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

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

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

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

patterns of carbonate authigenesis at the kouilou pockmarks on the congo deep sea fan abstract different types of seep carbonates were recovered from the kouilou pockmarks on the congo deep sea fan in approximately 3100 m water depth the carbonate aggregates are represented by pyritiferous nodules crusts and slabs tubes and filled molds the latter are interpreted to represent casts of former burrows of bivalves and holothurians the nodules consisting of high mg calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic anme 1 group whereas aragonitic crusts resulted from the activity of archaea of the anme 2 cluster evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts probably reflecting higher porosities of the host sediment during carbonate formation this finding is supported by lower 18 o values of crusts agreeing with precipitation from pore waters similar in composition to seawater the aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate rich pore waters similar to seawater moreover the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that anme 1 archaea tend to occur deeper in the sediment than members of the anme 2 group methane represents the predominant carbon source of all carbonates 13 c values as low as 58 9 v pdb and the encrusted archaeal biomarkers 13 c values as low as 140 v pdb oxygen isotope values of some nodular carbonates ranging from 3 9 to 5 1 v pdb are too high for precipitation in equilibrium with seawater probably reflecting the destabilization of gas hydrates which are particularly abundant at the kouilou pockmarks

cold seep carbonates and associated cold water corals at the hikurangi margin new zealand new insights into fluid pathways growth structures and geochronology the aim of this study is to provide combined new insights into the geochronological framework isotope geochemical signatures and structural observations of methane related authigenic carbonate settings and associated cold water corals from offshore new zealand the analysed samples are obtained from calcified sediments of three different cold seep areas at the hikurangi margin opouawe bank uruti and omakere ridge we focused the sub sampling on aragonitic precipitates in vein like structures partly still open fluid channel systems and related chemoherm structures in order to identify the timing and signature of focused marine methane emanation the presented initial u th age data set indicates different generations of intensified seep activity and related carbonate precipitation between 12 400 160 and 2090 850 years bp the youngest stage so far was identified as contemporaneous cold seep activity at the southernmost north tower opouawe bank and northernmost bear s paw omakere ridge sampling sites around 2300 years bp sharing the same water depth 1050 to 1100 m these sites imply regional margin wide tectonic or hydrological changes as controlling process an intermediate phase of vein and channel structures within the sediment was detected for a time interval between approximately 5000 and 4000 years bp with contemporaneous settings of focused seep activity around 4300 years bp at uruti ridge lm 10 and opouawe bank 13cpdb data reflect site and carbonate type specific signatures clustering around 52 uruti and omakere ridge and 47 for the fluid pathway system and the uppermost surface at north tower site opouawe bank late stage precipitates in chemoherm cavities of the latter reflect significantly heavier values of about 38 porous precipitates within open fluid channel systems are characterized by decreased 234u t values exceptional high th and u concentrations and slightly lighter 13cpdb signatures when compared to adjacent rim like and dense cements this specific kind of precipitate is interpreted as indicator for phases of less vigorous fluid seepage the observed occurrence of cold water corals seems to be mostly depending on the abundance of authigenic carbonates as a substrate exposed to erosive bottom water currents but seafloor observations combined with preliminary age data indicate a significant time gap between the inferred end of cold seep activity and coral colonization u th analyses of recent reef forming coral provided an initial 234u 0 value of 146 3 3 9 and 0 0013 0 0002 as starting 230th 234u activity ratio for coral growth in the bottom water

postglacial seismicity offshore mid norway with emphasis on spatio temporal magnitudal variations abstract norway and its continental margin in spite of being located in an intraplate environment is of great interest from a neotectonic perspective influence on neotectonics stems both from the fact that this is a recently deglaciated region in which significant rebound is still occurring and from its proximity both to a rifted continental margin and to failed rift zones passive continental margins and rift related structures are the most seismically active ones within stable continental regions with a number of well documented earthquakes of magnitude 7 and higher as observed around the globe indeed the disparity in seismicity between such regions and plate boundary regions is greater in terms of return times than in terms of maximum magnitude a recent detailed assessment of neotectonics in fennoscandia has shown strong bedrock related evidence for several large magnitude m 7 earthquakes in the northern areas lapland while few if any similar observations have been made farther south a possible explanation for this is that postglacial earthquakes have been occurring on hidden thrusts in the south in fact several major structures have recently been mapped and shown to have capabilities to accommodate m 6 5 7 earthquakes without breaking the surface given that the largest historically known earthquake in fennoscandia is only of the order of m 5 8 it is likely therefore that the potentially largest magnitude in this region may be well above this

authigenic carbonate formation and its impact on the biomarker inventory at hydrocarbon seeps a case study from the holocene black sea and the plio pleistocene northern apennines italy carbonates are widespread at methane and petroleum seeps and are often precipitated as consequence of an alkalinity increase due to the anaerobic oxidation of methane aom or less often reported of higher hydrocarbons these carbonates are taphonomic windows into earth s history because they excellently protect the in situ formed microbial signatures e g lipid biomarkers from diagenetic destruction a complication for paleoreconstructions however is that seep carbonates also encapsulate variable amounts of allochthonous organic matter sometimes even completely obscuring authigenic microbial signatures seep carbonates from the holocene black sea the pleistocene enza river and the pliocene san lorenzo both northern apennines italy provide hints to better understand i the importance of processes other than aom for the formation of seep carbonates and ii the controls of allochthonous and autochthonous contribution of biomarkers to organic matter in seep carbonates biomarker distributions in different parts of a black sea carbonate clearly demonstrate that high allochthonous organic matter is entrapped if aom carbonates are formed intrasedimentary particularly if methane supply is relatively low and external organic matter input high high allochthonous contributions were also found in the biomarker inventory of ancient seep carbonates from the italian northern apennines enza river and san lorenzo pointing at their precipitation within the sediment specific and complex conditions were indicated from our data for the enza river location carbonate facies and particularly biomarker compositions with abundant signatures of sulfate reducing bacteria suggest that sulfate reduction using alkaline and eventually sulfate and higher hydrocarbon enriched fluids triggered the growth of these seep carbonates our and other data suggest that this process has to be more considered if interpreting seep settings particularly where microbial processes rely on rising fluids from deep petroleum reservoirs

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

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

seismotectonics of the norwegian continental margin the norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide indicating a potential earthquake deficit the adjacent oceanic crust is mostly aseismic except for parts of the lofoten and norway basins which have experienced rapid deposition of glacial sediments the excess load has enhanced the local stress field and in turn the seismic activity the marginal highs along the continent ocean transition and the part of the more and voring basins which experienced crustal extension prior to the early tertiary breakup are practically aseismic this region is also underlain by high velocity lower crust emplaced during breakup suggesting crustal strengthening which also may increase the return periods for the largest events farther landward a spatial correlation of seismic activity and the glacial sediment wedge suggests a causal relationship expressed through preferential rejuvenation of late jurassic early cretaceous faults the locally high seismic activity in the coastal region occurs in areas where the continental crust is relatively thick 25 30 km but still thinner than in the shield area farther east 45 50 km although stress relations are complex we suggest that increased stress due to appreciable postglacial rebound gradients in the coastal region may be a contributing factor while the stress field along the margin complies in general with the ridge push force we infer that regional and local stress enhancement factors are not only present in this region but also necessary for explaining the earthquake activity the relative importance of local stress sources is supported by several cases of 90 stress rotations relative to the ridge push direction the superposition of regional and local stress together with the existence of weakness zones and faults yields potentials for earthquakes primarily through structural reactivation

ice sheet dynamics on the lofoten vesteralen shelf north norway from late mis 3 to heinrich stadial 1 abstract we present an account of the glacial dynamics from late marine isotope stage mis 3 to the heinrich stadial hs 1 on the lofoten vesteralen margin northern norway this account is based on an analysis of the sedimentary stratigraphy on the upper slope close to the ice sheet grounding line at the continental shelf edge and an analysis of the glacial morphology on the shelf areas our results point to a dynamic ice sheet that waxed and waned on the lofoten vesteralen continental shelf from the late mis 3 through the last glacial maximum lgm and the hs 1 glacigenic debris flows and deposition of laminated diamicts suggest that the ice sheet reached the shelf edge on several occasions these are dated to 25 5 ka bp hs3 between 21 1 and 21 7 ka bp 19 5 ka bp 15 6 ka bp and 14 4 ka bp uncorrected 14c ages a contouritic sorted sand sediment unit that formed in a relatively warm interval during late mis 3 suggests that a strong norwegian atlantic slope current also developed during glacial interstadials implying that the oceanic circulation in the eastern norwegian sea was similar as today the general pattern is that the ice sheet advanced onto the shelf during cold period with little influx of atlantic water to the norwegian sea and that it retreated toward the coast during intervals with higher temperatures and higher influx of warm atlantic water the last ice advance to the edge of the continental shelf occurred during hs 1

the andoya slide and the andoya canyon north eastern norwegian greenland sea based on gloria side scan sonar imagery echo sounder records 3 5 khz profiles multichannel seismics and gravity cores the andoya slide and andoya canyon north eastern norwegian greenland sea were mapped and interpreted the andoya slide covers an area of about 9700 km2 of which the slide scar area comprise ca 3600 km2 the slide has a total run out distance of about 190 km slope failure is inferred to have occurred during the holocene because the slide scar has prominent relief on the present sea floor the area of sediment removal is characterised by an irregular relief were relatively consolidated sediments are exposed at the sea floor little or no unconsolidated sediments overlies the slide deposits earthquake activity is inferred to have triggered the slide a holocene age of the andoya slide implies that three giant slides the storegga traenadjupet and andoya slides have occurred along the continental slope of norway during the last 10 000 years a large canyon the andoya canyon is located immediately south of the andoya slide on the upper slope the canyon has been incised about 1000 m in the bedrock and the maximum width at the bottom and between the canyon shoulders is 2 and 12 km respectively the andoya canyon represents the upper part of the lofoten basin channel based on analogy with other deep sea canyon channel systems the andoya canyon lofoten basin channel is possibly of pre quaternary age holocene sediments recovered from within the canyon and draping the flanking channel deposits indicate that the andoya canyon is not presently active and has probably not been active during the holocene during the holocene the canyon acted as a trap for sediments settling from the winnowing norwegian current

giant sandwaves in the hola glacial trough off vesteralen north norway abstract seabed data acquired from the hola glacial trough off vesteralen north norway reveal the morphology of spectacular sandwave fields and large sandwaves in great detail the sandwave fields are among the deepest known occurring in water depths of 200 260 m multibeam bathymetry and backscatter shallow seismic sediment samples and video data collected by the mareano programme and bottom current modelling results have been used to interpret morphology transport and distribution of the seabed sediments sand supply to the hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation more recently deposited bioclastic sand dominates the upper part of the sand unit which locally reaches 10 m thickness sand predominantly accumulates behind ridges where current strength decreases and in topographic lows straight crested and barchan type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0 7 0 8 m s 1 a large cluster of coral reefs occur immediately east of the barchan type sandwaves where modelling shows bottom current velocities up to c 1 m s 1 sandwaves occur in four major sandwave fields strong variation in the nature of sandwaves over short distances e g migration direction between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes the present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields

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

anaerobic methane oxidation and the formation of dolomite abstract we examine the link between organic matter degradation anaerobic methane oxidation amo and sulfate depletion and explore how these processes potentially influence dolomitization we determined rates and depths of amo and dolomite formation for a variety of organic rich sites along the west african margin using data from ocean drilling program odp leg 175 rates of amo are calculated from the diffusive fluxes of ch4 and so4 and rates of dolomite formation are calculated from the diffusive flux of mg we find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming indicating that the diffusive fluxes of mg and ca are not limiting based upon the calculated log iap values log ksp values for dolomite were found to narrowly range between 16 1 and 16 4 dolomite formation is controlled in part by competition between amo and methanogenesis which controls the speciation of dissolved co2 amo increases the concentration of co32 through sulfate reduction favoring dolomite formation while methanogenesis increases the pco2 of the pore waters inhibiting dolomite formation by regulating the pco2 and alkalinity methanogenesis and amo can regulate the formation of dolomite in organic rich marine sediments in addition to providing a mechanistic link between amo and dolomite formation our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming

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

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 carbonate precipitates from the ne black sea a mineralogical geochemical and lipid biomarker study carbonate precipitates recovered from 2 000 m water depth at the dolgovskoy mound shatsky ridge north eastern black sea were studied using mineralogical geochemical and lipid biomarker analyses the carbonates differ in shape from simple pavements to cavernous structures with thick microbial mats attached to their lower side and within cavities low 13c values measured on carbonates 41 to 32 v pdb and extracted lipid biomarkers indicate that anaerobic oxidation of methane aom played a crucial role in precipitating these carbonates the internal structure of the carbonates is dominated by finely laminated coccolith ooze and homogeneous clay layers both cemented by micritic high magnesium calcite hmc and pure botryoidal yellowish low magnesium calcite lmc grown in direct contact to microbial mats 18o measurements suggest that the authigenic hmc precipitated in equilibrium with the black sea bottom water while the yellowish lmc rims have been growing in slightly 18o depleted interstitial water although precipitated under significantly different environmental conditions especially with respect to methane availability all analysed carbonate samples show lipid patterns that are typical for anme 1 dominated aom consortia in the case of the hmc samples with significant contributions of allochthonous components of marine and terrestrial origin reflecting the hemipelagic nature of the primary sediment

sources of natural gases in middle cambrian reservoirs in polish and lithuanian baltic basin as determined by stable isotopes and hydrous pyrolysis of lower palaeozoic source rocks abstract origin of natural gases associated with oil and condensate accumulations within the middle cambrian sandstone reservoir of the polish and lithuanian baltic basin was characterised by means of molecular composition stable carbon isotopes of methane ethane propane butanes pentanes and carbon dioxide stable hydrogen isotopes of methane and stable nitrogen isotopes of gaseous nitrogen generated gas from potential upper cambrian tremadocian and llandovery source rocks by hydrous pyrolysis at 330 c and 355 c for 72 h was used to characterise thermogenic gas to evaluate correlation parameters based on molecular composition and stable isotopes the pyrolysis conditions represent gas generation during oil generation which appears to be the conditions represented by the natural gas accumulations and their low gors gas oil ratios the dryness of the pyrolysis and natural hydrocarbon compositions compare well but do not provide a means of distinguishing the contributions of each source rock to the natural gas accumulations the average 13 c value of the natural methane is 6 9 depleted in 13 c compared to methane generated in the hydrous pyrolysis experiments this difference is less for ethane and essentially nonexistent for propane butanes and pentanes tentatively this diminishing difference with increasing carbon number is attributed to kinetic effects resulting from higher experimental temperatures although the 13 c values of methane and ethane from the hydrous pyrolysis experiments are not useful in direct correlations with natural gas accumulations 13 c of propane butanes and pentanes is useful and indicates that the upper cambrian and tremadocian source rocks are the main contributors and that the llandovery source rocks are not significant contributors to the polish and lithuanian baltic natural gases polish natural gases with relatively higher methane and ethane are attributed to the mixing of drier more mature gases from deeper parts of the basin to the west carbon dioxide of natural gases was generated during thermogenic processes and gaseous nitrogen was generated from nh 4 rich illites of the clayey facies and from thermal transformation of organic matter of the lower palaeozoic strata hydrous pyrolysis gases have higher concentrations of co 2 h 2 s and h 2 than the natural gases this difference is attributed to reduction or loss of these highly reactive and soluble gases during migration and entrapment of the natural gases although co 2 concentrations between pyrolysis and natural gases are different the 13 c values of the former fall within the range of the latter

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

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

palaeo methane seepage history traced by biomarker patterns in a carbonate crust nile deep sea fan eastern mediterranean sea abstract we investigated lipid biomarkers in three depth intervals of a methane derived carbonate crust collected from a pockmark in the nile deep sea fan sampled during the nautinil cruise of the r v l atalante a diverse set of 13 c depleted archaeal and bacterial lipids were detected along this crust with 13 c values as low as 118 this provides evidence that anaerobic oxidation of methane was the main process that induced the formation of this carbonate crust archaeal lipid biomarker distributions revealed the presence of archaea belonging both to the anme 1 and anme 2 phylogenetic clusters with the former predominating the distribution and compound specific 13 c values of aom derived lipid biomarkers showed no significant variations with depth these results infer that the archaeal community structure and thus the local seepage environment remained relatively stable during the 5000 kyr period of the carbonate crust precipitation

dolomite formation in the dynamic deep biosphere results from the peru margin early diagenetic dolomite beds were sampled during the ocean drilling programme odp leg 201 at four reoccupied odp leg 112 sites on the peru continental margin sites 1227 684 1228 680 1229 681 and 1230 685 and analysed for petrography mineralogy 13c 18o and 87sr 86sr values the results are compared with the chemistry and 13c and 87sr 86sr values of the associated porewater petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate dolomite precipitation often pre dates the formation of framboidal pyrite most dolomite layers show 87sr 86sr ratios similar to the composition of quaternary seawater and do not indicate a contribution from the hypersaline brine which is present at a greater burial depth also the 13c values of the dolomite are not in equilibrium with the 13c values of the dissolved inorganic carbon in the associated modern porewater both petrography and 87sr 86sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment 30 m below the seafloor a significant depletion in the dissolved mg and ca in the porewater constrains the present site of dolomite precipitation which co occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate methane interface it has been hypothesized that microbial hot spots such as the sulphate methane interface may act as focused sites of dolomite precipitation varying 13c values from 15 to 15 for the dolomite are consistent with precipitation at a dynamic sulphate methane interface where 13c of the dissolved inorganic carbon would likewise be variable a dynamic deep biosphere with upward and downward migration of the sulphate methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter thus the study of dolomite layers in ancient organic carbon rich sedimentary sequences can provide a useful window into the palaeo dynamics of the deep biosphere

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

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

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

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

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

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

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

subduction zone earthquake as potential trigger of submarine hydrocarbon seepage methane is abundant in marine sediments analysis of sediment cores and seismic images of marine sediments obtained off the coast of pakistan show fracturing of gas hydrates and an increase in upward methane flux in the decades following a large earthquake in the arabian sea in 1945 suggesting that quakes can trigger hydrocarbon seepage

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

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