impact of iceberg calving on the retreat of thwaites glacier west antarctica over the next century with different calving laws and ocean thermal forcing

impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

continued retreat of thwaites glacier west antarctica controlled by bed topography and ocean circulation the amundsen sea sector is experiencing the largest mass loss glacier acceleration and grounding line retreat in antarctica enhanced intrusion of circumpolar deep water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat here we investigate the dynamics and evolution of thwaites glacier with a novel fully coupled ice ocean numerical model we obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model coupled simulations over the coming decades indicate a continued mass loss at a sustained rate uncoupled simulations using a depth dependent parameterization of sub ice shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat bed topography controls the pattern of grounding line retreat while oceanic thermal forcing impacts the rate of grounding line retreat the importance of oceanic forcing increases with time as thwaites grounding line retreats farther inland

antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

lithology and internal structure of the san andreas fault at depth based on characterization of phase 3 whole rock core in the san andreas fault observatory at depth safod borehole abstract we characterize the lithology and structure of the spot core obtained in 2007 during phase 3 drilling of the san andreas fault observatory at depth safod in order to determine the composition structure and deformation processes of the fault zone at 3 km depth where creep and microseismicity occur a total of approximately 41 m of spot core was taken from three separate sections of the borehole the core samples consist of fractured arkosic sandstones and shale west of the saf zone pacific plate and sheared fine grained sedimentary rocks ultrafine black fault related rocks and phyllosilicate rich fault gouge within the fault zone north american plate the fault zone at safod consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths two zones of serpentinite bearing clay gouge each meters thick occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation the gouge primarily is comprised of mg rich clays serpentinite lizardite chrysotile with notable increases in magnetite and ni cr oxides hydroxides relative to the surrounding host rock the rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith block in matrix and foliated cataclasite structure the blocks and clasts predominately consist of sandstone and siltstone embedded in a clay rich matrix that displays a penetrative scaly fabric mineral alteration veins and fracture surface coatings are present throughout the core and reflect a long history of syn deformation fluid rock reaction that contributes to the low strength and creep in the meters thick gouge zones

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

relationship between mega scale glacial lineations and iceberg ploughmarks on the bjornoyrenna palaeo ice stream bed barents sea abstract mega scale glacial lineations msgls are ridge groove corrugations aligned in the direction of the former ice flow tens of kilometers long and up to a few hundred meters wide they are the most striking subglacial features on the beds of former ice streams and play an important role in modulating ice flow through their influence on bed roughness and subglacial hydrology despite the importance of msgls their formation remains enigmatic most studies have tended to focus on assemblages of msgls and their relationship to other landforms up ice e g drumlins or bedrock features in ice stream onset zones but fewer studies have examined their characteristics and transition to other landforms towards ice stream grounding lines in this paper we investigate the relationship between an assemblage of msgls and ploughmarks on the bed of the former bjornoyrenna ice stream in the sw barents sea which occurs in the central part of the ice stream bed a sample of msgls is used to test their potential origin based on their metrics width length and diagnostic characteristics predicted by formation theories results show a down flow depth decrease of the msgl grooves and a shallowing tendency once they transition into ploughmarks their width shows an increasing tendency which we link mostly to the strong divergence of the trough and ice flow downstream the prominent continuity from linear to curvilinear features demonstrates that the grooves associated with msgls transition into iceberg ploughmarks this observation is consistent with the hypothesis that the msgls have formed through a mechanism of groove ploughing at least in part the continuity from msgls to iceberg ploughmarks resulted from detachment of large icebergs from the grounded ice wall or grounded ice shelf and their ploughing away from the ice margin

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

weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

sensitivity analysis of pine island glacier ice flow using issm and dakota assessing output errors of ice flow models is a major challenge that needs to be addressed if we are to increase our confidence level in projections of mass balance in antarctica and greenland major inputs to ice flow models include geometry ice thickness and surface elevation constitutive laws and boundary conditions geothermal flux basal drag coefficient surface temperature these inputs can be either measured in which case they carry errors due to instruments or inferred using inverse methods such as basal drag which is inverted using insar surface velocities in which case they carry additional errors generated by the inversion process itself in both cases these input errors will result in uncertainties that propagate throughout a forward model and that influence output diagnostics in order to estimate the resulting error margins on diagnostics such as mass flux we develop a new framework based on the design analysis kit for optimization and terascale applications dakota which we interface to the ice sheet system model issm we present results on the pine island glacier west antarctica for which we evaluate error margins of mass flux across the whole glacier given currently known error margins on ice thickness basal friction and ice hardness our results suggest errors in these inputs propagate linearly through the ice flow model providing a way to 1 calibrate measurement requirements for field campaigns collecting data such as bedrock or surface topography 2 quantify uncertainties in projections of mass balance and 3 assess the sensitivity of model outputs to input parameters this new error propagation model should help quantify confidence levels that we assign to model projections for the mass balance of antarctica and greenland which will ultimately improve our projections of future sea level rise in a warming climate copyright 2012 by the american geophysical union

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

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

bathymetry of the amundsen sea continental shelf implications for geology oceanography and glaciology the amundsen sea continental shelf is one of the most remote areas of coastal antarctica and was relatively unexplored until the late 1980s over the last two decades increased oceanographic and geological interest has led to several cruises that resulted in sufficient bathymetric data to compile a fairly detailed regional map of the amundsen continental shelf we have combined available multibeam and single beam bathymetry data from various sources and created a new regional bathymetry of the amundsen sea continental shelf and margin deep trough systems that dominate the inner shelf are aligned with present glaciers and separated by shallower ridges shaped by paleo ice streams these features merge into a small number of broader troughs on the middle shelf and shoal seaward they now serve as conduits and reservoirs for relatively warm circumpolar deep water this new compilation is a major improvement over previously available regional maps and should aid the numerical modeling of ocean circulation the reconstructions of paleo ice streams and the refinement of ice sheet models

dynamic in stability of thwaites glacier west antarctica 1 thwaites glacier west antarctica has the potential to directly contribute 1 m to sea level and currently is losing mass and thinning rapidly here we report on regional results for the sea level response to ice sheet evolution searise experiments and investigate the impact of i spatial resolution within existing data sets ii grounding zone processes and iii till rheology on the dynamics of this outlet glacier in addition to the searise data sets we use detailed aerogeophysical and satellite data from thwaites glacier as input to a coupled ice stream ice shelf ocean plume model that includes oceanic influences across a several kilometers wide grounding zone suggested by new high resolution data our results indicate that the ice tongue provides limited stability and that while future atmospheric warming will likely add mass to the surface of the glacier strong ice stream stabilization on bedrock highs narrower than the length of the grounding zone may be ephemeral if circulating waters substantially reduce basal resistance and enhance melting beneath grounded ice within this zone however we find that stability is significantly enhanced by effectively plastic till beds accurate projections of future sea level change relies on correct understanding of the till rheology as well as local basal processes near the grounding line

improved estimation of the mass balance of glaciers draining into the amundsen sea sector of west antarctica from the cecs nasa 2002 campaign in november december 2002 a joint airborne experiment by centro de estudios cientificos and nasa flew over the antarctic ice sheet to collect laser altimetry and radio echo sounding data over glaciers flowing into the amundsen sea a p 3 aircraft on loan from the chilean navy made four flights over pine island thwaites pope smith and kohler glaciers with each flight yielding 1 5 2 hours of data the thickness measurements reveal that these glaciers flow into deep troughs which extend far inland implying a high potential for rapid retreat interferometric synthetic aperture radar data insar and satellite altimetry data from the european remote sensing satellites ers 1 2 show rapid grounding line retreat and ice thinning of these glaciers using the new thickness data we have re evaluated glacier fluxes and the present state of mass balance which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium the revised total ice discharge of 241 5 km 3 a 1 exceeds snow accumulation by 81 17 km 3 a 1 of ice equivalent to a sea level rise of 0 21 0 04 mm a 1 this magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers this is confirmed by ice flow acceleration observed with insar we attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean which reduces buttressing of the glaciers and in turn accelerates them out of balance

first airborne gravity results over the thwaites glacier catchment west antarctica recent satellite observations of thwaites glacier in the amundsen sea embayment west antarctica have shown that the glacier is changing rapidly the causes of its dynamic behavior are uncertain but are of concern because this glacier has the most negative mass balance of all antarctic glaciers to better understand thwaites glacier s subglacial setting we conducted a multi instrumented aerogeophysical survey of its catchment and present here the first gravity results we employed a new gravimeter and it performed well despite extreme conditions and an unusual survey design the unleveled free air gravity anomalies have a 2 3 mgal rms error and a 9 km spatial resolution despite slightly higher than standard noise levels the free air anomalies correlate well with radar derived subglacial topography the new airborne gravity data assist in interpreting radar identified bedrock features and are an ideal basis for future studies of subglacial geology and its control on the dynamics of thwaites glacier

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

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

velocity map of the thwaites glacier catchment west antarctica the two dimensional surface velocity of thwaites glacier west antarctica was mapped with 23 ascending and 22 descending orbit european remote sensing satellite synthetic aperture radar ers sar interferograms time range 1995 2000 the velocity map covers 175 500 square km from the amundsen coast to the southern turning point of the satellite orbit and comprises over 80 of the thwaites catchment

potential seaways across west antarctica the west antarctic ice sheet wais has long been considered vulnerable to rapid retreat and today parts are rapidly losing ice projection of future change in wais is however hampered by our poor understanding of past changes especially during interglacial periods that could be analogs for the future but which undoubtedly provide an opportunity for testing predictive models we consider how ice loss would open seaways across wais these would likely alter southern ocean circulation and climate and would broadly define the de glacial state but they may also have left evidence of their existence in the coastal seas they once connected we show the most likely routes for such seaways and that a direct seaway between weddell and ross seas which did not pass through the amundsen sea sector is unlikely continued ice loss at present rates would open seaways between amundsen and weddell seas a w and amundsen and bellingshausen seas a b in around one thousand years this timescale indicates potential future vulnerability but also suggests seaways may have opened in recent interglacial periods we attempt to test this hypothesis using contemporary bryozoan species assemblages around antarctica concluding that anomalously high similarity in assemblages in the weddell and amundsen seas supports recent migration through a w other authors have suggested opening of seaways last occurred during marine isotope stage 7a 209 ka bp but we conclude that opening could have occurred in mis 5e 100 ka bp when antarctica was warmer than present and likely contributed to global sea levels higher than today

the geochemical and mineralogical fingerprint of west antarctica s weak underbelly pine island and thwaites glaciers abstract the marine based west antarctic ice sheet wais is considered the most unstable part of the antarctic ice sheet with particular vulnerability in the amundsen sea sector where glaciers are melting at an alarming rate far field sea level data and ice sheet models have pointed towards at least one major wais disintegration during the late quaternary but direct evidence for past collapse s from ice proximal geological archives remains elusive in order to facilitate geochemical and mineralogical tracing of the two most important glaciers draining into the amundsen sea i e pine island glacier pig and thwaites glacier tg we here provide the first multi proxy provenance analysis of 26 seafloor surface sediment samples from pine island bay our data show that the fingerprints of detritus delivered by pig and tg are clearly distinct near the ice shelf fronts of both ice stream systems for all grain sizes and proxies investigated glacial detritus delivered by pig is characterised by low end values 9 high 87sr 86sr ratios 0 728 low smectite content the geochemical and mineralogical fingerprints for pig and tg reported here provide novel insights into sub ice geology and allow us to trace both drainage systems in the geological past under environmental conditions more similar to those envisioned in the next 50 to 100 years

ice flow sensitivity to geothermal heat flux of pine island glacier antarctica model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation bedrock position or surface temperatures among others of all these inputs geothermal heat flux is the one for which uncertainty is greatest in the area of pine island glacier antarctica available data sets differ by up to a factor of 2 5 here we evaluate the impact of such uncertainty on ice flow using sampling analyses based on the latin hypercube method first we quantify the impact of geothermal heat flux errors on ice hardness a thermal parameter that critically controls the magnitude of ice flow second we quantify the impact of the same errors on mass balance specifically on the mass flux advecting through thirteen fluxgates distributed across pine island glacier we contrast our results with similar uncertainties generated by errors in the specification of ice thickness model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5 7 with maximum uncertainty reaching 15 resulting uncertainties in mass balance remain however below 1 we discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction viscous and geothermal heating we also show that comparatively errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux especially for fast flowing ice streams 2012 american geophysical union all rights reserved

constraining past accumulation in the central pine island glacier basin west antarctica using radio echo sounding the potential for future dynamical instability of pine island glacier west antarctica has been addressed in a number of studies but information on its past remains limited in this study we use airborne radio echo sounding res data acquired over pine island glacier to investigate past variations in accumulation pattern in the dataset a distinctive pattern of layers was identified in the central part of the glacier basin we use these layers as chronological identifiers in order to construct elevation maps of the internal stratigraphy the observed internal layer stratigraphy is then compared to calculated stratigraphy from a three dimensional ice flow model that has been forced with different accumulation scenarios the model results indicate that the accumulation pattern is likely to have changed at least twice since the deposition of the deepest identified layer additional res data linked to the byrd ice core provide an approximate timescale this timescale suggests that the layers were deposited at the beginning of or during the holocene period thus the widespread changes occurring in the coastal extent of thewest antarctic ice sheet at the end of the last glacial period could have been accompanied by changes in accumulation pattern

growth and decay of a marine terminating sector of the last british irish ice sheet a geomorphological reconstruction the boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins this paper uses the glacial landscape in western scotland to reconstruct changes in the british irish ice sheet that accompanied the growth and decay of a marine sector over the malin shelf ice advanced from a restricted mountain ice sheet with tidewater margins after 35 ka bp and reached the continental shelf in 7 ka average rate of 30 m a 1 early ice flow had been directed through north south geologically controlled over deepened fjords that were carved during previous restricted glaciations this flow regime was abandoned with development of the malin shelf ice sheet sector ice flow direction switched by 90 degrees and was drawn westwards towards the shelf edge the marine ice sheet phase saw episodes of west east ice divide migration by up to 60 km over west central scotland possibly linked to ice streaming and calving events at the ice sheet margin however permanent and stationary ice divides and zones of cold based ice associated with subglacial topographic highs also characterised the marine glacial stage over western scotland the north channel ice divide remained a constant though migratory feature while the biis occupied the malin shelf it finally collapsed at the end of the killard point stadial when the irish ice sheet began to rapidly decay 16 5 ka bp this permitted the scottish ice sheet to temporarily advance over north east ireland previously identified as the east antrim coastal readvance before it too retreated at rates in the order of 10 2 m a 1 although the imprint of extensive shelf edge ice sheet glaciation exists in the coastal landscape of western scotland the dominant landscape features relate to a restricted marine proximal mountain ice sheet with markedly different flow configurations similar first order geomorphological features relating to restricted glacial conditions are likely to be preserved in subglacial highlands under interior parts of modern ice sheets

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

impact of bedrock description on modeling ice sheet dynamics recent glaciological surveys have revealed a significant increase of ice discharge from polar ice caps into the ocean in parallel ice flow models have been greatly improved to better reproduce current changes and forecast the future behavior of ice sheets for these models surface topography and bedrock elevation are crucial input parameters that largely control the dynamics and the ensuing overall mass balance of the ice sheet for obvious reasons of inaccessibility only sparse and uneven bedrock elevation data is available this raw data is processed to produce digital elevation models dems on a regular 5 km grid these dems are used to constrain the basal boundary conditions of all ice sheet models here by using a full stokes finite element code we examine the sensitivity of an ice flow model to the accuracy of the bedrock description in the context of short term ice sheet forecast we show that in coastal regions the bedrock elevation should be known at a resolution of the order of one kilometer conversely a crude description of the bedrock in the interior of the continent does not affect modeling of the ice outflow into the ocean these findings clearly indicate that coastal regions should be prioritized during future geophysical surveys they also indicate that a paradigm shift is required to change the current design of dems describing the bedrock below the ice sheets they must give users the opportunity to incorporate high resolution bedrock elevation data in regions of interest

flow and retreat of the late quaternary pine island thwaites palaeo ice stream west antarctica multibeam swath bathymetry and sub bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo ice stream that carried the combined discharge from the parts of the west antarctic ice sheet now occupied by the pine island and thwaites glacier basins sets of highly elongated bedforms show that at the last glacial maximum the route of the pine island thwaites palaeo ice stream arced north northeast following a prominent cross shelf trough in this area the grounding line advanced to within similar to 68 km of and probably reached the shelf edge minimum ice thickness is estimated at 715 m on the outer shelf and we estimate a minimum ice discharge of similar to 108 km 3 yr 1 assuming velocities similar to today s pine island glacier similar to 2 5 km yr 1 additional bed forms observed in a trough northwest of pine island bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior the style of ice retreat is also evident in five grounding zone wedges which suggest episodic deglaciation characterized by halts in grounding line migration up trough stillstands occurred in association with changes in ice bed gradient and phases of inferred rapid retreat correlate to higher bed slopes supporting theoretical studies that show bed geometry as a control on ice margin recession however estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat our findings show that the ice stream channeled a substantial fraction of west antarctica s discharge in the past just as the pine island and thwaites glaciers do today

inversion of basal friction in antarctica using exact and incomplete adjoints of a higher order model basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics here we use a complete map of surface velocity of the antarctic ice sheet to infer the basal friction over the entire continent by combining these observations with a three dimensional thermomechanical higher order ice sheet numerical model from the ice sheet system model open source software we demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization at a spatial resolution as high as 3 km along the coastline we compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation the results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80 of the ice sheet the basal friction coefficient which relates basal friction to basal velocity is however significantly heterogeneous it is low on fast moving ice and high near topographic divides areas with low values extend far out into the interior along glacier and ice stream tributaries almost to the flanks of topographic divides suggesting that basal sliding is widespread beneath the antarctic ice sheet 2013 american geophysical union all rights reserved

thwaites glacier grounding line retreat influence of width and buttressing parameterizations major ice loss has recently been observed along coastal outlet glaciers of the west antarctic ice sheet mainly due to increased melting below the ice shelves however the behavior of this marine ice sheet is poorly understood leading to significant shortcomings in ice sheet models attempting to predict future sea level rise the stability of a marine ice sheet is controlled by the dynamics at the grounding line the boundary between the grounded ice stream and the floating ice shelf one of the largest contributors to current sea level rise is the fast flowing thwaites glacier which flows into the amundsen sea here we use an ice stream ice shelf model and perform a number of experiments along a central flowline to analyze the sensitivity of its grounding line on centennial timescales in the absence of width and buttressing effects we find that the grounding line retreats by 300 km in 200 years from the present day rate of 1 5 km a 1 with variable glacier width implemented in the model flow convergence slows the retreat of thwaites grounding line at 0 3 1 2 km a 1 the parameterization of ice shelf buttressing according to different observed scenarios further reduces the glacier retreat and can even lead to a slight advance in the most buttressed case

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

sharad sounding radar on the mars reconnaissance orbiter 1 sharad shallow radar is a sounding radar provided by agenzia spaziale italiana asi as a facility instrument on the mars reconnaissance orbiter mission its 20 mhz center frequency and 10 mhz bandwidth complement the lower frequency relatively narrower bandwidth capability of the marsis sounding radar a joint italian u s team has guided the experiment development and is responsible for data analysis and interpretation the radar transmits signals at a 700 hz pulse repetition frequency prf and collects reflections from both the surface and near subsurface of mars vertical and horizontal resolutions are respectively 15 m free space and 3 6 km cross track by 0 3 1 km along track the scientific objective of sharad is to map in selected locales dielectric interfaces to at least several hundred meters depth in the martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials including competent rock soil water and ice a signal to noise ratio of 50 db for a specular surface return is achieved with 10 w of radiated power by using range and azimuth focusing in ground data processing preprocessed data as well as range and azimuth focused data will be formatted according to planetary data system pds standards and be made available from the asi science data center asdc and from the geosciences node of the planetary data system pds important targets for sharad include the polar layered deposits sedimentary stacks especially in terra meridiani buried channel systems buried impact craters volcanic complexes and shallow ice deposits in equilibrium with the atmosphere

topographic and hydrological controls on subglacial lake ellsworth west antarctica 1 subglacial lake ellsworth sle was identified using reconnaissance data collected in the 1970s here we present more detailed surveys sle lies beneath 3 2 km of ice in a subglacial valley in west antarctica it has an area of only 18 km2 is dissimilar to the large tectonically controlled lakes beneath east antarctica and is a strong candidate for in situ exploration our analysis indicates that the ice above sle is floating on a fluid whose density is 950 1013 kg m 3 this could indicate freshwater but certainly precludes seawater or high salt acid or clathrate content the water in the lake is unlikely to be produced solely by local melt it is more likely delivered via subglacial drainage our surveys show no identifiable hydrological barrier to outflow meaning sle is effectively full new water entering the lake is likely balanced by outflow which would drain into another lake that we have also identified

intermittent structural weakening and acceleration of the thwaites glacier tongue between 2000 and 2018 evolving conditions at the terminus of thwaites glacier will be important in determining the rate of its future sea level contribution over the coming decades here we use remote sensing observations to investigate recent changes 2000 2018 in the structure and velocity of thwaites glacier and its floating tongue we show that the main trunk of thwaites glacier has accelerated by 38 over this period while its previously intact floating tongue has transitioned to a weaker melange of fractured icebergs bounded by sea ice however the rate of structural weakening and acceleration was not uniform across the observational period and we identify two periods of rapid acceleration and structural weakening 2006 2012 2016 2018 separated by a period of deceleration and re advance of the structurally intact shear margin boundary 2012 2015 the timing of these accelerations decelerations strongly suggests a link to variable ocean forcing the weakened tongue now has some dependency on landfast sea ice for structural integrity and is vulnerable to changes in landfast ice persistency future reductions in landfast sea ice could manifest from changes in climate and or the imminent removal of the b 22a iceberg from the thwaites embayment such changes could have important implications for the integrity of the ice tongue and future glacier discharge

antarctic elevation change from 1992 to 1996 satellite radar altimeter measurements show that the average elevation of the antarctic ice sheet interior fell by 0 9 0 5 centimeters per year from 1992 to 1996 if the variability of snowfall observed in antarctic ice cores is allowed for the mass imbalance of the interior this century is only 0 06 0 08 of the mean mass accumulation rate

subglacial melt channels and fracture in the floating part of pine island glacier antarctica 1 a dense grid of ice penetrating radar sections acquired over pine island glacier west antarctica has revealed a network of sinuous subglacial channels typically 500 m to 3 km wide and up to 200 m high in the ice shelf base these subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf above the apex of most channels the radar shows isolated reflections from within the ice shelf comparison of the radar data with acoustic data obtained using an autonomous submersible confirms that these echoes arise from open basal crevasses 50 100 m wide aligned with the subglacial channels and penetrating up to 1 3 of the ice thickness analogous sets of surface crevasses appear on the ridges between the basal channels we suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and if the channels were formed sufficiently rapidly would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets we conclude that ice shelf basal melting plays a role in determining patterns of surface and basal crevassing increased delivery of warm ocean water into the sub ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf perhaps as a prelude to eventual collapse

deformation styles at the appalachian structural front western newfoundland implications of new industry seismic reflection data new seismic reflection data gathered during hydrocarbon exploration in and adjacent to the external humber zone western newfoundland have important implications for the interpretation of structural style at the appalachian front these new data indicate that the structural front is influenced by both thin skinned and thick skinned structures where the structural front is thin skinned it is characterized by a triangle zone or tectonic wedge similar to structures observed at the southeastern margin of the canadian cordillera and at other orogenic fronts the thin skinned tectonic wedge is overridden locally by thick skinned thrusts which are generally emergent but are locally blind forming a thick skinned tectonic wedge timing relationships indicate that although initial motion occurred during the early to middle ordovician taconian orogeny the thin skinned allochthonous slices in western newfoundland were not emplaced until devonian time the acadian orogeny thick skinned deformation which p

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

analysis of deep penetrating radar surveys of west antarctica us itase 2001 1 we collected 1850 km of ground based radio echo sounding res data in west antarctica along the 2001 us leg of the international trans antarctic scientific expedition traverse us itase these data represent the first high resolution measurements of ice thickness and internal stratigraphy for portions of this area that bear on questions about the stability of the west antarctic ice sheet wais internal layers depicting isochrones have been traced continuously for much of the profile length and a date from the byrd deep core is ascribed to one of these isochrones from which we estimate vertical velocities across the traverse route comparison of our ice thickness measurements with bedmap show good agreement in regions where the database is well constrained by data coverage where the bedmap dataset was forced to interpolate differences are as high as 45 of the ice thickness

basement control on past ice sheet dynamics in the amundsen sea embayment west antarctica abstract the development of landscapes and morphologies follows initially the tectonic displacement structures of the basement and sediments such fault zones or lineaments are often exploited by surface erosional processes and play therefore an important role in reconstructing past ice sheet dynamics observations of bathymetric features of the continental shelf of the amundsen sea embayment and identification of tectonic lineaments from geophysical mapping indicate that the erosional processes of paleo ice stream flows across the continental shelf followed primarily such lineaments inherited from the tectonic history since the cretaceous break up between new zealand and west antarctica three major ice flow trends correspond to different tectonic phases in east west northwest southeast and north south directions east west oriented basement trends correlate with coastline trends and overlay tectonic lineaments caused by former rift activities directional trends with northwest southeast orientation are observed for the glacial troughs of the western embayment outer shelf the western pine island bay coastal zones and the inner pine island glacial trough and are associated with a distributed southern plate boundary zone of the former bellingshausen plate the north south trend of the main pine island glacial trough and the north northeast trend of the abbot ice shelf trough on the outer shelf follow the predicted lineation trend of an eastern branch of the west antarctic rift system extending from the thwaites drainage basin northward into pine island bay an understanding of this context helps better constrain the geometries and sea bed substrate conditions for regional paleo ice sheet models

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

state of the antarctic and southern ocean climate system this paper reviews developments in our understanding of the state of the antarctic and southern ocean climate and its relation to the global climate system over the last few millennia climate over this and earlier periods has not been stable as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in antarctic ice core proxies for past climate two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies also known as the southern annular mode between 6000 and 5000 years ago and since 1200 1000 years ago following the last of these is a period of major trans antarctic reorganization of atmospheric circulation and temperature between ad1700 and 1850 the two earlier antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the north atlantic and the end of the more recent event is coincident with reorganization of atmospheric circulation in the north pacific improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols special attention is given to the climate of the past 200 years which was recorded by a network of recently available shallow firn cores and to that of the past 50 years which was monitored by the continuous instrumental record significant regional climate changes have taken place in the antarctic during the past 50 years atmospheric temperatures have increased markedly over the antarctic peninsula linked to nearby ocean warming and intensification of the circumpolar westerlies glaciers are retreating on the peninsula in patagonia on the sub antarctic islands and in west antarctica adjacent to the peninsula the penetration of marine air masses has become more pronounced over parts of west antarctica above the surface the antarctic troposphere has warmed during winter while the stratosphere has cooled year round the upper kilometer of the circumpolar southern ocean has warmed antarctic bottom water across a wide sector off east antarctica has freshened and the densest bottom water in the weddell sea has warmed in contrast to these regional climate changes over most of antarctica near surface temperature and snowfall have not increased significantly during at least the past 50 years and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years furthermore the total sea ice cover around antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s despite large but compensating regional changes the inhomogeneity of antarctic climate in space and time implies that recent antarctic climate changes are due on the one hand to a combination of strong multi decadal variability and anthropogenic effects and as demonstrated by the paleoclimate record on the other hand to multi decadal to millennial scale and longer natural variability forced through changes in orbital insolation greenhouse gases solar variability ice dynamics and aerosols model projections suggest that over the 21st century the antarctic interior will warm by 3 4 1oc and sea ice extent will decrease by 30 ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level considering the potentially major impacts of a warming climate on antarctica vigorous efforts are needed to better understand all aspects of the highly coupled antarctic climate system as well as its influence on the earth s climate and oceans

evidence for rapid retreat and mass loss of thwaites glacier west antarctica thwaites glacier the second largest ice stream in west antarctica drains an area of 166 500 2000 km2 which accumulates 55 5 gt a 1 or 60 6 km3 ice a 1 into the amundsen sea unrestrained by an ice shelf using interferometric synthetic aperture radar insar data collected by the european remote sensing satellites ers 1 and 2 in 1996 an output flux of 71 7 gt a 1 or 77 8 km3 ice a 1 is estimated at the grounding line where ice thickness is deduced from hydrostatic equilibrium a similar flux 70 7 gt a 1 or 76 8 km3 ice a 1 is obtained at a gate located 20 km upstream where ice thickness was measured in 1978 by ice sounding radar total accumulation in between the two gates is 1 6 gt a 1 or 1 8 km3 ice a 1 ice discharge therefore exceeds mass accumulation by 30 15 and thwaites glacier must be thinning and retreating at present the insar data show that the glacier floating ice tongue exerts no back pressure on the inland ice calves into tabular icebergs along a significant fraction of its grounding line and has a grounding line thickness which exceeds a prior calculated limit for stability glacier thinning is confirmed at the coast by the detection of a 1 4 0 2 km retreat of its grounding line between 1992 and 1996 with insar which implies 3 2 0 6 m ice a 1 thinning at the glacier center and less near the sides these results complement the decimeter scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line the magnitude of ice thinning estimated at the coast however rules out temporal changes in accumulation as the explanation for surface lowering ice thinning must be due to changes in ice flow

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

a new bathymetric compilation for the south orkney islands region antarctic peninsula 49 39 w to 64 59 s insights into the glacial development of the continental shelf we present a new high resolution 300 m bathymetric grid of the continental shelf surrounding the south orkney islands northeast of the antarctic peninsula the new grid derived from a compilation of marine echo sounding data improves previous regional bathymetric representations and helps to visualize the morphology of the shelf in unrivalled detail the compilation forms important baseline information for a range of scientific applications and end users including oceanographers glacial modelers biologists and geologists in particular due to the limited understanding of glacial history in this region the bathymetry provides the first detailed insights into past glacial regimes the continental shelf is dominated by seven glacially eroded troughs marking the pathways of glacial outlets that once drained a former ice cap centered on the south orkney islands during previous glacial periods grounded ice extended to the shelf edge north of the islands a large 250 km long sediment depocenter interpreted as a maximum former ice limit of one or more cenozoic glaciations suggests that ice was only grounded to the 300 350 m contour in the south hypsometric analyses support this interpretation indicating that a significant proportion of the shelf has been unaffected by glacial erosion using these observations we propose a preliminary ice cap reconstruction for maximum glaciation of the south orkney plateau suggesting an ice coverage of about 19 000 km2 the timing of maximum ice extent number of past advances and pattern of subsequent deglaciation s remain uncertain and will require further targeted marine geological and geophysical investigations to resolve

ice sheet retreat dynamics inferred from glacial morphology of the central pine island bay trough west antarctica abstract pine island glacier drains portions of the west antarctic ice sheet into the amundsen sea during the last glacial maximum the glacier extended nearly 500 km from its present location onto the outer continental shelf unusually restricted sea ice cover during the austral summer of 2010 allowed for a systematic multibeam swath bathymetric and chirp sonar survey of the mid shelf section of pine island trough the mapped glacial landforms reveal new information about the paleo pine island ice stream s dynamic retreat from the mid shelf area and confirm previous suggestion of a retreat in distinct steps the periods of grounding line stability during the overall retreat phase are marked by sediment accumulations i e grounding zone wedges these wedges are here mapped in sufficient detail to characterize spatial dimensions and estimate the volume of deposited sediment considering a range of sediment flux rates from the paleo pine island ice stream we estimate that the largest and most clearly defined grounding zone wedge located at about 73 s in the surveyed area took between 600 and 2000 years to form the ice stream retreated landward of this wedge before 12 3 cal ka bp the swath bathymetric imagery of landforms in pine island trough includes glacial features that suggest that retreat between periods of grounding line stability may be associated with episodes of ice shelf break up the depths of grounding line wedges decrease in a landward direction from 740 to 670 m and record elevation of the grounding line as it stepped landward in all the grounding line elevation varied by only 80 m over a distance of just over 100 km implying a low ice sheet profile during retreat finally we revisited seismic reflection profile nb9902 acquired along pine island trough in 1999 in combination with the newly acquired swath bathymetric imagery from 2010 together these data show that the ice stream paused during its retreat to form grounding zone wedges at an area in central pine island trough where a high in dipping bedrock strata exists and the glacial trough is narrow forming a bathymetric bottle neck

ice stream flow switching during deglaciation of the southwestern barents sea ice streams dominate the discharge of continental ice sheets recent observations and reconstructions have revealed that large scale reorganizations in their flow trajectory flow switching can occur over relatively short time scales however the underlying causes of such behavior and the extent to which they are predictable are poorly known this paper documents a major episode of ice stream flow switching during the late weichselian deglaciation of the southwestern barents sea and explores various hypotheses for its causation regional bathymetric data show that two ice streams that had similar adjoining topographically constrained source areas had very different trajectories and dynamics on the outer shelf at the late weichselian maximum the hakjerringdjupet ice stream flowed westward along the cross shelf trough of hakjerringdjupet while the soroya trough ice stream flowed northward into ingoydjupet forming a tributary of the bjornoyrenna ice stream initial retreat of the hakjerringdjupet ice stream was rapid but with episodic periods of grounding as it retreated onto the higher rougher topography of the inner shelf we infer a reduction in ice velocity and a dramatic decrease in the pace of retreat as recorded by nested sequences of recessional moraines following and probably in response to this we suggest that there was a short lived surge readvance of an adjacent lobe onto fugloybanken in contrast the adjacent soroya trough ice stream remained active throughout deglaciation before retreating rapidly with no stillstands or readvances we argue that the different retreat histories of the ice streams were determined by variations in bed topography bathymetry which modulated the grounding line response to sea level variation such a mechanism is likely to be an important control on the long term behavior of marine based ice streams and outlet glaciers in antarctica and greenland and suggests that gathering data on their subglacial topography should be a priority

new boundary conditions for the west antarctic ice sheet subglacial topography of the thwaites and smith glacier catchments airborne radar sounding over the thwaites glacier tg catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the west antarctic ice sheet wais and reveals that tg is underlain by a single broad basin fed by a dendritic pattern of valleys while smith glacier lies within an extremely deep narrow trench subglacial topography in the tg catchment slopes inland from a broad low relief coastal sill to the thickest ice of the wais and makes deep connections to both pine island glacier and the ross sea embayment enabling dynamic interactions across the wais during deglaciation simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation aside from an island chain near the present day ross amundsen ice divide the lack of topographic confinement along tg s eastern margin implies that it may continue to widen in response to grounding line retreat

bathymetry of the amundsen sea embayment sector of west antarctica from operation icebridge gravity and other data we employ airborne gravity data from nasa s operation icebridge collected in 2009 2014 to infer the bathymetry of sub ice shelf cavities in front of pine island thwaites smith and kohler glaciers west antarctica we use a three dimensional inversion constrained by multibeam echo sounding data offshore and bed topography from a mass conservation reconstruction on land the seamless bed elevation data refine details of the pine island sub ice shelf cavity a slightly thinner cavity beneath thwaites and previously unknown deep 1200 m channels beneath the crosson and dotson ice shelves that shallow 500 m and 750 m respectively near the ice shelf fronts these sub ice shelf channels define the natural pathways for warm circumpolar deep water to reach the glacier grounding lines melt the ice shelves from below and constrain the pattern of past and future glacial retreat

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

a recent volcanic eruption beneath the west antarctic ice sheet a strong radar reflection in the west antarctic ice sheet is related to the eruption of the newly identified hudson mountains subglacial volcano dated to 207 bc this eruption probably caused short term changes in regional glacial and meltwater flow indirect evidence suggests that volcanic activity occurring beneath the west antarctic ice sheet influences ice flow and sheet stability1 2 3 however only volcanoes that protrude through the ice sheet4 and those inferred from geophysical techniques1 2 have been mapped so far here we analyse radar data from the hudson mountains west antarctica5 that contain reflections from within the ice that had previously been interpreted erroneously as the ice sheet bed we show that the reflections are present within an elliptical area of about 23 000 km2 that contains tephra from an explosive volcanic eruption the tephra layer is thickest at a subglacial topographic high which we term the hudson mountains subglacial volcano the layer depth dates the eruption at 207 bc 240 years which matches exceptionally strong but previously unattributed conductivity signals in nearby ice cores the layer contains 0 019 0 31 km3 of tephra which implies a volcanic explosive index of 3 4 production and episodic release of water from the volcano probably affected ice flow at the time of the eruption ongoing volcanic heat production may have implications for contemporary ice dynamics in this glacial system

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

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

surface slope control on firn density at thwaites glacier west antarctica results from airborne radar sounding we derive the surface density variations over thwaites glacier from a statistical analysis of airborne high capability radar sounder surface returns acquired in the 2004 2005 summer we produce a 5 km gridded map with an estimated 12 5 kg m 3 accuracy the background pattern of densities decreases inland from 480 kg m 3 to 350 kg m 3 a remarkable 30 km wide vein shaped anomaly of up to 570 kg m 3 is located 100 km from the coastline density anomalies correspond with surface slope breaks but are not necessarily coincident with smaller slopes they could result from complex wind driven snow redistribution and or refreezing of former snowmelt this inversion technique can significantly improve surface mass balance calculations to understanding of glacier dynamics at regional scales and is valuable to verify and improve antarctic climate models it is also a promising approach for future surface analysis of icy moons by planetary radars

new airborne laser altimetry over the thwaites glacier catchment west antarctica 1 a new airborne altimetry data set collected over thwaites glacier one of antarctica s most active ice streams demonstrates the improvement in publicly available digital elevation models dems of the antarctic ice sheet the airborne altimetry comprises 35 000 line km sampled at 20 m along track the full data set has a relative error of 20 cm a reference subset has an error of 8 cm these data are offset from icesat observations by 20 cm we find that a recently released icesat dem provides a good model of the surface of thwaites glacier despite cloud cover and wide track spacings however the icesat dem s accuracy is an order of magnitude less than that of the icesat profile data our airborne data will serve as an additional temporal reference for the evolution of thwaites glacier s surface as well as aid the construction of future high resolution dems

calibrated prediction of pine island glacier retreat during the 21st and 22nd centuries with a coupled flowline model a flowline ice sheet model is coupled to a box model for cavity circulation and configured for the pine island glacier an ensemble of 5000 simulations are carried out from 1900 to 2200 with varying inputs and parameters forced by ocean temperatures predicted by a regional ocean model under the a1b business as usual emissions scenario comparison is made against recent observations to provide a calibrated prediction in the form of a 95 confidence set predictions are for monotonic apart from some small scale fluctuations in a minority of cases retreat of the grounding line over the next 200 yr with huge uncertainty in the rate of retreat full collapse of the main trunk of the pig during the 22nd century remains a possibility

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

why is it hard to predict the future of ice sheets ice sheet retreat hypotheses of instability could be missing important processes that limit the rate or extent of retreat or conversely ice sheet behavior is strongly influenced by processes at its margin and base observations of rapid changes at these boundaries are helping modelers to improve predictions of future changes

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

antarctic topography at the eocene oligocene boundary we present a reconstruction of the antarctic topography at the eocene oligocene ca 34 ma climate transition this provides a realistic key boundary condition for modeling the first big antarctic ice sheets at this time instead of using the present day bedrock topography which has changed significantly from millions of years of tectonism and erosion we reconstruct topography using a set of tools including ice sheet erosion models models of thermal subsidence and plate movement erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty our results point to a significant upland area in the ross sea marie byrd land and weddell sea sectors in addition east antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets parts of east antarctica have not changed since the e o boundary because they were protected under non erosive cold based ice the reconstructions provide a better defined boundary condition for modeling that seeks to understand interaction between the antarctic ice sheet and climate along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations

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

changes in west antarctic ice stream dynamics observed with alos palsar data the advanced land observation system alos phased array synthetic aperture radar palsar is an l band frequency 1 27 ghz radar capable of continental scale interferometric observations of ice sheet motion here we show that palsar data yield excellent measurements of ice motion compared to c band 5 6 ghz radar data because of greater temporal coherence over snow and firn we compare palsar velocities from year 2006 in pine island bay west antarctica with those spanning years 1974 to 2007 between 1996 and 2007 pine island glacier sped up 42 and ungrounded over most of its ice plain smith glacier accelerated 83 and ungrounded as well their largest speed up are recorded in 2007 thwaites glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed total ice discharge from these glaciers increased 30 in 12 yr and the net mass loss increased 170 from 39 15 gt yr to 105 27 gt yr longer term velocity changes suggest only a moderate loss in the 1970s as the glaciers unground into the deeper smoother beds inland the mass loss from this region will grow considerably larger in years to come copyright 2008 by the american geophysical union

the internal layering of pine island glacier west antarctica from airborne radar sounding data this paper presents an overview of internal layering across pine island glacier west antarctica as measured from airborne radar data acquired during a survey conducted by the british antarctic survey and the university of texas in the 2004 05 season internal layering is classified according to type continuous discontinuous missing and the results compared with insar velocities several areas exhibit disruption of internal layers that is most likely caused by large basal shear stresses signs of changes in flow were identified in a few inter tributary areas but overall the layering classification and distribution of layers indicate that only minor changes in ice flow regime have taken place this is supported by bed topography data that show the main trunk of the glacier as well as some of the tributaries are topographically controlled and located in deep basins

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

occurrence and use of mineral and thermal waters in poland mineral and thermal waters are defined according to polish geological and mining law their occurrence in particular hydrogeological provinces of the country is characterized besides their use in balneotherapeutics their role as a heat carrier and source of chemical raw materials is stressed topics mentioned requiring further research are the origin of saline formation waters and the origin of co2 in carbonated waters

observation of low shear wave velocity at the base of the polar ice sheets evidence for enhanced anisotropy summary we analyse seismic data from the broad band stations located on the antarctic and greenland ice sheets to determine the large scale seismic parameters of the polar ice sheets the p to s converted waves at the ice rock interface and inside the ice sheets and their multiples the p receiver functions are used to estimate the in situ p velocity vp and the p to s velocity ratio vp vs of the polar ice the thickness of the whole ice layer is precisely known either from radio echo soundings or from ice core drillings allowing thus an accurate determination of vp and vp vs at some places in and near the wilkes basin a sedimentary layer is probably squeezed between the ice and the bedrock we find that the polar ice caps have a two layer structure the upper layer of variable thickness about 2 3 of the total thickness with velocities very close to the ice standard values and the lower layer preserving a standard vp but with about 25 per cent smaller shear wave velocity and a more or less constant thickness the shear velocity drop in the lower layer may be the evidence of a strong anisotropy induced by preferred orientation of ice crystals and by fine layering of soft and hard ice layers a large variation of ice viscosity with depth is therefore expected and heterogeneous flowing of the polar ice sheet this heterogeneous flowing may invalidate the use at great depth of the ice dating models based on monotonic layer thinning

meltwater intensive glacial retreat in polar environments and investigation of associated sediments example from pine island bay west antarctica abstract modern pine island and thwaites glaciers which both drain into pine island bay are among the fastest changing portions of the cryosphere and the least stable ice streams in antarctica here we show that the uppermost sediment unit in pine island bay was deposited from a meltwater plume a plumite during the late stages of ice sheet retreat 7 8 6 k cal yr bp and argue that this deposit records episodes of meltwater intensive sedimentation the plumite is a hydraulically sorted glacially sourced draping deposit that overlies proximal glacimarine sediments and thickens towards the modern grounding line the uppermost sediment unit is interpreted as a product of non steady state processes in which low background sedimentation in large bedrock carved basins alternates with episodic purging of sediment laden water from these basins the inner part of pine island bay contains several basins that are linked by channels with a storage capacity on the order of 70 km 3 of stagnant water and significant sediment storage capacity purging of these basins is caused by changes in hydraulic potential and glacial reorganization the sediment mobilized by these processes is found here to total 120 km 3 this study demonstrates that episodes of meltwater intensive sedimentation in pine island bay occurred at least three times in the holocene the most recent episode coincides with rapid retreat of the grounding line in historical time and has an order of magnitude greater flux relative to the entire unit we note that the final phase of ice stream retreat in marguerite bay was marked by a similar sedimentary event and suggest that the modern thwaites glacier is poised for an analogous meltwater intensive phase of retreat