seismic moulin tremor through glacial moulins meltwater is routed from the glacier surface to its base moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics but their geometry remains poorly known here we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry we present modeling results of hour long seimic tremors emitted from a vertical moulin shaft observed with a seismometer array installed at the surface of the greenland ice sheet the tremor was triggered when the moulin water level exceeded a certain height which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column the amplitude of the tremor signal changed over each tremor episode in close relation to the amount of inflowing water the tremor spectrum features multiple prominent peaks whose characteristic frequencies are distributed like the resonant modes of a semiopen organ pipe and were found to depend on the moulin water level consistent with a source composed of resonant tube waves water pressure waves coupled to elastic deformation of the moulin walls along the water filled moulin pipe analysis of surface particle motions lends further support to this interpretation the seismic wavefield was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin the former was found to dominate the wave field at close distance and the latter at large distance to the moulin

fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

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

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

supraglacial lakes on the larsen b ice shelf antarctica and at paakitsoq west greenland a comparative study we acknowledge the support of the u s national science foundation under grant ant 0944248

a preliminary investigation of the influence of basal and surface topography on supraglacial lake distribution near jakobshavn isbrae western greenland the spatial distribution of supraglacial lakes has been hypothesized to be dominantly controlled by the component of surface roughness influenced by basal topography basal topography and surface roughness profiles within the jakobshavn isbrae drainage basin in western greenland acquired from an ice penetrating radar echo sounder were analysed through fourier and wavelet decompositions spectral analyses of basal to surface transfer under a range of ice thickness were compared with spatial distribution of lakes mapped from high resolution landsat imagery fourier analysis identifies dominant signals in both the basal and surface profiles to be ranging between wavelengths of 1 25 12 5 km the strongest peaks of transfer of basal signals to the surface were identified at wavelengths 11 5 km wavelet analysis identifies these peaks with thicker 1200 1400 m and thinner 500 700 m ice respectively and also identifies surface frequencies not present in the basal signal indicating some influence from other factors spatial autocorrelation analysis of supraglacial lake distribution indentifies high correlations at 1 9 5 6 11 24 and 30 km lags lags at 5 6 and 11 km correspond to dominant frequencies present in the basal and surface profiles over thinner and thicker ice respectively these 5 and 11 km frequency components present in the basal topography surface roughness and lake distribution are within the theoretical transfer window of basal to surface transfer as a function of ice thickness the transect analysed in this study does not contain wavelengths in the surface structure that are less than the ice thickness even though there are relatively higher frequencies present in the surface profile between 0 and 15 km that have less power in the basal profile copyright 2011 john wiley sons ltd

characterizing englacial drainage in the ablation zone of the greenland ice sheet rapid local drainage of surface meltwater to the base of the greenland ice sheet is thought to result in surface velocity variations as far inland as the equilibrium zone zwally and others 2002 ice penetrating radar surveys throughout this region allow us to characterize englacial drainage features that appear as vertically stacked diffraction hyperbolae in common offset profiles these data are used with a radar simulation model which allows for variations in geometry penetration depth and infill material to understand the characteristics of these hyperbolae and the likelihood that they are produced by moulins we find only a moderate correlation between the locations of these possible moulins and supraglacial lakes indicating that many lakes drain over the surface of the ice sheet or do not contain sufficient water to reach the bed through moulin formation we find a strong correlation between moulin location in the ablation region and elevated along flow tension due to flow over rough bedrock which generates surface crevassing and provides an entry point for meltwater although theory suggests that moulins may form anywhere on the ice sheet given sufficient meltwater input our data suggest that they are far more common in the ablation zone than near or inland from the equilibrium line

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

seasonal acceleration of inland ice via longitudinal coupling to marginal ice we use an ice flow model to demonstrate how flow variations initiated in the marginal zone of an ice sheet affect flow farther inland through longitudinal along flow coupling our findings allow for an alternate interpretation of seasonal accelerations observed near the equilibrium line of the greenland ice sheet zwally and others 2002 we demonstrate that these observations can be explained by accelerations initiated up to 12 km closer to the margin where the ice is 40 thinner is heavily crevassed experiences a seasonal doubling of velocity and where the ablation rate surface meltwater flux and ice temperature are likely higher our modeling and observations suggest that conditions and processes normally found near ice sheet margins are adequate for explaining the observations of zwally and others 2002 this and considerations of the likely subglacial hydrology in the marginal zone lead us to suggest that seasonal accelerations may have limited impact on ice sheet mass balance even in the face of climate warming

a comparison of supraglacial lake observations derived from modis imagery at the western margin of the greenland ice sheet supraglacial lakes sgls affect the dynamics of the greenland ice sheet by storing runoff and draining episodically we investigate the evolution of sgls as reported in three datasets each based on automated classification of satellite imagery although the datasets span the period 2001 10 there are differences in temporal sampling and only the years 2005 07 are common by subsampling the most populous dataset we recommend a sampling frequency of one image per 6 5 days in order to minimize uncertainty associated with poor temporal sampling when compared with manual classification of satellite imagery all three datasets are found to omit a sizeable 29 48 and 41 fraction of lakes and are estimated to document the average size of sgls to within 0 78 0 48 and 0 95km2 we combine the datasets using a hierarchical scheme producing a single optimized dataset this combined record reports up to 67 more lakes than a single dataset during 2005 07 the rate of sgl growth tends to follow the rate at which runoff increases in each year in 2007 lakes drain earlier than in 2005 and 2006 and remain absent despite continued runoff this suggests that lakes continue to act as open surface bed conduits following drainage

controls on rapid supraglacial lake drainage in west greenland an exploratory data analysis approach the controls on rapid surface lake drainage on the greenland ice sheet gris remain uncertain making it challenging to incorporate lake drainage into models of gris hydrology and so to determine the ice dynamic impact of meltwater reaching the ice sheet bed here we first use a lake area and volume tracking algorithm to identify rapidly draining lakes within west greenland during summer 2014 second we derive hydrological morphological glaciological and surface mass balance data for various factors that may influence rapid lake drainage third these factors are used within exploratory data analysis to examine existing hypotheses for rapid lake drainage this involves testing for statistical differences between the rapidly and non rapidly draining lake types as well as examining associations between lake size and the potential controlling factors this study shows that the two lake types are statistically indistinguishable for almost all factors investigated except lake area thus we are unable to recommend an empirically supported deterministic alternative to the fracture area threshold parameter for modelling rapid lake drainage within existing surface hydrology models of the gris however if improved remotely sensed datasets e g ice velocity maps climate model outputs were included in future research it may be possible to detect the causes of rapid drainage

tides modulate crevasse opening prior to a major calving event at bowdoin glacier northwest greenland retreat of calving glaciers worldwide has contributed substantially to sea level rise in recent decades mass loss by calving contributes significantly to the uncertainty of sea level rise projections at bowdoin glacier northwest greenland most calving occurs by a few large events resulting from kilometre scale fractures forming parallel to the calving front high resolution terrestrial radar interferometry data of such an event reveal that crevasse opening is fastest at low tide and accelerates during the final 36 h before calving using the ice flow model elmer ice we identify the crevasse water level as a key driver of modelled opening rates sea water level variations in the range of local tidal amplitude 1 m can reproduce observed opening rate fluctuations provided crevasse water level is at least 4 m above the low tide sea level the accelerated opening rates within the final 36 h before calving can be modelled by additional meltwater input into the crevasse enhanced ice cliff undercutting by submarine melt ice damage increase due to tidal cyclic fatigue crevasse deepening or a combination of these processes our results highlight the influence of surface meltwater and tides on crevasse opening leading to major calving events at grounded tidewater glaciers such as bowdoin

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

response of debris covered glaciers in the mount everest region to recent warming and implications for outburst flood hazards abstract in areas of high relief many glaciers have extensive covers of supraglacial debris in their ablation zones which alters both rates and spatial patterns of melting with important consequences for glacier response to climate change wastage of debris covered glaciers can be associated with the formation of large moraine dammed lakes posing risk of glacier lake outburst floods glofs in this paper we use observations of glaciers in the mount everest region to present an integrated view of debris covered glacier response to climate change which helps provide a long term perspective on evolving glof risks in recent decades debris covered glaciers in the everest region have been losing mass at a mean rate of 0 32 m yr 1 although in most cases there has been little or no change in terminus position mass loss occurs by 4 main processes 1 melting of clean ice close to glacier elas 2 melting beneath surface debris 3 melting of ice cliffs and calving around the margins of supraglacial ponds and 4 calving into deep proglacial lakes modelling of processes 1 and 2 shows that everest region glaciers typically have an inverted ablation gradient in their lower reaches due to the effects of a down glacier increase in debris thickness mass loss is therefore focused in the mid parts of glacier ablation zones causing localised surface lowering and a reduction in downglacier surface gradient which in turn reduce driving stress and glacier velocity so the lower ablation zones of many glaciers are now stagnant model results also indicate that increased summer temperatures have raised the altitude of the rain snow transition during the summer monsoon period reducing snow accumulation and ice flux to lower elevations as downwasting proceeds formerly efficient supraglacial and englacial drainage networks are broken up and supraglacial lakes form in hollows on the glacier surface ablation rates around supraglacial lakes are typically one or two orders of magnitude greater than sub debris melt rates so extensive lake formation accelerates overall rates of ice loss most supraglacial lakes are perched above hydrological base level and are susceptible to drainage if they become connected to the englacial drainage system speleological surveys of conduits show that large englacial voids can be created by drainage of warm lake waters along pre existing weaknesses in the ice roof collapses can open these voids up to the surface and commonly provide the nuclei of new lakes thus by influencing both lake drainage and formation englacial conduits exert a strong control on surface ablation rates an important threshold is crossed when downwasting glacier surfaces intersect the hydrological base level of the glacier base level lakes formed behind intact moraine dams can grow monotonically and in some cases can pose serious glof hazards glacier termini can evolve in different ways in response to the same climatic forcing so that potentially hazardous lakes will form in some situations but not others additionally the probability of a flood is not simply a function of lake volume but depends on the geometry and structure of the dam and possible trigger mechanisms such as ice or rockfalls into the lake satellite based measurements of glacier surface gradient and ice velocities allow probable future locations of base level lakes to be identified a base level lake has begun to grow rapidly on ngozumpa glacier west of mount everest and could attain a volume of 10 8 m 3 within the next 2 or 3 decades unless mitigation efforts are undertaken this lake could pose considerable glof hazard potential

temperature distribution and thermal anomalies along a flowline of the greenland ice sheet englacial and basal temperature data for the greenland ice sheet gris are sparse and mostly limited to deep interior sites and ice streams providing an incomplete representation of the thermal state of ice within the ablation zone here we present 11 temperature profiles at five sites along a 34 km east west transect of west greenland these profiles depict ice temperatures along a flowline and local temperature variations between closely spaced boreholes a temperate basal layer is present in all profiles increasing in thickness in the flow direction where it expands from 3 of ice height furthest inland to 100 at the margin temperate thickness growth is inconsistent with modeled heat contributions from strain heating heat conduction and vertical extension of the temperate layer we suggest that basal crevassing facilitated by water pressures at or near ice overburden pressure is responsible for the large temperate ice thicknesses observed high temperature kinks at 51 85m depth are likely remnants from the thermal influence of partially water filled crevasses up ice sheet steep horizontal temperature gradients between closely grouped boreholes suggest the recent thermal influence of a moulin these profiles demonstrate the ability of meltwater to rapidly alter ice temperatures at all depths within the ablation zone

towards a general state variable constitutive relation to describe granular deformation triaxial compression experiments were performed on cylindrical quartz sand packs to explore the universality of statevariable constitutive relations used to describe granular deformation samples were subjected to relaxation creep experiments similar to slide hold slide tests from friction studies for relaxation creep tests a constant rate of axial shortening was interrupted by time intervals for which the applied shortening rate was set to zero during holds loadforce decayed exponentially with time and the total stress relaxation scales with the logarithm of hold time consistent with friction tests on reloading after holds stress monotonically increases to the pre hold steady state levels without displaying the restrengthening of static friction often observed from shear tests on granular layers we find that the existing rate and state variable friction laws can model the transient strength behavior during relaxation creep and subsequent reloading by using simple assumptions about the deformation of cylindrical sand packs in triaxial compression by analogy with the rate and state friction constitutive relations we propose a generalized state variable expression for granular deformation in simple and pure shear although additional experiments under a variety of load boundary geometries and load paths will be needed such a general constitutive relation may ultimately describe granular deformation under a broad range of conditions e g for large variations in loading rate shear stress normal stress mean stress and may be applicable to a variety of research problems that involve deformation within granular media d 2005 elsevier b v all rights reserved

new constraints on greenland ice sheet dynamics during the last glacial cycle evidence from the uummannaq ice stream system this paper presents the first assessment of the uummannaq ice stream system uiss in west greenland the uiss drained 6 of the greenland ice sheet gris at the last glacial maximum lgm the onset of the uiss is a function of a convergent network of fjords which feed a geologically controlled trough system running offshore to the shelf break mapping cosmogenic radiogenic nuclide crn dating and model output reveal that glacially scoured surfaces up to 1266 m above sea level asl in fjord head areas were produced by warm based ice moving offshore during the lgm with the elevation of warm based ice dropping westwards to 700 m asl as the ice stream trunk zone developed marginal plateaux with allochthonous blockfields suggest that warm based ice produced till and erratics up to 1200 m asl but crn ages and weathering pits suggest this was pre lgm with only cold based ice operating during the lgm deglaciation began on the outer shelf at 14 8 cal kyrs b p with ubekendt ejland becoming ice free at 12 4 ka the uiss then collapsed with over 100 km of retreat by 11 4 ka 10 8 cal kyrs b p a rapid and complex response to bathymetric deepening trough widening and sea level rise coinciding with rapidly increasing air temperatures and solar radiation but which occurred prior to ocean warming at 8 4 cal kyrs b p local fjord constriction temporarily stabilized the unzipped uiss margins at the start of the holocene before ice retreat inland of the current margin at 8 7 ka

basal crevasses on the larsen c ice shelf antarctica implications for meltwater ponding and hydrofracture a key mechanism for the rapid collapse of both the larsen a and b ice shelves was meltwater driven crevasse propagation basal crevasses large scale structural features within ice shelves may have contributed to this mechanism in three important ways i the shelf surface deforms due to modified buoyancy and gravitational forces above the basal crevasse creating 10 m deep compressional surface depressions where meltwater can collect ii bending stresses from the modified shape drive surface crevassing with crevasses reaching 40 m in width on the flanks of the basal crevasse induced trough and iii the ice thickness is substantially reduced thereby minimizing the propagation distance before a full thickness rift is created we examine a basal crevasse 4 5 km in length 230 m in height and the corresponding surface features in the cabinet inlet sector of the larsen c ice shelf using a combination of high resolution 0 5 m satellite imagery kinematic gps and in situ ground penetrating radar we discuss how basal crevasses may have contributed to the breakup of the larsen b ice shelf by directly controlling the location of meltwater ponding and highlight the presence of similar features on the amery and getz ice shelves with high resolution imagery 2012 american geophysical union all rights reserved

validation of a method for determining the depth of glacial melt ponds using satellite imagery in situ measurements of water depth and optical properties of a melt pond in east greenland were collected to verify a previously developed algorithm for determining supraglacial water depths using satellite imagery that algorithm made five simplifying assumptions which we have tested using the in situ data and laboratory analysis of water samples we conclude that three assumptions were justified one was not and the remaining one substrate homogeneity requires further study but probably has a minor effect on the retrieved water depths and volumes measured water depths of 0 2 3 0m agree well with those derived from a satellite image using the algorithm numerically modeled depths also agree well with those from the satellite image this new analysis demonstrates the validity of our algorithm as a means for determining meltwater volumes in supraglacial ponds and lakes

modeling moulin distribution on sermeq avannarleq glacier using aster and worldview imagery and fuzzy set theory abstract a fuzzy set overlay model is used to analyze the distribution of moulins vertical meltwater conduits on sermeq avannarleq dead glacier in west greenland in 1985 and 2008 09 input data is derived from a historical topographic map based on airborne visible imagery and more recent worldview 1 panchromatic imagery as well as an advanced spaceborne thermal emission and reflection radiometer aster digital elevation model dem a non parametric best fit model approach using a monte carlo simulation is used to derive the membership functions for moulin location based on three independent variables elevation slope and aspect and to test for the robustness of the model we determine that there is a topographic setting independent of time that favors the development of moulins in this region using the membership functions and an optimal alpha cut derived for 1985 we could correctly predict the locations of 88 of the moulins in 2008 09 the model accounts for increased surface melt in 2008 09 in comparison to 1985 our results demonstrate the potential of a fuzzy set based approach to improve models of ice sheet hydrology in western greenland by providing more reliable spatial distributions of entry points of meltwater into the ice based on remotely sensed datasets of the ice surface which are readily available

mechanisms of englacial conduit formation and their implications for subglacial recharge abstract ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by shreve 1972 movement of water in glaciers journal of glaciology 11 62 205 214 the shreve model is based on three main assumptions 1 englacial drainage is in steady state 2 englacial water will flow along the steepest hydraulic gradient within the glacier and 3 pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls the shreve model has been widely adopted as a fundamental component of englacial drainage theory there is no evidence however that the model provides a realistic picture of actual glacial drainage systems to evaluate shreve s theory we used speleological techniques to directly survey englacial conduits we mapped a total of 8 25 km of passage in 27 distinct englacial conduits in temperate polythermal cold based and debris covered glaciers between 2005 and 2008 new information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 176 m using speleological techniques in all cases englacial drainage systems consisted of a single unbranching conduit englacial conduit morphologies were found to be intimately linked to the orientation of a glacier s principal stresses or the presence of pre existing lines of high hydraulic conductivity if a sufficient supply of water is available hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive on unfractured glacier surfaces relatively shallow subhorizontal conduits with migrating nickpoints form by cut and closure provided channel incision is significantly faster than surface lowering conduits can also form along permeable debris filled crevasse traces that connect supraglacial lake basins of different potential our results suggest that shreve type englacial drainage systems do not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed where supraglacial water bodies either intersect or are advected through zones of acceleration

shrimp u pb ages of detrital zircons in metasedimentary rocks of the central ogcheon fold thrust belt korea evidence for tectonic assembly of paleozoic sedimentary protoliths abstract we analyzed detrital zircons in four metasandstones and two metadiamictites of the central ogcheon metamorphic belt omb in order to provide geochronologic constraints on the time of deposition and protolith age distributions shrimp u pb ages of the newly analyzed zircons range from archean to early paleozoic and their age patterns permit the definition of four lithotectonic slices upper and lower pibanryeong units consisting primarily of quartzite and metasandstone an upper poeun unit of black slate containing early permian plant fossils and a lower poeun unit made up of metasandstone and metadiamictite a quartzite of the upper pibanryeong unit yielded two major zircon age populations at 965 11 ma and 447 5 ma whereas two lower pibanryeong metasandstones show major age concentrations at 2 5 ga 1 0 0 9 ga and 545 535 ma the youngest components of these units suggest maximum ages of sedimentation as late ordovician and early cambrian respectively these upper and lower pibanryeong zircon age distribution patterns are consistent with those of ordovician and devonian sedimentary strata respectively from the south china craton scc on the other hand three samples of the lower poeun unit yielded two distinct age populations at 1 87 ga and 755 7 ma in addition a metadiamictite contains abundant mesoproterozoic zircons with two broad peaks centered at 1 3 and 1 6 ga protolith ages and distribution patterns are profoundly different between the pibanryeong and poeun lithotectonic packages suggesting the presence of a major thrust fault juxtaposing allochthons that are also characterized by large p t contrasts our detrital zircon results for the pibanryeong unit are compatible with derivation from the scc in apparent contrast with the poeun occurrence of early permian plant fossils most likely correlative with the north china craton ncc taken together the suturing of paleozoic metasedimentary terranes in the omb is a product of permo triassic orogeny reflecting collision of the north and south china cratons

observations of englacial water passages a fracture dominated system to test models of the hydraulics and geometry of englacial conduits 48 holes 3900 m of ice were drilled into storglaciaren sweden in search of conduits about 79 of the holes intersected a hydraulically connected englacial feature a video camera was used to examine the features and measure local water flow rates because of the extremely clear ice that surrounded most features their geometry could not be discerned of the remaining features 80 36 were fracture like 16 6 were of complex geometry and 4 2 exhibited a conduit like geometry the fracture like features exhibited steep plunges 708 narrow openings 40 mm and slow water flow speeds 10 mm s 1 we argue that these fracture like features are indeed englacial fractures of unknown origin the depth to fractures intersection varied from near the glacier surface to 96 of local ice depth with a maximum depth of 131 m few hydraulically connected fractures exhibited water motion indicating some preferential flow pathways exist we found one traditional englacial conduit after an intentional search in a field of moulins these results suggest that englacial water flow is conveyed through a ubiquitous network of fractures and that conduits are relatively rare

an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier bingham r g hubbard a l nienow p w sharp m j 2008 an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier journal of geophysical research earth surface 113 f2 f02006 sponsorship nerc nserc the university of alberta circumpolar institute and northern science training programme the geological society of america

a cut and closure origin for englacial conduits in uncrevassed regions of polythermal glaciers on uncrevassed regions of polythermal glaciers englacial conduits can form by incision of supraglacial stream channels followed by roof closure the origin and evolution of examples in longyearbreen svalbard and khumbu glacier nepal were determined by speleological survey the development of perennial incised channels requires that incision is significantly faster than glacier surface ablation and thus will be favoured by high meltwater discharges in combination with cool climatic conditions or thick debris cover incised canyons can become blocked by drifted winter snow refrozen meltwater ice rafting from non local sources allochthonous breccias and roof collapses autochthonous breccias conduit closure can also occur in response to ice creep particularly at depth following isolation from the surface englacial conduits continue to evolve by vadose incision down to local base level in the case of longyearbreen incision allowed the channel to reach the glacier bed but on khumbu glacier deep incision is prevented because an effectively impermeable terminal moraine provides a high base level for the glacier drainage system during our period of observations deeper parts of the longyearbreen conduit became blocked by a combination of ice accumulation and creep causing the stream course to be re routed to higher levels on that glacier incision blockage and upward re routing are cyclic we conclude that cut and closure is the dominant mechanism of englacial conduit formation on uncrevassed regions of polythermal glaciers

