global hydrocarbon seep carbonate precipitation correlates with deep water temperatures and eustatic sea level fluctuations since the late jurassic the anaerobic oxidation of methane aom in marine sediments most obviously at cold seeps is a major sink for this important greenhouse gas and is associated with the precipitation of carbonates the geological record of seep carbonates provides insights into the long term dynamics of this process since the late jurassic rates of seep carbonate precipitation have been high during times of low sea levels and cold deep water temperatures and vice versa one possibility is that sea level fall decreases the thickness of the methane hydrate stability zone in the sediment so that gas hydrates decompose in deeper sediments thereby increasing seepage and carbonate precipitation alternatively low deep water temperature facilitates gas hydrate formation on continental slopes and may thereby increase methane availability for aom performing consortia and associated carbonate precipitation these correlations and their potential causes may be of interest for the modelling of global carbon budgets and of past and future climates

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

review of fossil chemosynthetic assemblages in japan abstract seventy five fossil chemosynthetic assemblages of cretaceous to holocene age have been discovered in japan many are interpreted to have been cold seep dependent and one is associated with a whale fall fossil hydrothermal vent assemblages have not been found in japan non seep assemblages of solemyids lucinids and or thyasirids are not included in this study various combinations of five diagnostic features characterize cold seep assemblages 1 the abundant and often exclusive occurrence of large invertebrate taxa suspected of harboring chemosynthetic bacteria 2 associated authigenic carbonates greatly depleted in 13c 3 a three dimensional distribution of the assemblage that is laterally restricted 4 associated pockmark or diapir related sedimentary structures and 5 the presence of conduit related tectonic or sedimentological structures such as faults slumps or debris flows excluding the whale fall assemblage we can divide cenozoic fossil chemosynthetic assemblages in japan into three types regardless of geochronologic age type i is dominated by vesicomyid bivalves and or tube worms characterized by its allochthonous or parautochthonous mode of occurrence in deep water greater than 1000 m settings along with tectonically or sedimentologically induced structures such as faults slumps diapirs or debris flows likely dependent on seepage controlled by subduction related tectonism and or on methane coming from the drastic decomposition of gas hydrate type ii is dominated by vesicomyid bivalves in most cases the species differ from those in type i and is characterized by autochthonous occurrences in muddy sediments from less than 1000 m water depth type iii is dominated by lucinoma and or conchocele and characterised by autochthonous occurrences in muddy sediments from depths of less than 300 m a living community similar to the type ii and iii assemblages has not yet been observed in the seas around japan it is noteworthy that no tectonically or sedimentologically induced structures have been observed in and around the type ii and iii fossil localities although the muddy sediments sometimes show brecciated facies that suggest an explosive effusion of methane

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

a paleogene deep sea methane seep community from honshu japan an isolated limestone deposit occurs within late eocene to early oligocene submarine fan deposits of the tanamigawa formation in the outskirts of kushimoto town in southern honshu japan the petrography the very negative carbon isotope signature of early diagenetic cements and the abundance of chemosymbiotic bivalves namely thyasirids vesicomyids and bathymodiolins clearly identify this carbonate block as the first paleogene methane seep deposit in honshu from a biogeographical point of view distinctive features of paleogene seep faunas in japan are the apparently endemic vesicomyid hubertschenckia ezoensis and the scarcity of lucinid bivalves whereas the bivalves conchocele bisecta and bathymodiolus spp and the gastropod cryptonatica were widespread in north pacific seep communities of this age although cenozoic seep communities generally consists of members of the same families that inhabit seeps today marked differences in the genus level composition of the major chemosymbiotic bivalve families such as vesicomyidae and lucinidae and the subfamily bathymodiolinae are noted when paleogene seep communities in japan are compared to those of early neogene age on the one hand and to paleogene seep communities elsewhere on the other

a seismic reconnaissance survey of the northern congo fan abstract to study the time varying influence of the congo river and the benguela current on the deposition at the angola continental margin a high resolution reflection seismic survey was carried out on the northern congo fan four seismostratigraphic units have been defined for the upper 800 m 1000 ms twt of the data the units record different depositional environments ranging from pre establishment of the congo river drainage system to the influence of the benguela current an indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material the ascent of salt is recorded up to the pliocene gas that has migrated out of lower cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes in a few places this gas even ascends to the ocean floor along structural pathways through the topmost unit

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

u th dating and post depositional alteration of a cold seep carbonate chimney from the campos basin offshore brazil abstract dating authigenic cold seep carbonates allows determining the timing of active seepage of hydrocarbon rich fluids at continental margins and ultimately the detection of the geological factors affecting it u th dating has been selected in this study the method having been successfully used before however incorporation of initial thorium in substantial amounts of detrital material due to the precipitation of carbonates within the seabed and post depositional alteration potentially pose a major problem violating the basic assumptions of the method here we report results from several samples from the campos basin off the coast of brazil they are unusual in their high 13 c values mean of 10 0 vpdb high detrital contents and being dominantly dolomite elevated 230 th 238 u activity ratios indicate that most of the carbonates lost uranium during diagenesis isochrones are obtained from the unaffected samples yielding an age of 140 4 3 1 ka vertical growth of the chimney must have been short and rapid since results from the top and bottom parts of the chimney concur within analytical uncertainties furthermore profiles of u th radioisotope specific activity 234 u and 232 th specific activity on two quarter slices of a carbonate chimney reveal the occurrence of three distinct diagenetic processes u loss u exchange with seawater and th gain

late triassic mollusk dominated hydrocarbon seep deposits from turkey deep sea hydrothermal vents and hydrocarbon seeps host unique ecosystems relying on geochemical energy rather than photosynthesis whereas the fossil and evolutionary history of these ecosystems is increasingly well known from the cretaceous onward their earlier history remains poorly understood and brachiopods are considered to have played a dominant role during the paleozoic and mesozoic here we report five new hydrocarbon seep deposits from the upper triassic kasimlar shales in southern turkey the pyritiferous seep limestones predominantly consist of 13 c depleted micrite with 13 c values as low as 10 4 and contain only sparse 13 c depleted rim cement 13 c as low as 12 0 interpreted to result from the recrystallization of banded and botryoidal crystal aggregates of fibrous cement the geologic ages of the studied seep deposits were determined as late carnian and early norian using conodonts the associated fauna is dominated by modiomorphid and anomalodesmatan bivalves and also includes a diversity of gastropods and the dimerelloid brachiopod halorella these faunal assemblages allow a comparison between seep faunas from the two major triassic ocean basins the present assemblages being from tethys and the only previously known examples being from eastern panthalassa and indicate that a cosmopolitan seep restricted fauna as in the present day oceans has existed since the late triassic with almost 20 species the seep fauna of the kasimlar shales approaches the diversity of cretaceous to present day seep faunas further emphasizing the ecological similarity of seep faunas since the early mesozoic our findings also highlight that brachiopods and bivalves had a more complex history of coexistence at seeps than currently appreciated

u th dating of cold seep carbonates an initial comparison authigenic carbonates from hydrocarbon seeps are unique archives of past seepage and associated environmental parameters in order to constrain the ages of a set of seep carbonates and the time involved in carbonate formation we applied uranium thorium u th dating to samples from the gulf of mexico the congo fan and the black sea the resulting u th ages indicate that environmental conditions must have been favorable for enhanced methane rich fluid seepage during the time intervals of 53 4 to 1 7 ka bp for the gulf of mexico and 45 5 to 3 0 ka bp for the congo fan the seep carbonates from the black sea formed at 1 6 to 1 1 ka bp our results suggest that enhanced fluid flow during these time intervals was closely related to 1 sea level variations associated with glacial interglacial cycles and 2 environmental alterations in the course of late quaternary climate change including variations in bottom water temperatures that affected the stability of gas hydrate reservoirs

determinants of extinction in the fossil record the causes of mass extinctions and the nature of biological selectivity at extinction events are central questions in palaeobiology it has long been recognized however that the amount of sedimentary rock available for sampling may bias perceptions of biodiversity1 2 3 4 5 6 7 and estimates of taxonomic rates of evolution5 6 7 8 this problem has been particularly noted with respect to the principal mass extinctions5 6 7 8 9 10 11 12 here we use a new compilation of the amount of exposed marine sedimentary rock to predict how the observed fossil record of extinction would appear if the time series of true extinction rates were in fact smooth many features of the highly variable record of apparent extinction rates within marine animals can be predicted on the basis of temporal variation in the amount of exposed rock although this result is consistent with the possibility that a common geological cause determines both true extinction rates and the amount of exposed rock it also supports the hypothesis that much of the observed short term volatility in extinction rates is an artefact of variability in the stratigraphic record

hydrocarbon seep carbonates associated with mesozoic environmental perturbations example from the lower bajocian of morocco abstract in the lower bajocian of the jebel bou kendill section central high atlas mountain three peculiar levels of authigenic carbonates are observed within the carbonate poor agoudim iii member morphological sedimentological and geochemical evidences indicate that the two upper levels have been deposited at a hydrocarbon seep site whereas the lowermost level likely underlines the zone where hydrocarbon fluids were generated the first seep carbonate level is made of mineralized burrows that have acted as fluid conduits leading to a conspicuous cap carbonate bed on top of which several chimneys and chemoherm structures are embedded hence this carbonate bed is a fossilized sea floor carbonate crust the second seep carbonate level is made of chemoherm structures buttressed against the first carbonate beds of the agoudim iv member they were probably formed within the sedimentary column close to the sea floor similar seep carbonates are observed regionally within the agoudim iii member in the highly subsiding central high atlas troughs a relationship between these seep carbonates occurrence and the late early bajocian global environmental perturbation can be deduced as this latter has led to the deposition of a relatively organic matter rich sedimentary sequence in these troughs via seawater eutrophication the decomposition by bacterial activity of this organic matter aided by rapid burial rates has led to the generation of alkaline rich fluids promoting the precipitation of the seep carbonates a similar relationship between transient environmental perturbations and seep carbonates occurrence is observed in numerous mesozoic examples and can be explained by the fact that hydrocarbon seeps are supply limited systems transient deviation from the mesozoic greenhouse climate norm either toward hothouse or coldhouse climates favours the deposition of organic rich sediments or transient methane storage respectively hence increasing the likelihood for the formation of hydrocarbon seepage

isotopic signals from late jurassic early cretaceous volgian valanginian sub arctic belemnites yatria river western siberia this contribution presents the first detailed oxygen and carbon isotope record from the latest jurassic early cretaceous interval of the yatria river subpolar urals siberia isotopic compositions have been determined on well preserved belemnite samples from the genera lagonibelus cylindroteuthis and acroteuthis these new data indicate a shift to lower temperatures from the late volgian into the late valanginian with some warmer phases recognized within the ryazanian and earliest valanginian the lowest temperatures of the late valanginian consistent with subfreezing polar temperatures are coincident with an inferred eustatic sea level fall a late valanginian positive shift in carbon isotopes correlates with the carbon isotope excursion recorded from tethyan successions the most positive carbon isotope values correspond to the most positive oxygen isotope values and hence lowest palaeotemperatures in the absence of widespread valanginian organic rich black shale deposition the carbon isotope excursion may point to increased storage of organic carbon in coastal areas and or enhanced preservation within stratified waters in high latitude basins at these higher latitudes where rates of weathering were presumably much lower because of the prevalent cold climate the isotopic data may point to pulses of productivity being brought about by increased riverine nutrient transfer and also by nutrients being released by the melting of ice the correlation between positive carbon isotopes and cool climates may indicate the effectiveness of these high latitude carbon sinks and their ability to draw down atmospheric co 2 resulting in an inverse greenhouse effect

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

outcrop analogues of pockmarks and associated methane seep carbonates a case study from the lower cretaceous albian of the basque cantabrian basin western pyrenees abstract late albian deep water sediments of the black flysch group in the basque cantabrian basin western pyrenees preserve a fossil pockmark field including methane seep carbonates and associated macrofauna the geometry of the pockmarks is reconstructed from repeated lens shaped turbidite deposits with centrally located carbonate bodies early diagenetic carbonate phases such as clotted micrite and yellow calcite with 13c values as low as 41 6 and hydrocarbon biomarkers e g 2 6 10 15 19 pentamethylicosane with strong depletions in 13c indicate that the carbonates precipitated due to anaerobic oxidation of methane the pockmarks probably formed due to subsidence induced by dewatering and degassing of the gas charged seabed perhaps enhanced by the weight of the carbonate bodies the macrofauna resembles that of other late mesozoic deep water methane seeps world wide and is dominated by large lucinid and caspiconcha bivalves and hokkaidoconchid gastropods during late diagenesis the carbonate 18o values were reset to a narrow range of 12 to 10 the remaining pore spaces and fissures were filled with pyrobitumen and additional carbonate phases precipitated potentially due to thermochemical sulfate reduction processes

evidence of climatic control on hydrocarbon seepage in the miocene of the northern apennines the case study of the vicchio marls abstract the vicchio outcrop in the tuscan apennines contains excellent exposures of a miocene methane derived carbonate system made up of more than 80 carbonate bodies enclosed in marly sediments facies analysis composition and a detailed biostratigraphic study of the carbonates and enclosing vicchio marls allowed us to document the role of climatic changes and eustasy on seepage in these ancient deposits results of our study indicate that the stratigraphic horizon bearing seep carbonates is constrained by two planktonic foraminiferal events the acme end ae of turborotalita cf t quinqueloba 13 75 ma and the acme 1 beginning a 1 b of paragloborotalia siakensis 13 32 ma encompassing about 400 000 years the ae of t cf t quinqueloba approximates the mid miocene global cooling event mi3b as defined by the 18 o maxima 13 78 ma the glacio eustatic sea level drop associated with this cooling event is estimated to be approximately 60 m assuming that the highest rates of glacio eustatic sea level fall coincide with 18 o maxima the timing of the seepage onset at about 13 75 ma is concomitant with the phase of sea level lowering the ascent and emission of methane rich fluids may have been triggered by pressure drop due to the eustatic fall associated with the mi3b event the moderate intensity of fluid expulsion is suggested by the lack of brecciation by the scarcity of detrital particles in the carbonates and by the pervasive occurrence of carbonate in the enclosing marls this type of seepage differs from focused fault confined fluid flows typical of other tectonically controlled apenninic seep carbonates

hydrate ridge a natural laboratory for the study of microbial life fueled by methane from near surface gas hydrates hydrate ridge cascadia convergent margin is characterized by abundant methane hydrates at and below the seafloor active venting of fluids and gases chemosynthetic communities composed of giant sulfide oxidizing bacteria and clams authigenic carbonates and some of the highest methane oxidation rates ever found in the marine environment fluid flow rates vary over six orders of magnitude among closely spaced settings at the crest of hydrate ridge the distribution of benthic communities is mainly related to the sulfide flux from the subsurface sediments produced by anaerobic oxidation of methane aom even at high flux rates aom removes most of the methane seeping from the subsurface less than 50 of the methane escapes from bacterial mats from calyptogena fields 15 and from acharax beds no methane is emitted to the water column the precipitation of carbonate derived from aom is a significant and permanent carbon sink hence compared to the amount of gas hydrates and methane derived carbonates stored at the summit of hydrate ridge the emission of methane is relatively low due to the efficient filtering capacity of methanotrophic microbial communities this present review updates current knowledge of the geology of hydrate ridge summarizes recently published data on geomicrobiology and biogeochemistry and derives a local methane budget

estimating the rock volume bias in paleobiodiversity studies to interpret changes in biodiversity through geological time it is necessary first to correct for biases in sampling effort related to variations in the exposure of rocks and recovery of fossils with age data from new zealand indicate that outcrop area is likely to be a reliable proxy of rock volume in both stable cratonic regions where the paleobiodiversity record is strongly correlated with relative sea level and on tectonically active margins in contrast another potential proxy the number of rock formations is a poor predictor of outcrop area or sampling effort in the new zealand case

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

using 87sr 86sr ratios to date fossil methane seep deposits methodological requirements and an example from the great valley group california abstractmethane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems for this information to be useful a reliable determination of the carbonates stratigraphic ages is required but this is not always available here we investigate the using strontium isotope stratigraphy to date fossil methane seep carbonates via detailed petrographic and geochemical investigation of the different carbonate phases in biostratigraphically well dated seep carbonates of paleozoic mesozoic and cenozoic age the best results are obtained from banded botryoidal rim cements from carbonate phases showing a weak or no cathodoluminescence signal an oxygen isotope signature close to that of seawater and the lowest mn concentrations we then applied the method to a presumably late jurassic seep carbonate from the great valley group in california strontium isotope ratios of the least diagenetically altered carbonate phases indicate a tithonian late jurassic

holocene ooids of aitutaki atoll cook islands south pacific although oolitic sands are widespread throughout the phanerozoic carbonate rock record and they are abundant in some modern shallow marine carbonate systems recent ooids have not been recognized from any locality in the pacific basin on aitutaki atoll cook islands south pacific oolitic sands occur in water depths of 2 1 6 3 m within the lagoon and less abundantly on shallower reef aprons most ooids are between 150 and 300 m in diameter have between 1 and 25 concentric to irregular laminae and occur with peloids and skeletal grains cortical laminae include mg calcite crystals with radial tangential and random orientations the occurrence and distribution of oolitic sands on aitutaki are facilitated by a convergence of hydrodynamic and chemical factors wave driven currents combined either with flow separation over a sharp increase in depth at the reef apron lagoon interface or with oceanward flow in the lee of islands serve to transport ooids without flushing them from the system here ph alkalinity and carbonate supersaturation are at some of the highest levels in the tropical pacific the restricted occurrence of holocene oolitic sands in areas of the pacific and atlantic basins with elevated ph and total alkalinity reflects the importance of carbonate saturation thresholds in limiting the spatial distribution of these sands in modern shallow marine settings these recent examples are consistent with interpretations of the importance of these variables in determining the distribution abundance and cortical mineralogy of oolitic sands throughout the phanerozoic stratigraphic record of carbonate accumulation

composition and origin of authigenic carbonates in the krishna godavari and mahanadi basins eastern continental margin of india abstract the mineralogical and stable isotopic composition of authigenic carbonates from the krishna godavari kg and mahanadi basin provide a deeper insight into the processes inducing carbonate formation in the sediments of the eastern continental margin of india in the bay of bengal authigenic carbonate cements micro nodules bioturbation casts and tubes from 12 core locations drilled during the indian national gas hydrate program nghp expedition 01 were investigated for this study three main processes responsible for authigenic carbonate precipitation are identified organoclastic sulfate reduction anaerobic oxidation of methane and methanogenesis evidence of vigorous methane seepage is indicated in carbonates recovered at sites 7 10 12 kg basin and 19 mahanadi basin these methane derived carbonates display typical paragenetic carbonate mineralogies aragonite high mg calcite with 15 mol mgco 3 ca rich dolomite two separate horizons of methane derived carbonates are correlated between 4 drill holes up to 16 km apart the upper horizon has been dated with 14 c 40 100 51 600 a bp 1950 and clearly indicates that methane seepage has been much more vigorous in the past possibly due to the effect of lower sea level on the gas hydrate stability zone across the margin

correlating shelf carbonate evolutive phases with fluid expulsion episodes in the foredeep miocene northern apennines italy abstract in the miocene of the northern apennines of italy evolutionary phases of shallow water carbonates in wedge top basins are coeval with methane rich fluid expulsion episodes in the inner foredeep we identified four main correlative events primarily controlled by synsedimentary tectonics sediment input and climate cooling 1 a burdigalian tectonic phase determined the onset of shelfal sedimentation in wedge top areas and is correlated with an important episode of fluid expulsion in marly sediments of the inner foredeep croce di moggiona seep carbonates 2 the decrease of shelfal carbonate production and the increase of detrital input in the early langhian correspond with a second important episode of seep carbonate precipitation in the inner foredeep castagno d andrea 3 the demise of the carbonate shelf and the coeval expulsion of methane rich fluids in the inner foredeep vicchio and m citerna seep carbonates at the langhian serravallian boundary approximate the mi3b cooling event the lower temperatures associated with increased siliciclastic weathering led to the crisis of shallow water carbonate production whereas the pressure drop due to the eustatic fall may have triggered the gas charged fluid expulsion 4 the tectonic related uplift of frontal sectors of the wedge top basin led to the wide erosion of the burdigalian langhian carbonates and caused the precipitation of authigenic seep carbonates deruta seep both in the slope and the adjacent foredeep

theoretical simulation of the evolution of methane hydrates in the case of northern south china sea since the last glacial maximum the distribution and inventory of gas hydrates in a region is determined by the sediment characteristics methane supply and evolution of the reservoir in recent decades the geo environmental constraints and sources of methane have been intensively investigated and numerous experimental and numerical simulation tools have been developed to evaluate the inventory of hydrates however information regarding the evolution of hydrate reservoirs remains limited this study developed a simulator to theoretically model the evolution of specific reservoirs since the last glacial maximum lgm the lgm was a recent cold epoch that occurred approximately 18 000 years ago since the lgm the earth s climate system has experienced a continuous increase of surface temperature and rising sea level given a sufficient supply of methane and a transport system hydrates may form in marine sediments if sea level rises or melt if the temperature increases a one dimensional simulator that represents the sediment was developed and uses the current hydrate profiles as the initial conditions and reliable paleoenvironmental data obtained in sites located in the northern shelf of the south china sea scs as boundary conditions two types of hydrate profiles were reversely simulated till the lgm 1 a gaussian profile which was observed in the shenhu area and 2 a trapezoidal profile which was observed in the dongsha area scs the evolution and past quantities of local hydrate reservoirs were estimated the model results demonstrated that shallow 500 700 m below the seafloor or mbsf moderate saturation 50 pore volume or v v hydrate deposits will form if they are subjected to recent climate changes the inventory of hydrates in the nscs increased by only 0 7 over the past 18 000 years under various scenarios a sensitivity analysis was performed to identify the most pertinent parameters that control the formation and dissociation of hydrates including the grain size temperature gradient and deposition depth the distribution and depth of the reservoir were determined to be the most critical factors in the evolution of the studied hydrates

balancing the deglacial global carbon budget the hydrate factor abstract the discovery that methane from dissociation of gas hydrates could be an important factor in the global carbon cycle resolves the major discrepancy in estimates of the increase of terrestrial biomass from the last glacial period to the present carbon isotope budgeting using the marine carbon isotopic record results in an estimate centered around 500 gtc whereas palaeovegetation reconstruction with biosphere models gives averages around 1000 gtc the discrepancy may be resolved by considering release of isotopically light methane through destabilization of gas hydrates this provides a unique means of estimating the contribution of gas hydrates to the deglacial rise in atmospheric methane a release of 120 gtc methane makes a biospheric carbon transfer of 1000 gtc compatible with the marine carbon isotope data this however represents less than 30 of the enhanced atmospheric methane production between 18 and 8 ka observed in ice cores supporting the theory that glacial interglacial variations in atmospheric methane were driven primarily by changes in the extent of tropical and temperate wetlands and not by methane release from clathrates by balancing the deglacial carbon budget we demonstrate that global carbon models will have to incorporate glacial interglacial vegetation shifts of at least 1000 gtc which many currently find difficult

the beast burrowed the fluid followed crustacean burrows as methane conduits abstract an extensive pockmark field with associated methane seep carbonates has recently been reported from the late albian lower cretaceous ubidepea mudstone of the black flysch group in the basque country northern spain but the exact pathways of the migrating methane rich fluids remained elusive here we provide petrographic stable carbon and oxygen isotope evidence that abundant crustacean burrows in the surrounding mudstone preserved as the trace fossil thalassinoides and the seep carbonates themselves have acted as long lasting fluid conduits in this system the thalassinoides infill generations show a diagenetic parasequence often starting with a distinctive lining followed by one to several phases of cementing of the burrow walls and burrow infills and finally late diagenetic sparry calcite that filled the remaining open cavities and conduits the micrite cementing the burrow walls shows negative 13 c values similar to those reported from the seep carbonates indicating that methane oxidation was involved in their precipitation rather than or in addition to the oxidation of other organic matter even the late diagenetic sparry calcite shows 13 c values as low as 30 4 versus pdb providing evidence that these burrows served as fluid conduits for an extended period of time our study supports the growing body of evidence that crustacean burrows play an important role in channeling seeping fluids in such systems we anticipate that this phenomenon is much more widespread than currently appreciated especially in examples where other types of fluid conduits are absent

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

fossil seep structures of the monterey bay region and tectonic structural controls on fluid flow in an active transform margin abstract the subsurface plumbing system of ancient seeps of hydrocarbon rich gases and fluids is partially preserved by seep deposits in miocene rocks of the coast ranges and on the seafloor of the monterey bay region in central california the fossil seep related deposits are of two types authigenic carbonates and intrusions of oil saturated sandstones both types occur in fractured and faulted siliceous mudstone and porcelanite layers of the organic rich miocene santa cruz mudstone they are associated with tectonic structures indicating deformation and are located in the proximity of faults the morphology of the seep carbonates is characterized by strong asymmetries and by holes pipes and other types of fluid conduits with orientations parallel to the directions of the local fracture and fault sets thus suggesting structural control over pathways of fluid migration the dominant direction of the conduits in the seep carbonates and of the intrusive dikes northeast southwest differs from the dominant orientation of the tectonic structures of the san andreas fault system northwest southeast previous interpretations of these ancient seep related deposits suggested that lateral changes of the thickness of the cenozoic marine host sediments favored fluid migration from basin depocenters located offshore toward basin margins different driving forces for fluids have been hypothesized including hydrocarbon generation differential compaction and tectonic compression most of the fossil seeps are located on structural highs of the basement and adjacent to offshore basins that form elongated depressions oriented northwest southeast previously unidentified structures that offset the basement along northeast southwest trends cross setting faults paralleling the orientations of the conduits of the seep carbonates and the dikes of oil saturated sandstones were also recognized tectonic deformation block rotations documented by local clockwise rotations of paleomagnetic directions in the coastal ranges and cross faults were probably produced during the late miocene and pliocene to accommodate the shear stress between conjugate faults during eastward shifting of the activity of the san andreas fault system comparison with previous studies on ancient seep deposits regional geology and offshore tectonic data suggest that the fluids that sourced the seeps were driven by episodes of inverse tectonics during which previously extensional depositional basins were involved in episodes of tectonic compression tectonic deformation created cross faults in rotated blocks which could have acted as important conduits for the flow of hydrocarbon rich fluids and gases and fostered the injection of liquefied sands

formation of methane related authigenic carbonates within the bioturbated zone an example from the upwelling area off vietnam abstract in the biologically high productive area off vietnam authigenic carbonates formed in the bioturbated zone while methane was oxidized microbially under anaerobic conditions open crustacean burrows spongeliomorpha connected to the seafloor acted as conduits for methane as suggested by intense cementation of burrow walls having a light c isotope signature although seawater circulated within these open inhabited tubes conditions favorable for anaerobic oxidation of methane aom occurred trypanites cementation started at several spots that coalesced later to form the nodule carbon isotopes indicate a biogenic methane source 13 c carb reaching 49 to 40 v pdb while higher 13 c values imply mixing with carbon from other sources that is very likely within the bioturbated zone the cement is generally fine grained 18 o values of 5 1 reflect the general 18 o enriched isotope signature of the bottom water in south china sea especially during glacial times but influence of water released from decaying gas hydrate cannot be excluded the studied nodule was found on top of a slump that displaced it

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

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

glendonites from an early jurassic methane seep climate or methane indicators abstract an early jurassic late pliensbachian section margaritatus to spinatum ammonite zone northeast of cremlingen germany revealed three horizons with abundant glendonites calcite pseudomorphs after ikaite caco3 6h2o as well as authigenic carbonates these pseudomorphs have previously been used as indicators for organic and phosphate rich paleoenvironments with near freezing bottom water temperatures stable isotope measurements clearly show that the ikaite glendonite was precipitated at a methane seep location with methanotrophic sulfate reduction as the triggering geochemical process for ikaite formation at the sulfate methane interface the termination of methane seepage probably led to the disintegration of the primary mineral ikaite and successive pseudomorph formation reconstructed paleo bottom water temperatures ca 10 c from belemnites and ostracods indicate temperatures well above the previously observed range of ikaite stability near freezing raising the question if methane seeps are distinct geochemical sites promoting ikaite formation at higher temperatures