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

uranium series dating of pedogenic silica and carbonate crater flat nevada a 230th 234u 238u dating study on pedogenic silica carbonate clast rinds and matrix laminae from alluvium in crater flat nevada was conducted using small sample thermal ionization mass spectrometry tims analyses on a large suite of samples though the 232th content of these soils is not particularly low mostly 0 1 9 ppm the high u content of the silica component mostly 4 26 ppm makes them particularly suitable for 230th u dating on single 10 to 200 mg totally digested samples using tims we observed that 1 both micro within rind and macro stratigraphic mappable deposit order of the 230th u ages were preserved in all cases 2 back calculated initial 234u 238u fall in a restricted range typically 1 67 0 19 so that 234u 238u ages with errors of about 100 kyr 2 could be reliably determined for the oldest 400 to 1000 ka rinds and 3 though 13 of the samples were 350 ka only three showed evidence for an open system history even though the sensitivity of such old samples to isotopic disruption is very high an attempt to use leach residue techniques to separate pedogenic from detrital u and th failed yielding corrupt 230th u ages we conclude that 230th u ages determined from totally dissolved multiple sub mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples chemical enhancement of 238u 232th or isochrons

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

u th dating of carbonate nodules from methane seeps off joetsu eastern margin of japan sea abstract we performed u th radioactive disequilibrium analyses of carbonate nodules and sediment samples recovered from methane seep sites off joetsu of the eastern margin of japan sea to decipher the active period of the methane seep the carbonates contain 230th part of which is located in detritus such as silicate and organics at the time of precipitation the initial 230th renders accurate dating with u th radioactive disequilibrium method difficult we assessed the feasibility of correction using radioactive disequilibrium data of ambient sediment to overcome this difficulty a 230th 232th 234u 232th isochron drawn by three chips divided from a carbonate nodule pc05 04 50 passed through data points of local sediments we conclude that the problem of initial 230th can be resolved by measurements of local sediments results show that carbonate nodules include local sediment as impurities furthermore the results of trace element analyses such as rb zr nb ree pb and th also support the idea in all 18 carbonate samples were dated with correction of initial 230th using the mean value of local sediment in this study the u th correction ages show 12 35ka with an isochron age of 26 3ka results indicate that during the time interval of u th ages from 12ka to 35ka environmental conditions must have been favorable for enhanced methane flux through sediment the extensive methane flow period at 20ka accords with the lowest stand sea level during the last glacial age results of this study also suggest that u th ages of carbonate are useful as a reliable chronometer with regard to methane seep activation in order to acquire u th ages of carbonate at methane seep sites however it is important to evaluate the amount of initial 230th accurately using the value of sediment

subsurface seepage dynamics and flow types in a messinian paleoseep system maiella mts central italy abstract the subsurface feeder complexes of mud volcanoes and seepage systems potentially provide valuable information on the evolution of seepage dynamics ancient seepage systems in outcrops allow for the observation of expulsion features which are otherwise commonly beyond seismic resolution in the maiella hydrocarbon seep area the evolution of fluid migration through the sedimentary column during the messinian salinity crisis generated distinctive seep plumbing features geological responses and geochemical signatures in the brecciated limestones unit the fluid migration pathways evolved from funnel shaped feeder channels flow mobilized sediments into hydrofractured microbial carbonate buildups limestone buildups and finally into blow out micropipes in the host sediment patchy limestones as overpressure dissipated through the plumbing system the flow mobilized sediments 13c down to 24 5 pdb 1 correspond to the highest flow rates in the whole area whereas the patchy limestones 13c down to 39 3 pdb 1 correspond to the slowest flow rates within the intrusive zone the limestone buildups show different degrees of hydrofracturing that reflect different flow rates 13c down to 27 5 pdb 1 the fluid transport mechanisms evolved from focused venting through neoformed feeder channels where sediments elutriated from depths were carried out the sediment prone response to fluid migration to high rate seepage triggering high hydrofracturation in the microbial buildups while the hydrocarbon rich fluids contemporaneously triggered authigenic precipitation the mineral prone response to fluid migration progressive upward and lateral flow deceleration resulted in gradually weaker hydrofracturing of the microbial buildups and finally only local cementation in the form of carbonate patches within the host sediments

relative precipitation rates of aragonite and mg calcite from seawater temperature or carbonate ion control the temperature and degree of carbonate mineral supersaturation co3 2 ion concentration of seawater are the two most likely controlling variables on the compositions of recent marine carbonate cements the relative importance of these variables is difficult to assess in nature because they have similar trends with depth 0 1500 m and latitude in modern oceans we carried out laboratory experiments to investigate the relative growth rates of calcite mg calcite and aragonite in seawater as functions of both temperature 5 25 and 37 c and of carbonate ion concentration 2 5 to 15 times supersaturated with respect to calcite the precipitation rates of aragonite relative to those of calcite increase strongly with increasing temperature and are not affected greatly by changes in saturation state at 5 c calcite precipitation rates are nearly equivalent to those of aragonite regardless of the degree of saturation at both 25 and 37 c aragonite precipitation rates are much more rapid than those of calcite up to a factor of 4 except at very low saturation states calcite compositions vary from less than 5 mol mgco3 at 5 c to 14 mol mgco3 at 37 c our results suggest that the well documented shift toward precipitation of lower mol mg calcite and the decrease in abundance of aragonite cements with increasing oceanic depth and latitude can be attributed largely to lower temperatures

ch 4 consuming microorganisms and the formation of carbonate crusts at cold seeps to understand the role played by microorganisms in the formation of cold seep carbonates we conducted an integrated microbial mineralogical and organic geochemical study of methane related authigenic carbonate crusts formed on eastern mediterranean mud volcanoes we show that supersaturation with respect to carbonate minerals is induced by microbial anaerobic oxidation of methane combined lipid biomarker analysis and 16s rrna gene surveys identified a highly diversified methane consuming archaeal community possibly comprising novel species implying that the anaerobic oxidation of methane is phylogenetically widespread and directly implicating these organisms in the process of crust precipitation moreover pore water sulphate gradients produced by co occurring methane based sulphate reduction exert the main control on aragonite versus magnesian calcite precipitation we propose that this may be the dominant mode of carbonate crust formation at cold seeps world wide in agreement with aquatic chemistry predictions and explaining carbonate mineralogy

calyptogena cemented rocks and concretions from the eastern part of nankai accretionary prism age and geochemistry of uranium calyptogena valves included in a carbonate rich cement and fragments of a carbonate rich chimney have been examined for their stable isotopic c and o composition 14c activity and uranium series disequilibrium the fossil shells were formed essentially with seawater carbon and a negligible contribution of cold seepage organic carbon as shown by their 13c values this allows the14c method to be used to determine their age a fairly good concordance between the14c and230th234u ages of the youngest shells gives confidence in the dating of the older samples using the latter technique thus thecalyptogena are dated at ca 150 000 and 20 000 yrs b p they have been preserved from dissolution by rapid cementation by a supersaturated carbonate solution the cement is especially rich in uranium as high as 75 ppm whose source is seawater the enrichment is due to local reducing conditions brought about by the bacterial decomposition of the soft tissues of the bivalves shortly after death thecalyptogena that probably developed between these two events the events of ca 20 000 and 150 000 yrs have not been preserved from dissolution because as is presently the case the cold seepages were undersaturated with calcium carbonate the two events probably represent periods of intense fluid venting connected with tectonic activity

