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

persistent flow acceleration within the interior of the greenland ice sheet we present surface velocity measurements from a high elevation site located 140 km from the western margin of the greenland ice sheet and 50 km into its accumulation area annual velocity increased each year from 51 78 0 01 m yr 1 in 2009 to 52 92 0 01 m yr 1 in 2012 a net increase of 2 2 these data also reveal a strong seasonal velocity cycle of up to 8 1 above the winter mean driven by seasonal melt and supraglacial lake drainage sole et al 2013 recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer faster summers our data extend this analysis and reveal a year on year increase in annual velocity above the equilibrium line altitude where despite surface melt increasing it is still sufficiently low to hinder the development of efficient drainage under thick ice

greenland ice sheet motion insensitive to exceptional meltwater forcing changes to the dynamics of the greenland ice sheet can be forced by various mechanisms including surface melt induced ice acceleration and oceanic forcing of marine terminating glaciers we use observations of ice motion to examine the surface melt induced dynamic response of a land terminating outlet glacier in southwest greenland to the exceptional melting observed in 2012 during summer meltwater generated on the greenland ice sheet surface accesses the ice sheet bed lubricating basal motion and resulting in periods of faster ice flow however the net impact of varying meltwater volumes upon seasonal and annual ice flow and thus sea level rise remains unclear we show that two extreme melt events 98 6 of the greenland ice sheet surface experienced melting on july 12 the most significant melt event since 1889 and 79 2 on july 29 and summer ice sheet runoff 3 9 above the 1958 2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009 however despite record summer melting subsequent reduced winter ice motion resulted in 6 less net annual ice motion in 2012 than in 2009 our findings suggest that surface melt induced acceleration of land terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios

geometry bed topography and drainage system structure of the haut glacier d arolla switzerland as part of an integrated study of the hydrology meltwater quality and dynamics of the haut glacier d arolla switzerland the glacier s drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier this structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by a the subglacial hydraulic potential surface and b the subglacial bedrock surface these reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio echo sounding techniques the two reconstructions simulate the drainage system structures expected for a closed channels in which water is pressurized by the overlying ice and b gravity driven open channel flow the closed channel model provides the best fit to the observed structure even though theoretical calculations suggest that under summer discharge conditions open channel flow may be widespread beneath the glacier possible reasons for this apparent discrepancy are discussed

spectral properties of halite rich mineral mixtures implications for middle infrared remote sensing of highly saline environments abstract middle infrared reflectance of minerals is often used to qualitatively predict their spectral emittance characteristics reflectance spectra of particulate minerals dispersed in a nonabsorbing material such as halite nacl can be very different from those of the isolated minerals and often resemble transmittance spectra to illustrate this behavior diffuse reflectance spectra 4 3 m 14 3 m of particulate samples of calcite gypsum quartz and montmorillonite in various mixtures with nacl are compared with reflectance and transmittance spectra of the pure minerals diffuse reflectance of a natural efflorescence associated with a sample of mojave desert playa material is also illustrated similarities in the spectra of the field sample and certain of the laboratory mixtures suggest that high concentrations of nacl could produce ambiguities for remotely sensed observations in certain saline environments however these preliminary laboratory measurements need supporting field studies to determine their significance in actual remote sensing situations

seasonal speedup of the greenland ice sheet linked to routing of surface water we use interferometric synthetic aperture radar observations recorded in a land terminating sector of western greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow our data reveal a non uniform pattern of late summer ice speedup that in places extends over 100 km inland we show that the degree of late summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey and that the pattern of late summer speedup follows that of water routed at the ice sheet surface in late summer ice within the largest catchment flows on average 48 faster than during winter whereas changes in smaller catchments are less pronounced our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup

influence of subglacial drainage conditions on the velocity distribution within a glacier cross section substrate properties and hydrological conditions at the base of haut glacier d arolla switzerland are nonuniform the thickness and grain size of subglacial sediment vary on length scales less than one ice thickness whereas hydrological conditions vary seasonally and on length scales of about one ice thickness transverse to ice motion there is a close relationship between the annually averaged velocity field and this nonuniformity of bed conditions basal motion dominates in an area with large water level variations in boreholes whereas ice deformation contributes significantly to total movement elsewhere localized enhanced basal motion occurs primarily in summer especially during a july spring event but this motion is barely discernible in annually averaged surface velocity measurements because transverse coupling suppresses surface expression of the basal motion discontinuity these results highlight the need to include representations of bed nonuniformity in models of glacier flow and to consider ice deformation and basal motion as interdependent processes

basal drainage system response to increasing surface melt on the greenland ice sheet surface meltwater reaching the bed of the greenland ice sheet imparts a fundamental control on basal motion sliding speed depends on ice bed coupling dictated by the configuration and pressure of the hydrologic drainage system in situ observations in a four site transect containing 23 boreholes drilled to greenland s bed reveal basal water pressures unfavorable to water draining conduit development extending inland beneath deep ice this finding is supported by numerical analysis based on realistic ice sheet geometry slow meltback of ice walls limits conduit growth inhibiting their capacity to transport increased discharge key aspects of current conceptual models for greenland basal hydrology derived primarily from the study of mountain glaciers appear to be limited to a portion of the ablation zone near the ice sheet margin

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

winter motion mediates dynamic response of the greenland ice sheet to warmer summers 1 we present ice velocities from a land terminating transect extending 115 km into the western greenland ice sheet during three contrasting melt years 2009 2011 to determine whether enhanced melting accelerates dynamic mass loss we find no significant correlation between surface melt and annual ice flow there is however a positive correlation between melt and summer ice displacement but a negative correlation with winter displacement this response is consistent with hydro dynamic coupling enhanced summer ice flow results from longer periods of increasing surface melting and greater duration ice surface to bed connections while reduced winter motion is explicable by drainage of high basal water pressure regions by larger more extensive subglacial channels despite mean interannual surface melt variability of up to 70 mean annual ice velocities changed by 7 5 increased summer melting thereby preconditions the ice bed interface for reduced winter motion resulting in limited dynamic sensitivity to interannual variations in surface melting

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

modeling channelized and distributed subglacial drainage in two dimensions 1 we present a two dimensional glacier drainage system model glads that couples distributed and channelized subglacial water flow distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness channelized flow occurs through rothlisberger channels that can form on any of the edges of a prescribed unstructured network of potential channels water storage is accounted for in an englacial aquifer and in moulins which also act as point sources of water to the subglacial system solutions are presented for a synthetic topography designed to mimic an ice sheet margin for low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge the model exhibits a channelization instability which leads to the formation of a self organized channel system the random orientation of the network edges allows the channel system geometry to be relatively unbiased in contrast to previous structured grid based models under steady conditions the model supports the classical view of the subglacial drainage system with low pressure regions forming around the channels under diurnally varying input water flows in and out of the channels and a rather complex spatiotemporal pattern of water pressures is predicted we explore the effects of parameter variations on the channel system topology and mean effective pressure the model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model the results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings

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

hydrological controls on diurnal ice flow variability in valley glaciers 1 this paper uses a combination of field data and three dimensional modeling to investigate the spatial variability in basal conditions required to induce observed fluctuations in diurnal ice velocity at haut glacier d arolla switzerland a network of surface velocity markers was observed at intervals of as little as four hours over diurnal cycles in both winter and late summer winter motion showed limited diurnal variability presumably due to the absence of supraglacial meltwater inputs by contrast diurnal fluctuations in ice motion were recorded in summer across the lower and upper glacier in the lower glacier surface velocities were intimately linked to hydrological forcing in the vicinity of a subglacial channel previously observed diurnal excursions of meltwater away from the channel should reduce areas of basal drag adjacent to the channel thereby impacting on ice dynamics using a first order ice flow approximation we investigated the distribution of basal shear traction adjacent to the channel necessary to replicate the observed surface velocity field during periods of rapid ice motion the modeling suggests that the observed variations in diurnal velocity will only occur with extensive reductions in basal drag across a transverse zone of up to 560 m across well beyond the immediate vicinity and previously observed extent of diurnal excursions of meltwater away from the subglacial channel

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

feedbacks between coupled subglacial hydrology and glacier dynamics on most glaciers and ice sheet outlets the majority of motion is due to basal slip a combination of basal sliding and bed deformation the importance of basal water in controlling sliding is well established with increased sliding generally related to high basal water pressure but the details of the interactions between the ice and water systems has not received much study when there is coupling between the systems here we use coupled subglacial hydrology and ice dynamics models within the community ice sheet model to investigate feedbacks between the ice and water systems the dominant feedback we find is negative sliding over bedrock bumps opens additional cavity space which lowers water pressure and in turn sliding we also find two small positive feedbacks basal melt increases through frictional heat during sliding which raises water pressure and strain softening of basal ice during localized speedup causes cavities to close more quickly and maintain higher water pressures our coupled modeling demonstrates that a sustained input of surface water to a distributed drainage system can lead to a speedup event that decays even in the absence of channelization due to increased capacity of the system through opening of cavities which is enhanced through the sliding opening feedback we find that the negative feedback resulting from sliding opening is robust across a wide range of parameter values however our modeling also argues that subglacial channelization is required to terminate speedup events over timescales that are commensurate with observations of late summer slowdown on mountain glaciers

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

meiofauna abundance and community patterns along a transatlantic transect in the vema fracture zone and in the hadal zone of the puerto rico trench abstract despite the increasing sampling effort that occurred in the deep sea environment during the last decades knowledge about meiofauna ecology in trenches and fracture zones is still scarce based on the lack of this information a longitudinal transect across the vema fracture zone in the north atlantic was sampled to test whether meiofauna abundances differ between northeast and northwest atlantic basins separated by the mid atlantic ridge also for examination of meiofauna depth pattern the puerto rico trench floor its upper trench slope and the western north atlantic abyssal were investigated in this study meiofauna communities were dominated by nematoda 93 and copepoda 4 the highest total abundance of meiofauna was found in the puerto rico trench and the lowest in the western basin we found significant differences between the eastern and western atlantic basins which were potentially caused by differences in current regimes stronger currents observed in the western basin possibly led to the coarser sediment grain size observed in this region and consequently to the lower abundances of the major groups found there besides grain size the total abundance of meiofauna was significantly correlated with total nitrogen total organic carbon and water depth moreover our study reveals a trend of increasing abundance of total meiofauna with increasing water depth in the puerto rico trench also significant differences between the western abyssal and the puerto rico trench were discovered generally the meiofauna abundance in the investigated area decreased from east to west but increased with increasing water depth in the puerto rico trench due to funnelling of organic sediments increased food availability towards deeper regions in trenches could occur and promote higher abundance

high resolution bed topography mapping of russell glacier greenland inferred from operation icebridge data detailed maps of bed elevation and ice thickness are essential for understanding and projecting the evolution of the ice sheets such maps are traditionally obtained using airborne radarsounding profiler data interpolated onto regular grids using geostatistical tools such as kriging here we compare three mapping techniques applied to a dense radar survey of russell glacier west greenland by nasa operation icebridge 1 radar tomography rt processing of the radar data to map the bed elevation 2 interpolation of radar derived thickness by ordinary kriging kr and 3 reconstruction of ice thickness based on the principles of mass conservation mc combining radar sounding profiler and ice motion data rt eliminates ambiguities caused by off nadir reflections but is spatially limited kr yields a standard error in bed elevation of 35 m but large errors 300m a 1 in flux divergence when combined with ice motion data mc yields a comparable performance in bed elevation mapping and errors smaller than 1m a 1 in flux divergence when the number of radar sounding tracks is reduced the performance of kr decreases more rapidly than for mc our study site shows that mc is capable of maintaining precision levels of 60m at 400m posting with flight tracks separated by 5 km 2014 publishing technology

evolution of the subglacial drainage system beneath the greenland ice sheet revealed by tracers predictions of the greenland ice sheet s response to climate change are limited in part by uncertainty in the coupling between meltwater lubrication of the ice sheet bed and ice flow1 2 3 this uncertainty arises largely from a lack of direct measurements of water flow characteristics at the bed of the ice sheet previous work has been restricted to indirect observations based on seasonal and spatial variations in surface ice velocities4 5 6 7 and on meltwater flux8 here we employ rhodamine and sulphur hexafluoride tracers injected into the drainage system over three melt seasons to observe subglacial drainage properties and evolution beneath the greenland ice sheet up to 57 km from the margin tracer results indicate evolution from a slow inefficient drainage system to a fast efficient channelized drainage system over the course of the melt season further inland evolution to efficient drainage occurs later and more slowly an efficient routing of water was established up to 41 km or more from the margin where the ice is approximately 1 km thick overall our findings support previous interpretations of drainage system characteristics thereby validating the use of surface observations as a means of investigating basal processes

ice flow dynamics and surface meltwater flux at a land terminating sector of the greenland ice sheet fitzpatrick a a w hubbard a l joughin i quincey d j van as d mikkelsen a doyle s h hasholt b jones g a 2013 ice flow dynamics and surface meltwater flux at a land terminating sector of the greenland ice sheet journal of glaciology 59 216 687 696

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

spatial and temporal melt variability at helheim glacier east greenland and its effect on ice dynamics this is the publisher s version also available electronically from http onlinelibrary wiley com

the glaciers climate change initiative methods for creating glacier area elevation change and velocity products glaciers and their changes through time are increasingly obtained from a wide range of satellite sensors due to the often remote location of glaciers in inaccessible and high mountain terrain satellite observations frequently provide the only available measurements furthermore satellite data provide observations of glacier characteristics that are difficult to monitor using ground based measurements thus complementing the latter in the glaciers cci project of the european space agency esa three of these characteristics are investigated in detail glacier area elevation change and surface velocity we use a data from optical sensors to derive glacier outlines b digital elevation models from at least two points in time c repeat altimetry for determining elevation changes and d data from repeat optical and microwave sensors for calculating surface velocity for the latter the two sensor types provide complementary information in terms of spatio temporal coverage while c and d can be generated mostly automatically a and b require the intervention of an analyst largely based on the results of various round robin experiments multi analyst benchmark studies for each of the products we suggest and describe the most suitable algorithms for product creation and provide recommendations concerning their practical implementation and the required post processing for some of the products area velocity post processing can influence product quality more than the main processing algorithm

greenland ice sheet annual motion insensitive to spatial variations in subglacial hydraulic structure we present ice velocities observed with global positioning systems and terrasar x tandem x in a land terminating region of the southwest greenland ice sheet gris during the melt year 2012 2013 to examine the spatial pattern of seasonal and annual ice motion we find that while spatial variability in the configuration of the subglacial drainage system controls ice motion at short timescales this configuration has negligible impact on the spatial pattern of the proportion of annual motion which occurs during summer while absolute annual velocities vary substantially the proportional contribution of summer motion to annual motion does not these observations suggest that in land terminating margins of the gris subglacial hydrology does not significantly influence spatial variations in net summer speedup furthermore our findings imply that not every feature of the subglacial drainage system needs to be resolved in ice sheet models

surface ice motion deviating toward the margins during speed up events at gornergletscher switzerland 1 high frequency ice flow measurements during speed up events in gornergletscher switzerland revealed intriguing ice motion which has never been reported in detail before during the summer 2005 more than a 100 flow speed increase was observed three times at four gps stations installed across gornergletscher the speed ups were accompanied by a decimeter scale surface uplift two of the events were triggered by intensive surface melt and rainfall while the third one was due to the outburst of gornersee a glacier dammed lake located 2 km upglacier an interesting observation was ice motion deviating toward the side margins during the events as the glacier accelerated a transverse cross glacier velocity component was generated turning the flow direction away from the central flow line toward the margins when the glacier decelerated the transverse velocity component reversed so that the ice flowed back to the azimuth of the initial flow direction in the most significant case the trajectory of the survey stake deviated from the original track by 0 2 m in the transverse direction we hypothesize that the observed lateral ice motion was caused by locally elevated subglacial water pressure when the ice sole decoupled from the bed at a part of the glacier a point source of vertical displacement was transmitted to the surface through viscous ice this caused the transverse as well as vertical surface motion as observed in ground motion during magma intrusion the hypothesis was tested with a two dimensional ice flow model applied to the transverse glacier cross section the model confirmed that the surface ice would move toward the margins as observed in gornergletscher if subglacial water pressure exceeded the ice overburden pressure over a limited part of the bed

hydrological controls on patterns of surface internal and basal motion during three spring events haut glacier d arolla switzerland three early melt season high velocity events or spring events occurred on haut glacier d arolla switzerland during the melt seasons of 1998 and 1999 the events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures however differences in spatial patterns of surface velocity internal ice deformation rates the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls the data from two of the events suggest widespread ice bed decoupling particularly along a subglacial drainage axis creating the highest rates of basal motion and plug flow in the overlying ice the other event showed evidence of less extensive ice bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas we suggest that 1 plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag 2 under certain circumstances enhanced surface motion may be due in part to non locally forced enhanced bed deformation and 3 subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

large and rapid melt induced velocity changes in the ablation zone of the greenland ice sheet continuous global positioning system observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the greenland ice sheet within days ice velocity reacts to increased meltwater production and increases by a factor of 4 such a response is much stronger and much faster than previously reported over a longer period of 17 years annual ice velocities have decreased slightly which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input which results in a more or less constant ice flux over the years the positive feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades

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

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