methane fluxes and carbonate deposits at a cold seep area of the central nile deep sea fan eastern mediterranean sea abstract high acoustic seafloor backscatter signals characterize hundreds of patches of methane derived authigenic carbonates and chemosynthetic communities associated with hydrocarbon seepage on the nile deep sea fan ndsf in the eastern mediterranean sea during a high resolution ship based multibeam survey covering a 225 km2 large seafloor area in the central province of the ndsf we identified 163 high backscatter patches at water depths between 1500 and 1800 m and investigated the source composition turnover flux and fate of emitted hydrocarbons systematic parasound single beam echosounder surveys of the water column showed hydroacoustic anomalies flares indicative of gas bubble streams above 8 of the high backscatter patches in echosounder records flares disappeared in the water column close to the upper limit of the gas hydrate stability zone located at about 1350 m water depth due to decomposition of gas hydrate skins and subsequent gas dissolution visual inspection of three high backscatter patches demonstrated that sediment cementation has led to the formation of continuous flat pavements of authigenic carbonates typically 100 to 300 m in diameter volume estimates considering results from high resolution autonomous underwater vehicle auv based multibeam mapping were used to calculate the amount of carbonate bound carbon stored in these slabs additionally the flux of methane bubbles emitted at one high backscatter patch was estimated 0 23 to 2 3 106 mol a 1 by combined auv flare mapping with visual observations by remotely operated vehicle rov another high backscatter patch characterized by single carbonate pieces which were widely distributed and interspaced with sediments inhabited by thiotrophic chemosynthetic organisms was investigated using in situ measurements with a benthic chamber and ex situ sediment core incubation and allowed for estimates of the methane consumption 0 1 to 1 106 mol a 1 and dissolved methane flux 2 to 48 106 mol a 1 our comparison of dissolved and gaseous methane fluxes as well as methane derived carbonate reservoirs demonstrates the need for quantitative assessment of these different methane escape routes and their interaction with the geo bio and hydrosphere at cold seeps

a new approach to discern the hydrocarbon sources oil vs methane of authigenic carbonates forming at marine seeps abstract numerous marine hydrocarbon seeps have been discovered in the past three decades the majority of which are dominated by methane rich fluids however an increasing number of modern oil seeps and a few ancient oil seep deposits have been recognized in recent years oil seepage exerts significant control on the composition of the seep dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized yet distinguishing oil seep from methane seep deposits is difficult in cases where 13ccarb values are higher than approximately 30 due to mixing of different carbon sources here we present a comparative study of authigenic carbonates from oil dominated site gc232 and methane dominated site gc852 seep environments of the northern gulf of mexico aiming to determine the geochemical characteristics of the two types of seep carbonates we analyzed 1 major and trace element compositions of carbonates 2 total organic carbon toc total nitrogen tn and carbon isotope 13ctoc of residue after decalcification 3 sulfur isotope signatures of chromium reducible sulfur crs 34scrs and residue after crs extraction 34stos as well as 4 sulfur contents tos of residue after crs extraction carbonates from the studied oil seep are dominated by aragonite and exhibit lower 34scrs values suggesting carbonate precipitation close to the sediment surface in addition oil seep carbonates are characterized by higher toc and tos contents and higher toc tn ratios as well as less negative 13ctoc values compared to methane seep carbonates probably reflecting a contribution of residual crude oil enclosed in oil seep carbonates very low 13ctoc values as low as 68 7 vpdb and low toc tn ratios of methane seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota this study presents new geochemical data that will allow the discrimination of oil seep from methane seep deposits although some of the geochemical patterns are likely to be affected by late diagenesis if applied with caution such patterns can be used to discern the two end member types of seepage oil seeps and methane seeps in the geological record

highly isotopically depleted isoprenoids molecular markers for ancient methane venting abstract we propose that organic compounds found in a miocene limestone from marmorito northern italy are source markers for organic matter present in ancient methane vent systems cold seeps the limestone contains high concentrations of the tail to tail linked acyclic c20 isoprenoid 2 6 11 15 tetramethylhexadecane crocetane a c25 homolog 2 6 10 15 19 pentamethylicosane pme and a distinctive glycerol ether lipid containing 3 7 11 15 tetramethylhexadecyl phytanyl moieties the chemical structures of these biomarkers indicate a common origin from archaea their extremely 13c depleted isotope compositions 13c 108 to 115 6 pdb suggest that the respective archaea have directly or indirectly introduced isotopically depleted methane derived carbon into their biomass we postulate that a second major cluster of biomarkers showing heavier isotope values 13c 88 is derived from sulfate reducing bacteria srb the observed biomarkers sustain the idea that methanogenic bacteria in a syntrophic community with srb are responsible for the anaerobic oxidation of methane in marine sediments marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined two membered bacterial consortium 1 archaea that perform reversed methanogenesis by oxidizing methane and producing co2 and h2 and 2 srb that consume the resulting h2 furthermore the respective organic molecules are unlike other compounds tightly bound to the crystalline carbonate phase the marmorito carbonates can thus be regarded as cold seep microbialites rather than mere authigenic carbonates

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

concretionary methane seep carbonates and associated microbial communities in black sea sediments abstract gas seeps in the euxinic northwestern black sea provide an excellent opportunity to study anaerobic methane based ecosystems with minimum interference from oxygen dependent processes an integrated approach using fluorescence and electron microscopy fluorescence in situ hybridization lipid biomarkers stable isotopes 13c and petrography revealed insight into the anatomy of concretionary methane derived carbonates currently forming within the sediment around seeps some of the carbonate concretions have been found to be surrounded by microbial mats the mats harbour colonies of sulphate reducing bacteria dss group and archaea anme 1 putative players in the anaerobic oxidation of methane isotopically depleted lipid biomarkers indicate an uptake of methane carbon into the biomass of the mat biota microbial metabolism sustains the precipitation of concretionary carbonates significantly depleted in 13c the concretions consist of rectangularly orientated rod to dumbbell shaped crystal aggregates made of fibrous high mg calcite the sulphate reducing bacteria exhibit intracellular storage inclusions and magnetosomes with greigite fe3s4 indicating that iron cycling is involved in the metabolism of the microbial population transfer of fe3 into the cells is apparently mediated by abundant extracellular vesicles resembling known bacterial siderophore vesicles marinobactine in size 20 to 100 nm and structure

a climatic trigger for the giant troll pockmark field in the northern north sea abstract pockmarks are seafloor craters usually formed during methane release on continental margins however the mechanisms behind their formation and dynamics remain elusive here we report detailed investigations on one of the world s largest pockmark fields located in the troll region in the northern north sea seafloor investigations show that 7000 pockmarks are present in a 600 km2 area a similar density of pockmarks is likely present over a 15 000 km2 region outside our study area based on extensive monitoring coring geophysical and geochemical analyses no indications of active gas seepage were found still geochemical data from carbonate blocks collected from these pockmarks indicate a methanogenic origin linked to gas hydrate dissociation and past fluid venting at the seafloor we have dated the carbonates using the u th method in order to constrain the pockmark formation the carbonates gave an isochron age of 9 59 1 38 ka i e belonging to the initial holocene moreover radiocarbon dating of microfossils in the sediments inside the pockmarks is consistent with the ages derived from the carbonates based on pressure and temperature modelling we show that the last deglaciation could have triggered dissociation of gas hydrates present in the region of the northern part of the norwegian channel causing degassing of 0 26 mtch4 km2 at the seafloor our results stress the importance of external climatic forcing of the dynamics of the seafloor and the role of the rapid warming following the younger dryas in pacing the marine gas hydrate reservoir

