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

decadal scale variations in ice flow along whillans ice stream and its tributaries west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

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

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

a variational approach to ice stream flow ice sheets are susceptible to the formation of ice streams or narrow bands of fast flowing ice whose high velocities are caused by rapid sliding at the contact between ice and the underlying bed based on recent geophysical work which has shown that the sliding motion of ice streams may be described by a coulomb friction law we investigate how the location of ice streams depends on the geometry of an ice sheet and on the mechanical properties of the underlying bed more generally this problem is relevant to the flow of thin films with coulomb or solid friction laws applied at their base by analogy with friction problems in elasticity we construct a variational formulation for the free boundary between ice streams where bed failure occurs and the surrounding ice ridges where there is little or no sliding this variational problem takes the form of a non coercive variational inequality and we show that solutions exist provided a force and moment balance condition is satisfied in that case solutions are also unique except under certain specialized circumstances which are unlikely to arise for a real ice sheet further we show how the variational formulation of the ice flow problem can be exploited to calculate numerical solutions and to simulate the effect of changing ice geometry and bed friction on the location and velocities of streaming flow lastly we study the effect of ice shelf buttressing on the flow of ice streams whose spatial extent is determined by the yield stress distribution of the bed in line with previous studies of ice shelf buttressing we find that the removal of an ice shelf can cause an ice stream feeding the ice shelf to speed up considerably which underlines the important role ice shelves may play in controlling the dynamics of marine ice sheets

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

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

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

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

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

ongoing margin migration of ice stream b antarctica the rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone the method relies on the assumption that the velocity profile is a characteristic feature of the margin and that this profile is shifted laterally as the margin migrates application of the method to ice stream b antarctica indicates that the southern margin is moving outward into the inland ice at a rate ofat least 9 7 1 1 m a 1

hydraulics of subglacial cavities modele theorique du comportement a l etat stationnaire des cavites remplies d eau et interconnectees sur le lit du glacier

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

significant groundwater contribution to antarctic ice streams hydrologic budget satellite observations have revealed active hydrologic systems beneath antarctic ice streams but sources and sinks of water within these systems are uncertain here we use numerical simulations of ice streams to estimate the generation flux and budget of water beneath five ice streams on the siple coast we estimate that 47 of the total hydrologic input 0 98 km3 yr 1 to whillans wis mercer mis and kamb kis ice streams comes from the ice sheet interior and that only 8 forms by local basal melting the remaining 45 comes from a groundwater reservoir an overlooked source in which depletion significantly exceeds recharge of the total input to bindschadler bis and macayeal macis ice streams 0 56 km3 yr 1 72 comes from the interior 19 from groundwater and 9 from local melting this contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

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

energy balance of ice streams analysis of the cross flow transmission of force from the central parts of a well lubricated ice stream to its margins shows that there is a corresponding shift in the lateral location of motion induced heat generation the rate of basal heat generation in the center can be substantially smaller than the local rate of potential energy loss given by driving stress times the speed of downslope motion the basal heating is a maximum for an intermediate level of lubrication for which speed is about 40 of the speed over a frictionless bed and base stress is about 25 of the driving stress stable and unstable balances between meltwater production and drainage on the bed are identified a stable steady state with a speed less more than that giving maximum heat generation is termed drainage production limited since an increase in speed would lead to increased decreased basal melting and must need not be balanced by increased drainage it is shown that gradual evolution of the basal water drainage system and the factors affecting basal melting can cause discontinuous jumps between fast and slow moving states a simplified analysis applied to six cross sections of west antarctic ice streams b d e and rutford ice stream shows them to be diverse in the level of support from the sides and corresponding shift of mechanical heating sideward from their central parts the cross sections of ice stream b near upstream b may be production limited because of especially high lubrication and related support from the sides cross sections in the upper part of ice stream d ice stream e and rutford ice stream are in a drainage limited condition substantial reduction of basal heat generation by side drag in most cases and expected high heat flow into the basal ice associated with low thickness in some cases tends to favor basal freezing nevertheless all of the examined cross sections except one are expected to experience basal melting with a modest geothermal heat flux density of 60 mw m or less in some cases the lower part of ice stream b is an exception where the analysis indicates that geothermal flux density must exceed 80 100 mw m to maintain melting if this high geothermal flux is not present then the base of the lower part of ice stream b may be freezing which would suggest continued deceleration of this part of ice stream b

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

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

numerical investigations of the slow down of whillans ice stream west antarctica is it shutting down like ice stream c we investigate whether the recent slow down of whillans ice stream wis west antarctica may lead to its complete stoppage in the near future using a numerical model basal resistance to ice stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing we implement a basal drainage system which acts to hinder ice stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing the ice module is a standard flowline model with parameterized key out of plane effects raymond 1996 the most important result of our modeling effort is that we never obtained a slow down that would not be followed by a complete stoppage within 5100years wis slow down canbe avoided in the model if large basal water input rates are assumed to satisfy basal freezing in com parison thetestedperturbationsinwiswidthandmassbalancehada relativelysmalleffect onthetendencyof the simulated ice streamto slowdown these resultsunderscorethe need for morequantitativeconstraints ontheefficiencyof sub ice streamwaterdrainage we con jecture that the present day slow down of wis will evolve to shut down in the next few decades unless anadditionof basalwaterprevents freeze on drivenbed strengthening

sequential stagnation of kamb ice stream west antarctica 1 ice penetrating radar data confirm the presence of two relict northern margins of kamb ice stream kis west antarctica in an area called the duckfoot between the two relict margins deep layers are preserved and buried crevasses remain at a constant depth until roughly 1 5 km from the innermost youngest margin suggesting that this outer portion of the ice stream stagnated suddenly 10 years stagnation was likely accomplished through changes in basal conditions i e reduced lubrication beneath this outer portion of the ice stream and resulted in a narrowing of the ice stream trunk by 27 and a reduction in flow speed 200 years prior to complete stagnation flowstripe deformation as a result of duckfoot stagnation provides an estimate of kis paleo velocity of 210 m a after duckfoot stagnation

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

observed velocity fluctuations on a major antarctic ice stream most of the antarctic ice sheet is drained by systems comprising an area of slow sheet flow which converges into a much faster valley glacier or ice stream on reaching the coast this ice then either enters an ice shelf confined by embayments and ice rises or calves off into the ocean the higher speed of ice streams enables them to respond to changes in their environment faster than the slower ice sheet in the west antarctic where the ice sheet moves across bedrock well below sea level the ice streams have the potential to rapidly increase or decrease the ice discharge into the sea1 the timescale for these changes has been placed at a few hundred years if this is true then changes should be detectable by careful measurements on a decadal timescale by comparing recent ice velocity measurements with those collected ten years ago we establish that the ice in the mouth of ice stream b has decelerated by 20 we discuss the possible causes of this deceleration on the basis of our knowledge of the current regional dynamics and the possible ramifications of this deceleration on future ice stream behaviour

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

on the impact of topography and building mask on time varying gravity due to local hydrology we use 3 yr of surface absolute gravity measurements at three sites on the larzac plateau france to quantify the changes induced by topography and the building on gravity time series with respect to an idealized infinite slab approximation indeed local topography and buildings housing ground based gravity measurement have an effect on the distribution of water storage changes therefore affecting the associated gravity signal we first calculate the effects of surrounding topography and building dimensions on the gravity attraction for a uniform layer of water we show that a gravimetric interpretation of water storage change using an infinite slab the so called bouguer approximation is generally not suitable we propose to split the time varying gravity signal in two parts 1 a surface component including topographic and building effects 2 a deep component associated to underground water transfer a reservoir modelling scheme is herein presented to remove the local site effects and to invert for the effective hydrological properties of the unsaturated zone we show that effective time constants associated to water transfer vary greatly from site to site we propose that our modelling scheme can be used to correct for the local site effects on gravity at any site presenting a departure from a flat topography depending on sites the corrected signal can exceed measured values by 5 15 gal corresponding to 120 380 mm of water using the bouguer slab formula our approach only requires the knowledge of daily precipitation corrected for evapotranspiration therefore it can be a useful tool to correct any kind of gravimetric time series data

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

the gulf stream rainband a recurrent and almost stationary region of banded clouds and precipitation has been identified over the gulf stream off cape hatteras north carolina in winter this rainband is attributed to the large fluxes of heat and moisture from the warm waters of the gulf stream to the colder overlying air it is speculated that the gulf stream rainband could play a role in cyclogenesis which is common off the carolina coast

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

basal melt rates beneath whillans ice stream west antarctica basal water lubricates and enables the fast flow of the west antarctic ice streams which exist under low gravitational driving stress identification of sources and rates of basal meltwater production can provide insight into the dynamics of ice streams and the subglacial hydrology which remain insufficiently described by glaciological theory combining measurements and analytic modeling we identify two regions where basal meltwater is produced beneath whillans ice stream west antarctica downstream of the onset of shear crevasses strong basal melt 20 50 mm a 1 i s concentrated beneath the relatively narrow shear margins farther upstream melt rates are consistently 3 7 mm a 1 across the width of the ice stream we show that the transition in melt rate patterns is coincident with the onset of shear margin crevassing and streaming flow and related to the development of significant lateral shear resistance which reorganizes the resistive stress regime and induces a concentration of basal resistance adjacent to the shear margin finally we discuss how downstream freeze on in the ice stream center coupled with melt beneath the shear margin might result in a slowing but widening ice stream

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