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

glacier sliding at subfreezing temperatures theorie de nye modifiee par l incorporation du modele de gilpin d un film liquide le long des inclusions solides dans la glace effets des temperatures sur la regulation de vitesse applications aux glaciers que l on classe en glaciers a exces de fusion et ceux a deficit de fusion

effective stress profiles and seepage flows beneath glaciers and ice sheets the resistance to sliding and the extent of till deformation beneath soft bedded glaciers depend on the spatially averaged level of effective stress n which is controlled by the distribution of water pressure at the bed major subglacial conduits that facilitate large scale water transport are expected to be predominantly aligned with the direction of maximum hydraulic gradient which is normally parallel to the slope of the glacier surface when the basal heat flow promotes net melting or freezing seepage transport can enable water exchange between these conduits and the rest of the basal surface area for a simple glacier geometry with subglacial conduits that are aligned parallel to a uniform slope the seepage transport is driven primarily by gradients in effective stress balance equations determine how n varies with conduit spacing and the heat flow regime considerations of thermodynamic equilibrium require that ice penetrates the pore space at high effective stress even when the glacier base experiences net melting for a given heat flow regime there are limits on the conduit spacing that can be attained before a finite till layer becomes partially frozen throughout during net freezing the resistance to flow through partially frozen sediments limits the steady state conduit spacing the partially frozen zone can actually be restricted to smaller thicknesses when the freezing rate is greater

seasonal acceleration of inland ice via longitudinal coupling to marginal ice we use an ice flow model to demonstrate how flow variations initiated in the marginal zone of an ice sheet affect flow farther inland through longitudinal along flow coupling our findings allow for an alternate interpretation of seasonal accelerations observed near the equilibrium line of the greenland ice sheet zwally and others 2002 we demonstrate that these observations can be explained by accelerations initiated up to 12 km closer to the margin where the ice is 40 thinner is heavily crevassed experiences a seasonal doubling of velocity and where the ablation rate surface meltwater flux and ice temperature are likely higher our modeling and observations suggest that conditions and processes normally found near ice sheet margins are adequate for explaining the observations of zwally and others 2002 this and considerations of the likely subglacial hydrology in the marginal zone lead us to suggest that seasonal accelerations may have limited impact on ice sheet mass balance even in the face of climate warming

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

on the rheology of till the deformation of subglacial till is instrumental in causing certain glacier surges the motion of rapid ice streams in ice sheets and ice sheet surges which are associated with heinrich events and consequent rapid climatic shifts during the last ice age it may also be the means whereby drumlins are formed and these in turn may act as a brake on large scale ice flow it is therefore important in building models to understand the rate at which till deforms and how this controls the basal ice velocity in recent years two paradigms have emerged on the one hand theoreticians have tended to use a viscous rheology though this lacks quantitative support on the other hand field and laboratory studies suggest that till behaves plastically in this paper i will examine some of the dynamic consequences of this latter assumption and show how the dichotomy between viscous and plastic may be less clearcut than previously thought

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

subglacial hydrology for an ice sheet resting on a deformable aquifer glissements et deformation de la calotte de glace en relation avec le materiau de l aquifere et la densite des chenaux de drainage

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

site formation and chronology of the new paleolithic site sima de las palomas de teba southern spain abstract the newly identified paleolithic site sima de las palomas de teba hosts an almost seven m thick sediment profile investigated here to elucidate the rock shelter s chronostratigraphy and formation processes at its base the sediment sequence contains rich archeological deposits recording intensive occupation by neanderthals luminescence provides a terminus ante quem of 39 4 2 6 ka or 44 9 4 1 ka osl and 51 4 8 4 ka tl this occupation ended with a rockfall event followed by accumulation of archeologically sterile sediments these were covered by sediments containing few middle paleolithic artifacts which either indicate ephemeral occupation by neanderthals or reworking as suggested by micromorphological features above this unit scattered lithic artifacts of undiagnostic character may represent undefined paleolithic occupations sediment burial ages between about 23 0 1 5 ka osl and 40 5 3 4 ka pirir provide an upper paleolithic chronology for sediments deposited above the rockfall finally a dung bearing holocene layer in the uppermost part of the sequence contains a fragment of a human mandible dated to 4032 39 14 c yr bp overall the sequence represents an important new site for studying the end of neanderthal occupation in southern spain

soft bed experiments beneath engabreen norway regelation infiltration basal slip and bed deformation to avoid some of the limitations of studying soft bed processes through boreholes a prism of simulated till 1 8 m 1 6 m 0 45 m with extensive instrumentation was constructed in a trough blasted in the rock bed of engabreen a temperate glacier in norway tunnels there provide access to the bed beneath 213 m of ice pore water pressure was regulated in the prism by pumping water to it during experiments lasting 7 12 days the glacier regelated downward into the prism to depths of 50 80 mm accreting ice infiltrated till at rates predicted by theory during periods of sustained high pore water pressure 70 100 of overburden ice commonly slipped over the prism due to a water layer at the prism surface deformation of the prism was activated when this layer thinned to a sub millimeter thickness shear strain in the till was pervasive and decreased with depth a model of slip by ploughing of ice infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high slip at low effective pressures resulted from water layer thickening that increased non linearly with decreasing effective pressure if sufficiently widespread such slip over soft glacier beds which involves no viscous deformation resistance may instigate abrupt increases in glacier velocity

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

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

glacier changes in the karlik shan eastern tien shan during 1971 72 2001 02 glacier changes in the karlik shan eastern tien shan from 1971 72 to 2001 02 were monitored in this study topographic maps of 1 50000 scale based on aerial photographs from 1971 72 and satellite images landsat tm landsat etm and aster from 1992 2001 and 2002 were used to map glacier extent through a process of manual digitizing the total glacier area decreased by 5 3 from 1971 72 to 2001 02 the rate of glacier area shrinkage was 0 13 a 1 between 1972 and 1992 but it was 0 27 a 1 from 1992 to 2001 02 suggesting accelerated glacier retreat in recent decades glacier changes in the region are a response to summer temperature increase annual precipitation also showed an upward trend but this could not compensate for the mass loss due to ablation

characteristics of subglacial drainage systems deduced from load cell measurements rock tunnels beneath engabreen northern norway permit access to the ice bedrock interface beneath a 210 m thick glacier eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect with some interruptions the load cells have been logged at 15 min intervals since december 1992 here we analyse the records until 2003 load cell signals measure stresses acting normal to the bedrock and usually log the pressure of the thin water film between the basal ice and the bed occasionally there are distinct pressure events characterized by short lived hours local minima often followed by a maximum before decaying to background load cell pressure the amplitudes of these pressure events are of the order of 0 01 1 mpa and depend on the placement of the sensor and the state of the subglacial drainage system we identify winter and summer pressure regimes the winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells a lag of 0 6 days is observed between surface forcing rain and pressure events during winter the summer regime typically has periods of daily pressure events of low amplitude no delay is seen between surface forcing and pressure events during the summer regime in summer the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record whereas no such relation is found during the winter regime the transition from winter to summer may june is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating r channel system the autumn transition is less clearly defined but has usually occurred by the beginning of november stress bridging an increase in bed pressure at the edge of low pressure channels is recorded during the summer regime water pressures at the bed are connected or unconnected to the drainage system pressure increases in the connected system beyond local normal stress values lead to an uplift of the connected system and a pressure drop in the unconnected system the occurrence of pressure events is determined by the capacity of the drainage system uplift is controlled by local normal stress values and not mean ice overburden pressure

subglacial drainage by groundwater channel coupling and the origin of esker systems part 1 glaciological observations this work is presented in two parts part i presents observations on the coupling between subglacial channel flow and groundwater flow in determining subglacial hydraulic regime and creating eskers from an icelandic glacier that is suggested as an analogue for many parts of pleistocene ice sheets part ii develops a theory of perennial subglacial stream flow and the origin of esker systems and models the evolution of the subglacial stream system and associated groundwater flow in a glacier of the type described in part i it is suggested that groundwater flow may be the predominant mechanism whereby meltwater at the glacier bed finds its way to the major subglacial streams that discharge water to glacier margins

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

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