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

spatially integrated geodetic glacier mass balance and its uncertainty based on geostatistical analysis application to the western svartisen ice cap norway estimates of glacier mass balance using geodetic methods can differ significantly from estimates using direct glaciological field based measurements to determine if such differences are real or methodological there is a need to improve uncertainty estimates in both methods in this paper we focus on the uncertainty of geodetic methods and describe a geostatistical technique that takes into account the spatial correlation of the elevation differences when calculating spatially averaged elevation changes we apply this method to the western svartisen ice cap norway using elevation differences from the surrounding bedrock derived from stereophotogrammetry we show that the uncertainty is not only dependent on the standard error of the individual elevation differences but is also dependent on the size of the averaging area and the scale of the spatial correlation to assess if the geostatistical analysis made over bedrock is applicable to glacier surfaces we use concurrent photogrammetrical and laser scanning data from bedrock and a range of glacier surfaces to evaluate the dependency of the geostatistical analysis on the surface type the estimated geodetic mass balance and its uncertainty is 2 6 0 9 m w e for the period 1968 85 and 2 0 2 2 m w e for 1985 2002

melt under cutting and buoyancy driven calving from tidewater glaciers new insights from discrete element and continuum model simulations the simple calving laws currently used in ice sheet models do not adequately reflect the complexity and diversity of calving processes to be effective calving laws must be grounded in a sound understanding of how calving actually works here we develop a new strategy for formulating calving laws using a the helsinki discrete element model hidem to explicitly model fracture and calving processes and b the continuum model elmer ice to identify critical stress states associated with hidem calving events a range of observed calving processes emerges spontaneously from hidem in response to variations in ice front buoyancy and the size of subaqueous undercuts calving driven by buoyancy and melt under cutting is under predicted by existing calving laws but we show that the location and magnitude of hidem calving events can be predicted in elmer ice from characteristic stress patterns our results open the way to developing calving laws that properly reflect the diversity of calving processes and provide a framework for a unified theory of the calving process continuum

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

a geometric analysis of surface deformation implications for the tectonic evolution of ganymede abstract a visual nonalignment of the furrows and the circularity of impact craters are used to study surface deformation on ganymede the furrow system is examined to test the hypothesis that lateral motion has taken place between areas of dark terrain results show that while lateral motion cannot be ruled out it may not be required to explain the geometry of the system initial nonconcentricity of the furrows or an early period of penetrative deformation shortly after furrow formation could also account for the present configuration centers of curvature of the furrows in galileo and marius regiones are numerically determined and it is shown that if lateral movement did occur it is not possible to determine the amount of displacement the axial ratios of impact craters in the uruk sulcus region which separates galileo and marius regiones are determined and show that large scale shear deformation has not occured in that area since bright terrain was emplaced deformation of impact craters within galileo regio suggests that ganymede s lithosphere has behaved rigidly throughout most of the satellite s evolution the shapes and orientations of impact craters in dark terrain around wedges of bright terrain are used to place an upper limit on the amount of extension associated with bright terrain formation

constraints on the expansion of ganymede and the thickness of the lithosphere grooves and furrows have been interpreted as grabens resulting from extension of ganymede s lithosphere geologically reasonable estimates of the dip 90 60 and displacement less than 1 km to a few hundred meters of faults bounding grooves on ganymede allow calculation of the extension and surface area increase 0 04 2 required for the formation of all grooved terrain the implied radius increase is thus limited between 0 02 and 1 1 planetary expansion is the maximum possible since formation of the grooved terrain began similar estimates for faults bounding furrows the oldest preserved tectonic features on ganymede indicate a maximum surface area increase of 0 5 for the best developed furrow system which is located in galileo regio if equivalent furrow systems existed on all of the cratered terrain then a maximum of 0 5 increase in surface area of ganymede is possible after the formation of the lithosphere and prior to the formation of grooved terrain faults bounding simple graben originate at a subsurface mechanical discontinuity and propagate upwards resulting in grabens with similar widths and similar spacings between members of a set the depth to this mechanical discontinuity for furrows and grooves on ganymede must give at least a minimum lithosphere thickness because it is impossible for faults to originate and propagate upwards from a point below the lithosphere therefore the minimum lithosphere thickness on ganymede is 5 km and 9 km in marius and galileo regiones respectively at the time of furrow formation and 4 km at the time of groove formation

global geological mapping of ganymede abstract we have compiled a global geological map of ganymede that represents the most recent understanding of the satellite based on galileo mission results this contribution builds on important previous accomplishments in the study of ganymede utilizing voyager data and incorporates the many new discoveries that were brought about by examination of galileo data we discuss the material properties of geological units defined utilizing a global mosaic of the surface with a nominal resolution of 1 km pixel assembled by the usgs with the best available voyager and galileo regional coverage and high resolution imagery 100 200 m pixel of characteristic features and terrain types obtained by the galileo spacecraft we also use crater density measurements obtained from our mapping efforts to examine age relationships amongst the various defined units these efforts have resulted in a more complete understanding of the major geological processes operating on ganymede especially the roles of cryovolcanic and tectonic processes in the formation of might materials they have also clarified the characteristics of the geological units that comprise the satellite s surface the stratigraphic relationships of those geological units and structures and the geological history inferred from those relationships for instance the characteristics and stratigraphic relationships of dark lineated material and reticulate material suggest they represent an intermediate stage between dark cratered material and light material units

