the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

archaea mediate anaerobic oxidation of methane in deep euxinic waters of the black sea we evaluate anaerobic oxidation of methane aom in the black sea water column by determining distributions of archaea specific glyceryl dialkyl glyceryl tetraethers gdgts and 13c isotopic compositions of their constituent biphytanes in suspended particulate matter spm sinking particulate matter collected in sediment traps and surface sediments we also determined isotopic compositions of fatty acids specific to sulfate reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate reducing bacteria in carrying out aom bicyclic and tricyclic gdgts and their constituent 13c depleted monocyclic and bicyclic biphytanes down to 67 indicative of archaea involved in aom were present in spm in the anoxic zone below 700 m depth in contrast gdgt 0 and crenarchaeol derived from planktonic crenarchaeota dominated the gdgt distributions in the oxic surface and shallow anoxic waters fatty acids indicative of sulfate reducing bacteria i e iso and anteiso c15 were not strongly isotopically depleted e g 32 to 25 although anteiso c15 was 5 more depleted in 13c than iso c15 our results suggest that either aom is carried out by archaea independent of sulfate reducing bacteria or those sulfate reducing bacteria involved in a syntrophy with methane oxidizing archaea constitute a small enough fraction of the total sulfate reducing bacterial community that an isotope depletion in their fatty acids is not readily detected sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upper water column archaea aom specific gdgts and 13c depleted biphytanes characteristic of the spm in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments this observation suggests that sediments may not always record aom in overlying euxinic water columns and helps explain the absence of aom derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean

hydrocarbon sources of cold seeps off the vesteralen coast northern norway abstract we investigated active methane seeps in a water depth of 200 m in the hola area off the coast of vesteralen northern norway to assess 1 hydrocarbon sources 2 migration pathways and 3 the influence of hydrocarbon seepage on sediment pore water and water column chemistry the seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nm ca 5 m above the seafloor pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate methane transition zone smtz between 80 cm and 250 cm indicating spatially heterogeneous methane ascent the isotopic composition of methane 13 c from 40 to 63 and 2 h from 191 to 225 and 13 c depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin consistent with 13 c values of c 2 to c 4 hydrocarbons isotope data also indicate considerable biodegradation of propane seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying mesozoic sedimentary rocks we propose that these act as migration pathways for the hydrocarbons from late jurassic to early cretaceous source rocks

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

a seafloor observatory to monitor gas hydrates in the gulf of mexico gas hydrates are solid structures composed of gas molecules encased in cages of water molecules most interesting to the energy community are hydrates that contain hydrocarbon gases in the northern gulf of mexico hydrates of this type form in water depths greater than 450 m and often occur in mounds located where faults intersect the seafloor typically these hydrates consist of seawater from which the salt has been excluded and gases that have migrated up faults from buried hydrocarbon reservoirs in addition to hydrates the mounds contain large amounts of calcium carbonate and various other minerals precipitated by microbes that extract energy from hydrocarbon fluids

the mar vema fracture zone intersection surveyed by deep submersible nautile one of the two objectives of the vemanaute cruise of the french deep submersible nautile was the geological study of the eastern intersection area between the mid atlantic ridge mar and the vema fracture zone in the equatorial atlantic fourteen dives were conducted that allowed detailed geological survey and sampling of the main morphostructural units of this area the northern and southern walls of the fracture zone the median ridge the northern and southern troughs and the nodal basin in situ observations of recent tectonic features such as furrows ridges and circular depressions concentrated within the southern trough allowed us to establish the location and the size of the present day displacement zone geological investigations have shown that the nodal basin is entirely floored by basalts thus contrasting with other equivalent areas such as the kane and oceanographer fracture zone mar eastern intersections finally this study stresses the great opposition between the relatively old and tectonically inactive northern part of the fracture and the southern part which shows active tectonics and recent volcanic activity

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

complex gas hydrate system in a gas chimney south china sea abstract gas hydrates and deep sea cold seeps are of interest to industry and academia for several reasons including their implications for offshore geohazards and unconventional resources and for global climate change and carbon cycling guangzhou marine geological survey gmgs implemented its fifth gas hydrate drilling expedition in the qiongdongnan sea area qdn sa of the northern south china sea scs from june to september 2018 multidisciplinary investigations including logging while drilling lwd pressure and non pressure coring gas composition analyses in situ temperature and permeability measurements pore water geochemical analyses and a regional seismic survey reveal a complex distribution of gas hydrate within a vertical chimney structure lwd profiles record high resistivity anomalies and high p wave velocity variations at 9 174 mbsf suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney hydrate bound and void gas is an admixture of methane 70 1 98 3 and heavier hydrocarbons c2h6 0 16 17 59 c3h8 0 05 9 88 i c4h10

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

u th stratigraphy of a cold seep carbonate crust on continental margins oxidation of methane rich fluids from the sediment often leads to formation of authigenic carbonate pavements on the seafloor the biogeochemical processes involved during this carbonate precipitation are increasingly understood but little is known about the duration or mode of carbonate crust formation here we report uranium and thorium concentrations and isotope compositions for a set of 14 samples drilled across an authigenic carbonate pavement which provide the first stratigraphy for a cold seep carbonate crust the 5 5 cm thick crust nl7 cc2 was collected by submersible on the nile deep sea fan in an area of active fluid venting u th analyses must be corrected for initial th and measurement of co existing sediments indicates the presence of both scavenged and detrital initial 230th which must be considered during this correction the calculated 230th u age depth profile for nl7 cc2 provides evidence for continuous downward carbonate precipitation at the studied location over the last 5000 years three distinct phases can be distinguished from top to bottom with average growth rates of 0 4 5 and 0 8 cm kyr respectively corresponding to carbonate precipitation rates ranging from 7 to 92 mol m2 h 1 rates consistent with previous estimates high resolution 13c profiles gontharet s pierre c blanc valleron m m rouchy j m fouquet y bayon g foucher j p woodside j mascle j the nautinil scientific party 2007 nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the nile deep sea fan eastern mediterranean sea deep sea res ii 54 1292 1311 and major elements across nl7 cc2 show that the variations in carbonate precipitation rates were also accompanied by changes in carbonate mineralogy and fluid composition we suggest that these changes primarily reflect modification of the diagenetic environment i e a progressive depletion of dissolved sulphate through anaerobic oxidation of methane caused by the initial carbonate crust formation and the resulting reduction in bioirrigation overall u th dating of cold seep carbonates offers a promising tool to bring new insights into biogeochemical processes at cold seeps and to assess the timing and duration of fluid venting on continental margins

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

development of diagenetic seals in carbonates and sandstones abstract diagenetic seals effectively block the movement of reservoir hydrocarbons in many sandstone and carbonate rock units diagenetic processes that create these trapping seals include 1 chemical compaction through pressure solution of silicate and carbonate minerals 2 concentration of insoluble clay minerals and organic matter during chemical compaction 3 cementation by authigenic minerals 4 volume increase of rock constituents resulting from hydration or replacement 5 coalescive recrystallization 6 mechanical deformation of ductile constituents and 7 implacement of immobile organic residue derived from crude oil and natural gas sealing cements include silica minerals clays zeolites carbonates sulfates chlorides and subordinately several other mine al groups the sealing capacity of the rocks is related to 1 the amount of residual porosity 2 the pore geometry 3 the modulus of elasticity 4 the resealing capacity and 5 the phases and physical properties of pore filling media diagenetic seals develop in a wide array of diagenetic environments during eodiagenesis before burial mesodiagenesis during burial and telodiagenesis after burial diagenesis can convert any sandstone or carbonate lithology into a sealing rock the direct factors that control the formation of diagenetic seals include 1 textural and mineralogical composition 2 degree of lithification 3 burial history 4 fluid regime and history of chemical composition pressure and migration of pore filling media 5 thermal history and 6 tectonic stress these direct factors are in turn controlled by other parameters such as the lithology of intercalated sediments tectonic history structural position and spatial relationship to unconformities or faults diagenetic seals in sandstones and carbonate rocks encase reservoir end page 545 rocks with either depositional or diagenetic porosity diagenetic reservoir porosity may originate before or after the establishment of an effective diagenetic seal hydrocarbon traps with diagenetic seals may conform in their geometry as well to structure or stratigraphy as to diagenetic facies therefore some structural and stratigraphic traps may in part or entirely depend on diagenetic seals an example of such sealing is the recinus cardium a pool in this field lateral top and bottom sands have formed as a result of mesogenetic cementation at the margins of the sand body the limiting injection pressure in these seals is approximately 550 psi 3 790 kpa mercury against air which translates to a pore throat radius of 0 195 microns this seal could effectively withstand a hydrocarbon water injection pressure of 51 psi 352 kpa which represents a trapped hydrocarbon column of between 200 and 500 ft 61 and 92 m this clearly indicates that diagenetic seals can be tremendously effective detailed analysis of diagenetic seals in sandstones and carbonate rocks can considerably improve our ability to predict their occurrence and to recognize their spatial and temporal relationship to reservoir rocks and hydrocarbon migration end of article last page 546

time integrated variation of sources of fluids and seepage dynamics archived in authigenic carbonates from gulf of mexico gas hydrate seafloor observatory article i nfo authigenic carbonate rocks recovered from the gulf of mexico gas hydrate seafloor observatory in mississippi canyon block 118 mc118 at approximately 900 m water depth were studied using mineralogical bulk geochemical and lipid biomarker analyses carbonate rocks occurred as fractured blocks and nodular masses incorporated in carbonate breccias the carbonates were comprised mainly of high mg calcite and aragonite the stable carbon isotope composition

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

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

hydrocarbon gas hydrates in sediments of the mississippi canyon area northern gulf of mexico the gulf of mexico hydrates research consortium has begun installing a seafloor observatory to monitor gas hydrate outcrops and the hydrate stability zone in mississippi canyon area lease block 118 relevant background information concerning the mississippi canyon area and gas hydrate occurrences in the northern gulf of mexico is presented microbial influences and possible scenarios of hydrate accumulation are considered the design of the observatory was based on field data recorded in the mississippi canyon area principally lease block 118 mc118 and the vicinity of lease block 798 mc798 swath bathymetry by autonomous underwater vehicle played a large part as did seismic imaging within the hydrate stability zone and core sampling these data and the results of their analyses are discussed in detail discussion and interim conclusions are presented in an effort to expand the information base on how natural gas hydrates form in sediments and how they affect seafloor stability the university of mississippi organized the gulf of mexico hydrates research consortium in 1999 its principal goal is to develop and install a seafloor observatory capable of monitoring a hydrate outcrop and its environment more or less continuously for a period of 5 10 years in 2006 the consortium included researchers from more than 30 universities and corporations a board of scientific advisers oversees project management site selection geochemical aspects microbial aspects seismoacoustic aspects data quality control and data archiving a summary of project history an overview of observatory design and a list of organizations that participate in the scientific board is given by mcgee 2006 a number of possible sites for the observatory were considered by the site selection committee data presented here were collected in the vicinity of two sites of known hydrate occurrence in the mississippi canyon area sites lease block 118 mc118 and lease block 798 mc798 a lease block in the northern gulf of mexico comprises a square of sides 3 statute miles 5 km consortium members voted in october 2004 to select a prominent seafloor mound located in mc118 as the observatory site the minerals management service mms of the us department of interior reserved the vicinity of the mound exclusively for consortium activities pre installation studies began in january 2005 and the first monitoring components were installed during may of the same year additional components were scheduled for deployment during the following september but the aftermath of hurricane katrina forced a delay until june 2006 in addition to information on regional contexts this contribution includes early results and tentative conclusions produced by consortium activities prior to the resumption of the installation quaternary geology of the mississippi

authigenic carbonates at cold seeps in the marmara sea turkey a lipid biomarker and stable carbon and oxygen isotope investigation abstract cold seeps along the north anatolian fault in the marmara sea turkey have been recently explored during international expeditions through submersible surveys seafloor mapping and extensive sampling of sediments biota carbonates and pore waters in this study we investigated the main mineralogy carbon and oxygen isotopic compositions and lipid biomarkers in authigenic carbonates retrieved from active seepage areas in the marmara sea abundant microbial lipid biomarkers with light 13 c values as low as 113 as well as light 13 c caco 3 values of bulk carbonate 34 to 21 provided evidence that anaerobic oxidation of methane aom mediated by methanotrophic archaea anme and their bacterial partners is the major process leading to sequestration of methane derived carbon diagnostic lipid distributions indicated that anme 2 archaea and associated sulphate reducing bacteria srb as the prevalent aom mediating microbes diverse non isoprenoid dialkyl glycerol ethers dages including tentatively identified novel compounds were identified with similar 13 c values as lipid biomarkers of srb 105 to 68 though dages are often assigned to srb other unknown bacteria closely involved in aom are likely as source organisms as well despite dominant aom related lipid biomarker patterns the stable carbon isotopic composition of bulk carbonates in two samples from south cinarcik basin and central high in combination with higher hydrocarbon signatures and environmental parameters suggested that oxidation of non methane hydrocarbons including oil may be a contributor to the dissolved inorganic carbon pool and may also be involved in carbonate authigenesis at hydrocarbon seepage settings as previously proposed in the gulf of mexico

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

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

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

authigenic carbonates from an active cold seep of the northern south china sea new insights into fluid sources and past seepage activity abstract site f also named formosa ridge represents the most vigorous cold seep on the northern south china sea continental slope in order to constrain the fluid sources and intensities of seepage we investigated the petrography mineralogy stable carbon and oxygen isotopic compositions element geochemistry and radiocarbon dating of authigenic carbonate rocks retrieved from the seafloor carbonate rocks mainly occurred as crusts nodules and nodular masses incorporated in carbonate breccias the carbonates were comprised mainly of high mg calcite and aragonite the 13 c of authigenic carbonate varied from 55 3 to 34 3 mean 48 5 n 47 vs v pdb suggesting biogenic methane is the dominant carbon source fuelling the system the 18 o carbonate values were from 3 6 to 4 8 mean 3 9 n 47 the observed 18 o enrichement in relation to calculated equilibrium values in the carbonates probably reflects dissolution of gas hydrates combination of seafloor observations and the obtained ams 14 c ages suggest that 1 initiation of methane seepage from at least 10 6 ka ago 2 environmental conditions may have been favorable for enhanced fluid seepage around 6 ka bp and 3 relatively low intensity of seepage from 2 ka bp till today

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

biogeochemical controls on glycerol dialkyl glycerol tetraether lipid distributions in sediments characterized by diffusive methane flux the tex86 tetraether index of tetraethers consisting of 86 carbon atoms is a proxy for sea surface temperature sst based on the distribution of isoprenoidal glycerol dialkyl glycerol tetraether gdgt membrane lipids synthesized by marine pelagic thaumarchaeota one of the caveats of this proxy is the production of additional gdgts by sedimentary euryarchaeota involved in anaerobic oxidation of methane aom that occurs at deep sea methane seeps but is also widespread in many continental shelf settings here gdgt distributions are investigated through the sulfate methane transition zone smtz in aarhus bay denmark to examine the extent the tex86 proxy is compromised in such a continental shelf setting where aom is characterized by a diffusive rather than rapid advective methane flux both free extractable and non extractable lipid fractions were analyzed as it was expected that pelagic derived and sediment derived gdgts could become incorporated into the molecular and sedimentary matrix to a different extent the results show a large change of tex86 values mainly due to the relative high amounts of gdgt 2 produced by aom related archaea both in the free and non extractable lipid fractions this additional gdgt input renders calculation of ssts based on tex86 inappropriate at the smtz the aom related gdgt signature however did not persist into deeper sediments perhaps reflecting rapid remineralization of the gdgts at the smtz although the process of aom at aarhus bay might not be representative for all continental margin settings it illustrates that aom in a variety of settings not just cold seeps can influence sedimentary gdgt distributions

the role of geochemical self organization in the migration and trapping of hydrocarbons abstract many sedimentary basins of the world contain abnormally pressured compartments surrounded by impermeable seals which may serve as hydrocarbon traps observations of the many types of seals that may exist point to an origin due to self organization self organization is the development of repetitive or other low symmetry patterns from an initially unpatterned system in the absence of an externally imposed template and involves feedback generated by the interaction of transport chemical reactions and grain growth and dissolution herein we focus on the self organizing mechanisms of reaction driven advection as they pertain to the formation of diagenetic traps

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

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

hypersaline conditions during deposition of the calcare di base revealed from archaeal di and tetraether inventories abstract the calcare di base is a widespread messinian carbonate formation in the mediterranean especially in sicily and calabria italy the environmental conditions under which these limestones were deposited their formation mechanism and the timing of deposition are a matter of debate in order to shed new light on their formation the lipid biomarker inventories of calcare di base from four locations in sicily and calabria were studied following petrographical and mineralogical characterization of the limestone samples the mineralogy is chiefly calcite the microfabric is mostly peloidal and partly laminated or brecciated deposition took place at high salinity as confirmed by numerous pseudomorphs after halite long chain n fatty acids were the most abundant lipid biomarkers in samples from all sites but isoprenoid alcohols were also abundant the most prominent archaeal compounds were diphytanyl glycerol diethers dgds and glycerol dibiphytanyl glycerol tetraethers gdgts among these archaeol and gdgt 0 caldarchaeol predominated respectively other gdgts were virtually absent minor amounts of c 20 25 dgd extended archaeol a biomarker for halophilic archaea were detected in all samples the 13 c values of caldarchaeol avg 20 and archaeol avg 11 varied widely indicating that these compounds derived from distinct archaeal precursor lipids based on lipid biomarker patterns the calcare di base had been previously considered to represent a normal marine deposit in contrast to the evaporitic gypsum and halite deposits in messinian successions the archaeal biomarker inventories from the calcare di base samples reported here in conjunction with the occurrence of pseudomorphs after halite reveal that deposition took place in a hypersaline environment

petrographic and geochemical characterization of seep carbonate from bush hill gc 185 gas vent and hydrate site of the gulf of mexico abstract authigenic carbonates are common at cold seep sites as a result of microbial oxidation of hydrocarbons seep carbonate samples were collected from the surface of the bush hill green canyon block 185 gulf of mexico a mound containing gas hydrate the carbonates consisted of oily porous limestone slabs and blocks containing bioclasts and matrix analysis by x ray diffraction shows that aragonite is the dominant mineral 89 99 wt with an average of 94 wt in the matrix of seep carbonate this cement occurs in microcrystalline microspar and sparite forms the moderate 13 c depletion of the seep carbonate the most depleted one has 13 c value of 29 4 and 26 of 38 subsamples have 13 c values 20 0 indicates that the non methane hydrocarbons was incorporated during seep carbonate precipitation relative enrichment of 18 o may be related to localized destabilization of gas hydrate or derived from 18 o enriched pore water originated from smectite illite transition in the deep sediments the total content of rare earth elements ree of the 5 hno 3 treated solution of the carbonates is from 0 40 ppm to 30 9 ppm the shale normalized ree patterns show varied ce anomalies from significantly negative slightly negative and no to positive ce anomalies variable content of trace elements total ree and ce anomalies in different samples and even in the different carbonate mineral forms microcrystalline microspar and sparite of the same sample suggest that the formation condition of the bush hill seep carbonate is variable and complex which is possibly controlled by the rate of fluid flux

authigenic carbonates from methane seeps of the northern congo fan microbial formation mechanism abstract authigenic carbonates were collected from methane seeps at hydrate hole at 3113 m water depth and diapir field at 2417 m water depth on the northern congo deep sea fan during rv meteor cruise m56 the carbonate samples analyzed here are nodules mainly composed of aragonite and high mg calcite abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix the 13c values of the hydrate hole carbonates range from 62 5 to 46 3 pdb while the 13c values of the diapir field carbonate are somewhat higher ranging from 40 7 to 30 7 pdb indicating that methane is the predominant carbon source at both locations relative enrichment of 18o 18o values as high as 5 2 pdb are probably related to localized destabilization of gas hydrate the total content of rare earth elements ree of 5 hno3 treated solutions derived from carbonate samples varies from 1 6 ppm to 42 5 ppm the shale normalized ree patterns all display positive ce anomalies ce ce 1 3 revealing that the carbonates precipitated under anoxic conditions a sample from hydrate hole shows a concentric lamination corresponding to fluctuations in 13c values as well as trace elements contents these fluctuations are presumed to reflect changes of seepage flux

geological imprint of methane seepage on the seabed and biota of the convergent hikurangi margin new zealand box core and grab carbonate results abstract short box cores to 30 cm bsf and seafloor carbonate grab samples were acquired at mapped hydrocarbon seep sites 600 1200 m water depths during the 2007 rv sonne so191 cruise on the hikurangi margin offshore eastern north island new zealand to evaluate the influence of methane seepage on sedimentologic biotic mineralogic and stable isotopic attributes of seabed sediments sedimentary horizons in the box cores consist of siliciclastic silts and sands shell beds and nodular microcrystalline aragonite bands up to 15 cm thick the megafauna is dominated by infaunal to semi infaunal chemosymbiotic bivalves calyptogena lucinoma and acharax as well as associated worms and carnivorous and grazing gastropods burrows in silts some occupied by worms or juvenile acharax mainly have simple morphologies more typical of high energy nearshore settings than deep sea environments while a few are large and sparsely branched with wall scratch marks inferred to be of decapod crustacean origin the box core silts and nodular carbonate samples vary in toc content from 0 2 to 0 9 wt carbonate content from 4 to 78 and 13 c and 18 o values from 50 3 to 0 6 pdb and 0 77 to 3 2 pdb respectively low carbonate content silt samples have the most enriched 13 c values implying a seawater source for their pore water bicarbonate negative 13 c and positive 18 o values typify the nodular microcrystalline aragonite bands indicating formation during microbially mediated sulphate dependent anaerobic oxidation of methane aom in a cold near seafloor environment as is also supported by lipid biomarker data a clear isotopic mixing trend of decreasing 13 c and increasing 18 o and carbonate content in the fine seafloor grab carbonate samples show greater textural variability and are divided into two types fresh appearing grey to white blocks yield similar mineralogic isotopic and biomarker signatures to the nodular carbonate bands in the cores a largely seep related epifauna affiliated with these grab samples include chemosymbiotic bathymodiolin mussels siboglinid tube worms methanotrophic suberitid sponges and grazing limpets which in places are entombed within the carbonate by contrast some dark reddish to brown iron stained microcrystalline dolomitic slabs and tubular concretions support abundant non seep epifaunal encrusters from the surrounding deep sea indicative of long exposure on the seafloor distinct stable isotopic signatures of these exhumed dolostones 13 c c 20 pdb 18 o up to 7 pdb suggest derivation from an oxidised methane pool and pore fluid freshening by gas hydrate dissociation or from mobilized burial fluids that have undergone clay mineral dehydration

using chemical compositions of sediments to constrain methane seepage dynamics a case study from haima cold seeps of the south china sea abstract cold seeps frequently occur at the seafloor along continental margins the dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction which can significantly impact the global carbon and sulfur cycles the circulation of methane rich fluids at margins is highly variable in time and space and assessing past seepage activity requires the use of specific geochemical markers in this study we report multiple sedimentary proxy records for three piston gravity cores qdn 14a qdn 14b and qdn 31 from the haima seep of the south china sea scs by combining total organic carbon toc total inorganic carbon tic total nitrogen tn total sulfur ts acid insoluble carbon and sulfur isotope 13corganic carbon and 34sacid insoluble and 34s values of chromium reducibility sulfur 34scrs as well as carbon isotopes of tic 13ctic in sediments our aim was to provide constraints on methane seepage dynamics in this area we identified three sediment layers at about 260 300 cm 380 420 cm and 480 520 cm sediment depth characterized by particular anomalies of low 13ctic values and high ts content high ts and crs contents and high 34sacid insoluble and 34scrs values respectively on this basis we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record while the exact mechanisms accounting for the presence or absence of these particular geochemical signals in the sediment are not known we propose that they correspond to variations in methane flux and their duration through time overall our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time

spatial distribution of gas hydrates from high resolution seismic and core data woolsey mound northern gulf of mexico abstract integration of new jumbo piston cores with high resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep water cold seep the study area woolsey mound is a carbonate hydrate mound of thermogenic origin in the northern gulf of mexico where salt tectonics dominate the regional structure two typologies of unconventional seismo acoustic data were used in this study 1 surface source deep receiver ssdr data and 2 auv borne chirp sub bottom profiler data correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening high frequency scattering present in the ssdr records may indicate the presence of solid hydrates this suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep water settings coring results revealed the nature of the shallow gas hydrate system to be dominated by fine grained sediments gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault we present a model for woolsey mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons this dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site

seafloor oxygen consumption fuelled by methane from cold seeps the leakage of cold methane rich fluids from subsurface reservoirs to the sea floor sustains some of the richest ecosystems on the sea bed these cold seep communities consume around two orders of magnitude more oxygen than the surrounding sea floor as a result of the microbial consumption of seep methane

diagenetic traps in sandstones diagenesis may be an important agent in trapping hydrocarbons in sandstones by formation of reservoirs or seals reservoirs are formed by the creation of secondary porosity during burial seals are formed by heavy cementation which may allow sandstones to retain large hydrocarbon columns formation of a diagenetic trap requires that parts of the sandstone unit react differently from each other during burial this can be caused by differences in 1 detrital mineralogy resulting from grain size or depositional environment controls 2 early diagenetic mineralogy largely depositional environment controlled 3 burial history structural movement induced and 4 fluid content hydrocarbon or water saturation each of these factors can lead to differences in porosity and permeability of the sandstone sufficient to form reservoirs and seals in the correct configuration diagenetic traps may be formed basin center gas accumulations result from diagenetic trapping in some instances

formation of low magnesium calcite at cold seeps in an aragonite sea this study investigates the conditions of occurrence and petrographic characteristics of low mg calcite lmc from cold seeps of the gulf of mexico at a water depth of 2340m such lmc mineral phases should precipitate in calcite seas rather than today s aragonite sea the c 13 depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments the occurrence of lmc may result from brine fluid flows brines are relatively ca2 enriched and mg2 depleted mg ca mole ratio 0 7 relative to seawater where the mg ca mole ratio is 5 which drives high mg calcite and aragonite precipitation the dissolution of aragonitic mollusk shells grains and cements was observed aerobic oxidation of hydrocarbons and h2s is the most likely mechanism to explain carbonate dissolution these findings have important implications for understanding the occurrence of lmc in deep water marine settings and consequently their counterparts in the geological record

microbial activity in surficial sediments overlying acoustic wipeout zones at a gulf of mexico cold seep 1 down core concentration gradients of dissolved methane and sulfate isotope gradients of methane dissolved inorganic carbon and authigenic carbonate and organic matter elemental ratios are incorporated into a vent evolution model to describe spatial and temporal variability of sedimentary microbial activity overlying acoustic wipeout zones at mississippi canyon mc 118 gulf of mexico we tested the hypothesis that these zones indicate areas where sediments are exposed to elevated fluid flux and therefore should contain saturated methane concentrations and enhanced microbial activity from sulfate reduction sr anaerobic oxidation of methane aom and methanogenesis mp thirty surficial cores between 22 and 460 cm deep were collected from sediments overlying and outside the wipeout zones and analyzed for pore water and solid phase constituents outside the wipeout zones sulfate and methane concentrations were similar to overlying water values and did not vary with depth indicating low microbial activity above the wipeouts nine cores showed moderate activity with gently sloping sulfate and methane concentration gradients methane concentrations 20 m and isotope depth gradients indicative of organic matter oxidation in stark contrast to this moderate activity four cores showed high microbial activity where sulfate concentrations were depleted by 50 cm below seafloor maximum methane concentrations in the decompressed cores were above 4 mm and down core profiles of 13c ch4 and 13c dissolved inorganic carbon dic indicated distinct depth zones of sr aom and mp bulk organic matter analysis suggested that the high activity was supported by an organic source that was enriched in carbon c n 15 and depleted in 15n and 13c compared to other activity groups possibly due to the influx of petroleum or chemosynthetically fixed carbon within high activity cores the 13c dic values were similar to the 13c caco3 values a result expected for authigenic carbonate recently precipitated however these values were dissimilar in moderate activity cores suggesting that microbial activity was higher in the past this study provides evidence that the fluid flux at mc 118 varies over time and that the microbial activity responds to such variability it also suggests that sediments overlying wipeout zones are not always saturated with respect to methane which has implications for the formation and detection of gas hydrate

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

13c depleted biphytanic diacids as tracers of past anaerobic oxidation of methane abstract 13 c depleted biphytanic diacids containing 0 2 pentacyclic rings are abundant biomarkers in cenozoic methane seep limestones the strong 13 c depletion 132 to 96 suggests that they derive from euryarchaea metabolizing methane they occur with other biomarkers typically associated with anaerobic oxidation of methane such as crocetane pentamethylicosane pmi archaeol hydroxyarchaeol and glycerol dibiphytanyl glycerol tetraethers gdgts in contrast to gdgt derived biphytane hydrocarbons the biphytanic diacids appear to be almost exclusively derived from methanotrophic euryarchaea

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

characteristics and dynamics of gas hydrate systems in the northwestern south china sea results of the fifth gas hydrate drilling expedition abstract in 2018 guangzhou marine geological survey conducted the fifth gas hydrate drilling expedition gmgs5 in the northwestern continental slope of south china sea we use pore water geochemistry pressure core ct scanning degassing quantification seismic data and in situ temperature test at four drill sites w01 w07 w08 and w09 to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system based on the ct scanning and pressure core degassing we demonstrate the coexistence of pore filling and vein like gas hydrates gas hydrate saturation estimated based on the chloride concentration 0 92 is higher than that based on the degassing quantification 0 52 9 the saturation discrepancy is mainly resulted from the sampling discrimination based on the average gas hydrate saturation estimated using chloride concentration and degassing the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites w07 w08 and w09 are calculated to be 13 7 m3 26 82 m3 and 22 24 m3 and 3 03 m3 14 30 m3 and 16 25 m3 the geothermal gradients estimated based on the in situ temperature measurements at sites w07 w08 and w09 102 111 c km are more than 50 higher than that at the background site w01 65 c km it is inferred that the fractures and faults in the gas chimney at sites w07 w08 and w09 are the major contributors for the efficient fluid pathways and high geothermal gradients the pore water sulfate profiles and inferred sulfate reduction methane oxidation interface smi depths indicate that methane flux is lowest at site w01 and highest at site w08 the smi depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution kick type sulfate profiles at sites w07 and w09 indicate an ongoing increase of methane flux and corresponding shoaling smi depths in addition chloride enrichment in the shallow sediment at site w08 indicates a recent fast gas hydrate formation process the kick type sulfate profiles positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi stage gas hydrate evolution on time scale from months to thousands of years understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling

formation of carbonate chimneys in the mediterranean sea linked to deep water oxygen depletion marine sediments at ocean margins vent substantial amounts of methane1 2 microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3 4 the factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained here we characterize the petrology and geochemistry of rocks sampled from metre size build ups of methane derived carbonate chimneys located at the amon mud volcano on the nile deep sea fan we find that these carbonates comprise porous structures composed of aggregated spherules of aragonite and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the black sea5 using u series dating we show that the amon carbonate build ups formed between 12 and 7 thousand years ago contemporaneous with the deposition of organic rich sediments in the eastern mediterranean the so called sapropel layer s1 we propose that the onset of deep water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor and thus the formation of carbonate chimneys

biogeochemical signatures and microbial activity of different cold seep habitats along the gulf of mexico deep slope abstract microorganisms and the processes they mediate serve as the metabolic foundation of cold seeps we characterized a suite of biogeochemical constituents and quantified rates of two key microbial processes sulfate reduction sr and anaerobic oxidation of methane aom to assess variability between habitats at water depths exceeding 1000 m in the northern gulf of mexico rates of sr were highest in sediments beneath microbial mats lower in brine influenced and oil influenced sediments and lowest in animal habitats sediments collected near tubeworms had the highest sr rates for animal habitats rates of aom generally were low but higher rates were associated with brine influenced oil influenced tubeworm and urchin inhabited sediments rates of both sr and aom were orders of magnitude lower at deep slope sites compared to upper slope sites examined previously as observed at upper slope sites sr and aom rates were often loosely coupled at one site aom rates exceeded sr rates suggesting that an alternate electron acceptor for aom is possible extremely depleted 13c values in methane illustrated the broad significance of biogenic methane production at deep slope sites brine influenced habitats were characterized by extremely high concentrations of ammonium and dissolved organic carbon serving as important focused sources of these chemicals to adjacent environments

lipid biomarker patterns of methane seep microbialites from the mesozoic convergent margin of california abstract in order to reconstruct biogeochemical pathways at mesozoic methane seeps a set of late jurassic tithonian to early cretaceous aptian albian 13 c depleted seep limestones from forearc strata in western california were subjected to detailed molecular isotopic biomarker analyses two of the microbial carbonate deposits are turbidite hosted fault related whereas one is hosted in serpentinite in a diapir related setting the limestones contain 13 c depleted archaeal lipid biomarkers such as crocetane 13 c 80 and pmi 100 indicative of an involvement of anaerobic oxidation of methane aom in carbonate precipitation isotopically depleted crocetane in the tithonian sample represents the oldest reported occurrence of this compound at methane seeps in the set of samples a series of strongly 13 c depleted regular c 21 to c 24 isoprenoids possibly results from diagenetic alteration of archaeal sesterterpanylglycerol diethers as suggested by the presence of the putative intermediate 3 7 11 15 19 pentamethylicosanoic acid 13 c depleted 17 h 21 h and 17 h 21 h hopanes c 30 c 34 with 22 s and 22 r isomer couplets c 31 are present in all samples in distributions indicative of a moderate thermal maturity low 13 c values 78 to 60 suggest that these are derived from anaerobic bacteria involved in aom notably 22 s isomers are consistently enriched in 13 c relative to their 22 r counterparts our samples represent 70 myr of seepage activity and aom along the mesozoic margin of western california filling the gap between the currently oldest methane seep biomarker record from the oxfordian late jurassic and the more widely recognised cenozoic examples

biogeochemical controls on authigenic carbonate formation at the chapopote asphalt volcano bay of campeche abstract the campeche knolls in the bay of campeche southern gulf of mexico were investigated through detailed seafloor mapping rov surveys and sediment and pore water sampling the knolls are elongated submarine hills created by salt tectonics with a positive relief of up to 800 m above the surrounding seafloor several of the knolls are associated with sea surface oil slicks identified from satellite data indicating the presence of hydrocarbon seeps on the seafloor one of the knolls named chapopote was studied in detail during two international research expeditions so174 2 and m67 2 and is characterized by extensive hydrocarbon seepage including large asphalt flows oil and gas seeps and seafloor gas hydrate deposits chemosymbiotic biological communities and authigenic carbonate deposits are associated with the seeps and are the result of both biogeochemical turnover and the interaction between downward diffusing seawater and hydrocarbon rich pore fluids at shallow sediment depth authigenic carbonates are characterized by aragonite exhibit a porous texture and are cemented by a matrix of microsparitic to sparitic aragonite macropores of the carbonates were completely filled with liquid oil carbonate microfabrics include peloidal or clotted fabrics that may indicate the existence of microenvironments resulting from microbial metabolism banded botryoidal aragonite cements line the intra and bioclasts and incompletely fill the pore spaces the stable carbon isotopic composition of authigenic aragonite varies between 28 6 and 17 9 pdb identifying oil oxidation as the primary source of carbon to the dic pool while lipid biomarker data demonstrate the concurrent existence of microbial communities responsible for anaerobic oxidation of methane aom these observations indicate the presence of additional aom independent reactions responsible for carbon sequestration at hydrocarbon seeps and demonstrate the complexity of biogeochemical processes at seep sites in the gulf of mexico basin oxygen isotope data of authigenic aragonite vary from 2 5 to 3 8 pdb indicating carbonate precipitation in slight disequilibrium with the surrounding pore fluids

an archaeal biomarker record of paleoenvironmental change across the onset of the messinian salinity crisis in the absence of evaporites piedmont basin italy abstract a sudden change from normal marine to extreme paleoenvironmental conditions occurred approximately 6 ma ago in the mediterranean basin at the onset of the late miocene messinian salinity crisis one of the most severe ecological crises in earth history strong evaporation and tectonics led to hypersaline conditions resulting in widespread deposition of evaporites and the apparent annihilation of the marine metazoan biosphere in contrast to the prominent occurrence of evaporites elsewhere in the mediterranean evaporites did not form in the deeper part of some marginal basins at the onset of the crisis the strata of the pollenzo section piedmont basin nw italy studied here were deposited in such a paleogeographic setting instead of evaporites a cyclic succession of organic rich shales and carbonates formed during the early phase of the crisis in the study area these sediments record a sharp increase in the contents of archaeal molecular fossils that are mostly represented by isoprenoidal dialkyl glycerol diethers dgds and isoprenoidal glycerol dialkyl glycerol tetraethers gdgts such an expansion of archaeal biomass is best explained by a change of the archaeal communities at the onset of the messinian salinity crisis in particular the appearance of molecular fossils of extremophilic archaea mostly producing dgds archaeol and extended archaeol suggests the emergence of halophilic archaea at the same time lipids of planktonic thaumarchaeota especially crenarchaeol are present across the entire section suggesting the local persistence of normal marine conditions in agreement with the sudden appearance of tetrahymanol in the upper part of the section the persistence of crenarchaeol indicates the establishment of water column stratification after the advent of the messinian salinity crisis to further investigate the piedmont basin paleoenvironmental conditions we test the archaeol caldarchaeol ecometric ace a proxy developed for identifying high paleosalinities in waters and possibly in sediments despite high ace values found for the messinian salinity crisis samples these values are in contrast with the absence of any lithological evidence for high salinities as well with other biomarker independent information indicating low to normal seawater salinities this apparent contradiction is likely explained by the complexity of the sources of archaeal lipids especially of caldarchaeol and archaeol limiting the utility of the ace for the reconstruction of paleosalinities for messinian strata

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

archaeal lipids record paleosalinity in hypersaline systems abstract we present an ecologically based biomarker method for estimating past salinity especially in hypersaline conditions the relative amounts of acyclic diether and tetraether membrane lipids synthesized by archaea correlate with salinity from 0 250 practical salinity units psu in modern settings we examined the preservation of this lipid biomarker salinity relationship in ancient sedimentary organic matter using samples from two sequences of marls and diatomites deposited just prior to the messinian salinity crisis salinity estimates were consistent with expected absolute salinity as well as the amplitude of variations leading up to the messinian salinity crisis this lipid biomarker approach to salinity reconstruction complements existing paleosalinity proxies because i archaea survive and thrive over a broad salinity range well beyond that of haptophyte algae and other plankton which form the microfossil record and ii it provides fine salinity resolution for the wide range broadly defined as hypersaline with the proxy there is the potential to provide novel insights into salinity variation within desiccating basins in climatically sensitive seas e g dead sea permian delaware basin evolution of brines timing of onset of hypersaline conditions and evaporite deposition

paleo environmental controls on cold seep carbonate authigenesis in the sea of marmara abstract the factors controlling fluid emission dynamics at ocean margins are poorly understood in particular there are significant uncertainties on how fluid seepage at cold seeps may have responded to abrupt environmental changes in the geological past this study reports on a detailed geochemical investigation of seafloor carbonate crusts sampled at cold seeps along the submerged part of the north anatolian fault system in the sea of marmara an inland sea which has experienced major paleo environmental changes over the last deglaciation period we also analyzed a series of authigenic carbonate concretions recovered from two sediment cores at the western high ridge an active fluid venting area the ages of seafloor carbonate crusts derived from isochron u th dating cover the last 7 kyr suggesting that fluid activity along the fault system remained continuous over that time interval in the sediment cores carbonate concretions are concentrated at the lacustrine to marine transition which corresponds to the period when mediterranean waters flowed into the marmara basin about 12 14 kyr ago u th isotopic data indicate that most of these concretions formed later during the holocene around 9 10 kyr ago a period coinciding with an important anoxic event that led to the deposition of a sapropel layer in the sea of marmara based upon these results we suggest that the absence of carbonate concretions in the lacustrine sediment unit indicates that dissolved sulfate concentrations in the marmara lake pore waters during glacial time were too low to promote significant anaerobic methane oxidation thereby preventing sedimentary carbonate authigenesis in contrast the progressive inflow of mediterranean waters into the glacial marmara lake after 15 ka provided a source of dissolved sulfate that allowed anaerobic oxidation of methane to proceed within the anoxic sediment importantly the synchronism between the main phase of authigenic carbonate precipitation at the studied sites average 9 4 1 8 ka n 16 and the regional anoxic sapropel event support the idea that the drop in bottom water dissolved oxygen content was probably a key factor to enhance microbial activity and associated carbonate precipitation at that time overall these results provide straightforward evidence that fluid emission dynamics and hydrocarbon oxidation at cold seeps can be directly related to changing environmental conditions through time

quantifying gas hydrate from microbial methane in the south china sea abstract methane hydrate has been confirmed to be present in the south china sea scs based on drilling expeditions and geophysical indications it is necessary to access the methane hydrate inventory because of its role in the energy resources and climate change the aim of this study is to constrain the inventory of methane hydrates in the scs formed by the microbial degradation of organic matter within the gas hydrate stability zone ghsz bathymetry seafloor temperature and geothermal gradient datasets enabled us to estimate the ghsz at a resolution of 30 30 for the scs we conclude that the ghsz occurs at water depths of 500 m in most areas of the scs the methane hydrate distribution formed by microbial methane production in the scs was computed using the parameterized transfer function recently proposed by wallmann et al 2012 the result indicates widespread accumulations of methane hydrate along the continental slopes in the scs the microbial methane hydrate is distributed over a 7 69 1011 m2 area and the total abundance of methane hydrate in the scs is estimated to be 42 8 gt c

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

cold seep status archived in authigenic carbonates mineralogical and isotopic evidence from northern south china sea abstract cold seep carbonates are precipitated under high alkalinity conditions created by the anaerobic oxidation of methane in cold seep sites multiple ca mg carbonate phases are identified including aragonite low mg calcite lmc high mg calcite hmc protodolomite and dolomite these phases result from different conditions that are related with cold seep activities here we report on the relationship between the ca mg carbonate phases and the cold seep status authigenic carbonates were sampled from northern slope of south china sea carbon isotopic compositions of samples from shenhu area are lower than 40 indicating methane derived carbon the 13c values of samples from southwest sw taiwan area range from 30 to 20 which is the result of the mixture of methane carbon and seawater carbon carbonate phases were identified according to the composition and structure results samples from shenhu area are composed of protodolomite and hmc three zones were discovered from the center to the rim of the cross section of the tube like sample from sw taiwan area from the external to the internal zones the carbonate phases are hmc lmc and protodolomite hmc respectively the intensity of superstructure reflections of the protodolomite from shenhu area is stronger than that from sw taiwan area indicating higher mgco3 content based on the formation conditions of ca mg carbonates from lmc to dolomite those with higher mgco3 content are formed in more active cold seep environment according to the distribution of carbonate phases in each sample the cold seep flux was high in shenhu area and was sustained for a long time by contrast the flux in sw taiwan area was relatively low and not stable it once became higher but finally returned to low

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

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

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

formation of seep carbonates along the makran convergent margin northern arabian sea and a molecular and isotopic approach to constrain the carbon isotopic composition of parent methane abstract authigenic carbonate deposits have been sampled with the remotely operated vehicle marum quest 4000 m from five methane seeps between 731 and 1823 m water depth along the convergent makran continental margin offshore pakistan northern arabian sea two seeps on the upper slope are located within the oxygen minimum zone omz ca 100 to 1100 m water depth the other sites are situated in oxygenated water below the omz below 1100 m water depth the carbonate deposits vary with regard to their spatial extent sedimentary fabrics and associated seep fauna within the omz carbonates are spatially restricted and associated with microbial mats whereas in the oxygenated zone below the omz extensive carbonate crusts are exposed on the seafloor with abundant metazoans bathymodiolin mussels tube worms galatheid crabs aragonite and mg calcite are the dominant carbonate minerals forming common early diagenetic microcrystalline cement and clotted to radial fibrous cement the 18 o carbonate values range from 1 3 to 4 2 v pdb indicating carbonate precipitation at ambient bottom water temperature in shallow sediment depth extremely low 13 c carbonate values as low 54 6 v pdb point to anaerobic oxidation of methane aom as trigger for carbonate precipitation with biogenic methane as dominant carbon source prevalence of biogenic methane in the seepage gas is corroborated by 13 c methane values ranging from 70 3 to 66 7 v pdb and also by back calculations considering 13 c methane values of carbonate and incorporated lipid biomarkers these calculations 13 c methane carbonate 13 c anme methane 13 c mox methane prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place such an approach represents a useful tool to reconstruct fluid composition of ancient seeps aom is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane pentamethylicosane pmi and sn2 hydroxyarchaeol strongly depleted in 13 c 13 c values as low as 127 v pdb biomarkers of sulphate reducing bacteria are also abundant showing slightly less negative 13 c values but still significantly 13 c depleted average values as low as 101 other bacterial biomarkers such as bacteriohopanepolyols bhps hopanols and hopanoic acids are detected in most carbonates but are particularly common in seep carbonates from the non omz sites the bhp patterns of these carbonates and their low 13 c values resemble patterns of aerobic methanotrophic bacteria in the shallower omz sites bhps revealed much lower contents and varying compositions most likely reflecting other sources than aerobic methanotrophic bacteria 230 th u carbonate ages indicate that aom induced carbonate precipitation at the deeper non omz seeps occurred mainly during the late pleistocene holocene transition i e between 19 and 15 ka before present when the global sea level was lower than today

sr ca and mg ca ratios in niger delta sediments implications for authigenic carbonate genesis in cold seep environments we report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the niger deep sea fan our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid phase sediment geochemistry we show that sedimentary sr ca and mg ca ratios can be used to infer the presence of authigenic aragonite sr rich and mg rich carbonate phases high mg calcite dolomite in cold seep settings using sr ca and mg ca ratios the proportion wt of authigenic carbonates in niger fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material biogenic calcite aragonite and high mg calcite this approach was applied to high resolution geochemical profiles along sediment cores recovered from various cold seep settings mud volcano diapirs pockmarks gas hydrate bearing sediments our data reveal that authigenic carbonates occur as discrete phases in sediments from gas hydrate bearing areas suggesting that such carbonate rich sediment layers may represent paleo indicators for ancient methane seepage in marine sediments possibly associated to gas hydrate reservoirs

distributions and carbon isotopic compositions of lipid biomarkers in authigenic carbonate crusts from the nordic margin norwegian sea abstract we investigated the distributions and 13c values of lipid biomarkers in authigenic carbonate crusts from the nordic margin norwegian sea abundant highly 13c depleted archaeal and bacterial biomarkers revealed that anaerobic oxidation of methane aom is associated with carbonate authigenesis at the hakon mosby mud volcano hmmv and the nyegga pockmark field no conclusive evidence for aom was found in the crust sample from the storegga slide anme 2 methanotrophic archaea and sulfate reducing bacteria from the desulfosarcina desulfococcus cluster were inferred from lipid fingerprints distinct patterns of 13c depleted non isoprenoid dialkyl glycerol diethers dages are possibly associated with unknown microbial partners in the aom

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

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

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

observation of sections of oceanic crust and mantle cropping out on the southern wall of kane fz n atlantic the objective of the 20 nautile dives of the recent kanaut cruise was to study the southern wall of the kane fracture zone from its eastern intersection with the mid atlantic ridge mar to 5 myr in age the geological mapping shows four successive massifs wrench faulted and slightly tilted the transform facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites gabbros and basalts the entire crustal exposure is cataclased and metamorphosed to greenschist facies

variations of b values at the western edge of the ryukyu subduction zone north east taiwan using earthquakes relocated in north east taiwan we estimated b value distribution along a cross section located near the ryukyu slab edge and four b value anomalous areas are evidenced 1 a high b value body lying on top of a low vp low vs and high vp vs sausage like body was considered as a region of enhanced partial melt or water supply above which seismicity occurs 2 beneath the ilan plain an anomalous area characterized by b values slightly higher than 1 1 might give evidence to the magma conduits to the kueishantao island 3 above the ryukyu wadati benioff zone at depths ranging from 90 to 110 km a high b value anomaly might correspond to the depth where dehydration occurs in the subducting oceanic plate and 4 a low b value area located within the ryukyu slab at depths ranging from 70 to 90 km might be linked to the compressive mechanisms shown by focal mechanisms and the bending of the subducting plate

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

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

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

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

methane index a tetraether archaeal lipid biomarker indicator for detecting the instability of marine gas hydrates abstract gas hydrates represent one of the largest pools of readily exchangeable carbon on earth s surface releases of the greenhouse gas methane from hydrates are proposed to be responsible for climate change at numerous events in geological history many of these inferred events however were based on carbonate carbon isotopes which are susceptible to diagenetic alterations here we propose a molecular fossil proxy i e the methane index mi to detect and document the destabilization and dissociation of marine gas hydrates mi consists of the relative distribution of glycerol dibiphytanyl glycerol tetraethers gdgts the core membrane lipids of archaea the rational behind mi is that in hydrate impacted environments the pool of archaeal tetraether lipids is dominated by gdgt 1 2 and 3 due to the large contribution of signals from the methanotrophic archaeal community our study in the gulf of mexico cold seep sediments demonstrates a correlation between mi and the compound specific carbon isotope of gdgts which is strong evidence supporting the mi methane consumption relationship preliminary applications of mi in a number of hydrate impacted and or methane rich environments show diagnostic mi values corroborating the idea that mi may serve as a robust indicator for hydrate dissociation that is useful for studies of global carbon cycling and paleoclimate change

cold seep carbonates of the middle and lower continental slope northern gulf of mexico authigenic carbonates from cold seeps on the middle and lower continental slope of the northern gulf of mexico gom exhibit a wide range of mineralogical and stable isotopic compositions these carbonates consist of concretions and nodules in surface sediments hardgrounds of crusts and isolated slabs and mounded buildups of blocks and slabs of up to over 10 meters in relief above the surrounding seafloor mineralogically the carbonates are dominated by high mg calcite hmc and aragonite however low levels 5 wt of dolomite are present in most samples petrographically mg calcite peloidal matrix and acicular to botryoidal aragonitic void filling cements are the most frequent associations the carbon isotopic compositions of the carbonates range from 60 8 to 14 0 pdb indicating complex carbon sources that include 13c depleted biogenic and thermogenic methane biodegraded crude oil seawater co2 and 13c enriched residual co2 from methanogenesis a similarly large variability in 18o values 2 5 to 6 7 pdb demonstrates the geochemical complexity of the slope with some samples pointing toward an 18o enriched oxygen source that is possibly related to advection of 18o enriched formation water and or to the decomposition of gas hydrate a considerable range of mineralogical and isotopic variations in cold seep carbonate composition was noted even within individual study sites however common trends occur across multiple geographic areas this situation suggests that local controls on fluid and gas flux types of seep hydrocarbons the presence or absence of gas hydrate in the near surface sediment and chemosynthetic communities as well as the temporal evolution of the local hydrocarbon reservoir all may play a part in determining carbonate mineralogy and isotope geochemistry the carbon isotope data clearly indicate that between site variation is greater than within site variation seep carbonates formed on the middle and lower continental slope of the gom do not appear to be substantially different from those found on the upper slope 1000 m water depth the highly variable fluids and gases that leave their geochemical imprints on seep carbonate of the middle and lower continental slope are similar to their outer shelf and upper slope counterparts

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

u th systematics and ages of authigenic carbonates from hydrate ridge cascadia margin recorders of fluid flow variations uranium u concentrations and activity ratios 234u of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon rich fluids cold seeps at hydrate ridge off the coast of oregon usa the low u concentrations mean 1 3 0 4 g g and high 234u values 165 317 of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor their 230th 234u ages span a time interval from 0 8 to 6 4 ka and cluster around 1 2 and 4 7 ka in contrast chemoherm carbonates precipitate from marine bottom water marked by relatively high u concentrations mean 5 2 0 8 g g and a mean 234u ratio of 166 3 their u isotopes reflect the 234u ratios of the bottom water being enriched in 234u relative to normal seawater simple mass balance calculations based on u concentrations and their corresponding 234u ratios reveal a contribution of about 11 of sedimentary pore water to the bottom water from the u pore water flux and the reconstructed u pore water concentration a mean flow rate of about 147 68 cm a can be estimated 230th 234u ages of chemoherm carbonates range from 7 3 to 267 6 ka 230th 234u ages of two chemoherms alvin and se knoll chemoherm correspond to time intervals of low sealevel stands in marine isotope stages mis 2 4 5 6 7 and 8 this observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments the 18o pdb ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times deviations of the chemoherm 18o values from the marine 18o record can be interpreted as to reflect temporally and spatially varying bottom water and or vent fluid temperatures during carbonate precipitation between 2 6 and 8 6 c