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

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

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

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

a preliminary investigation of the influence of basal and surface topography on supraglacial lake distribution near jakobshavn isbrae western greenland the spatial distribution of supraglacial lakes has been hypothesized to be dominantly controlled by the component of surface roughness influenced by basal topography basal topography and surface roughness profiles within the jakobshavn isbrae drainage basin in western greenland acquired from an ice penetrating radar echo sounder were analysed through fourier and wavelet decompositions spectral analyses of basal to surface transfer under a range of ice thickness were compared with spatial distribution of lakes mapped from high resolution landsat imagery fourier analysis identifies dominant signals in both the basal and surface profiles to be ranging between wavelengths of 1 25 12 5 km the strongest peaks of transfer of basal signals to the surface were identified at wavelengths 11 5 km wavelet analysis identifies these peaks with thicker 1200 1400 m and thinner 500 700 m ice respectively and also identifies surface frequencies not present in the basal signal indicating some influence from other factors spatial autocorrelation analysis of supraglacial lake distribution indentifies high correlations at 1 9 5 6 11 24 and 30 km lags lags at 5 6 and 11 km correspond to dominant frequencies present in the basal and surface profiles over thinner and thicker ice respectively these 5 and 11 km frequency components present in the basal topography surface roughness and lake distribution are within the theoretical transfer window of basal to surface transfer as a function of ice thickness the transect analysed in this study does not contain wavelengths in the surface structure that are less than the ice thickness even though there are relatively higher frequencies present in the surface profile between 0 and 15 km that have less power in the basal profile copyright 2011 john wiley sons ltd

centennial scale variability of the british ice sheet implications for climate forcing and atlantic meridional overturning circulation during the last deglaciation evidence from paleoclimatic archives suggests that earth s climate experienced rapid temperature changes associated with pronounced interhemispheric asymmetry during the last glacial period explanations for these climate excursions have converged on nonlinear interactions between ice sheets and the ocean s thermohaline circulation but the driving mechanism remains to be identified here we use multidecadal marine records of faunal oxygen isotope and sediment proxies from the northeast atlantic proximal to the western margins of the last glacial british ice sheet bis to document the coupling between ice sheet dynamics ocean circulation and insolation changes the core data reveal successions of short lived 80 100 years high amplitude ice rafted debris ird events that were initiated up to 2000 years before the deposition of detrital carbonate during heinrich events he 1 and 2 progressive disintegration of the bis 19 16 kyr before present b p occurred in response to abrupt ocean climate warmings that impinged on the northeast atlantic during the early deglaciation peak ird deposition recurs at 180 220 year intervals plausibly involving repeated breakup of glacial tidewater margins and fringing marine ice shelves the early deglaciation culminated in a major meltwater pulse at 16 3 kyr b p followed by another discharge associated with he1 some 300 years after we conclude that temperature changes related to external forcing and marine heat transport caused a rapid response of the bis and possibly other margins of the eurasian ice sheet massive but short lived meltwater surges influenced the atlantic meridional overturning circulation thereby contributing to north atlantic climate variability and bipolar climatic asymmetry

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

partitioning effects from ocean and atmosphere on the calving stability of kangerdlugssuaq glacier east greenland we use the 7 km retreat of kangerdlugssuaq glacier kg east greenland to examine the mechanisms interactions and relative significance of atmospheric forcing and ice ocean interactions hydrographic data from 1991 1993 and 2004 show that subtropical waters are common in kangerdlugssuaq fjord kf and that surface waters were warm in 2004 relative to 1991 and 1993 the main water column was nonetheless warmest in 1991 we contend that while flow of subtropical waters into fjords provides a setting in which rapid glacier retreat can occur the triggering of retreat depends on additional environmental factors the climatic variables standing out in our study of kg and kf are air temperature and katabatic winds both had strong positive anomalies during winter 2004 05 when kg retreated we show that proglacial ice melange was absent and that fjord freeze up did not occur until 11 april 2005 due to warm and windy conditions we demonstrate that this setting is unusual and hypothesize that exposure to open water in winter months caused the retreat calculation of ice front melt rates shows that discharge of basal meltwater first from runoff and subsequently from frictional basal heating should intensify the interaction between glacier and fjord

rapid ice discharge from southeast greenland glaciers interferometric synthetic aperture radar insar observations of southeast greenland glaciers acquired by the earth remote sensing satellites ers 1 2 in 1996 were combined with ice sounding radar data collected in the late 1990s to estimate a total discharge of 46 3 km 3 ice per year between 62 n and 66 n which is significantly lower than a mass input of 29 3 km 3 ice per year calculated from a recent compilation of snow accumulation data further north helheim glacier discharges 23 1 km 3 yr vs 30 3 km 3 yr accumulation kangerdlugssuaq glacier discharges 29 2 km 3 yr vs 23 km 3 yr and daugaard jensen glacier discharges 10 5 0 6 km 3 yr vs 10 5 1 km 3 yr the mass balance of east greenland glaciers is therefore dominated by the negative mass balance of southeast greenland glaciers 17 4 km 3 yr equivalent to a sea level rise of 0 04 0 01 mm yr warmer and drier conditions cannot explain the imbalance which we attribute to longterm changes in ice dynamics copyright 2004 by the american geophysical union

force perturbation analysis of recent thinning and acceleration of jakobshavn isbrae greenland observations between 1997 and 2001 of a 30 velocity increase and up to 60 m thinning of downstream parts of jakobshavn isbrae greenland immediately following calving of about 4 km of its 15 km floating ice tongue suggest that acceleration may have been initiated by the calving assuming that the force perturbation associated with such weakening is swiftly transmitted far up glacier i develop equations to estimate the perturbation initially the observed changes are consistent with the comparatively small perturbation associated with the calving thereafter it was probably sustained by thinning of the remaining ice tongue at rates of about 80 m a 1 otherwise the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned with velocities dropping to their former values the calculated force perturbation increases to a maximum about 10 km inland of the grounding line consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards further inland it progressively decreases probably because marginal drag increased as the glacier accelerated both here and on the floating tongue marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and or fabric more recent observations show continued acceleration and thinning and most of the remaining ice tongue calved away in april 2003 so thinning is likely to continue

rapid and synchronous ice dynamic changes in east greenland two major outlet glaciers in east greenland have suddenly begun to accelerate and retreat the speeds of kangerdlugssuaq and helheim remained steady during the 1990s despite progressive and substantial thinning but have abruptly increased within the last two years more than doubling ice flux to the ocean had it been an isolated example the comparable 1998 speed up of jakobshavn isbra in west greenland might have been explained simply by its chance retreat past a pinning point now that two further greenland outlets have exhibited similar behavior a common process seems likely a remarkable correspondence in the inter annual patterns of speed and ice front variation between kangerdlugssuaq and helheim implies a significant sensitivity to regional environmental factors the period of continued warming and thinning appears to have primed these glaciers for a step change in dynamics not included in current models we should expect further greenland outlet glaciers to follow suit copyright 2006 by the american geophysical union

satellite archives reveal abrupt changes in behavior of helheim glacier southeast greenland rapid changes in helheim glacier and other greenland outlet glaciers since 2000 are well known but knowledge on earlier decades is fragmentary here we exploit the satellite image archives to produce and analyze a monthly to seasonal record of helheim glacier front position 1980 2011 statistical analysis identifies decadal periods with abrupt changes in variability and mean the record also reveals evidence of volatile advance retreat behavior in the 1980s in one of several cases of large amplitude subannual changes the glacier front surged forward in 1984 85 advancing 6 km within a few months surpassing its little ice age maximum position and afterward retreated 5 km within a few weeks these findings challenge the prevailing view of front position stability in the decades before the multi year retreat in the early 2000s cold conditions including rigid ice melange appear to be a factor in the high amplitude seasonal advances in the 1980s however the magnitude and abruptness of the changes in the record cannot be explained solely as a climatic response such that glacio dynamics must be invoked further the volatile advance retreat behavior in the cold 1980s resulted in increased dynamic ice loss complicating the interpretation of increased calving activity as a response to warming

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

seasonal variability in warm water inflow toward kangerdlugssuaq fjord seasonal variability in pathways of warm water masses toward the kangerdlugssuaq fjord glacier system kf kg southeast greenland is investigated by backtracking lagrangian particles seeded at the fjord mouth in a high resolution regional ocean model simulation in the ice free and the ice covered seasons the waters at kf are a mixture of atlantic origin water advected from the irminger basin ff for faxafloi the deep waters from the denmark strait and the waters from the arctic ocean both represented by the kogur section ko below 200m depth the warm water is a mixture of ff and ko water masses and is warmer in winter than in summer we find that seasonal differences in pathways double the fraction of ff particles in winter causing the seasonal warming and salinification seasonal temperature variations at the upstream sections ff and ko have a negligible impact on temperature variations near the fjord successful monitoring of heat flux to the fjord therefore needs to take place close to the fjord and cannot be inferred from upstream conditions

jakobshavn glacier west greenland 30 years of spaceborne observations early 1960 s reconnaissance satellite images are compared to more recent image and map data in an inter annual and seasonal study of west central greenland margin fluctuations from 1962 to 1992 ice sheet margins to the north and south of jakobshavn glacier retreated despite a decline in average summer temperatures the retreat may be reversing along the southern flank of the ice stream where regional mass balance estimates are positive from 1950 to fluctuated 2 5 km around its annual mean position the total calving flux during the summer is more than six times that during winter we identified that summer melting and the break up of sea ice and icebergs in the fjord are important in controlling the rate of iceberg production if correct calving rates may be expected to increase should climate become warmer in the near future

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

the treatment of meltwater retention in mass balance parameterisations of the greenland ice sheet retention of melt water runoff by percolation and or refreezing in the snowpack cannot be neglected when studying the surface mass balance of the greenlandice sheet in this paper we make a detailed comparison of several treatments proposed in the literature to account for this process in large scale mass balanceparameterizations the melt on the greenland ice sheet is calculated with a revised degree day model using updated datasets of surface elevation and precipitationrate on a 5 km grid crucial model parameters are recalibrated by comparing mass balance characteristics with available observations on a regional basis wediscuss the role of meltwater retention in the light of the overall mass balance of the greenland ice sheet and its sensitivity to climatic change and displaypatterns of effective retention fractions for the various methods as a main conclusion it appears that overall results are quite similar for the various models butthat meltwater retention has a large spatial variation not described by the simple treatments using the most comprehensive retention model the sensitivity of therunoff is found to be 0 35 mm deg c of sea level change per year we also present a new map of the different zones facies that characterize the accumulationarea of the greenland ice sheet that is useful for interpreting field data and calibrating satellite observations

ice sheet dynamics and ice streaming along the coastal parts of northern norway abstract in this paper regional and high resolution bathymetric data are used to reconstruct the pattern and dynamics of ice sheet flow in the fjord and shelf areas of northern norway interpretation of the morphology of the sea floor reveals a series of glacigenic features which are mainly related to the most recent glaciation of the area the shelf area of troms county comprises four cross shelf troughs with intermediate shallow banks glacial features within the cross shelf troughs mega scale glacial lineations msgl lateral shear margin moraines grounding zone wedges are interpreted to have been formed under fast flowing ice streams on the shallow bank areas there are far fewer ice flow indicators and here relatively passive ice acted as interstream ridges outside the finnmark coast to the north and east a coast parallel depression djuprenna deflected rapidly flowing ice from the fjords in a generally east to west direction and then farther out through ingoydjupet and into the bear island trough the fjords guided most of the ice flow and functioned as tributaries to the major ice streams on the shelf including the vast bear island trough ice stream at the last glacial maximum the landform distribution and their cross cutting relationships indicate that major switches in ice stream flow direction took place during deglaciation additionally grounding zone wedges are found within the cross shelf troughs and these reveal temporal variability in ice stream dynamics in different troughs during deglaciation the distribution of glacigenic features on the ice stream beds also reveals a downstream evolution that has been reported by other workers whereby small bedrock features progressively merge into more elongate forms and msgl this downstream evolution in subglacial geology subglacial bedforms implies that the force balance e g basal shear stress of the ice streams also evolves in the downstream direction and more crucially that the flow mechanism of the ice stream is spatially variable along its length

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

greenland ice sheet increased coastal thinning repeated laser altimeter surveys and modelled snowfall summer melt show average ice loss from greenland between 1997 and 2003 was 80 12 km3 yr 1 compared to about 60 km3 yr 1 for 1993 4 1998 9 half of the increase was from higher summer melting with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation velocities of one large glacier almost doubled between 1997 and 2003 resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003 copyright 2004 by the american geophysical union

where glaciers meet water subaqueous melt and its relevance to glaciers in various settings glacier change is ubiquitous but the fastest and largest magnitude changes occur in glaciers that terminate in water this includes the most rapidly retreating glaciers and also several advancing ones often in similar regional climate settings furthermore water terminating glaciers show a large range in morphology particularly when ice flow into ocean water is compared to that into freshwater lakes all water terminating glaciers share the ability to lose significant volume of ice at the front either through mechanical calving or direct melt from the water in contact here we present a review of the subaqueous melt process we discuss the relevant physics and show how different physical settings can lead to different glacial responses we find that subaqueous melt can be an important trigger for glacier change it can explain many of the morphological differences such as the existence or absence of floating tongues subaqueous melting is influenced by glacial runoff which is largely a function of atmospheric conditions this shows a tight connection between atmosphere oceans and lakes and glaciers subaqueous melt rates even if shown to be large should always be discussed in the context of ice supply to the glacier front to assess its overall relevance we find that melt is often relevant to explain seasonal evolution can be instrumental in shifting a glacier into a different dynamical regime and often forms a large part of a glacier s mass loss on the other hand in some cases melt is a small component of mass loss and does not significantly affect glacier response

grace observes small scale mass loss in greenland using satellite gravity data between february 2003 and january 2008 we examine changes in greenland s mass distribution on a regional scale during this period greenland lost mass at a mean rate of 179 25 gt yr equivalent to a global mean sea level change of 0 5 0 1 mm yr rates increase over time suggesting an acceleration of the mass loss driven by mass loss during summer the largest mass losses occurred along the southeastern and northwestern coast in the summers of 2005 and 2007 when the ice sheet lost 279 gt and 328 gt of ice respectively within 2 months in 2007 a strong mass loss is observed during summer at elevations above 2000 m for the first time since the start of the observations

basin scale patterns of subglacial sediment mobility implications for glaciological inversion modelling abstract patterns of glacial bedforms and their internal sediments are described from the omagh basin north central ireland here drumlins and ribbed moraines were variously formed preserved and remoulded during several late devensian ice flow stages and their patterns can be used as input into glaciological inversion models more detailed examination of the morphology and internal sediments of these bedforms shows a complex pattern of reworking driven by local scale basal thermal and hydrological regime sediment lithofacies associations and patterns of erratic carriage can be used to better evaluate ice bed conditions and basal processes than bedforms alone the sedimentary evidence suggests that bedforms were unlikely to have been formed during a single glacial episode as is implicit in glaciological inversion models and more likely formed during several episodes that cannot be resolved by crosscutting or superimposed bedform relationships alone

potassium argon dating of bed i olduvai gorge 1961 1972 abstract potassium argon dating of bed i in olduvai gorge was done or supervised by g h curtis and j f evernden in the department of geology and geophysics at the university of california at berkeley this work proceeded in three phases the first of which was the dating of a small number of tuff samples by evernden and curtis the dates ranged from 1 57 to 1 89 ma averaging 1 73 ma for tuffs interstratified with hominid bearing deposits this dating was controversial because of the long span indicated for the pleistocene and for the antiquity of early tool making hominids koenigswald et al 1962 obtained a date of 1 3 0 1 ma for a sample of basalt from below the dated tuffs of bed i and argued that the dates from tuffs were in error evernden and curtis then dated the basalts at 4 4 ma which was revised to 1 92 ma supporting the earlier dates on tuffs the stratigraphy of bed i was worked out over the period of dating and some dates that made no sense early in the stratigraphic work fell into place later the second phase of dating was primarily intended to check the earlier date of 1 75 ma for hominid remains and stone artifacts of bed i this work was based on a large number of tuff samples and yielded a wide scatter of ages contamination by older materials was recognized in some samples and inferred in others and criteria were established for accepting and rejecting dates using these criteria 15 dates were considered as valid from the total of 41 dates obtained from tuffs above the basaltic lavas the mean age of the valid dates was 1 74 0 05 ma confirming the antiquity of the early hominids the 15 valid dates were however difficult to interpret in terms of the stratigraphy and further selection was made among the valid dates to estimate a time span of 350 000 years for the interval between the basalts 1 92 0 06 ma and the highest tuff dated 1 57 0 05 ma the third phase involved dating samples from previously undated horizons and re evaluating all work done previously on bed i samples the highest priority for new dating was tuff if at the top of bed i but the results were unsatisfactory apparently because of undetected contamination by older volcanic materials remeasurement of the earlier mass spectograms led to changes in some of the published dates and the re evaluated dates except for those of tuff ib still showed a wide scatter indicating that most of the sampled tuffs were unsuited for accurate k ar dating only tuff ib with a mean age of 1 786 0 029 ma gave results with a precision comparable to laboratory standards the re evaluated bed i dates showed no trend toward younger ages upward suggesting a relatively short time span for bed i recent 40 ar 39 ar dates based on analyses of single crystals mark a great improvement over the k ar dating of bed i dates on all of the marker tuffs fall in the correct stratigraphic order and the age span of bed i is 0 27 ma from 2 02 to 1 75 ma components yielding two or more ages were identified in several tuffs thus accounting for some and perhaps most of the variability in the k ar dates

glacier velocity measurements in the eastern yigong zangbo basin tibet china we apply the feature tracking method to l band synthetic aperture radar sar images to derive detailed motion patterns of glaciers in the yigong zangbo basin during summer 2007 the results indicate that the flow patterns are generally constrained by the valley geometry and terrain complexity the mean velocities of the 12 glaciers were 15 206 m a 1 with a maximum of 423 m a 1 for glacier no 5a the majority of the glaciers exhibited high and low velocities in their upper and lower sections respectively the glacier area ranges from 3 to 42 km 2 it is found that velocity shows a positive correlation with the glacier area and length many small scale temporal spatial variations in the glacier flow patterns were observed along the central glacier flowline

the potential of satellite radar interferometry and feature tracking for monitoring flow rates of himalayan glaciers abstract the stagnation of himalayan glaciers will only be exacerbated by a warming climate increasing hazards associated with glacier lake development frontal recession is a poor indicator of negative balance in this region where most glaciers are debris covered but reduced glacier flow rates provide a valuable warning of stagnation in situ measurements of velocity are understandably rare and remote sensing measurements have hitherto been limited by severe topography and lack of suitable data in this paper we compare two techniques to measure flow rates in everest region glaciers firstly satellite radar interferometry sri has recently become more viable because of improved digital elevation data and precision orbit models which now allow snapshots of velocities to be retrieved from ers imagery in the mid 1990s without ground control information secondly feature tracking between synthetic aperture radar sar images is found to be successful even on the small glaciers of this region because of the patterning associated with debris cover this technique extends through the 1990s and beyond the time frame over which velocities can be measured using the examples of the khumbu glacier flowing south into nepal and the kangshung glacier flowing east into china from the everest massif we compare sri and sar feature tracking in ers images consider the errors in these techniques and examine the potential to monitor flow rates across the region the techniques are found to be highly complementary and appropriate for monitoring flow rates in this region suggesting that routine monitoring of himalayan glaciers is possible the kangshung glacier appears to sustain relatively high flow rates of up to 10 cm day which along with a convex topographic profile indicates a healthy glacier not exhibiting significant negative mass balance in comparison much of the khumbu glacier appears to have stagnated the surface velocity at gorak shep is only 20 of a field based measurement in the 1970 s and the concave surface profile confirms the downwasting associated with a debris covered glacier in recession

ice front and flow speed variations of marine terminating outlet glaciers along the coast of prudhoe land northwestern greenland satellite images were analyzed to measure the frontal positions and ice speeds of 19 marine terminating outlet glaciers along the coast of prudhoe land northwestern greenland from 1987 to 2014 all the studied glaciers retreated over the study period at a rate of between 12 and 200 m a 1 with a median mean retreat rate of 30 40 m a 1 the glacier retreat began in the year 2000 which coincided with an increase in summer mean air temperature from 1 4 to 5 5 c between 1996 and 2000 in this region ice speed near the front of the studied glaciers ranged between 20 and 1740 m a 1 in 2014 and many of them accelerated in the early 2000s in general the faster retreat was observed at the glaciers that experienced greater acceleration as represented by tracy glacier which experienced a retreat of 200 m a 1 and a velocity increase of 930 m a 1 during the study period a possible interpretation of this observation is that flow acceleration induced dynamic thinning near the termini resulting in enhanced calving and rapid retreat of the studied glaciers we hypothesize that atmospheric warming conditions in the late 1990s triggered glacier retreat in northwestern greenland since 2000

a sar record of early 21st century change in greenland glaciers in greenland are changing rapidly to better understand these changes we have produced a series of seven synthetic aperture radar sar backscatter mosaics for seven winters during the period from 2000 to 2013 six of the mosaics were created using radarsat fine beam data and the seventh used alos palsar fine beam single polarization data the radarsat mosaics are radiometrically calibrated and capture changes in the backscatter coefficient related to melt and other events particularly the strong melting in the summer of 2012 comparison of features in the ascending orbit alos mosaic and the descending orbit radarsat mosaics indicate that in areas of smooth to moderate topography their locations are consistent to within a few 10s of meters the locations of features identifiable in the radarat mosaics which were collected with the same imaging parameters generally agree to within better than the 20 m posting of the data with such geometric accuracy these data establish a record of change in greenland for the early part of the 21st century thus providing a baseline that can be compared with new radar and optical data sets

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

subaerial salt extrusions in iran as analogues of ice sheets streams and glaciers abstract ice h20 and salt halite nacl share many physical properties and resemble each other in hand specimens and subaerial gravity driven flows however while most significant bodies of ice accumulate in cold highlands and gravity spread where and soon after they form most significant bodies of salt accumulate in tropical marine basins and have to be buried by 1 km of other rocks before they flow buried salt is driven by differential loading into various categories of piercing structures known as diapirs many diapirs extrude onto the surface as sheets of allochthonous out of place salt thousands of sheets of allochthonous salt have been interpreted in over 35 basins worldwide in the last 25 years mainly in the toes of passive continental margins and in orogenic belts where some are 103 km2 in area most former salt sheets are now submarine or subsurface but several active examples are beautifully exposed in iran these were compared to ice glaciers soon after they were introduced to western science a comparison that has been neglected since here we update this analogy and use modern understanding of flowing ice and salt to examine the similarities and differences that might be mutually beneficial to both fields of study as well as to extraterrestrial scientists the profiles internal structures and fabrics in flowing bodies of ice and salt are sensitive gauges of the histories of their budgets of supply and loss however whereas snow merely compacts where it accumulates salt sheets are fed from below by already deformed salt when salt diapirs first emerge on land they extrude domes that mature to the profiles of viscous fountains that often feed glacier like flows known as namakiers after locally exhausting their deep source layers salt fountains spread to the profiles of viscous droplets normal for ice caps ice typically deforms at 80 usually 90 of its absolute melting temperature while most salt deforms at

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

widespread acceleration of tidewater glaciers on the antarctic peninsula over the last half century the antarctic peninsula ap has been among the most rapidly warming regions on earth this has led to increased summer snowmelt loss of ice shelves and retreat of 87 of marine and tidewater glacier fronts tidewater glacier flow is sensitive to changes in basal water supply and to thinning of the terminus and faster flow leads directly to sea level rise the flow rates of most ap tidewater glaciers have never been measured however and hence their dynamic response to the recent changes is unknown we present repeated flow rate measurements from over 300 glaciers on the ap west coast through nine summers from 1992 to 2005 we show that the flow rate increased by similar to 12 on average and that this trend is greater than the seasonal variability in flow rate we attribute this widespread acceleration trend not to meltwater enhanced lubrication or increased snowfall but to a dynamic response to frontal thinning we estimate that as a result the annual sea level contribution from this region has increased by 0 047 0 011 mm between 1993 and 2003 this contribution together with previous studies that assessed increased runoff from the area and acceleration of glaciers resulting from the removal of ice shelves implies a combined ap contribution of 0 16 0 06 mm yr 1 this is comparable to the contribution from alaskan glaciers and combined with estimated mass loss from west antarctica is probably large enough to outweigh mass gains in east antarctica and to make the total antarctic sea level contribution positive

interannual changes of the floating ice shelf of petermann gletscher north greenland from 2000 to 2012 petermann gletscher northwest greenland drains 4 of the greenland ice sheet into nares strait its floating ice shelf retreated from 81 to 48 km in length during two large calving events in 2010 and 2012 we document changes in the three dimensional ice shelf structure from 2000 to 2012 using repeated tracks of airborne laser altimetry and ice radio echo sounding icesat laser altimetry and modis visible imagery the recent ice shelf velocity measured by tracking surface features between flights in 2010 and 2011 is 1 25 km a 1 15 30 faster than estimates made before 2010 the steady state along flow ice divergence represents 6 3 gt a 1 mass loss through basal melting 5 gt a 1 and surface melting and sublimation 1 0 gt a 1 airborne laser altimeter data reveal thinning both along a thin central channel and on the thicker ambient ice shelf from 2007 to 2010 the ice shelf thinned by 5 m a 1 which represents a non steady mass loss of 4 1 gt a 1 we suggest that thinning in the basal channels structurally weakened the ice shelf and may have played a role in the recent calving events

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

glacier velocities from time lapse photos technique development and first results from the extreme ice survey eis in greenland automated digital cameras were installed in may june 2007 beside major west greenland marine terminating glaciers as part of the extreme ice survey eis eis cameras began imaging the lowest 4 km 2 of the glacier at hourly intervals throughout sunlit periods of the year this study presents the development of techniques for quantifying glacier velocity from a single camera perspective a multi image multi chip matching procedure yields higher matching skill than conventional matching and facilitates false match rejection via a clustering scheme the matching of motionless on land features facilitates compensating camera motion ray projection to a known terrain elevation allows the assigning of scale to convert pixel displacements to velocity units with the 10 2 megapixel camera system velocities on relatively fast glaciers can be resolved at distances up to 4 km at a distance of 2 km a demonstrated precision of 0 5 pixels yields a 0 5 m footprint size daily velocities indicate progressive multi day velocity accelerations associated with calving deceleration trends are associated with the regrowth of resistive stress after major calving the higher observation frequency available to terrestrial photogrammetry indicates higher observed intra seasonal velocity range than observable by the at best weekly satellite snapshots

seasonal variation in velocity before retreat of jakobshavn isbrae greenland 1 using repeat pass satellite feature tracking the dynamic behaviour of jakobshavn isbrae which drains around 7 of the greenland ice sheet is investigated in improved detail through its recent period of thinning acceleration and retreat the departure in 1995 from the normal seasonal invariance in velocity near the grounding line suggests a possible change in dynamics well before the subsequent thinning phase the initiation of the acceleration and retreat is pinpointed to spring 1998 and may have been prompted by the same mechanism that gave rise to the spring 1995 flow increase

substantial thinning of a major east greenland outlet glacier aircraft laser altimeter surveys in 1993 and 1998 over kangerdlugssuaq glacier in east greenland reveal thinning over the 5 year interim of several meters for all surveyed areas within 70 km of the seaward ice front rising to 50 meters in the final 5 km such rapid thinning is best explained by increased discharge velocities and associated creep thinning most probably caused by enhanced lubrication of the glacier bed the calving ice front over the past decade has occupied approximately the same location as in 1966 velocity estimates for 1995 96 are about the same as those for 1966 and 1988 but significantly less than for 1999 suggesting that major thinning began after 1995

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

evidence for the asynchronous retreat of large outlet glaciers in southeast greenland at the end of the last glaciation recent rapid changes in the marine terminating sectors of the greenland ice sheet gris have prompted concerns about the future stability of the ice sheet long term records of ice sheet behaviour provide valuable context to assess the magnitude of current change and may help resolve the mechanisms driving deglaciation we report 23 new 10be exposure ages which constrain the deglacial history of two large fjord systems in southeast se greenland we compare our chronologies with existing data from the centre of the sector to examine the timing and style of deglaciation at a regional scale glacial erratic 10be exposure ages demonstrate that kangerdlugssuaq fjord deglaciated at 11 8 ka at the end of the younger dryas yd 12 8 11 6 ka retreat at kangerdlugssuaq fjord coincided with known incursion of the warm irminger current ic onto the continental shelf this is inferred to have initiated retreat comparison with recently published results from sermilik fjord and new 10be ages from bernstorffs fjord indicates deglaciation occurred 1 ka later in the south of the se region sermilik fjord 10 9 ka and bernstorffs fjord 10 4 ka deglaciated later retreat likely occurred in response to dramatic climatic amelioration at the termination of the yd stadial we suggest that the disparate timing of deglaciation across the se region may be primarily explained by the varying influence of the warm ic glaciers in southern se greenland were isolated from warm atlantic waters during the yd by complex shelf bathymetry in all fjord settings ice retreat was rapid and persistent consistent with the absence of geomorphological evidence for stillstand or readvance events ice retreat was accompanied by rapid thinning and likely continued to well within present day ice sheet margins glacial erratic 10be age determinations and geomorphological observations show no evidence for holocene readvance events prior to the little ice age lia

understanding ice sheet mass balance progress in satellite altimetry and gravimetry satellite remote sensing has come to dominate the measurement of glacier and ice sheet change three independent methods now exist for assessing ice sheet mass balance and we focus on progress in two satellite altimetry icesat and gravimetry grace with improved spatial and temporal sampling and synergy with ice flow measurements both the mechanisms and causes changing mass balance can be investigated we present examples of mass losses due to widespread intensifying glacier dynamic thinning in northwest greenland but local ablation rates in the northeast that are unchanged for decades advances in grace processing reveal greenland net ice sheet mass loss continuing into 2010 at 19530 gt a 1 a similarly negative trend in the gulf of alaska has significant spatial and temporal variation that highlights the importance of intense summer melting here strong summer melt on the antarctic peninsula also precipitated recent ice shelf collapse and prompted rapid dynamic thinning of tributary glaciers at up to 70ma 1 thinning continued for years to decades after collapse and propagated far inland while understanding of the physical mechanisms of change continues to improve estimates of future behaviour and in particular the near future glacial sea level contribution still rely on projections from such observations we introduce the suite of new sensors that will monitor the ice sheets into the future