flow of anisotropic ice from the epica core a new test apparatus our objective is to measure the mechanical properties of anisotropic ice of the epica ice core from dome concordia antarctica through the full depth of the borehole in deformation experiments under simulated ice sheet conditions of temperature and pressure we propose to undertake experiments on the epica core using both a conventional triaxial test apparatus and a new true triaxial test apparatus incorporating automated tomographic imaging of the ice fabric evolution during deformation we present the design of this new apparatus together with our testing methodology for epica ice the new apparatus is capable of deforming an ice specimen up to 200 mm x 100 mm x 40 mm under servo controlled biaxial loading with a superimposed confining pressure of 50 mpa and at temperatures down to 40 degreesc highly unusual problems arising from the true triaxial nature of the apparatus and tomographic imaging are discussed

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

fabric and texture at siple dome antarctica preferred c axis orientations are present in the firn at siple dome west antarctica and recrystallization begins as shallow as 200 m depth in ice below 20 c based on digital analysis of c axis fabrics grain sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer long siple dome ice core the shallowest section analyzed from 22 m shows clustering of c axes toward the vertical by 200 m depth girdle fabric and other features of recrystallized ice are evident in layers or regions separated by layers regions of typically finer grained ice lacking evidence of recrystallization ice from about 700 780m depth which was deposited during the last ice age is especially fine grained with strongly vertical c axes but deeper ice shows much larger crystals and strong evidence of recrystallization azimuthal asymmetry of some c axis fabrics trends in grain size and other indicators reveal additional information on processes and history of ice flow at siple dome

summit of the east antarctic ice sheet underlain by thick ice crystal fabric layers linked to glacial interglacial environmental change ice cores in antarctica and greenland reveal ice crystal fabrics that can be softer under simple shear compared with isotropic ice owing to the sparseness of ice cores in regions away from the ice divide we currently lack information about the spatial distribution of ice fabrics and its association with ice flow radio wave reflections are influenced by ice crystal alignments allowing them to be tracked provided reflections are recorded simultaneously in orthogonal orientations polarimetric measurements here we image spatial variations in the thickness and extent of ice fabric across dome a in east antarctica by interpreting polarimetric radar data we identify four prominent fabric units each several hundred metres thick extending over hundreds of square kilometres by tracing internal ice sheet layering to the vostok ice core we are able to determine the approximate depth age profile at dome a the fabric units correlate with glacial interglacial cycles most noticeably revealing crystal alignment contrasts between the eemian and the glacial episodes before and after the anisotropy within these fabric layers has a spatial pattern determined by ice flow over subglacial topography

dynamic implications of discontinuous recrystallization in cold basal ice taylor glacier antarctica crystallographic investigations have been conducted of cold 17 c debris bearing ice from the base of an antarctic outlet glacier taylor glacier the 4 m thick sequence studied has been retrieved from a 20 m long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer aifa the top and bottom of the sequence consists of clean meteoric ice englacial facies whereas alternating debris rich and clean bubbly ice layers are found in the middle part stratified facies ice from the englacial facies displays a polygonal texture and a strong c axis clustering toward the vertical denoting recrystallization through subgrain rotation sgr in contrast clean ice from the stratified facies shows sgr fabrics which are delimited at the contact with debris rich layers by large interlocking grains organized in ribbons these two distinct textures within the stratified facies are associated with looser c axis patterns at the scale of single thin sections which is interpreted as resulting from migration recrystallization mr the change from sgr to mr trends marks a clear increase in grain boundary and nucleation kinetics hence the term discontinuous recrystallization and may be associated with strain localization at rheological interfaces during basal ice genesis analogies with bottom ice from deep polar ice sheets where temperature is commonly higher than at the studied site are highlighted two recrystallization scenarios are proposed accounting for the development of both types of fabrics it is shown that by controlling the repartition of stress and strain energy within basal ice the rheology of debris bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces we also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets provided that strain accumulation has been high enough in the former this challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm stagnant annealed ice copyright 2008 by the american geophysical union

the crossover stress anisotropy and the ice flow law at siple dome west antarctica we used observations and modeling of siple dome west antarctica a ridge ice divide to infer the importance of linear deformation mechanisms in ice sheet flow we determined the crossover stress a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms by combining measurements of ice properties with in situ deformation rate measurements and a finite element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal orientation fabric we found that a crossover stress of 0 18 bar produces the best match between predicted and observed deformation rates for siple dome this means that including a linear term in the flow law is necessary but generally the flow is still dominated by the nonlinear glen n 3 term the pattern of flow near the divide at siple dome is also strongly affected by crystal fabric measurements of sonic velocity which is a proxy for vertically oriented crystal fabric suggest that a bed parallel shear band exists several hundred meters above the bed within the ice age ice

crystal orientation fabrics within the antarctic ice sheet revealed by a multipolarization plane and dual frequency radar survey 1 to investigate the viscosity structure of ice sheets induced by crystal orientation fabric cof we carried out a multipolarization plane and dual frequency radar survey in east antarctica radar surveys were done along a 670 km long flow line from dome fuji toward the coast and two transverse lines of 300 km and 20 km length respectively the radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60 of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier when the polarization was perpendicular to the ice flow echoes were about 10 db stronger than when the polarization was parallel to the ice flow this feature was not clear in the upper part of this convergent flow sector farther inland where ice flow is divergent or parallel the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations dual frequency data showed that the cause of the reflections was changes in cof multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches respectively with the aid of ice core based studies on cof we show that ice is composed of stacked layers of single pole and vertical girdle fabrics in the lower reaches in contrast we argue that changes of single pole clustering cause isotropic reflectivities in the upper reaches we also discuss on the development of cof along ice flow and its implication to ice sheet dynamics

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

potential for stratigraphic folding near ice sheet centers lack of agreement between the deep portions of the greenland icecore project grip and greenland ice sheet project ii gisp2 ice cores from central greenland suggests that folds may disrupt annual layering even near ice divides we use a simple kinematic flow model to delineate regions where slope disturbances wrinkles introduced into the layering could overturn into recumbent folds and where they would flatten leaving the stratigraphic record intact wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties rather than from residual surface structures sastrugi or from open folds associated with transient flow over bed topography if wrinkles are preferentially created in anisotropic ice under divides where the resolved shear stress in the easy glide direction can be weak and variable then the deep intact climate record at dye 3 may result from its greater distance from the divide alternatively the larger simple shear at dye 3 may rapidly overturn wrinkles so that they are not recognizable as folds the ice core record from siple dome may be intact over a greater fraction of its depth compared to the central greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide

a flow line model for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet a flow line model is presented for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet with given base elevation profile ice thickness at the dome divide flow law parameters mass balance distribution and convergence divergence conditions along the flow line the model which is based on a quasi similarity hypothesis as regards the horizontal velocity depth profiles accounts for changes along the flow line in the depth distributions of temperature normal stress deviators and possible enhanced flow of deep ice of wisconsin origin a curvilinear coordinate system is applied with horizontal axes along flow lines and surface elevation contours respectively the flow equations are reduced to two differential equations one for the surface elevation profile and the other for a profile function that determines the depth distributions of velocities and strain rates the two equations are coupled through a profile parameter that communicates the influence of velocity profile changes to the surface profile equation it is shown that the variation along the flow line of this parameter should also be considered when deriving flow law parameters from ice sheet flow line data for a symmetric dome explicit expressions are derived for the depth distributions of the vertical velocity strain rates and stresses the strain rate profiles display an inflection about half way down the ice sheet and in the case of isothermal ice have surface values 2 2 times their depth averaged values the depth distribution of the vertical velocity indicates that a relatively thick layer of almost stagnant ice is present at the ice sheet base below a dome

a flow law for anisotropic ice and its application to ice sheets a new formulation of the flow law of polycrystalline ice was derived for an rbitrary c axis orientation fabric pattern the rate factor of the flow law is given by a geometrical factor that can be calculated with c axis fabric data and stress condition a theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field according to this flow model of polycrystalline ice if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain whereas simple shear makes the ice body softer with strain this may significantly affect ice sheet dynamics

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

ice flow induced scattering zone within the antarctic ice sheet revealed by high frequency airborne radar to better understand how internal radar echoes depend on ice flow conditions and radar polarization we surveyed two basins in east antarctica using 179 mhz airborne radar we compared radar echoes from three ice flow conditions parallel sheet flow in the main stream of a basin convergent flow towards an ice stream and longitudinal compression by nunataks we detected a distinct zone of high radar scattering several hundred meters thick at middle depths in the latter two regions this high scattering zone was detected only when the radar polarization plane was parallel to the compression axis in ice such a high scattering zone was not found in the parallel flow region regardless of the polarization using a recently developed theory of radar scattering in ice we interpret the high scattering zone as being caused by crystal orientation fabric alternations among adjacent ice layers due to difference in horizontal strain components we argue that the spatial variation of the high scattering zone is crucial for understanding past and present flow features

application of a continuum mechanical model for the flow of anisotropic polar ice to the edml core antarctica we present an application of the newly developed caffe model continuum mechanical anisotropic flow model based on an anisotropic flow enhancement factor to the epica ice core at kohnen station dronning maud land antarctica referred to as the edml core a one dimensional flow model for the site is devised which includes the anisotropic flow law and the fabric evolution equation of the caffe model three different solution methods are employed 1 computing the ice flow based on the flow law of the caffe model and the measured fabrics 2 solving the caffe fabric evolution equation under the simplifying assumption of transverse isotropy and 3 solving the unrestricted caffe fabric evolution equation method 1 demonstrates clearly the importance of the anisotropic fabric in the ice column for the flow velocity the anisotropic enhancement factor produced with method 2 agrees reasonably well with that of method 1 even though the measured fabric shows a girdle structure which breaks the transverse isotropy in large parts of the ice core for method 3 we find that the measured fabric is reproduced well by the model down to 2100 m depth systematic deviations at greater depths are attributed to the disregard of migration recrystallization in the model

structures and fabrics in glacial ice a review abstract glaciers ice sheets and ice caps represent tectonic systems driven by gravity their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements all glacial ice has primary stratification exhibited by variations in grain size bubble content and debris content as it deforms with deformation dominated by plastic flow and recrystallization accompanied locally by fracture under tension a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten foliation develops from inherited mostly stratification or introduced mostly ice veins or fracture traces fabric elements and from dynamic recrystallization it is largely dependent on the accumulated strain which is highest at the base and near the margins of glaciers ice sheets and ice streams folds develop largely passively due to initial amplification of irregularities in the primary stratification to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures buckle folds and boudinage mostly on a small scale occur where viscosity contrast is large mostly in basal ice thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures

deformation properties of subfreezing glacier ice role of crystal size chemical impurities and rock particles inferred from in situ measurements to improve understanding of the deformation properties of subfreezing polycrystalline glacier ice and in particular the role of crystal size chemical impurities and rock particle impurities we analyze in situ strain rates of the basal layers of meserve glacier antarctica strain rates were monitored on the walls of a subglacial tunnel where down flow shear stress is relatively uniform and ice properties were measured texture fabric and impurity content we propose a simple empirical model describing strain rate variations due to variations in crystal size and impurity content and we use all relevant meserve data to constrain model parameters we conclude that there is a direct dependence of strain rate on crystal size which reflects an important role for a grain size sensitive deformation mechanism such as grain boundary sliding or diffusion chemical impurities are found to enhance the grain size sensitive deformation and are found to be an important control on strain rate variations in the very impure ices of meserve glacier however the per molar sensitivity of strain rate to chemical impurity content is shown to be very low such that in the ice age ices of the greenland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement though we cannot exclude chemical enhancements as high as 1 3 there our analyses detect no direct rheologic effect of rock particles in the meserve ices which suggests that rock content is not directly responsible for the low viscosity of dirty basal layers

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

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

the stress distribution related to the boudinage of a visco elastic material examples from a polar outlet glacier abstract quantitative stress measurements related to the development of mesoscale structures in rock are difficult if not impossible a method for determining the stress distribution and history during the development of mesoscale boudinage structures in ice is introduced here boudinage structures in fracture trace ice have been observed in the outlets glaciers of the framnes mountains east antarctica fracture traces are preserved when creasses fill with surface melt water which freezes to form coarse grained columnar ice a 4 0 km flow parallel traverse across the central ice stream of the framnes mountains is presented to illustrate the boudinage of fracture traces with a mean width of 0 30 m field based measurement of the geometric evolution of boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the boudinage structures formed the strain rate measurements provide boundary constraints on several two dimensional finite difference models that have been used to analyse the stress distribution related to the formation of boudinage structures in a visco elastic solid the results reveal the development of pressure shadows during the boundinage of layered rocks and demonstrate the degree of refraction of stress across rheological boundaries which have important ramifications for the analysis of planar and linear fabrics in rocks

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

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

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

ice crystal properties of amber ice and strain enhancement at the base of cold antarctic glaciers to improve our understanding of the deformation properties of cold based polar glaciers we examine here some of the factors leading to the localization of strain within the amber ice facies we present a crystallographic case study of amber ice a fine grained bubbly ice containing a relatively high impurity content sampled at the base of two antarctic glaciers the crystal fabrics and textures of amber ice were computed by application of a recently developed automated method to date it was tedious and awkward to determine amber ice facies accurately because of the sub millimetric crystal size and relatively high debris content of this facies the authomatic analytical method applied in this study allows not only for improving analytical accuracy in this task but also for considerably reducing the time of analysis our investigations reveal highly homogeneous crystallographic properties for the studied amber ice the ice crystals are mainly polygonal equant and sub millimetric and show a strong lattice preferred orientation these properties beside the relatively high impurity content are likely to exert a major control on strain enhancement in amber ice when this facies is present at the base of cold glaciers

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

a novel tilt sensor for studying ice deformation application to streaming ice on jarvis glacier alaska we developed a tilt sensor for studying ice deformation and installed our tilt sensor systems in two boreholes drilled close to the shear margin of jarvis glacier alaska to obtain kinematic measurements of streaming ice we used the collected tilt data to calculate borehole deformation by tracking the orientation of the sensors over time the sensors tilts generally trended down glacier with an element of cross glacier flow in the borehole closer to the shear margin we also evaluated our results against flow dynamic parameters derived from glen s exponential flow law and explored the parameter space of the stress exponent n and enhancement factor e comparison with values from ice deformation experiments shows that the ice on jarvis is characterized by higher n values than that is expected in regions of low stress particularly at the shear margin 3 4 the higher n values could be attributed to the observed high total strains coupled with potential dynamic recrystallization causing anisotropic development and consequently sped up ice flow jarvis n values place the creep regime of the ice between basal slip and dislocation creep tuning e towards a theoretical upper limit of 10 for anisotropic ice with single maximum fabric reduces the n values by 0 2

instruments and methods crystal orientation measurements using transmission and backscattering a method has been devised and tested for measuring the c axis orientation of crystal grains in thin sections of glacier ice the crystal orientation and grain size of ice are of great interest to glaciologists since these parameters contain information on the prior thermal and flow history of the ice the traditional method of determining c axis orientation involves a transmission measurement through an ice sample a process that is time consuming and therefore impractical for obtaining a continuous record a reflection or backscatter based method could potentially be used inside boreholes with bubbles as reflectors to avoid such drawbacks the concept demonstration of this paper is performed on ice slices enabling a direct comparison of accuracy with traditional methods measurements of the crystal orientations in 11 grains showed an average error of 0 8 in with no grain error 1 4 measurements ofshowed an average error of 8 2 on ten grains with unexplained disagreement on the remaining grain although the technique is applied specifically to glacier ice it should be generally applicable to any transparent birefringent polycrystalline material

the role of crystal fabric in flow near an ice divide polycrystalline ice near an ice divide typically shows a crystal fabric crystal preferred orientation with c axes clustered vertically we explore the effect of this fabric on the large scale flow pattern near an ice divide we incorporate an analytical formulation for anisotropy into a non linear flow law within a finite element ice sheet flow model with four different depth profiles of crystal fabric we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than 25 for a steady state divide the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non linearity of ice flow a vertically oriented fabric tends to increase the size of the isochrone arch never to reduce it also non random fabric has little effect on the ice divide flow pattern when ice is modeled as a linear newtonian fluid finally when we use a crystal fabric profile that closely approximates the measured profile for siple dome west antarctica the model predicts concentrated bed parallel shearing 300 m above the bed

the evolution of crystal fabric in ice sheets and its link to climate history the evolution of preferred crystal orientation fabrics is strongly sensitive to the initial fabric and texture a perturbation in climate can induce variations in fabric and texture in the firn feedbacks between fabric evolution and ice deformation can enhance these variations through time and depth in an ice sheet we model the evolution of fabric under vertical uniaxial compression and pure shear regimes typical of ice divides using an analytic anisotropic flow law applied to an aggregate of distinct ice crystals the model evolves the fabric and includes parameterizations of crystal growth polygonization and migration recrystallization stress and temperature history drive the fabric evolution using this model we explore the evolution of a subtle variation in near surface fabric using both constant applied stress and a stress temperature history based on data from taylor dome east antarctica our model suggests that a subtle variation in fabric caused by climate perturbations will be preserved through time and depth in an ice sheet the stress history and polygonization rate primarily control the magnitude of the preserved climate signal these results offer the possibility of extracting information about past climate directly from ice fabrics

isotropic and anisotropic flow relations for ice dynamics a flow relation for the minimum strain rates for isotropic ice is presented this flow relation has been compiled from laboratory tests in shear and compression and covers the temperature and stress ranges typical of natural ice masses however the ice in natural ice masses is mostly anistropic three cores drilled at law dome antarctica are used as examples to illustrate some of the different anisotropies that develop in ice masses in relation to the stress configurations and accumulated strains for the shear or compression stress configurations anistropic flow rates can be derived as enhancements of the minimum isotropic flow rates for the particular stress temperature and stress configuration for compression this enhancement asymptotically approaches 3 for shear the enhancement may be as high as 8 for combinations of shear and compression further tests are required

the microstructure of polar ice part i highlights from ice core research polar ice sheets play a fundamental role in earth s climate system by interacting actively and passively with the environment active interactions include the creeping flow of ice and its effects on polar geomorphology global sea level ocean and atmospheric circulation and so on passive interactions are mainly established by the formation of climate records within the ice in form of air bubbles dust particles salt microinclusions and other derivatives of airborne impurities buried by recurrent snowfalls for a half century scientists have been drilling deep ice cores in antarctica and greenland for studying such records which can go back to around a million years experience shows however that the ice sheet flow generally disrupts the stratigraphy of the bottom part of deep ice cores destroying the integrity of the oldest records for all these reasons glaciologists have been studying the microstructure of polar ice cores for decades in order to understand the genesis and fate of ice core climate records as well as to learn more about the physical properties of polar ice aiming at better climate record interpretations and ever more precise models of ice sheet dynamics in this part i we review the main difficulties and advances in deep ice core drilling in antarctica and greenland together with the major contributions of deep ice coring to the research on natural ice microstructures in particular we discuss in detail the microstructural findings from camp century byrd dye 3 grip gisp2 northgrip vostok dome c edml and dome fuji besides commenting also on the earlier results of some pioneering ventures like the jungfraujoch expedition and the norwegian british swedish antarctic expedition among others in the companion part ii of this work faria et al 2014 the review proceeds with a survey of the state of the art understanding of natural ice microstructures and some exciting prospects in this field of research

two orthotropic models for strain induced anisotropy of polar ice as polar ice descends from the free surface to depth in a large ice sheet it undergoes deformations which give rise to the formation and subsequent evolution of a fabric and associated anisotropy in this paper two orthotropic models of such strain induced anisotropy are considered model a is based on analysis ofthe microscopic behaviour of an individual ice crystal with transversely isotropic response and assumed uniform stress in a polycrystal the macroscopic response of the ice aggregate is then derived by applying the concept of an orientation distribution function and the resulting viscous law relates the strain rate to the stress and three structure tensors in model b it is assumed that the macroscopic response of ice is determined by the fabric induced entirely by macroscopic deformations and all microprocesses taking place at the grain level are ignored a constitutive relation is derived from a general frame indifferent law for orthotropic materials and expresses the stress in terms of the strain rate strain and three structure tensors the two models are applied to determine the viscous response of ice to continued uniaxial compression and simple shearing in order to compare the predictions of both theories

palaeointensity results from the permo carboniferous kiaman reversed superchron the great whin and midland valley sills of the northern united kingdom summary fifty thellier palaeointensity determinations were made from the great whin sill gws of northern england and the midland valley sill mvs of central scotland which form the major part of a late carboniferous quartz dolerite complex palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the permo carboniferous reversed superchron palaeomagnetic results suggest that the two sills may not be exactly contemporaneous the mvs was intruded rapidly some time during the emplacement of the gws which was itself emplaced over a considerable time period the mvs may well represent the northernmost expression of the instrusive activity a mean palaeointensity of 22 9 2 6 t was calculated for the great whin sill with a corresponding vdm value of 5 9 1022 a m2 the palaeointensity value calculated from the midland valley sill was much lower 13 0 0 5 t with a corresponding vdm value of 3 3 1022 a m2 detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state multidomain grains are less important in the midland valley sill and thus greater confidence is attached to palaeointensity results from this sill vdm values from this and other studies of the permo carboniferous reversed superchron p crs indicate that the dipole strength showed similar variations to that witnessed for the past 5 ma there is no conclusive support in favour of either the weak or strong field models that have been suggested for superchrons more palaeointensity results are needed from the termination of the p crs and the late permian early triassic in order to understand how the dipole field evolved during the p crs and its relationship to the so called mesozoic dipole low

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

a user friendly anisotropic flow law for ice sheet modelling for accurate ice sheet flow modelling the anisotropic behaviour of ice must be taken fully into account however physically based micro macro m m models for the behaviour of an anisotropic ice polycrystal are too complex to be implemented easily in large scale ice sheet flow models an easy and efficient method to remedy this is presented polar ice is assumed to behave as a linearly viscous orthotropic material whose general flow law golf depends on six parameters and its orthotropic fabric is described by an orientation distribution function odf depending on two parameters a method to pass from the odf to a discrete description of the fabric and vice versa is presented considering any available m m model the parameters of the golf that fit the response obtained by running this m m model are calculated for any set of odf parameters it is thus possible to tabulate the golf over a grid in the space of the odf parameters this step is performed once and for all ice sheet flow models need the general form of the golf to be implemented in the available code once then during each individual run to retrieve the golf parameters from the table by interpolation as an example author i have changed from application ok the golf is tabulated using three different m m models and used to derive the rheological properties of ice along the grip ice core

a numerical study of anisotropic low reynolds number free surface flow for ice sheet modeling few ice sheet flow models have been developed that solve the complete set of mechanical equations until now these models were limited to isotropic conditions we present here a two dimensional finite difference method capable of solving the equations for the steady flow of a viscous incompressible anisotropic fluid with a free surface under isothermal conditions it is not a standard method especially with respect to the time discretization of the numerical scheme and converges for very low reynolds numbers this method is applied here to the planar flow of anisotropic ice over flat or irregular bedrock with no slip boundary conditions at the ice bedrock interface the results are presented here for newtonian behavior in the vicinity of an ice divide the ice is assumed to be isotropic at the ice sheet surface with continuous and prescribed development of anisotropy with increasing depth going from isotropic to anisotropic situations our results indicate that the free surface becomes flatter and the shear strain rates larger and more concentrated near the bedrock the flow is less sensitive to variations of the bedrock topography in the anisotropic case than in the isotropic case furthermore a new phenomenon appears in the anisotropic case the partial stagnation of ice in the holes of the bedrock these effects have significant consequences when dating the ice the isochrones obtained in the anisotropic case are flatter and the anisotropic ice is more than 10 younger above the bumps and more than 100 older within the holes than for the isotropic ice

full field predictions of ice dynamic recrystallisation under simple shear conditions understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large scale ice dynamics and thus our ability to predict future changes of ice sheets polar ice behaves anisotropically during flow which can lead to strain localisation in order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets we present a series of numerical simulations of ice polycrystals deformed under simple shear conditions the models explicitly simulate the evolution of microstructures using a full field approach based on the coupling of a viscoplastic deformation code vpfft with dynamic recrystallisation codes the simulations provide new insights into the distribution of stress strain rate and lattice orientation fields with progressive strain up to a shear strain of three our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure grain size and shape while the development of lattice preferred orientations lpo appears to be less affected activation of non basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration therefore the finite strain history is non directly reflected by the final microstructure masked strain localisation can be recognised in ice cores such as the edml from the presence of stepped boundaries microshear and grains with zig zag geometries

response timescales for martian ice masses and implications for ice flow on mars abstract on earth and on mars ice masses experience changes in precipitation temperature and radiation in a new climate state flowing ice masses will adjust in length and in thickness and this response toward a new steady state has a characteristic timescale however a flowing ice mass has a predictable shape which is a function of ice temperature ice rheology and surface mass exchange rate in addition the time for surface shape adjustment is shorter than the characteristic time for significant deformation or displacement of internal layers within a flowing ice mass as a result surface topography is more diagnostic of flow than are internal layer shapes because the shape of gemina lingula north polar layered deposits indicates that it flowed at some time in the past we use its current topography to infer characteristics of those past ice conditions or past climate conditions in which ice flow rates were more significant than today a plausible range of near basal ice temperatures and ice flow enhancement factors can generate the characteristic geometry of an ice mass that has been shaped by flow over reasonable volume response timescales all plausible ice flow scenarios require conditions that are different from present day mars if the basal layers are pure ice

an analytical approach to deformation of anisotropic ice crystal aggregates deformation rates of single hexagonal crystals deforming by glide on the basal plane are described as a function of stress state and crystal orientation these results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric this type of fabric is approximated by a uniform distribution of c axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis for simple shear stress acting on a single maximum fabric there is a slight de enhancement for cone angles of 60 90 in uniaxial compression a maximum enhancement of 1 7 occurs at a cone angle of 57 a pure shear stress state has similar features with the additional complication that it causes a non zero transverse strain rate except for perfect vertical alignment of crystals and isotropic fabric in combined states of stress the contribution of each stress component to the strain rate depends on fabric a single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states

ground penetrating radar 2 d and 3 d subsurface imaging of a coastal barrier spit long beach wa usa abstract the ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar gpr the gpr method which is based on the propagation and reflection of pulsed high frequency electromagnetic energy provides high resolution cm to m scale and shallow subsurface 0 60 m near continuous profiles of many coarser grained deposits sediments of low electrical conductivity this paper presents 2 d and 3 d gpr results from an experiment on a regressive modern barrier spit at willapa bay wa usa the medium grained sand spit is 38 km long up to 2 3 5 km wide and is influenced by a 3 7 m tidal range spring as well as high energy longshore transport and high wave energy depositional processes the spit has a freshwater aquifer recharged by rainfall the gpr acquisition system used for the test was a portable digital pulseekko system with antennae frequency ranging from 25 to 200 mhz and transmitter voltages ranging from 400 to 1000 v step sizes and antennae separation varied depending on the test requirements in addition 100 mhz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data 50 50 m sampled every meter the 2 d digital profiles were processed and plotted using pulseekko software the 3 d datasets after initial processing were entered into a landmark workstation that allowed for unique 3 d perspectives of the subsurface to provide depth near surface velocity measurements were calculated from common midpoint cmp surveys results from the present study demonstrate higher resolution from the 200 mhz antennae for the top 5 6 m whereas the 25 and 50 mhz antennae show deeper penetration to 10 m for the study site 100 mhz antennae provided acceptable resolution continuity of reflections and penetration the dip profiles show a shingle like accretionary depositional pattern whereas strike profiles show a horizontal and subhorizontal nearly continuous reflection pattern results from the gpr experiment reveal upper shoreface reflections with dip towards the ocean at about 1 2 the loss of signal from below a depth of 6 8 m indicates a lithofacies change because of the storm wave base the parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy continuity and depth of penetration the cross polarization test exhibits reduced continuity of reflections and less depth of penetration but dipping reflections are apparent the grid pattern data provided a detailed view of 3 d geometry of individual reflections high quality data were obtained processed and directly exported into a landmark workstation for interpretation the resulting interpretations of the upper shoreface beds from the test cube 50 50 m total 2600 traces are shown as vertical sections slices horizontal sections time slices contour maps 3 d representations of individual beds and an isopach map the 3 d depositional framework allows a more detailed interpretation than widely spaced 2 d profiles

ice fabric in an antarctic ice stream interpreted from seismic anisotropy here we present new measurements of an anisotropic ice fabric in a fast moving 377 ma 1 ice stream in west antarctica we use 6000 measurements of shear wave splitting observed in microseismic signals from the bed of rutford ice stream to show that in contrast to large scale ice flow models which assume that ice is isotropic the ice in rutford ice stream is dominated by a previously unobserved type of partial girdle fabric this fabric has a strong directional contrast in mechanical properties shearing 9 1 times more easily along the ice flow direction than across flow this observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers suggesting it is essential to consider anisotropy in data driven models to correctly predict ice loss and future flow in these regions we show how passive microseismic monitoring can be effectively used to provide these data

isothermal flow of an anisotropic ice sheet in the vicinity of an ice divide simulations of glacier flow are commonly based on the assumption that ice has an isotropic viscosity here we examine the plane flow of ice in the special region of an ice divide using a constitutive relation for an anisotropic incompressible viscous body that is orthotropic and transversally isotropic ice is assumed to be isotropic at the ice sheet surface with the continuous development of a vertical single maximum c axis fabric with increasing depth we consider the theoretical case of an isothermal ice sheet over a horizontal bedrock with no slip at the ice bedrock interface the ice sheet surface elevation is imposed and the flow corresponding to the steady state is calculated using a two dimensional finite difference model based on the resolution of a pressure poisson equation in this model all components of the stress and strain rate tensor are calculated the main conclusion is that for a fixed surface elevation the general flow pattern accelerates when the anisotropic behavior of the ice is taken into account due to the greater fluidity with respect to shear stress the downward motion of the ice is faster despite a higher resistance to vertical deformation as a result the dominance of shear strain rate in the flow of polar ice is stronger in the anisotropic case than in the isotropic case the shear stresses are slightly relaxed while the longitudinal stresses are significantly increased in the anisotropic case

an anisotropic flow law for ice sheet ice and its implications abstract a new flow law or anisotropic polycrystalline ice is presented the strain rate tensor is related by a geometrical actor tensor g to the stress tensor the g factor tensor can be obtained from the c axis fabric data and stress condition this new flow law describes well the direction dependent mechanical properties of anisotropic ice which cannot be demonstrated by glen s flow law for example the new flow law can explain the fact that a strong single maximum fabric ice such as dye 3 vvisconsin ice can deform several times aster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress i ve also show that at a deeper part of an ice sheet where a single maximum fabric develops a positi e vertical strain rate can be produced with only a horizontal shear stress 1 introduction ice sheet modelling and studies on ice sheet dynamics carried out so far have been based on glen s flow law

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