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

modeling the evolution of antarctic ice sheet over the last 420 000 years implications for altitude changes in the vostok region a new thermomechanical three dimensional model designed to simulate the evolution of the antarctic ice sheet over long time periods is presented this model incorporates the various types of ice flow found in antarctica relatively slow inland ice flow that is essentially due to ice deformation fast ice flow in the regions with ice streams and ice shelf flow by coupling these three types of flow it is possible to predict grounding line migration simulations covering four glacial interglacial cycles have been conducted by forcing this model with a temperature record from vostok and a sea level record from marine cores our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the antarctic ice volume on the other hand the altitude of the ice sheet surface at vostok is driven by accumulation rate variations the amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the antarctic plateau

role of transition zones in marine ice sheet dynamics 1 it has been suggested that rapid thinning of the west antarctic ice sheet is due to increased melting under ice shelves caused by a gradual ocean warming shepherd et al 2004 payne et al 2004 showed that such melting could lead to an acceleration of grounded ice flow in this paper we analyze the response of a marine ice sheet to different perturbations near the grounding line using a numerical ice sheet model that takes into account longitudinal stress coupling and grounding line migration at subgrid precision based on a novel technique results show that stress transmission or longitudinal coupling across the grounding line plays a decisive role the grounding line migration is a function of the length scale over which the basal conditions change from frozen to the bed to floating the transition zone we demonstrate that thinning of the ice shelf due to bottom melting has a negligible effect on the grounded ice mass only perturbations at the grounding line or reduction in buttressing of the ice shelf substantially thins the grounded ice sheet inclusion of lateral drag does not alter these results qualitatively marine ice sheets with large transition zones such as ice streams seem highly sensitive to such perturbations compared to ice sheets with small transition zones such as an abrupt ice sheet ice shelf junction

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

the role of subglacial water in ice sheet mass balance subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets depending on the geometry and capacity of the subglacial hydrologic system increased surface melting in greenland over the coming decades may influence the ice sheet s mass balance furthermore subglacial lakes in antarctica can modulate ice velocities and act as nucleation points for new fast flowing ice streams in the coming decades significant changes in the polar regions will increase the contribution of ice sheets to global sea level rise under the ice streams and outlet glaciers that deliver ice to the oceans water and deformable wet sediments lubricate the base facilitating fast ice flow the influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system water moving rapidly through a well connected system of conduits or channels will have little impact on ice sheet velocities but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet and thereby trigger increased ice sheet velocities in greenland the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice sheet mass balance in antarctica subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice flow tributaries climate models of ice sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics

investigating the stability of subglacial lakes with a full stokes ice sheet model despite the large amount of subglacial lakes present underneath the east antarctic ice sheet and the melt processes involved the hydrology beneath the ice sheet is poorly understood changes in subglacial potential gradients may lead to subglacial lake outbursts discharging excess water through a subglacial drainage system underneath the ice sheet such processes can eventually lead to an increase in ice flow in this paper a full stokes numerical ice sheet model was employed which takes into account the ice flow over subglacial water bodies in hydrostatic equilibrium with the overlying ice sensitivity experiments were carried out for small perturbations in ice flow and basal melt rate as a function of ice thickness general surface slope ice viscosity and lake size in order to investigate their influence on the subglacial potential gradient and the impact on subglacial lake drainage experiments clearly demonstrate that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events lake drainage can therefore be regarded as a common feature of the subglacial hydrological system and may influence to a large extent the present and future behavior of large ice sheets

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

high frequency ground motion and source characteristics of the 2008 wenchuan and 2013 lushan china earthquakes the 2008 mw7 9 wenchuan and the 2013 mw6 6 lushan earthquakes which both occurred on the longmen shan thrust belt show some interesting similarities and differences whereas the wenchuan earthquake entailed a rupture zone that extended about 300 km northeastward with fault slip extending to the surface the lushan earthquake was the result of a buried and much more compact zone of rupture the high frequency ground motions however for these two earthquakes as measured by the peak ground acceleration were evidently influenced by neither the extent of rupture nor the presence or absence of surface rupture the source parameters for these two earthquakes tend to confirm the idea that high frequency ground motion is controlled by stress changes in the rupture zone that give rise to the radiated ground acceleration the apparent stresses for the wenchuan and lushan earthquakes are about 0 5 and 0 75 mpa respectively and the stress drops in the same order are about 2 5 and 3 5 mpa the ratios of average stress drop to apparent stress are in the range 4 5 5 for both events consistent with expectations based on the brune j geophys res 75 26 4997 5009 1970 source model

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

understanding glacier flow in changing times subannual lurches of the greenland and antarctic ice sheets may reduce uncertainties about climate change effects on sea level rise

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

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

homogeneous climate variability across east antarctica over the past three glacial cycles recent ice core studies have raised the disturbing possibility that glacial interglacial climate changes may be non uniform across antarctica1 2 these findings have been confined to records from the ross sea sector of the continent but significant deviations in other areas would call into question the widely assumed validity of the climate record obtained from vostok east antarctica on large spatial scales3 here we present an isotopic profile from a core drilled at dome fuji4 5 situated 1 500 km from vostok in a different sector of east antarctica the two records show remarkable similarities over the past three glacial cycles the extent of the dome fuji record in both large amplitude changes such as terminations interglacials and interstadials and more subtle glacial events even when the origin of precipitation is accounted for our results indicate that antarctic climate is essentially homogeneous at the scale of the east antarctic plateau possibly as a consequence of the symmetry of the plateau and the adjacent ocean

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

large scale ice sheet modelling as a means of dating deep ice cores in greenland the three dimensional ice sheet model sicopolis is used to simulate the dynamic thermodynamic behaviour of the entire greenland ice sheet from 250 000 a bp until today external forcing consists of a surface temperature history constructed from 18 o data of the grip core a snowfall history coupled linearly to that of the surface temperature a piecewise linear sea level scenario and a constant geothermal heat flux the simulated greenland ice sheet is investigated in the vicinity of summit the position where the maximum elevation is taken and where the two drill sites grip and gisp 2 are situated 28 km apart from each other in the region the aggreement between modelled and observed topography and ice temperature tuns out to be very good computed age depth profiles for grip and gisp2 are presented which can be used to complete the dating of these cores in the deeper regions where annual layer counting is not possible however artififical diffusion influences the computed ages in a near basel boundary lar of approximately 15 of theice thickness so that the age at the bottom of the cores cannot be predicted yet

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

antarctic subglacial lakes abstract antarctic subglacial lakes were first identified by robin et al 1970 after airborne radio echo sounding res investigations of the ice sheet interior recently satellite altimetry was used to measure anomalous near flat regions on the ice sheet surface that represent a manifestation of the subglacial lake beneath using res and satellite altimetry the location and extent of antarctic subglacial lakes can be identified the largest subglacial lake exists beneath vostok station and is 14 000 km2 in area the combined area of additional subglacial lakes beneath dome c is 15 000 km2 and at least 15 000 km2 of lake surface lies beneath the remainder of the ice sheet the water depth of subglacial lakes can be estimated through seismic investigations although data exist only for lake vostok and consideration of the bedrock slopes that border subglacial lakes the depths of many subglacial lakes are of the order of 10 s 100 s of metres the total volume of water held beneath the ice sheet is estimated between 4000 and 12 000 km3 to date there are six known examples of radio echo reflections from the lake floors at a depth of no more than 20 m since e m attenuation through water is related to the salinity these data indicate that subglacial water is very pure and fresh some near flat surface regions that usually occur over lakes have been observed where no lakes exist such features are may be caused by water saturated basal sediments rather than subglacial lakes finally the spatial variation in geothermal heat flux around the central regions of antarctica can be established estimated by employing a simple thermal model of the ice sheet under an assumption that the basal ice temperature above subglacial lakes is equal to the pressure melting value calculations indicate that the geothermal heat flux varies spatially over the antarctic plate between 37 and 65 mw m 2

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

temperature profile for glacial ice at the south pole implications for life in a nearby subglacial lake airborne radar has detected 100 lakes under the antarctic ice cap the largest of which is lake vostok international planning is underway to search in lake vostok for microbial life that may have evolved in isolation from surface life for millions of years it is thought however that the lakes may be hydraulically interconnected if so unsterile drilling would contaminate not just one but many of them here we report measurements of temperature vs depth down to 2 345 m in ice at the south pole within 10 km from a subglacial lake seen by airborne radar profiling we infer a temperature at the 2 810 m deep base of the south pole ice and at the lake of 9 c which is 7 c below the pressure induced melting temperature of freshwater ice to produce the strong radar signal the frozen lake must consist of a mix of sediment and ice in a flat bed formed before permanent antarctic glaciation it may like siberian and antarctic permafrost be rich in microbial life because of its hydraulic isolation proximity to south pole station infrastructure and analog to a martian polar cap it is an ideal place to test a sterile drill before risking contamination of lake vostok from the semiempirical expression for strain rate vs shear stress we estimate shear vs depth and show that the icecube neutrino observatory will be able to map the three dimensional ice flow field within a larger volume 0 5 km3 and at lower temperatures 20 c to 35 c than has heretofore been possible

mass balance of the lambert glacier amery ice shelf system east antarctica a comparison of computed balance fluxes and measured fluxes we combine european remote sensing satellite ers 1 radar altimeter surface elevations fricker and others 2000 with six different accumulation distributions to compute balance fluxes for the lambert glacier amery ice shelf drainage system these interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the lambert glacier basin traverse and at 11 stations further downstream to assess the system s state of balance for the upstream line we obtain a range of imbalance estimates from 23 8 to 19 9 of the observed flux reflecting the sensitivity to the accumulation distributions for some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line imbalance estimates for the downstream line range from 17 7 to 70 2 with four of the estimates exceeding 30 again reflecting the sensitivity of the result to input accumulation and strongly suggesting that the mass balance of the region between the two lines is positive our results confirm the importance of accurate estimates of accumulation in ice sheet mass balance studies furthermore they suggest that it is not possible to accurately determine the state of balance of large antarctic drainage basins on the basis of currently available accumulation distributions

influence of subglacial vostok lake on the regional ice dynamics of the antarctic ice sheet a model study we applied a newly developed three dimensional time dependent thermomechanical ice sheet model including higher order stress gradients to simulate the ice flow across subglacial vostok lake east antarctica simulations of both lake and no lake conditions by treating the ice lake interface as a stress free surface similar to an ice shelf demonstrate the effects of the subglacial lake such as pronounced surface flattening and ice flow turning on the overall ice dynamics in the vicinity although subglacial lake dynamics are not treated explicitly when buoyancy forces and hydrostatic equilibrium of the ice above the lake are taken into account the along lake surface slope is preserved and the ice flow pattern is in accord with sparse observations model experiments point to a local ice speed up in the northern part of the lake which can be associated with the onset of an enhanced ice flow feature more precisely the onset of the totten glacier catchment

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

modeling compressional wave velocity and attenuation in carbonate sediments the biot poroelastic theory is used to model ultrasonic 1 mhz compressional wave velocity vp and specific attenuation qp 1 for carbonate sediment sequences from three deep sea drilling project dsdp sites 288 289 and 316 on the ontong java plateau in the western pacific ocean the sediments are assumed to be represented by a granular frame containing tortuous capillary tubes of a constant radius experimental data for the variation of shear velocity vs and bulk density with depth are used as input parameters for the model assumptions of the frame poisson s ratio b grain bulk modulus kg and tortuosity are necessary the low frequency asymptote of the biot theory is adequate in modeling the variation of vp with depth for sites 288 and 289 modeled velocities agree to within 8 of experimental the sensitivity of the modeled velocities to the uncertainty in the input parameters is investigated the equations of motion are partially differentiated with respect to each parameter and the result

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

drilling comparison in warm ice and drill design comparison for the deep ice core drilling community the 2005 06 antarctic season was an exciting and fruitful one in three different antarctic locations dome fuji epica dml and vostok deep drillings approached bedrock the ice water interface in the case of vostok emulating what had previously been achieved at northgrip greenland summer 2003 and 2004 and at epica dome c2 antarctica season 2004 05 for the first time in ice core drilling history three different types of drill kems jare and epica simultaneously reached the depth of warm ice under high pressure after excellent progress at each site the drilling rate dropped and the drilling teams had to deal with refrozen ice on cutters and drill heads drills have different limits and perform differently in this comparative study we examine depth pressure temperature pump flow and cutting speed finally we compare a few parameters of ten different deep drills

balance velocities and measured properties of the antarctic ice sheet from a new compilation of gridded data for modelling this paper presents a new compilation of gridded data sets for three dimensional modeling of the antarctic ice sheet these are for surface elevation icethickness bedrock elevation and accumulation rate as interpolated on a 281 x 281 mesh with 20 km spacing and encompass all of the ice sheet and thesurrounding continental shelf data sources include the bamber digital elevation model from ers 1 radar altimeter data a redigitization of available ice thicknessdata the giovinetto accumulation data recent ice thickness data from british and german expeditions as well as accumulation data from german and norwegianexpeditions in particular new data were incorporated for the filchner ronne schelfeis and for dronning maud land arising from the epica pre site survey special attention was devoted to carefully match the various data sources both among themselves as across the grounding line and below the ice shelves toenable ice sheet expansion and retreat in dynamic situations as an application the balance flow is calculated over the entire ice sheet using a two dimensionalfinite difference scheme and compared with a previous assessment this brought to light the existence of ice streaming features extending well inland of the icesheet a detailed zoom over dronning maud land exhibits the general flow characteristics of interest for locating a future deep drilling site as a by product anupdated value of 26 4 x 106 km3 was obtained for the total volume of the ice sheet and ice shelves or equivalent to 61 1 m of global sea level rise after removalof the ice sheet and subsequent oceanic invasion and isostatic rebound the total accumulation over the grounded ice sheet including the antarctic peninsula is1924 gta 1 or between 5 and 20 higher than earlier estimates including all of the ice shelves the value is 2344 gta 1

the role of membrane like stresses in determining the stability and sensitivity of the antarctic ice sheets back pressure and grounding line motion membrane stresses act along thin bodies which are relatively well lubricated on both surfaces they operate in ice sheets because the bottom is either sliding or is much less viscous than the top owing to stress and heat softening of the basal ice ice streams flow over very well lubricated beds and are restrained at their sides the ideal of the perfectly slippery bed is considered in this paper and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces propagation distances depend sensitively on the rheological index and can be very large for ice type rheologies a new analytical solution for ice shelf profiles and grounded tractionless stream profiles is presented which show blow up of the profile in a finite distance upstream at locations where the flux is non zero this is a feature of an earlier analytical solution for a floating shelf the length scale of decay of membrane stresses from the grounding line is investigated through scale analysis in ice sheets such effects decay over distances of several tens of kilometres creating a vertical boundary layer between sheet flow and shelf flow where membrane stresses adjust bounded physically reasonable steady surface profiles only exist conditionally in this boundary layer where bounded steady profiles exist adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur neutral equilibrium if no solution in the boundary layer can exist the ice sheet profile must change the conditions for existence can be written in terms of whether the basal rate factor sliding or internal deformation is too large to permit a steady solution the critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line high ice velocity and high stress both favour the existence of solutions and stability changes in these parameters can cause the steady solution existence criterion to be traversed and the ice sheet dynamics to change a finite difference model which represents both neutral equilibrium and the dynamical transition is presented and preliminary investigations into its numerical sensitivity are carried out evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem which will hamper predictability

the gamburtsev mountains and the origin and early evolution of the antarctic ice sheet the manner in which the antarctic ice sheet first formed at a time of rapid global climate change about 34 million years ago has been the subject of considerable speculation several lines of evidence point to central antarctic mountain ranges as the site of the initial ice sheet but our knowledge of the present ice sheet topography is highly restricted in the very zone where ice sheet initiation is most likely along the subglacial gamburtsev mountain range at the centre of the present ice sheet now a detailed survey and analysis of the morphology of the gamburtsev mountains has been performed based on an ice penetrating radar study conducted over two seasons the data reveal a mountain landscape initially incised by rivers then over deepened by ice movement the topography beneath up to 3 000 metres of ice resembles an exploded version of classic alpine valley topography and seems to have developed more than 34 million years ago when mean summer temperatures were about 3 c the nature of initial glaciation on antarctica about 34 million years ago is a mystery results from an intensive radar survey now show that the gamburtsev mountains at dome a at the centre of the present ice sheet were initially incised by rivers and then subsequently overdeepened by ice movement this is suggestive of topographical development before 34 million years ago when mean summer temperatures were about 3 c ice sheet development in antarctica was a result of significant and rapid global climate change about 34 million years ago1 ice sheet and climate modelling suggest reductions in atmospheric carbon dioxide less than three times the pre industrial level of 280 parts per million by volume that in conjunction with the development of the antarctic circumpolar current led to cooling and glaciation paced by changes in earth s orbit2 based on the present subglacial topography numerical models point to ice sheet genesis on mountain massifs of antarctica including the gamburtsev mountains at dome a the centre of the present ice sheet2 3 our lack of knowledge of the present day topography of the gamburtsev mountains4 means however that the nature of early glaciation and subsequent development of a continental sized ice sheet are uncertain here we present radar information about the base of the ice at dome a revealing classic alpine topography with pre existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 c this landscape is likely to have developed during the initial phases of antarctic glaciation according to antarctic climate history estimated from offshore sediment records the gamburtsev mountains are probably older than 34 million years and were the main centre for ice sheet growth moreover the landscape has most probably been preserved beneath the present ice sheet for around 14 million years

on the numerical computation of temperature in an ice sheet quadrature solutions for temperature fields in the central regions of ice sheets where flow is by internal deformation are presented and compared with numerical computations and the european ice sheet modelling initiative eismint benchmark standards the solution appears to be more accurate than the eismint benchmark standards for thermomechanically uncoupled temperature distributions the ability of finite difference and pseudo spectral methods to reproduce this solution is considered errors in the eismint benchmark are larger than expected the possibility that they could arise from inaccurate evaluation of the vertical velocity is considered and found to be unlikely formulae for computing the vertical velocity in jenssen s coordinate are complicated and may lead to programming errors a simple form of the heat transport equation in the coordinate for flow by sliding and internal deformation is derived arid presented and it is shown how this form is particularly suitable for finite element computations it is also shown that only two quadratures are necessary to compute the vertical and horizontal heat transport and the ice flux