shallow burial dolomitization and dedolomitization of cenozoic cool water limestones southern australia geochemistry and origin abstract the gambier limestone a bryozoan rich cool water carbonate of late eocene to early miocene age in the otway basin of southeastern australia is about one third variably dolomitized limestone and dolostone individual dolomite crystals range from 10 to 500 m and are of three types 1 silt size crystals 4 64 m diameters having a brightly luminescent core 10 20 m of nearly stoichiometric dolomite with 1 mol mnco3 and sio2 but no fe surrounded by a thin dull or alternating dark and bright cl cortex that is nonstoichiometric with 44 46 mol mgco3 but sharp to diffuse zoning in mn and fe 2 sand size crystals which also have a brightly luminescent and stoichiometric core and dark and bright non stoichiometric thin inner cortex but are mostly a thick outer cortex at ca 50 m diameter that is nonluminescent and nonstoichiometric with some fe but no mn and 3 identical sand size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement these three crystal populations represent two distinct dolomites and their alteration product the smallest of the silt size crystals which are rarely preserved are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition the 18o values of dolomite rhombs 1 6 to 2 9 correlate positively with 13c values 0 7 to 3 0 with the lowest values from silt size or small sand size crystals and the highest values from large sand size crystals with leached unfilled cores low and variable 18o and 13c values in conjunction with high and variable mn and fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater higher values for the outer cortex in sand size crystals are typical of those expected for precipitation from normal or near normal seawater low sr mn and fe contents in the largest crystals in conjunction with fluid inclusions of seawater salinity in the outer cortex are also consistent with the bulk of the large crystals having precipitated from seawater using the sr isotope evolution curve for cenozoic seawater sr isotope ratios of the dolomites can be divided into four distinct groups which correspond to times of major transgressions this cenozoic example in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression regression offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained

inferred gas hydrates and clay diapirs near the storegga slide on the southern edge of the voring plateau offshore norway this paper presents a new data set including single channel airgun seismic lines okean long range side scan sonar data and gravity cores acquired on the edge of the voring plateau and in the region of the storrega slide the data acquisition was part of the 8th training through research ttr 8 expedition with r v professor logachev the acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom simulating reflectors bsrs at about 350 ms twt subbottom depth the lower bsr zone is located on the slope of the voring plateau in the immediate vicinity of the northern headwall of the storegga slide and in some places below the slide deposits this zone runs parallel to the general trend of the continental slope the spatial distribution of the upper bsr zone located upslope in an area where diapir like structures are found does not demonstrate any topographic control interpretation of high backscatter patches on the okean sonographs associates the observed structures with fluid escape features on the seabed most of them are pockmarks but in a few places diapirs are cropping out and form dome like elevations after analysing the behaviour of the bsr sometimes crosscutting and its acoustic characteristics reversed polarity and after applying seismic inversion processing to estimate the acoustic velocity change across the bsr this reflector is interpreted to represent the base of the local gas hydrate stability field ghsf this information was used to derive the regional geothermal gradient spatial variations of the inferred geothermal field appeared to be negligible the observation of two separated bsr zones suggests different environmental controls for the growth of the hydrates enhanced reflectors observed in the intermediate zone can be explained by the presence of strata bound free gas accumulations and migration combined with overlying permeability barriers therefore a model for gas hydrate formation in relation to directional fluid migration processes fluid escape features and the presence of diapiric structures is proposed in this paper our model is supported by sedimentological analyses of seafloor samples which demonstrate the presence of stiff clays with evidence of gas migration and by paleontological studies of the cores retrieved from the pockmarks the presence of a bsr in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event this observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm year finally the effect of hydrate dissociation on slope instability is considered assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re distributed through the whole sedimentary section above the paleo bsr with consuming heat and increasing pore pressure dissociation tends to shift the pt conditions back to equilibrium values it appears that the dissociation of hydrates due to sliding would cause only 0 2 increase of the pore pressure which would hardly contribute to further slope instability

tubular structures of northern wairarapa new zealand as possible examples of ancient fluid expulsion in an accretionary prism evidence from field and petrographical observations

slickenside kinematic indicators abstract a new classification of slickenside kinematic indicators is presented based on 61 criteria these slickensides have been subdivided into eleven major groups v or crescentic markings steps fractures trains of inclined planar structures trailed material asymmetric elevations deformed elements mineralogical crystallographic orientations asymmetric plan view features asymmetric cavities and asymmetric folds this classification constitutes a useful tool for geologists interested in the determination of the shear sense in fault surfaces bearing slickensides examples of application of this classification to natural fault surfaces at different scales are presented

constraining core undertones using stacked ida gravity records a search is made for oscillatory gravity signals possibly associated with core undertones in the frequency range between the diurnal and semidiurnal tidal bands ida tidal data recorded during 6 month intervals following four large earthquakes were used and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l 4 the detection level of a signal in the diurnal semidiurnal frequency band was roughly 10 20 ngal but the only signal detected was a 9 ngal signal of 9 54 hr period whose origin remains unexplained

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

instabilities along ocean margins a geomorphological and geotechnical perspective due to the growing development of offshore activities along margins in the search for more natural resources there has been a greater need for understanding the mechanics of slope formation in these areas one aspect of this relates to the problems associated with slope stability issues this article presents an perspective view of some geomorphological and geotechnical aspects related to instabilities along ocean margins in order to properly address the issues of slope stability and other mass movements we need to improve our technology in the field of sampling high resolution seismic and in situ measurements ultimately the goal is to be able to carry out proper risk assessment analysis pertaining to submarine mass movement this could be achieved by integrating the geotechnical characterisation of mass movements into a risk assessment methodology which can then be applied on a regional basis with the initiation of a project on continental slope stability costa during the year 2000 major advances are expected on our understanding of the mechanics of submarine mass movements

the mid cenozoic challenger rift system of western new zealand and its implications for the age of alpine fault inception analysis of the structure and sedimentary geology of western new zealand has identified a middle eocene to early miocene continental rift system 1 200 km long and 100 200 km wide named here the challenger rift system four phases of rift development occurred 1 infra rift subsidence 2 active axial trough subsidence 3 expanded rift subsidence involving collapse of the rift shoulders and 4 incipient sea floor spreading the spatial and temporal distribution of these phases identifies a north island and a south island rift segment and shows that rifting propagated toward the center of the rift from both ends the northern segment shows a simple pattern of rifting that is comparable with vink9s model of rift propagation the southern segment with locked zones and rift nucleation segments is comparable to courtillot9s model of rift propagation the sea floor spreading history of the southwest pacific shows that the northern rift segment probably linked with a sea floor spreading center in the norfolk basin and the southern segment linked with the southeast indian ridge this is corroborated by the good correlation between the ages of sea floor magnetic anomaly lineations that are aligned with the rift and the biostratigraphic ages of rifting the probable continuity of the rift system in its early development precludes pre miocene transcurrent displacement on the alpine fault an early miocene 23 m y b p age of alpine fault inception is indicated by the age and pattern of rift disruption attributed to compression that originated at the australia pacific plate boundary the modern australia and pacific plates were not discrete entities therefore until the early miocene

miocene tubular concretions in east coast basin new zealand analogue for the subsurface plumbing of cold seeps the uplifted accretionary prism of east coast basin in hikurangi margin north island new zealand exposes late miocene slope mudrocks whangaehu mudstone 10 carbonate in coastal cliffs north of cape turnagain that contain conspicuous tubular carbonate concretions 50 85 carbonate supporting near central conduits pipe and bulbous morphologies dominate ranging in exposed length up to 5 m and up to 1 m in diameter the concretions were formed by the precipitation of micritic dolomite and calcite cement within the host mudstone at shallow burial depths 100 m 13c values of the cement range from 22 to 13 pdb and are interpreted to reflect carbonate precipitation from either the extensive anaerobic oxidation of methane aom and or mixing of microbial methane and methanogenic co2 aom is confirmed by lipid biomarker evidence indicating that methane oxidation occurred in the sediments at the time of carbonate precipitation the mixed dolomite calcite mineralogies and the trend of 13c in the tubular concretions from strongly negative to strongly positive values are interpreted to reflect methane oxidation from the onset of ascent through to the end of a migration event depleted and enriched 18o values suggest an evolved fluid source influenced by the dissociation of gas hydrates collectively our results indicate that the tubular concretions within the upper slope mudstones delineate parts of the subsurface plumbing network of a cold seep system on the late miocene paleo hikurangi margin in which the fluids were sourced from ascending methane the intermediate location of the whangaehu concretions between older early miocene seep carbonates to the west and modern ones offshore to the east indicates a progressive eastwards shift with time of a long lived if only periodically active seep system the concretionary plumbing features at whangaehu provide a conceptual model for subsurface fluid pathways and seep related processes beneath the modern hikurangi margin seabed and possibly also for other modern and ancient cold seep carbonate systems

hydrocarbon seep carbonates of a miocene forearc east coast basin north island new zealand an ancient hydrocarbon seep province of 14 isolated authigenic carbonate deposits has been identified in fine grained deep marine siliciclastic strata of the miocene east coast basin north island new zealand these forearc sediments have been uplifted and complexly deformed into accretionary ridges adjacent to the still active hikurangi convergent margin older active and passive margin strata mid cretaceous to oligocene in age underlie the neogene sequence and contain oil and gas prone source rocks older mesozoic meta sedimentary rocks constitute the backstop against which the current phase of subduction related sedimentation has accumulated 24 ma present the seep carbonates up to 10 m thick 200 m across archive methane signatures in their depleted carbon isotopes to 13c 51 7 pdb and contain chemosynthesis based paleocommunities e g worm tubes bathymodioline mussels and vesicomyid lucinid and thyasirid bivalves typical of other cenozoic and modern seeps northern and southern sites are geographically separated and exhibit distinct lithological and faunal differences structural settings are variable seep associated lithologies also are varied and suggest carbonate development in sub seafloor seafloor and physically reworked diapiric expansion gas explosion gravity slide or debris flow settings similar to italian apennine seep deposits of overlapping ages peculiar attributes of the new zealand miocene seep deposits are several including digitate thrombolites of clotted microbial micrite encased in thick isopachous horizons and botryoids of aragonite seep plumbing features are also well exposed at some sites displaying probable gas explosion breccias filled with aragonite tubular concretions fluid conduits and carbonate cemented thin sandstone beds and burrows within otherwise impermeable mudstones a few seeps were large enough to develop talus debris piles on their flanks which were populated by lucinid bivalves and terebratulid brachiopods firmgrounds and hardgrounds were common as evidenced by trace fossil associations or caryophyllid coral thickets atop some seep carbonate deposits thus the new zealand examples show strong evidence of formation in sediments at or just beneath the seafloor but some were clearly exhumed by erosion to sustain later non seep epifaunal and boring paleocommunities

methane in aging hydrothermal plumes methane concentration and the stable carbon isotopic composition 13 c ch4 were measured in aging hydrothermal plume waters originating at the endeavour segment juan de fuca ridge ch4 concen trations decreased systematically from 600 nm at on axis stations to 11 nm at a distal station 15 km off axis background ch4 concentrations are 2 nm ch4 behaves nonconservatively in plume waters and does not correlate with conservative parameters such as temperature anomaly but is highly correlated with nh4 and total dissolvable mn 13 c ch4 values for plume depth samples varies inversely and significantly r 2 0 89 with methane concentrations normalized to some 13 c ch4 values 1 8 and 10 9 measured at the 15 km off axis station are among the heaviest yet reported from a natural marine environment less than 5 of original hydrothermal methane remains in the plume at this station the data are consistent with extensive microbial methane oxidation a narrow range of fractionation factors rc 1 0072 to 1 0077 was calculated for the endeavour samples these fractionation factors are less than those reported by coleman et al 1981 but fall near the trend line of their rc versus temperature data when extrapolated to plume water temperature 2 c copyright 2002 elsevier science ltd

gas hydrate dissociation in the lorca basin se spain during the mediterranean messinian salinity crisis abstract the tortonian marls which were deposited in the lorca basin se spain prior to the messinian tripoli formation contain discontinuous levels of indurated nodules and massive layers up to a few meters thick of diagenetic dolomite the oxygen and carbon isotopic compositions of these dolomites indicate that the carbonate diagenesis occurred mostly in 13c rich and 18o rich to 18o poor solutions the vertical distribution of the values of these dolomites is interpreted as the result of the past occurrence of gas hydrate reservoirs where the methane source probably originated from the organic rich marls we postulate that the gas hydrates contained in the tortonian marls were destabilized during the water level drops related to the messinian salinity crisis associated sedimentary structures include numerous syn sedimentary deformations and brecciation which are the expression of important sedimentary instabilities which could have been generated during the gas hydrates decomposition

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

a multidisciplinary study of middle miocene seep carbonates from the northern apennine foredeep italy several pelitic intervals are intercalated at various levels within the marly arenaceous turbiditic successions of the middle miocene northern apennine foredeep they range in thickness from 30 to 200 m and represent sedimentation on top of ephemeral structural highs related to blind faults sediments are made up of hemipelagites and fine grained turbidites and include 13c depleted carbonates related to the rising of methane rich fluids hydrocarbon seep carbonates large portions of pelitic intervals are involved in chaotic masses by soft sediment deformation slumps slides intraformational breccias revealing an intense sediment instability during middle miocene a stratigraphic mineralogic and geochemical study was conducted on two of these pelitic intervals castagno d andrea mt citerna in order to reconstruct carbonate development the composition of fluids and to document the connections between fluid seepage and syndepositional tectonics this multidisciplinary approach has allowed us to discriminate between the two examined pelitic intervals in terms of age depositional rates and conditions organic carbon and post depositional processes seep carbonates are characterized by chemosymbiotic fossil communities autoclastic fractures and brecciation carbonate bodies show complex facies relationships as indicative of different stages in chemoherm growth the compositional study evidences the peculiar chemistry of chemoherm carbonates calcite low in mg and sr compared with carbonates in associated enclosing pelites and with modern chemoherms in general the non carbonate components within the chemoherms are enriched in detrital minerals and depleted in phyllosilicates with respect to the enclosing pelites the mineralogical changes in the clay component within the brecciated unit of the castagno d andrea chemoherms suggest authigenic precipitation of the mg rich phases isotopic analyses show the distinct carbon signature in the chemoherms from the two examined intervals castagno d andrea chemoherms more depleted from 15 8 up to 41 3 pdb than mt citerna from 5 2 up to 16 7 and the transitional 13c depletion trend observed moving from chemoherms to the enclosing pelites moderately depleted and te bouma sequence turbidites in the range of marine carbonates a slight but significant enrichment in 18o up to 1 4 pdb is observed for all chemoherms when compared to values of carbonate phases present in enclosing pelites geochemical data indicate that the brecciated facies of seep carbonates are related to an explosive release of gaseous fluids probably associated with the rise of deep hypersaline fluids

lower eocene carbonate cemented chimneys varna ne bulgaria formation mechanisms and the a biological mediation of chimney growth abstract in the area of pobiti kamani varna northeast bulgaria massive carbonate cemented columns chimneys up to 1 5 m diameter and 8 m high and horizontal interbeds 1 5 m thickness occur in dispersed outcrops over an area of 70 km2 within loose lower eocene sands field observations and petrographical and stable isotope geochemical characterisation of four studied locations reveal a relationship between these structures and processes of ancient hydrocarbon seepage column and interbed structures both consist of similar well sorted silt to sand sized nummulitic host sediments predominantly cemented by early diagenetic low magnesium calcite filamentous textures about 10 m in diameter and 80 650 m long are only locally detected within interparticle calcite cement of columns column samples from two sites reveal a similar linear and inverse covariant trend of 13c 18o values which was interpreted as a mixing trend between two end member fluid precipitation conditions i e 1 a methane and or higher hydrocarbon derived carbon member characterised by 13c values as low as 43 and marine controlled precipitation conditions with 18o of 1 0 5 v pdb and 2 a member with less contribution of methane which was mixed most likely with less depleted carbon sources explaining 13c values ranging up to 8 v pdb the corresponding depleted 18o values with many samples clustering around 8 v pdb are interpreted in terms of precipitation at elevated temperatures this suggests the venting system was not a true cold seep sensu stricto furthermore column cross transects often document an internal pattern consisting of concentric zones with distinct isotopic signatures which vary between the two end members the mixing and internal pattern of column isotopic data together with petrographical observations are qualitatively interpreted as evidence of alternating precipitation conditions controlled by varying seepage rates of a single fluid source at depth during build up of individual chimney pipes near the sediment surface based on several field observations migration of the hydrocarbon charged fluids in lower eocene times was possibly channelled along ne oriented faults isotopic signatures of calcite cemented horizontal interbeds with depleted 18o ratios as low as 8 88 v pdb and variable 13c 1 to 16 mainly around 5 to 7 suggest that ascending fluids contributed to their cementation or resetted the calcite cement isotopic signature predominantly during periods of active seepage of warmer fluids only few petrographical and preliminary lipid biomarker evidence has been found pointing to the presence or possibly former activity of microbiota involved in carbon cycling and calcite precipitation typical of cold seep settings this may result from diagenetic alteration of organic components however considering the processes of chimney formation a cementation process governed by the inorganic oxidation of hydrocarbons in which interstitial oxygen is rapidly consumed without bacterial mediation is considered

the place of tubular concretions in hydrocarbon cold seep systems late miocene urenui formation taranaki basin new zealand the late miocene urenui formation cropping out on the eastern margin of taranaki basin currently new zealand s only commercially exploited hydrocarbon basin is dominated by siliciclastic slope mudstone 5 carbonate that locally includes a variety of tubular concretions 50 85 carbonate dominated by commonly large pipe and bulbous varieties as much as 0 5 to more than 10 m 1 6 to 33 ft long and 5 to 70 cm 2 to 28 in across these concretions formed from the precipitation of fine grained calcite or dolomite cement within siliciclastic mud in the shallow subsurface likely 100 m burial most support a central conduit that may be empty or filled with sediment or multiple generations of post shallow burial carbonate cements depleted 13c values of the concretionary calcite cements 40 to 25 pdb indicate that their carbon was sourced from oxidized methane in contrast enriched 13c values of the concretionary dolomite cements 10 to 11 pdb indicate that their carbon was derived from methanogenic co2 and or extensive methane oxidation the range of 18o values for the concretionary cement types 2 to 5 pdb supports precipitation from fluids both depleted and enriched with respect to new zealand miocene marine waters which are suggested to reflect episodes of gas hydrate formation and dissociation respectively the tubular carbonate concretions are interpreted to mark the subsea floor pathways of focused methane infused upward fluid escape and as such they represent a part of the shallow subsurface plumbing system of a cold seep system developed on the continental margin bounding the taranaki basin in the late miocene this margin has been strongly influenced by periodic movements along the major taranaki fault thrust system which coincidentally directly underlies the urenui concretion occurrences and was probably the most likely avenue for active fluid migration within the hydrocarbon seep system the tubular concretions experienced periodic intervals of exhumation and burial caused by margin failure because of greater slope instability from injected fluids and resulted in conglomeratic channel fill deposits within the urenui formation that include mass transported tubular concretions

anatomy of seep carbonates ancient examples from the miocene of the northern apennines italy ancient hydrocarbon seep carbonates of miocene age have been reported from marine sedimentary successions of the northern apennines in the form of irregularly shaped micritic masses containing abundant fossil remains of oligotypic assemblages they are recognized by their peculiar biological sedimentological and geochemical features and represent an excellent on land example of carbonate bodies interpreted as the remains of ancient cold seeps in the apenninic chain the abundance and the extent of the outcrops provide a rare opportunity to study the geometry facies distribution and internal structures of fossil methane derived carbonates the present study reports an integrated view of the sedimentology mineral paragenesis and geochemistry of these seep carbonates in two different tectono sedimentary settings foredeep and satellite basins the main result of this study is the recognition of 17 facies in the investigated seepcarbonates the detailed facies and microfacies analysis led to the identification of different structures which occur in welldefined parts of the carbonate masses the recurring and broadly similar stratigraphy and facies association from the two considered geological environments is diagnostic of the different stages of seep carbonate formation and evolution d 2005 elsevier b v all rights reserved

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