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

annual down glacier drainage of lakes and water filled crevasses at helheim glacier southeast greenland supraglacial lake drainage events are common on the greenland ice sheet observations on the west coast typically show an up glacier progression of drainage as the annual melt extent spreads inland we use a suite of remote sensing and modelling techniques in order to study a series of lakes and water filled crevasses within 20km of the terminus of helheim glacier south east greenland automatic classification of surface water areas shows a down glacier progression of drainage which occurs in the majority of years between 2007 and 2014 we demonstrate that a linear elastic fracture mechanics model can reliably predict the drainage of the uppermost supraglacial lake in the system but cannot explain the pattern of filling and draining observed in areas of surface water downstream we propose that the water levels in crevasses downstream of the supraglacial lake can be explained by a transient high pressure wave passing through the subglacial system following the lake drainage we support this hypothesis with analysis of the subglacial hydrological conditions which can explain both the position and interannual variation in filling order of these crevasses similar behaviour has been observed in association with jokulhaups surging glaciers and antarctic subglacial lakes but has not previously been observed on major outlets of the greenland ice sheet our results suggest that the behaviour of near terminus surface water may differ considerably from that of inland supraglacial lakes with the potential for basal water pressures to influence the presence of surface water in crevasses close to the terminus of tidewater glaciers

a new regional high resolution map of basal and surface topography for the greenland ice sheet margin at paakitsoq west greenland in 2005 an airborne survey was carried out from a twin otter aircraft at pakitsup akuliarusersua paakitsoq near ilulissat in west greenland the survey aimed to measure ice thickness with a 60 mhz coherent radar and surface elevation with a scanning laser altimeter positioning information came from multiple on board differential gps units and an inertial navigation system the region surveyed covers 80 km along the ice margin and has a total area of 2700 km 2 with varying density of measurements the between track distance was 1 km near the margin increasing to 3k m away from the margin regional high resolution maps of basal topography under the greenland ice sheet are useful for resolving important glaciological and hydrological questions and for enhancing related process studies such as the influence of basal meltwater on ice dynamics the ice sheet margin in this region is also currently under consideration for hydropower development and has a long and continuing history of glaciological investigations lately with emphasis on the connection between surface meltwater formation and surface velocity of the ice sheet here we present a new regional map of the surface and basal topography of the ice sheet margin and discuss some of the implications for reported observations at swiss camp

modelling the delivery of supraglacial meltwater to the ice bed interface application to southwest devon ice cap nunavut canada the transfer of surface generated meltwater to the subglacial drainage system through full ice thickness crevassing may lead to accelerated glacier velocities with implications for ice motion under future climatic scenarios accurate predictions of where surface meltwater accesses the ice bed interface are therefore needed in fully coupled hydrodynamic ice sheet models we present a spatially distributed modelling routine for predicting the location and timing of delivery of surface derived meltwater to the ice bed interface through moulins and supraglacial lake drainage the model is explained as it is applied to the croker bay glacial catchment of devon ice cap canada the formation of moulins drainage of lakes and the transfer of meltwater through the full ice thickness are modelled for the 2004 and 2006 ablation seasons through this case study we assess the model s sensitivity to degree day factors fracture toughness tensile strength and crevasse width and confirm that parameters influencing the rate at which water fills a crevasse are the most significant controls on the ability of a crevasse to reach the bed increased surface melt production therefore has the potential to significantly influence the spatial and temporal transfer of meltwater through surface to bed connections in a warmer climate

constraints on the lake volume required for hydro fracture through ice sheets 1 water filled cracks are an effective mechanism to drive hydro fractures through thick ice sheets crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water filled until it reaches the bed in this study we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water filled cracks beneath supraglacial lakes we find that the cross sectional area of water filled cracks increases non linearly with ice sheet thickness using these results we place volumetric constraints on the amount of water necessary to drive cracks through 1 km of sub freezing ice for ice sheet regions under little tension lakes larger than 0 25 0 80 km in diameter contain sufficient water to rapidly drive hydro fractures through 1 1 5 km of subfreezing ice this represents 98 of the meltwater volume held in supraglacial lakes in the central western margin of the greenland ice sheet

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

englacial drainage structures in an east antarctic outlet glacier ground penetrating radar data acquired in the 2016 17 austral summer on sorsdal glacier east antarctica provide evidence for meltwater lenses within porous surface ice that are conceptually similar to firn aquifers observed on the greenland ice sheet and the arctic and alpine glaciers these englacial water bodies are associated with a dry relict surface basin and consistent with perennial drainage into an interconnected englacial drainage system which may explain a large englacial outburst flood observed in satellite imagery in the early 2016 17 melt season our observations indicate the rarely documented presence of an englacial hydrological system in antarctica with implications for the storage and routing of surface meltwater future work should ascertain the spatial prevalence of such systems around the antarctic coastline and identify the degree of surface runoff redistribution and storage in the near surface to quantify their impact on surface mass balance

subglacial hydrology at rink isbrae west greenland inferred from sediment plume appearance marine terminating outlet glaciers discharge most of the greenland ice sheet s mass through frontal ablation and meltwater runoff while calving can be estimated by in situ and remote sensing observations submarine melting and subglacial meltwater transport are more challenging to quantify here we investigate the subglacial hydrology of rink isbrae a fast flowing west greenland tidewater glacier using time lapse photography modeled runoff estimates and daily satellite imagery from 2007 to 2011 we find that sediment plumes appear episodically at four distinct locations across the terminus and last between 2 h and 17 d this suggests short term variability in discharge and the existence of persistent pathways the seasonal onset of sediment plumes occurs before supraglacial lake drainages shortly after the onset of runoff and only after the wintertime ice melange has begun disintegrating plumes were also visible after the cessation of runoff 23 5 d which is indicative of subglacial storage the lack of either a seasonal velocity change or a correspondence between meltwater availability and plume occurrence suggests that the subglacial system persists in a state of inefficient drainage subglacial hydrology at tidewater glaciers is of critical importance in understanding dynamics at the ice front

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

detection of a subglacial lake in glacier de tete rousse mont blanc area france the processes that form intraglacial lakes are poorly understood because of the difficulty in detecting and assessing such hidden lakes extensive geophysical surveys were performed between 2007 and 2010 in order to reassess the risk of an outburst flood from glacier de tete rousse where outburst flooding from an intraglacial lake caused 175 fatalities in 1892 our geophysical survey combined ground penetrating radar measurements and nuclear magnetic resonance imaging we found a subglacial water filled reservoir with a volume of 55 000 m 3 artesian outpourings occurred when the subglacial cavity was reached by two borehole drillings indicating that the hydrostatic water pressure exceeded the ice pressure at the bottom of the cavity on the basis of these geophysical and glaciological findings we warned the public authorities in july 2010 of the risk facing the 3000 inhabitants downstream of the glacier the subglacial reservoir was drained artificially this example demonstrates how geophysical surveys can be used to detect this type of hazard when it is suspected in particular when no hydraulic outlet from the snout exists numerous ice temperature measurements have shown that the tongue of this glacier is cold based this thermal regime could explain the accumulation of water in this glacier

supraglacial forcing of subglacial drainage in the ablation zone of the greenland ice sheet 1 we measure hydrological parameters in meltwater draining from an outlet glacier in west greenland to investigate seasonal changes in the structure and behaviour of the hydrological system of a large catchment in the greenland ice sheet gris our data reveal seasonal upglacier expansion and increase in hydraulic efficiency of the subglacial drainage system across a catchment 600 km2 to distances 50 km from the ice sheet margin this expansion occurs episodically in response to the drainage of surface meltwaters into a hitherto inefficient subglacial drainage system as new input locations become active progressively further upglacier this system is similar to alpine glaciers these observations provide the first synopsis of seasonal hydrological behaviour in the ablation zone of the gris

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

rapid response of modern day ice sheets to external forcing abstract the great ice sheets covering antarctica and greenland were traditionally believed to take thousands of years to respond to external forcing recent observations suggest however that major changes in the dynamics of parts of the ice sheets are taking place over timescales of years these changes were not predicted by numerical models and the underlying cause s remains uncertain it has been suggested that regional oceanic and or atmospheric warming are responsible but separating the influence and importance of these two forcings has not been possible in most cases the role of atmospheric versus oceanic control remains uncertain here we review the observations of rapid change and discuss the possible mechanisms in the light of advances in numerical modelling and our understanding of the processes that may be responsible

mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 1 observations the data presented in part 1 of this paper meier et al this issue are here used to assess the role of water input output water storage and basal water pressure in the rapid movement of columbia glacier alaska consistently high basal water pressures mostly in the range from 300 kpa below to 100 kpa above the ice overburden pressure are responsible in an overall way for the high glacier flow velocities 3 5 9 m d 1 which are due mainly to rapid basal sliding caused by the high water pressure diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site the water pressure fluctuation tracks the time integrated water input less a steady withdrawal as expected for the diurnally fluctuating storage of water in the glacier far from the terminus at the downglacier site the situation is more complex diurnal peaks in water level which are directly related to intraglacial water storage as well as to basal water pressure are shifted forward in time by 4 hours probably as a result of the effect of diurnal fluctuation in water output from the glacier which affects the local water storage fluctuations near the terminus because of the forward shift in the basal water pressure peaks which at the downglacier site lead the velocity peaks by 6 hours a mechanical connection between water pressure and sliding there would have to involve a 6 hour quarter period delay however the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single consistent control mechanism at both sites the velocity variations in nondiurnal speed up events caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier it appears that small variations in water pressure 100 kpa sufficient to produce the observed velocity variations 15 30 are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity at the achieved level of observation the clearest though not complication free control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

moulin density controls drainage development beneath the greenland ice sheet this work was funded through a uk natural environment research council doctoral training grant lcag 133 a bowring junior research fellowship st catharine s college cambridge and a leverhulme newton trust early career fellowship all awarded to a f b i j h was supported by a marie curie fp7 career integration grant within the 7th european union framework programme

a decadal investigation of supraglacial lakes in west greenland using a fully automatic detection and tracking algorithm abstract the sudden drainage of supraglacial lakes has been previously observed to initiate surface to bed hydrologic connections which are capable of enhancing basal sliding in regions of the greenland ice sheet where ice thickness approaches 1 km in this study we develop a robust algorithm which automatically detects and tracks individual supraglacial lakes using visible satellite imagery to document the evolution of a population of west greenland supraglacial lakes over ten consecutive melt seasons validation tests indicate that the algorithm is highly accurate 99 0 of supraglacial lakes can be detected and tracked and 96 3 of reported lakes are true supraglacial lakes with accurate lake properties such as lake area and timing of formation and drainage investigation of the interannual evolution of supraglacial lakes in the context of annual melt intensity reveals that during more intense melt years supraglacial lakes drain more frequently and earlier in the melt season additionally the lake population extends to higher elevations during more intense melt years exposing an increased inland area of the ice sheet to sudden lake drainage events these observations suggest that increased surface meltwater production due to climate change will enhance the spatial extent and temporal frequency of lake drainage events it is unclear whether this will ultimately increase or decrease the basal sliding sensitivity of interior regions of the greenland ice sheet

three dimensional glacial flow and surface elevation measured with radar interferometry outlet glaciers which serve to drain ice from ice sheets seem to be dynamically less stable in north greenland than in south greenland1 2 3 storstrommen a large outlet glacier in northeastern greenland which surged between 1978 and 1984 ref 2 has been well studied in general neither glacier surge mechanisms nor the geographical distribution of the surges are well known conventional satellite radar interferometry can provide large scale topography models with high resolution4 and can measure the radar line of sight component of ice flow vectors5 but cannot map full vector flow fields here we present an interferometry method that combines observations from descending and ascending satellite orbits which assuming ice flow parallel to the topographic surface allows us to use the differing view angles to estimate full three dimensional surface flow patterns the accuracy of our technique is confirmed by the good agreement between our radar based flow model and in situ global positioning system gps reference data at storstrommen radar measurements such as these made regularly and at high spatial density have the potential to substantially enhance our understanding of glacier dynamics and ice sheet flow as well as improve the accuracy of glacier mass balance estimates

