sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

relationships between sulphur organic carbon and iron in the modern sediments of the black sea abstract the ratio of sedimentary s to organic c has been used as a diagnostic criterion for recognizing anoxic environments in the sedimentary record here we examine the relationship between pyrite s and organic c in the modern unit 1 sediments of the black sea to re evaluate this suggested relationship in box cores from shallow oxic or near oxic water pyrite s contents ranging from 0 1 and 2 5 by weight do not correlate with organic c contents in cores from the deep anoxic basin pyrite s concentrations range between 1 0 and 2 5 and are linearly related to organic c contents with an intercept that is not significantly different from zero the s c ratio is around 0 31 in these sediments which is somewhat lower than the ratio of 0 36 considered to be characteristic of normal marine oxic sediments thus the black sea anoxic sediments appear to be depleted in s relative to organic c the degree of pyritization dop of the sediments may be a better measure of anoxic conditions of sedimentation the dop values measured here show that pyrite formation is fe limited the correlation between organic c and pyrite s probably being induced by a positive correlation between fine grained sediment components the original fe source for pyrite formation in the sediments and organic matter such fe limitation is probably characteristic of anoxic basins where pyrite formation can take place in the water column and fe oxyhydroxides do not accumulate in the surface sediments the results on the sample set examined here are at variance with earlier data and interpretations for the black sea in particular the suggested s enrichment of anoxic black sea sediments was not found because of fe limitation

windblown sand on venus preliminary results of laboratory simulations abstract small particles and winds of sufficient strength to move them have been detected from venera and pioneer venus data and suggest the existence of aeolian processes on venus the venus wind tunnel vwt was fabricated in order to investigate the behavior of windblown particles in a simulated venusian environment preliminary results show that sand size material is readily entrained at the wind speeds detected on venus and that saltating grains achieve velocities closely matching those of the wind measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from martian and terrestial simulations results are in general agreement with theory although certain discrepancies are apparent which may be attributed to experimental and or theoretical modeling procedures present findings enable a better understanding of venusian surface processes and suggest that aeolian processes are important in the geological evolution of venus

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

vent type taxa in a hydrocarbon seep region on the louisiana slope recent discoveries on the northern gulf of mexico continental slope have altered our understanding of biological and chemical processes occurring in the deep ocean a biological community of hydrothermal vent type organisms was recently discovered at the base of the florida escarpment1 where the fauna are apparently nourished by hydrogen sulphide rich hypersaline water seeping out onto the sea floor dense colonies of deep living chemosynthetic benthic organisms were first discovered during investigations of warm water anomalies along the axis of the galapagos rift in the pacific ocean in 19772 4 and this first discovery of clusters of clams tube worms and other filter feeders in the immediate proximity of warm water vents has been followed by the discovery of a number of other hydrothermal vent sites for example guaymas basin east pacific rise at 21 n the dense population assemblages at these sites are apparently restricted to small areas of the ocean floor where hydrogen sulphide rich water is escaping from spreading centres but the florida escarpment discovery indicates that these communities can also exist on passive margins here we report the discovery of dense biological communities associated with regions of oil and gas seepage on the louisiana continental slope these communities of large epi and infaunal organisms are similar to those associated with the vents of the pacific and the hypersaline brine seeps of the florida escarpment carbon isotope analyses suggest that these communities are chemosynthetic and derive their energy from hydrogen sulphide and or hydrocarbons similar communities may be widely distributed on the sea floor in other oil producing regions of the ocean

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

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

cratering mechanics on venus pressure enhancement by the atmospheric ocean the mass per unit area of the venusian atmosphere is 106 kg m2 equivalent to a 1 km deep water ocean an important effect of this ocean on cratering is pressure enhancement due to multiple reverberations during the compression stage we consider impactors with diameters d less than a scale height 15 km such impactors drive before them bow shocks with standoff distances of 0 05 d into the lowermost atmosphere reflection of this shock at normal incidence from the basalt surface raises the pressure in the atmosphere and surface but initially not as much as a direct impact would several shock reverberations should occur however as the shock crossing time in the trapped compressed atmospheric layer is much less than either the shock crossing time in the impactor or the time for the impactor to penetrate the atmospheric layer with each reverberation we estimate new hugoniots for shocked atmosphere impactor and target the first is treated as an ideal gas of 1 2 1 3 and the latter are modeled with murnaghan equations of state and gruneisen s 1 the asymptotic pressure enhancement for a 20 km s carbonaceous chondrite impactor modeled as serpentine striking basalt at 20 km s relative to the atmosphereless 1 d impedance match pressure 550 gpa is a factor of 1 4 this maximum enhancement factor increases with velocity reaching 1 6 for serpentine moving at typical venus impacting cometary velocities 65 km s and is greater for more compressible impactors comets each reverberation sends a shock pulse into the surface or impactor that overtakes the previous ones hence at the end of the compression stage both impactor and target are shocked to a higher pressure than would occur in the absence of the atmosphere the impactor is effectively denser or staffer than in the atmosphereless case but as the energy and momentum delivered are essentially the same entering efficiency should be unaffected to first order however fusion and vaporization efficiency should be enhanced for impactors whose impact velocities are below whether initially or due to atmospheric drag the threshold for complete melting or vaporization respectively in the absence of an atmosphere

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

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

the role of magnesium in the crystallization of calcite and aragonite in a porous medium abstract morphological development of calcite crystals is related to supersaturation conditions during growth crystallization of calcium carbonate calcite and aragonite as well as mg calcite was studied under controlled supersaturation conditions by the counter diffusion of ca2 and co32 ions through a porous transport medium a column of silica gel under our experimental conditions where ion transport is constrained to be diffusion controlled nucleation and growth take place under conditions of high supersaturation the actual threshold value of the supersaturation depending on the supersaturation gradient in the pure caco3 system calcite grows at lower supersaturation than aragonite the calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites presence of mg2 in the interstitial fluid inhibits nucleation increasing the threshold supersaturation at which crystallization begins the resulting mg calcite crystals show a range of morphologies depending on the mg content and the supersaturation at the point of crystallization at high values of supersaturation up to 15 mol mgco3 is incorporated into the calcite and the crystals form spheres at lower supersaturations mg content decreases and morphologies change progressively through a well defined and reproducible sequence from spheres to dumbbell like forms to wheat sheaf like bundles and eventually single crystals with steep rhombohedral faces the crystals are compositionally zoned showing both sector and oscillatory zoning the compositional evol tion is related to the supersaturation and interface roughness during crystal growth

rare earth elements in cold seep carbonates from the southwestern dongsha area northern south china sea the circulation of methane rich fluids at cold seeps often leads to the precipitation of seep carbonates close to the seafloor along continental margins which can be used as records of past fluid seepage rare earth element ree concentrations in seep carbonates have been used to trace fluid sources and provide information on associated biogeochemical processes at cold seeps the ree concentrations of a series of carbonates collected from cold seeps in the southwestern dongsha area of the northern south china sea are analyzed in this study the total ree contents sigma ree of the seep carbonates analyzed show a wide variation from 17 ppm to 523 ppm with an average sigma ree value of 54 ppm which are higher than the carbonates from other cold seep areas a positive correlation between fe mn content and sigma ree was observed these results suggest that the seep carbonates of this study were primarily controlled by the methane derived fluid from which they precipitated the fe rich dolomite and siderite which are the main components of the carbonates are responsible for the enrichment of the ree a slight positive ce anomaly observed in the shale normalized ree patterns of the studied seep carbonates suggests that they formed in anoxic conditions and the correlations between ce ce and la n sm n ce ce and dy n sm n ce ce and sigma ree further reveal that the ree characteristics of most seep carbonate samples preserve the original redox conditions in which they precipitated and late diagenesis has had little effect on the ree however the ree characteristics of sub samples ds2 2b ds1 6a and ds1 7a are very different from those of the other sub samples indicating a greater impact of late diagenesis and post oxidation favored ree enrichment c 2014 elsevier ltd all rights reserved

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

fluid seepage associated with slope destabilization along the zambezi margin mozambique abstract evidences for active fluid seepages have been discovered along the zambezi continental slope offshore southern mozambique these seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization fluid seepages are interpreted as a potential trigger for the slope destabilization acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone exploration with the scampi towed camera system in the widest pockmark diameter 200 m wide has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats these fluid seepages are also associated to the presence of chemiosynthetic organisms vesicomyidae and thyasiridae bivalves siboglinidae tubeworms the sampled gas in the sediment corresponds mainly to ch4 of microbial origin generated by hydrogenotrophic methanogenesis from a substrate of organic origin i e a conventional process of genesis of microbial gas in the marine domain no evidence for thermogenic gas was detected another type of pockmarks has been observed within the core of the slope destabilization zone most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries they probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around the spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes this proposed evolution started by scattered seepages of formation water with dissolved gas then free gas seepages appeared notably in the upper part of the slope this was followed by progressive shallow deformation in the sediments downslope of the main gas seepages finally the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys

an authigenic calcite layer in the sediments of the sea of marmara a geochemical marker horizon with paleoceanographic significance abstract three sediment cores taken from the central sea of marmara were sedimentologically and geochemically investigated all the cores show a continuous sedimentation record according to isotopic age dating measurements two of the cores cover an age range of more than 20 000 years since the sea of marmara was an isolated lacustrine system during the last glacial maximum about 20 ka bp the two cores should include the transition from lacustrine to marine environmental conditions the third core has a very high sedimentation rate 70 cm ka and therefore does not reach the transitional section in two cores however a very calcite rich layer unit c was observed carbon isotope investigations show that this calcite is not of diagenetic or biogenic origin but may represent a direct inorganic precipitate out of a deepwater layer in order to prove this hypothesis we used a geochemical model based on thermodynamic calculations this model shows that the calcite precipitation can only be explained by mixing of anoxic deep lacustrine water with oxic marine mediterranean water the necessary aqueous conditions are also obtained the model presented thus provides an important contribution to the evolution of the sea of marmara during the transition from lacustrine to marine conditions the thickness of the unit c layer indicates that this environmental change lasted approximately 1 5 ka

an abrupt drowning of the black sea shelf abstract during latest quaternary glaciation the black sea became a giant freshwater lake the surface of this lake drew down to levels more than 100 m below its outlet when the mediterranean rose to the bosporus sill at 7 150 yr bp 1 saltwater poured through this spillway to refill the lake and submerge catastrophically more than 100 000 km2 of its exposed continental shelf the permanent drowning of a vast terrestrial landscape may possibly have accelerated the dispersal of early neolithic foragers and farmers into the interior of europe at that time

a late miocene methane seep deposit bearing methane trapping silica minerals at joetsu central japan abstract the modern japan sea is characterized by active methane seeps associated with gas hydrates but their ancient counterparts are not fully understood this study describes a newly discovered methane seep carbonate block the nakanomata seep deposit from the upper miocene nodani formation in joetsu city central japan the age of this deposit is constrained to 7 5 6 5 ma based on its fossil diatom assemblage the deposit contains molluscan fossils typical of methane seeps including vesicomyid and bathymodiolin bivalves and provannid gastropods and it retains an almost entirely aragonitic mineralogy despite its miocene age it is composed of clotted microcrystalline aragonite containing nodules and intraclasts and is crosscut by vein like networks of voids and cavities rimmed with acicular aragonite the 13c values of the carbonate phases are as low as 41 1 and the presence of lipid biomarkers pentamethylicosane and crocetane suggests that the deposit originated from the anaerobic oxidation of methane it is suggested that an initially diffuse methane seepage formed the micritic nodules followed by a more rapid and intense methane seepage that led to the development of abundant voids in the sediment finally the sediment was cemented by microcrystalline aragonite and void lining acicular aragonite the seep deposit also contains peculiar globular silica minerals and authigenic quartz during their precipitation these globular silica minerals may have trapped methane gas bubbles and the minerals may be pseudomorphs after silica clathrate sufficient increase in ph and supersaturation of silica which led to the dissolution and subsequent precipitation of these silica minerals could have resulted from the degassing of carbon dioxide promoted by an effective supply of methane and its supersaturation thus forming gas bubbles

