modeling ground displacement above reservoirs undergoing fluid withdrawal injection based on an ellipsoidal inhomogeneity model abstract we propose a procedure to efficiently monitor the geomechanical behavior of underground porous reservoirs undergoing fluid withdrawal injection we apply the inhomogeneity problem and invert geodetic data on subsidence uplift to calibrate and specify parameters used for geomechanical models based on pressure changes we first calculate the poroelastic strain of the reservoirs then we test it assuming uniaxial deformation on a three dimensional ellipsoidal inhomogeneity with different semi axes in a full space isotropic elastic domain and finally we apply a simple half space approximation to account for the tractions free ground surface despite its simplicity and the initial full space approximation the semi analytical model derived in this study replicates well poroelastic strains inferred by geodetic data or obtained with other analytical and numerical techniques our model is a good approximation for predicting the profile of surface deformation i e geodetic strain and once calibrated its magnitude

distributed shear of subglacial till due to coulomb slip in most models of the flow of glaciers on till beds it has been assumed that till behaves as a viscoplastic fluid despite contradictory evidence from laboratory studies in accord with this assumption displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by coulomb friction motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress these fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing newton s second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed consequent displacement profiles are convex upward similar to those measured by boulton and colleagues at breidamerkur jokull iceland the model results when considered together with the long term and widespread empirical support for coulomb models in soils engineering indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

modelling the coupling of flood discharge with glacier flow during jokulhlaups we explore a mathematical model that couples together a thermomechanically evolving subglacial channel distributed cavity drainage and basal sliding along a subglacial flood path fed by a jokulhlaup lake it allows water transfer between channel and cavities and a migrating subglacial water divide or seal to form between floods notably it accounts for full coupling between the lake and subglacial drainage in terms of both discharge and pressure unlike models that neglect the pressure coupling by imposing a known history of lake discharge at the channel inlet this means that flood hydrographic evolution and its impact on glacier motion are consistently determined by our model numerical simulations for a model alpine lake yield stable limit cycles simulating repeating jokulhlaups with the channel drawing water from the cavities at a varying rate that modulates basal sliding during each flood a wave of fast sliding propagates down glacier at flood initiation followed by deceleration as the growing channel sucks water from the cavities these behaviours cannot be correctly simulated without the full coupling we show that the flood s peak discharge its initiation threshold and the magnitude of the fast sliding wave decrease with the background water supply to the cavities

lumped element analysis of subglacial hydraulic circuits observations of the subglacial hydrological system reveal a richness of responses that are thought to reflect variations in the water supply morphology and connectivity of the system to contribute insight into the physical controls that result in qualitative variations in system response a lumped element formalism is developed and used to simulate the behavior of simple hydraulic circuits the circuit elements for an idealized subglacial water system differ from those used to analyze conventional hydraulic flows but the motivation is similar lumped systems are governed by ordinary differential equations whereas continuous systems are governed by partial differential equations the accompanying examples illustrate the capture property of rothlisberger channels the concept of morphological switching the role of water storage geometry in contributing to system nonlinearity and the effects of seasonal variations in storage capacity and hydraulic resistance

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

a new approach to inferring basal drag and ice rheology in ice streams with applications to west antarctic ice streams drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers simultaneously inferring these parame ters is challenging since basal drag and ice viscosity are coupled in the momen tum balance which governs ice flow here we test the ability of adjoint based inverse methods to infer the slipperiness coefficient in a power law sliding law and the flow rate parameter in the constitutive relation we modify existing inversions by including surface strain rates into the regularization of the in version using synthetic data generated with physically motivated variations in basal drag and ice rheology we show that this allows for more accurate in ferences we apply this method to bindschadler and macayeal ice streams in west antarctica our results show relatively soft ice in the shear margins and spatially varying basal drag with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag we interpret the former to reflect a combination of heating through viscous dissipation and changes in the crystalline structure these results suggest that adjoint based inverse methods can provide inferences of basal drag and ice rheology when the regularization is informed by strain rates

seismicity induced by gas production ii lithology correlated events induced stresses and deformation a 3d relocation technique permits precise locations of induced earthquakes geostatistical processing using the data of 87 boreholes provides the basis of a precise 3d structure with a dome geometry conventional laboratory mechanical tests performed on deep rock samples 1000 m to 5000 m define the rock properties at depths similar to those of the seismic events 1 ml 4 2 that range from 1 to 7 km

a theory for ice till interactions and sediment entrainment beneath glaciers and frozen onto or melted from the glacier base can achieve a steady state that is in balance with the rate that latent heat is transported to or from the basal interface at constant n when a gradual increase in heat flow from the glacier base causes the rate of melting to decrease h increases and continues to do so when the heat flow is great enough to produce freezing as freezing becomes more rapid and h increases further the rate of fluid supply to the glacier base reaches a maximum when the effective permeability is sufficiently reduced by the partial ice saturation in the fringe larger h can be achieved with slower freezing at the glacier base but steady states with larger h are unstable the maximum rate of fluid supply to the glacier base is greater at lower n higher temperature gradients and for sediments with higher permeabilities unsteady behavior can lead to large changes in h when there is a mismatch between the rate that latent heat can be extracted and the rate that fluid is supplied to the glacier base transient behavior driven by abrupt changes in n is characterized by rapid variations in freezing rate followed by slower adjustments to h that are limited by the timescale for the conduction of latent heat the resulting patterns of sediment deformation are expected to commonly be distributed over finite depth ranges even when shear is perfectly localized at any single instant in time

modelling distributed and channelized subglacial drainage the spacing of channels models are proposed for channelized and distributed flow of meltwater at the base of an ice sheet the volumes of both channel and distributed systems evolve according to a competition between processes that open drainage space e g sliding over bedrock melting of the ice and processes that close it e g viscous creep of the ice due to a positive effective pressure channels are generally predicted to have lower water pressure and therefore capture water from the surrounding regions of distributed flow there is a natural length scale associated with the distributed system that determines the width of the bed from which water can be drawn into a channel it is suggested that this determines the spacing between major channels and that this may be reflected in the spacing of eskers a more permeable distributed system results in more widely spaced and therefore larger channels calculations of the flow into the head of a channel reveal that there is a critical discharge necessary for it to form and provide a criterion for where channels can exist

effects of ice deformation on rothlisberger channels and implications for transitions in subglacial hydrology along the base of glaciers and ice sheets the sliding of ice over till depends critically on water drainage in locations where drainage occurs through rothlisberger channels the effective pres sure along the base of the ice increases and can lead to a strengthening of the bed which reduces glacier sliding the formation of rothlisberger channels depends on two competing effects 1 melting from tur bulent dissipation opens the channel walls and 2 creep flow driven by the weight of the overlying ice closes the channels radially inward variation in downstream ice velocity along the channel axis re ferred to as an antiplane shear strain rate decreases the effective viscosity the softening of the ice increases creep closure velocities in this way even a modest addition of antiplane shear can double the size of the rothlisberger channels for a fixed water pressure or allow channels of a fixed radius to operate at lower effective pressure potentially decreasing the strength of the surrounding bed furthermore we show that rothlisberger channels can be deformed away from a circular cross section under applied antiplane shear these results can have broad impacts on sliding velocities and po tentially affect the total ice flux out of glaciers and ice streams

seismic detectability of anomalous structure at the top of the earth s outer core with broadband array analysis of smks phases abstract i have examined precisely the differential travel times and waveforms of smks seismic phases propagating under the southern indian ocean obtained from african broadband seismic arrays the smks phases analyzed in this study travel in the mantle with weak heterogeneity confirmed by a global tomographic study for the distance range of 115 135 the smks differential times were obtained from a vespagram a stack intensity on a time slowness diagram and comparison with the vespagram created from synthetic waveforms with prem gives the travel time residual for each event array pair although the residuals of s3ks s2ks times exhibit apparently a systematic dependence on epicentral distance this is likely due to small scale heterogeneity beneath the oceania where is covered by the smks ray entering points at the cmb waveform modeling was applied to a record section with a small travel time residual that suggests a small effect from the mantle heterogeneity on the data set i found that a low velocity zone in the outermost 50 km in the core rather than prem can explain an additional arrival detected just after the s3ks phase this result is still inconclusive because of the small number of data and non uniqueness of the model and ambiguity due to mantle structure however accumulation of the precise measurement described in this study may help the reduction of uncertainty and trade offs

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

seismicity induced by gas production i correlation of focal mechanisms and dome structure from august 1976 to june 1979 more than 220 seismic events were recorded by a 7 station network operating over an active gas field in france 120 earthquakes with magnitudes greater than 1 0 were selected for a detailed study in order to understand the seismic response to a decrease in the gas reservoir pressure of 50 mpa composite focal mechanisms were determined for these events we found that where preexisting fractures were known from well logs faults with the most favorable orientation were reactivated in the part of the gas field where no preexisting faults were known orientation of the fracture planes was close to the orientations of stratigraphic interfaces our kinematic model constrained by the distribution of the earthquakes and their focal mechanisms consists of the downward dropping of a block located close to the top of the gas field the main faults as defined by the spatial density of the seismic activity define the limits of this block the displacements observed along these faults can be either normal or reverse mechanism dependent on the preexisting orientation of each fault another important result is the evidence that the depletion of the gas reservoir exerts the major influence on local seismic displacements obscuring the effect of regional setting

deformation induced melting in the margins of the west antarctic ice streams flow of glacial ice in the west antarctic ice sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice but our understanding of the physical processes that generate this morphology is incomplete here we study the thermal and mechanical properties of ice stream margins where flow transitions from rapid to stagnant over a few kilometers our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation induced melting we propose a 2 d model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction we limit temperature to the melting point to estimate melt rates based on latent heat using rheology parameters as constrained by laboratory data and observations we conclude that a zone of temperate ice is likely to form in active shear margins

temperate ice in the shear margins of the antarctic ice sheet controlling processes and preliminary locations abstract the flux of grounded ice from the antarctic ice sheet ais primarily occurs through narrow fast flowing outlet glaciers and ice streams shearing generates heat in the lateral glacier margins where there is a transition from fast flow to near stagnant ice or rock this heat warms the ice and can form zones of water saturated porous ice at the melting temperature known as temperate ice here we derive a one dimensional thermomechanical model to infer which ais shear margins may contain temperate ice our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone both of these quantities are described by two nondimensional numbers the brinkman and peclet numbers that are functions of observable parameters such as shear strain rate surface temperature surface accumulation rate and ice thickness the development of temperate zones is incited by shear strain rates brinkman number and suppressed by the advection of cold ice from the surface peclet number which scales with surface accumulation rate we also include a parametrization for lateral advection which can suppress the formation of temperate ice applying recent observations and outputs from a regional climate model we show that many active glacier shear margins could contain temperate ice the spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates

chaotic dynamics of a glaciohydraulic model a model subglacial drainage system coupled to an ice dammed reservoir that receives a time varying meltwater input is described and analysed in numerical experiments an ice marginal lake drains through a subglacial channel producing periodic floods and fills with meltwater at a rate dependent on air temperature which varies seasonally with a peak value of tm the analysis reveals regions of tm parameter space corresponding to mode locking where flood repeat time is independent of tm resonance where decreasing tm counter intuitively increases flood size and chaotic dynamics where flood cycles are sensitive to initial conditions never repeat and exhibit phase space mixing bifurcations associated with abrupt changes in flood size and timing within the year separate these regions this is the first time these complex dynamics have been displayed by a glaciohydraulic model and these findings have implications for our understanding of ice marginal lakes moulins and subglacial lakes

tidally induced variations in vertical and horizontal motion on rutford ice stream west antarctica inferred from remotely sensed observations to better understand the influence of stress changes over floating ice shelves on grounded ice streams we develop a bayesian method for inferring time dependent 3 d surface velocity fields from synthetic aperture radar sar and optical remote sensing data our specific goal is to observe ocean tide induced variability in vertical ice shelf position and horizontal ice stream flow thus we consider the special case where observed surface displacement at a given location can be defined by a 3 d secular velocity vector a family of 3 d sinusoidal functions and a correction to the digital elevation model used to process the sar data using nearly 9 months of sar data collected from multiple satellite viewing geometries with the cosmo skymed 4 satellite constellation we infer the spatiotemporal response of rutford ice stream west antarctica to ocean tidal forcing consistent with expected tidal uplift inferred vertical motion over the ice shelf is dominated by semidiurnal and diurnal tidal constituents horizontal ice flow variability on the other hand occurs primarily at the fortnightly spring neap tidal period m sf we propose that periodic grounding of the ice shelf is the primary mechanism for translating vertical tidal motion into horizontal flow variability causing ice flow to accelerate first and most strongly over the ice shelf flow variations then propagate through the grounded ice stream at a mean rate of 29 km d and decay quasi linearly with distance over 85 km upstream of the grounding zone

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

high basal melting forming a channel at the grounding line of ross ice shelf antarctica antarctica s ice shelves are thinning at an increasing rate affecting their buttressing ability channels in the ice shelf base unevenly distribute melting and their evolution provides insight into changing subglacial and oceanic conditions here we used phase sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable ross ice shelf melt rates of 22 2 0 2 m a 1 2500 the overall background rate were observed 1 7 km seaward of mercer whillans ice stream grounding line close to where subglacial water discharge is expected laser altimetry shows a corresponding steadily deepening surface channel two relict channels to the north suggest recent subglacial drainage reorganization beneath whillans ice stream approximately coincident with the shutdown of kamb ice stream this rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

a model for polythermal ice incorporating gravity driven moisture transport the flow of ice sheets and glaciers dissipates significant amounts of heat which can result in the formation of temperate ice a binary mixture of ice and small amounts of melt water that exists at the melting point many ice masses are polythermal in the sense that they contain cold ice below the melting point as well as temperate ice temperature and melt water or moisture content conversely affect the flow of these ice masses through their effect on ice viscosity and sliding behaviour ice flow models therefore require a component that can solve for temperature and moisture content and determine the free boundary between the cold and temperate subdomains we present such a model based on the theory of compacting partial melts by contrast with other models we describe gravity and pressure gradient driven drainage of moisture while maintaining a divergence free ice flow at leading order we also derive the relevant boundary conditions at the free cold temperate boundary and find that the boundary behaves differently depending on whether ice enters or exits the temperate region the paper also describes a number of test cases used to compare with a numerical solution and investigates asymptotic solutions applicable to the limit of small compaction pressure gradients in the temperate ice regions a simplified enthalpy gradient model is finally proposed which captures most of the behaviour of the full model in this limit

flow variability and ongoing margin shifts on bindschadler and macayeal ice streams west antarctica ice streams on the ross sea side of the west antarctic ice sheet are known to experience flow variability on hourly annual and multicentury time scales we report here on observations of flow variability at the decade scale on the bindschadler and macayeal ice streams bis and macis our analysis makes use of archived ice velocity data and new mappings from composited landsat 7 and landsat 8 imagery that together span the interval from 1985 to 2014 both ice streams speedup and slowdown in a range of about 5 m a 2 over our various comparison intervals the rates of change are variable in both time and space and there is no evidence of external forcing at work across the two streams widespread changes are most likely linked to instability in the subglacial till and or subglacial water flow sticky spots near the confluence of the two ice streams are loci for speed changes these relatively young and slow flowing features appear to be forcing shifts in margin position near the outlets of both streams the margin jumps reduce the effective outlet widths of the streams by 20 and 30 on bis and macis respectively those magnitudes are similar to the outlet narrowing experienced by kamb ice stream prior to its stagnation

subglacial hydrology and ice stream margin locations fast flowing ice streams in west antarctica are separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins it is presently uncertain what mechanisms control the location of shear margins and possibly allow them to migrate in this paper we show how subglacial hydrological processes can select the shear margin location leading to a smooth transition from a slipping to a locked bed at the base of an ice stream our study uses a two dimensional thermomechanical model in a cross section perpendicular to the direction of flow we confirm that the intense straining at the shear margins can generate large temperate regions within the deforming ice assuming that the melt generated in the temperate ice collects in a drainage channel at the base of the margin we show that a channel locally decreases the pore pressure in the subglacial till therefore the basal shear strength just outside the channel assuming a coulomb plastic rheology can be substantially higher than that inferred under the majority of the stream results show that the additional basal resistance produced by the channel lowers the stress concentrated on the locked portion of the bed matching the model to surface velocity data we find that shear margins are stable when the slipping to locked bed transition occurs less than 500 m away from a channel operating at an effective pressure of 200 kpa and for a hydraulic transmissivity equivalent to a basal water film of order 0 2 mm thickness

