crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

stress concentrations and failure modes in horizontal wells accounting for elastic anisotropy of shale formations abstract this study presents a systematic analysis of stress changes near the horizontal well sections drilled in shale and clay formations using a new approach most of the prior analyses of stress concentrations and wellbore stability in anisotropic shale formations use the traditional lekhnitskii amadei solutions we present a faster set of analytical stress functions based on largely overlooked solutions by green and taylor first the full theoretical range of possible compliances for transverse isotropic materials is evaluated using data for 5 shale clay samples and 12 additional test materials contour plots show the principal stress trajectory patterns and variation in the spatial stress concentration due to deviation from isotropic elasticity next we show how uni axial far field stress solutions can be mutually superposed and combined with stress function solutions for local stress concentration due to internal pressure on the wellbore the reappraised set of green and taylor equations is then applied to horizontal wells in three andersonian stress states for compressional strike slip and extensional basins the stress concentrations in the horizontal wellbore are evaluated for well depths between 0 and 10 000 ft using the elastic stiffnesses from five shale and clay samples to show the stress deviations from the isotropic reference case finally a full wellbore stability evaluation model combines the stress concentrations around the wellbore with failure criteria to determine the safe drilling window at each well depth for each andersonian stress state practical graphs and supporting equations are provided to assess wellbore stability in transverse isotropic shale

the hydrochemistry of lake vostok and the potential for life in antarctic subglacial lakes our understanding of lake vostok the huge subglacial lake beneath the east antarctic ice sheet has improved recently through the identification of key physical and chemical interactions between the ice sheet and the lake the north of the lake where the overlying ice sheet is thickest is characterized by subglacial melting whereas freezing of lake water occurs in the south resulting in 210 m of ice accretion to the underside of the ice sheet the accreted ice contains lower concentrations of the impurities normally found in glacier ice suggesting a net transfer of material from meltwater into the lake the small numbers of microbes found so far within the accreted ice have dna profiles similar to those of contemporary surface microbes microbiologists expect however that lake vostok and other subglacial lakes will harbour unique species particularly within the deeper waters and associated sediments the extreme environments of subglacial lakes are characterized by high pressures low temperatures permanent darkness limited nutrient availability and oxygen concentrations that are derived from the ice that provides the meltwater copyright 2002 john wiley sons ltd

paleomagnetism of the borden dykes of baffin island and its bearing on the grenville loop northern baffin island is intersected by spectacular swarms of predominantly northwesterly trending diabase dykes cross cutting relationships k ar ages and paleomagnetic contact tests indicate that these dykes represent at least two major episodes of hadrynian igneous activity the borden dykes were emplaced during the earlier of these episodes they are about 950 ma old and have a paleomagnetic pole position of 153 3 e 26 7 s m 8 6e p 6 1 the later episode saw the emplacement of the previously defined franklin dykes which are about 750 ma old the newly defined borden pole lies at the southern extremity of the grenville loop and has an age consistent with poles of similar position that were derived from rocks of the grenville province all of the dykes probably reflect tension acting in an east northeast west southwest direction the directions of thick northwest trending dykes which form a considerable angle to that expected from east northeast tension are thought to have been controlled

air content paleo record in the vostok ice core antarctica a mixed record of climatic and glaciological parameters under present day climatic conditions the air content of ice shows a high sensitivity to the atmospheric pressure and hence to the elevation at the surface of the ice sheet this observation has been used to infer past ice sheet thickness variations of antarctica and greenland a high resolution air content profile more than 1000 measurements covering approximately the last 200 000 years was obtained along the 2546 m long vostok ice core three analytical techniques were used leading to consistent results which show large amplitude and rapid air content variations the vostok results support thicker thinner ice in the central part of east antarctica during warm cold periods

geomechanics of fracture caging in wellbores this study highlights the occurrence of so called fracture cages around underbalanced wellbores where fractures cannot propagate outwards due to unfavourable principal stress orientations the existence of such cages is demonstrated here by independent analytical and numerical methods we explain the fracture caging mechanism and pinpoint the physical parameters and conditions for its control this new insight has great practical relevance for the effectiveness and safety of drilling operations in general and hydraulic fracturing in particular fracture caging runaway poses a hazard for drilling operations in overpressured formations recognition of the fracture caging mechanism also opens up new opportunities for controlled engineering of its effects by the manipulation of the frac number in wells in order to bring more precision in the fracking process of tight formations

magnetic and tectonic history of the late proterozoic upper little dal and coates lake groups of northwestern canada abstract a pre to syn rift sequence upper little dal group coates lake group in the mackenzie mountains of northwestern canada reveals paleomagnetic directions in pigmental hematite thb rrb that are similar to hematite overprint directions p2 of underlying late proterozoic strata paleopoles of these magnetic components from the thundercloud and overlying redstone river formations of the coates lake group are respectively 202 w 27 n n 6 sites a95 9 and 206 w 32 n n 4 sites a95 10 the primary paleopole for the basal thundercloud formation tha at 210 w 01 n n 6 sites a95 21 is close to a previously reported pole 222 w 2 n from diabases dated by u pb at 778 2 ma that intrude strata below little dal group the large error in thai caused by scatter along a small circle is here attributed mainly to tectonic rotation of azimuths of section mean directions based on this interpretation the tha pole position suggests a minimal unconformity below the thundercloud formation contemporaneity of the sills and the basalts that cap the little dal group and uncomformity between thundercloud and redstone river formations anomalous directions from the little dal basalts and sequential directions in magnetic phases of the redstone river formation are also interpreted to record local block rotations during trans tensional tectonism the p2 disseminated pigmental hematite magnetization is interpreted to immediately postdate magnetizations from most other hematite phases as well as magnetizations from reduced sulphide magnetite beds at the base of the coppercap formation upper coates lake group p2 is absent from and likely predates deposition of the upper coppercap and overlying formations some poles p3 that postdate p2 poles and indicate high paleonatitudes are here interpreted as late precambrian negative fold tests suggest that most other high latitude poles from late proterozoic units of the region are of cretaceous laramide age

paleomagnetism of the late proterozoic tsezotene formation of northwestern canada paleomagnetic evidence from 37 sites of the partly red pigmented siliciclastic tsezotene formation supports a recently proposed apparent polar wander path for the hadrynian mackenzie mountains supergroup mms the probable primary remanence has a direction at d i 271 24 k 15 95 8 with an associated pole ta 12 n 214 w n 23 specimens p m 5 9 ta becomes the oldest pole from the mms it helps bridge the gap between the apparently youngest poles of the grenville loop about 0 88 ga and the suggested younger poles from the mms a secondary pole tb 23 n 198 w n 18 sites p m 3 5 derived from a magnetic direction in hematite pigment d i 263 48 k 73 95 4 supports a magnetization found in the overlying copper cycle and in younger units of the mms as a pervasive overprint another secondary pole tc 63 n 141 w n 29 sites p m 6 6 derived from a magnetization d i 317 87 k 89 95 3 partly residing in another phase of hematite pigment is of postfolding age post paleocene this study demonstrates the importance of using several treatment methods singly and in combination when analyzing complex magnetizations

raman spectroscopy of gaseous inclusions in edml ice core first results microbubbles measurements of n2 o2 ratios inside individual air bubbles in various depths of the edml ice core are presented here the small bubbles diameter below 200 m in deeper ice are signi cantly enriched in o2 compared to the larger bubbles the n2 o2 ratios show a systematic dependence on bubble size which is not the case for bubbles in shallower ice this is interpreted as an e ect of pressure relaxation during storage of the cores

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

air hydrate inclusions in fresh ice core one of the characteristic features of polar glacier ice is the abundance of air bubbles1 3 with increasing depth from the surface of the ice sheet the size of the bubbles generally decreases in response to the change of overburden pressure below a certain depth the bubbles start to shrink faster than expected and finally become invisible although not visible to the naked eye unit mass of the bubble free clear ice includes almost the same volume of air as unit mass of bubbly ice4 5 it is hypothesized that the air exists in the ice structure probably in the form of clathrate hydrates as suggested by miller6 7 but until now the location of the air has been unknown direct observations of air hydrate inclusions were carried out for the first time in fresh ice cores from dye 3 greenland when the ice structure surrounding air hydrate was melted an explosive transition occurred releasing a gas bubble into the melt cavity

comprehensive atlas of stress trajectory patterns and stress magnitudes around cylindrical holes in rock bodies for geoscientific and geotechnical applications abstract this study presents a comprehensive atlas comprising maps and graphs which quantify the wide range of possible stress states around circular cylindrical cavities when subjected to the full range of far field stress conditions and internal net pressures a greater understanding of the principal stress trajectories and related stress magnitudes around stressed cylindrical holes in rock formations significantly aids the design of safer and more stable wellbores advances our capacities in hydraulic fracturing and provides additional insights into the nature and stability of volcanic dikes salt domes and caverns used for hydrocarbon and other fluid storage the algorithms used in our study systematically non dimensionalize the analytical kirsch equations using two critical parameters to scale the variable boundary conditions 1 the bi axial stress scalar describes the far field stresses at all depths and for each of the formations penetrated by a cylindrical borehole or natural liquid filled pipe by describing the anisotropy of the two far field principal stresses and 2 the frac number f specifies the net pressure on the cylindrical hole as it relates to the natural background stress in the host rock and the pressure in the penetrated formation at each depth manipulation of these two governing parameters f enables us to view and monitor the stress trajectories in the host rock around a cylindrical hole at any given depth additionally the neutral point locations and their effects on the principal stress orientations near the cylindrical hole have been more thoroughly investigated than in earlier studies the neutral points form the boundaries for the stress cage in overbalanced cylindrical cavities and for the complimentary fracture cage in underbalanced cavities the understanding of stresses induced around cylindrical holes in a rock formation is paramount in geotechnical analyses and stability appraisals of manmade cylindrical holes as well as for the geoscientific interpretation of piercing by cylindrical fluid filled pipes our stress atlas aims to provide fundamental insight and practical support for such analyses

post tectonic magnetizations from the cordova gabbro ontario and palaeozoic reactivation in the grenville province summary the cordova gabbro of southern ontario intrudes 1300 myr old volcanic rocks of the hastings lowlands in the grenville structural province three distinct vector magnetizations a b and c have been isolated using a combination of stable endpoints subtracted vectors from orthogonal vector plots and converging remagnetization circles the a magnetization with mean direction d 294 i 55 5 k 42 ag5 5 5 n 18 sites is a high coercivity high blocking temperature remanence recorded by 49 samples the b magnetization was isolated in 33 samples and has a mean direction d 305 5 1 1 5 k 24 ag5 9 5 n 11 sites b has lower coercivities and blocking temperatures than a where the two are superimposed the a and b palaeopoles 151 e 10 5 s dp 6 dm 8 and 165 5 e 24 n dp so dm 9 5 fall on the grenville track around 900 and 820 ma respectively the a and b magnetizations thus date from uplift and cooling following the grenvillian orogeny the third magnetization the c component has been isolated in 23 samples its mean direction is d 180 i 27 5 k 18 ag5 10 5 n 12 sites the c is a low coercivity low blocking temperature overprint of a and b its palaeopole 102 e 31 n dp 6 5 dm 12 is unlike post 1300 precambrian poles for cratonic north america but matches silurian and late ordovician poles 40ar 39ar plateau ages of 446 and 447 ma determined by lopez martinez and york for plagioclases from one of the cordova samples confirm this age assignment the c magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in palaeozoic time long after the grenvillian orogeny

