self organization in the thermomechanical flow of ice sheets this paper uses a three dimensional thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets ice streams are distinguished within general ice sheet flow by their relatively fast of the order of kilometers per year velocity and are thought to be crucial in determining the response of ice sheets to climatic change we show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature inhomogeneities in topography and or geology are not therefore necessary conditions for ice stream development this spatial patterning replaces the large scale surging reported from previous modeling work in one horizontal dimension we suggest that surging is restricted to situations where ice flow is channelized e g valley glaciers and that it is replaced by streaming where horizontal ice flow is unconstrained ice sheets the ice streams in some of our experiments show switching behavior reminiscent of that known from the siple coast ice streams west antarctica the cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams this may lead to the gradual reduction of a stream s discharge below the threshold necessary to generate enough frictional heat to maintain warm viscous ice at the ice stream bed in this case the stream will stagnate abruptly which then prompts localized ice thickening and the initiation of a new stream in a different location

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

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 1 theory we employ a continuum mixture framework to incorporate ice streams in a three dimensional thermomechanical model of the laurentide ice sheet the ice mass is composed of a binary mixture of sheet ice which deforms by viscous creep and stream ice which flows by sliding and or sediment deformation at the bed dynamic and thermal evolutions are solved for each component in the mixture with coupling rules to govern transfer between flow regimes we describe two different transfer mechanisms 1 creep exchange the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet and 2 bed exchange the activation growth and deactivation of ice streams perpetrated by transfers of bed area between flow constituents this paper develops the underlying mixture theory we express the governing equations for mass momentum and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics a companion paper in this issue explores mixture and ice stream behavior in applications with the laurentide ice sheet

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

permanent fast flow versus cyclic surge behaviour numerical simulations of the austfonna ice cap svalbard a large part of the ice flux within ice caps occurs through spatially limited fast flowing units some of them permanently maintain fast flow whereas others operate in an oscillatory mode characterized by short lived active phases followed by long quiescent phases this surge type behaviour results from intrinsic rather than external factors thus complicating estimates of glacier response to climate change here we present numerical model results from austfonna an ice cap on svalbard that comprises several surge type basins previous studies have suggested a thermally controlled soft bed surge mechanism for svalbard we systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow surge type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure melting point and enhanced sliding of marine grounded ice irrespective of the dynamic regime the absence of considerable volumes of temperate ice both in the observed and simulated ice cap indicates that fast flow is accomplished by basal motion over a temperate bed given an idealized present day climate the equilibrium ice cap size varies significantly depending on the chosen parameters

new interpretations of the facies of the rhenish brown coal of west germany abstract new ideas concerning the understanding of the facies of the rhenish brown coal of west germany are discussed these new interpretations are based on a significantly larger number of samples and refined procedures for analysis within the field of paleobotany palynology coal petrology and organic geochemistry than were available to p w thomson and m teichmuller the light and dark bands in the coal are mainly the result of different degrees of plant decomposition the influence of the peat forming plant communities plays a subordinate role in the petrographical composition of these particular strata of the rhenish brown coal

a multicomponent coupled model of glacier hydrology 1 theory and synthetic examples 1 basal hydrology is acknowledged as a fundamental control on glacier dynamics especially in cases where surface meltwater reaches the bed for many glaciers at midlatitudes basal drainage is influenced by subaerial englacial and subsurface water flow one of the major shortcomings of existing basal hydrology models is the treatment of the glacier bed as an isolated system we present theoretical and computational models that couple glacier surface runoff englacial water storage and transport subglacial drainage and subsurface groundwater flow each of the four model components is represented as a two dimensional vertically integrated layer that communicates with its neighbors through water exchange governing equations are derived from the law of mass conservation and are expressed as a balance between the internal distribution of water and external sources the numerical exposition of this theory is a time dependent finite difference model that can be used to simulate glacier drainage in this paper we outline the theory and conduct simple tests using an idealized glacier geometry in the companion paper the model is tailored to trapridge glacier yukon territory canada where results are compared with measurements of subglacial water pressure

glacier surge propagation by thermal evolution at the bed bakaninbreen southern svalbard began a prolonged surge during 1985 in 1986 an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm unfrozen and cold frozen bed ground penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection interpreted as the top of a thick layer of sediment rich basal ice down glacier of the present surge front features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer the base of a permafrost layer and local ice lenses this indicates that this region of the bed is cold up glacier of the surge front a scattering zone above the basal reflection is interpreted as warm ice there is no evidence for this warm zone down glacier of the surge front nor do we see basal permafrost up glacier of it thus as in early surge phase the location of the surge front is now at the transition between warm and cold ice at the glacier bed we suggest that the propagation of the front is associated with this basal thermal transition throughout the surge because propagation of the front occurs rapidly and generates only limited heat basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice bed interface

mathematical modeling and numerical simulation of polythermal glaciers 1 a mathematical model for polythermal glaciers and ice sheets is presented the enthalpy balance equation is solved in cold and temperate ice together using an enthalpy gradient method to obtain a relationship between enthalpy temperature and water content we apply a brine pocket parameterization scheme known from sea ice modeling the proposed enthalpy formulation offers two advantages 1 the discontinuity at the cold temperate transition surface is avoided and 2 no treatment of the transition as an internal free boundary is required fourier s law and fick type diffusion are assumed for sensible heat flux in cold ice and latent heat flux in temperate ice respectively the method is tested on storglaciaren northern sweden numerical simulations are carried out with a commercial finite element code a sensitivity study reveals a wide range of applicability and defines the limits of the method realistic temperature and moisture fields are obtained over a large range of parameters

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

mechanisms for rainfall concurrent lava dome collapses at soufriere hills volcano 2000 2002 the evolution of rainfall concurrent dome collapses at soufriere hills volcano is followed using a limit equilibrium model for rain infiltration into a hot lava carapace magma infusing into the dome both supplies heat and builds the slopes the dome rocks are cooled by episodic rain infiltration and climatic cooling rainfall infiltrates fractures that develop in the hot dome carapace occludes the void space and staunches effusive gas flow gases may originate from juvenile de gassing of the dome interior or result from the vaporization of infiltrating water gas pressures build in cracks blocked off by rain and may destabilize the dome the effects of dome growth heating by magma infusion and cooling by rain infiltration and climatic influences are combined to follow the growth of the dome towards ultimate collapse for a fixed suite of strength and transport parameters and for measured magma influx rates the evolution of instability may be followed the evolving factor of safety tracks the observed march 2000 and july 2001 rainfall concurrent collapse events which evolve over months however the resolution of the hindcast is unable to discriminate between the effects of closely timed rainfall events order of hours the heightening of the dome is shown to exert the principal influence on average slopes and in the evolution of instability collapse removes the over heightened dome and temporarily restores stability

the surges of variegated glacier alaska u s a and their connection to climate and mass balance one of the questions still unanswered concerning the surge behavior of glaciers concerns their quasi periodic occurrence some results on the phenomenological connection between local cumulative balance and surge initiation of variegated glacier alaska u s a are discussed here based on climate data from neighboring weather stations an empirical relation between precipitation temperature and local mass balance is established and used to reconstruct the annual balance at a location in the accumulation area back to 1905 between the last four surges in 1946 47 1964 65 1982 83 and 1994 95 the ice equivalent cumulative balance was 43 5 m on average with a 1 error of 1 2 m although the existence of a surge level cannot be directly interpreted in physical terms it explains the variable length of the quiescent periods of variegated glacier by variations in the accumulation rate prior to the surge we use the surge level to hindcast former unobserved surges to compare the results with other surge datings obtained from photographs and to establish a complete surge history for variegated glacier for the 20th century

intermittent ice sheet discharge events in northeastern north america during the last glacial period the 3d ice sheet model of marshall and clarke which includes both dynamics and thermodynamics is used to successfully simulate millennial scale oscillations within an ice sheet under steady external forcing such internal oscillations are theorized to be the main cause of quasi periodic large scale ice discharges known as heinrich events an analysis of the mechanisms associated with multi millennial oscillations of the laurentide ice sheet including the initiation and termination of sliding events is performed this analysis involves an examination of the various heat sources and sinks that affect the basal ice temperature which in turn determines the nature of the ice sheet movement the ice sheet thickness and surface slope which affect the pressure melting point and strain heating respectively are found to be critical for the formation and development of fast moving ice streams which lead to large iceberg calving although the main provenance for heinrich events is thought to be from hudson bay and hudson strait we show that the more northerly regions around lancaster strait and baffin island may also be important sources for ice discharges during the last glacial period

flow thermal structure and subglacial conditions of a surge type glacier temperature measurements in a subpolar surge type glacier reveal a distinctive thermal structure associated with the boundary between the ice reservoir and receiving areas in the receiving area the glacier is cold based but bottom temperature has increased as much as 0 5 c between 1981 and 1982 and the basal heat flux is roughly 10 times the expected geothermal flux water percolation through permeable subglacial material is the probable energy source deformation of the substrate could destroy this drainage system and trigger a surge

a thermomechanical model of ice flow in west antarctica this study uses a three dimensional thermo mechanical model to investigate the internal flow dynamics of the west antarctic ice sheet wais the model allows ice thickness flow and temperature to interact freely its domain is prescribed as that of the present day grounded wais realistic present day climatic and topographical boundary conditions are employed the analysis of a series of experiments pays particular attention to the location and dynamics of concentrations of ice flow ice streams underlying topographic troughs are crucial in determining the strength and location of these concentrations of flow the flow pattern generated by subglacial troughs is made more distinct by the inclusion of ice flow temperature coupling the inclusion of sliding leads to the generation of limit cycles in the ice flow they are concentrated around the present day ice streams b and c of the siple coast and have a period of 5 to 10 ky there appears to be competition between several preferred ice flow pathways in this area the two end members of the flow regime are a strong ice stream c with a weakened ice stream a b complex and strong ice streams a and b with a dormant ice stream c ice streams appear to require ice discharges above a certain threshold in order to maintain frictional heat generation and fast flow individual ice streams can therefore interact through changes in catchment area size a reduction in catchment area reduces the volume of ice entering a stream and can cause stagnation as the amount of frictional heating falls

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

large scale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model 1 heinrich events related to large scale surges of the laurentide ice sheet represent one of the most dramatic types of abrupt climate change occurring during the last glacial here using a coupled atmosphere ocean biosphereice sheet model we simulate quasi periodic large scale surges from the laurentide ice sheet the average time between simulated events is about 7 000 yrs while the surging phase of each event lasts only several hundred years with a total ice volume discharge corresponding to 5 10 m of sea level rise in our model the simulated ice surges represent internal oscillations of the ice sheet at the same time our results suggest the possibility of a synchronization between instabilities of different ice sheets as indicated in paleoclimate records index terms 1827 hydrology glaciology 1863 1620 global change climate dynamics 3309 3344 meteorology and atmospheric dynamics paleoclimatology 5416 planetology solid surface planets glaciation citation calov r a ganopolski v petoukhov m claussen and r greve largescale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model geophys res lett 29 24 2216 doi 10 1029 2002gl016078 2002

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

thermally controlled glacier surging bakaninbreen in svalbard and trapridge glacier in yukon territory canada are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime both glaciers have developed large bulges which have propagated forward as travelling wave fronts and which are thought to divide relatively stagnant downstream cold based ice from faster moving warm based upstream ice additionally both glaciers are underlain by a wet metres thick layer of deforming till we develop a simple model for the cyclic surging behaviour of these glaciers which interrelates the motion of the ice and till through a description of the subglacial hydrology we find that oscillations surges can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier we therefore interpret the travelling wave fronts on both trapridge glacier and bakaninbreen as manifestations of surges in addition we find that the violence of the surge in the model is associated with the resistance to ice flow offered by undulations in the bed and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole

changes in the west antarctic ice sheet since 1963 from declassified satellite photography comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the ross ice shelf ice stream b has widened by 4 kilometers at a rate much faster than suggested by models and has decreased in speed by 50 percent the ice ridge between ice streams b and c has eroded 14 kilometers these changes along with changes in the crevassing around crary ice rise imply that this region s velocity field shifted during this century

is there a single surge mechanism contrasts in dynamics between glacier surges in svalbard and other regions 1 during the 1990s monacobreen a 40 km long tidewater glacier in svalbard underwent a major surge we mapped the surge dynamics using ers synthetic aperture radar images differential dual azimuth interferometry and intensity correlation tracking a series of 11 three dimensional 3 d velocity maps covering the period 1991 1997 show a months long initiation and years long termination to the surge with no indication of a surge front travelling downglacier during the surge the front of the glacier advanced 2 km the velocity and derived strain rate increased by more than an order of magnitude and maximum ice flow rates measured during 1994 were 5 m d 1 the spatial pattern of both velocity and strain rate was remarkably consistent and must therefore be controlled by spatially fixed processes operating at the glacier bed we combine these results with those published in the literature to construct a typical svalbard glacier surge cycle and compare this to surge dynamics of glaciers from other cluster regions especially those of variegated glacier in alaska the strong contrast in dynamics suggests that there exist at least two distinct surge mechanisms

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

does englacial water storage drive temperate glacier surges hydrological studies of surge type and steady flow glaciers combined with recent space borne synthetic aperture radar interferometry measurements of the motion of bagley ice valley alaska u s a during its 1993 95 surge suggest a temperate glacier surge hypothesis that is consistent with observational evidence and appears capable of shedding light on several aspects of surge behavior we propose that the fundamental driver of temperate glacier surges is englacial storage of water combined with gravity driven movement of stored water to the bed during winter whether a given glacier is surge type is a matter not of whether but of the degree to which these processes occur a surge type glacier must have sufficient storage capacity for continued downward movement of englacially stored water during winter to finally overwhelm the constricted basal drainage system thereby forcing pervasive failure of the subglacial till or alternatively widespread and rapid basal sliding thus initiating a surge we further propose that the sufficient storage capacity requirement is most easily met by glaciers with large thickness which are therefore likely to be long and to have on average low surface slopes the average length of the surge cycle in a given region appears to be a function of the mass balances which after each surge determine the time required to restore glaciers to their pre surge geometries we suggest that the stochastic timing of surge onset for a particular glacier however is a result of the uncertainty of the meteorological conditions required to cause englacial storage of a sufficiently large volume of water

controls on the distribution of surge type glaciers in svalbard we analyzed the possible controls on the distribution of surge type glaciers in svalbard using multivariate logit models including 504 glaciers and a large number of glacial and geological attributes specifically we examined the potential effect of geological boundaries mass balance conditions and thermal regime on surging it was found that long glaciers with relatively steep slopes overlying young fine grained sedimentary lithologies with orientations in a broad arc clockwise from northwest to southeast are most likely to be of surge type no relation between lithological boundaries and surge potential could be established possible explanations for length being conducive to surging are transport distance related substrate properties distance related attenuation of longitudinal stresses and the possible relation between thermal regime and glacier size analysis of glaciers with recorded radioecho sounding reveals that a polythermal regime accumulation area ratios close to balance and a large elevation span increase the surge potential the logit models also enabled us to detect 19 new surge type glaciers to reclassify six glaciers as normal and to identify unusual surge type glaciers our model results suggest that a polythermal regime and fine grained potentially deformable beds are conducive to the surge potential of svalbard glaciers

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

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

numerical simulations of cyclic behaviour in the parallel ice sheet model pism numerical experiments are conducted on a synthetic topography with a three dimensional thermomechanically coupled ice sheet model the parallel ice sheet model pism within the model combined stress balances are connected to evolving thermodynamics and hydrology the sensitivity of cyclic behaviour to changes in sliding law parameters and the climate input is studied multiple types of oscillations were found with strong variations in both amplitude and frequency a physical description is given in which these variations and transitions from one oscillation type to another are linked to the interplay of stresses heat transport and hydrological processes high frequency oscillations period 114 169 years which are shown to have a major impact on ice velocities and a small effect on the ice volume are related to variations in the water distribution at the base low frequency cycles period 1000 years which have a major impact on both velocities and ice volume are linked to changes in the thermal regime oscillation characteristics are shown to be strongly sensitive to changes in sliding law parameters and the prescribed surface temperature and mass balance incorporating a surface height dependence of the mass balance is shown to provide an additional feedback which may induce longperiod oscillations

results from the ice sheet model intercomparison project heinrich event intercomparison ismip heino results from the heinrich event intercomparison heino topic of the ice sheet model intercomparison project ismip are presented ismip heino was designed to explore internal large scale ice sheet instabilities in different contemporary ice sheet models these instabilities are of interest because they are a possible cause of heinrich events a simplified geometry experiment reproduces the main characteristics of the laurentide ice sheet including the sedimented region over hudson bay and hudson strait the model experiments include a standard run plus seven variations nine dynamic thermodynamic ice sheet models were investigated one of these models contains a combination of the shallow shelf ssa and shallow ice approximation sia while the remaining eight models are of sia type only seven models including the sia ssa model exhibit oscillatory surges with a period of 1000 years for a broad range of parameters while two models remain in a permanent state of streaming for most parameter settings in a number of models the oscillations disappear for high surface temperatures strong snowfall and small sediment sliding parameters in turn low surface temperatures and low snowfall are favourable for the ice surge cycles we conclude that further improvement of ice sheet models is crucial for adequate robust simulations of cyclic large scale instabilities

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

mass balance change as a control on the frequency and occurrence of glacier surges in svalbard norwegian high arctic the end of the little ice age lia in svalbard 76 81 n a climate sensitive region at the northern extreme of strong poleward heat transfer was marked by an abrupt increase in mean annual air temperature of up to 5 c around 1920 glacier mass balance has been consistently negative since this time and large cumulative net losses of mass have occurred at most glaciers energy balance modelling confirms the sensitivity of svalbard glaciers to climate change predicting a negative shift in net mass balance of up to 0 8 m a 1 water equivalent per degree temperature rise this climate related shift in glacier mass balance has reduced the intensity of glacier surge activity in svalbard one glacier known to have surged since the end of the lia has since failed to accumulate the mass required to re initiate the surge cycle and is also now cold at its base and incapable of rapid flow by basal sliding three overviews of the total number of actively surging glaciers in svalbard between 1936 90 show a decrease from 18 to 5 this is significant compared with the expected numbers of surges based on lia conditions post lia climate change in svalbard has therefore affected not only glacier extent but also ice dynamics this is trend will probably continue given co2 induced climate warming

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

exact solutions to the thermomechanically coupled shallow ice approximation effective tools for verification we describe exact solutions to the thermomechanically coupled shallow ice approximation in three spatial dimensions although artificially constructed these solutions are very useful for testing numerical methods in fact they allow us to verify a finite difference scheme that is to show that the results of our numerical scheme converge to the correct continuum values as the grid is refined in three dimensions comparison of numerical results with exact solutions has helped us to precisely quantify and understand some of the numerical errors we are making our verified numerical scheme shows the basal temperature spokes which arose in the eismint european ice sheet modelling initiative ii intercomparison payne and others 2000 a careful analysis describes these warm spokes as numerical errors which occur when the derivative of the strain heating term with respect to the temperature is large on the other hand the appearance of basal temperature spokes in a verified numerical scheme strongly suggests that they are a feature of the eismint ii experiment f continuum problem in fact they are clear evidence of an unstable equilibrium point of the continuum problem this paper is a sequel to bueler and others 2005 which addresses exact solutions and verification in the isothermal case