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

modeling channelized and distributed subglacial drainage in two dimensions 1 we present a two dimensional glacier drainage system model glads that couples distributed and channelized subglacial water flow distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness channelized flow occurs through rothlisberger channels that can form on any of the edges of a prescribed unstructured network of potential channels water storage is accounted for in an englacial aquifer and in moulins which also act as point sources of water to the subglacial system solutions are presented for a synthetic topography designed to mimic an ice sheet margin for low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge the model exhibits a channelization instability which leads to the formation of a self organized channel system the random orientation of the network edges allows the channel system geometry to be relatively unbiased in contrast to previous structured grid based models under steady conditions the model supports the classical view of the subglacial drainage system with low pressure regions forming around the channels under diurnally varying input water flows in and out of the channels and a rather complex spatiotemporal pattern of water pressures is predicted we explore the effects of parameter variations on the channel system topology and mean effective pressure the model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model the results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings

thermally driven migration of ice stream shear margins ice stream shear margins are the lateral boundaries of narrow fast flowing bands of ice within an ice sheet we develop a theory for the migration of shear margins over time driven by viscous dissipation of heat within the ice focusing on widening of the ice stream the location of the margin is modelled as a transition from a cold to a temperate ice sheet bed and simultaneously as the transition from no slip to free slip at the same location the temperature field in the ice is affected by intense shear heating as well as by the migration velocity of the margin i e by the widening rate of the ice stream if migration is too fast there is little time for the ice to warm up and the margin remains cold causing the bed to freeze this suppresses widening conversely if the migration speed is too slow the ice in the margin warms up causing the bed on the far side of the cold temperate transition to reach the melting point and migration to speed up using a wiener hopf method we show that for a given far field shear stress geothermal heat flux and ice geometry there is a single migration velocity that balances the two effects and permits widening at a steady rate this velocity increases with the far field lateral shear stress imposed by the ice stream which controls shear heating in the margin our results also indicate that i a region of temperate ice must form in the margin and that ii lateral advection of ice may play a significant role in controlling migration speeds

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

subglacial hydrology as a control on emergence scale and spacing of ice streams observations have long associated ice streams with the presence of meltwater at the bed more recently theoretical models have been able to reproduce ice stream behavior as a consequence of the coupled dynamics of ice and subglacial meltwater in this paper we analyze the properties of ice streams that form in a coupled model of ice flow and subglacial hydrology we see that there is a natural length scale defining ice stream separation and width this arises as a result of the balance between effective pressure gradients driving meltwater away from ice streams and the enhanced water production in the streams due to the fast ice flow we further discuss how the model interacts with topography and we show that small perturbations to a uniform bed have a strong effect on where ice streams emerge in the model however in many cases ice streams then evolve to be closer to the dimensions defined by the natural length scale of the unperturbed system the nondimensional parameter that defines this length scale is therefore of fundamental importance in the model

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

shear heating and weakening of the margins of west antarctic ice streams ice streams are fast flowing bands of ice separated from stagnant ridges by shear margins the mechanisms controlling the location of the margins remain unclear we use published ice deformation data and a simple one dimensional thermal model to show that west antarctic ice stream margins have temperate ice over a substantial fraction of their thickness a condition that may control their width the model predicts a triple valued relation between the thickness averaged lateral shear stress and the lateral shear strain rate observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream this requires enhanced basal resistance near the margin we suggest in agreement with the limited observations the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice till interface thus increasing the shear stress acting on the yielding coulomb plastic bed

transient effective stress variations forced by changes in conduit pressure beneath glaciers and ice sheets the resistance to sliding beneath soft bedded glaciers depends on the effective stress n which is controlled by the subglacial hydrological system while large scale water transport is primarily through conduits the effective stress profile beneath most of the glacier base is expected to be controlled by seepage flows through the permeable sediments models of the response to forced changes of the near conduit effective stress demonstrate that perturbations in n decay over a characteristic lateral distance that is typically only a few times the sediment thickness this implies that only relatively small changes to the spatially averaged effective stress n can be produced if conduit spacing is comparatively large and the glacier and sediments remain in contact to produce larger changes to n it is likely that flotation must be achieved beneath a significant portion of the glacier base at higher values spatial variations in n can produce gradients in the thickness h of a fringe of ice infiltrated sediments immediately adjacent to the glacier base this has implications for the development of glacial landforms when sliding causes sediment transport

impacts of warm water on antarctic ice shelf stability through basal channel formation the antarctic ice sheet is buttressed by floating ice shelves remote sensing data show extensive basal channels particularly in west antarctica which grow quickly in response to warm water intrusion antarctica s ice shelves provide resistance to the flow of grounded ice towards the ocean if this resistance is decreased as a result of ice shelf thinning or disintegration1 acceleration of grounded ice can occur increasing rates of sea level rise loss of ice shelf mass is accelerating especially in west antarctica where warm seawater is reaching ocean cavities beneath ice shelves2 here we use satellite imagery airborne ice penetrating radar and satellite laser altimetry spanning the period from 2002 to 2014 to map extensive basal channels in the ice shelves surrounding antarctica the highest density of basal channels is found in west antarctic ice shelves within the channels warm water flows northwards eroding the ice shelf base and driving channel evolution on annual to decadal timescales our observations show that basal channels are associated with the development of new zones of crevassing suggesting that these channels may cause ice fracture we conclude that basal channels can form and grow quickly as a result of warm ocean water intrusion and that they can structurally weaken ice shelves potentially leading to rapid ice shelf loss in some areas

a boundary layer model for ice stream margins the majority of antarctic ice is discharged via long and narrow fast flowing ice streams at ice stream margins the rapid transition from the vertical shearing flow in the ice ridges surrounding the stream to a rapidly sliding plug flow in the stream itself leads to high stress concentrations and a velocity field whose form is non trivial to determine in this paper we develop a boundary layer theory for this narrow region separating a lubrication type ice ridge flow and a membrane type ice stream flow this allows us to derive jump conditions for the outer models describing ridge and stream self consistently much of our focus is however on determining the velocity and shear heating fields in the margin itself ice stream margins have been observed to change position over time with potentially significant implications for ice stream discharge our boundary layer model allows us to extend previous work that has determined rates of margin migration from a balance between shear heating in the margin and the cooling effect of margin migration into the colder ice of the surrounding ice ridge solving for the transverse velocity field in the margin allows us to include the effect of advection due to lateral inflow of ice from the ridge on margin migration and we demonstrate that this reduces the rate of margin migration as previously speculated

on the mechanics of ice stream shear margins we investigate the mechanics of ice stream shear margins based on the assumption that the underlying bed behaves plastically sliding is assumed to occur if a prescribed locally defined yield stress is attained while no sliding is assumed possible if basal shear stress is lower than the yield stress mathematically the ice flow problem takes the form of a contact problem in which the zones of sliding are part of the solution and cannot be prescribed arbitrarily simplistic assumptions about the location of till failure or about mechanical conditions at the bed predict stress singularities at the margins which lead to corresponding singularities in the basal melt rate the ice flow problem is solved using a complex variable method and an associated quasi static thermal problem is also solved using a green s function high stress concentrations which coincide with high rates of strain heating are found on the ice stream side of the margins where basal melting is also greatest our results further indicate that a temperate zone may form over time above the bed in the margins these findings differ from earlier studies based on dif ferent sliding laws suggesting a high sensitivity of margin behaviour to basal conditions

bed reflectivity beneath inactive ice streams in west antarctica radio echo sounding res techniques are used to examine spatial changes inbed reflectivityacrossrelictice streams inwest antarctica measurementsfrom adjacent interstream ridgesare usedto correctthe measuredpower returned fromthebed for attenuation and losses due to geometric spreading scattering and absorption res measurements near boreholes drilled on ice stream c isc indicate high coefficients of bed reflectivity r40 1 in locations where the bed wasthawed and boreholes connected to thebasalwatersystem andlow reflectivitycoefficients r50 02 atlocationsthat were frozen and not connected intermediate values of bed reflectivity were measured at locations where the connection to the basal water systemwas weak measurements across four relictmargins show thatbedreflectivity usuallyjumpsfromlow tohighvaluesseveral kilometers inside the outermost buried crevasses we interpret this to be a transition from frozento thawedbasal conditions anddiscuss implications of these observations

wet subglacial bedforms of the ne greenland ice stream shear margins we describe elongate wet subglacial bedforms in the shear margins of the ne greenland ice stream and place some constraints on their formation lateral shear margin moraines have been observed across the previously glaciated landscape but little is known about the ice flow conditions necessary to form these bedforms here we describe in situ sediment bedforms under the ne greenland ice stream shear margins that are observed in active source seismic and ground penetrating radar surveys we find bedforms in the shear margins that are 500 m wide 50 m tall and elongated nearly parallel to ice flow including what we believe to be the first subglacial observation of a shear margin moraine acoustic impedance analysis of the bedforms shows that they are composed of unconsolidated deformable water saturated till we use these geophysical observations to place constraints on the possible formation mechanism of these subglacial features

seasonal changes in ice sheet motion due to melt water lubrication abstract a numerical model is used to calculate how the motion of an idealized ice sheet margin is affected by the subglacial drainage of melt water from its surface the model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure the results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed the growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin but high water pressure and ice velocity are maintained through much of the summer further inland annual mean ice velocity increases with the total quantity of melt water except close to the margin where it is almost insensitive to the amount of melting short term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input linked cavity and poroelastic models for the distributed drainage system are compared and their relative merits discussed two different sliding laws are considered and the need for a holistic description of hydraulically controlled sliding is highlighted

processes controlling the downstream evolution of ice rheology in glacier shear margins case study on rutford ice stream west antarctica ice rheology governs how glaciers flow and respond to environmental change the rheology of glacier ice evolves in response to a variety of mechanisms including damage heating melting and the development of crystalline fabric the relative contributions of these rheological mechanisms are not well understood using remotely sensed data and physical models we decouple the influence of each of the aforementioned mechanisms along the margins of rutford ice stream a laterally confined outlet glacier in west antarctica we show that fabric is an important control on ice rheology in the shear margins with an inferred softening effect consistent with a single maximum fabric fabric evolves to steady state near the onset of streaming flow and ice progressively softens downstream almost exclusively due to shear heating the rate of heating is sensitive to local shear strain rates which respond to local changes in bed topography as ice is squeezed through the basal trough the impact of shear heating on the downstream evolution of ice rheology in a laterally confined glacier suggests that the thermoviscous feedback wherein faster ice flow leads to higher rates of shear heating further softening the ice is a fundamental control on glacier dynamics

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

thermal effects on the location of ice stream margins west antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion intervening zones of slow moving ice are probably freezing at their bases where heat generation is absent heat is also generated in the shear zone between the slow and fast moving ice through modeling of coupled heat and mass flow we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed above a threshold speed in the ice stream heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream below this threshold heat generation at the edge of the thawed zone is insufficient to maintain melting leading to freezing and narrowing of the stream the threshold speed appears to be a function of the temperature at the base of slow ice relative to the melting point and is also highly sensitive to the inflow of colder ice from the slow zone for environmental conditions typical of west antarctica the threshold speed is about 100 m yr 1 in the absence of inflow and increases to typical ice stream speeds 500 m yr 1 for inflow speeds of a few meters per year thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream

bed radar reflectivity across the north margin of whillans ice stream west antarctica and implications for margin processes surface based ice penetrating radar profiles were made across the active north margin the snake of the upper part of whillans ice stream formerly ice stream b branch b2 west antarctica at three locations low frequency about 2 mhz and the ground deployment of the radar allowed penetration through the near surface zone of fracturing to detect internal layering and bed reflection characteristics on continuous profiles spanning from the slow moving ice of engelhardt ridge well into the chaotic zone of the shear margin internal layers were tracked beneath the chaotic zone where they are warped but remain continuous the energy returned from internal layers showed no systematic changes associated with the transition from the undisturbed surface of the slow moving ice into the fractured surface of the shear margin thus indicating little effect from the surface crevasses on the penetration of the radar signal based on this calibration of the near surface effects and corrections for path length spreading and attenuation we examine the spatial variation of bed reflectivity low bed reflectivity found under engelhardt ridge extends under the chaotic zone of the margin into fast moving ice we argue that the fast motion in a band along the margin is mediated by processes other than deformation of thick dilated till that is the source of lubrication allowing fast motion in the interior of the ice stream

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

drainage through subglacial water sheets 1 subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed here we study the flow of water in thin macroporous sheets between ice and bed previous work shows that small perturbations in depth of a nearly parallel sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof we argue that in the presence of bed protrusions bridging a water sheet downward motion of the ice roof can stabilize this sheet stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation we therefore modify existing theory to include protrusions that partially support the overlying glacier differences in the pressure on protrusions relative to water pressure drive roof closure the mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice bed gap in order to account for multiple protrusion sizes along the bed for instance resulting from an assortment of various sized sediment grains we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet partitioning is dependent on the amount of ice protrusion contact and therefore water depth this method allows prediction of roof closure rates we then investigate stable steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures as is appropriate for ice streams moreover we find that multiple steady sheet thicknesses exist raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

laboratory study of the frictional rheology of sheared till 1 deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength experiments were conducted on till a mixed size granular material from matanuska glacier alaska and the scioto ohio lobe of the laurentide ice sheet caesar till we deformed till in double direct shear under fixed shear velocity or shear stress creep normal stress ranged from 50 kpa to 5 mpa at shearing rates ranging from 1 to 300 m s for 1 cm thick samples creep was induced via small step perturbations in stress fabric development within till layers was investigated by varying shear strain prior to creep tests in velocity controlled experiments till deforms as a nearly coulomb plastic material with slight velocity strengthening corresponding to a stress exponent n 60 creep experiments conducted well below the shear strength indicate lower n values increasing as shear stress increases with increasing initial strain and inferred fabric development the creep strain rate decreases while n increases experiments at a normal stress of 1 mpa and no initial strain show n 6 8 increasing to n 17 5 at higher shear strains however strain rate was still decreasing and thus these values represent maximum estimates our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less at low strains n likely depends on consolidation state permeability and dilation deformation is nearly rate insensitive coulomb plastic at shear stress near the shear strength or at high strain

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

glacier sliding seismicity and sediment entrainment the evolution of glaciers and ice sheets depends on processes in the subglacial environment shear seismicity along the ice bed interface provides a window into these processes such seismicity requires a rapid loss of strength that is typically ascribed to rate weakening friction i e decreasing friction with sliding or sliding rate many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds to our knowledge however these studies have all found rate strengthening friction here we investigate the possibility that rate weakening rock on rock friction between sediments frozen to the bottom of the glacier and the underlying water saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds we test this entrainment seismicity hypothesis using targeted laboratory experiments and simple models of glacier sliding seismicity and sediment entrainment these models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity we propose that stagnation at the whillans ice stream west antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins our results suggest that basal shear seismicity may indicate geomorphic activity

rapid ice flow rearrangement induced by subglacial drainage in west antarctica ice streams are corridors of rapid ice flow draining the ice sheets they can exhibit astonishing spatial variability on annual to centennial time scales we propose that changes in the subglacial drainage of meltwater could induce these sudden rearrangements of ice streams we develop a two dimensional thermomechanical model representing an ice stream cross section and couple it to a plastically deforming bed with spatially variable meltwater influx we find that where ice flows over deformable sediments and lacks significant topographic control the efficiency of subglacial water drainage exerts direct control on the velocity location and width of ice streams this implies that meltwater percolation at the meter scale could have a significant effect on the short term variability in ice loss from a continental scale ice sheet we verify our model against previous analytical results and validate it against surface observations from the siple coast of west antarctica

plastic bed beneath hofsjokull ice cap central iceland and the sensitivity of ice flow to surface meltwater flux author s minchew b simons m bjornsson h palsson f morlighem m seroussi h larour e hensley s abstract the author s 2016 the mechanical properties of glacier beds play a fundamental role in regulating the sensitivity of glaciers to environmental forcing across a wide range of timescales glaciers are commonly underlain by deformable till whose mechanical properties and influence on ice flow are not well understood but are critical for reliable projections of future glacier states using synoptic scale observations of glacier motion in different seasons to constrain numerical ice flow models we study the mechanics of the bed beneath hofsjokull a land terminating ice cap in central iceland our results indicate that the bed deforms plastically and weakens following incipient summertime surface melt combining the inferred basal shear traction fields with a coulomb plastic bed model we estimate the spatially distributed effective basal water pressure and show that changes in basal water pressure and glacier accelerations are non local and non linear these results motivate an idealized physical model relating mean basal water pressure and basal slip rate wherein the sensitivity of glacier flow to changes in basal water pressure is inversely related to the ice surface slope

inversion of a glacier hydrology model the subglacial hydrologic system exerts strong controls on the dynamics of the overlying ice yet the parameters that govern the evolution of this system are not widely known or observable to gain a better understanding of these parameters we invert a spatially averaged model of subglacial hydrology from observations of ice surface velocity and outlet stream discharge at kennicott glacier wrangell mountains ak usa to identify independent parameters we formally non dimensionalize the forward model after specifying suitable prior distributions we use a markov chain monte carlo algorithm to sample from the distribution of parameter values conditioned on the available data this procedure gives us not only the most probable parameter values but also a rigorous estimate of their covariance structure we find that the opening of cavities due to sliding over basal topography and turbulent melting are of a similar magnitude during periods of large input flux though turbulent melting also exhibits the greatest uncertainty we also find that both the storage of water in the englacial system and the exchange of water between englacial and subglacial systems are necessary in order to explain both surface velocity observations and the relative attenuation in the amplitude of diurnal signals between input and output flux observations