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

abrupt change of antarctic moisture origin at the end of termination ii the deuterium excess of polar ice cores documents past changes in evaporation conditions and moisture origin new data obtained from the european project for ice coring in antarctica dome c east antarctic ice core provide new insights on the sequence of events involved in termination ii the transition between the penultimate glacial and interglacial periods this termination is marked by a north south seesaw behavior with first a slow methane concentration rise associated with a strong antarctic temperature warming and a slow deuterium excess rise this first step is followed by an abrupt north atlantic warming an abrupt resumption of the east asian summer monsoon a sharp methane rise and a co 2 overshoot which coincide within dating uncertainties with the end of antarctic optimum here we show that this second phase is marked by a very sharp dome c centennial deuterium excess rise revealing abrupt reorganization of atmospheric circulation in the southern indian ocean sector

distribution of stable isotopes in surface snow along the route of the 1990 international trans antarctica expedition this paper presents the distri bution of average od and 6180 values for the top i m of surface snow for a large area of antarctica the samples were collected on the 1990 internatio nal trans antarctica expedition which crossed the continent from the northern tip of the antarctic penins ula to mirny station the empirical rel ationships between the isotopes temperature and elevation are computed the slopes of od with respect to the surface temperature for the segments west patriot hills v ostok and east ofvostok komso molskaya mirny are not signifi cantly different to that obse rved between dumont d urville and dome c lorius and merlivat 1977 which are often used to inte rpret deep ice core isotopic profiles there is however a noti ceable shift between the two regression lines

effects of firn ventilation on isotopic exchange a new model of isotopic diffusion in the upper few meters of firn tracks the isotopic composition of both the ice matrix and the pore space vapor through time in two dimensions stable isotopes in the vapor phase move through the firn by diffusion along concentration gradients and by advection wind driven ventilation carries atmospheric water vapor into the firn where it mixes with existing pore space vapor unlike previous models our model allows disequilibrium between pore space vapor and the surrounding snow grains we also calculate the isotopic effects of ventilation driven sublimation and condensation in the firn model predictions of isotopic diffusion in firn compare favorably with existing diffusion models model results quantify what other investigators have suggested isotopic change in the upper few meters is more rapid than can be explained by the whillans and grootes 1985 model isotopic equilibration with atmospheric vapor is an important component of post depositional isotopic change and ventilation enhances isotopic exchange by creating regions of relatively rapid sublimation and condensation in the firn

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

spatial and temporal variability in isotope composition of recent snow in the vicinity of vostok station antarctica implications for ice core record interpretation continuous detailedisotope ddandd 18 o profileswereobtainedfrom eight snow pits dug in the vicinity of vostok station antarctica during the period1984 2000 in addition snow samples taken along the 1km long accumulation stake profile were measured to determine spatial variability in isotope composition of recent snow the stacked dd time series spanning the last 55years shows only weak correlation with the mean annual air temperature recorded at vostok station significant oscillations of both snow accumulation and snow isotope composition with the periods 2 5 5 20 and possibly 10 2 years observed at single points are interpreted in terms of drift of snow accumulationwaves of various scales onthe surface of the ice sheet

basal melt at northgrip modeled from borehole ice core and radio echo sounder observations from temperature measurements down through the 3001 m deep borehole at the north greenland icecore project northgrip drill site it is now clear that the ice at the base 3080 m below the surface is at the pressure melting point this is supported by the measurements on the ice core where the annual layer thicknesses show there is bottom melting at the site and upstream from the borehole surface velocity measurements internal radio echo layers borehole and ice core data are used to constrain a time dependent flow model simulating flow along the north northwest trending ice ridge flow line leading to the northgrip site also time dependent melt rates along the flowline are calculated with a heat flow model the results show the geothermal heat flow varies from 50 to 200 mw m 2 along the 100km section of the modeled flowline the melt rate at the northgrip site is 0 75 cm a 1 but the deep ice in the northgrip core originated 50 km upstream and has experienced melt rates as high as 1 1 cm a 1

temperature gradient and initial snow density as controlling factors in the formation and structure of hard depth hoar we investigate how temperature gradient and initial density influence depth hoar growth in snow and seek to better define the range of conditions under which cohesive or hard depth hoar forms samples of 400 kg m 3 sieved snow were exposed to temperature gradients of 20 80 c m 1 and samples of four different densities were exposed to a 40 c m temperature gradient following exposure to temperature gradients penetrometer tests were made on samples to determine the presence of solid and or hard depth hoar grain bond orientation was analyzed in section planes by two dimensional stereological techniques where hard depth hoar developed results indicate that hard cohesive depth hoar forms from rounded grain snow having a density of 400 kg m 3 or greater following exposure to a temperature gradient of 20 c m 1 or greater hard depth hoar appears to consist of solid type depth hoar grains connected by necks with vertically preferred directions of grain elongation and organization of grain to grain chains this work corroborates atikaya s 1974 results but extends his observation of formation of hard depth hoar to weaker temperature gradients for high density snow our results also indicate that hard depth hoar is composed of faceted solid type anhedral grains

