effects of birefringence within ice sheets on obliquely propagating radio waves in this paper effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side looking radar data a formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals ice fabrics we applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray path configurations ice fabrics and radar frequency predicted frequency dependence of the bed return power replicates prominent features observed at greenland ngrip ice core site results show that birefringence in ice of 1 km or more thickness with strong weak fabric can reduce the power returned from the bed 2 db or more at frequencies higher than 200 mhz 20 mhz as compared to isotropic ice this suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets application of this formalism also suggests a radar frequency range usable for attenuation measurements possible effects of fabric on synthetic aperture radar processing and a feasibility of remote sensing of ice fabric

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

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

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

comparison of fractal models using nmr and ct analysis in low permeability sandstones abstract fractal geometry provides an effective method for characterization of the complex and irregular pore structure of eocene shahejie low permeability sandstones in the raoyang sag the bohai bay basin china laboratory measurements including porosity permeability scanning electron microscope sem thin sections nuclear magnetic resonance measurements nmr and x ray computed tomography ct technology are used to provide insights into the fractal characteristics of pore structure in sandstones of eocene shahejie formation in the bohai bay basin china quantitative ct analysis reveals the pore radius is not always linear to t2 transverse relaxation time value obtained from the nmr tests but instead power function of t2 fractal analysis was performed on the t2 distribution using various fractal models and the related fractal dimensions are calculated the fractal dimensions calculated using various fractal models are correlated with nmr parameters and permeability the fractal curves break into two segments at the t2cutoff t2 separating the immovable and movable fluids value or smaller when using fractal model and fractal model and only the large scale pore networks can be described by the fractal geometry mostly the entire pore size distributions micro pores to large scale pore networks can be described by the fractal model and the calculated fractal dimensions are in accordance with the ct scanning and thin section data and are strongly correlated with the t2gm geometric mean of t2 permeability and bvi bulk volume of immovable fluids the fractal behaviors of pore size distributions from nmr analysis have implications for pore structure evaluation in low permeability sandstones with similar geological settings

ice flow induced scattering zone within the antarctic ice sheet revealed by high frequency airborne radar to better understand how internal radar echoes depend on ice flow conditions and radar polarization we surveyed two basins in east antarctica using 179 mhz airborne radar we compared radar echoes from three ice flow conditions parallel sheet flow in the main stream of a basin convergent flow towards an ice stream and longitudinal compression by nunataks we detected a distinct zone of high radar scattering several hundred meters thick at middle depths in the latter two regions this high scattering zone was detected only when the radar polarization plane was parallel to the compression axis in ice such a high scattering zone was not found in the parallel flow region regardless of the polarization using a recently developed theory of radar scattering in ice we interpret the high scattering zone as being caused by crystal orientation fabric alternations among adjacent ice layers due to difference in horizontal strain components we argue that the spatial variation of the high scattering zone is crucial for understanding past and present flow features

new boundary conditions for the west antarctic ice sheet subglacial topography beneath pine island glacier 1 predictions about future changes in the amundsen sea sector of the west antarctic ice sheet wais have been hampered by poorly known subglacial topography extensive airborne survey has allowed us to derive improved subglacial topography for the pine island glacier basin the trunk of this glacier lies in a narrow 250 km long 500 m deep sub glacial trough suggesting a long lived and constrained ice stream two tributaries lie in similar troughs others lie in less defined shallower troughs the lower basin of the glacier is surrounded by bedrock which after deglaciation and isostatic rebound could rise above sea level this feature would impede ice sheet collapse initiated near the grounding line of this glacier and prevent its progress into the deepest portions of wais the inland slope of the bed beneath the trunk of the glacier however confirms potential instability of the lower basin containing sufficient ice to raise global sea by 24 cm

spatial patterns in mass balance of the siple coast and amundsen sea sectors west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

late jurassic coral microbial reefs from the northern paris basin facies palaeoecology and palaeobiogeography abstract during the late middle oxfordian patch reefs grew on the northern margin of the paris basin according to the facies analysis of the reef and inter reef sediments the environment was a warm clear and agitated sea with highly episodic sedimentation the bioherms were a short lived phenomenon during the third phase of regional reefal development sequence stratigraphically they are associated with a highstand system tract volumetrically and trophically dominant organisms were microbes now represented by massive clotted leiolite stalactitic hemispheroids with purely thrombolitic texture are restricted to open caves corals were of structural reef building importance due to their rapid upward growth the patch reefs are characterised by thickets of ramose corals which developed a very open framework in the vicinity of these patch reefs though in hydrodynamically higher energy environments grew thickets of more stoutly branched corals however they are rarely preserved in situ and are generally represented as abundant coral rubble the reef taxa are characterised by the notable absence of several groups e g oysters serpulids bryozoans pectinids occurring at other localities where reefs of similar age developed in similar environmental conditions the reefs also have strikingly modern aspects to them in particular the presence of cryptic elements within caves and a sponge dominated borer association dwellers belong to various life form types although encrusting taxa are exceedingly rare this may be explained by the presence of soft microbial films on most surfaces the palaeoecological analysis suggests that the major controls on faunal composition and high diversity were elevated nutrient levels highly episodic sedimentation and probably seasonal environmental disturbances structural and functional aspects of the reef community grazers trigger framebuilders borers trigger binders binders hamper borers allow ecological comparisons to be made with contemporaneous as well as recent reefs the unique combination of ecological factors resulted in a specialised previously undescribed community which differs from both tethyan and northern localities in various aspects these include cavities with cryptofauna prominence of grazing gastropods and high faunal diversity in a microbially dominated build up

complex fabric development revealed by englacial seismic reflectivity jakobshavn isbrae greenland this is the published version copyright 2008 american geophysical union all rights reserved

holocene accumulation and ice sheet dynamics in central west antarctica 1 we derive depth age relationships across the ice divide between the ross and amundsen seas by tracking radar detected layers from the byrd ice core and a dated 105 m core near the divide the depth age relationships and an ice flow model are used to establish histories of accumulation and ice sheet dynamics over the past 8000 years results show that accumulation was approximately 30 higher than today from 5000 to 3000 years ago antarctic climate variability today is dominated by periodic fluctuations in strength of the circumpolar vortex which raises the possibility that the vortex was systematically weaker during the period of high accumulation accumulation today decreases almost linearly across the divide it is unlikely that this pattern has changed through the holocene the radar detected stratigraphy shows no evidence of the arched layers that are expected beneath a stable divide that is frozen to its bed implying that the divide has also been migrating and or the basal ice has been sliding through the holocene we cannot rule out the possibility of sliding because the basal ice is near its pressure melting point other evidence indicates that divide migration is likely the ross sea sector is now near steady state but it had a strong negative imbalance 200 years ago when kamb ice stream was active in contrast recent speedups of pine island and thwaites glaciers have likely caused the mass balance of the amundsen sea sector to become negative the divide is likely migrating toward the ross sea today

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

decoding the dipstick thickness of siple dome west antarctica at the last glacial maximum ice thickness in west antarctica at the last glacial maximum lgm is poorly known yet is key information for understanding ice streams and interpreting ice cores although trim lines moraine limits and exposure age dating provide geologic constraints on ice thickness near the transantarctic mountains and in marie byrd land lack of exposed bedrock hampers traditional geologic methods in a central 2 3 10 6 km 2 region here we infer ice sheet thickness changes in the central ross sea embayment by using a tran sient ice flow model to find combinations of accumulation rate and ice sheet thickness histories that match the depth age relationship and the measured layer thickness pattern in the siple dome ice core after we reject unreasonable accumulation rate histories the remaining history pairs indicate thinning of 200 400 m since the lgm our estimate is lower than previous reconstructions that were constrained by geologic evidence from the transantarctic mountains and by marine data from the ross sea floor which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period low surface elevations in the central ross sea embayment during the lgm do not preclude thicker ice along its boundaries however if this grounded ice sheet came over 1000 km from interior west antarctica as is usually assumed then it had very low surface slope requiring a very slippery bed alternatively the grounded ross sea ice might have flowed from the transantarctic mountains and western marie byrd land

a dynamic early east antarctic ice sheet suggested by ice covered fjord landscapes the east antarctic ice sheet has played a fundamental part in modulating climate and sea level during the past 30 million years understanding its history is crucial to evaluating its future behaviour and response to global warming airborne ice penetrating radar studies now reveal a fjord like landscape beneath several kilometres of ice in the east antarctic aurora subglacial basin the data confirm and provide a new constraint on the magnitude and dynamics of the oscillations of the east antarctic ice sheet during the late cenozoic which had previously been supported only by marine cores the first cenozoic ice sheets initiated in antarctica from the gamburtsev subglacial mountains1 and other highlands as a result of rapid global cooling 34 million years ago2 in the subsequent 20 million years at a time of declining atmospheric carbon dioxide concentrations2 and an evolving antarctic circumpolar current2 sedimentary sequence interpretation3 and numerical modelling4 suggest that cyclical periods of ice sheet expansion to the continental margin followed by retreat to the subglacial highlands occurred up to thirty times these fluctuations were paced by orbital changes and were a major influence on global sea levels5 ice sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of antarctic topographic lowlands here we show that the basal topography of the aurora subglacial basin of east antarctica at present overlain by 2 4 5 km of ice is characterized by a series of well defined topographic channels within a mountain block landscape the identification of this fjord landscape based on new data from ice penetrating radar provides an improved understanding of the topography of the aurora subglacial basin and its surroundings and reveals a complex surface sculpted by a succession of ice sheet configurations substantially different from today s at different stages during its fluctuations the edge of the east antarctic ice sheet lay pinned along the margins of the aurora subglacial basin the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level although the timing of the channel incision remains uncertain our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions each of which would have over deepened existing topographic depressions reversing valley floor slopes

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

rapid development of anisotropic ice crystal alignment fabrics inferred from englacial radar polarimetry central west antarctica 1 anisotropy in englacial radar power was measured using 60 mhz and 179 mhz copolarized pulse modulated radar at 19 sites in central west antarctica the study region is a 100 300 km2area near the west antarctic ice sheet divide that separates ice flow toward the ross and amundsen embayments the frequency dependence of the returned power indicates that most of the radar data are affected by vertical variations in the crystal orientation fabric cof though the 60 mhz data are more affected by acidity contrasts in the top 1000 m significant polarimetric variations occur at most sites likely due to effects of the anisotropic cof patterns more anisotropic variations occur at sites with significant horizontal strain whereas more isotropic variations occur at sites where vertical compression dominates azimuthal shifts with depth of the principal axes of cof were found in shallow ice near the current flow divide and at greater depths over locations of rough bed the former indicates that the divide has differentially migrated resulting in a rotation of the principal cof axes nevertheless the regionally consistent radar signatures suggest that the first order ice properties in this area have remained constant and that no major changes in the strain configuration or ice topography have occurred for the past five to eight thousand years we conclude that shallow polarimetric features can be related to the current strain configurations and that englacial polarimetric features can help constrain current ice rheology and evolution of the ice topography

development of an antarctic digital elevation model by integrating cartographic and remotely sensed data a geographic information system based approach we present a high resolution digital elevation model dem of the antarctic it was created in a geographic information system gis environment by integrating the best available topographic data from a variety of sources extensive gis based error detection and correction operations ensured that our dem is free of gross errors the carefully designed interpolation algorithms for different types of source data and incorporation of surface morphologic information preserved and enhanced the fine surface structures present in the source data the effective control of adverse edge effects and the use of the hermite blending weight function in data merging minimized the discontinuities between different types of data leading to a seamless and topographically consistent dem throughout the antarctic this new dem provides exceptional topographical details and represents a substantial improvement in horizontal resolution and vertical accuracy over the earlier continental scale renditions particularly in mountainous and coastal regions it has a horizontal resolution of 200 m over the rugged mountains 400 m in the coastal regions and approximately 5 km in the interior the vertical accuracy of the dem is estimated at about 100 130 m over the rugged mountainous area better than 2 m for the ice shelves better than 15 m for the interior ice sheet and about 35 m for the steeper ice sheet perimeter the antarctic dem can be obtained from the authors

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

potential mechanisms for anisotropy in ice penetrating radar data radar data center frequency 150 mhz collected on the antarctic plateau near the epica deep drilling site in dronning maud land vary systematically in backscattered power depending on the azimuth antenna orientation backscatter extrema are aligned with the principal directions of surface strain rates and change with depth in the upper 900 m backscatter is strongest when the antenna polarization is aligned in the direction of maximal compression while below 900m the maxima shift by 90 pointing towards the lateral flow dilatation we investigate the backscatter from elongated air bubbles and a vertically varying crystal orientation fabric cof using different scattering models in combination with ice core data we hypothesize that short scale variations in cof are the primary mechanism for the observed anisotropy and the 900m boundary between the two regimes is caused by ice with varying impurity content observations of this kind allow the deduction of cof variations with depth and are potentially also suited to map the transition between holocene and glacial ice

ice elevation near the west antarctic ice sheet divide during the last glaciation 1 interior ice elevations of the west antarctic ice sheet wais during the last glaciation which can serve as benchmarks for ice sheet models are largely unconstrained here we report past ice elevation data from the ohio range located near the wais divide and the onset region of the mercer ice stream cosmogenic exposure ages of glacial erratics that record a wais highstand 125 m above the present surface date to 11 5 ka the deglacial chronology prohibits an interior wais contribution to meltwater pulse 1a our observational data of ice elevation changes compare well with predictions of a thermomechanical icesheet model that incorporates very low basal shear stress downstream of the present day grounding line we conclude that ice streams in the ross sea embayment had thin lowslope profiles during the last glaciation and interior wais ice elevations during this period were several hundred meters lower than previous reconstructions citation ackert r p jr s mukhopadhyay b r parizek and h w borns 2007 ice elevation near the west antarctic ice sheet divide during the last glaciation geophys res lett 34 l21506 doi 10 1029 2007gl031412

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

past and present accumulation rate reconstruction along the dome fuji kohnen radio echo sounding profile dronning maud land east antarctica we used internal ice layers from a radio echo sounding profile between the kohnen and dome fuji deep drilling sites to infer the spatio temporal pattern of accumulation rate in this sector of dronning maud land east antarctica continuous internal reflection horizons can be traced to about half of the ice thickness and have a maximum age of approximately 72 7 ka bp to infer palaeo accumulation rates from the dated layers we derived the thinning functions from a flow calculation with a high resolution higher order model of dronning maud land embedded into a three dimensional thermomechanical model of the antarctic ice sheet the method takes into account complex ice flow dynamics and advection effects that cannot be dealt with using traditional local approaches we selected seven time intervals over which we determine the average accumulation rate and average surface temperature at the place and time of origin of the layer particles our results show lower accumulation rates along eastern parts of the profile for the late holocene 0 5 ka bp than are shown by existing maps which had no surface control points during the last glacial period we find a substantially lower accumulation rate than predicted by the usual approach linking palaeo accumulation rates to the condensation temperature above the surface inversion layer these findings were used to fine tune the relation between accumulation rate and temperature

recent thinning and migration of the western divide central west antarctica antarctica is currently thinning 0 08 m a 1 and migrating toward the ross sea at 10 m a 1 the asymmetric pattern of thickness change across the divide is not caused by changes in the accumulation gradient but rather by dynamical thinning that is stronger in the amundsen sea sector than in the ross sea sector available geological and glaciological data indicate that this pattern of thinning has persisted for at least two millennia with increased asymmetry likely over the past few centuries our data however are not sufficient to determine whether the present day migration of the western divide is a response to long term millennial forcing shorter term centennial forcing or both citation conway h and l a rasmussen 2009 recent thinning and migration of the western divide central west antarctica geophys res lett 36 l12502 doi 10 1029 2009gl038072

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

polar radar for ice sheet measurements prism abstract we provide an overview of a project to develop and demonstrate radars for imaging the ice bed interface mapping shallow and deep internal layers and measuring ice thickness we developed an ultra wideband very high frequency vhf radar 120 to 300 mhz for simultaneously measuring ice thickness mapping internal layers at depth and imaging the ice bed interface and an ultra high frequency uhf ultra wideband radar 500 to 2000 mhz for fine resolution mapping of near surface internal layers we used these radars to collect data at summit camp greenland during july 2005 and the west antarctic ice sheet wais deep drill core site during january 2006 these results provide us with an unprecedented view of basal characteristics and internal layers we also developed an outreach program a communications system and rovers to facilitate geophysical measurements in the future

radio wave depolarization and scattering within ice sheets a matrix based model to link radar and ice core measurements and its application crystal orientation fabric cof has a large influence on ice sheet flow earlier radar studies have shown that cof based birefringence occurs within ice sheets radio wave scattering in polar ice results from changing physical properties of permittivity and conductivity that arise from differing values of density acidity and cof we present an improved mathematical model that can handle all these phenomena together we use this matrix based model to study the two way propagation of depolarized radio waves that scatter at both isotropic and anisotropic boundaries based on numerical simulations we demonstrate how cof affects the radar signals in terms of radar polarization and frequency we then compare the simulated features with vhf radar data obtained at two contrasting inland sites in east antarctica where cof is known from ice core studies these two sites are dome fuji located near a dome summit and mizuho located in a converging ice flow region data at dome fuji are dominated by typical features resulting from birefringence in contrast both birefringence and anisotropic scattering affect the radar data at mizuho we argue that radar methods can be used to determine principal axes and strength of birefringence in the ice sheets

rigorous gps data processing strategies for glaciological applications global positioning system gps data are now routinely used for many glaciological applications in some common cases systematic errors are unmodelled at the data processing stage although they are often presumed insignificant in this paper i investigate these assumptions for three different scenarios 1 measurements on a moving glacier 2 measurements on a floating ice shelf and 3 precise height determination over large elevation ranges such as for aircraft positioning in lidar laser altimeter missions in each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the gps processing software under certain conditions if these coordinate biases go undetected short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques more rigorous processing strategies are discussed that allow for bias free parameter estimation