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

basal dynamics of kronebreen a fast flowing tidewater glacier in svalbard non local spatio temporal response to water input we evaluate the variability in basal friction for kronebreen svalbard a fast flowing tidewater glacier we invert 3 years 2013 15 of surface velocities at high temporal resolution generally 11 days to estimate the changing basal properties of the glacier our results suggest that sliding behaviour of kronebreen within a year is primarily influenced by changes in water input patterns during the meltwater season and basal friction is highly variable from a year to another at present models usually employ parameterisations to encompass the complex physics of glacier sliding by mathematically simulate their net effect for such ice masses with strong seasonal variations of surface melt the spatio temporal patterns of basal friction imply that it is neither possible nor appropriate to use a parameterisation for bed friction that is fixed in space and or time at least in a timescale of a few years basal sliding may not only be governed by local processes such as basal topography or summer melt but also be mediated by factors that vary over a larger distance and over a longer time period such as subglacial hydrology organisation ice thickness changes or calving front geometry

ocean forcing of the greenland ice sheet calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers 1 we have developed an automatic method to identify changes in the position of calving glacier margins using daily modis imagery application of the method to 32 ocean terminating glaciers in east greenland produced 26 802 margin positions for a 10 year long period 2000 2009 we report these high resolution data and show that the glaciers exhibit seasonal cycles with magnitudes of advance and retreat proportional to glacier width despite similar seasonality there is a distinct difference between the interannual trends of calving front positions north and south of 69 n all glaciers above this latitude showed very limited or no change when seasonality was excluded while glaciers south of 69 n retreated significantly between 2001 and 2005 2 3 km on average approximately 26 of the retreat of southern glaciers was regained by readvance from 2005 to 2009 to explain the latitudinal boundary of glacier dynamics we review basic climatic factors including summer and winter atmospheric forcing sea ice conditions and ocean temperature we conclude that the southern retreats were strongly influenced by warm oceanic conditions associated with increased transport of subtropical waters to the irminger sea and to fjords and coastal regions south of 69 n northern glaciers remained stable despite significant increase in runoff in this region because fjords at latitudes higher than 69 n are less exposed to subtropical waters the southern retreats illustrate sensitive behavior of calving glaciers and we hypothesize that the calving fronts retreated because they were exposed to rapid ice front melting

greenland marine terminating glacier area changes 2000 2010 area changes at 39 of the widest greenland marine terminating glacier outlets are measured in consecutive annual end of melt season moderate resolution imaging spectroradiometer modis scenes spanning ten annual intervals 2000 10 the rates of cumulative area change for glaciers and ice shelves are well represented by linear least squares fits r 0 99 and r 0 94 with average rates of 70 and 65 km 2 a 1 respectively collectively during this decade the 39 glaciers lost a cumulative area of 1368 km 2 more than three quarters of the total area change occurred north of 72 n the largest 11 year area change for a single glacier during the survey period is the 311 km 2 loss at humboldt glacier the largest annual change for a single glacier was extreme compared with the others where petermann glacier retreated 17 km between 3 and 5 august 2010 for the 10 year sample on average the count of glaciers retreating is twice that advancing a larger distinction is evident considering area change with the ratio of retreat and advance on average nine times the gain for glaciers with ice shelves we find no year with collective area gain the area change data from this study are posted at http bprc osu edu jbox data gac

surface melt driven laurentide ice sheet retreat during the early holocene received 21 september 2009 revised 3 november 2009 accepted 30 november 2009 published 30 december 2009 1 to better understand mechanisms of ice sheet decay we investigate the surface mass balance of the laurentide ice sheet lis during the early holocene a period of known rapid lis retreat we use a surface energy mass balance model embm driven with conditions derived from an equilibrium atmosphere ocean general circulation model 9 kilo years ago simulation our embm indicates a net lis surface mass balance of 0 67 0 13 m yr 1 with losses primarily due to enhanced boreal summer insolation and warmer summers this rate of loss compared to lis volume reconstructions suggests that surface ablation accounted for 74 22 of the lis mass loss with the remaining loss likely driven by dynamics resulting in basal sliding and calving thus surface melting likely played a governing role in the retreat and disappearance of this ice sheet citation carlson a e f s anslow e a obbink a n legrande d j ullman and j m licciardi 2009 surface melt driven laurentide ice sheet retreat during the early holocene geophys res lett 36 l24502 doi 10 1029 2009gl040948

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

modelling the evolution of supraglacial lakes on the west greenland ice sheet margin we present a model study investigating the summer evolution of supraglacial lakes on the greenland ice margin using a one dimensional 1 d model we calculate the surface ablation for a bare ice surface and beneath supraglacial lakes for 30 days in the summers of 1999 and 2001 the surface ablation beneath the lake was enhanced by 110 in 1999 and 170 in 2001 compared with the ablation for bare ice we then use the results from the 1 d model to further model the vertical and horizontal evolution of the supraglacial lakes the results of which are compared with satellite images within the region of the ice sheet where supraglacial lakes presently occur the area covered by supraglacial lakes is found to be more or less independent of the summer melt rate but controlled by topography we therefore predict that inside this region the area covered by supraglacial lakes will remain constant even in a warmer climate however in a warmer climate surface melting will occur higher on the ice sheet where small surface slopes favour formation of large supraglacial lakes enhanced surface melting beneath such lakes is a hitherto overlooked feedback mechanism related to climate warming

determining the three dimensional geometry of a dike swarm and its impact on later rift geometry using seismic reflection data dike swarm emplacement accommodates extension during rifting and large igneous province lip formation with ancient dike swarms serving to localize strain during later tectonic events deciphering three dimensional 3 d dike swarm geometry is critical to accurately calculating magma volumes and magma assisted crustal extension allowing syn emplacement mantle and tectonic processes to be interrogated it is also important for quantifying the influence of ancient dike swarms on post emplacement faulting however the essentially 2 d nature of earth s surface combined with the difficulties in imaging subvertical dikes in seismic reflection data and the relatively low resolution of geophysical data in areas of active diking means our understanding of dike swarm geometry at depth is limited we examine an 25 km wide 100 km long west southwest trending dike swarm imaged due to post emplacement rotation to shallower dips in high quality 2 d and 3 d seismic reflection data offshore southern norway tuned reflection packages correspond to thin 75 m thick closely spaced dikes these data provide a unique opportunity to image and map an ancient dike swarm at variable structural levels crosscutting relationships indicate emplacement occurred in the late carboniferous early permian and was linked to the formation of the ca 300 ma skagerrak centered lip dike swarm width increases with depth suggesting that magma volume and crustal extension calculations based on surface exposures are dependent on the level of erosion during the mesozoic rift related faults localized above and exploited mechanical anisotropies within the dike swarm we demonstrate that seismic reflection data are a powerful tool in understanding dike swarm geometry and the control of dikes on subsequent faulting

seasonal evolution of water contributions to discharge from a greenland outlet glacier insight from a new isotope mixing model the greenland ice sheet gris subglacial hydrological system may undergo a seasonal evolution with significant geophysical and biogeochemical implications we present results from a new isotope mixing model to quantify the relative contributions of surface snow glacial ice and delayed flow to the bulk meltwater discharge from a small 5k m 2 land terminating gris outlet glacier during melt onset may and at peak melt july we use radioactive 222 rn and stable isotopes 18 o deuterium to differentiate the water source contributions atmospherically derived 7 be further constrains meltwater transit time from the glacier surface to the ice margin we show that 1 222 rn is a promising tracer for glacial waters stored at the bed and 2 a quantitative chemical mixing model can be constructed by combining 222 rn and the stable water isotopes applying this model to the bulk subglacial outflow from our study area we find a constant delayed flow stored component from melt onset through peak melt this component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land terminating sections of the gris undergoes a seasonal evolution from a distributed to a channelized system

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

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

modeling supraglacial water routing and lake filling on the greenland ice sheet 1 during more intense melt years supraglacial lakes on the greenland ice sheet are found to fill more rapidly and to drain earlier in the melt season they also develop at higher latitudes and elevations since the rapid drainage of lakes has been shown to enhance basal sliding and since the lake volume can play an important role in determining if and when it drains understanding and modeling the processes which control lake filling rates forms a key development in successfully modeling the possible impacts of lake drainage events on ice motion we have developed a surface water routing and lake filling model and applied it to a 100 km2area of the paakitsoq region west greenland the model takes the time series of calculated runoff melt minus refreezing over the area and calculates flow paths and water velocities over the snow ice covered surface routing the water into topographic depressions which can fill to form lakes runoff is calculated from a distributed surface energy balance model coupled to a subsurface model which calculates changes in temperature density and water content in the snow firn and upper ice layers and hence refreezing and therefore net runoff the model is calibrated against field measurements of a filling lake in our study area during june 2011 and can be used to calculate the filling rate of the instrumented lake with a high degree of accuracy the filling rate of the instrumented modeled lake depends on melt and routing from the immediate lake catchment and from overflowing lakes in upstream catchments