eemian interglacial reconstructed from a greenland folded ice core efforts to extract a greenland ice core with a complete record of the eemian interglacial 130 000 to 115 000 years ago have until now been unsuccessful the response of the greenland ice sheet to the warmer than present climate of the eemian has thus remained unclear here we present the new north greenland eemian ice drilling neem ice core and show only a modest ice sheet response to the strong warming in the early eemian we reconstructed the eemian record from folded ice using globally homogeneous parameters known from dated greenland and antarctic ice core records on the basis of water stable isotopes neem surface temperatures after the onset of the eemian 126 000 years ago peaked at 8 4 degrees celsius above the mean of the past millennium followed by a gradual cooling that was probably driven by the decreasing summer insolation between 128 000 and 122 000 years ago the thickness of the northwest greenland ice sheet decreased by 400 250 metres reaching surface elevations 122 000 years ago of 130 300 metres lower than the present extensive surface melt occurred at the neem site during the eemian a phenomenon witnessed when melt layers formed again at neem during the exceptional heat of july 2012 with additional warming surface melt might become more common in the future

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

temperature measurements and heat transfer in near surface snow at the south pole to study near surface heat flow on the antarctic ice sheet snow temperatures were measured at south pole station to a depth of 3 m at 15 min intervals during most of 1992 solar heating and water vapor transport were negligible during the 6 month winter as was inter grain net thermal radiation leaving conduction as the dominant heat transport mechanism the rate of temperature change at depth over 15 min intervals was smaller than that at the surface by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth a finite difference model with conduction as the only heat transfer mechanism and measured temperatures as the upper and lower boundary conditions was applied to foursets of three thermistors each the thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor the thermal conductivity obtained at shallow depths above 40 cm was lower than that given by existing parameterizations based on density probably because the snow grains were freshly deposited cold and poorly bonded a model using only vertical conduction explains on average 87 ofthe observed 15 min temperature changes at less than 60 cm depth and 92 below 60 cm the difference between modeled andmeasured temperature changes decreased with depth the discrepancies between model and observation correlated more strongly with the air snow temperature difference than with the product of that difference with the square of the wind speed suggesting that the residual errors are due more to non vertical conduction and to sub grid scale variabilis of the conductivity than to windpumping the residual heating rate not explained by the model of vertical conduction exceeds 0 2 w m 3 only in the top 60 cm of the near surface snow

little ice age cold interval in west antarctica evidence from borehole temperature at the west antarctic ice sheet wais divide 1 the largest climate anomaly of the last 1000 years in the northern hemisphere was the little ice age lia from 1400 1850 c e but little is known about the signature of this event in the southern hemisphere especially in antarctica we present temperature data from a 300 m borehole at the west antarctic ice sheet wais divide results show that wais divide was colder than the last 1000 year average from 1300 to 1800 c e the temperature in the time period 1400 1800 c e was on average 0 52 0 28 c colder than the last 100 year average this amplitude is about half of that seen at greenland summit grip this result is consistent with the idea that the lia was a global event probably caused by a change in solar and volcanic forcing and was not simply a seesaw type redistribution of heat between the hemispheres as would be predicted by some ocean circulation hypotheses the difference in the magnitude of the lia between greenland and west antarctica suggests that the feedbacks amplifying the radiative forcing may not operate in the same way in both regions

snow metamorphism under alternating temperature gradients morphology and recrystallization in surface snow 1 the composition of the lower atmospheric boundary layer can be strongly influenced by snow photochemistry surface snow where air snow interactions take place is viewed as slowly changing quasi isothermal snow implied by the observation of mainly rounded snow crystals the role of snow for photochemistry is therefore expected to be purely geometric however large temperature gradients are often observed in these layers which would imply high recrystallization and faceting in controlled laboratory experiments we showed that temperature gradients on the order of 100 k m 1 do not lead necessarily to faceting if their sign changes with a daily cycle the shape of snow crystals does not reflect necessarily the metamorphic process with time lapse x ray tomography recrystallization rates as high as 60 of the total ice mass were observed during 12 hours the high recrystallization rates in apparently isothermal snow not only contradict the current understanding of snow metamorphism they also might influence chemical air snow interactions

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

variable relationship between accumulation and temperature in west antarctica for the past 31 000 years the antarctic contribution to sea level is a balance between ice loss along the margin and accumulation in the interior accumulation records for the past few decades are noisy and show inconsistent relationships with temperature we investigate the relationship between accumulation and temperature for the past 31 ka using high resolution records from the west antarctic ice sheet wais divide ice core in west antarctica although the glacial interglacial increases result in high correlation and moderate sensitivity for the full record the relationship shows considerable variability through time with high correlation and high sensitivity for the 0 8 ka period but no correlation for the 8 15 ka period this contrasts with a general circulation model simulation which shows homogeneous sensitivities between temperature and accumulation across the entire time period these results suggest that variations in atmospheric circulation are an important driver of antarctic accumulation but they are not adequately captured in model simulations model based projections of future antarctic accumulation and its impact on sea level should be treated with caution

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

climate archives from 90 to 250 ka in horizontal and vertical ice cores from the allan hills blue ice area antarctica abstract terrestrial meteorite ages indicate that some ice at the allan hills blue ice area ah bia may be as old as 2 2 ma as such ice from the ah bia could potentially be used to extend the ice core record of paleoclimate beyond 800 ka we collected samples from 5 to 10 cm depth along a 5 km transect through the main icefield and drilled a 225 m ice core s27 at the midpoint of the transect to develop the climate archive of the ah bia stable water isotope measurements d of the surface chips and of ice core s27 yield comparable signals indicating that the climate record has not been significantly altered in the surface ice measurements of 40 ar atm and 18 o atm taken from ice core s27 and eight additional shallow ice cores constrain the age of the ice to approximately 90 250 ka our findings provide a framework around which future investigations of potentially older ice in the ah bia could be based

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

ice temperature and high geothermal flux at siple dome west antarctica from borehole measurements a verlical temperature profile through the west antarctic ice sheet wais at the summit of siple dome reveals an elevated geothermal flux this could be the root cause for the existence of a dynamic ice stream system in the wais siple dome is still frozen on its bed but adjacent ice streams have bed temperatures at the pressure melting point of ice although present day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice indirect ellbcts like a rapid disintegration of the floating ice shelves or additional melt water input at the surface could give rise to speed up of the ice streams with an ensuing rise in sea level ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration within a relatively short period of time of at least part of the wais

isotopic diffusion in cold snow and firn molecular mixing in cold snow and firn is ordinarily controlled by vapor diffusion diffusion through or along the ice matrix is slow whereas diffusion from the air to within separate grains is comparatively rapid thus mixing occurs in the vapor phase and exchange with the local ice involves all the water molecules a quantification of these concepts is applied to the diffusion of oxygen isotopic depth profiles in southern greenland and at dome c antarctica it successfully describes the smoothing of measured profiles the smoothing rate is strongly dependent on temperature and density of the firn

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

cold conditions in antarctica during the little ice age implications for abrupt climate change mechanisms abstract the little ice age lia is one of the most prominent climate shifts in the past 5000 yrs it has been suggested that the lia might be the most recent of the dansgaard oeschger events which are better known as abrupt large scale climate oscillations during the last glacial period if the case then according to broecker 2000a 2000b antarctica should have warmed during the lia when the northern hemisphere was cold here we present new data from the ross sea antarctica that indicates surface temperatures were 2 c colder during the lia with colder sea surface temperatures in the southern ocean and or increased sea ice extent stronger katabatic winds and decreased snow accumulation whilst we find there was large spatial and temporal variability overall antarctica was cooler and stormier during the lia although temperatures have warmed since the termination of the lia atmospheric circulation strength has remained at the same elevated level we conclude that the lia was either caused by alternative forcings or that the sea saw mechanism operates differently during warm periods

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

meteorological conditions over antarctic blue ice areas and their influence on the local surface mass balance this paper addresses the causes of the prevailing meteorological conditions observed over an antarctic blue ice area and their effect on the surface mass balance over blue ice areas net accumulation is zero and ablation occurs mainly through sublimation sublimation rates are much higher than over adjacent snowfields the meteorological conditions favourable for high sublimation rates warm dry and gusty are due to the specific orographic setting of this blue ice area with usually a steep upwind mountainous slope causing strong adiabatic heating diabatic warming due to radiation and entrainment of warm air from aloft into the boundary layer augment the warming the prevailing warm dry conditions explain roughly 50 of the difference in sublimation and the different characteristics of blue ice mainly its lower albedo the other 50 most of the annual sublimation 70 takes place during the short summer mainly in daytime with winter ablation being restricted to occasional warm dry fohn like events the additional moisture is effectively removed by entrainment and horizontal advection which are maximum over the blue ice area low frequency turbulent motions induced by the upwind mountains enhance the vertical turbulent transports strong gusts and high peak wind speeds over blue ice areas cause high potential snowdrift transports which can easily remove the total precipitation thereby maintaining zero accumulation

solar heating rates and temperature profiles in antarctic snow and ice observations of temperature maxima at about 10 cm depth in cold antarctic snow during summer have previously been explained by proposing that solar heating is distributed with depth whereas thermal infrared cooling is localized at the surface the solid state greenhouse an increase in temperature from the surface to 10 cm depth t 4 k found by rusin 1961 on the antarctic plateau was successfully reproduced by schlatter 1972 in a combined radiative transfer and heat transfer model

physically based modeling of atmosphere to snow to firn transfer of h2o2 at south pole quantitative interpretation of ice core chemical records requires a detailed understanding of the transfer processes that relate atmospheric concentrations to those in the snow firn and ice a unique 2 year set of year round surface snow samples at south pole and snow pits with associated accumulation histories were used to test a physically based model for atmosphere to firn transfer of h2o2 the model which extends our previous transfer modeling at south pole into the snowpack is based on the advection dispersion equation and spherical diffusion within representative snow grains required physical characteristics of the snowpack such as snow temperature and ventilation were estimated independently using established physical models the surface snow samples and related model simulations show that there is a repeatable annual cycle in h2o2 in the surface snow at south pole it peaks in early spring and surface snow concentration is primarily determined by atmospheric concentration and temperature with some dependence on grain size the snow pits and associated model simulations point out the importance of accumulation timing and annual accumulation rate in understanding the deposition and preservation of h2o2 and 18o at south pole long term snowpack simulations suggest that the firn continues to lose h2o2 to the atmosphere for at least 10 12 years 3 m after burial at current south pole temperatures and accumulation rates

spatial variations in heat at the base of the antarctic ice sheet from analysis of the thermal regime above subglacial lakes antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure melting point we have used a one dimensional vertical heat transfer equation to determine theoretical temperature values for the ice sheet base above 77 subglacial lakes identified from airborne radio echo sounding data covering 50 of antarctica variations in our temperature results to below the pressure melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from a internal ice deformation and b basal sliding in the thermal model our results indicate that when the geothermal heat flux is set at 54 mw m 2 the ice sheet base above 70 of the known antarctic subglacial lakes is calculated to be at the pressure melting value these lakes are located mainly around dome c ridge b and vostok station for the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide using the same thermal model loss of heat due to vertical advection is calculated to be relatively high in such regions if the ice sheet base is at the pressure melting point heat lost due to vertical advection must be supplemented by heat from other sources for the three lakes beneath terre adelie and george v land for instance the basal thermal gradient calculated to produce pressure melting at the ice sheet base is equivalent to 1 5 2 times the value obtained when 54 mw m 2 of geothermal heat is used as the sole basal thermal component we suggest that as distance from the ice divide increases so too does the amount of heat due to internal ice deformation and basal sliding moreover by considering the ice sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline we demonstrate empirically that the heat due to these horizontal ice motion terms varies pseudo exponentially with distance from the ice divide the location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large scale basal sliding

carbonyl sulfide hydrolysis in antarctic ice cores and an atmospheric history for the last 8000 years 2014 american geophysical union all rights reserved carbonyl sulfide cos was measured in antarctic ice core samples from the byrd siple dome taylor dome and west antarctic ice sheet divide sites covering the last 8000 years of the holocene cos levels decrease downcore in most of these ice cores the magnitude of the downcore trends varies among the different ice cores and is related to the thermal histories of the ice sheet at each site we hypothesize that this is due to the temperature dependent hydrolysis of cos that occurs in situ we use a one dimensional ice flow and heat flux model to infer temperature histories for the ice core samples from different sites and empirically determine the kinetic parameters for cos hydrolysis we estimate e folding lifetimes for cos hydrolysis ranging from 102 years to 106 years over a temperature range of 0 c to 50 c the reaction kinetics are used to estimate and correct for the in situ cos loss allowing us to reconstruct paleoatmospheric cos trends during the mid to late holocene the results suggest a slow long term increase in atmospheric cos that may have started as early as 5000 years ago given that the largest term in the cos budget is uptake by terrestrial plants this could indicate a decline in terrestrial productivity during the late holocene

isotopic diffusion in polar firn implications for interpretation of seasonal climate parameters in ice core records with emphasis on central greenland if it were possible to properly extract seasonal information from ice core isotopic records paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice core proxy records it is widely recognized however that the diffusional smoothing of the seasonal record makes a proper extraction very difficult in this paper we examine the extent to which seasonal information specifically the amplitude and shape of the seasonal cycle is irrecoverably destroyed by diffusion in the firn first we show that isotopic diffusion firn is reasonably well understood we do this by showing that a slightly modified version of the whillans and grootes 1985 theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the gisp2 site though a small adjustment to one parameter significantly improves the prediction further we calculate the amplitude decay at various other ice core sites and show that these predictions compare favorably with published data from south pole and locations in southern and central greenland and the antarctic peninsula we then present numerical experiments wherein synthetic ice core records are created diffused sampled reconstituted and compared to the original these show that alter diffusive mixing in the entire fini column seasonal amplitudes can be reconstructed to within about 20 error in central greenland but that all information about sub annual signals is permanently lost there furthermore most of the error in the amplitude reconstructions is due to the unknowable variations in the sub annual signal finally we explore how these results can be applied to other locations and suggest that dye 3 has a high potential for meaningful seasonal reconstructions while siple dome has no potential at all an optimal ice core site for seasonal reconstructions has a high accumulation rate and a low temperature

snow precipitation at four ice core sites in east antarctica provenance seasonality and blocking factors snow precipitation is the primary mass input to the antarctic ice sheet and is one of the most direct climatic indicators with important implications for paleo climatic reconstruction from ice cores provenance of precipitation and the dynamic conditions that force these precipitation events at four deep ice core sites dome c law dome talos dome and taylor dome in east ant arctica were analysed with air mass back trajectories cal culated using the lagrangian model and the mean composite data for precipitation geopotential height and wind speed field data from the european centre for med ium range weather forecast from 1980 to 2001 on an annual basis back trajectories showed that the atlantic indian and ross pacific oceans were the main provenances of precipitation in wilkes land 80 and victoria land 40 respectively whereas the greatest influence of the ice sheet was on the interior near the vostok site 80 and in the southwest ross sea 50 an effect that decreased towards the coast and along the antarctic slope victoria land received snowfall atypically with respect to other antarctica areas in terms of pathway eastern instead of western seasonality summer instead of winter and velocity old air age geopotential height patterns at 500 hpa at low 10 days and high 2 6 days frequen cies during snowfall cycles at two core sites showed large positive anomalies at low frequencies developing in the tasman sea eastern indian ocean at higher latitudes 60 70s than normal this could be considered part of an atmospheric blocking event with transient eddies acting to decelerate westerlies in a split region area and accelerate the flow on the flanks of the low frequency positive anomalies

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

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

mass balance and accumulation rate across siple dome west antarctica snow accumulation rates and rates of ice thickness change mass balance are studied at several sites on siple dome west antarctica accumulation rates are derived from analyses of grossbeta radioactivity in shallow firn cores located along a 60 km transect spanning both flanks and the crest of the dome there is a north south gradient in snow accumulation rate across the dome that is consistent with earlier radar mappingof internalstratigraphy orographicprocessesprobablycontrolthisdistribution mass balance is inferred from the difference between global positioning system gps derived vertical velocities and snow accumulation rates for sites close to the firn core locations results indicatethatthere is virtually no netthicknesschangeat fourof thefive sites the exception is at the northernmost site where a small amount of thinning is detected that appearstobe inconsistentwith other studies a possiblecause ofthisanom alousthinning is recent retreatof the grounding line of ice stream d

space and time variation of δ18o and δd in antarctic precipitation revisited 1 to better understand the variations over time of precipitation water isotopes measured in polar ice cores we have developed an intermediate complexity model icm of atmospheric water vapor transport and associated isotopic distillation the model builds directly from earlier work by d fisher and m hendricks and is calibrated against the measured modern spatial distribution of d and deuterium excess d model improvements include a correction to the equation governing advective transport which solves a major puzzle arising from the earlier work the new model shows isotopic data to be consistent with dominance of eddy diffusive transport at high latitudes model experiments are used to show that a wide variety of climate changes in subtropical source regions can affect antarctic d and that such d changes are consistently anticorrelated with changes of d magnitudes of d d are 1 to 5 and are much higher at the ice sheet margin than in the interior of the continent changes in the relative dominance of diffusive and advective transport are also shown to cause anticorrelated changes of similar magnitude isotopic sensitivities to temperature change d t are shown to also vary spatially with low values in marginal zones and higher values inland effects of ocean surface relative humidity on antarctic d are shown to be nearly uniform across the continent in contrast to earlier results given by petit et al 1991 but quantitatively small the method used by vimeux et al 2001 for folding humidity effects into source temperature effects is supported we draw attention to a pervasive misapplication of marine composition corrections to antarctic d records arguments are made in support of simple interpretations of calibrated ice core isotopic time series in terms of temperature changes despite the potential complexities in the system

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

evidence for late pleistocene thinning of siple dome west antarctica 1 we use a two dimensional full stress thermomechanical flow band model to investigate late pleistocene and holocene histories of ice thickness and ice flow at siple dome in west antarctica measurements obtained from an ice core and borehole near the ice divide and measurements of radar detected internal stratigraphy are used as targets for modeling models that account for the climate history accumulation rate and temperature alone yield a poor match to observations histories of ice dynamics ice thickness ice divide location and ice flow enhancement also need to be included our favored model includes 350 m of thinning at siple dome between 15 000 and 14 000 years ago onset of divide flow 3000 years ago ongoing divide migration starting 2500 years ago and evolution of ice fabric over the past 30 000 years the timing of thinning is earlier than that inferred from geologic evidence from the margins but is coincident with a 20 m rise in global sea level about 14 500 years ago meltwater pulse 1a this opens the possibility that thinning was a result of increased activity of the ice streams in the late pleistocene forced by meltwater pulse 1a the contribution to meltwater pulse 1a as a result of this thinning was likely small

a seasonal deuterium excess signal at law dome coastal eastern antarctica a southern ocean signature the snow isotopic composition 18o and d of two shallow cores from the high accumulation summit region of law dome east antarctica was measured at monthly resolution over the 1980 1992 period while the 18o or d signals clearly reflect the local temperature cycle the deuterium excess d d 8 18o is shifted with respect to 18o cycle by a 4 months lag interpretation of this phase lag is investigated using both an atmospheric general circulation model agcm which includes the water isotopic cycles and a simple isotopic model which better describes the microphysical processes within the cloud using this dual approach we show that the seasonality of 18o and d at law dome summit results from a combination of the southern ocean temperature cycle shifted by 2 3 months with respect to the local insolation and seasonal moisture origin changes due to a strong contribution of the local ocean when ice free both approaches are consistent with a dominant temperate to subtropical moisture origin we thus demonstrate from our present day subseasonal study that the record of d in the dome summit south dss deep ice core represents a potential tool for identifying changes in southern ocean temperatures and or sea ice cover at the scale of the past thousand years

a new volcanic province an inventory of subglacial volcanoes in west antarctica abstract the west antarctic ice sheet overlies the west antarctic rift system about which due to the comprehensive ice cover we have only limited and sporadic knowledge of volcanic activity and its extent improving our understanding of subglacial volcanic activity across the province is important both for helping to constrain how volcanism and rifting may have influenced ice sheet growth and decay over previous glacial cycles and in light of concerns over whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the west antarctic ice sheet here we use ice sheet bed elevation data to locate individual conical edifices protruding upwards into the ice across west antarctica and we propose that these edifices represent subglacial volcanoes we used aeromagnetic aerogravity satellite imagery and databases of confirmed volcanoes to support this interpretation the overall result presented here constitutes a first inventory of west antarctica9s subglacial volcanism we identified 138 volcanoes 91 of which have not previously been identified and which are widely distributed throughout the deep basins of west antarctica but are especially concentrated and orientated along the 3000 km central axis of the west antarctic rift system

abrupt climate change around 22 ka on the siple coast of antarctica a new ice core from siple dome antarctica suggests the surface temperature increased by 6 c in just several decades at approximately 22 ka bp this abrupt change did not occur 500 km away in the byrd ice core or in climate proxy records in the siple dome core indicative of the mid latitude pacific this demonstrates there was significant spatial heterogeneity in the response of the antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in antarctica

extreme snow metamorphism in the allan hills antarctica as an analogue for glacial conditions with implications for stable isotope composition understanding physical processes in near zero accumulation areas can help us to better understand polar ice core records particularly during periods when accumulation rates were lower than today we report measurements from a 5 m firn core from the allan hills antarctica which include physical properties using computer tomography stable isotope ratios d and 18 o and 210 pb activity the core shows a highly metamorphosed firn with homogeneous and stable structure but with discrete layers near the surface the observed firn structure is caused by a combination of unique depositional and post depositional processes the irregular d and 18 o signal does not follow the stratigraphic sequence and implies post depositional modification caused by microscopic pressure gradients in the firn that can result from either forced ventilation over rough surfaces in the presence of wind or alternating temperature gradients between the firn and the atmosphere our results also indicate impact snow deposition under high winds and with a high initial density and air exchange between the atmosphere and the snowpack 210 pb activity below 0 3 m falls below the detection limit implying that most of the core is more than 100 years old we conclude that the allan hills record provides a unique opportunity to investigate important processes that would have affected ice core records from glacial periods

extreme firn metamorphism impact of decades of vapor transport on near surface firn at a low accumulation glazed site on the east antarctic plateau snow and firn properties control the transport of vapor gases and water between the atmosphere and the underlying strata an understanding of this transport and the properties that control it is important for predicting air snow transfer of chemical species and for interpreting ice cores remote sensing images of east antarctica show large areas of alternating light and dark bands these low amplitude long wavelength features have glazed downwind faces and rough upwind faces and are called megadunes the first linked measurements of the permeability and the associated microstructure for a glazed area within a well defined megadune area are reported in this paper permeability and density were measured along with grain scale properties derived from digital image processing of preserved thick sections at this cold low accumulation glazed site a clear layering pattern exists in the top meter the firn density ranges from 0 24 to 0 50 g cm 3 permeability measurements range from 50 x 10 10 to 200 x 10 10 2 several times greater than corresponding profiles from warmer higher accumulation sites like siple dome antarctica it is shown that buoyancy driven natural convection may be important in post depositional processes in very cold low accumulation sites like this

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

new insights on chinese cave δ18o records and their paleoclimatic significance abstract in the last two decades stalagmite oxygen isotope 18o records have been widely used to study the evolution of the paleomonsoon nevertheless interpreting cave 18o records in eastern monsoonal china as an indicator of east asian summer monsoon easm rainfall has been intensively debated this study provides new insights into chinese cave 18o records and their paleoclimatic significance based on recent research progress the following points are highlighted 1 chinese cave 18o records do not reflect changes in summer rainfall throughout the whole of eastern monsoonal china which is evident by comparing monitored summer rainfall amount in eastern china and an integrated chinese cave 18osyn proxy 2 chinese cave 18o records can document millennial scale easm events due to the sensitive response to large scale atmospheric circulation changes 3 during the early holocene before 7 ka a decoupled relationship between chinese cave 18o records and easm rainfall as represented by precipitation in north china is attributed to the reduction of rainfall in north china due to the influence of remnant melting ice sheets in the northern high latitude region 4 when the ice sheet boundary conditions were stable and ice sheets were small after 7 ka there is an indirect correlation between the chinese cave 18o and rainfall changes in north china 5 the first continuous speleothem 18o record spanning the entire holocene is synthesized for the core region of the indian summer monsoon ism domain comparison of this integrated ism proxy with chinese cave 18o records reveals that in china cave 18o is primarily a signal of changes in the tropical monsoon and related large scale circulation the coupled relationship between rainfall in north china and tropical monsoon ism rainfall is attributed to the response of downstream circulation to changes in circulation upstream while their decoupled relationship is due to the possible effects of northern high latitude ice sheets on reducing rainfall in north china 6 due to the imprecise definition of monsoon intensity we suggest that cave 18o can be used as an easm proxy if monsoon intensity is defined by changes in atmospheric circulation

meteoritic event recorded in antarctic ice during systematic sampling of volcanic ash tephra layers at a well known antarctic meteorite collection site the allan hills main ice field a band of unusually dark and rounded many spheroidal particles was discovered this debris layer bit 58 extends parallel to the stratigraphy of the ice established from the tephra bands apparently marking a single depositional event the shapes internal texture major element composition and levels of cosmogenic nuclides of particles from within bit 58 all strongly suggest that this material represents ablation debris from the passage of a large h group ordinary chondrite preliminary cosmogenic isotope dating suggests an age of 2 8 ma implying that the east antarctic ice sheet has been stable since that time the relationship of the bit 58 layer to known impact events is not clear

snow isotopic content change by sublimation we present results from cold laboratory observations of changes in isotopic d 18 o and dd content by sublimation in snow and ice samples under nearly isothermal conditions the results show large increases in observed d 18 o and dd in snow samples within several centimeters of the surface they contradict the assumption of a non changing isotopic content due to layer by layer transport mechanisms driven by sublimation desublimation processes the data also do not support the idea that isotopic changes of snow and firn are limited by the possibility that the ice matrix incorporates the atmospheric water vapor and that forced water vapor diffusion in the pore space wind pumping is a requirement for isotopic content change the observations show that sublimation from ice samples results in much lower increases in heavy isotope content in the first several millimetres near the sublimating surface over the same time period despite sublimation intensities similar to those of the snow samples the results suggest that continuous phase transitions inside snow recrystallization are the process responsible for the isotopic content change because they are the primary mass exchange mechanism between the snow mass and the surrounding environment modeling the isotopic content of the ice matrix therefore requires inclusion of a two stage process fractionation at the ice matrix surface due to repetitive phase transitions and fractionation due to preferable diffusion of light water isotopes in the pore space for interpretation of the observed natural isotopic profiles in snow the first process can be linked to the time a snow layer undergoes recrystallization while the second process is related to the total ice snow mass gain loss determined by the external environmental conditions

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

evidence from ice shelves for channelized meltwater flow beneath the antarctic ice sheet subglacial meltwater channels beneath the antarctic ice sheet have been reported but the nature and distribution of these meltwater pathways are unclear remote sensing observations reveal persistent channelized features beneath the filchner ronne ice shelf in west antarctica suggesting widespread channelized flow driven by melting

depth and time dependent vertical strain rates at siple dome antarctica as part of a project to investigate the flow of ice at low effective stress two independent strain gauge systems were used to measure vertical strain rate as a function of depth and time at siple dome antarctica the measurements were made from january 1998 until january 2002 at the ice divide and a site 7 km to the northeast on the flank the strain rate profiles place constraints on the rheology of ice at low stress show the expected differences between divide and flank flow with some structure due to firn compaction and probably ice stratigraphy and suggest that the flow of the ice sheet has not changed much in the last 8 6 kyr the strain rates show an unexpected time dependence on scales ranging from several months to hours including discrete summer events at the divide time dependence in strain rate water pressure seismicity velocity and possibly basal motion has been seen previously on the siple coast ice streams but it is especially surprising on siple dome because the bed is cold

deglacial temperature history of west antarctica the most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how earth s climate responds to various forcings including a rise in atmospheric co2 this transition has left a direct thermal remnant in the polar ice sheets where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings for antarctica the deglacial warming has previously been constrained only by the water isotopic composition in ice cores without an absolute thermometric assessment of the isotopes sensitivity to temperature to overcome this limitation we measured temperatures in a deep borehole and analyzed them together with ice core data to reconstruct the surface temperature history of west antarctica the deglacial warming was formula see text c approximately two to three times the global average in agreement with theoretical expectations for antarctic amplification of planetary temperature changes consistent with evidence from glacier retreat in southern hemisphere mountain ranges the antarctic warming was mostly completed by 15 kybp several millennia earlier than in the northern hemisphere these results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on antarctic temperature although climate models perform well on average in this context some recent syntheses of deglacial climate history have underestimated antarctic warming and the models with lowest sensitivity can be discounted

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

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

snow replica method for three dimensional x ray microtomographic imaging visualization and quantification of snow structures at a scale of a few millimetres is important in understanding the mechanical thermal and electromagnetic properties of snow surface sections and to an even greater degree three dimensional 3 d reconstructions of cast snow samples are difficult to prepare and automatic image processing is notoriously difficult and often requires manual evaluation here we present a new method to measure the 3 d structure of cast snow samples snow samples cast with diethyl phthalate dep and frozen are cut to a sample size a few centimetres in diameter and up to 10 cm in height the ice of these samples is then sublimated in high vacuum and the remaining negative structure replica is imaged using x ray microtomography micro ct the accuracy of the method is demonstrated by comparing micro ct scans of the original snow structure and the replica the method described here allows easy transportation of samples requires little manual interaction has a very high spatial resolution of up to 10 m and is environmentally friendly

the elevation history of ice streams and the spatial accumulation pattern along the siple coast of west antarctica inferred from ground based radar data from three inter ice stream ridges measurements of the surface and internal layer geometry from ice penetrating radar and global positioning system surveys on three inter ice stream ridges in west antarctica siple dome ridge de and ridge bc are examined with ice flow models to infer 1 the history of the divide position at each site and 2 the spatial pattern of accumulation across the ridges we find that the divide position is most steady at siple dome somewhat steady at ridge de and highly variable at ridge bc data from siple dome and ridge de show evidence for steady northward motion of the ice divide for the past few thousand years the layers beneath ridge bc suggest a 5 km northward shift of the divide position within the past several hundred years assuming the divide shifts are all due to changing elevation of the bounding ice streams we infer the relative elevation history for segments of ice streams b e the northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from e to b with most dramatic recent thinning 100 m in 103 years of ice stream b relative to ice stream c analysis of the internal layer pattern across the ridges indicates a south north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases the inferred accumulation distribution is nearly uniform on the northern flanks decreases sharply within a few ice thicknesses across the divides and then decreases gradually farther to the south the north south decrease is strongest for ridge de and weakest for ridge bc this spatial pattern and the reduction in gradient strength with distance from the amundsen sea is consistent with the hypothesis that storms from the amundsen sea carry moisture first south then west over west antarctica and deposit more snow on the windward side of ridges due to orographic lifting this pattern has been stable for at least the past several thousand years