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

using laser light scattering to measure impurities bubbles and imperfections in ice cores 1 laser light scattering lls on polar ice or on polar ice meltwater is an accepted method for measuring the concentration of water insoluble aerosol deposits dust in the ice lls on polar ice can also be used to measure water soluble aerosols as well as imperfections air bubbles and cavities in the ice lls was originally used for measuring the dust concentration in polar ice meltwater but was later also applied to solid ice for measuring the dust concentration profile along the deep bubble free sections of the greenland ice sheet project 2 gisp2 ice core from central greenland in this paper we extend the usability of lls on ice into the realm of the nontransparent bubbly polar ice and put previous empirical findings on a theoretical footing for lls on clear bubble free polar ice we studied numerically the scattering of light by soluble and insoluble aerosol particles embedded in the ice to complement previous experimental studies for air bubbles in polar ice we calculated the effects of multiple light scattering using mie theory and monte carlo simulations and found a method for determining the bubble size and number concentration using lls on bubbly ice we also demonstrated that lls can be used on bubbly ice to rapidly measure annual layers

a new paleomagnetic pole for the neoproterozoic uinta mountain supergroup central rocky mountain states usa abstract hematite cemented strata of the neoproterozoic age 800 750 ma uinta mountain group umg provide a new well defined mid neoproterozoic paleomagnetic pole for laurentia thermal demagnetization results and rock magnetic experiments reveal the presence of two discrete magnetizations in umg rocks both magnetizations are carried by hematite the dominant characteristic remanent magnetization chrm is an east directed or antipode shallow positive to negative inclination magnetization that is typically unblocked over a narrow range of high laboratory temperatures between 660 and 680 c the second magnetization exclusively of normal polarity is north to northeast directed and has a moderate to steep positive inclination and probably represents a recent or modern day viscous remanent magnetization the response of umg samples to thermal and chemical demagnetization comparison with previously obtained data from umg rocks in the area results from rock magnetic experiments and the inferred low latitude of laurentia during umg deposition in the neoproterozoic are all consistent with the interpretation that the west or east directed shallow magnetization is a near primary remanence the presence of dual polarities of the chrm with the west directed remanence inferred to be of normal polarity and the east directed of reverse polarity suggests that magnetization acquisition in the umg rocks took place over a sufficient amount of time to average secular variation of the geomagnetic field the mean paleomagnetic pole derived from the inferred primary magnetizations from umg strata 0 8 n 161 3 e 95 4 6 n 9 sampling localities consisting of 79 sites is fully consistent with recently reported results from rocks of mid neoproterozoic age for southwest laurentia which define a tight counterclockwise loop located in the south pacific in the apparent polar wander path for laurentia during the neoproterozoic

geochemistry of neohelikian nauyat plateau basalts borden rift basin northwestern baffin island canada subaerial basalt flows of the neohelikian nauyat formation from northwestern baffin island northwest territories constitute an approximately 360 m thick unit that overlies an archean aphebian metamorphic basement the lavas have undergone a low grade regional metamorphism that affected the abundances of na k and related trace elements the basalts are continental tholeiites possessing some characteristics of mid ocean ridge basalts they underwent fractional crystallization of clinopyroxene plagioclase and olivine mantle normalized patterns show an enrichment of the lithophile elements including th and light rare earth elements relative to the high field strength elements and a distinct depletion of nb and sr the parental magma of the basalts was derived either from oceanic type mantle and subsequently affected by lower crustal contamination or from the subcontinental lithospheric mantle the nauyat basalts are probably related to the initial stage of the opening of the poseidon proto arctic o

air clathrate crystals from the grip deep ice core greenland a number size and shape distribution study we performed microscopic observations and a statistical study of the number size and shape distribution of clathrates in the grip greenland ice core project deep ice core using 185 samples from a depth range of 1016 3014m spanning a period of 6 to 110 ka sp and encompassing the holocene wisconsin and eemian periods the number concentration of the clathrates varied considerably with climatic changes it was possible to detect the rapid climatic oscillations in the last glacial between 13 and 110 ka bp the dansgaard oeschger cycles in the number concentration profile of clathrates the mean volume of clathrates is less clearly influenced by climatic factors with a tendency towards greater volumes in warmer periods but also a growth of clathrates with depth could be detected this growth rate was calculated to be 3 1 x 10 12 cm 3 a 1 the amount of gases captured in the clathrates is estimated to be significantly smaller than the total amount of air determined by gas concentration measurements this points to diffusion processes of atmospheric gases within the ice matrix

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

raman tomography of natural air hydrates cryo raman tomography allows us for the first time to determine accurate morphologies and volumes of natural air hydrates in antarctic ice cores the measurements show complex growth structures that are not accounted for by the available models of hydrate formation

comment on a new model for the paleogene motion of greenland relative to north america plate reconstructions of the davis strait and nares strait regions between canada and greenland by g n oakey and j a chalmers

variation in n2 o2 ratio of occluded air in dome fuji antarctic ice ancient atmospheric gases are trapped in polar ice sheets the gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate significant gas fractionation has been found from measurements of the depth profile of the n2 o2 composition ratios in clathrate hydrates and air bubbles of vostok antarctic ice to investigate the effect of the ice condition on the fractionation process we measured the n2 o2 ratios in clathrate hydrates and air bubbles from dome fuji antarctic ice using raman spectroscopy the results showed that the n2 o2 ratios in the clathrate hydrates of the dome fuji ice are slightly lower than those of the vostok ice although the tendency of the variation of the n2 o2 ratio with depth is similar the difference in the n2 o2 ratio between the dome fuji ice and the vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate on the other hand it is concluded that the difference in the bubble free ice zone was caused by gas loss from the ice core after coring the n2 o2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of o2 than of n2 our data suggest that the effect of gas loss in the dome fuji ice is relatively small and so the gas composition in the dome fuji ice can be a precise paleoenvironmental indicator

visual stratigraphic dating of the gisp2 ice core basis reproducibility and application annual layers are visible in the greenland ice sheet project 2 ice core from central greenland allowing rapid dating of the core changes in bubble and grain structure caused by near surface primarily summertime formation of hoar complexes provide the main visible annual marker in the holocene and changes in cloudiness of the ice correlated with dustiness mark wisconsinan annual cycles both markers are evident and have been intercalibrated in early holocene ice layer counts are reproducible between different workers and for one worker at different times with 1 error over century length times in the holocene reproducibility is typically 5 in wisconsinan ice age ice and decreases with increasing age and depth cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50 000 years visible observations are not greatly degraded by brittle ice or many other core quality problems allowing construction of long consistently sampled time series high accuracy requires careful study of the core by dedicated observers

a new device for the measurement of air content in polar ice a new device for measuring air content in polar ice has been designed built and tested with 22 samples of antarctic ice the new technique is based on the barometric method which implies 1 air extraction under vacuum by melting refreezing of the ice sample placed in a calibrated cell and 2 accurate air pressure and temperature measurements the new apparatus simplifies the experimental procedures decreases the duration of measurement and can eventually be used in the field it provides results with an accuracy equivalent to or better than other methods previously used

air bubble to clathrate hydrate transformation in polar ice sheets a reconsideration based on the new data from dome fuji ice core 1 mapping the spatial distribution of air bubbles and clathrate hydrates in thick sections of antarctic ice from dome fuji station reveals a considerable redistribution of the inclusions in terms of number and volume in the bubble to hydrate transition zone analysis of the data indicated that more than 10 of the hydrates nucleated outside the preceding bubbles and more than 30 of the bubbles completely expired due to the gas outflow toward the coexisting hydrates these features of the gas hydrate ice nonequilibrium system are of major importance for modeling the global bubble to hydrate transformation in polar ice sheets and thus the results may help improve our ability to extract paleoclimatic information from air inclusions in ice cores

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

developing a bubble number density paleoclimatic indicator for glacier ice past accumulation rate can be estimated from the measured number density of bubbles in an ice core and the reconstructed paleotemperature using a new technique density increase and grain growth in polar firn are both controlled by temperature and accumulation rate and the integrated effects are recorded in the number density of bubbles as the firn changes to ice an empirical model of these processes optimized to fit published data on recently formed bubbles reconstructs accumulation rates using recent temperatures with an uncertainty of 41 p 0 05 for modern sites considered here no statistically significant trend exists between mean annual temperature and the ratio of bubble number density to grain number density at the time of pore close off optimum modeled accumulation rate estimates require an eventual 2 02 0 08 p 0 05 bubbles per close off grain bubble number density in the grip greenland ice core is qualitatively consistent with independent estimates for a combined temperature decrease and accumulation rate increase there during the last 5 kyr

on the age vs depth and optical clarity of deep ice at south pole the first four strings of phototubes for the amanda high energy neutrino observatory are now frozen in place at a depth of 800 to 1000 m in ice at the south pole during the 1995 96 season an addit

grain size measurements of natural gas hydrates abstract methane hydrates are present in marine seep systems and occur within the gas hydrate stability zone very little is known about their crystallite sizes and size distributions because they are notoriously difficult to measure crystal size distributions are usually considered as one of the key petrophysical parameters because they influence mechanical properties and possible compositional changes which may occur with changing environmental conditions variations in grain size are relevant for gas substitution in natural hydrates by replacing ch 4 with co 2 for the purpose of carbon dioxide sequestration here we show that crystallite sizes of gas hydrates from some locations in the indian ocean gulf of mexico and black sea are in the range of 200 400 m larger values were obtained for deeper buried samples from odp leg 204 the crystallite sizes show generally a log normal distribution and appear to vary sometimes rapidly with location

physical properties of the wais divide ice core the wais west antarctic ice sheet divide deep ice core was recently completed to a total depth of 3405 m ending 50 m above the bed investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large scale overturning or discontinuity the climate record developed from this core is therefore likely to be continuous and robust measured grain growth rates recrystallization characteristics and grain size response at climate transitions fit within current understanding significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size bubble number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice where bubble elongation is observed the direction of elongation is preferentially parallel to the trace of the basal 0001 plane preferred crystallographic orientation of grains is present in the shallowest samples measured and increases with depth progressing to a vertical girdle pattern that tightens to a vertical single maximum fabric this single maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest warmest ice a strong dependence of the fabric on the impurity mediated grain size is apparent in the deepest samples

bubbly ice densification in ice sheets i theory dry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure this stage extends to the transition zone where due to high pressures and low temperatures air trapped in bubbles and ice begins to form the mixed air clathrate hydrates while the gas phase progressively disappears here we develop a model of bubbly ice rheology and ice sheet dynamics taking into account glacier ice compressibility the interaction between hydrostatic compression of air bubbles deviatoric uniaxial compressive deformation of the ice matrix and global deformations of the glacier body is considered the ice matrix pressure and the absolute load pressure are distinguished similarity theory and scale analysis are used to examine the resultant mathematical model of bubbly ice densification the initial rate of bubble compression in ice sheets appears to be relatively high so that the pressure density relaxation process takes place only 150 200 m in depth below pore close off to reach its asymptotic phase wherein the minimal drop between bubble and ice pressures is governed by the rate of loading ice accumulation this makes it possible to consider densification under stationary present day conditions of ice formation as a special case of primary interest the computational tests performed with the model indicate that both ice porosity and bubble pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice however only the bubble pressure profile distinguishes between the rheological properties at low and high stresses the porosity profile at the asymptotic phase is mostly determined by the air content in the ice in the companion paper lipenkov and others 1997 we apply the model to experimental data from polar ice cores and deduce through an inverse procedure the rheological properties of pure ice as well as the mean air content in holocene and glacial ice sediments at vostok station antarctica

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

polar ice structure and the integrity of ice core paleoclimate records abstract polar ice is a unique archive of the climatic conditions in the past however ice sheets flow and this flow may affect the integrity of paleoclimate records a useful method to analyze the effect of ice flow upon climate records is the combination of microstructure mapping with ice core line scanning microstructure and stratigraphy have been mapped along the entire epica dml ice core with this combined method on the macroscale the stratigraphy seems perfectly preserved down to ca 1700 m depth mis4 below which minor undulations start to develop layers inclined up to 15 and millimeter scale z folds are observed below 2050 m depth notwithstanding the epica dml climate record appears not seriously disturbed down to ca 2400 m depth which marks the climatic transition from the last interglacial mis5e to the mis6 glacial period below this depth the synchronization with the epica dome c record is lost and stratigraphic disturbances appear up to the meter scale on the microscopic scale we observe dynamic recrystallization already in deep firn leading to substantial microstructural changes prior to bubble close off the concentration of visible micro inclusions in the holocene part of the core seems to increase with depth which could possibly indicate post depositional formation of salts in glacial period ice the concentration of visible micro inclusions in certain layers is so high that these strata appear as light scattering bands often called cloudy bands another interesting stratigraphic feature is what we call bubble free bands viz millimeter thick strata deprived of bubbles and rich in clathrate hydrates which are found within the bubble hydrate transition zone 800 1200 m depth and are probably caused by anomalously fast clathration while there is hardly any interaction between visible micro inclusions and grain boundaries down to 2500 m depth in the deeper ice warmer than 10 c many grain boundaries seem to harvest micro inclusions here we discuss these observations and their relevance for the interpretation of climate records

the nanogranular acoustic signature of shale a multiscale micromechanics model has been developed for the prediction of anisotropic acoustic properties of shale the model is based on the recently identified nanogranular mechanical response of shale through indentation experiments it recognizes the dominant role of the anisotropic elastic properties of compacted clay in the anisotropic elasticity of shale at different length scales compared to contributions of shape and orientation of particles following a thorough validation at multiple length scales using mineral elasticity data nanoindentation experiment results and ultrasonic pulse velocity tests the model predictions compare adequately with measurements on kerogen free and kerogen rich shales and shaley sandstones the acoustic signature of shale thus is found to be controlled by two volumetric parameters that synthesize the porosity and mineralogy information the clay packing density and the silt inclusion volume fraction through a series of dimensionless isoparametric plots the micromechanics model predicts trends of increasing elastic anisotropy with increasing clay packing density or decreasing porosity which correspond to the intrinsic mechanical response of unfractured shale and quantifies the stiffness reduction induced by the presence of kerogen

height changes over subglacial lake vostok east antarctica insights from gnss observations height changes of the ice surface above subglacial lake vostok east antarctica reflect the integral effect of different processes within the subglacial environment and the ice sheet repeated gnss global navigation satellite systems observations on 56 surface markers in the lake vostok region spanning 11 years and continuous gnss observations at vostok station over 5 years are used to determine the vertical firn particle movement vertical marker velocities are derived with an accuracy of 1 cm yr or better repeated measurements of surface height profiles around vostok station using kinematic gnss observations on a snowmobile allow the quantification of surface height changes at 308 crossover points the height change rate was determined at 1 5 mm yr thus indicating a stable ice surface height over the last decade it is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period the continuous gnss observations demonstrate that the particle heights vary linearly with time nonlinear height changes do not exceed 1 cm at vostok station and constrain the magnitude of spatiotemporal lake level variations icesat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters assuming the ice sheet to be in steady state over the entire lake estimates for the surface accumulation on basal accretion melt rates and on flux divergence are derived

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

the effect of kerogen on the elastic anisotropy of organic rich shales abstractthe elastic properties of reservoir rocks are important for geomechanics applications the most important of which are analysis of stress changes due to production analysis of rock deformation and failure wellbore trajectory optimization and the design of hydraulic fractures organic rich shales are often observed to be strongly anisotropic due to the partial alignment of anisotropic clay minerals and the bedding parallel lamination of organic material within the shale neglecting shale anisotropy may lead to incorrect estimates of the in situ stress or stress changes resulting from production as a result isotropic models may fail to describe geomechanical behavior correctly the distribution of the organic phase plays an important role in determining the elastic properties of organic rich shales and this has a significant effect on production induced stress changes the presence of kerogen leads to a decrease in all of the elastic moduli and has a significant effect on the geomechanical b

bubbly ice densification in ice sheets ii applications a mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the vostok central antarctica ice core two ice porosity profiles obtained by independent methods and a bubble pressure profile obtained by direct measurements of air pressure within individual bubbles the rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem the model and the inferred ice flow law are then validated using porosity profiles from seven other ice cores drilled in antarctica and greenland in the temperature range from 55 to 20 c the following expression is adopted for the constitutive law 2e 1 2 exp q 1 t s 1 t r s where e and are the effective strain rate and stress respectively is the creep exponent taken as 3 5 r s is the gas constant and t t s is the temperature standard temperature the numerical values obtained for the linear and non linear viscosities are 1 2 9 1 3 mpayear and 2 0 051 0 019 mpa year and the apparent activation energy q is confirmed to be 60 kj mole 1 the corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier ice deformation when the model is constrained by the porosity and bubble pressure profiles from vostok the mean air content in holocene ice is inferred to be about 0 088 cm 3 g 1 the corresponding mean air pressure in bubbles at the end of pore closure is about 0 083 mpa whereas the atmospheric pressure at this depth level would be 0 063 mpa the influence of the climatic change on the ice porosity profile is discussed it resulted in an increased air content in ice at vostok during the last glacial maximum 0 096 cm 3 g 1

kinetics of conversion of air bubbles to air hydrate crystals in antarctic ice the depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the south pole are modeled with diffusion growth data from the laboratory taking into account the dependence of age and temperature on depth in the ice the model fits the available data on bubbles in cores from vostok and byrd and on scattering length in deep ice at the south pole it explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below 1400 meters the amanda neutrino observatory at the south pole will operate unimpaired by light scattering from bubbles

flow induced mixing in the grip basal ice deduced from the co2 and ch4 records this paper documents a larger degree of mixing in ice near the bottom of an ice sheet than described or suspected previously it shows thanks to favourable circumstances due to co2 and ch4 production underneath the ice that flow induced mixing within the basal ice has taken place at the scale of a few centimeters in the grip core such a mechanism must be considered when interpreting the ice properties in the bottom part of ice sheets and must be taken into account as a potential process of layer disruption in the low levels of the central greenland ice cores

water exchange between the subglacial lake vostok and the overlying ice sheet it has now been known for several years that a 200 km long lake called lake vostok lies beneath the ice sheet on which sits vostok station in antarctica the conditions at the base of the ice sheet above this subglacial lake can provide information about the environment within the lake including the likelihood that it supports life here we present an analysis of the ice sheet structure from airborne 60 mhz radar studies which indicates that distinct zones of basal ice loss and accretion occur at the ice water interface subglacial melting and net ice loss occur in the north of the lake and across its 200 km long western margin whereas about 150 m of ice is gained by subglacial freezing in the south this indicates that significant quantities of water are exchanged between the base of the ice sheet and the lake waters which will enrich the lake with gas hydrates cause sediment deposition and encourage circulation of the lake water

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

physical and climatic parameters which influence the air content in polar ice abstract under present day climatic conditions the air content of polar ice v generally shows a high sensitivity to the atmospheric pressure and hence to the surface elevation of the ice sheet wherethe ice is formed the results presented here are from sixteen different sites fourteen in antartica one in greenland and one in the yukon territories canada and they allow a better understanding of the parameters which influence the air content in ice it is demonstrated the v can be expressed very simply as a function of the atmospheric pressure p i the temperature t i and the porous volume of ice v i at which the air in the firn becomes isolated in terms of pressure from the atmosphere during the process of pore close off our results confirm a v i increase with temperature and show no clear v i dependence on snow accumulation rate the possible non linearity of the v i t i relation we obtain could be due to a wind influence on v i and or to a second order effect of the accumulation rate our v i results are compared to measurements of the volume of closed pores versus depth in the firn to close we discuss the parameterization that we can obtain to interpret the air content variations observed in deep ice cores over periods covering glacial interglacial transitions in terms of paleoclimatic conditions prevailing at the surface of the ice sheet

palaeomagnetism and 40ar 39ar age determinations of the ediacaran traps from the southwestern margin of the east european craton ukraine relevance to the rodinia break up a palaeomagnetic study and age determinations have been performed on ediacaran basalts from the northwestern ukraine whole rock 40 ar 39 ar age determination revealed plateau ages at 590 560 ma and 393 ma the latter probably reflecting a resetting of the radiometric system palaeomagnetic poles have been calculated from five basalt flows two of which a poles are considered reliable with ages that range from 580 to 560 ma tentative poles b poles calculated from most probably primary magnetizations have ages estimated at 580 545 ma secondary magnetizations possibly of late ediacaran or devonian age have also been isolated c poles based on the new poles baltica drifted together with laurentia from an equatorial position at c 750 ma to occupy high southern latitude positions at c 580 ma baltica during that time period was joined to laurentia in a similar relative position to that at 750 ma the two shields then split up from each other and from c 550 ma baltica drifted at moderately high latitudes and rotated some 180 during the final opening of the iapetus ocean this reconstruction suggests that during the ediacaran glaciation baltica occupied high latitude positions which contradicts the high obliquity model to explain low latitude neoproterozoic glaciations

microstructure mapping a new method for imaging deformation induced microstructural features of ice on the grain scale this work presents a method of mapping deformation related sublimation patterns formed on the surface of ice specimens at microscopic resolution 3 4 mm pixel 1 the method is based on the systematic sublimation of a microtomed piece of ice prepared either as a thick or a thin section the mapping system consists of an optical microscope a ccd video camera and a computer controlled xy stage about 1500 images are needed to build a high resolution mosaic map of a 4 5 9 cm section mosaics and single images are used to derive a variety of statistical data about air inclusions air bubbles and air clathrate hydrates texture grain size shape and orientation and deformation related features subgrain boundaries slip bands subgrain islands and loops pinned and bulged grain boundaries the most common sublimation patterns are described and their relevance for the deformation of polar ice is briefly discussed

multistage dolomitization in the society cliffs formation northern baffin island northwest territories canada dolostones of the upper proterozoic society cliffs formation on northern baffin island are host to the nanisivik lead zinc sulfide deposits two distinct stages of dolomitization have occurred distinguished by their petrographic cathodoluminescence and isotopic characteristics 1 massive dolomitization of precursor carbonates and 2 late stage cementation sabkha facies of the lower member of the formation and laminated algal stromatolitic to massive dolostones of the upper member are isotopically similar 18o ranges from 1 5 to 6 5 13c from 1 7 to 3 1 pdb and 87sr 86sr averages 0 70761 petrographic and geochemical evidence indicates that these dolostones formed chiefly by seawater thermal convection beneath shale of the overlying victor bay formation although mixing of meteoric and marine water also could have occurred later fracture filling dolomitic cements have consistently lower 18o but 13c is similar to that in the host dolostones microtraverses of single fractures filled wit

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

high resolution variations in size number and arrangement of air bubbles in the epica dml antarctica ice core abstract we investigated the large scale 10 1000m and small scale mm cm variations in size number and arrangement of air bubbles in the epica dronning maud land edml antarctica ice core down to the end of the bubble hydrate transition bht zone on the large scale the bubble number density shows a general correlation with the palaeo temperature proxy d18o and the dust concentration which means that in holocene ice there are fewer bubbles than in glacial ice small scale variations in bubble number and size were identified and compared above the bht zone there exists a strong anticorrelation between bubble number density and mean bubble size in glacial ice layers of high number density and small bubble size are linked with layers with high impurity content identified as cloudy bands therefore we regard impurities as a controlling factor for the formation and distribution of bubbles in glacial ice the anticorrelation inverts in the middle of the bht zone in the lower part of the bht zone bubble free layers exist that are also associated with cloudy bands the high contrast in bubble number density in glacial ice induced by the impurities indicates a much more pronounced layering in glacial firn than in modern firn

paleomagnetism and u pb geochronology of the clarence head dykes arctic canada orthogonal emplacement of mafic dykes in a large igneous province the north south trending clarence head dyke swarm located on devon and ellesmere islands in the canadian high arctic has a trend orthogonal to that of the neoproterozoic franklin swarm that surrounds it the clarence head dykes are dated by the u pb method on baddeleyite to between 716 1 and 713 1 ma ages apparently younger than but within the published age range of the franklin dykes alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages but a comparison of the u pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the clarence head dykes are indeed a distinctly younger event within the period of franklin magmatism the clarence head dykes represent a large swarm tangential to and cogenetic with a giant radiating dyke swarm 800 km from the indicated source the preferred mechanism for the emplacement of the clarence head dykes is the exploitation of concentric zones of e

very low oxygen concentration in basal ice from summit central greenland oxygen concentration as low as 3 has been detected in the basal silty ice of the grip core such values were never observed in ice from ice sheets they are most probably the consequence of organic matter oxidation in ice developed in a peaty deposit when the greenland ice sheet was not present at the site flow induced mixing has further incorporated this ice into glacier ice during the ice sheet build up the part of the local ice component in the mixing process diminishes with the distance from the bed this is the process which explains the oxygen profile

characteristics of air bubbles and hydrates in the dome fuji ice core antarctica air bubbles trapped near the surface of an ice sheet are transformed into air hydrates below a certain depth their volume and number varies partly with environment and climate air bubbles and hydrates at 120 2200 m depth in the dome fuji dome f ice core were examined with a microscope this depth range covers the holocene last glacial last interglacial previous glacial periods no air bubbles were seen below about 1100 m depth and air hydrates began to appear from about 600 m the observed number of air bubbles and hydrates was similar to that found in the vostok ice core for the ice covering the last glacial maximum period however the hydrate concentration in the dome f core is about half that of the vostok core reference to snow metamorphism and packing does not explain this finding

formation of air clathrate hydrates in polar ice sheets heterogeneous nucleation induced by micro inclusions to investigate factors influencing nucleation of air clathrate hydrates in polar ice sheets we have performed high resolution mapping of the distributions of soluble impurities air bubbles and air hydrate crystals versus depth in the dome fuji antarctic ice significant correlation observed between the concentrations of air inclusions and impurities in ice along with frequent occurrence of impurities inside hydrate crystals suggest that micro inclusions promote hydrate nucleation in the ice matrix our observations also show that the diffusive macroscopic scale redistribution of air constituents in ice in the bubble hydrate transition zone is controlled by the original sedimentary layering of soluble impurities acting as nucleation helpers the results of this study are important for the correct interpretation of high resolution gas analyses of ice cores and for better understanding the global bubble to hydrate transformation process in polar ice sheets

