influence of glacier hydrology on the dynamics of a large quaternary ice sheet the influence of glacier hydrology on the time dependent morphology and flow behaviour of the late weichselian scandinavian ice sheet is explored using a simple one dimensional ice sheet model the model is driven by orbitally induced radiation variations ice albedo feedback and eustatic sea level change the influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet where a marginal zone of rapid sliding thin ice and low surface slopes develops such a zone is absent when hydrology is omitted from the model and its formation results in earlier and more rapid deglaciation than occurs in the no hydrology model the final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr bp channelised subglacial drainage develops only episodically and is associated with relatively low meltwater discharges and high hydraulic gradients the predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surface profile

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

characteristics of the heat balance of the greenland ice sheet for modelling les donnees du bilan thermique recoltees au cours de l egig de 1959 a 1967 sont appliquees au calcul du deplacement de la ligne d equilibre du au changement climatique

residual circulation in the gulf of suez egypt abstract a three dimensional non linear hydrodynamical numerical model has been used to study the water movement in the shallow gulf of suez at the northern end of the red sea the computational grid used to schematize the gulf has a horizontal resolution of 3 3 km and levels at 5 15 30 and 60 m in the vertical the simulations were carried out separately for tidal forcing and wind forcing further in each case stratified conditions were considered to represent the state of the gulf during late summer periods in the case of wind induced circulations a uniform wind speed of 5 m s 1was applied over the gulf blowing from nw n and ne directions the results suggest that the dynamics of the gulf of suez are governed by the interaction of wind and density forcing only occasionally for instance in the vicinity of headlands and islands tidal residuals are of similar importance for the overall residual currents it was found that the typical quasi baroclinic two layered inverse estuarine circulation observed in the gulf of suez can be produced under the combined action of m2 tide and winds blowing steadily from the north eastern direction north western and northern winds produced the reverse pattern

sub catchment melt and long term stability of ice stream d west antarctica 1 the apparent long term persistence and short term variability of the siple coast ice streams of west antarctica are tied to regional thermal conditions and local basal lubrication numerical simulations indicate that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of fast moving modern and ice age ice streams despite their tendency to freeze to the bed and would allow additional thinning and grounding line retreat

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

ice sheet modelling at different spatial resolutions focus on the grounding zone a high resolution time dependent two dimensional flowline model was developed which takes into account all relevant stresses i e both shear and longitudinal stresses and which is solved on a fixed finite difference grid special attention was paid to the computational efficiency a comparison is made with a similar model that solves the velocity field according to the shallow ice approximation both models were applied to an east antarctic flowline for which the data on surface and bedrock elevation were sampled at different spatial resolutions the numerical analysis deals with the importance of longitudinal stresses at the ice sheet ice shelf interface at different grid sizes and the importance of basal drag versus driving stress in basal sliding laws at a grid resolution of 5 to 10 km the force balance indicates that all stress components are of equal importance in an area that is larger than the nominal grid size when sliding becomes dominant this transition zone considerably widens furthermore the longitudinal stress deviator is found to vary considerably with depth due to the thermomechanical coupling fpattyn pmg nipr ac jp

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

organic geochemistry of sediments in new zealand part i a biomarker study of the petroleum seepage at the geothermal region of waiotapu abstract the saturated hydrocarbons from the oil seepage at waiotapu in new zealand were examined by gc and gcms techniques of molecular fingerprinting the petroleum was characterised by a high ratio of phytane nc18 2 37 and low ratios of pr ph 0 64 and saturates aromatics 0 60 this study revealed that the oil was derived from terrigenous organic matter and it had neither migrated from its source nor suffered biodegradation the source rocks were deposited under anoxic conditions and these rocks lay within pleistocene sediments which were immature with respect to oil generation the location of the source rocks within a geothermal field had accelerated the generation of petroleum since the source rocks would have been too young and shallow to generate petroleum in the absence of geothermal energy

changes in the west antarctic ice sheet since 1963 from declassified satellite photography comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the ross ice shelf ice stream b has widened by 4 kilometers at a rate much faster than suggested by models and has decreased in speed by 50 percent the ice ridge between ice streams b and c has eroded 14 kilometers these changes along with changes in the crevassing around crary ice rise imply that this region s velocity field shifted during this century

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

flow variability in the scandinavian ice sheet modelling the coupling between ice sheet flow and hydrology abstract there is increasing geologic evidence for periodic flow variability within large ice sheets manifested as spatially and temporally variable areas of fast ice flow and resulting in the very complex patterns of lineations observed in formerly glaciated areas however many ice sheet models do not replicate this behaviour a possible reason for this is that such models do not include a detailed treatment of basal hydrology changes in the character of sub glacial drainage systems are believed to cause surges in valley glaciers recent ice sheet models which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed often show flow variability however these models have typically assumed a deformable bed or have made no assumptions about the nature of the bed whilst these assumptions seem applicable to areas close to the former margins of quaternary ice sheets they are less applicable to interior areas these areas typically show thin or scanty till cover over eroded bedrock and the presence of eskers which are indicative of drainage in sub glacial tunnels we have developed a two dimensional time dependent ice sheet model that includes hard bed basal hydrology this allows calculation of sub glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities when used to simulate the weichselian scandinavian ice sheet with late quaternary climate and sea level as forcing functions this model develops localised areas of fast flowing ice which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally the behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow due to the resulting variations in the subglacial hydraulic potential which drive the water flow bedrock topography also has some influence but fast flow areas are not confined to obvious topographic troughs a relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow generally two to four separate areas of fast flow can be recognised and these are similar in size and shape to the lobes identified in some geologically based reconstructions of the scandinavian ice sheet within the fast flowing areas sub glacial drainage is typically in a cavity based system however tunnel based drainage is predicted to have extended up to 150 km from the ice sheet margin particularly during deglaciation because of the changes in ice sheet topography associated with fast flow and the resulting changes in the pattern of sub glacial water flow the model predicts that these fast flowing lobes interacted in complex ways and exhibited quasi periodic switching between fast and slow flow

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

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

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval