plio pleistocene megacycles record of climate and tectonics abstract paleoclimatic records from the deep sea floor show that in the last 3 m y more than 100 oscillations of mild and cool global climate took place with a larger amplitude than any experienced during the holocene variations of dry and wet climates in the chinese loess plateau and in the loess belt of central europe have similar frequency it has been convincingly proved that the periodicity of the past gross climate changes agrees with that of the earth s orbital perturbations however while the amplitude of the orbital cycles and of the oceanic oxygen isotope variations is relatively uniform several of the cold climate episodes recorded on land appear more severe than the remainder they correspond to chinese loess units l1 l2 l5 l6 and l9 correlating with central european loess of cycles b c f h and j these units correspond to oxygen isotope stages 2 6 12 16 and 22 and to the weichselian saalian and elsterian ice advances in addition chinese loess layers l13 l15 wl1 wl2 wl3 and wl4 appear to represent exceptionally dry and cold episodes apparently correlative with the inter bavelian cooling and with menapian eburonian and praetiglian glacials of the rhine delta based on this observation we propose to distinguish within the last 2 5 m y ten megacycles describe them in the chinese loess sequences and name them in the order of increasing age mc1 mc10 each megacycle starts with an unusually thick loess unit interpreted as a deposit of an exceptionally severe glaciation and terminates with a cluster of well developed soils indicative of warm humid climates interrupted by relatively mild glacials the megacycles of the last 2 5 m y lasted between 200 and 340 millennia each with an average duration of 255 thousand years when correlated with the stratigraphic record of the chinese loess plateau the wurm terrace as well as the saale glacier advances the eem marine transgression the weichsel ice advance and the holocene correspond to a still incomplete megacycle mc1 the riss terrace the elster ice advance holstein transgression and subsequent mild and cool intervals ending with the schoningen interglacial sensu urban 1992 belong to the megacycle mc2 recognition of four megacycles within brunhes resolves the long standing discrepancy between the limited number of classical continental glacial and interglacial units and the frequent climate oscillations recorded in the deep sea sediments and the loess sequences only in the holsteinian and eemian interglacials was the crustal rebound delayed enough to permit oceanic transgressions reaching deep inland there is no apparent astronomic reason for the exceptional intensity and deep southward penetration of continental ice sheets during only some of the cold intervals since the differences are much more pronounced on land than in the ocean and since the major ice advances are closely linked with terraces it seems that the megacycles were preconditioned by tectonic movements intermittent pulses of rapid uplift of mountain ranges could have impacted climate worldwide by deflecting atmospheric circulation in the sense of ruddiman and raymo 1988 it is also probable that the tectonically changing shape of the sea floor sills reoriented the oceanic circulation which then could have affected plio pleistocene climates world wide the loess covered terraces in the lanzhou area demonstrate that the tectonic movements in the northern part of the tibet plateau correlate closely with the advances of continental ice in europe suggesting that the uplift of the tibet plateau significantly contributed to the gradual climate deterioration of the last million years

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

reconstruction of lake balkhash levels and precipitation evaporation changes during the last 2000 years from fossil diatom assemblages environmental changes in arid central asia are prominently manifest through changes in the water balance lake balkhash the largest lake in kazakhstan is a shallow lake situated in a closed basin we obtained a 600 cm long core from the western part of the lake in 2007 and carried out analyses of fossil diatoms and ostracods geochemistry and grain size of the sediments the age control of the core is based on radiocarbon ages of fossil ostracods and in the uppermost part the ad 1963 137cs activity peak and the first appearance of 137cs in ad 1952 we recognized two main diatom assemblages by detrended correspondence analysis one characterized by the dominance of freshwater planktonic species and the other dominated by brackish planktonic brackish and marine benthic and freshwater benthic species the first assemblage suggests higher lake levels and the second suggests lower lake levels the observed assemblage changes are consistent with lake level observations recorded during the last 120 years the diatom assemblage data from the core indicate that there were seven periods when the lake level was low during the last 2000 years moreover the ph of the lake changed together with the lake level in addition during periods when the lake level was low from ad 1260 2000 the abundance of fossil ostracods and the ca and toc contents were low in comparison with the periods before ad 1260 the causes of low water levels were essentially the same between lake balkhash and the aral sea during the studied period one likely cause of the low water levels especially those during the last 1200 years is changes in solar activity furthermore some environmental changes during the past 40 years can be explained by human activities

genetic and temporal relations between formation waters and biogenic methane upper devonian antrim shale michigan basin usa abstract controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane natural gas in most conventional deposits has migrated from a source rock to a reservoir rarely remaining associated with the original or cogenetic formation waters we investigated an unusual gas reservoir the late devonian antrim shale in which large volumes of variably saline water are coproduced with gas the antrim shale is organic rich of relatively low thermal maturity extensively fractured and is both source and reservoir for methane that is generated dominantly by microbial activity this hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas providing a unique opportunity to characterize methane generating mechanisms the well developed fracture network provides a conduit for gas and water mass transport within the antrim shale and allows invasion of meteoric water from overlying aquifers in the glacial drift steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases dilute waters grade into dense brines 300 000 ppm over lateral distances of less than 30 km radiogenic 14c and 3h and stable isotope 18o and d analyses of shallow antrim shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20 000 yr bp carbon isotope compositions of methane from antrim shale wells are typical of the established range for thermogenic or mixed gas 13c 47 to 56 however the unusually high 13c values of co2 coproduced with methane 22 and dissolved inorganic carbon dic in formation waters 28 require bacterial mediation the d values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis methane ethane propane ratios and 13c values for ethane indicate 1 the presence of a thermogenic gas component that increases basinward and 2 progressive bacterial oxidation of ethane as the antrim shale subcrop is approached multiple episodes of pleistocene glaciation over northern michigan appear critical to the development of these gas deposits loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water the physical erosion cycle of repeated glacial advances and retreats exhumed the antrim shale around the northern margin of the michigan basin subjecting it to near surface physiochemical and biochemical processes the chemical and hydrologic relations demonstrated in the antrim shale reservoir suggest a dynamic connection between pleistocene glacial history of the midcontinent region and development of recoverable microbially generated natural gas reserves