hydrothermal origin of devonian conical mounds kess kess of hamar lakhdad ridge anti atlas morocco various interpretations have been proposed for the origin of peculiar cone shaped finely crystalline devonian carbonate mounds of the hamar lakhdad ridge anti atlas morocco ranging from shallow water reefs to deep water mud mounds formed by biogenic and or hydrodynamic processes this study is the first integrated sedimentological and geochemical analysis of these structures the mounds are cone shaped steep sided circular to subelliptical in plan view and exhibit internal crude bedding parallel to the mound9s outer surface they occur in a cluster of 48 mounds on top of a volcanic massif stable isotope analyses of first stage isopachous nonluminescent cement have yielded marine values in contrast the finely crystalline carbonate that makes up the bulk of the mound and the internally sedimented mud between crusts of early marine cements have significantly lower 18 o values whereas values of 13 c are similar to those of the early marine cement strontium isotope ratios also clearly distinguish the nonluminescent early marine cements from the finely crystalline material the 87 sr 86 sr values for the former 0 707 934 0 709 392 are in or near the devonian marine range whereas the ratios for the latter 0 708 515 0 709 656 indicate a more radiogenic sr source on the basis of their lithology architecture isotope geochemistry and clustered occurrence on top of a volcanic pile we propose that the finely crystalline material that forms the mounds and the intermound beds was precipitated from hydrothermal fluids and that accretion of the material was driven by hydrothermal venting

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

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

stable isotopic ratios and origins of the carbonates associated with cold seepage at the eastern nankai trough two types of cold seeps were located at the eastern nankai trough 1 seeps on sediment covered seafloor of 3800 3600 m in depth at the toe of the accretionary prism and 2 those between 1900 and 2100 m situated on the backthrust of the accretionary prism covered with rough outcrops of calcareous sandstones fragments of sandstone pipes or chimneys cemented with calcite were recovered from type 1 seeps whereas in type 2 seeps massive carbonate cemented sandstones and silty sandstones were obtained many of which contain cemented fragments of different lithologies and textures including dense aggregates of fossilcalyptogena type clam shells the carbon isotopic ratios of the carbonates in these sediments range from 36 to 56 pdb the carbonates formed by mixing between isotopically light carbonate ions derived from biogenic methane carried from depth in advecting fluids and heavy marine sedimentary carbonates sharp isotopic gradients in carbon and sometimes also in oxygen from the mixing processes are still retained across the walls of the sandstone pipes from type 1 seeps whereas the isotopic ratios of the samples from type 2 seeps exhibit variation patterns related to lithologies and textures and confirm the presence of several generations of carbonate cement within a single specimen these facts and the oxygen isotopic ratios of live and fossil clam shells recovered from the survey areas suggest that the cold seeps started at least during the last glaciation and have experienced repeated destruction and reformation due to episodic supply of methane rich fluid of different isotopic ratios and or temperature

cold seeps and authigenic carbonate formation in monterey bay california abstract authigenic carbonate associated with modern cold seep biological communities and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of monterey bay although such variations in ancient samples have been used to infer differing tectonic settings these carbonates all formed within a faulted continental margin environment and chemical variations reflect local differences in the sources and flux of carbon to sediment pore fluids the slow seepage of fluid and with it dissolved carbon along the transform faulted continental margin results in discrete areas of enhanced microbial sulfate reduction oxidation of methane from both biogenic and thermogenic origins as well as the active precipitation of both high mg calcite hmc and dolomite the authigenic carbonates include semicontinuous pavements of shallow cemented sediments surrounding benthic communities circular or pipe like chimneys interpreted as cemented conduits formed as a result of methane gas expulsion centimeter to meter scale rings doughnuts or slabs winnowed from variable depths within sediments and carbonate veins ankerite or calcite or cements in faulted basement rocks draped with bacterial mat abundant pyrite framboids preferentially filling the tests of the benthic foraminifer uvigerina peregrina and characteristic of the hmc bearing samples are products of a zone of shallow microbial sulfate reduction a process fundamental to the nourishment of the chemosynthetic cold seep communities sites on a sedimented ridge west of the san gregorio fault zone have carbon isotopic values between 35 and 56 that are strongly influenced by carbon derived from methane the higher values of 18 o more enriched in 18 o are found closer to the active fault zone carbonates from sites within the san gregorio fault zone or from monterey canyon floor exposures of miocene sediments have carbon isotope values between 7 and 26 that are mixtures of sedimentary organic carbon and microbially oxidized hydrocarbons from organic rich petroleum source rocks dolomite bearing samples are more enriched in both 18 o and 13 c bulk carbonate samples with heavier 18 o 18 o 3 5 result from the abundance of authigenic dolomite and low temperatures of formation local destabilization of clathrates might also create pore fluids that are more enriched in both 13 c and 18 o

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

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

accelerating exhumation in the eocene north american cordilleran hinterland implications from detrital zircon u th he pb double dating

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

holocene methane derived dolomite cemented sandstone pillars from the kattegat denmark abstract dolomite cemented sandstone pillars up to 4 m high and 1 m in diameter are recorded from 10 12 m of water in the kattegat denmark the framework of the sandstone pillars is similar to that of the adjacent sediments i e medium grained well sorted quartz sand the carbonate cement consists of equicrystalline ca rich dolomite mg47 ca53 co3 10 30 m in diameter exhibiting a sucrosic texture the sr content is relatively constant at approximately 1000 ppm whereas the fe and mn contents are highly variable with mean values of 1870 and 320 ppm respectively the carbon isotopic signature of the dolomite cement displays negative 13c values in the range of 44 to 40 the oxygen isotope composition shows 18o values in the range of 3 7 to 3 9 and is enriched in 18o by approximately 3 in comparison with co existing high mg calcite the negative 13c values suggest that the dolomite has derived its carbon via methane oxidation the genetic connection between dolomite cement formation and methane oxidation is visually supported by gas seeps from the seabed and vents in the sandstone pillars themselves it is believed that the methane originates from shallow gas in late quaternary marine deposits which led to a discharge of methane in the diagenetic zones of the topmost sedimentary column the dolomite precipitation most probably took place as a result of anaerobic methane oxidation subsequently the lithified sandstone pillars were exposed on the seafloor by large scale submarine erosion

u th dating and post depositional alteration of a cold seep carbonate chimney from the campos basin offshore brazil abstract dating authigenic cold seep carbonates allows determining the timing of active seepage of hydrocarbon rich fluids at continental margins and ultimately the detection of the geological factors affecting it u th dating has been selected in this study the method having been successfully used before however incorporation of initial thorium in substantial amounts of detrital material due to the precipitation of carbonates within the seabed and post depositional alteration potentially pose a major problem violating the basic assumptions of the method here we report results from several samples from the campos basin off the coast of brazil they are unusual in their high 13 c values mean of 10 0 vpdb high detrital contents and being dominantly dolomite elevated 230 th 238 u activity ratios indicate that most of the carbonates lost uranium during diagenesis isochrones are obtained from the unaffected samples yielding an age of 140 4 3 1 ka vertical growth of the chimney must have been short and rapid since results from the top and bottom parts of the chimney concur within analytical uncertainties furthermore profiles of u th radioisotope specific activity 234 u and 232 th specific activity on two quarter slices of a carbonate chimney reveal the occurrence of three distinct diagenetic processes u loss u exchange with seawater and th gain

magellan initial analysis of venus surface modification initial magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes ejecta deposits extending up to 1000 kilometers to the west of several impact craters windblown deposits and features in areas where there are both obstacles and a source of particulate material and evidence for slow steady degradation by atmosphere surface interactions and mass movements

shock implantation of martian atmospheric argon in four basaltic shergottites a laser probe 40ar 39ar investigation spatially resolved argon isotope measurements have been performed on neutron irradiated samples of two martian basalts los angeles and zagami and two martian olivine phyric basalts dar al gani dag 476 and north west africa nwa 1068 with a similar to 50 mu m diameter focused infrared laser beam it has been possible to distinguish between argon isotopic signatures from host rock matrix minerals and localized shock melt products pockets and veins the concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured j values ar 40 ar 39 ratios and k2o wt in each phase melt pockets contain on average 10 times more gas 7 24 ppb ar 40 than shock veins and matrix minerals 0 3 3 ppb ar 40 the ar 40 ar 36 ratio of the martian atmosphere estimated from melt pocket argon extractions corrected for cosmogenie ar 36 is los angeles similar to 1852 zagami similar to 1744 and nwa 1068 similar to 1403 in addition los angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs 1 martian atmosphere in melt pockets and 2 a trapped component possibly martian interior ar 40 ar 16 480 490 in matrix minerals average apparent ar 40 ar 39 ages determined for matrix minerals in the four analyzed meteorites are 1290 ma los angeles 692 ma zagami 515 ma nwa 1068 and 1427 ma dag 476 these ar 40 ar 39 apparent ages are substantially older than the similar to 170 0474 ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40ar cosmic ray exposure cre ages were measured using spallogenic ar 16 and ar 38 production los angeles 3 1 0 2 ma zagami 2 9 0 4 ma and nwa 1068 2 0 0 5 ma yielded ages within the range of previous determinations dag 476 however yielded a young cre age 0 7 0 25 ma attributed to terrestrial alteration the high spatial variation of argon indicates that the incorporation of martian atmospheric argon into near surface rocks is controlled by localized glass bearing melts produced by shock processes in particular the larger mm size melt pockets contain near end member martian atmospheric argon based on petrography composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection three processes may have led to atmosphere incorporation 1 argon implantation due to atmospheric shock front collision with the martian surface 2 transformation of an atmosphere filled cavity into a localized melt zone and 3 shock implantation of atmosphere trapped in cracks pores and fissures

radiometric dating of submarine hydrocarbon seeps in the gulf of mexico massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid bathymodiolus sp containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and u series isotopes stable carbon isotopes and 87 sr 86 sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon rich fluids samples from five seepage sites on the northern gulf of mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to 2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives in general paired 14 c and 230 th dates of carbonate buildups consisting of aragonite high mg calcite and dolomite mineralogies are highly discordant the cause of the discordance established on the basis of paired 14 c and 13 c values 14 c 898 to 992 13 c 9 5 to 53 3 for n 27 lies with the impairment of the radiocarbon dates resulting from dilution of the 14 c pool with fossil hydrocarbon derived carbon the validity of the ionium dates based on u rich 2 4 7 6 ppm samples is demonstrated by the concordance between 234 u 238 u and 230 th 234 u evolution in time and by the 234 u 238 u o activity ratios that are generally within the range of sea water value of 1 14 0 04 2 sigma some dolomite rich samples are exceptional because their 234 u 238 u o ratios are significantly higher 1 22 1 38 than sea water suggesting deposition from anoxic pore waters where the soluble u reached anomalous 234 u 238 u ratios the formation of the carbonates from sea water derived fluids rather than from formation fluids advecting from deep aquifers is supported by the 87 sr 86 sr composition of the samples mean 0 709145 19 10 6 n 14 that compares well with modern nonseep marine carbonates and the ambient gulf of mexico sea water 0 709171 8 10 6 calcareous shells the 13 c values of which indicate a carbon source in sea water 13 c 4 3 to 1 1 yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates isotope migration attested by the observed u uptake in the fossil shells is likely to affect the accuracy of their ionium dates radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late pleistocene time 195 13 ka ages derived from nascent seep sites at mid bathyal and abyssal depths 12 3 0 0 ka indicate that currently vigorous seepage was initiated at the end of the last deglaciation these radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor

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

a stable isotope study of pyrite formation in the late pleistocene and holocene sediments of the black sea the abundance and isotopic composition of total and pyrite sulphur have been determined in a core in the central black sea pyrite varies in concentration from roughly 2 wt in the sapropel unit 2 to 0 8 wt in the modern horizon unit 1 and ca 0 1 wt in the lowermost horizons of unit 3 lake beds an intermediate mud flow horizon and the upper part of the lake beds have similar values to those in the modern sediments the degree of pyritization dop is lowest 0 05 in the lower lake beds and highest 0 55 in unit 1 the sapropel has values of ca 0 48 34spyrite varies from a minimum of 37 in the sapropel to 33 in unit 1 up to 15 in the upper lake beds and 0 in the lower lake beds the unit 1 and sapropel values are similar to the 34s values reported by fry et al 1991 of dissolved sulphide immediately below the water column oxic anoxic interface 36 to 38 but significantly heavier than those in the deep waters of the basin ca 41 these results are interpreted as a reflection of the presence of four different types of pyrite in the sediments of the black sea 1 pyrite in units 1 and 2 that is formed within the upper part of the water column immediately below the oxic anoxic interface where 34s of dissolved sulphide is 36 2 pyrite in the mud flow layer lying between unit 1 and the sapropel unit 2 with a significantly heavier isotopic composition that was formed around the margins of the basin under conditions that were probably more closed with respect to dissolved sulphate than those in which the pyrite in units 1 and 2 were formed 3 pyrite in the upper part of the lake beds unit 3 that was formed under closed system conditions by reaction between excess reducible fe remaining in these freshwater sediments and downward diffusing sulphate and or sulphide from the overlying organic rich sapropel and 4 pyrite in the lower part of the unit 3 representing a phase formed from the ambient sulphate in the freshwaters of the late pleistocene lake the isotopic information supports earlier suggestions that pyrite in the modern sediments of the black sea forms mainly in the water column with only minor amounts forming in the sediment because of the severe fe limitation of the bottom sediments that accumulate 2 000 m below the main fe and hs reaction zone at the oxic anoxic interface at shallow depths in the water column

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

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

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

anaerobic methane oxidation by bacteria employing 14c methane uncontaminated with 14c carbon monoxide 14c labelled methane biologically prepared by methanobacterium thermoautotrophicum is widely used to determine methane oxidation rates however m thermoautotrophicum synthesizes carbon monoxide as a by product during methanogenesis in this study sulfate reducing bacteria utilizing the acetyl coa carbon monoxide dehydrogenase pathway were able to form 14co2 from 14ch4 containing 14co 14c labelled carbon monoxide was removed from 14ch4 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution measurable formation of 14co2 from purified 14c labelled methane by sulfate reducing bacteria was not observed therefore reported anaerobic methane oxidation rates in marine habitats measured with 14ch4 from m thermoautotrophicum are inclined to include carbon monoxide oxidation rates anaerobic oxidation of 14ch4 by sulfate reducing and acetogenic bacteria and methanogenic archaebacteria was tested only methanogenic species produced up to 900 ppm 14co2 from 14ch4 applied this observation and the absence of methane oxidation by sulfate reducing bacteria sustain the hypothesis that methanogenic archaebacteria in a syntrophic community might be responsible for the oxidation of methane in anaerobic habitats

wind interaction with falling ejecta origin of the parabolic features on venus unusual parabolic features associated with impact craters have been observed by magellan on venus a strong correlation exists between the orientation of the features and the zonal winds on venus we propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater as the ejecta particles fall through the atmosphere the winds transport them downwind from their entry point smaller particles being transported a greater distance since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater the winds blow the particles on the upwind side back upon one another leading to a pile up of material on the downwind side the winds disperse the ejecta particles and no pile up occurs the resulting thickness distribution on the venusian surface matches the observed parabolic features closely the dual parabolic feature associated with the crater carson is also explained by this model

dynamic accumulation of gas hydrates associated with the channel levee system in the shenhu area northern south china sea abstract research on the formation and distribution of submarine channel systems and associated gas bearing fluids is of great significance for gas hydrate exploration disseminated gas hydrates with high saturation up to 65 were recovered from a submarine ridge equivalent to the levee of the channel levee system in the shenhu area northern south china sea sedimentary deposits in the submarine ridge were dominated by fine grained silt and clay rich silt gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content although abundant foraminifera fossils may have increased reservoir pore space their presence was not a necessary condition for high saturation hydrates higher levels of pyrite appeared in the reservoirs corresponding to high saturation hydrates which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher saturation hydrates because of the migration of the channel levee system different channels formed their respective depositional systems composed of channel filling buried channel filling erosion grooves and slumped turbidities relatively coarse grained deposits were identified in the channel fillings and levees and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone ghsz ghsz modeling based on in situ measurements indicated that erosion and sedimentation as well as variations of the geothermal gradient resulted in the upward downward migration of bottom simulating reflectors bsrs on the erosion flank of the channel the strata thinned and rapid erosion was likely to destroy the shallower bsr causing gas hydrate decomposition and methane release and may have caused turbidite slumping and seepage whereas the strata thickened on the deposition flank of the channel the bsr in the channel levee system would gradually move toward the new ghsz eventually forming a new bsr parts of the bsr that formed under the original p t conditions have remained and double bsrs occurred in the seismic profile the thermal fluid that moved upward through a gas chimney may also have caused the migration of the ghsz resulting in the emergence of double bsrs during the lateral migration of the channel and the vertical migration of the gas bearing fluid there was a dynamic adjustment relationship between the ghsz and the erosion deposition process of the channel resulting in the dynamic accumulation of hydrates in the shenhu area a model to demonstrate the relationship between channel migration and variation of the bsr was established which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates

stable isotope evidence for chemosynthesis in an abyssal seep community communities of abundant organisms similar to those which surround the hydrothermal vents of the east pacific rise epr occur in 3 266 m of water in the gulf of mexico they were discovered with the alvin at 26 02 n 84 55 w in an area where pore waters seep from the cretaceous limestones of the adjacent platform at the contact between the florida escarpment and the holocene hemipelagic sediments of the abyssal gulf because of the unusual abundance of organisms in this deep sea environment and the taxonomic similarities to the chemosynthetically based communities which surround the hydrothermal vents on the epr the seep communities are suspected to exist on a local non photosynthetic food source1 we have now measured isotope ratios in tissues of organisms which surround these saline seeps to determine the origin of the local food chain the tissues have extremely negative 13c 42 to 77 pdb and 15n 2 72 to 9 34 air values such highly fractionated carbon and nitrogen isotopes are unknown in food chains based on photosynthesis suggesting that these communities are chemosynthetic the cause of this extreme fractionation is attributed either to assimilation from local supplies of isotopically depleted methane and ammonium or to biochemical fractionation the tissues of these animals contain significant amounts of 14c 60 modern so the source of this fractionated carbon is not predominantly fossil methane

carbon sulfur iron systematics of the uppermost deep water sediments of the black sea box cores recovered during leg 4 of the 1988 r v knorr black sea oceanographic expedition from deep water regions of the basin were dominated by coccolith rich microlaminated unit 1 sediment and muddy gray turbidite layers both organic carbon oc and pyrite sulfur values for unit 1 display narrow ranges with mean concentrations of 5 3 1 1 1 wt and 1 3 0 3 wt respectively unit 1 is not enriched in pyrite s relative to sediments deposited under oxygenated bottom waters normal marine sediments with comparable oc concentrations carbon sulfur relationships evaluated on a calcium carbonate free basis to avoid spurious correlations resulting from dilution effects demonstrate that oc and pyrite s are essentially decoupled these observations combined with the persistence of elevated pore water sulfide to depth and a strong correlation between pyrite s and the detrital fe component argue strongly for limitation of pyrite formation in unit 1 by the availability of reactive fe unit 1 fe limitation is further indicated by degree of pyritization dop studies a measure of the extent to which the original potentially reactive fe has been transformed to pyrite these studies show sulfidation of reactive fe ranging from 57 to 78 with dop values independent of oc concentration unit 1 dop profiles suggest that the majority of the pyrite is formed in the sulfidic water column and or very close to the sediment water interface pyrite s concentrations of unit 1 when compared with the particulate reduced sulfur fluxes measured in time series sediment traps are compatible with predominantly water column pyrite formation because of the limitations in the supply of reactive fe associated with the comparatively high supply of oc the microlaminated sediment is characterized by c s ratios greater than those typical of holocene oxically deposited sediments the turbidite muds of the deep basin display high reduced s values relative to holocene normal marine sediments in samples with low oc concentrations low c s ratios this reflects pyrite formation under anoxic sulfidic bottom water conditions in a probable upper slope source region as well as during transport and final deposition intermediate dop values for the turbidites in part a product of their rapid rate of deposition reveal that fe limitation is ultimately not a factor and that further pyrite should form during burial however the very rapid rate at which unit 1 pyrite forms suggests fundamental differences between the overall reactivities of the fe phases associated with the microlaminated and turbiditic sediments the signature of water column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high dop values of unit 1 and the comparatively high levels of s associated with the low concentrations of oc of the turbidite muds this agrees with similar conclusions based on studies of ancient sedimentary rocks

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

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

trace element systematics in cold seep carbonates and associated lipid compounds abstract seeping of methane rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates some trace elements play an important role in the biogeochemical processes operating at cold seeps especially as specific enzymatic co factors related to methanogenesis and the anaerobic oxidation of methane aom however it is unclear whether microbial trace metal utilization can be traced by the geochemical composition of seep carbonates in this study we analyzed a series of authigenic carbonate samples recovered from various seep settings worldwide and report for the first time trace element concentrations for total lipid fractions combined with biomarker analyses and determination of elemental abundances in associated inorganic mineral phases carbonate phases sulfides organic compounds and detrital fractions our results indicate marked enrichments of co ni cu mo and w in the archaeal and bacterial lipids associated with authigenic carbonates which can all be ascribed to previously identified enzymatic pathways in addition to the microbial communities involved in aom which most likely control specific lipid bound enrichments of co ni mo and w in seep carbonates cu was found to display higher concentrations in the lipid fractions extracted from a few authigenic carbonate samples formed closer to the sediment water interface hence possibly related to the presence of aerobic methane oxidizing bacterial assemblages in the near seafloor environment while the above mentioned trace metals are relatively enriched in all studied inorganic and organic fractions the very low w concentrations measured in carbonate phases combined with their pronounced enrichment in associated lipid fractions and inferred microbial requirement suggest that tungsten depletion in pore waters could possibly act as a limiting factor on aom at cold seeps finally two other trace elements li and ti also displayed particular enrichments in studied lipid fractions which despite no reported evidence could possibly indicate that they are also involved as metalloenzymes in microbial methane oxidation processes at cold seeps

atmospheric effects on cratering on venus a paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock induced modifications surrounding venusian impact craters we apply two dimensional oblique shock dynamics to describe a three dimensional paraboloidal shaped bow shock impinging upon an assumed incompressible venusian surface the effects of the hypersonic atmospheric shock acting on the venusian surface are considered in terms of induced maximum gas pressure density particle velocity and temperature for varying angles and velocities of impact the maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered the present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity and radii of impactors for a 1 km radius stony meteorite impacting normally at 20 km s the radius of the disturbed area extends 10 17 times the 3 5 km crater radius the perturbed surface affects the surface radar properties and the present results can provide an explanation of the wide dark bright halos surrounding some of the venusian impact craters observed via magellan imagery for example a 50 km radius bright halo surrounding a 20 km dark halo is observed around the 3 1 km radius crater located at 16 5 north latitude and 334 4 longitude the average value of the radar backscatter cross section of the 20 km radius dark halo indicates that 50 cm thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface the bright halo indicates that the surface roughness in this region is 30 greater than that of the surrounding original surface these features can be induced by atmospheric shock waves the present model can relate the observed crater halo radii to the impact parameters such as projectile radius and density and the impact velocity and angle

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

rb sr mineral ages for the grenvillian metamorphic development of spilites from the dalsland supracrustal group sw sweden rb sr isotope systematics have been investigated of epidote actinolite albite and biotite concentrates from 4 spilite samples from the precambrian basement directly w of lake vanern in sw sweden the rocks belong to the dalsland supracrustal group whose basal conglomerate was deposited on an erosion surface with a maximum age of c 1 200 ma epidote wr and actinolite wr tie lines in a nicolaysen diagram give ages with an average of 990 10 la ma which is thought to represent the metamorphic culmination biotite wr lines yield an average age of 900 15 1 ma which indicates the time at which the rock complex cooled through the isotopic closure temperature of biotite estimated at c 250 c the average cooling rate in this period was very low 1 2 c ma initial sr isotopic ratios of the 4 spilite samples vary from 0 7040 to 0 7080 and are thought to reflect interaction of mantle derived basalts with seawater

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

impact cratering on venus physical and mechanical models results are presented on laboratory theoretical and numerical simulations of various phenomena related to impact cratering processes specific to the venusian environment such processes are influenced mainly by two factors the high atmospheric density and the high surface temperature model results are presented for the influence of the atmosphere on high velocity meteoroids the influence of high velocity meteoroids on the atmosphere the influence of atmospheric shock waves on the venusian surface and the influence of high crustal temperature on impact melting and melt cooling new modeling techniques based on the two dimensional free lagrangian method have been employed and show that an icy body of 1 km radius may reach the bottom of the venusian atmosphere in a partially dispersed state and form an impact crater approximately 20 km in diameter a significant amount of energy is transferred to the venusian atmosphere during meteoroid transit and the gas dynamic flow will be similar to a strong explosion modeling shows that a significant wake of low density will form behind the meteoroid and have an important influence on crater formation and ejecta distribution according to our estimates the boundary of the flow of dense gas may correspond to the region of hummocky ejecta deposition observed for venusian impact craters the collapse of the gas rock debris column on the boundary between the low and high density regions may result in a turbiditelike flow outside the hummocky ejecta area we carried out preliminary laboratory simulations of the interaction of meteoroid shock waves with the surface considerable surface disruption is possible even under the trajectory of an oblique impact the possibility of back venting caused by shock wave pressuring and depressuring in ground pores may provide an explanation for the origin of dark margins the differences between venus and earth for impact melting and melt cooling were examined melt generation on venus should be 20 to 40 greater than on earth for the same crater diameter melt volumes should stay molten about an order of magnitude longer on venus leading to substantial melt flows and smooth crater floors

implications from stereo derived topography of venusian impact craters using radargrammetry we have created high resolution topographic maps of 74 venusian craters including all bright floored craters over 12 km in diameter covered by magellan stereo imagery our trend for rim floor depths rf as a function of diameter d for bright floored craters in the volcanic plains is rf 0 345 0 05 d0 235 0 05 and for dark floored craters in the plains rf 0 5 0 1 d0 0035 0 07 rim heights rh for bright floored craters in the plains are rh 0 06 0 03 d0 4 0 2 d 15 km and for dark floored craters in the plains rh 0 027 0 015 d0 4 0 2 these trends indicate that bright floored craters 30 km in diameter are on average deeper than dark floored craters by 180 m from rim to floor and have a 140 m higher rim and at 90 km in diameter they are 380 m deeper from rim to floor and have a 220 m higher rim the bright and dark floored populations are different at a 99 confidence level for both rim floor depths and rim heights the interpretation most consistent with our data and previous work by others is that venusian craters with radar dark floors have been partially filled and had their ejecta blankets embayed by regional scale lava flooding when the topography is examined in conjunction with the imagery it is clear that many dark floored craters have been surrounded by lavas that rose nearly to the crater rim even though a substantial portion of the crater s ejecta blanket was retained most previous analyses of the resurfacing history of venus have relied on past interpretations that only a small percentage of venusian craters are embayed by exterior volcanism because most craters on venus have dark floors our data indicate that the majority of venusian craters have been surrounded and partially filled by postimpact lavas and consequently those previous analyses may have significantly underestimated the amount of volcanism on the venusian surface over the past few hundred million years rim floor depths for venusian craters are consistent with the inverse gravity trend observed for the terrestrial planets and they are 50 deeper than current estimates for complex craters on the earth unlike the other terrestrial planets neither terrain floor depths nor central structure heights increase with increasing crater diameter an interesting trend for which we have no explanation is that on venus the moon mars and ganymede central peaks generally rise to within a constant elevation relative to the surrounding terrain but that elevation is lower on the moon and mars than on venus and ganymede

radiocarbon dating of danube delta deposits abstract the holocene history of the danube delta has been studied using 14c analyses of faunal material the principal phases of development include 1 initial letea caroarman spit 11 700 9800 yr b p in its central part 8800 5500 yr b p in its southern part 2 sf gheorghe i delta 8900 7200 yr b p 3 sulina delta 7200 2000 yr b p and 4 sf gheorghe ii delta chilia delta 2000 yr b p present other smaller scale features have also been dated including secondary deltas cosna and sinoe deltas and littoral bars age determinations carried out on whole samples were erratic and it was found that individual species had to be separated and dated in all cases the older dates were of transported material whereas the younger dates gave the true age of the formation

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

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

anomalous sea floor backscatter patterns in methane venting areas dnepr paleo delta nw black sea the relation between acoustic sea floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single beam echosounder this study is further supported by side scan sonar recordings high resolution 5 khz seismic data pore water analysis grain size analysis and visual sea floor observations the datasets were acquired during the 2003 and 2004 expeditions of the ec funded crimea project in the dnepr paleo delta area northwestern black sea more than 600 active methane seeps were hydroacoustically detected within a small 3 96 km by 3 72 km area on the continental shelf of the dnepr paleo delta in water depths ranging from 72 m to 156 m multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area seeps generally occur within medium to high backscatter areas which often coincide with pockmarks high resolution seismic data reveals the presence of an undulating gas front i e the top of the free gas in the subsurface which domes up towards and intersects the sea floor at locations where gas seeps and medium to high backscatter values are detected pore water analysis of 4 multi cores taken at different backscatter intensity sites shows a clear correlation between backscatter intensity and dissolved methane fluxes all analyzed chemical species indicate increasing anaerobic oxidation of methane aom from medium to high backscatter locations this is confirmed by visual sea floor observations showing bacterial mats and authigenic carbonates formed by aom grain size analysis of the 4 multi cores only reveals negligible variations between the different backscatter sites integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane derived authigenic carbonates mdacs caused by aom the carbonate formation also appears to lead to a gradual self sealing of the seeps by cementing fluid pathways horizons followed by a relocation of the bubble releasing locations