air bubbles and clathrate hydrates in the transition zone of the ngrip deep ice core abstract field studies of air bubble and clathrate inclusions in the ngrip ice core reveal two stages of clathrate formation in the transition zone translucent primary clathrates with rough surfaces and irregular shapes formed from individual bubbles split up and form smaller specimens deeper down in the transition zone 1100 m clathrate metamorphosis sets in giving rise to the formation of transparent secondary clathrates with smooth and regular often poly hedral or faceted shapes rod like crystals of great length 2 mm often show signs of separation into smaller units that fragment into smaller specimens these findings imply that the simplified picture of one bubble forming one clathrate has to be modified

lakes beneath the ice sheet the occurrence analysis and future exploration of lake vostok and other antarctic subglacial lakes abstract airborne geophysics has been used to identify more than 100 lakes beneath the ice sheets of antarctica the largest lake vostok is more than 250 km in length and 1 km deep subglacial lakes occur because the ice base is kept warm by geothermal heating and generated meltwater collects in topographic hollows for lake water to be in equilibrium with the ice sheet its roof must slope ten times more than the ice sheet surface this slope causes differential temperatures and melting freezing rates across the lake ceiling which excites water circulation the exploration of subglacial lakes has two goals to find and understand the life that may inhabit these unique environments and to measure the climate records that occur in sediments on lake floors the technological developments required for in situ measurements mean however that direct studies of subglacial lakes may take several years to happen

crystal growth of air hydrates over 720 ka in dome fuji antarctica ice cores microscopic observations of morphological changes below 2000 m depth air hydrate crystals store most of the ancient air contained in deep ice sheets we carried out microscopic observations of air hydrate crystals below 2000 m depth within the ice core from dome fuji antarctica to obtain their number and size distributions we found that the number density continuously decreased with depth whereas the average size increased in contrast to findings from shallower depths in addition the characteristic perturbations in both number density and average size distribution with climatic changes almost disappeared although they are clearly observed in shallow cores these results indicate that the air hydrate crystals grow considerably in deeper parts of the ice sheet and this growth is accompanied by the diffusion of air molecules in the ice the permeation coefficient of the air molecules in the ice sheet was estimated from the geometric parameters of the air hydrate distributions this is the first practical evidence comparable to the previous model estimations it allows us to evaluate the impacts of the air molecule migration in the ice sheet on the paleoclimatic information recorded in the deep ice cores

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

ice core age dating and paleothermometer calibration based on isotope and temperature profiles from deep boreholes at vostok station east antarctica an interpretation of the deuterium profile measured along the vostok east antarctica ice core down to 2755 m has been attempted on the basis of the borehole temperature analysis an inverse problem is solved to infer a local geophysical metronome the orbital signal in the surface temperature oscillations expressed as a sum of harmonics of milankovich periods by correlating the smoothed isotopic temperature record to the metronome a chronostratigraphy of the vostok ice core is derived with an accuracy of 3 0 4 5 kyr the developed timescale predicts an age of 241 kyr at a depth of 2760 m the ratio 13d 13t i between deuterium content and cloud temperature fluctuations at the top of the inversion layer is examined by fitting simulated and measured borehole temperature profiles the conventional estimate of the deuterium temperature slope corresponding to the present day spatial ratio 9 per mil oc is confirmed in general however the mismatch between modeled and measured borehole temperatures decreases noticeably if we allow surface temperature responsible for the thermal state of the ice sheet to undergo more intensive precession oscillations than those of the inversion temperature traced by isotope record with this assumption we obtain the long term temporal deuterium temperature slope to be 5 8 6 5 per mil oc which implies that the glacial interglacial temperature increase over central antarctica was about 15oc in the surface temperature and 10oc in the inversion temperature past variations of the accumulation rate and the corresponding changes in the ice sheet surface elevation are simultaneously simulated

late holocene climate evolution at the wais divide site west antarctica bubble number density estimates a surface cooling of 1 78c occurred over thetwo millennia prior to 1700 ce at the west antarctic ice sheet wais divide site based on trends in observed bubble number density of samples from the wdc06a ice core and on an independently constructed accumulation rate history using annual layer dating corrected for density variations and thinning from ice flow density increase and grain growth in polar firn are both controlled by temperature and accumulation rate and the integrated effects are recorded in the number density of bubbles as the firn changes to ice number density is conserved in bubbly ice following pore close off allowing reconstruction of either paleotemperature or paleo accumulation rate if the other is known a quantitative late holocene paleoclimate reconstruction is presented for west antarctica using data obtained from the wais divide wdc06a ice core and a steady state bubble number density model the resultant temperature history agrees closely with independent reconstructions based on stable isotopic ratios of ice the 1 78c cooling trend observed is consistent with a decrease in antarctic summer duration from changing orbital obliquity although it remains possible that elevation change at the site contributed part of the signal accumulation rate and temperature dropped together broadly consistent with control by saturation vapor pressure

regulation of ice stream flow through subglacial formation of gas hydrates localized subglacial zones of high traction help to regulate ice sheet flow geophysical data from a palaeo ice stream suggest that methane gas accumulation and hydrate formation beneath ice sheets can produce such high traction sticky spots

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity

crystalline texture of the 2083 m ice core at vostok station antarctica crystalline texture and c axis orien tation of the 2083 m ice core at vostok station covering more than 150 kyear reveal the existence of strong anisotropies changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries a smaller growth rate appears to be associated with cold periods a gradual increase in the horizontal elongation of grains was observed between 350 and 680 m but the mean value of the coefficient of the linear dimensional orientation of grains does not cha nge below 700 m the e axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains forming a vertical girdle patt ern this charac teristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longit udi nal tension the rotation of grains was calcu lated with respect to the total strain simulating the form ation of the girdle fabric pattern the fabric enhance ment factor was calculated at various depths it appears that vostok ice hardens gradua lly