rheology of ice at the bed of engabreen norway a three dimensional finite element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of engabreen a temperate glacier in northern norway the ice is modeled as an incompressible power law fluid with viscosity b 2 1 n 2n d where d is the second invariant of the stretching tensor and b and n are two parameters using measurements obtained in 1996 and 1997 two values of b are obtained one using the measured normal stress difference across the obstacle and the other using the measured bed parallel force over the instrument these two values are not equal probably owing to small frictional forces at the bed unaccounted for in the numerical model hence b ranges between 1 9 10 7 and 3 2 10 7 pa s 1 3 in 1996 and between 2 2 10 7 and 4 1 10 7 pa s 1 3 in 1997 these values are smaller than measured elsewhere for clean glacier or laboratory ice field measurements of water content fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice near isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

site information and initial results from deep ice drilling on law dome antarctica the aim of deep ice drilling on law dome antarctica has been to exploit the special characteristics of law dome summit i e low temperature and high accumulation near an ice divide to obtain a high resolution ice core for climatic environmental studies of the holocene and the last glacial maximum lgm drilling was completed in february 1993 when basal ice containing small fragments of rock was reached at a depth of 1196 m accurate ice dating obtained by counting annual layers revealed by fine detail 18 o peroxide and electrical conductivity measurements is continuous down to 399 m corresponding to a date of ad 1304 sulphate concentration measurements made around depths where conductivity tracing indicates volcanic fallout allow confirmation of the dating for agung in 1963 and tambora in 1815 or estimates of the eruption date from the ice dating for the kuwae vanuatu eruption 1457 the lower part of the core is dated by extrapolating the layer counting using a simple model of the ice flow at the lgm ice fabric measurements show a large decrease 250 to 14 mm 2 in crystal size and a narrow maximum in c axis verticality the main zone of strong single pole fabrics however is located higher up in a broad zone around 900 m oxygen isotope 18 o measurements show holocene ice down to 1113 m the lgm at 1133 m and warm 18 o about the same as holocene ice near the base of the ice sheet the lgm holocene 18 o shift of 7 0 only 1 larger than for vostok indicates that law dome remained an independent ice cap and was not overridden by the inland ice sheet in the glacial

rifting evolution in geological history abstract rift and rift like zones global linear extensional structures of the crust have appeared during most periods of the earth s history rifting as a geological process has undergone a complex evolution with its historical roots in the early precambrian the development of the oldest linear mobile tectonic zones archean greenstone belts and granulite belts combined the features of both rift and geosynclinal processes in the following epochs early proterozoic mobile zones protogeosynclinal regions and protorift zones evolved under similar tectonic regimes of crustal destruction in the early stages and compression in the late stages of their development but different in size scale and intensity of the deformations and thermal processes experienced typical rift structures have existed since the late proterozoic in the late proterozoic and paleozoic continental intracratonic rifting in the form of aulacogen and aulacogeosynclinal zone development was a process related and subordinated to a geosynclinal process in the adjacent mobile belts in the paleozoic the first epiorogenic postorogenic rift zones appeared within the epigeosynclinal folded belts during the meso cenozoic epoch of the earth s history rifting increased considerably and became independent of geosynclinal processes it is believed to be one of the main forms of continental crust destruction and formation of the oceanic basins and probably occurred under conditions of some general expansion of the earth

the buildup of the ice sheet in central greenland a study of the isotopic and gas composition of the basal silty ice recovered by the greenland ice core project grip core indicates that local ice formed in the absence of the greenland ice sheet is still preserved at summit such ice developed most probably within a peat deposit in a permafrost environment this local ice was subsequently intimately mixed with glacier ice from an advancing ice sheet progressing on the site this is in agreement with the highland origin and windward growth hypothesis for ice sheet development not for an in situ or regional growth from snowbanks the basal ice from the grip core possibly dates back to the original buildup of the greenland ice sheet 2 4 million years ago

modelled stress distributions at the dome summit south borehole law dome east antarctica a comparison of anisotropic ice flow relations in this study we compare the anisotropic flow relations for polycrystalline ice of azuma and goto azuma 1996 thorsteinsson 2002 placidi and others 2010 and budd and others 2013 observations from the dome summit south dss ice coring site at law dome east antarctica are used to model the vertical distribution of deviatoric stress components at the borehole site the flow relations in which the anisotropic rheology is parameterized by a scalar function so that the strain rate and deviatoric stress tensor components are collinear provide simple shear and vertical compression deviatoric stress profiles that are most consistent with laboratory observations of tertiary creep in combined stress configurations those flow relations where 1 the anisotropy is derived from the magnitude of applied stresses resolved onto the basal planes of individual grains and 2 the macroscopic deformation is obtained via homogenization of individual grain responses provide stress estimates less consistent with laboratory observations this is most evident in combined simple shear and vertical compression flow regimes where shear is dominant our results highlight the difficulties associated with developing flow relations which incorporate a physically based description of microdeformation processes in particular this requires that all relevant microdeformation recrystallization and recovery processes are adequately parameterized