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

significance of localized pore pressures to the genesis of septarian concretions the burial stress and hydrologic conditions existing during concretion formation in mudrocks are evaluated and integrated into a model for the genesis of septarian cracks initial concretion cement formation will lower concretion permeability through the filling of pre existing pore space during progressive burial this may lead to increased excess pore pressure localized within the concretion body causing a reduction of the effective stress analysis of the stress conditions and crack morphology suggests that cracks in septarian concretions result from tensional failure sub critical crack growth as a consequence of this localized excess pore pressure conditions suitable for crack formation will depend upon the magnitude of the excess pore pressure and the stress corrosion limit of the concretion body a review of the likely strength of such concretions indicates that cracking could be initiated at depths less than 10 m a variety of observed crack morphologies can be explained with this model depending upon the spatial distribution of strength and effective stress in the concretion crack orientations mostly reflect stress anisotropy but are also influenced by directional anisotropy in the crack growth rates locally increased pore pressure also likely occurs in non septarian concretions but is not sufficient to cause cracking this enhanced local pressure may assist the crystal surface growth reactions of the carbonate cement through this enhancement process the shape of concretions may be a response to the local anisotropic pore pressure contours which reflect the permeability anisotropy of the concretion and surrounding mudrock

structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the harstad basin southwest barents sea the harstad basin is a structural block on the continental shelf of sw barents sea where gas hydrates likely occurred below the grounded ice sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites we present an integrated study of fluid flow systems in the harstad basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane derived carbonate crusts more than 190 acoustic gas flares were registered in water column many of them in association with pockmarks and carbonate crust fields however weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation in the western harstad basin seeps of microbial methane occur mainly above tertiary formations that are pinching out below the glacial sediments high amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within tertiary deposits in contrast gas seeping in the eastern harstad basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted mesozoic rocks occurring below glacial sediments this spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts with 13 c values typically between 55 and 42 and 40 and 20 vpdb for carbonate crusts associated with microbial and thermogenic fluids respectively u th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor

sr and nd isotopes of cold seep carbonates from the northern south china sea as proxies for fluid sources abstract in this study sr isotopes combined with nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids we presented sr and nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the south china sea scs based on the evidence for negligible detrital contamination and diagenetic alteration carbonate samples are considered dependable archives which record sr and nd isotope signals of the original pore waters from which carbonates have precipitated excellent correlation between 87sr 86sr ratios and 1 sr observed in carbonate samples indicates two end members with distinct sr isotopic signatures including modern seawater as a major source and a lesser contribution from methane rich seep fluid driven from a deep source besides these two end members release of nd from organic matter and fe mn oxyhydroxide is an additional contribution to pore water nd furthermore sr and nd isotopic compositions suggest water rock interactions between upward seep fluid and detrital component in sediments which resulted in the isotopic shift towards pronounced 143nd depleted and slightly 87sr enriched in the fluid phase this study provides insight into the knowledge for tracing seepage fluid circulation by sr nd isotope system at modern seep systems

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

the upper layer circulation of the black sea its variability as inferred from hydrographic and satellite observations quasi synoptic hydrographic data and satellite imagery are used to describe the circulation and the structural variability of the black sea with particular emphasis on the turkish coast the circulation is indicated to involve a variable cyclonic circulation with no apparent central locus and a well defined cyclonic rim current containing meanders and interacting eddy fields confined to the shelf slope interspersed between the coastal eddies are filaments and intense jets often with dipole eddies at their termina the extension of these features across the shelf slope into the central basin offshore waters implies important dynamical processes related to the shelf deep basin exchanges these features are often steered by the topography and evolve continuously through the mixed baroclinic barotropic instability of the rim current

using 87sr 86sr ratios to date fossil methane seep deposits methodological requirements and an example from the great valley group california abstractmethane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems for this information to be useful a reliable determination of the carbonates stratigraphic ages is required but this is not always available here we investigate the using strontium isotope stratigraphy to date fossil methane seep carbonates via detailed petrographic and geochemical investigation of the different carbonate phases in biostratigraphically well dated seep carbonates of paleozoic mesozoic and cenozoic age the best results are obtained from banded botryoidal rim cements from carbonate phases showing a weak or no cathodoluminescence signal an oxygen isotope signature close to that of seawater and the lowest mn concentrations we then applied the method to a presumably late jurassic seep carbonate from the great valley group in california strontium isotope ratios of the least diagenetically altered carbonate phases indicate a tithonian late jurassic

dolomite chimneys as possible evidence of coastal fluid expulsion uppermost otago continental slope southern new zealand abstract a suite of carbonate chimneys slabs and ridges occur on a 1 km 2 220 m deep terrace on the uppermost continental slope east of the otago peninsula new zealand carbonate content and petrographic observations suggest cementation took place just below the sediment water interface encrustation borings and the abundance of chimneys and carbonate debris observed today on the uppermost slope suggest that the chimneys have been exhumed and are relict features with a maximum radiocarbon age of 33 000 550 yr b p negative 13 c values of 4 9 to 12 8 pdb indicate that the carbon contained within the cement was derived from the biogenic degradation of organic matter a 18 o composition of 4 9 to 5 4 pdb suggests a marine origin palaeotempertures of precipitation are consistent with modern bottom water temperatures cement in the chimneys is considered to be derived from aquifer forced ground water marine in character and sourced from the exposed inner middle shelf during glacially lowered sea level this contrasts with the tectonically induced fluid flow proposed for vent sites on convergent margins