hot water drilling and bore hole closure in cold ice drilling bore holes in deep cold ice masses by hot water methods and maintaining these holes with sufficient diameter to allow down hole experimentation poses a major obstacle to the investigation of conditions beneath ice sheets and ice streams closure of the water filled holes by refreezing is the dominant difficulty in this paper we describe calculations of heat transfer from the drilling system to the ice and the subsequent time dependent motion of the phase boundary defining the bore hole wall results are presented with the view of optimizing the bore hole radius at depth for a fixed drill performance and a variable rate of drilling calculation of melting refreezing rates at the bore hole wall requires the use of a one dimensional time dependent numerical heat flow model with a distorting mesh which follows the changing hole size the delay of hole closure is discussed with a view to keeping holes open long enough to allow instruments to be lowered to the glacier bed while realizing that drilling system performance may be marginal because of logisitical and or expenditure constraints the relative merits of drilling a large hole which is very time consuming with a small drill and the use of water soluble antifreezes which have a history of creating plugs of ice slush are discussed a method of creating a stable hole filled with antifreeze in which ice slush does not occur is described the recent application of these theoretical ideas to the planning and implementation of successful hot water drilling programs in antarctica and greenland is also presented

landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

englacial latent heat transfer has limited influence on seaward ice flux in western greenland surface meltwater can refreeze within firn layers and crevasses to warm ice through latent heat transfer on decadal to millennial timescales earlier work posited that the consequent softening of the ice might accelerate ice flow potentially increasing ice sheet mass loss here we calculate the effect of meltwater refreezing on ice temperature and softness in the p kitsoq near swiss camp and jakobshavn isbrae regions of western greenland using a numeric model and existing borehole measurements we show that in the jakobshavn catchment meltwater percolation within the firn warms the ice at depth by 3 5 c by contrast meltwater refreezing in crevasses cryo hydrologic warming at depths of 300 m warms the ice in p kitsoq by up to 10 c but this causes minimal increase in ice motion 10 m a 1 p kitsoq is representative of western greenland s land terminating ice where the slow movement of ice through a wide ablation zone provides ideal conditions for cryo hydrologic warming to occur we find that only 37 of the western greenland ice flux however travels through such areas overall our findings suggest that cryo hydrologic warming will likely have only a limited effect on the dynamic evolution of the greenland ice sheet

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

polythermal three dimensional modelling of the greenland ice sheet with varied geothermal heat flux computations over 50 000 years into steady state with greve s polythermal ice sheet model and its numerical code are performed for the greenland ice sheet with today s climatological input surface temperature and accumulation function and three values of the geothermal heat flux 42 54 6 29 4 mw m 2 it is shown that through the thermomechanical coupling the geometry as well as the thermal regime in particular that close to the bed respond surprisingly strongly to the basal thermal heat input the most sensitive variable is the basal temperature field but the maximum height of the summit also varies by more than 100m furthermore some intercomparison of the model outputs with the real ice sheet is carried out showing that the model provides reasonable results for the ice sheet geometry as well as for the englacial temperatures

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

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

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

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

regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the cordilleran ice sheet in british columbia canada and northern washington state usa abstract subglacial landsystems in and around okanagan valley british columbia canada are investigated in order to evaluate landscape development subglacial hydrology and cordilleran ice sheet dynamics along its southern margin major landscape elements include drumlin swarms and tunnel valleys drumlins are composed of bedrock diamicton and glaciofluvial sediments their form truncates the substrate tunnel valleys of various scales km to 100s km length incised into bedrock and sediment exhibit convex longitudinal profiles and truncate drumlin swarms okanagan valley is the largest tunnel valley in the area and is eroded 300 m below sea level over 600 m of late wisconsin age sediments consisting of a fining up sequence of cobble gravel sand and silt fill okanagan valley landform substrate relationships landform associations and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds they are best explained by meltwater erosion and deposition during ice sheet underbursts during the late wisconsin glaciation okanagan valley functioned as part of a subglacial lake spanning multiple connected valleys few 100s km of southern british columbia subglacial lake development started either as glaciers advanced over a pre existing sub aerial lake catch lake or by incremental production and storage of basal meltwater high geothermal heat flux geothermal springs and or subglacial volcanic eruptions contributed to ice melt and may have triggered along with priming from supraglacial lakes subglacial lake drainage during the underburst s sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys progressive flow channelization focused flows toward major bedrock valleys in okanagan valley most of the pre glacial and early glacial sediment fill was removed a fining up sequence of boulder gravel and sand was deposited during waning stages of the underburst s and bedrock drumlins in okanagan valley were enhanced or wholly formed by this underburst s subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes the prevailing conceptual model for growth and decay of the cis suggests significantly thicker ice in valleys compared to plateaus subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner due to melting and enhanced sliding with significant transfer of ice toward the ice sheet margin subglacial lake drainage may have hastened deglaciation by melting ice lowering ice surface elevations and causing lid fracture this paper highlights the importance of ice sheet hydrology its control on ice flow dynamics distribution and volume in continental ice masses

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

estimation of hydraulic properties of subglacial till from ploughmeter measurements force variations on a ploughmeter and fluctuations in subglacial water pressure have been measured in the same borehole at storglaciaren sweden to investigate hydraulic properties of the basal till layer a strong inverse correlation of the pressure and force records in conjunction with a significant time lag between the two signals suggests that pore water pressures directly affect the strength of the till variations in subglacial water pressure result in potential gradients across the water till interface at the bottom of the borehole that drive pressure waves downwards through the till layer when the borehole water level is high and back upwards when the water level is low analysis of the propagation velocity of this pressure wave indicates that the hydraulic diffusivity of storglaciaren till is in the range 1 9 3 6 10 6 m 2 s 1 in good agreement with estimates obtained in the laboratory hydraulic conductivity values associated with these diffusivities are between 10 and 10 m s 1 and thus are well within the range of values for other glacial tills

derived bedrock elevations strain rates and stresses from measured surface elevations and velocities jakobshavns isbrae greenland jakobshavns isbrae 69 10 n 49 59 w drains about 6 5 of the greenland ice sheet and is the fastest ice stream known the jakobshavns isbrae basin of about 10 000 km 2 was mapped photogrammetrically from four sets of aerial photography two taken in july 1985 and two in july 1986 positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation and surface velocity vectors were computed from the positions the two flights in 1985 yielded the best results and provided most common points 716 for velocity determinations and are therefore used in the modeling studies the data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation the field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses using the flow law of ice for rectilinear coordinates x y pointing eastward and northward and curvilinear coordinates l t pointing longitudinally and transversely to the changing ice flow direction ice surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding our calculated ice thicknesses are in fair agreement with an ice thickness map based on seismic sounding and supplied to us by k echelmeyer ice thicknesses were subtracted from measured ice surface elevations to map bed topography our calculation shows that basal sliding is significant only in the 10 15 km before jakobshavns isbrae becomes afloat in jakobshavns isfjord

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

impurity influence on normal grain growth in the gisp2 ice core greenland intercept analysis of approximately bi yearly vertical thin sections from the upper part of the gisp2 ice core central greenland shows that grain size ranges increase with increasing age this demonstrates that something in the ice affects grain growth rates and that grain size cannot be used directly in paleothermometry as has been proposed correlation of grain growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations and especially slow growth in forest fire layers containing abundant ammonium however the impurity grain growth relations are quite noisy little correlation is found between growth rate and isotopic composition of ice

oscillating quaternary water levels of the marmara sea and vigorous outflow into the aegean sea from the marmara sea black sea drainage corridor abstract detailed interpretation of single channel air gun and deep tow boomer profiles demonstrates that the marmara sea turkey experienced small amplitude 70 m fluctuations in sea level during the later quaternary limited in magnitude by the sill depth of the strait of dardanelles moderate subsidence along the southern shelf and quaternary glacio eustatic sea level variations created several stacked deltaic successions separated by major shelf crossing unconformities which developed during the transitions from global glacial to interglacial periods near the strait of dardanelles a series of sand prone deposits are identified beneath an uppermost holocene transparent mud drape the sandy deposits thicken into mounds with the morphology and cross sectional geometries of barrier islands sand waves and current generated marine bars all cross stratification indicates unidirectional flow towards the dardanelles prior to the deposition of the transparent drape which began 7000 years bp in strong support of the notion that the marmara sea flowed westwards into the aegean sea through the dardanelles at times of deglaciation in northern europe the global sea level curve shows that at 11 000 and 9500 years bp sea level rose to the sill depths of the straits of dardanelles and bosphorus respectively the effect from 11 000 to 9500 years bp was seawater incursion into the marmara sea drowning and formation of algal serpulid bioherms atop lowstand barrier islands and transgression of shelves and lowstand deltas at 9500 years bp glacial meltwater temporarily stored in the black sea lake developed into a vigorous southward flow toward the aegean sea forming west directed sandy bedforms in the western marmara sea and initiating deposition of sapropel s1 in the aegean sea this strong outflow persisted until 7000 years bp after which a mud drape began to accumulate in the marmara sea and euryhaline mediterranean mollusks successfully migrated into a progressively more saline black sea where sapropel deposition began most eastern mediterranean sapropels from s1 to s11 appear to correlate with periods of rising sea level and breaching or near breaching of the bosphorus sill these events are believed to coincide with times of vigorous outflow of low salinity fresh surface waters transiting the black sea marmara sea corridor and ultimately derived from melting of northern european ice sheets

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

tidal flexure of jakobshavns glacier west greenland jakobshavns glacier a floating outlet glacier on the west greenland coast was surveyed during july 1976 the vertical displacements of targets along two profiles perpendicular to the fjord wall bounding the north margin of the glacier were analyzed to determine the effect of flexure caused by tidal oscillations within the fjord an analysis based on the assumption that vertical displacements of the glacier reflected pure elastic bending yielded the conclusion that the effective thickness of the ice i e the thickness which remained unaffected by surface and basal cracking and which behaved as a continuum was 160 m 2 6 km upglacier from the calving front and 110 m 0 6 km from the calving front an analysis based on the more realistic assumption that observed bending reflected elastic and viscoplastic deformation yielded the conclusion that the average effective thickness of the ice was 316 74 m 40 of the estimated 800 m total thickness 2 6 km from the calving front and 160 48 m 21 of the estimated 750 m total 0 6 km from the calving front a constitutive relationship appropriate for hard glide during flexure was used

electrical measurements on the greenland ice sheet project 2 core the greenland ice sheet project 2 gisp2 electrical conductivity measurement ecm record is an indication of the concentration of h in the core the ecm detected seasonal variations in the nitrate concentration of the core which were used to assist in dating the core by annual layer counting volcanic eruptions that produce acidic aerosols are recorded in the ecm record evidence of biomass burning is detected by the ecm because fire related ammonium emissions neutralize the acids in the core rapid climate transitions associated with the younger dryas and dansgaard oeschger interstadial events alter the concentration of alkaline dust and are detected by the ecm the ecm has been used to develop stratigraphic ties between the gisp2 and the greenland ice core project cores users of the data should be aware of some instrument related artifacts in the ecm record

estimation of subglacial hydraulic properties from induced changes in basal water pressure a theoretical framework for borehole response tests in this paper we develop a theoretical model describing water motion in a coupled borehole subglacial flow system the theory applies to basal drainage systems having multiple and extensive interconnected flow paths within this domain it encompasses a broad range of flow regimes from laminar darcian flow in a thick permeable unit to turbulent sheet flow in a very thin layer important terms in the model are highlighted by recasting the problem in dimensionless form the non dimensional formulation indicates that there are four free parameters in the coupled system these parameters characterize skin friction in the borehole and diffusion transmissivity and turbulent transport in the subglacial flow layer dimensionless results show that under most circumstances the effects of skin friction in the borehole are negligible diffusion transmissivity and especially turbulent transport in the basal layer are found to influence subglacial water flow strongly we use our model to predict fluctuations of borehole water levels that result from different types of disturbances we show how this framework can be used to estimate subglacial hydraulic properties by comparing model results with data collected during field experiments on trapridge glacier yukon territory canada in 1989 and 1990

the eismint benchmarks for testing ice sheet models we present a series of benchmark experiments designed for testing and comparing numerical ice sheet models following the outcome of two eismintworkshops organized to intercompare large scale ice sheet models currently in operation model benchmark experiments are described for ice sheets underfixed and moving margin conditions these address both steady state and time dependent behavior under schematic boundary conditions and with prescribedphysics a comparison was made of each model s prediction of basic geophysical variables such as ice thickness velocity and temperature consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice sheet modeling codes can be assessed

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