kinetic modelling of petroleum formation and cracking implications from the high pressure high temperature elgin field uk north sea in this paper we use a case study to test the simulation of generation migration and cracking of petroleum fluids by a 2d compositional basin simulator using kinetic models based on chemical classes of compounds the selected area is the high pressure 1100 bar high temperature 190 c elgin field in the north sea this study is mainly focused on kinetic modelling of secondary cracking because in this zone paraffinic condensates with almost 50 c6 alkanes were recently discovered whereas only gas c5 was expected given the high temperature conditions the detailed hydrocarbon composition observed today in the fulmar sands reservoir of the elgin field is compared with the results of several simulations aimed at testing the influence of jurassic source rocks retention properties kinetic parameters and temperature variations on the predicted composition of fluids the influence of pressure was neglected in the modelling on the basis of results published in the literature results show that in this north sea area primary cracking of the kerogen into oil is completed whichever source rock is considered and that secondary cracking of accumulated petroleum compounds in the reservoir is presently in progress under these conditions source rock type and retention during primary and secondary migration have little influence on the present oil composition the occurrence of a monophasic paraffinic fluid in the elgin reservoir despite the high temperature is due to its very recent increase from 160 c to over 180 c in the last million years the fluid composition evolves rapidly and our compositional kinetic scheme accounts correctly for the presence of c6 hydrocarbons in these geological conditions first results using the compositional kinetic scheme of behar et al behar f kressman s rudkiewicz j l vandenbroucke m 1991 experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking organic geochemistry 19 1 3 173 189 for secondary cracking showed a reasonably good fit between calculated and observed detailed fluid composition except for methylated aromatics which are overestimated and methane which is underestimated a recent comparison of kinetic parameters obtained from pyrolysis experiments on model compounds showed that it is not realistic to assume as done previously the same frequency factor for all chemical classes of compounds the higher stability of methylated aromatics relative to saturates observed under laboratory conditions would be extended to geological conditions a second set of simulations using frequency factors and activation energies obtained on these model compounds gave a better fit for methylated aromatics however methane is still strongly underestimated and reasons which could explain this discrepancy are discussed

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

the utility of methane clumped isotopes to constrain the origins of methane in natural gas accumulations methane clumped isotope compositions provide a new approach to understanding the formational conditions of methane from both biogenic and thermogenic sources under some conditions these compositions can be used to reconstruct the formational temperatures of the gas and this capability can be applied to common subsets of both biogenic and thermogenic systems additionally there are examples in which clumped isotope compositions do not reflect gas formation temperatures but instead mixing effects and kinetic phenomena such kinetic effects also occur in common and recognizable subtypes of biogenic and thermogenic gases here we review the use of methane clumped isotope measurements for understanding the origin of methane in the subsurface we review methane clumped isotope measurements from numerous biogenic and thermogenic natural gas reservoirs we then place these measurements in the context of common frameworks for identifying the formational conditions of methane including the use of methane 13 c and d values and c 1 c 2 3 ratios finally we propose a framework for how methane clumped isotopes can be used to identify the origin of methane accumulations

thermal cracking of kerogen in open and closed systems determination of kinetic parameters and stoichiometric coefficients for oil and gas generation abstract the purpose of the study is to compare artificial maturation of kerogens representative of the main types of organic matter kerogen types i ii ii s and iii in open and anhydrous closed pyrolysis systems the generated compounds are fractionated according to their thermal stability into six chemical classes c 1 c 2 c 5 c 6 c 14 c 15 saturates c 15 aromatics and nsos which include resins and asphaltenes in both systems primary cracking of kerogen can be described by two main reactions oil generation together with early gas generation followed by late gas production mass balances obtained in the two pyrolysis systems are reasonably similar although some secondary cracking of the nsos and underestimation of methane potential due to incomplete pyrolysis may occur in the open system kinetic parameters of thermal cracking are derived from experiments in the open system using an optimization procedure which determines a unique frequency factor and a weight distribution of chemical classes for a discrete series of activation energies the resultant frequency factors are checked with those obtained for specific molecular tracers such as n c 12 and n c 24 alkanes generated from kerogens in open and closed systems under isothermal conditions the results show that using a unique frequency factor as determined by optimization is correct for type i ii and ii s kerogens this factor is more questionable however when the distribution of activation energies is very broad such as for type iii coals based on this study a strategy for determination of kinetic parameters on any new source rock sample is proposed

application of dynamic programming to the correlation of paleoclimate records 1 signal matching is a powerful tool frequently used in paleoclimate research but it is enormously time consuming when performed by hand previously proposed automatic correlation techniques require very good initial fits to find the correct alignment of two records a new technique presented in this paper utilizes dynamic programming to find the globally optimal alignment of two records geological realism is instilled in the solution through the definition of penalty functions for undesirable behavior such as unlikely changes in accumulation rate examples with synthetic and real data demonstrate that the dynamic programming technique produces accurate high resolution results with much less effort than hand tuning or preexisting automated correlation techniques

types of crust beneath the ligurian sea the structural setting beneath the ligurian sea resujts from several tectonic events reflected in the nature of the crust the central western sector called the ligurian basin is part of the northwestern mediterranean it is a marginal basin that was generated in oligocene miocene time by subduction of the adriatic plate beneath the european plate and by the eastward drift of the corsica sardinia block the eastern sector belongs to the tyrrhenian basin system and is characterized by extensional activity which since tortonian time superimposed an earlier compressional regime our effort has been addressed in particular towards simplifying the complex nature of the crust of the ligurian basin by modelling its genesis using uniform extension and sea floor depth variation with age in the rift stage of the basin s evolution the initial subsidence reaches the isostatic equilibrium level of the asthenosphere by a thinning factor of 3 15 the additional passive process corresponding to the cooling of the lithosphere since 21 ma leads to a total tectonic subsidence of 3 4 km representing the boundary of the extended continental crust for values up to 4 1 km a transitional type crust is expected whereas for higher tectonic subsidence values a typical oceanic crust should exist after setting these constraints the boundaries of the different crust types have been drawn based on total tectonic subsidence observations deduced from bathymetry and post rift sediment thickness although there is a general agreement with the previous reconstructions deduced from other experimental data the oceanic realm has wider extent and more complex shape the northernmost part of this realm shows crust of sub oceanic type altemating basement highs with lower subsidence values the observed surface heat flux is consistent with the predicted geothermal held in the alpine provencal continental margin and in the oceanic domain however a characteristic thermal asymmetry is clearly visible astride the basin due to the enhanced heat flux of the corsica margin even if the uniform extension model accounts well at a regional level for the present basement depth a remarkable tectonic subsidence excess has been found in the alpine provencal continental margin this evidence agrees with the reprise in compression of the margin the direction of the greatest principal stress is n120 e on average

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

the isotopic structures of geological organic compounds organic compounds are ubiquitous in the earth s surface sediments and many rocks and preserve records of geological geochemical and biological history they are also critical natural resources and major environmental pollutants the naturally occurring stable isotopes of volatile elements d 13 c 15 n 17 18 o 33 34 36 s provide one way of studying the origin evolution and migration of geological organic compounds the study of bulk stable isotope compositions i e averaged across all possible molecular isotopic forms is well established and widely practised but frequently results in non unique interpretations increasingly researchers are reading the organic isotopic record with greater depth and specificity by characterizing stable isotope structures the proportions of site specific and multiply substituted isotopologues that contribute to the total rare isotope inventory of each compound most of the technologies for measuring stable isotope structures of organic molecules have been only recently developed and to date have been applied only in an exploratory way nevertheless recent advances have demonstrated that molecular isotopic structures provide distinctive records of biosynthetic origins conditions and mechanisms of chemical transformation during burial and forensic fingerprints of exceptional specificity this paper provides a review of this young field which is organized to follow the evolution of molecular isotopic structure from biosynthesis through diagenesis catagenesis and metamorphism

timing and structure of the mid pleistocene transition records from the loess deposits of northern china abstract the mid pleistocene transition mpt from obliquity to eccentricity forced climate variations has been extensively documented within the oceanic realm using a collection of orbitally tuned records of palaeoclimatic change from the loess sequences of northern china we have analysed terrestrial palaeomonsoon behaviour since 1 5 ma and characterised the structure of the mpt wavelet analysis of a stacked magnetic susceptibility record from the loess sequences indicates that the onset of 100 kyr cyclicity was synchronous in both global ice volume and the east asian monsoon system however analysis in the time domain reveals that during the mpt interim period the east asian winter monsoon appears to have weakened whilst global ice volume was increasing indicating that the palaeomonsoon was not the primary forcing mechanism responsible for driving the global climatic change

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

coarse fraction fluctuations in pelagic carbonate sediments from the tropical indian ocean a 1500 kyr record of carbonate dissolution appendix table al is available with entire article onmicrofiche order from the american geophysical union 2000florida avenue n w washington d c 20009 documentp94 001 2 50 payment must accompany order we examined coarse fraction contents of pelagic carbonates deposited between 2000 and 3700 m water depth in the tropical indian ocean using ocean drilling program odp sites 722 owen ridge arabian sea and 758 ninetyeast ridge eastern equatorial indian ocean and four giant piston cores collected by the french r v marion dufresne during the seymama expedition over the last 1500 kyr coarse fraction records display high amplitude oscillations with an irregular wavelength on the order of 500 kyr these oscillations can be correlated throughout the entire equatorial indian ocean from the seychelles area eastward to the ninetyeast ridge and into the arabian sea changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation the well known mid bruhes dissolution cycle represents the last part of this irregular long term dissolution oscillation the origin of this long term oscillation is still poorly understood our observations suggest that it is not a true cycle it has an irregular wavelength and we propose that it may result from long term changes in ca flux to the ocean sites 722 and 758 18o records provide a high resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr site 722 2030 m lies above the present and late pleistocene lysocline the lysocline shoaled to the position of site 758 2925 m only during the interglacial intervals that occurred between about 300 and 500 ka peterson and prell 1985a despite these supralysoclinal positions of the two sites short term changes in coarse fraction contents are correctable from one site to another and probably result from regional or global dissolution pulses by stacking the normalized coarse fraction records from sites 722 and 758 we constructed a composite coarse fraction index ccfi curve in which most of the local signals cancelled out the last 800 kyr of this curve appear to compare extremely well with the composite dissolution index curve from core v34 53 ninetyeast ridge which unambiguously records past variations of carbonate dissolution in the equatorial indian ocean peterson and prell 1985a in the late pleistocene the ccfi variations are mainly associated with glacial interglacial changes they show strong 100 and 41 kyr periodicities but no clear precession related periodicities as proposed earlier by peterson and prell 1985a the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation we cannot totally reject the possibility however that low temporal resolution and or bioturbation degrade somehow the precessional signal at odp sites 722 and 758 in contrast spectral density of dissolution cycles in the giant 53 m long piston core md900963 maldives area displays clear maxima centered on the precession frequencies 23 and 19 kyr 1 as well as on the kyr 1 frequency but shows little power at the 100 kyr 1 frequency these high frequency changes most probably result from changes in surface productivity associated with monsoon variability dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak sw monsoon and or increased dry ne monsoon

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

comparison of petroleum generation kinetics by isothermal hydrous and nonisothermal open system pyrolysis this study compares kinetic parameters determined by open system pyrolysis and hydrous pyrolysis using aliquots of source rocks containing different kerogen types kinetic parameters derived from these two pyrolysis methods not only differ in the conditions employed and products generated but also in the derivation of the kinetic parameters i e isothermal linear regression and non isothermal nonlinear regression results of this comparative study show that there is no correlation between kinetic parameters derived from hydrous pyrolysis and open system pyrolysis hydrous pyrolysis kinetic parameters determine narrow oil windows that occur over a wide range of temperatures and depths depending in part on the organic sulfur content of the original kerogen conversely open system kinetic parameters determine broad oil windows that show no significant differences with kerogen types or their organic sulfur contents comparisons of the kinetic parameters in a hypothetical thermal burial history 2 5 c my show open system kinetic parameters significantly underestimate the extent and timing of oil generation for type iis kerogen and significantly overestimate the extent and timing of petroleum formation for type i kerogen compared to hydrous pyrolysis kinetic parameters these hypothetical differences determined by the kinetic parameters are supported by natural thermal burial histories for the naokelekan source rock type iis kerogen in the zagros basin of iraq and for the green river formation type i kerogen in the uinta basin of utah differences in extent and timing of oil generation determined by open system pyrolysis and hydrous pyrolysis can be attributed to the former not adequately simulating natural oil generation conditions products and mechanisms

recent advances in petroleum isotope geochemistry abstract organic matter is isotopically heterogeneous lipids tend to be depleted in the heavy carbon and hydrogen isotopes as compared to carbohydrates and lignite the c and h isotopic composition of kerogens is determined by many factors such as environmental conditions during formation and deposition and variability of sources of organic matter stable isotope concentrations in mature kerogens are not significantly altered during maturation and hence reflect the variability of the organic matter during deposition isotope variations in kerogens therefore reflect to some extent the paleogeographic situation and can be used for paleoreconstruction a basin facies with anoxia and a marginal facies with detrital input can be differentiated isotopically in the toarcian of central europe coals are isotopically heterogeneous due to different isotopic composition of macerals inertinite and vitrinite tends to be enriched in deuterium as compared to exinite also boghead coals tend to be depleted in dueterium which possibly reflects various lipid contents of respective precursor materials petroleums can be genetically classified through the patterns of covariance of their compound classes carbonate and shale sourced petroleums may be additionally differentiated through their deuterium content the general world wide applicability of such genetic differentiations needs still to be proved carbon and hydrogen isotope variations in methane are genetically controlled bacterial methane in freshwater environments is characteristically depleted in deuterium even complex origins of natural gases can be unraveled using compositional and isotopic variations new analytical developments led to the application of carbon isotope analyses of gaseous hydrocarbons desorbed from sediments for geochemical surface exploration a discussion reveals that processes such as isotope effects during degassing and bacterial oxidation as well as indigenous formation of hydrocarbons may completely obscure the results

regional seismic phases across the ligurian sea lg blockage and oceanic propagation abstract the ligurian sea west mediterranean is an example of a narrow oceanic basin which stops lg wave propagation locating the exact point of lg extinction was one of the goals of the sisbalig ii seismological experiment which deployed both inland stations in provence and corsica and ocean bottom seismometers obs in the ligurian sea a precise analysis of all available regional seismological data using energy diagrams reveals the role of the provence margin in the lg phase extinction for seismic events occurring in the southwestern alps the extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the provence margin the extinction is accompanied by the generation of two types of diffracted waves the mantle phase sn v s 3 5 km s 1 and very low velocity s waves appearing both near the provence margin and in the middle of the ligurian basin we performed numerical simulation of sh wave propagation in two dimensional 2d media to investigate the mechanisms of lg extinction the simulations confirm that simple crustal thinning at the continent ocean boundary alone cannot cause the observed extinction of lg waves although the change in moho depth does induce the diffraction of a part of the incident lg wavetrain into mantle waves giving rise to the sn phase on the other side of the basin the existence of soft oceanic sediments is a very important factor in explaining the extinction as a large part of the lg energy is transferred into slow s waves trapped in the sediments moreover comparison of synthetic seismograms and observed data shows that the observed two packets of low velocity waves are an indication of the asymmetry of the ligurian basin with the existence of two shallow basins with very low s velocity 1 0 km s 1 near the provence margin and in the middle of the ligurian sea however the attenuation of lg waves in the vicinity of the provence margin is more abrupt in the data than in the simulations this discrepancy could result from the complex geometry of the continent ocean transition zone and its possibly very diffractive nature two factors that could add to lg extinction at the margin and are impossible to take into account in models

microbial production and modification of gases in sedimentary basins a geochemical case study from a devonian shale gas play michigan basin an expanded data set for gases produced from the antrim shale a devonian black shale in the michigan basin united states has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid gas liquid systems that discriminate between microbial and thermogenic gas origin in the antrim shale economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network the most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water along the basin margins there is also a systematic enrichment in 13c of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components microbial oxidation accounts not only for the shift in 13c values for ethane but also in part for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed this oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate reducing bacteria the results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water one unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced co2 gas in contrast the 13c value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas in addition the confounding isotopic and compositional overprint of microbial oxidation increasing the values to typically thermogenic values may obscure the distinction between methanogenic and thermogenic gas

heat flow and lithospheric thinning in the ligurian basin n w mediterranean we present preliminary results of 78 new heat flow measurements collected on a transect across the central ligurian basin south france to corsica margins the basin wide flux has an average of 79 21 mwm 2 with significant local and regional variability correction for the deposition of up to 5 km of post rift sediment gives a mean value of 102 mwm 2 which is similar to values for normal oceanic crust of the same age 21 ma the total tectonic subsidence tts calculated for the region with probable oceanic crustal structure is comparable to or somewhat greater than 21 ma crust of the major ocean basins the heat flow data overall are compatible with the observed tts in showing substantial lithospheric thinning but do not tightly constrain thermal models of continental extension and rifting

simple kinetic models of petroleum formation part ii oil gas cracking kinetic schemes describing oil gas cracking are a required component of models which predict the masses compositions and phases of petroleum expelled from source rocks an additional application is in predicting the temperature zone over which reservoired oil deposits will be degraded to gas this simple first order kinetic model calibrated on closed system laboratory pyrolysis of 16 different source rock samples describes the bulk conversion of oil c6 molecular range to gas c1 5 molecular range cracking rates vary markedly between samples depending on the saturate to aromatic ratio of the generated oil the initial hydrogen index hl0 of the sample taken as an indirect measure of the saturate to aromatic ratio of the oil it generates correlates strongly with optimized rate constants for oil cracking in individual samples thus cracking rates are predictable using a simple routinely performed geochemical screening measurement source rocks with high hl0 tend to generate saturate rich oils with a high mean activation energy and a tight distribution of bond energies which crack relatively slowly over a high but relatively narrow temperature range source rocks with low hl0 generate aromatic rich oils with a low mean activation energy and a broad distribution of bond energies which crack relatively rapidly over a lower but relatively wide temperature range this is an important contributing factor in the observed gas proneness of type iii source rocks with low hl0 the projected cracking window defined limits at 10 90 of initial oil degraded reference heating rate 2 c ma 1 varies from 155 to 205 c in very high quality source rocks hl0 600 mg g c 1 to 115 205 c in very poor quality source rocks hl0 50 mg g c 1 an order of magnitude increase decrease in heating rate elevates depresses these temperature windows by ca 15 c the extrapolations to the geological subsurface are subject to a confidence limit no better than 7 c extrapolation to the reservoir environment requires caution because kerogen an important potential catalyst and hydrogen donor allowing 100 cracking efficiency in the source rock is absent furthermore fractionation during expulsion causes saturate enrichment of expelled ultimately reservoired oils thus in reservoir cracking should be at least as slow as cracking in good quality source rocks and coke or pyrobitumen formation is needed to conserve the hydrogen balance a case history based on deep petroleum pools of the uk central north sea confirms these projections demonstrating no positive evidence of significant in reservoir cracking at temperatures as least as high as 174 c and perhaps even 195 c previous studies have overestimated the importance of in reservoir cracking future studies aiming to predict petroleum composition in traps should concentrate on understanding the charge composition expelled from the kitchen

insolation regime in siberia as a major factor controlling diatom production in lake baikal during the past 800 000 years abstract a unique diatom biostratigraphy of the brunhes chron in lake baikal revealed by detailed studies of pelagic diatom paleo assemblages indicates that dramatic changes in insolation during the past 800 ka bp produced surprisingly rapid diatom speciation and extinctions the orbitally tuned age model allows the diatom assemblages to be compared with individual marine isotopic stages and substages during the brunhes most of the interglacial periods are characterized by explosive speciation and appearances of new pelagic planktonic diatom species in contrast most of the glacial periods are characterized by spectacular extinctions three broad periods characterized by different degrees of stability in the pelagic diatom assemblages are distinguished during the brunhes chron in the lake baikal sedimentary record mis 19 15e mis 15a 11 and mis 9 5 these periods are apparently associated with the long term changes in the amplitude of precessional insolation variations examination of individual interglacial periods further reveals the sensitivity of the baikal ecosystem to the regional energy balance virtually every precessional insolation peak during the early brunhes is characterized by a unique and peculiar paleo assemblage for the first time the siliceous microfossil record has demonstrated such a high degree of sensitivity to insolation forcing of regional climate despite the dramatic changes in the pelagic plankton driven by extreme climatic changes diatoms never went fully extinct during any glacial period although greatly or completely renewed during each subsequent interglacial the pelagic diatom assemblage still managed to return dominated by the new baikal endemic species the diatom assemblage observed in modern lake baikal despite being mostly endemic has formed during the past 50 100 ka and is therefore rather young these results demonstrate the remarkable capacity of this large freshwater system to sustain the dynamic stability on the large time scales

earthquake relocation in the ligurian sea western mediterranean geological interpretation abstract relocated epicenters are computed with a model based on complex local structure i e oceanic floor in the central ligurian basin and thinned continental margins by superposing these relocated earthquakes on a basement bathymetric map the structural relations are clearly demonstrated compressive stress towards the west northwest is confirmed in the northern ligurian basin by focal mechanism study according to our results most of the earthquakes appear to be shallow and linked to rejuvenation of older distensive and strike slip structures on margins as in the basin by a compressional motion

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

oxygen isotope evidence for a stronger winter monsoon current during the last glaciation a variety of palaeoclimate data from the northern indian ocean has shown that the asian summer monsoon was weaker during the last glacial maximum 18 000 years ago than it is today1 5 it has also been inferred from these data that the winter monsoon was stronger at that time5 here we present evidence that the transport of low salinity water from the bay of bengal to the arabian sea caused by the winter monsoon6 was higher during the last glacial maximum this inference is based on a negative excursion up to about 1 in magnitude in the oxygen isotope ratios of three planktonic foraminiferal species detected in a sediment core from the arabian sea by high resolution 1 2 cm sampling our results place an upper limit of about 4 000 years on the length of time during which the winter monsoon was stronger than today

field experiments to study seafloor seismoacoustic response results of a 5 year program to measure the seismoacoustic properties of unlithified sediments in the seafloor are presented a field technique utilizing an impulsive source and a geophone array located on the seafloor was used to obtain detailed travel time curves which permit a number of different kinds of analysis a new signal source powered by electrically detonated 8 gauge shotgun shells was used to focus energy into the bottom and eliminate much of the high frequency water borne noise that accompanies the usual explosive sources both interface waves and wide angle refractions were analyzed a variety of data inversion techniques including slant stacking and cross multiplication were used to obtain dispersion curves and constrained least squares inversion using partial derivatives was used to develop an iterative inversion method to obtain velocity and attenuation as a function of depth examples of data from a wide angle of different kinds of sediment ranging from soft holocene mud to stiff over

carbon isotope composition of marine crude oils 1 a histogram of isotope 13 c values of 621 post ordovician marine oils shows a trimodal distribution within this distribution four groups of oils can be recognized on the basis of isotope 13 c values in conjunction with pristane phytane ratios and sulfur contents oils with isotope 13 c values of 32 0 to 28 0 o oo are mainly marine shale oils older than oligocene oils with isotope 13 c values of 28 0 to 23 5 o oo are mainly deltaic oils of varying geologic age or mesozoic carbonate oils and oils with isotope 13 c values heavier than 23 5 o oo are mainly marine shale oils of miocene age the isotope 13 c variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric co 2 concentration since 25 ma which has resulted in decreased isotope fractionation by marine plankton during photosynthesis deltaic oils are mostly derived from terrestrial organic matter and their isotope 13 c values reflect the time invariant average isotope 13 c value of land plant carbon at about 25 o oo mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded processes that occur after oil generation generally have smaller effects on isotope 13 c values of oils as a result isotope 13 c val es of oils are useful for determining oil oil and oil source rock relationships and in conjunction with other geochemical properties can indicate the possible age and depositional environment of source rocks

clumped isotope geochemistry the study of naturally occurring multiply substituted isotopologues clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials that is it examines the extent to which rare isotopes d 13 c 15 n 18 o etc bond with or near each other rather than with the sea of light isotopes in which they swim abundances of isotopic clumps in natural materials are influenced by a wide variety of factors in most cases their concentrations approach within ca 1 relative the amount expected for a random distribution of isotopes deviations from this stochastic distribution result from enhanced thermodynamic stability of heavy isotope clumps slower kinetics of reactions requiring the breakage of bonds between heavy isotopes the mass dependence of diffusive and thermo gravitational fractionations mixing between components that differ from one another in bulk isotopic composition biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms and in some cases other processes we do not yet understand although clumped isotope geochemistry is a young field several seemingly promising applications have already emerged most importantly it appears that proportions of 13 c 18 o bonds in carbonate minerals are sensitive to their growth temperatures independent of bulk isotopic composition thus clumped isotope analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the 18 o of waters from which carbonates grew this approach has been used to reconstruct marine temperatures across the phanerozoic reaching back to the silurian terrestrial ground temperatures across the cenozoic thermal histories of aqueously altered meteorites among other applications clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of co 2 and should be capable of placing analogous new constraints on the budgets of other atmospheric gases finally this field could be extended to encompass sulfates volatile hydrocarbons organic moieties and other materials

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

monospecific planktonic diatom assemblages in the paleo kathmandu lake during the middle brunhes chron implications for the paradox of the plankton abstract the almost monospecific assemblages of planktonic diatoms that occurred in ancient lakes during the middle brunhes chron are an important research issue for lake ecosystems because their existence implies that the famous paradox of the plankton was no paradox at that time to investigate their cause i examined the assemblage structure and diversity of planktonic diatoms in the paleo kathmandu lake at nepal himalaya over the past 600 kyr the assemblage structure changes frequently after 300 ka but remains nearly stable before then additionally planktonic species show a relatively high average diversity after 300 ka but show an extremely low average before then those abnormal ecological conditions before 300 ka reflect the formation of almost monospecific planktonic diatom assemblages of cyclotella kathmanduensis and puncticulata versiformis the assemblage structure and species diversity of the planktonic diatoms were controlled by lake level fluctuations especially lake level declines as ecological disturbances after 300 ka fluctuations in lake levels occur in about 20 ka intervals which probably helped to sustain diverse planktonic diatom assemblages in contrast much longer intervals occurred between lake level fluctuations before 300 ka and these were of a frequency that was too low to sustain diverse planktonic diatom assemblages this may have allowed c kathmanduensis and p versiformis to form monospecific assemblages

a 350 000 year summer monsoon multi proxy stack from the owen ridge northern arabian sea five summer monsoon proxies from the northern arabian sea are combined using stacking and principal components analysis pca to create two very similar multi proxy records of summer monsoon variability the five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer monsoon indicators e g preservation dissolution diagenesis sediment reworking as such stacking and pca average out non monsoon variance yielding a more pure monsoon signal these stacked and pca records hereafter summer monsoon stack and summer monsoon factor allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands these two parameters are less reliable when estimated from individual proxy records in fact the summer monsoon factor smf accounts for only 33 of the total variance in the five records suggesting that relative amplitude variations in each individual proxy time series are influenced by non monsoon processes the summer monsoon stack sms and smf are spectrally very similar dominated by variance in the 41 k y obliquity and 23 k y precession bands there is very little variance at the 100 k y eccentricity band associated with large scale changes in global ice volume indeed equally strong monsoons occur in both glacial and interglacial intervals within the 23 k y precession cycle monsoon maxima fall at 121 relative to precession minima june 21 perihelion maximum northern hemisphere nh summer insolation this phase falls midway between 18o minima 78 and december 21 perihelion 180 indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band 1 sensible heating of the asian plateau which is maximized at times of ice volume minima 78 and 2 latent heat export from the southern subtropical indian ocean which is maximized at times of december 21 perihelion 180 the seasonal cycle at december 21 perihelion is characterized by warm southern hemisphere sh summers followed by cold sh winters a combination that preconditions the ocean to export latent heat during the boreal summer monsoon season summer monsoon winds transport this latent heat into asia where it is released during precipitation enhancing the asian monsoon low within the 41 k y obliquity cycle monsoon maxima are in phase with obliquity maxima this indicates that two mechanisms quite similar to those in the precession band influence the timing of strong summer monsoons in the obliquity band 1 sensible heating of the asian plateau but with no ice volume delay and 2 latent heat export from the southern subtropical indian ocean which is maximized at times of obliquity maxima again the seasonal cycle at obliquity maximum is characterized by warm sh summers followed by cold sh winters ideal for maximizing latent heat export during the boreal summer monsoon

propagation and attenuation characteristics of the crustal phase lg thelg wave consists of the superposition ofs waves supercritically reflected and thus trapped in the crust this mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed the numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models in regions with strong lateral variations the influence of large scale heterogeneities on thelg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure

paleoclimatic changes during the last 2 5 myr recorded in the kathmandu basin central nepal himalayas abstract palynological and sedimentological studies of a series of slimes collected from a 284 m long drill well from the kathmandu basin reveal paleoclimatic records and environmental changes within the kathmandu valley during the last 2 5 myr the slimes are composed of fluvio deltaic and lacustrine sediments comprising sand beds of 66 3 m and mud beds of 218 m in length pollen analyses show quercus and cyclobalanopsis are predominant with frequencies exceeding 70 pinus alnus and gramineae are the next dominant taxa three fossil pollen zones were discriminated each zone reflects major climatic change zone i the oldest stage indicates a cool and rather wet climate during 400 kyr from ca 2 5 to 2 1 ma zone ii the middle stage reflects a warm and relatively dry climate without remarkable fluctuation zone iii is characterized by seven cycles of warm and wet and cold and dry climate which reflect the alternation of glacial and interglacial periods the last cold maximum 11 m deep corresponds to the last glacial age around 20 kyr bp judging from the 14 c dating of the uppermost part of the lacustrine sediments the paleo kathmandu lake is likely to have been initiated at around 2 1 ma and to have been filled with black organic mud the kalimati clay the top of the kalimati clay is eroded and was overlain by fluvial sand after the last glacial age the abrupt appearance of a 4 m thick fossiliferous sand bed at the top of the middle member suggests a lowering of water level at around 1 ma

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

the egt bouguer gravity compilation abstract two colour plates showing the bouguer gravity anomalies along the european geotraverse egt were produced by compiling all available bouguer gravity data and where possible by applying a standard density of 2 67 mg m3 for the topographic correction the data points used to create the contours are printed on the reverse side of the maps so that an analyst using a light table can quickly identify artifacts produced by uneven data distribution the data are available on magnetic tape for those interested in working on interpretation along the egt

methane clumped isotopes in the songliao basin china new insights into abiotic vs biotic hydrocarbon formation abiotic hydrocarbon gas typically generated in serpentinized ultramafic rocks and crystalline shields has important implications for the deep biosphere petroleum systems the carbon cycle and astrobiology distinguishing abiotic gas produced by chemical reactions like sabatier synthesis from biotic gas produced from degradation of organic matter or microbial activity is sometimes challenging because their isotopic and molecular composition may overlap abiotic gas has been recognized in numerous locations on the earth although there are no confirmed instances where it is the dominant source of commercially valuable quantities in reservoir rocks the deep hydrocarbon reservoirs of the xujiaweizi depression in the songliao basin china have been considered to host significant amounts of abiotic methane here we report methane clumped isotope values 18 and the isotopic composition of c 1 c 3 alkanes co 2 and helium of five gas samples collected from those xujiaweizi deep reservoirs some geochemical features of these samples resemble previously suggested identifiers of abiotic gas 13 c enriched ch 4 decrease in 13 c 12 c ratio with increasing carbon number for the c1 c4 alkanes abundant apparently non biogenic co 2 and mantle derived helium however combining these constraints with new measurements of the clumped isotope composition of methane and careful consideration of the geological context suggests that the xujiaweizi depression gas is dominantly if not exclusively thermogenic and derived from over mature source rocks i e from catagenesis of buried organic matter at high temperatures methane formation temperatures suggested by clumped isotopes 167 213 c are lower than magmatic gas generation processes and consistent with the maturity of local source rocks also there are no geological conditions e g serpentinized ultramafic rocks that may lead to high production of h 2 and thus abiotic production of ch 4 via co 2 reduction we propose that the songliao gas is representative of an atypical type of thermogenic gas that can be mistaken for abiotic gas such gases may be encountered more frequently in future exploration of deep or over mature petroleum systems

several location methods for underwater shots in the gulf of genoa western mediterranean structural implications abstract during the egt survey 50 underwater shots were detonated on a n s section through the gulf of genoa running from the northern margin 44 12 n towards corsica 42 90 n along the 9 e meridian the aim of this work is in a first stage to establish the propagation model of regional phases in this region and consequently to improve the location of epicentres and afterwards to refine our structural knowledge of this complex crustal area the relocation of these shots was carried out with arrival times of seismic signals recorded at land stations we have used different location methods all of which led to similar crustal models they reveal a low p n velocity area in the northern ligurian sea and in the gulf of genoa with a 20 km thick crust in the southern part approximately south of latitude 43 7n a 4 km thick oceanic crust covered by 8 km of sediments is clearly confirmed using these models a significant improvement of epicentre locations was obtained these improvements to propagation models lead to the two main structural conclusions 1 1 in the southern part of the investigated area the spread of an oceanic type domain as far as the foot of the continental margin 2 2 a sharp transition between the two domains of different stretching rate the northern one with a moho depth of about 20 km and the southern one with an average moho depth of about 14 km these observations substantiate the presence of a transform margin between the rifted gulf of genoa and the oceanic type ligurian basin

muted climate variations in continental siberia during the mid pleistocene epoch the large difference in carbon and oxygen isotope data from the marine record between marine oxygen isotope stage 12 mis 12 and mis 11 spanning the interval between about 480 and 380 kyr ago has been interpreted as a transition between an extremely cold glacial period and an unusually warm interglacial period with consequences for global ice volume sea level and the global carbon cycle1 2 3 4 the extent of the change is intriguing because orbital forcing is predicted to have been relatively weak at that time5 here we analyse a continuous sediment record from lake baikal siberia which reveals a virtually continuous interglacial diatom assemblage a stable littoral benthic diatom assemblage and lithogenic sediments with interglacial characteristics for the period from mis 15a to mis 11 from about 580 to 380 kyr ago from these data we infer significantly weaker climate contrasts between mis 12 and 11 than during more recent glacial interglacial transitions in the late pleistocene epoch about 130 to 10 kyr ago for the period from mis 15a to mis 11 we also infer an apparent lack of extensive mountain glaciation

ecological variations in diatom assemblages in the paleo kathmandu lake linked with global and indian monsoon climate changes for the last 600 000 years abstract variations in fossil diatom assemblages and their relationship with global and indian monsoon climate changes for the last 600 000 yr were investigated using a core of ancient lake paleo kathmandu lake sediments drilled at the kathmandu basin nepal himalaya chronological scales of the core were constructed by tuning pollen wet and dry index records to the specmap 18o stack record examinations of biogenic silica contents and fossil diatom assemblages revealed that variations in productivity and compositions of diatom assemblages were closely linked with global and indian monsoon climate changes on glacial and interglacial time scales when summer monsoonal rainfall increased during interglacials interstadials diatom productivity increased because of increased inputs of terrestrial nutrients into the lake when summer monsoonal rainfall reduced and or winter monsoonal aridification enhanced during glacials stadials productivity of the diatoms decreased and lake level falling brought about changes in compositions of diatom assemblages monospecific assemblages by unique cyclotella kathmanduensis and puncticulata versiformis appeared during about 590 to 390 ka this might be attributed to evolutionary fine tuning of diatom assemblages to specific lake environmental conditions additionally low amplitude precessional variations in monsoon climate and less lake level changes may have also allowed both species to dominate over the long periods

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

ecological collapse of lake baikal and lake hovsgol ecosystems during the last glacial and consequences for aquatic species diversity sedimentary sections from lake baikal in siberia and lake hovsgol in mongolia show that during the last glacial maximum lgm planktonic benthic diatoms chrysophyte cysts sponge spicules and zooplankton remains did not accumulate in the lake sediments the combined data suggest a drastic lowering of photosynthetic production and perhaps the collapse of the ecosystems of both lakes due to significant environmental changes during the lgm both lakes were almost uninhabited during the lgm starting from 12 000 to 13 000 14c years ago the planktonic communities redeveloped and primary production increased in both lakes as a result holocene sediments contain abundant and well preserved planktonic benthic diatoms stomatocysts sponge spicules and zooplankton remains the ecosystem stress in lake baikal during glacial time appears to be due to a decrease in nutrient loading from the watershed lowering of surface water temperature and very low transparency of water and ice we believe that endemic species in baikal survived during glacial ecological catastrophes in refugia that could have been a source of species that invaded and repopulated baikal after intense and long glacial winters the ecosystem stress in lake hovsgol during glacial time appears to be due primarily to drastic lake level falls and changes in water chemistry at the beginning of the holocene lake hovsgol was reoccupied again by cosmopolitan species probably from surrounding small lakes and rivers in addition to the lgm results we also show that lake baikal planktonic diatom species diversity during the last 350 ka is closely related to solar insolation fluctuations caused by the 23 and 41 ka cycles of the earth s precession and obliquity our results also indicate that baikal s planktonic diatom species diversity as measured by the shannon weiner index correlates with the oxygen isotope record that represents changes in global ice volume

saline ground waters and brines in the canadian shield geochemical and isotopic evidence for a residual evaporite brine component abstract saline ca na cl type groundwaters and brines sampled in deep mines over an extensive area of the canadian precambrian shield have elevated br cl ratios which may indicate that the chlorinity of these waters was derived from the infiltration of residual evaporitic brines remnants of the great marine incursions of the paleozoic era boron concentrations in these waters are generally low i e 11 b 10 b ratios of these waters are significantly greater than the average value for continental crustal rocks with the highest values 4 19 approaching that of present day seawater 4 20 moreover the boron isotopic ratios generally trend to higher values with increasing chlorinity which supports the conclusion from the br cl relationship that most of the chloride in shield brines is of marine origin rather than a product of water rock interactions if this is correct crystalline rocks must then be sufficiently permeable on a regional scale to have allowed the brine to infiltrate to depths of several kilometers where it now resides

microbial generation of economic accumulations of methane within a shallow organic rich shale although methane of bacterial origin is ubiquitous in marine and freshwater sediments economic accumulations of bacterial gases occur mainly at depths of several kilometres in tertiary basins that had high sedimentation rates1 2 here we present an integration of geochemical and isotopic data from gas and water extracted from the upper devonian antrim shale along the northern margin of the michigan basin which demonstrates that significant volumes of bacterial gas have been generated in organic rich shales at depths of less than 600 metres the antrim shale is mainly a self sourced reservoir in contrast to conventional gas deposits that have migrated from a source to a reservoir and has become one of the most actively exploited gas reservoirs3 in the united states the gas forming processes operating at shallow depths in the antrim shale are not unique4 and an understanding of these processes should lead to the identification and development of other economic non conventional gas deposits around the world

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

fractured shale gas systems the first commercial united states natural gas production 1821 came from an organic rich devonian shale in the appalachian basin understanding the geological and geochemical nature of organic shale formations and improving their gas producibility have subsequently been the challenge of millions of dollars worth of research since the 1970s shale gas systems essentially are continuous type biogenic predominant thermogenic or combined biogenic thermogenic gas accumulations characterized by widespread gas saturation subtle trapping mechanisms seals of variable lithology and relatively short hydrocarbon migration distances shale gas may be stored as free gas in natural fractures and intergranular porosity as gas sorbed onto kerogen and clay particle surfaces or as gas dissolved in kerogen and bitumen five united states shale formations that presently produce gas commercially exhibit an unexpectedly wide variation in the values of five key parameters thermal maturity expressed as vitrinite reflectance sorbed gas fraction reservoir thickness total organic carbon content and volume of gas in place the degree of natural fracture development in an otherwise low matrix permeability shale reservoir is a controlling factor in gas producibility to date unstimulated commercial production has been achievable in only a small proportion of shale wells those that intercept natural fracture networks in most other cases a successful shale gas well requires hydraulic stimulation together the devonian antrim shale of the michigan basin and devonian ohio shale of the appalachian basin accounted for about 84 of the total 380 bcf of shale gas produced in 1999 however annual gas production is steadily increasing from three other major organic shale formations that subsequently have been explored and developed the devonian new albany shale in the illinois basin the mississippian barnett shale in the fort worth basin and the cretaceous lewis shale in the san juan basin in the basins for which estimates have been made shale gas resources are substantial with in place volumes of 497 783 tcf the estimated technically recoverable resource exclusive of the lewis shale ranges from 31 to 76 tcf in both cases the ohio shale accounts for the largest share

thickness trends and sequence stratigraphy of the middle devonian marcellus formation appalachian basin implications for acadian foreland basin evolution analysis of more than 900 wireline logs indicates that the middle devonian marcellus formation encompasses two third order transgressive regressive t r sequences mss1 and mss2 in ascending order compositional elements of the marcellus formation crucial to the successful development of this emerging shale gas play including quartz clay carbonate pyrite and organic carbon vary predictably within the proposed sequence stratigraphic framework thickness trends of marcellus t r sequences and lithostratigraphic units reflect the interplay of acadian thrust load induced subsidence short term base level fluctuations and recurrent basement structures rapid thickening of both t r sequences especially mss2 toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the acadian orogeny however local variations in t r sequence thickness in the western more distal area of the basin may reflect the reactivation of inherited eocambrian basement structures including the rome trough and northwest striking cross structural discontinuities induced by acadian plate convergence episodes of block displacement locally warped the basin into northeast southwest trending regions of starved sedimentation and or erosion adjacent to depocenters in which regressive systems tract deposits were ponded block movement appears to have initiated in late early devonian time resulting first in thinning and local erosion of the oriskany sandstone in northwest pennsylvania this study in addition to providing the basis for a predictive sequence stratigraphic model that can be used to further marcellus exploration tells of a foreland basin more tectonically complex than accounted for by simple flexural models

pleistocene rapid uplift of the himalayan frontal ranges recorded in the kathmandu and siwalik basins abstract core drilling project carried out in the southern margin of the kathmandu basin revealed that muddy debris flow deposits dammed up the proto bagmati river to form the paleo kathmandu lake during the jaramillo subchron from 1 07 to 0 97 ma subsequent deposition of the alluvial fanglomerate derived from the uplifting mahabharat range to the south raised the dam deepening the lake water after 1 ma in the southern part of the basin palaeo current directions changed from southward to northward and deposition of gneissose and granitic detritus are replaced by meta sediments derived from the mahabharat range during the same time at about 1 ma the boulder conglomerates were deposited on top of the siwalik group as piggy back basins in front of an intra basinal high along the main dung thrust in nepal and nw india onset of movement of the main dung thrust is dated back to 3 to 2 4 ma mugnier j l huyge p leturmy p jouanne f 2003 episodicity and rates of thrust sheet motion in the himalayas western nepal am assoc petrol geol mem 82 1 24 the main frontal thrust is most active at present suggesting that imbricated structure of the siwalik group was formed by convergence of the indian plate during the last 3 myr the accretionary wedge of the siwalik group stacked beneath the main boundary thrust might have started to jack up the frontal range of the lesser himalaya since 1 ma coeval uplift and erosion of the frontal range of the lesser himalaya and the intra basinal high in the siwalik since 1 ma are possible causes of an abrupt increase in both sedimentation rate and grain size of detrital quartz and changes in composition of clay minerals recorded in the sediments of the bengal deep sea fan at 0 9 ma

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction