flow dynamics of the rutford ice stream ice drainage basin west antarctica from radar stratigraphy rutford ice stream west antarctica drains a catchment of 45 000 km 2 into the ronne ice shelf through a 26 km wide 2 4 km deep subglacial trough adjacent to the ellsworth mountains forty two per cent of its catchment boundary is common with pine island glacier where rapid change to ice dynamics is currently underway these changes may eventually affect adjacent catchments such as rutford radar sounding data were acquired over the rutford ice drainage basin that show the internal structure in particular distinctive reflector groups were identified that mark the boundaries between four different flow elements the flow margin reflector groups include curvilinear events that cross cut isochrones and are therefore likely to be post depositional these reflections may arise from crystal orientation fabrics generated by localized strain in a flow margin one of the sectors of the ice drainage basin supplies the largest share 38 of the ice volume flux through the main trunk of rutford ice stream this sector may be preferentially affected by continuing surface lowering in the pine island glacier catchment

tide model accuracy in the amundsen sea antarctica from radar interferometry observations of ice shelf motion 1 this study assesses the accuracy of tide model predictions in the amundsen sea sector of west antarctica tide model accuracy in this region is poorly constrained yet tide models contribute to simulations of ocean heat transfer and to the removal of tidal signals from satellite observations of ice shelves we use two satellite based interferometric synthetic aperture radar insar methods to measure the tidal motion of the dotson ice shelf at multiple epochs a single difference technique that measures tidal displacement and a double difference technique that measures changes in tidal displacement we use these observations to evaluate predictions from three tide models tpxo7 1 cats2008a opt and fes2004 all three models perform comparably well exhibiting root mean square deviations from the observations of 9 cm single difference technique and 10 cm double difference technique care should be taken in generalizing these error statistics because 1 the dotson ice shelf experiences relatively small semidiurnal tides and 2 our observations are not sensitive to all tidal constituents an error analysis of our insar based methods indicates measurement errors of 7 and 4 cm for the single and double difference techniques respectively a model based correction for the effect of fluctuations in atmospheric pressure yields an 6 improvement in the agreement between tide model predictions and observations this study suggests that tide model accuracy in the amundsen sea is comparable to other antarctic regions where tide models are better constrained these methods can be used to evaluate tide models in other remote antarctic waters

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

influence of ionospheric electron density fluctuations on satellite radar interferometry evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry sri obtained with the radarsat ers and jers 1 satellites fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar sar imagery which can be detected using sri measurements of azimuth shift in sri can help to differentiate ionospheric from tropospheric propagation problems and to understand better the impact of the ionosphere on spaceborne sar further sri azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

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

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

trends in antarctic ice sheet elevation and mass fluctuations in antarctic ice sheet elevation and mass occur over a variety of timescales owing to changes in snowfall and ice flow here we disentangle these signals by combining 25 years of satellite radar altimeter observations and a regional climate model from these measurements patterns of change that are strongly associated with glaciological events emerge while the majority of the ice sheet has remained stable 24 of west antarctica is now in a state of dynamical imbalance thinning of the pine island and thwaites glacier basins reaches 122 m in places and their rates of ice loss are now five times greater than at the start of our survey by partitioning elevation changes into areas of snow and ice variability we estimate that east and west antarctica have contributed 1 1 0 4 and 5 7 0 8 mm to global sea level between 1992 and 2017

repeated advance and retreat of the east antarctic ice sheet on the continental shelf during the early pliocene warm period abstract diatom analysis of a sediment core recovered at iodp site u1358 on the continental shelf off the adelie coast indicated that the lower section of the core contained an assemblage dating back to the thalassiosira innura zone of the lower pliocene that ranges from 4 2 to 5 12 ma based on lithological descriptions at both a macro and micro scale of this early pliocene part of the core four facies were interpreted from the diamictons representing the progressive advance and retreat of the grounding line over the site facies 1a and 1b contain a distinct directional signal from the orientation of the a axis of clasts with several phases of fabric development along with both brittle and ductile deformation features that are common in sediments that have been subglacially deformed facies 1c and 1d are finely laminated and were deposited in open marine conditions the four facies within the depositional model provide for the first time direct evidence for ice advancing across the shelf adjacent to the wilkes subglacial basin on at least four occasions separated by three periods of open marine conditions indicating retreat of grounded ice inland of the site during a warmer than present early pliocene the times of open marine conditions are correlated with previous findings from the neighbouring rise sites that also indicated an oscillating ice margin this has significant implications because firstly it suggests a dynamic east antarctic ice sheet eais that is probably far more sensitive to climatic and oceanic forcing even during relatively short time periods than had previously been thought secondly it suggests that proxies used to interpret the advance and retreat of the grounding line from the rise can be linked with direct evidence of grounding line migration from the shelf it also has important implications for the future behaviour and sensitivity of the eais under present continuing warming conditions together with results from the rise this paper provides a crucial ice extent target for a new ice sheet model of this region during the pliocene

flow of glaciar moreno argentina from repeat pass shuttle imaging radar images comparison of the phase correlation method with radar interferometry high resolution radar images of glaciar moreno argentina acquired by the shuttle imaging radar c sir c on 9 and 10 october 1994 at 24 cm wavelength l band are utilized to map the glacier velocity both interferometrically and using the phase correlation method the precision of the interferometric ice velocities is 1 8 cm d 1 6 m a 1 1 the phase correlation method measures ice velocity with a precision of 14 cm d 1 50 m a 1 with image data at a 6 m sample spacing acquired 1 day apart averaged strain rates are measured with a precision of 10 4 d 1 at a 240 m sample spacing with the phase correlation method and 10 5 d 1 with radar interferometry the phase correlation method is less precise than radar interferometry but it performs better in areas of rapid flow is more robust to temporal changes in glacier scattering and measures the glacier velocity in two dimensions with only one image pair using this technique we find that glaciar moreno flows at 400 m a 1 in the terminal valley and 800 m a 1 at the calving front in agreement with velocities recorded a decade ago assuming steady state flow conditions the vertical strain rates measured by sir c are combined with prior data on mass ablation to estimate the glacier thickness and ice discharge the calculated discharge is 0 6 0 2 km3 ice a 1 at 300 m elevation and 1 1 0 2 km3 ice a 1 at the equilibrium line elevation 1150 m which yields a balance accumulation of 6 1m ice a 1

influence of subglacial conditions on ice stream dynamics seismic and potential field data from pine island glacier west antarctica 1 we interpret seismic reflection and airborne potential field data acquired on pine island glacier west antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics immediately beneath the glacier is a mixture of soft deforming sediments and harder non deforming sediments beneath this a sedimentary basin lies under part of the main glacier with another under one of its slower moving tributaries a tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south which also include a thick rift related magmatic intrusion the boundary correlates with changes in the basal roughness ice flow speed and basal drag smoother bed faster flow and lower basal drag characterize the thicker sedimentary sequences to the north but there is no corresponding lateral change in the acoustic properties of the bed changes in the sub bed i e deeper than the ice bed interface lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary leading to increased basal drag and reduced ice flow in the future we conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future

boron and magnesium rich marine brines at the origin of giant unconformity related uranium deposits δ11b evidence from mg tourmalines in giant unconformity related u deposits from the athabasca basin canada the origin of the ore fluids and the source of elements typically associated with u ore such as mg and b are highly controversial this paper presents a petrographic and boron isotopic study of mg tourmalines dravites coeval with u oxides and of primary tourmalines from basement rocks the heavy b isotopic compositions of dravites 11 b 19 6 36 5 contrast with the light composition of basement tourmalines 11 b 8 1 3 3 and clearly show that most of the b was brought to the ore system by basinal brines of marine origin because the latter are also typically mg rich a similar origin for mg is proposed these results support the idea that marine derived brines were the only fluids involved in the formation of the deposits and therefore challenge the previous models in which the b and mg were leached from sedimentary and or basement rocks or carried by a basement derived fluid

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

trends and connections across the antarctic cryosphere satellite observations have transformed our understanding of the antarctic cryosphere the continent holds the vast majority of earth s fresh water and blankets swathes of the southern hemisphere in ice reductions in the thickness and extent of floating ice shelves have disturbed inland ice triggering retreat acceleration and drawdown of marine terminating glaciers the waxing and waning of antarctic sea ice is one of earth s greatest seasonal habitat changes and although the maximum extent of the sea ice has increased modestly since the 1970s inter annual variability is high and there is evidence of longer term decline in its extent this paper discusses how antarctic ice has changed over recent decades and how these changes have been recorded in satellite observations

energy balance of ice streams analysis of the cross flow transmission of force from the central parts of a well lubricated ice stream to its margins shows that there is a corresponding shift in the lateral location of motion induced heat generation the rate of basal heat generation in the center can be substantially smaller than the local rate of potential energy loss given by driving stress times the speed of downslope motion the basal heating is a maximum for an intermediate level of lubrication for which speed is about 40 of the speed over a frictionless bed and base stress is about 25 of the driving stress stable and unstable balances between meltwater production and drainage on the bed are identified a stable steady state with a speed less more than that giving maximum heat generation is termed drainage production limited since an increase in speed would lead to increased decreased basal melting and must need not be balanced by increased drainage it is shown that gradual evolution of the basal water drainage system and the factors affecting basal melting can cause discontinuous jumps between fast and slow moving states a simplified analysis applied to six cross sections of west antarctic ice streams b d e and rutford ice stream shows them to be diverse in the level of support from the sides and corresponding shift of mechanical heating sideward from their central parts the cross sections of ice stream b near upstream b may be production limited because of especially high lubrication and related support from the sides cross sections in the upper part of ice stream d ice stream e and rutford ice stream are in a drainage limited condition substantial reduction of basal heat generation by side drag in most cases and expected high heat flow into the basal ice associated with low thickness in some cases tends to favor basal freezing nevertheless all of the examined cross sections except one are expected to experience basal melting with a modest geothermal heat flux density of 60 mw m or less in some cases the lower part of ice stream b is an exception where the analysis indicates that geothermal flux density must exceed 80 100 mw m to maintain melting if this high geothermal flux is not present then the base of the lower part of ice stream b may be freezing which would suggest continued deceleration of this part of ice stream b

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

in situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit pardo and clement townships ontario canada extensive pyrite bearing auriferous cobble and boulder conglomerates are present in the basal 30 in of the mississagi formation in pardo and clement townships ontario canada the sedimentology of the conglomerates combined with regional geology indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows with material derived from a highly restricted catchment area postdepositional overprinting of the conglomerates is related to the penokean orogeny at 1 85 to 1 5 ga and alkali metasomatism at similar to 1 7 ga several pyrite varieties including detrital and postdepositional recrystallized and altered grains are present in the conglomerates detailed in situ laser ablation inductively coupled plasma mass spectrometry la icpms analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains as invisible au with lesser amounts of free gold postdepositional pyrite and pyrite overgrowths have very low au content elements such as pb bi sb te ag and hg together with au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains small scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains in contrast ni co and as were not mobilized external fluid infiltration is negligible and is indicated only by minor compositionally distinct late sulfide veinlets crosscutting the conglomerates sulfur isotope analysis of detrital pyrite yields generally positive delta 34 s values 0 97 9 26 0 the delta 34 s sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains suggesting a near closed s system during postd epositional processes however the detrital pyrite tends to have slightly negative delta 33 s while the postdepositional and overgrowth pyrite are either neutral or slightly positive the s isotope composition of the detrital pyrite is compatible with an origin of s as dissolved sulfate in an ocean under a low oxygen atmosphere the potential source of the au bearing detrital pyrite appears to have been an as yet undiscovered archean deposit located within 1 to 8 kin of the placer deposit

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood

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

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

accelerated sea level rise from west antarctica recent aircraft and satellite laser altimeter surveys of the amundsen sea sector of west antarctica show that local glaciers are discharging about 250 cubic kilometers of ice per year to the ocean almost 60 more than is accumulated within their catchment basins this discharge is sufficient to raise sea level by more than 0 2 millimeters per year glacier thinning rates near the coast during 2002 2003 are much larger than those observed during the 1990s most of these glaciers flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates providing exit routes for ice from further inland if ice sheet collapse is under way

interferometric estimation of three dimensional ice flow using ascending and descending passes satellite radar interferometry sri provides an important new tool for determining ice flow velocity interferometric measurements made from a single track direction are sensitive only to a single component of the three component velocity vector observations from along three different track directions would allow the full velocity vector to be determined a north south looking synthetic aperture radar sar could provide these observations over large portions of the globe but not over large areas of the polar ice sheets the authors develop and demonstrate a technique that allows the three component velocity vector to be estimated from data acquired along two track directions ascending and descending under a surface parallel flow assumption this technique requires that there are accurate estimates of the surface slope which are also determined interferometrically to demonstrate the technique the authors estimate the three component velocity field for the ryder glacier greenland their results are promising although they do not have yet ground truth data with which to determine the accuracy of their estimates

ocean excited plate waves in the ross and pine island glacier ice shelves ice shelves play an important role in buttressing land ice from reaching the sea thus restraining the rate of grounded ice loss long period gravity wave impacts excite vibrations in ice shelves that can expand pre existing fractures and trigger iceberg calving to investigate the spatial amplitude variability and propagation characteristics of these vibrations a 34 station broadband seismic array was deployed on the ross ice shelf ris from november 2014 to november 2016 two types of ice shelf plate waves were identified with beamforming flexural gravity waves and extensional lamb waves below 20 mhz flexural gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow water gravity wave speeds 70 m s 1 in the 20 100 mhz band extensional lamb waves dominate and propagate at phase speeds 3 km s 1 flexural gravity and extensional lamb waves were also observed by a 5 station broadband seismic array deployed on the pine island glacier pig ice shelf from january 2012 to december 2013 with flexural wave energy also detected at the pig in the 20 100 mhz band considering the ubiquitous presence of storm activity in the southern ocean and the similar observations at both the ris and the pig ice shelves it is likely that most if not all west antarctic ice shelves are subjected to similar gravity wave excitation

a mini surge on the ryder glacier greenland observed by satellite radar interferometry satellite radar interferometry reveals that the speed of the ryder glacier increased roughly threefold and then returned to normal 100 to 500 meters year over a 7 week period near the end of the 1995 melt season the accelerated flow represents a substantial though short lived change in ice discharge during the period of rapid motion meltwater filled supraglacial lakes may have drained which could have increased basal water pressure and caused the mini surge there are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in greenland but because most other outlet glaciers are at lower latitudes they should experience more extensive melting making them more susceptible to meltwater induced surges

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

deglaciation of a major palaeo ice stream in disko trough west greenland recent work has confirmed that grounded ice reached the shelf break in central west greenland during the last glacial maximum lgm here we use a combination of marine sediment core data including glacimarine lithofacies and ird proxy records and geomorphological and acoustic facies evidence to examine the nature of and controls on the retreat of a major outlet of the western sector of the greenland ice sheet gris across the shelf retreat of this outlet which contained the ancestral jakobshavns isbrae ice stream from the outer shelf in disko trough was rapid and progressed predominantly through iceberg calving however minor pauses in retreat tens of years occurred on the middle shelf at a trough narrowing forming subtle grounding zone wedges by 12 1 cal kyr bp ice had retreated to a basalt escarpment and shallow banks on the inner continental shelf where it was pinned and stabilised for at least 100 years during this time the ice margin appears to have formed a calving bay over the trough and melting became an important mechanism of ice mass loss fine grained sediments muds were deposited alternately with ird rich sediments diamictons forming a characteristic deglacial lithofacies that may be related to seasonal climatic cycles high influxes of subglacial meltwater emanating from the nearby ice margins deposited muddy sediments during the warmer summer months whereas winters were dominated by iceberg calving leading to the deposition of the diamictons this is the first example of this glacimarine lithofacies from a continental shelf setting and we suggest that the calving bay configuration of the ice margin plus the switching between calving and melting as ablation mechanisms facilitated its deposition by channelling meltwater and icebergs through the inner trough the occurrence of a major stillstand on the inner shelf in disko trough demonstrates that the ice dynamical response to local topography was a crucial control on the behaviour of a major outlet in this sector of the gris during retreat

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

uneven onset and pace of ice dynamical imbalance in the amundsen sea embayment west antarctica we combine measurements acquired by five satellite altimeter missions to obtain an uninterrupted record of ice sheet elevation change over the amundsen sea embayment west antarctica since 1992 using these data we examine the onset of surface lowering arising through ice dynamical imbalance and the pace at which it has propagated inland by tracking elevation changes along glacier flow lines surface lowering has spread slowest 6 km yr along the pope smith and kohler psk glaciers due to their small extent pine island glacier pig is characterized by a continuous inland spreading of surface lowering notably fast at rates of 13 to 15 km yr along tributaries draining the southeastern lobe possibly due to basal conditions or tributary geometry surface lowering on thwaites glacier thg has been episodic and has spread inland fastest 10 to 12 km yr along its central flow lines the current episodes of surface lowering started approximately 10 years before the first measurements on psk around 1990 on pig and around 2000 on thg ice dynamical imbalance across the sector has therefore been uneven during the satellite record

gps determination of the velocity and strain rate fields on schirmacher glacier central dronning maud land antarctica global positioning system gps campaigns were conducted during the 2003 and 2004 austral summer seasons to obtain insight into the velocity and strain rate distribution on schirmacher glacier central dronning maud land east antarctica gps data were collected at 21 sites and analyzed to estimate the site coordinates baselines and velocities the short term precision of the base station mait is estimated from the daily coordinate repeatability solutions during the two years all gps points on the glacier were constrained with respect to mait and nearby international gps service stations horizontal velocities of the glacier sites lie between 1 89 0 01 and 10 88 0 01 m a 1 to the north northeast with an average velocity of 6 21 0 01 m a 1 the principal strain rates provide a quantitative measurement of extension rates which range from 0 11 0 01 10 3 to 1 48 0 85 10 3 a 1 and shortening rates which range from 0 04 0 02 10 3 to 0 96 0 16 10 3 a 1 the velocity and strain rate distributions across the gps network in schirmacher glacier are spatially correlated with topography subsurface undulations fracture zones crevasses and the partial blockage of the flow by nunataks and the schirmacher oasis

acceleration of pine island and thwaites glaciers west antarctica recent satellite investigations revealed that in the 1990s the grounding line of pine island and thwaites glaciers west antarctica retreated several km the ice surface on the interior of the basins lowered 10 cm a 1 and pine island glacier thinned 1 6 m a 1 these observations however were not sufficient to determine the cause of the changes here we present satellite radar interferometry data that show the thinning and retreat of pine island glacier are caused by an acceleration of ice flow of about 18 2 in 8 years thwaites glacier maintained a nearly constant flow regime at its center but widened along the sides and increased its 30 15 mass deficit by another 4 in 4 years the combined mass loss from both glaciers if correct contributes an estimated 0 08 0 03 mm a 1 global sea level rise in 2000

ice shelf changes in pine island bay antarctica 1947 2000 aerial photographs from 1947 and 1966 satellite optical imagery from 1973 and 1980 and interferometric synthetic aperture radar insar data from 1992 1996 and 2000 are employed to detect ice shelf changes in pine island bay antarctica the front position of the fast flowing central ice shelf did not migrate discernibly over the past 50 years new cracks and rifts appeared in the 1990s however that reveal a major weakening of the ice shelf at the grounding line center the ice shelf thinned 21 m in 8 years the northern slow moving ice shelf also shows signs of decay 1 its calving front is retreating at an accelerating rate and 2 the ice shelf is slowly unpinning from its bedrock anchors these changes are taking place in a region well beyond the temperature dependent limit of viability of ice shelves and hence differ from those observed along the antarctica peninsula they are likely due to a change in oceanic forcing not to a change in air temperature one possibility is that the documented intrusion of warm circumpolar deep water on the continental shelf has increased basal melting compared to that required to maintain the ice shelf in a state of mass balance and that this has triggered a general retreat of ice in this sector

dynamics and mass balance of four large east antarctic outlet glaciers this is the publisher s version also available electronically from http www ingentaconnect com