basal melting over subglacial lake ellsworth and its catchment insights from englacial layering deep water stable subglacial lakes likely contain microbial life adapted in isolation to extreme environmental conditions how water is supplied into a subglacial lake and how water outflows is important for understanding these conditions isochronal radio echo layers have been used to infer where melting occurs above lake vostok and lake concordia in east antarctica but have not been used more widely we examine englacial layers above and around lake ellsworth west antarctica to establish where the ice sheet is drawn down towards the bed and thus experiences melting layer drawdown is focused over and around the northwest parts of the lake as ice flowing obliquely to the lake axis becomes afloat drawdown can be explained by a combination of basal melting and the weertman effect at the transition from grounded to floating ice we evaluate the importance of these processes on englacial layering over lake ellsworth and discuss implications for water circulation and sediment deposition we report evidence of a second subglacial lake near the head of the hydrological catchment and present a new high resolution bed dem and hydropotential model of the lake outlet zone these observations provide insight into the connectivity between lake ellsworth and the wider subglacial hydrological system

geomorphology of the onset area of a paleo ice stream marguerite bay antarctic peninsula geomorphic features indicate that both glacial and melt water erosion characterize the onset area of the ancestral marguerite ice stream the large size of these features indicates that they formed over repeated glacial cycles most recently during the last glacial maximum ice drainage within the bay and on the inner continental shelf was strongly influenced by tectonic fabric deep isolated basins surrounded by rugged bedrock bathymetry characterize the innermost part of the bay drumlins and other streamlined features occur in the floors of these basins at depths of up to 900 m the outer bay has three large interconnected basins drumlins and megaflutings within these basins indicate ice was grounded at water depths up to 1000 m the orientations of these features show convergence of drainage from the northeast east and south into the marguerite paleo ice stream on the inner continental shelf the ice converged into a single wide trough dominated by mega scale glacial lineations this transition in geomorphic features from drumlins and megaflutings to mega scale glacial lineations occurs at the location on the continental shelf where sedimentary strata blanket bedrock and marks a zone of acceleration of the ice stream the glacially sculptured geomorphic features within marguerite bay co exist with anastomosing radial and relatively straight channels which become increasingly focused in a seaward direction this implies that a well organized subglacial drainage system existed within the bay at some point in the past copyright 2008 john wiley sons ltd

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

antarctic subglacial lake exploration first results and future plans after more than a decade of planning three attempts were made in 20122013 to access measure in situ properties and directly sample subglacial antarctic lake environments first russian scientist

jokulhlaups a reassessment of floodwater flow through glaciers 1 in glaciated catchments glacier generated floods jokulhlaups put human activity at risk with large sporadic jokulhlaups accounting for most flood related fatalities and damage to infrastructure in studies of jokulhlaup hydrodynamics the view predominates that floodwater travels within a distinct conduit eroded into the underside of a glacier however some jokulhlaups produce subglacial responses wholly inconsistent with the conventional theory of drainage by focusing on icelandic jokulhlaups this article reassesses how floodwater flows through glaciers it is argued that two physically separable classes of jokulhlaup exist and that not all jokulhlaups are an upward extrapolation of processes inherent in events of lesser magnitude and smaller scale the hydraulic coupling of multiple nonlinear components to the flood circuit of a glacier can induce extreme responses including pressure impulses in subglacial drainage representing such complexity in mathematical form should be the basis for upcoming research as future modeling results may help to determine the glaciological processes behind heinrich events moreover such an approach would lead to more accurate predictive models of jokulhlaup timing and intensity

hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

estimating the depth and shape of subglacial lake vostok s water cavity from aerogravity data 1 we use aerogravity data to estimate the water depth of subglacial lake vostok in east antarctica the inversion produces the first bathymetry map covering the entire lake lake vostok consists of two sub basins separated by a ridge with very shallow water depths the deeper southern sub basin is approximately double the spatial area of the smaller northern sub basin the close correlation between the pattern of basal melting and freezing and the bathymetric structure has important ramifications for the water circulation and the sediment deposition we estimate the lake volume to be 5400 1600 km3

is vostok lake in steady state stable isotope d da ndd 18 o data from the vostok east antarctica ice core are used to explore whether or not subglacial vostok lake is in isotopic steady state a simple box model shows that the lake is likely to be in steady state on time scales of the order of 10 4 10 5 years three to four residence times of the water in the lake given our current knowledge of north south and east west gradients in the stable isotopic composition of precipitation in the vicinity of vostok station and ridge b however the lake may not be in perfect steady state depending on the precise location of the melting area which determines the source region of inflowing ice and on the magnitude of the east west gradient in isotopic compositions in the vicinity of vostok station and ridge b

circulation in lake vostok a laboratory analogue study 1 the waters of sub glacial lake vostok are thought to represent a unique biological habitat that has been out of contact with the atmosphere for millions of years although the water circulation within the lake will determine how nutrients are redistributed and hence where life may exist the handful of existing studies conflict regarding whether the lake is stratified or well mixed whether there is a clockwise or anti clockwise circulation or whether the sloping roof leads to a flow intensified along the eastern boundary here an experimental analogue model of lake vostok is used to show a qualitatively different mode of circulation than described previously the dominant mode of dispersion and hence bio and chemical redistribution is controlled by the presence of columnar vortex structures that arise from rotating convection this form of circulation does not occur in any other lake on earth we estimate that a water parcel takes 20 30 days to travel from the base to the roof of lake vostok and will experience minimal lateral mixing during this time as the nutrient supply to the lake via melting ice is predicted to be less than that required for active growth within the entire lake we speculate that this form of slow columnar convection would lead to the presence of thin biological boundary layers within the waters of the lake this has important implications for the interpretation of any direct water samples taken near the roof of lake vostok

cauldron subsidence and subglacial floods ice cauldrons are depressions which form at the surface of ice sheets when an underlying subglacial lake empties notable examples of such cauldrons occur on the surface of the vatnajokull ice cap in iceland and in particular are formed when subglacial volcanic eruptions occur more generally cauldrons will form when a subglacial lake empties during a jokulhlaup the rate of subsidence of the ice surface is related to the rate at which the subglacial water empties from the lake we use a viscous version of classical beam theory applied to the ice sheet to determine the relation between the subsidence rate and flood discharge and we use the results to make inferences concerning ring fracture spacings in cauldrons the consequent effect on flood discharge dynamics and the likely nature of subsidence events in the antarctic ice sheet

a large deep freshwater lake beneath the ice of central east antarctica in 1974 75 an airborne radio echo survey of ice depths over central east antarctica led to the discovery of a sub ice lake of unknown depth and composition with an area of about 10 000km2 and lying beneath 4km of ice1 in 1993 altimetric data from satellite measurements2 provided independent evidence of the lake s areal extent thus confirming it to be the largest known sub ice lake by an order of magnitude here we analyse new altimetric and radio echo data along with existing seismic data3 to show that the lake is deep mean depth of 125 m or more and fresh and that it has an area that exceeds previous estimates by about 50 dimensions comparable with those of lake ontario we estimate that the residence time of the water in the lake is of the order of tens of thousands of years and that the mean age of water in the lake since deposition as surface ice is about one million years regional ice dynamics can be explained in terms of steady state ice flow along and over the lake

retreat patterns and dynamics of the sentralbankrenna glacial system central barents sea accepted manuscript version published version available at https doi org 10 1016 j quascirev 2017 06 004 accepted manuscript version licensed cc by nc nd 4 0

a decade of west antarctic subglacial lake interactions from combined icesat and cryosat 2 altimetry we use cryosat 2 interferometric satellite radar altimetry over the mercer and whillans ice streams west antarctica to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013 we validate cryosat 2 elevation measurements trends and spatial patterns of change using satellite image differencing and in situ vertical movement from global positioning system gps data two subglacial lake discharge events occur in the same subglacial hydrological catchment within a 9 month period using gps measurements that are spanning the gap between the ice cloud and land elevation satellite and cryosat 2 missions we cross calibrate the two missions to establish the efficacy of cryosat 2 altimetry to measure dynamic changes on the ice sheets

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

ice and bedrock characteristics underneath dome c antarctica from radio echo sounding data analysis the radio echo sounding res system is one of the most widely used active remote sensing techniques for polar ice sheet exploration including bedrock morphology studies and subglacial lake investigations recently bedrock characterization has been improved through the analysis of radar echo strength in this paper the analysis of the res signal amplitude has been used to collect information about the controversial problem of electromagnetic ice absorption to highlight areas of high reflectivity variation ascribable to wet ice bedrock interfaces a method to distinguish a wet or dry bedrock ice interface using a model to describe the internal ice absorption is proposed and discussed moreover the comparison between the ice absorption rates from res measurements and from european project for ice coring in antarctica dome c antarctica ice core conductivity data the signal amplitude contributions of internal ice layers and different kinds of rock interface is evaluated encouraged by the results the data analysis led to obtaining a bedrock reflectivity variation map of the dome c area this map outlined a wide dispersion of wet dry rock interfaces in the studied area indicating the possibility of flowing water along both sides of the concordia trench

evidence of a hydrological connection between the ice divide and ice sheet margin in the aurora subglacial basin east antarctica subglacial hydrology in east antarctica is poorly understood yet may be critical to the manner in which ice flows data from a new regional airborne geophysical survey icecap have transformed our understanding of the topography and glaciology associated with the 287 000 km2 aurora subglacial basin in east antarctica using these data in conjunction with numerical ice sheet modeling we present a suite of analyses that demonstrate the potential of the 1000 km long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin we present results from our analysis of basal topography bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures although no clear cut subglacial lakes are found within the aurora basin itself dozens of lake like reflectors are observed that in conjunction with other results reported here support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the dome c ice divide to reach the coast via the totten glacier

a microbial driver of chemical weathering in glaciated systems glaciological processes under ice sheets provide sustainable ecosystems for microbes forming an aquatic environment through basal melting and providing nutrients and energy from bedrock microbes facilitate solute production in most earth surface environments but the balance of biotic and abiotic weathering in subglacial environment is presently unknown this study demonstrates an up to eightfold increase in dissolved major cations in biotic relative to abiotic weathering experiments using glacial sediments and meltwater this conclusion greatly expands our view of earth s biogeochemically active weathering zone by incorporating the large wet based portions of glaciated continents both at present and during earth s history the profound environmental significance is that microbial processes have the ability to maintain terrestrial chemical weathering rates in cooling climates during glacial advance

morphometry of bedrock meltwater channels on antarctic inner continental shelves implications for channel development and subglacial hydrology abstract expanding multibeam bathymetric data coverage over the last two decades has revealed extensive networks of submarine channels incised into bedrock on the antarctic inner continental shelf the large dimensions and prevalence of the channels implies the presence of an active subglacial hydrological system beneath the past antarctic ice sheet which we can use to learn more about inaccessible subglacial processes here we map and analyse over 2700 bedrock channels situated across 100 000 km2 of continental shelf in the western antarctic peninsula and amundsen sea to produce the first inventory of submarine channels on the antarctic inner continental shelf morphometric analysis reveals highly similar distributions of channel widths depths cross sectional areas and geometric properties with subtle differences between channels in the western antarctic peninsula compared to those in the amundsen sea at 75 3400 m wide 3 280 m deep 160 290 000 m2 in cross sectional area and typically 8 times as wide as they are deep the channels have similar morphologies to tunnel valleys and meltwater channel systems observed from other formerly glaciated landscapes despite differences in substrate geology and glaciological regime we propose that the antarctic bedrock channels formed over multiple glacial cycles through the episodic drainage of at least 59 former subglacial lakes identified on the inner continental shelf

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

origin of the adventure subglacial trench linked to cenozoic extension in the east antarctic craton abstract the antarctic plate occupies a unique geodynamic setting being mostly surrounded by divergent or transform margins major intracontinental basins and highlands characterize its bedrock buried under the 34 ma east antarctic ice sheet eais their formation atop of the cratonic lithosphere in the interior of east antarctica remains a major open question post mesozoic intraplate extensional tectonic activity has been proposed for their development and is supported by this work here we focus on the adventure subglacial trench ast whose origin is poorly constrained and controversial as currently available geophysical models suggest either extensional or compressional tectonic origin the ast is an over 250 km long 60 km wide subglacial trough elongated in the nnw sse direction adjacent to the westernmost flank of the wilkes subglacial basin and is parallel to regional scale alignments of magnetic and gravimetric anomalies geophysical campaigns allowed better definition of the ast physiography showing its typical half graben geometry the rounded morphology of the western flank of the ast was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect a preserved relict of the tectonic processes affecting the interior of east antarctica in the cenozoic due to the negligible erosion deposition capability of the eais the bedrock morphology was replicated through the activity of the listric adventure fault characterized by a basal detachment at the base of the crust 34 km and a vertical displacement of 2 5 km this length suggests its regional crustal importance the predicted displacement is interpreted either as a newly formed fault or as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary the extensional regime in the ast is part of a more extensive 800 km long intraplate corridor characterized by nearly along strike extension in cenozoic times with a left lateral transpressional component this corridor may represent the effect of far field stresses induced by plate motions

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

incremental subglacial meltwater sediment deposition and deformation associated with repeated ice bed decoupling a case study from the island of funen denmark abstract on the island of funen denmark sediments that are 4 7 m below the land surface consist of a 2 4 m thick sequence of stacked and folded glaciofluvial sediments overlain by a 2 3 m thick basal till the glaciofluvial sediments consist of tabular beds of planar stratified massive and cross stratified sand and gravel with localized channel fills of open framework gravel sand and gravel beds exhibit curvilinear sheath folding often leading to eye folds this folding is attributed to repeated glacier bed decoupling and recoupling with subglacial deposition of sand and gravel by meltwater occurring during the decoupling phase and deformation occurring during recoupling sand and gravel beds are typically separated by silty clay beds and laminations recording slackwater deposition within subglacial cavities folds frequently display attenuated upper limbs reflecting shearing associated with overlying ice movement on lower fold limbs only localized thrusting is observed suggesting water saturation of sands and gravels during deformation and relatively low strain rates folds are frequently truncated by overlying sand and gravel beds no evidence of shearing is found along these contacts which are interpreted as erosional surfaces thus we propose that the glaciofluvial sediment sequence records incremental step wise subglacial deposition and deformation rather than wholesale deformation following emplacement of the entire glaciofluvial sequence specifically we envisage that individual beds were deposited and folded subglacially with ice bed decoupling occurring during sediment deposition followed by recoupling and shearing of the upper bed surface by the ice base to produce folds with attenuated upper limbs incremental deformation and folding were facilitated by elevated porewater pressure within the sediments silty clay beds helped to maintain elevated porewater pressure by acting as local aquitards a general absence of till from the glaciofluvial sequence save for localized cm scale blocks suggests either non deposition or reworking into the glaciofluvial sediments deposition of tabular sand and gravel beds and open framework gravel filling scours within single beds suggest broad sheet like subglacial flows 10 100 m wide that become increasingly channelized we propose that the sedimentary sequence records a highly dynamic glaciohydraulic system characterized by repeated localized subglacial cavity development and storage and release of meltwater leading to glacier decoupling that may have facilitated fast flow of the baltic ice stream

insights into hydrological regime of lake vostok from differential behavior of deuterium and oxygen 18 in accreted ice 1 we use isotopic data deuterium and oxygen 18 of the recently recovered deepest vostok ice core section down to 3650 m depth to study processes leading to the formation of lake ice and the hydrological regime of subglacial lake vostok the significant variability of the lake ice isotopic content implies fluctuations in physical conditions of ice formation mainly volume and or growth rate of frazil ice crystals as well as variations of the isotope composition of the freezing water the latter implies a poor mixing of the source waters glacier melt and hydrothermal water with the water of the main lake body poor mixing within the lake may have important consequences for the lake s chemical and gas balance and particularly for its microbiological content a poorly mixed lake may provide ecological niches where microbial life can hide from high oxygen concentrations likely typical for the lake we also show that the isotopic content of the main lake s input meltwater significantly differs from that of the output lake ice which can be explained by the contribution of an additional hydrothermal source this latter conclusion is supported by the observed noncovariant behavior of deuterium and oxygen 18 isotopes in the lake ice

origin and fate of lake vostok water frozen to the base of the east antarctic ice sheet the subglacial lake vostok may be a unique reservoir of genetic material and it may contain organisms with distinct adaptations but it has yet to be explored directly the lake and the overlying ice sheet are closely linked as the ice sheet thickness drives the lake circulation while melting and freezing at the ice sheet base will control the flux of water biota and sediment through the lake here we present a reconstruction of the ice flow trajectories for the vostok core site using ice penetrating radar data and global positioning system gps measurements of surface ice velocity we find that the ice sheet has a significant along lake flow component persistent since the last glacial maximum the rates at which ice is frozen accreted to the base of the ice sheet are greatest at the shorelines and the accreted ice layer is subsequently transported out of the lake using these new flow field and velocity measurements we estimate the time for ice to traverse lake vostok to be 16 000 20 000 years we infer that most vostok ice analysed to date was accreted to the ice sheet close to the western shoreline and is therefore not representative of open lake conditions from the amount of accreted lake water we estimate to be exported along the southern shoreline the lake water residence time is about 13 300 years

modeled subglacial water flow routing supports localized intrusive heating as a possible cause of basal melting of mars south polar ice cap the discovery of an 20 km wide area of bright subsurface radar reflections interpreted as liquid water beneath the martian south polar layered deposits spld in data from the mars advanced radar for subsurface and ionosphere sounding marsis instrument and the discovery of two geologically recent potential eskers landforms produced by subglacial melt associated with viscous flow features in martian midlatitudes has suggested recent basal melting of martian ice deposits may be feasible possibly due to locally elevated geothermal heating locations of terrestrial subglacial lakes and major drainage axes have been successfully predicted from subglacial hydraulic potential surfaces calculated from surface topography and ice thickness here we use surface topography from the mars orbiter laser altimeter and spld bed elevations derived from marsis data to calculate the subglacial hydraulic potential surface beneath the spld and determine whether the observed high reflectance area coincides with predicted subglacial lake locations given the sensitivity of terrestrial predictions of lake locations to basal topography we derive over 1 000 perturbed topographies using noise statistics from the marsis data to infer the most likely locations of possible subglacial water bodies and drainage axes our results show that the high reflectance area does not coincide with any substantial predicted lake locations three nearby lake locations are robustly predicted however we interpret this result as suggesting that the high reflectance area assuming the interpretation as liquid is correct is most likely a hydraulically isolated patch of liquid confined by the surrounding cold based ice rather than a topographically constrained subglacial lake

trends and connections across the antarctic cryosphere satellite observations have transformed our understanding of the antarctic cryosphere the continent holds the vast majority of earth s fresh water and blankets swathes of the southern hemisphere in ice reductions in the thickness and extent of floating ice shelves have disturbed inland ice triggering retreat acceleration and drawdown of marine terminating glaciers the waxing and waning of antarctic sea ice is one of earth s greatest seasonal habitat changes and although the maximum extent of the sea ice has increased modestly since the 1970s inter annual variability is high and there is evidence of longer term decline in its extent this paper discusses how antarctic ice has changed over recent decades and how these changes have been recorded in satellite observations

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

glacial curvilineations new glacial landforms produced by longitudinal vortices in subglacial meltwater flows abstract the glacial landscape of the dobrzyn plateau in central poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans the term glacial curvilineations is proposed to describe these enigmatic bedforms that occur as fields of parallel sinuous ridges separated by troughs the regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau we propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel forming flows parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau understanding curvilineation genesis may shed light on glacier hydrologic processes tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater such as drainage of subglacial and or supraglacial lakes the extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet like flows as wide as a few kilometres

antarctic elevation change from 1992 to 1996 satellite radar altimeter measurements show that the average elevation of the antarctic ice sheet interior fell by 0 9 0 5 centimeters per year from 1992 to 1996 if the variability of snowfall observed in antarctic ice cores is allowed for the mass imbalance of the interior this century is only 0 06 0 08 of the mean mass accumulation rate

breaking the seal at grimsvotn iceland of several problems associa ted wit h theoretica l expla nation s of the jokul hla ups which emerge from the outlet gla cie r skeioarar jokull of the ice ca p vatna jokull in sou theast iceland the mechani sm of flood init iation is one that has hi therto defied expl anat ion we provide such an expl ana tion based on a car efu l analysis of the cla ssica l nye ro thlisbe rger model near the subglacial lake grimsvotn the hyd raulic potenti al gradie nt is towards the lake and there is theref ore a catchme nt boun dary under the ice whose location dep ends on the subglacial mel twater drai nage charac terist ics as the con dit ions for a flood approa ch we show that the wat er divi de migra tes towards the lake while at the sa me time the lake pre ssure in creases when the hydra ulic potential gradient to wards the lake is low and the refillin g rate is sl ow the se al will break whe n the cat chment boun dary reaches the lake while the la ke level is st ill below flotati on pressu re whereas if refillin g is rapid flo tation can be achie ved bef ore a flood is initia ted this theory can thus explain why the seal is normally broken when the lake le vel at grimsvotn is still some 60 m below flotation level in addit ion we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved and we show how the cyeli cit y and magnitude ofjoku lhla ups can be explained wit hin this theory

height changes over subglacial lake vostok east antarctica insights from gnss observations height changes of the ice surface above subglacial lake vostok east antarctica reflect the integral effect of different processes within the subglacial environment and the ice sheet repeated gnss global navigation satellite systems observations on 56 surface markers in the lake vostok region spanning 11 years and continuous gnss observations at vostok station over 5 years are used to determine the vertical firn particle movement vertical marker velocities are derived with an accuracy of 1 cm yr or better repeated measurements of surface height profiles around vostok station using kinematic gnss observations on a snowmobile allow the quantification of surface height changes at 308 crossover points the height change rate was determined at 1 5 mm yr thus indicating a stable ice surface height over the last decade it is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period the continuous gnss observations demonstrate that the particle heights vary linearly with time nonlinear height changes do not exceed 1 cm at vostok station and constrain the magnitude of spatiotemporal lake level variations icesat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters assuming the ice sheet to be in steady state over the entire lake estimates for the surface accumulation on basal accretion melt rates and on flux divergence are derived

sensitivity of subglacial lake vostok s flow regime on environmental parameters abstract subglacial lakes provide unique habitats but the exact nature of physical and geochemical conditions are still a matter of debate and await direct sampling of water due to its isolation from external atmospheric forcing other environmental parameters influence the flow characteristics within the lake in this study we use an improved treatment of the physical processes at the ice water boundary interface to identify and quantify the impact of 1 the geothermal heat flux 2 the heat flux from the lake into the ice 3 the influence of the salinity of the lake water and 4 the ice thickness on the size of the freezing area and the freeze melt rates we show that the modelled basal mass imbalance that is the produced melt water minus the re frozen water depends on the geothermal heating as well as the heat flux into the ice the circulation and the temperature distribution within subglacial lake vostok are rather stable against variations of geothermal heat flux heat flux into the ice sheet salinity of the lake and small changes of the ice thickness above the lake however the flow regime for any subglacial lake with less than 2000 m ice thickness above will be substantially different from those that experience higher pressures this is because the buoyancy temperature relationship reverses at this depth

modelling flow and accreted ice in subglacial lake concordia antarctica more than 150 subglacial lakes have been discovered in antarctica so far due to obvious challenges with exploration numerical modelling remains one of the major tools to acquire information about those hard to access objects until now only the huge lake vostok has been investigated in detail this paper focuses on lake concordia the second largest subglacial lake in antarctica over which substantial geophysical data has been collected this lake is covered by about 4000 m ice and is located near dome c in order to apply numerical models to the hard to access antarctic subglacial lakes decent geometries and boundary conditions are required in this study we present the results of airborne gravity inversion suggesting that this lake has an area of 617 km2 a volume of 31 km3 and a maximum water column thickness of 126 m this bathymetry is used as geometry input for an established 3d numerical lake flow model to simulate the circulation and basal mass balance compared to our model studies of subglacial lake vostok we obtain a general circulation pattern that is significantly weaker due to the smaller size of the lake and of reversed orientation due to the reversed ice surface tilt the modelled mean horizontal and vertical velocities are in the order of 0 2 mm s and 0 5 m s respectively the larger molecular convective velocity estimations 1 35 0 13 mm s and 0 81 0 08 mm s are similar to lake vostok s the modelled average melting and freezing rates are 4 3 1 1 mm a and 1 1 0 3 mm a respectively and the corresponding fresh water gain is 58 27 dm3 s integration of the modelled freezing and melting along prescribed ice flow lines allows us to calculate the distribution and thickness of accreted ice at the ice sheet bottom we estimate a volume of 2 6 2 0 km3 8 3 8 2 of the total lake volume occupying the north eastern corner of the lake covering an area of 159 48 km2 26 9 of the total lake area with about 16 800 7600 yr the residence time of the lake s water is significantly shorter than lake vostok s

subglacial water storage and drainage beneath the fennoscandian and barents sea ice sheets abstract subglacial hydrology modulates how ice sheets flow respond to climate and deliver meltwater sediment and nutrients to proglacial and marine environments here we investigate the development of subglacial lakes and drainage networks beneath the fennoscandian and barents sea ice sheets over the late weichselian utilizing an established coupled climate ice flow model we calculate high resolution spatio temporal changes in subglacial hydraulic potential from ice sheet build up 37 ka bp to complete deglaciation 10 ka bp our analysis predicts up to 3500 potential subglacial lakes the largest of which was 658 km2 and over 70 of which had surface areas

grounding line variability and subglacial lake drainage on pine island glacier antarctica we produced a 6 year time series of differential tidal displacement for pine island ice shelf antarctica using speckle tracking methods applied to fine resolution terrasar x data these results reveal that the main grounding line has maintained a relatively steady position over the last 6 years following the speedup that terminated in 2009 in the middle of the shelf there are grounded spots that migrate downstream over the 6 year record examination of high resolution digital elevation models reveals that these grounded spots form where deep keels thickness anomalies advect over an approximately flow parallel bathymetric high maintaining intermittent contact with the bed these data sets also reveal several subsidence and uplift events associated with subglacial lake drainages in the fast flowing region above the grounding line although these drainages approximately double the rate of subglacial water flow over periods of a few weeks they have no discernible effect on horizontal flow speed

wissard at subglacial lake whillans west antarctica scientific operations and initial observations a clean hot water drill was used to gain access to subglacial lake whillans slw in late january 2013 as part of the whillans ice stream subglacial access research drilling wissard project over 3 days we deployed an array of scientific tools through the slw borehole a downhole camera a conductivity temperature depth ctd probe a niskin water sampler an in situ filtration unit three different sediment corers a geothermal probe and a geophysical sensor string our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics subglacial water is about two orders of magnitude less saline than sea water 0 37 0 41 psu vs 35 psu and two orders of magnitude more saline than pure drill meltwater 0 002 psu it reaches a minimum temperature of 0 558c consistent with depression of the freezing point by 7 019 mpa of water pressure subglacial water was turbid and remained turbid following filtration through 0 45 mm filters the recovered sediment cores which sampled down to 0 8 m below the lake bottom contained a macroscopically structureless diamicton with shear strength between 2 and 6 kpa our main operational recommendation for future subglacial access through water filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing

detecting himalayan glacial lake outburst floods from landsat time series abstract several thousands of moraine dammed and supraglacial lakes spread over the hindu kush himalayan hkh region and some have grown rapidly in past decades due to glacier retreat the sudden emptying of these lakes releases large volumes of water and sediment in destructive glacial lake outburst floods glofs one of the most publicised natural hazards to the rapidly growing himalayan population despite the growing number and size of glacial lakes the frequency of documented glofs is remarkably constant we explore this possible reporting bias and offer a new processing chain for establishing a more complete himalayan glof inventory we make use of the full seasonal archive of landsat images between 1988 and 2016 and track automatically where glofs left shrinking water bodies and tails of sediment at high elevations we trained a random forest classifier to generate fuzzy land cover maps for 2491 images achieving overall accuracies of 91 we developed a likelihood based change point technique to estimate the timing of glofs at the pixel scale our method objectively detected ten out of eleven documented glofs and another ten lakes that gave rise to previously unreported glofs we thus nearly doubled the existing glof record for a study area covering 10 of the hkh region remaining challenges for automatically detecting glofs include image insufficiently accurate co registration misclassifications in the land cover maps and image noise from clouds shadows or ice yet our processing chain is robust and has the potential for being applied on the greater hkh and mountain ranges elsewhere opening the door for objectively expanding the knowledge base on glof activity over the past three decades

transient subglacial hydrology of a thin ice sheet insights from the chasm esker british columbia canada abstract glacier drainage systems are never steady state instead they fluctuate over time and space in response to variation in water input in order to improve numerical models of glacier hydrology it is important to fully understand the processes controlling subglacial drainage system evolution but contemporary ice sheet beds are typically inaccessible thus esker distribution morphology and sedimentology have been used to infer the dynamics and hydrology of former ice sheets however debate remains as to the processes responsible for esker formation and most theoretical investigations have assumed that they formed due to processes that operated beneath thick ice despite many field investigations to the contrary we investigate esker formation during the final stages of the thin inactive and rapidly decaying cordilleran ice sheet cis in interior british columbia canada a combination of geomorphological ground penetrating radar gpr and electrical resistivity tomography ert data suggest esker ridge sedimentary architecture is consistent with synchronous formation during a glacial lake outburst flood glof these data reveal esker ridge deposition most likely took place within an ice tunnel that evacuated late waning stage flow following erosion and partial fill of a broader meltwater corridor esker ridge sedimentary architecture reveals this depositional environment was dynamic reflecting complex interaction between ice thickness ice structure ice tunnel geometry flow conditions and sediment supply under these thin inactive ice conditions ice tunnel location was initially governed by structural weaknesses in the ice and or equipotential gradient because creep closure rates were low the ice tunnel evolved through feedbacks between conduit growth via frictional melting mechanical ice excavation and conduit closure due to sediment infilling rather than ice creep this resulted in a non uniform ice tunnel that enlarged in an upglacier direction and locally unroofed to form open channels during its final stages these data suggest that eskers may record unsteady rather than steady flows in a drainage network and so the assumptions often made when using eskers to inform hydrological components of ice sheet models may not be applicable to all ice sheet settings

clathrate formation and the fate of noble and biologically useful gases in lake vostok antarctica 1 lake vostok is a large lake located 4 km beneath the east antarctic ice sheet that should be supersaturated with dissolved gases in equilibrium with clathrate present in the water column here we show that if the age of the lake is such that the lake water mass has been cycled over 30 times then the total dissolved gas equilibrates at about 2 5 liters stp of gas per kg of water high enough to have important implications for drilling into this deep subglacial lake different air gases are preferentially incorporated into the clathrate and thus the molar ratios in the water column will reflect the presence of clathrate and indicate a more precise age of the lake preferential incorporation of co2 into the clathrate would result in the clathrate sinking if the carbon input is 1 of the air input and the lake water is fresh water the redox state of the lake is set by the high oxygen concentration which is 50 times more than air equilibrated water and may be a severe biological stress

parameterization of directional absorption of orographic gravity waves and its impact on the atmospheric general circulation simulated by the weather research and forecasting model abstractin this work a new parameterization scheme is developed to account for the directional absorption of orographic gravity waves ogws using elliptical mountain wave theory the vertical mom

ultra low rare earth element content in accreted ice from sub glacial lake vostok antarctica this paper reports the first rare earth element ree concentrations in accreted ice refrozen from sub glacial lake vostok east antarctica ree were determined in various sections of the vostok ice core in order to geochemically characterize its impurities samples were obtained from accreted ice and for comparison from the upper glacier ice of atmospheric origin undisturbed disturbed and glacial flour ice ree concentrations ranged between 0 8 56 pg g 1 for ce and 0 0035 0 24 pg g 1 for lu in glacier ice and between 0 1 24 pg g 1 for ce and 0 0004 0 02 pg g 1 for lu in accreted ice interestingly the ree concentrations in the upper accreted ice ac1 characterized by visible aggregates containing a mixture of very fine terrigenous particles and in the deeper accreted ice ac2 characterized by transparent ice are lower than those in fresh water and seawater respectively we suggest that such ultra low concentrations are unlikely to be representative of the real ree content in lake vostok but instead may reflect phase exclusion processes occurring at the ice water interface during refreezing in particular the uneven spatial distribution on the order of a few cm and the large range of ree concentrations observed in ac1 are consistent with the occurrence absence of the aggregates in adjacent ice and point to the presence of solid phase concentration exclusion processes occurring within separate pockets of frazil ice during ac1 formation interestingly if the lree enrichment found in ac1 was not produced by chemical fractionation occurring in lake vostok water this may reflect a contribution of bedrock material possibly in combination with aeolian dust released into the lake by melting of the glacier ice collectively this valuable information provides new insight into the accreted ice formation processes the bedrock geology of east antarctica as well as the water chemistry and circulation of lake vostok

subaerial salt extrusions in iran as analogues of ice sheets streams and glaciers abstract ice h20 and salt halite nacl share many physical properties and resemble each other in hand specimens and subaerial gravity driven flows however while most significant bodies of ice accumulate in cold highlands and gravity spread where and soon after they form most significant bodies of salt accumulate in tropical marine basins and have to be buried by 1 km of other rocks before they flow buried salt is driven by differential loading into various categories of piercing structures known as diapirs many diapirs extrude onto the surface as sheets of allochthonous out of place salt thousands of sheets of allochthonous salt have been interpreted in over 35 basins worldwide in the last 25 years mainly in the toes of passive continental margins and in orogenic belts where some are 103 km2 in area most former salt sheets are now submarine or subsurface but several active examples are beautifully exposed in iran these were compared to ice glaciers soon after they were introduced to western science a comparison that has been neglected since here we update this analogy and use modern understanding of flowing ice and salt to examine the similarities and differences that might be mutually beneficial to both fields of study as well as to extraterrestrial scientists the profiles internal structures and fabrics in flowing bodies of ice and salt are sensitive gauges of the histories of their budgets of supply and loss however whereas snow merely compacts where it accumulates salt sheets are fed from below by already deformed salt when salt diapirs first emerge on land they extrude domes that mature to the profiles of viscous fountains that often feed glacier like flows known as namakiers after locally exhausting their deep source layers salt fountains spread to the profiles of viscous droplets normal for ice caps ice typically deforms at 80 usually 90 of its absolute melting temperature while most salt deforms at

geodetic observations of ice flow velocities over the southern part of subglacial lake vostok antarctica and their glaciological implications summary in the austral summer seasons 2001 02 and 2002 03 global positioning system gps data were collected in the vicinity of vostok station to determine ice flow velocities over lake vostok ten gps sites are located within a radius of 30 km around vostok station on floating ice as well as on grounded ice to the east and to the west of the lake additionally a local deformation network around the ice core drilling site 5g 1 was installed the derived ice flow velocity for vostok station is 2 00 m a 1 0 01 m a 1 along the flowline of vostok station an extension rate of about 10 5 a 1 equivalent to 1 cm km 1 a 1 was determined this significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline with these lower velocities compared to earlier studies and hence larger transit times the basal accretion rate is estimated to be 4 mm a 1 along a portion of the vostok flowline an assessment of the local accretion rate at vostok station using the observed geodetic quantities yields an accretion rate in the same order of magnitude furthermore the comparison of our geodetic observations with results inferred from ice penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years

extensive storage of basal meltwater in the onset region of a major west antarctic ice stream a major meltwater body exists beneath a tributary of bindschadler ice stream west antarctica in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body the water is trapped by a local reversal in ice air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water strong variations in the water content of the sediments upglacier of the water body arise from a similar process these results are revealed by a novel application of the amplitude variation with offset avo seismic technique the existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology lubrication of ice stream motion and sediment transport

hypothesis for mega outburst flooding from a palaeo subglacial lake beneath the east antarctic ice sheet subglacial outburst floods at the margins of the antarctic and laurentide ice sheets have been linked to changes in global ocean circulation and climate the impact of palaeo hydrological systems beneath the east antarctic ice sheet eais has however remained elusive by analysing bed morphology in the wilkes subglacial basin we hypothesise the occurrence of a major palaeo subglacial lake and associated outburst floods in the interior of east antarctica at similar to 70 km wide and over 100 km long the inferred area of outburst flooding is the most extensive in antarctica and the palaeo subglacial lake from which the floods originated is the second largest after lake vostok the scale of inferred outburst flooding is similar to mega floods beneath the laurentide ice sheet we suggest that this major hydrological system developed during expansion of the eais in the middle miocene and probably affected ice sheet stability ocean circulation and climate evolution

glaciology ice sheet plumbing in antarctica it s not easy to work out what is going on beneath four kilometres of ice but remote imaging has enabled the discovery of the long distance discharge of water from one subglacial lake to another in antarctica

the sediment infill of subglacial meltwater channels on the west antarctic continental shelf subglacial meltwater plays a significant yet poorly understood role in the dynamics of the antarctic ice sheets here we present new swath bathymetry from the western amundsen sea embayment west antarctica showing meltwater channels eroded into acoustic basement their morphological characteristics and size are consistent with incision by subglacial meltwater to understand how and when these channels formed we have investigated the infill of three channels diamictons deposited beneath or proximal to an expanded grounded west antarctic ice sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation the sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation the presence of deformation till at one core site and the absence of typical meltwater deposits e g sorted sands and gravels in all three cores suggest that channel incision pre dates overriding by fast flowing grounded ice during the last glacial period given the overall scale of the channels and their incision into bedrock it is likely that the channels formed over multiple glaciations possibly since the miocene and have been reoccupied on several occasions this also implies that the channels have survived numerous advances and retreats of grounded ice

predictive modeling of the seismic cycle of the greater san francisco bay region the seismic cycle for the san francisco bay region is synthesized by a model combining the pre and post 1906 seismic histories the long term acceleration of seismic release seismic moment benioff strain release or event count in the seismic cycle and the shorter term accelerations preceding the larger earthquakes within that cycle are modeled using an empirical predictive technique called time to failure analysis in which rate of seismic release is proportional to an inverse power of the remaining time to failure the exponent of time to failure in the accelerating sequences appears to be scale invariant and the length of the full cycle is estimated at 269 50 years the 1989 loma prieta earthquake which is the culmination of the first subcycle in the present long term seismic cycle should have been predictable with an uncertainty of 2 years in time and 0 5 in magnitude although the specific location at loma prieta was not predictable by this technique if our model is correct and if the loma prieta earthquake is the culmination of a subcycle the san francisco bay region should be entering a relatively long 20 50 years period of seismic quiescence above magnitude 6 a great earthquake such as the 1906 san francisco event would appear to be more than a century in the future

geological setting of the concordia trench lake system in east antarctica summary this study presents the interpretation of radio echo sounding res data collected during the 2003 geophysical campaign of pnra italian national research project in antarctica which focused on the exploration of the concordia trench lake system in east antarctica the data allow us to identify a new lake itl 28 at the southern edge of the concordia trench and a series of n s trending subglacial troughs cutting through the belgica highlands we have mapped the bedrock morphology at 3 km resolution which led to an improved geographical and geomorphological characterization of the concordia trench concordia ridge concordia lake and south hills improved knowledge of the concordia trench allowed us to model the 3 d geometry of the concordia fault suggesting that it played a role in governing the morpho tectonic evolution of the bedrock in the dome c region and to propose a cenozoic age for its activity we recognize the importance of catchment basin morphology in hosting subglacial lakes and discuss the role played by tectonics glacial scouring and volcanism in the origin of the trench lakes basin lakes and relief lakes respectively

estimation of ice sheet motion using satellite radar interferometry method and error analysis with application to humboldt glacier greenland satellite radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets we examine the sources of error in interferometrically derived ice motion measurements including those errors due to inaccurate estimates of the interferometric baseline several simulations are used to assess baseline accuracy in terms of tie point error and the number and distribution of tie points these results give insight into how best to select tie points and also demonstrate the level of accuracy that can be achieved examination of two representative cases likely to occur in mapping ice sheet motion leads to the conclusion that with adequate tie point information ice velocity can be measured accurately to within a few meters per year a method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed finally we estimate the single component velocity field for an area on humboldt glacier northern greenland using interferograms formed from ers i sar images we estimate that these velocity measurements are accurate to within 2 3 m year 1

evidence for the long term sedimentary environment in an antarctic subglacial lake lakes beneath the antarctic ice sheet are of fundamental scientific interest for their ability to contain unique records of ice sheet history and microbial life in their sediments however no records of subglacial lake sedimentation have yet been acquired from beneath the interior of the ice sheet and understanding of sediment pathways processes and structure in subglacial lake environments remains uncertain here we present an analysis of seismic data from subglacial lake ellsworth showing that the lake bed comprises very fine grained sediments deposited in a low energy environment with low water and sediment fluxes minimum sediment thickness is 6 m the result of prolonged low sedimentation rates based on the few available analogues we speculate this sediment age range is a minimum of 150 ka and possibly 1 ma sediment mass movements have occurred but they are rare and have been buried by subsequent sedimentation we present a new conceptual model of subglacial lake sedimentation allowing a framework for evaluating processes in subglacial lake environments and for determining future lake access locations and interpreting subglacial lake samples