rapid circulation of warm subtropical waters in a major glacial fjord in east greenland the recent rapid increase in mass loss from the greenland ice sheet is primarily attributed to an acceleration of outlet glaciers oceanographic data obtained in summer 2008 show that subtropical waters that reside year round in the shelf ocean off greenland continuously enter a large glacial fjord in east greenland and contribute to melting at the glacier terminus the recent rapid increase in mass loss from the greenland ice sheet1 2 is primarily attributed to an acceleration of outlet glaciers3 4 5 one possible cause of this acceleration is increased melting at the ice ocean interface6 7 driven by the synchronous warming8 9 10 of subtropical waters offshore of greenland however because of the lack of observations from greenland s glacial fjords and our limited understanding of their dynamics this hypothesis is largely untested here we present oceanographic data collected in sermilik fjord east greenland by ship in summer 2008 and from moorings our data reveal the presence of subtropical waters throughout the fjord these waters are continuously replenished through a wind driven exchange with the shelf where they are present all year the temperature and renewal of these waters indicate that they currently cause enhanced submarine melting at the glacier terminus key controls on the melting rate are the volume and properties of the subtropical waters on the shelf and the patterns of along shore winds suggesting that the glaciers acceleration has been triggered by a combination of atmospheric and oceanic changes our measurements provide evidence for a rapid advective pathway for the transmission of oceanic variability to the ice sheet margins

subaqueous melting of store glacier west greenland from three dimensional high resolution numerical modeling and ocean observations we present three dimensional high resolution simulations of ice melting at the calving face of store glacier a tidewater glacier in west greenland using the massachusetts institute of technology general circulation model we compare the simulated ice melt with an estimate derived from oceanographic data the simulations show turbulent upwelling and spreading of the freshwater laden plume along the ice face and the vigorous melting of ice at rates of meters per day the simulated august 2010 melt rate of 2 0 0 3 m d is within uncertainties of the melt rate of 3 0 1 0 m d calculated from oceanographic data melting is greatest at depth above the subglacial channels causing glacier undercutting melt rates increase proportionally to thermal forcing raised to the power of 1 2 1 6 and to subglacial water flux raised to the power of 0 5 0 9 therefore in a warmer climate store glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge key points simulated melt agrees with estimates from a hydrographic survey of the glacier with more runoff glaciers will melt faster at constant ocean temperature 2013 american geophysical union all rights reserved

ice flux divergence anomalies on 79north glacier greenland 1 the ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness here we combine high resolution ice surface velocity observations of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in north greenland with a dense grid of ice thickness data collected with an airborne radar sounder in 1998 to examine its ice flux divergence we detect large variations up to 100 m yr in flux divergence on grounded ice that are incompatible with what we know of the glacier surface mass balance basal mass balance and thinning rate we examine the hypothesis that these anomalies are due to the three dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three dimensional numerical models with its surface equivalent we find that three dimensional effects have only a small contribution to the observed anomalies on the other hand if we degrade the spatial resolution of the data to 10 km the anomalies disappear further analysis shows that the source of the anomalies is not the ice velocity data but the interpolation of multiple tracks of ice thickness data onto a regular grid using a scheme here block kriging that does not conserve mass or ice flux this problem is not unique to 79north glacier but is common to all conventional ice thickness surveys of glaciers and ice sheets and fundamentally limits the application of ice thickness grids to high resolution numerical modeling of glacier flow

continuity of ice sheet mass loss in greenland and antarctica from the grace and grace follow on missions abstract we examine data continuity between the grace and grace fo missions over greenland and antarctica using independent data from the mass budget method mbm which calculates the difference between ice sheet surface mass balance and ice discharge at the periphery for both ice sheets we find consistent grace grace fo time series across the data gap at the continental and regional scales and the data gap is confidently filled with mbm data in greenland the grace fo data reveal an exceptional summer loss of 600 gigatonnes in 2019 following two cold summers in antarctica ongoing high mass losses in the amundsen sea embayment of west antarctica the antarctic peninsula and wilkes land in east antarctica cumulate to 2130 560 and 370 gigatonnes respectively since 2002 a cumulative mass gain of 980 gigatonnes in queen maud land since 2009 however led to a pause in the acceleration in mass loss from antarctica after 2016

asynchronous behavior of outlet glaciers feeding godthabsfjord nuup kangerlua and the triggering of narsap sermia s retreat in sw greenland we thank w dryer and d podrasky for assistance with fieldwork and l kenefic for assisting with digitizing glacier front positions ch2 m hill polar services and air greenland provided logistics support the spot 5 images used for the 2008 dem were provided by the spirit program centre national d etudes spatiales france the digitalglobe worldview images used for the 2014 dem were obtained from p morin terminus positions were derived from landsat images courtesy of the u s geological survey funding was provided by the us national science foundation nsf office of polar programs opp grants nsf plr 0909552 and nsf plr 0909333 cassotto is supported by nasa under the earth and space science fellowship program grant nnx14al29h k k kjeldsen acknowledges support from the danish council research for independent research grant no dff 409000151 k kjaer is thanked for his support during the earlier phases of this study on ice weather stations are operated by geus denmark within the programme for monitoring of the greenland ice sheet promice j mortensen acknowledges support from iiknn greenland defrost project of the nordic centre of excellence program interaction between climate change and the cryosphere and the greenland ecosystem monitoring programme www g e m dk s rysgaard was funded by the canada excellence research chair programme additional funding was provided by the geophysical institute university of alaska fairbanks and greenland climate research centre scientific editor h fricker and reviewers h jiskoot and g cogley provided very constructive feedback that helped improve the paper

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

distributed subglacial discharge drives significant submarine melt at a greenland tidewater glacier this is the publisher s final pdf the article is copyrighted by american geophysical union and published by john wiley sons inc it can be found at http agupubs onlinelibrary wiley com agu journal 10 1002 28issn 291944 8007

constraints on global mean sea level during pliocene warmth reconstructing the evolution of sea level during past warmer epochs such as the pliocene provides insight into the response of sea level and ice sheets to prolonged warming1 although estimates of the global mean sea level gmsl during this time do exist they vary by several tens of metres2 4 hindering the assessment of past and future ice sheet stability here we show that during the mid piacenzian warm period which was on average two to three degrees celsius warmer than the pre industrial period5 the gmsl was about 16 2 metres higher than today owing to global ice volume changes and around 17 4 metres when thermal expansion of the oceans is included during the even warmer pliocene climatic optimum about four degrees celsius warmer than pre industrial levels 6 our results show that the gmsl was 23 5 metres above the present level with an additional 1 6 metres from thermal expansion we provide six gmsl data points ranging from 4 39 to 3 27 million years ago that are based on phreatic overgrowths on speleothems from the western mediterranean mallorca spain this record is unique owing to its clear relationship to sea level its reliable u pb ages and its long timespan which allows us to quantify uncertainties on potential uplift our data indicate that ice sheets are very sensitive to warming and provide important calibration targets for future ice sheet models7 using phreatic overgrowths on speleothems sea level during the mid piacenzian warm period which was about two to three degrees celsius warmer than the pre industrial period is shown to have been about 16 metres higher than today

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

fast retreat of zachariae isstrom northeast greenland after 8 years of decay of its ice shelf zachariae isstrom a major glacier of northeast greenland that holds a 0 5 meter sea level rise equivalent entered a phase of accelerated retreat in fall 2012 the acceleration rate of its ice velocity tripled melting of its residual ice shelf and thinning of its grounded portion doubled and calving is now occurring at its grounding line warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward sloping marine based bed its equal ice volume neighbor nioghalvfjerdsfjorden is also melting rapidly but retreating slowly along an upward sloping bed the destabilization of this marine based sector will increase sea level rise from the greenland ice sheet for decades to come

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

characteristics of ocean waters reaching greenland s glaciers interaction of greenland s marine terminating glaciers with the ocean has emerged as a key term in the ice sheet mass balance and a plausible trigger for their recent acceleration our knowledge of the dynamics however is limited by scarcity of ocean measurements at the glacier ocean boundary here data collected near six marine terminating glaciers 79 north kangerdlugssuaq helheim and petermann glaciers jakobshavn isbrae and the combined sermeq kujatdleq and akangnardleq are compared to investigate the water masses and the circulation at the ice ocean boundary polar water of arctic origin and atlantic water from the subtropical north atlantic are found near all the glaciers property analysis indicates melting by atlantic water aw found at the grounding line depth near all the glaciers and the influence of subglacial discharge at depth in summer aw temperatures near the glaciers range from 4 58c in the southeast to 0 168c in northwest greenland consistent with the distance from the subtropical north atlantic and cooling across the continental shelf a review of its offshore variability suggests that aw temperature changes in the fjords will be largest in southern and smallest in northwest greenland consistent with the regional distribution of the recent glacier acceleration

modeling of store gletscher s calving dynamics west greenland in response to ocean thermal forcing glacier front dynamics is an important control on greenland s ice mass balance warmer ocean waters trigger ice front retreats of marine terminating glaciers and the corresponding loss in resistive stress leads to glacier acceleration and thinning here we present an approach to quantify the sensitivity and vulnerability of marine terminating glaciers to ocean induced melt we develop a plan view model of store gletscher that includes a level set based moving boundary capability a parameterized ocean induced melt and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt we find that the glacier is stabilized by a sill at its terminus the glacier is dislodged from the sill when ocean induced melt quadruples at which point the glacier retreats irreversibly for 27 km into a reverse bed the model suggests that ice ocean interactions are the triggering mechanism of glacier retreat but the bed controls its magnitude

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

deep glacial troughs and stabilizing ridges unveiled beneath the margins of the antarctic ice sheet the antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers conditioned by ocean temperature and bed topography glaciers retreating along retrograde slopes that is the bed elevation drops in the inland direction are potentially unstable while subglacial ridges slow down the glacial retreat despite major advances in the mapping of subglacial bed topography significant sectors of antarctica remain poorly resolved and critical spatial details are missing here we present a novel high resolution and physically based description of antarctic bed topography using mass conservation our results reveal previously unknown basal features with major implications for glacier response to climate change for example glaciers flowing across the transantarctic mountains are protected by broad stabilizing ridges conversely in the marine basin of wilkes land east antarctica we find retrograde slopes along ninnis and denman glaciers with stabilizing slopes beneath moscow university totten and lambert glacier system despite corrections in bed elevation of up to 1 km for the latter this transformative description of bed topography redefines the high and lower risk sectors for rapid sea level rise from antarctica it will also significantly impact model projections of sea level rise from antarctica in the coming centuries a high resolution update of antarctic bed topography using mass conservation reveals broad stabilizing ridges for glaciers flowing across the transantarctic mountains and stabilizing slopes beneath moscow university totten and lambert glacier system

holocene mean annual air temperature maat reconstruction based on branched glycerol dialkyl glycerol tetraethers from lake ximenglongtan southwestern china abstract here we investigate the sources of branched glycerol dialkyl glycerol tetraethers brgdgts in lake ximenglongtan from southwestern china and present a brgdgts based holocene 9 4 cal kyr bp temperature reconstruction the similarities in the distributions of brgdgts between surface sediments and catchment soils indicate that soil is the dominant source of brgdgts this contrasts with the recent emphasis on lacustrine gdgts in other regional studies holocene temperature evolution is characterized by an early cool phase with a mean annual air temperature maat of 12 5 c prior to 7 6 cal kyr bp followed by a rapid warming towards the local thermal maximum maat 13 8 c from 7 6 to 5 5 cal kyr bp and a subsequent long term cooling that ended at 1 5 cal kyr bp temperature changes after 1 5 cal kyr bp show high variability and low correspondence to global climate events such as the medieval warm period overall holocene temperature variation has been primarily controlled by boreal summer insolation changes the larger amplitude 1 5 4 0 c in annual cooling during the early holocene in maat contrasts with summer temperature records from this region range of cooling 0 3 1 0 c and indicates the likely influence of lower winter temperature on maat variation we suggest that substantial remnants of the northern hemisphere ice sheets during the early holocene enhanced the winter westerlies and east asian winter monsoon eawm and decreased the winter temperature in southwestern china leading to a cold early holocene climate

greenland ice sheet retreat history in the northeast baffin bay based on high resolution bathymetry new swath bathymetric data acquired in 2010 and 2015 indicate a variety of glacial landforms in cross shelf troughs of the melville bay northeast baffin bay these landforms reveal that at their maximum extent ice streams in the troughs crossed the shelf all the way to the shelf edge moraines grounding zone wedges gzws and subglacial till lobes on the continental shelf define a pattern of variable ice stream retreat in the individual troughs on the outer shelf in the northern cross shelf trough ice stream retreat was slow compared to more episodic retreat in the central at least one stabilization on the outer shelf and southern cross shelf trough re advances at the shelf edge and fast retreat thereafter large gzws on the mid to inner shelf of the troughs indicate periods of grounding zone stabilization according to glacial landforms the final retreat across the inner shelf before 8 41 ka bp was episodic to slow furthermore evidence has been found for localized ice domes with minor ice streams on inter trough banks the glacial landforms in melville bay thus indicate the varying and discontinuous ice sheet retreat history across the northwest greenland continental shelf