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

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

sr isotopic compositions of cold seep carbonates from the south china sea and the panoche hills california usa and their significance in palaeooceanography 87sr 86sr ratios 13c and 18o values and concentrations of sr and mn in cold seep carbonates collected from the south china sea and the panoche hills california usa are determined in this study the carbon isotopic values of carbonates reflect different carbon sources carbonate samples with high sr concentrations and low mn sr ratios are considered to have very little or no influence by diagenetic alteration sr isotopes of seep carbonates from both the south china sea and the panoche hills record the information on the original seawater or pore water together with the published sr isotopic data of cold seep carbonates in italy we applied the sr isotope stratigraphy sis and sis lowess version 3 curve to calibrate the ages for the cold seep carbonates these ages are consistent with their sedimentary strata indicating that the cold seep carbonates with high sr concentration and low mn sr ratio are good candidates for dating by their sr isotopic compositions

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

new 230th u and 14c ages from lake lahontan carbonates nevada usa and a discussion of the origin of initial thorium five sets of coeval lacustrine carbonate samples from pleistocene lake lahontan in western nevada were dated by both the ams 14c and 230th u isochron methods all five groups of samples were analyzed for u th isotopes by alpha spectrometry and one of the groups was additionally measured by thermal and secondary ionization mass spectrometry tims and sims for comparison the 14c ages were corrected to calendar years using the calibration curve recommended by bard et al 1992 without local reservoir correction on the 14c ages mean 230th u isochron ages of some sets are apparently older than their calendar corrected 14c ages by up to 2300 years modern carbon contamination of these carbonate samples through recrystallization or deposition of secondary calcite is likely to be responsible for part of the age discrepancies we explored additional biases associated with the isochron ages maybe produced by the presence of initial th coprecipitated from the lake water it can be shown that if dissolved hydrogenous th is directly incorporated into the pure carbonates then the three component mixing among 1 detrital th 2 hydrogenous th adsorbed on detritus and 3 hydrogenous th incorporated by the carbonate can introduce a positive age bias we have developed an approach to estimate the magnitude of this bias of the lake lahontan carbonates the preliminary estimates suggest a positive age bias of 1000 to 2000 years for two sets of the samples

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

origins of hydrocarbon gas seeping out from offshore mud volcanoes in the nile delta abstract this paper discusses the origin of gas seepages free gas or dissolved gas in ground water or brine sampled with the nautile submarine during the nautinil cruise at the seafloor of the deep water area of the nile turbiditic system on different mud volcanoes and brine pools generally the gas is wet and includes c1 c2 c3 ic4 nc4 co 2 these gas samples show no evidence of biodegradation which is not the case of the gas present in the deep hydrocarbon accumulations at depth it indicates that the gas expelled by the mud volcanoes is not issued from direct leakages from deep gas fields the collected gas samples mainly have a thermogenic origin and show different maturities some samples show very high maturities indicating that these seepages are sourced from great depths below the messinian salt moreover the different chemical compositions of the gas samples reflect not only differences in maturity but also the fact that the gas finds its origin in different deep source rocks carbon dioxide has an organic signature and cannot result from carbonate decomposition or mantle fluids the crustal derived radiogenic isotopes show that the analyzed gas samples have suffered a fractionation processes after the production of the radiogenic isotopes due either to oil occurrence at depth interacting with the flux of gas and or fractionation during the fluid migration

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

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

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

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

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

pockmark formation and evolution in deep water nigeria rapid hydrate growth versus slow hydrate dissolution in previous works it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water nigeria it was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics new results obtained by drilling into gas hydrate bearing sediments with the mebo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution on one hand positive temperature anomalies free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth on the other hand slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the gas hydrate occurrence zone

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

response of anaerobic methanotrophs and benthic foraminifera to 20 years of methane emission from a gas blowout in the north sea highlights high abundance of active anaerobic methanotrophs in sediments of the blowout crater suggests adaptation to methane seepage within at most two decades fast exchange processes in permeable surface sediments prevent sulfate depletion and probably methane derived carbonate precipitation methane seepage impacts isotopic and assemblage composition of benthic foraminifera abstract methane emissions from marine sediments are partly controlled by microbial anaerobic oxidation of methane aom aom provides a long term sink for carbon through precipitation of methane derived authigenic carbonates mdac estimates on the adaptation time of this benthic methane filter as well as on the establishment of related processes and communities after an onset of methane seepage are rare in the north sea considerable amounts of methane have been released since 20 years from a man made gas blowout offering an ideal natural laboratory to study the effects of methane seepage on initially pristine sediment sediment cores were taken from the blowout crater and a reference site 50 m distance in 2011 and 2012 respectively to investigate porewater chemistry the aom community and activity the presence of authigenic carbonates and benthic foraminiferal assemblages potential aom activity up to 3060 nmol cm 3 sediment d 1 or 375 mmol m 2 d 1 was detected only in the blowout crater up to the maximum sampling depth of 18 cm card fish analyzes suggest that monospecific anme 2 aggregates were the only type of aom organisms present showing densities up to 2 2 107 aggregates cm 3 similar to established methane seeps no evidence for recent mdac formation was found using stable isotope analyzes 13c and 18o in contrast the carbon isotopic signature of methane was recorded by the epibenthic foraminifer cibicides lobatulus 13c 0 66 surprisingly the foraminiferal assemblage in the blowout crater was dominated by cibicides and other species commonly found in the norwegian channel and fjords indicating that these organisms have responded sensitively to the specific environmental conditions at the blowout the high activity and abundance of aom organisms only at the blowout site suggests adaptation to a strong increase in methane flux in the order of at most two decades high gas discharge dynamics in permeable surface sediments facilitate fast sulfate replenishing and stimulation of aom the accompanied prevention of total alkalinity build up in the porewater thereby appears to inhibit the formation of substantial methane derived authigenic carbonate at least within the given time window

