cyclic 100 ka glacial interglacial migration of subseafloor redox zonation on the peruvian shelf the coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood we examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial interglacial cycles in a sedimentary section drilled on the peruvian shelf ocean drilling program leg 201 site 1229 multiple and distinct layers of diagenetic barite and dolomite i e minerals that typically form at the sulfate methane transition smt occur at much shallower burial depth than the present smt around 30 meters below seafloor these shallow layers co occur with peaks of 13c depleted archaeol a molecular fossil of anaerobic methane oxidizing archaea present day non steady state distributions of dissolved sulfate also suggest that the smt is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the smt reaction transport modeling substantiates our hypothesis that shallow smts occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales e g glacial interglacial periods however our study demonstrates how dynamically sediment biogeochemistry of the peru margin has responded to glacial interglacial change and how these changes are now preserved in the geological record such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling

generalization of gas hydrate distribution and saturation in marine sediments by scaling of thermodynamic and transport processes gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins these compounds form in pore space between the seafloor and a sub bottom depth where appropriate stability conditions prevail however the amount and distribution of gas hydrate within this zone and free gas below can vary significantly at different locations to understand this variability we develop a one dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time the model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments the effect of salinity on gas hydrate distribution is also included the simulations delineate basic modes of gas hydrate distribution in marine sediment including systems with no gas hydrate gas hydrate without underlying free gas and gas hydrate with underlying free gas below the gas hydrate stability zone for various methane sources the results are scaled using combinations of dimensionless variables particularly the peclet number and damkohler number such that the dependence of average hydrate saturation on numerous parameters can be summa rized using two contour maps one for a biogenic source and one for upward flux from a deeper source simulations also predict that for systems at steady state large differences in parameters like seafloor depth seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables although important caveats exist our model presents a unified picture of hydrate accumulations that can be used to understand well characterized gas hydrate systems or to predict steady state average hydrate saturation and distribution at locations for which seismic or core data are not available

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

foraminiferal isotope anomalies from northwestern pacific marginal sediments 1 we evaluated the influence of methane release from methane hydrate reservoirs in the northwestern pacific continental margin on stable isotope values we analyzed stable isotopes of foraminifera from a piston core collected at 1066 m water depth off eastern hokkaido northern japan carbon isotope signals indicated that planktonic and benthic foraminifera in several glacial sediment layers in the core were highly depleted in 13c both the planktonic and benthic foraminiferal 13c values ranged from about 10 to 2 most foraminiferal tests in these horizons were brown as a result of postdepositional alteration foraminiferal oxygen isotopes fluctuated abnormally in the glacial sediment layers showing small about 0 5 positive shifts relative to normal glacial values we attributed the positive shifts to authigenic carbonate formation in the foraminiferal tests the authigenic carbonates have formed when rising methane from methane hydrate in the sediment became oxidized as it came in contact with dissolved sulfate near the seafloor episodic methane release events may be associated with great plate boundary earthquakes with epicenters near the coring site

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

macro and micro fauna from cold seeps in the palmahim disturbance israeli off shore with description of waisiuconcha corsellii n sp bivalvia vesicomyidae abstract chemosymbiotic micro and macro fauna related to cold seep sites were recovered in the palmahim disturbance pd offshore israel during eu eurofleets2 semseep cruise by box coring and remotely operated vehicle rov dives no live macrofauna was identified in the collected sediments with the exception of the seep related crustacean calliax lobata de gaillande and lagardere 1966 numerous calliax claws testify the past colonization of these soft bottoms by several generations of this ghost shrimp after sediment sieving on 1 mm we identified gastropods belonging to the families trochidae eucyclidae unassigned seguenzioidea genus anekes rissoidae elachisinidae raphitomidae mangeliidae architectonicidae orbitestellidae and acteonidae the identified bivalves belong to the families nuculidae yoldiidae mytilidae lucinidae thyasiridae semelidae kelliellidae vesicomyidae xylophagidae and cuspidariidae a seep related group of chemosymbiotic molluscs was detected including taranis moerchii malm 1861 lurifax vitreus waren and bouchet 2001 idas ghisottii waren and carrozza 1990 lucinoma kazani salas and woodside 2002 thyasira biplicata philippi 1836 isorropodon perplexum sturany 1896 and the newly described vesicomyid species waisiuconcha corsellii n sp that represents also the first record of the genus waisiuconcha in recent mediterranean sediments the rov dives recorded local patches of several m2 of seafloor covered by dead shells of l kazani with a density of up to about 200 loose shells per square meter the potential occurrence of seep related foraminifera among low oxygen tolerant species was explored by comparison with previously sampled adjacent localities and lead to the identification of chilostomella oolina globobulimina affinis and g pseudospinescens as potential foraminiferal seep indicators in the southeastern mediterranean sea the absence of live seep related fauna in surface sediments in the pd where seepage has been confirmed suggests intermittent activity and a pause or decline of the investigated seeps

paleofloods and historical floods of the ardeche river france 1 better understanding of flood occurrences and long term floodplain planning and flood risk assessment is achieved by integration of gauged historical and paleoflood data the ardeche river is ideal for this historical flood paleoflood study because its historical flood levels record dates back as early as a d 587 and useful data date back to a d 1522 its systematic gauging record is over 100 years long and the geologic and geomorphic settings are optimal for paleoflood studies three sites provide three different thresholds for flood stages and swd accumulation according to our one dimensional 1 d step backwater calculations these three thresholds are 5200 5700 m3 s 1 4900 5400 m3 s 1 and 3600 4000 m3 s 1 recording 6 9 and 19 large holocene floods respectively dating the deposits enabled a correlation with the historical record these paleoflood studies indicate that there are long gaps in flood occurrences on the ardeche river the floods are not randomly distributed in time but are clustered they also indicate that the recent nineteenth century floods were the largest at the millennial timescale

redox dependent distribution of early macro organisms evidence from the terminal ediacaran khatyspyt formation in arctic siberia abstract the ediacaran period witnessed the first appearance of macroscopic animal life in earth s history however the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain in part due to the dearth of integrated paleobiological and chemostratigraphic datasets in this study a comprehensive geochemical analysis was conducted on the fossiliferous khatyspyt formation in arctic siberia in order to gain insights into the ediacaran paleoenvironments this study was designed to specifically address the relationship between paleoredox conditions and ediacaran fossil occurrences in the khatyspyt formation our data reveal a dramatic shift in pyrite sulfur isotope compositions 34 s pyrite from ca 20 to ca 55 and this shift is intriguingly associated with the first occurrence of ediacara type macrofossils at the studied section suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms based on multiple lines of sedimentological and geochemical evidence we propose that the development of oceanic euxinia which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal ediacaran period may have locally prohibited the colonization of ediacara type organisms and resulted in low 34 s pyrite values in the lower khatyspyt formation in the middle and upper khatyspyt formation progressive secular transition from euxinic to non euxinic and more habitable conditions may have allowed for the colonization of ediacara type and other macro organisms

anaerobic oxidation of methane in a cold water coral carbonate mound from the gulf of cadiz the gulf of cadiz is an area of mud volcanism and gas venting through the seafloor in addition several cold water coral carbonate mounds have been discovered at the pen duick escarpment amidst the el arraiche mud volcano field on the moroccan margin one of these mounds named alpha mound has been studied to examine the impact of the presence of methane on pore water geochemistry potential sulphate reduction sr rate and dissolved inorganic carbon dic budget of the mound in comparison with off mound and off escarpment locations pore water profiles of sulphate sulphide methane and dic from the on mound location showed the presence of a sulphate to methane transition zone at 350 cm below the sea floor this was well correlated with an increase in sr activity 13c depleted dic at the transition zone 21 9 vs vienna pee dee belemnite indicated that microbial methane oxidation significantly contribute to the dic budget of the mound the alpha mound thus represents a new carbonate mound type where the presence and anaerobic oxidation of methane has an important imprint on both geochemistry and dic isotopic signature and budget of this carbonate mound

stable isotope patterns of coexisting pyrite and gypsum indicating variable methane flow at a seep site of the shenhu area south china sea aggregates of authigenic pyrite and gypsum were found at site hs373 in the shenhu seepage area south china sea most of the pyrite occurs as tubes consisting of framboids gypsum aggregates are mainly represented by rosettes and some of them grew over pyrite revealing that gypsum precipitation postdated pyrite formation at seeps extensive sulfate driven anaerobic oxidation of methane aom commonly leads to the accumulation of 34s enriched sulfide resulting in the formation of 32s depleted pyrite the sulfur isotopic composition of pyrite at site hs373 reveals a great variability of 75 5 in 34s values spanning from 37 6 to 37 9 v cdt with a distinct trend towards higher values with increasing depth this trend indicates an increase of the effect of sulfate driven aom with depth whereas pyrite at shallow depth apparently resulted mostly from organiclastic sulfate reduction to date reports on coexisting pyrite and gypsum in seep environments are limited and therefore the mechanism of gypsum formation is insufficiently understood this is the first time that the sulfur and oxygen stable isotope composition of gypsum has been analyzed to understand its formation at seeps the 34s values of gypsum vary from 24 5 to 0 9 revealing a similar trend with depth like pyrite the 18ogypsum values range from 5 3 to 12 3 v smow with a scattered trend to lower values with depth the combined evidence reveals that gypsum formation was linked to the establishment of more oxidizing conditions probably caused by waning seepage this led to the oxidation of pyrite and the associated increase in pore water sulfate and acidity which favored carbonate mineral dissolution providing the calcium ions required for gypsum formation the observed isotopic variability of gypsum suggests that oxidation occurred by different mechanisms at different depth whereby pyrite oxidation was dominated by aerobic processes at shallow depth involving dissolved molecular oxygen and by anaerobic processes at greater depth involving electron acceptors like ferric iron taken together the stable isotope compositions of pyrite and gypsum suggest that the redox conditions within the studied sediments were variable such variability was controlled by complex factors among which changes in seepage activity have been instrumental for the vertical displacement of the sulfate methane transition zone

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

high intensity gas seepage causes rafting of shallow gas hydrates in the southeastern black sea abstract submarine gas hydrates are a major global reservoir of the potent greenhouse gas methane since current assessments of worldwide hydrate bound carbon vary by one order of magnitude new technical efforts are required for improved and accurate hydrate quantifications here we present hydrate abundances determined for surface sediments at the high flux batumi seep area in the southeastern black sea at 840 m water depth using state of the art autoclave technology pressure sediment cores of up to 2 65 m in length were recovered with an autoclave piston corer backed by conventional gravity cores quantitative core degassing yielded volumetric gas bulk sediment ratios of up to 20 3 proving hydrate presence the cores represented late glacial to holocene hemipelagic sediments with the shallowest hydrates found at 90 cmbsf calculated methane concentrations in the different cores surpassed methane equilibrium concentrations in the two lowermost lithological black sea units sampled the results indicated hydrate fractions of 5 2 of pore volume in the sapropelic unit 2 and mean values of 21 pore volume in the lacustrine unit 3 we calculate that the studied area of 0 5 km2 currently contains about 11 3 kt of methane bound in shallow hydrates episodic detachment and rafting of such hydrates is suggested by a rugged seafloor topography along with variable thicknesses in lithologies we propose that sealing by hydrate precipitation in coarse grained deposits and gas accumulation beneath induces detachment of hydrate sediment chunks floating hydrates will rapidly transport methane into shallower waters and potentially to the sea atmosphere boundary in contrast persistent in situ dissociation of shallow hydrates appears unlikely in the near future as deep water warming by about 1 6 c and or decrease in hydrostatic pressure corresponding to a sea level drop of about 130 m would be required because hydrate detachment should be primarily controlled by internal factors in this area and in similar hydrated settings it serves as source of methane in shallow waters and the atmosphere which is mainly decoupled from external forcing

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

characteristics and possible origin of native aluminum in cold seep sediments from the northeastern south china sea abstract although native aluminum al has been reported to occur in various geological settings for more than 20 locations but its mechanism of formation still remains to be elucidated we report the occurrence and characterization of al particles recovered from the surface sediment cf4 and a short core sediment rov g 37 cm length obtained at cold seeps in the northeastern continental slope ncs of the south china sea scs x ray diffraction analysis shows that the collected particles are metallic aluminum with the unit cell edge a of 4 059 0 005 a cf4 2 and 4 029 0 004 a t26 28 2 rov g the al particles occur as spherules irregular plates and elongated forms with typical lamellar structures their chemical compositions are 95 07 99 84 al the average values is 98 42 with very small amounts of si fe ti s zn mg ca k na cu co and p and are similar to al particles from the east pacific rise and the central indian basin but differ markedly to those from other locations after ruling out several possibilities of al sources and mechanism of formation an alternative cold seep mechanism is proposed for the origin of al in the scs during the processes of anaerobic oxidation of methane aom and pyrite formation high h2s and h2 partial pressures result in strong reducing micro environments under which al oh 4 is reduced to its metallic state by the microbial bacterial processes

submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins in order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins six sediment cores were retrieved off central chile at water depth between 800 and 4000 m the sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate sulfide total alkalinity major cations chloride bromide iodide hydrocarbons as well as the carbon isotopic composition of methane because of negligible weathering on land surface sediments off central chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent andes deep sourced fluxes of alkalinity cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial comparison of deep sourced signals with data from nearby ocean drilling program sites reveals two different types of weathering processes in shallow tens of meters methanic sediments of slope basins with high organic carbon burial rates reactive silicate minerals undergo incongruent dissolution through reaction with co2 from methanogenesis at greater burial depth hundreds of meters silicate weathering is dominated by authigenic smectite formation this process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water deep seated smectite formation is more widespread than shallow silicate dissolution as it is independent from the availability of co2 from methanogenesis although solute transport is not focused enough to form cold seeps in the proper sense tectonically induced diffuse fluid flow transfers the deep seated signal of smectite formation into the shallow sediments the temperature controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments depth of kilometers however in agreement with other studies at active margins e g aleutians cascadia nankai trough and despite ubiquitous evidence for smectite formation little evidence for seafloor seepage of dehydration fluids could be found off central chile we argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin wide scale according to this rationale pore water freshening at seafloor seeps preferentially occurs at lower latitudes central america barbados mediterranean ridge where terrestrial weathering is more intense thus leading to external i e detrital smectite and thus freshwater inputs to the subduction system

pore water dissolved inorganic carbon sources and cycling in the shallow sediments of the haima cold seeps south china sea abstract pore water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane derived carbon in sediments in this study six sediment push cores were collected by manned submersible at the center and edge of the active haima cold seeps and a nearby background site pore water compositions of the so42 ca2 mg2 sr2 nh4 dissolved inorganic carbon dic and 13c dic at each sediments core were measured the very shallow depths of sulfate methane transition zone smtz suggest that high methane flux and significant anaerobic oxidation of methane aom activities occur in the shallow sediments reaction transport modelling results show that the relative contributions of aom and organoclastic sulfate reduction osr consuming sulfate are 84 1 and 15 9 at site hm 2 6 and 94 6 and 5 4 at site hm 3 3 respectively furthermore extremely low 13c dic values at the central sites hm 2 6 45 67 and hm 3 3 53 87 indicate an external methane derived dic based on the 13c mass balance we quantify the proportions of pore water dic derived from organic matter degradation and aom as 24 7 and 65 3 at site hm 2 6 and 9 3 and 80 7 at site hm 3 3 respectively an increase in pore water dic concentration leads to carbonate precipitation plots of sr ca vs mg ca ratios reveal that the carbonate phases in sediments are dominantly high mg calcite in this region our study demonstrates that aom can serve as a dominant biogeochemical process through incorporation of methane derived carbon into dissolved and solid inorganic carbon phases significantly influencing dic cycling within the shallow sediments of the haima cold seep area

geophysical and geochemical survey of a large marine pockmark on the malin shelf ireland 1 marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface a large composite pockmark on the malin shelf off the northern coast of ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface geophysical including acoustic and electromagnetic data confirmed the subsurface presence of signatures typical of fluids within the sediment shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards at least recently however there are no chimney structures observed in the sub bottom data and the migration pathways are not apparent electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark suggesting changes in physical properties of the sediment nuclear magnetic resonance nmr experiments were employed to characterize the organic component of sediments from selected depths very strong microbial signatures were evident in all nmr spectra but microbes outside the pockmark appear to be much more active than inside these observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present

using ammonium pore water profiles to assess stoichiometry of deep remineralization processes in methanogenic continental margin sediments 1 in many continental margin sediments a deep reaction zone exists which is separated from remineralization processes near the sediment surface here methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate however the methane sources that drive this anaerobic oxidation of methane aom in the sulfate methane transition zone smt may vary among sites in particular these sources can be thought of as either i internal sources from in situ methanogenesis regardless of where it occurs in the sediment column that are ultimately coupled to organic matter deposition and burial or ii external sources such as hydrocarbon reservoirs derived from ancient source rocks or deeply buried gas hydrates both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface using a modeling approach we examine the relationship between different methane sources and pore water sulfate methane dissolved inorganic carbon dic and ammonium profiles we show that pore water ammonium profiles through the smt represent an independent tracer of remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with aom and carbonate precipitation i e dic methane and sulfate pore water dic profiles also show an inflection point in the smt based on the type of deep methane source and the presence absence of accompanying upward dic fluxes with these results we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments

paleo cold seep activity in the southern south china sea evidence from the geochemical and geophysical records of sediments abstract over the past decade more than 30 cold seep sites have been reported along the continental slope of the south china sea scs these seep sites have all been located in the northern scs and no methane seepage has been discovered in the southern scs here we report on the analysis results of the geochemical and geophysical of the marine core sediments 58s collected from the southern continental slope of the scs we suggest that the sedimentary total sulfur ts total inorganic carbon tic 34scrs values of chromium reducible sulfur crs and carbonate 13ctic values of the bulk sediments coupled with the ratio of nrmaf60mt nrmaf80mt nrm can be used to trace the methane release events in a methane rich environment at least two methane release events were identified at depths of 5 5 6 2 m and 6 8 7 2 m moreover according to the characteristic of the ts toc and nrmaf60mt nrmaf80mt nrm ratio of sediments we suggest the methane flux has changed episodically this inference is supported by the occurrence of the strongly negative carbonate 13ctic values of bulk sediments and the positive 34scrs values which indicate the occurrence of anaerobic oxidation of methane aom and the location of a past or present sulfate methane transition zone smtz respectively ultimately these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment

pore water profiles and authigenic mineralization in shallow marine sediments above the methane charged system on umitaka spur japan sea umitaka spur situated on an unusual collisional plate boundary along the eastern margin of the japan sea hosts gas seeps pock marks collapse structures and gas hydrates piston cores were recovered from this ridge to understand carbon cycling pore fluid gradients and authigenic mineralization above a methane charged system we present the chemistry of fluids and solids from three cores adjacent to seep locations high fluxes of ch4 and alkalinity transport carbon from a deep zone of methanogenesis toward the seafloor methane however reacts with so42 across a shallow sulfate methane transition smt which generates additional alkalinity and hs a fraction of these ch4 oxidation products form authigenic carbonate and pyrite these minerals are not readily apparent from visual inspection of split cores because they exist as micritic coatings on microfossils or as framboidal pyrite they are however readily observed in chemical analyses as peaks of labile ca sr ba or s in sediment at or near the smt carbon inputs and outputs nicely balance across the smt in all three cores if one considers four relevant fluxes loss of alkalinity to the seafloor addition of methane from below addition of alkalinity from below and carbonate precipitation importantly in all cores the magnitude of the fluxes decreases in this order although some carbon rising from depth forms authigenic carbonate most 80 escapes to the ocean as alkalinity nonetheless authigenic fronts in sediment on umitaka spur are a significant reservoir of inorganic carbon given calculated pore fluid fluxes for ca and sr the fronts require tens of thousands of years to form suggesting that the present state and loss of carbon represent long lived processes

porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate bearing system from the taixinan basin the south china sea abstract anaerobic oxidation of methane aom is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling though aom has been extensively investigated the variations of its microorganisms and lipid biomarkers are still not fully understood in this study the porewater geochemical and lipid biomarker evidence for aom was systematically investigated in the hydrate bearing system from the taixinan basin the south china sea aiming to better expound the vertical variations of aom related biomarker signatures and causes of the weak lipid biomarker patterns accordingly the sulfate methane interface smi depth of 1 000 cmbsf and onset of aom were determined on the basis of the vertical profiles of porewater parameters the occurrence of 13c depleted 2 6 10 15 19 pentamethylicosane pmi with the 13c ratios of 77 7 to 131 4 near the smi are in accord with the porewater parameters despite the absence of archaeol and sn2 hydroxyarchaeol and low abundances of pmi meanwhile the strongly 13c enriched iso c15 0 and anteiso c15 0 fatty acids suggest that non methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate reducing bacteria due to the presence of unresolved complex mixtures ucms and odd even predominance oep values about 1 0 the weak in situ signal of aom related biomarkers may be attributed to the low flux methane supply and dilution effect from the high input of allochthonous and autochthonous organic matters the great difference in biomarkers between the taixinan basin and previously reported sites should be further clarified on the basis of more extensive studies in various aom related settings

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

linear analysis of melt band formation in a mid ocean ridge corner flow imposing external shear on systems of partial melt can cause compaction of the solid matrix and concentration of the interstitial liquid melt thereby generating bands of contrasting high and low porosity these shear induced porosity bands have been proposed to channel melt beneath a mid ocean ridge mor in this contribution we impose a shear flow that evolves as the bands are transported along streamlines of mor corner flow we evaluate the suitability of porosity band formation as a mechanism for melt channeling beneath a mor using a linear instability analysis with three different matrix viscosity conditions isotropic strain rate independent isotropic strain rate dependent and anisotropic our analysis shows that the largest amplitude bands channel melt away from the ridge axis toward the base of the plate at the lithosphere asthenosphere boundary while the effectiveness of channeling through the bands is highly dependent on the mantle bulk viscosity