implications of till deformation on glacier dynamics the dynamics of glacier motion are governed to a large extent by the properties of the basal interface in this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of black rapids glacier alaska u s a we also investigate whether or not a till layer has a clear surface observable signature in the dynamics of the glacier towards this end we use a finite element ice flow model with a coulomb failure criterion within the basal till layer we find that simple till physics can be used to describe decadal seasonal and short term hours to days velocity variations and possibly uplift events mechanisms for each of these variations involve an increase in the extent of till at failure a transfer of shear stress across the bed and a consequent increase in ice deformation effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models our analyses however have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier

rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

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

the response of black rapids glacier alaska to the denali earthquake rock avalanches 1 we describe the impact of three simultaneous earthquake triggered rock avalanches on the dynamics of black rapids glacier alaska by using spaceborne radar imagery and numerical modeling we determined the velocities of the glacier before and after landslide deposition in 2002 by using a combination of ers 1 ers 2 tandem radarsat 1 and alos palsar synthetic aperture radar data ice velocity above the debris covered area of the glacier increased up to 14 after the earthquake but then decreased 20 by 2005 within the area of the debris sheets mean glacier surface velocity increased 44 within 2 years of the landslides at the downglacier end of the lowest landslide where strong differential ablation produced a steep ice cliff velocities increased by 109 over the same period by 2007 ice velocity throughout the debris area had become more uniform consistent with a constant ice flux resulting from drastically reduced ablation at the base of the debris without further analysis we cannot prove that these changes resulted from the landslides because black rapids glacier displays large seasonal and interannual variations in velocity however a full stokes numerical ice flow model of a simplified glacier geometry produced a reversal of the velocity gradient from compressional to extensional flow after 5 years which supports our interpretation that the recent changes in the velocity field of the glacier are related to landslide induced mass balance changes

influence of subglacial drainage system evolution on glacier surface motion haut glacier d arolla switzerland 1 the relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at haut glacier d arolla switzerland with the onset of high and variable melt season discharges subglacial drainage changed from a predominantly distributed system to a predominantly channelized system the change occurred later farther up glacier during the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge the magnitude and spatial pattern of the glacier s velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity increasing discharge through incipient channels below moulins not yet connected to the main channel caused more localized basal forcing and slightly increased glacier velocity increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event the relative importance of these modes changed through the melt season suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing

episodic reactivation of large scale push moraines in front of the advancing taku glacier alaska 1 taku glacier an advancing former tidewater glacier in alaska has been actively pushing its proglacial sediments along part of its terminus over the last 50 years producing so called push moraines the mobilization of these sediments which were locally lifted more than 20 m above sea level by 2004 has happened episodically rather than steadily the last major event of proglacial sediment deformation occurred in 2001 presumably caused by sliding along a basal detachment layer since then most deformation has been localized within a few meters of the terminus including impressive deformational features of the terminal ice where slabs of ice tens of centimeters in thickness have undercut proglacial sediments and vegetation and lifted them up between 2002 and 2004 surface velocities and horizontal displacements were measured across the terminus and in the proglacial push moraine area sediment displacement was highest between the end of march and mid june a decrease in displacement with distance from the terminus revealed that the sediments were deforming internally rather than along a

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

laboratory study of fabric development in shearing till the importance of effective pressure and shearing rate abstract herein we present data on the shearing rate glacier velocity and effective pressure difference between the ice overburden pressure and pore water pressure in the development of magnetic fabric anisotropy of magnetic susceptibility using a rotary ring shear device a wisconsin age basal till was used in the experiments and deformed to its critical state at shear strains as high as 93 we also present data from hysteresis and high temperature susceptibility experiments to identify the magnetic carrier in the basal till results showed little change in fabric strength when varying the shearing rate in the speed range of 110 860 m year 1 moreover the effective pressure tests also showed an inconsistency in fabric between 30 and 150 kpa however a slight strengthening effect was documented thus the k 1 magnetic fabric strength is independent of the shearing rate and effective pressure this suggests that the fabric strength upon these variables cannot be used as a benchmark for estimating shear deformation to the geological record the k 1 fabric strength in this study however remained consistent with respect to other till particle fabric methods e g sand and pebble in which the same conclusion was drawn all particles align parallel to the direction of shear and plunge mildly up glacier

glacier motion dominated by processes deep in underlying till black rapids glacier is a 40 km long surge type glacier in the central alaska range in spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large the drilling revealed a layer of subglacial till up to 7 m thick that is believed to be water saturated at one location a string of instruments containing three dual axis tiltmeters and one piezometer was successfully introduced into the till the tiltmeters monitored the inclination of the borehole at the ice till interface and at 1 and 2 m into the till for 410 days they showed that no significant deformation occurred in the upper 2 m of the till layer and no significant amount of the basal motion was due to sliding of the ice over the till the measured surface velocity at the drill site is about 60 m a 1 of which 20 30 m a can be accounted for by ice deformation almost the entire amount of basal motion 30 40 m a 1 was taken up at a depth of 2 m in the till possibly in discrete shear layers or as sliding of till over the underlying bedrock we propose that the large scale mobilization of such till layers is a key factor in initiating glacier surges

dynamic controls on glacier basal motion inferred from surface ice motion 1 current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events such as lake drainage rainfall or surging this may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field both of which are not accounted for in typical sliding laws we use multiple time series of surface motion over a 66 day period at brei amerkurjokull iceland to infer changes in bed separation and longitudinal force budget we observe multiple distinct periods of increased surface motion and uplift corresponding to periods of rainfall and or increased temperatures we find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion

a three dimensional numerical model of the confluence area of unteraargletscher bernese alps switzerland with the use of a numerical three dimensional 3 d model the flow dynamics of the confluence area of unteraargletscher bernese alps switzerland are studied previous predictions based on conceptual two dimensional models about flow characteristics at confluence areas are tested against results from the fully 3 d model measured winter velocities are used for model verification despite some consistent systematic differences good overall agreement between measured and calculated surface velocities is obtained the calculated vertical strain rate variation with depth is in good agreement with available measurements from boreholes the ice is found to be almost three times stiffer than standard estimates of rheological parameters for glacier ice would predict the model predicts a complicated yet realistic pattern of vertical velocity variation along the surface the most noticeable features of the vertical velocity distribution across the surface are listed and their relation to topographic surface undulations and the overall dynamics of the confluence discussed in accordance with previous results from analytical models a strongly localized surface trough and a concomitant negative downward orientation vertical velocity anomaly develop at the junction point although depth integrated strain rates are positive extension the basal layer is compressed vertically the ice cored medial moraine is formed by differential ablation the flow mechanics of the confluence area play only an indirect role by enabling transfer of debris covered marginal ice towards the confluence center in the absence of differential ablation an elongated surface depression would be formed in the down glacier direction from the junction point instead of an elevated ice cored medial moraine

the flow of fireweed rock glacier alaska u s a fireweed rock glacier is a large rock glacier in south central alaska u s a it flows relatively fast with velocities up to 3 5 ma 1 and exhibits both seasonal and annual velocity variations some of which are related to periodic terminus calving and increased rainfall our analysis reveals that motion is likely concentrated in a pseudo rectangular channel within the larger parabolic channel with a shear plane at 27 m depth there is likely motion along the shear plane as well as internal deformation above it we estimate that the ice rock mixture is up to seven times softer than clean glacier ice with a temperature of 2 c calving at the terminus is an important component of the mass balance of this rock glacier

water storage and subglacial drainage conditions inferred from borehole measurements on gornergletscher valais switzerland measurements of change in length of a borehole or displacements of poles and of subglacial water pressure were combined with drainage tests and electrode tests in boreholes in experiments near the northern margin of gornergletscher valais switzerland at the upstream side of an overdeepening the measurements suggest that the subglacial drainage system consisted of discrete conduits at that location presumably linked cavities on clean bedrock changes in subglacial water pressure were followed by variations in subglacial water storage together with similar variations in elevation of a pole the variations in subglacial water storage caused changes in cross sections of subglacial passageways and thereby changed the frequencies of slug test oscillations similar experiments conducted near the center line of the glacier revealed different subglacial conditions impeded drainage through a sediment layer and different depths of water levels in different boreholes these results are discussed in relation to previous studies

stress balances of ice streams in a vertically integrated higher order formulation one challenge in improving our understanding of ice stream dynamics is to develop models of the spatial and temporal transition from ice sheet to ice stream flow we address this with a new vertically integrated higher order formulation for ice sheet dynamics that captures the leading order physics of low aspect ratio viscous fluid flow regardless of the amount of slip at the bed the theory introduces a parameter which approximates the ratio of the basal stress to the shear stress scale providing a measure of the relative importance of sliding and internal deformation our model is able to simultaneously describe the dynamics of both a slow moving sheet and rapidly flowing ice streams to test the formulation we apply a triple valued sliding law as the basal boundary condition and obtain numerical solutions that can be compared with previous work we investigate the sensitivity of flow regimes and shear margin width to parameter variation deriving a scaling for the latter we also consider a double valued sliding law which enforces a constant low basal stress beneath the ice stream comparisons of the resultant stress fields illustrate the different stress balances that can maintain

short term variations in glacier flow controlled by subglacial water pressure at lauteraargletscher bernese alps switzerland short term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in lauteraargletscher bernese alps switzerland horizontal surface flow speed oscillated diurnally showing a correlation with the water level in a borehole flow speed increased as a function of the water level with an asymptote at the ice overburden level this observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions detailed examination of the diurnal variations however showed that the speed was larger when the pressure was increasing than when it was decreasing greater speed with increasing pressure was interpreted by subglacial watercavity opening and or longitudinal stress coupling with the upper reaches of the glacier upward surface movements were observed when the glacier flow speed increased simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources vertical straining of ice and volume increase of subglacial water cavities the vertical surface movement was largely affected by the vertical strain and the uplift events could not be simply attributed to cavity opening

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

diurnal fluctuations in glacier ice deformation haut glacier d arolla switzerland mair d hubbard b nienow p willis i fischer u h 2008 diurnal fluctuations in glacier ice deformation haut glacier d arolla switzerland earth surface processes and landforms 33 8 1272 1284 sponsorship carnegie trust for the universities of scotland

in situ measurements of till deformation and water pressure a newly developed hammer was used to insert two autonomous probes 0 8 m and 2 1 m into clast rich subglacial till under black rapids glacier alaska usa both probes were instrumented with a dual axis tilt sensor and a pore water pressure transducer the data are compared to a 75 day record of surface velocities till deformation at depth was found to be highly seasonal it is significant during an early season speed up event but during long periods thereafter measured till deformation rates are negligible both tilt records show rotation around the probe axis which indicates a change in tilt direction of about 30 the tilt records are very similar suggesting spatial homogeneity on the scale of the probe separation 4 m horizontal and 3 3 m vertical there is evidence that during much of the year sliding of ice over till or deformation of a thin till layer 20 cm accounts for at least two thirds of total basal motion basal motion accounts for 50 70 of the total surface motion the inferred amount of ice till sliding is larger than that found at the same location in a previous study when surface velocities were about 10 lower we suggest that variations in ice till coupling account for the observed variations in mean annual speed

hydrological discharges and motion of fels and black rapids glaciers alaska u s a implications for the structure of their drainage systems characteristics of the hydrology and motion of black rapids and fels glaciers alaska were observed from 1986 to 1989 hydrological measurements included stage electrical conductivity and suspended sediment concentration in the discharge stream of each glacier and were made at 0 5 1 h intervals continuously through most of the melt seasons variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time lapse photography 1 d time resolution strain meters 0 5 1 h resolution and seismometers set up to count acoustic emissions both glaciers show similar seasonal diurnal and short term event changes in hydrological discharges and ice speed the hydrological behavior is analyzed in terms of a fast sub system composed of surface streams moulins and large tunnels with discharge that responds rapidly and a slow sub system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day the timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10 40 of the water is routed directly into the fast system the remaining 90 60 of the water enters the slow system dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related a small time lag from discharge maximum minimum to solute minimum maximum suggests that the fast system is confined to roughly the lowermost 30 40 of the full glacier length the residence time of water in the fast system is short compared to 1 d the slow system contains both short and long residence time passages characteristics of the diurnal cycles are somewhat variable through the melt season but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur which suggests parallel evolution of the variables that control the response of the fast system events were characterized by contemporaneous increases in suspended sediment concentration in the discharge water and distinct changes in straining on the glaciers events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water the correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products hydrological differences between black rapids and fels glaciers can be explained by differences in the size of the glaciers if there is a difference in bed structure that explains the difference in dynamics surge black rapids glacier versus non surge fels glacier it does not affect the hydrological parameters that were observed

marine ice sheet profiles and stability under coulomb basal conditions the behavior of marine terminating ice sheets such as the west antarctic ice sheet is of interest due to the possibility of rapid grounding line retreat and consequent catastrophic loss of ice critical to modeling this behavior is a choice of basal rheology where the most popular approach is to relate the ice sheet velocity to a power law function of basal stress recent experiments however suggest that near grounding line tills exhibit coulomb friction behavior here we address how coulomb conditions modify ice sheet profiles and stability criteria the basal rheology necessarily transitions to coulomb friction near the grounding line due to low effective stresses leading to changes in ice sheet properties within a narrow boundary layer ice sheet profiles taper off towards a flatter upper surface compared with the power law case and basal stresses vanish at the grounding line consistent with observations in the coulomb case the grounding line ice flux also depends more strongly on flotation ice thickness which implies that ice sheets are more sensitive to climate perturbations furthermore with coulomb friction the ice sheet grounds stably in shallower water than with a power law rheology this implies that smaller perturbations are required to push the grounding line into regions of negative bed slope where it would become unstable these results have important implications for ice sheet stability in a warming climate

a laboratory study of ploughing a new laboratory device is used to investigate the resistance to clast ploughing at the base of glaciers in experiments in which a ploughing tip is dragged at different velocities and effective normal stresses through water saturated sediment from unteraargletscher switzerland pore pressures above and below the hydrostatic level develop around the tip the absolute magnitude of these non hydrostatic pore pressures increases with the ploughing velocity but remains small compared to the sediment yield strength so that the pore pressures do not significantly weaken the sediment the shear stress on the tip is independent of the velocity but scales with the applied effective normal stress in agreement with a coulomb plastic behavior of the sediment the results indicate that depending upon position close to the object both sediment compaction and dilation can influence the pore pressure distribution and thus the sediment yield strength comparison with other studies of clast ploughing suggests that the significance of sediment weakening in front of ploughing clasts may depend on the relative magnitudes of the non hydrostatic pore pressures therefore depending on the dominant pore pressure response of the deforming sediment clast ploughing may have the potential to either trigger ice flow instabilities or stabilize glacier motion

the influence of sub glacial bed evolution on ice extent a model based evaluation of the last glacial maximum pukaki glacier new zealand abstract a potential complication in using glacier extent to estimate paleoclimatic conditions is the influence of glacier bed evolution on changes in ice extent over time here we examine this issue through model based reconstructions of the last glacial maximum lgm pukaki glacier southern alps new zealand whose lgm extents are exceptionally well defined by lateral and terminal moraines using the well dated moraine limits as an empirical constraint on maximum ice extents we employ a one dimensional glacier flowline model driven by a mass balance model in order to evaluate the influence of climate and glacier bed profile on the extent of the lgm pukaki glacier the lgm glacier bed is buried by lateglacial and post glacial sediment so we calculate bed profiles using a modified version of the one dimensional model and available geologic constraints a best fit to the moraine record occurs with a lgm temperature of 7 8 c cooler than present with precipitation ranging from 80 to 100 of present levels modeling of the bed profile evolution indicates that bed changes alone could account for kilometer scale changes in glacier width and length

spatial patterns of summer speedup on south central alaska glaciers seasonal changes in glacier basal motion are attributable to variations in subglacial hydrology and cause variations in both ice discharge and glacier erosion we develop a novel workflow based upon landsat 8 feature tracking to document differences between spatial patterns of summer and winter glacier surface speed which reflect changes in the distribution of basal motion we identify and characterize summer speedups on 13 of 19 land terminating glaciers in alaska s wrangell st elias ranges the speedups are relatively uniform over much of the ablation zones and the speedup magnitudes vary by only a factor of 2 between glaciers whose velocities span an order of magnitude summer speedups extend up to 30 km up glacier from termini and often end at the bases of icefalls these data provide systematic observation of the spatial pattern of enhanced summer glacier basal motion and suggest the possibility of its parameterization in glacier models

carbon dioxide glaciers on mars products of recent low obliquity epochs abstract three localized sets of small arcuate ridges associated with slopes in the northern polar area of mars 70 n latitude are morphologically similar to sets of drop moraines left by episodes of advance and retreat of cold based glaciers comparison with other glacial features on mars shows that these features differ in important aspects from those associated with water ice flow instead we interpret these features to be due to perennial accumulation and flow of solid carbon dioxide during recent periods of very low spin axis obliquity

evolution of variegated glacier alaska u s a prior to its surge on presente l evolution saison apres saison des changements dans la geometrie et la vitesse du glacier et on tente de rattacher ces variations a la deformation de la glace et au glissement basal

effects of bedrock lithology and subglacial till on the motion of ruth glacier alaska deduced from five pulses from 1973 2012 a pulse is a type of unstable glacier flow intermediate between normal flow and surging using landsat mss tm and etm imagery and feature tracking software a time series of mostly annual velocity maps from 1973 to 2012 was produced that reveals five pulses of ruth glacier alaska peaks in ice velocity were found in the 1981 1989 1997 2003 and 2010 approximately every 7 years during these peak years the ice velocity increased 300 from approximately 40 m yr to 160 m yr and occurred in an area of the glacier underlain by sedimentary bedrock based on the spatio temporal behavior of ruth glacier during the pulse cycles we suggest the pulses are due to enhanced basal motion via deformation of a subglacial till the cyclical nature of the pulses is theorized to be due to a thin till with low permeability that causes incomplete drainage of the till between the pulses followed by eventual recharge and dilation of the till these findings suggest care is needed when attempting to correlate changes in regional climate with decadal scale changes in velocity because in some instances basal conditions may have a greater influence on ice dynamics than climate

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

caterpillar like ice motion in the ablation zone of the greenland ice sheet current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the greenland ice sheet we present measurements of ice deformation subglacial water pressure and surface velocity that show periodic and episodic variations on several time scales seasonal multiday and diurnal these variations observed with gps and sensors at different depths throughout the ice column are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity with the help of a full stokes ice flow model these observations are explained as ice motion in a caterpillar like fashion caused by patches of different basal slipperiness horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior ice flow in the ablation zone of the greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season leading to caterpillar like ice motion superposed on the classical shear deformation

ice volume and subglacial topography for western canadian glaciers from mass balance fields thinning rates and a bed stress model abstracta method is described to estimate the thickness of glacier ice using information derived from the measured ice extent surface topography surface mass balance and rate of thinning or thickening of the ice column shear stress beneath an ice column is assumed to be simply related to ice thickness and surface slope as for an inclined slab but this calculation is cast as a linear optimization problem so that a smoothness regularization can be applied assignment of bed stress is based on the flow law for ice and a mass balance calculation but must be preceded by delineation of the ice flow drainage basin validation of the method is accomplished by comparing thickness estimates to the known thickness generated by a numerical ice dynamics model once validated the method is used to estimate the subglacial topography for all glaciers in western canada that lie south of 60 n adding the present ice volume of each glacier gives the estimated total volume as 2320 km3 equivalent to 5 8 mm of sea leve

a parallel high order accurate finite element nonlinear stokes ice sheet model and benchmark experiments 1 the numerical modeling of glacier and ice sheet evolution is a subject of growing interest in part because of the potential for models to inform estimates of global sea level change this paper focuses on the development of a numerical model that determines the velocity and pressure fields within an ice sheet our numerical model features a high fidelity mathematical model involving the nonlinear stokes system and combinations of no sliding and sliding basal boundary conditions high order accurate finite element discretizations based on variable resolution grids and highly scalable parallel solution strategies all of which contribute to a numerical model that can achieve accurate velocity and pressure approximations in a highly efficient manner we demonstrate the accuracy and efficiency of our model by analytical solution tests established ice sheet benchmark experiments and comparisons with other well established ice sheet models

quiescent phase evolution of a surge type glacier black rapids glacier alaska u s a black rapids glacier a surge type glacier in the alaska range most recently surged in 1936 37 and is currently in its quiescent phase mass balance ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994 the annual speed has undergone cyclical fluctuations of as much as 45 about the mean speed ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation which indicates that they are being driven by changes in basal motion the behavior of black rapids glacier during this quiescent phase is significantly different from that variegated glacier another well studied surge type glacier in alaska the present medial moraine configuration of black rapids glacier indicates that a surge could occur at any time however ice velocity data indicate that the next surge may not be imminent we believe that there is little chance that the next surge will cross and dam the delta river

feedbacks between coupled subglacial hydrology and glacier dynamics on most glaciers and ice sheet outlets the majority of motion is due to basal slip a combination of basal sliding and bed deformation the importance of basal water in controlling sliding is well established with increased sliding generally related to high basal water pressure but the details of the interactions between the ice and water systems has not received much study when there is coupling between the systems here we use coupled subglacial hydrology and ice dynamics models within the community ice sheet model to investigate feedbacks between the ice and water systems the dominant feedback we find is negative sliding over bedrock bumps opens additional cavity space which lowers water pressure and in turn sliding we also find two small positive feedbacks basal melt increases through frictional heat during sliding which raises water pressure and strain softening of basal ice during localized speedup causes cavities to close more quickly and maintain higher water pressures our coupled modeling demonstrates that a sustained input of surface water to a distributed drainage system can lead to a speedup event that decays even in the absence of channelization due to increased capacity of the system through opening of cavities which is enhanced through the sliding opening feedback we find that the negative feedback resulting from sliding opening is robust across a wide range of parameter values however our modeling also argues that subglacial channelization is required to terminate speedup events over timescales that are commensurate with observations of late summer slowdown on mountain glaciers

reversal of ice motion during the outburst of a glacier dammed lake on gornergletscher switzerland during the outburst flood of a glacier dammed lake on gornergletscher switzerland in july 2004 the drained lake water triggered anomalous glacier motion at the onset of the outburst the ice flow direction in the vicinity of the lake became closer to the central flowline when the lake discharge magnitude decreased the flow direction altered such that the ice moved back to the azimuth of the initial motion at one of the survey points where the ice flows parallel to the central flowline the ice accelerated along the pre event flow direction followed by a 1808 backward motion that lasted over 2 days these observations indicate the impact of the lake outburst on the subglacial and englacial stress conditions however the reversal in the flow direction is difficult to explain by drawing on our current understanding of glacier mechanics the timing and the timescale of the flow direction changes suggest that the elastic glacier motion and its rebound played a role under the rapidly changing stress conditions but the young s modulus of ice is too large to cause the observed ice motion other processes including basal separation and subglacial sediment deformation are discussed as possible mechanisms for the reversal of the ice motion

seismic detection of transient changes beneath black rapids glacier alaska u s a ii basal morphology and processes using changes observed in daily seismic reflections we have investigated the basal morphology of black rapids glacier alaska u s a the englacial drainage of ice marginal lakes caused significant changes in the daily reflections as well as dramatic increases in basal motion changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge type glacier rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients but that a decrease in till saturation is likely we therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure higher water pressures within the hydraulic system cause that region to support more of the glacier s weight leaving the remaining areas to support less any till within these areas of decreased normal stress would experience a consequent decrease in pore water pressure causing gas to exolve thus decreasing saturation this decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics

mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 1 observations the data presented in part 1 of this paper meier et al this issue are here used to assess the role of water input output water storage and basal water pressure in the rapid movement of columbia glacier alaska consistently high basal water pressures mostly in the range from 300 kpa below to 100 kpa above the ice overburden pressure are responsible in an overall way for the high glacier flow velocities 3 5 9 m d 1 which are due mainly to rapid basal sliding caused by the high water pressure diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site the water pressure fluctuation tracks the time integrated water input less a steady withdrawal as expected for the diurnally fluctuating storage of water in the glacier far from the terminus at the downglacier site the situation is more complex diurnal peaks in water level which are directly related to intraglacial water storage as well as to basal water pressure are shifted forward in time by 4 hours probably as a result of the effect of diurnal fluctuation in water output from the glacier which affects the local water storage fluctuations near the terminus because of the forward shift in the basal water pressure peaks which at the downglacier site lead the velocity peaks by 6 hours a mechanical connection between water pressure and sliding there would have to involve a 6 hour quarter period delay however the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single consistent control mechanism at both sites the velocity variations in nondiurnal speed up events caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier it appears that small variations in water pressure 100 kpa sufficient to produce the observed velocity variations 15 30 are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity at the achieved level of observation the clearest though not complication free control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

discrimination of the flow law for subglacial sediment using in situ measurements and an interpretation model 1 subglacial hydrological and mechanical processes play a critical role in determining the flow characteristics and stability of glaciers and ice sheets to study these processes we have measured simultaneously basal water pressure pore water pressure sediment deformation glacier sliding and sediment strength beneath trapridge glacier yukon territory canada to interpret these data we have developed a simple hydromechanical model of processes beneath a soft bedded alpine glacier the glacier bed is divided into soft bedded regions that are hydraulically connected to the subglacial drainage system soft bedded but hydraulically unconnected regions and hard bedded regions each region is represented as a one dimensional column the columns are coupled by a simple ice dynamics model that accounts for water pressure driven changes in basal shear stress distribution synthetic responses for subglacial instruments are calculated from the modeled basal conditions providing a framework for improving interpretation of field records the model is used to determine which of several till flow laws best represents conditions beneath trapridge glacier investigated are linear viscous nonlinear viscous nonlinear bingham and coulomb plastic tills pore water pressures sediment deformation profiles and sliding rates are calculated for each flow law comparison of synthetic and field instrument responses suggests that till behavior is best represented as coulomb plastic model results also suggest that the ploughmeter is the most diagnostic in situ indicator of till behavior currently available and that using long term observations of sediment deformation profiles in regions of varying pore water pressure can result in an underestimation of flow law nonlinearity

how much do we really know about glacier surging some of the ideas about glacier surging are considered mainly but not entirely in the light of observations of temperate glaciers in alaska u s a made within the last 15 years climate has an influence on surge recurrence interval climate and weather also affect surge initiation termination and magnitude regional studies lead to the speculation that subglacial till plays a key role in surging and it has been found under all surge type glaciers studied so far including black rapids and variegated glaciers alaska in most of the glaciers studied till deformation processes dominate the motion in quiescence the linked cavity model of surge triggering and rapid motion is not consistent with these observations but the limited coverage of the observations does not rule it out under parts of the glaciers studied the till observations in alaska raise old questions about the interaction between till and the hydraulic systems of temperate glaciers the role of stored water which observations show to be active even in winter on black rapids glacier is noted

the galloping glacier trots decadal scale speed oscillations within the quiescent phase a 28 year record of annual surface speeds has revealed oscillations with a period of roughly 12 years during the quiescent phase of black rapids glacier alaska u s a the original galloping glacier these oscillations are hypothesized to be the manifestation of slowly propagating waves of till failure and till healing with at least the second cycle being initiated by the anomalous advance of a tributary glacier observations support the idea that such dynamics may have occurred but are not conclusive in the conceptual model describing the mechanisms of till failure and healing temporal variations in longitudinal stress gradients are proposed to be more important in causing till failure than temporal variations in effective pressure

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

geodetic mass balance of surge type black rapids glacier alaska 1980 2001 2010 including role of rockslide deposition and earthquake displacement we determine the geodetic mass balance of surge type black rapids glacier alaska for the time periods 1980 2001 and 2001 2010 by combining modern interferometric synthetic aperture radar insar derived digital elevation models dems dems derived from archival aerial imagery laser altimetry data and in situ surface elevation measurements our analysis accounts for both the large rockslides and terrain displacements caused by the 2002 m7 9 earthquake on the denali fault which runs through black rapids glacier to estimate uncertainties we apply monte carlo simulations for the earthquake triggered rockslides we find a volume of 56 62 2 86 106 m3 equivalent to an average debris thickness of 4 44 0 24 m across the 11 7 km2 deposit area on the glacier terrain displacement due to the earthquake corresponds to an apparent glacier volume change of 53 1 106 m3 which would cause an apparent specific mass balance of 0 19 meter water equivalent mwe if not taken into account the geodetic mass balance of black rapids glacier is 0 48 0 07 mwe a 1 for the entire 30 year period but more negative for the period 2001 2010 0 64 0 11 mwe a 1 than the period 1980 2001 0 42 0 11 mwe a 1 in agreement with trends indicated by in situ mass balance measurements elevation data indicate no net thickening of the surge reservoir between 1980 and 2010 in contrast to what is expected during the quiescent phase a surge of black rapids glacier in the near future is thus considered unlikely

modulation of tropical cyclogenesis location and frequency over the indo western north pacific by the intraseasonal indo western pacific convection oscillation during the boreal extended summer abstractthe influence of the intraseasonal indo western pacific convection oscillation ipco on tropical cyclone tc genesis location and frequency over the indo western north pacific wnp during the boreal extended summer may october is explored observational analysis shows that the impacts of the intraseasonal ipco on tcs over the indo wnp include an evident phase lock of tc genesis location and distinct differences in tc frequency in the wnp in the positive intraseasonal ipco phase the atmosphere gains heat through the release of latent heat in cumulus convective condensation and the anomalous cyclonic circulation weakens the western pacific subtropical high wpsh and enhances tc genesis thereby tending to produce many more tcs moreover the diminished wpsh and the westward shift of the centers of anomalous cyclonic circulations lock tc genesis locations to the west wnp and lower latitudes around 5 20 n especially in the south china sea the almost opposite situation occurs in a nega

the role of ice stream dynamics in deglaciation since the mid pleistocene transition deglaciation has occurred only after ice sheets have grown large while experiencing several precession and obliquity cycles indicating that large ice sheets are more sensitive to milankovitch forcing than small ice sheets are observations and model simulations suggest that the development of ice streams in the laurentide ice sheet played an as yet unknown role in deglaciations in this study we propose a mechanism by which ice streams may enhance deglaciation and render large ice sheets more sensitive to milankovitch forcing we use an idealized configuration of the parallel ice sheet model that permits the formation of ice streams when the ice sheet is large and ice streams are sufficiently developed an upward shift in equilibrium line altitude commensurate with milankovitch forcing results in rapid deglaciation while the same shift applied to an ice sheet without fully formed ice streams results in continued ice sheet growth or slower deglaciation rapid deglaciation in ice sheets with significant streaming behavior is caused by ice stream acceleration and the attendant enhancement of calving and surface melting at low elevations ice stream acceleration is ultimately the result of steepening of the ice surface and increased driving stresses in ice stream onset zones which come about due to the dependence of surface mass balance on elevation these ice sheet simulations match the broad features of geomorphological observations and add ice stream dynamics that are missing from previous model studies of deglaciation

comparison of a three dimensional model for glacier flow with field data from haut glacier d arolla switzerland a three dimensional finite difference model based on a first order solution of the ice flow equations is applied to haut glacier d arolla switzerland the numerical model successfully converges at horizontal resolutions down to 70 m so a number of detailed comparisons with field data can be made modelled surface velocities with no basal sliding component are compared with surface velocities observed on the glacier over four different time periods the best fit is achieved with over winter surface velocities r 2 0 75 using a rate factor a in glen s flow law of 0 063 a 1 bar 3 surface zones of maximum computed effective stress display a high level of coincidence with observed crevassing the orientation of which is successfully predicted by the direction of the tensile component of the computed principal surface stress comparison of the relative magnitude and direction of computed principal stresses with principal strains measured at the ice surface also correspond closely in an attempt to simulate the observed annual velocity distribution within a cross section of the glacier tongue we incorporate two basal motion patterns into the model by treating net annual ice motion as a time weighted composite of three separate flow situations normal sliding enhanced sliding and no sliding we are able to reproduce the key features of the observed cross sectional ice and basal slip velocity distributions these experiments indicate there may be substantial decoupling taking place along an elongated narrow zone at the bed of haut glacier d arolla and that this decoupling interacts in a complex manner with the englacial stress and strain field

oxygen isotopes in glacier river water austre okstindbreen okstindan norway there is no discernible river discharge from austre okstindbreen norway in winter any water formed by basal melting is likely to be stored in isolated cavities or sediments at the bed in summer a baseflow component of discharge relatively depleted of 18 o is diluted by water more enriched in 18 o glacier ice meltwater with a high 18 o value passes rapidly through channelized systems and emerges in the glacier river within a few hours headward extension of the systems in late summer may tap water stored within the glacier since the previous summer s close down stored water also is released during early melt season events when the low capacity drainage systems cannot accommodate inputs the high 18 o value indicates that the probable source of the released water is glacier ice or low altitude snowfall stratigraphic variations of 18 o values resulting from changing weather conditions characterize the accumulating winter snowpack during the melt season the mean 18 o value of the residual pack rises the water leaving it depleted of 18 o passes slowly through the glacier smoothing out variations of supply 18 o values of rain vary between and within events and their effect on glacier river 18 o values is unpredictable

subglacial drilling at black rapids glacier alaska u s a drilling method and sample descriptions we employed a commercial wireline drill rig to investigate the subglacial conditions of black rapids glacier a well studied surge type glacier in the central alaska range the four main goals were to assess the capabilities of the commercial drilling industry for sampling subglacial material to investigate the basal morphology to determine the subglacial geology and to emplace borehole instruments the drilling was done in an area where seasonal and secular variations in speed are large and where seismic studies suggested the presence of a till layer four holes were drilled at three locations to a maximum depth of 620 m three holes yielded samples of basal ice and till although recovery of the latter was generally poor bedrock was sampled in one or possibly two of the holes in the area sampled the glacier is underlain by a till layer some 4 7 m thick confirming the seismic interpretation it consists ofa sandy matrix at least 20 30 of which comprises larger clasts limited samples ofthe matrix indicate that near the top ofthe till the porosity is 40 and that some of the pore water is frozen geologic studies suggest that the drilling area lies to the north of the denali fault a major tectonic boundary followed by the glacier and that most of the till is locally derived with transport distances of 2 km

variational methods for glacier flow over plastic till we investigate the mechanics of ice streams and glaciers flowing over a bed consisting of coulomb plastic subglacial sediment or more generally of channel flows with coulomb or solid friction laws at the boundary sliding is assumed to occur if shear stress at the glacier bed attains a prescribed locally defined yield stress while no sliding is assumed possible below that yield stress importantly the location of regions of slip and no slip at the bed is not known a priori but forms part of the solution by analogy with friction problems in elasticity we derive a weak formulation as a semi coercive variational inequality which admits a unique solution provided a solvability condition ensuring force balance is satisfied the variational formulation is then exploited to calculate numerical solutions and we investigate the effect of variations in subglacial water pressure ice thickness and surface slope on the discharge of a valley glacier with a plastic bed significant differences are found between the behaviour of wide and narrow as well as steep and shallow angled glaciers and our results further indicate the need to develop models capable of accounting for longitudinal stresses

surface ice motion deviating toward the margins during speed up events at gornergletscher switzerland 1 high frequency ice flow measurements during speed up events in gornergletscher switzerland revealed intriguing ice motion which has never been reported in detail before during the summer 2005 more than a 100 flow speed increase was observed three times at four gps stations installed across gornergletscher the speed ups were accompanied by a decimeter scale surface uplift two of the events were triggered by intensive surface melt and rainfall while the third one was due to the outburst of gornersee a glacier dammed lake located 2 km upglacier an interesting observation was ice motion deviating toward the side margins during the events as the glacier accelerated a transverse cross glacier velocity component was generated turning the flow direction away from the central flow line toward the margins when the glacier decelerated the transverse velocity component reversed so that the ice flowed back to the azimuth of the initial flow direction in the most significant case the trajectory of the survey stake deviated from the original track by 0 2 m in the transverse direction we hypothesize that the observed lateral ice motion was caused by locally elevated subglacial water pressure when the ice sole decoupled from the bed at a part of the glacier a point source of vertical displacement was transmitted to the surface through viscous ice this caused the transverse as well as vertical surface motion as observed in ground motion during magma intrusion the hypothesis was tested with a two dimensional ice flow model applied to the transverse glacier cross section the model confirmed that the surface ice would move toward the margins as observed in gornergletscher if subglacial water pressure exceeded the ice overburden pressure over a limited part of the bed

the basal speed of valley glaciers an inverse approach geophysical inverse methods are used to calculate the basal motion of a glacier they are applied to a one dimensional forward model that can be linearized to make the analysis simpler the inverse method finds a solution that fits the data within a given error it selects for smooth solutions to discriminate against unrealistic oscillations the method is applied to a simple model glacier of uniform shape and thickness to test how well a given basal motion field can be reconstructed it shows as expected that optimizing for smoothness lowers maxima and increases minima of the solution a step change in basal velocity is drawn out in the inversion over a distance that is given by the half width of a resolving function this is typically about three times the ice thickness but is also affected by the sampling rate of the data the method is then applied to two glaciers where suitable data are available brown glacier on heard island southern indian ocean and mccall glacier in the brooks range alaska u s a the mccall results agree well with earlier estimates of basal motion

measurements of ice deformation at the confluence area of unteraargletscher bernese alps switzerland an experimental study of ice deformation at the confluence area of unteraargletscher bernese alps switzerland has been carried out surface velocities were measured by repeatedly surveying stakes and with the use of remote sensing methods the variation of the vertical strain rates with depth was measured in boreholes the confluence center line is subjected to longitudinal horizontal extension which is exceeded in magnitude by a concomitant transverse compression vertical strain rates change from positive extension at the surface to negative compression in the lowest layers of the glacier

seismic detection of transient changes beneath black rapids glacier alaska u s a i techniques and observations to gain new insight into the mechanisms of basal motion we have demonstrated the feasibility of an active seismic technique to measure temporal changes in basal conditions on sub hourly time scales one region of the bed of black rapids glacier alaska u s a was monitored for a period of 45 days using seismic reflections the majority ofthese reflections were nearly identical however three significant anomalies were recorded several days apart these corresponded with the englacial drainage of two ice marginal lakes and one supraglacial pothole each up glacier of the study site as well as dramatic increases in basal motion two of these seismic anomalies revealed identical changes over 1 km 2 of the bed despite the fact that their drainage events occurred at different locations further these two seismic anomalies were followed by records identical to the non anomalous state showing that the seismic changes were reversible in one of these events two records taken 36 min apart revealed that the transition between the anomalous and normal states occurred completely within this short interval

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

evidence for temporally varying sticky spots at the base of trapridge glacier yukon territory canada during the 1992 summer field season we installed arrays of plough meters and water pressure transducers beneath trapridge glacier yukon territory canada to study hydromechanical coupling at the ice bed interface diurnal signals recorded with two of these ploughmeters appear to correlate with fluctuations in subglacial water pressure these diurnal variations can be explained by changes in basal resistance to sliding as mechanical conditions at the bed vary temporally in response to changes in the subglacial hydrological system we propose that a lubricating water film associated with high water pressures promotes glacier sliding whereas low pressures cause increased basal drag resulting in sticky areas using a theoretical model we analyze the sliding motion of glacier ice over a flat surface having variable basal drag and show that a consistent explanation can be developed results from our model calculations provide strong support for the existence of time varying sticky spots which are associated with fluctuations in subglacial water pressure

the impact of jokulhlaups on basal sliding observed by sar interferometry on vatnajokull iceland we have analyzed insar data from the ers 1 ers 2 tandem mission to study the ice dynamics of vatnajokull iceland during jokulhlaups from the skaftacauldrons and the grimsvot n geothermal area which drained under the tungnaarjokull and skei ararjokull outlets respectively during the initial phase of a grimsvotn jokulhlaup in march 1996 the velocity of skei ararjokull increased up to three fold relative to observed velocities in december 1995 over an area up to 8 km wide around the subglacial flood path accumulation of water was observed at one location in the flood path during a small jokulhlaup from the skaftacauldrons in october 1995 the velocity on tungnaarjokull increased up to four fold over a 9 km wide area the velocity increase was observed 1 5 days before the floodwater was detected in the river skafta a reduced glacier speed as the flood peaked in skaftaindicates evolution of the subglacial drainage system from sheet to tunnel flow the glacier acceleration and local uplift observed in the early phase of both jokulhlaups supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels thus reducing the coupling of ice with the glacier bed

an iterative scheme for determining glacier velocities and stresses there are no direct methods to measure the boundary condition at the base of a glacier here we propose a method that works by iteratively changing boundary conditions until a satisfactory fit to surface observations is obtained the method is an accelerated version of one known as kozlov maz ya iteration we apply it to the problem of ice flow through a transverse cross section and show it to be effective by solving the inverse problem for a sequence of synthetic data we also apply the method to two real glaciers athabasca and perito moreno one of which has a known basal velocity distribution

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

formation and deformation of basal till during a glacier surge elisebreen svalbard elisebreen a retreating valley glacier in nw svalbard experienced a surge in the past possibly during the little ice age when ice was thicker and the bed warmer the surge produced an assemblage of subglacial landforms and sediments now appearing in front of the downwasting ice margin we have analysed an array of sedimentary properties of basal till from glacial flutings ploughing marks an interflute ground moraine and meandering ridges in order to infer processes of till formation and deformation during the unrecorded surge till in flutings and ploughing marks reveals higher mechanical abrasion than in other landforms but grain size distribution and mineralogy are largely the same everywhere micromorphological characteristics are also very similar and comprise ductile and brittle deformation structures indicating fluctuating porewater pressures frequent micrograin fracturing suggests stress concentration along grain bridges during the surge till was formed by a combination of lodgement and deformation caused mainly by clast ploughing meandering ridges containing till with water escape structures indicate that surge termination was facilitated by rapid water evacuation from the ice bed interface through r type channels micromorphological similarity of till found in flutings ploughing marks and interflute areas suggests that the resolution of the micromorphological signature is insufficient to constrain the different processes that led to the formation of those landforms

the determination of glacier speed by time lapse photography under unfavorable conditions two practical problems in the use of time lapse photography for the measurement of speed were encountered during the recent surge of west fork glacier in the central alaska range alaska u s a the first is severe rotational camera instability we show how natural unsurveyed features of the valley wall can be used to make the necessary corrections the second problem is the computation of absolute speed when many different unsurveyed glacier surface features are used as targets

glacial geomorphology and geographic information systems recent developments in the field of glacial geomorphology have dramatically increased the need to acquire maintain manipulate and analyze large amounts of landform landscape and sediment data the use of geographic information systems gis has provided new platforms and tools for analysis and visualization of geomorphic data glacial geomorphologists have used gis to integrate multi source data manage multi scale studies identify previously unrecognized spatial and temporal relationships and patterns in geomorphic data and to link landform data with numerical models as part of model calibration and verification gis based analyses associated with numerical modeling are improving our understanding of glacial landscape evolution and are allowing new quantitative and systematic examinations of spatial and temporal patterns of glacial landforms and processes this has allowed for the development of insights and concepts that would be unlikely to arise using traditional methods alone key recommendations for future research and applications in glacial geomorphology include enhanced gis education and dissemination the development of standards and conventions for glacial geomorphic data community projects to collect data into readily accessible databases and enhanced use of linked gis model frameworks to address major issues in glacial geomorphology