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

the retreat of a tidewater glacier observations and model calculations on hansbreen spitsbergen based on observations and model calculations the retreat over the last two decades of hansbreen a tidewater glacier in southern spitsbergen svalbard is investigated the observations of the calving front position between 1982 and 1998 show an abrupt retreat in 1990 which is suggested to be related to a depression in the glacier bed the observed seasonal variations of the front position are mainly due to variations of the calving rate the observations of hansbreen further indicate that during periods of slow front position changes melting at the water line may play an important role in triggering the process of calving the evolution of hansbreen between 1982 and 1998 is simulated with a numerical model for the dynamics of tidewater glaciers using a flotation criterion for calving in which for each time step the part of the glacier terminus which is below a critical height above buoyancy is removed we are able to reproduce the observed rapid retreat of hansbreen through the depression in the glacier bed from the observations and model calculations we conclude that the rapid retreat is mainly an effect of basal topography in the terminus region and not a direct response to a change in mass balance

grounding line movement and ice shelf buttressing in marine ice sheets 1 understanding the dynamics of marine ice sheets is integral to studying the evolution of the antarctic ice sheet in both the short and long terms an important component of the dynamics grounding line migration has proved difficult to represent in numerical models and most successful attempts have made use of techniques that are only readily applicable to flow line models however to capture the stress transmission involved in another important component the buttressing of a marine ice sheet by its ice shelf the transverse direction must also be resolved we introduce a model that solves the time dependent shelfy stream equations and makes use of mesh adaption techniques to overcome the difficulties typically associated with the numerics of grounding line migration we compare the model output with a recent benchmark for flow line models and show that our model yields an accurate solution while using far less resources than would be required without mesh adaption we also show that the mesh adapting techniques extend to two horizontal dimensions experiments are carried out to determine how both ice shelf buttressing and ice rises affect the marine instability predicted for an ice sheet on a foredeepened bed we find that buttressing is not always sufficient to stabilize such a sheet but collapse of the grounded portion is still greatly delayed we also find that the effect of an ice rise is similar to that of narrowing the ice shelf

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

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

ice flux divergence anomalies on 79north glacier greenland 1 the ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness here we combine high resolution ice surface velocity observations of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in north greenland with a dense grid of ice thickness data collected with an airborne radar sounder in 1998 to examine its ice flux divergence we detect large variations up to 100 m yr in flux divergence on grounded ice that are incompatible with what we know of the glacier surface mass balance basal mass balance and thinning rate we examine the hypothesis that these anomalies are due to the three dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three dimensional numerical models with its surface equivalent we find that three dimensional effects have only a small contribution to the observed anomalies on the other hand if we degrade the spatial resolution of the data to 10 km the anomalies disappear further analysis shows that the source of the anomalies is not the ice velocity data but the interpolation of multiple tracks of ice thickness data onto a regular grid using a scheme here block kriging that does not conserve mass or ice flux this problem is not unique to 79north glacier but is common to all conventional ice thickness surveys of glaciers and ice sheets and fundamentally limits the application of ice thickness grids to high resolution numerical modeling of glacier flow

rossby waves mediate impacts of tropical oceans on west antarctic atmospheric circulation in austral winter recent studies link climate changearound antarctica to the sea surface temperature of tropical oceans with teleconnections from the pacific atlantic and indian oceans making different contributions to antarctic climate in this study the impacts of each ocean basin on the wintertime southern hemisphere circulation are identified by comparing simulation results using a comprehensive atmospheric model an idealized dynamical core model and a theoretical rossby wave model the results herein show that tropical atlantic ocean warming indian ocean warming and eastern pacific cooling are all able to deepen the amundsen sea low located adjacent to west antarctica while western pacific warming increases the pressure to the west of the international date line encompassing the ross sea and regions south of the tasman sea in austral winter these tropical ocean basins work together linearly to modulate the atmospheric circulation around west antarctica further analyses indicate that these teleconnections critically depend on stationary rossby wave dynamics and are thus sensitive to the background flow particularly the subtropical midlatitude jet nearthesejets wind shearis amplified which strengthens the generation of rossby waves on the other hand near the edges of the jets the meridional gradient of the absolute vorticity is also enhanced as a consequence of the rossby wave dispersion relationship the jet edge may reflect stationary rossby wave trains serving as a waveguide the simulation results not only identify the relative roles of each of the tropical ocean basins in the tropical antarctica teleconnection but also suggest that a deeper understanding of teleconnections requires a better estimation of the atmospheric jet structures

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

submarine melting at the terminus of a temperate tidewater glacier leconte glacier alaska u s a heat fresh and sea water balances indicate that the late summer rate of submarine melting at the terminus of tidewater leconte glacier alaska u s a in 2000 was about 12md 1 w e averaged over the submerged face this is 57 of the estimated total ice loss at the terminus calving plus melting at this time submarine melting may thusprovidea significantcontributiontotheoverallablationofatidewaterglacier oceano graphic measurements conductivity temperature depth made 200 500m from the ter minus identified an isohaline 27ppt and isothermal 7 2 c layer extending from 130m depthto the fjord floor capping this is a 40mthick overflow plume distinguishedby high outflow rates low salinity 22 25ppt and lower temperatures 5 6 c mixing models indicate that fresh water comprised about11 of this plume it originates mostly as sub glacial discharge whose buoyancy drives convection at the terminus deep warm saline watersareincorporatedintotheplumeasitascends causingsubstantialmeltingoficealong the submarine face the calving terminus undergoes seasonal changes that coincide with changes in subglacial discharge and fjord water temperatures and we suggest that these fluctuations interminuspositionare directly relatedto changes in submarine melting

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

a numerical investigation of surface crevasse propagation in glaciers using nonlocal continuum damage mechanics 1 we use a nonlocal viscoelastic damage model to investigate the conditions that enable water free surface crevasse propagation in grounded marine terminating glaciers our simulations on idealized rectangular ice slabs in contact with the ocean show that crevasses propagate faster in thicker ice slabs we find that 1 the fraction of ice slab thickness penetrated by surface crevasses decreases with increasing seawater depth near the terminus 2 a no slip fixed basal boundary condition retards crevasse growth and 3 crevasses form closer to the terminus when the seawater depth is larger or when the glacier base is fixed to the bedrock which could lead to calving of smaller icebergs however water free surface crevasses can penetrate nearly the entire ice thickness only in thicker ice slabs terminating in shallow seawater depths this leads us to the conclusion that surface crevasses alone are not responsible for calving events in marine terminating and thin glaciers

extraction of spudcan foundations in single and multilayer soils abstractdifficulties are often encountered in a variety of seabed sediments during extraction of spudcan foundations of mobile jack up rigs this paper reports results from centrifuge model tests undertaken to provide insight into spudcan foundation behavior during vertical extraction through single double and multilayer soils the model tests included half spudcan tests against a transparent window allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry piv analysis and separate full spudcan tests to measure the uplift resistance the observed soil failure mechanisms provide insight with respect to the profile of extraction resistance the mechanisms reveal evolving failure modes at different spudcan extraction depths and in particular changes in the failure patterns because of varying layer geometry soil types and properties for single double and multilayer clays suction at the spudcan base shearing along a trunc

experimental investigation of spudcan penetration in multi layer clays with interbedded sand layers highly layered soils are prevalent in several areas of current offshore activity and in particular in emerging provinces and fields however no investigation has been carried out for spudcan penetration in stratified sediments with more than two layers this paper reports centrifuge modelling of spudcan foundations penetrating through 3 6 clay layers with interbedded stronger clay silica sand and carbonate sand layers the aim was to provide insight into the behaviour of spudcan foundations in stratified sediments comprising layers of different drainage conditions and mineralogy the motivation for this paper emanated directly from the future needs identified by the recent revised version of iso 19905 1 it was found that the downward soil deformation was extended into 3 4 underlying layers of sand and clay with thickness ratios t d less than 0 6 the limiting squeezing depth was 0 17 0 18d when approaching stiff clay moderate clay layers and 0 28d when approaching dense sand stiff clay layers these

modeling of store gletscher s calving dynamics west greenland in response to ocean thermal forcing glacier front dynamics is an important control on greenland s ice mass balance warmer ocean waters trigger ice front retreats of marine terminating glaciers and the corresponding loss in resistive stress leads to glacier acceleration and thinning here we present an approach to quantify the sensitivity and vulnerability of marine terminating glaciers to ocean induced melt we develop a plan view model of store gletscher that includes a level set based moving boundary capability a parameterized ocean induced melt and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt we find that the glacier is stabilized by a sill at its terminus the glacier is dislodged from the sill when ocean induced melt quadruples at which point the glacier retreats irreversibly for 27 km into a reverse bed the model suggests that ice ocean interactions are the triggering mechanism of glacier retreat but the bed controls its magnitude

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

a modeling study of the effect of runoff variability on the effective pressure beneath russell glacier west greenland basal sliding is a main control on glacier flow primarily driven by water pressure at the glacier base the ongoing increase in surface melting of the greenland ice sheet warrants an examination of its impact on basal water pressure and in turn on basal sliding here we examine the case of russell glacier in west greenland where an extensive set of observations has been collected these observations suggest that the recent increase in melt has had an equivocal impact on the annual velocity with stable flow on the lower part of the drainage basin but accelerated flow above the equilibrium line altitude ela these distinct behaviors have been attributed to different evolutions of the subglacial draining system during and after the melt season here we use a high resolution subglacial hydrological model forced by reconstructed surface runoff for the period 2008 to 2012 to investigate the cause of these distinct behaviors we find that the increase in meltwater production at low elevation yields a more efficient drainage system compatible with the observed stagnation of the mean annual flow below the ela at higher elevation the model indicates that the drainage system is mostly inefficient and is therefore strongly sensitive to an increase in meltwater availability which is consistent with the observed increase in ice velocity

sulfur isotope and chemical compositions of the wet precipitation in two major urban areas seoul and busan korea abstract the seasonal variations of 34s together with major ions in snow and rainwater samples were measured in urban areas precipitation samples were collected from metropolitan city seoul and large port city busan korea for about one year from june 2002 to june 2003 the 34s values of sulfate in precipitation ranged from 4 3 to 6 6 mean 3 72 and 1 0 to 18 6 mean 5 55 in seoul and busan respectively even though both of the study areas have large populations and transportation infrastructure 34s values of sulfate in seoul precipitation originate more from anthropogenic sources relative to that of busan the elevated anthropogenic sources in seoul might be influenced by human activities from surrounding industrial cities while the scatter of 34s values in busan may be influenced by marine air during the study 34s values are high in the fall season and low in the spring season it may be inferred from this data that spring time 34s values are affected more by biogenic sources compared to the fall season as such seoul and busan showed significant differences in chemical compositions such as 34s and nss so 4 2 especially 34s values in the seoul show very good relationship with anthropogenic emissions e g emission from coal combustion and 34s values in the busan may be influenced by sea spray from the oceanic surface micro layer this correlation and the 34s values indicate that the major source in this region is likely to be anthropogenic source in seoul and sea spray in busan rather than traditionally suggested sources such as soil resuspension stratospheric air intrusion volcanic emissions and biogenic emissions

antarctic ice shelf thickness from satellite radar altimetry ice shelf thickness is an important boundary condition for ice sheet and sub ice shelf cavity modelling it is required near the grounding line to calculate the ice flux used to determine ice sheet mass balance by comparison with the upstream accumulation in this mass budget approach the accuracy of the ice thickness is one of the limiting factors in the calculation we present a satellite retrieval of the ice thickness for all antarctic ice shelves using satellite radar altimeter data from the geodetic phases of the european remote sensing satellite ers 1 during 1994 95 supplemented by icesat data for regions south of the ers 1 latitudinal limit surface elevations derived from these instruments are interpolated on to regular grids using kriging and converted to ice thicknesses using a modelled firn density correction the availability of a new spatial variable firn density correction significantly reduces the error in ice thickness as this was previously the dominant error source comparison to airborne data shows good agreement particularly when compared to soar casertz data on the largest ice shelves biases range from 13 0 m for areas where the assumption of hydrostatic equilibrium breaks down to 53 4 m in regions where marine ice may be present

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

using a thin sand layer to ease spudcan extraction in clay difficulties are encountered in soft clay and silt sediments during extraction of the spudcan foundations of mobile jack up rigs this paper reports results from centrifuge model tests undertaken to provide a simple effective and practical means for easing spudcan extraction in soft normally consolidated and lightly overconsolidated clay the tests were carried out on model spudcans of two different geometries typically used in the field the spudcans were extracted from penetrations of 2 3 diameters and after an operation period of 19 months the measured pore pressure at the bottom face of the spudcan during extraction process provided insight into the development of suction at the interface between the spudcan base and the underlying soil the tests were carried out with and without a sand layer placed locally in between the spudcan and the mudline at the onset of penetration in the field a gravel layer can be placed by means of a flexible fall pipe system the presence of a sand layer with a

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

temperate ice in the shear margins of the antarctic ice sheet controlling processes and preliminary locations abstract the flux of grounded ice from the antarctic ice sheet ais primarily occurs through narrow fast flowing outlet glaciers and ice streams shearing generates heat in the lateral glacier margins where there is a transition from fast flow to near stagnant ice or rock this heat warms the ice and can form zones of water saturated porous ice at the melting temperature known as temperate ice here we derive a one dimensional thermomechanical model to infer which ais shear margins may contain temperate ice our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone both of these quantities are described by two nondimensional numbers the brinkman and peclet numbers that are functions of observable parameters such as shear strain rate surface temperature surface accumulation rate and ice thickness the development of temperate zones is incited by shear strain rates brinkman number and suppressed by the advection of cold ice from the surface peclet number which scales with surface accumulation rate we also include a parametrization for lateral advection which can suppress the formation of temperate ice applying recent observations and outputs from a regional climate model we show that many active glacier shear margins could contain temperate ice the spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates

sensitivity of the lambert amery glacial system to geothermal heat flux geothermal heat flux ghf is one of the key thermal boundary conditions for ice sheet models we assess the sensitivity of the lambert amery glacial system in east antarctica to four different ghf datasets using a regional ice sheet model a control solution of the regional model is initialised by minimising the misfit to observations through an optimisation process the lambert amery glacial system simulation contains temperate ice up to 150 m thick and has an average basal melt of 1 3 mm a 1 with maximum basal melting of 504 mm a 1 the simulations which use a relatively high ghf compared to the control solution increase the volume and area of temperate ice which causes higher surface velocities at higher elevations which leads to the advance of the grounding line the grounding line advance leads to changes in the local flow configuration which dominates the changes within the glacial system to investigate the difference in spatial patterns within the geothermal datasets they were scaled to have the same median value these scaled ghf simulations showed that the ice flow was most sensitive to the spatial variation in the underlying ghf near the ice divides and on the edges of the ice streams

predicting the resistance profile of a spudcan penetrating sand overlying clay assessment of the risk of punch through failure of spudcan foundations on sand overlying clay requires prediction of the full penetration resistance profile from touchdown and through punch through to equilibrium of the vertical resistance at depth in the underlying clay layer this study uses the coupled eulerian lagrangian approach a large deformation finite element analysis method to model the complete penetration resistance profile of a spudcan on sand overlying clay the sand is modeled using the mohr coulomb model while the clay is modeled using a modified tresca model to account for strain softening the numerical method is then used to simulate a series of spudcan penetration tests performed in a geotechnical centrifuge on medium dense sand overlying clay the punch through behavior observed in the experiments is replicated and the penetration resistance profiles from numerical analyses are generally a reasonable match to the experimental measure ments the influences of the sand layer height to foundation diameter ratio sand clay interface shear strength and strength gradient in clay on the penetration resistance profiles are explored in a complementary parametric study the penetration resistance in the underlying clay layer is well predicted using a simple linear expression for the bearing capacity factor for the spudcan and underlying sand plug this expression is combined with an existing failure stress dependent model for predicting peak resistance to form a simplified method for prediction of the full penetration resistance profile this new method provides estimates of the vertical penetration that the spudcan will run during the punch through event it is validated against both medium dense and dense sand centrifuge tests

estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays difficulties in obtaining high quality soil samples from deepwater sites have necessitated increasing reliance on piezocone t bar and ball penetration tests to determine soil properties for design purposes this paper reports the results of an international collaborative project in which a worldwide high quality database of lightly overconsolidated clays was assembled and used to evaluate resistance factors for the estimation of intact and remoulded undrained shear strength from the penetration resistance of each device the derived factors were then compared with existing theoretical solutions to evaluate the influence of particular soil characteristics the overall statistics showed similar levels of variability of the resistance factors with low coefficients of variation for all three types of penetrometer however correlations of the resistance factors with specific soil characteristics indicated that the resistance factors for the piezocone were more influenced by soil stiffness or rigidity inde

shmip the subglacial hydrology model intercomparison project subglacial hydrology plays a key role in many glaciological processes including ice dynamics via the modulation of basal sliding owing to the lack of an overarching theory however a variety of model approximations exist to represent the subglacial drainage system the subglacial hydrology model intercomparison project shmip provides a set of synthetic experiments to compare existing and future models we present the results from 13 participating models with a focus on effective pressure and discharge for many applications e g steady states and annual variations low input scenarios a simple model such as an inefficient system only model a flowline or lumped model or a porous layer model provides results comparable to those of more complex models however when studying short term e g diurnal variations of the water pressure the use of a two dimensional model incorporating physical representations of both efficient and inefficient drainage systems yields results that are significantly different from those of simpler models and should be preferentially applied the results also emphasise the role of water storage in the response of water pressure to transient recharge finally we find that the localisation of moulins has a limited impact except in regions of sparse moulin density

future sea level rise due to projected ocean warming beneath the filchner ronne ice shelf a coupled model study a general ocean circulation model is coupled with a 3d thermodynamical ice sheet shelf model to simulate the response of the filchner ronne ice shelf fris antarctica and coastal parts of its catchment basin to a postulated inflow of warm deep water into the ice shelf cavity on a 1000 yr timescale prescribed ocean warming based on climate projections enters the ice shelf cavity in the up to 1500 m deep filchner trough and penetrates deep into the sub ice cavity increasing basal melt rates induce geometry changes of the cavity which in turn have an impact on the ocean circulation and therefore the modelled melt rates highest melt rates of about 20 m yr 1 follow the up to 180 km retreating grounding line basal mass loss reaches about 250 km3 yr 1 doubling the present day value the most vulnerable areas below the fris are the bailey ice stream and the area between the institute and moeller ice streams where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock the potential additional contribution to the eustatic sea level rise due to the grounded ice loss simulated in an ensemble approach against a transient control experiment is about 0 05 mm yr 1 during the first 500 yr and about 0 17 mm yr 1 thereafter

emissions of alkaline elements calcium magnesium potassium and sodium from open sources in the contiguous united states models of dust emissions by wind erosion including winds associated with regional activity as well as dust devils and vehicular disturbances of unpaved roads were developed calibrated and used to estimate alkaline dust emissions from elemental soil and road composition data emissions from tillage of soils were estimated from the work of previous researchers the area of maximum dust production by all of those sources is the area of the old dust bowl of the 1930s the panhandles of texas and oklahoma eastern new mexico and colorado and western kansas the areas of maximum alkaline dust production are the arid southwest the dust bowl and the midwestern mideastern states from iowa to pennsylvania our calculations show that calcium is the dominant alkaline element produced by open sources sources too great in extent to be controlled by enclosure or ducting although the largest dust mass source is wind erosion by winds associated with regional activity and convective activity the largest producer of the alkaline component is road dust because the abundance of alkaline materials in road coverings which include crushed limestone is significantly higher than for soils comparing the above estimated sources of alkaline material with inventories of so2 and nox emissions by previous investigators gives the rough approximation that alkaline emission rates are of the order of the so2 nox emissions in the western united states and that they are much smaller than so2 nox in the eastern united states this approximation is substantiated by data on ca so4 no3 for wet deposition for national atmospheric deposition program sites

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

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

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

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

widespread rifting and retreat of ice shelf margins in the eastern amundsen sea embayment between 1972 and 2011 the major outlet glaciers that drain the eastern sector of the amundsen sea embayment smith haynes thwaites and pine island are among the largest fastest flowing and fastest thinning glaciers in west antarctica their recent ice flow acceleration is linked to ocean induced ice shelf thinning but may also arise from additional losses of ice shelf buttressing that are not well understood here we present a comprehensive history of coastal change in the eastern amundsen sea embayment between 1972 and 2011 derived mostly from landsat imagery the termini of all four major outlet glaciers have retreated but retreat is most rapid along the ice shelf margins where progressive rifting has occurred this pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice shelf buttressing the observed pattern of margin led gradual ice shelf disintegration appears to be common in accelerating ocean terminating outlet glaciers we hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern amundsen sea embayment

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

experimental observations of the penetration of spudcan footings in silt mobile drilling platforms perform the majority of the world s drilling in water depths up to approximately 120 m the installation of their large inverted conical spudcan footings into fine grain

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

evaluation of ice stream model sensitivities for parameter estimation abstract large ensemble perturbed parameter forward ice flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice flow parameters inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling because ice flow depends on many such parameters with their associated uncertainties which may interact in nonlinear ways full uncertainty assessment for parameter estimates is challenging with rising computational power it is increasingly practicable to explore co dependencies and sensitivities here we use a well characterized higher order flowline model configured for a well lubricated shelfy ice stream to run large ensembles perturbing the magnitude and spatial pattern of basal drag basal topography and input flux we find for steady state that ice stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end but with greater local correlation during transients the modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local perturbations of input ice fluxes interact with the others in interesting ways these insights point to the value of inversions informed by concentrated observations during forced transients such as lake drainage events accumulation rate fluctuations or ice shelf losses and to the care needed when interpreting local results of some inversions for basal drag parameters

multiple oxygen 16o 17o and 18o and sulfur 32s 33s 34s and 36s isotope signatures of the dissolved sulfate from deception island antarctic peninsula implications on sulfate formation transportation and deposition in the antarctic region abstract oxygen 16 o 17 o and 18 o and sulfur 32 s 33 s 34 s and 36 s isotope ratios of and major ion na ca 2 cl no 3 and so 4 2 concentrations in lakes ponds and creeks from deception island antarctic peninsula were analyzed to study the sources of sulfate its oxidation and the surficial processes of the dissolved sulfate the positive relationship between the 34 s sulfate 8 1 to 17 3 and the cl so 4 2 molar ratio suggests mixing of sulfate from atmospheric deposition and from oxidation of sulfide minerals the average sea salt fraction 28 and 34 s nss values from 5 6 to 15 9 indicate that a combination of sea salt and marine biogenic sulfide provide the high 34 s end member of the dissolved sulfates the relatively low 18 o sulfate from 4 6 to 0 7 of deception island water suggests a role of local water in the formation of sulfate slightly negative but mass dependent 17 o sulfate values imply that atmospheric oxidation by o 3 and h 2 o 2 are negligible while these values might suggest a significant role of oxidation by molecular oxygen and oh the distinctly low 34 s sulfate value of two samples dcw 2 and dcw 3 suggests the input of sulfate from sulfide oxidation slight elevation of 34 s sulfate values up to 17 3 compared to a typical atmospheric value indicates a minimal role for dissimilatory microbial sulfate reduction of deception island water and sediments both 33 s sulfate and 36 s sulfate values are homogeneous and near zero implying that the dominant atmospheric oxidation process is tropospheric and that there are minimal to no contributions of stratospheric sulfate to deception island water

a mass conservation approach for mapping glacier ice thickness the traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging while this approach provides continuous maps of ice thickness it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations here we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations we apply this approach to the case of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in northeast greenland that has been relatively stable in recent decades the results show that our mass conserving method removes the anomalies in mass flux divergence yields interpolated data that are within about 5 of the original data and produces thickness maps that are directly usable in high spatial resolution high order ice flow models we discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness copyright 2011 by the american geophysical union

organization of ice flow by localized regions of elevated geothermal heat flux the impact of localized regions of elevated geothermal heat flux on ice sheet dynamics is largely unknown simulations of ice dynamics are produced using poorly resolved and low resolution estimates of geothermal heat flux observations of crustal heat production within the continental crust underneath the lambert amery glacial system in east antarctica indicate that high heat flux regions of at least 120 mw m 2 exist here we investigate the influence of simulated but plausible localized regions of elevated geothermal heat flux on ice dynamics using a numerical ice sheet model of the lambert amery glacial system we find that high heat flux regions have a significant effect across areas of slow moving ice with the influence extending both upstream and downstream of the geothermal anomaly while fast moving ice is relatively unaffected our results suggest that localized regions of elevated geothermal heat flux may play an important role in the organization of ice sheet flow

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

centrifuge model study of spudcan penetration in sand overlying clay a series of centrifuge model tests has been conducted to investigate the bearing resistance of spudcan foundations of offshore jack up rigs in sand overlying normally consolidated clay the spudcan bearing resistance profiles measured in most of the tests recorded a peak resistance at a shallow depth within the upper sand layer followed by an abrupt post peak reduction in resistance causing the spudcan to plunge into the underlying soft clay this phenomenon is commonly termed as spudcan punch through hazard which potentially leads to severe structural damage of jack up rigs in the field the centrifuge test results revealed that the ratio of upper sand layer thickness over spudcan diameter and the ratio of bearing resistance between the upper sand and underlying clay affect the development of spudcan bearing resistance the limitations of existing design methods which were derived for the ultimate bearing capacity of pre embedded shallow foundation to assess the spudcan bearing resistance depth prof

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

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

extraction response of skirted foundations and a spudcan on sand over clay deposits this paper presents a comparison of extraction resistance between spudcan and skirted foundations in sand over clay deposits a series of centrifuge model tests was undertaken on a spudcan and two skirted foundations varying the length of the skirt relative to the foundation diameter the thickness of the sand layer was also varied to cover a range of practical interest it was found that the ultimate breakout forces of the spudcan and sealed skirted foundations in the sand over clay deposits are lower than that of a spudcan in a single layer clay deposit on the sand over clay deposits when the skirted foundations are sealed the breakout forces are 60 61 higher than that of the spudcan however when the skirted foundations are vented this difference becomes minimal regardless of the skirt length and the effective sand layer thickness beneath the foundation with the height of the sand plug left in the clay layer being similar to the thickness of the sand layer

flow dynamics of tidewater glaciers a numerical modelling approach the dynamics of grounded tidewater glaciers is investigated with a time dependent numerical flow model which solves the full equations for the stress and velocity fields and includes a water pressure dependent sliding law the calving criterion implemented in the model shifts the calving front at each time step to the position where the frontal ice thickness exceeds flotation height by a prescribed value with this model the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus but not for situations of rapid changes length changes of tidewater glaciers i e especially rapid changes are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass balance change thus accurate information on the near terminus bed topography is required for reliable prediction of the terminus changes due to climate changes the results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice flow direction

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

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

can seismic observations of bed conditions on ice streams help constrain parameters in ice flow models we investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface to bed inversion using a stokes ice flow model using high resolution measurements along several seismic profiles on pine island glacier pig we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag however there is a stronger correlation when comparing average values along the individual profiles we hypothesize that the correlation appears at the length scales over which basal variations are important to large scale ice sheet flow although the seismic technique is sensitive to the material properties of the bed at present there is no clear way of incorporating high resolution seismic measurements of bed properties on ice streams into ice flow models we conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice bed interface from acoustic impedance measurements can be of direct use to ice sheet models

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying clay the safe and economic use of mobile jack up structures is still hindered by limited understanding of the installation of their large conical spudcan footings in layered sand over clay sites this paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally consolidated clay digital images were captured continuously by installing a half spudcan against a transparent window and analysed using a particle image velocimetry technique coupled with close range photogrammetry correction as the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case the experimental evidence provides a failure modes at different spudcan penetration depths b the transitional failure mechanism at the event of peak bearing resistance and c the changes in overall failure mechanism due to varying geometric and streng

processes controlling the downstream evolution of ice rheology in glacier shear margins case study on rutford ice stream west antarctica ice rheology governs how glaciers flow and respond to environmental change the rheology of glacier ice evolves in response to a variety of mechanisms including damage heating melting and the development of crystalline fabric the relative contributions of these rheological mechanisms are not well understood using remotely sensed data and physical models we decouple the influence of each of the aforementioned mechanisms along the margins of rutford ice stream a laterally confined outlet glacier in west antarctica we show that fabric is an important control on ice rheology in the shear margins with an inferred softening effect consistent with a single maximum fabric fabric evolves to steady state near the onset of streaming flow and ice progressively softens downstream almost exclusively due to shear heating the rate of heating is sensitive to local shear strain rates which respond to local changes in bed topography as ice is squeezed through the basal trough the impact of shear heating on the downstream evolution of ice rheology in a laterally confined glacier suggests that the thermoviscous feedback wherein faster ice flow leads to higher rates of shear heating further softening the ice is a fundamental control on glacier dynamics

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

modeling the dynamic response of outlet glaciers to observed ice shelf thinning in the bellingshausen sea sector west antarctica satellite observations of gravity anomalies ice surface elevation and glacier velocity show significant increases in net grounded ice mass loss over the past decade along the bellingshausen sea sector bss west antarctica in areas where warm 1 c sea water floods the continental shelf these observations provide compelling but indirect evidence that mass losses are driven primarily by reduced buttressing from the floating ice shelves caused by ocean driven ice shelf thinning here we combine recent observations of ice velocity thickness and thickness changes with an ice flow model to study the instantaneous dynamic response of bss outlet glaciers to observed ice shelf thinning alone our model results show that multiple bss outlet glaciers respond instantaneously to observed ice shelf thinning particularly in areas where ice shelves ground at discrete points increases in modeled and observed dynamic mass losses however account for 5 of the mass loss rates estimated from gravity anomalies and changes in ice surface elevation suggesting that variations in surface mass balance may be key to understanding recent bss mass loss our approach isolates the impact of ice shelf thinning on glacier flow and shows that if ice shelf thinning continues at or above current rates total bss mass loss will increase in the next decade

rapid subsurface warming and circulation changes of antarctic coastal waters by poleward shifting winds the southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s this wind trend is projected to persist under continued anthropogenic forcing but the impact of the changing winds on antarctic coastal heat distribution remains poorly understood here we show that a poleward wind shift at the latitudes of the antarctic peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 c at 200 700 m depth the model simulated warming results from a rapid advective heat flux induced by weakened near shore ekman pumping and is associated with weakened coastal currents this analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around antarctica with potentially significant ramifications for global sea level rise

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

relating bed character and subglacial morphology using seismic data from thwaites glacier west antarctica abstract seismic measurements on thwaites glacier show a spatially variable bed character with implications for ice sheet stability the west antarctic ice sheet is losing mass rapidly through outlet glaciers and ice streams in the amundsen sea embayment including thwaites glacier where limited observations and modeling suggest that ice flow rates depend on bed properties here we characterize bed properties of thwaites glacier based on amplitude analysis of reflection seismic data from a 40 km long profile collected in the approximate flow direction and two 10 km long profiles transverse to flow the upstream portion of the seismic profile reveals a 12 km long sedimentary basin with ice flow aligned bedforms capped by a continuous till layer that is likely soft and deforming porosity 0 4 0 45 with several locations where water has pooled at the bed downstream of the sedimentary basin the bed rises by 400 m over 25 km into subglacial highlands our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography 200 m amplitude at 2 to 5 km wavelength resembling crag and tails till on the stoss sides facing upglacier of topographic highs is more consolidated porosity 0 3 0 35 or lower whereas the lee sides facing downglacier and flat regions exhibit porosity similar to the till of the upstream sedimentary basin modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of thwaites glacier resulting in more realistic models of future grounding line retreat our findings highlight the need for more geophysical constraints on bed properties for important outlets in antarctica and greenland

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

bearing capacity on sand overlying clay soils a simplified conceptual model the derivation of a new conceptual model for predicting the peak penetration resistance of circular footings installed vertically into sand overlying soft clay is presented in this paper based on visualisation experiments and finite element analyses a failure mechanism of a frustum of sand being forced into the underlying clay with the outer angle reflecting the dilation in the sand is assumed the analytical basis of the new conceptual model follows the approach for silo analysis and takes into account the stress level and dilatant response of the sand a comprehensive analysis of the performance of the new model and other existing analytical methods in retrospectively predicting the peak resistance of a database of 49 geotechnical centrifuge tests is provided significant improvements over existing punching shear and load spread models are shown as a result of incorporating the strength properties of the sand in a consistent manner the new model has application to the offshore mobile jack up

influence of base suction on extraction of jack up spudcans operators of mobile jack up rigs often face difficulties when extracting spudcan foundations of jack up rigs in the field centrifuge model tests were carried out to evaluate the uplift resistance of a jack up spudcan in normally consolidated clay during extraction the top and bottom faces of the model spudcan were instrumented with total pressure and pore pressure transducers these pressure readings and pore pressure changes in the soil beneath the spudcan were monitored throughout the installation operation and extraction of the spudcan in the present study the operation stage was simply simulated by maintaining a constant vertical load on the spudcan tests were conducted to examine the effects of duration of operation period and ratio of operational working load to installation load on the uplift resistance of the spudcan the test results revealed that suction developed at the interface between the spudcan base and the underlying soil and the magnitude of base suction increased with the operati

flow speed within the antarctic ice sheet and its controls inferred from satellite observations accurate dynamical models of the antarctic ice sheet with carefully specified initial conditions and well calibrated rheological parameters are needed to forecast global sea level by adapting an inverse method previously used in electric impedance tomography we infer present day flow speeds within the ice sheet this inversion uses satellite observations of surface velocity snow accumulation rate and rate of change of surface elevation to estimate the basal drag coefficient and an ice stiffness parameter that influences viscosity we represent interior ice motion using a vertically integrated approximation to incompressible stokes flow this model represents vertical shearing within the ice and membrane stresses caused by horizontal stretching and shearing combining observations and model we recover marked geographical variations in the basal drag coefficient relative changes in basal shear stress are smaller no simple sliding law adequately represents basal shear stress as a function of sliding speed low basal shear stress predominates in central east antarctica where thick insulating ice allows liquid water at the base to lubricate sliding higher shear stress occurs in coastal east antarctica where a frozen bed is more likely examining thwaites glacier in more detail shows that the slowest sliding often coincides with elevated basal topography differences between our results and a similar adjoint based inversion suggest that inversion or regularization methods can influence recovered parameters for slow sliding and finer scales on broader scales we recover a similar pattern of low basal drag underneath major ice streams and extensive regions in east antarctica that move by basal sliding

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years