offshore india gas hydrate systems as revealed by hydrocarbon gas geochemistry abstract the national gas hydrate program expedition 01 nghp 01 targeted gas hydrate accumulations offshore of the indian peninsula and along the andaman convergent margin the primary objectives of coring were to understand the geologic and geochemical controls on the accumulation of methane hydrate and their linkages to underlying petroleum systems four areas were investigated 1 the kerala konkan basin in the eastern arabian sea 2 the mahanadi and 3 krishna godavari basins in the western bay of bengal and 4 the andaman forearc basin in the andaman sea upward flux of methane at three of the four of the sites cored during nghp 01 is apparent from the presence of seafloor mounds seismic evidence for upward gas migration shallow sub seafloor geochemical evidence of methane oxidation and near seafloor gas composition that resembles gas from depth the kerala konkan basin well contained only co2 with no detectable hydrocarbons suggesting there is no gas hydrate system here gas and gas hydrate from the krishna godavari basin is mainly microbial methane with 13c values ranging from 58 9 to 78 9 with small contributions from microbial ethane 52 1 and co2 gas from the mahanadi basin was mainly methane with lower concentrations of c2 c5 hydrocarbons c1 c2 ratios typically 1000 and co2 carbon isotopic compositions that ranged from 70 7 to 86 6 for methane and 62 9 to 63 7 for ethane are consistent with a microbial gas source however deeper cores contained higher molecular weight hydrocarbon gases suggesting a small contribution from a thermogenic gas source gas composition in the andaman basin was mainly methane with lower concentrations of ethane to isopentane and co2 c1 c2 ratios were mainly 1000 although deeper samples were

modeling sulfate reduction in methane hydrate bearing continental margin sediments does a sulfate methane transition require anaerobic oxidation of methane the sulfate methane transition smt a biogeochemical zone where sulfate and methane are metabolized is commonly observed at shallow depths 1 30 mbsf in methane bearing marine sediments two processes consume sulfate at and above the smt anaerobic oxidation of methane aom and organoclastic sulfate reduction osr differentiating the relative contribution of each process is critical to estimate methane flux into the smt which in turn is necessary to predict deeper occurrences of gas hydrates in continental margin sediments to evaluate the relative importance of these two sulfate reduction pathways we developed a diagenetic model to compute the pore water concentrations of sulfate methane and dissolved inorganic carbon dic by separately tracking dic containing 12c and 13c the model also computes 13c dic values the model reproduces common observations from methane rich sediments a well defined smt with no methane above and no sulfate below and a 13c dic minimum at the smt the model also highlights the role of upward diffusing 13c enriched dic in contributing to the carbon isotope mass balance of dic a combination of osr and aom each consuming similar amounts of sulfate matches observations from site u1325 integrated ocean drilling program expedition 311 northern cascadia margin without aom methane diffuses above the smt which contradicts existing field data the modeling results are generalized with a dimensional analysis to the range of smt depths and sedimentation rates typical of continental margins the modeling shows that aom must be active to establish an smt wherein methane is quantitatively consumed and the 13c dic minimum occurs the presence of an smt generally requires active aom

anaerobic methane oxidation on the amazon shelf anaerobic methane oxidation on the amazon shelf is strongly controlled by dynamic physical sedimentation processes rapidly accumulating physically reworked deltaic sediments characteristic of much of the shelf typically support what appear to be low rates of steady state anaerobic methane oxidation at depths of 5 8 m below the sediment water interface methane oxidation in these cases is responsible for 10 m methane charged sediment directly to seawater in this nonsteady state situation methane is a major source of recently produced co2 and an important reductant for sulfate these observations suggest that authigenic sedimentary carbonates derived from anaerobic methane oxidation may sometimes reflect physically enhanced nonsteady state exposure of methane to sulfate in otherwise biogeochemically unreactive deposits the concentration profiles of ch4 so 4 and co2 in the eroded deposit were reproduced by a coupled reaction transport model this area of the shelf was reexposed to seawater approximately 5 10 years ago based on the model results and the assumption that the erosion of the deposit occurred as a single event that has now ceased the necessary second order rate constant for anaerobic methane oxidation was 0 1 mm 1 d 1

influence of total organic carbon deposition on the inventory of gas hydrate in the indian continental margins abstract total organic carbon toc content of marine sediments represents residual carbon originally derived from terrestrial and marine sources which has survived seafloor and shallow subseafloor diagenesis ultimately its preservation below the sulfate reduction zone in marine sediments drives methanogenesis within the gas hydrate stability zone ghsz methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate in this paper we examine the inventory and sources of toc in sediments collected from four regions within the ghsz along the indian continental margins the recovered sediments vary in age from oligocene to recent mean toc abundance is greatest in the krishna godavari k g basin and decreases progressively to the mahanadi basin andaman wedge and kerala konkan k k basin this decrease in toc is matched by a progressive increase in biogenic caco3 and increasing distance from terrestrial sources of organic matter and lithogenic materials organic carbon sources inferred from c n and 13ctoc range from terrestrial k g basin to mixed marine and terrestrial mahanadi basin to marine dominant andaman wedge and k k basin in the k g basin variation in the bulk 13ctoc is consistent with changes in c3 and c4 vegetation driven by monsoon variability on glacial interglacial timescales whereas in the mahanadi basin a shift in the 13ctoc likely reflects the onset of c4 plant deposition in the late miocene a large shift the 13ctoc in the k k basin is consistent with a change from c3 to c4 dominated plants during the middle miocene we observe a close relationship between toc content and gas hydrate saturation but consider the role of sedimentation rates on the preservation of toc in the zone of methanogenesis and advective flow of methane from depth although toc contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data seismic borehole log pressure core and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions although toc content may be a first order indicator for gas hydrate potential the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur

gas hydrate systems at hydrate ridge offshore oregon inferred from molecular and isotopic properties of hydrate bound and void gases abstract we report and discuss molecular and isotopic properties of hydrate bound gases from 55 samples and void gases from 494 samples collected during ocean drilling program odp leg 204 at hydrate ridge offshore oregon gas hydrates appear to crystallize in sediments from two end member gas sources deep allochthonous and in situ as mixtures of different proportions in an area of high gas flux at the southern summit of the ridge sites 1248 1250 shallow 0 40 m below the seafloor mbsf gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic c2 gases which migrated vertically and laterally from as deep as 2 to 2 5 km depths in contrast deep 50 105 mbsf gas hydrates at the southern summit sites 1248 and 1250 and on the flanks of the ridge sites 1244 1247 crystallize mainly from microbial methane and ethane generated dominantly in situ a small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth e g sites 1244 and 1245 non hydrocarbon gases such as co2 and h2s are not abundant in sampled hydrates the new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at hydrate ridge formed from gas derived from decomposition of deeper and older gas hydrates gas hydrate formation at the southern summit is explained by a model in which gas migrated from deep sediments and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone ghsz free gas migrated into the ghsz when the overpressure in gas column exceeded sealing capacity of overlaying sediments and precipitated as gas hydrate mainly within shallow sediments the mushroom like 3d shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit the decrease of gas solubility in shallow sediment and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure

towards constraining h2 concentration in subseafloor sediment a proposal for combined analysis by two distinct approaches molecular hydrogen h 2 is a central metabolite that couples organic matter degradation and terminal electron accepting processes h 2 levels in natural environments are often regulated by microbial syntrophy therefore pore water h 2 concentration is a useful parameter for studying biogeochemical processes in sediments however little is known about h 2 concentrations in marine subsurface sediments previous studies applying either a headspace equilibration technique or an extraction method for the analysis of pore water h 2 in deeply buried sediments have generated results that sometimes contradict the principles established based on studies of microbial culture and surface sediments in this study we first evaluated and optimized an extraction method which was then applied in combination with a headspace equilibration method to determine concentrations of pore water h 2 in subseafloor sediments along a transect of five sites of different water depths and geochemical regimes at the continental margin off namibia se atlantic the two methods generated depth profiles with some similarities in curve shape but the extraction method yielded higher h 2 values than the headspace equilibration technique by comparing the two data sets with thermodynamic calculations of potential terminal electron accepting processes we were able to provide a first evaluation of syntrophic conditions in subseafloor sediment from the perspective of h 2 biogeochemistry we observed that in the sulfate reduction zone the h 2 concentrations are higher than the h 2 threshold allowed for the next most favorable terminal metabolism methanogenesis suggesting relaxation of coupling between h 2 producing and h 2 consuming activities at these depths in contrast the h 2 concentrations in the upper methanogenic zone are low enough for methanogens to outcompete co 2 reducing acetogens our findings suggest the existence of varied extents of syntrophic h 2 coupling in subseafloor sediment

stable isotopic and molecular composition of desorbed coal seam gases from the walloon subgroup eastern surat basin australia this study used compositional and stable isotopic analysis to test hypotheses on the distribution and origins of walloon subgroup coal seam gas csg in the eastern surat basin queensland australia the middle jurassic walloon subgroup play differs from many other low rank csg plays particularly in methane carbon isotopic signature i e the csg is not as microbial as could be expected the carbon isotope compositions of desorbed methane from three cored appraisal wells fall within the generally accepted range for thermogenic or mixed gas 13c 58 5 to 45 3 the 13c ch4 values from stratigraphically placed coal core samples increased became more thermogenic from the top of the upper juandah coal measures to the base of the tangalooma sandstone below the tangalooma sandstone in the lower taroom coal measures the 13c ch4 values decreased with increasing depth these positively parabolic 13c profiles tracked total measured gas content in two out of the three wells studied the third well displayed lower variance of 13c ch4 and gas content increased uniformly with depth a genetic classification based on methane stable carbon isotopes alone might interpret this pattern as a transition from microbially to thermogenically sourced methane in the central coal seams however a 13c co2 profile for one well tracks total gas content and 13c ch4 and exhibits an inverse relationship with d ch4 these results together with the mostly dry nature of the gas samples c1 c2 c3 ratios up to 10 000 and relatively uniform d ch4 values d 238 to 202 suggest that microbial co2 reduction is the primary source of walloon subgroup methane as such stratigraphic variations in gas content mainly reflect the extent of microbial methanogenesis we suggest that peak microbial utilisation of h2 co2 occurred at the tangalooma sandstone level enriching the residual co2 pool and derived methane in 13c carbon 13c co2 ch4 and deuterium isotopic differences d h2o ch4 and cross plots of d h2o and 18o h2o are also consistent with kinetic isotope fractionation during microbial mediated carbonate reduction the results are relevant for applying microbially enhanced coal bed methane mecom in the surat basin

the effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern cascadia accretionary margin analytical challenges in obtaining high quality measurements of rare earth elements rees from small pore fluid volumes have limited the application of rees as deep fluid geochemical tracers using a recently developed analytical technique we analyzed rees from pore fluids collected from sites u1325 and u1329 drilled on the northern cascadia margin during the integrated ocean drilling program iodp expedition 311 to investigate the ree behavior during diagenesis and their utility as tracers of deep fluid migration these sites were selected because they represent contrasting settings on an accretionary margin a ponded basin at the toe of the margin and the landward tofino basin near the shelf s edge ree concentrations of pore fluid in the methanogenic zone at sites u1325 and u1329 correlate positively with concentrations of dissolved organic carbon doc and alkalinity fractionations across the ree series are driven by preferential complexation of the heavy rees simultaneous enrichment of diagenetic indicators doc and alkalinity and of rees in particular the heavy elements ho to lu suggests that the heavy rees are released during particulate organic carbon poc degradation and are subsequently chelated by doc ree concentrations are greater at site u1325 a site where shorter residence times of poc in sulfate bearing redox zones may enhance ree burial efficiency within sulfidic and methanogenic sediment zones where ree release ensues cross plots of la concentrations versus cl li and sr delineate a distinct field for the deep fluids z 75 mbsf at site u1329 and indicate the presence of a fluid not observed at the other sites drilled on the cascadia margin changes in ree patterns the presence of a positive eu anomaly and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous crescent terrane located east of the drilled transect

culture based investigations of key microbial functional groups in gas hydrate bearing sediments of the krishna godavari basin in offshore india abstract results of national gas hydrate program 02 nghp 02 expedition have confirmed the resource potential of sand hosted gas hydrate accumulations in the deep offshore of krishna godavari basin kg basin nghp 02 has identified two types of gas hydrate accumulations one is thick layer type units developed in sand rich facies at depths of a few 100 m below sea floor mbsf and the other fracture type unit of variable thickness at shallow depths fracture type and layer type gas hydrates were observed in areas b c and e in the kg basin as defined under nghp 02 the importance of each type of gas hydrate occurrence as discovered during nghp 02 has led to the need for microbial profiling different classes of microbes anaerobic fermentative microbes afm methanotrophs and sulphate reducing bacteria srb were studied to understand the microbial system for biogenic gas generation in sedimentary sequences encountered in eight core holes 4 in area b 3 in area c and 1 in area e as established during nghp 02 this study reveals the availability of highest concentration of methane generating microbes that are closely associated with high total organic carbon toc content in the identified gas hydrate bearing sedimentary units cored in each borehole the presence of microbial consortia for methane generation through both processes of reduction of co2 and fermentation of formate acetate has been confirmed stratigraphic correlation between drill sites have also been generated to better understand the spatial relationship of various microbial processes an observation from the study is that the entire late miocene age sediments has potential to generate biogenic gas in holes nghp 02 05b 17b 22b and 23b late to early pleistocene age sediments are showing methane generating activity in holes nghp 02 01c 08b and 09b while early pleistocene to late pliocene age sediments contains methane generating microbes in holes nghp 02 05b 17b 19b 22b and 23b the existence of methane generating biogenic microbial system in areas b c and e of kg basin has been confirmed

slipstream an early holocene slump and turbidite record from the frontal ridge of the cascadia accretionary wedge off western canada and paleoseismic implications slipstream slump a well preserved 3 km wide sedimentary failure from the frontal ridge of the cascadia accretionary wedge 85 km off vancouver island canada was sampled during canadian coast guar