using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions abstract seepage of hydrocarbon rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time in this study we investigated the potential of mossbuaer spectroscopy and fe contents of seep related authigenic carbonates from the congo fan the gulf of mexico and the black sea for reconstructing past redox conditions and fluid seepage activity at cold seeps the fe speciation observed by mossbauer spectroscopy and fe contents suggest that 1 the congo fan carbonates precipitated in a sulfidic environment 2 the formation conditions of seep carbonates were variable at the gulf of mexico seep site ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone and 3 the stratified water column of the black sea or suboxic condition resulted in low fe contents of black sea carbonates the study reveals that fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage similarly mossbauer spectroscopy particularly when used in combination with the analysis of redox sensitive elements is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps

late jurassic early cretaceous hydrocarbon seep boulders from novaya zemlya and their faunas the paper describes late jurassic early cretaceous seep carbonate boulders from the russian arctic island of novaya zemlya collected in 1875 by a e nordenskiold during his expedition to siberia the carbonates are significantly depleted in heavy carbon isotopes 13c values as low as ca 40 and show textures typical for carbonates formed under the influence of hydrocarbons such as fibrous carbonate cements and corrosion cavities the rocks contain index fossils of late oxfordian early kimmeridgian late tithonian jurassic and latest berriasian early valanginian cretaceous age the fossil fauna is species rich and dominated by molluscs with subordinate brachiopods echinoderms foraminifera serpulids and ostracods most of the species including two chemosymbiotic bivalve species likely belong to the background fauna only a species of a hokkaidoconchid gastropod and a possible abyssochrysoid gastropod can be interpreted as restricted to the seep environment other seep faunas with similar taxonomic structure are suggestive of rather shallow water settings but in case of novaya zemlya seep faunas such structure might result also from high northern latitude

an integrated view of the methane system in the pockmarks at vestnesa ridge 79 n accepted manuscript version published version available at https doi org 10 1016 j margeo 2017 06 006 accepted manuscript version licensed cc by nc nd 4 0

authigenic carbonate formation revealed by lipid biomarker inventory at hydrocarbon seeps a case study from the okinawa trough abstract authigenic carbonates were recovered from the northern okinawa trough at 540 700 m water depth development of microbial communities and seepage dynamics driving the precipitation of authigenic carbonates remains poorly constrained even though the source of methane rich fluids formation of fe rich carbonates and the potential driving forces were previously reported here petrologic observations stable carbon and oxygen isotopic compositions mineralogy and lipid inventories of authigenic carbonates from the northern okinawa trough were analyzed the carbonate minerals were comprised predominantly of aragonite high magnesium calcite and siderite the presence of molecular fossils diagnostic for anaerobic methane oxidizing archaea anme and sulfate reducing bacteria srb with pronounced 13c depletions as low as 134 together with highly negative 13ccarb values as low as 55 8 suggest that the carbonates precipitated from anaerobic oxidation of methane aom with microbial methane as the predominant carbon source the difference between measured and calculated 18ocarb values based on 0 of sea water 18o vs v smow was higher than 3 in one of the carbonates ggd16 reflecting an origin from gas hydrate dissociation most of the carbonates revealed 18o offsets between measured and calculated values within 3 or a lower measured 18o than calculated value suggesting a mixture of methane derived carbonates low 13c high 18o and detrital origin carbonates high 13c low 18o biomarker patterns varied significantly among samples a suite of 13c depleted biomarkers indicated the predominance of an anme 1 assemblage for all samples suggesting carbonates formed at low to medium methane flux the predominance of anme 1 abundant detrital minerals and allochthonous biomarkers as well as low carbonate contents indicate that most samples formed at greater depth within the sediment column only one carbonate sample in this study is interpreted to have precipitated at relatively shallower depth as indicated by the occurrence of crocetane dominance of aragonite and higher overall carbonate content

changing redox conditions at cold seeps as revealed by authigenic carbonates from alaminos canyon northern gulf of mexico anaerobic oxidation of methane in anoxic sediments at cold seeps often leads to formation of authigenic carbonates close to the seafloor along continental margins recent work however indicated that the redox conditions in sediments may vary to some degree during seepage activity in order to shed new light on the extent of this variability authigenic carbonates from alaminos canyon lease block 645 of the northern gulf of mexico have been characterized by means of inorganic and organic geochemistry the carbonates were collected from seep deposits representing various seafloor morphologies including extensive pavements mounds fractured carbonate slabs surrounded by dense bivalve shells and vestimentiferan tubeworm colonies the deposits almost entirely consist of aragonite the 18o values of aragonite vary from 2 6 to 5 8 v pdb suggesting precipitation in slight disequilibrium with the surrounding pore fluids the 13c values of aragonite between 33 9 and 20 4 v pdb agree with variable amounts of carbonate derived from oxidation of thermogenic methane and crude oil methane was primarily oxidized in an anaerobic process as revealed by the presence of 13c depleted molecular fossils of methane oxidizing archaea 13c values as negative as 118 and sulfate reducing bacteria 13c values as negative as 97 the syntrophic partners in the anaerobic oxidation of methane the observed inventories of molecular fossils in the authigenic carbonates mirror those of known consortia of anaerobic methane oxidizing archaea anme and sulfate reducing bacteria namely the anme 2 desulfosarcina desulfococcus dss and anme 3 desulfobulbus dbb consortia in contrast the same carbonates exhibit shale normalized rare earth elements patterns that all display real negative ce anomalies ce ce 0 78 suggesting that at least temporarily oxic conditions prevailed the episodic occurrence of oxic conditions is confirmed by the presence of molecular fossils of aerobic methanotrophic bacteria including 4 methylcholesta 8 14 24 dien 3 ol and two bacteriohopanepolyols aminotetrol and aminotriol the 13c values of the biomarkers of aerobic methanotrophs are as negative as 58 and are consequently less 13c depleted than the molecular fossils of the prokaryotes performing anaerobic oxidation of methane a pattern in accord with culture experiments overall our results suggest that redox conditions at cold seeps are variable this variability probably reflects changes in seepage flux the combination of an inorganic and an organic geochemical approach used here is promising to better assess the variability and diversity of past fluid and gas expulsion at seeps

methane related authigenic carbonates from the black sea geochemical characterisation and relation to seeping fluids during ttr11 cruise 2001 three areas of active fluid venting and mud volcanism were investigated in the black sea below the oxic zone at depths varying between 800 and 2200 m authigenic carbonates often associated with microbial mats were recovered from the sea floor and the shallow subsurface structural and petrographic observations allowed the distinction of five different types of authigenic carbonates three of these consist of carbonate cemented layered hemipelagic sedimentary units while the other two consist of carbonate cemented mud breccia sediment and authigenic micrite slabs the carbonate cements consist predominantly of micritic mg calcite their d 13 ccaco3 varies between 8 5x and 46 9x at the different sampling locations indicatingthatauthigenic carbonatesincorporate variableproportions ofcarbonderivedfromthe anaerobicoxidationof methane aom the oxidation of organic matter and from sea water methane is the dominant component among other hydrocarbon gases in these sediments its relative amount varies from 99 9 to 95 1 of total hydrocarbon gases and its d 13 c values range fromi40xtoi74x methane in sediments associated with the carbonate crusts shows carbon isotopic values 25 30x lighter than the authigenic carbonates at all the studied sites indicating that methane present in the seeping fluids confers a distinct isotopic signature to the carbonate deposits at each location models proposed for the formation of carbonate slabs in the subsurface imply methane seepage impeded by homogenous clayey laminae or by pre existing slabs coupled with microbial activity oxidising methane and organic matter present in the sediment mud breccia crust pavements on the sea floor form by carbonate cementation of methane charged sediment gas saturation of the sediment is confirmed by the presence of gas hydrates whose shape indicates an association with authigenic carbonates supporting the idea that sedimentary structures can control gas distribution

patterns of carbonate authigenesis at the kouilou pockmarks on the congo deep sea fan abstract different types of seep carbonates were recovered from the kouilou pockmarks on the congo deep sea fan in approximately 3100 m water depth the carbonate aggregates are represented by pyritiferous nodules crusts and slabs tubes and filled molds the latter are interpreted to represent casts of former burrows of bivalves and holothurians the nodules consisting of high mg calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic anme 1 group whereas aragonitic crusts resulted from the activity of archaea of the anme 2 cluster evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts probably reflecting higher porosities of the host sediment during carbonate formation this finding is supported by lower 18 o values of crusts agreeing with precipitation from pore waters similar in composition to seawater the aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate rich pore waters similar to seawater moreover the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that anme 1 archaea tend to occur deeper in the sediment than members of the anme 2 group methane represents the predominant carbon source of all carbonates 13 c values as low as 58 9 v pdb and the encrusted archaeal biomarkers 13 c values as low as 140 v pdb oxygen isotope values of some nodular carbonates ranging from 3 9 to 5 1 v pdb are too high for precipitation in equilibrium with seawater probably reflecting the destabilization of gas hydrates which are particularly abundant at the kouilou pockmarks

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

neogene sedimentation history of the congo fan abstract high resolution seismic reflection data were interpreted to develop a model for the neogene sedimentation in the congo fan area identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the early and the late quaternary a single sediment source system during the late paleogene early neogene gradually changes into a two sediment source system in the late neogene since the late quaternary the congo river has been the prevailing sediment source the evolving benguela current further modified the system and led to a high biogenic productivity evidence for gas in the sediments can be found in the whole area of investigation additionally indications for a tectonic control of the congo canyon have been found in the distribution of sedimentary units and unit thicknesses

in reservoir mixing of mantle derived co2 and metasedimentary ch4 n2 fluids noble gas and stable isotope study of two multistacked fields pannonian basin system w hungary abstract several multistacked gas fields located in very different parts of hungary display a large variation in their gas composition the shallow position of the mantle beneath the basement of the pbs is responsible for the high heat flux it facilitates that the co 2 intensely released from the ascending upper mantle derived melts could reach the basin system in this work the results of a geochemical study of the main components and noble gases of the multistacked repcelak co 2 hc n 2 and mihalyi co 2 fields are presented and discussed the vertical variations of gas composition in the repcelak and mihalyi fields fit well with in reservoir mixing of two end members a mantle related magmatic co 2 fluid and a n 2 rich hc gas fluid of crustal origin however it is very likely that other processes modified to some extent the gas composition of the two fields carbon isotopic composition of the co 2 of the samples studied agrees with both the carbonate decomposition and mantle degassing origins it is very likely that the marked difference in helium isotopic composition between the neighbouring repcelak and mihalyi fields also reflects this heterogeneity

seafloor facies related to upward methane flux within a giant pockmark of the lower congo basin the origin of the cold fluid venting from a giant pockmark within the lower congo basin has been elucidated based upon results of precise mapping submersible dives gravity coring and isotopic analyses realized under a total ifremer cooperative project zaiango and biozaire projects during four dives of the ifremer rov victor 6000 the bottom was filmed hard and soft samples were lifted from the sediment and water samples were collected with a ctd rosette system the detailed dip map shows that the 800 m wide giant pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological mineralogical and chemical features on the seafloor leading to a seafloor anomaly recorded on the multibeam imagery methane rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate rich circulating sea water to produce hydrogen sulphide and bicarbonate ions this situation leads to a concentrical arrangement of the sedimentary facies with methane related features in the centre and sulphide related features at the periphery this organization is correlated with high levels of methane up to 20 mol l measured in the centre of the giant pockmark responsible for the crystallization of gas hydrates at the bottom in this model the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux

fluid sediment interactions at eastern mediterranean mud volcanoes a stable isotope study from odp leg 160 abstract pore fluids from two odp sites at eastern mediterranean mud volcanoes have been analyzed for their cl concentration and their 18o and d isotopic composition the cl data span a wide range of concentrations from extremely depleted with respect to seawater as low as 60 mm at the crest of milano dome site 970 to strongly enriched up to 5 4 m at napoli dome site 971 chloride enrichment is known to be due to dissolving messinian evaporites whereas the source of the low cl fluid is deduced from stable isotope data presented here the isotopic composition of the endmember fluid is found to be 10 for 18o and 32 for d for low as well as for high cl waters from this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water several other processes altering the 18o d composition of pore waters are discussed and considered to be of only negligible influence the process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration mainly smectite illite transformation corresponding to a depth range of 3 5 7 km and an elevated temperature of about 120 165 c a quantitative estimate shows that this reaction is capable of producing the observed extreme cl depletion

diffusive methane seepage in ancient deposits examples from the neogene shin etsu sedimentary basin central japan abstract along continental margins various types of methane seeps with a variety of flux rates spatial extents and durations occur although numerous studies have investigated spatially extensive high flux rate seeps seeps with ephemeral and or weak flows known as diffusive seeps have received less attention this study investigates ancient diffusive methane seeps in the shin etsu sedimentary basin central japan aiming to deduce their spatial extent and their sedimentological paleontological and geochemical properties the seeps were identified as centimeter scale carbonate concretions in two outcrops of neogene siltstones in the basin a comprehensive examination of the spatial distribution of the carbonates their associated molluscan fossils petrography carbon and oxygen isotopic compositions and lipid biomarker contents of the concretions revealed that they originated from the anaerobic oxidation of methane aom and formed via a weak diffusive fluid flow the small size and irregular or burrow shaped morphologies of the concretions indicate that methane bearing fluids diffused through small pore spaces such as burrows associated molluscan fossils consist of vesicomyids and other bivalves typical of methane seeps fossils of predator species were also identified but epifaunal seep organisms are notably absent the carbonate concretions are composed of strongly 13c depleted micrite 13c values as low as 46 0 vs vpdb with minor 13c rich void filling sparry cements the presence of lipid biomarkers pentamethylicosane pmi with 13c values as low as 119 and biphytane and the absence of crocetane suggests that the aom was performed by an anme 1 dominated archaeal community the insights of diffusive methane seepage in the geological record gained from this study can help to better understand depositional processes of seep deposits and variations in fluid flows in both ancient and modern methane seeping systems

the archaeal lipid composition of partially lithified cold seep mats the anaerobic oxidation of methane aom by converting methane to bicarbonate which is then precipitated as extensive carbonate crusts is an important methane sink in the earth s ocean systems here we employ a multidisciplinary approach to investigate the role of microorganisms in carbonate precipitation using biomarker analysis scanning electron microscopy sem and x ray diffraction we examined two microbial mats from the black sea and found that one comprised carbonate in both aragonite and mg calcite forms and most likely anme 1 archaea whereas the other contained only mg calcite and most likely anme 2 archaea we conclude as have others that the different microbial communities could impart different influences on carbonate mineralogy and morphology although further research is needed this is a contribution to our understanding of those relationships which could prove critical in the interpretation of ancient sedimentary deposits

the chimney forest of the deep montenegrin margin south eastern adriatic sea a spectacular field of columnar carbonates has been discovered on the montenegrin margin in the southern adriatic sea at a depth of about 450 m the site exposes many columnar carbonates protruding from the substrate or abated on the bottom such carbonates attain maximum visible lengths of ca 60 cm with diameters up to 20 cm display an annular growth and are either hollow or plugged by indurated sediment petrographic and geochemical analyses document the pervasive presence of dolomite and 13c values as low as 30 vpdb these chimneys are therefore interpreted as former conduits related to hydrocarbon expulsion in this sector of the adriatic basin however available data suggest that hydrocarbon flows at this site have ceased our results show that chimneys formed inside the local depositional units glacial pleistocene shelf shelly rich muddy sediment and were successively exhumed from the host sediment today the chimneys offer substrate to benthic life including cold water corals and sponges the u series dating of these carbonate concretions is complicated by the presence of a significant fraction of detrital sediment which represents a major source of initial 230th ams 14c and 87sr 86sr dating of shells embedded in one of the chimney provided ages beyond the range of radiocarbon dating and 600 kyrs indicating a pleistocene age of the host sediment uncorrected u series dating of the carbonate chimneys yielded and age of ca 250 270 kyrs providing a temporal upper limit for conduit formation driven by lithification caused by the degradation of seeping hydrocarbons in addition u th dating of cold water coral bases settled on chimneys indicates a holocene age for their first exposure after exhumation and subsequent function as hard substrate for benthic organisms the montenegrin chimney forest is a rare case where many such carbonate columnar concretions are still in their original vertical position

u th dating of cold seep carbonates an initial comparison authigenic carbonates from hydrocarbon seeps are unique archives of past seepage and associated environmental parameters in order to constrain the ages of a set of seep carbonates and the time involved in carbonate formation we applied uranium thorium u th dating to samples from the gulf of mexico the congo fan and the black sea the resulting u th ages indicate that environmental conditions must have been favorable for enhanced methane rich fluid seepage during the time intervals of 53 4 to 1 7 ka bp for the gulf of mexico and 45 5 to 3 0 ka bp for the congo fan the seep carbonates from the black sea formed at 1 6 to 1 1 ka bp our results suggest that enhanced fluid flow during these time intervals was closely related to 1 sea level variations associated with glacial interglacial cycles and 2 environmental alterations in the course of late quaternary climate change including variations in bottom water temperatures that affected the stability of gas hydrate reservoirs

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

seep deposits from northern istria croatia a first glimpse into the eocene seep fauna of the tethys region three isolated limestone deposits and their fauna are described from a middle eocene flysch succession in northwestern istria croatia the limestones are identified as ancient methane seep deposits based on fabrics and characteristic mineral phases 13 c carbonate values as low as 42 2 and 13 c depleted lipid biomarkers indicative of methane oxidizing archaea the faint bedding of the largest seep deposit the great dominance of authigenic micrite over early diagenetic fibrous cement as well as biomarker patterns indicate that seepage was diffusive rather than advective apart from methanotrophic archaea aerobic methanotrophic bacteria were present at the eocene seeps as revealed by 13 c depleted lanostanes and hopanoids the observed corrosion surfaces in the limestones probably reflect carbonate dissolution caused by aerobic methanotrophy the macrofauna consists mainly of chemosymbiotic bivalves such as solemyids acharax thyasirids thyasira and lucinids amanocina the middle eocene marks the rise of the modern seep fauna but so far the fossil record of seeps of this age is restricted to the north pacific region the taxa found at buje originated during the cretaceous period whereas taxa typical of the modern seep fauna such as bathymodiolin mussels and vesicomyid clams are absent although this is only a first palaeontological glimpse into the biogeography during the rise of the modern seep fauna it agrees with biogeographic investigations based on the modern vent fauna indicating that the dominant taxa of the modern seep fauna first appeared in the pacific ocean

late cretaceous maastrichtian shallow water hydrocarbon seeps from snow hill and seymour islands james ross basin antarctica fossil hydrocarbon seeps are present in latest cretaceous maastrichtian volcaniclastic shallow shelf sediments exposed on snow hill and seymour islands james ross basin antarctica the seeps occur in the snow hill island formation on snow hill island and are manifest as large sized cement rich carbonate bodies containing abundant thyasirid bivalves and rarer ammonites and solemyid bivalves these bodies have typical seep cement phases with 13c values between 20 4 and 10 7 and contain molecular fossils indicative of terrigenous organic material and the micro organisms involved in the anaerobic oxidation of methane including methanotrophic archaea and sulphate reducing bacteria on seymour island the seeps occur as micrite cemented burrow systems in the lopez de bertodano formation and are associated with thyasirid solemyid and lucinid bivalves and background molluscan taxa the cemented burrows also have typical seep cement phases with 13c values between 58 0 and 24 6 there is evidence from other data that hydrocarbon seepage was a common feature in the james ross basin throughout the maastrichtian and into the eocene the snow hill and seymour island examples comprise the third known area of maastrichtian hydrocarbon seepage but compared to most other ancient and modern seep communities the james ross basin seep fauna is of very low diversity being dominated by infaunal bivalves all of which probably had thiotrophic chemosymbionts but which were unlikely to have been seep obligates absent from the james ross basin seep fauna are typical obligate seep taxa from the cretaceous and the cenozoic reasons for this may have been temporal palaeolatitudinal palaeobathymetric or palaeoecological

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

formation of carbonate pipes in the northern okinawa trough linked to strong sulfate exhaustion and iron supply abstract the microbial anaerobic oxidation of methane aom a key biogeochemical process that consumes substantial amounts of methane produced in seafloor sediments can lead to the formation of carbonate deposits at or beneath the sea floor although fe oxide driven aom has been identified in cold seep sediments the exact mode by which it may influence the formation of carbonate deposits remains poorly understood here we characterize the morphology petrology and geochemistry of a methane derived fe rich carbonate pipe in the northern okinawa trough ot we detect abundant authigenic pyrites as well as widespread trace fe within microbial mat like carbonate veins in the pipe the in situ 34 s values of these pyrites range from 3 9 to 31 6 vcdt suggesting a strong consumption of seawater sulfate by sulfate driven aom at the bottom of sulfate reduction zone the positive 56 fe values of pyrite and notable enrichment of fe in the ot pipe concurrently indicate that the pyrites are primarily derived from fe oxides in deep sediments we propose that the fe rich carbonate pipe formed at the bottom of sulfate reduction zone below which fe driven aom rather than fe oxide reduction coupled to organic matter degradation might be responsible for the abundantly available fe 2 in the fluids from which pyrites precipitated the fe rich carbonate pipe described in this study probably represents the first fossil example of carbonate deposits linked to fe driven aom because fe rich carbonate deposits have also been found at other cold seeps worldwide we infer that similar processes may play an essential role in biogeochemical cycling of sub seafloor methane and fe at continental margins

lipid biomarker patterns reflect different formation environments of mussel and tubeworm dominated seep carbonates from the gulf of mexico atwater valley and green canyon abstract mussels and tubeworms thriving at many methane seeps typically live in symbiosis with chemosynthetic chiefly methanotrophic or thiotrophic bacteria it has been shown that the activities of chemosymbiotic animals can result in large differences in the sedimentary environments of their habitats here we put forward the concept that such environmental variability can be archived in the lipid biomarker inventories of authigenic carbonates forming in different locally confined environments at seeps mussel beds and tubeworm bushes in this case to test this hypothesis lipid biomarker patterns of carbonates from mussel and tubeworm environments from two seep sites atwater valley 340 and green canyon 852 of the gulf of mexico were analyzed previous work revealed stronger carbon isotope fractionation between the methane source and biomarkers of anaerobic methane oxidizing archaea 2 anme 2 sulfate reducing desulfosarcina desulfococcus dss consortia than for anme 1 dss consortia both performing anaerobic oxidation of methane aom similar 13cmethane values were found at the mussel and tubeworm sites from the same seeps and the local microbial consortia also appear to be largely similar based on the observed aom biomarker inventories yet a large average offset of 32 between the 13c values of molecular fossils of sulfate reducing bacteria srb involved in aom was observed with lower values typifying tubeworm carbonates than mussel carbonates this pattern is interpreted to reflect local effects on isotope fractionation caused by the chemosymbiotic metazoans at mussel and tubeworm dominated sites at tubeworm dominated sites the excess sulfate produced by thiotrophic symbionts of tubeworms and pumped down into the sediment results in persistent production of aom derived bicarbonate and the enrichment of 12c in sub surface sediments interestingly tubeworm carbonates also contain high amounts of non isoprenoidal dialkyl glycerol diethers dages with extreme 13c depletions representing compounds that derived from non dss cluster srb most likely 13c depleted aom derived organic intermediates were used as carbon sources by the dage producing non dss cluster srb possibly performing organoclastic sulfate reduction our study identifies significant variation in biomarker patterns between mussel and tubeworm carbonates at two seep sites in the gulf of mexico such variation allows to characterize different habitats at seeps which are shaped by the interaction of chemosymbiotic seep metazoans and their symbionts with the local environment metazoan community composition apparently controls geobiological interaction in seep ecosystems to a large degree which may allow tracing of the effects of chemosymbiosis into the rock record

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

authigenic carbonates from seeps on the northern continental slope of the south china sea new insights into fluid sources and geochronology seep carbonates represent excellent archives of seepage activities near seafloors in order to provide insights on sources of seeping fluids stable carbon and oxygen isotopes and sr 87 sr 86 ratios were applied to seep carbonates from shenhu sw dongsha and ne dongsha on the northern continental slope of the south china sea u th dating was selected to constrain the time involved in carbonate formation and the potential driving force the delta c 13 values of the shenhu and ne dongsha carbonates vary from 52 3 parts per thousand to 32 6 parts per thousand v pdb indicating that biogenic methane is the primary carbon source at both locations the delta c 13 values of the sw dongsha carbonates are much higher ranging from 18 8 parts per thousand to 11 parts per thousand v pdb suggesting that thermogenic methane is the predominant carbon source in this region a relatively deep origin of thermogenic methane for the sw dongsha seeps is also supported by somewhat lower sr 87 sr 86 ratios of the samples from 0 709025 to 0 709097 the delta o 18 values of the shenhu and ne dongsha carbonates vary from 1 4 to 5 1 v pdb while the delta o 18 values of the sw dongsha carbonates range from 2 5 to 3 4 v pdb many samples from shenhu and ne dongsha demonstrate an o 18 enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate u th ages of the shenhu carbonates show a large range from 330 ka to 152 ka bp and the ne dongsha carbonates have ages from 77 ka to 63 ka bp interestingly u th ages of most samples are either within times of sea level lowstands or correspond to periods when the sea level was falling taken together our data suggest that in the south china sea gas hydrate dissociation during sea level lowstands or falling stages resulted from reduced hydrastastic pressures which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor c 2013 elsevier ltd all rights reserved

a new constraint on the antiquity of anaerobic oxidation of methane late pennsylvanian seep limestones from southern namibia late pennsylvanian seep limestones ca 300 ma enclosed in the ganigobis shales in southern namibia formed by microbial activity the process that induced carbonate precipitation was the anaerobic oxidation of methane the presence of 13 c depleted pentamethylicosane pmi 113 and a mixture of crocetane and phytane 112 in concert with similarly 13 c depleted pseudohomologous series of regular isoprenoids reveals that methanotrophic archaea oxidized methane anaerobically at the ancient seep site biphytane and a c 39 pseudohomologue are other archaeal molecular fossils with 13 c values of 99 and 97 respectively the former presence of sulfate reducing bacteria as the syntrophic partners of methanotrophic archaea in the anaerobic oxidation of methane is indicated by isotopically depleted iso and anteiso alkanes these compounds most probably derive from non isoprenoidal monoethers and diethers synthates of sulfate reducing bacteria these findings show that anaerobic oxidation of methane is at least 300 m y old extending the record of this process for 140 m y as the molecular fossils of archaea and bacteria are preserved in a product of their own metabolic activity i e methane derived carbonates with 13 c values as low as 51 the syngenicity of molecular fossils and enclosing deposits is unambiguous this reveals that microbially formed rocks can represent excellent archives for studying past biogeochemical processes

palaeo methane seepage history traced by biomarker patterns in a carbonate crust nile deep sea fan eastern mediterranean sea abstract we investigated lipid biomarkers in three depth intervals of a methane derived carbonate crust collected from a pockmark in the nile deep sea fan sampled during the nautinil cruise of the r v l atalante a diverse set of 13 c depleted archaeal and bacterial lipids were detected along this crust with 13 c values as low as 118 this provides evidence that anaerobic oxidation of methane was the main process that induced the formation of this carbonate crust archaeal lipid biomarker distributions revealed the presence of archaea belonging both to the anme 1 and anme 2 phylogenetic clusters with the former predominating the distribution and compound specific 13 c values of aom derived lipid biomarkers showed no significant variations with depth these results infer that the archaeal community structure and thus the local seepage environment remained relatively stable during the 5000 kyr period of the carbonate crust precipitation

archaeal and bacterial lipids in authigenic carbonate crusts from eastern mediterranean mud volcanoes abstract we investigated the distributions and 13 c values of lipid biomarkers in authigenic methane related carbonate crusts formed on eastern mediterranean mud volcanoes a highly diverse suite of microbial membrane lipids was detected strongly 13 c depleted lipids diagnostic for archaea include isoprenoid glycerol diethers isoprenoid glycerol dialkyl glycerol tetraethers and several c 20 c 35 isoprenoid hydrocarbons bacterial lipids mainly consist of dialkyl glycerol diethers branched fatty acids and hopanoids showing important 13 c depletions albeit less than archaeal lipids the data provide further evidence for the involvement of archaea and bacteria in the anaerobic oxidation of methane aom and in the sequestration of methane derived carbon in authigenic carbonate crusts varying distributions and isotopic compositions of 13 c depleted lipids argue for heterogeneous aom related microbial consortia expanding on existing work at cold seeps distinctive biomarker features and inferred microbial assemblages are observed in these crusts in agreement with available phylogenetic data for the samples our results support the potential of lipids for distinguishing methanotrophic communities despite the observed variances conservative features in the abundances and isotopic offsets of microbial biomarkers occur likely reflecting the overall relationships between archaea and bacteria and the nature of carbon flow between them

carbonate formation by anaerobic oxidation of methane evidence from lipid biomarker and fossil 16s rdna carbonate chimneys and other carbonate structures occur widespread in the gulf of cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past possibly in the early pleistocene but it is unclear under what conditions and by which processes these carbonates were formed we studied a fossil methane related carbonate crust collected from the kidd mud volcano in the gulf concentrations of microbial lipids their stable carbon isotope composition sequences of fossil 16s rrna genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust this combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction archaeal lipid biomarkers and fossil 16s rrna gene sequence data revealed the dominance of archaeal anme 2 group and elevated methane partial pressures during the formation of the top part of the crust the lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of anme 1 archaea the combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo biogeochemical processes resulting in the formation of carbonate fabrics this interdisciplinary strategy may also be applied for other fossil methane derived carbonates generating new concepts and knowledge about past methane related carbonate systems

lipid biomarker patterns of authigenic carbonates reveal fluid composition and seepage intensity at haima cold seeps south china sea abstract authigenic carbonates retrieved from sites rov1 and rov2 of the haima hydrocarbon seeps of the south china sea at approximately 1390 m water depth were studied using lipid biomarker analyses abundant molecular fossils of anaerobic methane oxidizing archaea anme and sulfate reducing bacteria srb with strong 13c depletions 13c values as low as 126 in combination with low 13ccarbonate values 42 7 to 36 8 provide evidence that anaerobic oxidation of methane aom was the major process driving the precipitation of the studied seep carbonates the extremely low 13c values of archaeal biomarkers confirm that biogenic methane was the main carbon source but the seepage of accessory crude oil is also suggested by the presence of unresolved complex mixtures in the hydrocarbon fractions a suite of 13c depleted biomarkers indicate the predominance of anme 1 dss consortia at both sites which indicates that the studied carbonates formed during low to medium methane flux somewhat higher contents of archaeal biomarkers and their stronger 13c depletion at site rov2 probably reflect at least temporarily higher seepage intensities than at site rov1 abundant bacterial dialkyl glycerol diethers dages revealing a large offset of their 13c values compared to srb derived terminally branched fatty acids were possibly produced by distinct srb species other than members of the dss cluster the encountered hopanoids are attributed to aerobic methanotrophic bacteria based on their moderate 13c depletion the application of molecular fossils in combination with their compound specific isotope signatures is an efficient tool to reconstruct the composition of seepage fluids and seepage intensities

lipid geochemistry of methane seep related black sea carbonates abstract carbonates recovered from anoxic waters between 235 and 1555 m depth in the northwestern black sea were analyzed for lipid biomarkers and stable carbon isotopic compositions in addition a methane seep related microbial mat and a sample of surface sediment recovered from a non seep site were studied for comparison high concentrations of strongly 13 c depleted lipids attributed to bacteria and archaea mediating the anaerobic oxidation of methane aom were found in all samples except for the sediment differences of the dominant aom performing communities between the carbonates indicated by specific lipid patterns appear to be caused by the respective biogeochemical settings high proportions of anme 2 consortia are found at sites of assumingly high partial pressures of methane while anme 1 associations dominate at locations of moderate methane supply in the sedimentary concretion a complex mixture of biomarkers for terrestrial and planktonic organisms was found different molecular structures along with strong variations in the stable carbon isotopic compositions 13 c 20 2 to 94 3 allow for an estimation of the proportions of tetraether bound biphytanes derived from planktonic crenarchaeota and methanotrophic euryarchaeota our data imply that the shape of aom derived carbonate precipitates in black sea environments is crucially influenced by the respective methane supply active aom driven chimney like bioherms similar to those previously observed on the ukrainian shelf might also develop in the deep euxinic zone at 1555 m water depths

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

archaea mediating anaerobic methane oxidation in deep sea sediments at cold seeps of the eastern aleutian subduction zone cold seeps in the aleutian deep sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation this process is active in the anaerobic sediments at the sulfate reduction methane production boundary and is probably performed by archaea working in syntrophic co operation with sulfate reducing bacteria diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2 6 11 15 tetramethylhexadecane crocetane and 2 6 10 15 19 pentamethylicosane pmi as well as the glycerol ether lipid archaeol 2 3 di o phytanyl sn glycerol these biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates carbon isotopic compositions of the biomarkers are strongly depleted in 13c with values of 13c as low as 130 3 pdb the process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with 13c values of 39 2 and 41 8 and of the carbonate precipitates with values of 45 4 and 48 7 this suggests that methane oxidizing archaea have accumulated within the microbial community which is active at the cold seep sites the dominance of crocetane in sediments at one station indicates that probably due to decreased methane venting archaea might no longer be growing whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane the assemblages are apparently dependent on specific conditions found at each cold seep environment selective conditions probably include water depth temperature degree of anoxia and supply of free methane

distribution morphology and formation of pockmarks in lake constance germany pockmarks are well known in the marine environment but almost unreported from limnic systems in lake constance we investigated pockmarks near the mouth of the main tributary alpine rhine river using sediment cores and hydroacoustics sidescan sonar subbottom profiler horizontal scanning sonar multibeam echo sounder the maximum dimension of an individual pockmark is about 16 m in diameter and 3 4 m deep often with a round shape most pockmarks are located on top of morphological high points such as sediment waves or shoulders of old channels in front of the alpine rhine river they are collapse structures formed when a reservoir of biogenic methane rapidly empties into the water column ebullition of gas very often was observed indicating that biogenic methane still migrates toward pockmarks

biomarker and 16s rdna evidence for anaerobic oxidation of methane and related carbonate precipitation in deep sea mud volcanoes of the sorokin trough black sea many mud volcanoes were recently discovered in the euxinic bottom waters of the sorokin trough ne black sea three of them i e nioz odessa and kazakov were selected for a detailed biogeochemical investigation four methane related carbonate crusts covered with microbial mats and sediments dmud brecciat from these mud volcanoes were collected during the 11th training through research cruise ttr 11 in 2001 the first finding of methanotrophic microbial mats associated with authigenic carbonates in the deep black sea we measured the concentrations and 13c values of methane and specific archaeal and bacterial lipids and determined archaeal and bacterial 16s rrna gene sequences the 13c of the microbial lipids reflected the carbon isotopic values of the methane indicating that methane was the main carbon source for microorganisms inducing carbonate formation anaerobic oxidation of methane aom in these settings was performed by archaea affiliated with the so called anme 1 group none of the identified archaeal sequences were closely related to known methanogens the combined 16s rrna gene sequence and biomarker data revealed a distinct difference in archaeal assemblage between the carbonate crusts and mud breccias besides gene sequences of sulfate reducing bacteria dna analysis of bacterial communities revealed a diversity of bacteria with apparent contrasting metabolic properties the methane utilization via aom processes was detected in the uppermost sediments where it subsequently induces authigenic carbonate precipitation most probably below seafloor the results of integrated biomarker and 16s rrna gene study reveal a crucial role of aom processes in formation of authigenic carbonates in methane seep environments

a methane derived carbonate build up at a cold seep on the crimean slope north western black sea abstract a unique chimney shaped carbonate build up was produced by microbially mediated anaerobic oxidation of methane at a deep sea cold seep the build up was sampled from 1600 m water depth in the area of the lomonosov rise nw black sea crimean shelf slope the carbonate chimney grew free into the anoxic water column with its base attached to a steep slope composed of plagiogranite and void of sediments the perfectly preserved 1 5 m high chimney stores reliable records of the diversity of mineralogy geochemistry and stable isotope composition of deep sea methane related carbonates never studied before the build up consists of mg calcite mgco 3 9 13 mol with minor aragonite the carbonate matrix encloses organic matter and emiliania huxleyi coccoliths as well as minor framboidal pyrite gypsum barite and diatomite fragments the contribution of detrital silicate material is negligible micritic mg calcite in the inner zone of the chimney forms obtuse rhombohedrons clustered in hemispherical aggregates clots and hosts isolated prismatic aragonite 1 vol in the outer zone mg calcite exists as foliated crystals and spherulites while aragonite spherulites are restricted to the top surface the carbon isotope compositions of carbonates 13 46 5 to 33 0 vpdb and remnant bacterial mats 13 76 9 to 81 6 vpdb provide evidence of a biogenic methane source for the build up formation oxygen came mainly from sea water judging by a narrow range of 18 o values in carbonates 0 2 to 1 3 vpdb the prevalence of mg calcite throughout the build up indicates an anme 2 sbr consortium being the main component of the bacterial mats living at the seep aragonite grew mostly late in the seep history which hints to environmental changes and the ensuing predominance of the anme 1 sbr consortium the observed relationship between the prokaryotic communities and the main carbonate phases is common to the microbial mats of the black sea but is actually opposed to the pattern of aom community and aom related carbonates found in sediments different morphologies of carbonates in the outer and inner zones of the build up point to variations in the methane flux gradient which controlled the biomass density of the aom consortia the ree y patterns of mg calcite from the inner zone are typical of carbonates crystallised in a freshened sea water environment while both mg calcite and aragonite from the outer zone show distinct sea water signatures inasmuch as the time span of build up growth radiocarbon ages from 9000 180 to 7500 180 year bp overlaps the period when the black sea lake reconnected to the mediterranean sea the progressive arrival of sulphate rich waters can be inferred to be another possible control of ree y patterns in seep carbonates besides the methane flux gradient and related activity of aom consortia

3d morphology and timing of the giant fossil pockmark of beauvoisin se basin of france in the se basin of france drome fossil cold seep structures made of fossil rich carbonate lenses were identified in the oxfordian late jurassic terres noires formation about 30 years ago to date all seep sites have generally been studied individually without consideration of any link to neighboring or more distant sites based on a detailed fieldwork in the beauvoisin area and comparison with the modern active giant regab pockmark and sandbox experiments we suggest that this site can now be considered as a 800 m wide fossil analogue of a giant pockmark it comprises several 4 6 m deep 80 120 m wide coalesced sub seep sites a spatio temporal 3d reconstruction of the position of these sub sites shows that the carbonate lenses are organized in clusters with only one cluster active at a given period two periods of seep activity are separated by a period of quiescence due to the lateral shift of the feeder conduit beneath the structure this suggests that even if a seafloor pockmark appears to be inactive this could be due to the lateral shift of the feeder conduit meaning that the sub seafloor is still charged in gas

methane seepage intensities traced by biomarker patterns in authigenic carbonates from the south china sea authigenic carbonate rocks from an active seep site f located in the south china sea scs at a water depth of 1120 m were studied using mineralogical and lipid biomarker analyses carbonate mineral compositions in specific samples were predominantly aragonite high mg calcite hmc or a mixture of both abundant c 13 depleted lipid biomarkers diagnostic for archaea provide evidence that anaerobic oxidation of methane aom mediated by anaerobic methane oxidizing archaea anme and their bacterial partners are the major processes leading to formation of the carbonates high methane index dominance of anme 2 assemblages and the predominant contribution of aom related biomarkers in the aragonitic carbonate point to the strong impact of aom communities during its precipitation which is likely caused by high seepage intensities the relatively low methane index and abundant allochthonous input of the hmc carbonate are a result of the weak impact of aom microbial communities under conditions of low methane seepage evidence for this interpretation also comes from relatively higher carbonate content lesser contribution of allochthonous organic matter and lower pyrite content in the aragonitic carbonate than in the hmc carbonate high concentrations of c 13 depleted hopanoids were observed in the aragonitic sample which may be a result of the intermittent presence of oxic conditions in an overall anoxic condition possibly induced by changing seepage intensities our data suggests that lipid biomarkers in authigenic carbonate from seep sites can be helpful for improving assessments of the fluid dynamics in such a complex system c 2015 elsevier ltd all rights reserved

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

u th chronology and formation controls of methane derived authigenic carbonates from the hola trough seep area northern norway abstract we investigated methane derived authigenic carbonate mdac crusts and nodules from a cold seep site on the northern norwegian continental shelf in ca 220 m water depth to determine the timing and mode of their formation gas bubbling observed during remotely operated vehicle rov assisted sampling of mdac crusts revealed ongoing seep activity authigenic carbonates were present as crusts on the seafloor and as centimetre size carbonate cemented nodules at several intervals within an adjacent sediment core aragonite dominated mineralogy of the mdac crusts suggests formation close to the seafloor at higher rates of sulphate dependent anaerobic oxidation of methane aom in contrast dolomite cemented nodules are consistent with the formation at the sulphate methane transition zone deeper within the sediment at lower rates of aom the 13c carbonate values of bulk rock and of micro drilled aragonite samples vary between 22 2 and 34 6 vpdb we interpret the carbon in aragonite to be mainly derived from the anaerobic oxidation of thermogenic methane with a minor contribution from seawater dissolved inorganic carbon dic aom activity is supported by high concentrations of aom related biomarkers of archaea archaeol and 2 sn hydroxyarchaeol and sulphate reducing bacteria iso and anteiso c15 0 fatty acids in the crusts the dolomite nodules exhibit higher 13c carbonate values 12 vpdb suggesting a smaller amount of methane derived carbon presumably due to the contribution of dic migrating from depth and lower aom rates the latter is supported by orders of magnitude lower concentrations of archaeol and sn 2 hydroxyarchaeol in the sediment interval containing the largest dolomite nodules 18o values of pure aragonite samples and dolomite nodules indicate the precipitation of carbonate close to isotopic equilibrium with seawater and no influence of gas hydrate derived water u th dating of two mdac crusts shows that they formed between 1 61 0 02 and 4 39 1 63 ka bp and between 2 65 0 02 and 4 32 0 08 ka bp we infer both a spatial and temporal change in methane flux and related mdac formation at this seep site these changes might be caused by regional seismic events that can affect pore pressure or re activation of migration pathways thus facilitating fluid flow from deep sources towards the seabed

pockmarks in the northern congo fan area sw africa complex seafloor features shaped by fluid flow abstract three pockmarks named hydrate hole black hole and worm hole were studied in the northern congo fan area at water depths around 3100 m the cross disciplinary investigations include seafloor observations by tv sled sampling by tv guided grab and multicorer as well as gravity coring in addition to hydroacoustic mapping by a swath system a parametric sediment echosounder and a deep towed sidescan sonar the pockmarks are morphologically complex features consisting of one or more up to 1000 m wide and 10 15 m deep depressions revealed by swath mapping high reflection amplitudes in the sediment echosounder records indicate the presence of a 25 30 m thick shallow sediment section with gas hydrates which have been recovered by gravity corer hydrates chemosynthetic communities and authigenic carbonates clearly indicate fluid flow from depths which we propose to be mainly in the form of ascending gas bubbles rather than advection of methane rich porewater evidence for seepage at the seafloor is confined to small areas within the seafloor depressions and was revealed by characteristic backscatter facies small meter scale sized depressions signified as pits exist in or close to the pockmarks but seafloor observations did not reveal evidence for the presence of typical seep organisms or authigenic carbonates areas of intermediate backscatter were inhabited by vesicomyid clams in soft sediments high backscatter was associated with vestimentiferan tubeworms siboglinidae and authigenic carbonates we discuss the three different environments pits vesicomyid clams vestimentifera carbonate in the light of differences in the geochemical setting pits are probably formed by escaping gas bubbles but seepage is too transient to sustain chemosynthetic life vesicomyid clams are present in sediments with gas hydrate deposits however the hydrates occur several meters below the surface indicating a lower flux compared to the vestimentifera carbonate environment in the latter environment accumulated carbonates and clam shells indicate that fine grained particles have been eroded away gas hydrates were found in this environment at depths below about 50 cm suggesting the highest supply with methane compared to the other environments

molecular and isotopic analysis of anaerobic methane oxidizing communities in marine sediments convergent lines of molecular carbon isotopic and phylogenetic evidence have previously indicated hinrichs k u hayes j m sylva s p brewer p g delong e f 1999 methane consuming archaebacteria in marine sediments nature 398 802 805 that archaea are involved in the anaerobic oxidation of methane in sediments from the eel river basin offshore northern california now further studies of those same sediments and of sediments from a methane seep in the santa barbara basin have confirmed and extended those results mass spectrometric and chromatographic analyses of an authentic standard of sn 2 hydroxyarchaeol hydroxylated at c 3 in the sn 2 phytanyl moiety have confirmed our previous tentative identification of this compound but shown that the previously examined product was the mono tms rather than di tms derivative further analyses of 13 c depleted lipids appreciably more abundant in samples from the santa barbara basin have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13 c these are additional glycerol ethers and fatty acids the alkyl substituents in the ethers mostly monoethers with some diethers are non isoprenoidal the carbon number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar suggesting strongly that they are produced by the same organisms their structures n alkyl and methyl branched n alkyl require a bacterial rather than archaeal source the non isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic sulfate and nitrate reducing organisms which lie near the base of the rrna based phylogenetic tree based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate it appears likely that the non archaeal 13 c depleted lipids are products of one or more previously unknown sulfate reducing bacteria which grow syntrophically with the methane utilizing archaea their products account for 50 of the fatty acids in the sample from the santa barbara basin at all methane seep sites examined the preservation of aquatic products is apparently enhanced because the methane oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column

shallow water methane derived authigenic carbonate mounds at the codling fault zone western irish sea abstract methane derived authigenic carbonate mdac mound features at the codling fault zone cfz located in shallow waters 50 120 m of the western irish sea were investigated and provide a comparison to deep sea mdac settings carbonates consisted of aragonite as the major mineral phase with 13 c depletion to 50 and 18 o enrichment to 2 these isotope signatures together with the co precipitation of framboidal pyrite confirm that anaerobic oxidation of methane aom is an important process mediating methane release to the water column and the atmosphere in this region 18 o enrichment could be a result of mdac precipitation with seawater in colder than present day conditions or precipitation with 18 o enriched water transported from deep petroleum sources the 13 c depletion of bulk carbonate and sampled gas 70 suggests a biogenic source but significant mixing of thermogenic gas and depletion of the original isotope signature cannot be ruled out active seepage was recorded from one mound and together with extensive areas of reduced sediment confirms that seepage is ongoing the mounds appear to be composed of stacked pavements that are largely covered by sand and extensively eroded the cfz mounds are colonized by abundant sabellaria polychaetes and possible nemertesia hydroids which benefit indirectly from available hard substrate in contrast to deep sea mdac settings where seep related macrofauna are commonly reported seep specialist fauna appear to be lacking at the cfz in addition unlike mdac in deep waters where organic carbon input from photosynthesis is limited lipid biomarkers and isotope signatures related to marine planktonic production e g sterols alkanols were most abundant evidence for microbes involved in aom was limited from samples taken possibly due to this dilution effect from organic matter derived from the photic zone and will require further investigation