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

a century of stability of avannarleq and kujalleq glaciers west greenland explained using high resolution airborne gravity and other data the evolution of greenland glaciers in a warming climate depends on their depth below sea level flow speed surface melt and ocean induced undercutting at the calving front we present an innovative mapping of bed topography in the frontal regions of sermeq avannarleq and kujalleq two major glaciers flowing into the ice choked torssukatak fjord central west greenland the mapping combines a mass conservation algorithm inland multibeam echo sounding data in the fjord and high resolution airborne gravity data at the ice ocean transition where other approaches have traditionally failed we obtain a reliable precision 40 m solution for bed topography across the ice ocean boundary the results reveal a 700 m deep fjord that abruptly ends on a 100 300 m deep sill along the calving fronts the shallow sills explain the presence of stranded icebergs the resilience of the glaciers to ocean induced undercutting by warm atlantic water and their remarkable stability over the past century

a new velocity map for byrd glacier east antarctica from sequential aster satellite imagery this is the publisher s version also available electronically from http www ingentaconnect com

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

patterns and rates of erosion produced by high energy wave processes on hard rock headlands the grind of the navir shetland scotland abstract the grind of the navir is an ignimbrite headland on the exposed atlantic coast of the shetland islands scotland during storms offshore wave heights exceed 20 m and deep water close inshore allows high energy waves to impact on these cliffs the cliff top at 15 m above sea level is awash with wave water when wave heights exceed 8 m a condition met in storms in most years detailed mapping ground photography and patterns of lichen colonisation and growth allow the processes patterns and rates of erosion to be assessed on different parts of the headland over the past 100 years recent fractures in the rock of the cliff face and top indicate that wave impact forces exceed the 1 5 kpa tensile strength of the rock and fresh sockets low on the cliff face record the removal of fracture bounded blocks that are lost to seaward on the upper part of the cliff face upward moving water flow is capable of removing blocks from both vertical faces and stepped overhangs clefts in the cliff face are extended inwards and upwards by crack propagation and block removal leading to the development of slot caves the movement of waves across the cliff top in fast moving bores quarries blocks from the cliff face close to the edge and from rock steps on the cliff top promoting further rock fracturing socket sizes and block characteristics indicate that blocks of 1 m 3 are rotated from sockets on the cliff top platform and carried landwards for up to 60 m to be deposited in a series of boulder ridges on the cliff top multiple boulder movements during a storm in january 2005 generated impact marks and orientated striations that allow the pattern of water movement to be reconstructed across considerable areas of cliff top the average rate of erosion at the grind is estimated at 1 3 6 0 mm yr on the cliff face and 5 mm yr on the cliff top although erosion rates vary in both time and space although block movement on the cliff top occurs in most years erosion of rock by block removal is most active during major storms nevertheless some parts of the cliff top and face remain blackened by lichen and so have experienced little recent erosion over the last 70 years or so although erosion occurs at the base of the cliffs in recent decades it has been concentrated on upper part of the cliff face and top cliff top erosion is most intense where stepped geos act as channels for waves to access the cliff top the most active features on the cliff top are the boulder ridges where substantial changes can be traced to storms over the past century unlike conventional models of cliff erosion where erosion is concentrated by wave attack at the waterline wave impacts on this headland may occur on any part of cliff face and top and appear particularly effective on the upper cliff and cliff top platform

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

control of ocean temperature on jakobshavn isbrae s present and future mass loss author s bondzio jh morlighem m seroussi h wood mh mouginot j abstract 2018 american geophysical union all rights reserved large uncertainties in model parameterizations and input data sets make projections of future sea level rise contributions of outlet glaciers challenging here we introduce a novel technique for weighing large ensemble model simulations that uses information of key observables the approach is robust to input errors and yields calibrated means and error estimates of a glacier s mass balance we apply the technique to jakobshavn isbrae using a model that includes a dynamic calving law and closely reproduce the observed behavior from 1985 to 2018 by forcing the model with ocean temperatures only our calibrated projection suggests that the glacier will continue to retreat and contribute about 5 1 mm to eustatic sea level rise by 2100 under present day climatic forcing our analysis shows that the glacier s future evolution will strongly depend on the ambient oceanic setting

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

a deep subglacial embayment adjacent to the grounding line of institute ice stream west antarctica the institute ice stream iis in west antarctica may be increasingly vulnerable to melting at the grounding line through modifications in ocean circulation understanding such change requires knowledge of grounding line boundary conditions including the topography on which it rests here we discuss evidence from new radio echo sounding res data on the subglacial topography adjacent to the grounding line of the iis in doing so we reveal a previously unknown subglacial embayment immediately inland of the iis grounding zone which is not represented in the bedmap2 compilation we discuss whether there is an open water connection between the embayment and the ice shelf cavity the exact location of the grounding line over the embayment has been the subject of considerable uncertainty with several positions being proposed recently from our compilation of data we are able to explain which of these grounding lines is most likely and in doing so highlight the need for accurate bed topography in conjunction with satellite observations to fully comprehend ice sheet processes in this region and other vulnerable locations at the grounded margin of antarctica

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

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

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

focused synthetic aperture radar processing of ice sounder data collected over the greenland ice sheet the authors developed a synthetic aperture radar sar processing algorithm for airborne spaceborne ice sounding radar systems and applied it to data collected in greenland by using focused sar phase corrected coherent averaging they improved along track resolution by a factor of four and provided a 6 db processing gain over unfocused sar coherent averaging without phase correction based on a point target analysis for a greenland ice sounding data set also they demonstrated that the focused sar processing reduced clutter and enabled them to identify bedrock interface returns buried in clutter using focused sar technique they processed data collected over a key 360 km long portion of the 2000 m contour line of southwest greenland to the best of their knowledge these are the first high quality radar ice thickness measurements over this key location moreover these ice thickness measurements have been used for improving mass balance estimates of the greenland ice sheet

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

bed elevation of jakobshavn isbrae west greenland from high resolution airborne gravity and other data jakobshavn isbrae west greenland which holds a 0 6 m sea level volume equivalent has been speeding up and retreating since the late 1990s interpretation of its retreat has been hindered by difficulties in measuring its ice thickness with airborne radar depth sounders here we employ high resolution helicopter borne gravity data from 2012 to reconstruct its bed elevation within 50 km of the ocean margin using a three dimensional inversion constrained by fjord bathymetry data offshore and a mass conservation algorithm inland we find the glacier trough to be asymmetric and several 100 m deeper than estimated previously in the lower part from 1996 to 2016 the grounding line migrated at 0 6 km yr from 700 m to 1100 m depth upstream the bed drops to 1600 m over 10 km then slowly climbs to 1200 m depth in 40 km jakobshavn isbrae will continue to retreat along a retrograde slope for decades to come

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

magnitude and frequency of extra tropical north atlantic cyclones a chronology from cliff top storm deposits cliff top storm deposits ctsds are known to occur on cliffs at elevations of up to 50 m above sea level at exposed sites on the deep water coasts of the british isles eye witness accounts and time series field mapping of ctsds demonstrate their formation over the historical period and their continued modification during major storms with ctsds burying a range of modern human derived debris older deposits show surface lichen growth and weathering effects that relate to the relative age of ctsds over longer time spans buried sand and peat deposits provide dates on phases of ctsd emplacement and mobilisation and so provide an indication of the frequency and timing of the most severe storms two adjacent sites in shetland provide a combination of 14c dates on peat buried beneath storm deposited boulders 14c dates on marine shells buried inside ctsd boulder ridges and optically stimulated luminescence osl ages on sands intercalated with storm derived boulders although the precision is low this evidence identifies phases of exceptional storm activity spanning approximately 400 550 700 1050 1300 1900 and since 1950 ad by the very nature of cliff top erosion and deposition these records are fragmentary yet the timeline of major storms recorded in the shetland cliffs matches fairly well with the more general record of storminess in the north atlantic derived from the sea salt record in the gisp2 ice core as such ctsds represent an overlooked archive of storm sedimentation with the potential to elucidate local storm chronologies as well as informing debate surrounding the palaeo tempestology and impact of major cyclones and hurricanes

the site formation history of schoningen 13ii 4 germany testing different models of site formation by means of spatial analysis spatial statistics and orientation analysis abstract the lower palaeolithic middle pleistocene locality schoningen has been a focus of archaeological research for over two decades the locality is best known for the discovery of wooden spears in close association with numerous butchered remains of horses and other large mammals in the spear horizon scho 13ii 4 with an age of ca 300 kyr several site formation models have been proposed to explain the faunal accumulation at the site 1 single hunting event on a dried lake shore 2 multiple hunting events in the soft mud of a lake shore 3 deposition on an exposed delta plain 4 geogenic displacement by hydrological processes 5 hominin waste disposal and storage behaviour and 6 hominin butchering activities on a frozen lake surface visual spatial analyses allow for the subjective incorporation of archaeological knowledge in the interpretation of spatial data while spatial statistics allow for more objective and reproducible inferences about spatial patterns the combination of the two could thus provide a vital tool in disentangling complex site formation processes this study uses a combination of visual spatial analyses spatial statistics and orientation analyses in order to further disentangle the site formation history of scho 13ii 4 and to assess the impact of post depositional processes on the faunal assemblage this study revealed the existence of intra site and inter species differences in spatial distribution and orientation the results of this study are compared to the suggested site formation models for scho 13ii 4 to test which of these models is most parsimonious with the spatial distribution and orientation of the faunal assemblage it is concluded that the previously proposed site formation models are overly simplified and cannot be used to explain the site formation history of scho 13ii 4

the mechanisms behind jakobshavn isbrae s acceleration and mass loss a 3 d thermomechanical model study the mechanisms causing widespread flow acceleration of jakobshavn isbrae west greenland remain unclear despite an abundance of observations and modeling studies here we simulate the glacier s evolution from 1985 to 2016 using a three dimensional thermomechanical ice flow model the model captures the timing and 90 of the observed changes by forcing the calving front basal drag in the trough is low and lateral drag balances the ice stream s driving stress the calving front position is the dominant control on changes of jakobshavn isbrae since the ice viscosity in the shear margins instantaneously drops in response to the stress perturbation caused by calving front retreat which allows for widespread flow acceleration gradual shear margin warming contributes 5 to 10 to the total acceleration our simulations suggest that the glacier will contribute to eustatic sea level rise at a rate comparable to or higher than at present

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

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

a search in north greenland for a new ice core drill site a new deep ice core drilling site has been identified in north greenland at 75 12 n 42 30 w 316 km north northwest nnw of the grip drill site on the summit of the ice sheet the ice thickness here is 3085 m the surface elevation is 2919 m the north grip ngrip site is identified so that ice of eemian age 115 130 ka bp calendar years before present is located as far above bedrock as possible and so the thickness of the eemian layer is as great as possible an ice flow model similar to the one used to date the grip ice core is used to simulate the flow along the nnw trending ice ridge surface and bedrock elevations surface accumulation rate distribution and ratio echo sounding along the ridge have been used as model input the surface accumulation rate drops from 0 23 m ice equivalent year 1 at grip to 0 19 m ice equivalent year 1 50 km from grip over the following 300 km the accumulation is relatively constant before is starts decreasing again further north ice thickness up to 3250 m bring the temperature of the basal ice up to the pressure melting point 100 250 km from grip the ngrip site is located 316 km from grip in a region where the bedrock is smooth and the accumulation rate is 0 19 m ice equivalent year 1 the modeled basal ice here has always been a few degrees below the pressure melting point internal radio echo sounding horizons can be traced between the grip and ngrip sites allowing us to date the ice down to 2300 m depth 52 ka bp an ice flow model predicts that the eemian age ice will be located in the depth range 2710 2800 m which is 285 m above the bedrock this is 120 m further above the bedrock and the thickness of the eemian layer of ice is 20 m thicker than at the grip ice core site

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

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

automatic ice surface and bottom boundaries estimation in radar imagery based on level set approach accelerated loss of ice from greenland and antarctica has been observed in recent decades the melting of polar ice sheets and mountain glaciers has considerable influence on sea level rise in a changing climate ice thickness is a key factor in making predictions about the future of massive ice reservoirs the ice thickness can be estimated by calculating the exact location of the ice surface and subglacial topography beneath the ice in radar imagery identifying the locations of ice surface and bottom is typically performed manually which is a very time consuming procedure here we propose an approach which automatically detects ice surface and bottom boundaries using distance regularized level set evolution in this approach the complex topology of ice surface and bottom boundary layers can be detected simultaneously by evolving an initial curve in the radar imagery using a distance regularized term the regularity of the level set function is intrinsically maintained which solves the reinitialization issues arising from conventional level set approaches the results are evaluated on a large data set of airborne radar imagery collected during a nasa icebridge mission over antarctica and show promising results with respect to manually picked data

radio echo sounding of pine island glacier west antarctica aperture synthesis processing and analysis of feasibility from space airborne radio echo sounding of the west antarctic ice sheet over pine island glacier was performed in the austral summer of 2004 2005 under the national science foundation s west antarctic ice sheet initiative the british antarctic survey flew its newly developed 150 mhz ice sounding radar over pine island glacier and collected approximately 35 000 km of sounding data the synthetic aperture radar sar technique was applied to process those data in order to enhance radar signatures from the bed as a matter of fact airborne ice sounding radar systems are generally affected by surface clutter returns masking the echoes of internal layers and ice bedrock transition at a large depth focused and unfocused doppler filtering sar techniques were compared and their respective advantages disadvantages were analyzed enhancement of bedrock detection at a large depth 2000 m through sar processing is demonstrated finally a simulation analysis was performed for assessing the feasibility of ice sheet sounding from space it is shown that the gain in bed detection threshold is marginal in the satellite sounding geometry airborne radar antarctica ice sounding satellite remote sensing subglacial topography synthetic aperture radar sar processing west antarctic ice sheet

ground penetrating radar profiles of the mcmurdo shear zone antarctica acquired with an unmanned rover interpretation of crevasses fractures and folds within firn and marine ice abstractthe crevassed firn of the mcmurdo shear zone sz within the ross ice shelf may also contain crevasses deep within its meteoric and marine ice but the surface crevassing prevents ordinary vehicle access to investigate its structure geophysically we used a lightweight robotic vehicle to tow 200 and 400 mhz ground penetrating radar antennas simultaneously along 100 parallel transects over a 28 km2 grid spanning the sz width transects were generally orthogonal to the ice flow total firn and meteoric ice thickness was approximately 160 m firn crevasses profiled at 400 mhz were up to 16 m wide under snow bridges up to 10 m thick and with strikes near 35 40 to the transect direction from the top down 200 mhz profiles revealed firn diffractions originating to a depth of approximately 40 m no discernible structure within the meteoric ice a discontinuous transitional horizon and at least 20 m of stratified marine ice 28 31 m of freeboard found more marine ice exists based on 10 consecutiv

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

analysis of antarctic glacigenic sediment provenance through geochemical and petrologic applications abstract the number of provenance studies of glacigenic sediments in antarctica has increased dramatically over the past decade providing an enhanced understanding of ice sheet history and dynamics along with the broader geologic history such data have been used to assess glacial erosion patterns at the catchment scale flow path reconstructions over a wide range of scales and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments it is notable that even though most of the bedrock of the continent is ice covered and inaccessible provenance data can provide such valuable information about antarctic ice and can even be used to infer buried rock types along with their geo and thermochronologic history glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes and in this paper we review methods and examples from all size fractions that have been applied to the antarctic glacigenic sedimentary record interpretations of these records must take careful consideration of the choice of analytical methods uneven patterns of erosion and spatial variability in sediment transport and rock types which all may lead to a preferential identification of different elements of sources in the provenance analyses because of this we advocate a multi proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods

ice sheet velocity mapping a combined interferometric and speckle tracking approach the first and second radarsat antarctic mapping missions amm 1 and 2 have now acquired interferometric synthetic aperture radar sar overmuch of the ice sheet the radarsat 24 day repeat cycle is nearly ideal for measuring slow ice motion e g 1 but application of sar interferometry is limited in faster moving areas with a 1day repeat period ers 1 2 tandem sar data are much better matched to fast motion but are not always available fortunately several authors have demonstrated the ability to measure velocity in fast moving areas by tracking sar speckle from image to image which works well even in the absence of visible features while these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements they serve well in areas where there are no data suitable for conventional interferometry this paper describes algorithms i have developed for merging interferometric and speckle tracking data from multiple swaths to form a single seamless mosaic of velocity at each point in the mosaic all the available data are combined to produce estimates of the velocity and the associated error this technique is demonstrated using radarsat data collected over lambert glacier antarctica during amm 1 and 2

resolving bathymetry from airborne gravity along greenland fjords recent glacier mass loss in greenland has been attributed to encroaching warming waters but knowledge of fjord bathymetry is required to investigate this mechanism the bathymetry in many greenland fjords is unmapped and difficult to measure from 2010 to 2012 national aeronautics and space administration s operation icebridge collected a unique set of airborne gravity magnetic radar and lidar data along the major outlet glaciers and fjords in greenland we applied a consistent technique using the icebridge gravity data to create 90 bathymetric profiles along 54 greenland fjords we also used this technique to recover subice topography where warm or crevassed ice prevents the radar system from imaging the bed here we discuss our methodology basic assumptions and error analysis we present the new bathymetry data and discuss observations in six major regions of greenland covered by icebridge the gravity models provide a total of 1950 line kilometers of bathymetry 875 line kilometers of subice topography and 12 new grounding line depths

elevation change of the southern greenland ice sheet seasat and geosat satellite altimeter measurements for the greenland ice sheet south of 72 degreesn latitude show that surface elevations above 2000 meters increased at an average rate of only 1 5 0 5 centimeters per year from 1978 to 1988 in contrast elevation changes varied regionally from 15 to 18 centimeters per year seasonally by 15 centimeters and interannually by 8 centimeters the average growth rate is too small to determine if the greenland ice sheet is undergoing a long term change due to a warmer polar climate

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

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

direction of arrival analysis of airborne ice depth sounder data in this paper we analyze the direction of arrival doa of the ice sheet data collected over jakobshavn glacier with the airborne multichannel radar depth sounder mrds during the 2006 field season we extracted weak ice bed echoes buried in signals scattered by the rough surface of the fast flowing jakobshavn glacier by analyzing the doa of signals received with a five element receive antenna array this allowed us to obtain ice thickness information which is a key parameter when generating bed topography of glaciers we also estimated ice bed roughness and bed slope from the combined analysis of the doa and radar waveforms the bed slope is about 8 and the roughness in terms of rms slope is about 16

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