simulation of long term feedbacks from authigenic carbonate crust formation at cold vent sites fluid flow at cold vent sites is usually driven by the ascent of overpressured fluids from subsurface reservoirs porosity changes in surface sediments due to precipitation and dissolution of authigenic carbonates affects fluid flow and biogeochemical turnover from observations it is known that carbonate precipitates often occur in distinct layers in high concentrations surrounded by layers with low carbonate content using a non steady state model we simulate aragonite and calcite precipitation and dissolution in a 2 m long sediment column located under a bacterial mat at hydrate ridge cascadia accretionary margin assuming a constant pressure gradient over 7000 years fluid flow anaerobic oxidation of methane aom rates and carbonate precipitation and dissolution rates show strong oscillations evoked by changes in permeability and fluid flow over time the porosity reaches values below 0 35 in the carbonate layers that reduce the fluid flow velocity from an initial value of 30 cm a 1 to a minimum value of about 2 cm a 1 these significant changes in the fluid flow system displace the depth of sulfate penetration the simulation predicts cycles of carbonate crust formation and dissolution with a duration of 2000 2700 years resulting in several distinct carbonate layers during periods of high fluid flow aom reaches rates over 1000 mol cm 1 a 1 and methane fluxes out of the sediment reach 200 mol cm 1 a 1 during periods of low fluid flow aom is about 450 mol cm 1 a 1 and the methane flux into the bottom water vanishes completely the oscillations are dampened so that fluid flow and biogeochemical turnover slowly approach steady state after about 7000 years towards the end of the simulation period showing a 1 m thick area at the surface with carbonate concentrations of about 25 wt flow oscillations may also impact the colonization of chemoautotrophic larva and bacteria in and on the sediment the frequency of precipitation dissolution cycles of 2000 2700 years is long enough for vent biota to react to changes in the sediments for example explaining the occurrence of buried calyptogena and acharax mussel shells at former vent sites that are not presently active the evaluation of side scan sonar data reveals large areas of cemented sediments and carbonate pavements north of the investigated site the situation found in the investigated core may be characterized as intermediate stage of carbonate cementation this ongoing process will also form a solid carbonate pavement at this site in the future

the nile deep sea fan an example of interacting sedimentation salt tectonics and inherited subsalt paleotopographic features abstract structural analysis of the nile deep sea fan ndsf indicates that post miocene deformation is largely controlled by down slope movement above a messinian salt layer in contrast to other similar systems the ndsf shows significant lateral asymmetry and three distinct provinces can be recognized regional variations in salt thickness and base salt geometry within the system have led to the presence of structures typical of both gravity gliding and gravity spreading trends of structures within the western and central parts of the ndsf are consistent with forming due to gravity gliding processes this pattern contrasts with that in the eastern ndsf that seem to be much more consistent with deformation due to gravity spreading observed differences in structural style across the ndsf can be related directly to the paleomorphological evolution of the nile cone during messinian time which is perhaps partly related to features that have been inherited from early mesozoic evolution of the north egypt passive margin in the east part of the ndsf the structural evolution has been significantly influenced by the proximity of eratosthenes seamount this large topographic buttress has served to both limit and deflect northeastward allochthonous advance of the messinian evaporites and has thus severely complicated deformation of the overlying plio quaternary sedimentary cover within the broad corridor that runs nnw sse and cuts obliquely across the modern bathymetry of the nile cone

characterization of benthic biogeochemistry and ecology at three methane seep sites on the northern u s atlantic margin abstract several hundred methane seeps were recently discovered along the u s atlantic margin a passive margin without a major hydrocarbon basin these seeps represent a compelling opportunity to compare and contrast geochemical parameters as well as the distribution and composition of associated chemosynthetic ecosystems across geologically distinct seeps specifically this study characterized the physical chemical geological and biological features at veatch canyon new england and shallop canyon methane seep sites using a suite of state of the art sampling and communication tools auv sentry hov alvin a video equipped multicore and a real time telepresence connection to the inner space center isc at the university of rhode island water column backscatter data collected by auv sentry confirmed previously detected gas emission and further indicated the presence of 13 actively emitting gas seeps within the veatch canyon survey area 5 within the new england survey area and 15 within the shallop canyon survey area complementary high resolution seafloor bathymetry and backscatter mapping data indicated highly rugose morphology at all surveyed seep sites including the presence of extensive rock outcrops and bivalve beds video collected by hov alvin provided visual confirmation of gas plumes and the presence of chemosynthetic communities including microbial mats and mussel beds as well as large outcrops of authigenic carbonate rock surrounding locations of discrete gas emission bathymodiolus was the dominant species observed at the veatch canyon methane seep site while the deep sea red crab chaceon quinquedens dominated the new england and shallop canyon seep sites elevated pore water sulfide concentrations suggested stimulation of sulfate coupled anaerobic oxidation of methane aom and or degradation of organic matter in seep sediments relative to non seep sediments aom was observed directly within overlying authigenic carbonate rocks dna sequencing of the surface sediments revealed diverse sulfide oxidizing bacteria similar to the taxa described previously at other methane seeps including members of the thiotrichales and campylobacterales together these analyses provide one of the first interdisciplinary descriptions of sites within this massive seep system setting the stage for more targeted hypothesis driven investigations in this context telepresence facilitated interdisciplinary research between specialists at the isc and scientists and engineers at sea enabling researchers to transfer data and maximize research efforts with sampling tools at sea our findings suggest that although the geological setting is distinct the surficial microbial and macrofaunal communities closely resemble those of previously described methane seeps

analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope cascadia margin sediments contain a rich reservoir of carbon derived both from terrestrial input and sea surface productivity a portion of this carbon exists as solid gas hydrate within sediment pore spaces which previous studies have shown to be a methane reservoir of substantial size on both the vancouver island and oregon portions of the cascadia margin multichannel seismic reflection profiles on the cascadia margin show the widespread presence of bottom simulating reflectors bsrs within the sediment column indicating the gas hydrate reservoir extends from the deformation front at 3000 m depth to the upper limit of gas hydrate stability near 500 m water depth in this study we compile an inventory of methane bubble plume sites on the cascadia margin identified in investigations carried out for a range of interdisciplinary goals that also include sites volunteered by commercial fishermen high plume density anomalies are associated with both the continental shelf 180 m and the depth of the upper limit of methane hydrate stability depth mhsd that occurs near 500 m in the ne pacific the observed anomalies on the cascadia slope may be due to the warming of seawater at intermediate depths suggesting that modern climate change has begun to destabilize the climate sensitive hydrate reservoir within the cascadia margin sediments reanalysis of similar plume images on the north american atlantic slope suggests a lack of correlation between observed plume depths and the mhsd for much of the latitudinal range

a study of the chemistry of pore fluids and authigenic carbonates in methane seep environments kodiak trench hydrate ridge monterey bay and eel river basin analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the kodiak trench hydrate ridge monterey bay clam flats and the eel river basin coupled with the studies of the chemistry of the pore fluids have shown that these carbonates have grown within the sediment column geochemical profiles of pore fluids show that in deep water seeps kodiak trench 4450 m monterey bay 1000 m hydrate ridge 650 m 13c dic values are low isotopically light whereas in the eel river area 350 500 m 13c dic values are much higher isotopically heavier in all cases the 13c values indicate that processes of methane oxidation associated with sulfate reduction are dominant in the shallow sediments data on the isotopic composition of authigenic carbonates found at sites in kodiak trench eel river basin south and eel river basin north indicate a variable composition and origin in different geochemical environments some of the authigenic carbonates from the study sites show a trend in their 13c values similar to those of the pore fluids obtained in their vicinity suggesting formation at relatively shallow depths but others indicate formation at greater sediment depths the latter usually consist of high magnesium calcite or dolomite which from their high values of 13c up to 23 and 18o up to 7 5 suggest formation in the deeper horizons of the sediments in the zone of methanogenesis these observations are in agreement with observations by other workers at hydrate ridge in monterey bay and in the eel river basin

u th isotope constraints on gas hydrate and pockmark dynamics at the niger delta margin abstract the application of uranium thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins in this study we report u th isotope measurements for carbonate breccias collected from within a hydrate bearing pockmark located at the niger delta margin these concretions were extracted from a carbonate rich layer in the upper two meters of a sediment core n2 ks 44 1200 m water depth well above the present day sulphate methane transition zone about 3 m depth and the presence of gas hydrates in the sediment the stratigraphy of core n2 ks 44 was established by tuning its downcore al ti profile to a well dated nearby reference core and carbonate 230 th u ages were calculated using isochron methods our results indicate that a major event of aragonite precipitation occurred between about 13 and 2 5 ka at the studied location comparison of sediment accumulation rates at both core n2 ks 44 and the nearby reference site suggests that the initial stage of carbonate precipitation between 13 and 10 ka was associated with sediment winnowing probably related to intense fluid seepage in contrast our data indicate that sedimentation rates rapidly increased within the pockmark after 7 ka in agreement with the presence of carbonate breccias exhibiting u th ages older than their corresponding stratigraphic age this observation would suggest that sediment reworking took place after that time possibly caused by erosion of the surrounding sediment within the pockmark we hypothesize that the period of carbonate formation between 13 and 2 5 ka was related to an upward migration of gas hydrate reservoirs to the near seafloor environment after this pulse of enhanced fluid flow the diminution of methane fluxes at the base of the local gas hydrate occurrence zone would have led to hydrate dissolution in sub surface sediments and pockmark formation thereby explaining the progressive increase in sedimentation rates the absence of recent carbonate concretions and the deepening of the sulphate methane transition zone at site n2 ks 44 inferred from pore water data overall these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time

u and th concentrations and isotope ratios in modern carbonates and waters from the bahamas the short residence times of th and pa in seawater make them very responsive to changes in the ocean environment we use a new multi ion counting technique to make th and pa isotope measurements in seawaters from a near shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs the bahamas an unusual feature of the bahamas setting is the shallow depth of water residing on the bank tops these waters have significantly lower 232th 230th 10 000 than those immediately adjacent to the banks 24 000 31 000 and a 231pa 230th near the production ratio 0 1 the change in 232th 230th and 231pa 230th on the bank tops is explained by almost quantitative removal of th and pa by scavenging and their replacement with a mixture of 230th and 231pa alpha recoiled from the underlying carbonates together with th from dust dissolution analysis of a water profile in the tongue of the ocean which separates the great and little bahama banks allows us to trace the movement of bank top water to depth a distinct minimum in both 232th 230th 13 000 and 231pa 230th 0 5 is observed at 430 m and is interpreted to reflect density cascading of bank top water with entrained carbonate sediment these results suggest that th and pa can be used as water mass tracers in near shore environments uranium concentration measurements on the same waters demonstrate that u is conservative across a range in salinity of 2 psu with a concentration of 3 33 ppb at a salinity of 35 the incorporation of u and th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these bahamas waters such incorporation is critical for u th geochronology u isotope analyses demonstrate that seawater 234u averages 146 6 and does not vary by more than 2 5 o and that carbonates capture this value additional high precision measurements 1 o on modern carbonates confirm that all oceans have identical 234u modern marine carbonates are shown to have 232th 230th ratios that reflect the local seawater in which they formed

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

uranium series dating of marine and lacustrine carbonates of the possible uranium series dating schemes the most important and most widely applied to marine carbonates is 230th dating with 231pa dating playing an increasingly important role for this reason this review will focus on these two methods 230th dating also referred to as u th dating or 238u 234u 230th dating involves calculating ages from radioactive decay and ingrowth relationships among 238u 234u and 230th 232th is also typically measured as a long lived essentially stable index isotope over the time scales relevant to 230th dating at present 230th dating can in principle be used to date materials as young as 3 years and in excess of 600 000 years edwards et al 1987a 1993 edwards 1988 see stirling et al 2001 for an example of dating corals in excess of 600 000 years old 231pa dating also referred to as u pa dating involves calculating ages from the ingrowth of 231pa from its grandparent 235u at present 231pa dating can be used to date materials as young as 10 years and as old 250 000 years edwards et al 1997 230th dating covers all of the 231pa time range and more with somewhat higher precision and is therefore the method of choice if a single method is applied however the combination of 231pa and 230th dating is of great importance in assessing possible diagenetic mobilization of the pertinent nuclides and thereby the accuracy of the ages allegre 1964 ku 1968 even if the primary age exceeds the 250 000 year limit of 231pa dating the combined methods can be used to assess the degree to which the samples have remained closed over the past 250 000 years e g edwards et al 1997 thus

last glacial emplacement of methane derived authigenic carbonates in the sea of japan constrained by diatom assemblage carbon 14 and carbonate content abstract we studied diatom assemblages and caco 3 contents of methane derived authigenic carbonates from the eastern margin of the sea of japan and assessed the formation time of these samples radioactive 14 c date was determined in selected samples to obtain the maximum age of the time the results of our study suggest mass formation of carbonate nodules in a glacial period within 40 ky consistent with a published u th dating result of carbonate nodules in the study area diatom assemblages and contents in the carbonate nodules abundance of 10 6 g dominance of neritic littoral species warm cold water species ratio lower than 25 differ from the near seafloor sediments in the study area which have characteristics of holocene sediments in the sea of japan and suggest cementation of glacial sediments laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi enclosed ocean similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules glacial sediments with oceanic cold water species as low as holocene sediments restricts the sediment age to before 20 cal ky bp carbonate contents higher than 78 wt suggest the cementation of poorly compacted sediments near the seafloor and the date of carbonate cementation is therefore close to that of the cemented sediments most carbonate nodule samples in this study were formed in a glacial period and detection of 14 c restricts this period to within 40 ky