in situ carbon isotopic exploration of an active submarine volcano abstract the geologic and biogeochemical cycling of carbon in deep ocean environments has important implications for our understanding of the functioning of earth systems across a wide range of spatial and temporal scales to improve our ability to study the cycling and fluxes of carbon in the deep ocean new technologies are emerging for making in situ measurements of carbon compounds over a range of environmental contexts within many of these deep sea environments fluxes of carbon compounds often occur as either venting fluids or rising gas bubbles key compounds of interest include methane ch4 dissolved inorganic carbon dic and carbon dioxide co2 a component of dic in particular measurement of the carbon isotopic composition 13c of these pools can offer a better understanding of the nature of sources fluxes and cycling processes involving these compounds here we present the advancement of an in situ laser spectrometer initially developed for measurement of 13cch4 only into a sensor that can measure 13c of both ch4 and co2 in both deep sea bubble plumes as well as geologic fluids we present results of a 2014 telepresence enabled investigation of a back arc submarine volcano kick em jenny in the caribbean sea in situ isotopic analysis of both bubbles and fluids suggest a primarily thermogenic origin for ch4 and a magmatic origin for co2 yet highlight the occurrence of some heterogeneities indicating locally elevated contribution of organic matter to dic fluxes

rare earth elements in the terminal ediacaran bambui group carbonate rocks brazil evidence for high seawater alkalinity during rise of early animals abstract rare earth elements plus yttrium rey mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater here we investigate rey patterns in two carbonate sections from the ediacaran bambui group sao francisco basin brazil which comprise its two lowermost transgressive regressive second order sedimentary sequences shale normalised distributions vary with the sequence stratigraphy framework in the basal 2nd order sequence carbonate samples from the basal sequence transgressive systems tract display light rey lrey distributions slightly depleted to enriched that reflect input of freshwater possibly in a post glacial episode upwards carbonate rocks from the early highstand systems tract ehst yielded lrey enriched distributions which progressively turns into lrey shale normalized depleted distributions on samples from the late highstand systems tract lhst this portion of the sequence also displays y positive anomaly in some cases carbonate samples from the upper second order sequence do not display coherent patterns ce ce values 1 in most samples throughout the two sections suggest permanent anoxia of seawater the rey change from the ehst to lhst in the basal sequence marks an important paleoenvironmental overturn in the basin with increasing alkalinity in seawater driving rey fractionation and lrey depletion confinement of the basin in the inner areas of west gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent thus delivering a higher ionic influx to a restricted setting increasing alkalinity during lhst cloudina sp fragments were reported in this stage of the bambui group and in carbonate rocks with high sr mass fractions in other west gondwana basins supporting the hypothesis that the high alkalinity of seawater during late ediacaran may have driven the appearance of the first biomineralizing organisms

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

the role of low temperature organic matter diagenesis in carbonate precipitation within a marine deposit abstract carbonate minerals in veins can record paleo hydrogeological information that enables the reconstruction of groundwater history this paper investigates the cause of differences in the occurrence of carbonate veins in the koetoi and wakkanai formations both neogene mudstone units in northwestern hokkaido from the perspective of controls on co2 supply from the alteration of organic matter carbonate veins are rare in the koetoi formation but are widespread in the wakkanai formation this area is a region of oil and gas accumulation where deep groundwater is saturated mainly with ch4 and co2 the results show high 13c values in co existing ch4 32 6 and co2 31 0 gases an investigation of 13c d systematics among these gases indicates that isotopic fractionation was caused by microbial co2 reduction although total organic carbon content in the koetoi formation decreases with increasing depth total organic content in the wakkanai formation remains roughly constant with depth furthermore although 13c values also show depth dependence values from the wakkanai formation are higher than those from the koetoi formation this 13c enrichment could be explained by rayleigh fractionation in a closed system based on these results the processes behind the formation of the carbonate veins can be summarized as follows carbon dioxide behavior is thought to play an important role with respect to carbonate formation because co2 abundance is closely linked to ph and pressure in shallow sedimentary rocks such as the koetoi formation that have started to experience diagenetic alteration of organic matter co2 in groundwater is supplied by microbial decomposition of organic matter and is reduced to ch4 by methanogens in deep sedimentary rocks such as the wakkanai formation that have undergone diagenesis but have only experienced moderate temperatures so that thermal decomposition of organic matter has not yet begun microbial degradation of organic matter has proceeded too far for any more co2 to be produced thus carbonate precipitation is initiated when ph rises due to microbial co2 reduction the contrast between the occurrence of carbonate veins in the koetoi and wakkanai formations can be explained by our results which can also be applied to general carbonate behavior in marine sedimentary rocks

holocene changes in tropical wind intensity and rainfall evidence from southeast ghana abstract remnants of a fixed aeolian dune ridge occur along the southeast coast of ghana just behind the present shoreline aeolian sands also cover extensive areas of the accra plains no dunes are present here the sands mainly occurring as sheets which blanket an early holocene landscape the sediments are of mid holocene age and were deposited during the interval 4500 b p 3800 yr b p when the southwesterly winds were stronger than they are at present and much of tropical africa seems to have been subject to marked aridity the onset of drier windier conditions around 4500 yr b p brought to an end the more equable climates than had characterized much of west africa during the earlier holocene aridity intensified winds and desert expansion between 4500 and 3800 yr b p parallel environmental conditions in tropical continental areas at the height of the late pleistocene glaciation

gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the krishna godavari basin offshore india abstract the national gas hydrate program expedition 01 cored ten sites in the krishna godavari basin located on the southeastern margin of india a comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin in situ diagenetic and metabolic reactions and fluid migration and flow pathways gas hydrate was present at all of the sites cored and in situ microbial methanogenesis leads to estimates of depth integrated average gas hydrate saturations that are typically

generation and evolution of hydrothermal fluids at yellowstone insights from the heart lake geyser basin 1 we sampled fumaroles and hot springs from the heart lake geyser basin hlgb measured water and gas discharge and estimated heat and mass flux from this geothermal area in 2009 the combined data set reveals that diverse fluids share an origin by mixing of deep solute rich parent water with dilute heated meteoric water accompanied by subsequent boiling a variety of chemical and isotopic geothermometers are consistent with a parent water that equilibrates with rocks at 205 c 10 c and then undergoes 21 2 adiabatic boiling measured diffuse co2 flux and fumarole compositions are consistent with an initial dissolved co2 concentration of 21 7 mmol upon arrival at the caldera boundary and prior to southeast flow boiling and discharge along the witch creek drainage the calculated advective flow from the basin is 78 16 l s 1of parent thermal water corresponding to 68 14 mw or 1 of the estimated thermal flux from yellowstone helium and carbon isotopes reveal minor addition of locally derived crustal biogenic and meteoric gases as this fluid boils and degasses reducing the he isotope ratio rc ra from 2 91 to 1 09 the hlgb is one of the few thermal areas at yellowstone that approaches a closed system where a series of progressively boiled waters can be sampled along with related steam and noncondensable gas at other yellowstone locations steam and gas are found without associated neutral cl waters e g hot spring basin or cl rich waters emerge without significant associated steam and gas upper geyser basin

dual stable isotopes of ch 4 from yellowstone hot springs suggest hydrothermal processes involving magmatic co 2 abstract volcanism and post magmatism contribute significant annual methane ch 4 fluxes to the atmosphere on par with other natural sources such as forest fire and wild animal emissions and have been implicated in past climate change events the yellowstone hot spot is one of the largest volcanic systems on earth and is known to emit ch 4 as well as carbon dioxide co 2 and other gases but the ultimate sources of this ch 4 flux have not been elucidated here we use dual stable isotope analysis 2 h and 13 c of ch 4 sampled from ten high temperature geothermal pools in yellowstone national park along with other isotopic and gas analyses to evaluate potential sources of methane the average 13 c and 2 h values of ch 4 emitted from hot springs 26 7 2 4 and 236 9 12 0 respectively are inconsistent with microbial methanogenesis but do not allow distinction between thermogenic and abiotic sources correlation between 13 c ch4 and 13 c of dissolved inorganic c dic is consistent with dic as the parent c source for the observed ch 4 or with equilibration of ch 4 and dic methane formation temperatures estimated by isotopic geothermometry based on 13 c ch4 and 13 c co2 ranged from 250 350 c which is just below previous temperature estimates for the hydrothermal reservoir further the 2 h h2o of the thermal springs and the measured 2 h ch4 values are consistent with equilibration between the source water and the ch 4 at the formation temperatures though the ultimate origin of the ch 4 could be attributed to either abiotic of themorgenic processes with subsequent isotopic equilibration the c 1 c 2 composition of the gases is more consistent with abiotic origins for most of the samples thus our data support the hypothesis that subsurface rock water interactions are responsible for at least a significant fraction of the ch 4 flux from the yellowstone national park volcanic system

an extinct foraminifer endemic to hydrocarbon seeps modern seafloor hydrocarbon seeps are usually surrounded by an unusual macrobiota dominated by symbiont bearing endemic bivalves and worms numerous species of foraminifera shelled protists also live around hydrocarbon seeps but none have been found that are endemic to this environment an extinct species of benthic foraminifera amphimorphinella butonensis has been found in large numbers in a 15 m diameter patch of siltstone surrounding a miocene concretionary carbonate mound inferred to be a fossil methane seep in new zealand the tests exhibit highly negative 13 c values consistent with a methane rich environment of recrystallization on or just below the seafloor this extremely rare species has been recorded only once before from asphalt impregnated miocene muddy limestone in indonesia most likely also associated with hydrocarbon seepage is this the first record of a foraminiferal species that was specifically adapted and endemic to hydrocarbon seep environments

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

quaternary aeolian dust processes and sediments in the australian region abstract aeolian dust processes have played an important role in the evolution of the australian landscape yet little is known of the processes and their sediments two major dust transport pathways are described based upon paleo dune patterns and present day dust storms and related processes these dust pathways have been fed by alluvium deposited in internally draining river basins during the period leading up to and including the late pleistocene aridity rates of dust entrainment transport and deposition were significantly higher than at present the ocean sediments of the tasman sea the pacific and indian oceans received massive amounts of aeolian sediment dust contributions to some australian soils have also been considerable

stress history controls the spatial pattern of aftershocks case studies from strike slip earthquakes earthquake ruptures perturb stress within the surrounding crustal volume and as it is widely accepted now these stress perturbations strongly correlates with the following seismicity here we have documented five cases of the mainshock aftershock sequences generated by the strike slip faults from different tectonic environments of world in order to demonstrate that the stress changes resulting from large preceding earthquakes decades before effect spatial distribution of the aftershocks of the current mainshocks the studied mainshock aftershock sequences are the 15 october 1979 imperial valley earthquake mw 6 4 in southern california the 27 november 1979 khuli boniabad mw 7 1 the 10 may 1997 qa enat mw 7 2 and the 31 march 2006 silakhor mw 6 1 earthquakes in iran and the 13 march 1992 erzincan earthquake mw 6 7 in turkey in the literature we have been able to find only these mainshocks that are mainly characterized by dense and strong aftershock activities along and beyond the one end of their ruptures while rare aftershock occurrences with relatively lower magnitude reported for the other end of their ruptures it is shown that the stress changes resulted from earlier mainshock s that are close in both time and space might be the reason behind the observed aftershock patterns the largest aftershocks of the mainshocks studied tend to occur inside the stress increased lobes that were also stressed by the background earthquakes and not to occur inside the stress increased lobes that fall into the stress shadow of the background earthquakes we suggest that the stress shadows of the previous mainshocks may persist in the crust for decades to suppress aftershock distribution of the current mainshocks considering active researches about use of the coulomb stress change maps as a practical tool to forecast spatial distribution of the upcoming aftershocks for earthquake risk mitigation purposes in near real time it is further suggested that the background mainshocks along the neighbouring faults should be taken into account in producing the stress change maps for commenting on aftershock occurrences

methane sources and production in the northern cascadia margin gas hydrate system abstract the oceanographic and tectonic conditions of accretionary margins are well suited for several potential processes governing methane generation storage and release to identify the relevant methane evolution pathways in the northern cascadia accretionary margin a four site transect was drilled during integrated ocean drilling program expedition 311 the 13c values of methane range from a minimum value of 82 2 on an uplifted ridge of accreted sediment near the deformation front site u1326 1829 mbsl meters below sea level to a maximum value of 39 5 at the most landward location within an area of steep canyons near the shelf edge site u1329 946 mbsl an interpretation based solely on methane isotope values might conclude the 13c enrichment of methane indicates a transition from microbially to thermogenically sourced methane however the co existing co2 exhibits a similar trend of 13c enrichment along the transect with values ranging from 22 5 to 25 7 the magnitude of the carbon isotope separation between methane and co2 ec 63 8 5 8 is consistent with isotope fractionation during microbially mediated carbonate reduction these results in conjunction with a transect wide gaseous hydrocarbon content composed of 99 8 by volume methane and uniform dch4 values 172 8 that are distinct from thermogenic methane at a seep located 60 km from the expedition 311 transect suggest microbial co2 reduction is the predominant methane source at all investigated sites the magnitude of the intra site downhole 13c enrichment of co2 within the accreted ridge site u1326 and a slope basin nearest the deformation front site u1325 2195 mbsl is 5 at the mid slope site site u1327 1304 mbsl the downhole 13c enrichment of the co2 is 25 and increases to 40 at the near shelf edge site u1329 this isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13c enrichment the magnitude of the 13c enrichment of co2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate we suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation during burial thereby reducing the availability of metabolizable organic matter available for methane production this process is reflected in the occurrence and distribution of gas hydrate within the northern cascadia margin accretionary prism our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins

the origin of thermogenic gas hydrates on the northern cascadia margin as inferred from isotopic 13c 12c and d h and molecular composition of hydrate and vent gas the isotopic 13c and d and hydrocarbon compositions of hydrate bound and vent gas collected from the seafloor of barkley canyon northern cascadia margin offshore vancouver island canada were evaluated to characterize the gas and infer the type and maturity of the source rock kerogen the hydrate gas contained methane having 13c values from 43 4 to 42 6 d values from 143 to 138 and a large percentage 14 9 to 31 9 of c2 to c5 hydrocarbons these data are consistent with a thermogenic gas source the data from barkley canyon are interpreted within the context of similar data from the gulf of mexico and caspian sea thermogenic hydrates which occur in regions where the petroleum systems supporting gas and oil generation are better understood a stable carbon isotope based natural gas plot model and d h data from the hydrate gas indicate that the source rock for the barkley canyon hydrate and vent gas had primarily type iii kerogen mixed with a small fraction of type ii kerogen oil from a seep having a pristane phytane ratio of 3 2 was identified as a gas condensate which supports the conclusions drawn from the gas data a mechanism for explaining how fluids are conducted from the deep petroleum reservoir to the seafloor of barkley canyon is proposed

rare earth element cycling in the pore waters of the bering sea slope iodp exp 323 abstract we studied the diagenetic behavior of rare earth elements rees in a highly productive passive margin setting of the bering sea slope site u1345 was drilled during the integrated ocean drilling program expedition 323 at a water depth of 1008 m currently in the center of an oxygen minimum zone pore water concentrations of fourteen rees were determined down to 140 meters below the seafloor mbsf the ree concentrations were higher in the pore water than the deep seawater indicating that there was significant liberation from the sediments during diagenesis there was a major peak at 10 mbsf that was more pronounced for the heavy ree hree this peak occurred below the sulfate methane transition zone 6 3 mbsf and coincided with high concentrations of dissolved iron and manganese at 2 mbsf there was a minor peak in ree and mn contents below 40 mbsf the ree concentration profiles remained constant the ce anomaly was insignificant and relatively constant paas normalized ce ce 1 1 0 2 throughout the depth profile showing that the ce depleted in seawater was restored in the pore water hree enrichment was observed over the entire 140 m except for the upper 1 m where a middle ree mree bulge was apparent ree release in shallow depths 2 4 mbsf is attributed to the release of light rees lrees and mrees during the organoclastic reduction of mn oxides in anoxic sediments the high hree concentrations observed at 10 mbsf can be attributed to the reduction of fe and mn minerals tied to anaerobic oxidation of methane or less significantly to ferromagnesian silicate mineral weathering the upward diffusion flux across the sediment water interface was between 3 for tm and 290 for ce pmol m 2 y 1

origin of siltstones glacial grinding versus weathering siltstones are unusual rocks they are mainly made up of quartz glacial grinding is considered by some authors to be the main or even the only process generating silt this is supported by 1 the presence of silt in basal tills from present ice caps and above all 2 by the volumetric importance of quaternary loess however ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites or forming thin intercalations in pro and periglacial deposits these siltstones represent less than 5 of the total volume of upper proterozoic and upper ordovician glaciogenic sequences of west africa on the other hand silt is abundant in weathering profiles of tropical equatorial and mediterranean zones this silt is generated by splitting of minerals mainly quartz inherited from the parent rock quartz grains are partly dissolved in situ and their fragments cemented by iron oxi hydroxides in tropical and equatorial zones and by calcite in mediterranean zones silt is particularly abundant in tropical soils comprising up to 50 75 secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water weathering processes and especially those operating in tropical zones are the main phenomena generating silt glacial grinding would generate only a little silt a large part of the material of quaternary loess may be derived from glacially reworked weathering profiles

subsurface methane sources and migration pathways within a gas hydrate mound system gulf of mexico 1 geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon source migration and fate at a gulf of mexico carbonate gas hydrate mound woolsey mound three seafloor features were investigated in detail 1 major faults resulting from a rising salt body 2 an acoustic backscatter anomaly and 3 a pockmark associated with a major fault we analyzed sulfate chloride dissolved inorganic carbon and hydrocarbon concentrations and carbon isotopes in pore water extracted from 20 m piston cores to characterize gas source and calculate methane flux dissolved biogenic methane dominated the off fault sites while the contribution of thermogenic methane increased near a major fault where thermogenic gas hydrates were recovered within the pockmark methane concentrations were low and isotopes indicated a biogenic source since pockmarks are typically formed from expulsive fluid flow this suggests that either the pockmark is the legacy of a conduit that has become plugged or that the expulsed fluid is confined within the fault walls at the acoustic anomaly nonsteady state sulfate profiles suggested temporal variability in methane flux estimates from 75 gravity cores collected across woolsey mound since 2002 were mapped to display the spatial variability in methane flux relative to the faults methane flux to the seafloor was generally low but increased several fold near the faults suggesting that the faults may provide conduits for hydrocarbons to bypass the microbial biofilter and cross the sediment water interface

the nature and origin of the aeolian mantles of central northern nigeria abstract evidence is presented here to show that the upland soils of central northern nigeria are formed in aeolian mantles deposited by two aeolian processes aeolian suspension or dust deposition and aeolian saltation or dune sand deposition the soils to the south and west of kano are largely the product of harmattan dust deposition whereas the northern soils appear to originate from dune sand migration from the quaternary dune fields to the east of the study area the spatial pattern of particle size populations or quanta and quartz grain roundness characteristics within the mantles and dune sands indicates that considerable mixing of dune sands and dust has occurred in the formation of these mantles

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

depressurization experiment of pressure cores from the central ulleung basin east sea insights into gas chemistry abstract we performed depressurization experiments on pressure cores pcs recovered from holes ubgh1 10b and ubgh2 11b which were drilled during two separate expeditions in the ulleung basin ubgh1 and ubgh2 various lines of evidence such as convex curvature in the p v curves obtained by depressurization as well as x ray images and resistivity at bit rab data point to the presence of gas hydrates at these holes we estimated gas hydrate saturation sh at these holes to range from 5 23 based on the amount of gas released from pcs these sh values are in agreement with those reported at other high gas flux settings molecular c1 c2 and isotopic 13cch4 dch4 13cc2h6 indicators delineate a microbial source for the ch4 and c2h6 in core ubgh1 10b 5r the carbon isotopic fractionation factor between co2 and ch4 ec 13cco2 13cch4 was observed to increase from 50 to 68 as pressure was reduced during our experiments these changes illustrate the isotope fractionation that results from the preferential release of 13c depleted co2 during the early stages of depressurization the constant carbon isotope separation 10 20 between co2 in void gases and in the dissolved inorganic carbon dic pool shown in cores from the ulleung basin has also been observed in gas hydrate bearing sediments elsewhere it is important to consider this effect when using carbon isotopic data in carbon cycling models specifically when unraveling metabolic pathways of methane generation in situations where partial co2 degassing of a core is suspected we recommend the use of the carbon isotope ratio of dic instead of the gas measurements to best delineate the methane cycling pathways in marine systems

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

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

pore water sulfate alkalinity and carbon isotope profiles in shallow sediment above marine gas hydrate systems a numerical modeling perspective 1 both the concentration and the carbon isotope composition of dissolved inorganic carbon dic vary considerably across the sulfate methane transition smt in shallow marine sediment at locations with gas hydrate this variability has led to different interpretations for how carbon including ch4 cycles within gas charged sediment sequences over time we extend a one dimensional model for the formation of gas hydrate to account for downhole changes in dissolved ch4 so42 dic and ca2 and the 13c of dic the model includes advection diffusion and two reactions that consume so42 degradation of particulate organic carbon poc and anaerobic oxidation of methane aom using our model and site specific parameters steady state pore water profiles are simulated for two sites containing gas hydrate but different carbon chemistry across the smt site 1244 hydrate ridge dic 38 mm 13c of dic 22 5 pdb and site keathley canyon kc 151 3 gulf of mexico dic 16 mm 13c of dic 49 6 pdb the simulated profiles for ch4 so42 dic ca2 and 13c of dic resemble those measured at the sites and the model explains the similarities and differences in pore water chemistry at both sites an upward flux of ch4 consumes most net so42 at a shallow smt and calcium carbonate removes a portion of dic at this horizon however a large flux of 13c enriched hco3 enters the smt from depth at site 1244 but not at site kc151 3 this leads to a high concentration of dic with a 13c much greater than that of ch4 even though aom causes the smt the addition of hco3 from depth impacts the slope of certain concentration crossplots crucially neither the dic concentration nor its carbon isotope composition at the smt can be used to discriminate between sulfate reduction pathways

methanogenic biodegradation of petroleum in the west siberian basin russia significance for formation of giant cenomanian gas pools approximately 1700 tcf 48 trillion m3 of dry gas 99 methane reserves and resources occur in western siberia mostly in shallow 1500 m 4921 ft cenomanian pools in the northern part of the basin this dry gas constitutes about 11 of the world s conventional gas endowment and about 17 of the annual gas production the origin of the dry gas has been debated extensively over the last 45 yr but remains controversial widely discussed hypotheses on the origin include early mature thermogenic gas from coal primary microbial gas from dispersed organic matter or coal and thermogenic gas from deep source rocks however all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling here i present geochemical and geological evidence that a significant although yet not quantified part of the shallow dry gas in the northern west siberian basin originated from methanogenic biodegradation of petroleum circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many cenomanian gas pools as well as geochemical evidence of heavy to slight biodegradation in jurassic albian reservoirs commonly underlying the cenomanian pools direct evidence includes most importantly 13c enriched co2 in pools with biodegraded oil although data are limited which indicates 40 70 wt conversion of oil derived co2 to secondary microbial methane distinctive hydrocarbon molecular and isotopic compositions of most gases in cenomanian pools average dryness c1 sum c1 c5 is 0 9976 average 13c of methane is 51 8 suggest that they represent mixtures of biodegraded thermogenic gases from deep mainly jurassic source rocks and secondary microbial methane with an occasional small addition of primary microbial methane contribution of early mature coal derived gas is possible in areas with the most significant thermal stress of hauterivian aptian sediments but remains speculative review of petroleum habitats of five representative oil gas condensate fields in western siberia including the world s second largest gas field urengoyskoe suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields recognition of secondary microbial gas in western siberia helps explain the observed dominance of gas in the shallow cool northern part of the basin where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin secondary microbial gas has been recognized worldwide and may 1 represent a volumetrically significant exploration target in shallow reservoirs perhaps more significant than primary microbial gas and 2 indicate effective thermogenic petroleum systems in the deeper sections large volumes up to 66 500 tcf 1884 trillion m3 of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide although a part of that gas accumulated as oil dissolved free and hydrate bound gas most gas apparently escaped into the overburden atmosphere and ocean and could have affected global climate in the geologic past