subglacial bed form morphology controlled by ice speed and sediment thickness subglacial bed forms drumlins ribbed moraines and megascale glacial lineations are enigmatic repetitive flow parallel and flow transverse landforms common in glaciated landscapes their evolution and morphology are a potentially powerful constraint for ice sheet modeling but there is little consensus on bed form dynamics or formative mechanisms here we explore shallow sediment bed form dynamics via a simple model that iterates i down flow till flux ii pressure gradient driven till flux and iii entrainment and deposition of sediment under various boundary conditions replicas of subglacial bed forms readily emerge bed form dynamics mirror those in subaqueous and aeolian domains transitions between ribbed moraines and elongate flow parallel bed forms are associated with increasing ice speeds and declining sediment thickness these simulations provide quantitative flux estimates and suggest that widely observed transitions in shallow sediment subglacial bed forms e g ribbed moraines to drumlinoids to megascale glacial lineations are manifestations of subtle variations in ice velocity and sediment thickness

bed topography and subglacial landforms in the onset region of the northeast greenland ice stream the northeast greenland ice stream negis is an important dynamic component for the total mass balance of the greenland ice sheet as it reaches up to the central divide and drains 12 of the ice sheet the geometric boundary conditions and in particular the nature of the subglacial bed of the negis are essential to understand its ice flow dynamics we present a record of more than 8000 km of radar survey lines of multi channel ultra wideband radio echo sounding data covering an area of 24 000 km2 centered on the drill site for the east greenland ice core project egrip in the upper part of the negis catchment our data yield a new detailed model of ice thickness distribution and basal topography in the region the enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity potentially over several glacial interglacial cycles off nadir reflections from the ice bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms our new bed topography model will help to improve the basal boundary conditions of negis prescribed for ice flow models and thus foster an improved understanding of the ice dynamic setting

a model for spiral flows in basal ice and the formation of subglacial flutes based on a reiner rivlin rheology for glacial ice 1 flutes are elongated sediment ridges formed at the base of glaciers and ice sheets in this paper we show that flutes can be the product of a corkscrew like spiral flow in basal ice that removes sediment from troughs between flutes and deposits it at their crests as first suggested by shaw and freschauf in order to generate the type of basal ice flow required for this mechanism the viscous rheology of ice must allow for the generation of deviatoric normal stresses transverse to the main flow direction this type of behavior which is commonly observed in real nonlinearly viscous and viscoelastic fluids can be described by a reiner rivlin rheology here we develop a mathematical model that describes the role of these transverse stresses in generating spiral flows in basal ice and investigate how these flows lead to the amplification of initially small basal topography and the eventual formation of assemblies of evenly spaced subglacial flutes

realistic yet simple bottom boundary conditions for glaciers and ice sheets over a nondeformable and impermeable bed the sliding law has to be a relationship between the bottom shear stress b the sliding velocity u and the effective pressure n pi pw where pi is the ice load and pw is the pressure in the interconnected water cavities three horizontal scales are recognized 0 1 m 10 m and 10 km classical theories consider the smallest scale they are of dubious value because temperate ice is neither dry nor totally impermeable the intermediate scale yields sliding laws to be used in glacier modeling melting refreezing processes may then be ignored for modeling large cold ice sheets the sliding law at the largest scale is needed it is suggested that sliding occurs only when a bottom temperate layer exists how to tackle non newtonian rheology is discussed at the intermediate scale a microrelief model consisting of two superimposed bumpy profiles is favored if the smaller one had bumps of equal height b would be double valued but this model is unrealistic the ice avalanche at the allalingletscher in 1965 may be explained otherwise therefore a model with the smaller bumps of unequal height is adopted it yields an asymptotic sliding law at large sliding velocities b n cun1 n with and c as adjustable parameters a third one has to be introduced at low velocities although the correct interpolation function between both extreme cases remains unknown some qualitative results about kinematic waves are obtained

spatial organisation of drumlins ice sheets flowing over soft sediments produce undulations in the bed typically of metres in relief of which drumlins are the most abundant and widely investigated consensus regarding their mechanism of formation has yet to be achieved in this paper we examine the spatial organisation of drumlins in order to provide an improved description of the phenomenon and to guide hypotheses of their formation we review the literature highlighting contradictory findings regarding drumlin spatial organisation and then use this to motivate our study based on a large sample 42 488 of drumlins from canada britain and norway are there typical arrangements in drumlin positioning and are they organised in a regular spatial manner patterned or are they distributed randomly we recognise that drumlin fields are inherently patchy and therefore apply inhomogeneous spatial statistics in order to study their distribution this shows that whilst drumlins are occasionally randomly placed their main state is non random they exhibit a strong and statistically significant signal of regularity across lengths scales of 100 1200 m we conclude that patterning is a near ubiquitous property of drumlins this finding of regularity demonstrates spatial self organisation in the bedforming process with drumlins as an emergent manifestation of subglacial sediment mobility kilometre scale interactions between drumlins must occur as they evolve or interactions may arise as a consequence of growth or migration hypotheses or models are required that can explain the regular spacing of drumlins we highlight three suggestions for such self organisation instability in the coupling of ice flow sediment flux bed shape local feedback between sediment mobility and relief and coarsening by growth or migration

hindcast of the wave conditions along the west iberian coast this paper describes the development of a wave prediction system for the west iberian coast the implemented wave prediction system is based on two state of the art spectral wave models wam for the ocean area and swan for the nearshore however because of its extended geographical space the swan model will include some generation effects in the coarse swan simulations complemented by wave transformation effects near the coast the system was validated by means of extended hindcast runs in various regions belonging to the continental portuguese coastal environment which were compared with buoy data focusing on the extreme energetic events and both direct comparisons and statistical results are presented

integrating satellite observations with modelling basal shear stress of the filcher ronne ice streams antarctica using inverse methods constrained by recent satellite observations we have produced a comprehensive estimate of the basal shear stress beneath the filchner ronne ice streams the inversions indicate that a weak bed approx 4 20 kpa underlies much of these ice streams compared to the ross ice streams the distribution of weak subglacial till is more heterogeneous with sticky spots providing much of the resistance to flow a weak bed beneath recovery ice stream extends several hundred kilometres inland with flow along this ice stream discrepancies between thickness measurements and flux estimates suggest the existence of a deep 1400 m trough not resolved by existing maps of subglacial topography we hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice sheet evolution

rapid erosion drumlin formation and changing hydrology beneath an antarctic ice stream what happens beneath a glacier affects the way it flows and the landforms left behind when it retreats direct observations from beneath glaciers are however rare and the subglacial environment remains poorly understood we present new repeat observations from west antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record we interpret erosion at a rate of 1 m a 1 beneath a fast flowing ice stream followed by cessation of erosion and the formation of a drumlin from mobilized sediment we also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed all these changes occurred on time scales of a few years or less this variability suggests that an ice stream can reorganize its bed rapidly and that present models of ice dynamics may not simulate all the relevant subglacial processes

pressure dependent viscosity and interfacial instability in coupled ice sediment flow we study an interfacial instability in the coupled flow of ice and subglacial sediment both modelled as viscous media unlike other interfacial instabilities in coupled viscous flows at zero reynolds number the mechanism considered here does not rely on buoyancy or the effect of an upper free surface but on the pressure dependence of the sediment viscosity specifically the instability relies on sediment rheology being such that when sediment flows in simple shear sediment flux increases with compressive normal stress at the ice sediment interface when the velocity of the interface is kept constant when ice moves over a shallow bump in the interface it generates a higher compressive stress on the bump s upstream side than in its lee if in addition the effective sediment viscosity is low compared with that of ice interfacial velocity remains approximately constant and this then implies that more sediment flows into the bump than out of it causing it to grow modelling ice as a newtonian material we show that this mechanism works for a wide range of sediment rheologies including the highly nonlinear shear thinning ones typically thought most appropriate for the description of nearly plastic sediment the instabilities predicted are essentially two dimensional with infinite transverse wavelength and a nonlinear model shows that growth is unbounded until cavitation occurs in the lee of evolving bumps on the interface the instability mechanism does not seem to predict the formation of common glacial landforms but may explain the formation of water filled cavities on deformable glacier beds

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

decoding ice sheet behavior using englacial layer slopes the complex flow fields of the antarctic and greenland ice sheets deform layers deposited as snow at the ice sheet surface leaving a record of the regional flow history and or local transitions in basal boundary conditions within the geometry of ice sheet layers ice penetrating radar reveals these layers but interpretations are limited by the challenges of quantitatively and reproducibly comparing observations with model output we present a conceptual framework that relates along track reflector slope to gradients in the steady state velocity field of ice sheets this method makes effective use of englacial reflectors in regions where it is challenging to image continuous layers and avoids the error propagation inherent to tracer transport methods developing the potential for formal radar data assimilation in future modeling studies we apply our method to radar data collected at the grounding line of whillans ice stream where enhanced bed friction produces characteristic reflector slopes reproducible using a higher order ice flow model

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

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

thermomechanical forcing of deep rock slope deformation 1 conceptual study of a simplified slope 1 thermo elastic rock slope deformation is often considered to be of relatively minor importance and limited to shallow depths subject to seasonal warming and cooling in this study we demonstrate how thermomechanical tm effects can drive rock slope deformation at greater depths below the annual thermal active layer here in part 1 of two companion papers we present 2d numerical models of a simplified slope subject to annual surface temperature cycles the slope geometry and discontinuity sets are loosely based on the randa instability considered in detail in part 2 results show that near surface thermo elastic stresses can propagate to depths of 100 m and more as a result of topography and elasticity of the rock mass shear dislocation along discontinuities can have both a reversible component controlled by discontinuity compliance and provided that the stress state is sufficiently close to the strength limit an irreversible component i e slip induced slip increments are followed by stress redistribution resulting in the propagation of slip fronts thus deformation and progressive rock slope failure can be driven solely by thermomechanical forcing the influence of tm induced stress changes becomes stronger for increasing numbers of critically stressed discontinuities and is enhanced if failure of discontinuities involves slip weakening the net tm effect acts as a meso scale fatigue process involving incremental discontinuity slip and hysteresis driven by periodic loading

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

direct observation of frost wedging in alpine bedrock width and temperature of rock joints were automatically monitored in the japanese alps three years of monitoring on a sandstone rock face shows two seasonal peaks of joint widening in autumn and spring the autumn events are associated with short term freeze thaw cycles and the magnitude of widening reflects the freezing intensity and water availability the short term freezing can produce wedging to a depth of at least 20 cm the spring events follow a rise in the rock surface temperature to 0 c beneath the seasonal snowcover and likely originate from refreezing of meltwater entering the joint some of these events contribute to permanent enlargement of the joint two other joints on nearby rock faces experience only sporadic widening accompanying freeze thaw cycles and insignificant permanent enlargement observations indicate that no single thermal criterion can explain frost weathering the temperature range at which wedging occurs varies with the bedrock conditions water availability and duration of freezing copyright 2001 john wiley sons ltd

measurement of rock fracture toughness under modes i and ii and mixed mode conditions by using disc type specimens rock fracture mechanics has been widely applied to blasting hydraulic fracturing mechanical fragmentation rock slope analysis geophysics earthquake mechanics hot dry rock geothermal energy extraction and many other practical problems but a standard method to accurately determine fracture toughness of rocks one of the most important parameters in fracture mechanics as an intrinsic property of rock has not been yet well established to obtain rock fracture toughness disc type specimens were used in this study rock fracture toughness under mixed mode conditions was measured by using the straight through crack assumption stca applied to the cracked chevron notched brazilian disc ccnbd specimen and the semicircular bend scb specimen size effects in terms of specimen thickness diameter and notch length on fracture toughness were investigated from the mixed mode test results fracture envelopes were obtained by applying various regression curves the mixed mode test results were also compared with the three well known mixed mode failure criteria

do subglacial bedforms comprise a size and shape continuum understanding the evolution of the ice bed interface is fundamentally important for gaining insight into the dynamics of ice masses and how subglacial landforms are created however the formation of the suite of landforms generated at this boundary subglacial bedforms is a contentious issue that is yet to be fully resolved bedforms formed in aeolian fluvial and marine environments either belong to separate morphological populations or are thought to represent a continuum of forms generated by the same governing processes for subglacial bedforms a size and shape continuum has been hypothesised yet it has not been fully tested here we analyse the largest data set of subglacial bedform size and shape measurements ever collated 96 900 bedforms our results show that flutes form a distinct population of narrow bedforms however no clear distinction was found between drumlins and megascale glacial lineations msgls which form a continuum of subglacial lineations a continuum of subglacial ribs also exists with no clear size or shape distinctions indicating separate populations furthermore an underreported class of bedform with no clear orientation to ice flow quasi circular bedforms overlaps with the ribbed and lineation continua and typically occurs in spatial transition zones between the two potentially merging these three bedform types into a larger continuum

ice sheet bed 3 d tomography information on bed topography and basal conditions is essential to developing the next generation ice sheet models needed to generate a more accurate estimate of ice sheet contribution to sea level rise synthetic aperture radar sar images of the ice bed can be analyzed to obtain information on bed topography and basal conditions we developed a wideband sar which was used during july 2005 to perform measurements over a series of tracks between the gisp2 and grip cores near summit camp greenland the wideband sar included an eight element receive antenna array with multiple phase centers we applied the multiple signal classification music algorithm which estimates direction of arrival signals to single pass multichannel data collected as part of this experiment to obtain fine resolution bed topography this information is used for producing fine resolution estimates of bed topography over a large swath of 1600 m with a 25 m posting and a relative accuracy of approximately 10 m the algorithm derived estimate of ice thickness is within 10 m of the grip ice core length data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9 1 and 10 3 m for the parallel and perpendicular tracks respectively

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

drumlinization and drumlin forming instabilities viscous till mechanisms glacially induced flow naturally tends to thin and extended till cover through shock formation even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its surface and base a lower sliding velocity or deformation rate and above a critical thickness a decrease in wave velocity with thickness leading to reverse facing shocks moving downstream for sliding and for some theologies of internal deformation a decrease in sediment flux with thickness occurs implying backward moving kinematic waves and reverse facing reverse moving shock downstream facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally eventually shocks coalesce leaving an upstream lacing shock for sliding and a downstream facing shock for internal deformation it is observed that some drumlins have downstream blunt ends only fairly realistic three dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice sheet flow the fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic viscous transition scale

landform signature of the laurentide and cordilleran ice sheets across alberta during the last glaciation government geological survey maps and research publications have portrayed the distribution of glacial landforms associated with the advance and retreat of the laurentide and cordilleran ice sheets across alberta at a local regional and continental scale to date this information has not been systematically synthesized into a single compilation at a consistent scale although this original work provided valuable information to constrain reconstructions of former ice sheet extent configuration and flow geometry its derivation primarily from the interpretation of aerial photographs and the shuttle radar topography mission 90 m digital elevation model dem may result in methodological inconsistencies and spatial biases these biases together with challenges associated with geomorphic mapping in densely forested areas of western and northern alberta limit the usefulness of previous mapping when applied to inversion based ice sheet reconstructions which have specific input data demands recently ligh

radar images of the bed of the greenland ice sheet 1 in this paper we apply radar tomography methods to very high frequency airborne synthetic aperture radar data to measure the ice thickness field and to construct three dimensional basal image maps of a 5 20 km study area located along the southern flank of the jakobshavn glacier greenland unlike ice radar measurements typically made at nadir our approach uses radar echo phase and amplitude measured across an antenna array to determine the propagation angle and signal strength of pixel elements distributed on each side of the aircraft flight path that information combined with knowledge of aircraft position and the assumed dielectric properties of the glacier can be used to measure ice thickness and radar reflectivity across a 3 km wide swath combining ice thickness and surface topography data we estimate basal topography and basal drag we conclude that the glacier is sliding over the bed we use the three dimensional image maps of the bed to inspect the modern subglacial geomorphology and find for the first time beneath the greenland ice sheet assemblages of long ridge groove landforms that are oriented in the direction of the ice flow spatial dimensions 10 to 30 m depths 150 to 500 m spacing and lengths of 10 km or more and correlation with the current ice flow direction suggest that these are glacial erosional features similar to mega grooves observed on deglaciated terrain

relating bed character and subglacial morphology using seismic data from thwaites glacier west antarctica abstract seismic measurements on thwaites glacier show a spatially variable bed character with implications for ice sheet stability the west antarctic ice sheet is losing mass rapidly through outlet glaciers and ice streams in the amundsen sea embayment including thwaites glacier where limited observations and modeling suggest that ice flow rates depend on bed properties here we characterize bed properties of thwaites glacier based on amplitude analysis of reflection seismic data from a 40 km long profile collected in the approximate flow direction and two 10 km long profiles transverse to flow the upstream portion of the seismic profile reveals a 12 km long sedimentary basin with ice flow aligned bedforms capped by a continuous till layer that is likely soft and deforming porosity 0 4 0 45 with several locations where water has pooled at the bed downstream of the sedimentary basin the bed rises by 400 m over 25 km into subglacial highlands our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography 200 m amplitude at 2 to 5 km wavelength resembling crag and tails till on the stoss sides facing upglacier of topographic highs is more consolidated porosity 0 3 0 35 or lower whereas the lee sides facing downglacier and flat regions exhibit porosity similar to the till of the upstream sedimentary basin modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of thwaites glacier resulting in more realistic models of future grounding line retreat our findings highlight the need for more geophysical constraints on bed properties for important outlets in antarctica and greenland

environmental controls of frost cracking revealed through in situ acoustic emission measurements in steep bedrock 1 frost cracking the breakdown of rock by freezing is one of the most important mechanical weathering processes acting on earth s surface insights on the mechanisms driving frost cracking stem mainly from laboratory and theoretical studies transferring insights from such studies to natural conditions involving jointed bedrock and heterogeneous thermal and hydrological properties is a major challenge we address this problem with simultaneous in situ measurements of acoustic emissions used as proxy of rock damage and rock temperature moisture content the 1 year data set acquired in an alpine rock wall shows that 1 liquid water content has an important impact on freezing induced rock damage 2 sustained freezing can yield much stronger damage than repeated freeze thaw cycling and 3 that frost cracking occurs over the full range of temperatures measured extending from 0 down to 15 c these new measurements yield a slightly different picture than previous field studies where ice segregation appears to play an important role

bed type variability and till dis continuity beneath thwaites glacier west antarctica recent seismic measurements from upper thwaites glacier indicate that the bed type variability is closely related to the along flow basal topography in high relief subglacial highlands stoss sides of topographic highs have a relatively higher acoustic impedance hard bed with lower acoustic impedance soft till on lee sides this pattern is similar to observations of many deglaciated terrains subglacial hydraulic potential gradient and its divergence show a tendency for water to diverge over the stoss sides and converge into the lee sides convergence favors a thicker or more widespread water system which can more efficiently decouple ice from the underlying till under such circumstances till deformation does occur but fluxes are relatively small till carried from the lee sides onto stoss sides of downstream bumps should couple to the ice more efficiently increasing the ability for transport by till deformation in turn this suggests that steady state till transport can be achieved if the stoss side till layer is thin or discontinuous in addition the large basal shear stress generated in the highlands seems too high for a bed lubricated by a continuous although thin deforming till suggesting till discontinuity which would allow debris laden ice to erode bedrock on stoss sides supplying additional till for transport

advanced multifrequency radar instrumentation for polar research this paper presents a radar sensor package specifically developed for wide coverage sounding and imaging of polar ice sheets from a variety of aircraft our instruments address the need for a reliable remote sensing solution well suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution the sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance the instruments operate simultaneously over different frequency bands within the 160 mhz 18 ghz range the sensor package has allowed us to sound the most challenging areas of the polar ice sheets ice sheet margins and outlet glaciers to map near surface internal layers with fine resolution and to detect the snow air and snow ice interfaces of snow cover over sea ice to generate estimates of snow thickness in this paper we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance we also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform finally we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features

interpretation of topography and bed properties beneath thwaites glacier west antarctica using seismic reflection methods abstract thwaites glacier tg west antarctica is losing mass in response to oceanic forcing future evolution could lead to deglaciation of the marine basins of the west antarctic ice sheet depending on ongoing and future climate forcings but also on basal topography bathymetry basal properties and physical processes operating within the grounding zone hence it is important to know the distribution of bed types of tg s interior and grounding zone and to incorporate them accurately in models in order to improve estimates of retreat rates and stability here we estimate properties of the bed by determining its acoustic impedance from amplitude analysis of reflection seismic data we report on the results from two lines a longitudinal l line and a transverse n line on a central flowline of tg 100 km inland from the grounding zone the data show considerable spatial variability in bed forms and properties similar to results from a comparable survey farther inland notably we find the same pattern here of hard presumed bedrock material on the stoss side of bumps and soft presumed till on the lee side physical understanding indicates the basal flow law describing motion over different regions of tg s bed likely varies from nearly viscous over the bedrock regions to nearly plastic over till regions providing guidance for modeling

climatic controls on frost cracking and implications for the evolution of bedrock landscapes 1 in mountainous landscapes the role of periglacial processes in producing sediment is poorly defined despite evidence of abundant talus slopes ice growth in rock has long been recognized as an efficient erosion mechanism but the effects have not been readily applied to landscape evolution in response to tectonic and climatic forcing here we quantify how and where ice driven mechanical erosion occurs in cold bedrock dominated landscapes using a simple one dimensional numerical heat flow model in our model ice grows by water migration to colder regions in shallow rock by the reduction in chemical potential associated with intermolecular forces between ice and mineral surfaces a process called segregation ice growth the depth and intensity of frost cracking is primarily dependent on mean annual temperature mat with positive mat sites characterized by intense cracking in the top meter of the rock mass and a maximum frost penetration of 4 m in contrast negative mat areas have less intense cracking that primarily occurs at depths between 50 and 800 cm we compare the depth and intensity of frost cracking predicted by our model with measures of the intensity of frost processes determined in three studies the first measured the timing of rockfall in the canadian rockies niagara escarpment and japanese alps the second analyzed scree deposits in the southern alps new zealand and the third documented rockfall frequency in utah these natural examples show that rockfalls tend to nucleate at elevations that coincide with zones of intense frost cracking predicted by our model as such climatic variations associated with interglacial glacial cycles may impart a significant influence on the denudation of mountainous landscapes

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