rapid viscoelastic deformation slows marine ice sheet instability at pine island glacier author s kachuck sb martin df bassis jn price sf abstract 2020 american geophysical union all rights reserved the ice sheets of the amundsen sea embayment ase are vulnerable to the marine ice sheet instability misi which could cause irreversible collapse and raise sea levels by over a meter the uncertain timing and scale of this collapse depend on the complex interaction between ice ocean and bedrock dynamics the mantle beneath the ase is likely less viscous 1018 pa s than the earth s average mantle 1021 pa s here we show that an effective equilibrium between pine island glacier s retreat and the response of a weak viscoelastic mantle can reduce ice mass lost by almost 30 over 150 years other components of solid earth response purely elastic deformations and geoid perturbations provide less stability than the viscoelastic response alone uncertainties in mantle rheology topography and basal melt affect how much stability we expect if any our study indicates the importance of considering viscoelastic uplift during the rapid retreat associated with misi

variations of the antarctic ice sheet in a coupled ice sheet earth sea level model sensitivity to viscoelastic earth properties a coupled ice sheet solid earth sea level model is applied to long term variations of the antarctic ice sheet a set of radially varying viscoelastic profiles in the global earth model is used to explore feedbacks on ice sheet variability including one with a very weak upper mantle zone and thin lithosphere representative of west antarctic regions simulations are performed for 1 the deglacial retreat over the last 20 000 years 2 the future 5000 years with greenhouse gas scenario rcp8 5 and 3 the warm pliocene 3 ma for the deglacial period a large ensemble of 625 simulations is analyzed with a score computed for each run based on comparisons to geologic and modern data for each of the five earth profiles the top scoring combinations of the other model parameters in the ensemble are used to perform future and pliocene simulations for the last deglacial retreat the viscoelastic earth profiles produce relatively small differences in overall ice volume and equivalent sea level in contrast profiles with weak upper mantle and thin lithosphere produce strong negative feedback and less ice retreat in the future and pliocene runs this is due to the faster pace of ice sheet retreat in these runs leading to greater lags in the viscous bedrock rebound behind the unloading which allows for greater influence of the viscosity profiles however the differences in grounding line retreat are located primarily in east antarctic basins where a weak upper mantle and thin lithosphere may not be realistic emphasizing the need for lateral heterogeneity in the earth model

seamount loading and stress in the ocean lithosphere 2 viscoelastic and elastic viscoelastic models analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts the models investigated include layered elastic plates homogeneous viscoelastic plates and elastic over viscoelastic models in all cases the composite models overlie an inviscid half space the principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences authors

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

self inhibiting growth of the greenland ice sheet 1 the build up of the greenland ice sheet gris from ice free conditions is studied in an ice sheet model ism driven by fields from an atmospheric general circulation model gcm to demonstrate the importance of coupling between the two components experiments where the two are coupled off line are augmented by one where an intermediate ice sheet configuration is coupled back to the gcm forcing the ism with gcm fields corresponding to the ice free state leads to extensive regrowth which however is halted when the intermediate recoupling step is included this inhibition of further growth is due to a fohn effect of moist air parcels being lifted over the intermediate ice sheet and arriving in the low lying greenland interior with high temperatures this demonstrates that two way coupling between the atmosphere and the ice sheet is essential for understanding the dynamics and that large scale conditions cooler than those of today may be necessary for the gris to regrow to the present volume

the sensitivity of northern hemisphere ice sheets to atmospheric forcing during the last glacial cycle using pmip3 models the evolution of northern hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one general circulation model cosmos by comparing the simulated results to geological reconstructions we first show that the modelled climate is capable of capturing the main features of the ice sheet evolution however large deviations exist likely due to the absence of nonlinear interactions between ice sheet and other climate components the model uncertainties of the climate forcing are examined using the output from nine climate models from the paleoclimate modelling intercomparison project phase iii the results show a large variability in simulated ice sheets between the different models we find that the ice sheet extent pattern resembles summer surface air temperature pattern at the last glacial maximum confirming the dominant role of surface ablation process for high latitude northern hemisphere ice sheets this study shows the importance of the upper boundary condition for ice sheet modelling and implies that careful constraints on climate output is essential for simulating realistic glacial northern hemisphere ice sheets

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

coupled marine ice sheet earth dynamics using a dynamically consistent ice sheet model and a self gravitating viscous earth model we use a self gravitating viscoelastic model of the earth and a dynamically consistent marine ice sheet model to study the relationships between marine ice sheet dynamics relative sea level basal topography and bedrock dynamics our main conclusion is that sea level change and lithospheric coupling are likely to have played limited roles in the postglacial retreat of marine ice sheets the postglacial rise in sea level would only have caused at the most around 100 km of grounding line retreat for an ice sheet of similar dimensions to the west antarctic ice sheet compared with the several hundred km of retreat which has occurred in the ross sea there is no evidence that reverse slopes lead to instability incorporating coupling with lithospheric dynamics does not produce markedly different effects the implication of these studies is that marine ice sheet retreat is the result of physical mechanisms other than lithospheric coupling and sea level rise

the rheology of polymineralic rocks an approach deduced from empirical and theoretical work a model is proposed for the rheology of bi and polyphase rocks based on the rheology of the components their volume fractions and their geometrical distribution the model is formulated for the common case where some minerals deform by crystall plasticity at steady state and others stronger ones deform in the vincinity of the brittle ductile transition

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

coseismic slip on the southern cascadia megathrust implied by tsunami deposits in an oregon lake and earthquake triggered marine turbidites 1 we test hypothetical tsunami scenarios against a 4 600 year record of sandy deposits in a southern oregon coastal lake that offer minimum inundation limits for prehistoric cascadia tsunamis tsunami simulations constrain coseismic slip estimates for the southern cascadia megathrust and contrast with slip deficits implied by earthquake recurrence intervals from turbidite paleoseismology we model the tsunamigenic seafloor deformation using a three dimensional elastic dislocation model and test three cascadia earthquake rupture scenarios slip partitioned to a splay fault slip distributed symmetrically on the megathrust and slip skewed seaward numerical tsunami simulations use the hydrodynamic finite element model selfe that solves nonlinear shallow water wave equations on unstructured grids our simulations of the 1700 cascadia tsunami require 12 13 m of peak slip on the southern cascadia megathrust offshore southern oregon the simulations account for tidal and shoreline variability and must crest the 6 m high lake outlet to satisfy geological evidence of inundation accumulating this slip deficit requires 360 400 years at the plate convergence rate exceeding the 330 year span of two earthquake cycles preceding 1700 predecessors of the 1700 earthquake likely involved 8 9 m of coseismic slip accrued over 260 years simple slip budgets constrained by tsunami simulations allow an average of 5 2 m of slip per event for 11 additional earthquakes inferred from the southern cascadia turbidite record by comparison slip deficits inferred from time intervals separating earthquake triggered turbidites are poor predictors of coseismic slip because they meet geological constraints for only 4 out of 12 33 cascadia tsunamis

analogies between the high temperature deformation of polyphase rocks and the mechanical behavior of porous powder metal abstract in some rock types one or more mechanically similar minerals may constitute a loadcarrying framework while other subordinate phases carry little load if incompetent phases are more than one or two orders of magnitude weaker than the competent phases the load carrying framework behaves much like a porous solid the deformational properties of porous powder metal provide analogies for the deformation of such porous like rock types 1 the yield strength of porous powder metal is analogous to the power law flow stress for rock deformed at high temperatures 2 the correspondence principle relating elastic and viscoelastic deformation may be used to relate the young s modulus of porous powder metals to the flow stress for nabarro herring creep in rock where the incompetent fraction is 15 of the rock there should be a 50 reduction in power law flow stress for an incompetent fraction of 40 the power law stress is reduced by an order of magnitude and the nabarro herring creep stress is reduced by 80

fast computation of a viscoelastic deformable earth model for ice sheet simulations the model used by lingle and clark 1985 to approximate the deformation of the earth under a single ice stream is adapted to the purposes of continent scale ice sheet simulation the model combines a layered elastic spherical earth farrell 1972 with a viscous half space overlain by an elastic plate lithosphere cathles 1975 for the half space model we identify a new mathematical formulation essentially a time dependent partial differential equation which generalizes and improves upon the standard elastic plate lithosphere with relaxing asthenosphere model widely used in ice sheet simulation the new formulation allows a significantly faster numerical strategy a spectral collocation method based directly on the fast fourier transform we verify this method by comparing to an integral formula for a disk load we also demonstrate that the magnitudes of numerical errors made in approximating coupled ice flow earth deformation systems are significantly smaller than pairwise differences between several earth models our implementation of the lingle and clark 1985 model offers important features of spherical layered self gravitating viscoelastic earth models without the computational expense

a numerical model of interactions between a marine ice sheet and the solid earth application to a west antarctic ice stream a time dependent numerical model has been constructed that simulates retreat of a west antarctic ice stream from the edge of the continental shelf during the holocene period of rising sea level this paper describes a method for computing the deformation of the solid earth caused by changes in ice and water loading during retreat of the ice stream the relative sea level changes caused by earth deformation are incorporated as a feedback mechanism in the ice stream model elastic and viscous uplift of the earth caused by thinning of the ice stream and its catchment area is found to delay retreat of the grounding line relative to computed retreat when the ice stream was assumed to be resting on a rigid earth computed retreat of the grounding line began very slowly at 15 000 b p because of rising eustatic sea level retreat accelerated after about 30 km at 13 000 b p because of increasing water depth and a seabed sloping down toward the ice sheet interior by 8000 b p the feedback effects of earth deformation caused retreat of the grounding line to be delayed by about 1000 years relative to computed retreat on a rigid earth the elastic and viscous earth response exerted a moderating influence on the computed retreat rate because uplift at the grounding line partially counteracted the effect of increasing water depth computed retreat of the grounding line slowed and gradually stopped near the present position of the ice stream grounding line uplift of the seafloor caused the total retreat distance to be reduced by 80 km relative to retreat computed on a rigid earth a readvance of 20 km of the grounding line occurred between 3000 b p and the present because continuing viscous uplift caused sea depth to decrease resistance from the ross ice shelf was found to be of primary importance in bringing retreat of the grounding line to a stop in the ross embayment as suggested by earlier investigators within the context of a given ice shelf retreat history the feedback effects of earth deformation caused a reduction of the grounding line retreat rate a reduction of the total computed retreat distance and a readvance of the grounding line after eustatic sea level stopped rising

mountain building and the initiation of the greenland ice sheet the effects of a new hypothesis about mountain building in greenland on ice sheet initiation are investigated using an ice sheet model in combination with a climate model according to this hypothe

a flow band model of the ross ice shelf antarctica response to co2 induced climatic warming a time dependent model is applied to the ross ice shelf flow band discharging ice stream b west antarctica the model includes the effects of temperature depth dependent density and backpressure from the coasts of the ross embayment and crary ice rise data from the ross ice shelf geophysical and glaciological survey and the siple coast project are used as input data accuracy and stability are verified by reproducing the flow band for 10 000 model years with equilibrium distributions of accumulation surface temperature and basal balance sensitivity is tested by forcing the model with increasing accumulation rates surface temperatures and basal melt rates respectively while other factors are held constant the response of the ice shelf to three climatic scenarios that may result from increasing carbon dioxide and trace greenhouse gases is simulated the results range from slight thickening with moderately increased backpressure in the grounding zone to rapid thinning accompanied by rapidly decreasing backpressure during 175 to 600 year simulations depending primarily upon whether increasing surface temperatures and accumulaton rates are accompanied by increased rates of basal melting the central ice shelf about 400 km upglacier from the calving front thins by 22 in 600 years when basal melting is increased linearlymore to a maximum of 0 5 m yr after 150 years then held steady the ice shelf thins by 40 in 175 years at the same location when basal melting is increasedlinearly to 2 0 m yr after 150 years then held steady the present calculated equilibrium rate of basal melting averaged over the bottom surface of the flow band is 0 17 m yr less

multistability of the greenland ice sheet and the effects of an adaptive mass balance formulation the effect of a warmer climate on the greenland ice sheet as well as its ability to regrow from a reduced geometry is important knowledge when studying future climate here we use output from a general circulation model to construct adaptive temperature and precipitation patterns to force an ice flow model off line taking into consideration that the patterns change in a non uniform way both spatially and temporally as the geometry of the ice sheet evolves and as climate changes in a series of experiments we investigate the retreat from the present day configuration build up from ice free conditions of the ice sheet during a warmer than present climate and how the ice sheet moves between states the adaptive temperature and accumulation patterns as well as two different constant pattern formulations are applied and all experiments are run to steady state all results fall into four different groups of geometry regardless of the applied accumulation pattern and initial state we find that the ice sheet is able to survive and build up at higher temperatures using the more realistic adaptive patterns compared to the classic constant patterns in contrast decay occurs at considerably higher temperatures than build up when the other formulations are used when studying the motion between states it is clear that the initial state is crucial for the result the ice sheet is thus multistable at least for certain temperature forcings and this implies that the ice sheet not does not necessarily return to its initial configuration after a temperature excursion

late pleistocene interactions of east and west antarctic ice flow regimes evidence from the mcmurdo ice shelf we present new interpretations of deglaciation in mcmurdo sound and the western ross sea with observationally based reconstructions of interactions between east and west antarctic ice at the last glacial maximum lgm 16000 12 000 8000 and 4000 bp at the lgm east antarctic ice from mulock glacier split one branch turned westward south of ross island but the other branch rounded ross island before flowing southwest into mcmurdo sound this flow regime constrained by an ice saddle north of ross island is consistent with the reconstruction of stuiver and others 1981a after the lgm grounding line retreat was most rapid in areas with greatest water depth especially along the victoria land coast by 12 000 bp the ice flow regime in mcmurdo sound changed to through flowing mulock glacier ice with lesser contributions from koettlitz blue and ferrar glaciers because the former ice saddle north of ross island was replaced by a dome the modern flow regime was established 4000 bp ice derived from high elevations on the polar plateau but now stranded on the mcmurdo ice shelf and the pattern of the transantarctic mountains erratics support our reconstructions of mulock glacier ice rounding minna bluff but with all ice from skelton glacier ablating south of the bluff they are inconsistent with drewry s 1979 lgm reconstruction that includes skelton glacier ice in the mcmurdo sound through flow drewry s 1979 model closely approximates our results for 12 000 4000 bp ice sheet modeling holds promise for determining whether deglaciation proceeded by grounding line retreat of an ice sheet that was largely stagnant because it never approached equilibrium flowline profiles after the ross ice shelf grounded or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present day outlet glaciers after the ross ice shelf grounded

present day antarctic ice mass changes and crustal motion the peak vertical velocities predicted by three realistic but contrasting present day scenarios of antarctic ice sheet mass balance are found to be of the order of several mm a one scenario predicts local uplift rates in excess of 5 mm a these rates are small compared to the peak antarctic vertical velocities of the ice 3g glacial rebound model which are in excess of 20 mm a if the holocene antarctic deglaciation history portrayed in ice 3g is realistic and if regional upper mantle viscosity is not an order of magnitude below 10 exp 21 pa s then a vast geographical region in west antarctica is uplifting at a rate that could be detected by a future global positioning system gps campaign while present day scenarios predict small vertical crustal velocities their overall continent ocean mass exchange is large enough to account for a substantial portion of the observed secular polar motion omega m bar and time varying zonal gravity field j sub 1

glaciology and geological signature of the last glacial maximum antarctic ice sheet dynamical changes in contemporary ice sheets account for significant proportions of their current rates of mass loss but assessing whether or not these processes are a natural part of ice sheet evolution requires inference from palaeo glaciological records however a robust mechanism for translating sparse geological data into meaningful interpretations of past glacier dynamics at the continental scale is lacking since geological archives can be ambiguous and often their chronology is only poorly constrained to address this we combine the interpretation of high resolution antarctic ice sheet model results with continent wide geological evidence pertinent to the dynamical configuration of the ice sheet during the last and possibly preceding glacial maxima we first focus on the thermal regime of the ice sheet its pattern and velocity of flow variability in likely subglacial erosion and sediment transport and how these characteristics evolve during glacial transitions we show that rapid basal sliding was restricted to discrete outlets that eroded and advected sediment toward and across the continental shelf primarily during the early stages of advance and retreat of the ice sheet highlighting the need to consider time transgressive behaviour in the interpretation of geological archives secondly we present new modelling that attempts to improve the fit of our numerical model to geologically based reconstructions in the ross sea by accounting for locally enhanced ablation in mcmurdo sound our new simulation achieves a much closer fit to empirically derived flow patterns than previously growth of the modelled last glacial maximum ice sheet takes place primarily by marine ice accretion in the major embayments as a consequence of cooler ocean temperatures and reduced sub ice shelf melting and at its maximal extent represents a grounded ice volume excess above present of approximately 8 3 m sea level equivalent this figure thus provides an upper bound on the possible antarctic contribution to deglacial meltwater pulses

a numerical model of interactions between a polar ice stream and the ocean application to ice stream e west antarctica a time dependent numerical model has been developed to study the dynamics of a polar ice stream grounded below sea level the model was applied to ice stream e one of the two most active ice streams draining the ross sea sector of the west antarctic ice sheet the retarding force or back stress from the floating ross ice shelf needed to hold the model grounding line in dynamic equilibrium was found to compare closely with retarding force mapped from field data by earlier investigators this implies that the grounding line of ice stream e is in fact close to dynamic equilibrium neither advancing nor retreating rapidly if the ice stream and its catchment area are approximately in mass balance sensitivity tests showed that the model ice stream is more sensitive to changes in back stress from the ice shelf than to changes in the accumulation rate changes in back stress can be caused by changes in the average thickness of the ice shelf the model was used to simulate retreat of the ice stream from the edge of the continental shelf in the ross sea during the holocene period of rising sea level the sea floor was assumed to be isostatically depressed but to remain rigid during retreat the effect of the assumed retreat history of the ross ice shelf on the computed timing of grounding line retreat was investigated by considering several alternative ice shelf histories the results support the conclusion of earlier investigators that the ross ice shelf caused the grounding line to stop retreating near its present position after extensive retreat during the holocene in addition the timing of retreat of the calving front of the ross ice shelf exerted major influence on the timing of retreat of the ice stream grounding line

the deformational behaviour of bimineralic limestone halite aggregates abstract a compositional foliation banding develops within synthetic limestone halite aggregates experimentally deformed in axially symmetric compression and in simple shear the strength of the bulk aggregate decreases as the foliation becomes stronger foliation weakening in calcite rich specimens the development of foliation starts with the formation of cataclastic shear systems in calcite domains between isolated halite domains with increasing deformation halite is sheared into these systems the halite domains coalesce into halite rich shear systems and accommodate an increasing amount of the bulk deformation the bulk strength of bimineralic aggregates at a given amount of strain increases with decreasing halite content the influence of halite on the bulk strength is much greater than would be expected from its volume fraction and becomes dominant with increasing deformation due to the development of the foliation it is postulated that such processes also occur during natural deformation of polyphase rocks competent clasts within a weaker ductile matrix showing no recrystallization differ in shape and rotational behaviour from recrystallizing clasts such as feldspar augen these competent clasts denoted as type clasts may cause problems in deducing the sense and direction of shear when formed as almost exact mirror images of type recrystallizing clasts

on postglacial sea level summary an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non rotating earth models the method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late quaternary ice sheets we find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins if a quantity of ice equivalent to a uniform 100 m rise in sea level melts from the laurentide and fennoscandian ice sheets then in the south pacific the instantaneous rise in sea level can be as large as 120m in the north atlantic the instantaneous rise is always less than 100 m there is a zone in the north atlantic with almost no sea level change and near greenland and norway the sea level falls rather than rises by over 100 m one thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the south pacific into the north pacific suggesting that raised beaches should occur in the south pacific the gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice this extra load near the ice may have a significant influence on postglacial isostatic adjustment

a comparison of different ways of dealing with isostasy examples from modelling the antarctic ice sheet during the last glacial cycle the bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models this paperreviews the various methods normally used in the ice sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full earth model each of these bedrock treatments five in total are coupled with a three dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the antarctic ice sheet during the last glacial cycle the outputs of the simulations are compared on the basis of thetime dependent behavior for the total ice volume and the mean bedrock elevation during the cycle and of the present rate of uplift over antarctica thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns it furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle nevertheless when characteristicparameters are varied within their range of uncertainty differences within one single method are often of the same order as those between the variousmethods this overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements

effects of anhydrite and pressure on the mechanical behavior of synthetic rocksalt bimineralic polycrystalline rocksalt specimens ranging in composition from pure halite to pure anhydrite have been fabricated in hydrostatic compression at 150 mpa and 100 sup 0 c for 15 minutes resulting porosities of the compacted samples increased with increasing anhydrite content ranging from 02 for pure halite to 14 for pure anhydrite twenty three compacted samples were then deformed in triaxial compression experiments at confining pressures of 10 100 and 200 mpa a nominal strain rate of 10 sup 4 sec sup 1 and a temperature of 200 sup 0 c sample strength at 10 percent axial strain was directly related to both anhydrite content and pressure relatively small strength increases were observed for increases in anhydrite content up to 75 percent by weight and for all pressure increases much larger increases in strength were observed when the anhydrite content was increased from 75 to 100 percent ductilites of less than 10 percent were seen in only two sets of data the 100 and 75 percent anhydrite samples run at 10 mpa confining pressure and these data indicated a trend of increasing ductility with increasing halite content and or confining pressure all samples exhibited strain hardening except for the 75 percent halite samples at all three pressures andmore the 50 percent sample of 10 mpa which exhibited slight strain softening less

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change