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

comparison of 14c ages of hydrothermal petroleums abstract in order to set limits on the time frame of formation of hydrothermal petroleum we have obtained 14 c ages on samples from three diverse regions gulf of california guaymas basin northeast pacific ocean escanaba trough and middle valley and the east african rift tanganyika trough the results date the source of carbon and therefore provide maximum ages for the formation and emplacement of the hydrothermal petroleums the youngest petroleum occurs iin the souther trough of guaymas basin 3200 6600 yr mean 4692 yr in the northern trough the petroleum is slightly older 7400 yr significantly older hydrothermal petroleum occurs in escanaba trough 17 000 yr and middle valley 29 000 yr a continental example from the east african rift has an age of 25 000 yr comparable to the ages observed in the oceanic samples from the northeast pacific ocean these ages affirm that hydrothermal petroleum formation is a very rapid process and took place some time between the latest pleistocene and the present in these active hydrothermal systems

discovery of hydrothermal vent fields on alarcon rise and in southern pescadero basin gulf of california author s paduan jb zierenberg ra clague da spelz rm caress dw troni g thomas h glessner j lilley md lorenson t lupton j neumann f santa rosa del rio ma wheat cg abstract 2018 the authors hydrothermal vent fields located in the gap between known sites in guaymas basin and 21 n on the east pacific rise were discovered on the alarcon rise and in southern pescadero basin the alarcon rise spreading segment was mapped at 1 m resolution by an autonomous underwater vehicle individual chimneys were identified using the bathymetric data vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives the ja sit pericu and meyibo active fields are near the eruptive fissure of an extensive young lava flow vent fluids up to 360 c from meyibo have compositions similar to northern east pacific rise vents the tzab ek field is 850 m west of the volcanic axis and active chimneys rise up to 33 m above a broad sulfide mound the inactive field is 10 km north northeast along the rift axis and most sulfide chimneys are enriched in zn and associated elements that are transported at lower temperature compared to the more cu rich active fields in southern pescadero basin the auka field is on the margin of a sediment filled graben at 3 670 m depth discharging fluids are clear contain hydrocarbons and have neutral ph elevated salinity and temperatures up to 291 c they have deposited massive mounds of calcite with minor sulfide the fluids are compositionally similar to those in guaymas basin produced by high temperature basalt seawater interaction followed by reaction with sediment the paucity of sulfide minerals suggests subsurface deposition of metals

variable methane fluxes in shallow marine systems over geologic time abstract carbon cycling is an important but poorly understood process on passive continental margins in this study we use the ionic and stable isotopic composition of interstitial waters and the petrology mineralogy and stable isotopic composition of authigenic carbonates collected from ocean drilling program odp leg 174a sites 1071 and 1072 to constrain the origin of the carbonates and the evolution of methane on the outer new jersey shelf the pore fluids of the new jersey continental shelf are characterized by 1 a fresh brackish water plume and 2 organic matter degradation reactions which proceed through sulfate reduction however only minor methanogenesis occurs the oxygen isotopic composition of the pore fluids supports a meteoric origin of the low salinity fluids authigenic carbonates are found in nodules thin 1 cm layers and carbonate cemented pavements siderite is the most common authigenic carbonate followed by dolomite and calcite the oxygen isotopic composition of the authigenic carbonates i e 1 3 6 5 peedee belemnite pdb indicates an origin in marine pore fluids the carbon isotopic composition of dolomite cements range from 16 4 to 8 8 pdb consistent with formation within the zone of sulfate reduction siderite 13c values show a greater range 17 67 16 4 but are largely positive mean 2 8 and are interpreted to have formed throughout the zone of methanogenesis in contrast calcite 13c values are highly negative as low as 41 7 and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation pore water data show that despite complete sulfate reduction methanogenesis appears not to be an important process presently occurring in the upper 400 m of the outer new jersey shelf in contrast the carbon isotopic composition of the siderites and calcites document an active methanogenic zone during their formation the methane may have been either oxidized or vented from shelf sediments perhaps during sea level fluctuations if this unaccounted and variable methane flux is an areally important process during neogene sea level fluctuations then it likely plays an important role in long term carbon cycling on passive continental margins

