separation of high frequency pn phases and mantle refracted p phases at distances between 6 and 18 in the western pacific by ocean bottom seismograph array observations of high frequency hf pn phases by an oceanbottom seismometer obs array were made at distances between 6 and 18 in the north western pacific at this distance range mantle refracted p phases arrive earlier than hf pn phases these two phases are successfully separated for near surface earthquakes by applying appropriate filtering since the dominant frequencies of hf pn phases are higher than 6 hz and the frequencies of mantle refracted p phases are rather low about 3 hz the apparent velocities of hf pn phases in this distance range are estimated to be from 8 3 to 8 1 km sec or even lower while those of mantle refracted p waves are from 8 4 to 8 6 km sec the reported value of 8 33 km sec for hf pn phases appears to be an average of both hf pn and mantle refracted p phases therefore these observations suggest that a proposed additional hf pn wave guide for the higher velocity of 8 33 km sec is questionable

deep sea carbonates reading the carbon isotope signal the carbon isotope signal in deep sea sediments reflects a mix of 1 global changes in the rates of exchange of the ocean s carbon reservoir with biosphere soil and sediments 2 global and regional changes in surface water productivity 3 internal shifts in water mass structure and circulation basin basin fractionation oxygen minimum development and 4 organism specific fractionation effects due to changes in micro habitat and or ontogenic fractionation vital effects additional complications arise from differential preservation it is impossible to entirely isolate these various factors as a rule of thumb long period signals that are parallel for planktonic and benthic data reflect external global fractionation patterns whilst short period signals are more likely tied to internal patterns water mass fractionation the various approaches to interpretation are illustrated with three case studies the glacial holocene transition the messinian carbon shift and the miocene monterey excursion

carbon geochemistry of cold seeps methane fluxes and transformation in sediments from kazan mud volcano eastern mediterranean sea abstract despite growing concerns about potential enhancement of global warming and slope failure by methane produced by gas hydrate dissociation much uncertainty surrounds estimates of gas hydrate reservoir sizes as well as methane fluxes and oxidation rates at the sea floor for cold seep sediments of the eastern mediterranean sea depth dependent methane concentrations and rates of anaerobic oxidation of methane aom are constrained by modeling the measured pore water sulfate profile the calculated dissolved methane distribution and flux are sensitive to the advective flow velocity which is estimated from the depth distributions of conservative pore water constituents na b near complete anaerobic oxidation of the upward methane flux of 6 0 mol m 2 yr 1 is supported by the depth distributions of indicative biomarkers and the carbon isotopic compositions of organic matter and dissolved inorganic carbon pore water and solid phase data are consistent with a narrow depth interval of aom 14 18 cm below the sediment water interface based on an isotopic mass balance the biomass of the microbial population carrying out oxidation of methane coupled to sulfate reduction at the given methane flux represents 20 of the total organic carbon which is a significant pool of in situ formed organic matter model results indicate that the asymptotic methane concentration is reached a few meters below the sediment surface the predicted asymptotic concentration is close to the in situ saturation value with respect to gas hydrate suggesting that the rate of shallow gas hydrate formation is controlled by the ascending methane flux the proposed model approach can be used to predict the formation of gas hydrate and to quantify methane fluxes plus transformation rates in surface sediments where fluid advection is an important transport mechanism

a unique lacustrine mixed dolomitic clastic sequence for tight oil reservoir within the middle permian lucaogou formation of the junggar basin nw china reservoir characteristics and origin abstract the middle permian lucaogou formation in the jimusaer sag of the southeastern junggar basin nw china was the site of a recent discovery of a giant tight oil reservoir this reservoir is unusual as it is hosted by lacustrine mixed dolomitic clastic rocks significantly differing from other tight reservoirs that are generally hosted by marine lacustrine siliciclastic calcitic sequences here we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field petrological geophysical including seismic and logging and geochemical data field and well core observations indicate that the lucaogou formation is a sequence of mixed carbonate mainly dolomites and terrigenous clastic mainly feldspars sediments that were deposited in a highly saline environment the formation is divided into upper and lower cycles based on lithological variations between coarse and fine grained rocks in particular dolomites and siltstones are interbedded with organic rich mudstones in the lower part of each cycle whereas the upper part of each cycle contains few dolomites and siltstones tight oil accumulations are generally present in the lower part of each cycle and dolomites and dolomite bearing rocks are the main reservoir rocks in these cycles including sandy dolomite dolarenite dolomicrite and a few dolomitic siltstones optical microscope back scattered electron and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon related fluids

spectrum of pore types and networks in mudrocks and a descriptive classification for matrix related mudrock pores matrix related pore networks in mudrocks are composed of nanometer to micrometer size pores in shale gas systems these pores along with natural fractures form the flow path permeability network that allows flow of gas from the mudrock to induced fractures during production a pore classification consisting of three major matrix related pore types is presented that can be used to quantify matrix related pores and relate them to pore networks two pore types are associated with the mineral matrix the third pore type is associated with organic matter om fracture pores are not controlled by individual matrix particles and are not part of this classification pores associated with mineral particles can be subdivided into interparticle interp pores that are found between particles and crystals and intraparticle intrap pores that are located within particles organic matter pores are intrap pores located within om interparticle mineral pores have a higher probability of being part of an effective pore network than intrap mineral pores because they are more likely to be interconnected although they are intrap om pores are also likely to be part of an interconnected network because of the interconnectivity of om particles in unlithifed near surface muds pores consist of interp and intrap pores and as the muds are buried they compact and lithify during the compaction process a large number of interp and intrap pores are destroyed especially in ductile grain rich muds compaction can decrease the pore volume up to 88 by several kilometers of burial at the onset of hydrocarbon thermal maturation om pores are created in kerogen at depth dissolution of chemically unstable particles can create additional moldic intrap pores

the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

synthesis of realistic oceanic pn wave trains the synthesis of realistic oceanic pn phases was accomplished using wave number integration wave trains with durations of the order of 100 s following the initial compressional wave arrival were generated for a laterally homogeneous vertically inhomogeneous oceanic lithosphere model the excitation of these long wave trains is primarily a result of near receiver water and sediment reverberations and does not involve scattering a late arriving wave train with a group velocity of 1 6 km s having an emergent onset and substantial duration characteristic of oceanic abyssal t phases was also generated even with phase velocities restricted to be greater than 5 5 km s the synthetic pn wave train was compared to data collected in the southwest pacific during the ngendei seismic experiment many of the spectral characteristics associated with sediment and water reverberation observed in the synthetics were also observed in the data first it was found that the amplitude falloff rate of the pn wave train is a strongly increasing function of frequency in direct contrast to results predicted for forward scattered coda this leads to a predominance of low frequencies late in the wave train which is easily interpreted in terms of the constructive interference of different modes of reverberation second the presence of spectral peaks at frequencies associated with both sediment and water reverberations was observed the predicted frequencies were computed using values of the water depth sediment thickness and sediment velocity obtained in an independent study strong alignment of the spectral peaks with the predicted frequencies illustrates the reverberating nature of the pn wave train the presence of substantial power in excess of 15 hz can be attributed to the confinement of pn to the oceanic lithosphere the oceanic pn phase is thus characterized as a refraction from the lower lithosphere with multiple reverberations in the oceanic water column and sediment layer comprising the subsequent coda complicated oceanic lithosphere models are not needed to explain the gross characteristics of pn propagation

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

synthetic pn and sn phases and the frequency dependence of q of oceanic lithosphere the oceanic lithosphere is an extremely efficient waveguide for high frequency seismic energy in particular the propagation of the regional to teleseismic oceanic pn and sn phases is largely controlled by properties of the oceanic plates the shallow velocity gradient in the sub moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle the confinement of pn sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed the rapid disappearance of sn and the discontinuous drop in pn sn group velocity beyond a critical distance dependent upon the local thickness of the lithosphere are interpreted as a shadowing effect of the low q asthenosphere wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere the results were compared to data collected during the 1983 ngendei seismic experiment in the southwest pacific the pn sn coda is successfully modeled as a sum of leaky organ pipe modes in the sediment layer and oceanic water column while scattering is present to some degree it is not required to explain the long duration and complicated nature of the pn sn wave trains the presence of extremely high frequencies in pn sn phases and the greater efficiency of sn than pn propagation are interpreted in terms of an absorption band rheology a shorter high frequency relaxation time for p waves than for s waves results in a rheology with the property that q q at low frequency while q q at high frequency consistent with the teleseismic pn sn observations the absorption band model is to viewed as only an approximation to the true frequency dependence of q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

high resolution seismic stratigraphy sedimentary processes and the origin of seabed cracks and pockmarks at nyegga mid norwegian margin abstract densely spaced high resolution topas seismic profiles and em1002 bathymetric data reveal the presence of numerous pockmarks mound like structures and elongated seabed cracks at nyegga offshore mid norway the seabed cracks are located adjacent to the northern escarpment of the storegga slide appearing as graben like structures in the topas data unlike the cracks pockmarks and mound structures are largely associated with vertical zones of acoustic blanking at depth interpreted as pathways for vertically migrating gaseous fluids based on the topas data a new seismostratigraphic framework has been established and correlated to previously published age models of images cores md99 2291 and md99 2289 seismic facies interpretation suggests repeated and rapid deposition of up to 40 m thick glacigenic wedges in the eastern part of the study area around 18 2 14 c ka bp 21 8 cal ka 17 5 14 c ka bp 20 8 cal ka and 16 9 14 c ka bp 20 cal ka towards the west glacimarine deposition has prevailed characterized by progressively increasing sedimentation rates with peak values of 30 m ka during the period from 15 0 14 c ka bp 18 2 cal ka to 15 8 14 c ka bp 19 cal ka as the distribution of the nyegga pockmarks closely coincides with the main late weichselian sediment depocenters we suggest a relation between rapid and repeated sedimentation and periodic overpressure generation at depth ultimately leading to fluid expulsion at the seabed and the formation of the nyegga pockmark field in contrast seabed cracks at nyegga appear to have formed due to local extension which we relate to horizontal stress reduction as a consequence of the storegga slide event potentially this event has been accompanied by renewed vertical fluid migration and the most recent stage of pockmark development

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

high resolution records of the late paleocene thermal maximum and circum caribbean volcanism is there a causal link two recently drilled caribbean sites contain expanded sedimentary records of the late paleocene thermal maximum a dramatic global warming event that occurred at ca 55 ma the records document significant environmental changes including deep water oxygen deficiency and a mass extinction of deep sea fauna intertwined with evidence for a major episode of explosive volcanism we postulate that this volcanism initiated a reordering of ocean circulation that resulted in rapid global warming and dramatic changes in the earth s environment

cenozoic evolution of the northern voring margin the northern voring volcanic margin was initiated by late campanian paleocene rifting culminating with massive breakup related igneous activity near the paleocene eocene transition paleocene uplift of the central rift zone led to subaerial erosion and deposition in a restricted basin west of the fles fault complex the western source area was active toward the end of eocene time in the east the low relief land surface underwent modest relative uplift which led to the construction of an early oligocene delta system on the trondelag platform this event was followed by a period of margin subsidence and modest sedimentation until late pliocene time although miocene and early pliocene biosiliceous hemipelagic sediments dominate on the outer margin the influx of ice rafted detritus records the climatic deterioration and the establishment of glaciers in late miocene and early pliocene time since ca 2 6 ma ongoing epeirogenic uplift of fennoscandia and the onset of northern hemisphere glaciation increased the erosion potential a huge prograding wedge of glacial sediments was constructed from ca 2 6 to 1 0 ma when the glacial mode changed from moderate relatively stable icecaps to distinct glacial interglacial cycles the wedge overlies the base of the late pliocene horizon which marks pronounced changes in lithology and physical sediment properties and corresponds to a distinct regional velocity inversion the differential glacial sediment load over unconsolidated and mobile biosiliceous oozes may have caused abundant small offset faulting and diapirism in the western voring basin

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

pore characterization and shale facies analysis of the ordovician silurian transition of northern guizhou south china the controls of shale facies on pore distribution abstract the ordovician silurian wufeng longmaxi formations in the upper yangtze platform have been considered as the most promising shale play in china however few studies of detailed shale facies and their influence on pore distribution have been conducted therefore it is important to describe the heterogeneity of shale facies in detail and elucidate their relationship to pore networks and development seven distinct facies were identified on the basis of their characteristic physical biological and chemical attributes ion milled samples from all facies were examined with sem for identification of pore types point counting was used to quantify porosity four major pore types appear dominant framework pores phyllosilicate framework pf pores intrapores within inorganic grains and organic matter om pores due to variability in composition and depositional setting of the different facies pore networks are not uniform framework pores between detrital grains and pf pores are widespread in bioturbated claystones pores in black siliceous shales are dominated by framework pores supported by authigenic silica most of these are filled with kerogen bitumen that is host to foam and bubble type om pores intrapores within calcite or dolomite grains are common in fossiliferous mudstones and black dolomitic siltstones but are isolated and do not contribute to productive porosity in muddy siltstones pf pores are well developed within silty bands initial framework pores within silty beds or laminae however were subsequently cemented by calcite or destroyed by compaction as most of primary pf pores and primary frameworks pores are lost during compaction porosity is highly dependent on the population of framework pores between authigenic quartz grains secondary cracking of amorphous organic matter added porosity after initial infilling of porosity by oil and bitumen migration therefore porosity is mainly controlled by abundance of authigenic silica and toc in spite of differences of burial diagenesis among various facies understanding these main controls on porosity should be helpful to predict and find and more porous facies and have a significant impact on future gas exploration in the wufeng longmaxi formations

carbonate equilibria and ph buffering by organic acids response to changes in pco2 abstract organic acid anions such as acetate a natural product of the thermal breakdown of kerogen may buffer the ph of natural formation waters the extent of ph buffering by acetate should determine whether an increase in the partial pressure of carbon dioxide p co 2 will lead to calcite dissolution porosity enhancement or calcite precipitation porosity destruction successful prediction of porosity therefore requires a quantitative understanding of this process this paper presents computer simulations of the response of the acetic acid carbonate system to changes in p co 2 for typical reservoir fluid compositions and temperatures the effects of variations in temperature ionic strength initial ca 2 concentration and the magnitude of the change in p co 2 were investigated natural formation waters from cenozoic reservoirs in the gulf coast basin and california show maximum organic acid concentrations at 100 20 c at 100 c the simulations show that for concentrations of acetic acid up to 0 06 m 3600 mg l 1 increases in the p co 2 of initially calcite saturated solutions will promote calcite undersaturation at higher temperatures equivalent buffering responses occur at lower acetic acid concentrations but acetic acid concentrations in natural formation waters also decrease rapidly with increasing temperature since present day concentrations of acetic acid in formation waters from cenozoic reservoirs rarely exceed 0 06 m increases in p co 2 will generally promote calcite dissolution by these waters in the gulf coast basin variation in organic acid concentrations as a function of reservoir age suggests that reductions in dissolved organic acid concentrations over geologic time have been relatively small it can be concluded therefore that at any point in the histories of these reservoirs an increase in p co 2 would have promoted calcite dissolution despite the effects of ph buffering by acetate since increases in p co 2 will in most cenozoic basins promote calcite dissolution knowledge of the location and timing of co 2 production in the subsurface can more easily be applied to the prediction of calcite cementation and dissolution

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

distribution and occurrence of aliphatic acid anions in deep subsurface waters abstract this study reports 144 new analyses of short chain aliphatic acid anions acetate propionate butyrate and valerate in formation waters from eight localities eastern venezuelan basin denver basin eastern green river basin red desert and washakie basins wamsutter arch san juan basin piceance basin raton basin gulf coast basin and the western overthrust reservoir temperature does not predict total or relative abundance of aliphatic acid anions but does predict maximum total concentrations of these species maximum concentrations increase to 90 c above 90 c maximum concentrations decrease above 250 c maximum concentrations should not exceed 1 mg 1 the general order of dominance is acetate propionate butyrate valerate but for coal associated waters is propionate acetate butyrate valerate lack of longer chain aliphatic acid anion dominance over acetate at low reservoir temperatures may suggest hydrologie communication with deeper reservoirs

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

the geochemical characteristics of the hydrate bound gases from the nyegga pockmark field norwegian sea the nyegga pockmark field located south east of the voring plateau norwegian sea was investigated on the uob cruises gs07 148 2007 and gs08 155 2008 remotely operated vehicle rov monitoring and successful recovery of gas hydrate give insights to the origin of the hydrocarbon gases in the complex pockmark feature named g11 isotope values of methane 13c ranging from 72 4 to 66 2 and d from 202 0 to 198 0 in the hydrate bound gas suggest that the methane is microbially produced and originates from co2 reduction it is proposed that the ethane from the gas hydrate has mixed thermogenic and microbial contributions which is supported by the fact that the gas hydrate also contains propane and isobutane the composition of the hydrate bound gas thus leads to small inclusions of structure ii sii in the hydrate while the major part is structure i si the isotope data and gas composition together with the geological setting suggest that the hydrate bound gas originated predominantly from a free gas system located beneath the gas hydrate stability zone and accumulated under glaciated debris formations the free gas system is mainly microbial methane from in situ production however a contribution of thermogenic gas from deeper polygonal faults to the free gas system seems evident from the composition of the gas the rov monitoring strengthens previous suggestions that the fauna in the complex pockmark g11 are supported by ongoing micro seepage

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

petrologic and experimental evidence for the etching of garnets by organic acids in the upper jurassic morrison formation northwestern new mexico abstract the upper jurassic morrison formation in the southern part of the san juan basin contains uniquely etched detrital garnets characterized by smooth crystallographically controlled faceted surfaces the etched garnets occur in relatively homogeneous morrison sandstones in discrete stratigraphic zones that are bounded above and below by sandstones bearing unetched garnets diagenetic alterations associated with etched garnets include etched staurolite skeletal plagioclase regularly interstratified illite smectite iron rich chlorite ankerite albite diagenetically altered organic poor uranium ore and extensive secondary porosity etching of garnets and partial to complete dissolution of other aluminosilicate minerals were caused by high concentrations of organic acids generated during the maturation of epigenetic organic matter predominantly type iii kerogen in the morrison formation the presence of authigenic phases that form near 100 c indicates that temperatures were high enough during diagenesis to cause the thermal degradation of kerogen the subsequent release of reactive organic acids promoted a variety of water rock reactions this hypothesis was corroborated by experiments in which dicarboxylic acid solutions created facets on garnets identical to those developed on naturally etched morrison garnets the excellent complexing ability of organic acid anions accounts for the characteristic lack of alteration products on etched surfaces

formation of authigenic carbonates at a methane seep site in the middle okinawa trough east china sea abstract the formation of authigenic carbonates at cold seep is closely related to underlying methane seepage and anaerobic oxidation of methane aom and these processes can be reflected in the geochemical characteristics of pore water in the sediments in terms of petrological mineralogical and geochemical analyses of research materials including authigenic carbonates and pore water samples from the middle okinawa trough east china sea these results show that authigenic carbonate minerals are mainly composed of high magnesium calcite low magnesium calcite dolomite and iron dolomite and exhibit extremely negative carbon isotopic compositions 13c 52 30 to 31 10 v pdb and positive oxygen isotopic compositions 18o 3 36 to 7 27 v pdb geochemical analyses of pore water suggest linear sulfate gradients and steep increase of methane and total alkalinity concentrations with depth which indicate sulfate induced aom activities based on the sulfate concentration reduction carbon isotope characteristics of dissolved inorganic carbon dic and increased methane concentration the sulfate methane interface smi is considered to be shallow these data combined with the variations in ion concentrations in pore water suggest that the leakage of biogenic methane and intense aom resulted in the formation of abundant authigenic carbonates which inherited light carbon 12c enriched isotopic signature of methane in the shallow sediments combined geochemical characteristics of pore water and authigenic carbonates intense methane seep activities and associated aom can be inferred both in present day and ancient geological records on the western slope of the middle okinawa trough

methane seepage inferred from the porewater geochemistry of shallow sediments in the beikang basin of the southern south china sea abstract several tens of methane ch4 seepage sites have been discovered in the northern south china sea scs during the investigation and exploration of gas hydrates and cold seeps over the past decade however our knowledge about methane seepage in the southern scs remains scarce especially in the beikang basin on the southern slope of the scs four piston cores bh cl18 bh cl23 bh cl29 and bh cl35 were collected in the beikang basin to understand ch4 cycling in shallow sediments the extrapolated depths of the sulfate methane transition zones smtz of the four cores are between 5 3 and 8 8 mbsf by simulating the porewater profiles of so42 ch4 dic and po43 using the reaction transport model we estimated that the dissolved so42 was predominantly consumed by the anaerobic oxidation of methane aom at rates of 27 5 mmol m 2 yr 1 in bh cl18 28 1 mmol m 2 yr 1 in bh cl23 43 1 mmol m 2 yr 1 in bh cl29 and 39 4 mmol m 2 yr 1 in bh cl35 respectively due to intense aom almost all the methane sourced from subsurface sediments was depleted within the smtz resulting in negligible benthic methane fluxes the kink type depth so42 profile of bh cl23 was apparently attributable to an enhanced methane flux which was not captured by the steady state modelling furthermore highly depleted 13c values of the dissolved inorganic carbon dic in bh cl18 43 6 bh cl23 44 and bh cl29 51 2 also indicated a methane derived dic in contrast relatively higher 13c values in bh cl35 suggested a mixture of 13c enriched dic produced by methanogenesis as shown by the reversal toward more 13c enriched dic below 4 2 mbsf in bh cl29 to our knowledge this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern scs and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern scs

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to discussion by p d lundegard and l s land abstract macgowan and s urdam 1990a suggested some modifications to the model of lundegard and land 1989 to make it more geologically and geochemically reasonable the predictive power of such a geochemical model is wholly dependent on the species modeled and the constants used any model that excludes important species or important thermodynamic data or one that couples certain reactions in an unrealistic way may produce results which are not geologically or geochemically reasonable w k harrison pers commun 1988 y k kharaka pers commun 1991 we have long recognized that under early burial diagenetic conditions aluminosilicate hydrolysis generally controls formation water ph surdam and eugster 1976 mariner and surdam 1970 taylor and surdam 1981 and that during intermediate burial either aqueous co2 or caa species in the absence of aqueous s species or other weak conjugate acid base pairs will dominate formation water alkalinity and control ph surdam et al 1989c we reassert that the model of lundegard and land 1989 does not take into account the relative importance of pco2 and of concentrations of both ca2 and caa and their relative organic metal complexes to carbonate mineral stability in sandstones in the zone of intermediate burial clastic diagenesis cf the models of surdam et al 1984 and surdam and crossey 1985 the usefulness of such models is predicted on the completeness of the model and the use of the best most accurate thermodynamic data also geologically realistic concentrations of critical species are required for reasonable modeling to be done although their model is vigorously defended in the discussion of lundegard and land 1989 lundegard and land 1993 we continue to disagree that their analysis of their model conditions are either geologically or geochemically satisfying we agree with the fundamental approach and philosophy of lundegard and land 1989 lundegard and land 1993 it is of the utmost importance to determine from experimental geochemical petrographic and geological data what the controls on ph and alkalinity in formation waters are as well as the exact thermodynamic speciation of aqueous moieties and the stability of detrital and authigenic minerals lundegard and land 1993 raise an additional point about caa reaction with carbonate minerals in shales although fisher and lewan 1989 lewan 1989 and macgowan and surdam 1990b have demonstrated that caa generated in shale likely migrate in the oil phase along incipient shale microfractures to the sandstone reservoir and thus are likely to not react much with the shale finally we agree with lundegard and land that these areas require much additional experimental and field analysis and petrographic study

an effect of source on po so generation evidenced by a deep focus earthquake po so phases generated by a deep focus earthquake focal depth 360 km were observed at epicentral distances between 11 and 14 in the northwestern pacific by an ocean bottom seismometer obs array the apparent velocity of po was about 8 3 km s which is higher than is observed for shallow focus earthquakes the high frequency long duration features of po so wave trains appear to be related to observed high q s in the deep dipping seismic zone lateral propagation of po so associated with a deep focus earthquake seems to begin when the p wave travels up the subducting slab and reaches the upper lithosphere beneath the trench axis also a waveguide phenomenon occurs in the dipping subduction slab itself wave trains generated near the source should be taken into consideration in the theory of po so generation

seafloor facies related to upward methane flux within a giant pockmark of the lower congo basin the origin of the cold fluid venting from a giant pockmark within the lower congo basin has been elucidated based upon results of precise mapping submersible dives gravity coring and isotopic analyses realized under a total ifremer cooperative project zaiango and biozaire projects during four dives of the ifremer rov victor 6000 the bottom was filmed hard and soft samples were lifted from the sediment and water samples were collected with a ctd rosette system the detailed dip map shows that the 800 m wide giant pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological mineralogical and chemical features on the seafloor leading to a seafloor anomaly recorded on the multibeam imagery methane rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate rich circulating sea water to produce hydrogen sulphide and bicarbonate ions this situation leads to a concentrical arrangement of the sedimentary facies with methane related features in the centre and sulphide related features at the periphery this organization is correlated with high levels of methane up to 20 mol l measured in the centre of the giant pockmark responsible for the crystallization of gas hydrates at the bottom in this model the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux

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

magnetotactic bacterial response to antarctic dust supply during the palaeocene eocene thermal maximum distinct magnetic properties of marine sediments that record the palaeocene eocene thermal maximum petm have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the petm we document the dominance of bacterial magnetite in deep sea sediments from southern kerguelen plateau ocean drilling program hole 738c southern ocean not only during the petm but also before and after the thermal event this occurrence of magnetofossils throughout the petm indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect instead we suggest that it is due to sustained mild iron reducing conditions that dissolved the most labile aeolian derived iron which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite enhanced aeolian haematite abundances at the beginning of the petm indicate drier conditions on the neighbouring antarctic continent at those times our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern kerguelen plateau with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the petm increased iron supply from aeolian dust that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor along with mild iron reducing diagenetic conditions seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied petm record from southern kerguelen plateau our analyses of a deep sea petm record from hole 1051b at blake nose atlantic ocean failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin contrasting magnetic properties at these petm sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep sea sediments

turnover of larger foraminifera during the paleocene eocene thermal maximum and paleoclimatic control on the evolution of platform ecosystems the larger foraminifera turnover lft during the paleocene eocene transition constitutes an important step in paleogene larger foraminifera evolution involving a rapid increase in species diversity shell size and adult dimorphism a platform to basin transect in egypt provides new data on timing and causal mechanisms through correlation with planktic biozonations and through integration with regional paleoenvironmental data the lft coincides with the boundary between shallow benthic biozones sbz4 and sbz5 and closely correlates with the paleocene eocene thermal maximum petm enhanced oligotrophy from the late paleocene onward favored the diversification of k strategist larger foraminifera we suggest that a short term eutrophication during the petm led to a temporary decline of extreme k strategist larger foraminifera species providing opportunities for new taxa with different ecological strategies to develop during post petm oligotrophic conditions these new taxa were able to evolve rapidly and soon dominated early eocene larger foraminifera assemblages whereas many paleocene taxa gradually disappeared the success of larger foraminifera during the early paleogene appears climatically controlled because of the vulnerability of corals to high surface water temperatures the late paleocene to early eocene global warming may have favored larger foraminifera at the expense of corals as the main carbonate producing component on carbonate platforms at lower latitudes

comparison and implications from strikingly different authigenic carbonates in a nyegga complex pockmark g11 norwegian sea authigenic carbonates sampled in a complex pockmark g11 from the nyegga area south western voring basin reveal strikingly different morphologies a comparative study of macro and micro features of these neighbouring samples was approached in order to attempt a reconstruction of the pockmark dynamics the carbonate morphologies include thin 1 cm platy carbonate crusts blocky and massive carbonate ridges up to several metres in size and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity our observations indicate that three distinct mechanisms of fluid seepage and carbonate precipitation occurred within the pockmark petrography studies reveal that the samples contain three generations of predominantly aragonitic cements in addition zoned crystals with coexisting mg calcite and dolomite are abundant in one sample suggesting a trend of gradual increase of the sulphate concentrations in the pore fluids during carbonate precipitation isotope measurements on bulk carbonate cements show consistent values with 13 c as low as 52 1 18 o as high as 6 4 and 87 86 sr of 0 7092 anaerobic oxidation of methane close to seafloor conditions can explain these isotope data the carbonates have a common carbon source despite the differences in morphology methane probably originated from the dissolution of gas hydrates as supported from calculated 18 o enrichment in the pore fluids and the methanogenic carbon we propose a model where the differences in bulk carbonate morphology are related to local sediment properties depth of carbonate precipitation and the flow rate of the seeping fluids

natural flux of greenhouse methane from the timor sea to the atmosphere 1 methane gas bubbles from the cornea seep were sampled at the sea surface in the timor sea continental shelf area in june 2005 total bubble gas flux was 0 076 to 0 76 l m 2 h 1 during the 6 h d 1 periods of low neap tides in june 2005 this bubble gas contained an average of 26 mmol ch4 l 1 and about 0 16 and 0 006 mmol l 1 of ethane and propane we estimate the daily flux from the sea surface to the atmosphere to be 0 012 to 0 12 mol ch4 m 2 d 1 or 0 13 to 1 3 t ch4 d 1 from an area of about 0 7 km2 this methane flux came from a 500 1400 m carbonate pavement dome on the seafloor at 84 m water depth the seep hard ground was swath mapped and 3 5 khz subbottom profile data indicate that the seep dome was strongly reflective with poor penetration into the subsurface consistent with the presence of a carbonate hard ground carbon and deuterium isotope ratios 13c 41 to 42 d 157 to 158 of the seep bubble gas indicate that this methane had a thermogenic origin and was in the same isotopic range as gas within the late cretaceous cornea oil and gas field we could not detect inputs of fluids containing nutrients or short lived radium isotopes at this site commonly associated with other cold seeps tens to a hundred of kilometers seaward from the cornea seep site water column dissolved methane concentrations in this sector of the timor sea shelf and slope were 100 500 times supersaturated with respect to the atmosphere and thus the water column is expected to be degassing additional methane to the atmosphere perhaps there are thousands of other methane seeps of similar magnitude to the cornea seep on this shelf and slope to account for all the excess dissolved methane 86 000 t measured in the water column these measured and calculated fluxes provide evidence for the hypothesis that shallow sea seeps may be a significant source of atmospheric methane in contrast to deep sea vents where most of the methane is dissolved and oxidized in seawater and does not reach the atmosphere

the impact of organic fluids on the carbon isotopic compositions of carbonate rich reservoirs case study of the lucaogou formation in the jimusaer sag junggar basin nw china abstract the processes involved in the interaction between organic fluids and carbonates and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates here we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic clastic and organic rich sedimentary sequence within the middle permian lucaogou formation in the junggar basin of nw china the techniques used during this study include drillcore observations thin section petrography scanning electron microscopy sem and electron probe microanalysis and carbon and oxygen isotope analyses oil grades and total organic carbon toc contents represent the amount of oil charging and the abundance of organic fluids within a reservoir respectively and both negatively correlate with the whole rock 13c and 18o of the carbonates in the study area indicating that organic fluids have affected the reservoir rocks secondary carbonates including sparry calcite and dolomite overgrowths and cements are common within the lucaogou formation well developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite rich reservoir rocks in this formation comparing thin section petrology with 13c compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates the net change in whole rock 13c as a result of these secondary carbonates ranges from 1 8 to 4 6 with the secondary carbonates having calculated 13c compositions from 18 6 to 8 5 that are indicative of an organic origin the positive correlation between the concentration of fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the lucaogou formation was relatively closed the correlation between 13c and 18o in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids however good initial correlation between 13c and 18o of whole rock carbonates within the lucaogou formation resulted from the evaporitic sedimentary environment was reduced by organic fluids to some extent consequently the 13c 18o covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water the characteristics and 13c compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic co2 to the reservoir further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic co2 however dissolved carbonate reprecipitated as secondary carbonates meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality although this process did enhance the dissolution of feldspar and increase porosity this indicates that the 13c and 18o of secondary carbonates and their influence on whole rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate rich reservoir systems

coupled c s fe geochemistry in a rapidly accumulating marine sedimentary system diagenetic and depositional implications in the present study we have investigated the c s fe systematics in a sediment core md161 13 from the krishna godavari k g basin bay of bengal the core covers the late holocene period with high overall sedimentation rate of 573 cm kyr 1 pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone smtz is at 6 mbsf the ta ca mg so42 ratios suggest that both organoclastic degradation and anaerobic oxidation of methane aom drive sulfate reduction at the study site the positive correlation between total organic carbon content toc and chromium reducible sulfur crs content indicates marked influence of organoclastic sulfate reduction on sulfidization coupled occurrence of 34s enriched iron sulfide pyrite with 12c enriched authigenic carbonate zones is the possible records of paleo sulfate methane transition zones where aom driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone aluminum normalized poorly reactive iron fepr al and la yb ratios suggest increasing contribution from deccan basalts relative to that of archean proterozoic granitic complex in sediment flux of krishna godavari basin during the last 4 kyr

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis abstract carboxylic acid anions caa in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water eh and ph and thus substantially contribute to controls on carbonate mineral stability and their ability to complex and transport al and si from the site of aluminosilicate mineral dissolution during diagenesis carboxylic acid anions are also extremely important to the aqueous geochemistry of ca fe mn pb and zn some formation waters from sedimentary basins contain high concentrations of caas the analyses of 20 formation waters from the san joaquin basin california and 20 formation waters from the louisiana gulf coast basin presented in this study show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional cca by comparison previously reported analyses indicate monofunctional caa occir in concentrations up to 10 000 ppm and difunctional caa may occur in concentrations up to 2610 ppm analyses of drilling muds and scale soaps presented in this study show that few if any difunctional caa in the study area can be attributed to contamination from these sources additionally aqueous extracts of crude oils contain both mono and difunctional acid anions as do the aqueous and petroleum phases of hydrous pyrolysates previously unreported dissolution experiments equilibrium computer simulations and hydrous pyrolysis experiments support those already published and suggest that caa are generated during thermal maturation of kerogen and expelled from the shale along oil wet microfractures upon entering the water wet sandstone pore the hydrophilic caa partition into the aqueous phase organic inorganic reactions may occur which from caa metal complexes because the complexes are hydrophobic they partition in the petroleum phase where present carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity certainly no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of a1 as well as the carbonate mineral diagenetic successions observed in sand shale systems world wide the utility of modeling these observations of organic inorganic diagenesis is limited only by the ability to model the concentration and distribution of caa through space and time

acoustic imaging of gas hydrate and free gas at the storegga slide 1 analysis of p wave velocities of an ocean bottom cable obc data set demonstrates the existence of gas hydrates and free gas on the northern flank of the storegga slide the distribution and concentration of gas hydrates and free gas show significant variation over the length of the obc average gas hydrate saturation of pore space is 5 when modeled by effective medium theory with hydrates as a sediment frame component average gas saturation is 0 45 assuming homogeneous distribution the bottom simulating reflector bsr is generally identified as termination of enhanced reflection but in some places appears as a reflection proper amplitude anomalies related to the bsr are primarily caused by the gas and not the hydrates the gas hydrates at the storegga slide develop from gas rich fluids migrating into the gas hydrate stability zone ghsz from the sediments below development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area the physical and geological properties of the sediments control the distribution and concentration of gas which migrates predominantly along strata rather than along the base of the ghsz this migration mechanism controls the flux of gas rich fluids into the ghsz and hence the distribution of gas hydrates and the variation in gas hydrate concentration

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

rates of anaerobic oxidation of methane and authigenic carbonate mineralization in methane rich deep sea sediments inferred from models and geochemical profiles abstract pore water chemical data obtained from a 10 5 m long giant gravity core collected in methane rich sediments from 647 m water depth in the northern gulf of mexico n 28 04 00 w 89 43 15 defines sub bottom gradients in unprecedented detail this core penetrated the sulfate methane interface smi at 300 cm below the seafloor cmbsf at the smi dissolved inorganic carbon dic concentrations reach a maximum 13 5 mm and pore water 13 c dic 63 2 pdb and 13 c methane 89 5 pdb values are most negative below the smi pore water sulfate is nearly depleted methane concentrations rise sharply with simultaneous occurrence of a bubble textured sediment and fine grained methane derived authigenic carbonate nodules and cements are common the sharp peaks in dic concentration and isotope values centered at the smi indicate that dic is being produced by anaerobic oxidation of methane aom within a narrow zone centered at the smi the detailed sulfate and dic concentration profiles and dic 13 c values have enabled geochemical models to be constructed that explore the rate of dic formation by aom and its effect on pore water dic 13 c values model results closely match measured dic concentration and 13 c isotope profiles and indicate that microbiological conversion of methane carbon to dic is rapid in geologic terms and that aom is occurring at the present position of the smi isotope values for authigenic carbonate found immediately below the present day smi 13 c 60 2 0 7 pdb at 440 cmbsf are consistent with derivation of the carbonate carbon from methane via aom at the former location of a smi these observations and model results suggest that aom is occurring at rates that would generate the observed profiles and begin the precipitation of methane derived carbonate occur on time scales of centuries model results also show that the time needed to produce the resulting authigenic cements is an order of magnitude greater than that for aom to produce the observed dic profiles the metabolic rates for dic production by aom inferred from modeling the geochemical profiles compare favorably with available rate data obtained from laboratory microbial incubations and radiolabeled tracer experiments

submarine pingoes indicators of shallow gas hydrates in a pockmark at nyegga norwegian sea complex pockmarks up to 300 m wide and 12 m deep are located in the nyegga area in the norwegian sea bathymetric data and direct visual documentation and sampling with rovs remotely operated vehicles have shown that these pockmarks contain abundant methane derived authigenic carbonate rocks furthermore geochemical results and the finding of seep associated organisms including tubeworms and bacteria shows that the pockmarks are still active fluid flow locations hovland m svensen h forsberg c f johansen h fichler c fossa j h jonsson r rueslatten h 2005 complex pockmarks with carbonateridges off mid norway products of sediment degassing marine geology 218 191 206 here we report the discovery of localized pingo like sediment mounds up to 1 m high and 4 m wide they occur inside one of the nyegga complex pockmarks g11 all of the seven structures we investigated have four characteristics in common 1 they have a positive topography rounded mounds and cones 2 they are partly covered in bacterial mats indicating ongoing fluid flow 3 they are partly covered in a carpet of small living tubeworms polychaetes which utilize methane 4 they have distinct corrosion pits on their surfaces indicating fluidization and point source corrosion of the covering sediments probably caused by localized sub surface hydrate dissociation we interpret the features as true submarine pingoes formed by the local accumulation of hydrate ice below the sediment surface it is inferred that the pingoes are formed as documented hydrocarbon gases methane ethane propane and butane migrate upwards through distinctive sub surface channels or conduits inside the pockmark we suggest that these submarine hydrate pingoes manifest the exact locations where fluid flow through the seafloor is currently active and that they can therefore be used as small scale indicators of active seepage

sulfate reduction using methane in sediments beneath a bathyal cold seep giant clam community off hatsushima island sagami bay japan two sandy sediment cores cores d227 120 and d380 were collected from inside a deep sea giant clam calyptogena soyoae community off hatsushima island western sagami bay central japan 35 59 9 n 139 13 6 e 1160 m deep and a muddy sediment core core d227 202 was obtained from outside the community by the submersibleshinkai 2000 the chloride concentration of the pore waters is constant vertically and sulfate reduction using sedimentary organic matter occurs in core d227 202 21 cm long the chloride concentrations are lower by 7 at the 7 5 9 cm depth in core d227 120 9 cm long and by 3 at the 11 12 cm depth in core d380 16 cm long than those of the overlying bottom waters in the cores from inside of the community sulfate concentration decreases remarkably and dissolved inorganic carbon alkalinity ammonium n and hydrogen sulfide concentrations increase significantly with increasing depth in core d380 34s values of sulfate ions increase from 20 5 to 35 3 and 13c values of dissolved inorganic carbon decrease drastically from 7 0 to 45 with increasing depth from the top to the bottom of the core although the 13c values of the organic carbon of the sediments are 23 7 0 9 in core d380 these results indicate that sulfate reduction using methane is active within the sediments just beneath the living clams and that the hydrogen sulfide produced can be used by endosymbiotic sulfur oxidizing bacteria living in the gills ofc soyoae in the community

pore water dissolved inorganic carbon sources and cycling in the shallow sediments of the haima cold seeps south china sea abstract pore water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane derived carbon in sediments in this study six sediment push cores were collected by manned submersible at the center and edge of the active haima cold seeps and a nearby background site pore water compositions of the so42 ca2 mg2 sr2 nh4 dissolved inorganic carbon dic and 13c dic at each sediments core were measured the very shallow depths of sulfate methane transition zone smtz suggest that high methane flux and significant anaerobic oxidation of methane aom activities occur in the shallow sediments reaction transport modelling results show that the relative contributions of aom and organoclastic sulfate reduction osr consuming sulfate are 84 1 and 15 9 at site hm 2 6 and 94 6 and 5 4 at site hm 3 3 respectively furthermore extremely low 13c dic values at the central sites hm 2 6 45 67 and hm 3 3 53 87 indicate an external methane derived dic based on the 13c mass balance we quantify the proportions of pore water dic derived from organic matter degradation and aom as 24 7 and 65 3 at site hm 2 6 and 9 3 and 80 7 at site hm 3 3 respectively an increase in pore water dic concentration leads to carbonate precipitation plots of sr ca vs mg ca ratios reveal that the carbonate phases in sediments are dominantly high mg calcite in this region our study demonstrates that aom can serve as a dominant biogeochemical process through incorporation of methane derived carbon into dissolved and solid inorganic carbon phases significantly influencing dic cycling within the shallow sediments of the haima cold seep area

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

a type of continuous petroleum accumulation system in the shulu sag bohai bay basin eastern china abstract this study analyzed crude oils from the lower part of the third member of the eocene shahejie formation es 3 l and three prospective source rocks from the shulu sag bohai bay basin eastern china using a variety of organic geochemical methods biomarker characteristics were used to interpret source rock organic matter input and depositional environment and oil source rock correlation the biomarker data indicate that the crude oils originated from the es 3 l source rock which contains a mixture of plankton and land plant organic matters deposited in brackish fresh water under reducing conditions the oil in the es 3 l is self sourced instead of migrated from the overlying source rocks the petroleum generation potential of the es 3 l source rock was evaluated using organic geochemistry total organic carbon toc values for approximately 100 samples are between 1 02 and 4 92 wt and hydrogen indices range from 285 to 810 mg hydrocarbons g toc the es 3 l source rock contains mainly type ii and iii kerogen and most of the samples are thermally mature the data show that the es 3 l source rock has good potential for liquid hydrocarbon generation the es 3 l rock also acts as the oil reservoir having very low bulk porosity and permeability various types of storage space in the marlstone and carbonate rudstone in the es 3 l of the shulu sag include 1 fractures 2 intergranular pores 3 dissolution pores 4 organic matter pores 5 intragranular pores and 6 seams around gravels pore size ranges from nanometers to millimeters because the oil was generated and stored in es 3 l strata which lack any obvious trap and seal and have low permeability the unit represents a continuous petroleum accumulation

organic geochemistry and reservoir characterization of the organic matter rich calcilutite in the shulu sag bohai bay basin north china abstract although extensive studies have been conducted on unconventional mudstone shales reservoirs in recent years little work has been performed on unconventional tight organic matter rich fine grained carbonate reservoirs the shulu sag is located in the southwestern corner of the jizhong depression in the bohai bay basin and filled with 400 1000 m of eocene lacustrine organic matter rich carbonates the study of the organic matter rich calcilutite in the shulu sag will provide a good opportunity to improve our knowledge of unconventional tight oil in north china the dominant minerals of calcilutite rocks in the shulu sag are carbonates including calcite and dolomite with an average of 61 5 wt the carbonate particles are predominantly in the clay to silt size range three lithofacies were identified laminated calcilutite massive calcilutite and calcisiltite calcilutite the calcilutite rocks including all the three lithofacies in the third unit of the shahejie formation in the eocene es3 have total organic carbon toc values ranging from 0 12 to 7 97 wt with an average of 1 66 wt most of the analyzed samples have good very good or excellent hydrocarbon potential the organic matter in the shulu samples is predominantly of type i to type ii kerogen with minor amounts of type iii kerogen the temperature of maximum yield of pyrolysate tmax values range from 424 to 452 c with an average of 444 c indicating most of samples are thermally mature with respect to oil generation the calcilutite samples have the free hydrocarbons s1 values from 0 03 to 2 32 mg hc g rock with an average of 0 5 mg hc g rock the hydrocarbons cracked from kerogen s2 yield values in the range of 0 08 57 08 mg hc g rock with an average of 9 06 mg hc g rock and hydrogen index hi values in the range of 55 749 mg hc g toc with an average of 464 mg hc g toc the organic rich calcilutite of the shulu sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window the pores in the shulu calcilutite are of various types and sizes and were divided into three types 1 pores within organic matter 2 interparticle pores between detrital or authigenic particles and 3 intraparticle pores within detrital grains or crystals fractures in the shulu calcilutite are parallel to bedding high angle and vertical having a significant effect on hydrocarbon migration and production the organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the shulu sag

the potential volume of oceanic methane hydrates with variable external conditions gas hydrate abundance in marine sediment depends on gas concentration and the available pore space within certain stability limits potentially gas hydrates can occur between the seafloor and a locus of subbottom depths where geothermal gradients intersect gas gas hydrate pore water equilibrium curves perpendicular to a given continental margin the lens shaped area between these two bounding surfaces a sl varies according to seven basic parameters gas composition water activity a w bottom water temperature t b geothermal gradient g slope depth z slb slope gradient z and sea level relative to the shelf break z 0 assuming pure ch 4 gas 35 km 2 of sediment can host gas hydrate across an average continental margin at a pleistocene lowstand a w 0 981 t b 0 c g 0 c z slb 4000 m z 0 04 z 0 0 however this potential area would decrease with smaller a w higher t b greater g shallower z slb steeper z and lower z 0 and increase with opposite external conditions of the basic parameters temperature t b and g and bathymetry z slb and z can particularly influence the distribution of gas hydrate on continental slopes a hydrothermal gradient with e g surface temperatures t b will also decrease a sl although minimally especially if t b is warm the sum of parallel cross sectional areas along a margin combined with porosity o gives the potential volume of gas hydrate v assuming 200 000 km of continental margin with a o of 50 3 5 10 6 km 3 of pore space can contain gas hydrates at present day a volume that compares favorably with previous estimates 1 2 to 6 4 10 6 km 3 although underlying approaches differ fundamentally since the triassic v glob probably has increased significantly because t b has cooled while total margin length has grown this trend was likely punctuated by at least one major decrease nominally 1 5 to 0 7 10 6 km 3 when t b suddenly rose by 5 c during the latest paleocene thermal maximum lptm a prominent global negative 13 c excursion across the lptm may signify massive release of ch 4 associated with this theoretical drop in v glob

methane source linked to gas hydrate system at hydrate drilling areas of the south china sea porewater geochemistry and numerical model constraints abstract methane of various origins is widespread in the sediments of continental margins representing a key component of the carbon cycle in the ocean its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column however this type of methane is difficult to track due to complex reactions occurred during its long distance migration porewater geochemical analyses of two 7 m piston cores d17 15 and w01 16 combined with a reaction transport model were applied to quantify methane related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of shenhu and dongsha in the south china sea scs the model results revealed that anaerobic oxidation of methane aom is the predominant biogeochemical process in both cores consuming approximately 85 and 74 of sulfate in the modeled domain respectively the crossplot of dissolve inorganic carbon dic accumulation versus sulfate depletion of the system suggests that methane consumed during the aom is mainly originated from external methane sources rather than local methanogenesis using a 13cdic mass balance calculation together with model derived fractions of different dic sources we estimated that the 13c value of the external methane in core d17 15 is 66 6 which is in accordance with the previous reported 13c values of hydrate bound methane in the area these results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the scs is mainly originated from local gas hydrate reservoir the proposed approach has the potential to be used to differentiate the sources of methane which will provide constraints on dynamics of methane in gas hydrate bearing marine sediments

the impact of diffusive transport of methane on pore water and sediment geochemistry constrained by authigenic enrichments of carbon sulfur and trace elements a case study from the shenhu area of the south china sea abstract sulfate driven anaerobic oxidation of methane sd aom controls methane release from marine sediments to the ocean a variety of authigenic precipitates within sediments has been widely used to identify past occurrences of sd aom however the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of sd aom in past environments characterized by diffusive transport of methane carbon sulfur and trace element geochemistry of pore water and sediments was investigated at a site affected by upward methane diffusion in the shenhu area of the south china sea here the sulfate methane transition zone smtz is located 7 6 m below the seafloor based on sulfate and methane concentrations the slope of 34s vs 18o values of sulfate is consistent with diffusive transport of methane concentration and isotope profiles of pore water species point to diffusive rather than advective transport of solutes enhanced sulfate reduction inferred from 34s values of sulfate agrees with the local occurrence of relatively abundant 34s enriched iron sulfide minerals the time required to produce the observed authigenic iron sulfides around the smtz is estimated to be 1 1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate no enrichment of authigenic carbonate barium ba and molybdenum mo was detected in the studied sediments this observation is consistent with the calculation that the current fluxes of pore water calcium magnesium ba and mo are too low to allow for authigenic enrichments the low fluxes are largely controlled by diffusion which facilitates the formation of 34s enriched iron sulfide minerals the observed enrichment patterns unlike those of sediments affected by advective seepage are expected to be prevalent in modern and ancient continental margin sediments and may contribute to the identification of past methane rich zones and overlying sd aom zones as sinks for methane in the geological record

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to critical comment by j s hanor l s land and g l macpherson

the blast in the past the rapidity and amount of carbon being released through human agency have been thought to be unprecedented not so it emerges from the evidence of anomalies in the ratios of carbon isotopes in an ocean core it seems that around 55 million years ago there was an equivalently swift and massive blast of carbon into the ocean and atmosphere the source was probably methane hydrates that are usually locked up in huge deposits on continental shelves

stratigraphic development of the south voring margin mid norway since early cenozoic time and its influence on subsurface fluid flow abstract the cenozoic seismic stratigraphy and geological development of the south voring margin are analyzed to understand their relation to fluid flow and margin stability the regional stratigraphy and palaeomorphology of the more and voring basins indicate gradual changes in depositional environment and tectonic compression between 55 ma to 2 8 ma during brygge and kai periods and abrupt changes associated with glacial interglacial cycles from last 2 8 ma during naust period these changes resulted in deposition of various types of sediments and led to processes such as polygonal faulting and dewatering inter fingering of contouritic stratified and glacigenic sediments and margin progradation a gas hydrate related bottom simulating reflector bsr occurs at nyegga and within the central voring basin while pockmarks are observed at nyegga only diagentic reflectors due to opal a opal ct conversion dbsrs occur along a wider area beyond the shelf edge the dbsrs are located in oozes within the kai and late brygge formations the gas hydrate bsr occurrence is concentrated above eocene depocenters in hemipelagic and contouritic sediments deposited during late plio pleistocene the bsr overlies polygonal faults and dbsrs but are confined to the slope of anticlines indicating its formation being related to fluid pathways from methanogenic rocks through focused fluid flow microbial gas production in kai brygge and deeper formations may have supplied the gas for gas hydrate formation fluid expulsion due to dbsr formation and polygonal faulting in oozes may have created overpressure development in permeable layers belonging to the overlying naust formation slide headwalls are also located close to the anticlines in the study area implying that over pressured oozes and focussed fluid flow may have been important in creating weak surfaces in the overlying naust sediments promoting conditions for failures to occur

application of organic petrography in north american shale petroleum systems a review abstract organic petrography via incident light microscopy has broad application to shale petroleum systems including delineation of thermal maturity windows and determination of organo facies incident light microscopy allows practitioners the ability to identify various types of organic components and demonstrates that solid bitumen is the dominant organic matter occurring in shale plays of peak oil and gas window thermal maturity whereas oil prone type i ii kerogens have converted to hydrocarbons and are not present high magnification sem observation of an interconnected organic porosity occurring in the solid bitumen of thermally mature shale reservoirs has enabled major advances in our understanding of hydrocarbon migration and storage in shale but suffers from inability to confirm the type of organic matter present herein we review organic petrography applications in the north american shale plays through discussion of incident light photographic examples in the first part of the manuscript we provide basic practical information on the measurement of organic reflectance and outline fluorescence microscopy and other petrographic approaches to the determination of thermal maturity in the second half of the paper we discuss applications of organic petrography and sem in all of the major shale petroleum systems in north america including tight oil plays such as the bakken eagle ford and niobrara and shale gas and condensate plays including the barnett duvernay haynesville bossier marcellus utica and woodford among others our review suggests systematic research employing correlative high resolution imaging techniques and in situ geochemical probing is needed to better document hydrocarbon storage migration and wettability properties of solid bitumen at the pressure and temperature conditions of shale reservoirs

po so synthetics for a variety of oceanic models and their implications for the structure of the oceanic lithosphere summary polso synthetic seismograms were computed for a variety of oceanic structures in order to model data from an t zb 5 7 earthquake recorded during the oss iv seismic experiment satisfactory modelling of polso waveshapes and frequency content was achieved with no lateral heterogeneity but with a small random vertical heterogeneity in the mantle superimposed on a mean velocity structure which is consistent with seismic refraction data the random heterogeneity in the p and s velocities appears to be about 2 per cent with a scale length of 5 km this kind of heterogeneity is easily achieved by varying slightly the major mineral components in either the peridotite or eclogite mantle mineralogy and can be another cause of observed upper mantle anisotropy using a step function source time behaviour our numerical modelling also indicates that in the upper mantle qp is roughly proportional to rising from about 450 at f 1 hz to about 800 at f 15 hz and that q is roughly proportional to rising from about 900 at f 1hz to about 1800 at f 15hz a slight decrease in q from these values in any small zone of the upper mantle reduces so amplitudes drastically and we believe this is why so is sometimes missing from observed seismograms the computed synthetic record sections were also used to examine the relation between intrinsic q and the apparent q inferred from data in earlier studies apparent q was found to be lower than intrinsic q below about 5hz and higher than intrinsic q above 5 hz when these differences are accounted for the mantle q s of this study agree with the mantle q s of butler et al 1987 to within 10 per cent there can no longer be any doubt that oceanic upper mantle q s are very large above 5 hz and that q is about twice as large as qp

distribution and significance of dicarboxylic acid anions in oil field waters the origin distribution and interactions of low molecular weight organic acid anions have become an intensively studied field in geochemistry since their widespread occurrence in formation waters of sedimentary basins was first documented by carothers and kharaka 1978 high concentrations up to 10 000 mg 1 of monocarboxylic mainly acetate propionate and butyrate and dicarboxylic mainly oxalate malonate and succinate acid anions have been reported from many sedimentary basins with the highest values present in relatively young cenozoic age petroleum reservoir rocks at subsurface temperatures of 80 120 c see lundegard and kharaka 1990 for a recent review with references geochemical interest in these organic anions stems mainly from their important role in mineral diagenesis in sedimentary basins kharaka et al 1986 macgowan and surdam 1990 in particular these species act as sources or sinks of protons as a source of co2 and as ph and eh buffering agents lundegard and kharaka 1990 they also form complexes with cations and metals such as ca a1 fe pb and zn kharaka et al 1985 harrison and thyne 1992 data on the concentrations of monocarboxylic acid anions in subsurface waters from many sedimentary basins worldwide are available and are generally accepted as representing the aqueous concentrations at depth data on the concentrations of dicarboxylic acid anions are much more limited some reported values are controversial and the total values reported range widely from 0 to 2640 mg 1 surdam et al 1984 kharaka et al 1986 barth 1987 macgowan and surdam 1988 1990 because of the wide range in the reported concentrations and because dicarboxylic acid anions generally form stronger complexes with a1 fe and other cations than do monocarboxylic acid anions we resampled four oil wells in the san joaquin basin california in order to better assess the role of these acid anions in water rock interactions in sedimentary basins

predictive models for sandstone diagenesis abstract factors governing the evolutionary path that sandstone and shale sequences follow during burial diagenesis include provenance and depositionally controlled compositional and textural elements near surface redox reactions organic inorganic interactions within the zone of intermediate burial diagenesis deep diagenetic reactions which include abiotic thermal sulfate reduction carbonate mineral alteration and quartz cementation and the potential for meteoric influx and renewal of near surface processes due to uplift exposure and or base level fluctuation as a consequence the use of thermal exposure indicators to describe the extent of diagenesis such as r0 or time temperature history indicators such as tti must be used with caution though useful in predicting general trends in porosity loss due to simple compaction they have little utility in making forward predictions of anomalously high porosity e g preserved and or enhanced porosity or anomalously low porosity a more useful approach whereby the processes controlling porosity evolution are evaluated in terms of the depositional hydrologic burial and thermal histories is presented here

analysis of methane and sulfate flux in methane charged sediments from the mississippi canyon gulf of mexico abstract sediment porewater geochemical data so4 2 ch4 dic 13c dic and cl were obtained from piston cores collected in atwater valley gulf of mexico prior to 2005 drilling to study gas hydrates in the region the geochemical data were used for a study of shallow sediment ch4 cycling on a seafloor mound mound f where an apparent upward deflection of the bottom simulating reflector bsr suggested vertical fluid advection fifteen sediment cores ranging from 300 to 800 cm long were collected from locations on top of the mound and across a transect up to 3 5 km off the mound the sulfate methane transition smt was determined in each core from porewater so4 2 and ch4 concentration profiles and occurred at depths ranging from 0 to 410 cm below the seafloor cmbsf the shape of porewater so4 2 profiles plotted against depth also varied from linear to non linear along the transect diffusion rates estimated from linear so4 2 concentration gradients ranged from 20 4 to 249 1 mmol m 2 a 1 with the greatest rate measured in sediments on the mound the large variation in smt depth and so4 2 profiles along the transect indicates lateral differences in total vertical ch4 flux between locations results suggest steady state and non steady state ch4 fluxes both on the mound and transitioning off the mound and likely differences in the relative contribution of fluid advection to local shallow sediment ch4 cycling cores collected from on the mound had high porewater headspace ch4 concentrations up to 8 34 mm coupled with elevated cl concentrations up to 956 5 mm at shallow depths suggesting that salt diapirism in deep sediments may be inhibiting hydrate stability and increasing vertical ch4 flux

multiple episodes of fluid flow in the sw barents sea loppa high evidenced by gas flares pockmarks and gas hydrate accumulation abstract gas and gas hydrates have gained considerable attention in the last 20 yr due to their possible impact on global climate change from methane escalations our study area the sw barents sea is unique in its evolution due to the effect of glaciations which removed thick layers of sediment from the seabed unloading due to erosion and deglaciation resulted in opening of pre existing faults and creation of new ones facilitating fluid migration and eventually escape into the water from the subsurface expressions of hydrocarbon gas accumulation and fluid flow such as gas hydrates and pockmarks are widely distributed in the barents sea several gas flares some of them 200 m high in echograms occur along a segment of the ringvassoy loppa fault complex indicating open fractures and active fluid flow observation of gas flares along regional fault complexes outside pockmark regions indicates that present gas escape occurs along faults only the pockmark formation and gas flares belong to different episodes of fluid flow the pockmarks are absent outside soft sediment depocentres indicating that their existence is related to the presence of recording medium the thin sediment cover in pockmarks and their penetration down into glaciomarine sediments indicate that they formed after deposition of these sediments and that fluid escape was active to the very recent past methane hydrate stability zone mhsz modelling shows that after the last glacial maximum 18 000 20 000 14c years ago the mhsz has thinned from 600 m to zero in most parts of the sw barents sea the fluid expulsion probably happened after retreat of the glaciers causing release of methane from dissociation of methane hydrates through fluid escape processes which lasted until recently bottom simulating reflectors bsr observed are probably due to structure ii hydrates formed from gas containing higher order hydrocarbon components such as ethane propane etc along with methane the study area falls close to the new gigantic oil discoveries skrugard and havis and north of the snohvit hydrocarbon field

environmental perturbation in the southern tethys across the paleocene eocene boundary dababiya egypt foraminiferal and clay mineral records foraminiferal and clay mineral records were studied in the upper paleocene to lower eocene dababiya section egypt this section hosts the gssp for the paleocene eocene boundary and as such provides an expanded and relatively continuous record across the paleocene eocene thermal maximum petm deposition of illite smectite clay minerals is interpreted as a result of warm and arid conditions in the southern tethys during the latest paleocene benthic foraminiferal assemblages are indicative of seasonal variation of oxygen and food levels at the seafloor a sea level fall occurred in the latest paleocene followed by a rise in the earliest eocene foraminiferal diversity and densities decreased strongly at the p e boundary coinciding with the level of global extinction of benthic foraminifera bee and start of the carbon isotope excursion cie and petm in the lower cie the seafloor of the stratified basin remained nearly permanently anoxic and azoic a sudden increase in mixed clay minerals kaolinite and others suggests that warm and perennial humid conditions prevailed on the continent high levels of toc and phosphathic concretions in the middle cie are evidence for increased organic fluxes to the sea floor related to upwelling and to augmented continental runoff low densities of opportunistic taxa appeared indicating occasional ephemeral oxygenation and repopulation of the benthic environment the planktic community diversified although conditions remained poor for deep dwelling taxa an increase in illite smectite dominated clay association is considered to mark the return of a seasonal signature on climatic conditions during the late cie environmental conditions changed to seasonally fluctuating mesotrophic conditions and diverse and rich benthic and planktic foraminiferal communities developed post cie planktic faunas consisted of both deep and shallow dwelling taxa and buliminid dominated benthic assemblages reflect fluctuating mesotrophic conditions the frequent environmental perturbations during the cie petm at dababiya provided a rather specialized group of foraminiferal taxa i e anomalinoides aegyptiacus the opportunity to repopulate survive and subsequently dominate by a hypothesized capacity to switch to an alternative life strategy population dynamics habitat shift or different metabolic pathway the faunal record of dababiya provides insight into the cause and development of the bee various severe global changes during the petm e g ocean circulation caco3 dissolution productivity and temperature changes disturbed a wide range of environments on a geologically brief timescale explaining together the geographically and temporally variable character of the bee this allowed a number of specific but different foraminiferal assemblages composed of stress tolerant and opportunistic taxa to be successful during and after the periods of environmental perturbations associated with the petm

impact of anaerobic oxidation of methane on the geochemical cycle of redox sensitive elements at cold seep sites of the northern south china sea abstract cold hydrocarbon seepage is a frequently observed phenomenon along continental margins worldwide however little is known about the impact of seeping fluids on the geochemical cycle of redox sensitive elements pore waters from four gravity cores d 8 d 5 d 7 and d f collected from cold seep sites of the northern south china sea were analyzed for so42 mg2 ca2 sr2 dissolved inorganic carbon dic 13cdic dissolved fe mn and trace elements e g mo u the sulfate concentration depth profiles 13cdic values and dic ca2 mg2 so42 ratios suggest that organoclastic sulfate reduction osr is the dominant process in d 8 core besides osr anaerobic oxidation of methane aom is partially responsible for depletion of sulfate at d 5 and d 7 cores the sulfate consumption at d f core is predominantly caused by aom the depth of sulfate methane interface smi and methane diffusive flux of d f core are calculated to be 7 m and 0 035 mol m 2 yr 1 respectively the relatively shallow smi and high methane flux at d f core suggest the activity of gas seepage in this region the concentrations of dissolved uranium u were inferred to decrease significantly within the iron reduction zone it seems that aom has limited influence on the u geochemical cycling in contrast a good correlation between the consumption of sulfate and the removal of molybdenum mo suggests that aom has a significantly influence on the geochemical cycle of mo at cold seeps accordingly cold seep environments may serve as an important potential sink in the marine geochemical cycle of mo

assessing methane release from the colossal storegga submarine landslide 1 marine slope failure involving methane gas hydrate bearing sediments is one mechanism for releasing enormous quantities of methane to the ocean and atmosphere the storegga slide on the norwegian margin is the largest known holocene aged continental margin slope failure complex and is believed to have occurred in sediments that may have initially contained gas hydrate here we report pore water sulfate gradient measurements that are used as a proxy for the relative amounts of methane that exist in continental margin sediments associated with the colossal storegga slide these measurements suggest that a considerable inventory of methane occurs in sediments adjacent to and unaffected by the storegga slide events but indicate that methane is notably absent from sediments on the sole of the slide and distal deposits created by the slide events either methane was lost during previous pleistocene failure events or was never present in significant concentrations within the sediments that failed

water rock interaction in tertiary sandstones san joaquin basin california u s a diagenetic controls on water composition three water types are recognized among petroleum associated waters of the southern san joaquin basin 1 meteoric water 2 diagenetically modified connate water and 3 mixed meteoric modified connate water the diagenetically modified connate marine water can be further subdivided into three types 1 fully mg depleted mg 1 mole of the sum of na k ca and mg 2 partially mg depleted and 3 ca enriched enrichment in ca which is the sharpest chemical divide found for the diagenetically modified connate water is the result of feldspar albitization the ca enriched waters are also rich in total dissolved al up to 6 mg l 1 and exhibit lighter 18o values than other diagenetically modified connate waters the total dissolved al abundance in these waters may be due to the presence of organic complexing agents simple and aromatic organic acids and phenolics the lighter 18o values are probably due to both albitization and the lack of substantial clay diagenesis illitization of smectite

high resolution 3d seismic data indicate focussed fluid migration pathways above polygonal fault systems of the mid norwegian margin seismic attribute analysis and interpretation of high resolution 3d and 2d seismic data reveal focussed fluid flow processes through the gas hydrate stability zone ghsz at the northern flank of the giant storegga slide diffusive fluid migration predominantly starts from a widespread polygonal fault system in fine grained sediments of the miocene kai formation the overlying 600 700 m thick plio pleistocene naust formation shows spatially related soft sediment deformation and overlying fluid conduits a low relief antiform structure connects to an overlying 250 m high 300 m wide and 3 km elongated columnar zone where seismic signatures suggest self enhanced permeability i e natural hydraulic fracturing push down effects create an elongated depression caused by increased gas accumulations where a cluster of vertical cylindrical acoustic pipe structures originates these pipe clusters pierce the ghsz and indicate focussed fluid flow pathways originating from potentially overpressured sediments high seismic reflection amplitudes at the seafloor above the pipe structures may indicate pockmarks with authigenic carbonates and or gas hydrates the observed objects and seismic features presented are not stand alone indicators for fluid flow but a joint perspective illustrates that they are vertically tied together providing new insights to the effects of focussed fluid flow

oilfield waters and sandstone diagenesis abstract the evolution of the organic geochemistry and carbonate alkalinity of oilfield waters is apparently regular and predictable this evolution can be typified by five generalizations 1 at or near 80 c there appears to be an exponential rise in the concentration of organic acid anions 2 the maximum concentration of organic acid anions occurs over approximately the 80 100 c temperature interval 3 the highest concentrations of difunctional acid anions are associated with the other organic species maxima 4 difunctional acid anions are the first to be decarboxylated typically at temperatures of 100 110 c 5 with increasing temperature 110 130 c monofunctional acid anions begin to become decarboxylated resulting in a fluid alkalinity dominated by bicarbonate dissolution experiments using artificial and natural oilfield waters demonstrate that mono and difunctional carboxylic acid anions and hydroxybenzoic acid anions present in both oilfield waters and the aqueous phase of hydrous pyrolysates are capable of greatly enhancing al si fe and ca concentrations in solutions from dissolution of minerals by organometallic complexation this enhancement of mineral solubility has been called upon to explain aluminosilicate dissolution porosity which is quantitatively important in many subsurface reservoirs certainly no other viable mechanism has been proposed to explain aluminum transport in the subsurface when integrated into basin models the regular evolution of organic and carbonate alkalinity in oilfield waters and the changing mineral stabilities that accompany that evolution help explain commonly observed diagenetic sequences in clastic systems

authigenic carbon entombed in methane soaked sediments from the northeastern transform margin of the guaymas basin gulf of california extensive rov based sampling and exploration of the seafloor was conducted along an eroded transform parallel fault scarp on the northeastern side of the guaymas basin in the gulf of california to observe the nature of fluids venting from the seafloor measure the record left by methane venting on the carbonates from this area and determine the association with gas hydrate one gas vent vigorous enough to generate a water column gas plume traceable for over 800 m above the seafloor was found to emanate from a 10 cm wide orifice on the eroded scarp face sediment temperature measurements and topography on a sub bottom reflector recorded in a transform parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent active chemosynthetic biological communities cbcs and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform parallel scarp the carbon isotopic composition of methane bubbles flowing vigorously from the gas vent 53 6 0 8 pdb is comparable to methane found in sediment cores taken within the cbcs distributed along the scarp 51 9 8 1 pdb however the 13c value of the co2 in the vent gas 12 4 1 1 pdb is very distinct from those for dissolved inorganic carbon dic 35 8 to 2 9 pdb found elsewhere along the scarp including underneath cbcs the 13c values of the carbonate rich sediments and rocks exposed on the seafloor today also span an unusually large range 40 9 to 12 9 pdb and suggest two distinct populations of authigenic carbonate materials were sampled unconsolidated sediments and some carbonate rocks which have lithologic evidence for near seafloor formation have negative 13c values while carbonate rocks that clearly formed in the subsurface have positive 13c values up to 23 0 close to that measured for co2 in the vent gas there appears to be two carbon sources for the authigenic carbonates 1 deeply sourced isotopically heavy co2 12 and 2 isotopically light dic derived from local anaerobic oxidation of methane at the sulfate methane interface in the shallow subsurface addition of isotopically light methane derived carbon at the seafloor may completely mask the isotopically heavy co2 signature 12 4 in the underlying sediments thus the authigenic carbonates may have formed from the same methane and carbon dioxide bearing fluid but under different migration and alteration conditions depending on how it migrated through the sediment column

sources of methane inferred from pore water δ13c of dissolved inorganic carbon in pockmark g11 offshore mid norway abstract pockmark g11 is the most spectacular one among the pockmarks located at the southern border of voring plateau and 1 2 km away from the northern flank of the storegga slide mid norwegian continental margin for the first time detailed pore water geochemical studies were conducted to address methane hydrate occurrence methane seepage and associated geochemical processes and methane characteristics in the pockmark pore waters collected from five sediments cores inside and one sediment core outside the pockmark were analyzed for dissolved cl sulfate so42 total hydrogen sulfide h2s ca2 mg2 dissolved inorganic carbon dic 13cdic and 18o methane hydrates were recovered in all sediments below 0 75 m in a core inside pockmark g11 which is in good accordance with heavy oxygen isotope 1 9 to 2 3 smow and low cl concentrations 84 9 to 16 1 mm in pore waters pore water profiles indicate that upward migrating methane fluids are spatially variable in the pockmark with methane fluxes ranging from below detection in the center and outside to 0 30 0 54 mol m 2 a 1 inside in the cores with active methane fluxes maximum dic concentrations 19 4 to 21 5 mm and corresponding minimum 13cdic values 52 3 to 54 6 pdb occur within sulfate methane transition smt zones from 0 40 to 0 50 m below seafloor mbsf close to the seafloor simple mass balance modeling and 13cdic measurements within the smt zones suggest that methane in shallow sediments within pockmark g11 is microbial in origin pore water geochemistry and seabed observations suggest that methane fluxes inside pockmark g11 are episodic and derived mostly from the recycling of methane hydrate at depth during sediment burial

fluid geochemistry of the acqui terme visone geothermal area piemonte italy the main geothermal reservoir of acqui terme visone hosts na cl waters which are in chemical equilibrium at 120 130 c with typical hydrothermal minerals including quartz albite k feldspar illite chlorite or smectite anhydrite calcite and an unspecified ca al silicate in the acqui terme visone area these geothermal waters ascend along zones of high vertical permeability and discharge at the surface almost undiluted or mixed with cold shallow waters to the sw of acqui terme other ascending geothermal waters either undiluted or mixed with low salinity waters enter relatively shallow secondary reservoirs where they reequilibrate at 65 70 c both chemical and isotopic data indicate that bacterial so4 reduction affects all these waters especially those discharged by the secondary reservoirs therefore geothermal waters must get in contact with oil acquiring the relatively oxidized organic substances needed by so4 reducing bacteria this oil water interaction process deserves further investigations for potential economic implications

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

marcasite in black shales a mineral proxy for oxygenated bottom waters and intermittent oxidation of carbonaceous muds abstract black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions although bottom water anoxia favors deposition of black shales certain biological indicators such as benthic fossils and bioturbation suggest that bottom water conditions may not have been as persistently anoxic as commonly presumed examination of twelve marine carbonaceous shale bearing units ranging in age from middle proterozoic to cretaceous suggests that in addition to paleontological clues early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen bearing bottom waters marcasite requires low ph values to form and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition in the studied examples degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low ph values required for marcasite formation thus observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed the latter consideration may for example prove important for those instances where a black shale is associated with an oceanic anoxic event oae because these are commonly presumed to have been deposited under fully anoxic conditions

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis critical comment the analyses of carboxylic acid anions caas in san joaquin basin and louisiana gulf coast formation waters published by macgowan and scrdam 1990 do little to resolve the important question of whether caas are of great significance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity as advocated by these two authors comparison of their analyses with analyses of formation waters from the same wells previously published by three independent research groups hanor and workman 1986 land et al 1988 and land and macpherson 1989 and fisher and boles 1990 a body of data not discussed in their article reveals that most of the macgowan and surdam values are grossly inflated relative to the previous analyses table 1 fig 1 cedures of llco et al 1982 while field procedures for caas now normally include the addition of a bactericide to the sample an independent check on the upper limit of total caas which could have been present in this water is provided by the results of an alkalinity titration to ph 2 5 also published by hanor and workman 1986 which show that the total concentration of weak acid anions bicarbonate plus caas could not have exceeded approximately 428 mg 1 the alkalinity determinations were done shortly after collection on separate untreated aliquots stored in gas tight glass containers further details are given in workman 1985

structural evolution of the voring basin norway during the late cretaceous and palaeogene the voring basin forms an integral part of the passive margin off central norway three phases of extension and three phases of compression record the transition from rifting to sea floor spreading between norway and greenland the first regional extensional event is of early cenomanian age causing relative uplift of the gjallar ridge and development of a regional depocentre in the ras basin a second extensional event is represented by late campanian faulting on the nyk high and along the fles fault complex a compressional event in the late maastrichtian led to inversion of the r s basin this was followed by a regional extensional event in late palaeocene leading to the continental break up between norway and greenland in earliest eocene after the onset of sea floor spreading the voring basin has remained in overall compression with two major compressional events identified in early eocene and mid oligocene

the campo section pyrenees spain revisited implications for changing benthic carbonate assemblages across the paleocene eocene boundary abstract the campo section is a parastratotype section for the shallow water ilerdian and cuisian stages although a lot of work has been concentrated during the last years on biostratigraphy sequence stratigraphy magnetostratigraphy and geochemistry surprisingly little work has been published on the facies development here we evaluate the facies evolution with particular emphasis on the biogenic assemblages of the latest paleocene shallow benthic zonation 4 to the earliest eocene shallow benthic zonations 5 and 6 of the campo and the nearby serraduy section overall the facies show a deepening in the paleocene and the eocene interrupted by a terrestrial interval across the newly defined paleocene eocene boundary in the late paleocene altogether seven microfacies types were distinguished these are dominated by various algal taxa and subordinate corals and larger foraminifera the lower eocene carbonate platform is characterised by twelve microfacies types which are dominated by larger foraminifera such as alveolinids orbitolitids and nummulitids and subordinate corals this facies dichotomy recognized in the campo and serraduy sections can be extrapolated to the whole pyrenean region in the early paleogene and is even found in egypt the comparison of the two regions shows that the facies dichotomy is less pronounced in the pyrenean region this is interpreted to reflect a latitudinal trend where corals are absent in the low latitudes while in the middle latitudes they occur subordinately this gradient within the facies distribution can be explained by latitudinal temperature changes and by the long time warming in the early paleogene and the short time warming of the paleocene eocene thermal maximum hot temperatures in the low latitudes hamper the growth of temperature sensitive corals while in the middle latitudes the temperatures were still in a favourable temperature range