flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought

flow dynamics of the rutford ice stream ice drainage basin west antarctica from radar stratigraphy rutford ice stream west antarctica drains a catchment of 45 000 km 2 into the ronne ice shelf through a 26 km wide 2 4 km deep subglacial trough adjacent to the ellsworth mountains forty two per cent of its catchment boundary is common with pine island glacier where rapid change to ice dynamics is currently underway these changes may eventually affect adjacent catchments such as rutford radar sounding data were acquired over the rutford ice drainage basin that show the internal structure in particular distinctive reflector groups were identified that mark the boundaries between four different flow elements the flow margin reflector groups include curvilinear events that cross cut isochrones and are therefore likely to be post depositional these reflections may arise from crystal orientation fabrics generated by localized strain in a flow margin one of the sectors of the ice drainage basin supplies the largest share 38 of the ice volume flux through the main trunk of rutford ice stream this sector may be preferentially affected by continuing surface lowering in the pine island glacier catchment

influence of subglacial conditions on ice stream dynamics seismic and potential field data from pine island glacier west antarctica 1 we interpret seismic reflection and airborne potential field data acquired on pine island glacier west antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics immediately beneath the glacier is a mixture of soft deforming sediments and harder non deforming sediments beneath this a sedimentary basin lies under part of the main glacier with another under one of its slower moving tributaries a tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south which also include a thick rift related magmatic intrusion the boundary correlates with changes in the basal roughness ice flow speed and basal drag smoother bed faster flow and lower basal drag characterize the thicker sedimentary sequences to the north but there is no corresponding lateral change in the acoustic properties of the bed changes in the sub bed i e deeper than the ice bed interface lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary leading to increased basal drag and reduced ice flow in the future we conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future

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

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

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

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

traveltime inversion of offset vertical seismic profiles a feasibility study traveltimes from an offset vertical seismic profile vsp are used to estimate subsurface two dimensional dip by applying an iterative least squares inverse method tests on synthetic data show that inversion techniques are capable of estimating dips in the vicinity of a wellbore by using the traveltimes of the direct arrivals and the primary reflections the inversion method involves a layer stripping approach in which the dips of the shallow layers are estimated before proceeding to estimate deeper dips examples demonstrate that the primary reflections become essential whenever the ratio of source offset to layer depth becomes small traveltime inversion also requires careful estimation of layer velocities and proper statics corrections aside from these difficulties and the ubiquitous nonuniqueness problem the vsp traveltime inversion was able to produce a valid earth model for tests on a real data case modified journal abstract

glaciological studies on rutford ice stream antarctica rutford ice stream drains part of the west antarctic ice sheet into ronne ice shelf much of the drainage basin has a bed well below sea level and is therefore an area which could undergo substantial change if a climatic warming were to cause sustained thinning of ronne ice shelf snow accumulation data suggest that an accumulation rate of 0 51 0 12 mg m 2 yr 1 is required for balance an extensive ice movement survey network was established across the grounding line in 1978 1980 and was extended for more than 100 km upstream in 1984 1986 velocities at five sites have been calculated using doppler satellite position measurements when the velocities are combined with cross sectional areas of the ice stream measured by radio echo sounding mass flux figures show that drag at the sidewalls and on the base must decrease downstream toward the grounding line the increasing influence of buoyancy forces on ice stream motion is also shown by the steady rise in the ratio of bedrock depth to ice thickness going downstream along the network below the grounding line that was found to cross the middle part of the earlier network is a zone stretching for a further 100 km where it can now be shown that the ice stream is intermittently grounded in these grounded areas the glacier bed is raised with respect to the surrounding seabed and forms eskerlike features surface elevations over the grounded areas are less than 30 m above those needed for hydrostatic equilibrium

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

expansion and retreat of the jura ice sheet france during the last glacial maximum in continental environments while there is no uninterrupted record of glaciation glacier activity can be reconstructed from the nature and geometry of sediments trapped by lake filling examination of the combe d ain lake filling provides insight into the history of the jura mountains during the last glacial maximum lgm from the evidence of outcrops borehole cores and seismic reflection profiles of the lake filling we show that glaciation was marked by three stages of stabilization then recessional moraines marked five episodes of stabilization in the course of glacial retreat comparison with b 18 o data from ice cores of greenland makes possible to date the last glacial maximum at 25 500 22 000 cal bc and

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

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

bedform topography and basal conditions beneath a fast flowing west antarctic ice stream a grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath similar to 140 km 2 of rutford ice stream west antarctica the subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water saturated sediments the two troughs are filled with deforming sediment whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream the deforming bed is very flat along flow but undulates across flow sliding areas show rougher bed topography cross stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction these bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation in places they form finger like mounds of material which extend into the sliding region further downstream mean bedform height is 22 m mean width is 267 m and many of them extend for at least 1 2 km along flow we interpret most of these bedforms as drumlins and one as a mega scale glacial lineation the juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas variable both spatially and temporally any theory of subglacial sediment rheology must also be able to account for our conclusion that at any given time pervasive deformation extends at least a few metres into the bed and can persist over a considerable area many km 2 bedform geometry and basal conditions concur with interpretations of former ice streams with evidence for increasing elongation ratio with distance downstream however those studies also identified bedrock cropping out at the ice bed interface for which there is no evidence on rutford ice stream c 2008 elsevier ltd all rights reserved