bedmachine v3 complete bed topography and ocean bathymetry mapping of greenland from multibeam echo sounding combined with mass conservation greenland s bed topography is a primary control on ice flow grounding line migration calving dynamics and subglacial drainage moreover fjord bathymetry regulates the penetration of warm atlantic water aw that rapidly melts and undercuts greenland s marine terminating glaciers here we present a new compilation of greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach a new 150 m horizontal resolution bed topography bathymetric map of greenland is constructed with seamless transitions at the ice ocean interface yielding major improvements over previous data sets particularly in the marine terminating sectors of northwest and southeast greenland our map reveals that the total sea level potential of the greenland ice sheet is 7 42 0 05 m which is 7 cm greater than previous estimates furthermore it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which aw can access glaciers with marine based basins thereby highlighting sectors of greenland that are most vulnerable to future oceanic forcing

fjord insertion into continental margins driven by topographic steering of ice fjords line mountainous continental margins where icesheets and glaciers once stood a two dimensional model simulation suggests that fjords can be eroded within one million years primarily in response to topographic ice steering and erosion from ice discharge subsequent glaciers that form on these landscapes are smaller and exhibit greater responses to climate change

sediment supply on the west greenland passive margin redirection of a large pre glacial drainage system the mesozoic cenozoic separation of greenland and north america produced the small oceanic basins of the labrador sea and baffin bay connected via a complex transform system through the davis strait during rifting and partial breakup sedimentary basins formed that record the changing regional sediment supply the onshore and offshore stratigraphy of central west greenland outlines the presence of a major fluvial system that existed during the cretaceous and was later redirected in the early cenozoic by the formation of the west greenland igneous province hydrological analysis of greenland9s isostatically balanced basement topography outlines two major drainage systems that likely flowed across greenland prior to the onset of glaciation and emptied into the sisimiut basin within the davis strait offshore west greenland the course of the northern drainage system suggests that it initially flowed nw into the cretaceous palaeocene nuussuaq basin before being redirected sw around the west greenland igneous province in the mid palaeocene moreover characteristics of these two drainage systems suggest they acted as a single larger fluvial system prior to the onset of glaciation that was likely the primary source of sediment across central west greenland throughout the cretaceous and palaeogene this scenario provides a greater understanding of the west greenland margin9s late cenozoic evolution which differs from previous interpretations that hypothesize a period of considerable post rift tectonism and uplift this work highlights the importance of large pre glacial drainage systems across north atlantic passive margins and their relevance when studying post rift stratigraphy in rifted margin settings supplementary material isostatic modelling hydrological analysis and chi mapping is available at https doi org 10 6084 m9 figshare c 5050146

hagen brae a surging glacier in north greenland 35 years of observations we use remotely sensed ice velocities in combination with observations of surface elevation and glacier area change to investigate the dynamics of hagen brae north greenland in high detail over the last 35 years from our data we can establish for the first time that hagen brae is a surge type glacier with characteristics of both alaskan and svalbard type surging glaciers we argue that the observed surge was preconditioned by the glacier geometry and triggered by englacially stored meltwater at present the glacier is in a transitional state between active and quiescence phases and is not building up to its pre surge geometry we suggest that the glacier is adjusting to the loss of its floating section general thinning and changes in fjord conditions that occurred over the study period which are unrelated to the surge behavior the high temporal resolution of the ice velocity data gives insight to the sub annual glacier flow

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

oceanic forcing of penultimate deglacial and last interglacial sea level rise sea level histories during the two most recent deglacial interglacial intervals show substantial differences1 3 despite both periods undergoing similar changes in global mean temperature4 5 and forcing from greenhouse gases6 although the last interglaciation lig experienced stronger boreal summer insolation forcing than the present interglaciation7 understanding why lig global mean sea level may have been six to nine metres higher than today has proven particularly challenging2 extensive areas of polar ice sheets were grounded below sea level during both glacial and interglacial periods with grounding lines and fringing ice shelves extending onto continental shelves8 this suggests that oceanic forcing by subsurface warming may also have contributed to ice sheet loss9 12 analogous to ongoing changes in the antarctic13 14 and greenland15 ice sheets such forcing would have been especially effective during glacial periods when the atlantic meridional overturning circulation amoc experienced large variations on millennial timescales16 with a reduction of the amoc causing subsurface warming throughout much of the atlantic basin9 12 17 here we show that greater subsurface warming induced by the longer period of reduced amoc during the penultimate deglaciation can explain the more rapid sea level rise compared with the last deglaciation this greater forcing also contributed to excess loss from the greenland and antarctic ice sheets during the lig causing global mean sea level to rise at least four metres above modern levels when accounting for the combined influences of penultimate and lig deglaciation on glacial isostatic adjustment this excess loss of polar ice during the lig can explain much of the relative sea level recorded by fossil coral reefs and speleothems at intermediate and far field sites a reduction in the strength of the atlantic meridional overturning circulation initiated during the penultimate deglaciation led to excess polar ice losses contributing to higher sea levels during the last interglacial period

characteristics and dynamics of two major greenland glacial fjords the circulation regimes of two major outlet glacial fjords in southeastern greenland sermilik fjord sf and kangerdlugssuaq fjord kf are investigated using data collected in summer 2009 the two fjords show similar flow patterns with a time dependent vertically sheared flow structure dominating over the background estuarine flow driven by buoyancy input we show that this time dependent flow is consistent with circulation induced by density interface fluctuations at the fjord mouth often referred to as intermediary circulation one difference between the fjords is that the hydrographic and velocity structure below a surface modified layer is found to be three layer in kf in summer compared to two layer in sf outside each fjord large scale geostrophic currents dictate the stratification at the mouth although the way in which these large scale flows impinge on each fjord is distinct combining the observations with estimates from existing theories we find the magnitudes of the estuarine qe and intermediary qi circulation and show that qi qe although along fjord winds can also be significant we expect that the critical parameter determining qi qe is the sill depth compared to the fjord depth with shallower sills corresponding to weaker intermediary circulation finally we discuss the implications of strong intermediary circulation on calculating heat transport to the glacier face and its potential feedbacks on the background circulation in these highly stratified estuaries

interruption of two decades of jakobshavn isbrae acceleration and thinning as regional ocean cools jakobshavn isbrae has been the single largest source of mass loss from the greenland ice sheet over the last 20 years during that time it has been retreating accelerating and thinning here we use airborne altimetry and satellite imagery to show that since 2016 jakobshavn has been re advancing slowing and thickening we link these changes to concurrent cooling of ocean waters in disko bay that spill over into ilulissat icefjord ocean temperatures in the bay s upper 250 m have cooled to levels not seen since the mid 1980s observations and modelling trace the origins of this cooling to anomalous wintertime heat loss in the boundary current that circulates around the southern half of greenland longer time series of ocean temperature subglacial discharge and glacier variability strongly suggest that ocean induced melting at the front has continued to influence glacier dynamics after the disintegration of its floating tongue in 2003 we conclude that projections of jakobshavn s future contribution to sea level rise that are based on glacier geometry are insufficient and that accounting for external forcing is indispensable

spatio temporal variations in seasonal ice tongue submarine melt rate at a tidewater glacier in southwest greenland submarine melting of tidewater glaciers is proposed as a trigger for their recent thinning acceleration and retreat we estimate spring submarine melt rates smrs of kangiata nunaata sermia in southwest greenland from 2012 to 2014 by examining changes in along fjord freeboard and velocity of the seasonal floating ice tongue estimated smrs vary spatially and temporally near the grounding line with mean rates of 1 3 0 6 0 8 0 3 and 1 0 0 4 m d 1 across the tongue in 2012 2013 and 2014 respectively higher melt rates correspond with locations of emerging subglacial plumes and terminus calving activity observed during the melt season using time lapse camera imagery modelling of subglacial flow paths suggests a dynamic system capable of rapid re routing of subglacial discharge both within and between melt seasons our results provide an empirically derived link between the presence of subglacial discharge plumes and areas of high spring submarine melting and calving along glacier termini

resolving the internal and basal geometry of ice masses using imaging phase sensitive radar the phase sensitive radio echo sounder pres is a powerful new instrument that can measure the depth of internal layers and the glacier bed to millimetre accuracy we use a stationary 16 antenna pres array on store glacier in west greenland to measure the three dimensional orientation of dipping internal reflectors extending the capabilities of pres beyond conventional depth sounding this novel technique portrays the effectiveness of pres in deriving the orientation of dipping internal layers that may complement profiles obtained through other geophysical surveying methods deriving ice vertical strain rates from changes in layer depth as measured by a sequence of pres observations assumes that the internal reflections come from vertically beneath the antenna by revealing the orientation of internal reflectors and the potential deviation from nadir of their associated reflections the use of an antenna array can correct this assumption while the array configuration was able to resolve the geometry of englacial layers the same configuration could not be used to accurately image the glacier bed here we use simulations of the performance of different array geometries to identify configurations that can be tailored to study different types of basal geometry for future deployments

impact of bedrock description on modeling ice sheet dynamics recent glaciological surveys have revealed a significant increase of ice discharge from polar ice caps into the ocean in parallel ice flow models have been greatly improved to better reproduce current changes and forecast the future behavior of ice sheets for these models surface topography and bedrock elevation are crucial input parameters that largely control the dynamics and the ensuing overall mass balance of the ice sheet for obvious reasons of inaccessibility only sparse and uneven bedrock elevation data is available this raw data is processed to produce digital elevation models dems on a regular 5 km grid these dems are used to constrain the basal boundary conditions of all ice sheet models here by using a full stokes finite element code we examine the sensitivity of an ice flow model to the accuracy of the bedrock description in the context of short term ice sheet forecast we show that in coastal regions the bedrock elevation should be known at a resolution of the order of one kilometer conversely a crude description of the bedrock in the interior of the continent does not affect modeling of the ice outflow into the ocean these findings clearly indicate that coastal regions should be prioritized during future geophysical surveys they also indicate that a paradigm shift is required to change the current design of dems describing the bedrock below the ice sheets they must give users the opportunity to incorporate high resolution bedrock elevation data in regions of interest

multi phased deglaciation of south and southeast greenland controlled by climate and topographic setting abstract to put recent greenland ice sheet gris ice loss into a longer term context we must understand its behavior during late glacial and early holocene warming previous results seem to suggest that there is a large contrast in the timing of deglaciation between south and southeast greenland however because of lack of available data in particular in southeast greenland it is difficult to assess how the ice sheet responded to major late glacial and early holocene climate changes in this study we use 41 new 10be ages to constrain the deglaciation chronology in 12 new locations from the coast to the present ice margin in south and southeast greenland we find that south greenland south of 61 5 n deglaciated between 14 8 and 11 9 ka whereas southeast greenland 61 5 n to 68 2 n deglaciated between 11 4 and 11 3 ka the deglaciation of the coastal low intermediate topography in south greenland coincides with increased air surface temperatures during the bolling allerod with fjords continuing to deglaciate into the early holocene in contrast the ice sheet persisted at the coast until the late glacial and early holocene in southeast greenland likely because of increased precipitation in the high alpine topography and fjord geometry and bathymetry e g width of fjords and presence of sills this multi phased deglaciation demonstrates a contrasting response of the southern gris to changes in climate and variations in topographic setting and that the spatial deglaciation of the gris was complex and likely did not respond to a single external climate forcing

glacial to postglacial submarine landform assemblages in fiords of northeastern baffin island abstract fiord morphology plays a fundamental role in glacier flow dynamics and on ice margin stability as most of the present day margins of the greenland ice sheet lays in fiords there is a need for understanding short and long term glacial dynamics in fiord settings we investigate ice sheet retreat patterns in previously glaciated submarine terrain of northeastern baffin island fiords to provide analogues for modern and future ice sheet response to climate change and sea level rise geomorphological maps constructed from the interpretation of swath bathymetry imagery in fiords of northeastern baffin island reveal a wide range of glacial to postglacial landforms that allow the reconstruction of past ice sheet retreat dynamics ice flow landforms such as mega scale glacial lineations crag and tails and meltwater channels reveal the direction and behaviour of late foxe ice flow through the fiords the presence of undisturbed elongated landforms within the fiords suggests that ice streams have probably been active until the late stage of deglaciation landforms transverse to ice flow direction include grounding zone wedges frontal moraines grounding line fans recessional moraines and de geer moraines these landforms are interpreted as the result of former standstills of the ice margin during deglaciation the occurrence of grounding zones in deep 800 m part of the fiords contrasts with studies suggesting instability and rapid retreat of outlet glaciers over deep fiord basins sediment filled basins often characterised by the presence of turbidity channels gullies and mass movement scars occur in between the moraines sediment filled basins with a ponded architecture between sills illustrate that most of the sediment accumulation was ice proximal during deglaciation and characterised by gravity driven flows the proposed landform assemblage model for northeastern baffin fiords includes landforms typical of different fiord landsystems

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

subannual and seasonal variability of atlantic origin waters in two adjacent west greenland fjords greenland fjords provide a pathway for the inflow of warm shelf waters to glacier termini and outflow of glacially modified waters to the coastal ocean characterizing the dominant modes of variability in fjord circulation and how they vary over subannual and seasonal time scales is critical for predicting ocean heat transport to the ice here we present a 2 year hydrographic record from a suite of moorings in davis strait and two neighboring west greenland fjords that exhibit contrasting fjord and glacier geometry kangerdlugssuaq sermerssua and rink isbrae hydrographic variability above the sill exhibits clear seasonality with a progressive cooling of near surface waters and shoaling of deep isotherms above the sill during winter to spring renewal of below sill waters coincides with the arrival of dense waters at the fjord mouth warm salty atlantic origin water cascades into fjord basins from winter to midsummer we then use seaglider observations at davis strait along with reanalysis of sea ice and wind stress in baffin bay to explore the role of the west greenland current and local air sea forcing in driving fjord renewal these results demonstrate the importance of both remote and local processes in driving renewal of near terminus waters highlighting the need for sustained observations and improved ocean models that resolve the complete slope trough fjord ice system

the east greenland coastal current structure variability and forcing abstract the subtidal circulation of the southeast greenland shelf is described using a set of high resolution hydrographic and velocity transects occupied in summer 2004 the main feature is the east greenland coastal current egcc a low salinity high velocity jet with a wedge shaped hydrographic structure characteristic of other surface buoyancy driven currents the egcc was observed along the entire greenland shelf south of denmark strait while the transect north of the strait showed only a weak shelf flow this observation in conjunction with water mass considerations and other supporting evidence suggests that the egcc is an inner branch of the east greenland current egc that forms south of denmark strait it is argued that bathymetric steering is the most likely reason why the egc apparently bifurcates at this location repeat sections occupied at cape farewell between 1997 and 2004 show that the alongshelf wind stress can have an influence on the structure and strength of the egcc and egc on timescales of 2 3 days accounting for the wind induced effects the volume transport of the combined egcc egc system is roughly constant 2 sv over the study domain from 68 n to cape farewell near 60 n the corresponding freshwater transport increases by roughly 60 over this distance 59 96 msv referenced to a salinity of 34 8 this trend is consistent with a simple freshwater budget of the egcc egc system that accounts for meltwater runoff melting sea ice and icebergs and net precipitation minus evaporation

a mass conservation approach for mapping glacier ice thickness the traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging while this approach provides continuous maps of ice thickness it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations here we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations we apply this approach to the case of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in northeast greenland that has been relatively stable in recent decades the results show that our mass conserving method removes the anomalies in mass flux divergence yields interpolated data that are within about 5 of the original data and produces thickness maps that are directly usable in high spatial resolution high order ice flow models we discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness copyright 2011 by the american geophysical union

an extensive and dynamic ice sheet on the west greenland shelf during the last glacial cycle considerable uncertainty surrounds the extent and timing of the advance and retreat of the greenland ice sheet gis on the continental shelf bordering baffin bay during the last glacial cycle here we use marine geophysical and geological data to show that fast flowing ice sheet outlets including the ancestral jakobshavn isbrae expanded several hundred kilometers to the shelf edge during the last glaciation ca 20 ka retreat of these outlets was asynchronous initial retreat from the shelf edge was underway by 14 880 calibrated cal yr b p in uummannaq trough radiocarbon dates from the adjacent disko trough and adjoining trough mouth fan imply later deglaciation of jakobshavn isbrae and significantly an extensive readvance and rapid retreat of this outlet during the younger dryas stadial yd this is notable because it is the first evidence of a major advance of the gis during the yd on the west greenland shelf although the short duration suggests that it may have been out of phase with yd temperatures

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

increase in radon emission due to rock failure an experimental study 1 radon anomalies are commonly observed prior to dynamic failure in the crust and are interpreted as cracking of the medium thus attracting considerable attention in understanding the precursory phenomena of earthquakes and volcanic activity in this study we have compared the starting radon emissions from low porosity crystalline lava phonolite samples with those from damaged and failed samples the damaged sample was loaded up to just beyond the end of the linear elastic phase as evidenced by the output of ae energy the increase in total porosity and a decrease in p wave and s wave velocity relative to the intact sample whereas the failed sample showed deformation behaviour characteristically brittle with increasing values of ae output and porosity as the sample approached macroscopic failure radon measurements have evidenced that dilatational microcracking of deformed sample produced no significant variation in radon emanation with respect to the intact sample in contrast after macroscopic failure radon emanation drastically increased therefore major finding from this study is that in the case of low porosity and relatively high strength crystalline lavas the development of a macroscopic fracture provides new large exhaling surface resulting in a substantial increase in radon emission rate

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

ice front change of marine terminating outlet glaciers in northwest and southeast greenland during the 21st century the increasingly negative mass balance of the greenland ice sheet gris over the last 25 years has been associated with enhanced surface melt and increased ice loss from marine terminating outlet glaciers accelerated retreat during 2000 2010 was concentrated in the southeast and northwest sectors of the ice sheet however there was considerable spatial and temporal variability in the timing and magnitude of retreat both within and between these regions this behaviour has yet to be quantified and compared for all glaciers in both regions furthermore it is unclear whether retreat has continued after 2010 in the northwest and whether the documented slowdown in the southeast post 2005 has been sustained here we compare spatial and temporal patterns of frontal change in the northwest and southeast gris for the period 2000 2015 our results show near ubiquitous retreat of outlet glaciers across both regions for the study period however the timing and magnitude of inter annual frontal position change is largely asynchronous we also find that since 2010 there is continued terminus retreat in the northwest which contrasts with considerable inter annual variability in the southeast analysis of the role of glacier specific factors demonstrates that fjord and bed geometry are important controls on the timing and magnitude of glacier retreat

topographical evolution and glaciation history of south greenland constrained by paired 26al 10be nuclides abstract the pattern of cosmogenic nuclide inheritance within bedrock surfaces in previously glaciated regions can be used to assess the efficiency of glacial erosion and constrain topographic evolution here we present fifty new in situ cosmogenic 10be and 26al pairs in bedrock and boulder erratics from south greenland the data demonstrate rapid retreat of the south greenland ice sheet in the early holocene and a clear gradient in cosmogenic nuclide inheritance in bedrock with elevation sites 800 m above sea level m a s l generally show limited or no nuclide inheritance exposure from before the last glaciation in contrast the nuclide inheritance in samples from sites at higher elevations varies from negligible to 90 kyr of exposure before the last glaciation depending on the topographic setting our results suggest that steering of ice into troughs led to substantial erosion 2 6 m in the troughs over the last glacial cycle even at elevations above 1500 m a s l while bedrock on summit flats at similar elevations experienced much less erosion inverse markov chain monte carlo mcmc modelling of the nuclide inventories further indicates that selective linear erosion developed 1 myr ago at the summit flat closest to the present ice margin and effectively limited the subsequent erosion to 2 6 m erosion of the redekammen ridge some 40 km from the present ice margin was slightly more efficient over the last 1 myr indicating that selective linear erosion is less pronounced or developed later here than at the summit flats long term ice cover histories are generally less well constrained than the erosion histories the results suggest that the summit flats were possibly ice covered during large parts of the last 1 myr and experienced minimal erosion during this interval but it is also possible that they were only covered by ice during the coldest parts of the glacial periods and that a cover of regolith or overlying rock was stripped off during one of the latest major glaciations although inverse modelling cannot distinguish between these scenarios we identify the more likely ice cover scenarios for each site based on the geological setting and surface characteristics

martian global scale co2 exchange from time variable gravity measurements 1 the martian co2 cycle causes variations in the gravity field that can be detected by an orbiting spacecraft the time variable c20 and c30 reported from the radio tracking analysis of mars global surveyor are used to calculate the seasonal changes in the polar cap masses as well as in the global atmospheric pressure the results show a good agreement with the model dependent solutions of general circulation models and co2 thickness observations of the high energy neutron detector on board mars odyssey

a new bathymetry of the northeast greenland continental shelf constraints on glacial and other processes a new digital bathymetric model dbm for the northeast greenland neg continental shelf 74 n 81 n is presented the dbm has a grid cell size of 250 m 250 m and incorporates bathymetric data from 30 multibeam cruises more than 20 single beam cruises and first reflector depths from industrial seismic lines the new dbm substantially improves the bathymetry compared to older models the dbm not only allows a better delineation of previously known seafloor morphology but in addition reveals the presence of previously unmapped morphological features including glacially derived troughs fjords grounding zone wedges and lateral moraines these submarine landforms are used to infer the past extent and ice flow dynamics of the greenland ice sheet during the last full glacial period of the quaternary and subsequent ice retreat across the continental shelf the dbm reveals cross shelf bathymetric troughs that may enable the inflow of warm atlantic water masses across the shelf driving enhanced basal melting of the marine terminating outlet glaciers draining the ice sheet to the coast in northeast greenland knolls sinks and hummocky seafloor on the middle shelf are also suggested to be related to salt diapirism north south orientated elongate depressions are identified that probably relate to ice marginal processes in combination with erosion caused by the east greenland current a single guyot like peak has been discovered and is interpreted to have been produced during a volcanic event approximately 55 ma ago

strain rate and grain size effects in ice on etudie la resistance de compression de la glace polycristalline en fonction de la taille des grains et de la vitesse de deformation on note l apparition d une inversion dans l effet de la taille des grains lorsque le domaine des vitesses de deformations est etendu a 10 7 s 1

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

late quaternary ice flow in a west greenland fjord and cross shelf trough system submarine landforms from rink isbrae to uummannaq shelf and slope sea floor landforms and acoustic stratigraphic records allow interpretation of the past form and flow of a westward draining ice stream of the greenland ice sheet rink isbrae the late pliocene pleistocene glacial package is several hundred metres thick and down laps onto an upper miocene horizon several acoustic facies are mapped from sub bottom profiler records of the 400 km long uummannaq fjord shelf slope system an acoustically stratified facies covers much of the fjord and trough floor interpreted as glacimarine sediment from rain out of fine grained debris in turbid meltwater beneath this facies is a semi transparent deformation till unit which includes buried streamlined landforms landform distribution in the uummannaq system is used to reconstruct past ice extent and flow directions the presence of streamlined landforms mega scale glacial lineations drumlins crag and tails shows that an ice stream advanced through the fjord system to fill uummannaq trough reaching the shelf edge at the last glacial maximum beyond the trough there is a major fan built mainly of glacigenic debris flows turbidity current channels were not observed on uummannaq fan contrasting with well developed channels on disko fan 300 km to the south ice retreat had begun by 14 8 cal ka ago grounding zone wedges gzw in uummannaq trough imply that retreat was episodic punctuated by several still stands ice retreat between gzws may have been relatively rapid there is little sedimentary evidence for still stands in the inner fjords except for a major moraine ridge marking a little ice age maximum position on the shallow banks either side of uummannaq trough iceberg ploughing has reworked any morphological evidence of earlier ice sheet activity

high resolution mapping of offshore and onshore glaciogenic features in metamorphic bedrock terrain melville bay northwestern greenland abstract geomorphological studies of previously glaciated landscapes are important to understand how ice sheets and glaciers respond to rapidly changing climate melville bay in northwestern greenland contains some of the most sensitive but least studied ice sheet sectors in the northern hemisphere where the bathymetric knowledge previously was restricted to a few sparsely distributed single beam echo soundings we present here the results of high resolution geomorphological mapping of the offshore and onshore landscapes in melville bay using multibeam sonar and satellite data at 5 and 10 m resolutions respectively the results show a similar areally scoured bedrock dominated landscape with a glacially modified cnoc and lochan morphology on the inner shelf 150 500 m depth and on the nearby exposed coast this is manifested by the presence of u shaped troughs moutonee type elongated landforms stoss and lee forms and streamlined features the submarine landscape shows features that are characteristic of bedrock in folded faulted and weathered metamorphic terrain and to a lesser extent glacially molded bedforms while coastal landforms exhibit higher relief irregular shaped basins and more subdued fracture valleys although generally similar the onshore and offshore landscapes contain examples of distinctly different landform patterns which are interpreted to reflect a longer exposure to long term deep weathering as well as to more recent periglacial weathering processes on land the spatial variability in the distribution of landforms across the landscape in both study areas is mostly attributed to differences in lithological properties of the bedrock the lack of sediment cover on the inner shelf is likely a result of a capacity for sediment erosion and removal by the west greenland current flowing northward over the area in combination with limited sediment supply from long sea ice cover seasons the distribution and orientation of the landforms in the offshore part indicate ice movement toward the nw and suggests that this area acted as a tributary or onset region for the major paleo ice stream that formed the present day melville bay trough

radar mapping of isunnguata sermia greenland ice thickness estimates using advanced nadir sounding and tomographic radar processing techniques are compared and combined in a study of isunnguata sermia glacier greenland using an ensemble of operation icebridge flight lines spaced at 500m intervals and running approximately along the flow direction we find there is a statistically excellent comparison between subglacial terrains derived from two dimensional tomography and gridded nadir sounding analysis shows that tomographic data better capture short wavelength 1 2 km patterns in basal terrain but interpolated nadir sounding data yield more spatially extensive and continuous coverage across the glacier especially in deep subglacial troughs using derived surface and basal topography maps we find that driving stress and measured and modeled surface velocity comparisons indicate that basal sliding is an important component of the glacier motion but is also only weakly coupled to the detailed bed topography save for the deepest troughs as might be expected for this land terminating relatively slow moving glacier the subglacial and proglacial topography is similar suggesting the erosional processes acting on the modern glacier bed once helped sculpt the now exposed land

spreading of warm ocean waters around greenland as a possible cause for glacier acceleration we examine the pattern of spreading of warm subtropical origin waters around greenland for the years 1992 2009 using a high resolution 4km horizontal grid coupled ocean and sea ice simulation the simulation provided by the estimating the circulation and climate of the ocean phase ii ecco2 project qualitatively reproduces the observed warming of subsurface waters in the subpolar gyre associated with changes of the north atlantic atmospheric state that occurred in the mid 1990s the modeled subsurface ocean temperature warmed by 1 5 c in southeast and southwest greenland during 1994 2005 and subsequently cooled by 0 5 c modeled subsurface ocean temperature increased by 2 2 5 c in central and then northwest greenland during 1997 2005 and stabilized thereafter while it increased after 2005 by xu and others 2012 we show that the oceanic changes have very likely increased the subaqueous melt rates of the glacier fronts and in turn impacted the rates of glacier flow

sermeq model produces a realistic upper bound on calving retreat for 155 greenland outlet glaciers the rate of land ice loss due to iceberg calving is a key source of variability among model projections of 21st century sea level rise it is especially challenging to account for mass loss due to

undercutting of marine terminating glaciers in west greenland marine terminating glaciers control most of greenland s ice discharge into the ocean but little is known about the geometry of their frontal regions here we use side looking multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough deep glacier grounding enables contact with subsurface warm salty atlantic waters aw which melts ice at rates of meters per day we detect cavities undercutting the base of the calving faces at the sites of subglacial water sgw discharge predicted by a hydrological model the observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of sgw transport warm aw to the ice faces glacier undercutting likely enhances iceberg calving impacting ice front stability and in turn the glacier mass balance

high resolution ice thickness mapping in south greenland airborne radar sounding is difficult in south greenland because of the presence of englacial water which prevents the signal from reaching the bed data coverage remains suboptimal for traditional methods of ice thickness and bed mapping that rely on geostatistical techniques such as kriging because important features are missing here we apply two alternative approaches of high resolution 300 m ice thickness mapping that are based on the conservation of mass to two regions of south greenland 1 qooqqup sermia and kiattuut sermiat and 2 ikertivaq these two algorithms solve optimization problems for which the conservation of mass is either enforced as a hard constraint or as a soft constraint for the first region very few measurements are available but there is no gap in ice motion data whereas for ikertivaq more ice thickness measurements are available but there are gaps in ice motion data we show that mass conservation algorithms can be used as validation tools for radar sounding we also show that it is preferable to apply mass conservation as a hard constraint rather than a soft constraint as it better preserves elongated features such as glacial valleys and ridges

heat sources for glacial ice melt in a west greenland tidewater outlet glacier fjord the role of subglacial freshwater discharge the melting of tidewater outlet glaciers from the greenland ice sheet contributes significantly to global sea level rise accelerated mass loss is related to melt processes in front of calving glaciers yet the role of ocean heat transports is poorly understood here we present the first direct measurements from a subglacial plume in front of a calving tidewater outlet glacier surface salinity in the plume corresponded to a meltwater content of 7 which is indicative of significant entrainment of warm bottom water and according to plume model calculations significant ice melt energy balance of the area near the glacier showed that ice melt was mainly due to ocean heat transport and that direct plume associated melt was only important in periods with high meltwater discharge rates of 100 m3 s 1 ocean mixing outside of the plume area was thus the primary heat source for melting glacier ice

echo source discrimination in single pass airborne radar sounding data from the dry valleys antarctica implications for orbital sounding of mars the interpretation of radar sounding data from mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface this can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes we have developed general techniques for this using airborne radar data from the dry valleys of antarctica these data were collected in a single pass including taylor glacier ice covered lake bonney and an ice free area of taylor valley the pulsed radar 52 5 67 5 mhz was coherently recorded our echo discrimination techniques included a radar simulator using a digital elevation model dem to predict the location and shape of surface echoes in the radar data real and simulated echo strengths were used to calculate a signal to clutter ratio this was complemented by the cross track migration of radar echoes onto the surface these migrated echoes were superimposed on the dem and imagery in order to correlate with surface features using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes including a continuous reflector under the main trunk of taylor glacier and multiple reflectors beneath the terminus of taylor glacier surface based radar confirms the thickness of the glacier at three crossing points the results illustrate the importance of using complementary techniques the usefulness of a dem and the limitations of single pass radar sounding data

bathymetry data reveal glaciers vulnerable to ice ocean interaction in uummannaq and vaigat glacial fjords west greenland marine terminating glaciers play a critical role in controlling greenland s ice sheet mass balance their frontal margins interact vigorously with the ocean but our understanding of this interaction is limited in part by a lack of bathymetry data here we present a multibeam echo sounding survey of 14 glacial fjords in the uummannaq and vaigat fjords west greenland which extends from the continental shelf to the glacier fronts the data reveal valleys with shallow sills overdeepenings 1300 m from glacial erosion and seafloor depths 100 1000 m deeper than in existing charts where fjords are deep enough we detect the pervasive presence of warm salty atlantic water aw 2 5 degrees c with high melt potential but we also find numerous glaciers grounded on shallow 200 m sills standing in cold 1 degrees c waters in otherwise deep fjords i e with reduced melt potential bathymetric observations extending to the glacier fronts are critical to understand the glacier evolution

holocene break up and reestablishment of the petermann ice tongue northwest greenland abstract over the last decade two major calving events of the petermann ice tongue in northwest greenland have led to speculation on its future stability and contribution to further greenland ice sheet mass loss however it has been unclear if these events are anomalous or typical within the context of limited historical observations we extend the historical record of the floating ice tongue using the stratigraphy of petermann fjord sediments to provide a longer term perspective computed tomography ct scans x ray fluorescence xrf scans ice rafted debris ird counts and the magnetic properties of specific particle size fractions constrain changes in depositional processes and sediment sources at our core sites allowing for reconstructions of past behavior of the petermann ice tongue radiocarbon dating of foraminifera 210pb and paleomagnetic secular variation psv provide age control and help to address uncertainties in radiocarbon reservoir ages a floating ice tongue in petermann fjord formed in late glacial time as petermann glacier retreated from an advanced grounded position this paleo ice tongue broke up during the early holocene when high northern latitude summer insolation was higher than present after gradual regrowth of the ice tongue associated with regional cooling the ice tongue reached its historical extent only within the last millennium little or no ice tongue was present for nearly 5000 years during the middle holocene when decadal mean regional temperatures are estimated to be 0 8 2 9 c higher than preindustrial 1750 ce and seasonal sea ice in the lincoln sea was reduced this pre historical behavior shows that recent anthropogenic warming may already be in the range of ice tongue instability and future projected warming increases the risk of ice tongue break up by the mid 21st century

holocene history of the helheim glacier southeast greenland abstract helheim glacier ranks among the fastest flowing and most ice discharging outlets of the greenland ice sheet gris after undergoing rapid speed up in the early 2000s understanding its long term mass balance and dynamic has become increasingly important here we present the first record of direct holocene ice marginal changes of the helheim glacier following the initial deglaciation by analysing cores from lakes adjacent to the present ice margin we pinpoint periods of advance and retreat we target threshold lakes which receive glacial meltwater only when the margin is at an advanced position similar to the present we show that during the period from 10 5 to 9 6 cal ka bp the extent of helheim glacier was similar to that of todays after which it remained retracted for most of the holocene until a re advance caused it to reach its present extent at c 0 3 cal ka bp during the little ice age lia thus helheim glacier s present extent is the largest since the last deglaciation and its holocene history shows that it is capable of recovering after several millennia of warming and retreat furthermore the absence of advances beyond the present day position during for example the 9 3 and 8 2 ka cold events as well as the early neoglacial suggest a substantial retreat during most of the holocene

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

acoustic mapping of the ilulissat ice fjord mouth west greenland a ship based acoustic mapping campaign was conducted at the exit of ilulissat ice fjord of west greenland and in the sedimentary basin of disko bay west of the fjord mouth submarine landscape and sediment distribution patterns represented by five acoustic facies types represent glaciomarine sediment facies types that are related to variations in the past position and relative motion of the glacier front asymmetric ridges on the shelf that form a curved entity and a large sill at the fjord mouth represent moraines that depict at least two relatively stable positions of the ice front in the disko bay and at the fjord mouth comparable ice end features are not observed seaward of the east greenland kangerdlugssuaq glacier although both glaciers are comparable in their latitudinal position present size and present role for the ice discharge from the inland ice sheet apparently the retreat of the greenland inland ice after the last maximum expansion was a more discontinuous process on the west greenland shelf than on the east greenland shelf the iceberg bank a prominent sill at the fjord exit appears to play an important role for the sedimentation after the retreat of the ice front from the shelf was completed the retreat of the glacier behind the iceberg bank into the inner fjord is marked by a reorganization of sediment delivery in disko bay as most of the till is now deposited within the fjord two linear clusters of pockmarks in the center of the sedimentary basin seem to be linked to methane release due to dissociation of gas hydrates a process driven by fast crustal uplift of the greenland shelf the orientation of these clusters appears to reflect a migration path that is defined by a buried structure which we could not resolve

advanced multifrequency radar instrumentation for polar research this paper presents a radar sensor package specifically developed for wide coverage sounding and imaging of polar ice sheets from a variety of aircraft our instruments address the need for a reliable remote sensing solution well suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution the sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance the instruments operate simultaneously over different frequency bands within the 160 mhz 18 ghz range the sensor package has allowed us to sound the most challenging areas of the polar ice sheets ice sheet margins and outlet glaciers to map near surface internal layers with fine resolution and to detect the snow air and snow ice interfaces of snow cover over sea ice to generate estimates of snow thickness in this paper we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance we also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform finally we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features

bathymetry of southeast greenland from oceans melting greenland omg data southeast greenland has been a major participant in the ice sheet mass loss over the last several decades interpreting the evolution of glacier fronts requires information about their depth below

an aerial view of 80 years of climate related glacier fluctuations in southeast greenland both marine and land terminating glaciers in southeast greenland have experienced dramatic recent retreat an 80 year record of historical aerial photographs and satellite imagery shows that many land terminating glaciers in this region retreated more rapidly in the 1930s than today whereas marine terminating glaciers have retreated faster in the 2000s

glacier dynamics near the calving front of bowdoin glacier northwestern greenland to better understand recent rapid recession of marine terminating glaciers in greenland we performed satellite and field observations near the calving front of bowdoin glacier a 3 km wide outlet glacier in northwestern greenland satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for 20 years ice radar measurements showed that the glacier front is grounded but very close to the floating condition these results in combination with the results of ocean depth soundings suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat presumably glacier thinning due to atmospheric and or ocean warming triggered the initial retreat in situ measurements showed complex short term ice speed variations which were correlated with air temperature precipitation and ocean tides ice speed quickly responded to temperature rise and a heavy rain event indicating rapid drainage of surface water to the bed semi diurnal speed peaks coincided with low tides suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance these observations demonstrate that the dynamics of bowdoin glacier are sensitive to small perturbations occurring near the calving front