stable isotopes and rare earth element compositions of ancient cold seep carbonates from enza river northern apennines italy implications for fluids sources and carbonate chimney growth abstract cold seep carbonates are remarkable fossil records of hydrocarbon seepage and related conduits in the present research we analysed the carbon and oxygen isotopes rare earth element ree and trace element content of carbonate chimneys and concretions in the pleistocene argille azzurre formation northern apennines italy strong 13 c depletions values as low as 40 3 observed in both carbonate concretions and chimneys indicate that anaerobic oxidation of biogenic methane played a major role in the formation of these carbonates slightly higher 13 c values mostly found in the outer layers of the carbonate chimneys reflect a contribution of inorganic carbon from seawater ree patterns reveal the carbonates formed under mostly anoxic pore water conditions combined ree and trace element data reveal the growth of chimneys occurred from the outside in with the outer chimney walls experiencing rapid growth excellent correlations between ce ce ree contents and la n sm n and y ho ratios observed in the chimney walls provide additional evidence for the influence of seawater in the formation of the chimneys meanwhile o isotopic compositions of the samples suggest an 18 o depleted fluid was involved in carbonate precipitation this study has major implications for tracking fluid sources of seep carbonates and patterns of carbonate chimney growth

methane rich plumes on the carolina continental rise associations with gas hydrates seafloor venting of microbial gases occurs at 2167 m water depth over the blake ridge diapir gas rich plumes were identified acoustically in the water column up to 320 m above a pockmarked sea floor associated with active chemosynthetic biological communities plumes and venting fluids emanate from near a small fault that extends downward toward a dome in the bottom simulating reflector indicating that fluid and or gas migration is associated with gas hydrate bearing sediment below these plumes might be caused by gas bubbles or buoyant dumps of gas hydrate that float upward from the seafloor 18 refs 3 figs

formation of carbonate concretions in surface sediments of two mud mounds offshore costa rica a stable isotope study the surface sediments of two mud mounds mound 11 and mound 12 offshore southwest costa rica contain abundant authigenic carbonate concretions dominated by high mg calcite 14 20 mol mgco3 pore fluid geochemical profiles sulfate sulfide methane alkalinity ca and mg indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane aom at variable depths the current location of the authigenic carbonate concretions is however not related to the present location of the aom zone suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18ocarbonate values ranging between 34 0 and 37 7 vienna standard mean ocean water vsmow and d13ccarbonate values from 52 2 to 14 2 vienna pee dee belemnite vpdb assuming that no temperature changes occurred during mineral formation the authigenic carbonate concretions have been formed at in situ temperature of 4 5 c the d18ocarbonate values suggest mineral formation from seawater derived pore fluid d18oporefluid 0 vsmow for mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid d18oporefluid 5 in mound 11 a positive correlation between d13ccarbonate and d18ocarbonate is observed indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds the carbon of these sources are 1 marine bicarbonate d13cporefluid 0 and 2 bicarbonate which formed during the aom d13cporefluid 70 furthermore the d18oporefluid composition with values up to 4 7 vienna standard mean ocean water vsmow is interpreted to be affected by the presence of emanating freshened and boronenriched fluids earlier it has been shown that the origin of 18o enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature d13cch4 38 a combination of present geochemical data with geophysical observations indicates that mounds 11 and 12 represent a single fluid system interconnected by deep seated fault s

acoustically detected hydrocarbon plumes rising from 2 km depths in guaymas basin gulf of california plumes extending nearly 1000 m from the 1500 2000 m deep seafloor of guaymas basin were detected from below the 23 5 khz inverted echo sounder of the scripps deep two vehicle individual sound reflectors bubbles or drops rise at approximately 17 cm s in one plume the deep tow side scan records provide more information on the plumes structure at the altitude of the vehicle 75 m where some form multiple side scan targets one 20 m across near bottom 4 khz profiles show that plumes overlie young fault traces associated with extensional faulting at the basin s spreading centers of outcrops of tilted beds beside strike slip faults we infer from analysis of the deep tow observations field relationships and knowledge of the geology of this basin that the plumes are made of light hydrocarbons perhaps mainly methane that emanate from seabed seeps one of the acoustically detected plumes was at a spreading axis hydrothermal field which has many buoyant acoustically transparent thermal plumes some of which are rich in dissolved hydrocarbons

distribution of chemosynthetic biological communities in monterey bay california we report the first quantitative evaluation of the distribution of seafloor chemosynthetic biological communities on a regional scale the results are based on the analysis of video images and navigation from 792 benthic remotely operated vehicle dives conducted on the continental margin in monterey bay california these communities are common occurring within 5 of the 25 m square grid cells within which there have been bottom observations within 45 km of the bay9s head and within 9 of the visited cells that are below 550 m water depth although it has been previously assumed that these communities are associated with fluid seepage from faults they are not more common within known fault zones surprisingly the communities in monterey bay occur preferentially on steep slopes which are commonly sites of recent erosion

authigenic carbonates in upper miocene sediments of the tertiary piedmont basin nw italy vestiges of an ancient gas hydrate stability zone a wide array of carbonate rich rocks has been recognized in the tertiary piedmont basin nw italy hosted in lower messinian slope deposits carbonate cements show negative 13 c values and positive 18 o values suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabilization two groups of rocks have been distinguished 1 lucina bearing mud breccias representing the seafloor product of an ancient seepage site and 2 lucina free concretions originating below the sediment water interface within this group two subtypes have been further distinguished stratiform concretions and cylindrical concretions stratiform concretions result from precipitation of dolomite in the pores of muddy sediments some of them display a brecciated structure others show a network of septarian like cracks that are empty filled with sediments or zoned carbonate cements their internal features are related to the formation of gas hydrates within the sediments and to their destabilization thus these rocks mark a portion of the sedimentary column located within a paleo gas hydrate stability zone cylindrical concretions represent ancient fluid conduits related to the upward migration of ch 4 rich fluids subsequent to gas hydrate destabilization the carbonate rich rocks of the tertiary piedmont basin stand as one of the first examples of methane derived rocks that record successive episodes of dissociation and re formation of gas hydrates and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone

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

deep water hydrocarbon seeps in guaymas basin gulf of california abstract acoustically discovered hydrocarbon seeps along a transform fault zone on the sonoran margin of guaymas basin and in the southern trough of guaymas basin were examined and sampled during dives of dsv alvin seepage of methane and heavier hydrocarbons occurs through shallow pockmarks along the eroding crest of a steep anticline 1600 m below sea level extensive ledges of aragonite crop out around the rims of the pockmarks isotopic analysis indicates that carbonate precipitation is a result of methane oxidation seepage zones within the pockmarks support dense communities of calyptogena half buried in the mud tubeworms lamellibracheae sp have colonized many of the aragonite outcrops though the margin site is only a few kilometers from a high temperature sulfide precipitating and petroleum discharging vent system at a nearby center its structural setting is more akin to pockmarks described from continental shelves and its chemosynthetic fauna is more like that around low temperature seeps on other continental slopes and margins e g oregon u s a and japan the seep in the southern trough is a condensate type petroleum c 1 c 40 and rises as a plume from a young vent mound system about 2000 m below sea level

a transform continental margin rich in hydrocarbons gulf of california conventional and near bottom geophysical surveys mapped shallow deformational structures and sediment accumulation patterns along part of the actively shearing continental margin of sonora including the intersection with a guaymas basin spreading center the principal strike slip faults occupy a zone 1 2 km 0 6 1 2 mi wide along the lower slope with a ridge of tightly folded sediments raised by uplift of sheared basement at the abrupt boundary between continental and oceanic crust this transform ridge and a continental rise on its seaward slope grows as the fault zone ages away from the spreading center intersection shoaling at a net rate of 500 m m y 1 600 ft m y despite erosion of its crest a clathrate horizon in the upper 80 m 260 ft of sediment is inferre from bottom simulating reflectors in the rise and lower slope hydrocarbon seeps occur at 1 600 m 1 mi below sea level along the crest of the transform ridge and patchy gas accumulation and seepage are recognized on a young marginal plateau

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

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

surface expression of fluid venting at the toe of the nankai wedge and implications for flow paths abstract cold seeps have been observed with manned submersibles at many sites on the tokai margin most sites are found on the hanging wall of major thrust faults within 1 2 km of their outcrop few sites relate to purely erosional features this relationship between thrusting and venting may be explained either by fluid flow along fault zones by fluid flow along permeable layers outcropping at the fault scarp or by diffuse upward flow of fluids released during footwall compaction we use deep towed side scan sonar and sediment sounder profiles acquired at the toe of the nankai accretionary wedge to identify the context of seepage at a smaller scale modes of fluid expulsion inferred from observations in this zone include flow channelling along strata along thrusts along gravity faults and along joint fractures as well as diffuse flow we then combine visual observation and monitoring of electrochemical potential eh to assess the relative importance of seepage sites we show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites cemented by authigenic carbonates these were sampled and have permeability high enough to channel fluid flow 1 4 10 14 m2 we suggest they are the main conduits near the seafloor however seepage sites are also observed directly on the outcrop of major thrusts indicating that they do play a role as fluid conduits we observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance

nd isotope composition of seep carbonates towards a new approach for constraining subseafloor fluid circulation at hydrocarbon seeps abstract the carbonates forming at deep sea hydrocarbon seeps provide an archive for the source and migration pathways of the seeping fluids interpretation of the archived isotopic and elemental signatures is however not straightforward because of mixing between the signals of fluids and ambient seawater limited understanding of subseafloor fluid circulation and effects of diagenetic alteration the nd isotope system is among the most widely used tracers of circulation patterns in modern and past oceans but to date nd isotopes have been rarely used in investigations of fluid migration pathways at hydrocarbon seeps to test the sensitivity of nd isotope signals of seep carbonates to record past interactions between the seeping fluids and 143nd enriched volcanic derived mineral components we performed nd sr and stable isotope analyses of the lower cretaceous barremian seep carbonates found in baska czech carpathians these seep deposits are directly underlain by a complex of mafic igneous rocks which the methane charged fluids must have passed through on their way to the seafloor the end t values measured in the seep carbonates are significantly more radiogenic than the inferred signal of contemporaneous local seawater and shifted towards the composition typical of the underlying basaltoids this is in agreement with the 87sr 86sr ratios which are markedly lower that the coeval seawater signature consistent co variance trends are observed among the measured end t 87sr 86sr and 13c values attesting increasing role of the volcanic derived fluid component with increasing contribution of methane derived carbon the results of the present approach reveal the potential of nd isotopes to fingerprint exotic fluid end members present in plumbing systems of both fossil and modern methane discharges the method can be of particular use in studies of seeps associated with plate margins where 143nd enriched igneous materials are commonly encountered in the basement or in the sediment column

stromatolitic fabric of authigenic carbonate crusts result of anaerobic methane oxidation at cold seeps in 4 850 m water depth methane seepage leads to mg calcite and aragonite precipitation at a depth of 4 850 m on the aleutian accretionary margin stromatolitic and oncoid growth structures imply encrustation of microorganisms microbial mats in the host sediment with a unique growth direction downward into the sediment forming crust shaped lithologies biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol which contain extremely low 13c values this indicates the presence of methane consuming archaea and 13c values of 42 to 51 pdb indicate that methane is the carbon source for the carbonate crusts thus it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate reducing bacteria and that carbonate precipitation proceeds downward into the sediment where ascending cold fluids provide a methane source strontium and oxygen isotope analyses as well as 14c ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3 000 years ago

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

isotopically heavy oxygen containing siderite derived from the decomposition of methane hydrate authigenic siderite from the miocene and pliocene sediments of the blake outer ridge formed in a methane fermentation zone in equilibrium with anomalously heavy oxygen water as evidenced by its heavy carbon 1 6 to 12 5 13c and heavy oxygen 0 1 to 7 9 18o values blake outer ridge sediments are impregnated with methane hydrate which is generally enriched in 18o relative to ambient water this suggests that decomposition of methane hydrate and subsequent release of large amounts of methane and heavy oxygen water affected the formation of the 18o enriched siderite gas hydrate reaction in sediments is an important process for causing a dramatic variation of oxygen isotopic composition of the interstitial water and of authigenic carbonates formed during burial diagenesis

authigenic carbon entombed in methane soaked sediments from the northeastern transform margin of the guaymas basin gulf of california extensive rov based sampling and exploration of the seafloor was conducted along an eroded transform parallel fault scarp on the northeastern side of the guaymas basin in the gulf of california to observe the nature of fluids venting from the seafloor measure the record left by methane venting on the carbonates from this area and determine the association with gas hydrate one gas vent vigorous enough to generate a water column gas plume traceable for over 800 m above the seafloor was found to emanate from a 10 cm wide orifice on the eroded scarp face sediment temperature measurements and topography on a sub bottom reflector recorded in a transform parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent active chemosynthetic biological communities cbcs and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform parallel scarp the carbon isotopic composition of methane bubbles flowing vigorously from the gas vent 53 6 0 8 pdb is comparable to methane found in sediment cores taken within the cbcs distributed along the scarp 51 9 8 1 pdb however the 13c value of the co2 in the vent gas 12 4 1 1 pdb is very distinct from those for dissolved inorganic carbon dic 35 8 to 2 9 pdb found elsewhere along the scarp including underneath cbcs the 13c values of the carbonate rich sediments and rocks exposed on the seafloor today also span an unusually large range 40 9 to 12 9 pdb and suggest two distinct populations of authigenic carbonate materials were sampled unconsolidated sediments and some carbonate rocks which have lithologic evidence for near seafloor formation have negative 13c values while carbonate rocks that clearly formed in the subsurface have positive 13c values up to 23 0 close to that measured for co2 in the vent gas there appears to be two carbon sources for the authigenic carbonates 1 deeply sourced isotopically heavy co2 12 and 2 isotopically light dic derived from local anaerobic oxidation of methane at the sulfate methane interface in the shallow subsurface addition of isotopically light methane derived carbon at the seafloor may completely mask the isotopically heavy co2 signature 12 4 in the underlying sediments thus the authigenic carbonates may have formed from the same methane and carbon dioxide bearing fluid but under different migration and alteration conditions depending on how it migrated through the sediment column

chemical biological and hydrological controls on the 14c content of cold seep carbonate crusts numerical modeling and implications for convection at cold seeps understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations increasing the sediment ocean methane flux evidence for the occurrence of convection at cold seeps however is still limited we use the concentration of 14c d14c in carbonate crusts formed at cold seeps of the eastern mediterranean sea as a tracer for convective recirculation of seawater derived fluids a numerical model is applied to investigate the controls on 14c incorporation in cold seep carbonates our simulations show that increased amounts of ch4 in the expelled fluids result in elevated crust d14c while high ca2 and hco3 concentrations produce the opposite effect convection is the only transport process that can significantly increase crust d14c advection bioirrigation eddy diffusion and bioturbation instead have little effect on or produce a decrease of crust d14c in addition the presence of old or modern carbon mc in host sediments prior to cementation and the 14c decay associated to the time needed to form the crust contribute in defining the d14c of carbonate crusts we then use the model to reproduce the 14c content of the eastern mediterranean sea crusts to constrain the chemical and hydrological conditions that led to their formation some crusts contain relatively low amounts of 14c 945 0 d14c 930 2 which assuming no ageing after crust formation can be reproduced without considering convection other crusts from two sites the amsterdam and napoli mud volcanoes instead have a very high 14c content 899 0 d14c 838 4 which can only be reproduced by the model if convection mixes deep fluids with seawater order of magnitude calculations using the rayleigh criterion for convection suggest that the slow seepage about 10 cm year 1 of low salinity 20 fluids at the amsterdam sites could trigger haline convection there on the napoli mud volcano where high density brines are expelled density driven convection cannot take place and other processes possibly involving the rapid movement of free gas in the sediment could be important

electron spin resonance esr dating of pre quaternary faults in the sichuan basin sw china abstract the electron spin resonance esr dating method is thought to be limited to the quaternary period in this study we attempt to expand the esr dating range to the pre quaternary using three fault zone samples collected from faults in the sichuan basin and its surroundings we adopt a ray radiation dose of up to 2 104 gy which is much greater than that typically employed and find that the esr intensity of the e1 center a type of signal for esr dating in quartz continuously increases with the ray dose and is slightly influenced by the radiation dose rate this indicates the potential of esr to date pre quaternary faults by simulating environmental radiation under geological burial conditions we use esr to date the latest extension event in the western hubei and hunan thrust belt to 9 17 0 76 ma and the thrusting of the longmenshan fault zone toward the sichuan basin to approximately 6 14 0 73 ma conversely the saturated esr intensity of fault zone samples from the daba fault zone suggests a lack of strong tectonic activity in the southern daba fault zone in the last 10 ma our results are consistent with fission track ages and the regional tectonic evolution history therefore esr is a practical method for dating pre quaternary faults in structural and petroleum geology applications

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

patterns of subsurface fluid flow at cold seeps the hikurangi margin offshore new zealand based on multichannel seismic geoacoustic and methane sensor data four different areas along the hikurangi margin show multiple indications for seep activity including bright spots transparent zones vertical chimneys and the occurrence and distribution of bottom simulating reflectors locations where these features reach the seafloor are characterised by high backscatter intensity on sidescan sonar images and transparent zones in sediment echosounder profiles while methane sensors show episodic elevated methane concentrations near the seep sites methane discharge is facilitated by reduced hydrostatic pressure during low tides the greatest number of seeps at opouawe bank correlates with the highest methane activity along the hikurangi margin high heat flow values on flanks of ridges and low heat flow values on anticlines reflect a topographic effect on subsurface temperatures elevated heat flow occurs in the vicinity of seeps on opouawe bank we propose that there are two drivers behind methane seepage with respect to the migration pathways of methane through the gas hydrate stability zone ghsz to the seafloor 1 structurally controlled and 2 stratigraphically controlled in the structural model vertical chimneys are the major pathways for methane through the ghsz part of the upwardly migrating methane forms gas hydrate within the chimney in the stratigraphic model methane migration is stratigraphically controlled beneath seeps that are located on bathymetric highs and or where subsurface anticlines occur beneath seeps the structurally controlled seeps produce higher methane escape at the seafloor than those that are stratigraphically controlled a combination of both driving mechanisms results in the highest methane seepage rates at the tui seep on opouawe bank

new constraints on equatorial temperatures during a late neoproterozoic snowball earth glaciation intense glaciation during the end of cryogenian time 635 million years ago marks the coldest climate state in earth history a time when glacial deposits accumulated at low tropical paleolatitudes the leading idea to explain these deposits the snowball earth hypothesis predicts globally frozen surface conditions and subfreezing temperatures with global climate models placing surface temperatures in the tropics between 20 c and 60 c however precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined here we make new geologic observations of tropical periglacial aeolian and fluvial sedimentary structures formed during the end cryogenian marinoan glaciation in south australia these observations allow us to constrain ancient surface temperatures we find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith the wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2 4 m below the ground surface these observations paired with a model of ground temperature forced by solar insolation constrain the local mean annual surface temperature to within a few degrees of freezing this temperature constraint matches well with our observations of fluvial deposits which require temperatures sufficiently warm for surface runoff although this estimate coincides with one of the coldest near sea level tropical temperatures in earth history if these structures represent peak marinaon glacial conditions they do not support the persistent deep freeze of the snowball earth hypothesis rather surface temperatures near 0 c allow for regions of seasonal surface melting atmosphere ocean coupling and possible tropical refugia for early metazoans if instead these structures formed during glacial onset or deglaciation then they have implications for the timescale and character for the transition into or out of a snowball state

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

episodic methane concentrations at seep sites on the upper slope opouawe bank southern hikurangi margin new zealand along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere with numerous cold seeps already known the convergent hikurangi margin east of north island new zealand was selected as a target area for further detailed multidisciplinary investigation of cold seeps within the new vents and associated projects methane and temperature sensors mets were deployed at selected seep sites on the opouawe bank off the southeastern tip of north island and near the southern end of the imbricate thrust hikurangi margin together with seismic ocean bottom stations they remained in place for about 48 h while seismic data were collected the seeps were associated with seep related seismic structures methane concentrations were differing by an order of magnitude between neighbouring stations the large differences at sites only 300 m apart demonstrate that the seeps were small scale structures and that plumes of discharged methane were very localised within the bottom water high methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow methane discharge from the seafloor was episodic which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides the strong semi diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater despite dispersal by currents fluid flow through fissures fractures and faults close to the mets positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the mets small earthquakes do not appear to be correlated with seawater methane anomalies