testing crevasse depth models a field study at breiðamerkurjokull iceland interest in crevasses and associated ice fracture processes has recently increased due to recognition of the importance of calving glaciers to the mass balance of the cryosphere as well as the importance of fractures in glacier hydrology recently developed calving criteria make use of models which predict crevasse depth from surface strain rates but these models have rarely been tested against observations in this study we present data on crevasse depth and surface strain rates and compare the measured values with results of two crevasse depth models a simple function proposed by nye and a linear elastic fracture mechanics lefm model developed by van der veen our results indicate that both models predict crevasse depths within the correct order of magnitude the lefm model incorporating measured values of crevasse spacing and tuned for fracture toughness performs better in predicting crevasse depths but where lack of input data precludes such tuning the results are similar to nye s model predictions we conclude that both models may be used to calculate crevasse depths in calving models although the nye function is undoubtedly much simpler to implement within an ice dynamics model

mechanisms of subglacial cavity filling in glacier de tete rousse french alps the deadliest outburst flood from an englacial cavity occurred on glacier de tete rousse in the mont blanc area french alps in 1892 a subglacial reservoir was discovered in the same glacier in 2010 and drained artificially in 2010 2011 and 2012 to protect the 3000 inhabitants downstream the mechanism leading to the spontaneous refilling of the cavity following these pumping operations has been analyzed for this purpose the subglacial water volume changes between 2010 and 2013 were reconstructed the size of the cavity following the pumping was found to have decreased from 53 500 m 3 in 2010 to 12 750 m 3 in 2013 creep and the partial collapse of the cavity roof explain a large part of the volume loss analysis of cavity filling showed a strong relationship between measured surface melting and the filling rate with a time delay of 4 6 hours a permanent input of 15 m 3 d 1 not depending on surface melt was also found the meltwater and rain from the surface is conveyed to bedrock through crevasses and probably through a permeable layer of rock debris at the glacier bed the drainage pathway permeability was estimated at 0 054 m s 1 from water discharge measurements and dye tracing experiments

response of glacier basal motion to transient water storage the speed of a glacier is affected most by sudden jumps in the water supply to the glacier but it goes back to previous levels if high water inputs are sustained because the glacier s plumbing system adjusts

modelling channelized surface drainage of supraglacial lakes supraglacial lakes can drain to the bed of ice sheets affecting ice dynamics or over their surface relocating surface water focusing on surface drainage we first discuss observations of lake drainage in particular for the first time lakes are observed to drain 70 km across the nivlisen ice shelf east antarctica inspired by these observations we develop a model of lake drainage through a channel that incises into an ice sheet surface by frictional heat dissipated in the flow modelled lake drainage can be stable or unstable during stable drainage the rate of lake level drawdown exceeds the rate of channel incision so discharge from the lake decreases with time this can prevent the lake from emptying completely during unstable drainage discharge grows unstably with time and always empties the lake model lakes are more prone to drain unstably when the initial lake area the lake input and the channel slope are larger these parameters will vary during atmospheric warming induced ablation area expansion hence the mechanisms revealed by our analysis can influence the dynamic response of ice sheets to warming through their impact on surface water routing and storage

fracture propagation as means of rapidly transferring surface meltwater to the base of glaciers 1 propagation of water filled crevasses through glaciers is investigated based on the linear elastic fracture mechanics approach a crevasse will penetrate to the depth where the stress intensity factor at the crevasse tip equals the fracture toughness of glacier ice a crevasse subjected to inflow of water will continue to propagate downward with the propagation speed controlled primarily by the rate of water injection while the far field tensile stress and fracture toughness determine where crevasses can form once initiated the rate of water driven crevasse propagation is nearly independent of these two parameters thus rapid transfer of surface meltwater to the bed of a cold glacier requires abundant ponding at the surface to initiate and sustain full thickness fracturing before refreezing occurs

a mini surge on the ryder glacier greenland observed by satellite radar interferometry satellite radar interferometry reveals that the speed of the ryder glacier increased roughly threefold and then returned to normal 100 to 500 meters year over a 7 week period near the end of the 1995 melt season the accelerated flow represents a substantial though short lived change in ice discharge during the period of rapid motion meltwater filled supraglacial lakes may have drained which could have increased basal water pressure and caused the mini surge there are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in greenland but because most other outlet glaciers are at lower latitudes they should experience more extensive melting making them more susceptible to meltwater induced surges

an assessment of surge induced crevassing and the formation of crevasse squeeze ridges 1 evidence for extensive crevassing is preserved on the deglaciated forelands of many surging glaciers as crevasse squeeze ridges csrs at some point these crevasses make direct connection with the bed in order to become sediment filled and full depth connections have been inferred from turbid water up wellings in crevasses and the formation of concertina eskers the dynamics of seven surging glaciers are assessed using a linear elastic fracture mechanics approach to determine the likely directions of fracture and controlling parameters for mode i crevasses extensional surface strain rates are insufficient to promote top down full depth penetration for small crevasse spacing 5 m surface strain rates are sufficient for top down crevassing to depths of 4 12 m explaining the extensive surface crevassing associated with glacier surging as has been shown in other settings top down full depth crevassing is only possible when water is added and approaches 97 of the crevasse depth the provision of sufficient meltwater to facilitate this is problematic due to the extensive surface crevassing unless water can move along connected crevasses to a dominant water capturing crevasse for ice thicknesses greater than 200 m basal water pressures in excess of 80 90 of flotation are required for full depth bottom up crevassing field evidence suggests that this is the default for surging glaciers and that on occasion water pressures may even become artesian csrs found across many surging glacier forelands and ice margins most likely result from the infilling of basal crevasses driven for the most part bottom up by high basal water pressures

seasonal evolution of supraglacial lakes on a floating ice tongue petermann glacier greenland supraglacial lakes are known to trigger antarctic ice shelf instability and break up however to date no study has focused on lakes on greenland s floating termini here we apply lake boundary area and depth algorithms to landsat 8 imagery to analyse the inter and intraseasonal evolution of supraglacial lakes across petermann glacier s 81 n floating tongue from 2014 to 2016 while also comparing these lakes to those on the grounded ice lakes start to fill in june and quickly peak in total number volume and area in late june early july in response to increases in air temperatures however through july and august total lake number volume and area all decline despite sustained high temperatures these observations may be explained by the transportation of meltwater into the ocean by a river and by lake drainage events on the floating tongue further as mean lake depth remains relatively constant during this time we suggest that a large proportion of the lakes that drain do so completely likely by rapid hydrofracture the mean areas of lakes on the tongue are only 20 of those on the grounded ice and exhibit lower variability in maximum and mean depth differences likely attributable to the contrasting formation processes of lakes in each environment

seismic characterization of a rapidly rising jokulhlaup cycle at the a p olsen ice cap ne greenland rapidly rising jokulhlaups or glacial outburst floods are a phenomenon with a high potential for damage the initiation and propagation processes of a rapidly rising jokulhlaup are still not fully understood seismic monitoring can contribute to an improved process understanding but comprehensive long term seismic monitoring campaigns capturing the dynamics of a rapidly rising jokulhlaup have not been reported so far to fill this gap we installed a seismic network at the marginal ice dammed lake of the a p olsen ice cap apo in ne greenland episodic outbursts from the lake cause flood waves in the zackenberg river characterized by a rapid discharge increase within a few hours our 6 months long seismic dataset comprises the whole fill and drain cycle of the ice dammed lake in 2012 and includes one of the most destructive floods recorded so far for the zackenberg river seismic event detection and localization reveals abundant surface crevassing and correlates with changes of the river discharge seismic interferometry suggests the existence of a thin basal sedimentary layer we show that the ballistic part of the first surface waves can potentially be used to infer medium changes in both the ice body and the basal layer interpretation of time lapse interferograms is challenged by a varying ambient noise source distribution

evolution of drainage system morphology at a land terminating greenlandic outlet glacier 1 the influence of meltwater on the dynamics and geomorphic impact of the greenland ice sheet is strongly controlled by the morphology of the ice sheet s drainage system however this system and its evolution through the melt season remain poorly understood here we present the results of an intensive programme of dye tracing experiments undertaken along the lower 14 km of a land terminating greenlandic outlet glacier over a period of four months during the 2010 melt season these data are interpreted in conjunction with observations of proglacial discharge englacial water storage surface melt rates and ice velocity to produce a detailed picture of the changing hydrology of the glacier following the onset of melt in the spring inputs to the drainage system regularly exceed outputs causing the englacial water level to rise to the ice sheet surface during this time there is a rapid transition from distributed to channelized drainage in those parts of the drainage system closed by ice deformation over winter as the melt season progresses channel efficiency increases and englacial storage and ice velocity decrease high velocity events continue to be observed following the channelization of the drainage system however indicating that hydrological forcing of ice velocity occurs despite the existence of channels during periods when meltwater inputs exceed the capacity of the subglacial drainage system

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

supraglacial lake spatial structure in western greenland during the 2007 ablation season 1 increases in surface melt rates along the greenland ice sheet margin have accelerated meltwater production resulting in supraglacial lakes that can drain rapidly and enhance basal sliding in this study supraglacial lakes in western greenland are mapped from multitemporal landsat panchromatic imagery during the 2007 melt season the analysis models lake spatial structure and establishes sequencing of drainage from 6 june through 9 august 2007 results indicate high spatial clustering at lower elevations between 100 and 700 m with minimum lake to lake distances of 100 500 m and lake sizes 0 5 km2 at higher elevations 700 1200 m clustering decreases with minimum distances on the order of 900 1400 m and lake sizes between 0 5 and 1 km2 lakes at elevations above 1200 m exhibit less clustering and smaller areas clustering was spatially heterogeneous within elevation bands with high clustering between ice streams and fewer lakes along shear margins spatial patterns of lake drainage throughout the 2007 melt season exhibit strong spatial coherency the magnitude of clustering reveals patterns caused by surface melt production and water routing through surface streams as well as in situ drainage cluster patterns are influenced by proximity to crevasses fields shear margins as well as topographic structure

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

surge propagation constrained by a persistent subglacial conduit bakaninbreen paulabreen svalbard glacier surges tend to be initiated in relatively small regions then propagate down glacier up glacier and or across glacier the processes controlling patterns and rates of surge propagation however are incompletely understood in this paper we focus on patterns of surge propagation in two confluent glaciers in svalbard and examine possible causes one of these glaciers bakaninbreen surged in 1985 95 the surge propagated 7 km down glacier but did not cross the medial moraine onto the other glacier paulabreen when paulabreen surged between 2003 and 2005 the surge wave travelled several km down glacier but its lateral boundary stayed very close to the medial moraine the confluent glaciers formerly extended into a fjord and bathymetric mapping and historical observations show that an active subglacial conduit has existed between bakaninbreen and paulabreen since at least the early 20th century the existence of a persistent subglacial conduit below the medial moraine was confirmed when we entered and mapped a nye channel at the confluence of bakaninbreen and paulabreen we argue that the conduit acts as a barrier to surge propagation if pressurized water below one branch of the glacier system reaches the conduit water can be readily evacuated preventing its propagation into the other branch

the role of hydrologically driven ice fracture in drainage system evolution on an arctic glacier 1 observations from ellesmere island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically driven propagation of fractures from the surface to the bed where ice is 150 m thick water ponded to a depth of 6 9 m within a supraglacial stream system before establishing a permanent bed connection multiple premonitory drainage events preceded the final drainage of ponded water implying that fracturing is necessary but insufficient to establish a permanent link between surface and subglacial drainage refreezing of water that penetrates the first fractures to form may reseal the connection while flow resistance within the subglacial system may delay the onset of continuous through flow a large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis