ice flow velocities on rutford ice stream west antarctica are stable over decadal timescales surface ice flow velocities measured at stakes on rutford ice stream west antarctica covering a period of similar to 25 years are analysed for evidence of temporal variations in flow no indications of significant long term changes in flow are found earlier observations have shown significant tidally related variations in flow speed we conclude that temporal variability on rutford ice stream west antarctica appears limited to tidal periods of days and weeks and weaker interannual variation possibly related to long period tides or seasonal effects while long term decadal changes in flow speed are either absent or smaller than similar to 0 1 a 1

decadal gps derived ice surface velocity along the transect from zhongshan station to and around dome argus east antarctica 2005 16 using repeat gps measurements during 2005 16 we calculated and updated two dimensional high resolution decadal ice surface velocity estimates along the traverse route from zhongshan station to and around dome argus east antarctica along the 71 sites of the transect the magnitudes of ice velocity increased from near 0 in dome argus to 1 10 and 100 m a 1 at the sites dt416 dt333 and lt980 respectively the comparison between gps and interferometric synthetic aperture radar insar derived results agree well when the magnitude of the ice surface velocities is faster than 5 m a 1 and disagree for slower flow velocities a scale value 1 15 and 0 12 can be applied to insar derived results over this region with ice surface velocity larger and 1 respectively we attributed the cause of the discrepancy to the insensitivity of insar to the magnitude of low ice surface velocities thus confirming the importance of gps fieldwork based ground truth high resolution ice velocity estimates to constrain ice sheet dynamics

ice shelf flexure and tidal forcing of bindschadler ice stream west antarctica abstract viscoelastic models of ice shelf flexure and ice stream velocity perturbations are combined into a single efficient flowline model to study tidal forcing of grounded ice the magnitude and timing of ice stream response to tidally driven changes in hydrostatic pressure and or basal drag are found to depend significantly on bed rheology with only a perfectly plastic bed allowing instantaneous velocity response at the grounding line the model can reasonably reproduce gps observations near the grounding zone of bindschadler ice stream formerly ice stream d on semidiurnal time scales however other forcings such as tidally driven ice shelf slope transverse to the flowline and flexurally driven till deformation must also be considered if diurnal motion is to be matched

bedform topography and basal conditions beneath a fast flowing west antarctic ice stream a grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath similar to 140 km 2 of rutford ice stream west antarctica the subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water saturated sediments the two troughs are filled with deforming sediment whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream the deforming bed is very flat along flow but undulates across flow sliding areas show rougher bed topography cross stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction these bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation in places they form finger like mounds of material which extend into the sliding region further downstream mean bedform height is 22 m mean width is 267 m and many of them extend for at least 1 2 km along flow we interpret most of these bedforms as drumlins and one as a mega scale glacial lineation the juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas variable both spatially and temporally any theory of subglacial sediment rheology must also be able to account for our conclusion that at any given time pervasive deformation extends at least a few metres into the bed and can persist over a considerable area many km 2 bedform geometry and basal conditions concur with interpretations of former ice streams with evidence for increasing elongation ratio with distance downstream however those studies also identified bedrock cropping out at the ice bed interface for which there is no evidence on rutford ice stream c 2008 elsevier ltd all rights reserved

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

tidally driven stick slip motion in the mouth of whillans ice stream antarctica we show that the ice plain in the mouth of whillans ice stream formerly ice stream b antarctica moves by stick slip motion during a spring tide period rapid motions regularly occur near high tide and during falling tide this correlation is weaker during a neap tide period when the tidal magnitudes are less precise timing of these motion events suggests that they propagate through the region with a mean velocity of 88 ms 1 we hypothesize that this speed is associated with the propagation of shear waves through a wet subglacial till motion events are also seen on more smoothly flowing floating ice event delays are very short between grounded and floating stations suggesting the events propagate through the ice shelf as an elastic wave we further hypothesize the events are caused by the interaction of a sticky bed the accumulation of stored elastic strain through the compression of ice by upstream inflow and tidal forcing motion events seem to be triggered either by reduction of vertical normal stresses at high tide or by the increase of shear stresses from sub shelf ocean currents during falling tide event magnitudes are not related to the length of the preceding quiescent period suggesting significant viscous dissipation within the till

model investigations of inland migration of fast flowing outlet glaciers and ice streams recent observations of increased discharge through fast flowing outlet glaciers and ice streams motivate questions concerning the inland migration of regions of fast flow which could increase drawdown of the ice sheet interior to investigate one process that could lead to inland migration we conduct experiments with a two dimensional full stress transient ice flow model an initial steady state is perturbed by initiating a jump in sliding speed over a fraction of the model domain as a result longitudinal stress gradients increase frictional melting upstream from the slow to fast sliding transition and a positive feedback between longitudinal stress gradients basal meltwater production and basal sliding causes the sliding transition to migrate upstream over time the distance and speed of migration depend on the magnitude of the perturbation and on the degree of non linearity assumed in the link between basal stress and basal sliding larger perturbations and or higher degrees of non linearity lead to farther and faster upstream migration migration of the sliding transition causes the ice sheet to thin over time and this change in geometry limits the effects of the positive feedback ultimately serving to impede continued upstream migration

landslide movement in southwest colorado triggered by atmospheric tides the slumgullion landslide in the western united states has been moving almost continuously for over 100 years observations and numerical modelling show that air pressure changes from atmospheric tides trigger daily movement

megatides in the arctic ocean under glacial conditions 1 over the history of the earth changes in ocean depth and coastal configuration have led to considerable variations in the pattern and amplitude of ocean tides here we perform global simulations of ocean tides for the last glacial maximum using new data sets for both ocean depth and density stratification we show how the configuration of the arctic ocean which was almost entirely enclosed by continents at that time leads to the near resonant excitation of large semi diurnal tides under certain conditions this previously unidentified arctic tide is massively amplified in the canadian archipelago such tides may have played a role in destabilizing the coastal margins of north american ice sheets with implications for rapid changes in the earth s climate and ocean circulation

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

flow switching and water piracy between rutford ice stream and carlson inlet west antarctica rutford ice stream and carlson inlet are neighbouring glaciers in west antarctica rutford ice stream flows at speeds greater than 350m a 1 whereas carlson inlet which has some similar dimensions and supports a similar driving stress flows 10 50 times slower we discuss a range of observations concerning carlson inlet and conclude that there is good indirect evidence that it is a relict ice stream which ceased streaming more than 240 years bp but sufficiently recently that its surface morphology basal water content and basal morphology still retain characteristics produced by streaming an analysis of expected subglacial drainage pathways indicates that carlson inlet is not streaming because it is currently starved of subglacial water which is currently directed beneath rutford ice stream this current state of water piracy by rutford ice stream is however sensitive to minor thickness changes on the ice streams a similar to 120 m 4 thickening of rutford ice stream would divert almost all the subglacial water in the system towards carlson inlet and could reactivate its flow the result highlights the importance of subglacial drainage in controlling ice stream evolution and the requirement for ice sheet models to couple ice flow with subglacial drainage

on the interpretation of ice shelf flexure measurements tidal flexure in ice shelf grounding zones has been used extensively in the past to determine grounding line position and ice properties although the rheology of ice is viscoelastic at tidal loading frequencies most modelling studies have assumed some form of linear elastic beam approximation to match observed flexure profiles here we use density radar and dinsar measurements in combination with full stokes viscoelastic modelling to investigate a range of additional controls on the flexure of the southern mcmurdo ice shelf we find that inclusion of observed basal crevasses and density dependent ice stiffness can greatly alter the flexure profile and yet fitting a simple elastic beam model to that profile will still produce an excellent fit estimates of the effective young s modulus derived by fitting flexure profiles are shown to vary by over 200 depending on whether these factors are included even when the local thickness is well constrained conversely estimates of the grounding line position are relatively insensitive to these considerations for the case of a steep bed slope in our study region by fitting tidal amplitudes only and ignoring phase information elastic beam theory can provide a good fit to observations in a wide variety of situations this should however not be taken as an indication that the underlying rheological assumptions are correct

five decades of strong temporal variability in the flow of brunt ice shelf antarctica data showing velocity changes on the brunt ice shelf bis antarctica over the last 55 years are presented and analysed during this period no large scale calving events took place and the ice shelf gradually grew in size ice flow velocities however fluctuated greatly increasing twofold between 1970 and 2000 then decreasing again to previous levels by 2012 after which velocities started to increase yet again in the observational period velocity changes in the order of 10 a 1 have commonly been observed and currently velocities are increasing at this rate by modelling the ice flow numerically we explore potential causes for the observed changes in velocity we find that a loss of mechanical contact between the bis and the mcdonald ice rumples following a local calving event in 1971 would explain both the increase and the subsequent decrease in ice velocities other explanations involving enlargement of observed rift structures are discounted as the effects on ice flow are found to be too small and the spatial pattern of velocity change inconsistent with data the most recent phase of acceleration remains unexplained but may potentially be related to a recent re activation of a known rift structure within the bis

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

geodetic imaging of time dependent three component surface deformation application to tidal timescale ice flow of rutford ice stream west antarctica we present a method for inferring time dependent three component surface deformation fields given a set of geodetic images of displacements collected from multiple viewing geometries displacements are parameterized in time with a dictionary of displacement functions the algorithm extends an earlier single component i e single line of sight framework for time series analysis to three spatial dimensions using combinations of multitemporal multigeometry interferometic synthetic aperture radar insar and or pixel offset po maps we demonstrate this method with a set of 101 pairs of azimuth and range po maps generated for a portion of the rutford ice stream west antarctica derived from data collected by the cosmo skymed satellite constellation we compare our results with previously published insar mean velocity fields and selected gps time series and show that our resulting three component surface displacements resolve both secular motion and tidal variability

ocean forced ice shelf thinning in a synchronously coupled ice ocean model the first fully synchronous coupled ice shelf ocean model with a fixed grounding line and imposed upstream ice velocity has been developed using the mitgcm massachusetts institute of technology general circulation model unlike previous asynchronous approaches to coupled modeling our approach is fully conservative of heat salt and mass synchronous coupling is achieved by continuously updating the ice shelf thickness on the ocean time step by simulating an idealized warm water ice shelf we show how raising the pycnocline leads to a reduction in both ice shelf mass and back stress and hence buttressing coupled runs show the formation of a western boundary channel in the ice shelf base due to increased melting on the western boundary due to coriolis enhanced flow eastern boundary ice thickening is also observed this is not the case when using a simple depth dependent parameterized melt as the ice shelf has relatively thinner sides and a thicker central bulge for a given ice shelf mass ice shelf geometry arising from the parameterized melt rate tends to underestimate backstress and therefore buttressing for a given ice shelf mass due to a thinner ice shelf at the boundaries when compared to coupled model simulations

tidal pressurization of the ocean cavity near an antarctic ice shelf grounding line mass loss from the antarctic ice sheet is sensitive to conditions in ice shelf grounding zones the transition between grounded and floating ice to observe tidal dynamics in the grounding zone we moored an ocean pressure sensor to ross ice shelf recording data for 54 days in this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone this implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kpa with respect to glaciostatic pressure at the ice shelf base phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides an effect that has not been incorporated into ocean models of tidal currents below ice shelves these tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters according to linear elastic fracture mechanics we find anomalously high tidal energy loss at the k1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography these observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub ice shelf cavities

how does the antarctic ice sheet affect sea level rise global sea levels are predicted to rise as a result of global warming but many contributions to this sea level rise are poorly understood the contribution of the antarctic ice sheet is particularly uncertain in his perspective vaughan 1 highlights the report by davis et al 2 who have used satellite data to compile an 11 year record of surface elevation change in antarctica the resulting maps show that the ice sheet is thickening in some areas and thinning in others vaughan cautions that the complex patterns of change in antarctica preclude prediction of which effect will dominate in the future 1 http www sciencemag org cgi content full 308 5730 1877 2 http www sciencemag org cgi content short 308 5730 1898

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

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

tidal influence on rutford ice stream west antarctica observations of surface flow and basal processes from closely spaced gps and passive seismic stations high resolution surface velocity measurements and passive seismic observations from rutford ice stream west antarctica 40 km upstream from the grounding line are presented these measurements indicate a complex relationship between the ocean tides and currents basal conditions and ice stream flow both the mean basal seismicity and the velocity of the ice stream are modulated by the tides seismic activity increases twice during each semi diurnal tidal cycle the tidal analysis shows the largest velocity variation is at the fortnightly period with smaller variations superimposed at diurnal and semi diurnal frequencies the general pattern of the observed velocity is two velocity peaks during each semi diurnal tidal cycle but sometimes three peaks are observed this pattern of two or three peaks is more regular during spring tides when the largest amplitude velocity variations are observed than during neap tides this is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream

structural characterization of kerogen in 3 4 ga archaean cherts from the pilbara craton western australia hydrogen lean kerogen atomic h c 0 46 isolated from the 3 4 ga strelley pool chert in the north pole area pilbara craton western australia were studied by vibrational spectroscopy fourier transform infrared ftir spectroscopy and raman spectroscopy nuclear magnetic resonance spectroscopy solid state 13 c nmr spectroscopy catalytic hydropyrolysis followed by gas chromatography mass spectrometry hypy gc ms and isotope ratio mass spectrometry irms the kerogen occurs in sedimentary rocks as clasts and clots deposited together with other detrital materials that are finely disseminated throughout a chert matrix the bulk kerogen 13 c values range from 28 3 to 35 8 solid state 13 c nmr spectroscopy and ftir spectroscopy reveals that the kerogen is highly aromatic fa varying from 0 90 to 0 92 and contains only minor aliphatic carbon or carbon oxygenated c o functionalities the raman carbon first order spectra for the isolated kerogens are typical of spectra obtained from disordered sp 2 carbons with low 2 d ordering biperiodic structure the implications of the raman results show low 2 d ordering throughout the carbonaceous network indicate the incorrect usage of the term graphite in the literature to describe the kerogen or carbonaceous material in the warrawoona cherts hydropyrolysates contain aromatic compounds consisting of 1 ring to 7 ring polycyclic aromatic hydrocarbons which were covalently bound into the kerogen as well as alkanes linear branched and cyclic which were most probably trapped in the microporous network of the kerogen these pahs have mainly c1 and c2 alkylation while c3 substitued aromatics are low in abundance and do not show a high degree of branched alkylation for the first time we have shown a correlation between elemental analysis h c atomic ratios raman spectroscopic parameters id1 ig id1 id1 ig and la and the degree of alkylation of bound polyaromatic molecular constituents generated from hypy for archaean kerogens similarities in molecular

tidal modulation of ice shelf flow a viscous model of the ross ice shelf three stations near the calving front of the ross ice shelf antarctica recorded gps data through a full spring neap tidal cycle in november 2005 the data revealed a diurnal horizontal motion that varied both along and transverse to the long term average velocity direction similar to tidal signals observed in other ice shelves and ice streams based on its periodicity it was hypothesized that the signal represents a flow response of the ross ice shelf to the diurnal tides of the ross sea to assess the influence of the tide on the ice shelf motion two hypotheses were developed the first addressed the direct response of the ice shelf to tidal forcing such as forces due to sea surface slopes or forces due to sub ice shelf currents the second involved the indirect response of ice shelf flow to the tidal signals observed in the ice streams that source the ice shelf a finite element model based on viscous creep flow was developed to test these hypotheses but succeeded only in falsifying both hypotheses i e showing that direct tidal effects produce too small a response and indirect tidal effects produce a response that is not smooth in time this nullification suggests that a combination of viscous and elastic deformation is required to explain the observations

slow slip events on the whillans ice plain antarctica described using rate and state friction as an ice stream sliding law the whillans ice plain wip antarctica experiences twice daily tidally modulated stick slip cycles slip events last about 30 minutes have sliding velocities as high as 0 5 mm s 15 km yr and have total slip 0 5 m slip events tend to occur during falling ocean tide just after high tide and just before low tide to reproduce these characteristics we use rate and state friction which is commonly used to simulate tectonic faulting as an ice stream sliding law this framework describes the evolving strength of the ice bed interface throughout stick slip cycles we present simulations that resolve the cross stream dimension using a depth integrated treatment of an elastic ice layer loaded by tides and steady ice inflow steady sliding with rate weakening friction is conditionally stable with steady sliding occurring for sufficiently narrow ice streams relative to a nucleation length stick slip cycles occur when the ice stream is wider than the nucleation length or equivalently when effective pressures exceed a critical value ice streams barely wider than the nucleation length experience slow slip events and our simulations suggest that the wip is in this slow slip regime slip events on the wip show a sense of propagation and we reproduce this behavior by introducing a rate strengthening region in the center of the otherwise rate weakening ice stream if pore pressures are raised above a critical value our simulations predict that the wip would exhibit quasi steady tidally modulated sliding as observed on other ice streams this study validates rate and state friction as a sliding law to describe ice stream sliding styles

three dimensional imaging of earth and planetary materials x ray computed tomography neutron computed tomography and magnetic resonance imaging are driving novel scientific advances by enabling rapid non destructive three dimensional examination and analysis of earth and planetary materials discoveries catalyzed by these approaches range across fields from environmental geology to petroleum geology hydrology to soil science paleontology to petrology and geodynamics to meteoritics they have impacted our understanding of hydrocarbon reservoirs contaminant transport climate change co2 sequestration the evolution of life crustal uplift mantle metasomatism planetary differentiation and more three dimensional imaging is likely soon to become an essential component of every investigator s toolkit especially as instruments and facilities emerge that are optimized for increasingly sophisticated geological applications of these techniques and as access to them expands

elastic dynamics and tidal migration of grounding lines modify subglacial lubrication and melting 1 we combine the dynamics of ice bed and ocean in a new elastic model for the tidal timescale migration of grounding lines on deformable foundations previous interpretations of tidal flexure using models of elastic ice shelves with fixed grounding lines were found to be inconsistent suggesting an elasticity of ice that varies spatially and temporally and that is significantly smaller than measured experimentally we argue here that with our model a consistent purely elastic interpretation can be made combining this new approach with remote sensing measurements we show that the grounding line migrates several kilometers during a tidal cycle that we can infer the effective elastic properties of the bed and that the elastic pressure of the ice leads to a hydrological barrier near the grounding line that controls subglacial hydrology our findings imply that subglacial lubrication and melting induced by the ocean thermal forcing can increase substantially during high tide

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

analysis of the internal structure of rocks and characterization of mechanical deformation by a non destructive method x ray tomodensitometry abstract tomodensitometry is a new non destructive method which reveals the internal structure of rocks through analysis of x ray images without any damage to the sample the scanner gives a radiological density which is a function of gravimetric density and mineralogy for limestones and sandstones an attempt is made to quantify the relation between radiological density and porosity spatial variations in the values of the radiological density measured on rock core specimens that are deformed and fractured in a triaxial cell are then related to the intensity of experimental volumetric deformation

tidal controls on the flow of ice streams the flow of many antarctic ice streams is known to be significantly influenced by tides in the past modeling studies have implemented the tidal forces acting on a coupled ice stream ice shelf system in a number of different ways but the consequences that this has on the modeled response of ice streams to tides have until now not been considered here we investigate for the first time differences in model response that are only due to differences in the way tidal forcings are implemented we find that attempts to simplify the problem by neglecting flexural stresses are generally not valid and forcing models with only changes in ocean back pressure will not capture either the correct amplitudes or length scale

tide modulated ice motion and seismicity of a floating glacier tongue in east antarctica we recorded the ice motion and icequakes on the floating part of langhovde glacier in east antarctica to better understand the dynamic behavior of ice shelves and floating tongues diurnal and semi diurnal variations in ice motion and seismicity were simultaneously observed at all four global navigation satellite system and three seismic stations over 2 weeks the short term along flow ice motion is explained by the elastic response of the glacier to ocean tide induced hydrostatic stress variations which decayed at a rate of 0 8 km 1 toward the grounding line we observed a large number of icequakes during mid rising and high tides that covered a broad frequency range and formed two major groups of events centered at 10 and 120 hz respectively the hourly occurrence rates were 500 events h 1 with the observed seismicity consistent with fracture due to floating tongue bending we also observed minor secondary peaks at high ice speeds which could reflect surface cracking due to stretching or basal friction our observation demonstrates that tidal modulation was the main factor to fracture the floating tongue of langhovde glacier

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

rifting processes and ice flow modulation observed on mertz glacier east antarctica we investigated the evolution of two major rifts cutting across mertz glacier tongue east antarctica using a combination of satellite images and 60 day sets of gps data from two stations deployed either side of the western rift in 2007 the eastern rift began to open in the early 1990s and the western rift initiated in 2002 in conjunction with the collision of a large iceberg with the tongue velocity time series derived from the 2007 gps data exhibited strong variations at tidal periods modulated by sea surface height and sea surface slope and reproduced here with a conceptually simple model we found that opening of the western rift in 2002 leads to a dramatic change in behavior of the tongue as the large range in velocity 700 2400 m a 1 observed in 2000 was largely reduced 1075 1225 m a 1 in 2007 opening of the western rift decoupled the glacier from the transverse loading on the tongue driven by east west tidal circulation this loading previously induced time varying lateral drag which caused the large velocity range our results suggest changes in the mechanical behavior of an ice tongue impact the dynamics of the outlet glacier system and should be considered in longer term mass balance evaluations

rapid sea level rise soon from west antarctic ice sheet collapse there has been recent concern over whether shrinkage of ice sheets in antarctica caused by climate change could lead to rapid global sea level rise in his perspective bentley discusses the processes at work in ice sheet behavior and concludes that in view of the evidence for ice sheet stability catastrophic changes are unlikely in the next century or two

glaciological investigations on union glacier ellsworth mountains west antarctica union glacier west antarctica was intensively mapped in december 2008 when an over snow traverse was conducted by cecs and ale with the aim to determine the ice dynamical characteristics of the glacier through mapping the crevasse fields and by providing a glaciological baseline for future studies a mean ice thickness of 1450 m was measured confirming the presence of a deep subglacial topography 900 m below sea level much deeper than previously estimated ice velocities were also measured at 21 stakes drilled into the ice at the narrowest gate of the glacier between december 2007 and december 2008 yielding a mean value of 22 6 m a 1 these velocities combined with the measured ice thicknesses and a numerical model yielded an ice flux of 0 10 0 03 km 3 a 1 w e considering the ice basin above this gate a mean surface mass balance of 0 18 0 05 m a 1 was estimated a value consistent with a mean snow accumulation for nearby ice streams these values indicate that the glacier is at present near equilibrium

ice velocity from static gps observations along the transect from zhongshan station to dome a east antarctica dome a the highest point on the antarctic ice sheet at just over 4000 m a s l is located near the centre of east antarctica chinese national antarctic research expeditions have studied ice sheet dynamics and mass balance along a traverse route from zhongshan station to dome a during the austral summers from 1996 97 to 2004 05 nineteen gps sites were occupied on at least two occasions at approximately 50 km intervals the purpose of the surveys was to provide accurate ice dynamics data a dual frequency gps receiver was used and each site was occupied for 1 12 hours gps data were processed using gamit globk software and horizontal accuracies were within 0 1 m repeat gps measurements provided ice velocities the horizontal surface ice velocities increase from the summit of the ice sheet to the coast in the dome a area the velocities are 10m a 1 in the plateau area velocities range from 8 to 24 m a 1 and reach about 98 2 m a 1 at a site lt980 near the coast the flow directions are roughly perpendicular to the ice sheet surface elevation contours primarily toward the lambert glacier basin

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

a joint inversion of receiver function and rayleigh wave phase velocity dispersion data to estimate crustal structure in west antarctica we determine crustal shear wave velocity structure and crustal thickness at recently deployed seismic stations across west antarctica using a joint inversion of receiver functions and fundamental mode rayleigh wave phase velocity dispersion the stations are from both the uk antarctic network ukanet and polar earth observing network antarctic network polenet anet the former include for the first time 4 stations along the spine of the antarctic peninsula 3 in the ellsworth land and 5 stations in the vicinity of the pine island rift within the west antarctic rift system wars we model a crustal thickness range of 18 28 km and show that the thinnest crust 18 km is in the vicinity of the byrd subglacial basin and bentley subglacial trench in these regions we also find the highest ratio of fast vs 4 0 4 3 km s likely mafic lower crust to felsic intermediate upper crust the thickest mafic lower crust we model is in ellsworth land a critical area for constraining the eastern limits of the wars although we find thinner crust in this region 30 km than in the neighbouring antarctic peninsula and haag ellsworth whitmore block hew the ellsworth land crust has not undergone as much extension as the central wars this suggests that the wars does not link with the weddell sea rift system through ellsworth land and instead has progressed during its formation towards the bellingshausen and amundsen sea embayments we also find that the thin wars crust extends towards the pine island rift suggesting that the boundary between the wars and the thurston island block lies in this region 200 km north of its previously accepted position the thickest crust 38 40 km we model in this study is in the ellsworth mountain section of the hew block we find thinner crust 30 33 km in the whitmore mountains and haag nunatak sectors of the hew consistent with the composite nature of the block in the antarctic peninsula we find a crustal thickness range of 30 38 km and a likely dominantly felsic intermediate crustal composition by forward modelling high frequency receiver functions we also assess if any thick low velocity subglacial sediment accumulations are present and find a 0 1 0 8 km thick layer at 10 stations within the wars thurston island and ellsworth land we suggest that these units of subglacial sediment could provide a source region for the soft basal till layers found beneath numerous outlet glaciers and may act to accelerate ice flow

the role of megatides and relative sea level in controlling the deglaciation of the british irish and fennoscandian ice sheets funding was provided by the natural environment research council nerc through grant ne i527853 1 phd studentship to s l w the research was supported by the climate change consortium of wales and the nerc britice chrono consortium grant ne j007579 1 jess vaughan and martyn roberts drafted figs 2 5

glacier dynamics near the calving front of bowdoin glacier northwestern greenland to better understand recent rapid recession of marine terminating glaciers in greenland we performed satellite and field observations near the calving front of bowdoin glacier a 3 km wide outlet glacier in northwestern greenland satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for 20 years ice radar measurements showed that the glacier front is grounded but very close to the floating condition these results in combination with the results of ocean depth soundings suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat presumably glacier thinning due to atmospheric and or ocean warming triggered the initial retreat in situ measurements showed complex short term ice speed variations which were correlated with air temperature precipitation and ocean tides ice speed quickly responded to temperature rise and a heavy rain event indicating rapid drainage of surface water to the bed semi diurnal speed peaks coincided with low tides suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance these observations demonstrate that the dynamics of bowdoin glacier are sensitive to small perturbations occurring near the calving front

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

synthetic aperture radar interferometry over rutford ice stream and carlson inlet antarctica we describe the calibration and interpretation of interferograms generated from ers i synthetic aperture radar single look complex sar slc images of the rutford ice stream area ground surveys provide over 100 tie points with which to optimise the interferometric baselines that separate nominally repeated satellite orbits covariant tie point errors are dealt with by constructing an error covariance matrix for the expected values of the unwrapped interferometric phases at the tie points with baseline parameters that minimise the weighted residual variance rms tie point residuals of less than i cm in slant range are obtained these are attributed to a combination of interferometric phase noise movement survey errors and inadequate slope information the image set used is inadequate for isolating the influences of topography and movement so the glaciological conclusions to be drawn are limited nevertheless the interferograms confirm that the whole of the upper 50 k m of carlson inlet flows at a speed less than a tenth of that of the neighbouring rutford ice stream also confirmed are the entry of faster moving ice into the lower reaches of carlson inlet and the position of part of the carlson inlet grounding line in general the distribution of the residuals suggests no significant differences in ice movement between 1978 and 1992 an exception is the neighbourhood of the shear margin between rutford ice stream and carlson inlet where inconsistencies between ground surveys over the periods 1984 86 and 1994 96 and interferograms from 1994 a n d 1996 suggest fluctuations in velocity of up to 10 m year 1

nonlinear interaction between ocean tides and the larsen c ice shelf system 1 we report on three gps records of flow of the larsen c ice shelf spanning 2 months to 2 years variations in speed are evident at periods from a few hours to 182 days including semi diurnal diurnal and 14 76 days at fortnightly periods the ice shelf varies by 10 from its long term speed but at diurnal timescales it is up to 100 a nonlinear relationship between ocean tides and velocity is required to explain the observed variations in flow as an initial examination we model flow as a non linear function of basal shear stress including tidal perturbations in the ice shelf grounding zone and show that the major features of the observations are reproduced notably the long period signal largely absent from the vertical tidal signal alternative explanations are discussed these observations demonstrate that the ice shelf system is highly sensitive to relatively modest changes in forcing at its boundaries

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

an improved model for tidally modulated grounding line migration understanding grounding line dynamics is necessary for predictions of long term ice sheet stability however despite growing observations of the tidal influence on grounding line migration this short timescale migration is poorly understood with most modeling attempts assuming beam theory to calculate displacements here we present an improved model of tidal grounding line migration that treats migration as an elastic fracture problem forced by the additional ocean water pressure from the tide this new model predicts that the grounding line cannot be assumed to be in hydrostatic equilibrium and furthermore that migration is inherently asymmetric and nonlinear with migration distances that are not proportional to the tidal load specifically for constant surface slope the grounding line migrates upstream approximately ten times further as the tide rises from mean sea level to high tide than it migrates downstream as the tide falls from mean sea level to low tide and migration distances are substantially larger than simple flotation arguments suggest numerical tests also show that the dependence of migration distance on elastic moduli and ice sheet thickness are inconsistent with predictions of beam theory for a range of realistic values finally applying the new model to observations in antarctica results in new estimates of bed slopes though these estimates remain uncertain due to imperfect knowledge of the relevant rheological parameters