transfer of basal sliding variations to the surface of a linearly viscous glacier on etudie pour une plaque a faces paralleles d epaisseur h et de viscosite lineaire l effet en surface des fluctuations de la vitesse de derapage

strain induced phase changes within cold basal ice from taylor glacier antarctica indicated by textural and gas analyses this paper reports detailed textural and gas measurements conducted in cold basal ice 178c from the margin of taylor glacier an outlet glacier of the east antarctic ice sheet the analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout the basal sequence exhibits two contrasting ice facies defined as the englacial and stratified facies on the one hand analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation especially within the stratified facies where significant dynamic recrystallization was detected on the other hand high resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies these gas composition changes are typical of melting refreezing processes but are not associated with any significant loss of gas volume given the specific subglacial thermal conditions at the margin of taylor glacier we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre melt phase it is argued that such processes may play an important role in the post genetic geochemical evolution of cold debris laden ice and may be enhanced through intense strain conditions

basal flow characteristics of a non linear flow sliding frictionless over strongly undulating bedrock the flow field of a medium sliding without friction over a strongly undulating surface is calculated numerically the results are used to elucidate the basal flow characteristics of glacier flow and they are discussed with reference to known analytical solutions extrusion flow is found to become increasingly pronounced as the value of n where n is a parameter in glen s flow law becomes larger for sinusoidal bedrock undulations a flow separation occurs if the amplitude to wavelength ratio exceeds a critical value of about 0 28 the main flow then sets up a secondary flow circulation within the trough and the ice participating in this circular motion theoretically never leaves it the sliding velocity is calculated numerically as a function of the mean basal shear stress the amplitude to wavelength ratio and the flow parameter n for moderate and high slope fluctuations the sliding velocity is significantly different from what would be expected from results based on the small slope approximation

direct observation of basal sliding and deformation of basal drift at sub freezing temperatures in a tunnel at the base of sub polar urumqi glacier no 1 china three new mechanisms of glacier flow at sub freezing temperatures have been observed taken individually or in combination these modes of flow can account for nearly all 60 80 of the overall glacier motion and yet they act only within the lowermost 1 2 of the effective glacier thickness these mechanisms are 1 enhanced deformation of the frozen and ice laden subglacial drift 2 motion across discrete shear planes or shear bands within the frozen drift or at the ice drift interface and 3 basal sliding at an ice rock interface at a temperature of nearly 5 c the ice laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice thus allowing very rapid shear deformation the observed rate or basal sliding at the ice rock interface agrees favorably with that predicted by the recent work of shreve 1984 if proper account is taken of the measured surface roughness and reduced ice viscosity

climate change during the last deglaciation in antarctica greenland ice core records provide clear evidence of rapid changes in climate in a variety of climate indicators in this work rapid climate change events in the northern and southern hemispheres are compared on the basis of an examination of changes in atmospheric circulation developed from two ice cores high resolution glaciochemical series covering the period 10 000 to 16 000 years ago from a central greenland ice core and a new site in east antarctica display similar variability these findings suggest that rapid climate change events occur more frequently in antarctica than previously demonstrated

ice sheet flow conditions deduced from mechanical tests of ice core uniaxial compression tests were performed on samples of the greenland ice gore project grip deep ice core both in the field and later in a cold room laboratory in order to understand the ice flow behavior of large ice sheets experiments were conducted under conditions of constant strain rate type a and constant load type b fifty four uniaxial compression test specimens from 1327 2922 m were selected each test specimen 25 mm x 25 mm x 90 mm was prepared with its uniaxial stress axis inclined 45 from the core axis in order to examine the flow behavior of strong single maximum ice core samples with basal planes parallel to the horizontal plane of the ice sheet the ice flow enhancement factors show a gradual increase with depth down to approximately 2000 m these results can be interpreted in terms of an increase in the fourth order schmid factor below 2000 m depth the flow enhancement factor increases to about 20 30 with a relatively high variability when the schmid factor was 0 46 the enhancement factor obtained was higher than expected from the axis concentrations measured the higher values of flow enhancement factor were obtained from specimens with a cloudy band structure it was revealed that cloudy bands affect ice deformation processes but the details remain unclear

the influence of basal processes on the dynamic behaviour of cold based glaciers abstract there exists within glaciology a widely held assumption that basal sliding and bed deformation do not operate beneath cold based ice and that their basal velocity is therefore zero irrespective of bed conditions consequently their ability to erode entrain and transport sediment and thereby alter the landscape is assumed to be limited consequently very little research has been focused towards describing and understanding the motion of cold based ice masses and our knowledge of their behaviour remains poor and the assumption of zero basal velocity unchallenged in this review paper it is argued that this assumption is not universally applicable and that in certain circumstances basal processes not only remain active at sub freezing temperatures but can significantly influence glacier motion this is particularly the case in glaciers where sub freezing basal thermal conditions coincide with the presence of fine grained ice rich subglacial sediments due to the lack of work undertaken on contemporary cold based glaciers much of the information used to support this argument is derived from field research in permafrost areas and on quaternary glacial sediments and from the laboratory testing of ice sediment mixtures it is concluded that the assumption of zero basal velocity beneath cold based ice is overly simplistic and that in reality the situation is likely to be much more complex work is therefore urgently required firstly to investigate the circumstances in which basal motion remains active at sub freezing temperatures and to determine their influence on glacier motion and secondly to examine the likely extent of subglacial permafrost in both the contemporary and quaternary glacial environment

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

extrusion flow demonstrated by bore hole deformation measurements over a riegel storglaciaren sweden inclinometry measurements in a 126 m bore hole slightly down glacier from the crest of a riegel on storglaciaren showed what appears to be indisputable extrusion flow during the last 3 weeks of july 1984 the velocity at the bed was about five times that at the surface and lay in a direction deviating 50 60 from that at the surface the increase in velocity with depth began more than 50 m above the bed in august and the first half of september the flow returned nearly to normal with only a slight tendency toward extrusion the flow in july appeared to be related to a period of large diurnal water pressure variation and cavity opening at the bed up glacier from the riegel decoupling of the ice from the bed on the lowest parts of the riegel may have permitted the extrusion the return to more normal conditions in august appears to have been related to a decrease in ice flux over the riegel and a widening of the zone of decoupling

paraglacial redistribution of glacial sediments in the vestfold hills east antarctica abstract the vestfold hills landscape is dominated by bare rock slopes and unconsolidated deposits form a minor component of the surficial geology the principal objectives of the study were to explain the distribution of unconsolidated deposits determine the nature and timing of sedimentary processes during and after deglaciation and the impact of these processes on landscape development observations of late quaternary glaciation and deglaciation of the area and contemporary depositional processes are reported and a conceptual model of geomorphological change since deglaciation is presented the model attempts to reconcile the types and rates of contemporary depositional processes with the distribution of deposits and landforms and to explain why unconsolidated deposits cover less than 20 of the hills despite the effects of recent glaciation the lack of unconsolidated deposit in the landscape can be attributed to the combined effects of low debris volumes in the ice and the redistribution of debris during and after deglaciation redistribution of debris can be explained by paraglacial processes which are non glacial sedimentary processes that are directly conditioned by glaciation paraglacial sedimentation is a response to rapid adjustment of the debris system after the dominance of glacial conditions and is characterized by disequilibrium between sediment supply and transportation of sediment by nonglacial processes evidence from the vestfold hills suggests paraglacial sedimentary processes are dominated by mass movement of glacial debris from slopes and the formation of stable valley fills and that the main sediment transfers occur within a few thousand years of deglaciation

regional scale controls on the spatial activity of rockfalls turtmann valley swiss alps a multivariate modeling approach abstract in mountain geosystems rockfalls are among the most effective sediment transfer processes reflected in the regional scale distribution of talus slopes however the understanding of the key controlling factors seems to decrease with increasing spatial scale due to emergent and complex system behavior and not least to recent methodological shortcomings in rockfall modeling research in this study we aim i to develop a new approach to identify major regional scale rockfall controls and ii to quantify the relative importance of these controls using a talus slope inventory in the turtmann valley swiss alps we applied for the first time the decision tree based random forest algorithm rf in combination with a principal component logistic regression pclr to evaluate the spatial distribution of rockfall activity this study presents new insights into the discussion on whether periglacial rockfall events are controlled more by topo climatic cryospheric paraglacial or and rock mechanical properties i both models explain the spatial rockfall pattern very well given the high areas under the receiver operating characteristic roc curves of 0 83 highest accuracy was obtained by the rf correctly predicting 88 of the rockfall source areas the rf appears to have a great potential in geomorphic research involving multicollinear data ii the regional permafrost distribution coupled to the bedrock curvature and valley topography was detected to be the primary rockfall control rockfall source areas cluster within a low radiation elevation belt 2900 3300 m a s l consistent with a permafrost probability of 90 the second most important factor is the time since deglaciation reflected by the high abundance of rockfalls along recently deglaciated the study demonstrates the benefit of combined statistical approaches for predicting rockfall activity in deglaciated permafrost affected mountain valleys and highlights the complex interplay between rock mechanical paraglacial and topo climatic controls at the regional scale

ice dynamics and thermal regime of taylor glacier south victoria land antarctica les donnees glaciologiques recueillies sur le taylor glacier ont servi a reconstituer les principaux traits de la dynamique et du regime thermique du glacier les etudes du bilan de masse montrent que le glacier est proche de l equilibre les conditions thermiques de la glace basale sur une grande partie de la zone d ablation ont ete calculees a partir d estimation du flux de chaleur geothermique et de mesures des temperatures de la glace pres de la surface des vitesses et de l epaisseur de la glace on a trouve que jusqu a 50 de la zone d ablation la glace basale pouvait etre fondante

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

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

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

inference of accumulation rate patterns from deep layers in glaciers and ice sheets the spatial pattern of accumulation rate can be inferred from internal layers in glaciers and ice sheets non dimensional analysis determines where finite strain can be neglected shallow layer approximation or approximated with a local one dimensional flow model local layer approxima tion and where gradients in strain rate along particle paths must be included deep layers we develop a general geophysical inverse procedure to infer the spatial pattern of accumulation rate along a steady state flowband using measured topography of the ice sheet surface bed and a deep layer a variety of thermomechanical ice flow models can be used in the forward problem to calculate surface topography and ice velocity which are used to calculate particle paths and internal layer shapes an objective tolerance criterion prevents over fitting the data after making site specific simplifications in the thermomechanical flow algorithm we find the accumulation rate along a flowband through taylor mouth a flank site on taylor dome antarctica using a layer at approximately 100 m depth or 20 of the ice thickness accumulation rate correlates with ice surface curvature at this site gradients along flow paths critically impact inference of both the accumulation pattern and the depth age relation in a 100 m core

direct shear tests of materials from a cold glacier implications for landform development abstract the mechanical properties of materials at the ice bed interface exert a major control on glacier behaviour and define the nature and rate of subglacial erosion this paper presents new data on the strength and behaviour of basal ice and substrate of the suess glacier south victoria land antarctica which is a small alpine dry based glacier that has a basal temperature of 17 c a tunnel excavated in the glacier revealed a substrate composed of frozen sand and gravel and a basal zone that was 3 8 m thick from bottom to top the basal zone was composed of 1 8 m of stratified complexly deformed layers of ice and debris laden ice that was overlain by a 0 9 m thick layer of frozen sediment and a 0 8 m thick layer of discoloured ice that lies immediately beneath clean glacier ice direct shear tests were performed in the field on 36 samples using a strain rate of 0 85 mm h and on replicate samples in a laboratory at strain rates of 0 08 and 0 01 mm h the experiments performed in the field show that the average peak shear strength of substrate samples was 2 53 mpa which is almost twice as strong as the average value for basal ice 1 28 mpa and the glacier ice samples 1 39 mpa the laboratory experiments show that the behaviour and peak strength of samples sheared at the higher strain rates are considerably greater than the peak and residual strengths measured during the lower strain rate tests the direct shear tests suggest that the glacier substrate is unlikely to deform at the current temperature

a mathematical model of the ocean boundary layer under drifting melting ice abstract a mathematical model of the ocean boundary layer under drifting melting ice is formulated verified and applied the model is based on the conservation equations for heat salt and momentum and uses turbulence models to achieve closure novel features of the model include a low reynolds number turbulence model for the viscous region and a discrete element approach to the parameterization of roughness basic verification studies using laboratory measurements include the budget of turbulent kinetic energy and the mean temperature profile all verification studies are focused on the viscous region good agreement with measurements is generally obtained the model is finally compared to field data obtained during mizex predicted melt rates are in good agreement with measurements an analysis of the fluxes of salt and heat within the boundary layer is also provided

basal flow characteristics of a linear medium sliding frictionless over small bedrock undulations the basal deformation of a gravity driven linear creeping flow sliding frictionless over slowly varying bed undulations in two dimensions is analysed analytically using results from second order perturbation theory one of the key results is that close to sinusoidal bedrock undulations up to two different spatial regions of local extrusion flow may arise the offset and onset of extrusion flow is controlled primarily by the amplitude to wavelength ratio above the crest of a sinusoidal bed line a local maximum of the surface parallel velocity develops for e ak 0 138 where a is the amplitude and k is the wave number as e increases from zer to this critical value the vertical position of the velocity maximum moves from kz 1 to kz 1 98 where z is the vertical distance above the mean bed line within and above the trough of a sinusoid a region of local minimum of the surface parallel velocity component develops which shifts from kz 1 towards the bed line as e increases front zero to 1 2 below this velocity minimum and for some distance above the velocity maximum the surface parallel velocity increases with depth this type of extrusion flow will cause a reversal of borehole inclination profiles close to the bedrock

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

bed topography and lubrication inferred from surface measurements on fast flowing ice streams observations of surface elevation s and horizontal velocity components u and v are inverted to infer the topography b and lubrication c at the bed of an ice stream based on a linearized perturbation theory of the transmission of flow disturbances through the ice thickness synthetic data are used to illustrate non uniqueness in the inversion but also demonstrate that effects of b and c can be separated when s u and v are specified even with added noise to simulate measurement errors we have analyzed prominent short horizontal scale 2 km features in topography and velocity pattern in a local 64 km by 32 km area of the surface of ice stream e west antarctica our preferred interpretation of bed conditions beneath the most prominent features on the surface identifies a deep trough in the basal topography with low lubrication in the base of the trough

influence of debris rich basal ice on flow of a polar glacier at taylor glacier a cold based outlet glacier of the east antarctic ice sheet observed surface speeds in the terminus region are 20 times greater than those predicted using glen s flow law for cold 17 c thin 100 m ice rheological properties of the clean meteoric glacier ice and the underlying deformable debris rich basal ice can be inferred from surface velocity and ablation rate profiles using inverse theory here with limited data we use a two layer flowband model to examine two end member assumptions about the basal ice properties 1 uniform softness with spatially variable thickness and 2 uniform thickness with spatially variable softness we find that the basal ice contributes 85 98 to the observed surface velocity in the terminus region we also find that the basal ice layer must be 10 15 m thick and 20 40 times softer than clean holocene age glacier ice in order to match the observations because significant deformation occurs in the basal ice our inverse problem is not sensitive to variations in the softness of the meteoric ice our results suggest that despite low temperatures highly deformable basal ice may dominate flow of cold based glaciers and rheologically distinct layers should be incorporated in models of polar glacier flow

application of control methods for modelling the flow of pine island glacier west antarctica the distribution of basal traction on a transect along pine island glacier west antarctica is estimated by inverting observed surface velocities using a control method and a simple numerical stream flow model this model calculates the horizontal flow along a transect based on the assumptions that the horizontal flow is independent of ice depth and that the driving stresses are balanced by resistive forces at the glacier bed and margin and by gradients in longitudinal stress basal traction is assumed to be linearly related to the basal velocity for the lateral shear traction a parameterization based on an inversion of glen s flow law is used the applicationof the control method allowsus to calculate the set of model parameters e g the basal friction coefficient that gives the best fit betweenmodelled andobserved surface velocities themodel is used to investigate the stress regime of pine island glacier in particular to estimate the importance of basal lateral and longitudinal stresses relative to each other in the flat region just behind the grounding line basal drag lateral drag and the longitudinal stress gradient are the same order of magnitude in the steep region up glacier from the grounding line the driving stresses are highest and balanced predominantly by basal resistive stresses further upstream in the trunk of the glacier lateral and basal drag predominate

entrainment at cold glacier beds here we present measurements of the gas content and isotopic composition of debris rich basal layers of a polar glacier meserve glacier antarctica which has a basal temperature of 17 c these measurements show that debris entrainment has occurred without alteration of the glacial ice and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers without bulk freezing of water entrainment at subfreezing temperatures may have formed the u shaped trough containing meserve glacier in addition to possibly allowing some cold based glaciers to be important geomorphic agents entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets which are rheologically distinct and can limit the time span of ice core analyses furthermore accumulating evidence suggests that geomorphologists should abandon the assumption that cold based glaciers do not slide and abrade their beds