setting a chronology for the basal ice at dye 3 and grip implications for the long term stability of the greenland ice sheet abstract the long term stability of the greenland ice sheet gis is an important issue in our understanding of the climate system limited data suggest that the northern and southern sections extend well back into the pleistocene but most age constraints do not definitively date the ice here we re examine the grip and dye 3 ice cores to provide direct ice core observations as to whether the gis survived previous interglacials known to be warmer 130 ka or longer 430 ka than the present interglacial we present geochemical analyses of the basal ice from dye 3 1991 2035 m and grip 3020 3026 m that characterize and date the ice we analyzed the elemental and isotopic composition of o2 n2 and ar of trapped air in these two cores to assess the origin of trapped gases in silty ice dating of the trapped air was then achieved by measuring the paleoatmospheric 40ar 38ar and the 17o anomaly 17 of o2 the resulting age is a lower limit because the trapped air may be contaminated with crustal radiogenic 40ar the oldest average age of replicates measured at various depths is 970 140 ka for the grip ice core and 400 ka 170 ka for dye 3 17 data from dye 3 also argue strongly that basal ice in this core predates the eemian this confirms that the greenland ice sheet did not completely melt at southern greenland during the last interglacial nor did it completely melt at summit greenland during the unusually long interglacial 430 kyr before present

a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we have constructed a numerical model of basal ice formation for glacier ice in contact with subglacial sediment the model predicts four different ice facies whose formation is controlled by availability of subglacial water to satisfy the basal freeze on rate clean or clotted facies may result from congelation or frazil ice growth occurring when supercooled meltwater separates ice base from substrate or if a groundwater source can supply water to the ice base at a velocity equal to the freezing rate laminated facies develops when the supply of subglacial water is sufficiently constrained for cryostatic suction to raise the subglacial effective stress above a threshold for intrusion of ice into sediment by regelation debris laminas 1 mm are entrained by short periodic regelation events of a few hours separated by longer periods days to months of congelation further meltwater limitation produces a massive dirty ice facies due to stacking of debris laminas the model predicts growth of solid dirty ice facies when the bed is meltwater depleted with fast freezing 5 mm yr 1 causing enhanced erosion 30 mm yr 1 we find that basal ice facies and sediment entrainment are controlled mainly by the ratio of freezing rate to water supply rate the predicted ice facies compare favorably with borehole camera imagery of the basal ice layer in kamb ice stream west antarctica facies variability in this layer suggests complex hydrologic history for the west antarctic ice sheet with significant changes occurring over a period of several thousand years

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

convection in ice i shells and mantles with self consistent grain size 1 the viscosity of ice i is grain size dependent for temperature and stress conditions appropriate for ice i shells and mantles of large and midsized icy satellites satellite thermal evolution heat flux critical shell thickness for convection brittle ductile transition temperature and potential for surface deformation are therefore grain size dependent using measured grain sizes from terrestrial ice sheets experiencing temperature and strain rate conditions similar to convecting ice shells in icy satellites we develop two end member models of grain size and its evolution in the absence of non water ice impurities grain size in a convecting ice shell of a large icy satellite will evolve over time to an equilibrium value which we determine to be between 30 and 80 mm because of dynamic recrystallization such large grains imply viscous ice so convection in an ice shell thinner than 35 km in a large icy satellite such as ganymede and callisto would be sluggish at best and may cease after a few convective overturns if it starts soluble ions and silicate microparticles may keep grains small however of order 1 5 mm and permit convection in shells as thin as 30 km in large icy satellites the dynamic recrystallization model provides a plausible upper limit on grain size in a geodynamically evolved ice shell whereas grain sizes from dust contaminated terrestrial ice cores provide a plausible lower limit on grain size in an icy satellite on the basis of these constraints grain sizes in evolved ice shells of large water ice satellites in the absence of tidal forcing should be of order 1 mm to 10 cm

the tertiary creep of polycrystalline ice experimental evidence for stress dependent levels of strain rate enhancement laboratory creep deformation experiments have been conducted on initially isotropic laboratory made samples of polycrystalline ice steady state tertiary creep rates ter were determined at strains exceeding 10 in either uniaxial compression or simple shear experiments isotropic minimum strain rates min determined at 1 strain provide a reference for comparing the relative magnitude of tertiary creep rates in shear and compression through the use of strain rate enhancement factors e defined as the ratio of corresponding tertiary and isotropic minimum creep rates i e e ter min the magnitude of strain rate enhancement in simple shear was found to exceed that in uniaxial compression by a constant factor of 2 3 results of experiments conducted at octahedral shear stresses of o 0 04 0 80 mpa indicate a creep power law stress exponent of n 3 for isotropic minimum creep rates and n 3 5 for tertiary creep rates the difference in stress exponents for minimum and tertiary creep regimes can be interpreted as a o stress dependent level of strain rate enhancement i e e 1 2 o the implications of these results for deformation in complex multicomponent stress configurations and at stresses below those used in the current experiments are discussed

origin and development of textures and fabrics in basal ice at summit central greenland abstract detailed textural and crystallographic characteristics of the bottom 6 m of the grip core summit central greenland are presented the results are discussed in the light of present day fabric analogues ice fabrics are shown to retain the mark from the entrainment of locally formed ice by the growing ice sheet this mark is only partly overprinted by the present day stress configuration at the ice divide these results corroborate the model put forward by souchez et al 1 on stable isotopes and gas composition grounds in which the basal silty ice appears as a remnant of a growing stage of the ice sheet during this stage which probably represents the original build up ice of local origin formed at the ground surface was overridden by the main ice sheet and incorporated into its base preservation of remnants of ancient fabric patterns and occurrence of annealing recrystallization fabrics just above the first layer of silty ice are used amongst other arguments to demonstrate that this basal ice is not presently subject to large cumulative strains the implications for flow and dating models are discussed

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

fabrics in polar ice sheets development and prediction fabrics in polar ice sheets provide a record of deformational history and control the viscosity of ice during further deformation they affect geophysical sensing of ice sheets and provide an accessible analogue to fabric development during deformation of other geological and engineering materials a new synthesis of experimental and theoretical results shows that c axis fabrics are quantitatively related to cumulative strain and stress state in ice sheets for the full range of likely flow patterns basal shear divergent flow and parallel flow cause c axes to rotate toward the vertical axis whereas convergent flow causes c axes to rotate toward a vertical plane transverse to flow

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

on the nature of the dirty ice at the bottom of the gisp2 ice core abstract we present data on the triple ar isotope composition in trapped gas from clean stratigraphically disturbed ice between 2800 and 3040 m depth in the gisp2 ice core and from basal dirty ice from 3040 to 3053 m depth we also present data for the abundance and isotopic composition of o2 and n2 and abundance of ar in the basal dirty ice the ar n2 ratio of dirty basal ice the heavy isotope enrichment reflecting gravitational fractionation and the total gas content all indicate that the gases in basal dirty ice originate from the assimilation of clean ice of the overlying glacier which comprises most of the ice in the dirty bottom layer o2 is partly to completely depleted in basal ice reflecting active metabolism the gravitationally corrected ratio of 40ar 38ar which decreases with age in the global atmosphere is compatible with an age of 100 250 ka for clean disturbed ice in basal ice 40ar is present in excess due to injection of radiogenic 40ar produced in the underlying continental crust the weak depth gradient of 40ar in the dirty basal ice and the distribution of dirt indicate mixing within the basal ice while various published lines of evidence indicate mixing within the overlying clean disturbed ice excess ch4 which reaches thousands of ppm in basal dirty ice at grip is virtually absent in overlying clean disturbed ice demonstrating that mixing of dirty basal ice into the overlying clean ice if it occurs at all is very slow order of magnitude estimates indicate that the mixing rate of clean ice into dirty ice is sufficient to maintain a steady thickness of dirty ice against thinning from the mean ice flow the dirty ice appears to consist of two or more basal components in addition to clean glacial ice a small amount of soil or permafrost plus preglacial snow lake or ground ice could explain the observations

ice flow between the greenland ice core project and greenland ice sheet project 2 boreholes in central greenland the ice flow between the greenland ice core project grip and greenland ice sheet project 2 gisp2 boreholes in the summit region of central greenland is modeled with a steady state finite element model the model calculates the free ice sheet surface and the coupled ice flow and temperature fields along the flow line between the boreholes three dimensional effects are expressed in terms of the divergence of the flow and included in the coupled stress and temperature fields in addition to ice core data several geophysical surface programs have provided data that are used to constrain the model the modeled isochrones resemble the bedrock structure along the flow line but rise at the divide the rise at the divide is not seen in internal layers found by surface radio echo sounding an ice particle originally deposited at the surface moves through different stress and temperature regimes a set of trajectory lines are followed from the surface to the boreholes in order to follow the variation of stress deviators temperature and deformation along with the ice movement these variations along a trajectory line constitute a history of the corresponding ice layer the difference between the history of ice layers found in the grip and gisp2 boreholes is mainly due to the shear stress which is 3 4 times higher for gisp2 ice than for grip ice furthermore the grip ice experiences a nonzero longitudinal stress deviator much longer than the gisp2 ice

on the special rheological properties of ancient microparticle laden northern hemisphere ice as derived from bore hole and core measurements on observe des differences dans le fluage des glaces a forte concentration en microparticules wisconsin recent qui sont plus douces que celles de la glace contemporaine ou holocene on analyse les parametres suivants concentration ou microparticules 18 o taille des cristaux axe c ca na conductivite et densite dans des carottes prelevees a agassiz iles ellesmere sur l ile devon et au groenland

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

a renewed argument for crystal size control of ice sheet strain rates at present it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content here we challenge this view because it is not consistent with recent results from analyses of deformation at meserve glacier and instead ascribe residual enhancement in the ice sheets to variations in crystal size we resurrect the idea that variations of crystal size can be an important part of the total shear enhancement in the ice sheets though agree with paterson that this effect is generally dominated by variations of crystal fabric we propose that the enhanced shear strain rate of ice age ice in southern greenland as inferred from tilt of the dye 3 borehole can be explained as a result of combined fabric variations and crystal size variations with these two ice properties accounting for roughly 70 and 30 of the average enhancement respectively permitting a grain size dependence of ice viscosity also resolves the quandary concerning closure and tilt of the agassiz ice cap borehole

magnetic fabrics in the basal ice of a surge type glacier 1 anisotropy of magnetic susceptibility ams has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments here we present results from debris rich basal glacier ice to examine deformation associated with glacier motion basal ice samples were collected from tunabreen a polythermal surge type glacier in svalbard the magnetic fabrics recorded show strong correlation with structures within the ice such as sheath folds and macroscopic stretching lineations thermomagnetic low temperature susceptibility varying field susceptibility and isothermal remanent magnetism acquisition experiments reveal that the debris rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier an up glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of noncoaxial strain during surge propagation the results suggest that ams can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed

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

experimental work on the effect of pre existing anisotropy on fabric development in glaciers abstract localization of deformation in ice is known to be important at all scales in deforming glaciers however relatively little is known of the significance of shear localization and the influence of fabric development in anisotropic ice at microscopic scales c axis preferred orientation and the inclination of the primary layering in anisotropic ice masses during both plane strain compression and combined simple shear compression have been examined a series of creep tests in the temperature range of 5 to 1 c over a range of shortening strains varying from 10 to 40 and compressive stresses ranging from 0 to 0 7 mpa have been undertaken significant variations in the strain rate and microstructural development have been observed in the plane strain compression experiments that reflects the varying orientations of the anisotropy and its relationship to easy glide directions in the ice mass in the unconfined combined compression and shear experiments minimum shear stress rates vary between the variously oriented anisotropic ice masses and deviate from the normal power flow law for isotropic ice where annealing occurs such ice masses preserve the pre existing c axis fabric and hence may reflect a contribution from both the recrystallized and inherited ice components

meteoric and marine ice crystal orientation fabrics from the amery ice shelf east antarctica the northwestern sector of the amery ice shelf east antarctica has a layered structure due to the presence of both meteoric ice and a marine ice layer resulting from sub shelf freezing processes crystal orientation fabric and grain size data are presented for ice cores obtained from two boreholes 70 km apart on approximately the same flowline multiple maxima crystal orientation fabrics and large mean grain sizes in the meteoric ice are indicative of stress relaxation and subsequent grain growth in ice that has flowed into the amery ice shelf strongly anisotropic single maximum crystal orientation fabrics and rectangular textures near the base of the 200 m thick marine ice layer suggest accretion occurs by the accumulation of frazil ice platelets crystal orientation fabrics in older marine ice exhibit vertical large circle girdle patterns influenced by the complex stress configurations that exist towards the margins of the ice shelf post accumulation grain growth and fabric development in the marine ice layer are restricted by a high concentration of brine and insoluble particulate inclusions differences in the meteoric and marine ice crystallography are indicative of the contrasting rheological properties of these layers which must be considered in relation to large scale ice shelf dynamics

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

grain growth in polar ice ii application grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials microparticles bubbles and dissolved impurities are characterized fully dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice the high concentration of soluble impurities in wisconsinan ice from the dome c antarctica ice core and perhaps other ice cores probably causes the small grain sizes observed in that ice microparticles have little effect on grain growth in ordinary ice in ice layers that appear dirty owing to concentrations of volcanic tephra such as in the byrd station antarctica ice core or of morainal material micro particles reduce grain growth rates significantly the relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks so grain growth in firn is limited by boundary migration rather than by neck growth bubbles formed by pore close off at the firn ice transition are less mobile than grain boundaries causing bubble boundary separation whenever geometric constraints are satisfied however such separation reduces grain growth rates by only about 10 the observed linear increase of grain area with time is thus predicted by theory but the growth rate depends on soluble impurity concentrations as well as on temperature

stable isotopes in the basal silty ice preserved in the greenland ice sheet at summit environmental implications modelling ice sheet behaviour in the context of climatic changes depends on initial and boundary conditions which can be better defined by studying the composition of basal ice this study deals with basal ice reached by deep drilling at summit in central greenland grip core the isotopic composition of this ice indicates that ice formed at the ground surface in the absence of the ice sheet largely contributed to its formation the basal silty ice is a remnant of a growing stage of the ice sheet possibly the original build up

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

debris entrainment at the ice bedrock interface in sub freezing temperature conditions terre adelie antarctica the debris rich ice from the bottom 6 m of the 82 m deep caroline coastal antarctic record of last interglacial natural environment ice core reaching bedrock and from five 2 m long surface cores at moraine prudhomme in terre adelie antarctica is described and compared to debris laden ice from the core drilling site d10 isotopic total gas content co2 concentration and sem investigations of embedded particles together with ice textures and fabrics rule our pressure melting regelation around bed obstacles or freezing on as possible mechanisms for the debris entrainment at the ice bedrock interface it is suggested that the debris entrapment by purely mechanical means e g shearing is an efficient process in forming basal ice layers bil at sub freezing temperatures from authors

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

gas isotopes in ice reveal a vegetated central greenland during ice sheet invasion 1 environmental conditions prevailing during build up of the greenland ice sheet gis are not yet established here we use results from gas isotopes from basal ice of the grip ice core to show that central greenland was vegetated during invasion by the ice sheet the 13cch4 and 18o of o2 values obtained were never encountered before in ice core studies such values are indeed the signature of a significant biological activity

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

permian clear fork group north robertson unit integrated reservoir management and characterization for infill drilling part ii petrophysical and engineering data located in the permian basin of west texas north robertson unit nru produces from highly heterogeneous carbonate reservoirs of the permian leonardian clear fork group and lowermost glorieta formation discovered in 1955 the productive area encompasses 5366 ac 2173 ha along the northern border of the central basin platform and contains an estimated 262 mmbbl original oil in place ooip total recovery from the unit including 30 yr of primary production and 10 yr of waterflood production is only 10 1 ooip a recent integrated reservoir characterization effort in nru suggests that previous recovery efforts were inadequate to achieve maximum production from the reservoir analysis of petrophysical properties in the clear fork has allowed for pore level modeling leading to identifying rock types and delineating flow units maps constructed of various reservoir parameters including ooip primary and secondary ultimate recovery and flow capacity were compared with rock type distribution within individual flow units and the results of reservoir simulation to estimate portions of the unit suitable for targeted 10 ac 4 ha infill development related data also suggested the importance of completion techniques in helping improve conformity between producers and injection intervals in pay intervals in nearby wells results of 14 producing wells and 4 injector wells drilled on the basis of the new reservoir model and improved completion techniques were impressive and confirmed the value of using such an approach to optimize the economics and production of infill development in such a complex reservoir

significant marine ice accumulation in the ablation zone beneath an antarctic ice shelf high resolution crystallographic salinity and isotopic analyses of a 45 m ice core reveal the presence of a thick layer of marine ice near the grounding line of the nansen ice shelf antarctica the anomalous formation of marine ice in a zone assumed to be the site of active basal melting leads us to propose the hypothesis of large basal crevasses as a favorable environment for important marine ice accretion this hitherto unexplored possibility is supported by the overall field configuration and by the discrepancy in some ice properties between this core and the marine ice sections of previous drilling projects these findings could have important implications for the general stability of ice shelves and their disintegration processes the specific properties of this core reveal that marine ice is post genetically deformed