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

past methane release events and environmental conditions at the upper continental slope of the south china sea constraints by seep carbonates authigenic carbonates and seep biota are archives of seepage history and record paleo environmental conditions at seep sites we obtained the timing of past methane release events at the northeastern slope of the south china sea based on u th dating of seep carbonates and seep bivalve fragments from three sites located at 22 02 22 09 n 118 43 118 52 e water depths from 473 to 785 m also we were able to reconstruct the paleo bottom water temperatures by calculating the equilibrium temperature using the ages the corresponding past 18o of seawater 18osw and the 18o of the selected samples formed in contact with bottom seawater with negligible deep fluid influence a criterion consists of mineralogy redox sensitive trace elements and u th isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid our results show that all methane release events occurred between 11 5 0 2 and 144 5 12 7 ka when sea level was about 62 104 m lower than today enhanced methane release during low sea level stands seems to be modulated by reduced hydrostatic pressure increased incision of canyons and increased sediment loads the calculated past bottom water temperature at one site site 3 water depth 767 771 m during low sea level stands 11 5 and 65 ka ago ranges from 3 3 to 4 0 c i e 1 3 to 2 2 c colder than at present the reliability of 18o of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18o enriched fluids that might be emitted from gas hydrate and or clay dehydration our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the south china sea

the geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters pore water profiles of uranium and thorium isotopes in the muddy sediments of buzzards bay ma permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements uranium shows a pronounced minimum pore water concentration 1 2 dpm kg near the sediment water interface 0 3 cm which coincides with the pore water fe maximum u concentrations increase with depth to a broad maximum which is greater than the overlying seawater value then decrease laboratory sediment tank experiments maintained without macrofauna also show the near interface minimum and increase and sediment incubation experiments show removal of u from the pore water to a constant value by about 40 days the 234u238u activity ratio is equal to the seawater value in all samples oxidation state separations of pore water u in hcl solution using anion exchange demonstrate that u is present in the pore water as u vi the coincidence of the pore water uranium and fe maximum is consistent with the hypothesis of reduction of u vi to u iv and removal from solution at about the same depth or pe as fe is reduced increases in pore water u with depth appear to be related to release of authigenic seawater u vi to solution possibly as alkalinity and ph increase and organic matter with which the u is associated is oxidized lower concentrations at depth in the sediment column may be linked to increases of effective u reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna in contrast pore water profiles of the long lived th isotopes 232th and 230th show low activities 02 dpm kg and relatively little change with depth although values are greater than those in the overlying water solid phase activities are similarly constant with depth and the distribution may be controlled by a sorption equilibrium between pore water and the solid phase the short lived isotope 234th shows greatest pore water activities in the upper 5 cm and is present at virtually zero activities elsewhere in the sediment column the upper few centimeters of the sediment generally contain excess 234th scavenged from the overlying water column and distributed by bioturbation as well as pore water fe and mn profiles which show maxima indicating reduction and dissolution of fe and mn oxyhydroxides the pore water 234th profile can be explained by release of 234th from the solid phase as femn oxide surface coatings are reduced and dissolved

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

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

geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field east pacific rise 18 s an inactive vent field comprised of dead chimneys was discovered on the ultrafast east pacific rise epr at 18 degrees s during the research campaign naudur with the r v le nadir in december 1993 one of these chimneys was sampled studied and found to be largely composed of silica mineralized bacterial like filaments the filaments are inferred to be the result of microbial activity leading to silica fe oxyhydroxide precipitation the chimney grew from the most external layer precipitated 226 4 yr b p towards the central chimney conduit hydrothermal activity ceased 154 13 yr b p and the chimney conduit was completely sealed mixing between an end member hydrothermal fluid and seawater explains the sr nd isotopic composition of the chimney seawater was the major source of sr to the chimney whereas the dominant nd source was the local mid ocean ridge basalt morb leached by the hydrothermal fluids the mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions the proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85 pb isotope data indicates that all of the pb in the chimney was derived from the underlying morb the precipitation temperatures of the chimney layers varied between 55 and 71 degrees c and were a function of the seawater end member hydrothermal fluid mixing ratio delta si 30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the si isotope composition of the chimney concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end member hydrothermal fluid the inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater hydrothermal fluid mixing ratio which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids the silica chimney is significantly enriched in u likely a result of bacterial concentration of u from the seawater dominated vent fluid the chimney is poor in rare earth elements ree it inherited its ree distribution patterns from the parent end member hydrothermal fluids the dilution of the hydrothermal fluid with over 85 seawater could not obliterate the particular ree features positive eu anomaly of the hydrothermal fluids

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

preliminary video spatial analysis of cold seep bivalve beds at the base of the continental slope of israel palmahim disturbance abstract seafloor video surveys were carried out with the rov max rover in the palmahim disturbance pd area offshore israel in september 2016 during the eurofleets2 semseep cruise on board the r v aegaeo preliminary observations of distribution and frequencies of bivalve accumulations show that they are mostly composed by lucinoma kazani shells valves are distributed mainly along the base of authigenic carbonate crusts in association with bacterial mats and inside inactive chimney like structures sometimes associated with crabs and living tubeworms however no living molluscs were observed the relation between distribution of bivalve beds and seepage activity is not straightforward in the pd area however the presence of these accumulations and the size of l kazani suggest that seepages may have lasted for at least one decade before its reduction cessation that induced l kazani mortality the presence of few articulated valves of this species indicates that they were alive up to one year before the rov observations and their different preservation state may imply different seepage phases this research shows that chemosymbiotic benthic communities may be used as a proxy for better understanding the past dynamics of fluid emissions at the seafloor however further quantitative studies are needed to confirm these observations

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

u th dating of marine isotope stage 7 in bahamas slope sediments in order to understand the driving forces for pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing in contrast to the last interglacial marine isotope stage mis 5 the timing of the penultimate interglacial mis 7 is poorly constrained this study constrains its timing and structure by precise u th dating of high resolution n 18 o records from aragonite rich bahamian slope sediments of odp leg 166 sites 1008 and 1009 the major glacial interglacial cycles in n 18 o are distinct within these cores and some mis 7 substages can be identified these sediments are well suited for u th dating because they have uranium concentrations of up to 12 ppm and very low initial 230 th contributions with most samples showing 230 th 232 th activity ratio of s 75 u and th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry with the latter providing dramatically better precision twenty nine of the 41 samples measured have a n 234 u value close to modern seawater suggesting that they have experienced little diagenesis ages from 27 of the 41 samples were deemed reliable on the basis of both their u and their th isotope ratios ages generally increase with depth although we see a repeated section of stratigraphy in one core extrapolation of constant sedimentation rate through each substage suggests that the peak of mis 7e lasted from v237 to 228 ka and that 7c began at 215 ka this timing is consistent with existing low precision radiometric dates from speleothem deposits the beginning of both these substages appears to be slightly later than in orbitally tuned timescales the end of mis 7 is complex but also appears to be somewhat later than is suggested by orbitally tuned timescales although this event is not particularly well defined in these cores fl 2002 elsevier science b v all rights reserved

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

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

multi disciplinary investigation of fluid seepage on an unstable margin the case of the central nile deep sea fan we report on a multidisciplinary study of cold seeps explored in the central nile deep sea fan of the egyptian margin our approach combines in situ seafloor observation geophysics sedimentological data measurement of bottom water methane anomalies pore water and sediment geochemistry and 230 th u dating of authigenic carbonates two areas were investigated which correspond to different sedimentary provinces the lower slope at 2100 m water depth indicates deformation of sediments by gravitational processes exhibiting slope parallel elongated ridges and seafloor depressions in contrast the middle slope at not vert 1650 m water depth exhibits a series of debris flow deposits not remobilized by post depositional gravity processes significant differences exist between fluid escape structures from the two studied areas at the lower slope methane anomalies were detected in bottom waters above the depressions whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities carbonate u th age dates 8 kyr bp pore water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past in contrast large 1 km2 carbonate paved areas were discovered in the middle slope with u th isotope evidence for ongoing carbonate precipitation during the late holocene since 5 kyr bp at least our results suggest that fluid venting is closely related to sediment deformation in the central nile margin it is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of fossil carbonate ridges whereas pervasive diffuse flow prevails at the unfailed middle slope

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

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

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

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

seepage response along an alluvial valley in a semi arid catchment in north central california over a period of 12 months soil moisture content and potential was monitored in an annual grass dominated 20 ha catchment in order to determine flow paths leading to exfiltration at the catchment outlet water was found to enter the catchment valley either through flow originating in the slopes or through surface infiltration during rainfall events although subsurface flow from the slopes to the catchment outlet occurred throughout the year surface recharge was restricted to a few events during the wet season in the deeper saturated profile of the valley flow was directed upwards along the valley edges and gradually became horizontal towards the central axis of the valley during the peak of the rainfall season horizontal flow close to the catchment outlet intercepted the gradually sloping surface resulting in exfiltration plants influenced the hydrology of the catchment by removing moisture from the root zone during spring and early summer resulting in evapotranspiration losses from the vadose zone heterogeneities within the valley soil were evident as variable permeability layers that resulted in a seasonally confined water table within the valley this investigation shows that the vadose zone plays an important role in redistributing surface recharge and emphasizes the importance of accounting for effective moisture in low yielding catchments with ephemeral surface runoff copyright 2002 john wiley sons ltd

fossil evidence for serpentinization fluids fueling chemosynthetic assemblages among the deep sea hydrothermal vent sites discovered in the past 30 years lost city on the mid atlantic ridge mar is remarkable both for its alkaline fluids derived from mantle rock serpentinization and the spectacular seafloor carbonate chimneys precipitated from these fluids despite high concentrations of reduced chemicals in the fluids this unique example of a serpentinite hosted hydrothermal system currently lacks chemosynthetic assemblages dominated by large animals typical of high temperature vent sites here we report abundant specimens of chemosymbiotic mussels associated with gastropods and chemosymbiotic clams in approximately 100 kyr old lost city like carbonates from the mar close to the rainbow site 36 n our finding shows that serpentinization related fluids unaffected by high temperature hydrothermal circulation can occur on axis and are able to sustain high biomass communities the widespread occurrence of seafloor ultramafic rocks linked to likely long range dispersion of vent species therefore offers considerably more ecospace for chemosynthetic fauna in the oceans than previously supposed

thorium isotopes in the western mediterranean sea an insight into the marine particle dynamics we present a detailed view of the 230 th 232 th systematics in the western mediterranean sea in order to constrain water and particle fluxes the conclusions obtained at the regional scale are also relevant at a more global scale although they may be more difficult to establish in the open ocean 230 th and 232 th were analyzed in size fractionated seawater and marine particle samples collected in the ligurian alboran and ionian seas the 230 thxs 230 th produced by the radioactive decay of 234 u in seawater profiles at dyfamed and in the alboran sea site are not linear due to the formation of deep waters the difference of 230 thxs concentration between these two sites is related to the aging of the water masses and to different scavenging conditions the 230 th 232 th ratio is used as a tracer during mixing processes to provide information on the pathways followed by the matter in the water column at dyfamed filtered 6 0 2 wm and ultrafiltered 6 1 kda solutions have similar 230 th 232 th ratios suggesting that an equilibrium exists between truly dissolved and colloidal th the change of the th flux and of the 230 th 232 th ratio of trapped particles observed in august between 200 m and 1000 m is best explained by aggregation of filtered large particles on the trapped particles the residence time of filtered large particles with respect to aggregation on trapped particles is of the order of 8 80 days the combined budget of 230 th and 232 th inputs to the western mediterranean sea requires the dissolution of 3 5 of the th associated with all the continental particulate inputs including particles sequestered in margins suggesting that eolian inputs are not the only source of refractory elements to the mediterranean sea and may be to the whole ocean fl 2002 elsevier science b v all rights reserved

fluid flow through carbonate platforms constraints from 234u 238u and cl in bahamas pore waters the geometry timing and rate of fluid flow through carbonate margins and platforms is not well constrained in this study we use u concentrations and isotope ratios measured on small volumes of pore water from bahamas slope sediment coupled with existing chlorinity data to place constraints on the fluid flow in this region and by implication other carbonate platforms these data also allow an assessment of the behaviour of u isotopes in an unusually well constrained water rock system we report pore water u concentrations which are controlled by dissolution of high u organic material at shallow depths in the sediment and by reduction of u to its insoluble 4c state at greater depths the dominant process influencing pore water 234 u 238 u is alpha recoil in holocene sediments the increase of pore water 234 u 238 u due to recoil provides an estimate of the horizontal flow rate of 11 cm year but with considerable uncertainty at depths in the sediment where conditions are reducing features in the u concentration and 234 u 238 u profiles are offset from one another which constrains the effective diffusivity for u in these sediments to be1 2 10 8 cm 2 s 1 at depths between the holocene and these reducing sediments pore water 234 u 238 u values are unusually low due to a recent increase in the dissolution rate of grain surfaces this suggests a strengthening of fluid flow probably due to the flooding of the banks at the last deglaciation and the re initiation of thermally driven venting of fluid on the bank top and accompanying recharge on the slopes interpretation of existing chlorinity data in the light of this change in flow rate constrain the recent horizontal flow rate to be 10 6 3 4 cm year estimates of flow rate from 234 u 238 u and cl are therefore in agreement and suggest flow rates close to those predicted by thermally driven models of fluid flow this agreement supports the idea that flow within the bahamas banks is mostly thermally driven and suggests that flow rates on the order of 10 cm year are typical for carbonate platforms where such flow occurs 1999 elsevier science b v all rights reserved

diagenetic mg calcite overgrowths on foraminiferal tests in the vicinity of methane seeps methane is a potent greenhouse gas and some episodes of past global warming appear to coincide with its massive release from seafloor sediments as suggested by carbon isotope records of foraminifera here we present structural geochemical and stable carbon isotope data from single foraminiferal calcite tests and authigenic mg calcite overgrowths in a sediment core recovered from an area of active methane seepage in western svalbard at ca 340 m water depth the foraminifera are from intervals in the core where conventional bulk foraminiferal 13c 13c values are as low as 11 3 mg ca analyses of the foraminiferal tests reveal that even tests for which there is no morphological evidence for secondary authigenic carbonate can contain mg rich interlayers with mg ca up to 220 mmol mol transmission electron microscopy tem of the contact point between the biogenic calcite and authigenic mg calcite layers shows that the two phases are structurally indistinguishable and they have the same crystallographic orientation secondary ion mass spectrometry sims analyses reveal that the mg rich layers are strongly depleted in 13c 13c 13c as low as 34 1 these very low 13c 13c values indicate that the authigenic mg calcite precipitated from pore waters containing methane derived dissolved inorganic carbon at the depth of the sulfate methane transition zone smtz as the depth of the smtz can be located several meters below the sediment seawater interface interpretation of low foraminiferal 13c 13c values in ancient sediments in terms of the history of methane seepage at the seafloor must be undertaken with care

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

uranium series isotopes in colloids and suspended sediments timescale for sediment production and transport in the murray darling river system abstract we have measured 238u 234u 230th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the murray darling river basin se australia continuous compositional trends of composition are observed for 234u 238u and 230th 238u activity ratios over the range of size fractions studied they are explained by the variable contribution of two components detrital alumino silicates and organic matter using relationships between activity ratios and the loss on ignition it is possible to estimate the u series isotopic composition of both detrital and organic end members which provide the best estimate for the composition of the true detrital phase i e controlled only by the removal of elements through weathering and the true dissolved phase i e colloid free respectively when performing a single filtration at 0 2 m it appears that size fractions and

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

decadal topographic change in the mcmurdo dry valleys of antarctica thermokarst subsidence glacier thinning and transfer of water storage from the cryosphere to the hydrosphere abstract recent local scale observations of glaciers streams and soil surfaces in the mcmurdo dry valleys of antarctica mdv have documented evidence for rapid ice loss glacial thinning and ground surface subsidence associated with melting of ground ice to evaluate the extent magnitude and location of decadal scale landscape change in the mdv we collected airborne lidar elevation data in 2014 2015 and compared these data to a 2001 2002 airborne lidar campaign this regional assessment of elevation change spans the recent acceleration of warming and melting observed by long term meteorological and ecosystem response experiments allowing us to assess the response of mdv surfaces to warming and potential thawing feedbacks we find that locations of thermokarst subsidence are strongly associated with the presence of excess ground ice and with proximity to surface or shallow subsurface active layer water subsidence occurs across soil types and landforms in low lying low slope areas with impeded drainage and also high on steep valley walls glacier thinning is widespread and is associated with the growth of fine scale roughness pond levels are rising in most closed basin lakes in the mdv across all microclimate zones these observations highlight the continued importance of insolation driven melting in the mdv the regional melt pattern is consistent with an overall transition of water storage from the local cryosphere glaciers permafrost to the hydrosphere closed basin lakes and ponds as well as the ross sea we interpret this regional melting pattern to reflect a transition to arctic and alpine style hydrologically mediated permafrost and glacial melt

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

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

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

fluid source and methane related diagenetic processes recorded in cold seep carbonates from the alvheim channel central north sea integrated petrography mineralogy geochronology and geochemistry of cold seep carbonate crusts and free gas from the alvheim channel elucidate diagenetic carbonate precipitation and related seepage histories in the central north sea free gas isotope characteristics coupled with carbonate 13c values as low as 66 vpdb indicate a predominantly microbial methane source with minor thermogenic contribution we estimate that 70 of the carbon sequestered into carbonate precipitates was derived from local oxidation of methane the early stage of crust growth is represented by microcrystalline aragonite and mg calcite 10 to 40 mol mgco3 cementing seafloor sediments consisting of clays quartz feldspar and minor detrital low mg calcite and dolomite typical association of aragonite cement with coarse grained detritus may reflect elevated fluid flow and flushing of fine particles prior to cementation close to the seafloor middle rare earth element enrichment in early generation microcrystalline cements containing framboidal pyrite indicates diagenetic precipitation within the zone of anaerobic methane oxidation contiguous to iron reduction the later generation diagenetic phase corresponds to less abundant radial fibrous and botryoidal aragonite which lines cavities developed within the crusts in contrast to early generation cements late generation cavity infills have rare earth elements and y patterns with small negative ce anomalies similar to seawater consistent with carbonate precipitation in a more open seawater dominated system aragonite u th ages indicate carbonate precipitation between 6 09 and 3 46 kyr bp in the northern part of the channel whereas in the southern part precipitation occurred between 1 94 and 0 81 kyr bp reflecting regional changes in fluid conduit position

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

u th dating of carbonate platform and slope sediments absolute chronology of marine sediment beyond the 14c age range provides a test for models of climate change and has many other applications u th techniques have been used for such chronology by dating corals but extending these techniques to marine sediment is complicated by the presence of significant initial 230th both in detrital material and scavenged from seawater in this study we investigate four methods of solving the initial 230th problem for a particular type of marine sediment the aragonite rich sediments of carbonate platforms and slopes bulk sediment u th analyses can be corrected for initial th to yield ages with 2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high uncertainty on the corrections causes inadequate precision for sediment from other periods however removal of scavenged th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches using heavy liquids to separate the various carbonate minerals found in bahamas sediment enables an isochron approach to correct for initial th but the presence of initial th from two sources requires correction or removal of one source of initial th before the other is deconvolved by the isochron quantitative removal of detrital material before isochron analysis proves a successful approach such isochron data demonstrate that although sediment remains closed to u th on a centimetre scale nuclides are moved from grain to grain by recoil such intergrain exchange is expected to be observed in all sediments containing mineral grains with different u concentrations measured 234u 238u allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr the accuracy of this approach has been proved by dating samples within the 14c age range sediments spanning the penultimate deglaciation have also been dated after a small correction for bioturbation the age for this event is found to be 135 2 3 5 ka this date is 8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics this isochron approach shows considerable promise for dating of sediments older than this event which will provide further information about the timing and mechanisms of global climate change

numerical modeling of carbonate crust formation at cold vent sites significance for fluid and methane budgets and chemosynthetic biological communities at many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments the range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear here we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate starting with the conditions prevailing at a previously investigated reference site located on hydrate ridge off oregon several parameters are systematically varied in a number of numerical experiments these parameters include coefficients of bioturbation and bioirrigation sedimentation rate fluid flow velocity methane concentration in the ascending vent fluids and ph and saturation state at the sediment water interface the simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane 50 mm and if bioturbation coefficients are low 50 cm ka 1 inhibit crust formation bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust crusts only form over a rather narrow range of upward fluid flow velocities 20 60 cm a 1 which is somewhat enlarged up to 90 cm a 1 if the overlying bottom waters are supersaturated with respect to calcite at higher flow rates methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment the formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years 100 500 a crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport therefore rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation due to the moderate flow rates and the slow diffusive transport only very little methane escapes into the bottom water overlying carbonate encrusted vent areas

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

methane derived authigenic carbonates on accretionary ridges miocene case studies in the northern apennines italy compared with modern submarine counterparts abstract we present new field data from three outcrops of miocene methane derived authigenic carbonates in the foredeep of the northern apennines that contain chemosynthetic fauna and record a long history 1 ma of shallow fluid seepage linked to seafloor anaerobic oxidation of methane the studied outcrops show similar features in terms of carbonate morphology facies spatial distribution and lateral and vertical contacts with the enclosing sediments methane derived carbonates occur in two structural positions 1 on the slope of the accretionary wedge in hemipelagites draping buried thrust related anticlines and 2 at the leading edge of the deformation front in the inner foredeep within fault related anticlines standing above the adjacent deep seafloor as intrabasinal ridges we compare fossil seeps with two extensively investigated modern analogues the hikurangi margin offshore new zealand and hydrate ridge on the cascadia margin offshore the u s a these analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane derived carbonate bodies and chemosynthetic fauna on the present day seafloor the comparison allows us to propose a model for the evolution of fluid seeps on thrust related ridges at the deformation front uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults mostly blind connected to the basal detachment favoring the migration of fluids toward the incipient anticline fold development generates extensional stresses in the hinge zone of the anticline promoting the development of normal faults fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve in the slope setting far from the deformation front thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor after reaching a mature stage within the wedge the structure is less active and buried in the slope environment of the evolved prism

using 234u 238u to assess diffusion rates of isotope tracers in ferromanganese crusts abstract ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography interpreting crust isotope profiles however requires an assessment of the rate of diffusion of each element within the crust we address this issue using u and th isotope profiles coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts a depth profile of 230thxs 234u for a north atlantic crust alv539 yields a growth rate of 3 05 mm myr consistent with an existing 10be 9be record but the growth rate implied by a 234u 238u profile in the same crust is 6 times faster due to diffusion of u in the crust the difference between the observed 234u 238u profile and that expected from 230thxs 234u enables the effective diffusivity of u to be calculated as 1 2 10 6 cm2 year recalculated literature data indicate that other crusts have diffusivities which range down to 4 7 10 8 cm2 yr this variation is to be expected given the range of porosity observed for ferromanganese crusts concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust these distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of u such diffusivities are assessed for crust alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts li os and sr have diffusivities of 2 10 4 3 10 5 and 2 10 5 cm2 year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions th nd pb and be have diffusivities less than 10 9 cm2 year and are highly immobile in crusts a reassuring result for th and be crust chronologies and for nd and pb isotope tracing of past water masses using crusts hf has a diffusivity of 3 10 8 in alv539 fast enough to perturb but not destroy its isotope history for studies of hf in this and other crusts diffusion should therefore be considered measuring u isotope profiles provides a potential way of assessing such diffusion for hf or for other elements with relatively low distribution coefficients into ferromanganese crusts

cold seep driven carbonate deposits at the central american forearc contrasting evolution and timing in escarpment and mound settings continuous surface cores of cold seep carbonates were recovered offshore pacific nicaragua and costa rica from 800 to 1 500 m water depths meteor 66 3 in order to decipher their evolution and methane enriched fluid emanation in contrasting geological settings cores from the mounds iguana perezoso baula v and from the jaco scarp escarpment were used for a multi method approach for both settings aragonite was revealed as dominant authigenic carbonate phase in vein fillings and matrix cementation followed by mg calcite as second most abundant this common precipitation process of caco3 polymorphs could be ascribed as indirectly driven by chemical changes of the advecting pore water due to anaerobic oxidation of methane a more direct influence of seep related microbial activity on the authigenic mineral assemblage in both settings is probably reflected by the observed minor amounts of dolomite and a dolomite like camg carbonate mgco3 42 13c data of jaco scarp samples are significantly lower 43 to 56 pdb than for mound samples 22 to 36 pdb indicating differences in fluid composition and origin noteworthy 18o values of scarp samples correlate most closely with the ocean signature at their time of formation documenting the archive potential a high resolution case study of a mound core implies at least 40 changes in fluid supply within a time interval of approximately 14 ky as most striking difference the age data indicate a late stage downward progressing cementation front for all three mound cap structures approx 2 5 cm ky but a significantly faster upward carbonate buildup in the bulging sediments on top of the scarp environment approx 120 cm ky the latter data set leads to the hypothesis of chemoherm carbonate emplacement in accord with reported sedimentation rates until decompression of the advective fluid system probably caused by the jaco scarp landslide and dating this to approximately 13 000 years ago

early diagenetic processes in labrador sea sediments uranium isotope geochemistry the concentration and isotopic composition of u dissolved in pore waters from hemipelagic sediments of the labrador sea were determined by thermal ionization mass spectrometry in two 30 cm long box cores the present fluxes of seawater u that diffuses across the sediment seawater interface are on the order of 2 4 g cm2 ka this diffusion imposes decreasing gradients of dissolved u downwards but the u concentration in pore waters immediately below the surface is much lower than that of open ocean seawater this is a primary feature that cannot be explained by carbonate precipitation due to decompression during core retrieval more likely it reflects the presence of a stagnant benthic boundary layer above the sediment water interface in which molecular diffusion of u is slower than in the overlying turbulently mixed seawater and or of microzones near the interface where u is bioaccumulated uranium is adsorbed at depths onto the solid phase in response to changes in the redox conditions within the