extensive liquid meltwater storage in firn within the greenland ice sheet surface melt water from the greenland ice sheet can become trapped in firn delaying its journey to the sea radar and ice core observations provide direct evidence of a perennial aquifer in the firn layer in southern greenland that represents a potentially significant contribution to the greenland mass budget

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

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

southeast greenland high accumulation rates derived from firn cores and ground penetrating radar despite containing only 14 of the greenland ice sheet by area the southeastern sector has the highest accumulation rates and hence receives 30 of the total snow accumulation we present accumulation rates obtained during our 2010 arctic circle traverse derived from three 50 m firn cores dated using geochemical analysis we tracked continuous internal reflection horizons between the firn cores using a 400 mhz ground penetrating radar gpr gpr data combined with depth age scales from the firn cores provide accumulation rates along a 70 km transect we followed an elevation gradient from 2350 to 1830m to understand how progressive surface melt may affect the ability to chemically date the firn cores and trace the internal layers with gpr from the firn cores we find a 52 0 43 m w e a 1 increase in average snow accumulation and greater interannual variability at the lower site than the upper site the gpr profiling reveals that accumulation rates are influenced by topographic undulations on the surface with up to 23 variability over 7 km these measurements confirm the presence of high accumulation rates in the southeast as predicted by the calibrated regional climate model polar mm5

over winter persistence of supraglacial lakes on the greenland ice sheet results and insights from a new model we present a newly developed 1 d numerical energy balance and phase transition supraglacial lake model glacierlake glacierlake incorporates snowfall in situ snow and ice melt incoming water from the surrounding catchment ice lid formation basal freeze up and thermal stratification snow cover and temperature are varied to test lake development through winter and the maximum lid thickness is recorded average wintertime temperatures of 2 to 30 circ rm c and total snowfall of 0 to 3 45 m lead to a range of the maximum lid thickness from 1 2 to 2 8 m after sim 250 days with snow cover exerting the dominant control an initial ice temperature of 15 circ rm c with simulated advection of cold ice from upstream results in 0 6 m of basal freeze up this suggests that lakes with water depths above 1 3 to 3 4 m dependent on winter snowfall and temperature upon lid formation will persist through winter these buried lakes can provide a sizeable water store at the start of the melt season expedite future lake formation and warm underlying ice even in winter

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

differentiating bubble free layers from melt layers in ice cores using noble gases melt layers are clear indicators of extreme summer warmth on polar ice caps the visual identification of refrozen meltwater as clear bubble free layers cannot be used to study some past warm periods because in deeper ice bubbles are lost to clathrate formation we present here a reliable method to detect melt events based on the analysis of kr ar and xe ar ratios in ice cores and apply it to the detection of melt in clathrate ice from the eemian at neem greenland additionally melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases or by altering gas transport in the firn which affects the gas chronology we find that the easily visible 1 mm thick bubble free layers in the wais divide ice core do not contain sufficient melt to alter the gas composition in the core and do not cause artifacts or discontinuities in the gas chronology the presence of these layers during winter and the absence of anomalies in soluble gases suggests that these layers can be formed by processes other than refreezing of meltwater consequently the absence of bubbles in thin crusts is not in itself proof of a melt event

automated mapping of local layer slope and tracing of internal layers in radio echograms an automated method is presented for tracing layers in radio echograms the method is designed to work with most radio echo sounding echograms and has been successfully tested with a 180 210 mhz multichannel coherent depth sounder to accurately trace layers first approximate layer positions are calculated by integrating the local layer slope which is inferred by the intensity response to a slanted filter then the positions are refined using an iterative optimization the layers are traced using an active contour model or snake which can be constrained to conserve both echogram features and smooth layers with this technique it is possible to trace internal layers over distances of several hundred kilometers the method was tested between two greenland deep ice cores where the age depth relation is known

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

acceleration of the contribution of the greenland and antarctic ice sheets to sea level rise 1 ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques over different time periods and at various levels of spatial detail considerable disparity remains between these estimates due to the inherent uncertainties of each method the lack of detailed comparison between independent estimates and the effect of temporal modulations in ice sheet surface mass balance here we present a consistent record of mass balance for the greenland and antarctic ice sheets over the past two decades validated by the comparison of two independent techniques over the last 8 years one differencing perimeter loss from net accumulation and one using a dense time series of time variable gravity we find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss in 2006 the greenland and antarctic ice sheets experienced a combined mass loss of 475 158 gt yr equivalent to 1 3 0 4 mm yr sea level rise notably the acceleration in ice sheet loss over the last 18 years was 21 9 1 gt yr2 for greenland and 14 5 2 gt yr2 for antarctica for a combined total of 36 3 2 gt yr2 this acceleration is 3 times larger than for mountain glaciers and ice caps 12 6 gt yr2 if this trend continues ice sheets will be the dominant contributor to sea level rise in the 21st century

the extreme melt across the greenland ice sheet in 2012 1 the discovery of the 2012 extreme melt event across almost the entire surface of the greenland ice sheet is presented data from three different satellite sensors including the oceansat 2 scatterometer the moderate resolution imaging spectroradiometer and the special sensor microwave imager sounder are combined to obtain composite melt maps representing the most complete melt conditions detectable across the ice sheet satellite observations reveal that melt occurred at or near the surface of the greenland ice sheet across 98 6 of its entire extent on 12 july 2012 including the usually cold polar areas at high altitudes like summit in the dry snow facies of the ice sheet this melt event coincided with an anomalous ridge of warm air that became stagnant over greenland as seen in melt occurrences from multiple ice core records at summit reported in the published literature such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the medieval warm period given its rarity the 2012 extreme melt across greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research

stratigraphic continuity in 400 mhz short pulse radar profiles of firn in west antarctica we track dated firn horizons within 400mhz short pulse radar profiles to find the continuous extent over which they can be used as historical benchmarks to study past accumulation rates in west antarctica the 30 40cm pulse resolution compares with the accumulation rates of most areas we tracked a particular set that varied from 30 to 90m in depth over a distance of 600 km the main limitations to continuity are fading at depth pinching associated with accumulation rate differences within hills and valleys and artificial fading caused by stacking along dips the latter two may be overcome through multi kilometer distances by matching the relative amplitude and spacing of several close horizons along with their pulse forms and phases modeling of reflections from thin layers suggests that the 37 to 50db range of reflectivity and the pulse waveforms we observed are caused by the numerous thin ice layers observed in core stratigraphy constructive interference between reflections from these close high density layers can explain the maintenance of reflective strength throughout the depth of the firn despite the effects of compaction the continuity suggests that these layers formed throughout west antarctica and possibly into east antarctica as well

firn cold content evolution at nine sites on the greenland ice sheet between 1998 and 2017 current sea level rise partly stems from increased surface melting and meltwater runoff from the greenland ice sheet multi year snow also known as firn covers about 80 of the ice sheet and retains part of the surface meltwater since the firn cold content integrates its physical and thermal characteristics it is a valuable tool for determining the meltwater retention potential of firn we use gap filled climatological data from nine automatic weather stations in the ice sheet accumulation area to drive a surface energy budget and firn model validated against firn density and temperature observations over the 1998 2017 period our results show a stable top 20 m firn cold content cc20 at most sites only at the lower elevation dye 2 site did cc20 decrease by 24 in 2012 before recovering to its original value by 2017 heat conduction towards the surface is the main process feeding cc20 at all nine sites while cc20 reduction occurs through low cold content fresh snow addition at the surface during snowfall and latent heat release when meltwater refreezes our simulations suggest that firn densification while reducing pore space for meltwater retention increases the firn cold content enhances near surface meltwater refreezing and potentially sets favourable conditions for ice slab formation

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

khmer sandstone quarries of kulen mountain and koh ker a petrographic and geochemical study abstract sandstone from several ancient quarries and natural outcrops located at the eastern foothill of kulen mountain and in koh ker northern cambodia has been characterized by means of petrographic analysis scanning electron microscopy and geochemical analysis the samples have been collected during a series of field surveys organized jointly by the metropolitan museum of art and the authority for the protection and management of angkor and the region of siem reap apsara the data show how the sandstone quarried from these sites is characterized by an overall petrographic and geochemical homogeneity and is consistent with the upper terms of the lower middle jurassic terrain rouge formation this result suggests that petrography and geochemistry of sandstone belonging to the terrain rouge formation might not be unambiguously characteristic of single quarry sites but rather representative of vast portions of the sedimentary sequence

meltwater penetration through temperate ice layers in the percolation zone at dye 2 greenland ice sheet meltwater retention in the firn layer of the greenland ice sheet is has the potential to buffer sea level rise due to ice sheet melt the capacity of the firn layer to store meltwater is unclear h