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

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

warming of the polar water layer in disko bay and potential impact on jakobshavn isbrae a number of recent studies have shown enhanced retreat of tidewater glaciers over much of southern and western greenland one of the fastest retreats has occurred at jakobshavn isbrae with the rapid retreat linked to the arrival of relatively warm and saline irminger water along the west coast of greenland similar links to changes in ocean water masses on the coastal shelf of greenlandwere also seen on the east coast this study presents hydrographic data from disko bay additionally revealing that there was also a significant warming of the cold polar water entering disko bay from the mid to late 1990s onward this layer which lies at a depth of 30 200m warmedby 18 28c the heat content of the polar water layer increased by a factor of 3 6 for the post 1997 period compared to the period prior to 1990 the heat content in the west greenland irminger water layer between the same periods increased only by a factor of 2 but contained more total heat the authors suggest that the changes in the polar water layer are related to circulation changes in baffin bay

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

viscous relaxation of ganymede s impact craters constraints on heat flux abstract measurement of crater depths in ganymede s dark terrain have revealed substantial numbers of unusually shallow craters indicative of viscous relaxation see companion paper singer k n schenk p m bland m t mckinnon w b 2017 relaxed impact craters on ganymede regional variations and high heat flow icarus submitted these viscously relaxed craters provide insight into the thermal history of the dark terrain the rate of relaxation depends on the size of the crater and the thermal structure of the lithosphere here we use finite element simulations of crater relaxation to constrain the heat flux within the dark terrain when relaxation occurred we show that the degree of viscous relaxation observed cannot be achieved through radiogenic heating alone even if all of the relaxed craters are ancient and experienced the high radiogenic fluxes present early in the satellite s history for craters with diameter 10 km heat fluxes of 40 50 mw m 2 can reproduce the observed crater depths but only if the fluxes are sustained for 1 gyr these craters can also be explained by shorter lived heat pulses with magnitudes of 100 mw m 2 and timescales of 10 100 myr at small crater diameters 4 km the observed shallow depths are difficult to achieve even when heat fluxes as high as 150 mw m 2 are sustained for 1 gyr the extreme thermal conditions required to viscously relax small craters may indicate that mechanisms other than viscous relaxation such as topographic degradation are also in play at small crater diameters the timing of the relaxation event s is poorly constrained due to the sparsity of adequate topographic information though it likely occurred in ganymede s middle history neither recently nor shortly after satellite formation the consistency between the timing and magnitude of the heat fluxes derived here and those inferred from other tectonic features suggests that a single event caused both ganymede s tectonic deformation and its crater relaxation future observations should permit more robust determination of the relative timing of the heating event that caused viscous relaxation

dark terrain on ganymede geological mapping and interpretation of galileo regio at high resolution abstract during its first two encounters with ganymede the galileo spacecraft obtained images of a 16 500 km 2 portion of galileo regio a large expanse of dark terrain at high resolution 76 86 m pixel through mapping of the g1 and g2 target sites within galileo regio we are able to characterize geological units based on their morphology and relative albedo we find three generally low albedo units an intermediate albedo plains unit a lower albedo plains unit and the lowest albedo unit which is found on furrow and crater floors we also find high albedo units which include crater rims furrow rims and isolated knobs and massifs other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta several processes are interpreted to have occurred within galileo regio these include tectonic deformation mass wasting sublimation resurfacing by impact ejecta and possibly cryovolcanism and isostatic adjustment we observe that the nw se trending furrows lakhmu fossae in galileo regio are degraded and are crosscut by the younger n s trending furrows zu fossae we also find several other tectonic features which may be minor faults or fractures related to one or other of these systems through mapping and crater size frequency distributions we are able to propose a stratigraphy for the galileo regio target site the oldest features in the area are high albedo knobs and massifs which are interpreted to be remnants of early impact related features and furrow rims these may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems the lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting much of the northeast portion of the target area was subsequently obscured by one of the youngest units ejecta from an impact just to the north we use our mapping of the high resolution images of galileo regio to evaluate three end member models for the formation of dark terrain 1 the crust is dark throughout 2 material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust and 3 dark material is distributed in small quantities throughout the crust and geological processes have acted to concentrate low albedo material on the surface although it is possible that elements of more than one of these models are present within the dark terrain we find that the third model that of a thin veneer of low albedo material best fits observations of galileo regio

fault offsets and lateral crustal movement on europa evidence for a mobile ice shell abstract right lateral structural offsets of 25 km have been identified on the icy galilean satellite europa these occur along dark lineaments oriented orthogonally to wedge shaped bands which are also 25 km wide wedge shaped bands are interpreted as dilated tension fractures which formed as crustal blocks or plates 50 100 km across separated and slipped past each other along flanking strike slip faults this style of deformation does not appear to be characteristic of other lineament types and with the exception of earth appears to be unique to europa together the subparallel wedge shaped b bands form a broad nw se trending belt 1500 km long and less than 500 km across near the anti jovian point this belt is interpreted as a major crustal fracture or rift zone with a pole of rotation determined by the strike slip faults near 47 s 144 w and an approximate ne sw direction of maximum tensile stress extension may have been areally compensated at agenor linea a bright band of possible compressional origin global expansion tidal distortion and nonsynchronous rotation do not explain the inferred minimum principal i e least compressive stress directions alternatively fracturing near the anti jovian point may be a result of i solid state convection in the lower ice crust possibly triggered by uneven heat flow from the silicate interior ii rotation of the icy shell about the sub and anti jovian points induced by latitudinal lithospheric thickness variations or iii preferential strain accumulation from the rest of the icy shell no significant distortion of the crustal blocks occurred during fracturing and rotation indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere not a silicate one and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior which we show is likely unless the ice shell rotates independently decoupling could have been due to either warm ice or liquid water near the base of the icy crust mechanical bounds on lithospheric thickness a few to 10 km lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water

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

greenland ice sheet surface temperature melt and mass loss 2000 06 author posting international glaciological society 2008 this article is posted here by permission of international glaciological society for personal use not for redistribution the definitive version was published in journal of glaciology 54 2008 81 93 doi 10 3189 002214308784409170

reverse glacier motion during iceberg calving and the cause of glacial earthquakes nearly half of greenland s mass loss occurs through iceberg calving but the physical mechanisms operating during calving are poorly known and in situ observations are sparse we show that calving at greenland s helheim glacier causes a minutes long reversal of the glacier s horizontal flow and a downward deflection of its terminus the reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front the downward motion results from a hydrodynamic pressure drop behind the capsizing berg which also causes an upward force on the solid earth these forces are the source of glacial earthquakes globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in greenland and antarctica

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

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

dynamic jamming of iceberg choked fjords we investigate the dynamics of ice melange by analyzing rapid motion recorded by a time lapse camera and terrestrial radar during several calving events that occurred at jakobshavn isbrae greenland during calving events 1 the kinetic energy of the ice melange is 2 orders of magnitude smaller than the total energy released during the events 2 a jamming front propagates through the ice melange at a rate that is an order of magnitude faster than the motion of individual icebergs 3 the ice melange undergoes initial compaction followed by slow relaxation and extension and 4 motion of the ice melange gradually decays before coming to an abrupt halt these observations indicate that the ice melange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence we therefore suspect that local jamming influences longer timescale ice melange dynamics and stress transmission

true polar wander on europa from global scale small circle depressions the tectonic patterns and stress history of europa are exceedingly complex and many large scale features remain unexplained true polar wander involving reorientation of europa s floating outer ice shell about the tidal axis with jupiter has been proposed as a possible explanation for some of the features this mechanism is possible if the icy shell is latitudinally variable in thickness and decoupled from the rocky interior it would impose high stress levels on the shell leading to predictable fracture patterns no satisfactory match to global scale features has hitherto been found for polar wander stress patterns here we describe broad arcuate troughs and depressions on europa that do not fit other proposed stress mechanisms in their current position using imaging from three spacecraft we have mapped two global scale organized concentric antipodal sets of arcuate troughs up to hundreds of kilometres long and 300 m to approximately 1 5 km deep an excellent match to these features is found with stresses caused by an episode of approximately 80 degrees true polar wander these depressions also appear to be geographically related to other large scale bright and dark lineaments suggesting that many of europa s tectonic patterns may also be related to true polar wander

localized plumes drive front wide ocean melting of a greenlandic tidewater glacier support was provided by the national science foundation nsf through plr 1418256 and plr 1744835 and through woods hole oceanographic institution whoi ocean and climate change institute occi and the clark foundation this work was also supported by a uk natural environmental research council nerc phd studentship ne l501566 1 and scottish alliance for geoscience environment society sages early career research exchange funding to d a s

tectonic and volcanic evolution of dark terrain and its implications for the internal structure and evolution of ganymede furrows in ancient dark terrain on ganymede form three systems that are each hemispheric or greater in scale the oldest of the systems designated system iii is dominated by approximately concentric troughs centered on about 60 n 50 w system i in the anti jovian hemisphere contains concentric and subradial furrows arrayed around a large degraded palimpsest centered at 15 s 165 w furrows in each system formed on and locally are buried by dark volcanic materials that embay and infill preexisting topographic features they crosscut extremely few well preserved older craters and they occur on surfaces having significantly different relative crater ages system ii also in the anti jovian hemisphere contains widely spaced radially arrayed furrows commonly 500 2000 km in length which are organized around a large area of dark smooth resurfacing material intense dark terrain fracturing and some of the globally oldest light material the total thickness of dark terrain resurfacing estimated using the stratigraphy of different dark material deposits and the rim heights of the largest crater size class whose density was depleted by each deposit is probably in the range of 3 8 km multiple models of the origin of each furrow system were tested using observed geologic features and patterns systems i and iii were found to be most consistent with reactivation of impact generated multiringed structures by endogenic global extension during a period of widespread dark material volcanism that obliterated a dense ancient crater population system ii was found to be most consistent with fracturing of a single circular isostatic uplift covering an entire hemisphere on the basis of geologic observations and interpretations and theoretical models of convection in spheres it is hypothesized that the uplift developed by long term warming of the upwelling current of a single axisymmetric convection cell in an initially cooler undifferentiated interior such wanning would also have created global expansion and supplied the tensional stress inferred to have formed systems i and iii this hypothesis is supported by the concentration around the center of system ii of intense fracturing and relatively young dark volcanic deposits suggestive of high lithospheric heat flow lithospheric thinning and stress concentration an observed long term decrease in the width of extensional tectonic features interpreted to be of endogenic origin is also consistent with lithospheric thinning due to warming of a cool interior

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

cometary nuclei and tidal disruption the geologic record of crater chains on callisto and ganymede prominent crater chains on ganymede and callisto are most likely the impact scars of comets tidally disrupted by jupiter and are not secondary crater chains we have examined the morphology of these chains in detail in order to place constraints on the properties of the comets that formed them and the disruption process in these chains intercrater spacing varies by no more than a factor of 2 and the craters within a given chain show almost no deviation from linearity although the chains themselves are on gently curved small circles all of these crater chains occur on or very near the jupiter facing hemisphere for a given chain the estimated masses of the fragments that formed each crater vary by no more than an order of magnitude the mean fragment masses for all the chains vary by over four orders of magnitude w b mckinnon and p m schenk 1995 geophys res lett 13 1829 1832 however the mass of the parent comet for each crater chain is not correlated with the number of fragments produced during disruption but is correlated with the mean mass of the fragments produced in a given disruption event also the larger fragments are located near the center of each chain all of these characteristics are consistent with those predicted by disruption simulations based on the rubble pile cometary nucleus model in which nuclei are composed on numerous small fragments weakly bound by self gravity and with those observed in comet d shoemaker levy 9 similar crater chains have not been found on the other icy satellites but the impact record of disrupted comets on callisto and ganymede indicates that disruption events occur within the jupiter system roughly once every 200 to 400 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

early thermal profiles and lithospheric strength of ganymede from extensional tectonic features abstract estimates of the brittle lithosphere thickness derived from the width and spacing of extensional tectonic features coupled with lithospheric strength envelopes brittle and ductile yield stress versus depth appropriate for ice allow the quantitative determination of early thermal profiles and lithospheric strength and stability on ganymede furrows and grooves indicate brittle lithospheric thicknesses of 5 10 and 2 5 km respectively assuming that their spacing is controlled by an extensional instability or that their width is controlled by the intersection depth of their bounding faults plots of the brittle and ductile yield stress versus depth for the icy lithosphere of ganymede show a linear increase in brittle strength with depth to a maximum at the brittle ductile transition followed by an exponential decrease in ductile yield stress with depth because the depth to the brittle ductile transition depends primarily on the thermal gradient the thickness of the brittle lithosphere can be used to calculate early thermal profiles of 1 5 6 and 4 5 20 km during the formation of the furrows and grooves respectively lithospheric strength the integral of the yield stress versus depth curve varied from 30 125 gpa m when the furrows formed to 5 30 gpa m when the grooves formed which correspond to maximum yield stresses of 6 11 and 2 5 6 mpa respectively these results indicate that the thermal gradient and lithospheric strength varied laterally by as much as a factor of 5 and that ganymede cooled in a highly inhomogeneous manner with significant lateral thermal anomalies finally this analysis provides a straightforward explanation for the stability of large remnants of cratered terrain such as galileo regio that had a low thermal gradient and strong lithosphere in contrast to small remnants of cratered terrain that were fractured and broken up by grooved terrain as a result of higher thermal gradients and weaker lithospheres

viscous relaxation as a prerequisite for tectonic resurfacing on ganymede insights from numerical models of lithospheric extension abstract ganymede s bright terrain formed during a near global resurfacing event or events that produced both heavily tectonized and relatively smooth terrains the mechanism s by which resurfacing occurred on ganymede e g cryovolcanic or tectonic and the relationship between the older dark and the younger bright terrain are fundamental to understanding the geological evolution of the satellite using a two dimensional numerical model of lithospheric extension that has previously been used to successfully simulate surface deformation consistent with grooved terrain morphologies we investigate whether large amplitude preexisting topography can be resurfaced erased by extension i e tectonic resurfacing using synthetically produced initial topography we show that when the total relief of the initial topography is larger than 25 50 m periodic groove like structures fail to form instead extension is localized in a few individual isolated troughs these results pose a challenge to the tectonic resurfacing hypothesis we further investigate the effects of preexisting topography by performing suites of simulations initialized with topography derived from digital terrain models of ganymede s surface these include dark terrain fresh relatively deep impact craters smooth bright terrain and a viscously relaxed impact crater the simulations using dark terrain and fresh impact craters are consistent with our simulations using synthetic topography periodic groove like deformation fails to form in contrast when simulations were initialized with bright smooth terrain topography groove like deformation results from a wide variety of heat flow and surface temperature conditions similarly when a viscously relaxed impact crater was used groove like structures were able to form during extension these results suggest that tectonic resurfacing may require that the amplitude of the initial topography be reduced before extension begins we emphasize that viscous relaxation may be the key to enabling tectonic resurfacing as the heat fluxes associated with groove terrain formation are also capable of reducing crater topography through viscous relaxation for long wavelength topography large craters viscous relaxation is unavoidable we propose that the resurfacing of ganymede occurred through a combination of viscous relaxation tectonic resurfacing cryovolcanism and at least in a few cases band formation variations in heat flow and strain magnitudes across ganymede likely produced the complex variety of terrain types currently observed

postglacial relative sea level history of the prince rupert area british columbia canada this paper presents a history of relative sea level rsl change for the last 15 000 years in the prince rupert region on the northern coast of british columbia canada one hundred twenty three radiocarbon ages of organic material from isolation basin cores sediment sequence exposures and archaeological sites having a recognized relation to past sea levels constrain postglacial rsl the large number of new measurements relating to past sea level provides a well constrained rsl curve that differs in significant ways from previously published results after deglaciation following the last glacial maximum the region experienced an isostatically induced rapid rsl drop from as much 50 m asl to as low as 6 3 m asl in as little as a few centuries between 14 500 bp and 13 500 bp after a lowstand below current sea level for about 2000 years during the terminal pleistocene rsl rose again to a highstand at least 6 m asl after the end of the younger dryas rsl slowly dropped through the holocene to close to its current position by 2000 1500 bp with some potential fluctuations between 3500 and 1500 bp this study highlights variation in rsl histories across relatively short distances which must be accounted for by local rsl reconstructions such as this one this rsl curve aided in the identification of an 8000 9000 year old archaeological site on a 10 12 m asl terrace which is currently the earliest dated archaeological site in the area and it provides guidance for searching for even older archaeological remains we highlight the utility and potential of this refined rsl history for developing surveys for other archaeological sites associated with paleoshorelines

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

a global system of furrows on ganymede indicative of their creation in a single impact event abstract furrows are a concentric system of tectonic troughs and are the oldest recognizable surface feature on ganymede we analyzed the distribution of furrows utilizing voyager and galileo images and found that furrows over ganymede s surface are part of a global concentric circular structure if this multi ring structure is impact origin this is the largest impact structure identified so far in the solar system deviations of the shapes of the furrows from the concentricity are small everywhere which implies that the relative location of the blocks of the dark terrains over the entire surface of ganymede has not changed appreciably even during formation of the bright terrains the estimate of the impactor size is difficult but an 150 km radius impactor is consistent with the observed properties of furrows the furrow forming impact should have significant effects on the satellite s geological and internal evolution which are expected to be confirmed by future explorations of jupiter s icy moons such as the juice jupiter icy moon explorer or europa clipper mission

a simple mechanical model of valhalla basin callisto the valhalla basin on callisto is a multiringed structu e that extends over much of the satellite s surface although its appearance differs in detail from lunar multiringed basins its origin may also be attributed to lithospheric fragmentation accompanying collapse of the transient crater formed by an impact event this paper explores the mechanics of the collapse process by treating the lithosphere as a thin elastic von mises plastic sheet plane geometry or shell spherical geometry flow of the underlying asthenosphere inward toward the crater cavity induces plastic failure of the lithosphere and produces a characteristic pattern of faults in the disrupted lithosphere the pattern and extent of faulting is a function of a single dimensionless parameter that involves the strength and thickness of the lithosphere the crater depth and diameter and the surface gravity of the planet the tectonic structures of valhalla correspond well with the failure pattern expected for a large crater produced in a thin circa 30 km weak strength 100 bar lithospheric shell

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

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

derived bedrock elevations strain rates and stresses from measured surface elevations and velocities jakobshavns isbrae greenland jakobshavns isbrae 69 10 n 49 59 w drains about 6 5 of the greenland ice sheet and is the fastest ice stream known the jakobshavns isbrae basin of about 10 000 km 2 was mapped photogrammetrically from four sets of aerial photography two taken in july 1985 and two in july 1986 positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation and surface velocity vectors were computed from the positions the two flights in 1985 yielded the best results and provided most common points 716 for velocity determinations and are therefore used in the modeling studies the data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation the field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses using the flow law of ice for rectilinear coordinates x y pointing eastward and northward and curvilinear coordinates l t pointing longitudinally and transversely to the changing ice flow direction ice surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding our calculated ice thicknesses are in fair agreement with an ice thickness map based on seismic sounding and supplied to us by k echelmeyer ice thicknesses were subtracted from measured ice surface elevations to map bed topography our calculation shows that basal sliding is significant only in the 10 15 km before jakobshavns isbrae becomes afloat in jakobshavns isfjord

geology and mapping of dark terrain on ganymede and implications for grooved terrain formation geological mapping of regional and high resolution galileo images reveals a variety of units and structures within ganymede s dark terrain we have made a detailed study of areas within galileo marius and nicholson regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain dark terrain appears to become fractured as a precursor to grooved terrain formation in places dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths tectonic deformation may be but is not always focused through preexisting weaknesses caused by impact craters and furrows a prominent groove lane anshar sulcus is inferred to have formed by the process of hanging wall rollover accompanied by a small amount of right lateral horizontal shear offset resulting from ne sw extension

possible breakup of dark terrain on ganymede by large scale shear faulting a system of arcuate furrows in galileo regio and marius regio two large blocks of ancient dark terrain has previously been proposed to be the remnant of an originally more concentric furrow set that was disrupted by shear estimates of the two regions poles of furrow concentricity indicate a considerable westward offset of the galileo regio pole these measurements suggest the possibility of about 500 km of left lateral offset of the two areas four additional independent structural indicators support this hypothesis there is evidence that shear deformation was concentrated along a major structural lineament which closely follows a small circle about 45 of arc of radius which encloses galileo regio and that a smaller amount of distributed shear occurred across an adjacent 500 to 1500 km wide band there is also morphologic evidence for zones of minor right lateral shear between central and southern marius regio and between barnard and nicholson regio and for a zone of minor left lateral shear in nun sulci stratigraphic relations indicate that any major shear offsets occurred before and during the earliest stages of grooved terrain formation before most grooves had formed and before virtually all light material had been emplaced however regionally dominant groove orientations are generally consistent with orientations expected for transtensional features either shear strain or the shear s driving mechanism may have had a long term effect on patterns of deformation of ganymede s lithosphere

the tectonics of ganymede the surface of ganymede the largest icy jovian satellite is composed of two distinct terrain types showing abundant evidence of global scale tectonics there is geological evidence consistent with the formation of younger bright terrain by the emplacement of relatively clean ice into rift zones developed in the more silicate rich dark terrain

high resolution dem generation from spaceborne and terrestrial remote sensing data for improved volcano hazard assessment a case study at nevado del ruiz colombia abstract volcanoes with rugged terrain remain a challenging target for generating high resolution digital elevation models dems especially in tropical areas with frequent cloud cover using nevado del ruiz volcano as an example we combined dems from the tandem x tdx satellite mission terrestrial radar interferometry tri and structure from motion sfm to generate a new dem with 10 m spatial resolution this is the first study combining satellite radar ground based radar photography and freely available global dems to generate a high resolution dem without data gaps tdx data from ascending and descending orbits were combined to generate the base dem instead of using a raster format to fuse dems generated from different data sets with different resolutions we developed a methodology based on 3 d point clouds 1 re georeference the 5 m tri and 1 m sfm dems to the 10 m tdx dem using the iterative closest point icp algorithm to minimize the horizontal and vertical discrepancy between dems then 2 merge the multiple point clouds to generate a final dem without data gaps using an adaptive algorithm that uses two search distances to smooth the transition at the edges of different data sets we assess the new 10 m dem by comparing simulated inundation zones obtained with two volcano flow models laharz for lahars and volcflow for pyroclastic flows and find significant differences with respect to the 30 m srtm dem our laharz simulation over the new dem shows a longer lahar run out distance for pyroclastic flows the volcflow simulation over the new dem produces highly channelized flows over the steep portions of a river channel and gives a larger extent of thicker deposits compared to those obtained with the 30 m srtm dem quantitative and qualitative geomorphic analysis suggests that up to date dems with high spatial resolution 10 m or even better need to be generated to improve volcano hazard assessment for active volcanoes

ring geometry on ganymede and callisto abstract arguments have been made based on geometry for both an impact and an internal origin for the ancient partially preserved major furrow system of ganymede zuber and parmentier concluded that furrows are not concentric but could be impact related if multiringed structures on icy satellites are initially noncircular we examine the geometry of the valhalla ring structure on callisto in order to assess the circularity of an unmodified ring system despite prominent local meandering the only gross deviations from concentricity in the valhalla system are found in the outer northeast quadrant of the system here a number of ring segments intersect small circles about the center at angles up to 30 the ganymede furrow system was remapped to make use of improvements in coordinate control the least squares center of curvature determined using natural weighting for all furrows in marius and galileo regio is 20 7 1 1 s 179 2 1 1 w 2 uncertainty furrows in marius and galileo regio are reasonably concentric are much more circular than previously estimated and probably once covered at least an entire hemisphere of ganymede in addition at least three other multiring systems of varying size are identified on ganymede indicative of a projectile population thus we find furrow geometry and occurrence are consistent with an impact origin deviations of some furrows from concentricity about the center of curvature on the scale of those found at valhalla do exist as in the case of valhalla these variations are principally confined to outer regions of the structure and are interpreted as inherent properties of multiringed structures on icy satellites the cause s of this may be in the ring formation mechanism itself but are more likely due to variations in preexisting lithospheric mechanical properties the perceived present nonalignment of the assumed originally concentric furrows has been used to argue for large scale lateral motion of dark terrain blocks in ganymede s crust presumably in association with bright terrain formation the overall alignment of furrows as well as the inherent scatter in centers of curvature for subregions of galileo and marius regio do not support this hypothesis

applications of land based high resolution seismic reflection analysis to quaternary and geomorphic research abstract the availability in the last few years of low cost microcomputers and multichannel seismographs has made possible the application of high resolution digital seismic reflection profiling to quaternary problems seismic analysis of the subsurface can provide continuous two dimensional sections containing detailed structural and stratigraphic information a review of the general principles of shallow seismic reflection methods includes the nature of seismic sources the use of shotgun shells the application of the optimum offset method data processing and the issue of site dependency a brief overview is provided of seismic and sequence stratigraphy four case studies illustrate the application of reflection seismic profiling 1 champlain sea sediments late wisconsinan on the flank of the gatineau hills near quyon quebec 2 the definition of bedrock valleys beneath pleistocene deposits near shawville quebec 3 the fill of a linear depositional basin okanagan valley british columbia 4 the seismic signatures of topset and foreset beds slope failures and a distributary channel of the fraser river delta british columbia

ground penetrating radar of northern lacustrine deltas ground penetrating radar gpr was used in several selected deltaic sedimentary environments to better understand subsurface stratigraphy and reconstruct former depositional environments the profiles provide high resolution continuous subsurface data on facies thickness and depths orientation of major sedimentary structures postdepositional failure planes and depth of peat deposits field experiments were carried out on six river deltas records from four of the deltas exhibit sedimentary facies a record from one delta shows a possible slump and records from another delta reveal the thickness and stratigraphic relationships of peat deposits the delta types are i sandy wave influenced ii sandy immature wave influenced steeper middle and lower shoreface iii sandy braided and iv gravelly fan foreset in areas of limited subsurface control stratigraphic logs from drill core cutbank exposure or geophysical logs radar profiles can provide big picture perspectives of the subsurface

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

galileo s first images of jupiter and the galilean satellites the first images of jupiter io europa and ganymede from the galileo spacecraft reveal new information about jupiter s great red spot grs and the surfaces of the galilean satellites features similar to clusters of thunderstorms were found in the grs nearby wave structures suggest that the grs may be a shallow atmospheric feature changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on io patchy emissions were seen while io was in eclipse by jupiter the outer margins of prominent linear markings triple bands on europa are diffuse suggesting that material has been vented from fractures numerous small circular craters indicate localized areas of relatively old surface pervasive brittle deformation of an ice layer appears to have formed grooves on ganymede dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution

sea level related architectural trends in coarse grained delta complexes distinct stratigraphical styles of delta development can be related to basically different patterns of relative sea level base level change 1 an overall rise with periods of standstill and without significant fall rs 2 an overall rise with superimposed falls rf 3 an overall fall with superimposed rises fr and 4 an overall fall with periods of standstill and no significant rises fs the related stratigraphical models contain either preserved elements of prototype deltas or show characteristic trends in development from one prototype delta into another each stratigraphical model is characterized by lateral and vertical lithofacies features which can be reasonably well established from outcrop in coarse clastic basin margins where age control is poor sedimentological and stratigraphical delta models as presented here may help refining reconstructions of basin fill patterns and basin subsidence kinematics

nass river on the move radar facies analysis of glaciofluvial sedimentation and its response to sea level change in northwestern british columbia the nass valley of northwestern british columbia is a glacial fiord containing extensive glaciomarine and glaciofluvial sediments two parallel braidplains separated by a bedrock ridge were deposited within the fiord mapping of these deposits led to the hypothesis that the braidplains must have terminated at deltas however a lack of surface exposures meant that ground penetrating radar was needed to investigate these deposits radar facies analysis aided in the identification of braidplain braid delta and glaciomarine depositional environments as well as underlying bedrock several deltas graded to different sea levels were discovered allowing inferences to be made about the relationship of falling sea level to sediment architecture the upper section of the western braidplain is graded to a sea level of 185 m above sea level asl indicating that the proto nass river flowed on the western side of the bedrock ridge when the sea was at that level however the river moved to the east side of the ridge as sea level fell depositing the extensive aiyansh braidplain braid delta which is graded to a 152 m sea level stand several other deltas also formed at this sea level stand avulsion occurred and the river flowed on the west side of the ridge again when sea level fell to 134 m asl the river remained in this position throughout late glacial time and eventually evolved into the modern nass river the coarse grained deposits are indicative of forced regression with both stepped top attached and detached stratal architecture present

fan deltas and braid deltas varieties of coarse grained deltas two types of coarse grained deltas are recognized fan deltas and braid deltas fan deltas are gravel rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland they occupy a space between the highland usually a fault bounded margin and the standing body of water in contrast braid deltas here introduced are gravel rich deltas that form where a braided fluvial system progrades into a standing body of water braid deltas have no necessary relationship with alluvial fans as exemplified by fluvioglacial braid deltas braid deltas have previously been classified as fan deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different braid deltas are a common present day geomorphic feature and are abundant in the geological record fan deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems the shoreline and subaqueous components of both are similar fan delta sequences have a subaerial component that is an alluvial fan facies comprising interbedded sheetflood debris flow and braided channel deposits fan deltas produce small a few tens of square kilometres wedge shaped bodies of sediment commonly displaying high variability in paleocurrent patterns and abrupt changes in facies the deposits are generally very coarse grained with large out sized clasts very poorly sorted matrix rich polymictic heterolithic partially cemented by penecontemporaneous carbonate and have low porosity and permeability braid deltas in contrast have a subaerial component consisting entirely of braided river or braidplain facies their deposits display better sorting roundness and clast orientation than do fan delta sediments they lack a muddy matrix they display size grading and bar migration they commonly have a sheet geometry with high lateral continuity tens to hundreds of square kilometres and they exhibit moderate to high porosity and permeability valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse grained deltaic systems are not differentiated

tidally driven ice speed variation at helheim glacier greenland observed with terrestrial radar interferometry we used a terrestrial radar interferometer tri at helheim glacier greenland in august 2013 to study the effects of tidal forcing on the terminal zone of this tidewater glacier during our study period the glacier velocity was up to 25 m d 1 our measurements show that the glacier moves out of phase with the semi diurnal tides and the densely packed melange in the fjord here detrended glacier displacement lags behind the forecasted tidal height by 8 hours the transition in phase lag between the glacier and the melange happens within a narrow 500 m zone in the fjord in front of the ice cliff the tri data also suggest that the impact of tidal forcing decreases rapidly up glacier of the terminus a flowline model suggests this pattern of velocity perturbation is consistent with weak ice flowing over a weakly nonlinear bed

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

calving laws sliding laws and the stability of tidewater glaciers a new calving criterion is introduced which predicts calving where the depth of surface crevasses equals ice height above sea level crevasse depth is calculated from strain rates and terminus position and calving rate are therefore functions of ice velocity strain rate ice thickness and water depth we couple the calving criterion with three sliding laws in which velocity is controlled by 1 basal drag 2 lateral drag and 3 a combination of the two in model 1 velocities and strain rates are dependent on effective pressure and hence ice thickness relative to water depth imposed thinning can lead to acceleration and terminus retreat and ice shelves cannot form in model 2 ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient velocities are strongly dependent on channel width and calving margins tend to stabilize at flow unit widenings model 3 exhibits the combined characteristics of the other two models and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow but stable if most resistance is from lateral drag ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale in combination the new calving criterion and the basal lateral drag sliding function model 3 can be used to simulate much of the observed spectrum of behaviour of calving glaciers and present new opportunities to model ice sheet response to climate change

the melt anomaly of 2002 on the greenland ice sheet from active and passive microwave satellite observations 1 active and passive microwave satellite data are used to map snowmelt extent and duration on the greenland ice sheet the passive microwave pm data reveal the extreme melt extent of 690 000 km2 in 2002 as compared with an average extent of 455 000 km2 from 1979 2003 a statistical analysis of the melt time series affirms an increased melt extent earlier in the melt season the quikscat qscat or am analysis for 1999 to 2004 confirms the extreme melt situation in 2002 qscat data show a significant increase in melt season length over several areas in 2003 ssm i and qscat melt data reveal that the later detects melt earlier and is more sensitive to surface melt qscat detects an earlier stage of melt and ssm i detects a later stage of melt the ssm i xgpr melt extent is approximately confined to the qscat melt areas experiencing 2 weeks or more of melting time

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

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

topographically independent ice flow in northwestern british columbia implications for cordilleran ice sheet reconstruction studies in southern british columbia have shown that cordilleran ice sheet flow was controlled by topograph even in full glacial time new ice flow evidence from the nass river region northern british columbia however indicates that ice was thicker there and that the continental ice sheet phase of glaciation was reached inspection of high elevation sites has revealed a suite of ice flow indicators mainly striae undetected by earlier work these suggest that at the last glacial maximum fraser glaciation ice flowed southwestward across the nass river region from an ice divide that probably was located in the skeena mountain area comparisons with adjacent work allow this divide to be mapped over a wide area the results suggest that maximum ice thicknesses in the northern part of the cordilleran ice sheet were larger than reported previously the location of storm tracks in full glacial time may have played an important role in the production of an ice sheet that was thicker in northern british columbia than it was in the southern half of the province during deglaciation ice thinned and gradually became confined to fiords and valleys resulting in numerous and variable ice flow directions at that time topographic control was thus exerted on ice flow only after the glacial maximum was reached despite the significant amount of relief in this region copyright 2002 john wiley sons ltd

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

brine volcanism and the interior structures of asteroids and icy satellites abstract cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites volcanic brine petrogenesis in ammonia deficient satellites should mainly involve eutectic melting in relevant salt water systems carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids c1 chondrites contain about one fifth by mass of salts in various states of hydration many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system h2o mgso4 na2so4 minor components include sulfates of k ni mn and ca chondrites also contain abundant carbonates but these are probably not very important in brine magmatism due to their low solubilities under expected conditions chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of cl s soluble salts may have profound effects on the geology and structure of icy satellites and asteroids in some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate rich mantle and crustal layers in other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities europa probably has a layered crust composed of anhydrous mg na sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated mg and na sulfates near the surface ganymede may have a crust about 300 km thick composed of a 10 1 ratio of ice mirabilite and a mantle 500 km thick composed of 50 ice phases plus 50 hydrated mg and na sulfates

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

crater densities and crater ages of different terrain types on ganymede abstract crater densities and size frequency distributions on ganymede were measured for separate areas of dark furrowed and dark smooth terrains reticulate terrain complex grooved terrain and groups of groved polygons and groove lanes having a variety of relative ages these measurements were used first together with results of previous studies to test alternate models of heliocentric and planetocentric crater forming bombardments of ganymede and callisto the spatial distribution of crater production functions on ganymede is consistent with a single impactor population having dominated cratering of that satellite s observed surface in addition relative crater ages of different ganymedean material deposits are better reconciled with the deposits stratigraphic relations if a heliocentric impactor population is assumed these results are consistent with results of previous studies interpreted here to indicate that the spatial distributions of most classes of craters on ganymede and younger craters on callisto are more consistent with heliocentric than with planetocentric bombardment the measurements were then used to infer the sequence and style of surface modification of ganymede bounds on absolute ages depend on the assumption of heliocentric bombardment dark terrain was emplaced during a prolonged period of volcanic resurfacing of older heavily cratered terrain probably at least 10 8 yrs in duration and at least 3 8 4 0 gyr ago furrows generally formed on blanket like deposits of dark material typically hundreds of meters or more in thickness soon after their emplacement furrow formation continued globally throughout the period of dark material deposition next dark terrain was deformed locally by reticulate terrain formation light terrain emplacement subsequently began at least regionally within complex grooved terrain it continued for an extended period perhaps several times 10 8 yrs in grooved lanes grooved polygons andsmooth terrain where a light material layer averaging several hundred meters or more in thickness was emplaced

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 geology of callisto the geology of callisto is not boring although cratered terrain dominates callisto a key end member of the jovian satellite system a number of more interesting features are apparent cratered terrain is broken into irregular mappable bright and dark subunits that vary in albedo by a factor of 2 and several relatively smooth units are depleted of small craters some of these areas may have been volcanically resurfaced lineaments including parallel and radial sets may be evidence for early global tectonism frost deposition occurs in cold traps and impact scars have formed from tidally disrupted comets geologic evidence suggests that callisto does have a chemically differentiated crust central pit and central dome craters and palimpsests are common the preferred interpretation is that a relatively ice rich material at depths of 5 km or more has been mobilized during impact and exposed as domes or palimpsests the close similarity in crater morphologies and dimensions indicates that the outermost 10 km or so of callisto may be as differentiated as on ganymede the geology of cratered terrain on callisto is simpler than that of cratered terrain on ganymede however orbital evolution and tidal heating may provide the answer to the riddle of why callisto and ganymede are so different malhotra 1991 we should expect a few surprises and begin to answer some fundamental questions when callisto is observed by galileo in late 1996