bed topography of jakobshavn isbrae greenland and byrd glacier antarctica this paper presents the bed topography of jakobshavn isbrae greenland and byrd glacier antarctica derived from sounding these glaciers with high sensitivity radars to understand the processes causing the speed up and retreat of outlet glaciers and to enable the development of next generation ice sheet models we need information on bed topography and basal conditions to this end we performed measurements with the progressively improved multichannel coherent radar depth sounder imager mcords i we processed the data from each antenna array element using synthetic aperture radar algorithms to improve radar sensitivity and reduce along track surface clutter we then applied array and image processing algorithms to extract the weak bed echoes buried in off vertical scatter cross track surface clutter at jakobshavn isbrae we observed 2 7 km thick ice 30 km upstream of the calving front and 850 m thick ice at the calving front we also observed echoes from multiple interfaces near the bed we applied the music algorithm to the data to derive the direction of arrival of the signals this analysis revealed that clutter is dominated by the ice surface at jakobshavn isbrae at byrd glacier we found 3 62 km thick ice as well as a subglacial trench 3 05 km below sea level we used ice thickness information derived from radar data in conjunction with surface elevation data to generate bed maps for these two critical glaciers the performance of current radars must be improved further by 15 db to fully sound the deepest part of byrd glacier unmanned aerial systems equipped with radars that can be flown over lines spaced as close as 5 m apart in the cross track direction to synthesize a two dimensional aperture would be ideal for collecting fine resolution data over glaciers like jakobshavn near their grounding lines

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

full depth englacial vertical ice sheet velocities measured using phase sensitive radar we describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling using a ground based phase sensitive radio echo sounder pres during seven antarctic field seasons we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on berkner island roosevelt island fletcher promontory and adelaide island recorded changes in reflector positions yield full depth profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides we interpret these variations by comparing them to the results of a full stokes simulation of ice divide flow qualitatively validating the model and demonstrating that we are directly detecting an ice dynamical phenomenon called the raymond effect using pres englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice we discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pres measured vertical velocities into age modeling

a new exploration tool quantitative core characterization we will describe a new laboratory system which was designed to be highly automated and portable while maintaining quality driving this design was the recognition of the temporal dependence of physical properties it becomes apparent that some sedimentary rocks particularly shales degrade and disaggregate so completely that mechanical or elastic properties cannot be measured this temporal dependence displays a time scale much shorter than normal weathering but greater than the time for stress relief a system was designed to permit field characterization of freshly recovered core material a benefit of automation and portability is a marked increase in measurement efficiency the attributes of this system permit rapid characterization of a large number of fresh cores in remote frontier exploration areas this feature can significantly reduce prospect evaluation time statistically significant rock property databases can be created in a short period of time

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

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

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

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

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

the state of stress in subducting slabs as revealed by earthquakes analysed by moment tensor inversion abstract we have used a data set of earthquakes whose mechanisms have been studied here or elsewhere by moment tensor inversion to confirm some earlier ideas concerning the state of stress in subducting slabs the moment tensor method offers the advantages of speed and objectivity over earlier methods usually based exclusively on first motion data deep earthquakes below 300 400 km tend to have compression axes aligned with the dip of the subducting slab the behavior of intermediate earthquakes from 70 to 300 km is less clear there is some evidence for down dip tension but the alignment is not as obvious as it is in the case of the compression axes of deep events tonga shows a tendency toward down dip compression at intermediate depths but other deep extending slabs as well as shallow extending slabs slabs with maximum depths shallower than 300 km seem to be closer to down dip tension

determination of three dimensional in situ stresses from anelastic strain recovery measurement of cores at great depth in order to examine whether the anelastic strain recovery asr method can be applied for determining the in situ stress in hard rocks at great depths the anelastic strain recovery of oriented cores was measured in six independent directions the core specimens were taken from four depths within the range of about 2400 4500 m md at the meti niitsu well in japan the rock materials were mudstone dolerite basalt and andesite for all the rocks the expansional anelastic strains were obtained the magnitude of the strains in various directions continuously measured for 1 or 2 weeks was of the order of 1000 10 6 in mudstone in contrast strains of the other cores did not exceed a few hundred microstrains these strains were used for a three dimensional analysis of the principal in situ stresses at the third depth the principal stress directions were considered to be affected by fractures pre existing near the core and showed the features of a very local stress state with the exception of this data the directions determined by the asr method were in agreement with those determined using other in situ measurement methods based on two assumptions i e i the rock stress in vertical direction is equal to density related gravitational overburden stress ii the ratio of anelastic strain recovery compliance of shear deformation mode and the compliance of volumetric deformation mode is equal to 2 the values of the three principal stresses were estimated the values of the minimum principal stress in the plane perpendicular to the well axis determined in this study were in agreement with those determined based on extended leak off tests elot or xlot conducted at the same well therefore it can be said that the asr method is well suited for use in directly determining the directions of principal in situ stresses in three dimensions and in estimating the magnitude of the stresses in isotropic rocks at great

automatic extraction and analysis of ice layering in radar sounder data nowadays the interest on the development of orbiting radar sounders for the observation of earth polar areas is increasing in this context the analysis of the structure of the ice stratigraphy is of primary importance for the study of the past history and for the prediction of the evpolution of icy environments however as proven by planetary missions orbiting radar sounders provide a huge amount of data thus the development of automatic techniques for the analysis of these data is of fundamental importance for proper data exploitation in this paper we propose a novel method for the automatic detection of subsurface linear features from radar sounder data acquired in icy regions showing extended layering the proposed method allows the estimation of the position of the linear features with subpixel accuracy moreover each detected linear interface is treated as a single object which is completely described by the position of its points the estimated local width and the contrast this allows the direct measurement of geometrical and radiometric parameters e g slope angle and intensity without the need of further postprocessing steps this paper also proposes some measurements for deriving from the output of the proposed technique important variables that can characterize quantitatively the properties of the detected linear features e g mean depth and mean intensity and their distribution e g number and density of layers the effectiveness of the proposed method is confirmed by the results obtained on several radargrams acquired by the shallow radar on the north pole of mars

deformation warming and softening of greenland s ice by refreezing meltwater when basal meltwater refreezes the resulting warm ice can influence the flow dynamics of the ice sheet above an analysis of airborne gravity and radar data identifies extensive basal ice units across the northern greenland ice sheet that coincide with areas of deformed ice and fast ice flow

greenland ice sheet melt extent 1979 1999 analysis of melt extent on the greenland ice sheet is updated to span the time period 1979 1999 and examined along with its spatial and temporal variability using passive microwave satellite data to acquire the full record the issue of continuity between the previous passive microwave sensors smmr ssm i f 8 and ssm i f 11 and the most recent ssm i f 13 sensor is addressed the f 13 cross polarized gradient ratio melt classification threshold is determined to be 0 0154 results show that for the 21 year record a positive melt trend of nearly 1 yr is observed but this trend falls just below the 90 significance level the observed melt increase does appear to be driven by conditions in the western portion of the ice sheet rather than the east where melt appears to have decreased slightly moreover the eruption of mount pinatubo in 1991 is likely to have had some impact on the melt but not so much as previously suspected the 1992 melt anomaly is 1 7 standard deviations from the mean finally the relationship between coastal temperatures and melt extent suggests an increase in surface runoff contribution to sea level of 0 31 mm yr for a 1 c temperature rise

draping or overriding the effect of horizontal stress gradients on internal layer architecture in ice sheets internal isochronic layers in ice sheets sensed by radar show two characteristic relationships to the basal topography either they override it with layers above the crests of rises lying essentially flat or they drape over it with the layers following rises and falls in basal topography a mechanical theory is presented which shows that overriding is the expected behavior when topographic wavelengths are comparable with or less than the ice thickness while draping occurs at longer wavelengths this is shown with analytical perturbation solutions for newtonian fluids numerical perturbation solutions for nonlinear fluids and finite element solutions for nonlinear fluids and large amplitude variations bed variation from topography and changes in the basal boundary condition are considered for fixed bed and sliding beds as well as three dimensional flows and thermomechanically coupled flows in all cases the dominant effect on draping overriding is the wavelength of the topography or variation in basal boundary conditions results of these full mechanical system calculations are compared with those from the shallow ice approximation and the longitudinal stress approximation some calculations are carried out for zero accumulation where the age of the ice and therefore isochrone geometry is not defined it is shown that there is a close relationship between isochrones and streamlines and that they behave similarly when bed wavelength divided by the ice thickness is small compared with the ratio of ice velocity and accumulation rate which is a useful approximation numerical comparisons of isochrones and streamlines show them to be virtually coincident

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

automated processing to derive dip angles of englacial radar reflectors in ice sheets we present a novel automated processing method for obtaining layer dip from radio echo sounding res data the method is robust easily applicable and can be used to process large several terabytes ground and airborne res datasets using modest computing resources we give test results from the application of the method to two antarctic datasets the fletcher promontory ground based radar dataset and the wilkes subglacial basin airborne radar dataset the automated res processing aresp method comprises the basic steps 1 res noise reduction 2 radar layer identification 3 isolation of individual layer objects 4 measurement of orientation and other object properties 5 elimination of noise in the orientation data and 6 collation of the valid dip information the apparent dip datasets produced by the method will aid glaciologists seeking to understand ice flow dynamics in greenland and antarctica aresp could enable a shift from selective regional case studies to ice sheet scale studies

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

radiostratigraphy and age structure of the greenland ice sheet 2015 the authors several decades of ice penetrating radar surveys of the greenland and antarctic ice sheets have observed numerous widespread internal reflections analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections here we present a comprehensive deep radiostratigraphy of the greenland ice sheet from airborne deep ice penetrating radar data collected over greenland by the university of kansas between 1993 and 2013 to map this radiostratigraphy efficiently we developed new techniques for predicting reflection slope from the phase recorded by coherent radars when integrated along track these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing core intersecting reflections were dated using synchronized depth age relationships for six deep ice cores additional reflections were dated by matching reflections between transects and by extending reflection inferred depth age relationships using the local effective vertical strain rate the oldest reflections dating to the eemian period are found mostly in the northern part of the ice sheet within the onset regions of several fast flowing outlet glaciers and ice streams reflections typically do not conform to the bed topography disrupted radiostratigraphy is also observed in a region north of the northeast greenland ice stream that is not presently flowing rapidly dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly this radiostratigraphy provides a new constraint on the dynamics and history of the greenland ice sheet

inference of accumulation rate patterns from deep layers in glaciers and ice sheets the spatial pattern of accumulation rate can be inferred from internal layers in glaciers and ice sheets non dimensional analysis determines where finite strain can be neglected shallow layer approximation or approximated with a local one dimensional flow model local layer approxima tion and where gradients in strain rate along particle paths must be included deep layers we develop a general geophysical inverse procedure to infer the spatial pattern of accumulation rate along a steady state flowband using measured topography of the ice sheet surface bed and a deep layer a variety of thermomechanical ice flow models can be used in the forward problem to calculate surface topography and ice velocity which are used to calculate particle paths and internal layer shapes an objective tolerance criterion prevents over fitting the data after making site specific simplifications in the thermomechanical flow algorithm we find the accumulation rate along a flowband through taylor mouth a flank site on taylor dome antarctica using a layer at approximately 100 m depth or 20 of the ice thickness accumulation rate correlates with ice surface curvature at this site gradients along flow paths critically impact inference of both the accumulation pattern and the depth age relation in a 100 m core

radar based subglacial lake classification in antarctica 1 subglacial lakes in east antarctica can be separated into four categories specified by radar reflection properties definite lakes are brighter than their surroundings by at least 2 db relatively bright and both are consistently reflective specular and have a reflection coefficient greater than 10 db absolutely bright dim lakes are relatively bright and specular but not absolutely bright indicating nonsteady ice dynamics fuzzy lakes are both relatively and absolutely bright but not specular and may indicate saturated sediments or swamps indistinct lakes are absolutely bright and specular but no brighter than their surroundings lakes themselves and the different classes of lakes are not arranged randomly throughout antarctica but are clustered around ice divides ice stream onsets and prominent bedrock troughs with each cluster demonstrating a different characteristic lake classification distribution the lake classification algorithm expands on previous studies and demonstrates a novel way to characterize ice water interactions in east antarctica

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

fast flow signature in the stagnated kamb ice stream west antarctica among the major ice streams that drain west antarctica kamb ice stream formerly called ice stream c is unique in that it stagnated 150 yr ago but its former fast flow conditions are virtually unknown here we presentsurface based radar profiles of the ice stream s undulating internal stratigraphy which records these conditions our analysis of the profiles indicates that pre stagnation flow velocities averaged over a period 740 yr exceeded 350 m yr 1 in the trunk of the ice stream this velocity constraint would be lower if the ice had been thickening higher if thinning but suggests mass loss from the ice stream catchment that is of sufficient magnitude to reverse the gain estimated for today s siple coast region analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present day velocities useful for evaluating how west antarctic ice drainage has evolved

traveling slippery patches produce thickness scale folds in ice sheets large complex stratigraphic folds that rise as high as 60 of the local ice thickness have been observed in ice sheets on antarctica and greenland here we show that ice deformation caused by heterogeneous and time variable basal sliding can produce the observed structures we do this using a thermomechanical ice sheet model in which sliding occurs when the base approaches the melting point and slippery patches develop these slippery patches emerge and travel downstream because of a feedback between ice deformation vertical flow and temperature our model produces the largest overturned structures comparable to observations when the patches move at about the ice column velocity we conclude that the history of basal slip conditions is recorded in the ice sheet strata these basal conditions appear to be dynamic and heterogeneous even in the slow flowing interior regions of large ice sheets

radio echo layering in polar ice sheets and past volcanic activity a recent study1 has shown how acidity profiles along dated greenland ice cores can reveal large volcanic eruptions in the northern hemisphere during the past 10 000 yr it has been suggested that these layers of acidic ice in the polar ice sheets may also be detected by airborne radio echo sounding res techniques as stratification echoes2 4 explosive volcanic eruptions eject large amounts of so2 into the stratosphere5 where it forms an h2so4 aerosol6 studies of antarctic snow7 and greenland ice cores2 show that this material can be deposited in layers of large areal extent on polar ice sheets calculations based on observations of these slightly acidic layers show that they should give rise to radar reflections of similar magnitude to those observed for stratification echoes new res data from the antarctic enables the present comparison to be made between observed layer power reflection coefficients prcs and calculated values for reflections from acidic ice layers and from layers of ice of changed density a gap in layering has been identified which seems to coincide with a similar gap reported in greenland at crete8 profiles of layer prc against age show a common pattern for many sites on the antarctic ice sheet this prc age profile may provide a record of explosive volcanic activity for the southern hemisphere over the past 150 000 yr

automated mapping of near bed radio echo layer disruptions in the greenland ice sheet abstract one of the key processes for modulating ice flow is the interaction between the ice and the bed but direct observations of the subglacial environment are sparse and difficult to obtain in this study we use information from an extensive radio echo sounding dataset to identify areas of the greenland ice sheet where internal layers have been influenced by near bed processes based on an automatic algorithm for calculating the slope of the internal radio echo layers we identify areas with disrupted layer stratigraphy we find that large parts of the northern portion of the ice sheet are influenced by locally confined mechanisms that produce up warping or folds in the layer stratigraphy inconsistent with the surface and bed topography this is particularly evident at the onset of ice streams although less dynamic areas close to the ice divide also contain imprints of layer disturbances our results show that the disturbances are found in many different flow and thermal regimes and underscore the need to understand the mechanisms responsible for creating them

mapping basal melt under the northern greenland ice sheet ice sheets in greenland and antarctica are based on continental rock and are coupled strongly to changes in global sea level their beds may be frozen or thawed but it has not been known how much or where basal melt occurs except in special cases where a borehole has reached the bed or where airborne radar has revealed subglacial lakes and reflective ice stream beds in antarctica we have used a previously published technique to detect subglacial melt water in the general case and have here applied it to radar sounder data collected over northern and central greenland we have found extensive subglacial water along between 13 and 20 of the flight paths this paper provides maps of the measured locations and probable extent of subglacial water