consideration on the construction process and the sandstone quarries during the angkor period based on the magnetic susceptibility abstract detailed magnetic susceptibility measurement was conducted on the sandstones used for the angkor monuments constructed in the period spanning the preah ko and angkor wat styles and the construction process of the buildings and quarries of the sandstones was considered combined with the previous study on the sandstones of the bayon style uchida e cunin o shimoda i suda c nakagawa t 2003 the construction process of the angkor monuments elucidated by the magnetic susceptibility of sandstone archaeometry 45 221 232 the magnetic susceptibility measurement revealed that there were 7 sandstone quarries corresponding to stages i ii iii iva v vii and viiib during the angkor period the sandstones used for the monuments belonging to stage i show average magnetic susceptibility values ranging from 1 1 to 2 3 10 3 si units in the bakheng style period stage ii the average magnetic susceptibility of the supplied sandstones decreased gradually from around 10 to 1 10 3 si units over time the magnetic susceptibility range of the sandstones used for the monuments of stage iii was from 2 3 to 3 0 10 3 si units in the early angkor wat style period ivb the average magnetic susceptibility of the supplied sandstones increased over time reflecting the supply of the sandstones from two different quarries that is the quarry corresponding to the khleang and baphuon style monuments stage iva with low average magnetic susceptibilities of 1 1 to 2 4 10 3 si units and the quarry corresponding to the main angkor wat style monuments stage v with high average magnetic susceptibilities ranging from 2 8 to more than 4 3 10 3 si units the sandstones of stage vii show low average magnetic susceptibility around 1 10 3 si unit the sandstones of stage vid is a mixture of sandstones of stages v and those of stage vii the sandstones with high magnetic susceptibilities are found in the monuments belonging to stage viiib reflecting the supply from the new sandstone quarry

nature and provenance of the sandstone used for bayon style sculptures produced during the reign of jayavarman vii under jayavarman vii 1182 83 ca 1218 ce the khmer empire reached its apex leaving a heritage of major construction works and unique artistic production the stone materials of several sculptures produced under his reign were characterized and compared to possible geological sources in northern and eastern cambodia the data suggest that a specific type of sandstone rich in volcanic detritus was deliberately selected and quarried from a triassic sedimentary sequence exposed far from angkor the main political and economic center at that time

a quasi geostrophic circulation model of the northeast pacific part ii effects of topography and seasonal forcing abstract a quasi geostrophic regional model of the northeast pacific is used to investigate the effects of bottom topography and seasonal wind forcing on the circulation of the alaskan gyre in a numerical experiment with a flat bottom and steady forcing the most energetic signal is due to mesoscale eddies with a 100 day period associated with barotropic wave propagation along the aleutian island arc bottom topography eliminates this signal and causes the flow fluctuations to be of lower frequency and primarily in the first baroclinic rather than the barotropic mode experiments with a climatological seasonal cycle in the wind field show that bottom topography has an important influence in moderating the intensity of the seasonal response it is suggested that the very substantial seasonal variations of the alaskan gyre transport obtained in previous numerical studies of the ne pacific are due to the failure of these models to include or to resolve adequately the bottom topography

direct evidence of meltwater flow within a firn aquifer in southeast greenland within the lower percolation zone of the southeastern greenland ice sheet meltwater has accumulated within the firn pore space forming extensive firn aquifers previously it was unclear if these aquifers stored or facilitated meltwater runoff following mixing of a saline solution into boreholes within the aquifer we observe that specific conductance measurements decreased over time as flowing freshwater diluted the saline mixture in the borehole these tests indicate that water flows through the aquifer with an average specific discharge of 4 3 10 6 m s 2 5 10 6 m s the specific discharge decreases dramatically to 0 m s defining the bottom of the aquifer between 30 to 50 m depth the observed flow indicates that the firn pore space is a short term 30 years storage mechanism in this region meltwater flows out of the aquifer likely into nearby crevasses and possibly down to the base of the ice sheet and into the ocean

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

quantifying vulnerability of antarctic ice shelves to hydrofracture using microwave scattering properties abstract recent ice shelf disintegrations on the antarctic peninsula and subsequent increases in ice sheet mass loss have highlighted the importance of tracking ice shelf stability with respect to surface melt ponding and hydrofracture in this study we use active microwave scatterometry in time series to estimate melt season duration and winter backscatter levels as a proxy for relative concentration of refrozen ice lenses in antarctic ice shelf firn we demonstrate a physical relationship between melt days and firn ice backscatter using scatterometry and field data from greenland and apply the observed relationship to derive and map a vulnerability index for antarctica s ice shelves the index reveals that some remaining antarctic peninsula ice shelves have already reached a firn state that is vulnerable to hydrofracture we also show that the progression of an ice shelf towards vulnerability is affected by many factors such as surface mass balance internal stresses and ice shelf geometry

dynamic perennial firn aquifer on an arctic glacier ice penetrating radar and gps observations reveal a perennial firn aquifer pfa on a svalbard ice field similar to those recently discovered in southeastern greenland a bright widespread radar reflector separates relatively dry and water saturated firn this surface the phreatic firn water table is deeper beneath local surface elevation maxima shallower in surface lows and steeper where the surface is steep the reflector crosscuts snow stratigraphy we use the apparent deflection of accumulation layers due to the higher dielectric permittivity below the water table to infer that the firn pore space becomes progressively more saturated as depth increases our observations indicate that pfas respond rapidly subannually to surface forcing and are capable of providing significant input to the englacial hydrology system

greenland ice sheet contribution to sea level rise buffered by meltwater storage in firn observations of firn structure and meltwater retention on the greenland ice sheet s percolation zone a region of the ice sheet that is perennially covered by snow and firn quantify the capacity of the firn to store future surface meltwater and to delay expansion of the area contributing to sea level rise the extent and the fate of surface melt on the greenland ice sheet are important factors contributing to sea level change the pore spaces in firn a type of partly compacted snow can be filled by meltwater and thus act as a temporary reservoir here joel harper et al show that the greenland ice sheet parts of which are permanently covered by snow and firn can buffer several hundred gigatonnes of meltwater for several decades although this buffering capacity is only temporary and cannot be easily regained it has the potential to delay the pace of near term sea level rise surface melt on the greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era1 2 3 records for melt were broken in 20054 20075 20106 and 20127 much of the increased surface melt is occurring in the percolation zone a region of the accumulation area that is perennially covered by snow and firn partly compacted snow the fate of melt water in the percolation zone is poorly constrained some may travel away from its point of origin and eventually influence the ice sheet s flow dynamics and mass balance and the global sea level whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects here we quantify the existing water storage capacity of the percolation zone of the greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage we collected in situ observations of firn structure and meltwater retention along a roughly 85 kilometre long transect of the melting accumulation area our data show that repeated infiltration events in which melt water penetrates deeply more than 10 metres eventually fill all pore space with water as future surface melt intensifies under arctic warming a fraction of melt water that would otherwise contribute to sea level rise will fill existing pore space of the percolation zone we estimate the lower and upper bounds of this storage sink to be 322 44 gigatonnes and gigatonnes respectively furthermore we find that decades are required to fill this pore space under a range of plausible future climate conditions hence routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea level rise although once the pore space is filled it cannot quickly be regenerated

prototype wireless sensors for monitoring subsurface processes in snow and firn the detection and monitoring of meltwater within firn presents a significant monitoring challenge we explore the potential of small wireless sensors etracer et to measure temperature pressure electrical conductivity and thus the presence or absence of meltwater within firn through tests in the dry snow zone at the east greenland ice core project site the tested sensor platforms are small robust and low cost and communicate data via a vhf radio link to surface receivers the sensors were deployed in low temperature firn at the centre and shear margins of an ice stream for 4 weeks and a bucket experiment was used to test the detection of water within otherwise dry firn the tests showed the et could log subsurface temperatures and transmit the recorded data through up to 150 m dry firn two vhf receivers were tested an autonomous phase sensitive radio echo sounder apres and a winradio the apres can combine high resolution imaging of the firn layers by radio echo sounding with in situ measurements from the sensors to build up a high spatial and temporal resolution picture of the subsurface these results indicate that wireless sensors have great potential for long term monitoring of firn processes

ice dammed lake drainage cools and raises surface salinities in a tidewater outlet glacier fjord west greenland the drainage of ice dammed lakes in the form of outburst floods in greenland is detected regularly by remote sensing and these events are expected to occur more frequently in a warmer climate however their impact on ice sheet stability and neighboring water bodies is still unknown in this interdisciplinary study we investigate lake drainages from the greenland ice sheet into a west greenland fjord by analyzing simultaneous time series of satellite observations and direct hydrographic measurements of temperature and salinity in the fjord satellite images show that in general lake drainages have occurred quasiperiodically during the last decade a particular sequence of drainage events was observed by satellite in 2009 and was analyzed together with the first direct hydrographic observations signs of ice dammed lake drainages were observed by a downstream mooring located just below the intertidal zone the release of freshwater occurred at the fjord subsurface at a tidewater outlet glacier the downstream in water sequence of property changes in relation to these drainage events was observed as an almost immediate decrease in surface layer temperature 2 c followed within a week by the arrival of a high saline pulse 5 units with elevated salinity lasting for several days during the passage during lake drainages large amounts of relatively warm and saline intermediate water are brought to the near surface layers by entrainment processes near the glacier front and this influences the hydrography of the fjord but also impacts the ecosystem through upwelling of nutrient rich intermediate water

a new high resolution digital elevation model of greenland fully validated with airborne laser altimeter data a new digital elevation model of the greenland ice sheet and surrounding rock outcrops has been produced at 1 km postings from a comprehensive suite of satellite remote sensing and cartographic data sets height data over the ice sheet were mainly from ers 1 and geosat radar altimetry these data were corrected for a slope dependent bias that had been identified in a previous study the radar altimetry was supplemented with stereophotogrammetric data sets synthetic aperture radar interferometry and digitized cartographic maps over regions of bare rock and where gaps in the satellite altimeter coverage existed the data were interpolated onto a regular grid with a spacing of 1 km the accuracy of the resultant digital elevation model over the ice sheet was assessed using independent and spatially extensive measurements from an airborne laser altimeter that had an accuracy of between 10 and 12 cm in a comparison with the laser altimetry the digital elevation model was found to have a slope dependent accuracy ranging from 1 04 1 98 m to 0 06 14 33 m over the ice sheet for a slope range of 0 0 1 0 the mean accuracy over the whole ice sheet was 0 33 6 97 m over the bare rock areas the accuracy ranged from 20 to 200 m dependent on the data source available the new digital elevation model was used as an input data set for a positive degree day model of ablation the new elevation model was found to reduce ablation by only 2 compared with using an older 2 5 km resolution model which suggests that resolution induced errors in estimating ablation are no longer important

firn densification in a late noachian icy highlands mars implications for ice sheet evolution and thermal response abstract recent modeling of a thicker early co2 martian atmosphere and late noachian climate predicts that for pressures beyond a fraction of a bar atmosphere surface thermal coupling occurs resulting in adiabatic cooling of high areas across mars this promotes the transport of water ice from relatively warmer low lying areas to the highlands where deposition and accumulation of water ice result in an icy highlands late noachian mars deposits will remain stable in the highlands under nominal late noachian conditions but the potential exists for punctuated heating by both top down e g impacts volcanism and bottom up e g elevated geothermal heat flux processes important in understanding melt generation from these processes is the state of the accumulated snow and ice through modeling of the firn densification process in the icy highlands framework we assess 1 the nature of snow accumulation and the physical growth and evolution of the predicted ice deposits and 2 the implications for the thermal properties of the ice sheets and the response to heating events analysis of the firn densification process in the icy highlands context indicates that 1 the upper layers of the ice sheet will be more vulnerable to melting from top down heating processes because they are comprised of the least dense and least thermally conductive ice and 2 even with a low thermal conductivity firn layer basal melting is only likely to occur through a combination of top down and bottom up heating this is because at the nominal mean annual surface temperatures and estimated effective thermal conductivities the predicted ice sheet thicknesses do not produce enough basal warming to initiate melting for plausible geothermal heat fluxes variations in spin axis orbital parameters alone are not predicted to cause widespread ablation melting and sublimation of the icy highlands ice sheets

ice thickness of the northern half of the patagonia icefields of south america from high resolution airborne gravity surveys author s gourlet p rignot e rivera a casassa g abstract 2015 american geophysical union all rights reserved we employ high resolution high precision helicopter borne gravity observations of the northern npi and southern patagonia icefields spi south america to infer ice thickness and bed topography using a three dimensional model constrained by fjord and lake bathymetry and a land ice mask the results reveal thicker ice than the reflecting horizon of radar sounders with 1 km deep ice for glaciar san rafael and colonia npi and 1 5 km deep ice for glaciar occidental spi these bedrock troughs channelize fast motion of ice from the plateaus combining ice motion and thickness we calculate balance accumulation levels of 3 6 m yr water equivalent for the plateaus bed elevation remains below sea lake level 15 20 km inland for jorge montt and o higgins which favors retreat and is at sea level for san rafael which halted its retreat the results demonstrate the utility of airborne gravity surveys for providing critical data on ice volume bed elevation and balance accumulation of temperate ice masses

antarctic grounding line mapping from differential satellite radar interferometry 1 the delineation of an ice sheet grounding line i e the transition boundary where ice detaches from the bed and becomes afloat in the ocean is critical to ice sheet mass budget calculations numerical modeling of ice sheet dynamics ice ocean interactions oceanic tides and subglacial environments here we present 15 years of comprehensive high resolution mapping of grounding lines in antarctica using differential satellite synthetic aperture radar interferometry dinsar data from the earth remote sensing satellites 1 2 ers 1 2 radarsat 1 and 2 and the advanced land observing system alos palsar for years 1994 to 2009 dinsar directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision at a sample spacing better than 50 m simultaneously over areas several 100 km wide in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data on stagnant and slow moving areas we find that breaks in surface slope are reliable indicators of grounding lines but on most fast moving glaciers and ice streams our dinsar results reveal that prior mappings have positioning errors ranging from a few km to over 100 km a better agreement is found with icesat s data also based on measurements of vertical motion but with a detection noise one order of magnitude larger than with dinsar overall the dinsar mapping of antarctic grounding lines completely redefines the coastline of antarctica

the abandoned ice sheet base at camp century greenland in a warming climate in 1959 the u s army corps of engineers built camp century beneath the surface of the northwestern greenland ice sheet there they studied the feasibility of deploying ballistic missiles within the ice sheet the base and its wastes were abandoned with minimal decommissioning in 1967 under the assumption they would be preserved for eternity by perpetually accumulating snowfall here we show that a transition in ice sheet surface mass balance at camp century from net accumulation to net ablation is plausible within the next 75 years under a business as usual anthropogenic emissions scenario representative concentration pathway 8 5 net ablation would guarantee the eventual remobilization of physical chemical biological and radiological wastes abandoned at the site while camp century and four other contemporaneous ice sheet bases were legally established under a danish u s treaty the potential remobilization of their abandoned wastes previously regarded as sequestered represents an entirely new pathway of political dispute resulting from climate change

analysis and modeling of melt water refreezing in dry snow a dynamic zone of thermal disequilibrium is described which separates wet snow at 0 c from dry sub freezing snow the dynamic zone tends to be eliminated by thermal equilibrium through freezing but is sustained and propagated into the sub freezing snow by water flow from the wet snow the width of the dynamic zone is controlled by the rate of water inflow and by the rate of freezing of water on to sub freezing ice grains which is in turn controlled by the ice water geometry two ice water geometries are investigated isolated ice spheres and capillary tubes of ice into which water is pulled by capillary suction the rate of freezing of water is calculated for the two models for various initial dimensions and temperatures equilibrium times are short typically about 0 5 s but depend on the assumed geometry which is poorly constrained by existing data equilibration times and freeze on mass fluxes are calculated for a variety of general conditions these results can be used in numerical models of wetting front propagation into cold snow

cryosphere bottom up restructuring of ice water at the base of ice sheets can lead to faster ice flow radar sounding of the greenland ice sheet reveals that refreezing of this water can also induce large changes in ice flow and structure

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

quarries and transportation routes of angkor monument sandstone blocks sandstone blocks are the main construction materials used in the angkor monuments in cambodia however a thorough study of the quarries has not yet been carried out we conducted a field investigation of sandstone quarries from the angkor period at the southeastern foot of mt kulen which is approximately 35 km northeast of the angkor monuments as a result we discovered more than 50 sandstone quarries on the basis of the measurements of magnetic susceptibilities and thicknesses step heights we found that they were quarried at different times these four quarrying areas were identified as the quarries d to g inferred by uchida et al 2007 in addition we investigated a canal that was identified on satellite images connecting quarry sites at the foot of mt kulen to the angkor monuments the field investigation suggests a high probability that the canal was used for the transportation of sandstone blocks from mt kulen

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

greenland ice sheet mass balance reconstruction part ii surface mass balance 1840 2010 abstractmeteorological station records ice cores and regional climate model output are combined to develop a continuous 171 yr 1840 2010 reconstruction of greenland ice sheet climatic surface mass balance bclim and its subcomponents including near surface air temperature sat since the end of the little ice age independent observations are used to assess and compensate errors melt water production is computed using separate degree day factors for snow and bare ice surfaces a simple meltwater retention scheme yields the time variation of internal accumulation runoff and bare ice area at decadal time scales over the 1840 2010 time span summer june august sat increased by 1 6 c driving a 59 surface meltwater production increase winter warming was 2 0 c substantial interdecadal variability linked with episodic volcanism and atmospheric circulation anomalies is also evident increasing accumulation and melt rates bare ice area and meltwater retention are driven by increasing sat as a con

cryosphere greenland s lurking aquifer runoff estimates from the greenland ice sheet carry uncertainty because the fate of surface melt in permanently snow covered regions is unconstrained in situ and airborne observations reveal large scale liquid water storage in buried layers of aged and compacted snow

surface meltwater impounded by seasonal englacial storage in west greenland the delivery of surface meltwater through englacial drainage systems to the bed of the greenland ice sheet modulates ice flow through basal lubrication recent studies in southeast greenland have identified a perennial firn aquifer however there are few observations quantifying the input or residence time of water within the englacial system and it remains unknown whether water can be stored within solid ice using hourly stationary radar measurements we present observations of englacial and episodic subglacial water in the ablation zone of store glacier in west greenland we find significant storage of meltwater in solid ice damaged by crevasses extending down to 48 m below the ice surface during the summer which is released or refrozen during winter this is a significant hydrological component newly observed in the ablation zone of greenland that could delay the delivery of meltwater to the bed changing the ice dynamic response to surface meltwater

persistent englacial drainage features in the greenland ice sheet 1 surface melting on the greenland ice sheet is common up to 1400 m elevation and in extreme melt years even higher water produced on the ice sheet surface collects in lakes and drains over the ice sheet surface via supraglacial streams and through the ice sheet via moulins water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally here we use ice penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred we find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years we find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years our observations indicate that once established these persistent moulins might be capable of establishing well connected meltwater drainage pathways

a model study of the response of dry and wet firn to climate change we study the response of firn to a stepwise surface temperature change using a firn model that includes meltwater hydrology and is driven by an idealized surface climate we find that adjustment of dry firn i e without surface melt to surface warming takes longer than a subsequent cooling to the original colder climate mainly because firn compacts faster at higher firn temperatures in contrast wet firn adjusts faster to a surface warming than to a cooling increased meltwater percolation enhances the downward transport of latent heat whereas there is no such mechanism that can enhance the downward transport of a cooling signal thus wastage of firn after surface warming is faster than its regeneration if the warming were reversed furthermore the response of wet firn to temperature change exhibits a complex relation between accumulation rate and the steady state deep firn temperature for high accumulation rates the deep firn temperature is higher because latent heat release upon refreezing is isolated by winter snow as a result the response of wet firn to a temperature change varies strongly with accumulation rate in general the magnitude and the rate of density change is larger in wet firn than in dry firn

a plot scale study of firn stratigraphy at lomonosovfonna svalbard using ice cores borehole video and gpr surveys in 2012 14 spatial heterogeneity of snow and firn properties on glaciers introduces uncertainty in interpretation of point and profile observations and complicates modelling of meltwater percolation and runof

fast draining lakes on the greenland ice sheet 1 the rapid drainage of supraglacial lakes around the ablation zone of the greenland ice sheet forms an important link between water at the surface and the ice sheet base allowing surface meltwater to reach the bed and hence increase glacial velocity the conduits formed by lake drainages may remain open during the remainder of the melt season providing a pathway for further meltwater to reach the base we investigated the drainage behavior of lakes from all regions of the greenland ice sheet for the period 2005 2009 we mapped the evolution of 2600 lakes from 3704 modis images detecting a mean of 263 drainage events per year of which 61 occurred in the south west region only 1 of lake drainages occurred in the rapidly thinning south east region our results show marked differences between the hydrology of the different regions of the ice sheet with few lake drainages occurring in the regions where the highest dynamic mass loss is occurring in the south west and north east lake drainages are common and could impact glacier dynamics in the south east they are rare and are thus unlikely to do so