gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

the messinian salinity crisis in the dacic basin sw romania and early zanclean mediterranean eastern paratethys high sea level connection new field observations and fossil analyses complete and clarify the strong impact of the mediterranean sea level changes linked to the peak of the messinian salinity crisis on the dacic basin in southwestern romania in addition to the gilbert type fan delta already evidenced along the danube river in the area of turnu severin a new gilbert type fan delta is described northward early zanclean bottomset beds are evidenced and dated based on nannofossils at the junction of the two coalescing gilbert type fan deltas a clear sedimentological morphological and chronologic differentiation is established in the area between the carpathians late miocene piedmont alluvial fans and the early zanclean gilbert type fan deltas the early zanclean age of the hinova clays where the bottomset beds of the gilbert type fan deltas are mostly developed is confirmed by the occurrence of nannofossil markers of subzone nn12b and a bosphorian mollusk macrofauna early zanclean inflow of mediterranean marine waters into the dacic basin is also supported by the record of planktonic foraminifers in the dacic basin the messinian salinity crisis resulted in the cutting of the iron gates by a carpathians river fluvial erosion also affected the residual pannonian basin and probably catched the paleo tisza river which contributed to the erosion of the iron gates and to the fluvial drainage of the partly desiccated dacic basin arguments are reinforced in favor of a marine gateway between the mediterranean and dacic basin through the balkans before and after the messinian salinity crisis

microbial fabrics from neogene cold seep carbonates northern apennine italy abstract a number of hydrocarbon seep carbonate masses are hosted within miocene and pliocene siliciclastic units along the apennine range and sicily and are mainly composed of micritic and microsparitic textures geomicrobiological analysis of some of these deposits shows that preserved microbial processes are few and the contribution of microbe derived textures to their construction is volumetrically very limited in the carbonate groundmass of two northern apennine miocene and pliocene seep carbonate deposits uncommon and well preserved features suggest direct and indirect evidence of microbial activity some of these features are interpreted as the mineralization products of microbial textures and include complex alveolar textures and fossilized biofilms that line and fill microconduits pores and other voids similar to living bacterial aggregations recovered in modern seep carbonates other mineral structures such as spheroids and rhombs with concentric fabrics composed of calcite dolomite alternations abound as cement components and suggest a nucleation via bacterial sulfate reduction processes a similar origin is also assumed for the acicular aragonite bundles that in the miocene seep carbonate mass enclose dense clusters of millimeter size filamentous morphologies of biological origin

origin of siderite concretions in microenvironments of methanogenesis developed in a sulfate reduction zone an exception or a rule abstract the mississippian shales from wardie scotland were deposited in a large normally brackish water basin they host siderite concretions with 13c values ranging from 3 5 to 12 1 which show that sulfate reduction and methanogenesis contributed to the production of bicarbonate fossils are found within each of the concretions suggesting that the decay of these organic remains provided a catalyst for concretionary growth the pore water was freshened which is indicated by the very negative 18o values from 12 8 to 4 7 many features indicate that the concretions are very early diagenetic precompactional precipitates the concretions are mostly homogeneous only a few exhibit complex concentric zoning the microstructure of the homogeneous concretions indicates a pervasive growth however their isotopic composition reveals a clear trend towards lighter 13c and 18o values from the center to the edge it is proposed that the concretions grew due to methanogenesis in microenvironments around large organic remains which acted as nuclei we suggest that this process may be one of the reasons for the very common occurrence of cements as mineral concretions not as disseminated precipitates along beds these microenvironments developed entirely within the sulfate reduction zone which resulted in the establishment of isotopic gradients in bicarbonate dissolved in the pore water across these microenvironments these gradients were caused by fast consumption of sulfate by the intensive oxidation of organic remains this resulted in the isotopic trends preserved in the homogeneous concretions the formation of the zoned concretions proceeded generally concentrically but detailed observations suggest that pervasive growth also contributed to their formation afterwards all the concretions were deeply buried probably within the oil window where late diagenetic phases crystallized mainly kaolinite quartz and sulfides

an analytical model of hanging wall and footwall deformation at ramps on normal and thrust faults abstract we determine stress strain and structural evolution for an analytical model of moderate angle 10 20 ramps developed on both normal faults normal ramps and thrust faults thrust ramps in the model a frictionless sliding surface with ramp configuration separates two incompressible isotropic viscous half spaces of different viscosity hanging wall and footwall ramps are planar but ramp flat corners are locally rounded deformation is due solely to translation across the ramp and regional extension or shortening is ignored sliding down a normal ramp causes vertical extension and horizontal shortening in both the hanging wall and the footwall this conforms with reverse faults seen in experimental models of normal ramps the stress in the footwall has the same magnitude as that in the hanging wall but the footwall strain rate is reduced in magnitude by the ratio both a hanging wall syncline and a footwall anticline form the normal ramp migrates in front of the rising footwall at a rate v ss v where v is the sliding velocity but the ramp does not change shape the continuous ramp motion in the model may correspond to the creation of an extensional duplex or chaos zone in brittle rocks at a normal ramp mean compressive stress is lowered and deviatoric stress is raised favoring local faulting and fracture sliding up a thrust ramp causes stresses strain rates and strains of the opposite sign horizontal extension and vertical shortening in both walls is consistent with normal faulting both a hanging wall anticline and a footwall syncline form passively although the deviatoric stress is raised at the ramp so is the mean compressive stress inhibiting fracturing however the mean compressive stress is lowered on the adjacent flats favoring fracturing there

petrology chemistry and diagenesis of calcite concretions in silurian shales from central sweden calcite concretions in the lower silurian rastrites shale from central sweden were formed within 1 m depth of the seafloor micrite 1 4 m was formed initially but recrystallized into microspar 4 50 m and subsequently transformed into pseudospar 50 150 m skeletal calcite in the concretions and in the host shale has been replaced by microspar which is chemically similar to that occurring in the concretion body the 13cpdb values mostly between 2 and 2 suggest that carbonates were mainly derived from seawater some of the carbon within calcite was derived from the oxidation of organic matter recrystallization has resulted in increasingly negative 18 opdb values 7 to 4 3 whereas the 13c was not changed significantly

clathrites archives of near seafloor pore fluid evolution δ44 40ca δ13c δ18o in gas hydrate environments aragonitic clathrites are methane derived precipitates that are found at sites of massive near seafloor gas hydrate clathrate accumulations at the summit of southern hydrate ridge cascadia margin these platy carbonate precipitates form inside or in proximity to gas hydrate which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation the clathrites record the preferential incorporation of 18o into the hydrate structure and hence the enrichment of 16o in the surrounding brine we measured 18o values as high as 2 27 relative to peedee belemnite that correspond to a fluid composition of 1 18 relative to standard mean ocean water the same trend can be observed in ca isotopes ongoing clathrite precipitation causes enrichment of the 44ca in the fluid and hence in the carbonates carbon isotopes confirm a methane source for the carbonates our triple stable isotope approach that uses the three main components of carbonates ca c o provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites this approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation thus recording the evolution of the geochemical environment of gas hydrate systems

the central paratethys during oligocene as an ancient counterpart of the present day black sea unique records from the coccolith limestones abstract four isochronous oligocene coccolith limestone horizons from the carpathians were examined in order to reconstruct paleoceanographic conditions in the central paratethys the dominance of small and size uniform pyrite framboids the occurrence of low diversity dinoflagellate cysts and coccolithophorids and the presence of biomarker molecule 28 30 dinorhopane indicate that the water column was stratified with the upper water column being relatively well oxygenated but the bottom water being anoxic the latter is confirmed by the fine and consistent horizontal laminations in various parts of the basin scarcity of benthic organisms and their trace fossils the limestones exhibit typical marine 13c values but are significantly depleted in 18o and enriched in 87sr relative to contemporaneous ocean water these isotopic compositions result from a decreased salinity of the surface waters caused by an increased riverine input this is confirmed by the lack or impoverishment of planktonic foraminifers presence to abundance of goniodomid dinoflagellate cysts and massive occurrence of low diversity nannoplankton assemblages which indicates decreased salinities as low as 17 and high productivity in the upper water column these observations indicate that the limestones were formed during periods when connection of the central paratethys with the global ocean was limited which impeded water exchange causing the development of low salinity conditions of surface water and bottom water anoxia during the deposition of the oldest tylawa horizon primary productivity was enhanced and chemocline was positioned exceptionally high in the water column moreover decreased 13c values in both carbonates and organic matter of this horizon suggest that widespread methane venting took place in the basin during np23 all these data show that during the oligocene the central paratethys experienced similar conditions to those of the current black sea which can be used as a modern analogue especially for the tylawa horizon therefore the tylawa horizon can be perceived as a potential effect of future post depositional processes of coccolith marls analogous to those having been deposited in the black sea for 2 7 kyr

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

variability in fault size scaling due to rock strength heterogeneity a finite element investigation abstract practical application of fault size scaling relationships usually involves extrapolation of a limited data set over a scale range beyond that observed or into adjacent unstudied areas here we investigate the validity of such extrapolations by considering the variability in the size frequency distributions of fault populations that develop under identical tectonic conditions using a numerical model to generate conjugate normal fault populations in cross section the deforming material is modelled using a strain softening von mises rheology with gaussian heterogeneity in yield strength distributed randomly throughout the mesh we present eight deformation experiments that differ only in the random spatial pattern of yield strengths we observe power law size frequency scaling i e n ax c where n is the cumulative number of faults and x is a measure of fault size but with a range of values of c the ensemble average value of c decreases with increasing percentage extension however for individual model runs the dependence of c on total strain shows significant variability that we can relate to small but important differences in fault growth and strain localisation at any particular strain the range of values of c is 10 times greater than the error estimate derived from least squares regression of the cumulative frequency data our results suggest therefore that large uncertainties should be associated with extrapolating fault population data from one scale or region to another even if the lithology and tectonic history are similar

the mediterranean mare nostrum of earth sciences the mediterranean area is one of the most appealing natural laboratories in the world to study geodynamic and paleoclimatic processes on different scales consequently the mediterranean sea can be considered as the mare nostrum our sea of earth sciences its semi enclosed land locked configuration in a convergent setting between africa in europe in combination with its latitudinal position makes the mediterranean extremely suitable to study both fundamental plate tectonic processes and astronomically induced oscillations in climate for many years to come the mediterranean will certainly remain a fascinating area where an integrated and multi disciplinary approach will increasingly contribute to our understanding of geological geophysical geodetical and geochemical processes in an accurate and high resolution time frame

free hydrocarbon gas gas hydrate and authigenic minerals in chemosynthetic communities of the northern gulf of mexico continental slope relation to microbial processes research submersibles and piston cores were used to sample two chemosynthetic communities in the gulf of mexico continental slope at 540 m water depth vent gas from the deep subsurface is the starting material from which other carbon pools are derived including gas hydrate free hydrocarbon gas in sediment and authigenic carbonate rock gas crystallizes as exposed mounds of structure ii gas hydrate and as massive vein fillings in hemipelagic mud venting rates from gas hydrate mounds periodically increase after the temperature of the bottom water increases gas hydrate decomposition is largely restricted to exposed hydrate and at shallow depth in sediment overall gas hydrate is accumulating not decomposing at study sites most free gas in sediment appears to be destroyed in situ by anaerobic microbial oxidation in chemosynthetic communities leading to sequestration of carbon as abundant authigenic carbonate rock depleted in 13c free methane is rapidly oxidized leaving residual methane enriched in 13c some in situ microbial co2 reduction occurs and methane depleted in 13c mixes with vent methane enriched in 13c the c2 c5 hydrocarbons of vent gas initially most depleted in 13c ethane isobutane isopentane are least affected by microbial oxidation whereas hydrocarbons initially enriched in 13c propane normal butane normal pentane are most affected anaerobic microbial oxidation of all c1 c5 hydrocarbon gases not only methane may be significant in chemosynthetic communities microbial processes contribute to the development and stability of chemosynthetic communities by providing required h2s anaerobic microbial processes lead to deposition of diagnostic authigenic minerals related to the carbon carbonate minerals and sulfur cycles pyrite elemental sulfur altering the seafloor seafloor cementation favors fauna such as chemosynthetic tubeworms and seep mussels by forming carbonate hardgrounds on an otherwise unfavorable mud dominated seafloor

carbon and oxygen isotope analysis of hydrate associated oligocene authigenic carbonates using nanosims and irms abstract petrography and stable c and o isotope compositions of methane derived carbonates from the polish outer carpathians were analyzed two methods were employed in isotopic determinations conventional irms measurements on powdered bulk samples were performed with high precision errors 0 06 for 13c and 0 08 for 18o 1 whilst in situ measurements were made using nanosims with a high spatial resolution 5 m but with low precision errors 3 0 for 13c and 2 3 for 18o 2 the nanosims study proved very useful in identifying the sources of the fluids precipitating the carbonates they were formed predominantly by the anaerobic oxidation of biogenic methane aom the very fine spatial resolution of the analyses revealed highly variable 13c values even in individual crystals from extremely negative related to aom to positive related to methanogenesis me other methods including laser ablation and standard sims would fail to detect this variability and would not indicate the true sources of the fluids because they would show average values therefore any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the 13c values are extreme i e resulting either from aom or me unusual clast like druses from the carbonates were analyzed in detail with nanosims they are filled with strongly 13c depleted down to 54 and 18o enriched up to 6 fringe calcite which precipitated on the walls of the druses from heavy oxygen containing water released from the dissociation of gas hydrates the rocks can thus be termed clathrites the bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks precipitation of the carbonate cements started when hydrates were still present so they are a type of melt and seal fabric the recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane derived carbonates the isotopic results have demonstrated that nanosims can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements this provides a basis for the application of nanosims to tracing diagenetic pathways from various even complexly formed authigenic precipitates

eustatic sea level changes interpreted from seismic stratigraphy a critique of the methodology with particular reference to the north sea jurassic record seismic stratigraphic methods for interpreting eustatic sea level change are based on generalized stratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning onlap and offlap seismic models may not indicate sea level change but flexural subsidence facies shifts or changes in sea level in the opposite direction to that proposed resolution of some of these problems may be achieved by publication of basic data now in confidential industry files the widely acclaimed vail curves should not be used as templates for basin correlation until such data have been provided and such use should be supported by appropriate conventional lithostratigraphic and biostratigraphic documentation from each new basin where such data have been made available as with the north sea jurassic serious questions of interpretation are raised

carbon isotopic evidence of methane oxidation through sulfate reduction in sediment beneath cold seep vents on the seafloor at nankai trough during the kaiko tokai project we took a total of 15 near surface sediment push cores from inside and outside seafloor clam colonies and bacterial mats in the nankai trough in this study we present the chemical and isotopic composition of pore fluids extracted from the sampled cores with special emphasis on those taken at the ryuyo canyon and discuss a the chemical relation between the seeping fluid and biological communities and b the origin of cold seep fluids compared with bottom seawater up to 5 depletion of chloride concentration extremely high ch4 concentration of more than 300 mol kg and slight dh2o depletion are observed in pore fluids at the ryuyo canyon which suggests that the ryuyo canyon is an active seep zone of low chloride ch4 rich low dh2o fluid from the sea bottom remarkable depletion of sulfate and significant enrichment of dissolved inorganic carbon and of hydrogen sulfide are also observed in the pore fluids particularly those taken within bacterial mats while the carbon isotopic composition of pore fluid dissolved methane 13cch4 shows large heterogeneity up to 30 in and around the vents these chemical and isotopic characteristics might be due to active anaerobic methane oxidation and hydrogen sulfide production methane oxidation through sulfate reduction in the sedimentary layer just below within 30 cm the vents on the seafloor the carbon isotopic composition of methane can be considered direct evidence of the mediation process between methane enriched cold seep fluid and the h2s dependent biological community on the cold seep vent after correcting for the effect of methane oxidation at the sediment surface we estimate the end member carbon isotopic composition of methane as less than 90 pdb and the end member ch4 c2h6 ratio as more than 103 which suggests a microbial shallow layer is the origin of methane in the fluid

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

pore water evolution during sediment burial from isotopic and mineral chemistry of calcite dolomite and siderite concretions abstract coal measures often contain concretions segregations of diagenetic minerals originally formed within unconsolidated sediments three different types calcite pyrite dolomite pyrite and siderite occurring spatially quite close together in the central pennine region of england vary widely in carbon isotope composition 10 35 13c 21 49 and in major cation chemistry ca mg fe mn within some siderite concretions very high mn fe ratios were found in central subsamples these were also most enriched in 13c the fe mg ratio decreases systematically from centre to edge early shallow to deeper later precipitation the calcite pyrite and dolomite pyrite concretions developed completely prior to significant burial both have high mn fe ratios but negative 13c values calcite 21 49 dolomite 8 67 to 10 48 all of these patterns can be equated precisely with theories of pore water evolution developed on the basis of geochemical investigations of modem sediments microbial processes sulphate reduction methanogenesis contributed significantly as did thermal decarboxylation to siderite precipitated at considerable burial depth mn iv and fe iii acted differentially as oxidants producing co2 and increasing alkalinity the interplay of fresh and marine depositional waters is seen most obviously in the presence or absence of sulphate reduction this controlled mineral type iron sulphide or carbonate as well as isotopic and mineral chemistry

decoding the mediterranean salinity crisis this historical narrative traces the steps to unravel over a span of 40 years an extraordinary event in which 5 of the dissolved salt of the oceans of the world was extracted in a fraction of a million years to form a deposit more than 1 million km 3 in volume a buried abyssal salt layer was identified with reflection profiling and sampled during the deep sea drilling project leg 13 the dolomite gypsum anhydrite and halite in the drill cores paint a surprising picture of a mediterranean desert lying more than 3 km below the atlantic ocean with brine pools that shrank and expanded by the evaporative power of the sun the desert drowned suddenly when the gibraltar barrier gave way the explanation of deep basin shallow water desiccation and the notion of a catastrophic zanclean flood had a mixed reception however the hypothesis became broadly accepted following subsequent drilling expeditions nevertheless as experts examined the evaporate facies and sequences of equivalent age in the terrestrial outcrops weaknesses appeared in the concept of repeated flooding and drying to account for the magnitude of the deposits the rosetta stone used to decipher conflicting interpretations turns out not to be the deposits but the erosion surfaces that enclose them these surfaces and their detritus formed in response to the drop in base level during evaporative drawdown extend to the basin floor evaporative drawdown began halfway through the salinity crisis when influx from the atlantic no longer kept up with evaporation prior to that time a million years passed as a sea of brine concentrated towards halite precipitation during the later part of this interval more than 14 cyclic beds of gypsum accumulated along shallow margins modulated by orbital forcing the thick salt on the deep seabed precipitated in just the next few cycles when drawdown commenced and the brine volume shrank upon closure of the atlantic spillway the remnants of the briny sea transformed into salt pans and endorheic lakes fed from watersheds of eurasia and africa the revised model of evaporative concentration now has shallow margin shallow water and deep basin deep water precursors to desiccation

in situ oxygen isotope micro analysis of faunal material and human teeth using a shrimp ii a new tool for palaeo ecology and archaeology a sensitive high resolution ion microprobe shrimp ii has been used to make high spatial resolution in situ micro analyses of oxygen isotopes in fish otoliths and teeth from fossil herbivores and a neanderthal large intra tooth variations in the oxygen isotopic composition up to 9 were observed in the enamel of herbivores from the neanderthal fossil site of payre consistent with preservation of seasonal cyclicity the range of isotopic compositions observed in neanderthal tooth enamel was much smaller w3 possibly the result of a longer enamel maturation time averaging out variability an archaeological otolith from a preceramic site in northern peru exhibited marked changes in d 18 o over life due either to the fish occasionally migrating from the sea to a lower salinity habitat or to short lived rises in sea water temperature a fish otolith from australia s willandra lakes world heritage area showed clear seasonal variations but also a general trend towards isotopically heavier and more saline water as indicated by higher d 18 o and sr ca values resulting from increased evaporation the results of these case studies are compared to results of oxygen isotope analysis using more conventional methods and demonstrate the ability of the shrimp ii to provide precise high spatial resolution in situ oxygen isotope analyses of a variety of biogenic materials this approach has major advantages over conventional methods it can provide rapid micro scale isotopic analyses of sub permil precision without the need for chemical preparation of the sample

source rock habitat and hydrocarbon potential of oligocene menilite formation flysch carpathians southeast poland an organic geochemical and isotope approach geochemical and isotope methods have been used to analyse the hydrocarbon potential and petroleum generation characteristics of the oligocene menilite formation the most important petroleum source rock in the overthrust part of the carpathian foreland basin in se poland the data show a large variability in quantity up to 18 toc quality type ii and ii iii and carbon isotope composition of the organic matter in samples of different lithology and from different stratigraphic and tectonic settings these differences reflect distinct sedimentary and tectonic histories of individual sub basins the maturity in the outer tectonic units is very low tmax 420 c ro 0 35 but increases towards the inner nappes and also laterally to the se the variability of the kinetic parameters indicates the likely occurrence of multiphase hydrocarbon generation the menilite formation forms a very good source rock in the complex petroleum system of the carpathian overthrust belt

fluid sediment interactions at eastern mediterranean mud volcanoes a stable isotope study from odp leg 160 abstract pore fluids from two odp sites at eastern mediterranean mud volcanoes have been analyzed for their cl concentration and their 18o and d isotopic composition the cl data span a wide range of concentrations from extremely depleted with respect to seawater as low as 60 mm at the crest of milano dome site 970 to strongly enriched up to 5 4 m at napoli dome site 971 chloride enrichment is known to be due to dissolving messinian evaporites whereas the source of the low cl fluid is deduced from stable isotope data presented here the isotopic composition of the endmember fluid is found to be 10 for 18o and 32 for d for low as well as for high cl waters from this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water several other processes altering the 18o d composition of pore waters are discussed and considered to be of only negligible influence the process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration mainly smectite illite transformation corresponding to a depth range of 3 5 7 km and an elevated temperature of about 120 165 c a quantitative estimate shows that this reaction is capable of producing the observed extreme cl depletion

the messinian salinity crisis in mallorca new insights for a western mediterranean stratigraphic scenario abstract the messinian salinity crisis msc was an extraordinary geological event that affected the whole mediterranean region as well as global marine circulation between 5 97 and 5 33 ma in the mediterranean the crisis was mainly characterized by isolation from the atlantic ocean resulting in a new hydrological budget leading to the accumulation of large amounts of evaporites and large scale erosion of its margins the nature of the majority of the evaporites and erosional surfaces is still unknown thus representing a challenge which forms the subject of debate in the scientific community the neogene mallorca basins balearic islands are well situated to provide a stratigraphic record which expresses the sequence of events related to the msc being especially suitable for testing possible geological models or scenarios new research on the island of mallorca provides data about the msc and zanclean reflooding in the central part of the western mediterranean balearic basin with obvious repercussions for both academia and industry the main aim of this study is to describe the features which characterize sedimentation during the miocene pliocene transition on the island of mallorca to establish how this stratigraphic record relates to the different proposed msc scenarios for the whole of the mediterranean in this regard a stratigraphic analysis was carried out on a total of 100 exposures and boreholes corresponding to the neogene basins of mallorca the results indicate that on the island of mallorca the development of the msc is most consistent with models that represent the main phase of desiccation occurring after the accumulation of marginal evaporites primary lower gypsum the terminal carbonate complex and the lago mare deposits

multiproxy approach for determination of nature and origin of carbonate micro particles so called micarb in pelagic sediments abstract carbonate fractions of pelagic and hemipelagic sediments may contain significant proportions of microcrystalline 6 m calcitic particles with no specific structure so called micarb depending on the authors this term is related to different descriptions and yet there is no clear consensus on their origin although micarb can dominate the sediment compositions in the pelagic realm their isolation from the other carbonate particles is not easy due to their small size the use of a granulometric separation method of the main carbonate particles allows the characterization and quantification of the compositions of the carbonate phases of the sediments and gives access to the geochemical signatures oxygen and carbon stable isotopes and mineralogical characteristics of each type of carbonate particles in the sediments this approach was applied to three sections the mid pliocene punta piccola section south sicily the upper campanian bidart section sw france and the aptian sediments from la marcouline quarry se france the quantifications showed quite important proportions up to 70 of the carbonated phases of micarb in the sections studied excepted in the mid pliocene punta piccola samples however depending on the sedimentary context it appears that those fine calcitic particles may result from fine biogenic fragmentation aptian la marcouline quarry authigenic precipitation in the water column mid pliocene punta piccola section or they may have an allochthonous origin upper campanian bidart section based on these interpretations it is then possible to estimate precisely the composition of the carbonate fractions in each studied section this work shows that except for the bidart sediments which are dominated by fine exported carbonate mud the other sections mainly consist in biogenic carbonates

messinian event in the black sea evidence of a messinian erosional surface in 1975 sediment cores from leg dsdp 42b sites 380a and 381 revealed a thin sediment layer in the black sea basin which points to a shallow water environment at the miocene pliocene boundary with these facts and in the wake of hypothesis of the messinian salinity crisis msc it was proposed that the black sea like the mediterranean sea suffered a desiccation period at the end of the messinian hsu k j and giovanoli f 1979 messinian event in the black sea palaeogeography palaeoclimatology palaeoecology 29 75 93 whereas the main topics of the msc in the mediterranean sea is now widely accepted the lack of evidence for a messinian erosional surface in the black sea left the debate about the messinian desiccation of this basin open until today the analysis of high resolution multi channel seismic data acquired during the blason surveys brings important new elements for this scientific debate 1 down the slope offshore the bosporus we show a clear erosional surface correlated to the top of the upper miocene shallow water environment unit of site dsdp 381 the overlying lower zanclean unit inevitably dates this erosional surface of the messinian event 2 a wide intra pontian erosional surface ipu is evidenced on the romanian shelf the ipu is characterized by a sharp decrease in the incision rate from outer deep canyons to inner shelf superficial incisions network according to the most recent paratethyan and mediterranean stratigraphic scale correlations the ipu erosional surface is considered as the analogue to the messinian erosional surface described down the slope offshore the bosporus in addition to recently discovered inland erosional signature the wide regional erosional surface we underline on the western black sea margins validates the black sea messinian desiccation hypothesis we also demonstrate that evaporative draw down of the black sea implied the installation in the basin of a negative hydrologic budget during the messinian the lack of a major messinian danube canyon on the romanian shelf supports the hypothesis of a messinian danube trapped in the dacic basin however the presence of a messinian superficial incision network connecting the location of the modern danube delta to deeply incised messinian canyons on the outer shelf ipu makes the hypothesis of a danube reaching the partially desiccated black sea still possible

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

messinian deposits and erosion in northern tunisia inferences on strait of sicily during the messinian salinity crisis outcrops offshore wells electric logs and seismic profiles from northern tunisia provide an opportunity to decipher the messinian salinity crisis in the strait of sicily messinian deposits including gypsum beds near the tellian range reveal two successive subaerial erosional surfaces overlain by breccias and marine zanclean clays respectively in the gulf of tunis messinian thick evaporites mostly halite are strongly eroded by a fluvial canyon infilled with zanclean clays the first erosional phase is referred to the intra messinian tectonic phase and is analogous to that found in sicily the second phase corresponds to the messinian erosional surface that postdates the marginal evaporites to which the entire sicilian evaporitic series must refer the western and eastern mediterranean basins were separated during deposition of the central evaporites

effects of pressure solution and fluid migration on initiation of shear zones and faults abstract constitutive relations for an isotropic poro power law fluid are proposed in which the effective viscosities in both shear and dilation have the same power law dependence on the second invariant of the deviatoric stress alone but the viscosity in dilation is 1 times the viscosity in shear the shear rate is otherwise proportional to the deviatoric stress and the dilation rate to an effective mean stress the behavior may either involve darcy flow of a pore fluid whose viscosity is much less than that of the solid matrix or involve dissolution diffusive transport and precipitation of a soluble phase as in pressure solution linearized relations for small additional rate of deformation and stress superposed on a basic state flow are obtained these are used to treat the deformation due to a shallow narrow indentation at the interface between a layer and an adjacent medium when both undergo bulk uniform layer parallel shortening or extension if the layer the medium or both are nonlinear fluids with large stress exponent shear band like structures extending from the indentation and reflecting from any weak slipping interface occur in both the instantaneous velocity field and in the strain field since the materials considered have constant and uniform properties e g they do not exhibit strain softening these features leave no imprint in the medium they persist over mean strains of 10 20 but are like the indentation ephemeral it is hypothesized that localization due to an interfacial irregularity of this sort would produce bona fide shear bands if the material were strain softening the development of shear bands is enhanced if the adjacent medium has a large stress exponent or if it is a poroviscous fluid with low resistance to dilation in a poroviscous medium dilation is concentrated in paired positive and negative lobes at the terminations of shear bands in the adjacent layer the development of shear bands in a poroplastic layer is mildly suppressed relative to that in an incompressible layer dilation is concentrated in paired loci at shear band terminations with weaker paired bands of positive and negative dilation on either side of the shear zone

radiometric dating of submarine hydrocarbon seeps in the gulf of mexico massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid bathymodiolus sp containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and u series isotopes stable carbon isotopes and 87 sr 86 sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon rich fluids samples from five seepage sites on the northern gulf of mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to 2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives in general paired 14 c and 230 th dates of carbonate buildups consisting of aragonite high mg calcite and dolomite mineralogies are highly discordant the cause of the discordance established on the basis of paired 14 c and 13 c values 14 c 898 to 992 13 c 9 5 to 53 3 for n 27 lies with the impairment of the radiocarbon dates resulting from dilution of the 14 c pool with fossil hydrocarbon derived carbon the validity of the ionium dates based on u rich 2 4 7 6 ppm samples is demonstrated by the concordance between 234 u 238 u and 230 th 234 u evolution in time and by the 234 u 238 u o activity ratios that are generally within the range of sea water value of 1 14 0 04 2 sigma some dolomite rich samples are exceptional because their 234 u 238 u o ratios are significantly higher 1 22 1 38 than sea water suggesting deposition from anoxic pore waters where the soluble u reached anomalous 234 u 238 u ratios the formation of the carbonates from sea water derived fluids rather than from formation fluids advecting from deep aquifers is supported by the 87 sr 86 sr composition of the samples mean 0 709145 19 10 6 n 14 that compares well with modern nonseep marine carbonates and the ambient gulf of mexico sea water 0 709171 8 10 6 calcareous shells the 13 c values of which indicate a carbon source in sea water 13 c 4 3 to 1 1 yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates isotope migration attested by the observed u uptake in the fossil shells is likely to affect the accuracy of their ionium dates radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late pleistocene time 195 13 ka ages derived from nascent seep sites at mid bathyal and abyssal depths 12 3 0 0 ka indicate that currently vigorous seepage was initiated at the end of the last deglaciation these radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor

geological imprint of methane seepage on the seabed and biota of the convergent hikurangi margin new zealand box core and grab carbonate results abstract short box cores to 30 cm bsf and seafloor carbonate grab samples were acquired at mapped hydrocarbon seep sites 600 1200 m water depths during the 2007 rv sonne so191 cruise on the hikurangi margin offshore eastern north island new zealand to evaluate the influence of methane seepage on sedimentologic biotic mineralogic and stable isotopic attributes of seabed sediments sedimentary horizons in the box cores consist of siliciclastic silts and sands shell beds and nodular microcrystalline aragonite bands up to 15 cm thick the megafauna is dominated by infaunal to semi infaunal chemosymbiotic bivalves calyptogena lucinoma and acharax as well as associated worms and carnivorous and grazing gastropods burrows in silts some occupied by worms or juvenile acharax mainly have simple morphologies more typical of high energy nearshore settings than deep sea environments while a few are large and sparsely branched with wall scratch marks inferred to be of decapod crustacean origin the box core silts and nodular carbonate samples vary in toc content from 0 2 to 0 9 wt carbonate content from 4 to 78 and 13 c and 18 o values from 50 3 to 0 6 pdb and 0 77 to 3 2 pdb respectively low carbonate content silt samples have the most enriched 13 c values implying a seawater source for their pore water bicarbonate negative 13 c and positive 18 o values typify the nodular microcrystalline aragonite bands indicating formation during microbially mediated sulphate dependent anaerobic oxidation of methane aom in a cold near seafloor environment as is also supported by lipid biomarker data a clear isotopic mixing trend of decreasing 13 c and increasing 18 o and carbonate content in the fine seafloor grab carbonate samples show greater textural variability and are divided into two types fresh appearing grey to white blocks yield similar mineralogic isotopic and biomarker signatures to the nodular carbonate bands in the cores a largely seep related epifauna affiliated with these grab samples include chemosymbiotic bathymodiolin mussels siboglinid tube worms methanotrophic suberitid sponges and grazing limpets which in places are entombed within the carbonate by contrast some dark reddish to brown iron stained microcrystalline dolomitic slabs and tubular concretions support abundant non seep epifaunal encrusters from the surrounding deep sea indicative of long exposure on the seafloor distinct stable isotopic signatures of these exhumed dolostones 13 c c 20 pdb 18 o up to 7 pdb suggest derivation from an oxidised methane pool and pore fluid freshening by gas hydrate dissociation or from mobilized burial fluids that have undergone clay mineral dehydration

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

cold seep systems in the south china sea an overview abstract three decades after the discovery of cold seep systems various sites of hydrocarbon seepage have been found in the south china sea scs over the past decade these sites have become model systems for understanding the variability of hydrocarbon seepage and associated biogeochemical processes in this review we describe the cold seep systems of the scs with an emphasis on seafloor manifestations fluid sources biogeochemical processes and macroecology seafloor features associated with seeps include mud volcanoes pockmarks and carbonate deposits a common characteristic of cold seeps is the occurrence of authigenic i e in situ precipitated carbonate minerals these carbonates commonly exhibit low 13c and high 18o values suggesting the incorporation of methane derived carbon and oxygen derived from gas hydrate water biogeochemical processes such as sulfate driven anaerobic oxidation of methane sd aom the key process at seeps have been studied in detail with the aim of establishing geochemical proxies to trace these processes into the geological past we also detail the features characterizing seep ecosystems understanding the impact of decomposing methane hydrate on the marine carbon budget remains challenging and requires additional seafloor observations as well as models predicting how gas hydrate responds to changing conditions such as temperature increase sea level rise and episodic mass wasting

insights into the velocity dependent geometry and internal strain in accretionary wedges from analogue models although the brittle material in analogue models is characterized by a linear navier coulomb behaviour and rate independent deformation the geometry and style of deformation in accretionary wedges is sensitive to shortening velocity in this study we have constructed a series of analogue models with various shortening velocities in order to study the influence of shortening velocity on the geometry and kinematics of accretionary wedges model results illustrate how shortening velocity has an important influence on the geometry and kinematics of the resulting wedge in general for models having similar bulk shortening the accretionary wedges with higher velocities of shortening are roughly steeper higher and longer as well as having larger critical wedge angles and height it accommodates a number of foreland vergent thrusts larger fault spacing and displacement rates than those of low to medium velocity shortening which indicates a weak velocity dependence in geometry of the wedge moreover models with a high velocity of shortening undergo larger amounts of volumetric strain and total layer parallel shortening than models with low to medium velocity shortening the former accommodate a greater development of back thrusts and asymmetric structures a backwards to forwards style of wedge growth therefore occurs in the frontal zone under high velocity shortening

insight into the silicate and organic reservoirs of the comet forming region abstract cometary interplanetary dust particles idps collected in earth s stratosphere currently represent the best way to sample outer solar system primordial dust the fine grained sub m minerals of idps show some strong similarities to the textures expected for primary condensates from the solar nebula in this study we have analysed a set of idps for combined bulk carbon nitrogen and hydrogen isotopes and high precision oxygen isotopes by nanosims 50l this study concentrates on combining isotopic analyses including high precision oxygen on the silicate and organic components within the same sample on fine grained primordial materials oxygen isotope analyses reveal that some idps are more 16 o rich than any bulk meteorite compositions extending to o isotope compositions in between chondritic and solar like values 17 o 20 18 o 20 the 16 o rich idps display more primitive organic signatures than the chondritic like 16 o poor idps but rather interestingly they also have lower presolar grain abundances the 16 o poor signatures probably indicate an abundant component of chondritic like material processed in the inner protoplanetary disk in order to explain the association of relatively processed material silicates and organics with high presolar grain abundances we propose a model whereby the initial dust component is 16 o rich solar like and originally present homogenously throughout the protoplanetary disk outward radial transport potentially associated with aerodynamic sorting led to an influx of processed silicates from the inner solar system with chondritic like isotopic signatures comet accretion that occurred late or at smaller heliocentric distance included higher abundances of this chondritic like component and hence had more 16 o poor o isotope compositions presolar grains whose extreme isotopic signatures were destroyed during early homogenisation of solar nebula dust continue to accrete onto the protoplanetary disk with time such that the later formed comets those with a higher proportion of processed silicates within their source also had time to accumulate a higher abundance of presolar grains a similar pattern is observed in the organics with decreasing d and c h from increased exchange with nebula gas in idps that contain more 16 o poor chondritic like material

erosional processes and paleo environmental changes in the western gulf of lions sw france during the messinian salinity crisis abstract current interpretation of the messinian salinity crisis msc involves partial desiccation of the mediterranean sea coupled with the deposition of thick evaporites in the deep basins new sets of seismic reflection profiles in the western part of the gulf of lions confirm the basinward extension of the messinian erosion and enable the mapping of distinctive seismic markers indicating the messinian erosional surface or messinian unconformity the basin margin detrital deposits and the deep evaporite sequence the geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the msc provide new information about the evolution of the study area during the messinian the messinian erosional surface mes commonly correlated with the desiccation phase and the deposition of deep evaporites during the apogee of the event is generally interpreted as a subaerial feature in the gulf of lions it is a complex diachronic polygenic erosional surface observed at the base of the prograding plio quaternary sequence beneath the shelf and slope it extends downslope beneath the deep basin upper evaporites and the salt and possibly correlates conformably with the base of the so called deep lower evaporites the whole morphology of the mes reflects a buried drainage pattern supporting the interpretation of fluvial erosion driven by a substantial drop in sea level our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of salt in the basin and prior to the upper evaporites consequently interbedded clastic deposits may partly account for the parallel reflectors of the lower evaporites since river erosion persisted throughout the msc the salt and upper evaporite units may also contain a large amount of detrital sediments the good quality of the new seismic data clearly reveals fan shaped messinian deposits in the downstream part of the main messinian valleys i e the nile var and spanish rivers the depositional scenarios generally involve a substantial sea level fall coupled with deltaic prodeltaic accumulations a chaotic seismic unit unit d filling messinian lows and extending beneath the salt within the study area is interpreted as a messinian clastic unit we propose a polyphase scenario of detrital fan deposition involving pre syn and post salt deposition in subaqueous subaerial environments in the gulf of lions a late miocene tectonic phase that affected the western shelf also played an important role in controlling a the pattern of the messinian fluvial network b the location of maximum erosion on the shelf and c the location of the detrital fan depocentre downslope

authigenic dolomites in the eocene oligocene organic carbon rich shales from the polish outer carpathians evidence of past gas production and possible gas hydrate formation in the silesian basin abstract eocene oligocene dolomite concretions and beds from the grybow and dukla units of the polish outer carpathians were studied these rocks occur in the organic carbon rich marine and fine grained deposits of hemipelagic or turbiditic origin mineralogic elemental and stable c and o isotopic composition of the dolomites was determined results indicate that the rocks were formed by precipitation of predominantly fe rich dolomite cement close to the sediment water interface prior to significant compaction the main source of bicarbonate for dolomite formation was bacterial methanogenesis as evidenced by the high 13c values up to 16 6 the main source of alkalinity was probably weathering of silicate minerals which might have also liberated ca and mg ions for the dolomites to form the distribution of these dolomites indicates that microbial methane production was widespread in the silesian basin moreover formation of some dolomites in the eastern part of the dukla unit was probably associated with gas hydrates as suggested by the elemental and oxygen isotopic composition of dolomitic matrix therefore the dolomites may serve as a proxy of areas where biogenic methane was produced where the rocks had high hydrocarbon potential and where hydrates could have existed detailed mineralogic and petrographic analyses allowed for the reconstruction of the diagenetic sequence and the evolution of pore fluids textural relationships between successive cement generations indicate that the central parts of the composite dolomite crystals experienced corrosion and that the latest ankerite cement filled the secondary intragranular cavities within those crystals this observation shows that reconstructions of pore fluid evolution based on core to rim analyses of such composite crystals may lead to wrong interpretations septarian cracks developed in the dolomites are often filled with multistage cements the earliest generations are ferroan dolomite and ankerite cements which precipitated within the cracks simultaneously to the ferroan dolomite and ankerite cements from the matrix of the dolomitic rocks which shows that septarian cracking occurred very early during the final stages of concretionary formation these cements were followed by the late diagenetic precipitates mainly quartz kaolinite and blocky calcite this calcite is commonly associated with bitumen which shows that it precipitated during or after oil migration in the decarboxylation zone

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

geochemical paleogradient in pore waters controlled by aom recorded in an oligocene laminated limestone from the outer carpathians abstract a 20 cm thick laminated limestone of oligocene age from the outer carpathians is composed mainly of coccoliths and authigenic calcite 13 c values unequivocally indicate that the authigenic calcite precipitated as a result of methane oxidation the 13 c and 18 o values change gradually from the base to the top of the bed from 38 0 to 17 6 and from 1 1 to 5 7 respectively this trend reflects a geochemical gradient which developed in the zone of anaerobic oxidation of methane aom due to the consumption of sulfate by methane this gradient was related to an inverse relationship between the concentrations of both compounds in the pore waters of the aom zone methane was the source of isotopically light carbon it was accompanied by heavy oxygen containing water liberated from decomposition of gas hydrates the methane bearing fluids migrated upwards to the base of the sulfate reduction sr zone where methane was oxidized ch 4 so 4 2 hco 3 hs h 2 o and the aom zone developed the bicarbonate produced in this reaction was significantly depleted in 13 c and 16 o in the sr zone organic matter was oxidized by sulfate 2ch 2 o so 4 2 h 2 s 2hco 3 the bicarbonate released in that reaction was the source of isotopically heavier c and lighter o the proportion of bicarbonate produced by oxidation of methane to bicarbonate produced by oxidation of organic matter decreased upwards the resultant isotopic composition of bicarbonate dissolved in the pore waters changed upwards towards higher 13 c and lower 18 o values and has been recorded in the limestone s isotopic composition

mediterranean sea level variations during the messinian salinity crisis 1 the mediterranean basin has not always been connected to the atlantic ocean during the messinian salinity crisis msc the mediterranean sea became progressively isolated by a complex combination of tectonic and glacio eustatic processes when isolated the mediterranean water level depends on the hydrological flux and is expected to vary significantly the amplitude and number of large water level fluctuations in the isolated mediterranean is still controversial despite numerous geological investigations the observation of 3 5 surfaces of erosion in the nile delta eastern basin provides new elements for understanding the dynamics of the msc our model demonstrates that numerous water level falls of short duration may explain the preservation of a discontinuous river profile at 500 m and 1500 m in the western basin as well as the existence of deep surfaces of erosion in the eastern basin

interaction between trench retreat and anatolian escape as recorded by neogene basins in the northern aegean sea the evolution of the north aegean sea is studied through the development of three deep basins the north aegean trough the north skyros basin and the ikaria basin bathymetric data a 2d seismic dataset and the well investigated stratigraphic records of the onshore deep basins of northern greece and western turkey were used to make structural and seismic stratigraphic interpretations the study area shows two sharp unconformities that correspond to the eocene oligocene transition and the miocene pliocene shift these discontinuities were used as marker horizons for a more detailed structural and seismic stratigraphic interpretation resulting in the identification of several seismic units a general seismic signature chart was established using onshore basin stratigraphy and well data which was then used to constrain the ages of the different seismic units the main features observed in the basins are interpreted as 1 trans tensional growth patterns in pliocene and quaternary sediments that combine ne sw trending and steeply dipping fault zones that likely correspond to strike slip corridors and e w wnw ese trending normal faults 2 regional erosional truncations of miocene sediments likely related to the messinian salinity crisis msc 3 thick delta turbidite deposits of neogene age only the north aegean trough shows evidence of earlier development and polyphase deformation through inversion structures and additional seismic units extension processes in the aegean region have been driven by the hellenic slab rollback since the middle eocene the widespread development of neogene basins at the whole aegean scale attests to a major tectonic change due to an acceleration of the trench retreat in the middle miocene the present study shows that the neogene basins of the north aegean sea developed in dextral transtension with the northward migration of the associated ne sw trending strike slip faults at regional scale this tectonic pattern indicates that the westward escape of anatolia started to interact with the trench retreat in the middle miocene around 10 myr before the arrival of the north anatolian fault in the north aegean sea

stable c and o isotopic study of the campanian chalk from the mielnik section eastern poland signals from bulk rock belemnites benthic foraminifera nannofossils and microcrystalline cements abstract exceptionally well preserved chalk from the mielnik section in eastern poland has been analyzed for sr c and o isotope composition recorded in bulk rock samples belemnite rostra hand picked benthic foraminifera and separated fine fractions incl calcareous nannoplankton and calcite cements the stratigraphic 13 c and 18 o trends are consistent with the global seawater variations during the campanian and confirm strong climatic perturbation accompanying the global sea level fall during the uppermost campanian chalk samples contain disseminated early diagenetic microcrystalline calcite cement having 13 c and 18 o values only slightly different by 13 c and 18 o values between different constituents measured from the same levels are discernible they are related with thermal gradients in the water column and with the habitat and vital effects of biogenic components benthic foraminifera reflect strong vital effects in 13 c and 18 o with a tendency to lower values than the other sample types the minimum strength of the disequilibrium fractionation effect for 18 o during foraminiferal shell secretion is estimated up to 2 belemnites show offsets in 13 c values which are difficult to estimate but their 18 o values appear to have recorded composition of seawater the large difference in 18 o values between belemnites and nannoplankton or bulk material suggests that belemnites were nectobenthic organisms living close to the seafloor the 87 sr 86 sr values and biostratigraphic position of the belemnites correspond well to those in the lagerdorf mcarthur et al 1993 the reference section for the european upper cretaceous epicontinental marine deposits but are not in agreement with the campanian stratigraphy defined in the geological time scale 2012 gradstein et al 2012 where positions of some biostratigraphic divisions are not well calibrated against the absolute age

controls on strain localization in a two dimensional elastoplastic layer insights into size frequency scaling of extensional fault populations 1 we use a two dimensional 2 d finite element code to investigate how mechanical properties and boundary conditions influence progressive strain localization during extension of a 2 d elastoplastic layer the deforming medium is modeled using a strain softening von mises rheology with a gaussian heterogeneity in yield strength distributed randomly in space specifically we examine the effects of changing 1 the thickness of the deforming layer 2 the basal boundary condition 3 the range of strengths in the deforming layer 4 the strength loss on failure and 5 the total accommodated strain discrete zones of plastic shear strain are observed to nucleate and grow progressively during each experiment in order to quantify and thus discriminate between the deformation patterns produced under differing conditions we calculate the size frequency distributions of the shear band populations size is defined as the total plastic strain represented by each discrete structure a wide range of size frequency distributions is observed including both power law and exponential as well as distributions that show breaks in scaling with a transition from power law at small sizes to exponential at large sizes we show that these variations in population statistics are directly reflecting a change in strain localization in space and time caused by changing model parameters furthermore by making an analogy between the model shear bands and tectonic faults we provide insights into key features of fault size frequency distributions that have been measured in continental and oceanic extensional settings and also observed in analogue experiments

isotopic compositions of diagenetic dolomites in the tortonian marls of the western mediterranean margins evidence of past gas hydrate formation and dissociation abstract previous studies in the lorca basin s e spain sediment geol 2002 have shown that the diagenetic dolomitic concretions from the tortonian marls are related to the past occurrence of gas hydrates which were rapidly dissociated when the sea level dropped during the messinian salinity crisis these conclusions were based on the oxygen and carbon isotopic compositions of these dolomites which indicated that the carbonate diagenesis occurred either in 13 c rich 18 o poor solutions as it is typical in gas hydrate systems or in 13 c poor 18 o rich solutions generated during gas hydrate decomposition we have recently discovered similar dolomitic nodules in other tortonian marly successions from s e spain and northern morocco the oxygen and carbon isotopic compositions of the diagenetic dolomites from these areas in spain and morocco are very similar to those from the lorca basin this demonstrates that the dolomitic nodules from the tortonian marls of the betic and rifian basins were all related to the same diagenetic environments where pore fluids were modified by gas hydrate formation dissociation processes these overall observations in s e spain and northern morocco suggest that the dissociation of gas hydrates during the messinian salinity crisis was not a local phenomenon but corresponded to a major event which affected both sides of the western mediterranean margins it might have been initiated during the first major restriction step that started around 6 7 ma in the mediterranean sea and probably climaxed at 5 9 ma when the mediterranean sea level dropped by more than 1000 m during the salt deposition stage

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

how much did climate force the messinian salinity crisis quantified climatic conditions from pollen records in the mediterranean region the latest miocene 5 96 to 5 33 ma is characterised by an outstanding event the desiccation of the mediterranean sea messinian salinity crisis it has been suggested that this was caused by a tectonic event with no climatic change playing a role in desiccation quantifying the climate of the region during this period will help support or refute this hypothesis an effective method for reconstructing the climate from neogene pollen data is the climatic amplitude method based on the modern climatic requirements of plants to interpret fossil data it has been conceived especially for periods devoid of modern vegetation analogue twenty messinian to lower zanclean pollen sequences are now available in the peri mediterranean region most of them do not cover the whole messinian interval particularly those along the mediterranean shorelines where sedimentation was interrupted during the sea s desiccation in contrast sedimentation was almost continuous in such areas as the atlantic side of morocco along the adriatic coast including the po valley and to a lesser extent the black sea the mediterranean sites nonetheless provide a reliable if not a discontinuous record of vegetation variability in time and space a first examination of the pollen diagrams reveals a high regional variability controlled by local conditions and throughout the interval a southward increase in herb pollen frequency in contrast to the tree pollen frequency this indicates that open and probably dry environments existed in the southern mediterranean region prior to during and after the salinity crisis trees developed in areas close to mountains such as in the po valley in cerdanya and in the black sea region most variations observed in the pollen diagrams are constrained by fluctuations of pinus pollen amounts indicating eustatic variations climatic quantification from pollen data does not show obvious climatic changes due to the desiccation of the mediterranean sea especially in the dry and warm southwestern mediterranean area sicily southern spain and north africa at maccarone along the adriatic sea a decrease in temperatures of the coldest month and less importantly a decrease in mean annual temperatures corresponding to a drastic vegetation change are reconstructed these temperature variations are assumed to be controlled by regional environmental changes massive arrival of waters in this basin rather than to reflect cooling because some authors link the second phase of evaporite deposition to a period of global warming some migrations of plants probably occurred as a response to mediterranean desiccation but the climatic contrast which has probably existed at that time between the central mediterranean and the peripheral areas might be amplified climatic reconstruction from pollen data in the western mediterranean area shows that climate is not the direct cause of the mediterranean desiccation as the mediterranean region had experienced continuously high evaporation long before the crisis therefore the main factor leading to this event seems to be the successive closures of the betic and rifian corridors isolating the mediterranean sea from the atlantic ocean

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

polyphasic carbonate precipitation in the shallow subsurface insights from microbially formed authigenic carbonate beds in upper miocene sediments of the tertiary piedmont basin nw italy article i nfo authigenic methane derived carbonates hosted in upper miocene slope sediments of the tertiary piedmont basin nw italy are studied by a multidisciplinary approach including petrography stable oxygen and car bon isotopes of carbonates as well as lipid biomarkers in order to explore the relationship between microbial activity and carbonate precipitation in the shallow subsurface the studied rocks show a bed parallel geom etry and are characterized by dolomitic intergranular cement which is typified by positive 13 c values as high as 6 2 vpdb a striking feature of some dolomite beds is an intricate network of septarian like cracks filled with both injected sediments and polyphasic carbonate cements prokaryotic molecular fossils in the dolomite beds comprise archaeol 13 c 40 vpdb and various bacterial dialkyl glycerol diethers dages 13 c 30 vpdb strongly suggesting that dolomite precipitation took place at the interface of the zones of archaeal methanogenesis and bacterial sulphate reduction in contrast extremely negative 13 c values of carbonate cements as low as 56 3 vpdb and various archaeal and bacterial molecular fossils e g pentamethylicosane pmi 106 vpdb are recorded in the crack filling carbonate cements these cements precipitated due to anaerobic oxidation of methane coupled to sulphate reduction we pro pose a scenario for the formation of the diagenetic beds suggesting that carbonate precipitation was the re sult of three microbially driven processes sulphate reduction methanogenesis and finally anaerobic oxidation of methane this unusual sequence was a consequence of a dynamic change of environmental geo chemical conditions and fluid circulation patterns that prevailed in the ancient subseafloor during early dia genesis of the unconsolidated sediments anaerobic oxidation of methane which usually predates methanogenesis during increasing burial postdates methanogenesis in case of the septarian like beds after the beds were affected by crack formation induced by overcritical pore pressure allowing the ingress of sulphate rich water from above and methane rich water from below

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

pore fluid compositions and inferred fluid flow patterns at the haima cold seeps of the south china sea abstract fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean which is critical in its environmental impact assessment here we report pore fluid compositions of three 8 m long piston cores qdn 14a qdn 14b and r1 collected from the newly discovered active haima cold seeps on the northwestern slope of the south china sea reaction transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow extremely low 13c values

dynamics of cementation in response to oil charge evidence from a cretaceous carbonate field u a e abstract oil charge is thought to inhibit the growth of cements within subsurface pore systems we explore this phenomenon in a giant cretaceous carbonate field from u a e where the oil filled crest porosity ranges from 10 to 50 and permeability from 0 08 to 830 md but coeval water leg porosity is reduced to 10 to 23 and permeability to 0 1 to 4 md only 5 of primary interparticle pores 30 m diameter in the crest are fully cemented compared to 99 of pores in the water leg syntaxial calcite burial cements 10 m diameter in the oil leg show 12 cathodoluminescence zones with oil inclusions n 27 occurring in four of the five final zones mean in situ ion microprobe 18ovpdb data from the oil leg cements range from 1 2 in the oldest zone decreasing to 10 3 in zone 11 returning to 7 7 in the final zone the oldest distinguishable cement zone in the water leg shows highly variable 18o from 3 6 to 9 3 with a mean of 7 3 and with subsequent zones decreasing to a mean value of 9 4 for the youngest cement zone decreasing 18o values are interpreted as indicating increasing temperature reflecting burial and the evolution of pore water composition broadly similar trends in the oil and water legs suggest precipitation under the same general conditions unlike the oil leg cements the final zone in the water leg occludes nearly all remaining pore space the 18ovpdb of bulk micrite from the water leg shows an average of 7 4 n 9 compared to 6 2 n 10 from the oil leg suggesting the precipitation of further micrite cement at greater burial depths we infer that burial cementation slowed in the presence of oil due to a reduction of potential nucleation sites as well as porewater and solute movement within weakly oil wet pores whereas continued flow and solute movement through all pores including the micropores

ramp initiation and spacing in a homogeneous thrust wedge 1 fold and thrust belts grow largely through sequential initiation of thrust faults and subsequent motion on them from hinterland to foreland i e away from the plate boundary while perturbations in basal geometry stratigraphy excess fluid pressure folding and surface slope have all been suggested as mechanisms for thrust ramp nucleation we show that preexisting inhomogeneities are not necessary we present a mechanically homogeneous finite element wedge model to assess the effects of variations in yield stress dilation angle and basal friction angle on foreland fold and thrust belt evolution computer experiments produce a broad foreland dipping plastic strain band that marks the topographic inflection produced by the previous ramp this band migrates toward the rigid base where the plastic strain is preferentially concentrated in a thrust ramp subsequent ramps develop toward the foreland in a self similar fashion the ramp formation process is systematic ramps form at the same location with respect to the wedge origin regardless of significant variations in yield stress dilation angle or basal friction however the backstop displacement required to initiate a new ramp is dependent on these variables we conclude that the wedge geometry is the primary factor in determining both thrust ramp location and spacing syntectonic erosion and deposition or any other process that changes wedge geometry may in some cases control the location of thrust ramps

alternate interpretation of the messinian salinity crisis controversy resolved a high resolution chronology of the mediterranean messinian salinity crisis is proposed two types of evaporite deposition may be distinguished those in marginal areas vs those in basinal ones their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major messinian erosional surface a two step model is proposed for the evolution of the salinity crisis through time during the first phase from 5 75 to 5 60 ma the deposition of marginal evaporites took place in response to a modest sea level fall in the second interval from 5 60 to 5 32 ma the mediterranean basin became isolated during this later period the deposition of basinal evaporites and the cutting of the mediterranean canyons took place

deformation behavior during blind thrust translation as a function of fault strength deformation partitioning between translation and distortion for a thrust sheet and for the roof sequence above a moving thrust horse were analyzed in terms of the role of fault strength using finite element models greater frictional resistance on the thrust decollement decreased fault translation increased distortion of the thrust sheet and roof sequence and increased the ratio of backthrusting to forethrusting for the roof sequence in all models with weak decollements 0 35 the ratio of tip line propagation to thrust displacement is very high during initial thrust movement but decreases significantly during later fault motion this observation supports interpretations in natural thrust systems where thrust faults propagate large distances before accumulating significant displacements the tip lines for weak decollements also propagated into the foreland well in advance of the distortion front in the overlying thrust sheet or roof sequence this result does not support interpretations from natural systems for the existence of ductile beads in front of propagating thrusts but rather suggests that recent models for fault arrest or displacement gradient folds may be mechanically valid in addition no model for roof sequences yielded a simple end member response of only backthrusting or forethrusting suggesting that the many blind thrust belts where only one response has been interpreted may need to be re examined finally in those cases where more than one thrust flat moves simultaneously in the models partitioning of displacement and distortion in space and time changes significantly this result supports interpretations from natural systems where more than one thrust is thought to move at once

genesis and geometry of the meiklejohn peak lime mud mound bare mountain quadrangle nevada usa ordovician limestone with submarine frost heave structures a possible response to gas clathrate hydrate evolution abstract during the early middle ordovician early whiterockian the meiklejohn peak lime mud mound a large whaleback or dolphin back dome grew on a carbonate ramp tens to hundreds of kilometres offshore this ramp extended from the northwest margin of laurentia into the open waters of the ancestral pacific ocean to the north the mound developed in an outer ramp environment in relatively deep and cold water a steep northern margin with a slope that exceeds 55 characterizes the mound this margin is split by a 14 m long vertical fracture that separates a zone of slumped drag folded and brecciated rocks from the main mass of the mound failure along this fracture occurred subcutaneously as highlighted by covering beds that are folded next to the mound brecciated blocks and clasts contain zebra and stromatactis structures indicating that these rocks and structures were lithified early in the history of the mound the southern end of the mound is less steep and is characterized by large echinodermal grainstone cross beds these deposits are part of a large subaqueous dune that grew northwards and preceded the main development of the mound southward dipping and downlapping layers of mud mound mudstone and wackestone overlie the dune these muddy limestone layers are cut in several places by injection dykes and are pierced near the contact with the underlying dune by a 25 m long pipe filled with rotated nodular and brecciated mud mound clasts this long pipe extends to the edge of the mound and appears to have been a conduit where fluidized materials that came from the mound s interior were vented the interior of the mound is typified by light grey limestone with zebra bands and stromatactis structures both structures represent former cavity systems that are filled with fibrous and bladed calcite and pelleted and laminated geopetal mudstone spar bands of zebra limestone often extend for several metres and appear to have been unsupported over these distances zebra banded rocks are also accompanied by snout and socket structures and in some instances are folded and sheared by curving kink bands zebra and stromatactis limestone structures found throughout the mud mound resemble frost heave and cryoturbation structures identified in both holocene and pleistocene cryosols and in laboratory experiments with advancing freezing fronts in clay size sediment significantly modern occurrences of methane clathrate hydrate methane charged ice display parallel and digitate layering similar in depositional appearance to that of zebra and stromatactis limestone from meiklejohn peak early carbonate cements are also commonly associated with these modern clathrate hydrate deposits consequently gas clathrate hydrates may have been the propping agent for zebra and stromatactis structures observed in the mud mound in this scenario carbonate cements would have precipitated and stabilized these structures both with the consolidation and dissociation of gas clathrate hydrates and with the oxidation and reduction of associated gases stable 13 c and 18 o isotope ratios collected from mudstone and spar of zebra and stromatactis structures indicate that they were lithified in equilibrium with ordovician seawater the 13 c isotope ratios recorded at meiklejohn peak are similar to 13 c isotopic ratios obtained from co 2 evolving from modern seafloor these isotopic ratios may indicate that frost heave structures in the meiklejohn peak mud mound are the result of consolidation and dissociation of carbon dioxide clathrate hydrates even though the bulk of gas clathrate hydrates identified to date in modern ocean floors are composed of methane carbon dioxide clathrate hydrates are known from the modern seafloor of the okinawa trough they may also be common in areas of abundant carbonate sediment accumulation as suggested by recent observations from the great australian bight

thermogenic hydrocarbons in surface sediments of the bransfield strait antarctic peninsula antarctica is the only continent from which petroleum deposits have not been encountered despite the occurrence of potential hydrocarbon basins onshore and around its margins we have discovered thermogenic hydrocarbons in unconsolidated recent sediments from the king george basin bransfield strait west antarctica these sediments possessed a marked petroliferous smell throughout the length of the 8 6 m gravity core the basin sediments are glacial marine deposits dominated by turbidites and contain abundant autochthonous organic matter as a result of high seasonal primary productivity in this most fertile part of the circumpolar ocean1 the maturation of this material into hydrocarbons may have been accelerated by the high geothermal gradient in the basin associated with back arc spreading created by the subduction of the drake plate into the south shetland trench2 sediments in this rifted basin are frequently intruded by volcanic sills and dykes3 giving rise to acoustic features shown in fig 1 the presence of these thermogenic hydrocarbons provides the first demonstration of active source rocks in antarctica

changes in the pelagic fine fraction carbonate sedimentation during the cretaceous paleocene transition contribution of the separation technique to the study of bidart section abstract the bidart section southwestern france displays an almost complete late maastrichtian early danian transition as the cretaceous paleocene k p crisis mainly affects carbonate producers the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary a new protocol for separation of fine carbonates into discrete size fractions was used 1 to precisely evaluate in weight of carbonate and not only number of particles per gram of rock the contribution of each carbonate component type foraminifera calcareous nannofossils and nonbiogenic carbonates calcite microparticles and calcareous macrocrystals and 2 to measure their isotopic signature and thus constrain the origin of these carbonate particles the maastrichtian pelagic carbonates are mostly biogenic 70 of the bulk carbonate but also contain nonbiogenic carbonates calcite microparticles 4 m due to their isotopic signature calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains and the most likely origin appears to be early diagenetic processes and or export from shallow water platforms the calcareous macrocrystals derive from an early diagenetic process the lowermost danian sediments are dominated by nonbiogenic carbonates between 50 and 80 of the bulk carbonate nevertheless two kinds of biogenic components are present thoracosphaera operculata and maastrichtian like calcareous nannofossils thoracosphaera operculata is an opportunistic calcareous dinoflagellate which frequently occurs at high abundances in basal danian sediments and is certainly in situ this is confirmed by it showing a danian isotopic signature the maastrichtian like calcareous nannofossils are nannofossils of taxa that indisputably occur in the maastrichtian but which do not occur consistently in later danian strata they show a maastrichtian type isotopic signature which strongly supports the inference that they are mainly reworked from the maastrichtian this study shows that nonbiogenic carbonates are important components of the pelagic carbonate even during periods of normal sedimentation in the crisis interval the biogenic carbonate production drops sharply while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest danian sediments finally the sediments return to a composition similar to that of the maastrichtian in m pseudobulloides foraminifera zone

shallow subsurface growth and burial alteration of middle devonian calcite concretions abstract organic rich shale hosted septarian and nonseptarian calcite concretions in the middle devonian givetian hamilton group of central new york state formed early in the shallow subsurface as indicated by detailed petrography sem x radiographic lithologic geochemical and isotopic analyses 13ccarb values suggest that carbon was derived from microbial breakdown of organic matter in the sulfate reduction zone at subsurface depths of less than 10 m however strongly depleted 18o 7 to 10 pdb suggests much greater burial depths the sup 18o data appear to represent an altered isotopic signature as indicated by their narrow depleted range and petrographic evidence for recrystallization of concretion carbonate evidence of primary porosity in concretions during growth indicates that early stage diagenetic alteration occurred in an open marine phreatic environment that homogenized isotopic and geochemical signatures of metastable carbonate minerals the association of septarian calcite and bitumen along with the depleted 18o values suggests that late stage diagenesis was a result of migration of warm basin derived connate waters through the open system preburial condensation of sediments in younger stratigraphic units within the hamilton group possibly caused by increased winnowing bioturbation and erosional events during shallowing of the hamilton sea has generated stratigraphic variations in concretion lithology and shape within any given unit however variation in characteristics of concretions remains a primary function of their timing of growth versus burial compaction unlike cenozoic carbonate concretions in organic rich marine sediments that possess a dolomitic matrix no dolomite concretions were discovered in the hamilton group preliminary data suggest that restriction of calcite as the host mineral for early to middle paleozoic concretions may be controlled by elevated sulfate concentrations that inhibited dolomite precipitation and or primary secular variations in carbonate mineralogy

stable isotope microsampling of speleothems for palaeoenvironmental studies a comparison of microdrill micromill and laser ablation techniques three different techniques are currently in use to microsample speleothem and tufa specimens at moderate to high spatial resolution manual microdrilling using a dentist drill semi automated micromilling and laser ablation while a separate handling step is involved in the first two techniques transferring the sample powder to an on line carbonate preparation system linked to an isotope ratio mass spectrometer the laser is directly interfaced with the mass spectrometer via a gas preparation device in this study we compare stable c and o isotope results obtained by these three preparation techniques using continuous he flow mass spectrometry on the same speleothem samples the overall consistency of the results is satisfactorily high in particular between the micromill and the laser data although the precision of the laser data is slightly lower than analyses obtained by the other two methods in particular with respect to 18o the speed of acquisition renders laser ablation a useful reconnaissance technique for rapidly analyzing long sections of speleothems micromilling is the method of choice to obtain highly spatially resolved isotope data of high analytical precision while microdrilling is still an low cost alternative for sampling at comparably low resolution e g of fast growing stalagmites

isotopically heavy oxygen containing siderite derived from the decomposition of methane hydrate authigenic siderite from the miocene and pliocene sediments of the blake outer ridge formed in a methane fermentation zone in equilibrium with anomalously heavy oxygen water as evidenced by its heavy carbon 1 6 to 12 5 13c and heavy oxygen 0 1 to 7 9 18o values blake outer ridge sediments are impregnated with methane hydrate which is generally enriched in 18o relative to ambient water this suggests that decomposition of methane hydrate and subsequent release of large amounts of methane and heavy oxygen water affected the formation of the 18o enriched siderite gas hydrate reaction in sediments is an important process for causing a dramatic variation of oxygen isotopic composition of the interstitial water and of authigenic carbonates formed during burial diagenesis

the messinian salinity crisis in the dardanelles region chronostratigraphic constraints abstract an intense controversy on chronostratigraphy of upper miocene lower pliocene deposits and the messinian salinity crisis in the dardanelles area led to a systematic investigation of calcareous nannoplankton content of 10 key sections representative of the most relevant regional kirazli and alcitepe formations our study shows clearly that the kirazli formation deposits predate the messinian salinity crisis while those of the alcitepe formation postdate this outstanding event which severely impacted the region as widely known around the mediterranean basin fluvial canyon cutting or gap in sedimentation linked to the peak of the messinian salinity crisis separates the two formations detailed palaeoenvironmental investigations based on the fluctuation and distribution pattern of dinoflagellate cysts pollen grains and calcareous nannoplankton allow us to reconstruct the regional palaeogeography before during and after the messinian salinity crisis the gathered data do not indicate any marine corridor between the eastern mediterranean sea and eastern paratethys through the marmara sea region at the time of the messinian salinity crisis

bacterial methane oxidation in sea floor gas hydrate significance to life in extreme environments samples of thermogenic hydrocarbon gases from vents and gas hydrate mounds within a sea floor chemosynthetic community on the gulf of mexico continental slope at about 540 m depth were collected by research submersible the study area is characterized by low water temperature mean 7 c high pressure about 5 400 kpa and abundant structure ii gas hydrate bacterial oxidation of hydrate bound methane ch sub 4 is indicated by three isotopic properties of gas hydrate samples relative to the vent gas from which the gas hydrate formed 1 methane bound methane is enriched in sup 13 c by as much as 3 8 pdb peedee belemnite 2 hydrate bound methane is enriched in deuterium d by as much as 37 smow standard mean ocean water and 3 hydrate bound carbon dioxide co sub 2 is depleted in sup 13 c by as much as 22 4 pdb hydrate associated authigenic carbonate rock is also depleted in sup 13 c bacterial oxidation of methane is a driving force in chemosynthetic communities and in the concomitant precipitation of authigenic carbonate rock that modifies sea floor geology bacterial oxidation of hydrate bound methane expands the potential boundaries of life in extreme environments

planktonic foraminiferal oxygen isotope analysis by ion microprobe technique suggests warm tropical sea surface temperatures during the early paleogene 1 cool tropical sea surface temperatures ssts are reported for warm paleogene greenhouse climates based on the 18o of planktonic foraminiferal tests these results are difficult to reconcile with models of greenhouse gas forced climate it has been suggested that this cool tropics paradox arises from postdepositional alteration of foraminiferal calcite yielding erroneously high 18o values recrystallization of foraminiferal tests is cryptic and difficult to quantify and the compilation of robust 18o records from moderately altered material remains challenging scanning electron microscopy of planktonic foraminiferal chamber wall cross sections reveals that the basal area of muricae pustular outgrowths on the chamber walls of species belonging to the genus morozovella contain no mural pores and may be less susceptible to postdepositional alteration we analyzed the 18o in muricae bases of morozovellids from the central pacific ocean drilling program site 865 by ion microprobe using 10 m pits with an analytical reproducibility of 0 34 2 standard deviations in situ measurements of 18o in these domains yield consistently lower values than those published for conventional multispecimen analyses assuming that the original 18o is largely preserved in the basal areas of muricae this new 18o record indicates early paleogene 49 56 ma tropical ssts in the central pacific were 4 8 c higher than inferred from the previously published 18o record and that ssts reached at least 33 c during the paleocene eocene thermal maximum this study demonstrates the utility of ion microprobe analysis for generating more reliable paleoclimate records from moderately altered foraminiferal tests preserved in deep sea sediments

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

neogene sequences in the pannonian basin hungary abstract the rifting in the extensional pannonian basin occurred in the middle miocene the subbasins within the pannonian system formed under the control of expansional tectonics during this time sequence stratigraphic analysis on well logs revealed a single third order synrift sedimentary sequence developed due to tectonically enhanced eustatic sea level change in shallow marine environments the pannonian basin became an isolated lake by its postrift phase during the middle late miocene boundary the first postrift sequence formed under peculiar conditions when the basin lost its marine connections and simultaneously a considerable differential subsidence began the large volume of freshwater input supplied by rivers and the rapid subsidence of the basement caused the basin to be a deep freshwater lake the lacustrine postrift infill was studied by sequence stratigraphy using seismic reflection well log and magnetostratigraphic data numerous unconformities can be revealed which represent relative lake level fluctuations of different magnitudes the water level and volume of the large inland lake was very sensitive to different effects the most important ones are lake water budget sediment influx and differential subsidence of the basement the style of extension and sedimentation show differences with respect to passive continental margins that are reflected in the sedimentary architecture a significant third order water level change has been discovered in the eastern part of the pannonian basin while the western side did not experience such a great change in lake level this fact gives credit to the view of different interplay of lake level influencing effects in different areas of the basin this major third order lake level change has been age dated using magnetostratigraphy and a very close match was found with the age of the mediterranean salinity crisis the pannonian basin offers the possibility to study the infilling style and stratigraphic evolution of an inland rifted basin detailed sequence analysis allows the recording of the apparent sea lake level motion which can be compared to those observed in the surrounding seas further tectono stratigraphic evaluations of the pannonian basin may reveal hidden facts on the nature of lake level fluctuations and contribute new data to the discipline of sequence stratigraphy

late cretaceous campanian carbon isotope events sea level change and correlation of the tethyan and boreal realms abstract carbon stable isotope stratigraphy provides unique insights into environmental change during the campanian the longest 83 5 71 3 ma but least well understood stage of the late cretaceous a new carbon isotope 13c profile for a 500 m thick campanian basal maastrichtian tethyan pelagic hemipelagic section near el kef northern tunisia is calibrated using data from a biostratigraphically well constrained succession at kalaat senan the general shapes of the tunisian 13c reference curve and published tethyan 13c profiles from elles tunisia and bidart sw france and a boreal curve for the trunch borehole eastern england are remarkably similar in all three areas a positive carbon isotope event of 0 2 13c in the mid campanian dated at 78 7 ma and a negative excursion of 0 4 in the upper campanian at 74 8 ma can be correlated between tunisia and england a positive excursion of 0 3 at 83 7 ma spans the santonian campanian boundary these isotope events enable precise inter regional correlations that are consistent with published nannofossil data review of campanian sea level data from north africa the middle east and northern europe indicates that major shifts in 13c profiles coincide with changes in eustatic sea level relatively stable 13c values in the lower campanian and their long term fall through the upper campanian reflect high and then falling eustatic sea levels and increased carbonate production short term 600 kyr positive excursions record greater organic productivity and or organic matter preservation and decreased carbonate fluxes during periods of rapid sea level rise and the drowning of carbonate platforms excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea level fall carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of campanian global biostratigraphic frameworks and improve our understanding of the nature and timing of late cretaceous sea level change

methane derived authigenic carbonates on accretionary ridges miocene case studies in the northern apennines italy compared with modern submarine counterparts abstract we present new field data from three outcrops of miocene methane derived authigenic carbonates in the foredeep of the northern apennines that contain chemosynthetic fauna and record a long history 1 ma of shallow fluid seepage linked to seafloor anaerobic oxidation of methane the studied outcrops show similar features in terms of carbonate morphology facies spatial distribution and lateral and vertical contacts with the enclosing sediments methane derived carbonates occur in two structural positions 1 on the slope of the accretionary wedge in hemipelagites draping buried thrust related anticlines and 2 at the leading edge of the deformation front in the inner foredeep within fault related anticlines standing above the adjacent deep seafloor as intrabasinal ridges we compare fossil seeps with two extensively investigated modern analogues the hikurangi margin offshore new zealand and hydrate ridge on the cascadia margin offshore the u s a these analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane derived carbonate bodies and chemosynthetic fauna on the present day seafloor the comparison allows us to propose a model for the evolution of fluid seeps on thrust related ridges at the deformation front uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults mostly blind connected to the basal detachment favoring the migration of fluids toward the incipient anticline fold development generates extensional stresses in the hinge zone of the anticline promoting the development of normal faults fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve in the slope setting far from the deformation front thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor after reaching a mature stage within the wedge the structure is less active and buried in the slope environment of the evolved prism

cenozoic deep sea temperatures and global ice volumes from mg ca in benthic foraminiferal calcite a deep sea temperature record for the past 50 million years has been produced from the magnesium calcium ratio mg ca in benthic foraminiferal calcite the record is strikingly similar in form to the corresponding benthic oxygen isotope 18o record and defines an overall cooling of about 12 c in the deep oceans with four main cooling periods used in conjunction with the benthic 18o record the magnesium temperature record indicates that the first major accumulation of antarctic ice occurred rapidly in the earliest oligocene 34 million years ago and was not accompanied by a decrease in deep sea temperatures

paleotemperature variations of early jurassic seawater recorded in geochemical trends of belemnites from the basque cantabrian basin northern spain abstract here we present new o isotope mg ca and sr ca data of early jurassic belemnites from the basque cantabrian basin of northern spain a succession deposited in a periodically anoxic restricted hemipelagic basin nearly 200 pliensbachian lower toarcian belemnites were collected and examined using optical cathodoluminescence and chemical techniques to constrain the effects of potential diagenetic alteration after a precise diagenetic screening of samples 176 belemnites were selected to provide records of changes in seawater geochemistry and temperature the records show a fairly good correlation of 18o with mg ca ratios r 0 65 and a weak correlation of 18o with sr ca ratios r 0 34 the oxygen isotope data if interpreted in terms of paleotemperature have revealed a period of relatively uniform and warm seawater temperatures during the early pliensbachian followed by a sharp temperature drop during the late pliensbachian earliest toarcian the onset of this cooler period occurred within the protogrammoceras celebratum subzone and the coolest seawater temperatures were reached at the end pliensbachian pleuroceras spinatum zone this was followed by a rapid seawater warming during the harpoceras serpentinus zone of the early toarcian in addition the data set provides new information regarding paleoceanographic conditions for anoxic events in a critical region of connection between the boreal and tethys realms it is the background to relatively high 18o values that prevailed during the anoxic episodes concerning the impact of the early toarcian anoxic event in northern spain the detected 18o and mg ca excursions reached their minimum and maximum respectively during the lower h serpentinus zone while the anoxic black shale interval developed earlier during the upper dactylioceras tenuicostatum lowermost h serpentinus zones