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 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

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

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

a modeling study of the effect of runoff variability on the effective pressure beneath russell glacier west greenland basal sliding is a main control on glacier flow primarily driven by water pressure at the glacier base the ongoing increase in surface melting of the greenland ice sheet warrants an examination of its impact on basal water pressure and in turn on basal sliding here we examine the case of russell glacier in west greenland where an extensive set of observations has been collected these observations suggest that the recent increase in melt has had an equivocal impact on the annual velocity with stable flow on the lower part of the drainage basin but accelerated flow above the equilibrium line altitude ela these distinct behaviors have been attributed to different evolutions of the subglacial draining system during and after the melt season here we use a high resolution subglacial hydrological model forced by reconstructed surface runoff for the period 2008 to 2012 to investigate the cause of these distinct behaviors we find that the increase in meltwater production at low elevation yields a more efficient drainage system compatible with the observed stagnation of the mean annual flow below the ela at higher elevation the model indicates that the drainage system is mostly inefficient and is therefore strongly sensitive to an increase in meltwater availability which is consistent with the observed increase in ice velocity

calculating the balance between atmospheric co2 drawdown and organic carbon oxidation in subglacial hydrochemical systems in order to constrain co2 fluxes from biogeochemical processes in subglacial environments we model the evolution of ph and alkalinity over a range of subglacial weathering conditions we show that subglacial waters reach or exceed atmospheric pco2 levels when atmospheric gases are able to partially access the subglacial environment subsequently closed system oxidation of sulfides is capable of producing pco2 levels well in excess of atmosphere levels without any input from the decay of organic matter we compared this model to published ph and alkalinity measurements from 21 glaciers and ice sheets most subglacial waters are near atmospheric pco2 values the assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies if the subglacial environment is assumed to be closed from any input of atmospheric gas large organic carbon inputs are required in nearly all cases these closed system assumptions imply that order of 10 g m 2 y 1 of organic carbon are removed from a typical subglacial environment a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon partial open system input of atmospheric gases is therefore likely in most subglacial environments the decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock in glaciers and ice sheets on silicate bedrock substantial long term drawdown of atmospheric co2 occurs

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

alpine subglacial hydrology abstract the size and shape of subglacial flow pathways exert a strong influence over the pressure and speed at which meltwaters are routed through glaciers these characteristics in turn influence glacier sliding velocity and the quantity and quality of bulk meltwaters discharging from glaciers in this paper we summarise four empirical research approaches that have been developed to improve our understanding of the subglacial hydrology of predominantly temperate based alpine glaciers investigations of bulk meltwater discharge and chemistry tracer studies proglacial bedrock investigations and borehole investigations as a result of linking these studies to conceptual or numerical models three arborescent and four non arborescent or distributed subglacial drainage configurations are described these drainage networks may be neither spatially nor temporally persistent but may compete with and replace each other on a seasonal basis a qualitative model of the temporal and spatial evolution of a typical alpine subglacial drainage system is advanced on the basis of research conducted at haut glacier d arolla switzerland

velocity discharge relationships derived from dye tracer experiments in glacial meltwaters implications for subglacial flow conditions repeated dye tracer tests were undertaken from individual moulins at haut glacier d arolla switzerland over a number of diurnal discharge cycles during the summers of 1989 1991 it was hoped to use the concepts of at a station hydraulic geometry to infer flow conditions in subglacial channels from the form of the velocity discharge relationships derived from these tests the results however displayed both clockwise and anticlockwise velocity discharge hysteresis in addition to the simple power function relationship assumed in the hydraulic geometry approach clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial channel and that discharges in the two channels vary out of phase with each other anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthrough curve and is best explained by hydraulic damming of moulins or sub englacial passageways despite the complex velocity discharge relationships observed some indication of subglacial flow conditions may be obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity discharge relationships are derived from dye injections conducted during periods when the supraglacial discharge entering the moulin and the bulk discharge vary in phase analyses based on this premise suggest that both open and closed channel flow occur beneath haut glacier d arolla and that flow conditions are highly variable at and between sites

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

a wireless subglacial probe for deep ice applications we present the design and first results from two experiments using a wireless subglacial sensor system wise that is able to transmit data through 2500 m thick ice energy consumption of the probes is minimized enabling the transmission of data for at least 10 years in july 2010 the first prototype of the system was used to measure subglacial pressure at the base and a temperature profile consisting of 23 probes in two 600 m deep holes at russell glacier a land terminating part of the west greenland ice sheet near kangerlussuaq the time series of subglacial pressure show very good agreement between data from the wise system and the wired reference system the wireless measured temperature data were validated by comparison with the theoretical decrease of melting point with water pressure inside the water filled hole directly after installation to test the depth range of the wise system a second experiment using three different probe types and two different surface antennas was performed inside the 2537 m deep hole at neem it is demonstrated that with the proper combination of transmission power and surface antenna type the wise system transmits data through 2500 m thick ice

variability in ice motion at a land terminating greenlandic outlet glacier the role of channelized and distributed drainage systems we use a combination of field observations and hydrological modelling to examine the mechanisms through which variability in meltwater input affects ice motion at a land terminating greenlandic outlet glacier we find a close agreement between horizontal ice velocity vertical ice velocity and modelled subglacial water pressure over the course of a melt season on this basis we argue that variation in horizontal and vertical ice velocity primarily reflects the displacement of basal ice during periods of cavity expansion and contraction a process itself driven by fluctuations in basal water pressure originating in subglacial channels this process is not captured by traditional sliding laws linking water pressure and basal velocity which may hinder the simulation of realistic diurnal to seasonal variability in ice velocity in coupled models of glacial hydrology and dynamics

subglacial drainage processes at a high arctic polythermal valley glacier dye tracer experiments undertaken over two summer melt seasons at polythermal john evans glacier ellesmere island canada were designed to investigate the character of the subglacial drainage system and its evolution over a melt season in both summers dye injections were conducted at several moulins and traced to a single subglacial outflow tracer breakthrough curves suggest that supraglacial meltwater initially encounters a distributed subglacial drainage system in late june the subsequent development and maintenance of a channelled subglacial network are dependent upon sustained high rates of surface melting maintaining high supraglacial inputs in a consistently warm summer 2000 subglacial drainage became rapidly and persistently channelled in a cooler summer 2001 distributed subglacial drainage predominated these observations confirm that supraglacial meltwater can access the bed of a high arctic glacier in summer and induce significant structural evolution of the subglacial drainage system they do not support the view that subglacial drainage systems beneath polythermal glaciers are always poorly developed they do suggest that the effects on ice flow of surface water penetration to the bed of predominantly cold glaciers may be short lived

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

seasonal changes in the morphology of the subglacial drainage system haut glacier d arolla switzerland dye tracing techniques were used to investigate the glacier wide pattern of change in the englacial subglacial drainage system of haut glacier d arolla during the ablation seasons of 1990 and 1991 analysis of breakthrough curve characteristics indicate that over the course of a melt season a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system by the end of the melt season this channel system extended at least 3 3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier the upper limit of the channel system closely followed the retreating snowline up glacier rates of headward channel growth reached c 65 m d 1 although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer it appears that the removal of snow with its high albedo and significant water storage capacity from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins and in the peakedness of daily runoff cycles this induced transient high water pressures within the distributed drainage system which caused it to evolve rapidly into a channelised system it is therefore likely that at a local scale channel growth occurred down glacier from moulins and that the overall up glacier directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice derived meltwater damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier 1998 john wiley sons ltd

trends of mesospheric gravity waves at northern middle latitudes during summer 1 recent investigations of the seasonal variation of the activity of gravity waves in the mesosphere lower thermosphere mlt at middle and high latitudes suggest a semiannual variation with maxima during winter and summer and minima during the equinoxes it is generally assumed that this annual cycle is determined by filtering processes due to the background winds in the stratosphere and lower mesosphere on the other side long term observations of mesospheric winds at juliusruh 55 n 13 e since 1990 indicate a stable increase of westward directed winds below 80 km negative trends during summer as e g clearly evident in monthly means in july here we are studying how these long term changes of winds are related to trends of the activity of gravity waves gw with periods between 3 6 hours our results show that the observed zonal wind trend at about 75 km during july goes along with an enhancement of the gw activity at altitudes above 80 km indeed also the year to year variation of maxima of the observed westward directed winds at altitudes near 75 km and the gw activity at about 80 km are significantly correlated our results stimulate the further study of long term wind changes and corresponding gravity wave trends

sedimentary record of a scandinavian ice sheet drainage system and till deposition over subglacial obstacles promoting basal sliding an example from southern poland abstract subglacial obstacles occurring in the path of advancing ice sheets generally generate higher longitudinal compression and higher frictional drag than a flat substrate however in the case of a soft sediment substratum they can have a very different effect on ice sheet behaviour this study concerns a substrate composed of very fine grained sediments with low permeability the relationship between subglacial obstacles and the overriding scandinavian ice sheet was studied in an area of southern poland where a small intervalley neogene clay ridge 40 m high was present based on sedimentological and structural analysis of subglacial till and gravelly sandy sediments the basal depositional processes and subglacial conditions and their influence on ice sheet behaviour were analysed the till and related deposits within the ridge reflecting high water pressure conditions and lack of glacitectonic deformations indicate that the clay ridge did not generate much resistance against the advancing ice sheet but instead favoured basal slip the impermeable substratum weakened the ice bed coupling and promoted ice detachment from the substratum gravelly sandy inclusions at the till clay contact indicate that during the first stage of ice sheet overriding a canal drainage system developed at the ice substrate interface varied geometry size and location of inclusions of sorted sediments suggest periodic instability of the canal system which could lead to its transformation from initially uniform to being composed of conduits of different sizes during later stages of ice sheet overriding a traction till was deposited and occasional drainage through a water film was sufficient to evacuate basal meltwater the resulting change in the character of subglacial drainage was probably related to variations in water pressure gradient during progressive ice sheet advance

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

character of the englacial and subglacial drainage system in the lower part of the ablation area of storglaciaren sweden as revealed by dye trace studies

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

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

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

geoscience the plumbing of greenland s ice observations of the water pressure in drilled boreholes and natural moulins on the greenland ice sheet show how its underlying plumbing system controls ice motion during the course of the summer melt season see letter p 80 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

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

amplified melt and flow of the greenland ice sheet driven by late summer cyclonic rainfall intense rainfall events significantly affect alpine and alaskan glaciers through enhanced melting ice flow acceleration and subglacial sediment erosion yet their impact on the greenland ice sheet has not been assessed here we present measurements of ice velocity subglacial water pressure and meteorological variables from the western margin of the greenland ice sheet during a week of warm wet cyclonic weather in late august and early september 2011 we find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice sheet interior we suggest that the late season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less efficient winter mode reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western greenland during this time and we observe a corresponding ice flow response at all land and marine terminating glaciers in these regions for which data are available given that the advection of warm moist air masses and rainfall over greenland is expected to become more frequent in the coming decades our findings portend a previously unforeseen vulnerability of the greenland ice sheet to climate change

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

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

why is it hard to predict the future of ice sheets ice sheet retreat hypotheses of instability could be missing important processes that limit the rate or extent of retreat or conversely ice sheet behavior is strongly influenced by processes at its margin and base observations of rapid changes at these boundaries are helping modelers to improve predictions of future changes

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

hydrologic exchange and chemical weathering in a proglacial watershed near kangerlussuaq west greenland abstract the exchange of proglacial river water with active layer pore water could alter water chemical compositions in glacial outwash plains and oceanic solute fluxes to evaluate effects of this exchange we sampled watson river and adjacent pore water during the 2013 melt season at two sandurs in western greenland one in sandflugtdalen and the other near the confluence with sondre stromfjord we measured temperature specific conductivity and head gradients between the river and bank over a week long period at sandflugtdalen as well as sediment hydraulic conductivity and chemical compositions of waters from both sites specific conductivity of pore water is four to ten times greater than river water as solutes are concentrated from weathering reactions cryoconcentration and evaporation pore water compositions are predominantly altered by carbonate dissolution and sulfide mineral oxidation high concentrations of hco3 and so4 result from solute recycling and dissolution of secondary ca mg carbonate sulfate salts initially formed by near surface evaporation in the summer and at depth by freeze in of the active layer and cryoconcentration in the winter high hydraulic conductivity 10 5 to 10 4 m s and diurnal fluctuations of river stage during our study caused exchange of river and pore water immediately adjacent to the river channel with a net loss of river water to the bank pore water 6 m from the river continuously flowed away from the river approximately 1 8 of the river discharge through the sandflugtdalen was lost to the river bank during our 6 75 day study based on calculations using darcy s law although not sampled some of this water should discharge to the river during low river stage early and late in the melt season elevated pore water solute concentrations in sandurs and water exchange at diurnal and seasonal frequency should impact fluxes of solutes to the ocean although understanding the magnitude of this effect will require long term evaluation throughout the melt season

an automated method for mapping geomorphological expressions of former subglacial meltwater pathways hummock corridors from high resolution digital elevation data abstract elongated tracts of hummocks or hummock corridors exposed on palaeo ice sheet beds are believed to represent former subglacial meltwater pathways here we present a method coded in matlab for automatically detecting and mapping hummock corridors from high resolution digital elevation models dems initially the dem is filtered to remove bed roughness outside the size range of hummocks a fast fourier transform is then performed to determine the dominant orientation of hummock corridors and remove misaligned features finally image segmentation is used to isolate and extract the hummock corridors as a binary mask the automated approach is tested visually and statistically against detailed manual mapping in three areas of canada and northern scandinavia results show that while the automated method does not perfectly reproduce the manual mapping it successfully captures the general configuration morphometry length and width and location of hummock corridors despite variation in expression across and between sites this technique is ideally suited to take advantage of newly available high resolution digital elevation data e g the arcticdem whose enormous volume makes large scale manual mapping prohibitively time consuming its application will enable efficient and comprehensive mapping of the spatial distribution of hummock corridors across palaeo beds that is necessary for deriving insights into their formation and the organisation of subglacial meltwater flow beneath ice sheets

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

carbon dating reveals a seasonal progression in the source of particulate organic carbon exported from the greenland ice sheet surface melt from the greenland ice sheet gris collects particulate organic carbon poc as it drains into subglacial environments and transports it downstream where it serves as a microbial substrate we hypothesized that older poc is entrained by meltwaters as the subglacial drainage network expands upglacier over the summer to test this poc samples were collected from a meltwater river exiting the gris over an ablation season and 14c dated resulting values were compared with meltwater hydrochemistry and satellite observations of the catchment area we found that poc ages increased from 5000 to 9000 years b p until peak discharge and catchment size afterward significant fluctuations in poc age were observed interpreted to result from periods of high and low subglacial hydrological pressure and sediment supply and subsequent exhaustion these observations suggest a seasonal progression in the source of poc exported from the gris and provide evidence for a seasonally evolving 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