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

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

influence of increasing surface melt over decadal timescales on land terminating greenland type outlet glaciers over recent decades greenland ice sheet surface melt has shown an increase both in intensity and spatial extent part of this water probably reaches the bed and can enhance glacier speed advecting a larger volume of ice into the ablation area in the context of a warming climate this mechanism could contribute to the future rate of thinning and retreat of land terminating glaciers of greenland these changes in ice flow conditions will in turn influence surface crevassing and thus the ability of water to reach the bed at higher elevations here using a coupled basal hydrology and prognostic ice flow model the evolution of a greenland type glacier subject to increasing surface melt is studied over a few decades for different scenarios of surface melt increase over the next decades the evolution of crevassed areas and the ability of water to reach the bed is inferred our results indicate that the currently observed crevasse distribution is likely to extend further upstream which will allow water to reach the bed at higher elevations this will lead to an increase in ice flux into the ablation area which in turn accelerates the mass loss of land terminating glaciers

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

seasonal evolution of supra glacial lakes on the greenland ice sheet abstract a survey of supra glacial lakes on the western margin of the greenland ice sheet reveals a seasonally driven hydrological system culminating in widespread lake drainage in late summer we used satellite imagery to study the evolution of 292 lakes across two sites totalling 22 000 km 2 in area during 2001 the lakes combined area increased to 75 5 km 2 by the beginning of july over the following 25 days an area totalling 36 3 5 km 2 drained from 216 lakes at one study site we used meteorological data and a positive degree day model to calculate the volume of water generated by melting in the lake catchments based on this estimate the mean depth of filling lakes surveyed rose from 1 5 0 7 m on 7th july to 3 9 1 1 m on 1st august in agreement with a value for one lake of 4 4 0 9 m we have derived from airborne altimetry during this 25 day period we estimate that 38 18 10 7 m 3 of water drained from the surface at this site and that there was an average water flux of 1 3 0 3 m 3 s 1 passing through each lake that drained completely

links between acceleration melting and supraglacial lake drainage of the western greenland ice sheet 1 the impact of increasing summer melt on the dynamics and stability of the greenland ice sheet is not fully understood mounting evidence suggests seasonal evolution of subglacial drainage mitigates or counteracts the ability of surface runoff to increase basal sliding here we compare subdaily ice velocity and uplift derived from nine global positioning system stations in the upper ablation zone in west greenland to surface melt and supraglacial lake drainage during summer 2007 starting around day 173 we observe speedups of 6 41 above spring velocity lasting 40 days accompanied by sustained surface uplift at most stations followed by a late summer slowdown after initial speedup we see a spatially uniform velocity response across the ablation zone and strong diurnal velocity variations during periods of melting most lake drainages were undetectable in the velocity record and those that were detected only perturbed velocities for 1 day suggesting preexisting drainage systems could efficiently drain large volumes of water the dynamic response to melt forcing appears to 1 be driven by changes in subglacial storage of water that is delivered in diurnal and episodic pulses and 2 decrease over the course of the summer presumably as the subglacial drainage system evolves to greater efficiency the relationship between hydrology and ice dynamics observed is similar to that observed on mountain glaciers suggesting that seasonally large water pressures under the ice sheet largely compensate for the greater ice thickness considered here thus increases in summer melting may not guarantee faster seasonal ice flow

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

modelling ice bed coupling during a glacier speed up event haut glacier d arolla switzerland fischer u h mair d kavanaugh j l willis i nienow p hubbard b 2011 modelling ice bed coupling during a glacier speed up event haut glacier d arolla switzerland hydrological processes 2599 1361 1372

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

subglacial water drainage storage and piracy beneath the greenland ice sheet meltwater drainage across the surface of the greenland ice sheet gris is well constrained by measurements and modeling yet despite its critical role knowledge of its transit through the subglac

automated remote sensing of sediment plumes for identification of runoff from the greenland ice sheet the viability of employing sediment plumes emanating from outlets along the western margin of the greenland ice sheet as indicators of runoff is assessed an automated sediment plume quantification system based on daily 250 m moderate resolution imaging spectroradiometer modis band 1 reflectance imagery is developed coherent plumes are identified using spectral thresholds and polygon tracing validation employs imagery quality control procedures and manual verification of plume areas outlets at land terminating margins with wide and straight fjord geometries deliver the most accurate and consistent results plume area observations are also possible at marine terminating margins with relatively static fronts and low proximal sea ice concentrations variability in plume area is examined with reference to special satellite microwave imager ssm i derived daily melt extent at the hydrologic catchment scale at annual timescales plume areas tend to co vary with surface melt extent indicating that more mass is lost by runoff during years of extensive melting some synchronicity in plume areas from different catchments is apparent at seasonal and daily timescales plumes from individual outlets primarily relate to catchment specific melting

accretion tectonics of the neotethyan ermioni complex peloponessos greece late cretaceous early tertiary terrigenous and calcareous turbidites exposed in the argolis peninsula of southern greece represent an accretionary complex assembled during the closure of a late cretaceous neotethyan ocean massive sulphide bodies thrust slices of cherts and serpentinized ultramafics are found imbricated within a tectonic stack of approximately 8 km thickness basalts with overlying cenomanian maastrichtian pelagic limestones and metalliferous sediments are found distributed throughout the complex the basalts give trace element chemical signatures consistent with an origin at a mid ocean ridge or back arc basin folding and shearing is locally intense in fault zones 10 20 m across resulting in a block in matrix fabric metamorphism within the complex is of low to high diagenetic grade with grade generally increasing structurally up section palaeo slope indicators suggest that the ocean basin lay to the present se the ssw to nne emplacement is at variance with the sw vergent early tertiary thrusting of the hellenides and may be explicable in terms of transpressional tectonics along the southeastern termination of the pelagonian micro continent the processes which assembled the ermioni complex appear to have operated in a similar fashion to those seen along the margins of major ocean basins although active over much shorter periods of time

thermal tracking of meltwater retention in greenland s accumulation area 1 poorly understood processes controlling retention of meltwater in snow and firn have important implications for greenland ice sheet s mass balance and flow dynamics here we present results from a 3 year 2007 2009 field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west greenland we installed one or two thermistor strings at 14 study sites each string having 32 sensors spaced between 0 and 10 m depth data from our network of over 500 sensors were collected at 15 60 min intervals for 1 2 years thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles we document three types of heating of firn related to different mechanisms of meltwater motion and freezing including heterogeneous breakthrough events wetting front advance and year round heating from freezing of residual deep pore water vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters the offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect the firn is 10 c warmer than the mean annual air temperature at the region where meltwater runoff initiates during 2007 2009 runoff was limited to elevations lower than 1500 m with no sharp runoff limit rather the ratio of retention to runoff transitioned from all retention to all runoff across a 20 km wide zone

sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice contact fan deposits a pleistocene example ireland a pit located near ballyhorsey 28 km south of dublin eastern ireland displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice contact fan deposits the coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies the lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space mainly controlled by the position of an overlying ice roof during ice bed decoupling these sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments the sorted sediments consist of fine grained laminated facies cross laminated sand and channelized gravels and are interpreted as subglaciofluvial sediments deposited within a subglacial de coupled space the subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds triggered by fluid overpressure and shear stress during episodes of ice bed recoupling clastic dykes and folds the upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake forming a thick sedimentary wedge erosively overlying the subglacial deposits gravel facies and large scale trough bedding sand are observed within this proximal wedge while normally graded sand beds with developed bedforms are observed further downflow the building of the prograding ice contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading normal faults this study facilitates the recognition of subglacial submarginal depositional environments formed in part during localized ice bed coupling episodes in the sedimentary record the sedimentary sequence exposed in ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation the impact on the subglacial drainage system and the consequences on the irish sea ice stream flow mechanisms

theoretical framework and diagnostic criteria for the identification of palaeo subglacial lakes abstract the antarctic ice sheet is underlain by numerous subglacial lakes which comprise a significant and active component of its hydrological network these lakes are widespread and occur at a range of scales under a variety of conditions at present much glaciological research is concerned with the role of modern subglacial lake systems in antarctica another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets this is challenging both conceptually in identifying whether and where subglacial lakes may have formed and also distinguishing the signature of former subglacial lakes in the geological record in this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo ice sheets this includes a discussion of the formative mechanisms that could trigger onset of or drain subglacial lakes these concepts provide a framework for discussing the probability that subglacial lakes exist ed beneath other palaeo ice sheets indeed we conclude that the former mid latitude ice sheets are likely to have hosted subglacial lakes although the spatial distribution frequency and type of lakes may have differed from today s ice sheets and between palaeo ice sheets given this possibility we propose diagnostic criteria for identifying palaeo subglacial lakes in the geological record these criteria are derived from contemporary observations hydrological theory and process analogues and provide an observational template for detailed field investigations

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

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

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

flotation and free surface flow in a model for subglacial drainage part 2 channel flow we present a new model of subglacial drainage incorporating flow in a network of channels and a porous sheet with water exchange between the two determined by pressure gradients the sheet represents the average effect of many linked cavities whilst the channels emerge from individual cavities that enlarge due to dissipationinduced melting the model distinguishes cases when the water pressure drops to zero in which case it allows for the drainage space to be only partially filled with water free surface flow and when the pressure reaches the ice overburden pressure in which case it allows for uplift of the ice to whatever extent is needed to accommodate the water flotation numerical solutions are found for a one dimensional flow line version of the model the results capture typically observed or inferred features of subglacial drainage systems including open channel flow at the ice margin seasonal channel evolution and high water pressures and uplift of the ice surface driven by rapid changes in water supply

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

intra annual and intra seasonal flow dynamics of a high arctic polythermal valley glacier measurements of surface dynamics onpolythermaljohn evans glacier nunavut canada over two winter periods and every 7 10 days throughout two melt seasons june july 2000 2001 provide new insight into spatio temporal patterns of high arctic glacier dynamics in the lower ablation zone mean annual surface velocities are 10 21ma 1 but peak velocities up to 50 higher are attained during latejune earlyjuly in the upper ablation zone and lower accumulation zone mean annual surface velocities are typically10 18ma 1 and peak velocities up to 40 higher occur during latejuly in the upper accumulation zone mean annual surface velocities are 2 9ma 1 and motion in mid to latejuly exceeds this by up to 10 rapid drainage of ponded supraglacial water in the upper ablation zone to an initially distributed subglacial drainage system in mid june may force excess surface motion in the warm based lower glacier the data indicate thatthe durationof the velocity response maybe relatedto the rate of channeliz ation of the basal drainage and the velocity response may be transmitted up glacier by longitudinal coupling an increase in surface velocities in the middle glacier in latejuly occurs in conjunction with the opening of two further moulins in the accumulation zone

spectral roughness and glacial erosion of a land terminating section of the greenland ice sheet spectral roughness offers a significant potential for understanding the evolution of glaciated landscapes here we present the first roughness study combining a high resolution 250 to 500 m dem

hydrologic drainage of the greenland ice sheet a simple hydrologic drainage network for the greenland ice sheet is modelled from available digital elevation models dems of bedrock and surface topography and assumptions of hydrostatic water pressure uniform hydraulic conductivity and no conduit flow within the ice sheet as such it is a first order model best suited for broad scale hydrological assessment results identify 293 distinct hydrologic basins 185 117 000 km2 together with their realized wet and unrealized dry drainage patterns intersection with 1991 2000 polar mm5 pmm5 mesoscale climate model hindcasts of meltwater runoff suggest that these basins route varying amounts of water to the ice edge ranging from 0 to 16 km3 annually and totalling 242 km3 year for the entire ice sheet regionally speaking average annual volumetric meltwater production km3 year is highest in southwest and lowest in northeast greenland with greater hydrologic activity in western regions than in eastern regions for a given latitude the extent to which meltwater truly reaches the ice margin as modelled is difficult to test however the simulated flow outlet locations show qualitative agreement with the locations of 460 observed meltwater outlets proglacial lakes streams and rivers and sediment plumes into fjords mapped continuously along the ice sheet perimeter on average about 36 of the modelled drainage network was activated i e received water over the 1991 2000 study period remaining areas barring dynamic changes to ice surface topography would presumably activate if surface melt penetrates deeper into the ice sheet interior both new datasets are freely available for scientific use at the national snow and ice data center ftp sidads colorado edu pub datasets parca nsidc 0372 hydrologic outlets ftp sidads colorado edu pub datasets parca nsidc 0371 hydrologic sub basins copyright 2009 john wiley sons ltd

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

insar observations of ice elevation and velocity fluctuations at the flade isblink ice cap eastern north greenland 1 the 8500 km2 flade isblink ice cap fiic 81 15 n 15 0 w is the largest ice cap in greenland we use repeat pass interferometric synthetic aperture radar insar techniques to investigate the form and flow of the fiic european remote sensing satellite ers 1 and ers 2 data acquired in winter 1996 were used to form a 100 m resolution digital elevation model dem which we constrained using ice cloud and elevation satellite icesat laser altimeter elevation measurements from 2007 this insar dem was used to isolate the phase due to motion from seven ers tandem 1 day pairs of sar scenes acquired between 15 august 1995 and 7 february 1996 to produce one wintertime and two summertime velocity maps five of the eight major outlet glaciers draining the fiic are marine terminating and two terminate at a lake margin a maximum ice velocity of 581 m yr 1 was observed in mid august 1995 six of the eight major outlet glaciers exhibit seasonal velocity variations between late summer and winter and flow speeds vary by up to 20 over a 10 day period in august 1995 our findings show that while marine terminating glaciers flow faster than land terminating glaciers there is no simple relationship between glacier type and seasonality of ice motion

remote sounding of greenland supraglacial melt lakes implications for subglacial hydraulics a supraglacial lake depth retrieval function is developed based on the correspondence between moderate resolution imaging spectroradiometer modis reflectance and water depth measured during raft surveys individual lake depth area and volume statistics including short term temporal changes for greenland s southwestern ablation region were compiled for 2000 05 the maximum area of an individual lake was found to be 8 9 km 2 the maximum volume 53 0 10 6 m 3 and the maximum depth 12 2 m sampling over 0 0625 km 2 pixel areas the total lake volume reaches 1 km 3 in this region by july each year the importance of melt lake reservoirs to greenland ice sheet flow may be a feedback between abrupt lake drainage events and ice dynamics lake outburst volumes up to 31 5 10 6 m 3 d 1 are capable of providing sufficient water via moulins to hydraulically pressurize the subglacial environment since the overburden pressure at the base of a flooded moulin is greater than that provided by ice lake outburst events seem capable of exerting sufficient upward force to lift the ice sheet locally if water flow in the subglacial environment is constrained laterally considering a moulin with a 10 m 2 cross sectional area basal pressurization can be maintained over lake outburst

spatial patterns of summer speedup on south central alaska glaciers seasonal changes in glacier basal motion are attributable to variations in subglacial hydrology and cause variations in both ice discharge and glacier erosion we develop a novel workflow based upon landsat 8 feature tracking to document differences between spatial patterns of summer and winter glacier surface speed which reflect changes in the distribution of basal motion we identify and characterize summer speedups on 13 of 19 land terminating glaciers in alaska s wrangell st elias ranges the speedups are relatively uniform over much of the ablation zones and the speedup magnitudes vary by only a factor of 2 between glaciers whose velocities span an order of magnitude summer speedups extend up to 30 km up glacier from termini and often end at the bases of icefalls these data provide systematic observation of the spatial pattern of enhanced summer glacier basal motion and suggest the possibility of its parameterization in glacier models

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

growth and collapse of the distributed subglacial hydrologic system of kennicott glacier alaska usa and its effects on basal motion nearly 100 days of hourly glacier motion hydrology and hydrochemistry measurements on kennicott glacier alaska usa demonstrate the complicated relationship between water and motion at the glacier bed our observations capture the transient glacier response to seasonal and daily melt cycles and to a jokulhlaup that prompts a sixfold increase in glacier speed sliding is promoted whenever the water inputs to the glacier exceed the capacity of the subglacial hydrologic system to transmit the water sensitivity of sliding to daily meltwater inputs varies strongly through the season implying that the state of the hydrologic system governs the sensitivity of basal sliding a numerical model constructed to explore these relationships reveals the roles of the effective pressure the exponent to which this is taken in the sliding law 0 1 0 6 glacier macroporosity 2 and the cavity generating capacity of the glacier bed which encapsulates the sizes and spacing of bed roughness elements temporal changes in the effective pressure associated with evolution of both water inputs and subglacial water transmission capacity can explain the varying strength of diurnal velocity fluctuations of kennicott glacier spatial patterns of glacier macroporosity and of basal roughness govern variation in sensitivity of sliding to water inputs

caterpillar like ice motion in the ablation zone of the greenland ice sheet current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the greenland ice sheet we present measurements of ice deformation subglacial water pressure and surface velocity that show periodic and episodic variations on several time scales seasonal multiday and diurnal these variations observed with gps and sensors at different depths throughout the ice column are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity with the help of a full stokes ice flow model these observations are explained as ice motion in a caterpillar like fashion caused by patches of different basal slipperiness horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior ice flow in the ablation zone of the greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season leading to caterpillar like ice motion superposed on the classical shear deformation

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

using pressure pulse seismology to examine basal criticality and the influence of sticky spots on glacial flow 1 here we report results of water pressure pulse studies conducted at storglaciaren sweden and west washmawapta glacier british columbia canada comparison of pressure pulse records with meteorological conditions at storglaciaren indicates that several periods of increased basal slip activity observed during a 10 day interval of summer 2008 were due to precipitation loading of the glacier surface rather than to infiltration of surface water to the glacier bed this indicates that the glacier bed was close to the failure strength for much of this interval pressure pulse magnitudes for the two glaciers were well fit by power law distributions similar to those earlier observed at trapridge glacier and similar in form to the gutenberg richter relationship commonly used in seismology suggesting that the mechanical processes that give rise to these distributions are robust features of soft bedded glaciers in contrast interevent time distributions for both glaciers diverge from those observed at trapridge glacier for short recurrence intervals suggesting that the factors that govern the rate at which these processes occur differ between glaciers an examination of pressure pulse characteristics at west washmawapta glacier indicates that the establishment of a basal drainage system in summer 2008 resulted in increased stability and reduced sensitivity to meltwater input suggesting that common assumptions about the relationship between meltwater production and ice flow are oversimplified these results demonstrate that water pressure pulse observations can provide valuable insight into the dynamics of soft bedded 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

evolution of melt pond volume on the surface of the greenland ice sheet 1 the presence of surface meltwater on ice caps and ice sheets is an important glaciological and climatological characteristic we describe an algorithm for estimating the depth and hence volume of surface melt ponds using multispectral aster satellite imagery the method relies on reasonable assumptions about the albedo of the bottom surface of the ponds and the optical attenuation characteristics of the ponded meltwater we apply the technique to sequences of satellite imagery acquired over the western margin of the greenland ice sheet to derive changes in melt pond extent and volume during the period 2001 2004 results show large intra and interannual changes in ponded water volumes and large volumes of liquid water stored in extensive slush zones

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

disappearance of the last tropical glaciers in the western pacific warm pool papua indonesia appears imminent the glaciers near puncak jaya in papua indonesia the highest peak between the himalayas and the andes are the last remaining tropical glaciers in the west pacific warm pool wpwp here we report the recent rapid retreat of the glaciers near puncak jaya by quantifying the loss of ice coverage and reduction of ice thickness over the last 8 y photographs and measurements of a 30 m accumulation stake anchored to bedrock on the summit of one of these glaciers document a rapid pace in the loss of ice cover and a 5 4 fold increase in the thinning rate which was augmented by the strong 2015 2016 el nino at the current rate of ice loss these glaciers will likely disappear within the next decade to further understand the mechanisms driving the observed retreat of these glaciers 2 32 m long ice cores to bedrock recovered in mid 2010 are used to reconstruct the tropical pacific climate variability over approximately the past half century on a quasi interannual timescale the ice core oxygen isotopic ratios show a significant positive linear trend since 1964 ce 0 018 0 008 per year p 0 03 and also suggest that the glaciers retreat is augmented by el nino southern oscillation processes such as convection and warming of the atmosphere and sea surface these papua glaciers provide the only tropical records of ice core derived climate variability for the wpwp

controls on short term variations in greenland glacier dynamics short term ice dynamical processes at greenland s jakobshavn and kangerdlugssuaq glaciers were studied using a 3 day time series of synthetic aperture radar data acquired during the 2011 european remote sensing satellite 2 ers 2 3 day repeat campaign together with modelled meteorological parameters the time series spans the period march july 2011 and captures the first 30 of the summer melting season in both study areas we observe velocity fluctuations at the lower 10 km of the glacier at jakobshavn isbrae where our dataset covers the first part of the seasonal calving front retreat we identify ten calving episodes with a mean calving front area loss of 1 29 0 4km2 significant glacier speed up was observed in the near terminus area following all calving episodes we identify changes in calving front geometry as the dominant control on velocity fluctuations on both glaciers apart from a

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

an increase in crevasse extent west greenland hydrologic implications 1 we compare high resolution 1985 and 2009 imagery to assess changes in crevasse extent in the sermeq avannarleq ablation zone west greenland the area occupied by crevasses 2 m wide significantly increased 13 4 over the 24 year period this increase consists of an expansion of existing crevasse fields and is accompanied by widespread changes in crevasse orientation up to 45 we suggest that a combination of ice sheet thinning and steepening are responsible for the increase in crevasse extent we examine the potential impact of this change on the hydrology of the ice sheet we provide a first order demonstration that moulin type drainage is more efficient in transferring meltwater fluctuations to the subglacial system than crevasse type drainage as enhanced basal sliding is associated with meltwater pulses an increase in crevasse extent can therefore be expected to result in a net decrease in basal sliding sensitivity an increase in crevasse extent may also accelerate cryo hydrologic warming and enhance surface ablation

glacier velocity and water input variability in a maritime environment franz josef glacier new zealand short term glacier velocity variations typically occur when a water input is accommodated by an increase in the subglacial water pressure although these velocity variations have been well documented on many glaciers few studies have considered them on glaciers where heavy rain and glacier melt occur year round this study investigates the relationship between water inputs and glacier velocity on franz josef glacier new zealand we installed six gnss stations across the lower glacier during austral summer 2010 11 and one station during summer 2012 13 glacier velocity remained elevated at all stations for 7 days following large rain events during diurnal melt events we find velocity variations in the early afternoon 12 00 16 00 at 600 m a s l and in the late evening 20 00 01 00 at 400 m a s l we hypothesize that the late evening velocity variations occurred as an upstream region of high subglacial water pressures and accelerated ice motion propagated downstream this mechanism may also explain the increased longitudinal compression and transverse extension across the lower glacier during speed up events our results indicate that the subglacial drainage system likely decreases in efficiency upstream and that the water input variability can still cause short term velocity variations despite the large year round water inputs

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

inland advance of supraglacial lakes in north west greenland under recent climatic warming the inland advance of supraglacial lakes sgls towards the interior regions of the greenland ice sheet gris may have implications for the water volumes reaching the subglacial drainage system and could consequently affect long term ice sheet dynamics here we investigate changes to the areas volumes and elevation distributions of over 8000 manually delineated sgls using 44 landsat images of a 6200 km 2 sector of north west greenland over three decades 1985 2016 our results show that sgls have advanced to higher maximum 418 m and mean 299 m elevations and that there has been a near doubling of total regional sgl areas and volumes over the study period accelerating after 2000 these changes were primarily caused by an increased sgl area and volume at high 1200 m a s l elevations where sgl coverage increased by over 2750 during the study period many of the observed changes particularly the post 2000 accelerations were driven by changes to regional surface temperature anomalies this study demonstrates the past and accelerating response of the gris s hydrological system due to climatic warming indicating an urgent need to understand whether the increasingly inland sgls will be capable of hydrofracture in the future thus determining their potential implications for ice sheet 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

rigorous gps data processing strategies for glaciological applications global positioning system gps data are now routinely used for many glaciological applications in some common cases systematic errors are unmodelled at the data processing stage although they are often presumed insignificant in this paper i investigate these assumptions for three different scenarios 1 measurements on a moving glacier 2 measurements on a floating ice shelf and 3 precise height determination over large elevation ranges such as for aircraft positioning in lidar laser altimeter missions in each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the gps processing software under certain conditions if these coordinate biases go undetected short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques more rigorous processing strategies are discussed that allow for bias free parameter estimation

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

does porewater or meltwater control tunnel valley genesis case studies from the hirnantian of morocco several ordovician tunnel valleys are exposed in the moroccan anti atlas mountains including the alnif and the foum larjamme tunnel valleys located 150 km away from each other sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations the alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface ball structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces clastic dykes dewatering structures folds and radial step normal faults the characteristics of the alnif tunnel valley can be explained by a porewater pressure driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed these processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley the foum larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation this second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion the undulating base of the foum larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of nye channels or alternatively by channels migrating laterally during episodic minor subglacial outbursts these two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys the distribution of palaeo ice streams in north africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter ice stream zones due to variations in meltwater availability the topography and bed lithological properties

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

seismicity on the western greenland ice sheet surface fracture in the vicinity of active moulins we analyzed geophone and gps measurements collected within the ablation zone of the western greenland ice sheet during a 35 day period of the 2011 melt season to study changes in ice deformation before during and after a supraglacial lake drainage event during rapid lake drainage ice flow speeds increased to 400 of winter values and icequake activity peaked at times 7 days after drainage this seismicity developed variability over both diurnal and longer periods 10 days while coincident ice speeds fell to 150 of winter values and showed nightly peaks in spatial variability approximately 95 of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near surface ice 330m deep that generated rayleigh waves icequakes occurring before and during drainage frequently were collocated with the down flow west end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage we interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage followed by enhanced seismicity and ice flow on the downstream side of the lake basin this region collocates with interferometric synthetic aperture radar measured speedup in previous years and could reflect the migration path of themeltwater supplied to the bed by the lake the diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction thereby promoting elevated strain in the surficial ice

greenland ice sheet mass balance reconstruction part iii marine ice loss and total mass balance 1840 2010 abstractgreenland ice sheet mass loss to the marine environment occurs by some combination of iceberg calving and underwater melting referred to here as marine ice loss lm this study quantifies the relation between lm and meltwater runoff r at the ice sheet scale a theoretical basis is presented explaining how variability in r can be expected to govern much of the lm variability over annual to decadal time scales it is found that r enhances lm through three processes 1 increased glacier discharge by ice warming softening and basal lubrication sliding 2 increased calving susceptibility through undercutting glacier front geometry and reducing ice integrity and 3 increased underwater melting from forcing marine convection applying a semiempirical lm f r parameterization to a surface mass balance reconstruction enables total ice sheet mass budget closure over the 1840 2010 period the estimated cumulative 171 yr net ice sheet sea level contribution is 25 10 mm the rise punctuated by periods

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