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

aragonite crystal fans in neoproterozoic cap carbonates a case study from brazil and implications for the post snowball earth coastal environment abstract neoproterozoic cap carbonates are characterized by anomalous sedimentary features including sea floor precipitates represented by aragonite pseudomorph cement crusts and crystal fans these features are found in restricted periods of geological history and their generation in the context of the neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation new sedimentary and geochemical results on well preserved neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the sete lagoas formation bambui group central brazil are reported the micrite settling facies shows the highest abundance of crystal fans arranged in centimeter scale layers interfingered with the micrite matrix this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters a numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pco2 in these conditions the mg ca ratio of the neoproterozoic seawater is calculated at ca 1 2 alkalinity oversaturation is monitored by a variable evaporation degree aragonite formation is kinetically favored and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pco2 hence alkalinity is higher these modeling conditions do not preclude the contribution of incomplete organic matter degradation to alkalinity as suggested by negative carbon isotope signatures in addition the large number of aragonite micrite pairs suggests a seasonal or paleoclimatic forcing other cases of abiotic aragonite precipitation through time are also briefly examined by comparison

highly isotopically depleted isoprenoids molecular markers for ancient methane venting abstract we propose that organic compounds found in a miocene limestone from marmorito northern italy are source markers for organic matter present in ancient methane vent systems cold seeps the limestone contains high concentrations of the tail to tail linked acyclic c20 isoprenoid 2 6 11 15 tetramethylhexadecane crocetane a c25 homolog 2 6 10 15 19 pentamethylicosane pme and a distinctive glycerol ether lipid containing 3 7 11 15 tetramethylhexadecyl phytanyl moieties the chemical structures of these biomarkers indicate a common origin from archaea their extremely 13c depleted isotope compositions 13c 108 to 115 6 pdb suggest that the respective archaea have directly or indirectly introduced isotopically depleted methane derived carbon into their biomass we postulate that a second major cluster of biomarkers showing heavier isotope values 13c 88 is derived from sulfate reducing bacteria srb the observed biomarkers sustain the idea that methanogenic bacteria in a syntrophic community with srb are responsible for the anaerobic oxidation of methane in marine sediments marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined two membered bacterial consortium 1 archaea that perform reversed methanogenesis by oxidizing methane and producing co2 and h2 and 2 srb that consume the resulting h2 furthermore the respective organic molecules are unlike other compounds tightly bound to the crystalline carbonate phase the marmorito carbonates can thus be regarded as cold seep microbialites rather than mere authigenic carbonates

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

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

microfacies diagenesis and hydrocarbon potential of the neoproterozoic cap carbonate of the southern amazon craton abstract the low latitude marinoan glaciation 635 ma record in the southern amazon craton has been one of the most important sites with strong evidence for the neoproterozoic snowball earth hypothesis in central brazil the 50 m thick carbonate succession of the post marinoan cap carbonate exposed in the tangara da serra region central brazil represents a complementary section of the classical occurrence of the puga cap carbonate in mirassol d oeste region that overlies glaciogenic deposits marking the well preserved cryogenian ediacaran boundary in south america the 8 m thick cap dolostone that represents the mirassol d oeste formation consists of pinkish microcrystalline peloidal dolomudstone exhibiting even parallel to low angle lamination locally tubestone and megaripple bedded dolopackstone the cap dolostone is succeeded upsection by the cap cementstone limestone basal guia formation comprising laminated lime mudstone rich in silt sized terrigenous grains quartz and subordinately bituminous shale dolomitic marls intraformational breccias and abundant calcite crystal fans and crusts crystal fans occur in isolated and decimetre scale arrays and are considered aragonite pseudomorphs primary precipitation of dolomite crystal fans and micrite and the formation of tubestone are related to depositional processes the cap dolostone was affected by the neomorphism of the dolomicrite development of vug and intercrystalline porosity euhedral dolomite precipitation synsedimentary fracturing calcite and quartz precipitation chemical compaction fe oxide substitution and precipitation the diagenetic processes of the cap cementstone limestone were pyrite precipitation calcite cementation neomorphism of micrite and crystal fan partial crystal fan dissolution dolomitization chemical compaction fracturing ferrous dolomite and calcite precipitation pyrite pseudomorph and micrite replacement with fe oxide and replacement of rhombohedral dolomite by mn oxides hydrocarbons are rare and its migration represents the last diagenetic event being found 1 in fractures and interpeloidal vug and intercrystalline pores 2 with low permeability and connectivity of the cap dolostone and 2 in intercrystalline pores fractures and rare pores in crystal fans of the cap cementstone limestone during the post marinoan sea level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm action the progressive transgression led the implantation of a deep anoxic and caco3 oversaturated aragonitic platform the similarities of cap carbonate microfacies among the post marinoan succession worldwide reflects an extraordinary and non recurring global precipitation event in geological history in contrast besides the majority of cap carbonate have the same textural and compositional framework the diagenetic history seems to be regional and completely dependent on the tectonic thermal and burial history of the depositional basins where they were deposited

methane derived carbonate cementation of marine sediments from the kattegat denmark geochemical and geological evidence abstract methane derived carbonate cementation of holocene marine sediment occurs at several locations in the kattegat denmark generally the carbonate cemented sandstones occur at the seafloor as individual slabs or widely distributed thinly lithified pavements in some areas the lithified sandstones take the form of large and spectacular vertical pillar and mushroom like bodies standing up from the seafloor the carbonate cement consists of aragonite high mg calcite and dolomite the high mg calcite is the most widespread carbonate solid phase found aragonite is primarily associated with shell accumulations in the sediments so far dolomite has only been recorded as cement in the large vertical lithified sandstone bodies the carbonates are remarkably depleted in 13c i e generally in the range of 55 to 45 pdb and are therefore interpreted as being the result of methane oxidation by sulphate reducing bacteria in the anaerobic diagenetic zone of the sediments however the oxygen isotopic composition and the concentrations of minor elements indicate that carbonate precipitation took place under relatively constant temperature and salinity conditions in normal marine pore fluids the habit and mineralogy of the carbonate cemented sandstone observed most likely are controlled by the effect of bacterial activity and upward gas migration on the interstitial geochemical microenvironment the lithified sediments were subsequently exposed as spectacular sandstone formations on the seafloor by submarine erosion data from onshore exploration wells reveal that methane is encountered in eemian early weichselian late quaternary marine deposits the chemical composition and the isotopic signatures of the methane suggest a gas of microbial origin and it is thought that the gas is generated within the host marine quaternary sediment the locations of submarine gas seeps along with those of the carbonate cemented sandstone formations and the onshore gas seeps show a linear distribution which largely coincides with the extension of the fennoscandian border zone thus the seeps may be controlled by the structural framework in the northeastern part of the danish subbasin

fluid seepage associated with slope destabilization along the zambezi margin mozambique abstract evidences for active fluid seepages have been discovered along the zambezi continental slope offshore southern mozambique these seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization fluid seepages are interpreted as a potential trigger for the slope destabilization acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone exploration with the scampi towed camera system in the widest pockmark diameter 200 m wide has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats these fluid seepages are also associated to the presence of chemiosynthetic organisms vesicomyidae and thyasiridae bivalves siboglinidae tubeworms the sampled gas in the sediment corresponds mainly to ch4 of microbial origin generated by hydrogenotrophic methanogenesis from a substrate of organic origin i e a conventional process of genesis of microbial gas in the marine domain no evidence for thermogenic gas was detected another type of pockmarks has been observed within the core of the slope destabilization zone most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries they probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around the spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes this proposed evolution started by scattered seepages of formation water with dissolved gas then free gas seepages appeared notably in the upper part of the slope this was followed by progressive shallow deformation in the sediments downslope of the main gas seepages finally the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys

structural evolution of the eastern balkans bulgaria abstract the balkans are an e w trending thrust belt mainly of palaeogene age generated by a left lateral transpression with maximum stress oriented n30 e the transpressive nature of the balkans may be deduced from the en echelon pattern of folds and thrusts which is due to oblique shortening with respect to pre existing mesozoic e w trending crustal anisotropies between the moesia platform and the balkans basin the front of the eastern balkans is marked by the chudnite skali ajtos thrust during the neogene and quaternary the balkans were inverted by right lateral transtensional tectonics which cross cut and dismembered the thrust belt the main transtensional segment is the southern margin of the balkans e g the emine line the steep transtensional faults disconnected the thrust planes of the balkans from the thrusts of the srednogorie to the south grabens are in general asymmetrical with master faults dipping 80 40 south westward and probably flattening into the middle crust the structural evolution of the balkans may also be traced into the black sea but the structural trends gradually change from the e w trend to a nw se strike indicating pure palaeogene compression and more recent neogene extension inverting and cross cutting the compressive features it is also interesting to note that the neogene tensional or transtensional inversion is not cylindrical it exhibits undulations of graben and transfer faults controlled by the pre existing compressive fabric the eastern balkans and their continuation in the black sea are morphologically and structurally deeper and they plunge eastward and south eastward towards the black sea this is due both to the deeper position of the balkans de collement planes in the black sea and to the thermal subsidence along the black sea continental margin

a miocene mud volcano and its plumbing system a chaotic complex revisited monferrato nw italy abstract chaotic deposits are frequently reported in the geological literature and are commonly interpreted as olistostromes or tectonic melanges a chaotic complex in the cenozoic succession of monferrato nw italy consists of interbedded mud breccia and burrowed silty clays that are pierced by sheared mud breccias and embed carbonate cemented blocks these may be represented by microcrystalline limestones or strongly cemented matrix supported breccias locally containing remains of chemosymbiotic organisms lucinid bivalves moreover cylindrical concretions up to 15 cm in diameter and 1 m long occur in the chaotic complex and crosscut bedding planes at high angles the cement of all these lithified portions is mainly dolomite characterized by low 13c values from 10 3 to 23 pdb and 18o values up to 7 pdb the 13c values testify to precipitation of carbonates induced by microbial oxidation of methane whereas the markedly positive 18o signature ubiquitous in the cylindrical concretions is the evidence for the presence and destabilization of gas hydrates the studied section provides a well exposed example of the geological record of the birth life and death of a mud volcano unsheared soft mud breccias represent mud flows along the flanks of the volcano whereas sheared mud breccias are the result of the injection of unconsolidated overpressured fine grained sediments both taking place during eruptive phases they were followed by more quiet stages of hemipelagic sedimentation burrowing and ch4 seeping the cylindrical concretions represent the first described ancient example of the chimneys observed in present day mud volcano settings they are the remnants of a cold seep plumbing network that crosscut the mud volcano edifice the chimneys were the pathway for the expulsion toward the sea floor of gas and sediment charged fluids likely originated from destabilization of methane gas hydrates the association of mud breccias and methane derived carbonates may not be due to mass gravity flows but can be primary and therefore is a diagnostic criterion for recognizing chaotic deposits due to mud volcano activity in the geological record

stable isotopic ratios and origins of the carbonates associated with cold seepage at the eastern nankai trough two types of cold seeps were located at the eastern nankai trough 1 seeps on sediment covered seafloor of 3800 3600 m in depth at the toe of the accretionary prism and 2 those between 1900 and 2100 m situated on the backthrust of the accretionary prism covered with rough outcrops of calcareous sandstones fragments of sandstone pipes or chimneys cemented with calcite were recovered from type 1 seeps whereas in type 2 seeps massive carbonate cemented sandstones and silty sandstones were obtained many of which contain cemented fragments of different lithologies and textures including dense aggregates of fossilcalyptogena type clam shells the carbon isotopic ratios of the carbonates in these sediments range from 36 to 56 pdb the carbonates formed by mixing between isotopically light carbonate ions derived from biogenic methane carried from depth in advecting fluids and heavy marine sedimentary carbonates sharp isotopic gradients in carbon and sometimes also in oxygen from the mixing processes are still retained across the walls of the sandstone pipes from type 1 seeps whereas the isotopic ratios of the samples from type 2 seeps exhibit variation patterns related to lithologies and textures and confirm the presence of several generations of carbonate cement within a single specimen these facts and the oxygen isotopic ratios of live and fossil clam shells recovered from the survey areas suggest that the cold seeps started at least during the last glaciation and have experienced repeated destruction and reformation due to episodic supply of methane rich fluid of different isotopic ratios and or temperature

cold seeps and authigenic carbonate formation in monterey bay california abstract authigenic carbonate associated with modern cold seep biological communities and their extinct analogues exhibit a broad range in stable isotope and mineral composition within the limited geographic area of monterey bay although such variations in ancient samples have been used to infer differing tectonic settings these carbonates all formed within a faulted continental margin environment and chemical variations reflect local differences in the sources and flux of carbon to sediment pore fluids the slow seepage of fluid and with it dissolved carbon along the transform faulted continental margin results in discrete areas of enhanced microbial sulfate reduction oxidation of methane from both biogenic and thermogenic origins as well as the active precipitation of both high mg calcite hmc and dolomite the authigenic carbonates include semicontinuous pavements of shallow cemented sediments surrounding benthic communities circular or pipe like chimneys interpreted as cemented conduits formed as a result of methane gas expulsion centimeter to meter scale rings doughnuts or slabs winnowed from variable depths within sediments and carbonate veins ankerite or calcite or cements in faulted basement rocks draped with bacterial mat abundant pyrite framboids preferentially filling the tests of the benthic foraminifer uvigerina peregrina and characteristic of the hmc bearing samples are products of a zone of shallow microbial sulfate reduction a process fundamental to the nourishment of the chemosynthetic cold seep communities sites on a sedimented ridge west of the san gregorio fault zone have carbon isotopic values between 35 and 56 that are strongly influenced by carbon derived from methane the higher values of 18 o more enriched in 18 o are found closer to the active fault zone carbonates from sites within the san gregorio fault zone or from monterey canyon floor exposures of miocene sediments have carbon isotope values between 7 and 26 that are mixtures of sedimentary organic carbon and microbially oxidized hydrocarbons from organic rich petroleum source rocks dolomite bearing samples are more enriched in both 18 o and 13 c bulk carbonate samples with heavier 18 o 18 o 3 5 result from the abundance of authigenic dolomite and low temperatures of formation local destabilization of clathrates might also create pore fluids that are more enriched in both 13 c and 18 o

the chimney forest of the deep montenegrin margin south eastern adriatic sea a spectacular field of columnar carbonates has been discovered on the montenegrin margin in the southern adriatic sea at a depth of about 450 m the site exposes many columnar carbonates protruding from the substrate or abated on the bottom such carbonates attain maximum visible lengths of ca 60 cm with diameters up to 20 cm display an annular growth and are either hollow or plugged by indurated sediment petrographic and geochemical analyses document the pervasive presence of dolomite and 13c values as low as 30 vpdb these chimneys are therefore interpreted as former conduits related to hydrocarbon expulsion in this sector of the adriatic basin however available data suggest that hydrocarbon flows at this site have ceased our results show that chimneys formed inside the local depositional units glacial pleistocene shelf shelly rich muddy sediment and were successively exhumed from the host sediment today the chimneys offer substrate to benthic life including cold water corals and sponges the u series dating of these carbonate concretions is complicated by the presence of a significant fraction of detrital sediment which represents a major source of initial 230th ams 14c and 87sr 86sr dating of shells embedded in one of the chimney provided ages beyond the range of radiocarbon dating and 600 kyrs indicating a pleistocene age of the host sediment uncorrected u series dating of the carbonate chimneys yielded and age of ca 250 270 kyrs providing a temporal upper limit for conduit formation driven by lithification caused by the degradation of seeping hydrocarbons in addition u th dating of cold water coral bases settled on chimneys indicates a holocene age for their first exposure after exhumation and subsequent function as hard substrate for benthic organisms the montenegrin chimney forest is a rare case where many such carbonate columnar concretions are still in their original vertical position

an early diagenetic deglacial origin for basal ediacaran cap dolostones abstract the beginning of the ediacaran period 635 ma is marked by conspicuous dolostone units that cap marinoan glacial deposits worldwide the extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over saturation coincided with deglacial sea level rise and ocean warming however the geochemical variability within cap dolostones both between continents across single continental margins and within individual stratigraphic sections has been difficult to reconcile with depositional models using a compilation of new calcium and magnesium isotope measurements in marinoan cap dolostone successions worldwide we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid and sediment buffered conditions dolostones from the outer platform formed under fluid buffered conditions whereas dolostones on the inner platform and foreslope environment formed under sediment buffered conditions this spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea level rise and development of a meltwater surface ocean using a numerical diagenetic model to evaluate the geochemical differences between sediment and fluid buffered cap dolostone units we constrain the chemical and isotopic composition of both the dolomitizing fluid glacial seawater 13 c 0 2 the meltwater lens 13 c 11 and the primary aragonite sediment 13 c 6 to 3 these model end members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal ediacaran cap dolostones our results provide a novel framework for understanding the geochemical variability of cap dolostone units including large excursions in carbon isotopes and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last snowball earth

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

how tillite weathering during the snowball earth aftermath induced cap carbonate deposition the development and aftermath of a snowball earth during precambrian time still raise questions particularly the process involved in the deposition of the associated cap carbonates and the time required for the earth to return to more stable and less extreme climatic conditions we present numerical simulations modeling the chemical weathering of regolith exposed to a co 2 rich atmosphere after deglaciation we consider the effects of an early cation proton exchange reaction that promotes fast initial dissolution of the regolith a reaction often omitted in the modeling approach but one that becomes particularly important under acidic conditions and in environments where glacial erosion can substantially refresh the rock surfaces calculations show that this dissolution step may contribute significantly to the riverine ionic input and may promote the deposition of thick cap carbonates over a short period of time the long term weathering of the regolith becomes predominant representing an efficient way of removing atmospheric co 2 over periods ranging from tens to hundreds of thousands of years depending on the grain size of the regolith material

the role of so4 in the switch from calcite to aragonite seas throughout the phanerozoic the primary inorganic marine calcium carbonate mineralogy oscillated between calcite and aragonite reflecting changes in seawater chemistry these variations in seawater composition also appear to have influenced the evolution of calcifying organisms however the processes controlling these mineralogical and biological changes are poorly constrained previous work has focused mainly on the mg ca ratio in seawater as the primary driver here we examine the role of dissolved so 4 in these processes by performing controlled laboratory precipitation experiments and geochemical modeling of evaporite formation we show that an increase in dissolved so 4 decreases the mg ca ratio at which calcite is destabilized and aragonite becomes the dominant caco 3 polymorph our data suggest that the mg ca and so 4 thresholds for formation of calcite seas are significantly lower than previous estimates and are mutually dependent this shows that our understanding of phanerozoic changes in seawater chemistry and the models relating this to primary caco 3 mineralogy need reevaluation 2011 geological society of america

aftermath of a snowball earth 1 using a simple 3 box model of the ocean atmosphere system we simulate the cycling of carbon and strontium in the aftermath of a global glaciation model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pco2 conditions as well as ocean mixing air sea gas exchange and biological productivity the 13c of the first carbonate precipitated after the glaciation depends on the pco2 temperature the saturation state of the surface ocean and kinetic effects associated with mineral precipitation with no biological productivity the model produces 13c values between 1 and 3 consistent with observations this is in direct contradiction with arguments by kennedy et al 2001a who suggest that the 13c value of dissolved carbon in a snowball ocean and directly afterward must be 5 kennedy et al assume the carbon isotope cycle is in steady state which does not apply to a global glaciation and also neglect any effect of high pco2 on the carbonate chemistry of seawater a major difference between our findings and the qualitative predictions of hoffman et al 1998 is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves as a result the 2 drop in the 13c observed in the cap dolostone is unlikely to be the product of rayleigh distillation of atmospheric co2 via silicate weathering instead we interpret the 2 drop in the 13c values as indicative of an increase in sea surface temperature which lowers the fractionation between co2 and carbonate kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important rayleigh distillation of atmospheric co2 via silicate weathering is a viable explanation for the continued drop in the 13c values in the limestone sequence above the cap dolostone with biological productivity and carbonate weathering driving a slow increase in 13c values once pco2 levels decline our study also simulates the cycling of strontium in seawater in contrast to the finding of jacobsen and kaufman 1999 and kennedy et al 2001a model simulations show a drop in 87sr 86sr of less than 0 001 during 5 million years of global glaciation and an increase of less than 0 001 over the entire episode of silicate weathering our calculations emphasize the importance of considering the changes in seawater chemistry due to high pco2 in evaluating the snowball earth hypothesis

