mineralogical and stable isotopic characterization of authigenic carbonate from a hydrocarbon seep site gulf of mexico slope possible relation to crude oil degradation abstract one of the characteristic features on the cold deep seafloor on atwater valley at block 425 1930 m water depth in the gulf of mexico is the occurrence of seepage of crude oil and hydrocarbon gases associated with thermogenic gas hydrate and authigenic carbonate deposits mineralogical petrographic and isotopic analyses revealed that the precipitation of these carbonates was related to the microbial oxidation of hydrocarbons carbonate deposits are composed of heavily oil stained small nodules to large cobbles of dense limestones of fine grained textures other carbonate fragments are composed of small intraclasts pellets and some bivalve shell fragments cemented together by aragonite cement xrd analysis and petrographic investigations revealed that the authigenic carbonate is predominantly composed of aragonite that occurs as microcrystalline matrix and cement microcrystalline sparitic and botryoidal acicular aragonite crystals are observed as void filling cement the 18o values of the authigenic carbonate 3 80 to 4 16 v pdb suggest that the precipitating fluid has equilibrated with the cold bottom seawater the calculated 18o values 0 18 to 0 54 smow of the precipitating fluid show subtle deviation from the values reported for the present bottom seawater this is attributed possibly to the precipitation from pore water with temperature ranged from 3 3 to 1 7 c which is less than the measured bottom seawater 4 c however the mixing of the marine pore water with some deep hydrocarbon rich brines or hydrate water would provide a source of 18o enrichment fluid for the precipitation of the authigenic carbonates based on the predominance of aragonite with botryoidal and clotted fabric the occurrence of pyrite and from the oxygen isotopic data it is plausible that the authigenic carbonates may have formed near the sediment water interface in an oxygen depleted environment with low temperature the narrow range of the 13c values of the authigenic carbonate 23 88 to 28 62 v pdb suggests that a single source of carbon is predominant based on the moderately low 13c values and the occurrence of spots of crude oil coatings of the carbonate samples in addition to the application of a carbon isotope mass balance model the carbon involving in the formation of the carbonates of the at 425 seep site could be mainly derived from microbial degradation of crude oil microbial oxidation of non methane thermogenic hydrocarbon gases such as ethane propane and isobutane would play some role as a source of carbonate carbon

archaeal lipid biomarkers and isotopic evidence of anaerobic methane oxidation associated with gas hydrates in the gulf of mexico abstract anaerobic oxidation of methane aom occurs in the gulf of mexico gas hydrate systems here we show lipid biomarker and isotopic evidence that archaea are involved in aom the estimated abundance of total archaeal lipids ranges from 44 8 to 60 4 g g dry sediment in hydrate bearing samples but is below detection limit in the hydrate free sample the 13c values of archaeal lipids range from 69 to 99 in hydrate bearing samples these results suggest that biomass of archaea is significantly enhanced through aom at the gas hydrate deposits these data also support a currently acknowledged mechanism of aom mediated by a consortium of sulfate reducing bacteria and archaea observed in a variety of methane rich marine settings anaerobic oxidation of oil hydrocarbons also occurs in the gulf of mexico gas hydrate systems as shown by degradation of n alkanes c15 in the anoxic sediments these processes convert hydrocarbons to carbon dioxide and increase pore water alkalinity which promote the precipitation of enormous volumes of authigenic carbonate rock depleted in 13c this long term geologic sequestration of carbon may affect models of global climate change

origin distribution and alteration of asphalts at chapopote knoll southern gulf of mexico abstract following the discovery of asphalt volcanism in the campeche knolls a research cruise was carried out in 2006 to unravel the nature of the asphalt deposits at chapopote the novel results support the concept that the asphalt deposits at the seafloor in 3000 m of water depth originate from the seepage of heavy petroleum with a density slightly greater than water the released petroleum forms characteristic flow structures at the seafloor with surfaces that are ropy or rough similar to magmatic lava flows the surface structures indicate that the viscosity of the heavy petroleum rapidly increases after extrusion due to loss of volatiles consequently the heavy petroleum forms the observed asphalt deposit and solidifies detailed survey with a remotely operated vehicle revealed that the asphalts are subject to sequential alterations e g volume reduction leading to the formation of visible cracks in the asphalt surface followed by fragmentation of the entire deposit while relatively fresh asphalt samples were gooey and sticky older fragmented pieces were found to be brittle without residual stickiness furthermore there is evidence for petroleum seepage from below the asphalt deposits leading to local up doming and sometimes to whip shaped extrusions extensive mapping by tv guided tools of chapopote asphalt volcano indicates that the main asphalt deposits occur at the south western rim that borders a central crater like depression the most recent asphalt deposit at chapopote is the main asphalt field covering an area of 2000 m2 asphalt volcanism is distinct from oil and gas seepage previously described in the gulf of mexico and elsewhere because it is characterized by episodic intrusions of semi solid hydrocarbons that spread laterally over a substantial area and produce structures with significant vertical relief as chapopote occurs at the crest of a salt structure it is inferred that asphalt volcanism is a secondary result of salt tectonism

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

petroleum degradation and associated microbial signatures at the chapopote asphalt volcano southern gulf of mexico abstract at the chapopote knoll in the southern gulf of mexico deposits of asphalt provide the substrate for a prolific cold seep ecosystem extensively colonized by chemosynthetic communities this study investigates microbial life and associated biological processes within the asphalts and surrounding oil impregnated sediments by analysis of intact polar membrane lipids ipls petroleum hydrocarbons and stable carbon isotopic compositions 13c of hydrocarbon gases asphalt samples are lightly to heavily biodegraded suggesting that petroleum derived hydrocarbons serve as substrates for the chemosynthetic communities accordingly detection of bacterial diester and diether phospholipids in asphalt samples containing finely dispersed gas hydrate suggests the presence of hydrocarbon degrading bacteria biological methanogenesis contributes a substantial fraction to the methane captured as hydrate in the shallow asphalt deposits evidenced by significant depletion in 13c relative to background thermogenic methane in sediments petroleum migrating from the subsurface stimulates both methanogenesis and methanotrophy at a sulfate methane transition zone 6 7 m below the seafloor in this zone microbial ipls are dominated by archaeal phosphohydroxyarchaeols and archaeal diglycosidic diethers and tetraethers bacterial ipls dominate surface sediments that are impregnated by severely biodegraded oil in the sulfate reduction zone diagnostic ipls indicate that sulfate reducing bacteria srb play an important role in petroleum degradation a diverse mixture of phosphohydroxyarchaeols and mixed phospho and diglycosidic archaeal tetraethers in shallow oil impregnated sediments point to the presence of anaerobic methane oxidizing anme 2 and anme 1 archaea respectively or methanogens archaeal ipls increase in relative abundance with increasing sediment depth and decreasing sulfate concentrations accompanied by a shift of archaeol based to tetraether based archaeal ipls the latter shift is suggested to be indicative of a community shift from anme 2 and or methanogenic archaea in shallower sediments to anme 1 methanogenic archaea and possibly benthic archaea in deeper sediments

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

a conceptual model for hydrocarbon accumulation and seepage processes around chapopote asphalt site southern gulf of mexico from high resolution seismic point of view 1 as part of the german r v meteor m67 2 expedition in 2006 to the southern gulf of mexico a set of 2d high resolution seismic profiles was acquired across the chapopote knoll to study seafloor asphalt occurrences on the basis of regional seismostratigraphic studies and dsdp drilling a more highly reflective coarse grained sediment unit of late miocene age is identified as a potential shallow hydrocarbon reservoir although a unit of that age would typically be buried by pliocene and pleistocene sediment cover at chapopote local salt tectonism has highly elevated the structure since the late miocene and the miocene reservoir is locally above present day regional datum the elevation resulted in a thin 100 200 m fine grained sediment cover on the crest of the knoll above the reservoir because oil and gas production can be expected at depth in jurassic cretaceous and tertiary source rocks the presence of high amplitude reflector packages within the reservoir unit is interpreted as an evidence of hydrocarbons this is variously supported by observations of crosscutting reflectors polarity reversal and drops in instantaneous frequency the thin seal above the reservoir unit facilitates leakage of trapped hydrocarbons hydrocarbons apparently invaded into the seal sediments in the wider vicinity around the crest of the knoll even extending beyond the area where seafloor asphalt is known the asphalt site thus may be a currently active spot while the rest of the crest may be temporarily sealed by solid phase hydrocarbons we propose that a shallow large reservoir with deeply sourced relatively heavy petroleum is principally responsible for the formation of asphalts on the seafloor

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

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

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

marine cold seeps and their manifestations geological control biogeochemical criteria and environmental conditions characteristics of cold seeps at different geologic settings are the subject of this review primarily based on results of the research consortium sfb 574 criteria are drawn from examples on the erosive convergent margin off costa rica the accretionary margin off chile supplemented by examples from the transform margin of the golf of cadiz and the convergent hikurangi margin off new zealand others are from well studied passive margins of the black sea the golf of mexico the eastern mediterranean sea and the south china sea seeps at all settings transport water and dissolved compounds to the ocean through the seafloor by different forcing mechanism and from different depths of the submerged geosphere 10s of meters to 10s of km the compounds sustain oasis type ecosystems by providing bioactive reductants sulfide methane and hydrogen hereby the interaction between fluid composition flux rates and biota results in a diagnostic hydrocarbon metazoan microbe carbonate association currently well over 100 active sites are known the single most important reaction is microbially mediated anaerobic oxidation of methane with secondary reactions involving s biogeochemistry and carbonate mineral precipitation seep fluids and their seafloor manifestations provide clues as to source depth fluid sediment rock interaction during ascent lifetime and cyclicity of seepage events but less so on the magnitude of return flow at erosive margins cl depleted and b enriched fluids from clay dehydration provide criteria for source depth and temperature the upward material flow generates mud volcanoes at the seafloor above the projected location of dehydration at depth at accretionary margins fluids are derived from more shallow depths by compaction of sediments as they ride on the incoming oceanic plate they are emitted through thrust faults at highly sedimented margins organic rich and evaporite containing strata when present determine the final fluid composition by emitting characteristically gas hydrate derived methane brine associated non methane hydrocarbons or leached elements and their isotopes li 7li b ba from host sediments smectite illite transformation and associated cl depletion from release of interlayer water is a pervasive process at these margins rare earth element pattern in conjunction with redox sensitive metals retained in seep carbonates indicate whether or not they precipitated in contact with oxic bottom water or suboxic fluids clear environmental characterization though currently remains inconclusive more deeply sourced fluids as in transform margins may be characterized by their 87sr 86sr ratios from interaction with oceanic crustal rocks below quantification of flow and reliable estimates of total volatile output from fore arcs remain a challenge to seep research as does understanding the role of geologically derived methane in the global methane cycle

tracing the composition and origin of fluids at an ancient hydrocarbon seep hollard mound middle devonian morocco a nd ree and stable isotope study abstract for the first time nd isotope signatures combined with rare earth element ree concentrations were used in investigations of ancient seep carbonates the study was performed on the fossil hydrocarbon seep deposit of the middle devonian hollard mound eastern anti atlas morocco where nd isotopes ree concentrations and carbon and oxygen isotope ratios were measured to investigate the origin former migration pathways and composition of fluids relatively high end values compared to local eifelian seawater as well as consistently appearing positive eu anomalies in mree enriched shale normalized ree patterns of the seep carbonates provided evidence for interaction between the seeping fluids and the lower devonian basaltic volcaniclastics underlying the studied seep deposit strongly reducing conditions and increased temperature of methane formation could have constituted an additional factor in the eu enrichment of the investigated carbonate phases the presence of exclusively negative ce anomalies in these carbonates is in line with observations of other workers that seep limestones may not necessarily display positive ce anomalies indicative of precipitation under anoxic conditions the negative ce anomalies are attributed here to mixing between anoxic pore waters and oxic ce depleted seawater necessary to enable carbonate precipitation at seeps the methane rich fluids ascended most likely from below the volcaniclastic unit and inherited the enriched end signatures and positive eu anomalies due to fluid rock interactions during their seepage to the seafloor the carbon isotope data are most consistent with thermogenic origin of methane although contribution of abiotic and biogenic methane sources cannot be excluded our results indicate that neodymium isotope and rare earth element analyses constitute one of the most valuable tools for reconstructing former fluid migration patterns the study shows also that nd isotopes and eu anomalies can serve as sensitive tracers of fluid rock interactions at submarine springs

rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps marine hydrocarbon seeps are dynamic systems which are typically reflected in complex lithologies of limestones forming at these sites in order to test if seep limestones with their multiple generations of carbonate phases reflect variable redox conditions the rare earth element ree patterns of limestones from five ancient hydrocarbon seep localities hollard mound middle devonian iberg mississippian beauvoisin oxfordian canyon and satsop river oligocene and marmorito miocene have been investigated the total ree contents vary widely 0 3 43 7 p p m early microcrystalline phases tend to show higher contents than spatially associated later phases microspar sparite and blocky cement the shale normalized ree patterns of the carbonates show both negative and positive ce anomalies even for individual deposits suggesting that the redox conditions varied significantly it is apparent that in addition to anoxic conditions oxic conditions were common in some ancient seep environments intermittent oxygenation is presumed to have resulted from abrupt changes in fluid flow

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

the effect of diagenesis and fluid migration on rare earth element distribution in pore fluids of the northern cascadia accretionary margin analytical challenges in obtaining high quality measurements of rare earth elements rees from small pore fluid volumes have limited the application of rees as deep fluid geochemical tracers using a recently developed analytical technique we analyzed rees from pore fluids collected from sites u1325 and u1329 drilled on the northern cascadia margin during the integrated ocean drilling program iodp expedition 311 to investigate the ree behavior during diagenesis and their utility as tracers of deep fluid migration these sites were selected because they represent contrasting settings on an accretionary margin a ponded basin at the toe of the margin and the landward tofino basin near the shelf s edge ree concentrations of pore fluid in the methanogenic zone at sites u1325 and u1329 correlate positively with concentrations of dissolved organic carbon doc and alkalinity fractionations across the ree series are driven by preferential complexation of the heavy rees simultaneous enrichment of diagenetic indicators doc and alkalinity and of rees in particular the heavy elements ho to lu suggests that the heavy rees are released during particulate organic carbon poc degradation and are subsequently chelated by doc ree concentrations are greater at site u1325 a site where shorter residence times of poc in sulfate bearing redox zones may enhance ree burial efficiency within sulfidic and methanogenic sediment zones where ree release ensues cross plots of la concentrations versus cl li and sr delineate a distinct field for the deep fluids z 75 mbsf at site u1329 and indicate the presence of a fluid not observed at the other sites drilled on the cascadia margin changes in ree patterns the presence of a positive eu anomaly and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous crescent terrane located east of the drilled transect

methane discharge into the black sea and the global ocean via fluid flow through submarine mud volcanoes during the margasch cruise m52 1 in 2001 with rv meteor we sampled surface sediments from three stations in the crater of the dvurechenskii mud volcano dmv located in the sorokin trough of the black sea and one reference station situated 15 km to the northeast of the dmv we analysed the pore water for sulphide methane alkalinity sulphate and chloride concentrations and determined the concentrations of particulate organic carbon carbonate and sulphur in surface sediments rates of anaerobic oxidation of methane aom were determined using a radiotracer 14ch4 incubation method numerical transport reaction models were applied to derive the velocity of upward fluid flow through the quiescently dewatering dmv to calculate rates of aom in surface sediments and to determine methane fluxes into the overlying water column according to the model aom consumes 79 of the average methane flux from depth 8 9 10 6 mol a 1 such that the resulting dissolved methane emission from the volcano into the overlying bottom water can be determined as 1 9 10 6 mol a 1 if it is assumed that all submarine mud volcanoes smvs in the black sea are at an activity level like the dmv the resulting seepage represents less than 0 1 of the total methane flux into this anoxic marginal sea the new data from the dmv and previously published studies indicate that an average smv emits about 2 0 10 6 mol a 1 into the ocean via quiescent dewatering the global flux of dissolved methane from smvs into the ocean is estimated to fall into the order of 10 10 mol a 1 additional methane fluxes arise during periods of active mud expulsion and gas bubbling occurring episodically at the dmv and other smvs

natural oil seep detection in the santa barbara channel california with shuttle imaging radar natural submarine oil seeps in the santa barbara channel california were detected by the first shuttle imaging radar sir a oil slicks on the ocean are seen in radar imagery as areas of decreased radar signal return that result from a damping of surface roughness orbital radar imagery shows promise as an effective and efficient means of mapping submarine oil seeps as sources of naturally occurring oil slicks

rare earth elements in cold seep carbonates from the niger delta abstract the circulation of methane rich fluids at cold seeps often leads to precipitation of authigenic carbonates mediated by microbial activity which can be used as archives of fluid seepage on continental margins in this study we have investigated the potential of using rare earth element ree concentrations in authigenic carbonates for tracing fluid sources and providing information on associated biogeochemical processes at cold seeps we have examined various chemical procedures for the analysis of ree in authigenic carbonates and present a robust method using dilute acetic acid solution that allows the extraction of ree from carbonate phases with reduced contamination from fe mn oxides and detrital minerals using this method we analysed a series of carbonate samples collected at various deep sea sites of active fluid seepage on the niger delta for their ree and trace element concentrations 13c and 18o values and 87sr 86sr isotope compositions we show that the ree composition of cold seep carbonates is controlled primarily by that of the fluid from which they have precipitated our results also suggest that significant ree fractionation can occur during carbonate precipitation which can be related to various degrees of ree complexation by organic versus carbonate ligands in response to changing pore water chemical composition for parameters such as carbonate alkalinity

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

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

gas hydrate destabilization enhanced dewatering benthic material turnover and large methane plumes at the cascadia convergent margin mixed methane sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the cascadia convergent margin vent fields from which methane charged low salinity fluids containing sulfide ammonia 4he and isotopically light co2 escape are associated with these exposures they characterize a newly recognized mechanism of dewatering at convergent margins where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward this process augments the convergence generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths the enhanced turnover results from the oxidation of methane hydrogen sulfide and ammonia by vent biota the injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide a large fraction of the methane is oxidized within the water column and generates 13c anomalies of the dissolved inorganic carbon pool

a comprehensive approach to the study of methane seep deposits from the lincoln creek formation western washington state usa a comprehensive approach using palaeontology petrography stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane seep carbonate deposits these deposits are in the oligocene part of the lincoln creek formation exposed along the canyon and satsop rivers in western washington each deposit preserves invertebrate fossils many representing typical seep biota authigenic carbonates with 1 3 c values as low as 51 pdb reveal that the carbon is predominately methane derived carbonates contain the irregular isoprenoid hydrocarbons 2 6 11 15 tetramethylhexadecane crocetane and 2 6 10 15 19 pentamethylicosane pmi lipid biomarkers diagnostic for archaea these lipids are strongly depleted in 1 3 c 1 3 c values as low as 120 pdb indicating that archaea were involved in the anaerobic oxidation of methane small filaments preserved in the carbonate may represent methanotrophic archaea archaeal methanogenesis induced the formation of a late diagenetic phase brownish calcite consisting of dumbbell shaped crystal aggregates that exhibit 1 3 c values as high as 7 pdb clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation downward directed carbonate aggregation in the seeps produced inverted stromatactoid cavities large filaments interpreted as green algae based on their size shape arrangement and biomarkers imply that deposition occurred in places in water no deeper than 210 m

microstructures of structure i and ii gas hydrates from the gulf of mexico gas hydrate samples from various locations in the gulf of mexico gom differ considerably in their microstructure distinct microstructure characteristics coincide with discrete crystallographic structures gas compositions and calculated thermodynamic stabilities the crystallographic structures were established by x ray diffraction using both conventional x ray sources and high energy synchrotron radiation the microstructures were examined by cryo stage field emission scanning electron microscopy fe sem good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses gas hydrate structure ii samples from the green canyon in the northern gom had methane concentrations of 70 80 and up to 30 of c2 c5 of measured hydrocarbons hydrocarbons in the crystallographic structure i hydrate from the chapopote asphalt volcano in the southern gom was comprised of more than 98 methane fairly different microstructures were identified for those different hydrates pores measuring 200 400 nm in diameter were present in structure i gas hydrate samples no such pores but dense crystal surfaces instead were discovered in structure ii gas hydrate the stability of the hydrate samples is discussed regarding gas composition crystallographic structure and microstructure electron microscopic observations showed evidence of gas hydrate and liquid oil co occurrence on a micrometer scale that demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix

raman spectrum of carbonaceous material a possible metamorphic grade indicator for low grade metamorphic rocks raman spectral analyses of carbonaceous material cm extracted from pelitic samples along two sections traversing the metamorphic belt of taiwan were carried out in the present study the results show similar spectral variations of cm with metamorphic grade as those documented in the literature however continuous sampling from zeolite facies through prehnite pumpellyite facies to greenschist facies metamorphic rocks in the present study does reveal some interesting features on the raman spectra of cm that were not noted before both the raman d disordered o ordered peak area i e integrated intensity ratio and the d o peak width i e full width at half maximum fwhm ratio of the cm decrease with progressive metamorphism but the most prominent change in the d o peak area ratio occurs in samples of lower greenschist facies metamorphic grade while the most significant decrease in the d o peak width ratio occurs in samples near the boundary of prehnite pumpellyite facies and greenschist facies this phenomenon is interpreted as a result of the decoupling of the changing rates of in plane crystallite size and degree of defects of cm with progressive metamorphism it is postulated that the raman spectrum of cm can serve as a metamorphic grade indicator to distinguish samples of prehnite pumpellyite facies metamorphic grade from those of greenschist facies metamorphic grade

formation processes through archaeobotanical remains the case of the bronze age levels in el mirador cave sierra de atapuerca spain el mirador site has a sequence formed by burnt dung resulting from pastoral activities during the bronze age and the neolithic period because there is a high sediment variation in the profile facies descriptions were used to guide the archaeologists in their work paleobotanical and mineralogical analyses were used to describe and understand the formation of the facies from the mir 4 level furthermore the paleobotanical results serve to reconstruct the landscape in atapuerca during the bronze age most seeds recovered are from triticum aestivum durum but other cereals and also leguminoseae and fruits have been identified charcoal analysis has yielded mainly deciduous and evergreen quercus pollen analysis has revealed a low arboreal cover with pinus quercus and riverside trees most of the pollen record corresponds to herbs such as poaceae and asteraceae all these results indicate a mosaic of different biota with forest pastureland and cultivated fields near the site most of the phytoliths come from the leaves and stems of festucoid grasses however there are some differences in the phytolith type and number the amount of faecal spherulites the mineralogical composition and the percentage of multicellular structures that indicate that not all the facies in the site have been formed in the same way thus the work carried out has demonstrated that some facies are related more to agricultural activities than to pastoralism activities and that other facies have been affected by incipient diagenetic processes

rare earth elements in authigenic methane seep carbonates as tracers for fluid composition during early diagenesis abstract authigenic carbonates forming at an active methane seep on the makran accretionary prism mainly consist of aragonite in the form of microcrystalline cryptocrystalline and botryoidal phases the 13 c carbonate values are very negative 49 0 to 44 0 v pdb agreeing with microbial methane as dominant carbon source the 18 o carbonate values are exclusively positive 3 0 to 4 5 v pdb and indicate precipitation in equilibrium with seawater at bottom water temperatures the content of rare earth elements and yttrium ree y determined by laser ablation inductively coupled plasma mass spectrometry la icp ms and solution icp ms varies for each aragonite variety with early microcrystalline aragonite yielding the highest cryptocrystalline aragonite intermediate and later botryoidal aragonite the lowest ree y concentrations shale normalised ree y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation microcrystalline aragonite shows high contents of middle rare earth elements mree reflecting ree patterns ascribed to anoxic pore water cryptocrystalline aragonite exhibits a seawater like ree y pattern at elevated total ree y concentrations indicating higher concentrations of rees in pore waters which were influenced by seawater botryoidal aragonite is characterised by seawater like ree y patterns at initial growth stages followed by an increase of light rare earth elements lree with advancing crystal growth reflecting changing pore fluid composition during precipitation of this cement conventional sample preparation involving micro drilling of carbonate phases and subsequent solution icp ms does not allow to recognise such subtle changes in the ree y composition of individual carbonate phases to be able to reconstruct the evolution of pore water composition during early diagenesis an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases high resolution analysis of seep carbonates from the makran accretionary prism by la icp ms reveals that pore fluid composition not only evolved in the course of the formation of different phases but also changed during the precipitation of individual phases

tracing the evolution of seep fluids from authigenic carbonates green canyon northern gulf of mexico authigenic carbonates from hydrocarbon seeps are unique long term archives of past fluid flow the studied samples were collected from green canyon block 140 at a water depth of 260 m in the gulf of mexico petrography x ray diffraction stable isotopes and c 14 dating were applied to assess the evolution of seep activity and potential driving forces the carbonates are dominated by high mg calcite hmc and aragonite with a minor amount of low mg calcite lmc and dolomite petrographically peloids clotted microfabric acicular aragonite and a variable content of bioclasts were observed three types of carbonates are recognized structure i carbonates with c 14 ages from 46 5 ka to 25 8 ka bp are characterized by delta c 13 values from 23 2 parts per thousand to 5 1 parts per thousand suggesting multiple carbon sources that include thermogenic methane biodegraded crude oil seawater and residual co2 from methanogenesis at greater depth in contrast structure ii carbonates formed between 17 6 ka and 11 7 ka bp and have delta c 13 values varying from 22 2 parts per thousand to 8 8 parts per thousand suggesting carbon sources similar to those of structure i carbonates but with a negligible influence of residual co2 from methanogenesis in addition the presence of lmc in this type of carbonate may be associated with brine seepage structure iii carbonates among the youngest of the samples analyzed with c 14 ages of 1 2 ka bp these carbonates have the most negative delta c 13 values ranging from 36 1 parts per thousand to 26 8 parts per thousand suggesting that thermogenic methane is the primary carbon source the majority carbonates of both structure i and ii are slightly o 18 enriched which is most likely related to the incorporation of water from dehydration of clay minerals the considerable range of mineralogical and isotopic variations of the studied carbonates highlights the local control of the seepage flux it is proposed that factors affecting the activity of hydrocarbon seeps are sea level changes and salt movement the combination of petrography stable isotopes and dating approach used here highlights that these are valuable tools to assess the variability of past fluid flow at hydrocarbon seeps c 2013 elsevier ltd all rights reserved

evidence for intense ree scavenging at cold seeps from the niger delta margin for many trace elements continental margins are the location of intense exchange processes between sediment and seawater which control their distribution in the water column but have yet to be fully understood in this study we have investigated the impact of fluid seepage at cold seeps on the marine cycle of neodymium we determined dissolved and total dissolvable td concentrations for ree and well established tracers of fluid seepage ch4 tdfe tdmn and nd isotopic compositions in seawater samples collected above cold seeps and a reference site i e away from any fluid venting area from the niger delta margin we also analyzed cold seep authigenic phases and various core top sediment fractions pore water detrital component easily leachable phases uncleaned foraminifera recovered near the hydrocast stations methane tdfe and tdmn concentrations clearly indicate active fluid venting at the studied seeps with plumes rising up to about 100 m above the seafloor depth profiles show pronounced ree enrichments in the non filtered samples td concentrations within plumes whereas filtered samples dissolved concentrations exhibit slight ree depletion in plumes relative to the overlying water column and display typical seawater ree patterns these results suggest that the net flux of ree emitted into seawater at cold seeps is controlled by the presence of particulate phases most probably fe mn oxyhydroxides associated to resuspended sediments at the reference site however our data reveal significant enrichment for dissolved ree in bottom waters that clearly relates to diffusive benthic fluxes from surface sediments neodymium isotopic ratios measured in the water column range from end 15 7 to 10 4 evidence that the end values for antarctic intermediate waters aaiw differed from those reported for the same water mass at open ocean settings shows that sediment water interactions take place in the gulf of guinea at each site however the bottom water end signature generally differs from that for cold seep minerals easily leachable sediment phases and detrital fractions from local sediments ruling out the possibility that seepage of methane rich fluids and sediment dissolution act as a substantial source of dissolved nd to seawater in the gulf of guinea taken together our data hence suggest that co precipitation of fe mn oxyhydroxide phases in sub surface sediments leads to quantitative scavenging of dissolved ree at cold seeps preventing their emission into bottom waters most probably it is likely that diffusion from suboxic surface sediments dominates the exchange processes affecting the marine nd cycle at the niger delta margin

natural oil seepage at kobuleti ridge eastern black sea abstract analysis of advanced synthetic aperture radar satellite images in combination with water column and seafloor investigations documented natural oil seepage from pechori mound and colkheti seep in 1000 1200 m water depth in the eastern black sea offshore georgia hydroacoustic imaging of the water column using multibeam echosounder evidenced numerous gas emissions from both structures gas bubbles rose as high as 45 m below sea surface it is proposed that oil coatings around gas bubbles hamper their dissolution allowing them to reach the sea surface where widespread oil slicks are formed slow rise velocities 14 1 cm s 1 of bubbles with radii of 2 6 0 3 mm n 101 derived from video data obtained with the remotely operated vehicle marum quest4000 corroborate the assumption that bubbles are oil coated high resolution seafloor mapping with the autonomous underwater vehicle marum seal5000 at colkheti seep revealed a crater strewn morphology whose formation is explained by frequent rafting of shallow gas hydrate deposits satellite imaging of oil slicks on the sea surface above both sites indicates that oil seepage is rather persistent since 2003 an order of magnitude estimation of minimum oil seepage rates suggests discharge rates of 40 liters per hour from both sites the data presented are the first comprehensive description of oil seepage in the black sea

organically bound metals and biomarkers in the monterey formation of the santa maria basin california abstract systematic variations in metal concentration ratios and biomarker ratios are observed with depth in cores of the monterey formation of the union leroy 51 18 well in the santa maria basin california specific metal ratios v v ni and mo mo cr correlate between kerogen and extractable organic matter further the organic matter in the two predominant lithofacies siliceous shales vs phosphatic shales can be distinguished based upon metal concentration ratios e g v v ni and certain biomarker characteristics e g sterane terpane mono triaromatic steroid hydrocarbon ratios maximum 28 30 bisnorhopane hopane ratios occur over a narrow range of v v ni ratios in kerogen concentrates and extractable organic matter this suggests that metal ratios in kerogen concentrates and eom may be useful for oil source rock correlation studies in the monterey further while kerogen parameters metals are discontinuous at the lithofacies boundary extract parameters biomarkers are gradational at this boundary suggesting about 30 ft 9 m of vertical hydrocarbon migration across the lithofacies contact

orientation of exsolution lamellae in mantle xenolith pyroxenes and implications for calculating exsolution pressures exsolution lamellae in pyroxene occurring in mantle lherzolite xenoliths in cenozoic basalts from the mingxi area fujian province china have been investigated by electron backscattered diffraction ebsd to determine epitaxial relationships between host and lamellae clinopyroxene diopside hosts developed two sets of lamellae one of orthopyroxene pigeonite enstatite lamellae and the other of clinopyroxene augite lamellae orthopyroxene enstatite hosts developed a single set of clinopyroxene augite lamellae a zone crossing method has been used to determine miller indices of lamellae which appear as linear traces on thin sections tested by ebsd in clinopyroxene hosts the index of orthopyroxene lamellae is 100 and that of clinopyroxene lamellae is 401 at 22 to the c axis in orthopyroxene hosts the index of clinopyroxene lamellae is 100 published high pressure crystallographic data for compositions approximating those of the lamellae and host are used to compare cell parameters of lamellae and hosts at different pressures exact phase boundary theory is applied to estimate the exsolution pressure and the data uncertainty of composition cell parameters and orientation of the lamellae have been analyzed uncertainties of composition and cell parameters give rise to only small uncertainties in the exsolution pressure but that of the orientation of the lamellae generates large uncertainty independent high accuracy measurement of the angle between lamellae and c axis by tem or other techniques combined with exact phase boundary theory would give more reliable estimates of exsolution pressure

discriminating stromatolite formation modes using rare earth element geochemistry trapping and binding versus in situ precipitation of stromatolites from the neoproterozoic bitter springs formation northern territory australia stromatolites consist primarily of trapped and bound ambient sediment and or authigenic mineral precipitates but discrimination of the two constituents is difficult where stromatolites have a fine texture we used laser ablation inductively coupled plasma mass spectrometry to measure trace element rare earth element ree y and th concentrations in both stromatolites domical and branched and closely associated particulate carbonate sediment in interspaces spaces between columns or branches from bioherms within the neoproterozoic bitter springs formation central australia our high resolution sampling allows discrimination of shale normalised ree patterns between carbonate in stromatolites and immediately adjacent fine grained ambient particulate carbonate sediment from interspaces whereas all samples show similar negative la and ce anomalies positive gd anomalies and chondritic y ho ratios the stromatolites and non stromatolite sediment are distinguishable on the basis of consistently elevated light rees lrees in the stromatolitic laminae and relatively depleted lrees in the particulate sediment samples additionally concentrations of the lithophile element th are higher in ambient sediment samples than in stromatolites consistent with accumulation of some fine siliciclastic detrital material in the ambient sediment but a near absence in the stromatolites these findings are consistent with the stromatolites consisting dominantly of in situ carbonate precipitates rather than trapped and bound ambient sediment hence high resolution trace element ree y th geochemistry can discriminate fine grained carbonates in these stromatolites from coeval non stromatolitic carbonate sediment and demonstrates that the sampled stromatolites formed primarily from in situ precipitation presumably within microbial mats biofilms rather than by trapping and binding of ambient sediment identification of the source of fine carbonate in stromatolites is significant because if it is not too heavily contaminated by trapped ambient sediment it may contain geochemical biosignatures and or direct evidence of the local water chemistry in which the precipitates formed

rare earth and trace elements of microbialites in upper jurassic coral and sponge microbialite reefs major trace y zr pb th and u rare earth element ree and sm nd isotope analyses were carried out on microbialites and detrital carbonate sediments collected in upper jurassic coral and sponge microbialite reef settings selected bioconstructions developed in various palaeogeographic settings and depositional environments on northwestern tethys and eastern atlantic margins pure carbonate lagoon carbonate dominated deep shelf to epicontinental basin and mixed carbonate siliciclastic shallow ramp and lagoon the proximity to terrigenous sources directly controlled ree patterns of carbonate samples lateral and intrareef sediments and to a lesser degree stromatolitic microbial crusts are more susceptible to contamination by terrigenous material than are microbialites of thrombolitic fabric in mixed carbonate siliciclastic settings microbialites display a flat shale normalized ree pattern in pure carbonate lagoonal settings microbialites have shale normalized ree patterns similar to those of modern seawaters with a negative ce anomaly ce ce up to 0 14 positive la and gd anomalies and light ree depletion relative to heavy ree these data imply that similar ree chemistry than modern seawater already occurred on the northwestern tethys margin at the upper jurassic strong negative ce anomalies b0 4 imply microbialite formation in well oxygenated seawaters both in coral reefs of pure carbonate lagoonal environments pagny sur meuse and sponge bioherms of deep shelf settings plettenberg absence of positive ce anomalies rules out an influence of alkaline and high ph seawaters thus ree patterns and nd isotopic signatures which are in perfect agreement with existing results for tethysian seawater confirm that microbialites can be used as proxies of palaeoenvironmental seawater chemical characteristics

chemoherms on hydrate ridge unique microbially mediated carbonate build ups growing into the water column two active chemoherm build ups growing freely up into the oceanic water column the pinnacle and the south east knoll chemoherms have been discovered at hydrate ridge on the cascadia continental margin these microbially mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle shaped morphology with steep flanks the recovered rocks are pure carbonates dominated by aragonite based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates the fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates the strongly 13c depleted carbon isotope values of the samples as low as 48 1 pdb are characteristic of methane as the major carbon source for the carbonate formation the methane rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom simulating reflector bsr and rise through fault systems the 18o values of the aragonitic chemoherm carbonates are substantially higher as high as 5 0 pdb than the expected equilibrium value for an aragonite forming from ambient seawater 3 5 pdb as a first approximation this indicates formation from glacial ocean water but other factors are considered as well a conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates this model explains the function of the consortium of archaea and sulfate reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane aom and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate dependent anaerobic oxidation of methane within an oxic environment the contact between beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro environment where carbonate formation takes place