jurassic tithonian radiolaria from la desirade lesser antilles preliminary paleontological and tectonic implications jurassic upper tithonian radiolaria recovered from bedded red ribbon cherts on la desirade guadeloupe are the oldest fossils yet discovered in the lesser antilles this age not only corroborates contested isotopic ages for the igneous basement of la desirade but also demonstrates that previously reported differences in basement ages for the central and eastern end of the island are invalid in addition la desirade chert yielded a higher latitude northern tethyan to southern boreal realm radiolarian assemblage indicating deposition at a minimum of 22 north or south of the jurassic paleoequator because northern tethyan or boreal jurassic oceanic crust did not exist in the spreading gap between north and south america or east to eurasia we conclude that la desirade oceanic crust formed in the pacific the fact that no upper jurassic red ribbon chert has been found in the atlantic ocean or in the caribbean aside from in displaced oceanic fragments in puerto rico and the dominican republic lends additional support to a pacific origin for the oldest crustal fragment in the lesser antilles

surface melt driven seasonal behaviour englacial and subglacial from a soft bedded temperate glacier recorded by in situ wireless probes we investigate the spatial and temporal englacial and subglacial processes associated with a temperate glacier resting on a deformable bed using the unique glacsweb wireless in situ probes embedded in the ice and the till combined with other techniques including ground penetrating radar gpr and borehole analysis during the melt season spring summer and autumn high surface melt leads to high water pressures in the englacial and subglacial environment winter is characterised by no surface melting on most days base apart from a series of positive degree days once winter begins a diurnal water pressure cycle is established in the ice and at the ice sediment interface with direct meltwater inputs from the positive degree days and a secondary slower englacial pathway with a five day lag this direct surface melt also drives water pressure changes in the till till deformation occurred throughout the year with the winter rate approximately 60 that of the melt season we were able to show the bed comprised patches of till with different strengths and were able to estimate their size relative percentage and temporal stability we show that the melt season is characterised by a high pressure distributed system and winter by a low pressure channelized system we contrast this with studies from greenland overlying rigid bedrock where the opposite was found we argue our results are typical of soft bedded glaciers with low englacial water content and suggest this type of glacier can rapidly respond to surface driven melt based on theoretical and field results we suggest that the subglacial hydrology comprises a melt season distributed system dominated by wide anastomosing broad flat channels and thin water sheets which may become more channelized in winter and more responsive to changes in meltwater inputs

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

influence of subglacial drainage system evolution on glacier surface motion haut glacier d arolla switzerland 1 the relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at haut glacier d arolla switzerland with the onset of high and variable melt season discharges subglacial drainage changed from a predominantly distributed system to a predominantly channelized system the change occurred later farther up glacier during the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge the magnitude and spatial pattern of the glacier s velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity increasing discharge through incipient channels below moulins not yet connected to the main channel caused more localized basal forcing and slightly increased glacier velocity increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event the relative importance of these modes changed through the melt season suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing

influence of glacier runoff and near terminus subglacial hydrology on frontal ablation at a large greenlandic tidewater glacier charlie bunce is supported by a nerc dtp studentship ne l002558 1 ben davison is funded by the scottish alliance for geoscience environment and society sages and university of st andrews we acknowledge field and research grants from the rgs ibg postgraduate research fund and mackay weir greenland fund university of edinburgh awarded to charlie bunce and rgs ibg postgraduate research fund mackay weir greenland fund university of edinburgh and centenary funding university of edinburgh awarded to alexis moyer university of edinburgh

anatomy of a meltwater drainage system beneath the ancestral east antarctic ice sheet the east antarctic ice sheet was larger than present during past cold periods seafloor geophysical data show that in the ross sea the extended ice sheet was underlain by an active hydrologic system during the glacial termination

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

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

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 the diurnal variation of tracer transit velocity through a subglacial channel 1 tracer injections into a moulin connecting to a subglacial channel at unteraargletscher were repeated at intervals of about two hours over two diurnal discharge cycles in august and september 2000 the pronounced hysteresis in the relationships between tracer transit velocity and proglacial discharge may be explained by adjustments of the channel cross section in response to discharge variations or alternatively by modulation of inflow from the tributary moulin into a main subglacial channel to test these hypotheses we employ a physically based time dependent model of channelized subglacial drainage in a series of numerical experiments we explore the general characteristics of each mechanism before the model is applied to diurnal discharge variations measured during the tracer tests at unteraargletscher we found that the ability of a rothlisberger channel to adjust its size to the prevailing hydraulic conditions can contribute to the hysteresis in the velocity discharge relationships however using plausible parameter values r channel hydraulics alone is not satisfactory in reproducing the magnitude of observed transit velocities the model further confirms that retardation of the tracer in the tributary moulin due to inflow modulation has major effects on transit velocities both in magnitude and timing with appropriate assumptions on channel sinuosity and moulin size a combination of both mechanisms can be found that is capable of reproducing the observed transit velocities

diurnal variability of subglacial drainage conditions as revealed by tracer experiments 1 the morphology of the drainage system of unteraargletscher switzerland and the diurnal variability of drainage conditions were investigated by conducting a series of tracer tests over a number of discharge cycles during the ablation season 2000 dye injections into a moulin were repeated at intervals of a few hours and were accompanied by simultaneous measurements of discharge of supraglacial meltwater draining into the moulin and bulk runoff in the proglacial stream fast transit velocities in conjunction with low dispersion values suggest that the tracer was routed through a hydraulically efficient channelized drainage system however detailed analyses reveal a large diurnal variability in terms of transit velocity and dispersion coefficient this finding underlines the difficulty of detecting a possible drainage system evolution based on single tracer tests conducted at coarse temporal intervals furthermore the obtained velocity discharge relationships display pronounced hysteresis we suggest that the evolution of the cross sectional area of an ice walled conduit and the modulation of inflow at the junction of a tributary moulin to a main subglacial channel are responsible for the observed behavior

borehole measurements indicate hard bed conditions kangerlussuaq sector western greenland ice sheet the hydrological processes and sliding dynamics of the greenland ice sheet are closely linked to the structural framework of its bed establishing whether the ice mass rests on bedrock relatively clear of rock debris a thick deformable till layer or some intermediate combination is therefore key to understanding the ice sheet response to changes in meltwater inputs here we report on direct observations of the ice bed interface along a flow line transect in the ablation zone of the western greenland ice sheet our measurements are derived from a network of 32 boreholes that vary from 100 m deep near the ice margin to 830 m deep 46 km upflow from the terminus we performed a suite of experiments and sampling techniques in the holes to investigate the bed conditions in contrast to previous geophysical and drilling studies elsewhere in greenland which have suggested that the ice rests on a thick layer of sediment we find no evidence for a thick sediment cover on the bed of the kangerlussuaq sector of the ice sheet our observations imply that this area has a relatively clean and hard bed and that this facilitates a basal hydraulic system governed by ice bedrock interactions the lack of sediment cover on the bed could be due to recent deglaciation and advance of this sector of the ice sheet and or variable rates of erosion and preferential routing of sediment through bedrock troughs

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

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

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

controls on the early holocene collapse of the bothnian sea ice stream new high resolution multibeam data in the gulf of bothnia reveal for the first time the subglacial environment of a bothnian sea ice stream the geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of bothnian sea ice during deglaciation here we apply a well established one dimensional flowline model to simulate ice flow through the gulf of bothnia and investigate controls on retreat of the ice stream during the post younger dryas deglaciation of the fennoscandian ice sheet the relative influence of atmospheric and marine forcings are investigated with the modelled ice stream exhibiting much greater sensitivity to surface melting implemented through surface mass balance and hydrofracture induced calving than to submarine melting or relative sea level change such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record the modelled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope but local variations in basal topography and ice stream width result in non linear retreat of the grounding line notably demonstrating points of short lived retreat slowdown on reverse bed slopes retreat of the ice stream was most likely governed by increased ice surface meltwater production with the modelled retreat rate less sensitive to marine forcings despite the marine setting

seasonal to multiyear variability of glacier surface velocity terminus position and sea ice ice melange in northwest greenland glacier ice discharge which depends on ice velocity and terminus fluctuations is a primary component of greenland ice sheet mass loss some research suggests that ice melange influences terminus calving in turn affecting glacier velocity the details and broad spatiotemporal consistency of these relationships however is undetermined focusing on 16 northwestern greenland glaciers during 2009 through summer 2014 we examined seasonal surface velocity changes glacier terminus position and sea ice and ice melange conditions for a longer term analysis we also produced extended records of four glaciers from 1999 to 2014 there is a strong correspondence between seasonal near terminus sea ice melange conditions and terminus change with rigid ice melange conditions associated with advance and open water associated with retreat extended sea ice free periods and reduced rigid melange are also linked with anomalously large terminus retreat in all but one case sustained multiyear retreat of greater than 1 km during both the 15 year and 6 year records was accompanied by interannual velocity increases seasonal velocity patterns however correspond more strongly with runoff changes than terminus behavior projections of continued warming and longer sea ice free periods around greenland indicate that notable retreat over wide areas may continue this sustained retreat likely will contribute to multiyear speedup longer melt seasons and earlier breakup of melange may also alter the timing of seasonal ice flow variability

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

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

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

greenland ice sheet flow response to runoff variability we use observations of ice sheet surface motion from a global positioning system gps network operating from 2006 2014 around north lake in west greenland to investigate the dynamical response of the greenland ice sheet s ablation area to inter annual variability in surface melting we find no statistically significant relationship between runoff season characteristics and ice flow velocities within a given year or season over the seven year time series annual velocities at north lake decrease at an average rate of 0 9 1 1 m yr 2 consistent with the negative trend in annual velocities observed in neighboring regions over recent decades we find that net runoff integrated over several preceding years has a negative correlation with annual velocities similar to findings from the two other available decadal records of ice velocity in western greenland however we argue that this correlation is not necessarily evidence for a direct hydrologic mechanism acting on the timescale of multiple years but could be a statistical construct finally we stress that neither the decadal slowdown trend nor the negative correlation between velocity and integrated runoff are predicted by current ice sheet models underscoring that these models do not yet capture all the relevant feedbacks between runoff and ice dynamics needed to predict long term trends in ice sheet flow

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

geometry and flow conditions of subglacial water at south cascade glacier washington state u s a an analysis of tracer injections tracers were injected into south cascade glacier to determine the flow condition and geometry of the subglacial water system results indicate that two distincts drainage basins on the glacier feed the two main streams flowing from the glacier in the largest basin two parallel drainage networks exist one englacial and the other subglacial

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

rock comminution as a source of hydrogen for subglacial ecosystems microbes live under glaciers that have persisted for millions of years without a clear energy supply analyses of crushed rocks suggest that interactions of glaciers with the rocks beneath can produce enough h2 to support methanogenic bacteria substantial parts of the beds of glaciers ice sheets and ice caps are at the pressure melting point1 the resulting water harbours diverse subglacial microbial ecosystems2 3 capable of affecting global biogeochemical cycles4 5 such subglacial habitats may have acted as refugia during neoproterozoic glaciations6 however it is unclear how life in subglacial environments could be supported during glaciations lasting millions of years because energy from overridden organic carbon would become increasingly depleted7 8 here we investigate the potential for abiogenic h2 produced during rock comminution to provide a continual source of energy to support subglacial life we collected a range of silicate rocks representative of subglacial environments in greenland canada norway and antarctica and crushed them with a sledgehammer and ball mill to varying surface areas under an inert atmosphere in the laboratory we added water and measured h2 production with time h2 was produced at 0 c in all silicate water experiments probably through the reaction of water with mineral surface silica radicals formed during rock comminution h2 production increased with increasing temperature or decreasing silicate rock grain size sufficient h2 was produced to support previously measured rates of methanogenesis under a greenland glacier we conclude that abiogenic h2 generation from glacial bedrock comminution could have supported life and biodiversity in subglacial refugia during past extended global glaciations

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

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

utility of 222rn as a passive tracer of subglacial distributed system drainage abstract water flow beneath the greenland ice sheet gris has been shown to include slow inefficient distributed and fast efficient channelized drainage systems in response to meltwater delivery to the bed via both moulins and surface lake drainage this partitioning between channelized and distributed drainage systems is difficult to quantify yet it plays an important role in bulk meltwater chemistry and glacial velocity and thus subglacial erosion radon 222 which is continuously produced via the decay of 226ra accumulates in meltwater that has interacted with rock and sediment hence elevated concentrations of 222rn should be indicative of meltwater that has flowed through a distributed drainage system network in the spring and summer of 2011 and 2012 we made hourly 222rn measurements in the proglacial river of a large outlet glacier of the gris leverett glacier sw greenland radon 222 activities were highest in the early melt season 10 15 dpm l 1 decreasing by a factor of 2 5 3 5 dpm l 1 following the onset of widespread surface melt using a 222rn mass balance model we estimate that on average greater than 90 of the river 222rn was sourced from distributed system meltwater the distributed system 222rn flux varied on diurnal weekly and seasonal time scales with highest fluxes generally occurring on the falling limb of the hydrograph and during expansion of the channelized drainage system using laboratory based estimates of distributed system 222rn the distributed system water flux generally ranged between 1 5 of the total proglacial river discharge for both seasons this study provides a promising new method for hydrograph separation in glacial watersheds and for estimating the timing and magnitude of distributed system fluxes expelled at ice sheet margins

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

quantifying supraglacial meltwater pathways in the paakitsoq region west greenland increased summer ice velocities on the greenland ice sheet are driven by meltwater input to the subglacial environment however spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood to further our understanding of surface drainage we apply a supraglacial hydrology model to the paakitsoq region west greenland for three contrasting melt seasons during an average melt season crevasses drain 47 of surface runoff lake hydrofracture drains 3 during the hydrofracturing events themselves while the subsequent surface to bed connections drain 21 and moulins outside of lake basins drain 15 lake hydrofracture forms the primary drainage pathway at higher elevations above 850 m while crevasses drain a significant proportion of meltwater at lower elevations during the two higher intensity melt seasons model results show an increase 5 and 6 of total surface runoff in the proportion of runoff drained above 1300 m relative to the melt season of average intensity the potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production

spatially distributed runoff at the grounding line of a large greenlandic tidewater glacier inferred from plume modelling understanding the drivers of recent change at greenlandic tidewater glaciers is of great importance if we are to predict how these glaciers will respond to climatic warming a poorly constrained component of tidewater glacier processes is the near terminus subglacial hydrology here we present a novel method for constraining near terminus subglacial hydrology with application to marine terminating kangiata nunata sermia in south west greenland by simulating proglacial plume dynamics using buoyant plume theory and a general circulation model we assess the critical subglacial discharge if delivered through a single compact channel required to generate a plume that reaches the fjord surface we then compare catchment runoff to a time series of plume visibility acquired from a time lapse camera we identify extended periods throughout the 2009 melt season where catchment runoff significantly exceeds the discharge required for a plume to reach the fjord surface yet we observe no plume we attribute these observations to spatial spreading of runoff across the grounding line persistent distributed drainage near the terminus would lead to more spatially homogeneous submarine melting and may promote more rapid basal sliding during warmer summers potentially providing a mechanism independent of ocean forcing for increases in atmospheric temperature to drive tidewater glacier acceleration

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

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

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

a cross validated three dimensional model of an englacial and subglacial drainage system in a high arctic glacier recent speleological surveys of meltwater drainage systems in cold and polythermal glaciers have documented dynamic englacial and in some cases subglacial conduits formed by the cut and closure mechanism investigations of the spatial distribution of such conduits often require a combination of different methods here we studied the englacial drainage system in the cold gla cier longyearbreen svalbard by combining speleological exploration of a 478 m long meltwater conduit with a high resolution ground penetrating radar gpr survey with two different centre frequencies 25 and 100 mhz the results yielded a 3 d documentation of the present englacial drainage system the study shows that the overall form of englacial conduits can be detected from velocity depth converted gpr data and that the 3 d model can facilitate a method to pinpoint the reflections in a radargram corresponding with the englacial drainage system although fine detail cannot be resolved visible reflections approximately parallel to the mapped englacial water drainage system likely result from sediment incorporated in the ice or from abandoned parts of the englacial drainage system less

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

rapid erosion beneath the greenland ice sheet the pleistocene ice sheets left a clear signature of erosion but the rate at which ice sheets erode is difficult to determine from either paleolandscapes or observations of contemporary processes here we use two years of sediment flux data derived from meltwaters emerging from an outlet glacier in west greenland to calculate an average rate of subglacial erosion across a catchment extending 50 km inland from the ice margin erosion in this zone occurs at 4 8 2 6 mm a 1 a rate 1 2 orders of magnitude greater than previous estimates of erosion rate beneath the greenland ice sheet our results suggest that where surface meltwaters are able to access the bed the rate of erosion by ice sheets is in keeping with the rapid erosion observed at temperate alpine glaciers during deglacial phases when meltwater was abundant ice sheet margins should therefore have acted as highly efficient agents of erosion

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

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

decadal slowdown of a land terminating sector of the greenland ice sheet despite warming ice flow along land terminating margins of the greenland ice sheet gis varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet such inputs lubricate the ice bed interface transiently speeding up the flow of ice greater melting results in faster ice motion during summer but slower motion over the subsequent winter owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice sheet bed that have a high basal water pressure however the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained here we show that annual ice motion across an 8 000 km 2 land terminating region of the west gis margin extending to 1 100 m above sea level was 12 slower in 2007 14 compared with 1985 94 despite a 50 increase in surface meltwater production our findings suggest that over these three decades hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion not a speed up as previously postulated increases in meltwater production from projected climate warming may therefore further reduce the motion of land terminating margins of the gis our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought

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

rapid mineral surface weathering beneath the greenland ice sheet shown by radium and uranium isotopes abstract glaciated watersheds are regions of intense physical and chemical weathering in order to gain new insight on subglacial weathering processes we measured uranium and radium isotopes from a proglacial river draining the greenland ice sheet gris time series samples were collected from the spring to mid summer a time period during which subglacial drainage pathways are thought to transition from slow inefficient distributed to fast efficient channelized systems the 228ra 226ra activity ratio of the dissolved load varied from 5 2 0 9 16 9 3 6 which was significantly higher than the 228ra 226ra ratio of a suspended sediment load sample of 2 1 0 07 and crustal values of 1 the high 228ra 226ra in the dissolved load relative to the source rock material is indicative of mineral surface weathering induced by rapid and continuous flushing of the subglacial drainage network during the course of the melt season and those prior the 234u 238u ratio 234u varied between 33 and 106 with a discharge weighted mean of 67 the seasonal evolution of 234u did not correlate to geochemical indicators of subglacial meltwater storage time an experiment designed to measure changes in 234u with increasing meltwater storage times found that 234u in the dissolved phase decreased rapidly with increasing storage time similarly samples collected along a transect moving downstream from the ice sheet terminus had decreasing 234u values from 63 to 15 further indicating that with increased weathering the 234u of meltwater decreases coupled with the relatively low 234u and high 228ra 226ra u appears to be impacted by rapid chemical weathering of subglacial and suspended sediments the leverett river discharge weighted u concentration was 0 13 nm if this system is considered representative of the broader gris then the total dissolved u flux from the gris would be on the order of 6 4 104 mol y using a similar set of assumptions the dissolved 228ra and 226ra flux from the gris was 1 1 1014 dpm y and 5 5 1013 dpm y respectively these estimates suggest that the 226ra flux to the ocean from the gris is globally significant and that the 228ra flux in particular is larger than most river inputs

subglacial drainage evolution modulates seasonal ice flow variability of three tidewater glaciers in southwest greenland b j d was funded in the form of a phd studentship provided by the scottish association for geosciences environment and society sages and the university of st andrews uk j m l is supported by a ukri future leaders fellowship grant no mr s017232 1 d f would like to acknowledge the support of this work through the epsrc and esrc centre for doctoral training on quantification and management of risk and uncertainty in complex systems environments grant no ep l015927 1

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

mechanisms of fast flow in jakobshavns isbrae west greenland part i measurements of temperature and water level in deep boreholes several holes were drilled to depths of 1500 1630 m along a profile across jakobshavns isbrae 50 km upstream from the calving front drilling was by hot water and required approximately 20 h the holes were rapidly closed by refreezing but it was possible to instrument them with thermistors and tilt sensors before this occurred near the margins of the ice stream the holes reached the bed and connected with the subglacial drainage system water level changes recorded in these holes are discussed in terms of the basal hydraulic system the temperature measurements show that the glacier is temperate based moreover extrapolation of a measured temperature profile and its curvature suggests that a temperate layer of substantial thickness may exist at the bed near the center of the ice stream there is a striking difference in the shapes of temperature profiles measured at different locations beneath the center line the temperature minimum is at a considerably smaller relative depth than near the margins but it is nearly the same in magnitude 22 1 c this may indicate a disproportionately large vertical stretching of the basal ice in the center of the ice stream since the basal ice is warmer and much less viscous than the ice above a thickening of that layer would cause a corresponding increase of surface velocity we presume that this mechanism contributes to the fast flow of jakobshavns isbrae

evolution of the internal drainage system in the lower part of the ablation area of storglaci aren sweden during the 1989 melt season ten tracer experiments were conducted to investigate seasonal diurnal and spatial variations in the internal drainage system in the ablation area of storglaciaren sweden dye was injected into moulins and dye concentration and discharge were monitored in two streams at the glacier terminus sydjakk and nordjakk dye emerged only in sydjakk transit velocities ranged from 0 07 to 0 29 m s implying that drainage was through a well defined conduit system velocity and discharge relations suggest that these conduits are part of a multi branched arborescent network of individually braided wide low passages a progressive decline in dispersivity during the early part of the melt season reflects structural changes in these conduits presumably including a decrease in the degree of braiding and an increase in size the system reached its late season configuration in early august by which time the energy dissipated by the flowing water would have sufficed to melt between 0 7 and 21 m 3 of ice per meter of conduit length depending on conduit size potential closure rates exceed melt rates on conduit walls throughout most of the drainage system and so conduits are probably full of water during periods of higher daily discharge with water pressures exceeding atmospheric pressure variations in tracer return curves and dispersivity on a diurnal time scale also appear to reflect changes in the degree of braiding in this case resulting from drowning of low divides between channels as discharge increases

short term variability in greenland ice sheet motion forced by time varying meltwater drainage implications for the relationship between subglacial drainage system behavior and ice velocity 1 high resolution measurements of ice motion along a 120 km transect in a land terminating section of the gris reveal short term velocity variations 1 day which are forced by rapid variations in meltwater input to the subglacial drainage system from the ice sheet surface the seasonal changes in ice velocity at low elevations 1000 m are dominated by events lasting from 1 day to 1 week although daily cycles are largely absent at higher elevations reflecting different patterns of meltwater input using a simple model of subglacial conduit behavior we show that the seasonal record of ice velocity can be understood in terms of a time varying water input to a channelized subglacial drainage system our investigation substantiates arguments that variability in theduration and rate rather than absolute volume of meltwater delivery to the subglacial drainage system are important controls on seasonal patterns of subglacial water pressure and therefore ice velocity we suggest that interpretations of hydro dynamic behavior in land terminating sections of the gris margin which rely on steady state drainage theories are unsuitable for making predictions about the effect of increased summer ablation on future rates of ice motion

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

seasonal variability in the dynamics of marine terminating outlet glaciers in greenland recent studies indicate that the dynamics of fast flowing marine terminating outlet glaciers of the greenland ice sheet may be sensitive to climate and ocean forcing on sub annual timescales observations of seasonal behavior of these glaciers at such high temporal resolution however are currently few here we present observations of front position flow speed near surface air temperature and ocean conditions for six large marine terminating glaciers in the uummannaq region of west greenland to investigate controls on short term glacier dynamics as proposed by other studies we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange our data suggest that high sea surface temperature anomalously low sea ice concentration and reduced melange formation in early 2003 have triggered multi year retreat of several glaciers in the study area which is consistent with other regions in greenland of the stable glaciers only rink isbrae exhibits a seasonal speed variation that correlates with variations in front position with the others undergoing mid summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution drainage of supraglacial lakes and water filled crevasses results in substantial decreases in speed 40 60 on fast flowing glaciers our results demonstrate that attempts to model ice sheet evolution must take into account short timescale flow dynamics resulting from drainage events and oceanographic conditions

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

geomorphic implications of subglacial drainage configuration rates of basal sediment evacuation controlled by seasonal drainage system evolution abstract although basal sediment evacuation by subglacial meltwater is widely assumed to dominate the sediment budget of most temperate glaciers and ice caps its potential influence for glacial geomorphic processes and landforms has largely been overlooked in the glacial literature data collected at haut glacier d arolla switzerland during the 1998 ablation season are used to test theoretical predictions that subglacial drainage configuration should exert a primary control on rates of basal sediment evacuation by subglacial meltwater seasonal evolution of the subglacial drainage system enables observation of sediment evacuation from both distributed and channelised drainage systems daily suspended sediment load exhibits an extreme nonlinear increase with respect to discharge between winter and summer periods which correspond to periods of predominantly distributed and channelised subglacial drainage respectively these results suggest that subglacial drainage configuration should significantly impact upon a rates of subglacial erosion and debris production b the nature of debris transport and the proportion of sediment in glacial versus fluvioglacial transport pathways and c rates and styles of ice marginal sedimentation in contrast to their distributed counterparts channelised drainage configurations are predicted to encourage subglacial erosion through the removal of basal sediment from the ice bedrock interface and with the exception of supraglacial transport will discourage sediment transport by alternative glacial mechanisms direct sedimentation at the glacier margin may therefore be reduced resulting in smaller moraines despite an overall increase in sediment production an appreciation of the coupling between glacier hydrology and geomorphic processes suggests new frameworks for the understanding of rates and styles of glacial erosion sediment transfer and deposition and the complex geomorphology of ice marginal environments

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

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

meltwater chemistry and solute export from a greenland ice sheet catchment watson river west greenland summary solute export from the greenland ice sheet gris to coastal regions around greenland is likely to increase in the future as a consequence of increasing icemelt production here we present hydrochemical characteristics solute and major ion exports and chemical denudation rates for 2007 2010 for the watson river sector of the gris that drains into the fjord kangerlussuaq the hydrochemistry is dominated by ca2 and hco3 with a relatively high molar k na ratio of 0 6 0 1 typical for meltwaters draining a gneissic lithology low molar ca2 na and mg2 na ratios indicate that weathering of disseminated carbonates contributes less than silicate weathering to the chemical composition the solute export varied between 33 103 2009 and 61 103 tons 2010 showing that increasing discharge leads to increasing solute export at the catchment scale deviations between ion yield estimates derived from use of discharge weighted and mean daily concentrations methods were generally less than 5 indicating that the choice of method is of less importance the chemical denudation rates ranged between 36 and 56 meq m 2 per year which are lower than previous records from glacierized catchments however when normalized by discharge the denudation rates are comparable to other arctic sites when extrapolating the results from the watson river catchment to the entire greenland for 2007 2010 the solute export from greenland meltwater varied between 7 1 106 and 7 8 106 tons whilst the major ion export was between 6 4 106 and 7 3 106 tons dissolved fe a potential biolimiting nutrient for primary productivity in the north atlantic had annual export rates from greenland between 15 103 and 52 103 tons

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

subglacial drainage system structure and morphology of brewster glacier new zealand a global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of brewster glacier these are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure pw i pw ice overburden ii pw half ice overburden iii pw atmospheric additionally 16 dye tracing experiments at 12 locations were performed through a summer melt season dye return curve shape together with calculations of transit velocity dispersivity and storage are used to infer the likely morphology of the subglacial drainage system taken together the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden by mid summer water pressures are closer to half ice overburden and the channelised drainage system is more hydraulically efficient surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system copyright 2008 john wiley sons ltd

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

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

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

modeling of subglacial hydrological development following rapid supraglacial lake drainage the rapid drainage of supraglacial lakes injects substantial volumes of water to the bed of the greenland ice sheet over short timescales the effect of these water pulses on the development of basal hydrological systems is largely unknown to address this we develop a lake drainage model incorporating both 1 a subglacial radial flux element driven by elastic hydraulic jacking and 2 downstream drainage through a linked channelized and distributed system here we present the model and examine whether substantial efficient subglacial channels can form during or following lake drainage events and their effect on the water pressure in the surrounding distributed system we force the model with field data from a lake drainage site 70 km from the terminus of russell glacier in west greenland the model outputs suggest that efficient subglacial channels do not readily form in the vicinity of the lake during rapid drainage and instead water is evacuated primarily by a transient turbulent sheet and the distributed system following lake drainage channels grow but are not large enough to reduce the water pressure in the surrounding distributed system unless preexisting channels are present throughout the domain our results have implications for the analysis of subglacial hydrological systems in regions where rapid lake drainage provides the primary mechanism for surface to bed connections

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

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

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

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

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

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

multiproxy evidence of middle and late pleistocene environmental changes in the loess paleosol sequence of bůhzdař czech republic abstract loess paleosol sequences preserve information that can be used to reconstruct paleoenvironments a dense network of reliably analyzed sequences from different geographic locations is crucial to address ecological and climatic trends which occurred during the pleistocene this paper focuses on a loess paleosol sequence in b hzda situated 9 km nw of prague czech republic geochemical approaches total organic carbon xrf elemental analyses xrd mineralogy 13c and 18o stable isotopes are combined with grain size distributions and optically stimulated luminescence osl to assess the climatic conditions at the time of formation of the strata the oldest loess at this location was formed circa 200 ka ago mis 7 making it the oldest loess in central bohemia for now the b hzda loess paleosol sequence was highly affected by several erosion events eemian paleosols mis 5e being completely missing the partly relocated paleosols situated at the lower part of the b hzda sequence represent a period of increased humidity during late mis 7 and mis 6 despite the fact that the b hzda profile is not particularly thick 5 m significant changes in paleoclimate reconstructions have been detected