twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

past rates of accumulation in central west antarctica 1 the spatial variation in ice accumulation is critical to the form and stability of ice sheets in west antarctica knowledge of present ice accumulation rates is restricted to sparse measurements and interpolations across data free regions while information on former ice accumulation is restricted to a small number of ice cores and numerical modeling results here isochronous internal ice sheet layers recorded by airborne radio echo sounding and dated at their stratigraphic position at the byrd ice core site are used to reconstruct the spatial pattern of accumulation across the main west antarctic ice divide for the past 16 000 years mean accumulation calculated for the last 3100 years matches well to present day accumulation rates which is compatible with previous observations on siple dome accumulation between 6400 and 16 000 years ago is calculated to be around half that at present in accordance with recent ice sheet modeling results index terms 1223 geodesy and gravity ocean earth atmosphere interactions 3339 1827 hydrology glaciology 1863 9310 information related to geographic region antarctica 9604 information related to geologic time cenozoic 3349 meteorology and atmospheric dynamics polar meteorology citation siegert m j and a j payne 2004 past rates of accumulation in central west antarctica geophys res lett 31 l12403 doi 10 1029 2004gl020290

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

glacial interglacial ice stream stability in the weddell sea embayment antarctica the resilience of the antarctic ice sheet and its effect on global sea level depends on the dynamics of ice streams antarctic ice streams are known to be responsive to changes at the ocean interface and as expected most have thinned in response to ocean warming and sea level rise since the last glacial maximum lgm here we provide direct and unexpected evidence that points toward the glacial interglacial stability of the slessor and recovery glaciers ice streams of the east antarctic ice sheet eais which merge with the west antarctic ice sheet wais to form the filchner ice shelf in the weddell sea embayment cosmogenic nuclide measurements in the shackleton range suggest that the slessor and recovery ice streams were not significantly thicker than today during the lgm we hypothesise that the glaciers did not thicken because the grounding line was not able to migrate seaward beyond the deep thiel crary trough beneath the filchner ice shelf immediately offshore this discovery reveals how a topographic threshold can affect the dynamics of ice streams it also reduces uncertainties on the thickness extent and volume of the antarctic ice sheet in a large but unknown sector of the antarctic ice sheet it constrains the potential sea level rise from antarctica it helps explain observed anomalies in glacio isostatic adjustment above all it suggests that the behaviour of the atlantic facing weddell sea sector of the wais contrasts with that of the pacific facing ross and amundsen sea sectors

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

late holocene ice flow reconfiguration in the weddell sea sector of west antarctica abstract here we report late holocene ice sheet and grounding line changes to the weddell sea sector of west antarctica internal radio echo layering within the bungenstock ice rise which comprises very slow flowing ice separating the fast flowing institute and moller ice streams reveals ice deformed by former enhanced flow overlain by un deformed ice the ice rise surface is traversed by surface lineations explicable as diffuse ice flow generated stripes which thus capture the direction of flow immediately prior to the creation of the ice rise the arrangement of internal layers can be explained by adjustment to the flow path of the institute ice stream during either a phase of ice sheet retreat not longer than 4000 years ago or by wholesale expansion of the grounding line from an already retreated situation not sooner than 400 years ago some combination of these events involving uplift of the ice rise bed during ice stream retreat and reorganisation is also possible whichever the case the implication is that the ice sheet upstream of the bungenstock ice rise which currently grounds over a 1 5 km deep basin has been and therefore may be susceptible to significant change

late quaternary grounded ice extent in the filchner trough weddell sea antarctica new marine geophysical evidence the last glacial maximum lgm ca 23 19 ka bp extent of grounded ice in the filchner trough a major cross shelf trough extending seaward from the filchner ice shelf in the southern weddell sea has been much debated here we present data from the first extensive multibeam swath bathymetry and sub bottom acoustic profiling surveys in the filchner trough that include several parallel survey lines with overlapping swaths we interpret these new data combined with published observations and radiocarbon dates from sediment cores as indicating that the grounding line in the filchner trough during the lgm advanced beyond the middle shelf probably to within 40 km of the shelf break and possibly reached the shelf break three different hypotheses are discussed that could reconcile this interpretation with interpretations based on ice coring and surface exposure age data that lgm ice surface elevations in areas draining into the filchner and ronne ice shelves were no more than a few hundred metres higher than today 1 ice plain conditions extended along most of the filchner trough 2 the ice shelf advanced and thickened so that it touched down on the continental shelf for a short period 3 lgm ice drainage pathways in the interior of the weddell sea embayment were different from those observed today

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

spatial stability of ice stream d and its tributaries west antarctica revealed by radio echo sounding and interferometry it has been shown recently that ice streams are fed by fast flowing tributaries occupying well defined subglacial troughs and with shared source areas here ice penetrating radio echo sounding res data are analyzed in conjunction with ice surface velocities derived from interferometric synthetic aperture radar insar to determine the englacial properties of tributaries feeding ice stream d west antarctica all of ice stream d s tributaries are coincident with buckled internal ice sheet layers most probably deformed by the processes responsible for enhanced ice flow between the tributaries well preserved internal layers occur the data reveal that no lateral migration of the ice stream tributaries has occurred recently this is consistent with thermomechanical ice flow modelling which indicates that the flow of ice stream d is controlled by a subglacial trough and is unaffected by changes to the flow of neighbouring ice stream c

dynamic ice stream retreat in the central sector of the last british irish ice sheet during the last glacial maximum lgm ice streams of the last british irish ice sheet biis controlled ice discharge from various dispersal centres deglaciation was characterised by shifts in ice divide location and changes in internal ice sheet dynamics resulting in substantial flow switches and significant ice stream reconfiguration and hence modification of their landform signatures we present new geomorphological mapping and 11 10be cosmogenic nuclide ages from northern england stainmore gap eden valley and vale of york that constrain regional dynamic ice stream retreat following the lgm we identify complex decoupling of competing ice lobes characterised by early retreat of the north sea lobe and a minor re advance of stainmore ice prior to 20 ka this was followed by rapid recession of the central stainmore gap which was ice free by 19 8 0 7 to 18 0 0 5 ka contemporaneous with the recession of the tyne gap ice stream in the southern vale of eden crossby ravensworth fell became exposed between 19 2 and 20 3 ka the northwards ice flow reversal in the vale of eden was associated with the development of ice domes across the northern pennines howgill fells and the lake district this shift in dispersal centres and ice divide migration likely triggered the rapid collapse of eastward ice stream corridors the central sector of the biis rapidly collapsed back up into upland dispersal centres between 20 and 17 5 ka this work highlights the role internal factors such as topography in driving ice divide migration and flow switches during externally and climatically forced ice sheet thinning

low post glacial rebound rates in the weddell sea due to late holocene ice sheet readvance many ice sheet reconstructions assume monotonic holocene retreat for the west antarctic ice sheet but an increasing number of glaciological observations infer that some portions of the ice sheet may be readvancing following retreat behind the present day margin a readvance in the weddell sea region can reconcile two outstanding problems i the present day widespread occurrence of seemingly stable ice streams grounded on beds that deepen inland and ii the inability of models of glacial isostatic adjustment to match present day uplift rates by combining a suite of ice loading histories that include a readvance with a model of glacial isostatic adjustment we report substantial improvements to predictions of present day uplift rates including reconciling one problematic observation of land sinking we suggest retreat behind present grounding lines occurred when the bed was lower and isostatic recovery has since led to shallowing ice sheet re grounding and readvance the paradoxical existence of grounding lines in apparently unstable configurations on reverse bed slopes may be resolved by invoking the process of unstable advance in accordance with our load modelling

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

three dimensional flow influences on radar layer stratigraphy variations in the depth of radar detectable englacial layers isochrones are commonly used to assess past variability in accumulation rates but little is known about the effect of internal and basal flow variations on isochrone deflections e g bumps troughs in this paper we show how the isochrones are affected by such variation using a three dimensional flow model to investigate changes in the flow mode and in increased basal melting we also investigate how transverse flows with lateral velocity gradients affect the development of isochrones we use the model to visualize how such variations will be seen in radar lines which cross the flow direction we show that in the presence of lateral gradients in the flow field we can produce bumps and troughs when viewed along transects perpendicular to the flow the model results show that the influences of flow convergence melting and changes in flow mode when coupled together affect isochrones over the whole depth of the ice sheet finally changes in the near surface layers cannot be solely attributed to spatial variation in the accumulation rate there can also be a strong signal from changes in the flow mode

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

major ice sheet change in the weddell sea sector of west antarctica over the last 5 000 years until recently little was known about the weddell sea sector of the west antarctic ice sheet in the last 10 years a variety of expeditions and numerical modelling experiments have improved knowledge of its glaciology glacial geology and tectonic setting two of the sector s largest ice streams rest on a steep reverse sloping bed yet despite being vulnerable to change satellite observations show contemporary stability there is clear evidence for major ice sheet reconfiguration in the last few thousand years however knowing precisely how the ice sheet has changed in the past and when would allow us to better understand whether it is now at risk two competing hypotheses have been established for this glacial history in one the ice sheet retreated and thinned progressively from its last glacial maximum position retreat stopped at or very near the present position in the late holocene alternatively in the late holocene the ice sheet retreated significantly upstream of the present grounding line and then advanced to the present location due to glacial isostatic adjustment and ice shelf and ice rise buttressing both hypotheses point to data and theory in their support yet neither has been unequivocally tested or falsified here we review geophysical evidence to determine how each hypothesis has been formed where there are inconsistencies in the respective glacial histories how they may be tested or reconciled and what new evidence is required to reach a common model for the late holocene ice sheet history of the weddell sea sector of west antarctica

investigations of the mass balance of the southeastern ronne ice shelf antarctica data from the filchner v campaign were used to investigate the mass balance conditions in the southeastern ronne ice shelf ris antarctica radio echo sounding and seismic measurements over this area show a maximum ice thickness of 2000 m close to the grounding line of foundation ice stream the measurements also revealed that the position of this grounding line is 40 km further south than previously thought new mass flux calculations result in an estimate or 51 km 3 a 1 for the ice stream transport from the ice sheet into the eastern ice shelf the mollereisstrom mes west of foundation ice stream shows a maximum ice thickness of 1100 1200 m in the grounding line area and a mass flux of 23 km 3 a 1 assuming steady state conditions mass balance calculations based on the new data result in a mean melt rate of about 1 m a 1 at the ice shelf base for the entire southeastern part of the ris the melt rate in the grounding line area of foundation ice stream exceeds 9 m a 1 in contrast other ice streams draining into the filchner ronne ice shelf show maximum melt rates from 1 2 m a 1 mes to 4 m a 1 rutford ice stream our calculations indicate that nearly all of the ice deposited in the drainage area of the eastern ris on the ice sheet does not reach the ice shelf front as original meteoric ice but is melted at the ice shelf base

ice flow reorganization in west antarctica 2 5 kyr ago dated using radar derived englacial flow velocities we date a recent ice flow reorganization of an ice divide in the weddell sea sector west antarctica using a novel combination of inverse methods and ice penetrating radars we invert for two dimensional ice flow within an ice divide from data collected with a phase sensitive ice penetrating radar while accounting for the effect of firn on radar propagation and ice flow by comparing isochronal layers simulated using radar derived flow velocities with internal layers observed with an impulse radar we show that the divide s internal structure is not in a steady state but underwent a disturbance potentially implying a regional ice flow reorganization 2 5 1 8 2 9 kyr b p our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary these findings increase our knowledge of the glacial history of a region that lacks dated constraints on late holocene ice sheet retreat and provides a key target for models that reconstruct and predict ice sheet behavior

on the effects of divide migration along ridge flow and basal sliding on isochrones near an ice divide 1 radar layer geometry in divide areas is strongly influenced by the operation of the raymond effect which causes upwarping of the layers as a consequence of the nonlinear rheology of ice the detailed geometry of these layers is known to store a record of change in the cryosphere of local thinning and of the age of formation of the divide and has been surmised to provide information about lateral motion of divides such lateral motion can be caused by changes in flanking ice streams and the divide area thereby contains a record of ice stream dynamics it has also been suggested that a large perturbation of divide position will obliterate the cumulative effects of the operation of the raymond mechanism leading to the disappearance of raymond bumps since the raymond effect has a strong influence on the age depth relation in ice cores knowledge of whether its operation is localized leading to strong bump formation or distributed is crucial in the interpretation of ice cores the detailed evolution of ice divide radar layer geometry remains poorly understood employing a full thermomechanically coupled transient model we qualitatively explore the effect of divide migration on radar layer geometry certain qualitative features emerge which can be used to infer history of cryosphere change in particular in areas distant from the usual sites of geological dating there remains uncertainty about the influence of sliding on the operation of the raymond effect under certain conditions the existence of sliding can damp or eliminate the operation of the raymond effect if this is generally true then dating of ice divides may simply be a date for the freezing of the divide bottom we show that sliding does not necessarily eliminate the formation of bumps dates of divide formation are likely to be dates for the location of the ridge at a particular spot raymond bump evolution is weakened by flow along the ridge we explore quantitatively the strength of this effect using a scaling analysis to show that the weakening can efficiently be described by one parameter the ratio of along ridge slope to a measure of the across divide curvature

early last interglacial ocean warming drove substantial ice mass loss from antarctica the future response of the antarctic ice sheet to rising temperatures remains highly uncertain a useful period for assessing the sensitivity of antarctica to warming is the last interglacial lig 129 to 116 ky which experienced warmer polar temperatures and higher global mean sea level gmsl 6 to 9 m relative to present day lig sea level cannot be fully explained by greenland ice sheet melt 2 m ocean thermal expansion and melting mountain glaciers 1 m suggesting substantial antarctic mass loss was initiated by warming of southern ocean waters resulting from a weakening atlantic meridional overturning circulation in response to north atlantic surface freshening here we report a blue ice record of ice sheet and environmental change from the weddell sea embayment at the periphery of the marine based west antarctic ice sheet wais which is underlain by major methane hydrate reserves constrained by a widespread volcanic horizon and supported by ancient microbial dna analyses we provide evidence for substantial mass loss across the weddell sea embayment during the lig most likely driven by ocean warming and associated with destabilization of subglacial hydrates ice sheet modeling supports this interpretation and suggests that millennial scale warming of the southern ocean could have triggered a multimeter rise in global sea levels our data indicate that antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice climate feedbacks that further amplify warming

distortion of isochronous layers in ice revealed by ground penetrating radar in addition to measuring ice sheet thickness ground penetrating radar can be used to delineate reflections in ice sheets1 2 3 these reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow here we present ground penetrating radar data from fletcher promontory antarctica which show arches and troughs in isochronous ice layers to a depth of 100 m we demonstrate that the origin of these features can be determined by their growth with depth and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope one arch appears to be the result of a local anomaly in vertical strain rate its proximity to the ice divide width and growth with depth indicate that this arch is one of a class of features postulated by raymond4 but not previously shown to exist in the field such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice flow

controls on last glacial maximum ice extent in the weddell sea embayment antarctica the weddell sea sector of the antarctic ice sheet is hypothesized to have made a significant contribution to sea level rise since the last glacial maximum using a numerical flowline model we investigate the controls on grounding line motion across the eastern weddell sea and compare our results with field data relating to past ice extent specifically we investigate the influence of changes in ice temperature accumulation sea level ice shelf basal melt and ice shelf buttressing on the dynamics of the foundation ice stream we find that ice shelf basal melt plays an important role in controlling grounding line advance while a reduction in ice shelf buttressing is found to be necessary for grounding line retreat there are two stable positions for the grounding line under glacial conditions at the northern margin of berkner island and at the continental shelf break global mean sea level contributions associated with these two scenarios are 50 mm and 130 mm respectively comparing model results with field evidence from the pensacola mountains and the shackleton range we find it unlikely that ice was grounded at the continental shelf break for a prolonged period during the last glacial cycle however we cannot rule out a brief advance to this position or a scenario in which the grounding line retreated behind present during deglaciation and has since re advanced better constraints on past ice sheet and ice shelf geometry ocean temperature and ocean circulation are needed to reconstruct more robustly past behavior of the foundation ice stream

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

flow at ice divide triple junctions 2 three dimensional views of isochrone architecture from ice penetrating radar surveys 1 ground based surveys of ice divide triple junctions in two antarctic ice rises the fletcher promontory and berkner island have been carried out using low frequency ice penetrating radars these surveys have focused on understanding the internal layer architecture at and around the triple junctions with a particular emphasis on understanding how the raymond effect operates a variety of features have been observed including raymond cupolas under the junctions bump stacks of varying amplitude with distance from the divide double bumps crooked stacks implying divide motion and the presence of ridges containing weak and strong bump stacks it was generally not possible to elucidate the three dimensional structure by correlation of pick crossovers so instead a least squares cross correlation technique was used which computes nominal age surfaces using radial basis function interpolation double bumps are absent from the central cupola of fletcher promontory permitting us to infer that the ice here has thinned by around 500 m in the past 5000 years estimates of the age scale suggest that there will be recoverable eemian ice a technique for inferring the velocity profile at divides using layer traces and assumed velocities in the flanks is presented

widespread persistent thickening of the east antarctic ice sheet by freezing from the base an international polar year aerogeophysical investigation of the high interior of east antarctica reveals widespread freeze on that drives substantial mass redistribution at the bottom of the ice sheet although the surface accumulation of snow remains the primary mechanism for ice sheet growth beneath dome a 24 of the base by area is frozen on ice in some places up to half of the ice thickness has been added from below these ice packages result from the conductive cooling of water ponded near the gamburtsev subglacial mountain ridges and the supercooling of water forced up steep valley walls persistent freeze on thickens the ice column alters basal ice rheology and fabric and upwarps the overlying ice sheet including the oldest atmospheric climate archive and drives flow behavior not captured in present models

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

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

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

west antarctic ice stream discharge variability mechanism controls and pattern of grounding line retreat west antarctic ice streams show pronounced flow variability in their downstream reaches with changes stranding formerly fast flowing ice and redirecting discharge a simple model in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach can explain this behavior downstream thinning steepens the temperature gradient near the bed increasing upward heat flow and the tendency toward basal freezing the basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning that is the fastest flowing ice the most successful rapid outflows are regions where basal water from elsewhere is likely to be consumed freezing here leads to episodic slow downs and redirections of flow the history of which appears in satellite imagery as ice rises distorted streaklines and margin jumps created when discharge migrates to areas with more favorable basal conditions one compelling consequence of this process is that it makes catastrophic collapse less likely if discharge currents are forced to slow when they become too fast thin then there may be an upper bound on the retreat rate and discharge flux of the west antarctic ice sheet wais ice stream system under the present climate

marine geological constraints for the grounding line position of the antarctic ice sheet on the southern weddell sea shelf at the last glacial maximum abstract the history of grounded ice sheet extent on the southern weddell sea shelf during the last glacial maximum lgm and the timing of post lgm ice sheet retreat are poorly constrained several glaciological models reconstructed widespread grounding and major thickening of the antarctic ice sheet in the weddell sea sector at the lgm in contrast recently published onshore data and modelling results concluded only very limited lgm thickening of glaciers and ice streams feeding into the modern filchner and ronne ice shelves these studies concluded that during the lgm ice shelves rather than grounded ice covered the filchner and ronne troughs two deep palaeo ice stream troughs eroded into the southern weddell sea shelf here we review previously published and unpublished marine geophysical and geological data from the southern weddell sea shelf the stratigraphy and geometry of reflectors in acoustic sub bottom profiles are similar to those from other west antarctic palaeo ice stream troughs where grounded ice had advanced to the shelf break at the lgm numerous cores from the southern weddell sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments these data sets give evidence that grounded ice had advanced across the shelf during the past thereby grounding in even the deepest parts of the filchner and ronne troughs radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited but indicate that the ice grounding occurred at the lgm and that ice retreat started before 15 1 corrected 14c kyrs before present bp on the outer shelf and before 7 7 corrected 14c kyrs bp on the inner shelf which is broadly synchronous with ice retreat in other antarctic sectors the apparent mismatch between the ice sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles similar to those of ice plains that had advanced through filchner trough and ronne trough at the lgm considering the global sea level low stand of 130 m below present a low surface slope of the expanded lgm ice sheet in the southern weddell sea can reconcile grounding line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland this scenario implies that ice sheet growth in the weddell sea sector during the lgm and ice sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea level fluctuations during these times

holocene stability of the amundsen weddell ice divide west antarctica we analyze holocene ice flow using global positioning system and radio echo sounding data acquired near the amundsen weddell ice divide between pine island glacier and the institute ice stream west antarctica the data show that this part of the west antarctic ice sheet wais has maintained a stable ice flow regime and ice divide position for similar to 7000 yr independent glacial geological data in support of this assertion suggest that the interior of the wais west of the ellsworth mountains has thinned by only a few hundred meters since the last glacial maximum lgm and that the position of the ice divide may be recurrent over longer more than 20 k y perhaps pre quaternary time scales we suggest that basal topography constrains ice flow in this sector of the wais making the ice divide relatively insensitive to perturbations around the margins of the ice sheet compared with other ice divides large scale forcing measured elsewhere in the wais as major reorganizations of the ice sheet surface and flowline pattern may be manifested only in simple vertical changes of the amundsen weddell interior

extensive retreat and re advance of the west antarctic ice sheet during the holocene to predict the future contributions of the antarctic ice sheets to sea level rise numerical models use reconstructions of past ice sheet retreat after the last glacial maximum to tune model parameters 1 reconstructions of the west antarctic ice sheet have assumed that it retreated progressively throughout the holocene epoch the past 11 500 years or so 2 4 here we show however that over this period the grounding line of the west antarctic ice sheet which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf retreated several hundred kilometres inland of today s grounding line before isostatic rebound caused it to re advance to its present position our evidence includes first radiocarbon dating of sediment cores recovered from beneath the ice streams of the ross sea sector indicating widespread holocene marine exposure and second ice penetrating radar observations of englacial structure in the weddell sea sector indicating ice shelf grounding we explore the implications of these findings with an ice sheet model modelled re advance of the grounding line in the holocene requires ice shelf grounding caused by isostatic rebound our findings overturn the assumption of progressive retreat of the grounding line during the holocene in west antarctica and corroborate previous suggestions of ice sheet re advance 5 rebound driven stabilizing processes were apparently able to halt and reverse climate initiated ice loss whether these processes can reverse present day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity parameters that are difficult to measure and to incorporate into ice sheet models

airborne radar evidence for tributary flow switching in institute ice stream west antarctica implications for ice sheet configuration and dynamics despite the importance of ice streaming to the evaluation of west antarctic ice sheet wais stability we know little about mid to long term dynamic changes within the institute ice stream iis catchment here we use airborne radio echo sounding to investigate the subglacial topography internal stratigraphy and holocene flow regime of the upper iis catchment near the ellsworth mountains internal layer buckling within three discrete topographically confined tributaries through ellsworth independence and horseshoe valley troughs provides evidence for former enhanced ice sheet flow we suggest that enhanced ice flow through independence and ellsworth troughs during the mid to late holocene was the source of ice streaming over the region now occupied by the slow flowing bungenstock ice rise although buckled layers also exist within the slow flowing ice of horseshoe valley trough a thicker sequence of surface conformable layers in the upper ice column suggests slowdown more than 4000 years ago so we do not attribute enhanced flow switch off here to the late holocene ice flow reorganization intensely buckled englacial layers within horseshoe valley and independence troughs cannot be accounted for under present day flow speeds the dynamic nature of ice flow in iis and its tributaries suggests that recent ice stream switching and mass changes in the siple coast and amundsen sea sectors are not unique to these sectors and that they may have been regular during the holocene and may characterize the decline of the wais

sensitivity of the divide position at siple dome west antarctica to boundary forcing a linearized perturbation about a two dimensional vialov nye icesheet profile is used to investigate the sensitivity of the divide position at siple dome west antarctica to small changes in the accumulation pattern and in the elevation of its lateral boundaries at the margins of ice streams c and d relaxation time scales for the icesheet surface and divide position are derived from the perturbation theory for siple dome these time scales are short 450 800 years for surface adjustment and 200 350 years for divide position adjustment these short time scales indicate that siple dome responds quickly to forcing at its boundaries therefore the recent migration of the siple dome divide determined from previous work is probably a response to an ongoing sustained forcing rather than a response to a long past climate event such as the transition from the last glacial maximum to the holocene based on our analysis the inferred rate of migration of the siple dome divide could be attained be

meteorological tower microbarograph array and sodar observations of solitary like waves in the nocturnal boundary layer abstract five short duration disturbances of the stable nocturnal boundary layer ranging in depth from about 15 m to over 500 m were observed using a 300 m meteorological tower 2 sodars and a 4 station microbarograph array four of the events showed characteristic pulse like sodar signatures the fifth occurred within the minimum range of the sodars we show that the events have the characteristics expected of strongly nonlinear deep fluid internal solitary waves that are transporting and recirculating cold air at a supercritical velocity the unique sodar signatures tracing out the form of the waves are attributed to small scale turbulent temperature inhomogeneities marking the boundary of the recirculating air as it is transported through the ambient nocturnal boundary layer by the wave all the events are shown to deviate from the benjamin davis ono bdo theory because of their amplitude wavelength and amplitude stable layer thickness ratios are considerably larger than normally thought appropr

reconstructing the last glacial maximum ice sheet in the weddell sea embayment antarctica using numerical modelling constrained by field evidence abstract the weddell sea embayment wse sector of the antarctic ice sheet has been suggested as a potential source for a period of rapid sea level rise meltwater pulse 1a a 20 m rise in 500 years previous modelling attempts have predicted an extensive grounding line advance in the wse to the continental shelf break leading to a large equivalent sea level contribution for the sector a range of recent field evidence suggests that the ice sheet elevation change in the wse at the last glacial maximum lgm is less than previously thought this paper describes and discusses an ice flow modelling derived reconstruction of the lgm ice sheet in the wse constrained by the recent field evidence the ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break a range of modelled ice sheet surfaces are instead produced with different grounding line locations derived from a novel grounding line advance scheme the ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea level estimates between approximately 1 4 and 3 m for this sector this paper describes the modelling procedure in detail considers the assumptions and limitations associated with the modelling approach and how the uncertainty may impact on the eustatic sea level equivalent results for the wse

deglacial history of the west antarctic ice sheet in the weddell sea embayment constraints on past ice volume change the retreat history of the west antarctic ice sheet wais since the last glacial maximum is important for understanding the process of rapid deglaciation constraining models that seek to predict the future trajectory of the ice sheet and for estimating rates of sea level change here we report new glacial geologic data from the southwestern weddell sea embayment that demonstrate that this part of the wais was thinner than previously suggested and that there was progressive thinning of the ice sheet by 230 480 m since ca 15 ka we use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the weddell sea at the last glacial maximum the volume of this ice would have added between 1 4 and 2 0 m to postglacial sea level rise and would not have been sufficient to contribute significantly to meltwater pulse 1a a rapid rise in sea level 14 200 yr ago

the structure of west antarctica from geophysical studies the subglacial topographical and geological configurations of west antarctica and the boundary zone between east and west antarctica are poorly known yet they are crucial to our understanding of the tectonic evolution of the pacific proto atlantic margin of gondwanaland preliminary results are presented of simultaneous airborne radio echo and magnetic sounding of 106 km2 of west antarctica including detailed maps of ice sheet and bedrock surfaces magnetics radio echo and other available geophysical data suggest interpretations of sub glacial geology

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

ice flow structure and ice dynamic changes in the weddell sea sector of west antarctica from radar imaged internal layering recent studies have aroused concerns over the potential for ice draining the weddell sea sector of west antarctica to figure more prominently in sea level contributions should buttressing from the filchner ronne ice shelf diminish to improve understanding of how ice stream dynamics there evolved through the holocene we interrogate radio echo sounding res data from across the catchments of institute and moller ice streams iis and mis focusing especially on the use of internal layering to investigate ice flow change as an important component of this work we investigate the influence that the orientation of the res acquisition track with respect to ice flow exerts on internal layering and find that this influence is minimal unless a res flight track parallels ice flow we also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice flow regime our findings suggest that ice in northern iis draining the ellsworth subglacial highlands has retained its present ice flow configuration throughout the holocene this contrasts with less topographically constrained ice in southern iis and much of mis whose internal layering evinces spatial changes to the configuration of ice flow over the past 10 000 years our findings confirm siegert et al s 2013 inference that fast flow was diverted from bungenstock ice rise during the late holocene and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the iis and mis catchments as the west antarctic ice sheet has thinned since the last glacial maximum

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse

glacial interglacial variations in central east antarctic ice accumulation rates past rates of ice accumulation in central east antarctica are defined poorly for the last glacial period ice cores from which current estimates are based are limited in number there is one deep ice core at vostok station one at dome f and two others currently being extracted at dome c and dronning maud land and are restricted spatially they are one dimensional here spatial variations in ice accumulation rates during the last glacial are calculated from isochronous internal ice sheet layering measured by airborne ice penetrating radar data the layers are used to identify the stratigraphy around five east antarctic ice domes isochrons are dated by linking them to the vostok ice core and in one case the epica dome c ice core this chronostratigraphy allows the depth age function to be identified across the ice sheet a simple ice flow model using the depth age measurements as input is then used to determine the spatial and temporal variation in east antarctic ice accumulation over the last glacial cycle the lowest rates of ice accumulation were predicted in the full glacial period to the south of dome a and ridge b there are two possible explanations for this 1 the ice divide topographies may have caused a precipitation shadow in the ice age 2 with the expansion of the ross ice shelf the area to the south of dome a and ridge b would be at the very centre of the ice sheet and hence at a maximum distance from the ocean moisture source r 2002 elsevier science ltd all rights reserved

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream