gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern taiwan abstract the accretionary wedge of offshore southwestern taiwan contains abundant deposits of gas hydrate beneath the sea floor high concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available to understand temporal variation of methane venting in sediments over geologic time a 23 m long calypso piston core md05 2911 was collected on the flank of the yung an ridge pore water sulfate dissolved sulfide dissolved iron methane sedimentary pyrite acid volatile sulfide reactive iron organic carbon and nitrogen as well as carbonate 13 c were analyzed three zones with markedly different pyrite concentration were found at the study site unit i sediments 20 mbsf were characterized with a high amount of pyrite 251 380 mol g and a 13 c depleted carbonate unit ii sediments 15 20 mbsf with a low pyrite 15 43 mol g and a high content of iron oxide mineral and unit iii sediments the oscillation records of pyrite concentrations are controlled by temporal variations of methane flux with an abundant supply of methane to unit i and iii anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation no aom signal was found in unit ii characterized by typical organically limited normal marine sediments with little pyrite formation the aom induced pyrite formation near the smi generates a marked pyrite signature rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment

bubble induced porewater mixing a 3 d model for deep porewater irrigation porewater data from vent sites of the northeastern shelf off sakhalin island sea of okhotsk exhibit bottom water concentrations down to a sediment depth of up to 300 cm below this depth solute concentrations rapidly change due to methanogenesis and anaerobic methane oxidation amo the profile shapes suggest an irrigation like process that mixes on a meter scale at these sites active gas emanation into the overlying water column and near surface gas hydrates are commonly observed we propose that methane gas bubbles rise through the soft surface sediments and cause mixing of the porewater mathematically the bubble induced irrigation can be described by eddy diffusion enhancing the diffusive transport of solutes by several orders of magnitude a 3 d numerical transport reaction model was developed to investigate the parameters defining the mixing process such as bubble rise velocity tube size tube distribution in the sediment and ebullition frequency model consistency with the field data requires eddy diffusivities 1 105 cm2 a tube densities of 4 tubes m2 equivalent to a tube spacing of 40 cm active gas seepage for more than a few weeks or months and moderate to low diagenetic reaction rates of solutes the corresponding methane gas fluxes that are predicted from the results of the model realizations range from 1 103 5 105 l m2 a due to bubble mixing solute fluxes in these sediments are increased by a factor of 3 and the maximum amo rate by a factor of 7

composition and exhalation flux of gases from mud volcanoes in taiwan many mud volcanoes are distributed along the tectonic sutures in southern taiwan and can be divided into five zones based on their relative positions in different tectonic domains most active mud volcanoes are exhaling methane dominated gases nevertheless some gases show unusual carbon dioxide dominated and or nitrogen excess compositions this implies that there are multiple sources for the gas compositions of mud volcanoes in taiwan

halogen and 129i systematics in gas hydrate fields at the northern cascadia margin iodp expedition 311 insights from numerical modeling we measured halogen concentrations and i 129 i ratios in five drilling sites of integrated ocean drilling program expedition 311 offshore vancouver island canada in order to identify potential sources of fluids and methane in gas hydrate fields iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope i 129 t 1 2 15 7 ma allows the age determination of organic sources for iodine here we report halogen concentrations in 135 pore water samples i concentrations in 48 sediment samples and i 129 i ratios measured in a subset of 20 pore water samples most i 129 i ratios fall into a range around 500 x 10 15 corresponding to a minimum age of 25 ma and the lowest ratio of 188 x 10 15 t min 47 ma was observed at 208 m below sea floor mbsf in site 1326 these ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the juan de fuca plate indicating that old accreted sediments in the accretionary wedge contribute a significant amount of iodide and by association of methane to the gas hydrate occurrences a geochemical transport reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide bromide and ammonia in the host sediments due to organic matter degradation the model was first tested with data from two well studied areas ocean drilling program site 1230 peru margin and site 1245 hydrate ridge the model results for the expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids carrying large amounts of old iodine from deep sources the comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry especially at the cascadia margin the iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields

clathrites archives of near seafloor pore fluid evolution δ44 40ca δ13c δ18o in gas hydrate environments aragonitic clathrites are methane derived precipitates that are found at sites of massive near seafloor gas hydrate clathrate accumulations at the summit of southern hydrate ridge cascadia margin these platy carbonate precipitates form inside or in proximity to gas hydrate which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation the clathrites record the preferential incorporation of 18o into the hydrate structure and hence the enrichment of 16o in the surrounding brine we measured 18o values as high as 2 27 relative to peedee belemnite that correspond to a fluid composition of 1 18 relative to standard mean ocean water the same trend can be observed in ca isotopes ongoing clathrite precipitation causes enrichment of the 44ca in the fluid and hence in the carbonates carbon isotopes confirm a methane source for the carbonates our triple stable isotope approach that uses the three main components of carbonates ca c o provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites this approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation thus recording the evolution of the geochemical environment of gas hydrate systems

thermodynamic and pore water halogen constraints on gas hydrate distribution at odp site 997 blake ridge abstract marine sediment sequences with ch4 hydrate are characterized by an atypical depth profile in dissolved cl squeezed from pore space a shallow subsurface cl maximum overlies a lengthy and pronounced cl minimum this pore water cl profile represents a combination of multiple processes including glacial interglacial variations in ocean salinity advection and diffusion of ions that are excluded during gas hydrate formation at depth and release of fresh water from dissociation of hydrate during core recovery in situ quantities of gas hydrate can be determined from a measured pore water cl profile provided the in situ pore water signature prior to core recovery can be separated ocean drilling program odp site 997 was drilled into a large ch4 hydrate reservoir on the blake ridge in the western atlantic ocean previously we have constructed a high resolution pore water cl profile at this location here we present a coupled chloride hydrate numerical model to explain basic trends in the cl profile and to isolate in situ cl concentrations the model is based on thermodynamic and ecological considerations and uses established equations for describing chemical behavior in marine sediment pore water systems the model incorporates four key concepts 1 most gas hydrate is formed immediately below the so42 reduction zone 2 fluid dissolved ions and gas advect upward through the sediment column 3 ch4 hydrate dissociates at the base of hydrate stability conditions and 4 seawater salinity fluctuates during glacial interglacial cycles of the late pliocene and quaternary rates of upward advection in the model are sufficient to account for measured br and i concentrations as well as ch4 oxidation at the base of the so42 reduction zone in situ pore water cl inferred from the model is similar to that determined by limited direct sampling in situ ch4 hydrate amounts inferred from the model an average of about 4 of porosity are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques the model also predicts production of substantial quantities of free ch4 gas bubbles 2 5 of porosity at a depth immediately below the lowest accumulation of ch4 hydrate in the sediment column our explanation for the pore water cl profile at site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water cl gas hydrate and free gas in a marine sediment column

temporal variations of gas compositions of fumaroles in the tatun volcano group northern taiwan article i nfo he 4 he geochemical monitoring hydrothermal activity is common in the tatun volcano group of northern taiwan helium isotopic compositions of fumarolic samples show that mantle component occupies more than 60 in the previous study along with recent seismic results a magma reservoir is inferred to have existed beneath the area of da you keng where fumarolic venting is the most active in tatun volcano group progressive increases of hcl concentrations and so2 h2s ratio in fumaroles from da you keng have been observed since august 2004 the hcl concentration changed from almost the detection limit to thousands of ppm even up to 30 000 ppm so2 h2s ratios varied from almost 0 to 3 hence so2 became the dominated s species in this area these variations were accompanied by rising temperature of fumaroles in the tatun volcano group especially in the area of da you keng from boiling point to 131 c meanwhile 3

global scale quantification of mineralization pathways in marine sediments a reaction transport modeling approach 1 the global scale quantification of organic carbon corg degradation pathways in marine sediments is difficult to achieve experimentally due to the limited availability of field data in the present study a numerical modeling approach is used as an alternative to quantify the major metabolic pathways of corg oxidation cox and associated fluxes of redox sensitive species fluxes along a global ocean hypsometry using the seafloor depth sfd as the master variable the sfd dependency of the model parameters and forcing functions is extracted from existing empirical relationships or from the noaa world ocean atlas results are in general agreement with estimates from the literature showing that the relative contribution of aerobic respiration to cox increases from 80 in deep sea sediments sulfate reduction essentially follows an inversed sfd dependency the other metabolic pathways denitrification mn and fe reduction only adding minor contributions to the global scale mineralization of corg the hypsometric analysis allows the establishment of relationships between the individual terminal electron acceptor tea fluxes across the sediment water interface and their respective contributions to the corg decomposition process on a global average simulation results indicate that sulfate reduction is the dominant metabolic pathway and accounts for approximately 76 of the total cox which is higher than reported so far by other authors the results also demonstrate the importance of bioirrigation for the assessment of global species fluxes especially at shallow sfd most of the teas enter the sediments via bioirrigation which complicates the use of concentration profiles for the determination of total tea fluxes by molecular diffusion furthermore bioirrigation accounts for major losses of reduced species from the sediment to the water column prohibiting their reoxidation inside the sediment as a result the total carbon mineralization rate exceeds the total flux of oxygen into the sediment by a factor of 2 globally

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

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

methane seepage inferred from the porewater geochemistry of shallow sediments in the beikang basin of the southern south china sea abstract several tens of methane ch4 seepage sites have been discovered in the northern south china sea scs during the investigation and exploration of gas hydrates and cold seeps over the past decade however our knowledge about methane seepage in the southern scs remains scarce especially in the beikang basin on the southern slope of the scs four piston cores bh cl18 bh cl23 bh cl29 and bh cl35 were collected in the beikang basin to understand ch4 cycling in shallow sediments the extrapolated depths of the sulfate methane transition zones smtz of the four cores are between 5 3 and 8 8 mbsf by simulating the porewater profiles of so42 ch4 dic and po43 using the reaction transport model we estimated that the dissolved so42 was predominantly consumed by the anaerobic oxidation of methane aom at rates of 27 5 mmol m 2 yr 1 in bh cl18 28 1 mmol m 2 yr 1 in bh cl23 43 1 mmol m 2 yr 1 in bh cl29 and 39 4 mmol m 2 yr 1 in bh cl35 respectively due to intense aom almost all the methane sourced from subsurface sediments was depleted within the smtz resulting in negligible benthic methane fluxes the kink type depth so42 profile of bh cl23 was apparently attributable to an enhanced methane flux which was not captured by the steady state modelling furthermore highly depleted 13c values of the dissolved inorganic carbon dic in bh cl18 43 6 bh cl23 44 and bh cl29 51 2 also indicated a methane derived dic in contrast relatively higher 13c values in bh cl35 suggested a mixture of 13c enriched dic produced by methanogenesis as shown by the reversal toward more 13c enriched dic below 4 2 mbsf in bh cl29 to our knowledge this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern scs and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern scs

early diagenetic processes fluxes and reaction rates in sediments of the south atlantic abstract porewaters recovered from sediment cores gravity corers box corers and multicorers from various subrogions of the south atlantic amazon river mouth equatorial upwelling congo river mouth benguela coastal upwelling area and angola basin were investigated geochemically objectives included determination of eh ph oxygen nitrate sulfate alkalinity phosphate ammonium fluoride sulfide cal mg sr fe mn and si in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes porewater profiles from the eastern upwelling areas of the south atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation whereas organic matter degradation by sulfate reduction is restricted to the near surface sediments further a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m far below the zone of bioturbation or bioirrigation because other sedimentological reasons seem to fail as explanations an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed porewater profiles from the sediments of the amazon fan area are mainly influenced by reactions with fe iii phases the remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments in contrast to the upwelling sediments the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5 3 m probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals a comparison of organic matter degradation rates from the different areas of the south atlantic show the expected relationship to primary productivity oxygen is the dominating oxidant whereas organic matter degradation by nitrate mn iv and probably also fe iii reduction is several orders of magnitude lower sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic rich sediments but may also include methane oxidation reoxidation of ammonium and mn2 by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone but may contribute to the nitrate reduction in suboxic layers

a quantitative assessment of methane cycling in hikurangi margin sediments new zealand using geophysical imaging and biogeochemical modeling takahe seep located on the opouawe bank hikurangi margin is characterized by a well defined subsurface seismic chimney structure ca 80 500 m2 in area sub seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure reaction transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas the dissolution of gas up to 3761 mmol m 2 yr 1 dwarfed the rate of methanogenesis within the simulated sediment column 2 6 mmol m 2 yr 1 dissolved methane is mainly consumed by anaerobic oxidation of methane aom at the base of the sulfate reduction zone and trapped by methane hydrate formation below it with maximum rates in the central part of the chimney 946 and 2420 mmol m 2 yr 1 respectively a seep wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of aom rates gas hydrate formation and benthic dissolved methane fluxes of 3 68 104 mol yr 1 73 85 104 mol yr 1and 1 19 104 mol yr 1 respectively a much larger flux of methane probably escapes in gaseous form through focused bubble vents the approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales

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

importance of submarine landslides for non steady state conditions in pore water systems lower zaire congo deep sea fan abstract most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone however a very marked change in this gradient has been observed several meters below the surface at many locations which has been attributed to anoxic sulfide oxidation or to non local transport mechanisms of pore waters the subject of this study is to investigate whether this feature could be better explained by non steady state conditions in the pore water system to this end data are presented from two gravity cores recovered from the zaire deep sea fan the sediments at this location can be subdivided into two sections the upper layer about 10 m thick consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr it is underlain by a turbidite sequence measuring several meters in thickness which contains large crystals of authigenic calcium carbonate ikaite caco3 6h2o ikaite 13c values are indicative of a methane carbon contribution to the co2 pool radiocarbon ages of these minerals as well as of the adjacent bulk sediments provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block therefore the emplacement of a relatively undisturbed sediment package is postulated pore water profiles show the depth of the sulfate methane transition zone within the turbiditic sediments by the adaptation of a simple transport reaction model it is shown that the change in the geochemical environmental conditions resulting from this slide emplacement and the development towards a new steady state are fully sufficient to explain all features related to the pore water profiles particularly so42 and dissolved inorganic carbon dic the model shows that the downslope transport took place about 300 yr ago

boron isotopic composition of mud volcano fluids implications for fluid migration in shallow subduction zones abstract mud volcanoes are important conduits for deep fluids to migrate upward providing an important window to study fluid sediment interactions at depth in accretionary prisms fluids emitting from mud volcanoes in southern and eastern taiwan were analyzed for dissolved chemical composition and b isotopes to delineate the sources and chemical alteration occurring during fluid migration concentrations of dissolved major elements cl na k ca mg and so4 in most of the mud volcano fluids were lower than seawater concentrations except for the high ca samples in eastern taiwan which were probably affected by fluids igneous rock interactions the high concentrations of b ba and li in the mud volcano fluids are consistent with a scenario that involves intense sediment alteration and clay dehydration at depth the reaction temperatures estimated using chemical thermometers fall within the range of 70 170 c with some variation by location in nearby mud volcano sites the concentration of dissolved major elements varied little but the b stable isotope compositions varied by as much as 20 supporting the argument that dissolved b concentrations were over printed with effects of sediment adsorption this explains the occurrence of b enriched and heavy 11b fluids at moderate depths in subduction zones it is probable that the fluids in mud volcanoes in taiwan originated from 2 to 5 km depth and are influenced by processes involving tectonic shortening sediment alteration and clay dehydration and subsequent retrograde adsorption during fluid migration

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

methane discharge into the black sea and the global ocean via fluid flow through submarine mud volcanoes during the margasch cruise m52 1 in 2001 with rv meteor we sampled surface sediments from three stations in the crater of the dvurechenskii mud volcano dmv located in the sorokin trough of the black sea and one reference station situated 15 km to the northeast of the dmv we analysed the pore water for sulphide methane alkalinity sulphate and chloride concentrations and determined the concentrations of particulate organic carbon carbonate and sulphur in surface sediments rates of anaerobic oxidation of methane aom were determined using a radiotracer 14ch4 incubation method numerical transport reaction models were applied to derive the velocity of upward fluid flow through the quiescently dewatering dmv to calculate rates of aom in surface sediments and to determine methane fluxes into the overlying water column according to the model aom consumes 79 of the average methane flux from depth 8 9 10 6 mol a 1 such that the resulting dissolved methane emission from the volcano into the overlying bottom water can be determined as 1 9 10 6 mol a 1 if it is assumed that all submarine mud volcanoes smvs in the black sea are at an activity level like the dmv the resulting seepage represents less than 0 1 of the total methane flux into this anoxic marginal sea the new data from the dmv and previously published studies indicate that an average smv emits about 2 0 10 6 mol a 1 into the ocean via quiescent dewatering the global flux of dissolved methane from smvs into the ocean is estimated to fall into the order of 10 10 mol a 1 additional methane fluxes arise during periods of active mud expulsion and gas bubbling occurring episodically at the dmv and other smvs

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

bromine and iodine in peru margin sediments and pore fluids implications for fluid origins at the peruvian convergent margin two distinct pore fluid regimes are recognized from differences in their cl concentrations the slope pore fluids are characterized by low cl concentrations but elevated br and i concentrations due to biogenic production the shelf pore fluids exhibit elevated cl and br concentrations due to diffusive mixing with an evaporitic brine in the slope pore fluids the br i and nh4 concentrations are elevated following bacterial decomposition of organic matter but the i concentrations are in excess of those expected based on mass balance calculations using nh4 and br concentrations the slope sediment organic matter which is enriched in iodine from oxidationreduction processes at the oxygenated sediment water interface is responsible for this enrichment the increases in dissolved i and the i enrichments relative to nh4 and br correlate well with sedimentation rates because of differential trapping following regeneration the pore fluid i br ratios suggest that membrane ion fiitration is not a major cause of the decreases in cl concentrations other possible sources for low cl water including meteoric water clathrate dissociation and or mineral dehydration reactions imply that the diluting component of the slope low cl fluids has flowed at least 1 km through the sediment the low bottom water oxygenation in the shelf is responsible for the low if any enrichment of iodine in the shelf sediments fluctuations in bottom water oxygen concentrations in the past however may be responsible for the observed variations in the sediment ibr ratios comparison of na cl and br cl molar ratios in the pore fluids shows that the shelf high cl fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution this dense brine located below the sediment sections drilled appears to have flowed a distance 500 km through the sediment

temperature dependence of organic matter remineralization in deeply buried marine sediments abstract assuming that the temperature dependence of sediment organic matter remineralization can be described by the arrhenius equation organic matter that is highly refractory at seafloor temperatures 2 3 c should become more reactive at sediment depths of several hundred meters due to burial and heating by the natural geothermal gradient results obtained using a coupled non linear reactive transport model support this suggestion for deeply buried marine sediments i e those found 100s of meters below the seafloor model results predict the occurrence of a deep zone of methanogenesis that is separated by a relatively thick region in which methane diffuses upwards to be oxidized by downward diffusing sulfate this depth zonation of biogeochemical processes in deeply buried sediments is in sharp contrast to that observed in nearshore marine sediments where sulfate reduction and methanogenesis generally occur in much closer vertical proximity model results presented here also provide a simple mechanistic explanation for the occurrence of linear pore water sulfate profiles that are common in many continental margin sediments linear sulfate gradients are often taken as an indirect indicator of gas hydrate occurrence although results presented here suggest that this does not necessarily always have to be the case application of the model to gas hydrate containing sediments on the blake ridge yields results that are in good agreement with sediment and pore water data and microbial rate studies in these sediments model results also suggest that in situ methanogenesis in and or near the gas hydrate stability zone may play an important role in supplying the methane found in blake ridge gas hydrates

cold seep systems in the south china sea an overview abstract three decades after the discovery of cold seep systems various sites of hydrocarbon seepage have been found in the south china sea scs over the past decade these sites have become model systems for understanding the variability of hydrocarbon seepage and associated biogeochemical processes in this review we describe the cold seep systems of the scs with an emphasis on seafloor manifestations fluid sources biogeochemical processes and macroecology seafloor features associated with seeps include mud volcanoes pockmarks and carbonate deposits a common characteristic of cold seeps is the occurrence of authigenic i e in situ precipitated carbonate minerals these carbonates commonly exhibit low 13c and high 18o values suggesting the incorporation of methane derived carbon and oxygen derived from gas hydrate water biogeochemical processes such as sulfate driven anaerobic oxidation of methane sd aom the key process at seeps have been studied in detail with the aim of establishing geochemical proxies to trace these processes into the geological past we also detail the features characterizing seep ecosystems understanding the impact of decomposing methane hydrate on the marine carbon budget remains challenging and requires additional seafloor observations as well as models predicting how gas hydrate responds to changing conditions such as temperature increase sea level rise and episodic mass wasting

melt topology and seismic anisotropy in mantle peridotites of the oman ophiolite this paper presents shape measurements of plagioclase and clinopyroxene inclusions assumed to reflect melt topology in peridotites of the uppermost mantle section of the oman ophiolite the plagioclase and clinopyroxene grains are devoid of any intracrystalline deformation in all samples in contrast the olivine in these rocks has recorded a high temperature plastic deformation with different strengths of the crystallographic preferred orientation cpo of the olivine grains individual melt pockets are first described by ellipses in two dimensions they are more elongated when they have a larger area and they are preferentially oriented parallel to the lineation the x structural axis of the sample with a better defined preferred orientation for samples that have a stronger cpo in a second step an average melt phase shape is defined in three dimensions for each sample using the image autocorrelation technique the average shape is nearly spherical for the samples with weak cpos and it is ellipsoidal with a long axis parallel to x and the short axis parallel to z normal to the foliation for samples with strong cpos the long axis of the ellipsoid is 3 times as long as the short axis for the sample with the strongest cpo we use an anisotropic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites the melt pockets were modelled as basalt filled inclusions with the average shape and orientation given by the image analysis the cpo of the olivine crystals was used to calculate the elastic properties of the anisotropic background medium the calculated p wave seismic anisotropies ranged from 5 to 15 with the anisotropy increasing with the cpo strength and melt fraction the maximum p wave velocities are found along x with velocities above 8 km s at 0 melt and an average 0 5 km s reduction for 10 of melt the minimum p wave velocities are found along z with velocities generally below 7 5 km s at 0 melt and an average reduction of 0 8 km s for 10 of melt 1998 elsevier science b v all rights reserved

deep sulfate reduction completely mediated by anaerobic methane oxidation in sediments of the upwelling area off namibia abstract porewater concentrations of sulfate methane and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off namibia which were recovered from the continental slope at water depths of 1300 and 2000 m at all four stations a distinct sulfate methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100 of deep sulfate reduction within the sulfate methane transition zone and consumes the total net diffusive sulfate flux a significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could therefore be excluded we state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction

evaluation of gas hydrate deposits in an active seep area using marine controlled source electromagnetics results from opouawe bank hikurangi margin new zealand several known gas seep sites along the hikurangi margin off the east coast of new zealand were surveyed by marine controlled source electromagnetic csem experiments a bottom towed electric dipole dipole system was used to reveal the occurrence of gas hydrate and methane related to the seeps the experiments were part of the international multidisciplinary research program new vents carried out on german r v sonne in 2007 cruise so191 to study key parameters controlling the release and transformation of methane from marine cold vents and shallow gas hydrate deposits two csem lines have been surveyed over known seep sites on opouawe bank in the wairarapa region off the se corner of the north island the data have been inverted to sub seafloor apparent resistivity profiles and one dimensional layered models clearly anomalous resistivities are coincident with the location of two gas seep sites north tower and south tower on opouawe bank a layer of concentrated gas hydrate within the uppermost 100 m below the seafloor is likely to cause the anomalous resistivities but free gas and thick carbonate crusts may also play a role seismic data show evidence of fault related venting which may also indicate the distribution of gas hydrates and or authigenic carbonate geochemical profiles indicate an increase of methane flux and the formation of gas hydrate in the shallow sediment section around the seep sites takahe is another seep site in the area where active venting higher heat flow shallow gas hydrate recovered from cores and seismic fault planes but only moderately elevated resistivities have been observed the reasons could be a the gas hydrate concentration is too low even though methane venting is evident b strong temporal or spatial variation of the seep activity and c the thermal anomaly indicates rather temperature driven fluid expulsion that hampers the formation of gas hydrate beneath the vent

methane source linked to gas hydrate system at hydrate drilling areas of the south china sea porewater geochemistry and numerical model constraints abstract methane of various origins is widespread in the sediments of continental margins representing a key component of the carbon cycle in the ocean its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column however this type of methane is difficult to track due to complex reactions occurred during its long distance migration porewater geochemical analyses of two 7 m piston cores d17 15 and w01 16 combined with a reaction transport model were applied to quantify methane related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of shenhu and dongsha in the south china sea scs the model results revealed that anaerobic oxidation of methane aom is the predominant biogeochemical process in both cores consuming approximately 85 and 74 of sulfate in the modeled domain respectively the crossplot of dissolve inorganic carbon dic accumulation versus sulfate depletion of the system suggests that methane consumed during the aom is mainly originated from external methane sources rather than local methanogenesis using a 13cdic mass balance calculation together with model derived fractions of different dic sources we estimated that the 13c value of the external methane in core d17 15 is 66 6 which is in accordance with the previous reported 13c values of hydrate bound methane in the area these results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the scs is mainly originated from local gas hydrate reservoir the proposed approach has the potential to be used to differentiate the sources of methane which will provide constraints on dynamics of methane in gas hydrate bearing marine sediments

evidence and age estimation of mass wasting at the distal lobe of the congo deep sea fan on continental margins sulfate reduction occurs within the sedimentary column it is coupled with the degradation of organic matter and the anaerobic oxidation of methane these processes may be significantly disturbed by sedimentary events leading to transient state profiles for the involved chemical species yet little is known about the impact of turbidity currents and mass wasting on the migration of chemical species and the redox reactions in which they are involved due to its connection to the river the congo deep sea fan continuously receives huge amount of organic matter rich sediments primarily transported by turbidity currents which impact on the development of the associated ecosystems rabouille et al 2016 thus it is well suited to better understand causal relationships between sedimentary events and fluid flow path with consequences on the zonation of early diagenesis sequences here we combined sedimentological observations with geochemical analyses of pore water and sediment samples to explore how sedimentary instabilities affected the migration of methane and the distribution of organic matter within the sedimentary column the results unveiled mass wasting processes affecting recent turbiditic and pelagic deposits and are interpreted as being slides slumps and debrites two slides were responsible for the exhumation of an organic matter rich sedimentary block of more than 5 m thick and the movement of a methane rich sedimentary block while turbidity currents enable the intercalation of sandy intervals within a pelagic clay layer the youngest slide promoted the development of two sulfate methane transition zones smtz and may have possibly triggered a lateral migration of methane numerical simulation of the sulfate profile indicates that the youngest sedimentary event has occurred around a century ago our study emphasizes that turbidity currents and sedimentary instabilities can significantly affect the transport paths and the distribution of both methane and organic matter in the terminal lobe complex with consequences on geochemical zonation of the sequential early diagenetic processes within the sedimentary column

towards quantifying the reaction network around the sulfate methane transition zone in the ulleung basin east sea with a kinetic modeling approach abstract we present a kinetic model based upon pore water data collected from eight sites drilled during the second ulleung basin gas hydrate drilling expedition ubgh2 in 2010 three sites were drilled at locations where acoustic chimneys were identified in seismic data and the rest were drilled on non chimney i e background environments our model coupled a comprehensive compositional and isotopic data set is used to illustrate the different biogeochemical processes at play in those two environments in terms of reactions around the sulfate methane transition zone smtz organic matter decomposition is an important process for production of methane dissolved inorganic carbon dic and consumption of sulfate in the non chimney sites whereas anaerobic oxidation of methane aom dominates both carbon and sulfur cycles in the chimney environment different sources of methane mediate aom in the two settings internally produced methane through co 2 reduction cr and methanogenesis fuels aom in the non chimney sites whereas aom is sustained by methane from external sources in the chimney sites we also simulate the system evolution from non chimney to chimney conditions by increasing the bottom methane supply to a non chimney setting we show that the higher ch 4 flux leads to a higher microbial activity of aom and more organic matter decomposition through methanogenesis a higher methanogenesis rate and a smaller cr contribution relative to aom in the chimney sites is responsible for the isotopically light dic and heavy methane in this environment relative to the non chimney sites

gas hydrate destabilization enhanced dewatering benthic material turnover and large methane plumes at the cascadia convergent margin mixed methane sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the cascadia convergent margin vent fields from which methane charged low salinity fluids containing sulfide ammonia 4he and isotopically light co2 escape are associated with these exposures they characterize a newly recognized mechanism of dewatering at convergent margins where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward this process augments the convergence generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths the enhanced turnover results from the oxidation of methane hydrogen sulfide and ammonia by vent biota the injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide a large fraction of the methane is oxidized within the water column and generates 13c anomalies of the dissolved inorganic carbon pool

analysis of methane and sulfate flux in methane charged sediments from the mississippi canyon gulf of mexico abstract sediment porewater geochemical data so4 2 ch4 dic 13c dic and cl were obtained from piston cores collected in atwater valley gulf of mexico prior to 2005 drilling to study gas hydrates in the region the geochemical data were used for a study of shallow sediment ch4 cycling on a seafloor mound mound f where an apparent upward deflection of the bottom simulating reflector bsr suggested vertical fluid advection fifteen sediment cores ranging from 300 to 800 cm long were collected from locations on top of the mound and across a transect up to 3 5 km off the mound the sulfate methane transition smt was determined in each core from porewater so4 2 and ch4 concentration profiles and occurred at depths ranging from 0 to 410 cm below the seafloor cmbsf the shape of porewater so4 2 profiles plotted against depth also varied from linear to non linear along the transect diffusion rates estimated from linear so4 2 concentration gradients ranged from 20 4 to 249 1 mmol m 2 a 1 with the greatest rate measured in sediments on the mound the large variation in smt depth and so4 2 profiles along the transect indicates lateral differences in total vertical ch4 flux between locations results suggest steady state and non steady state ch4 fluxes both on the mound and transitioning off the mound and likely differences in the relative contribution of fluid advection to local shallow sediment ch4 cycling cores collected from on the mound had high porewater headspace ch4 concentrations up to 8 34 mm coupled with elevated cl concentrations up to 956 5 mm at shallow depths suggesting that salt diapirism in deep sediments may be inhibiting hydrate stability and increasing vertical ch4 flux

origins of taiwan s mud volcanoes evidence from geochemistry abstract the supply of natural gas from mud volcanoes is commonly attributed to the migration of gas bearing fluids from a deeper source area however the relative importance of these gas sources is not well known onshore mud volcanoes in taiwan are mainly located in the foothills of southwestern taiwan and in the coastal range of eastern taiwan generally along structurally weak zones geochemical analyses of gases and sediments from 17 mud volcanoes were undertaken in this study using molecular and stable isotopic compositions hydrocarbon gases were characterized by type and source maturity source rock assessments and nanofossil analyses were carried out to evaluate the potential and to infer the origin of sediments a particular effort was directed towards determining the genetic relationships between mud volcanic gas and associated sediments gases from taiwan s mud volcanoes are predominantly ch4 enriched with the exception of a few mud volcanoes where co2 predominates the origins of hydrocarbon gases in these systems range from microbial to thermally over mature sources while excess co2 may be derived from an inorganic origin as indicated by measured 13 c co 2 values the integration of geochemistry and local geology has yielded models with which to evaluate possible mechanisms for the formation of mud volcanoes

production consumption and migration of methane in accretionary prism of southwestern taiwan to systematically quantify the production consumption and migration of methane along a continental margin 210 sediment cores were collected from offshore southwestern taiwan and analyzed for their gas and aqueous geochemistry these data combined with published results were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance to constrain deep microbial and thermogenic methane production rates within the accretionary prism the results showed that methane diffusive fluxes ranged from 2 71 10 3 to 2 78 10 1 and from 1 88 10 1 to 3 97 mmol m 2 d 1 at the sulfate methane transition zone smtz and sediment seawater interfaces respectively high methane fluxes tend to be associated with structural features suggesting a strong structural control on the methane transport a significant portion of ascending methane 50 is consumed by anaerobic oxidation of methane at the smtz at most sites indicating effective biological filtration gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater the flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1 512 43 096 tg myr 1 methane and could be likely accounted for by the sequestration of methane into hydrate forms and clay absorption

sulfate reduction and methane oxidation in continental margin sediments influenced by irrigation south east atlantic off namibia abstract sulfate reduction rates srr and concentrations of so 4 2 h 2 s pyrite sulfur total sulfur ch 4 and organic carbon were measured with high depth resolution through the entire length of the so 4 2 zone and well into the ch 4 zone at two continental slope stations in the eastern south atlantic benguela upwelling area the sediments were characterized by a high organic carbon content of approx 7 5 at geob 3703 and 3 7 at geob 3714 at geob 3703 so 4 2 concentrations decreased linearly with depth to about 40 m at the sulfate methane transition zone smt at 3 5 m while at geob 3714 so 4 2 remained at sea water concentration in the top 2 m of the sediment and then decreased linearly to about 70 m at the smt at 6 m direct rate measurements of srr 35 so 4 2 showed that the highest srr occurred within the surface 3 5 cm with peak rates of up to 20 and 7 nmol so 4 2 cm 3 day 1 at geob 3703 and geob 3714 respectively srr decreased quasi exponentially with depth at geob 3703 and the cumulative srr over the length of the so 4 2 zone resulted in an areal srr srr area of 1114 3493 mol m 2 day 1 median value 2221 mol m 2 day 1 at geob 3703 with more than 80 of the total sulfate reduction proceeding in the top 30 cm sediment at geob 3714 srr exhibited more scatter with a cumulative srr area of 398 1983 mol m 2 day 1 median value 1251 mol m 2 day 1 and with 60 of the total sulfate reduction occurring below a depth of 30 cm due partially to a deeply buried zone of sulfate reduction located between 3 and 5 m depths srr peaks were also observed in smt of both cores ostensibly associated with methane oxidation but with rates about 10 times lower than at the surface modeled srr balanced both methane oxidation rates and measured srr within the smt but severely underestimated by up to 89 the total srr area that were obtained from direct measurements modeled and measured srr were reconciled by including solute transport by irrigation described by a non local pore water exchange function which had values of up to 0 3 year 1 in the top sediment and decreased exponentially to zero i e no irrigation at 2 3 meters i e above smt these results suggested that co existing sulfate reduction processes and linear so 4 2 gradients can be maintained by a non local transport mechanism such as irrigation by which pore water in tubes or burrows is exchanged with bottom waters by activities of tube dwelling animals or some similar physical transport phenomenon i e bubble ebullition further support for an irrigation mechanism was found in the observations of open tubes of up to 8 mm id at depths down to 6 m which also contained fecal pellets indicating that these tubes were or had been inhabited

clastic injection dynamics during ice front oscillations a case example from solheimajokull iceland abstract soft sediment deformation structures are being increasingly used as a tool for reconstructing palaeoenvironments and porewater pressure conditions in glacial settings however the potential of hydrofractures and clastic injections in the reconstruction of ice dynamics remains poorly constrained this paper presents the results of a detailed study of a clastic injection network outcropping in the solheimajokull forefield south iceland sedimentological descriptions are combined with microscopic to macroscopic analyses of clastic injection geometries sediment fills and cross cutting relationships the 250 m long and 20 m high exposure observed along the east flank of the proglacial braid plain displays alternating glaciofluvial sediments and subglacial tills illustrating oscillations of the ice margins these sediments are cross cut by a dense network of injection composed of dykes propagating upward or downward sills and stepped sills these clastic injections result from processes of hydrofracturing and the sediment fills in these hydrofractures are generally laminated with an increase of grain size towards the centre of the injections these fracture fill characteristics suggest multiple injection phases within the hydrofractures and an increase of porewater pressure over time five main generations of clastic injections showing different senses of propagation and dip directions are determined and are interpreted as forming in different environments per descensum clastic dykes dipping down ice demonstrate subglacial hydrofracturing underneath flowing ice while sills and per ascensum clastic dykes form in submarginal to marginal environments due to the decrease of ice overburden pressure the integration of these results with the sedimentological characteristics allows the holocene ice front oscillations of the solheimajokull to be reconstructed this study demonstrates the importance of hydrofracture systems and their sediment fills in the reconstruction of palaeo ice dynamics

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

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

relating sulfate and methane dynamics to geology accretionary prism offshore sw taiwan geochemical data ch4 so42 i cl particulate organic carbon poc 13c ch4 and 13c co2 are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest taiwan the sampling stations covered three ridges tai nan yung an and good weather each characterized by bottom simulating reflectors acoustic turbidity and different types of faulting and anticlines sulfate and iodide concentrations varied little from seawater like values in the upper 1 3 m of sediment at all stations a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments below this depth sulfate was rapidly consumed within 5 10 m by anaerobic oxidation of methane aom at the sulfate methane transition carbon isotopic data imply a mainly biogenic methane source a numerical transport reaction model was used to identify the supply pathways of methane and estimate depth integrated turnover rates at the three ridges methane gas ascending from deep layers facilitated by thrusts and faults was by far the dominant term in the methane budget at all sites differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth integrated aom rates by comparison methane produced in situ by poc degradation within the modeled sediment column was unimportant this study demonstrates that the geochemical trends in the continental margins offshore sw taiwan are closely related to the different geological settings

laboratory experiment and numerical simulation on authigenic mineral formation induced by seabed methane seeps abstract in natural gas hydrate marine settings cold seepage of methane fluid is a widely observed phenomenon where authigenic minerals serve as an indication of potential gas hydrate bearing reservoirs at depth in this study based on the data from the site hd196 near dongsha island northern continental slope of south china sea laboratory experiments and numerical simulation studies were conducted to investigate the biogeochemical processes of authigenic mineral formation induced by methane seepage the bioreactor experimental results show that in response to methane flux ph increased to 8 5 after 20 days of reaction and eh declined rapidly first and then remained unchanged at about 100 mv the decrease in so42 hs and hco3 concentrations indicated the occurrence of anaerobic oxidation of methane coupled with sulfate reduction aom sr the depletion of fe2 implied the formation of iron bearing minerals e g iron sulfides carbonate minerals were also identified in the experimental phase of this study most iron sulfides existed as massive pieces and in some cases as spherical or rod shape pieces the calcium carbonates were observed as blocky pieces numerical simulations were also performed to reproduce the biogeochemical reactions that occurred in the reactor experiments based on experimental data kinetic parameters associated with the observed reactions were calibrated the model simulated results are general consistent with those obtained by the experiments conducted in this study the combination of simulation and experimental studies provided a powerful tool to investigate the biogeochemical processes in the methane leakage environment at different temporal and spatial scales this study gave a new perspective to understand the formation of cold seep authigenic minerals in marine sediments and was significant for future investigations on the effects of hydrate decomposition

methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the mid okinawa trough abstract widely distributed cold seeps are crucial sources of carbon to the seawater whereas anaerobic oxidation of methane aom and precipitation of authigenic carbonate might change the compositions of the methane rich fluids and thus reduce the outputs of dissolved carbon to seawater in this work we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the mid okinawa trough for the northern research area active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes sites c01 and c10 and strong emissions of gas and fluids may occur in the central mud volcanoes in a submarine canyon at the south c23 site exhibits rarely methane seepage while high rate transportation of methane from deep sediments and associated aom are identified at site c25 where small vertical faults and gas bearing layers are developed at the surrounding aom at site c25 is indicated by 1 quasi linearly decrease of sulfate concentrations with depths 2 intense increases of dissolved inorganic carbon dic concentrations and 3 significantly negative 13c values vary from 7 2 to 26 3 of dic organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of nh4 as well as negligible sulfate consumption at three of the four sites our data reveal that the cold systems are one of the dic sources the fluxes are ca 11 2 mmol m2 yr to the seawater in the okinawa trough which can be tracked by high depletion of 13c 13cdic ranges from 7 2 to 26 3 vpdb

removal of methane through hydrological microbial and geochemical processes in the shallow sediments of pockmarks along eastern vestnesa ridge svalbard the recent discovery of methane seeps in the arctic region requires a better understanding of the fate of methane in marine sediments if we are to understand the contributions of methane to arctic ecosystems and climate change to this goal we analyze pore water data from five sites along eastern vestnesa ridge a sediment drift off north west svalbard to quantify the consumption of dissolved methane across the sulfate methane transition zone which are 3 5 m below seafloor from the investigated sites we use transport reaction models to quantify the hydrology as well as the carbon mass balance in the sediments pore water profiles and our model results demonstrate that hydrological microbial and geochemical processes reactions efficiently remove methane carbon from fluid over different time scales we interpret the nonsteady state behavior of the first 50 70 cm of our pore water profiles from the active sites as an annual scale downward fluid flow due to a seepage related pressure imbalance such downward flow supplies sulfate which enhances methane consumption through anaerobic oxidation of methane aom within this depth range our steady state modeling confirms the efficiency of aom in consuming dissolved methane in the upper 0 8 1 2 m of sediments based on the phosphate profiles we estimate that aom at the active pockmarks may have been operating for the last two to four centuries precipitation of authigenic carbonate removes more than a quarter of the dissolved inorganic carbon produced by aom and fixes it as authigenic carbonate in the sediments a process that sequestrates methane carbon over geological time

rising methane gas bubbles form massive hydrate layers at the seafloor extensive methane hydrate layers are formed in the near surface sediments of the cascadia margin an undissociated section of such a layer was recovered at the base of a gravity core i e at a sediment depth of 120 cm at the southern summit of hydrate ridge as a result of salt exclusion during methane hydrate formation the associated pore waters show a highly elevated chloride concentration of 809 mm in comparison the average background value is 543 mm a simple transport reaction model was developed to reproduce the cl observations and quantify processes such as hydrate formation methane demand and fluid flow from this first field observation of a positive cl anomaly high hydrate formation rates 0 15 1 08 mol cm 2 a 1 were calculated our model results also suggest that the fluid flow rate at the cascadia accretionary margin is constrained to 45 300 cm a 1 the amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water thus most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from ch4 dissolved in the pore water

remote quantification of methane fluxes in gassy marine sediments through seismic survey a novel methodology for predicting upward diffusive fluxes of dissolved methane in gassy marine sediments is presented the predicted fluxes are derived from a set of theoretical simulation data gener ated using a diagenetic reaction transport model the model calculates the upward methane flux for a given free gas depth fgd below the seafloor and a given in situ gas solubility which together define the methane concentration gradient fluxes can thus be extracted from a nomogram of fgd and solubility parameter space because in general microorganisms anaerobically oxidize all dissolved methane before it can escape the sediment the estimated fluxes are equivalent to the amount of methane trapped by this subsurface microbial barrier a test of the approach using measured methane fluxes from aarhus bay denmark reveals a statistically significant correlation between the observed and predicted fluxes the predicted fluxes further show a low sensitivity toward enhanced sediment mixing by faunal activity as well as the deposition flux and reactivity of organic matter therefore only a limited amount of data at strategic coring sites is required to constrain the major physical and geochemical forcings for a particular study area in order to extrapolate fluxes at a regional scale because the fgd can be mapped over large areas of the seafloor from shipboard seismic survey the new approach represents a means to estimate regional methane flux budgets for gassy sediments in a cost efficient manner

initial rise of bubbles in cohesive sediments by a process of viscoelastic fracture 1 an understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid e g magmas and hydrofractures appear not to be applicable to bubbles in soft sediments and this paper presents a new model for gas bubble rise in soft fine grained cohesive sediments bubbles in such sediments are essentially dry little if any free water and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics which assume the existence of a spectrum of flaws within the sediment fabric by extending this theory we predict that bubbles initially rise by preferential propagation of a fracture in a sub vertical direction we present a criterion for initial bubble rise once rise is initiated the speed of rise is controlled by the viscoelastic response of the sediments to stress using this new bubble rise model we estimate rise velocities to be of the order of centimeters per second we again show that capillary pressure plays no substantive role in controlling bubble growth or rise