coupled ice shelf ocean modeling and complex grounding line retreat from a seabed ridge recent observations and modeling work have shown a complex mechanical coupling between antarctica s floating ice shelves and the adjacent grounded ice sheet a prime example is pine island glacier west antarctica which has a strong negative mass balance caused by a recent increase in ocean induced melting of its ice shelf the mass loss coincides with the retreat of the grounding line from a seabed ridge on which it was at least partly grounded until the 1970s at present it is unclear what has caused the onset of this retreat and how feedback mechanisms between the ocean and ice shelf geometry have influenced the ice dynamics to address these questions we present the first results from an offline coupling between a state of the art shallow ice flow model with grounding line resolving capabilities and a three dimensional ocean general circulation model with a static implementation of the ice shelf a series of idealized experiments simulate the retreat from a seabed ridge in response to changes in the ocean forcing and we show that the retreat becomes irreversible after 20 years of warm ocean conditions a comparison to experiments with a simple depth dependent melt rate parameterization demonstrates that such parameterizations are unable to capture the details of the retreat process and they overestimate mass loss by more than 40 over a 50 year timescale

volcano dome dynamics at mount st helens deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets the surface deformation field measured at volcanic domes provides insights into the effects of magmatic processes gravity and gas driven processes and the development and distribution of internal dome structures here we study short term dome deformation associated with earthquakes at mount st helens recorded by a permanent optical camera and seismic monitoring network we use digital image correlation dic to compute the displacement field between successive images and compare the results to the occurrence and characteristics of seismic events during a 6 week period of dome growth in 2006 the results reveal that dome growth at mount st helens was repeatedly interrupted by short term meter scale downward displacements at the dome surface which were associated in time with low frequency large magnitude seismic events followed by a tremor like signal the tremor was only recorded by the seismic stations closest to the dome we find a correlation between the magnitudes of the camera derived displacements and the spectral amplitudes of the associated tremor we use the dic results from two cameras and a high resolution topographic model to derive full 3 d displacement maps which reveals internal dome structures and the effect of the seismic activity on daily surface velocities we postulate that the tremor is recording the gravity driven response of the upper dome due to mechanical collapse or depressurization and fault controlled slumping our results highlight the different scales and structural expressions during growth and disintegration of lava domes and the relationships between seismic and deformation signals

present day vertical crustal deformations in west greenland from repeated gps observations summary a gps network consisting of 10 sites was established in the ice free area of west greenland and was observed for the first time in 1995 in 2002 a complete re observation was carried out these repeated gps observations served as a basis for the determination of vertical crustal deformations the data analysis was performed using the bernese software version 5 0 for the central site kangerlussuaq a negative uplift rate subsidence of 3 1 1 1 mm yr 1 was obtained related to the reference frame igb00 the regional pattern is characterized by an east west gradient of up to 4 mm yr 1 between the outer coast and the subsiding area along the present ice margin which can be explained to a great extent as a result of the late holocene re advance of the greenland ice sheet relative sea level changes could be calculated taking the present eustatic sea level rise into account the present day sea level rises at the outer coast between maniitsoq and paamiut and in the large fjords with increasing rates of more than 4m m yr 1 in their innermost parts these findings are in agreement with the general picture obtained from geomorphological and archaeological research for sisimiut and the disko bay area the present sea level change is almost zero whereas the crustal uplift rate for ilulissat was observed to be 1 6 mm yr 1 w econclude that this present vertical uplift rate in ilulissat is affected by the retreat of the ice margin during the last 150 yr and the present negative mass balance of the jakobshavn isbrae and its drainage basin

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

control of ocean temperature on jakobshavn isbrae s present and future mass loss author s bondzio jh morlighem m seroussi h wood mh mouginot j abstract 2018 american geophysical union all rights reserved large uncertainties in model parameterizations and input data sets make projections of future sea level rise contributions of outlet glaciers challenging here we introduce a novel technique for weighing large ensemble model simulations that uses information of key observables the approach is robust to input errors and yields calibrated means and error estimates of a glacier s mass balance we apply the technique to jakobshavn isbrae using a model that includes a dynamic calving law and closely reproduce the observed behavior from 1985 to 2018 by forcing the model with ocean temperatures only our calibrated projection suggests that the glacier will continue to retreat and contribute about 5 1 mm to eustatic sea level rise by 2100 under present day climatic forcing our analysis shows that the glacier s future evolution will strongly depend on the ambient oceanic setting

dynamics of glacier calving at the ungrounded margin of helheim glacier southeast greenland during summer 2013 we installed a network of 19 gps nodes at the ungrounded margin of helheim glacier in southeast greenland together with three cameras to study iceberg calving mechanisms the network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved data collection covered 55 days and many nodes survived in locations right at the glacier front to the time of iceberg calving the observations included a number of significant calving events and as a consequence the glacier retreated 1 5 km the data provide real time high frequency observations in unprecedented proximity to the calving front the glacier calved by a process of buoyancy force induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium calving then occurs back to the flexion zone this calving process provides a compelling and complete explanation for the data tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier interpretation of the gps data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving the flexion zones are probably formed by the exploitation of basal crevasses and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium thus this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in greenland

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

ice shelf grounding zone structure from icesat laser altimetry 1 we present a technique for investigating the grounding zone gz of antarctic ice shelves using laser altimetry from the ice cloud and land elevation satellite icesat most surface height variability in the gz is easily resolved by the icesat laser s 65 m footprint and 172 m along track spacing comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide forced ice flexure providing gz location and width information for each track using icesat data in the institute ice stream region of southern ronne ice shelf we demonstrate that the location of the gz based on feature identification in satellite imagery or digital elevation models may be in error by several km our results show that icesat will contribute significantly to improving knowledge of gz structure and to studies requiring accurate gz locations e g ice mass balance calculations and ice sheet ocean modeling

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

the recession of the inland ice margin during the holocene climatic optimum in the jakobshavn isfjord area of west greenland recent subsurface mapping of parts of the greenland inland ice margin in the region of jakobshavn isbrae indicates that the fjord system in the period of at least 2700 4700 calendar yr b p was more ice free than at present and that the front of the glacier was at least 15 km behind the present position the 14c datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for greenland s first settlers the sarqaq people who arrived in the region about 4000 yr ago to find hunting grounds 10 20 larger than the present

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

why does near ridge extensional seismicity occur primarily in the indian ocean abstract the possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest this effect is suggested by three key observations a decrease in seismicity with lithospheric age the fact that focal mechanisms show extension perpendicular to the spreading direction and a depth stratification of mechanism types a difficulty with this idea is that although thermoelastic stresses should be comparable in different regions the intraplate seismicity seems to occur in local concentrations in particular the ridge parallel extensional seismicity occurs preferentially in the central indian ocean region we explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects in particular the dramatic concentration of extensional seismicity in the central indian ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the himalayan collision and the various subduction zones the ridge parallel extensional stresses show a decrease with age similar to that of the seismicity as earthquakes in this area provide a major portion of the data for both ridge parallel extension and depth stratification these effects may be due more to the regional stress we thus propose that thermoelastic stresses provide a low level background in all plates but that the dominant effect is that of individual plate geometry and local boundary effects

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

recent changes on greenland outlet glaciers aircraft laser altimeter surveys during the 1990s showed near coastal parts of the greenland ice sheet to be thinning despite slow thickening at higher elevations the ice sheet lost mass to the ocean many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers indicating dynamic imbalance between glacier velocity and upstream snow accumulation results from more recent surveys presented here show that thinning rates have increased in most coastal regions for almost half of the surveys these increases might have resulted from increases in summer melting but rapid thinning on others is indicative of dynamic changes that increased with time in particular thinning rates on the three fastest glaciers increased to tens of m a 1 after 2000 and other observations show an approximate doubling in their velocities the deep beds of these glaciers appear to have a strong influence on rates of grounding line retreat and thickness change with periods of glacier acceleration and rapid thinning initiated by flotation and break up of lightly grounded glacier snouts or break up of floating ice tongues near simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause nearby glaciers without deep beds are thinning far more slowly suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on greenland outlet glacier acceleration

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

numerical modeling of iceberg capsizing responsible for glacial earthquakes the capsizing of icebergs calved from marine terminating glaciers generate horizontal forces on the glacier front producing long period seismic signals referred to as glacial earthquakes these forces can be estimated by broadband seismic inversion but their interpretation in terms of magnitude and waveform variability is not straightforward we present a numerical model for fluid drag that can be used to study buoyancy driven iceberg capsize dynamics and the generated contact forces on a calving face using the finite element approach we investigate the sensitivity of the force to drag effects iceberg geometry calving style and initial buoyancy we show that there is no simple relationship between force amplitude and iceberg volume and similar force magnitudes can be reached for different iceberg sizes the force history and spectral content varies with the iceberg attributes the iceberg aspect ratio primarily controls the capsize dynamics the force shape and force frequency whereas the iceberg height has a stronger impact on the force magnitude iceberg hydrostatic imbalance generates contact forces with specific frequency peaks that explain the variability in glacial earthquake dominant frequency for similar icebergs top out and bottom out events have significantly different capsize dynamics leading to larger top out forces especially for thin icebergs for realistic iceberg dimensions we find contact force magnitudes that range between 5 6 1011 and 2 1014 kg m consistent with seismic observations this study provides a useful framework for interpreting glacial earthquake sources and estimating the ice mass loss from coupled analysis of seismic signals and modeling results

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

bed elevation of jakobshavn isbrae west greenland from high resolution airborne gravity and other data jakobshavn isbrae west greenland which holds a 0 6 m sea level volume equivalent has been speeding up and retreating since the late 1990s interpretation of its retreat has been hindered by difficulties in measuring its ice thickness with airborne radar depth sounders here we employ high resolution helicopter borne gravity data from 2012 to reconstruct its bed elevation within 50 km of the ocean margin using a three dimensional inversion constrained by fjord bathymetry data offshore and a mass conservation algorithm inland we find the glacier trough to be asymmetric and several 100 m deeper than estimated previously in the lower part from 1996 to 2016 the grounding line migrated at 0 6 km yr from 700 m to 1100 m depth upstream the bed drops to 1600 m over 10 km then slowly climbs to 1200 m depth in 40 km jakobshavn isbrae will continue to retreat along a retrograde slope for decades to come

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

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

grounding line migration and high resolution calving dynamics of jakobshavn isbrae west greenland 1 we performed three field campaigns in 2004 2007 and 2010 at the southern margin of the jakobshavn isbrae west greenland in order to infer flow velocities and their changes from photogrammetric time lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface area wide analysis of more than 3000 three dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events from 2004 to 2010 the grounding line of jakobshavn isbrae retreated 3 5 0 2 km considering previously published results the grounding line retreat amounts to 6 km since 1985 the glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m d can be reached moreover large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event after the break up the glacier slowly adjusts to the new boundary conditions within the next 4 5 days flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large scale glacier dynamics

tidal motion ice velocity and melt rate of petermann gletscher greenland measured from radar interferometry over a floating glacier ice tongue or an ice shelf the glacier motion measured by a single repeat pass radar interferogram is difficult to analyze because the long term steady motion of the ice is intermixed with its cyclic downward motion induced by tidal forcing multiple interferograms and a quadruple difference technique are necessary to separate the tidal signal from the long term steady motion of the ice an example of application of this technique is given here using ers 1 radar images of petermann gletscher a major outlet glacier of northern greenland tidal displacements are measured with 5 mm statistical noise the long term ice velocity is measured with a precision of 1 m a 1 the inferred tidal displacements agree well with model predictions from a fixed elastic beam with an elastic damping factor of 0 47 0 01 km 1 the hinge line is mapped with a precision of 20 80 m combining the interferometric ice velocities with ice thickness data the glacier ice discharge is calculated at and below the hinge line at the hinge line the ice flux is 12 1 1 km3 a 1 at the ice front calf ice production is only 0 59 km3 a 1 meaning that 95 of the ice that crosses the grounding line melts before it reaches the calving front assuming steady state conditions the melt rate of the glacier tongue averages 12 1 m a 1 with peak values exceeding 20 m a 1 near the hinge line this high melt rate cannot be accommodated by surface ablation alone only about 2 3 m a 1 and is attributed to pronounced basal melting of the ice tongue basal melting often assumed to be negligible in greenland is the dominant process of mass release from the floating section of petermann gletscher

large scale changes in greenland outlet glacier dynamics triggered at the terminus the recent dramatic retreat of many outlet glaciers of the greenland ice sheet has raised concerns over greenland s contribution to future sea level rise simulations with a numerical ice flow model indicate that the recent rates of mass loss in greenland s outlet glaciers are transient and should not be extrapolated into the future

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

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

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

rapid changes in ice discharge from greenland outlet glaciers using satellite derived surface elevation and velocity data we found major short term variations in recent ice discharge and mass loss at two of greenland s largest outlet glaciers their combined rate of mass loss doubled in less than a year in 2004 and then decreased in 2006 to near the previous rates likely as a result of fast re equilibration of calving front geometry after retreat total mass loss is a fraction of concurrent gravity derived estimates pointing to an alternative source of loss and the need for high resolution observations of outlet dynamics and glacier geometry for sea level rise predictions

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

investigating tidal flexure on an ice shelf using kinematic gps the positions of ice stream grounding zones are uniquely sensitive to changes in the mass balance of the ice sheet present methods for locating groundingzone features are either imprecise or require considerable effort in interpretation and so are of little value for change studies we present a new method which uses the kinematic gps technique to locate the position of the limit of tidal flexure the method involves the collection of at least two surface elevation profiles along the same track through the grounding zone at different times during the tidal cycle the elevation profiles obtained coincide upstream of the limit of flexure but diverge downstream of the limit of flexure subtracting the profiles produces a tidal def1ection profile which shows directly the response of the ice shelf to the tidal forcing we present two examples of the use of this method both on rutford ice stream antarctica the first is across the grounding zone and shows that the method is capable of measuring grounding positions to around 200 m precision the second taken across an active shear margin shows a tidal deflection profile with an absence of steps that would indicate the presence of fracture planes penetrating from the ice base to sea level

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

an integrated structure from motion and time lapse technique for quantifying ice margin dynamics fine resolution topographic data derived from methods such as structure from motion sfm and multi view stereo mvs have the potential to provide detailed observations of geomorphological change but have thus far been limited by the logistical constraints of conducting repeat surveys in the field here we present the results from an automated time lapse camera array deployed around an ice marginal lake on the western margin of the greenland ice sheet fifteen cameras acquired imagery three times per day over a 426 day period yielding a dataset of 19 000 images from these data we derived 18 point clouds of the ice margin across a range of seasons and successfully identified calving events ranging from 234 to 1475 m2 in area and 815 8725 m3 in volume induced by ice cliff undercutting at the waterline and the collapse of spalling flakes low ambient light levels locally reflective surfaces and the large survey range hindered analysis of smaller scale ice margin dynamics nevertheless this study demonstrates that an integrated sfm mvs and time lapse approach can be employed to generate long term 3 d topographic datasets and thus quantify ice margin dynamics at a fine spatio temporal scale this approach provides a template for future studies of geomorphological change

a comparison of arctic and antarctic mesoscale vortices the mesoscale less than 100km vortices occurring in the two polar regions are considered in terms of their geographical and seasonal distribution satellite cloud signatures and forcing mechanisms environmental conditions important in the development of the vortices are considered including sensible heat flux stability throughout the troposphere and synoptic factors such as baroclinicity and upper air cold pools a scheme to classify the observed vortices in the two polar regions in terms of the physical mechanisms behind their formation and development is proposed the processes considered important are baroclinic instability convection and vorticity generation through cyclonic vorticity advection and topographic forcing the major difference between the systems observed in the two polar regions is the lack of deep convection in the southern hemisphere which precludes the development of many of the vigorous types of system found in the north and the major role that topography plays in the antarctic coastal region the most common type of vortex found in the antarctic occurs over the ice free ocean to the west of synoptic scale disturbances and is similar to the type of northern system know as a comma cloud

controls on short term variations in greenland glacier dynamics short term ice dynamical processes at greenland s jakobshavn and kangerdlugssuaq glaciers were studied using a 3 day time series of synthetic aperture radar data acquired during the 2011 european remote sensing satellite 2 ers 2 3 day repeat campaign together with modelled meteorological parameters the time series spans the period march july 2011 and captures the first 30 of the summer melting season in both study areas we observe velocity fluctuations at the lower 10 km of the glacier at jakobshavn isbrae where our dataset covers the first part of the seasonal calving front retreat we identify ten calving episodes with a mean calving front area loss of 1 29 0 4km2 significant glacier speed up was observed in the near terminus area following all calving episodes we identify changes in calving front geometry as the dominant control on velocity fluctuations on both glaciers apart from a

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

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

quantitative estimates of velocity sensitivity to surface melt variations at a large greenland outlet glacier this is the publisher s version also available electronically from http www ingentaconnect com

the statistical physics of iceberg calving and the emergence of universal calving laws determining a calving law valid for all glaciological and environmental regimes has proven to be a difficult problem in glaciology for this reason most models of the calving process are semi empirical with little connection to the underlying fracture processes in this study i introduce methods rooted in statistical physics to show how calving laws valid for any glaciological domain can emerge naturally as a large spatial scale long temporal scale limit of an underlying continuous or discrete fracture process an important element of the method developed here is that iceberg calving is treated as a stochastic process and that the probability an iceberg will detach in a given interval of time can be described by a probability distribution function using limiting assumptions about the underlying probability distribution the theory is shown to be able to simulate a range of calving styles including the sporadic detachment of large tabular icebergs from ice tongues and ice shelves and the more steady detachment of smaller sized bergs from tidewater outlet glaciers the method developed has the potential to provide a physical basis to include iceberg calving into numerical ice sheet models that can be used to produce more realistic estimates of the glaciological contribution to sea level rise

jakobshavn isbrae west greenland flow velocities and tidal interaction of the front area from 2004 field observations 1 during the summer of 2004 the front area of the jakobshavn isbrae was monitored using a geodetic photogrammetric survey with temporarily coincident precise observations of local ocean tides in the disko bay close to ilulissat the geodetic and photogrammetric observations were conducted at the southern margin of the glacier front the largest observed horizontal flow velocities are in the central part of the front with values up to 45 m d this is a factor of 2 greater than the average velocities at the front area observed in the last century our new observations confirm previous estimates of an acceleration of glacier flow during the last decade the photogrammetric survey provided flow trajectories for 4000 surface points with a time resolution of 30 min these flow trajectories were used to compare the vertical motion of the glacier with the observed tides the existence of a free floating glacier tongue in 2004 was confirmed by these data however it occupied only a small belt of at most a few 100 m width in the central part of the glacier front horizontal motion did not appear to depend on the tidal phase unlike some of the fast moving ice streams of west antarctica

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

pointcatcher software analysis of glacial time lapse photography and integration with multitemporal digital elevation models terrestrial time lapse photography offers insight into glacial processes through high spatial and temporal resolution imagery however oblique camera views complicate measurement in geographic coordinates and lead to reliance on specific imaging geometries or simplifying assumptions for calculating parameters such as ice velocity we develop a novel approach that integrates time lapse imagery with multi temporal digital elevation models to derive full 3d coordinates for natural features tracked throughout a monoscopic image sequence this enables daily independent measurement of horizontal ice flow and vertical ice melt velocities by combining two terrestrial laser scanner surveys with a 73 day sequence from solheimajokull iceland variations in horizontal ice velocity of 10 were identified over timescales of 25 days an overall surface elevation decrease of 3 0 m showed rate changes asynchronous with the horizontal velocity variations demonstrating a temporal disconnect between the processes of ice surface lowering and mechanisms of glacier movement our software pointcatcher is freely available for user friendly interactive processing of general time lapse sequences and includes monte carlo error analysis and uncertainty projection onto dem surfaces it is particularly suited for analysis of challenging oblique glacial imagery and we discuss good features to track both for correction of camera motion and for deriving ice velocities

mapping the grounding zone of the ross ice shelf antarctica using icesat laser altimetry we use laser altimetry from the ice cloud and land elevation satellite icesat to map the grounding zone gz of the ross ice shelf antarctica at 491 locations where icesat tracks cross the grounding line gl ice flexure in the gz occurs as the ice shelf responds to short term sea level changes due primarily to tides icesat repeat track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced we find that the icesat derived landward limits of tidal flexure are in many places offset by several km and up to 60 km from the gl mapped previously using other satellite methods we discuss the reasons why different mapping methods lead to different gl estimates including instrument limitations variability in the surface topographic structure of the gz and the presence of ice plains we conclude that reliable and accurate mapping of the gl is most likely to be achieved when based on synthesis of several satellite datasets

changes in ice front position on greenland s outlet glaciers from 1992 to 2007 1 previous studies in greenland show that retreat of tidewater glaciers may be linked to recent increases in ice loss raising greenland s contribution to sea level rise we examined ice front changes of 203 tidewater glaciers land terminating glaciers and glaciers terminating with ice shelves to understand greenland glacier behavior over three periods 1992 2000 2000 2006 and 2006 2007 we observed synchronous ice sheet wide increases in tidewater retreat during 2000 2006 relative to 1992 2000 coinciding with a 1 1 c increase in mean summer temperature at coastal weather stations rates of retreat for the southeast and east slowed during 2006 2007 when temperatures were slightly cooler than the 2000 2006 average our work suggests that regional greenland tidewater retreat responds strongly to climate change with higher temperatures corresponding to increasing retreat and helps confirm a link between ice thickness velocity and ice front position

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

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

rapid retreat of a scandinavian marine outlet glacier in response to warming at the last glacial termination abstract marine outlet glaciers on greenland are retreating yet it is unclear if the recent fast retreat will persist and how atmosphere and ocean warming will impact future retreat we show how a marine outlet glacier in hardangerfjorden retreated rapidly in response to the abrupt warming following the younger dryas cold period approximately 11 600 years before present this almost 1000 m deep fjord with several sills at 300 500 m depth hosted a 175 km long outlet glacier at the western rim of the scandinavian ice sheet we use a dynamic ice flow model constrained by well dated terminal and lateral moraines to simulate the reconstructed 500 year retreat of hardangerfjorden glacier the model includes an idealized oceanic and atmospheric forcing based on reconstructions but excludes the surface mass balance elevation feedback our simulations show a highly episodic retreat driven by surface melt and warming fjord waters paced by the fjord bathymetry warming air and ocean temperatures by 4 5 c during the period of retreat result in a 125 km retreat of hardangerfjorden glacier in 500 years retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a 1 generally close to 200 m a 1 punctuated by brief events of swift retreat exceeding 500 m a 1 each event lasting a few decades we show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes ice shelf length and fjord water depth is less important our results have implications for modern glacial fjord settings similar to hardangerfjorden where high retreat rates have been observed our findings imply that increasing air temperatures and warming subsurface waters in greenland fjords will continue to drive extensive retreat of marine outlet glaciers however the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

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

intermittent thinning of jakobshavn isbrae west greenland since the little ice age rapid thinning and velocity increase on major greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the jakobshavn effect hughes 1986 with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat to assess whether recent trends deviate from longer term behavior we measured glacier surface elevations and terminus positions for jakobshavn isbrae west greenland using historical photographs acquired in 1944 1953 1959 1964 and 1985 these results were combined with data from historical records aerial photographs ground surveys airborne laser altimetry and field mapping of lateral moraines and trimlines to reconstruct the history of changes since the little ice age lia we identified three periods of rapid thinning since the lia 1902 13 1930 59 and 1999 present during the first half of the 20th century the calving front appears to have been grounded and it started to float during the late 1940s the south and north tributaries exhibit different behavior for example the north tributary was thinning between 1959 and 1985 during a period when the calving front was stationary and the south tributary was in balance the record of intermittent thinning combined with changes in ice marginal extent and position of the calving front together with changes in velocity imply that the behavior of the lower parts of this glacier represents a complex ice dynamical response to local climate forcings and interactions with drainage from the interior

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

monitoring greenland ice sheet buoyancy driven calving discharge using glacial earthquakes since the 2000s greenland ice sheet mass loss has been accelerating followed by increasing numbers of glacial earthquakes ges at near grounded glaciers ges are caused by calving of km scale icebergs which capsize against the terminus seismic record inversion allows a reconstruction of the history of ge sources which captures capsize dynamics through iceberg to terminus contact when compared with a catalog of contact forces from an iceberg capsize model seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size giving errors up to 1 km 3 calving determined from ges recorded ateight glaciers in 1993 2013 accounts for up to 21 of the associated discharge and 6 of the greenland mass loss the proportion of discharge attributed to capsizing calving may be underestimated by at least 10 as numerous events could not be identified by standard seismic detections olsen and nettles 2018 while calving production tends to stabilize in east greenland western glaciers have released more and larger icebergs since 2010 and have become major contributors to greenland dynamic discharge production of ges and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water we illustrate how ges can help in partitioning and monitoring greenland mass loss and characterizing capsize dynamics

non linear glacier response to calving events jakobshavn isbrae greenland jakobshavn isbrae a tidewater glacier that produces some of greenland s largest icebergs and highest speeds reached record high flow rates in 2012 joughin and others 2014 we use terrestrial radar interferometric observations from august 2012 to characterize the events that led to record high flow we find that the highest speeds occurred in response to a small calving retreat while several larger calving events produced negligible changes in glacier speed this non linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position our observations indicate that a glacier s response to calving is a consequence of two competing feedbacks 1 an increase in strain rates that leads to dynamic thinning and faster flow thereby promoting destabilization and 2 an increase in flow rates that advects thick ice toward the terminus and promotes restabilization the competition between these feedbacks depends on temporal and spatial variations in the glacier s proximity to flotation this study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability and further suggests the latter may be limiting the current retreat due to the thick ice that occupies jakobshavn isbrae s retrograde bed

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis