full depth englacial vertical ice sheet velocities measured using phase sensitive radar we describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling using a ground based phase sensitive radio echo sounder pres during seven antarctic field seasons we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on berkner island roosevelt island fletcher promontory and adelaide island recorded changes in reflector positions yield full depth profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides we interpret these variations by comparing them to the results of a full stokes simulation of ice divide flow qualitatively validating the model and demonstrating that we are directly detecting an ice dynamical phenomenon called the raymond effect using pres englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice we discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pres measured vertical velocities into age modeling

density log of a 181 m long ice core from berkner island antarctica a 181 m long ice core was drilled at 79 36 51 s 45 43 28 w near the summit of berkner island antarctica 886 m a s l berkner island is located between the filchner and ronne ice shelves and the ice near the summit shows little lateral flow the density of the ice core was measured every 3 mm along its length using attenuation of a gamma ray beam which gave an absolute accuracy of 2 as expected there is a general density increase with depth the maximum densities of 900 kg m 3 being reached just above 100 m depth comparison with the electrical conductivity method ecm shows density variations with the same wavelength as the annual signals which can be seen in the ecm log higher acidity during summer in the shallowest part of the core the density of winter layers is higher than that of summer layers a relationship which is reversed at greater depth we assume that the densification rates for the two types of firn are different similar density phenomena were observed on ice cores from greenland showing that such phenomena are not a local effect

migration of the siple dome ice divide west antarctica the non linearity of the ice flow law or a local accumulation low over an ice divide can cause isochrones internal layers to be shallower under the divide relative to the flanks forming a divide bump in the internal layer pattern this divide signature is analyzed using ice flow models and inverse techniques to detect and quantify motion of the siple dome ice divide west antarctica the principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide the calculated migration rate is 0 05 0 50 m a 1 toward ice stream d and depends slightly on whether the divide bump is caused by the non linearity of ice flow or by a local accumulation low our calculations also suggest a strong south north accumulation gradient of 5 10 x 10 6 a 1 in a narrow zone north of the divide a consequence of divide migration is that pre holocene ice is thickest about 0 5 km south of the present divide position divide motion indicates that non steady processes possibly associated with activity of the bounding ice streams are affecting the geometry of siple dome the migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths this suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 10 4 years

late holocene ice flow reconfiguration in the weddell sea sector of west antarctica abstract here we report late holocene ice sheet and grounding line changes to the weddell sea sector of west antarctica internal radio echo layering within the bungenstock ice rise which comprises very slow flowing ice separating the fast flowing institute and moller ice streams reveals ice deformed by former enhanced flow overlain by un deformed ice the ice rise surface is traversed by surface lineations explicable as diffuse ice flow generated stripes which thus capture the direction of flow immediately prior to the creation of the ice rise the arrangement of internal layers can be explained by adjustment to the flow path of the institute ice stream during either a phase of ice sheet retreat not longer than 4000 years ago or by wholesale expansion of the grounding line from an already retreated situation not sooner than 400 years ago some combination of these events involving uplift of the ice rise bed during ice stream retreat and reorganisation is also possible whichever the case the implication is that the ice sheet upstream of the bungenstock ice rise which currently grounds over a 1 5 km deep basin has been and therefore may be susceptible to significant change

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

integrated sedimentary biotic and paleoredox dynamics from multiple localities in southern laurentia during the late silurian ludfordian extinction event abstract the silurian was a time of major climatic transition punctuated by multiple biotic crises and global carbon cycle perturbations the most severe of these biotic events was the late silurian ludfordian lau kozlowskii extinction event lke and the associated lau carbon isotope excursion cie although the extinction event and lau cie are globally documented the only records thus far of local and global marine paleoredox conditions through this interval are from a single region in scandinavia here we examine four sections along a bathymetric transect of mixed carbonate siliciclastic sediments from western tennessee usa a novel approach using a multi proxy dataset combining high resolution geochemical data and microfacies analyses from multiple localities explores the possibilities of local regional scale redox heterogeneities during a time of widespread environmental upheaval on a global scale paired positive excursions recorded in carbonate carbon isotopes and carbonate associated sulfate sulfur isotopes support recent work from carbonate and siliciclastic successions from scandinavia suggesting a global enhancement of organic carbon and pyrite burial driven by an expansion of euxinic anoxic and sulfidic water column conditions in the oceans during the mid ludfordian furthermore positive excursions in organic carbon isotopes and pyrite sulfur isotopes reflect the global changes in redox stratigraphic trends in i ca ratios imply a local expansion of low oxygen conditions with low but non zero values during the rising limb and peak of the cie the fossil assemblages vary across the shelf and through the cie interval stratigraphic changes in fossil assemblages and i ca are closely associated with local and global changes in oxygenation and sea level during the mid ludfordian the collective data indicate significant biotic reorganization in response to changes in marine redox conditions and in conjunction with sea level variation during the lke interval but detailed macroscopic biodiversity is currently unconstrained for this region

regional and temporal variation of accumulation around northgrip derived from ground penetrating radar during the summer of 2003 a ground penetrating radar survey around the north greenland icecore project northgrip deep ice core drilling site 75806 0 n 42820 0 w 2957 m a s l was carried out using a shielded 250 mhz radar system the drill site is located on an ice divide roughly 300 km north northwest of the summit of the greenland ice sheet more than 430 km of profiles were measured covering a 10 km by 10 km area with a grid centered on the drilling location and eight profiles extending beyond this grid seven internal horizons within the upper 120 m of the ice sheet were continuously tracked containing the last 400 years of accumulation history based on the age depth and density depth distribution of the deep core the internal layers have been dated and the regional and temporal distribution of accumulation rate in the vicinity of northgrip has been derived the distribution of accumulation shows a relatively smoothly increasing trend from east to west from 145 kg m 2 a 1 to 200 kg m 2 a 1 over a distance of 50 km across the ice divide the general trend is overlain by small scale variations on the order of 2 5 kg m 2 a 1 km 1 i e around 1 5 of the accumulation mean the temporal variations of the seven periods defined by the seven tracked isochrones are on the order of 4 of the mean of the last 400 years i e at northgrip 7k g m 2 a 1 if the regional accumulation pattern has been stable for the last several thousand years during the holocene and ice flow has been comparable to today advective effects along the particle trajectory upstream of northgrip do not have a significant effect on the interpretation of climatically induced changes in accumulation rates derived from the deep ice core over the last 10 kyr

in situquantification of ice rheology and direct measurement of the raymond effect at summit greenland using a phase sensitive radar 1 the glen exponent ncharacterizes the stress dependence of ice deformation directly influencing the rate at which ice masses respond to external forcing the slow deformation in large ice sheets makes laboratory rheometry at representative strain rates difficult we develop a new technique to estimaten in situ deploying a phase sensitive radar to measure vertical strain rates of around 10 4 yr 1within the top 1000 m of ice across ice divides at summit and neem greenland a fluid dynamical feature the raymond effect predicts strong vertical strain rate variation across divides over distances of a few ice thicknesses we achieve sufficient resolution to show this pattern enabling us to estimaten 4 5 by inverting our observations with flow modelling this is higher than values previously used but consistent with other indirect measurements implying laboratory measurements do not explore the full range of ice rheology and the consequent possibility of a greater sensitivity and responsiveness in ice sheet dynamics

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

dating ice flow change near the flow divide at roosevelt island antarctica by using a thermomechanical model to predict radar stratigraphy 1 radar detected internal layering contained in some ice divides shows upwarped arches termed raymond bumps the distribution of their amplitude with height can date the onset of divide flow reflecting changes in the basin structure of the ice sheet the distribution depends on rheology surface geometry accumulation rate and temperature conway et al 1999 used an isothermal parameterized ice flow model to estimate a date of 3200 years b p with no error estimate for the onset of divide flow in roosevelt island ross ice shelf which they associated with grounding line retreat of the west antarctic ice sheet no other retreat dating exists for an area of the ross ice shelf distant from geological exposures employing a full thermomechanically coupled transient model we use a direct search to determine which ice rheology best fits the observed bump distribution and surface profile estimating the sensitivity of the dating to model parameters and exploring possible reasons for the observed asymmetry of the surface profile our main results are as follows 1 a best estimate of the date of grounding line retreat near roosevelt island is 3000 years b p bounded by 2300 and 4200 years b p 2 standard rheology power n 3 can only match the observed bump distribution for unrealistic isothermal models 3 for thermomechanically coupled models a high power n 4 in glen s law is required 4 low power rheologies and wind scouring cannot produce correct bump amplitude distributions 5 asymmetric accumulation cannot explain the observed asymmetry of the surface profile 6 modeling isochrones in flanking regions also indicates changes in flow around 3000 years b p

modelling ice divide dynamics by perturbation methods two dimensional isothermal non stationary cold glacier dynamics are analysed by perturbation methods when the ice creep is described by glen s flow law this approach allows us to model the ice divide dynamics separately form that of the glacier and to pose a simplified problem for the ice divide to order of the glacier aspect ratio all unknown characteristics near the ice divide can be searched for in an infinite layer with parallel boundaries whose thickness coincides with taht found by the shallow ice approximation the problem for the ice divide is stationary and does not depend on the ice and glacier characteristics such as a flow law constant ice thickness and accumulation rate at the ice divide the ice upper surface curvature is finite and the shallow ice approximation is inadequate

on the effects of anisotropic rheology on ice flow internal structure and the age depth relationship at ice divides we use numerical modeling with a full system stokes solver to elucidate the effects of nonlinear rheology and strain induced anisotropy on ice flow at ice divides we find that anisotropic rheology profoundly affects the shape of both isochrone layering and surface topography anisotropic effects cause the formation of a downward curving fold i e a syncline in isochrones in the lower central area beneath the ice divide when the resulting syncline is superimposed on the well known raymond anticline a double peaked raymond bump is formed furthermore to each side of the raymond bump flanking synclines are formed in addition anisotropic effects are found to give rise to a subtle concavity in the surface profile to both sides of the summit the lower center syncline the flanking synclines and the near summit surface concavity have all previously been observed in nature but hitherto no explanation for the genesis of these features has been given we compare modeling results with radiograms collected from fuchs ice piedmont and kealey ice rise antarctica good overall agreement is found in particular we are able to reproduce all observed qualitative features of surface geometry and internal layering by including and only by including the effects of induced nonlinear rheological anisotropy on flow rheological anisotropy has the potential to profoundly affect the age distribution with depth and caution must be exercised when estimating age of ice from ice cores with an isotropic model the occurrence of linear features parallel to the ridge of ice divides often seen in satellite imagery is indicative of long term stability rather than signs of ongoing ice divide migration as previously suggested such ice divides are ideal locations for extracting ice cores

the james ross island and the fletcher promontory ice core drilling projects following on from the successful project to recover an ice core to bedrock on berkner island similar drilling equipment and logistics were used on two further projects to recover ice cores to bedrock in the antarctic peninsula at james ross island a ship and helicopter supported project drilled to bedrock at 363m depth in a single season while a twin otter supported project drilled to bedrock at 654m depth again in a single season from fletcher promontory in both new projects drilling was from the surface with the infrastructure enclosed in a tent using an uncased partially fluid filled borehole

ice age storm trajectories inferred from radar stratigraphy at taylor dome antarctica the taylor dome ice core east antarctica provides a climate record that extends through the previous interglacial we use ice dynamics modeling and geochemical records to infer the local accumulation rate history a pronounced accumulation reduction occurred during the last glacial maximum lgm the spatial gradient of accumulation across the surface of taylor dome indicates the modern local prevailing storm direction radar internal layering shows that this gradient was reversed during the lgm suggesting a reorganization of synoptic weather systems occurred possibly linked to the advance and retreat of the west antarctic ice sheet

isochrones and isotherms beneath migrating ice divides we use simple numerical and analytical models of ice flow and heat flow to characterize the shape of isochrones and isotherms beneath moving ice divides both non linear ice flow and reduced accumulation wind scouring at a divide can cause reduced downward flow in a region about one ice thickness wide under a divide greater downward velocities on the flanks cause isochrones and isotherms to become arched at depth the magnitudes and shapes of these arches depend on the history of divide position arch amplitudes decrease by approximately e for each increase in migration rate of 3 5 times the accumulation rate the arches become asymmetric with steeper leading edges and more gentle trailing edges and the arch apex lags behind the divide isochrone and isotherm shapes can be used to infer past divide motions in advection dominated ice sheets isochrone shapes record a longer history of divide position than do isotherm shapes the opposite is true for diffusion dominated ice sheets in which a spatial array of ice temperature measurements might extend the recorded history of divide position

on the effects of divide migration along ridge flow and basal sliding on isochrones near an ice divide 1 radar layer geometry in divide areas is strongly influenced by the operation of the raymond effect which causes upwarping of the layers as a consequence of the nonlinear rheology of ice the detailed geometry of these layers is known to store a record of change in the cryosphere of local thinning and of the age of formation of the divide and has been surmised to provide information about lateral motion of divides such lateral motion can be caused by changes in flanking ice streams and the divide area thereby contains a record of ice stream dynamics it has also been suggested that a large perturbation of divide position will obliterate the cumulative effects of the operation of the raymond mechanism leading to the disappearance of raymond bumps since the raymond effect has a strong influence on the age depth relation in ice cores knowledge of whether its operation is localized leading to strong bump formation or distributed is crucial in the interpretation of ice cores the detailed evolution of ice divide radar layer geometry remains poorly understood employing a full thermomechanically coupled transient model we qualitatively explore the effect of divide migration on radar layer geometry certain qualitative features emerge which can be used to infer history of cryosphere change in particular in areas distant from the usual sites of geological dating there remains uncertainty about the influence of sliding on the operation of the raymond effect under certain conditions the existence of sliding can damp or eliminate the operation of the raymond effect if this is generally true then dating of ice divides may simply be a date for the freezing of the divide bottom we show that sliding does not necessarily eliminate the formation of bumps dates of divide formation are likely to be dates for the location of the ridge at a particular spot raymond bump evolution is weakened by flow along the ridge we explore quantitatively the strength of this effect using a scaling analysis to show that the weakening can efficiently be described by one parameter the ratio of along ridge slope to a measure of the across divide curvature

a comparison of radio echo sounding data and electrical conductivity of the grip ice core the depth of reflecting layers in arctic ice sheets has been determined by electromagnetic echo sounding using a varying distance between transmitter and receiver to determine the radar wave velocity the depth of the radar reflecting layers is compared with a profile of electrical conductivity measurements ecms from the greenland ice core project grip ice core in order to determine the velocity of the radar waves in the ice cap by using several reflecting layers it is possible to isolate the firn correction of the wave velocity and to estimate the accuracy of the calculated electromagnetic wave velocity the measured firn correction is compared with the correction calculated from the density profile and a comparison between the depth profiles of ecm and radar based on the corrected electromagnetic wave velocity is presented this profile shows that acid layers which originate from major volcanic eruptions show up as reflecting radar horizons

ice flow reorganization in west antarctica 2 5 kyr ago dated using radar derived englacial flow velocities we date a recent ice flow reorganization of an ice divide in the weddell sea sector west antarctica using a novel combination of inverse methods and ice penetrating radars we invert for two dimensional ice flow within an ice divide from data collected with a phase sensitive ice penetrating radar while accounting for the effect of firn on radar propagation and ice flow by comparing isochronal layers simulated using radar derived flow velocities with internal layers observed with an impulse radar we show that the divide s internal structure is not in a steady state but underwent a disturbance potentially implying a regional ice flow reorganization 2 5 1 8 2 9 kyr b p our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary these findings increase our knowledge of the glacial history of a region that lacks dated constraints on late holocene ice sheet retreat and provides a key target for models that reconstruct and predict ice sheet behavior

distortion of isochronous layers in ice revealed by ground penetrating radar in addition to measuring ice sheet thickness ground penetrating radar can be used to delineate reflections in ice sheets1 2 3 these reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow here we present ground penetrating radar data from fletcher promontory antarctica which show arches and troughs in isochronous ice layers to a depth of 100 m we demonstrate that the origin of these features can be determined by their growth with depth and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope one arch appears to be the result of a local anomaly in vertical strain rate its proximity to the ice divide width and growth with depth indicate that this arch is one of a class of features postulated by raymond4 but not previously shown to exist in the field such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice flow

flow at ice divide triple junctions 2 three dimensional views of isochrone architecture from ice penetrating radar surveys 1 ground based surveys of ice divide triple junctions in two antarctic ice rises the fletcher promontory and berkner island have been carried out using low frequency ice penetrating radars these surveys have focused on understanding the internal layer architecture at and around the triple junctions with a particular emphasis on understanding how the raymond effect operates a variety of features have been observed including raymond cupolas under the junctions bump stacks of varying amplitude with distance from the divide double bumps crooked stacks implying divide motion and the presence of ridges containing weak and strong bump stacks it was generally not possible to elucidate the three dimensional structure by correlation of pick crossovers so instead a least squares cross correlation technique was used which computes nominal age surfaces using radial basis function interpolation double bumps are absent from the central cupola of fletcher promontory permitting us to infer that the ice here has thinned by around 500 m in the past 5000 years estimates of the age scale suggest that there will be recoverable eemian ice a technique for inferring the velocity profile at divides using layer traces and assumed velocities in the flanks is presented

reflection of seismic surface waves at the northern apennines abstract seismic surface waves with periods of 15 to 20 s are reflected laterally at the northern apennines for earthquakes originated and recorded in the wider alpine area a few hundred kilometres north of the apennines we can directly observe late arrivals of reflected regional surface waves these have a characteristic polarization of particle motion and often dominate the intermediate period surface wave coda love waves are the most prominent coda arrivals while reflected rayleigh waves show smaller amplitudes and become clear only after rotation of the recordings to the reflection incidence direction we can track the development of the love wave reflection along a temporary broad band transect across the northern apennines indicating the location of the reflector near to the highest topography from spectral element simulations of three dimensional wave propagation we attribute the reflection to a continuous 270 km long offset in the crust mantle boundary under the mountain belt with vertical throw of 20 km thus supporting a deep crustal root under the outer side of the apennines fold and thrust belt and significant crustal thinning on the inner side amplitude reflection coefficients for near normal incidence can be estimated as 7 for both love and rayleigh waves

glaciological and climatic significance of hercules dome antarctica an optimal site for deep ice core drilling 1 we present glaciological and climatological characteristics of hercules dome antarctica 86 s 105 w which demonstrate its potential as a deep ice core site annual layering in d ratios from a 72 m ice core collected by the us itase 2002 traverse indicate accumulation rates of 0 16 0 20 m yr ice equivalent over the last 300 years age control from stratigraphy seen in the radio echo sounding data collected during the same traverse suggests a rate of 0 09 0 11 m yr averaged over the past 18 000 years ice stratigraphy also indicates that the ice divide position has been stable through at least this period comparison of satellite derived temperature anomalies with atmospheric reanalysis data show that the site is sensitive to the two dominant patterns of climate variability in the high latitude southern hemisphere climate proxy data from a deep ice core at hercules dome would be indicative of changes in pacific southern hemisphere climate variability and may provide new information on rapid climate change events in antarctica the sensitivity of the site and the combination of relatively high accumulation rates low temperatures mean annual 35 c to 40 c and simple ice flow suggest that hercules dome is an ideal site for a future deep ice core

decoding the dipstick thickness of siple dome west antarctica at the last glacial maximum ice thickness in west antarctica at the last glacial maximum lgm is poorly known yet is key information for understanding ice streams and interpreting ice cores although trim lines moraine limits and exposure age dating provide geologic constraints on ice thickness near the transantarctic mountains and in marie byrd land lack of exposed bedrock hampers traditional geologic methods in a central 2 3 10 6 km 2 region here we infer ice sheet thickness changes in the central ross sea embayment by using a tran sient ice flow model to find combinations of accumulation rate and ice sheet thickness histories that match the depth age relationship and the measured layer thickness pattern in the siple dome ice core after we reject unreasonable accumulation rate histories the remaining history pairs indicate thinning of 200 400 m since the lgm our estimate is lower than previous reconstructions that were constrained by geologic evidence from the transantarctic mountains and by marine data from the ross sea floor which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period low surface elevations in the central ross sea embayment during the lgm do not preclude thicker ice along its boundaries however if this grounded ice sheet came over 1000 km from interior west antarctica as is usually assumed then it had very low surface slope requiring a very slippery bed alternatively the grounded ross sea ice might have flowed from the transantarctic mountains and western marie byrd land

climate variability in west antarctica derived from annual accumulation rate records from itase firn ice cores thirteen annually resolved accumulation rate records covering the last 200 years from the pine island thwaites and ross drainage systems and the south pole are used to examine climate variability over west antarctica accumulation is controlled spatially by the topography of the ice sheet and temporally by changes in moisture transport and cyclonic activity a comparison of mean accumulation since 1970 at each site to the long term mean indicates an increase in accumulation for sites located in the western sector of the pine island thwaites drainage system accumulation is negatively associated with the southern oscillation index soi for sites near the ice divide and periods of sustained negative soi 1940 42 1991 95 correspond to above mean accumulation at most sites correlations of the accumulation rate records with sea level pressure slp and the soi suggest that accumulation near the ice divide and in the ross drainage system may be associated with the mid latitudes the post 1970 increase in accumulation coupled with strong slp accumulation rate correlations near the coast suggests recent intensification of cyclonic activity in the pine island thwaites drainage system

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

radar surveys of the rutford ice stream onset zone west antarctica indications of flow in stability we present 1 and 100 mhz ground based radar data from the onset region of rutford ice stream west antarctica which indicate the form and internal structure of isochrones in the flow parallel lines modelled isochrone patterns reproduce the gross pattern of the imaged near surface layers assuming steady state flow velocity from gps records and the current accumulation rate for the last 200 years we interpret this as indicating overall stability in flow in the onset region of rutford ice stream throughout this period however in the cross flow lines some local variability in accumulation is seen in areas close to the ice stream margin where a number of tributaries converge towards the ice stream onset zone episodic surface lowering events are observed followed by rapid fill episodes the fill events indicate deposition towards the northwest most likely generated by storm winds which blow at an oblique angle to ice flow more problematic is explaining the generation of episodic surface lowering in this area we speculate this may be due to changing ice flow direction in the complex tributary area of the onset zone a change in basal sediments or sedimentary landforms a change in basal melt rates or water supply or episodic lake drainage events in the fjord systems of the ellsworth subglacial highlands the study highlights the difficulty of assessing flow stability in the complex onset regions of west antarctic ice streams

1000 year ice core records from berkner island antarctica two medium depth ice cores were retrieved from berkner island by a joint project between the alfred wegener institut and the british antarctic survey in the 1994 95 field season a151mdeep core from the northern dome reinwarthho he of berkner island spans 700 years while a 181m deep core from the southern dome thyssenho he spans approximately 1200 years both cores display clear seasonal cycles in electrical conductivity measurements allowing dating by annual layer counting and the calculation of accumulation profiles stable isotope measurements both d18o and dd together with the accumulation data allow us to estimate changes in climate for most of the past millennium the data show multi decadal variability around a generally stable long term mean in addition a full suite of major chemistry measurements is available to define the history of aerosol deposition at these sites again there is little evidence that the chemistry of the sites has changed over the past six centuries finally we suggest that the southern dome with an ice thickness of 950m is an ideal site from which to gain a climate history of the late stages of the last glacial and the deglaciation for comparison with the records from the deep antarctic ice cores and with other intermediate depth cores such as taylor dome and siple dome

the role of crystal fabric in flow near an ice divide polycrystalline ice near an ice divide typically shows a crystal fabric crystal preferred orientation with c axes clustered vertically we explore the effect of this fabric on the large scale flow pattern near an ice divide we incorporate an analytical formulation for anisotropy into a non linear flow law within a finite element ice sheet flow model with four different depth profiles of crystal fabric we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than 25 for a steady state divide the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non linearity of ice flow a vertically oriented fabric tends to increase the size of the isochrone arch never to reduce it also non random fabric has little effect on the ice divide flow pattern when ice is modeled as a linear newtonian fluid finally when we use a crystal fabric profile that closely approximates the measured profile for siple dome west antarctica the model predicts concentrated bed parallel shearing 300 m above the bed

deglacial history of the west antarctic ice sheet in the weddell sea embayment constraints on past ice volume change the retreat history of the west antarctic ice sheet wais since the last glacial maximum is important for understanding the process of rapid deglaciation constraining models that seek to predict the future trajectory of the ice sheet and for estimating rates of sea level change here we report new glacial geologic data from the southwestern weddell sea embayment that demonstrate that this part of the wais was thinner than previously suggested and that there was progressive thinning of the ice sheet by 230 480 m since ca 15 ka we use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the weddell sea at the last glacial maximum the volume of this ice would have added between 1 4 and 2 0 m to postglacial sea level rise and would not have been sufficient to contribute significantly to meltwater pulse 1a a rapid rise in sea level 14 200 yr ago

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

towards bedmap himalayas development of an airborne ice sounding radar for glacier thickness surveys in high mountain asia the thickness of glaciers in high mountain asia hma is critical in determining when the ice reserve will be lost as these glaciers thin but is remarkably poorly known because very few measurements have been made through a series of ground based and airborne field tests we have adapted a low frequency ice penetrating radar developed originally for antarctic over snow surveys for deployment as a helicopter borne system to increase the number of measurements the manoeuvrability provided by helicopters and the ability of our system to detect glacier beds through thick dirty temperate ice makes it well suited to increase greatly the sample of measurements available for calibrating ice thickness models on the regional and global scale the bedmap himalayas radar survey system can reduce the uncertainty in present day ice volumes and therefore in projections of when hma s river catchments will lose this hydrological buffer against drought

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape

geometry motion and mass balance of dyer plateau antarctica geodetic surveying and ground based radar profiling were used to determine geometry and surface motion of the ice sheet on the dyer plateau antarctica in the vicinity of an ice core site on a local dome vertical strain measurements in the core hole constrain the depth profile of vertical velocity these geophysical measurements are used to analyze the profiles of density and annual layer thickness measured on the ice core to estimate the current mass balance of the ice column and the past history of accumulation rate consideration of horizontal and vertical mass flow divergence shows that the profiles of density and vertical velocity are not fully consistent with steady state mean density of the firn layer appears to be increasing which leads to the deduction of a small rate of mass increase 0 02 m a 1 ice equivalent thickness over the last 200a there has been a gradual increase in accumulation rate from about 0 46 m a 1 to 0 54 m a 1 ice equivalent thickness in recent time

influence of a non uniform velocity field on isochrone geometry along a steady flowline of an ice sheet the relationship between velocity field and isochrone geometry along a steady flowline of an ice sheet is examined the method is analytical and based upon the stream function and its vertically normalized form the normalized stream function nsf we show that the slope of the isochrones is the slope of the iso nsf lines plus a path term which is the cumulative result of the past trajectory of the ice particles we illustrate this path term in three different examples varying basal melting varying basal sliding weertman effect and varying velocity profile around a divide raymond effect the path term generally counteracts the slope of the iso nsf lines in the case of the raymond effect it can even lead to depressions surrounding the bumps if the transition from dome to flank velocity profile is sufficiently abrupt

radar internal layers from the greenland summit ice penetrating radar measurements made over the summit region of greenland show returns from internal layers which can be used to augment the interpretation of climate information from the two deep cores recently recovered from this area these reflecting surfaces believed to represent isochrones give information about the stress regime near the summit and may aid in a better calibration of the age depth scale between the two cores particularly in the lowest 10 of ice thickness where there is currently disagreement the approximate depth at which internal echoes become discontinuous corresponds with the observations of steep inclinations and overturned folds on the scale of centimeters in the core samples however the deepest internal layers which can be distinguished in the profiles place constraints on the scale and location of high angle or overturned folds

modelling of kealey ice rise antarctica reveals stable ice flow conditions in east ellsworth land over millennia flow at ice divides their shape size and internal structure depend not only on local conditions but also on the flow regimes and past histories of the surrounding ice masses here we use field data from kealey ice rise ellsworth land west antarctica in combination with flow modelling to investigate any possible signs of transients in the flow of the surrounding ice masses kealey ice rise shows linear surface features running parallel to its ridge in satellite imagery and a conspicuous layering in the ground penetrating radar data known as double peaked raymond bumps through numerical modelling by using an anisotropic full stokes thermomechanical flow solver we analyse the evolution of kealey ice rise and the timescales involved we conclude that the features observed in the stratigraphy of kealey ice rise require at least 3 ka of near stationary flow conditions however we cannot exclude the possibility of a recent flow reorganization in the last century we stress that the signs of stationary flow in radar stratigraphy observed in kealey ice rise have been observed in other ice divides in the east ellsworth land area suggesting stationary flow conditions over a millennial timescale in the region

recent thinning and migration of the western divide central west antarctica antarctica is currently thinning 0 08 m a 1 and migrating toward the ross sea at 10 m a 1 the asymmetric pattern of thickness change across the divide is not caused by changes in the accumulation gradient but rather by dynamical thinning that is stronger in the amundsen sea sector than in the ross sea sector available geological and glaciological data indicate that this pattern of thinning has persisted for at least two millennia with increased asymmetry likely over the past few centuries our data however are not sufficient to determine whether the present day migration of the western divide is a response to long term millennial forcing shorter term centennial forcing or both citation conway h and l a rasmussen 2009 recent thinning and migration of the western divide central west antarctica geophys res lett 36 l12502 doi 10 1029 2009gl038072

characterizing the glaciological conditions at halvfarryggen ice dome dronning maud land antarctica we present a comprehensive approach including field data remote sensing and an anisotropic ice flow model to characterize halvfarryggen ice dome in coastal dronning maud land antarctica this is a potential drill site for the international partnerships in ice core sciences which has identified the need for ice cores covering atmospheric conditions during the last few millennia we derive the surface topography the ice stratigraphy from radar data and accumulation rates which vary from 400 to 1670 kg m 2 a 1 due to preferred wind directions and changing surface slope the stratigraphy shows anticlines and synclines beneath the divides we transfer dansgaard johnsen age depth scales from the flanks along isochrones to the divide in the upper 20 50 of the ice thickness and show that they compare well with the results of a full stokes anisotropic ice flow model which predicts 1 11 ka bp ice at 90 of the ice thickness 2 a temporally stable divide for at least 2700 4500 years 3 basal temperatures below the melting point 12 degrees c to 5 degrees c and 4 a highly developed crystal orientation fabric cof we suggest drilling into the apices of the deep anticlines providing a good compromise between record length and temporal resolution and also facilitating studies of the interplay of anisotropic cof and ice flow

analytical solutions for the effect of topography accumulation rate and lateral flow divergence on isochrone layer geometry the effect of spatial variations in ice thickness accumulation rate and lateral flow divergence on radar detected isochrone geometry in ice sheets is computed using an analytical method under assumptions of a steady state ice sheet geometry a steady state accumulation pattern and a horizontally uniform velocity shape function by using a new coordinate transform we show that the slope of the isochrones with a normalized vertical coordinate depends on three terms a principal term which determines the sign of the slope and two scale factors which can modify only the amplitude of the slope the principal term depends only on a local characteristic time ice thickness divided by accumulation rate minus melting rate between the initial and final positions of the ice particle for plug flow only the initial and final values have an influence further applications are a demonstration of how the vertical velocity profile can be deduced from sharp changes in isochrone slopes induced by abrupt steps in bedrock or mass balance along the ice flow we also demonstrate ways the new coordinate system may be used to test the accuracy of numerical flow models