abrupt climate change and transient climates during the paleogene a marine perspective detailed investigations of high latitude sequences recently collected by the ocean drilling program odp indicate that periods of rapid climate change often culminated in brief transient climates with more extreme conditions than subsequent long term climates two examples of such events have been identified in the paleogene the first in latest paleocene time in the middle of a warming trend that began several million years earlier the second in earliest oligocene time near the end of a middle eocene to late oligocene global cooling trend superimposed on the earlier event was a sudden and extreme warming of both high latitude sea surface and deep ocean waters imbedded in the latter transition was an abrupt decline in high latitude temperatures and the brief appearance of a full size continental ice sheet on antarctica in both cases the climate extremes were not stable lasting for less than a few hundred thousand years indicating a temporary or transient climate state geochemical and sedimentolog

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

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 carbon isotope challenge to the snowball earth the snowball earth hypothesis contends that most of the planet was covered with thick ice for millions of years about three quarters of a billion years ago a dramatic warming event thought to have been fuelled by high atmospheric carbon dioxide levels eventually defrosted earth a new isotopic analysis of rocks that were laid down after the initial glaciation challenges the idea of a hard snowball earth the data are consistent with a rather low partial pressure of atmospheric co2 not the orders of magnitude above current levels that would provide the extreme warming necessary for escape from snowball earth this supports the idea that the initial glaciation was much less severe than has been proposed the snowball earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the neoproterozoic era in a self enhanced glaciation caused by the high albedo of the ice covered planet in a hard snowball picture the subsequent rapid unfreezing resulted from an ultra greenhouse event attributed to the buildup of volcanic carbon dioxide co2 during glaciation1 high partial pressures of atmospheric co2 from 20 000 to 90 000 p p m v in the aftermath of the marinoan glaciation 635 myr ago have been inferred from both boron and triple oxygen isotopes2 3 these values are 50 to 225 times higher than present day levels here we re evaluate these estimates using paired carbon isotopic data for carbonate layers that cap neoproterozoic glacial deposits and are considered to record post glacial sea level rise1 the new data reported here for brazilian cap carbonates together with previous ones for time equivalent units4 5 6 7 8 provide estimates lower than 3 200 p p m v and possibly as low as the current value of 400 p p m v our new constraint and our re interpretation of the boron and triple oxygen isotope data provide a completely different picture of the late neoproterozoic environment with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard snowball earth

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

excellent source rocks discovered in the cryogenian interglacial deposits in south china geology geochemistry and hydrocarbon potential abstract the cryogenian 720 635 ma is characterized by the well known sturtian and marinoan glaciations which plays a key role in understanding remarkable climate and sea level changes and oxygenation events that occurred during the neoproterozoic era however less attention has been paid to the interglacial deposits recently we paid great attentions to the excellent hydrocarbon source rocks toc 2 in the cryogenian interglacial datangpo formation 663 4 ma 654 3 8 ma in south china which mainly consists of black shales with thin manganese interbeds making the interglacial interval a new focus to elucidate their characteristics and depositional environments petrological and geochemical studies were conducted on fresh rock samples collected from six sections gucheng sanlian xiaoxi gaodongyuan xiaochayuan yanglizhang in manganese mining areas at southeastern south china sedimentary reconstructions suggest a transition from restricted to open ocean environment during the deposition of the datangpo formation and this formation records variations from warm and humid to dry and cold climate black shales were deposited in its lowest member during the warm period and showed greatest thicknesses more than 20 m at depocenters microscopic observations of the black shale samples exhibited abundant benthic algae with great diversities indicating a gradual recovery of the ecosystem biomarker analyses further reveal that these algae combined with bacterial microorganisms prosperous in low salinity reducing environments were the dominant organic contributors to the source rocks which were inferred by low pristane pr to phytane ph ratio pr ph low gammacerane high tricyclic terpene and high c27 sterane contents trace element records combined with isotopic data suggest that the deposition of source rocks in datangpo formation were related to a burst in surface water productivity due to warm climate and better preservations resulted from anoxic waters within a stratified water column these findings indicate the cryogenian source rocks may provide a new potential field for future explorations of shale gas and deep oil and gas

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

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

cold seep deposits of beauvoisin oxfordian southeastern france and marmorito miocene northern italy microbially induced authigenic carbonates the relation of two well known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study archaea are found to be associated with the formation of oxfordian seep carbonates from beauvoisin and with a miocene limestone from marmorito tube worm limestone carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative 13ccarbonate values respectively highly positive values 15 reflect the use of 13c enriched co2 produced by methanogenesis low 13c values of the marmorito carbonates 30 indicate the oxidation of seepage derived hydrocarbons likewise the 13c content of specific tail to tail linked isoprenoids biomarkers for archaea was found to be strikingly depleted in these samples as low as 115 the isotopic signatures corroborate that archaea were involved in the cycling of seepage derived organic carbon at the ancient localities another miocene limestone marmorito limestone shows a strong imprint of methanotrophic bacteria as indicated by 13c values of carbonate as low as 40 and biomarker evidence epifluorescence microscopy and field emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation in addition to lucinid bivalves previously reported from both localities we infer that sponges from beauvoisin and tube worms from marmorito depended on chemosynthesis as well low 13c values of nodules related to sponge taphonomy 27 indicate that sponges might have been linked to an enhanced hydrocarbon oxidation tube worm fossils from marmorito closely resemble chemosynthetic pogonophoran tube worms from recent cold seeps and are embedded in isotopically light carbonate 13c 30

origin of cap carbonates an experimental approach abstract an experimental study was carried out to assess the abiotic natural reactions associated with carbonate precipitation leading to the formation of neoproterozoic cap carbonate sequences for this purpose synthetic seawater solutions were prepared reflecting the presumed composition of neoproterozoic seawater these solutions were heated to 40 c undergoing degassing and or evaporation while monitoring their chemical composition sulphate concentration mg ca molar ratio and identifying the precipitated minerals the experimental results are in good agreement with previous studies allowing us to constrain the type of carbonate precipitates formed in the ocean during the snowball earth aftermath among the phases precipitated during heating of the solutions mg bearing carbonate minerals i e mg calcite magnesite and dolomite remain of minor importance compared to ca carbonates especially aragonite consequently the basal dolostone units of the cap carbonates i e cap carbonates s s were originally in the form of caco3 and evolved spontaneously toward dolomite cap carbonates s l i e cap limestones containing aragonitic cement could result from the degassing of the whole ocean and the concomitant precipitation of aragonite the calculated thicknesses of carbonates formed in this way are of the order of several metres in line with the field observations although this process of abiotic carbonate precipitation does not rule out biologically mediated carbonate formation it may represent an alternate route to produce carbonate particularly in environments which are not yet colonized by bacteria after the global ice melting

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

seawater buffered diagenesis destruction of carbon isotope excursions and the composition of dic in neoproterozoic oceans carbonate sediments of nonglacial cryogenian 659 to 649 ma and early ediacaran 635 to 590 ma age exhibit large positive and negative 13ccarb excursions in a shallow water marine platform in northern namibia the same excursions are recorded in fringing deep sea fans and in carbonate platforms on other paleocontinents however coeval carbonates in the upper foreslope of the namibian platform and to a lesser extent in the outermost platform have relatively uniform 13ccarb compositions compatible with dissolved inorganic carbon dic in the modern ocean we attribute the uniform values to fluid buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection this attribution which is testable with paired ca and mg isotopes implies that large 13ccarb excursions observed in neoproterozoic platforms while sedimentary in origin do not reflect the composition of ancient open ocean dic

mineralogy geochemistry and petrography of methane derived authigenic carbonates from enza river northern apennines italy abstract a remarkable exposure of carbonate pipe like and slab concretions occurs along the enza riverbanks on the northern apennines foothills italy based upon geochemical and field evidences their genesis has been attributed to microbial governed carbonate precipitation from hydrocarbon enriched fluids the pipe like concretions are thus interpreted as former conduits chimneys marking sites of methane ascent onto the seafloor the resulting enza river chimney field is arranged in a palisade fashion a rare example of such mineralogical and petrographic analyses of some chimneys and slabs document that the dominant cement is dolomite although the chimneys show a rather homogeneous texture a clear zonation is observed in the relative distribution of major and minor elements in their internal and external parts the occurrence of sulphide minerals in the stratigraphically upper samples indicates possible renewal of fluids leakage after a major erosive event

vast fields of hydrocarbon derived carbonate chimneys related to the accretionary wedge olistostrome of the gulf of cadiz we report the first discovery and sampling of vast fields of hydrocarbon derived carbonate chimneys along the gulf of cadiz continental slope at depths between 500 and 1200 m a large variety of carbonate chimneys more than 200 samples were recovered from four different areas named the diasom tasyo gbf guadalquivir basin and esf east moroccan fields observations from an underwater camera revealed a spectacular high density of pipe like chimneys some of them longer than 2 m lying over the sea floor and some protruding from muddy sediment local fissures and alignment of isolated chimneys were also observed suggesting that their distribution is controlled by fault planes chimneys collected show a wide range of morphological types spiral cylindrical conical mushroom like and mounded with numerous nodule protuberances and ramified fluid channelways the chimneys are mainly composed of authigenic carbonates ankerite fe bearing dolomite and calcite with abundance of iron oxides forming agglomerates of pseudo pyrite framboids n 18 o isotopic values vary from 0 7 to 5 5x whereas n 13 c values indicate that chimney carbonates are moderately depleted in 13 c ranging from 346x to 320x pdb interpreted as formed from a mixture of deep thermogenic hydrocarbons and shallow biogenic methane these vast fields of carbonate chimneys imply new considerations on the importance of hydrocarbon fluid venting in the gulf of cadiz and moreover on the active role of the olistostrome accretionary wedge of the gibraltar arc b 2003 elsevier science b v all rights reserved

influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater a constant disequilibrium technique was used to determine the composition of magnesian calcite overgrowths precipitated on pure calcite seeds from artificial seawater at 5 25 and 40 c the amount of magnesium incorporated in the overgrowths at a given temperature is independent of the precipitation rate over a wide range of saturation states and is believed to correspond to a composition in true equilibrium with seawater the distribution coefficient of magnesium dmg2 c in the magnesian calcite overgrowths increases almost linearly with temperature being 0 0121 0 0013 at 5 c 0 0172 0 0022 at 25 c and 0 0271 0 0013 at 40 c these values apply only to magnesian calcites precipitated from standard composition seawater since a previous study has shown dmg2 c to be a function of the mg2 ca2 ratio in the parent solution results of this study are compared with values reported previously by other workers and with the compositional distribution of naturally occurring magnesian calcite cements and ooids found in seawater it appears that variations in temperature are not sufficient to account for the compositional variability of naturally occurring inorganic marine magnesian calcite cements

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

the fluctuations of fixed nh4 n content in the luochuan loess and its paleoclimatic significance abstract the fixed nh4 n content varies from 120 to 202 g g in luochuan loess paleosol sequences this content in the interstratified paleosols is significantly higher than that of the loess fixed nh4 n content in the loess l2 is the lowest varying between 144 to 147 g g whereas the highest in the paleosol s5 reaching 200 g g the distribution of fixed nh4 n is similar to that of the

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