spatial variation of radar derived basal conditions on kamb ice stream west antarctica radar profiles of bed echo intensity can survey conditions at the ice bed interface and test for the presence or absence of water however extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice we used the relationship between bed echo intensities from constant offset radar data and ice thickness to estimate depth averaged attenuation rates at several locations on and near kamb ice stream kis west antarctica we found values varying from 29 db km 1 at siple dome to 15 db km 1 in the main trunk region of kis in agreement with a previous measurement and models using these attenuation rate values we calculated the relative bed reflectivity throughout our kis surveys and found that most of the bed in the trunk has high basal reflectivities similar to those obtained in the location of boreholes that found water at the bed areas of lower bed reflectivity are limited to the sticky spot where a borehole found a dry bed and along the margins of kis these results support previous hypotheses that the basal conditions at locations like the sticky spot on kis control its stagnation and possible reactivation

an integrated lightweight ice penetrating radar system we describe a portable low frequency impulse radar system intended for ground based surveys that employs off the shelf hardware integrated with custom designed software the hardware comprises a 1 200 mhz transmitter digitizer computer and gps receiver which together weigh 1 5 kg the entire system including waterproof enclosures and batteries suited for 8 hours of continuous operation weighs 10 kg plus the weight of the antenna housing the system design is flexible permitting hardware components such as the digitizer or navigation device to be exchanged the software includes acquisition parameter control real time visual ice depth rendering and data management capabilities using a hierarchical data format the system described here has been successfully used to sound polythermal ice up to 220 m thick in ski based surveys in the yukon canada and temperate ice up to 550 m thick in machine based surveys in iceland

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

cryosphere fire and ice ice sheet stability is affected by a complex interplay between meltwater and the geological characteristics of the bedrock under the ice the identification of a recently active subglacial volcano in antarctica adds uncertainty to this system

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

a stationary impulse radar system for autonomous deployment in cold and temperate environments l mingo acknowledges the sr ed tax incentive program of the cra for offsetting some r d costs of the sipr g flowers and d bigelow are grateful to nserc cfi csa nstp pcsp and sfu for funding kluane first nation yukon government and parks canada for access to yukon field sites and s williams l goodwin a pulwicki j crompton f beaud and canadian astronaut d saint jacques for support and field assistance c schoof and c rada provided time lapse imagery and 2017 water pressure data a crawford and d mueller acknowledge funding from nserc transport canada pkc nstp and the garfield weston foundation as well as support from arctic net the ccgs amundsen crew and pilots o talbot a roy and g carpentier

a time marker at 17 5 kyr bp detected throughout west antarctica deep radar soundings as part of the international trans antarctic scientific expedition us itase traverses in west antarctica have revealed a bright internal reflector that we have imaged throughout widespread locations across the ice sheet the layer is seen in traverses emanating from byrd station in four directions and has been traced continuously for distances of 535 km toward the weddell sea drainage 500 km toward south pole 150 km toward the executive committee range and 160 km toward kamb ice stream former ice stream c the approximate area encompassed by the layer identified in these studies is 250 000 km 2 if the layer identification can also be extended to siple dome where we have additional radar soundings jacobel and others 2000 the approximate area covered would increase by 50 in many locations echo strength from the layer rivals the bed echo in amplitude even though it generally lies at a depth greater than half the ice thickness at byrd station where the layer depth is 1260 m an age of 17 5 kyr bp has been assigned based on the blunier and brook 2001 chronology hammer and others 1997 note that the acidity at this depth is 20 times the amplitude of any other part of the core the depiction of this strong and widespread dated isochrone provides a unique time marker for much of the ice in west antarctica we apply a layer tracing technique to infer the depth time scale at the inland west antarctic ice sheet divide and use this in a simple model to estimate the average accumulation rate

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

radar characterization of the basal interface across the grounding zone of an ice rise promontory in east antarctica radar power returned from the basal interface along a 42 km long profile over an ice rise promontory and the adjacent roi baudouin ice shelf dronning maud land east antarctica is analyzed to infer spatial variations in basal reflectivity and hence the basal environment extracting basal reflectivity from basal returned power requires an englacial attenuation model we estimate attenuation in two ways 1 using a temperature dependent model with input from thermomechanical ice flow models and 2 using a radar method that linearly approximates the geometrically corrected returned power with ice thickness the two methods give different results we argue that attenuation calculated using a modeled temperature profile is more robust than the widely used radar method especially in locations where depth averaged attenuation varies spatially or where the patterns of basal reflectivity correlate with the patterns of the ice thickness

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

assessment of rock quality variability due to smectitic alteration in basalt using x ray diffraction analysis abstract the quality of basalt rock in particular the distribution of deleterious smectitic alteration at blakebrook quarry northeast new south wales australia varies considerably and accurate resource assessment requires detailed mineralogical investigation x ray diffraction analysis was used to assess the primary and alteration mineralogy and was found to provide a rapid and cost effective tool for basalt exploration and resource assessment a smectitic alteration index sai relating smectite content to primary mineral crystallinity was devised as a basis of rock quality comparison samples from quarry faces and sub surface samples from core and percussion drilling were analysed leading to delineation of reserves on the basis of quality rock quality was found to relate closely to the succession of horizontal flows comprising layers of premium grade smectite free basalt second grade slightly altered smectitic basalt intercalated with layers of third grade moderately to highly altered smectitic basalts

limits to recharge of groundwater from tibetan plateau to the gobi desert implications for water management in the mountain front multiple isotopic and hydrogeochemical tracers were utilized to understand the limits to recharge from tibetan plateau to gobi desert the average direct recharge rate according to chloride mass balance is between 0 9 2 5 mm yr 1 based on an annual rainfall of 120 mm yr 1 indicating that this has negligible impact on groundwater the groundwater shows markedly depleted stable isotopic composition compared to modern rainfall the signature of groundwaters around 10 4 parts per thousand to 9 6 parts per thousand delta 18 o in several wells from dajing and south lake playa differ clearly from that of the alluvial fan and modern rainfall and has lower or undetected tritium activity implying that the aquifer in the desert is maintained by palaeowater this groundwater has an estimated 14 c age of similar to 4 kyr with a recharge temperature of 7 3 degrees c at current hydraulic head gradients transit time from the recharge zone wuwei gulang alluvial fan to south lake is approximately 2200 3500 yrs indicating that groundwater flow is relatively slow and that the water resources are non renewable the geochemical information is comparable to the isotopic results of limit recharge sources of groundwater from tibetan plateau to the gobi desert the buildup of dissolved solids through evaporation is a major control on groundwater composition and the dominant anion species change systematically from hco 3 so 4 2 to cl but the dissolved ions from albite calcite dolomite and gypsum also have a significant contribution it is advised to re consider seriously the immigration policy of south lake to meet the groundwater management in the mountain front to gobi desert c 2008 elsevier b v all rights reserved

ice and bedrock characteristics underneath dome c antarctica from radio echo sounding data analysis the radio echo sounding res system is one of the most widely used active remote sensing techniques for polar ice sheet exploration including bedrock morphology studies and subglacial lake investigations recently bedrock characterization has been improved through the analysis of radar echo strength in this paper the analysis of the res signal amplitude has been used to collect information about the controversial problem of electromagnetic ice absorption to highlight areas of high reflectivity variation ascribable to wet ice bedrock interfaces a method to distinguish a wet or dry bedrock ice interface using a model to describe the internal ice absorption is proposed and discussed moreover the comparison between the ice absorption rates from res measurements and from european project for ice coring in antarctica dome c antarctica ice core conductivity data the signal amplitude contributions of internal ice layers and different kinds of rock interface is evaluated encouraged by the results the data analysis led to obtaining a bedrock reflectivity variation map of the dome c area this map outlined a wide dispersion of wet dry rock interfaces in the studied area indicating the possibility of flowing water along both sides of the concordia trench

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

instruments and methods an improved transient type ice penetrating radar a low cost compact short pulse ice penetrating radar ipr system with a center frequency of 50 mhz for sounding glacier topography is presented the radar was developed to measure ice thickness and to image internal structures and basal conditions of glaciers and ice sheets with a maximum range of 16 000 ns and a depth resolution better than 2 5 m the receiver of the ipr system employs asynchronous operation mode avoiding the need for a cable between the transmitter and receiver a new sampling technology using a high speed field programmable gate array which implements a 256 trace stacking algorithm to realize the analog to digital conversion both simplifies the structure of the receiver and increases the sampling efficiency the power consumption of the whole receiver is 1 5 w which can be supplied by a laptop computer test measurements were made during the 5th china expedition to the grove mountains in east antarctica field tests show the capability of this system to measure ice thickness up to 650 m and to define internal layers within the ice body

radar absorption due to impurities in antarctic ice measurements of radar echo strength have been carried out on ronne and george vi ice shelves antarctica the differences in ice column absorption between the two sites cannot be resolved by considering ice temperatures alone we present an absorption model that resolves the discrepancy by considering both the ice temperature and the ice impurity concentrations in antarctic coastal sites the concentrations of acid and sea salt impurities both of which affect absorption have a marked spatial variability and can dominate the absorption budget using the absorption model impurity levels in the ice column at the george vi site are deduced from the radar strength measurements and found to be typical of levels measured in antarctic ice at similar elevations and distances from the sea in particular the measurements suggest that the concentrations of biogenically derived acids are much lower than the unusually high levels found on the east coast of the antarctic peninsula

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

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

measurements of time variable gravity show mass loss in antarctica using measurements of time variable gravity from the gravity recovery and climate experiment satellites we determined mass variations of the antarctic ice sheet during 2002 2005 we found that the mass of the ice sheet decreased significantly at a rate of 152 80 cubic kilometers of ice per year which is equivalent to 0 4 0 2 millimeters of global sea level rise per year most of this mass loss came from the west antarctic ice sheet

subglacial lake whillans ice penetrating radar and gps observations of a shallow active reservoir beneath a west antarctic ice stream abstract ice penetrating radar and kinematic gps observations from subglacial lake whillans slw west antarctica reveal a shallow lake that is confined by steep basal topographic features radar imaging of slw although indicating wet basal conditions is consistent with a water column depth of only 6 m or less during the near low stand state at the time of the survey kinematic gps profiles reveal that slw is generally defined by a 15 m surface depression centered at s 84 237 w 153 614 this point coincides with the area of lowest hydropotential in the lake basin and also the largest surface elevation range in icesat data therefore this location appears to be an opportune site for subglacial access drilling of this active subglacial lake a distinct basal topographic ridge on the grid south side of the basin is coincident with a strong contrast in relative basal reflectivity 6 db which we interpret as the lake boundary mapped hydropotential calculated assuming hydrostatic equilibrium shows that water enters the lake from the upstream direction and drains downstream we hypothesize that a lake level rise of 5 m plus flexural effects is sufficient to overtop a drainage divide thus slw acts as a temporary storage basin for water beneath whillans ice stream

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

prototype wireless sensors for monitoring subsurface processes in snow and firn the detection and monitoring of meltwater within firn presents a significant monitoring challenge we explore the potential of small wireless sensors etracer et to measure temperature pressure electrical conductivity and thus the presence or absence of meltwater within firn through tests in the dry snow zone at the east greenland ice core project site the tested sensor platforms are small robust and low cost and communicate data via a vhf radio link to surface receivers the sensors were deployed in low temperature firn at the centre and shear margins of an ice stream for 4 weeks and a bucket experiment was used to test the detection of water within otherwise dry firn the tests showed the et could log subsurface temperatures and transmit the recorded data through up to 150 m dry firn two vhf receivers were tested an autonomous phase sensitive radio echo sounder apres and a winradio the apres can combine high resolution imaging of the firn layers by radio echo sounding with in situ measurements from the sensors to build up a high spatial and temporal resolution picture of the subsurface these results indicate that wireless sensors have great potential for long term monitoring of firn processes

a framework for attenuation method selection evaluated with ice penetrating radar data at south pole lake all radar power interpretations require a correction for attenuative losses moreover radar attenuation is a proxy for ice column properties such as temperature and chemistry prior studies use either paired thermodynamic and conductivity models or the radar data themselves to calculate attenuation but there is no standard method to do so and before now there has been no robust methodological comparison here we develop a framework meant to guide the implementation of empirical attenuation methods based on survey design and regional glaciological conditions we divide the methods into the three main groups 1 those that infer attenuation from a single reflector across many traces 2 those that infer attenuation from multiple reflectors within one trace and 3 those that infer attenuation by contrasting the measured power from primary and secondary reflections to assess our framework we introduce a new ground based radar survey from south pole lake comparing selected empirical methods to the expected attenuation from a temperature and chemistry dependent arrhenius model based on the small surveyed area lack of a sufficient calibration surface and low reflector relief the attenuation methods that use multiple reflectors are most suitable at south pole lake

characteristics of the sticky spot of kamb ice stream west antarctica amplitude analysis of reflection seismic data reveals the presence of highly variable bed conditions under the main sticky spot and adjacent regions of the kamb ice stream kis formerly ice stream c the sticky spot which is a zone of bed that imparts high basal resistance to ice flow is situated on a local topographic high composed of consolidated sediments or sedimentary rock any meltwater draining from upglacier along the base of the ice is routed around the sticky spot the ice over the sticky spot includes in at least some places a seismically detectable basal layer containing a low concentration of debris which locally thickens to 40 m over a topographic low in the bed the ice contact basal material ranges from dilated and highly porous to more compacted and stiff and perhaps locally frozen the softer material is preferentially in topographic lows but there is not a one to one correspondence between basal character and basal topography we speculate that the 40 m thick frozen on debris layer formed by glaciohydraulic supercooling of lake drainage events along a basal channel during the former active phase of the ice stream we also speculate that loss of lubricating water perhaps from piracy upstream contributed to the slowdown of the ice stream with drag from the sticky spot playing an important role and with the basal heterogeneity greatly increasing after the slowdown of the ice stream

recovery lakes east antarctica radar assessment of sub glacial water extent 1 a fast flowing tributary of recovery ice stream penetrates more than 500 km into the interior of east antarctica recent satellite based studies found surface features in the onset area of this tributary that indicate a significant subglacial hydraulic system including four large smooth basins the typical surface expression of large subglacial lakes as well as eleven smaller areas over which ice sheet surface elevations rapidly changed by discharge filling of subglacial water here we present the first ice penetrating radar evidence of subglacial conditions in this area we identified a distinct ice water interface only over a limited area within the boundaries of the investigated large smooth basins previously hypothesized to be water filled lakes the radar characteristics in some areas are similar to those of a drained lake indicating that parts of the bed are wet but not a typical lake we also find evidence for discrete water bodies outside of the lake boundaries the lines of evidence indicate that the northern most two recovery lakes have recently drained

analysis of deep penetrating radar surveys of west antarctica us itase 2001 1 we collected 1850 km of ground based radio echo sounding res data in west antarctica along the 2001 us leg of the international trans antarctic scientific expedition traverse us itase these data represent the first high resolution measurements of ice thickness and internal stratigraphy for portions of this area that bear on questions about the stability of the west antarctic ice sheet wais internal layers depicting isochrones have been traced continuously for much of the profile length and a date from the byrd deep core is ascribed to one of these isochrones from which we estimate vertical velocities across the traverse route comparison of our ice thickness measurements with bedmap show good agreement in regions where the database is well constrained by data coverage where the bedmap dataset was forced to interpolate differences are as high as 45 of the ice thickness

first results from radar profiles collected along the us itase traverse from taylor dome to south pole 2006 2008 the 2006 07 and 2007 08 us itase traverses from taylor dome to south pole in east antarctica provided opportunities to survey the subglacial and englacial environments using 3 mhz and 200 mhz radar we present first results of these new ground based radar data a prominent basal deformation layer indicates different ice flow regimes for the northern and southern halves of the byrd glacier drainage buried dune stratigraphy that appears to be related to the megadunes towards the west occurs at depths of up to 1500 m at least two new water filled subglacial lakes were discovered while two recently drained lakes identified from repeat icesat surface elevation surveys appear to be devoid of water

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

the distribution of basal water between antarctic subglacial lakes from radar sounding antarctica s subglacial lakes have two end member geophysical expressions as hydraulically flat radar reflective regions highlighted in ice surface topography and radar sounding profiles definit

englacial and subglacial water flow at skalafellsjokull iceland derived from ground penetrating radar in situ glacsweb probe and borehole water level measurements we reconstruct englacial and subglacial drainage at skalafellsjokull iceland using ground penetrating radar gpr common offset surveys borehole studies and glacsweb probe data we find that englacial water is not stored within the glacier water content 0 0 3 instead the glacier is mostly impermeable and meltwater is able to pass quickly through the main body of the glacier via crevasses and moulins once at the glacier bed water is stored within a thin 1 m layer of debris rich basal ice 2 water content and the till the hydraulic potential mapped across the survey area indicates that when water pressures are high most of the year water flows parallel to the margin and emerges 3 km down glacier at an outlet tongue gpr data indicates that these flow pathways may have formed a series of braided channels we show that this glacier has a very low water storage capacity but an efficient englacial drainage network for transferring water to the glacier bed and therefore it has the potential to respond rapidly to changes in melt water inputs

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

radar reflections reveal a wet bed beneath stagnant ice stream c and a frozen bed beneath ridge bc west antarctica digital airborne radar data were collected during the 1987 88 antarctic field season in nine gridded blocks covering the downstream portions of ice stream b 6 km spacing and ice stream c 11 km spacing together with a portion of ridge bc between them an automated processing procedure was used for picking onset times of the reflected radar pulses converting travel times to distances interpolating missing data converting pressure transducer readings correcting navigational drift performing crossover analysis and zeroing remanent crossover errors interpolation between flight lines was carried out using the minimum curvature method maps of ice thickness estimated accuracy 20 m and basal reflection strength estimated accuracy 1 db were produced the ice thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features the reflection strength map shows pronounced contrasts between the ice streams and ridge bc and between the two ice streams themselves we interpret the reflection strengths to mean that the bed of ice stream c as well as that of ice stream b is unfrozen that the bed of ridge bc is frozen and that the boundary between the frozen bed of ridge bc and the unfrozen bed of ice stream c lies precisely below the former shear margin of the ice stream

transient slip events from near field seismic and geodetic data on a glacier fault whillans ice plain west antarctica 1 bidaily tidally modulated stick slip speed ups of the whillans ice plain wip provide insight into glacier dynamics and failure at a naturally repeating fault asperity we installed a network of continuously operating gps receivers in 2007 and deployed on ice broadband seismometers during the austral summer of 2008 on whillans ice stream wis west antarctica and recorded 26 glacier speed up events previous work during the 2004 field season suggested that these speed ups initiate as failure of an asperity on or near ice raft a that triggers rupture across the entire wip our results for 2008 locate the slip initiation farther to the south of this feature closer to the grounding line and the southernmost extent of the ross ice shelf the initiation may be controlled by a discontinuity in basal boundary conditions at the suture between two ice streams a strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event 30 min suggests slip predictable behavior i e the ability to forecast the eventual slip based on the first minute of seismic radiation successive slip events propagate with different rupture speeds 100 300 m s that strongly correlate r 2 0 73 with the interevent duration in addition the amount of slip achieved during each event appears to be correlated with the rupture speed we use these observations to constrain basal shear stress to be 4 kpa by calculating conditions for basal freezing our observations yield information regarding mechanics and dynamics of ice streambeds at the scale of tens to hundreds of kilometers subglacial processes are notoriously difficult to constrain on these large scales which are relevant to the understanding of regional and continental ice motion

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

constraining attenuation uncertainty in common midpoint radar surveys of ice sheets for common offset radar data there is no clear way to disentangle path effects from reflector characteristics so efforts to determine the physical properties at the bed using reflection amplitude are inherently limited by the constraints on englacial attenuation in this study we identify the theoretical considerations required for interpreting bistatic radar surveys and use data collected on the northeast greenland ice stream negis and kamb ice stream to compute local attenuation profiles we found that failing to correct for angle dependent controls on return power including antenna directivity the reflection coefficient and refractive focusing can bias the computed attenuation rates as much as 30 db km for reflectors at 1 km depth because the radiation characteristics are the dominant source of uncertainty in our data we recommend either a simplified survey design for the future where the antennae are decoupled from the ice surface or additional data collection to constrain the near field permittivity and its effect on the radiation pattern depth averaged attenuation rates computed using cmp methods for deep reflectors yield values 10 db km higher than attenuation rates computed using common offset techniques with the same data we attribute these anomolously high attenuation rates to additional wavenumber and therefore angle dependent interferences between sub wavelength reflectors

spatial variation of englacial radar attenuation modeling approach and application to the vostok flowline 1 constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the vostok ice core site antarctica the simplest model often used but rarely valid assumes a uniform attenuation rate everywhere along the flowline so that total attenuation is proportional to ice thickness the next simplest model uses spatially varying temperatures predicted by an ice flow model and assumes uniform chemistry additional models account for spatially varying chemistry using englacial stratigraphy we find that the roundtrip attenuation to the bed can easily differ by 10 db or more between the uniform attenuation rate model and models that account for variable ice temperature such differences are sufficient to confound the delineation of dry and wet beds also including spatial variations in chemistry produces smaller differences 10 db but the magnitude of these differences depends on the relative importance of dry and wet deposition of impurities in the past accounting for dry deposited impurities requires ice flow modeling and results in larger differences from all other models which assume uniform chemistry or wet deposition only these results indicate that modeling the spatial variation of attenuation requires a spatially varying temperature model in order to infer bed conditions from bed returned power accurately and that both ice core data and radar stratigraphy are also strongly desirable

assessment of electromagnetic absorption of ice from ice core measurements ice core drillings have been performed in various zones in antarctica and greenland to obtain climatological information study ice properties or analyze air and dust encapsulated in the ice during the quaternary period during these procedures a set of measurements to characterize the ice and to evaluate its physical and chemical properties are usually performed in situ in particular using known temperature and dielectric profiles dep measurements it is possible to evaluate the ice electromagnetic power absorption profile valid at the drilling site in the last decades bedrock characterization through radio echo sounding surveys has been improved by the analysis of the power of radar echoes this way analysis of the electromagnetic properties of bedrock interfaces makes it possible to assess the physical characteristics and to distinguish between wet and dry conditions power variation of the received echoes also depends on ice absorption and on bedrock reflectivity due to specific physical conditions of the ice in this paper the propagation of electromagnetic waves through the ice sheet is examined and in particular a new method for establishing the electromagnetic absorption profile for ice from core drilling measurements is proposed and discussed variation in the ice absorption is deduced starting from the analysis of ice core data from the european project for ice coring in antarctica epica at the concordia station antarctica and from the greenland ice core project grip site greenland this direct method of measurement is proposed with the aim of defining common characteristics of the ice absorption rate that are valid both in antarctica and in greenland

recent elevation changes on the ice streams and ridges of the ross embayment from icesat crossovers 1 we analyze differences in icesat elevation estimates at orbital crossover locations to determine short term rates of elevation change for small regions within the ross embayment of the west antarctic ice sheet a linear regression of crossover elevation difference against time difference gives an estimate of the mean elevation change rate during the icesat mission to date we observe prevalent elevation change in the south with uplift in the upstream end of kamb ice stream at 0 24 0 30 m a 1 and thinning in the parts of whillans ice stream mercer ice stream and the adjacent conway and engelhardt ice ridges at 0 05 0 18 m a 1 these rates of elevation change are too large to be explained by the 0 02 0 03 m a 1 formal regression error by seasonal height variations or by accumulation or densification rate variability suggesting that they reflect real variations stemming from ice dynamics of the region