till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

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

on the duration of west antarctic ice sheet grounding events in ross sea during the quaternary abstract a back stepping succession of three seismically defined grounding zone wedges gzws in the glomar challenger basin palaeo ice stream trough is usually assigned to the short time that elapsed since the west antarctic ice sheet retreat began at 11 ka 14c bp recent radiocarbon dates have however suggested an alternate interpretation in which the youngest of these three gzws the gray unit gzw on the middle shelf corresponds to deposition during the last glacial maximum lgm if so then the gray unit must represent an amalgamation of erosion and deposition spanning a much longer time interval i e the 100 ky interval between the last interglacial and the lgm to test these conflicting interpretations the gray unit sediment volume was mapped from seismic and multibeam data two end member durations were calculated because flux during ice sheet retreat is significantly higher than flux during ice sheet advance using the retreat mode flux the 1 47 ky grounding event estimate shows that the middle shelf gzw could have been deposited during the post lgm retreat however the 147 34 ky grounding event estimate based on the advance mode flux also demonstrates that the gzw might reasonably represent an amalgamation of erosion and deposition as grounded ice gradually advanced between mis5e and mis2 i e from the last interglacial to the last glacial maximum these data thus require that both interpretations of how the near surface stratigraphy relates to grounding line translations in the last glacial cycle be considered feasible working hypotheses

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

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

assessing subglacial processes from diatom fragmentation patterns reconstructing the size and glacial style of past ice sheet advances requires interpreting complex glacial sedimentary facies we use diatoms a major component of antarctic continental shelf deposits to infer the physical conditions under which these deposits were emplaced the degree of diatom fragmentation and the presence of diatoms of varying stratigraphic age in glacial sediments provide means to qualitatively gauge glacial mixing and transport here we report an experimentally calibrated index of diatom fragmentation that provides a simple but objective method of assessing the degree of subshearing imparted on marine glacial sedimentary deposits by using a ring shear device to subject diatomaceous sediment to stresses comparable to those beneath the ross ice streams we quantitatively assess patterns of diatom comminution resulting from compaction and from progressive shear stress elongate pennate diatoms are found to break disproportionately to discoid centric diatoms when subjected to shear stress thus a simple ratio of unbroken centric to pennate diatoms provides a reliable gauge of past shearing comparison of ring shear results with a suite of previously analyzed sediments that represent a variety of glacial glacial marine and hemipelagic settings of the ross sea and subglacial ross embayment demonstrates that this index can be employed for estimating relative subglacial stresses in this setting

satellite image derived velocity field of an antarctic ice stream the surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery variations of ice velocity are spatially related to surface undulations and transverse velocity variations of up to 30 percent occur such large variations negate the concept of plug flow and call into question earlier mass balance calculations for this and other ice streams where sparse velocity data were used the coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross correlation of image windows provide significant improvements in the use of satellite imagery for ice flow determination

hydraulic properties of subglacial sediment determined from the mechanical response of water filled boreholes freezing of water filled boreholes drives water into the subglacial bed and the associated pressure effects yield information about subglacial hydraulic properties a numerical model describing the mechanical response of an unconnected borehole and the bed beneath it to this freezing forcing was developed using a nonlinear transient visco elastic ice flow law and an approximate model of top down freezing the resulting system of equations was solved using the method of lines results agreed well with analytic solutions when parameters were correctly chosen forward modelling of pressure records from three 1992 boreholes and three from other years indicated that the till underlying trapridge glacier has a hydraulic conductivity of 1 35 7 0 10 9 m s 1 the model was also used to investigate the response of a borehole to sudden pressure changes the response is very fast compared to pressure sensor sampling rates thus the true basal signal is essentially unaffected by the presence of the borehole except during the initial freeze in

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

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

reply to comment by a w rempel et al on a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we are grateful to rempel et al 2007 hereinafter referred to as rww2007 for giving us this opportunity to discuss in greater detail the fascinating problem of developing quantitative models of subglacial freeze on and basal ice formation we find many of their comments valuable but disagree with their general negative assessment of our previous work on the subject christoffersen et al 2006 hereinafter referred to as ctcb2006 we find it particularly disappointing that rww2007 dismiss our model on the basis of misconstrued model physics rww2007 criticize our assumed treatment of the freezing process but never really give specific examples of how changing these assumptions would improve the ability of our model to realistically reproduce different facies of debris bearing basal ice here we lay out the details of our disagreement with their dismissal of our work 2 despite the claim made by rww2007 there is no sign error in equation 1b of christoffersen and tulaczyk 2003 hereinafter referred to as ct2003 if we switch the sign in this equation as rww2007 imply should be done then we would get pore water pressures higher than ice pressure at the freezing ice water interface this would result in water being driven away from the ice water interface rather than being drawn toward it as the phenomenon of frost heaving requires o neill and miller 1985 their claim that our equation is erroneous is the more surprising since it is straightforward to show that their equation for interfacial pressure rempel et al 2004 hereinafter referred to as rww2004 takes on the same form as equation 1b of ct2003 once the assumption is made that ice saturation is zero in the till underlying a freezing ice base i e no frozen fringe under this assumption the upper limit of integration in equation 3 4 of rww2004 is zl zf and the equation for interfacial pressure piw at a freezing icewater interface can be expanded to a form similar to our equation 1b ct2003

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

sliding with cavity formation we present a model for the determination of a sliding law in the presence of sub glacial cavitation this law determines the basal stress at a clean ice bedrock interf ace in terms of the velocity and effective pressure the method is based on an exact solut ion of the nye kamb linearly viscous sliding problem with cavities and uses ideas of lliboutry 979 to construct via re norma lization methods an approximate law for gene ral bedrock form we show that for a bedrock whose spectrum has a power law behaviour one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity and to a power of the effective pressure the effect of subgl acial cavitation on the drainage system is exam ined using recent ideas of kamb for sufficiently high velocities drainage through a r6thlisberger tunnel system is unstable and drai nage takes place through the linked system of cavities this leads to a reduction of the effective pressure and by taking account of this one can rewrite the sliding law in terms of st ress and velocity only this sliding law can be multi v alued and it is suggested that this unde rlies the dynamic phenomenon of surges

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

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

coupling between a glacier and a soft bed ii model results the relation between the local effective pressure and shear stress on till beneath storglaciaren sweden discussed in iverson and others 1999 provides an empirical basis for studying the processes that control the strength of the ice bed coupling particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms model calculations based on studies of cone penetration through fine grained sediment and sliding theory agree with the observed relation between shear stress and effective pressure if the water layer at the ice bed interface is assumed to thicken rapidly as the effective pressure approaches zero studies of the hydraulics of linked cavities provide support for this assumption if the mean thickness of the water layer reflects the extent of microcavity development at the interface comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath storglaciaren only at intermediate effective pressures at very low effective pressures like those inferred at the site of the tiltmeter discussed in iverson and others 1999 and at sufficiently high effective pressures ploughing and sliding should focus motion near the glacier sole a calculation using parameter values appropriate for ice stream b west antarctica suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth this conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down glacier from ploughing particles thereby weakening the bed near the glacier sole however given the apparent sensitivity of the ice bed coupling to basal conditions that may be highly variable any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism

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

waves of accelerated motion in a glacier approaching surge the mini surges of variegated glacier alaska u s a periods of dramatically accelerated motion in which the flow velocity increases suddenly from about 55 cm d to a peak of 100 300 cm d and then decreases gradually over the course of a day occurred repeatedly during june and july 1978 81 in variegated glacier alaska a surging type glacier that surged in 1982 83 these mini surges appear to be related mechanistically to the main surge the flow velocity peak propagates down glacier as a wave at a speed of about 0 3 km h over a reach of about 6 km in length it is accompanied by a propagating pressure wave in the basal water system of the glacier in which after a preliminary drop the pressure rises rapidly to a level greater than the ice overburden pressure at the glacier bed and then drops gradually over a period of 1 2 d usually reaching a new low for the summer the peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing it is also accompanied by a rapid uplift of the glacier surface amounting to 6 11 cm which then relaxes over a period of 1 2 d maximum uplift rate coincides with the peak in flow velocity the peak in accumulated uplift lags behind the velocity peak by 2 h the uplift is mainly due to basal cavitation driven by the high basal water pressure although the strain wave associated with the mini surge motion can also contribute basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal outflow stream in association with each mini surge in the down glacier reach where the mini surge waves are attenuating the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity but in the reach of maximum mini surge motion the strain wave has a form quite different possibly related to special features in the mini surge initiation process from that point up stream the flow acceleration in the mini surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction a preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives the mini surge wave propagation is controlled by the propagation of the water pressure wave in the basal water conduit system the propagation characteristics result from a longitudinal gradient up glacier increase in hydraulic conductivity of the basal water system in response to the up glacier increase of the basal water pressure in the mini surge wave the mini surge waves are initiated in a succession of areas situated generally progressively up glacier during the course of the summer season in these areas presumably melt water that has accumulated in subglacial reservoirs is released suddenly into the basal water system immediately below generating a pressure rise that propagates down stream from there relationships of the mini surges to the main surge are seen in the role of high basal water pressure in causing the rapid glacier motion in both phenomena in the pulse propagation features of both and in the high outflow turbidity associated with both the mini surges of variegated glacier have a strong resemblance to movement and uplift events observed in unteraargletscher and findelengletscher switzerland this bears on the question whether the mini surges are a particular characteristic of surge type glaciers prior to surge

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

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

deformation of the batestown till of the lake michigan lobe laurentide ice sheet deep pervasive shear deformation of the bed to high strains 100 may have been primarily responsible for flow and sediment transport of the lake michigan lobe of the laurentide ice sheet to test this hypothesis we sampled at 0 2 m increments a basal till from one advance of the lobe batestown till along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand grain preferred orientation unlike past fabric studies interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains fabric strengths indicate that more than half of the till sampled has a 5 probability of having been sheared to moderate strains 7 30 secular changes in fabric azimuth over the thickness of the till probably due to changing ice flow direction as the lobe receded indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters orientations of principal magnetic susceptibilities show that the state of strain was commonly complex deviating from bed parallel simple shear deformation is inferred to have been focused in shallow temporally variable patches during till deposition from ice

the experimental basis for interpreting particle and magnetic fabrics of sheared till particle fabrics of basal tills may allow testing of the bed deformation model of glacier flow which requires high bed shear strains 100 field studies however have not yielded a systematic relationship between shear strain magnitude and fabric development to isolate this relationship four basal tills and viscous putty were sheared in a ring shear device to strains as high as 714 fabric was characterized within a zone of shear deformation using the long axis orientations of fine gravel and sand particles and the anisotropy of magnetic susceptibility ams of small 5 8 cm3 intact samples results indicate that till particles rotate toward the plane of shearing with long axis orientations that become tightly clustered in the direction of shear 0 78 s1 0 94 for three dimensional data these strong steady state fabrics are attained at shear strains of 7 30 with no evidence of fabric weakening with further strain regardless of the specific till or particle size fraction under consideration these results do not support the jeffery model of particle rotation which correctly describes particle rotation in the viscous putty but not in the tills owing to fluid mechanical assumptions of the model that are violated in till the sensitivity of fabric development to shear strain magnitude indicates that for most till units where shear strain magnitude is poorly known attributing fabric variations to spatial differences in other variables such as till thickness or water content will be inherently speculative attributing fabric characteristics to particular basal till facies is uncertain because shear strain magnitude is unlikely to be closely correlated to till facies weak or spatially variable fabrics in the absence of post depositional disturbance or major deviations from unidirectional simple shear indicate that till has not been pervasively sheared to the high strains required by the bed deformation model strong flow parallel fabrics are a necessary but insufficient criterion for confirming the model copyright 2008 john wiley sons ltd

development of the topography controlled upper odra ice lobe scandinavian ice sheet in the fore mountain area of southern poland during the saalian glaciation abstract during the saalian drenthe glaciation the upper odra ice lobe constituted the most southern part of the scandinavian ice sheet in central europe the lobe was situated in an area with varied topography in the foreland of a mountain range the ice lobe had a complex form because it was subdivided into several smaller sublobes the primary factors that controlled the lobe were investigated it was found that the ice lobe development depended largely on the topography but the lithology of the ice sheet substratum was also highly important the topography controlled the main movement direction of the ice and caused a concentration of the ice flow in the axial part of the silesian lowland between subglacial morphological barriers which thereby formed a corridor of relatively fast flowing ice which was probably streaming and which supplied the lobe the topography also determined the ice flow within the lobe which strongly matched the valley system the presence of sediments with relatively low permeability at the base of the lobe the quaternary deposits are underlain by neogene clays caused high pore water pressure in the subglacial zone and triggered the basal movement of the ice sheet which promoted a rapid flow of the ice and favoured the development of the lobe the basal tills deposited from the active and passive ice and extant till patches without a fluvioglacial cover atop some parts of the studied area indicate spatially varying ice flow dynamics especially in the later stage of glaciation certain parts of the lobe underwent stagnation while in other parts the ice was probably still active the varied behaviour of the ice within the lobe was determined primarily by the topography and lithology of the substratum which produced spatially varied marginal compression and basal friction a model of the development of the lobe is proposed

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

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

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

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

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

wissard at subglacial lake whillans west antarctica scientific operations and initial observations a clean hot water drill was used to gain access to subglacial lake whillans slw in late january 2013 as part of the whillans ice stream subglacial access research drilling wissard project over 3 days we deployed an array of scientific tools through the slw borehole a downhole camera a conductivity temperature depth ctd probe a niskin water sampler an in situ filtration unit three different sediment corers a geothermal probe and a geophysical sensor string our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics subglacial water is about two orders of magnitude less saline than sea water 0 37 0 41 psu vs 35 psu and two orders of magnitude more saline than pure drill meltwater 0 002 psu it reaches a minimum temperature of 0 558c consistent with depression of the freezing point by 7 019 mpa of water pressure subglacial water was turbid and remained turbid following filtration through 0 45 mm filters the recovered sediment cores which sampled down to 0 8 m below the lake bottom contained a macroscopically structureless diamicton with shear strength between 2 and 6 kpa our main operational recommendation for future subglacial access through water filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing

ice bed coupling beneath and beyond ice streams byrd glacier antarctica 1 ice sheet thickness is determined mainly by the strength of ice bed coupling that controls holistic transitions from slow sheet flow to fast streamflow to buttressing shelf flow byrd glacier has the largest ice drainage system in antarctica and is the fastest ice stream entering ross ice shelf in 2004 two large subglacial lakes at the head of byrd glacier suddenly drained and increased the terminal ice velocity of byrd glacier from 820 m yr 1 to 900 m yr 1 this resulted in partial ice bed recoupling above the lakes and partial decoupling along byrd glacier an attempt to quantify this behavior is made using flowband and flowline models in which the controlling variable for ice height above the bed is the floating fraction of ice along the flowband and flowline changes in before and after drainage are obtained from available data but more reliable data in the map plane are required before byrd glacier can be modeled adequately a holistic sliding velocity is derived that depends on with contributions from ice shearing over coupled beds and ice stretching over uncoupled beds as is done in state of the art sliding theories

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

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

water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica geophysical studies and glaciological analyses suggest strongly that ice stream b west antarctica moves primarily by pervasive deformation of a meters thick subglacial till analysis of the longitudinal profile of the ice stream up stream of the ice plain suggests that basal sliding is slow everywhere that effective pressure decreases slowly down stream and that the strain rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure discrete shearing may occur beneath the pervasively deforming zone these and other hypotheses which build on the analyses of the first two papers in this series can be tested in the field

anisotropy and crystalline fabric of whillans ice stream west antarctica inferred from multicomponent seismic data crystal orientation fabric cof describes the intrinsic anisotropic nature of ice and is an important parameter for modeling glacier flow we present the results of three component active source seismic observations from the whillans ice stream wis a fast flowing ice stream in west antarctica surface wave dispersion analysis ray tracing and traveltime inversion of compressional p and shear s waves reveal the presence of transversely isotropic ice with a vertical axis of symmetry vti beneath approximately 65 m of isotropic firn the ice stream is characterized by weak anisotropy involving an average ice thickness of approximately 780 m the analysis indicates that about 95 of the ice mass is anisotropic and the crystalline c axes span within an average broad cone angle of 73 10 with respect to the vertical axis moreover the mean temperature t below the firn estimated from seismic data is 15 5 c these data do not show evidence of englacial seismic reflectivity which indicates the lack of abrupt changes in the cof the presence of azimuthal anisotropy due to transversely compressive flow or fractures aligned along a preferential direction is also excluded we suggest that the observed vti ice structure is typical of large ice streams in regions where basal sliding and bed deformation dominate over internal glacial deformation

estimation of subglacial hydraulic properties from induced changes in basal water pressure a theoretical framework for borehole response tests in this paper we develop a theoretical model describing water motion in a coupled borehole subglacial flow system the theory applies to basal drainage systems having multiple and extensive interconnected flow paths within this domain it encompasses a broad range of flow regimes from laminar darcian flow in a thick permeable unit to turbulent sheet flow in a very thin layer important terms in the model are highlighted by recasting the problem in dimensionless form the non dimensional formulation indicates that there are four free parameters in the coupled system these parameters characterize skin friction in the borehole and diffusion transmissivity and turbulent transport in the subglacial flow layer dimensionless results show that under most circumstances the effects of skin friction in the borehole are negligible diffusion transmissivity and especially turbulent transport in the basal layer are found to influence subglacial water flow strongly we use our model to predict fluctuations of borehole water levels that result from different types of disturbances we show how this framework can be used to estimate subglacial hydraulic properties by comparing model results with data collected during field experiments on trapridge glacier yukon territory canada in 1989 and 1990

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

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

sediment entrainment by a soft bedded glacier a model based on regelation into the bed experimental results indicate that wet based soft bedded glaciers may penetrate their substrates by regelation melting and refreezing and thereby entrain sediment in principle there should be a steady depth of penetration at which the downward regelation speed driven by the interfacial effective pressure equals the basal melt rate herein the magnitude and distribution of penetration are estimated for the case of a glacier resting on a deformable bed with channelized basal drainage the distance between channels and the distribution of effective pressure across the bed are calculated and special attention is paid to the interdependence of the basal melt rate and effective pressure a major uncertainty is whether fine grained tills may impede or prevent regelation as a result of high surface tension at the ice water interface predicted penetration depths range from millimetres to a few decimetres and thus dirty basal ice layers of such thicknesses might be expected predicted distances between channels agree well with those indicated by borehole measurements at ice stream b west antarctica effective pressures and hence penetration depths increase toward channels therefore the edges of interfluves bounded by anastomosing subglacial channels should be eroded preferentially this motivates the testable hypothesis that such erosion contributes to the formation of streamlined landforms such as drumlins copyright 2000 john wiley sons ltd

impact of floods versus routing events on the thermohaline circulation 1 the last deglaciation was interrupted by three major cooling events the younger dryas the preboreal oscillation and the 8 2 ka cold event as the laurentide ice sheet retreated different outlets from lake agassiz became available causing the proglacial lake to drain to a new level in a rather short timescale flood and divert continental runoff over hundreds of years routing here we use an earth system model to investigate the impact of floods short term high amplitude versus routing events long term low amplitude on the thermohaline circulation and the resulting climate change we show that the initial outbursts of freshwater are not strong enough to influence large scale climate and can therefore not be the cause of millennial scale climate variability however routing events have the potential of weakening the thermohaline circulation and therefore causing a cool event in the northern hemisphere

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

heterogeneous seawater phosphorus concentrations during the sturtian glaciation evidence from p fe ratios of fulu formation ironstone in south china abstract phosphorus p is one of the key bio limiting elements in the modern ocean and controls marine primary productivity over the geological time scales it is proposed that the ediacaran atmosphere oxygenation and animal evolution might have been triggered by high seawater p concentration in cryogenian oceans seawater p concentration can be reconstructed by the ratio of phosphorus to iron expressed as p fe 100 in ironstones here we measured the p fe ratios of the banded iron formation bif from the cryogenian fulu formation in south china the p fe 100 ratios of the fulu bif vary between 0 08 and 1 81 mean 0 45 n 22 considerably lower than the values of coeval bif deposited in other continents mean 1 24 n 74 variation of p fe ratio of ironstones may suggest a large seawater p concentration gradient during the sturtian glaciation which is comparable to or even higher than that in the modern ocean 1 2 m in north atlantic and 2 6 m in north pacific though being explained in different ways the heterogeneous seawater p concentration requires active organic matter production in the surface ocean and or decomposition in the ocean interior as well as possible onset of thermohaline circulation during the sturtian glaciation finally our study suggests that precipitation of bif in the completely ice covered ocean was unlikely

depth and time dependent vertical strain rates at siple dome antarctica as part of a project to investigate the flow of ice at low effective stress two independent strain gauge systems were used to measure vertical strain rate as a function of depth and time at siple dome antarctica the measurements were made from january 1998 until january 2002 at the ice divide and a site 7 km to the northeast on the flank the strain rate profiles place constraints on the rheology of ice at low stress show the expected differences between divide and flank flow with some structure due to firn compaction and probably ice stratigraphy and suggest that the flow of the ice sheet has not changed much in the last 8 6 kyr the strain rates show an unexpected time dependence on scales ranging from several months to hours including discrete summer events at the divide time dependence in strain rate water pressure seismicity velocity and possibly basal motion has been seen previously on the siple coast ice streams but it is especially surprising on siple dome because the bed is cold

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

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

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock