variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

decadal scale variations in ice flow along whillans ice stream and its tributaries west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

on thickening ice the large ice sheets in greenland and antarctica play an important role in global climate much attention has focused on the west antarctic ice sheet in his perspective alley 1 highlights the report by joughin et al 2 who show that in the region of the west antarctic drainage into the ross sea on average the ice sheet is thickening slowly earlier studies indicated a net thinning in this region the results may indicate the stabilization or readvance of the ice sheet but the system is complex and existing data are insufficient to identify a trend 1 http www sciencemag org cgi content full 295 5554 451 2 http www sciencemag org cgi content short 295 5554 476

tidally driven stick slip motion in the mouth of whillans ice stream antarctica we show that the ice plain in the mouth of whillans ice stream formerly ice stream b antarctica moves by stick slip motion during a spring tide period rapid motions regularly occur near high tide and during falling tide this correlation is weaker during a neap tide period when the tidal magnitudes are less precise timing of these motion events suggests that they propagate through the region with a mean velocity of 88 ms 1 we hypothesize that this speed is associated with the propagation of shear waves through a wet subglacial till motion events are also seen on more smoothly flowing floating ice event delays are very short between grounded and floating stations suggesting the events propagate through the ice shelf as an elastic wave we further hypothesize the events are caused by the interaction of a sticky bed the accumulation of stored elastic strain through the compression of ice by upstream inflow and tidal forcing motion events seem to be triggered either by reduction of vertical normal stresses at high tide or by the increase of shear stresses from sub shelf ocean currents during falling tide event magnitudes are not related to the length of the preceding quiescent period suggesting significant viscous dissipation within the till

unusual locomotion behaviour preserved within a crocodyliform trackway from the upper cretaceous bayanshiree formation of mongolia and its palaeobiological implications abstract crocodyliform tracks are reported from the upper cretaceous cenomanian santonian bayanshiree formation in southeastern mongolia ten tracks are preserved as natural casts forming a trackway with a quadrupedal gait pattern with a tail trail all tracks are short and wide and dominated by toe traces without plantar impressions pes tracks are characterized by four deep claw impressions and push back marks behind them manus tracks have shallow claw marks and long sub parallel scratch marks behind the preferential association of the scratch marks with only the distal digit impressions and irregular pattern of footfalls suggests that this trackway was made by a bottom walking and punting crocodyliform under water this trackway represents the first crocodyliform swim tracks in the late cretaceous of asia and the first evidence for punting behaviour of a fossil crocodyliform the swim tracks can be divided into two categories such as bottom walking tracks with punting for moving somewhat more quickly and subaqueous walking tracks without punting to be associated with slower underwater speeds the tracks show that crocodylians had adopted a bottom walking behaviour similar to extant crocodylians by cretaceous times

pleistocene collapse of the west antarctic ice sheet some glacial sediment samples recovered from beneath the west antarctic ice sheet at ice stream b contain quaternary diatoms and up to 108 atoms of beryllium 10 per gram other samples contain no quaternary diatoms and only background levels of beryllium 10 less than 10 6 atoms per gram the occurrence of young diatoms and high concentrations of beryllium 10 beneath grounded ice indicates that the ross embayment was an open marine environment after a late pleistocene collapse of the marine ice sheet

increased water storage at ice stream onsets a critical mechanism the interdependence of rapid ice flow surface topography and the spatial distribution of subglacial water are examined by linking existing theories the motivation is to investigate whether the acceleration of an ice stream tributary contains a positive feedback that encourages the retention of subglacial water that leads to faster flow periodically varying surface and bed topographies are related through a linear ice flow perturbation theory for various values of mean surface slope perturbation amplitude and basal sliding speeds the topographic variations lead to a periodic variation in hydraulic potential that is used to infer the tendency for subglacial water to be retained in local hydraulic potential minima if water retention leads to enhanced basal sliding a positive feedback loop is closed that could explain the transition from slower tributary flow to faster streaming flow and the sustained downstream acceleration along the tributary ice stream system a sensitivity study illustrates that the same range of topographic wavelengths most effectively transmitted from the bed to the surface also strongly influences the behavior of subglacial water a lubrication index is defined to qualitatively measure the heterogeneity of the subglacial hydrologic system application of this index to field data shows that the transition from tributary to ice stream closely agrees with the location where subglacial water may be first stored

new pterosaur tracks pteraichnidae from the late cretaceous uhangri formation southwestern korea numerous footprints of dinosaurs pterosaurs and birds together with arthropod tracks have been discovered in the upper cretaceous uhangri formation which crops out along the south western coastline of south korea this ichnofauna contains the first pterosaur tracks reported from asia the digitigrade tridactyl manus impressions exhibit features of a typical pterosaur hand print the pes impressions however show features that are different from pterosaur footprints reported pre viously there is no visible trace of impressions of individual digits and the toes are triangular or rounded in shape distally without distinct claw impressions as these features clearly distinguish the uhangri tracks from pteraichnus and purbeckopus we assign them to a new genus haenamichnus which accommodates the new ichnospecies haenamichnus uhangriensis the prints are five to six times larger than those of pteraichnus and are currently the largest pterosaur ichnites known they show virtually no trace of the 5th phalange of the pes indicating that they were made by pterodactyloids moreover features of the tracks suggest that they can be attributed to azhdarchids the commonest pterosaur of the late cretaceous the longest pterosaur trackway yet known from any track site length 7 3 m and consisting of 14 pairs of foot impressions was also found in the uhangri formation and suggests that azhdarchids at least were competent terrestrial locomotors the fossil track site at uhangri represents the first occurrence of the tracks of pterosaurs dinosaurs and web footed birds all on the same level this demonstrates that pterosaurs and birds visited the same habitat but the large size disparity suggests that they occupied different ecological niches

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

ice velocity mapping of ross ice shelf antarctica by matching surface undulations measured by icesat laser altimetry abstract we present a novel method for estimating the surface horizontal velocity on ice shelves using laser altimetry data from the ice cloud and land elevation satellite icesat 2003 2009 the method matches undulations measured at crossover points between successive campaigns elevation measurements are first relocated into a time varying moving coordinate system using an initial velocity e g from velmap and then crossover height differences are minimized with an adjustment vector errors in geolocation of the icesat tracks result in some error in the adjustment vectors but these are small relative to the velocity adjustment for fast moving ice shelves we use the algorithm to estimate changes in the ice velocity of ross ice shelf between an earlier mapping from velmap and the icesat period the new velocity field is compared with velocities from in situ measurements and satellite radar interferometry the slowdown of 98 34 m yr 1 23 is observed in the ice shelf downstream of whillans ice stream and the deceleration rate is 3 1 1 1 m yr 2 during last three decades the method can be expanded to the simultaneous mapping of ice horizontal velocity ice thickness change and surface deformation for antarctic ice shelves as well as a more accurate mapping using future icesat 2 measurements

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

multispectral imaging contributions to global land ice measurements from space global land ice measurements from space glims is an international consortium established to acquire satellite images of the world s glaciers analyse them for glacier extent and changes and assess change data for causes and implications for people and the environment although glims is making use of multiple remote sensing systems aster advanced spaceborne thermal emission and reflection radiometer is optimized for many needed observations including mapping of glacier boundaries and material facies and tracking of surface dynamics such as flow vector fields and supraglacial lake development software development by glims is geared toward mapping clean ice and debris covered glaciers terrain classification emphasizing snow ice water and admixtures of ice with rock debris multitemporal change analysis visualization of images and derived data and interpretation and archiving of derived data a global glacier database has been designed at the national snow and ice data center nsidc boulder colorado parameters are compatible with and expanded from those of the world glacier inventory wgi these technology efforts are summarized here but will be presented in detail elsewhere our presentation here pertains to one broad question how can aster and other satellite multispectral data be used to map monitor and characterize the state and dynamics of glaciers and to understand their responses to 20th and 21st century climate change our sampled results are not yet glaciologically or climatically representative our early results while indicating complexity are generally consistent with the glaciology community s conclusion that climate change is spurring glacier responses around the world mainly retreat whether individual glaciers are advancing or retreating the aggregate average of glacier change must be climatic in origin as nonclimatic variations average out we have discerned regional spatial patterns in glaciological response behavior these patterns are best attributed to climate change variability and to regional differences in glacier size and response times in many cases glacier length changes under represent the magnitude of glacier ablation because thinning sometimes without immediate length changes is also important an expanded systematic uniform analysis of many more glaciers is needed to isolate the glacier response components due to climatic and nonclimatic perturbations to produce quantitative measures of regional variation in glacier changes and to predict future regional glacier trends relevant to water resources glaciological hazards and global sea level this comprehensive assessment to be completed in stages is expected to lend a critically needed filter to identify successful climate models that explain recent glacier changes and change patterns and hence are apt to describe future changes and to eliminate unsuccessful models

changes in the west antarctic ice sheet since 1963 from declassified satellite photography comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the ross ice shelf ice stream b has widened by 4 kilometers at a rate much faster than suggested by models and has decreased in speed by 50 percent the ice ridge between ice streams b and c has eroded 14 kilometers these changes along with changes in the crevassing around crary ice rise imply that this region s velocity field shifted during this century

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

sedimentological record of subglacial conditions and ice sheet dynamics of the vistula ice stream north central poland during the last glaciation abstract deposits of the vistula ice stream draining the scandinavian ice sheet during the last glaciation were investigated at four field sites in north central poland using micro and macroscale features the study reveals several till units with specific structural textural and lithological characteristics the individual till units are either macroscopically massive or bedded and the contacts between the units are either sharp or transitional the nature of the contacts with the underlying sediments ductile deformation structures largely undeformed clayey clasts tectonic lamination thin horizontal stringers of sorted sediments ploughing marks boulder pavements and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions it is suggested that the till is a hybrid deposit generated by some combination of lodgement deformation and ploughing punctuated by periods of basal decoupling the depth of deformation at any point in time was thinner up to several decimetres than the maximum till thickness c 2 5 m the ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr 1 which suggests an unstable and highly dynamic ice lobe consistent with spatial variability of till characteristics sand wedges in the deposits beneath the till and the nature of the till bed interface indicate permafrost under the advancing ice sheet we suggest that under the increasing ice thickness a layer of thawed water saturated sediment formed on top of the still frozen ground due to inefficient drainage and contributed to ice streaming by promoting pervasive deformation and basal sliding

recent changes in the microwave scattering properties of the antarctic ice sheet time series satellite microwave data are used to monitor and quantify changes in the scattering properties of the antarctic ice sheet daily ers scatterometer escat and special sensor microwave imager ssm i image data acquired since 1992 are analyzed to understand the seasonal and interannual changes over the ice sheet for regions of the ice sheet where azimuthal modulation is negligible seasonal cycles are observed in both the escat amplitude spl sim 0 5 db and ssm i data amplitude spl sim 10 k these cycles are attributed to seasonal changes in surface temperature interannual variability in the time series signatures appears to be associated with accumulation there is also evidence to suggest that shifts in the wind direction can alter the backscatter through azimuthal modulation over the period 1992 97 large trends are observed in the escat 1 k yr sup 1 signatures over several regions in antarctica these changes typically occur over ice shelves and at the margins of the ice sheet where previous melt events have occurred and where accumulation is relatively high 300 mm yr sup 1 it is likely the large changes result from the successive burial of an efficient scattering layer formed by refreezing after a melt event prior to 1992 there is also evidence to suggest that similarly large changes can be observed in the interior of the ice sheet due to the burial of depth hear layers in order to monitor long term change in the properties of the antarctic ice sheet it is necessary to remove the seasonal cycle from time series microwave data such anomaly data can then be used to understand the link between escat and ssm i with accumulation and wind shifts

new velocity map and mass balance estimate of mertz glacier east antarctica derived from landsat sequential imagery automatic feature tracking on two landsat images acquired in january 2000 and december 2001 generates a complete velocity field of mertz glacier east antarctica this velocity field shows two main tributaries to the ice stream between the tributaries a likely obstruction feature in the bedrock results in a slow down of the flow a third landsat image acquired in 1989 and combined with the 2000 image permits the determination of the glacier mean velocity during the 1990s although some parts of the mertz glacier system show evidence of slight speed increase we conclude that the mertz flow speed is constant within our uncertainty 35 m a 1 using this complete velocity field new estimates of the ice discharge flux 17 8 km 3 a 16 4 gt a and of the basal melting of the tongue 11m a of ice are given our results lead to an apparent imbalance of the drainage basin ice discharge 3 5 km 3 a lower than the accumulation considering previous studies in the mertz glacier area we discuss the uncertainty of this imbalance and the problems with accumulation mapping for this region

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

application of vertebrate trace fossils to palaeoenvironmental analysis abstract this is a review of the main uses of vertebrate trace fossils ichnofabrics and ichnofacies in the palaeoenvironmental analysis of sedimentary sequences the article accounts for the significant developments produced in the last three decades including the application of the ichnofacies concept to vertebrate trace fossils recognition of footprints in cross sectional view and their distinction from inorganic structures and burrow fills is first discussed the response of different substrates showing contrasting water content and imprinted by different animals or devices is compared in terms of the morphology of the resultant footprint trackways with sand crescents are typical of aeolian cross strata and are absent in associated flat lying to low angle deposits thick packages of highly bioturbated sandy dune and interdune sediments have been interpreted as reflecting periods of increased rainfall neoichnological observations in modern lake basins suggest that distinct zones can be recognized in the margins of fossil ponds and lakes including onshore shoreline and shallow subaqueous zones abundant flamingo like footprints and flamingo nest mounds are good indicators of alkaline and or saline lake waters hippopotamus trails are found closely associated with modern and fossil freshwater wetlands dinosaur and pterosaur swim traces from lacustrine and fluvial deposits can be used to estimate water depth turtle crocodile amphibian hippopotamus and fish swim traces allow one to infer a subaqueous substrate certain modern intertidal fish feeding traces are oriented with the predominant tidal current and can be used as palaeocurrent indicators the preferential orientation of tetrapod trackways in lacustrine and fluvial deposits is analyzed vertebrate trace fossils can help to infer discharge variability in fluvial channels the descriptions of vertebrate ichnofabrics are commonly limited to heavily bioturbated beds due to trampling by vertebrates and to a few examples of ichnofabrics with discrete trace fossils the nature and implications of the recognized vertebrate ichnofacies are still being debated and have a limited utility in palaeoenvironmental analysis the distinction of a potential vertebrate burrow ichnofacies in carbonate bearing palaeosols is proposed to represent well drained soils developed under arid or semiarid climate

holocene deglaciation of marie byrd land west antarctica surface exposure ages of glacial deposits in the ford ranges of western marie byrd land indicate continuous thinning of the west antarctic ice sheet by more than 700 meters near the coast throughout the past 10 000 years deglaciation lagged the disappearance of ice sheets in the northern hemisphere by thousands of years and may still be under way these results provide further evidence that parts of the west antarctic ice sheet are on a long term trajectory of decline west antarctic melting contributed water to the oceans in the late holocene and may continue to do so in the future

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

mass balance and accumulation rate across siple dome west antarctica snow accumulation rates and rates of ice thickness change mass balance are studied at several sites on siple dome west antarctica accumulation rates are derived from analyses of grossbeta radioactivity in shallow firn cores located along a 60 km transect spanning both flanks and the crest of the dome there is a north south gradient in snow accumulation rate across the dome that is consistent with earlier radar mappingof internalstratigraphy orographicprocessesprobablycontrolthisdistribution mass balance is inferred from the difference between global positioning system gps derived vertical velocities and snow accumulation rates for sites close to the firn core locations results indicatethatthere is virtually no netthicknesschangeat fourof thefive sites the exception is at the northernmost site where a small amount of thinning is detected that appearstobe inconsistentwith other studies a possiblecause ofthisanom alousthinning is recent retreatof the grounding line of ice stream d

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

observed velocity fluctuations on a major antarctic ice stream most of the antarctic ice sheet is drained by systems comprising an area of slow sheet flow which converges into a much faster valley glacier or ice stream on reaching the coast this ice then either enters an ice shelf confined by embayments and ice rises or calves off into the ocean the higher speed of ice streams enables them to respond to changes in their environment faster than the slower ice sheet in the west antarctic where the ice sheet moves across bedrock well below sea level the ice streams have the potential to rapidly increase or decrease the ice discharge into the sea1 the timescale for these changes has been placed at a few hundred years if this is true then changes should be detectable by careful measurements on a decadal timescale by comparing recent ice velocity measurements with those collected ten years ago we establish that the ice in the mouth of ice stream b has decelerated by 20 we discuss the possible causes of this deceleration on the basis of our knowledge of the current regional dynamics and the possible ramifications of this deceleration on future ice stream behaviour

numerical simulations of cyclic behaviour in the parallel ice sheet model pism numerical experiments are conducted on a synthetic topography with a three dimensional thermomechanically coupled ice sheet model the parallel ice sheet model pism within the model combined stress balances are connected to evolving thermodynamics and hydrology the sensitivity of cyclic behaviour to changes in sliding law parameters and the climate input is studied multiple types of oscillations were found with strong variations in both amplitude and frequency a physical description is given in which these variations and transitions from one oscillation type to another are linked to the interplay of stresses heat transport and hydrological processes high frequency oscillations period 114 169 years which are shown to have a major impact on ice velocities and a small effect on the ice volume are related to variations in the water distribution at the base low frequency cycles period 1000 years which have a major impact on both velocities and ice volume are linked to changes in the thermal regime oscillation characteristics are shown to be strongly sensitive to changes in sliding law parameters and the prescribed surface temperature and mass balance incorporating a surface height dependence of the mass balance is shown to provide an additional feedback which may induce longperiod oscillations

dynamics of the siple coast ice streams west antarctica results from a thermomechanical ice sheet model most of the ice leaving the antarctic ice sheet is discharged to the sea by ice streams an understanding of the dynamics of these ice streams is therefore crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level the behavior of ice streams a to e of the siple coast west antarctica has attracted much attention with evidence of recent stagnation and numerous relict ice flow features here i present results from a three dimensional thermodynamic ice sheet model of the west antarctic ice sheet wais the model generates variability in the flow of the siple coast ice streams reminiscent of that mentioned above this is caused by competition between several preferred ice flow pathways in the area individual ice streams interact through changes in catchment area size a reduction in catchment area reduces the flow of ice entering a stream and can cause stagnation through the associated reduction in deformational heating

the posture of floating pterosaurs ecological implications for inhabiting marine and freshwater habitats abstract the highly pneumatic skeleton of the extinct flying pterosaurs suggests that they would float high up on open water but in a posture rather different to that of birds however the exact posture of the body and head remains unknown and would be critical for an ocean going pterosaur forced onto the waters surface or animals that alighted to feed using computational methods with recent models and body mass estimates for four pterosaur genera dimorphodon rhamphorhynchus pteranodon and dsungaripterus we show that the floating posture of pterodactyloid pterosaurs led to the head neck and body being horizontal with the ventral 1 4 to 1 3 being immersed and the external nares being almost at or potentially partially below the waterline that could have left them vulnerable to drowning the floatation methods were verified using a model of a canada goose branta canadensis that is able to successfully replicate the expected orientation and depth of immersion of the bird while there is convincing ev idence for a number of pterosaurs foraging in marine and freshwater environments these results suggest that many did not regularly rest on the surface of the water and if immersed would need to take off again rapidly the high numbers of fossils of juvenile pterosaurs compared to the terrestrial mesozoic dinosaurs suggest that this may be linked to their floating posture

the geodetic mass balance of eyjafjallajokull ice cap for 1945 2014 processing guidelines and relation to climate mass balance measurements of icelandic glaciers are sparse through the 20th century however the large archive of stereo images available allows estimates of glacier wide mass balance dot b in decadal time steps since 1945 combined with climate records they provide further insight into glacier climate relationship this study presents a workflow to process aerial photographs 1945 1995 spy satellite imagery 1977 1980 and modern satellite stereo images since 2000 using photogrammetric techniques and robust statistics in a highly automated open source pipeline to retrieve seasonally corrected decadal glacier wide geodetic mass balances in our test area eyjafjallajokull s iceland 70 km 2 we obtain a mass balance of cdata dot curr b curr 1945 curr 2014 curr 0 27 pm 0 03 rm curr m w rm curr e rm rm curr a rm ndash curr 1 with a maximum and minimum of dot curr b curr 1984 curr 1989 curr 0 77 curr pm 0 19 rm curr m curr w rm curr e rm curr rm curr a rm curr ndash 1 and dot curr b curr 1994 curr 1998 curr 1 94 curr pm 0 34 rm curr m w rm curr e rm curr rm curr a rm curr ndash 1 respectively attributed to climatic forcing and dot curr b curr 2009 curr 2010 curr 3 39 rm curr pm rm curr 0 43 rm curr m w rm curr e rm curr rm curr a rm curr ndash 1 mostly caused by the april 2010 eruption the reference surface mass balances correlate with summer temperature and winter precipitation and linear regression accounts for 80 of the mass balance variability yielding a static sensitivity of mass balance to summer temperature and winter precipitation of 2 1 0 4 m w e a 1 k 1 and 0 5 0 3 m w e a 1 10 1 respectively this study serves as a template that can be used to estimate the mass balance changes and glaciers response to climate

crevasse patterns at the onset to ice stream b west antarctica sequential satellite imagery and modeling are used to investigate crevasse patterns at the head of ice stream b tributary blb the crevasses informally called the chromosomes form at the upstream limit to blb s northern shear margin and chaotic crevasse zone we find that the onset to crevasse formation and by inference the onset to streaming flow has migrated upstream over time at a mean rate of 230 16 m a 1 a possible cause for that migration is changes in net basal friction due to changes in basal water production rate and storage

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

surface morphology and internal layer stratigraphy in the downstream end of kamb ice stream west antarctica satellite images of kamb ice stream formerly ice stream c west antarctica reveal several long curved linear features lineations oriented sub parallel to the ice flow direction we use ground based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound some lineations are relict ice stream shear margins identified by hyperbolic diffractors near the surface interpreted to be buried crevasses and highly disturbed internal layers at depth satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter ice stream ridges internal layers beneath these lineations are downwarped strongly into a syncline shape the internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past the deep hyperbolas are interpreted to be basal crevasses and the strongly downwarped internal layers mark the position of a relict grounding line our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins differentiation between these features is not possible using satellite images alone

lateral shear moraines and lateral marginal moraines of palaeo ice streams abstract an understanding of the nature of sedimentation at ice stream lateral margins is important in reconstructing the dynamics of former ice sheets and modelling the mechanisms by which sediment is transported beneath contemporary ice streams theories of the formation of ice stream lateral moraines islms have hitherto been based on a relatively limited number of terrestrial and marine examples here an inventory of islms is compiled from available studies together with independent analysis of seismic reflection and bathymetric datasets the locations and dimensions of 70 islms alongside a synthesis of their key architectural and geomorphic characteristics are presented two different types of islms are identified type 1 islms are up to 3 5 km wide and 60 m thick they maintain a constant width thickness and cross sectional shape along their length type 1 islms are interpreted and referred to as ice stream lateral shear moraines that form subglacially in the shear zone between ice streams and slower flowing regions of an ice sheet in contrast type 2 islms are up to 50 km wide and 300 m thick they are only identified close to the shelf break in the marine environment type 2 islms exhibit an increase in width and thickness along their length and their distal slopes become steeper in a seaward direction they contain internal dipping reflections that indicate sediment progradation away from the former ice stream type 2 islms are interpreted and referred to as ice stream lateral marginal moraines that were formed at the lateral boundary between ice streams and seafloor terrain that was free of grounded ice we suggest that using bathymetric images and acoustic profiles it is possible to differentiate between ice stream lateral shear moraines and lateral marginal moraines in the geological record this distinction is important for understanding the mechanisms of sediment transfer beneath ice streams and for making inferences about the conditions that existed beyond the lateral ice stream margin at the time of lateral moraine formation slow moving ice beyond lateral shear moraines and ocean water rather than ice beyond lateral marginal moraines

thickness changes on whillans ice stream and ice stream c west antarctica derived from laser altimeter measurements repeat airborne laser altimeter measurements are used to derive surface elevation changes on parts of whillans ice stream and ice stream c west antarctica elevation changes are converted to estimates of ice equivalent thickness change using local accumulation rates surface snow densities and vertical bedrock motions the surveyed portions of two major tributaries of whillans ice stream are found to be thinning almost uniformly at an average rate of 1 m a 1 ice stream c has a complicated elevation change pattern but is generally thickening these results are used to estimate the contribution of each surveyed region to the current rate of global sea level rise

transport of low ah2o dehydration products to melt sites via reaction zone networks milford sound new zealand the gabbroic dioritic pembroke hornblende granulite phg of milford sound displays a geometrically simple mesoscopic network of sub planar garnet reaction zones grz in which the meta igneous hornblende granulite has been depleted of na si and h2o and c 25 vol almandine rich garnet has formed some studies postulate the initial presence of melt along the centres of all grz explaining the frequent absence of feldspathic veins by selective melt loss a more parsimonious model is necessitated by structural evidence and together with chemical data suggests a relationship between mid range metasomatic transport and anatexis the pembroke outcrops show a process of incipient melting of gabbro diorite in an environment of relatively low ah2o in lithologies that have limited free quartz a non equilibrium steady state is proposed in which a sodic dehydration fluid moves some distance via the grz network towards areas of partial melting only in these areas are na and si reconstituted as albite with more garnet as byproduct having avoided the need for melt percolation the combined structural and chemical evidence directs a focus on mass transport in low ah2o gabbroic environments in subsequent events of shearing and complete transposition both sets of garnet the atypical grz residue and partial melt melanosomes were inherited by the milford gneiss facies of the phg

changes in the west antarctic ice sheet the portion of the west antarctic ice sheet that flows into the ross sea is thinning in some places and thickening in others these changes are not caused by any current climatic change but by the combination of a delayed response to the end of the last global glacial cycle and an internal instability the near future impact of the ice sheet on global sea level is largely due to processes internal to the movement of the ice sheet and not so much to the threat of a possible greenhouse warming thus the near term future of the ice sheet is already determined however too little of the ice sheet has been surveyed to predict its overall future behavior

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

dynamics of thermally induced ice streams simulated with a higher order flow model we use a new discretization technique to solve the higher order thermomechanically coupled equations of glacier evolution we find that under radially symmetric continuum equations small perturbations in symmetry due to the discretization are sufficient to produce the initiation of nonsymmetric thermomechanical instabilities which we interpret as ice streams in good agreement with previous studieswhich have indicated a similar instability we find that the inclusion of membrane stresses regularizes the size of predicted streams eliminating the ill posedness evident in previous investigations of ice stream generation through thermomechanical instability ice streams exhibit strongly irregular periodicity which is influenced by neighboring ice streams and the synoptic state of the ice stream ice streams are not always the same size but instead appear to follow a temperature dependent distribution of widths that is robust to grid refinement the morphology of the predicted ice streams corresponds reasonably well to extant ice streams in physically similar environments

depth and time dependent vertical strain rates at siple dome antarctica as part of a project to investigate the flow of ice at low effective stress two independent strain gauge systems were used to measure vertical strain rate as a function of depth and time at siple dome antarctica the measurements were made from january 1998 until january 2002 at the ice divide and a site 7 km to the northeast on the flank the strain rate profiles place constraints on the rheology of ice at low stress show the expected differences between divide and flank flow with some structure due to firn compaction and probably ice stratigraphy and suggest that the flow of the ice sheet has not changed much in the last 8 6 kyr the strain rates show an unexpected time dependence on scales ranging from several months to hours including discrete summer events at the divide time dependence in strain rate water pressure seismicity velocity and possibly basal motion has been seen previously on the siple coast ice streams but it is especially surprising on siple dome because the bed is cold

revealing the early ice flow patterns with historical declassified intelligence satellite photographs back to 1960s the reconnaissance argon satellites collected the earliest images of antarctica from space dating back to the 1960s providing valuable historical baseline information for studying polar ice sheets those photographs are underutilized for ice motion mapping due to lack of sufficient ground controls for image orthorectification in this study we orthorectified the argon photographs by fully exploiting the metric qualities of worldview satellite images very high spatial resolution and precise geolocation through a case study over larsen ice shelf we demonstrated that the camera model with bundle block adjustment can achieve geolocation accuracy of better than the nominal resolution 140 m for orthorectifying argon images with worldview imagery as ground control source this allowed us to extend the ice velocity records of larsen ice shelf back into 1960s 1970s for the first time the retrospective analysis revealed that acceleration of the collapsed larsen b occurred much earlier than previously thought

changes in the ice plain of whillans ice stream west antarctica data from the mouth of the decelerating whillans ice stream wis west antarctica spanning 42 years are reviewed deceleration has continued with local areas of both thinning and thickening occurring the mean thinning rate is 0 48 0 77m a 1 no consistent overall pattern is observed ice thickens immediately upstream of crary ice rise where deceleration and divergence are strongest suggesting expanded upstream influence of the ice rise thinning is prevalent on the ross ice shelf grounding line advance at a rate of 0 3 km a 1 is detected in a few locations basal stresses vary across an ice stream transect with a zone of enhanced flow at the margin marginal shear is felt at the ice stream center mass balance values are less negative but larger errors of earlier measurements mask any possible temporal pattern comparisons of the recent flow field with flow stripes suggest wis contributes less ice to the deep subglacial channel carved by mercer ice stream and now flows straighter the general lack of geometric changes suggests that the regional velocity decrease is due to changing basal conditions

the elevation history of ice streams and the spatial accumulation pattern along the siple coast of west antarctica inferred from ground based radar data from three inter ice stream ridges measurements of the surface and internal layer geometry from ice penetrating radar and global positioning system surveys on three inter ice stream ridges in west antarctica siple dome ridge de and ridge bc are examined with ice flow models to infer 1 the history of the divide position at each site and 2 the spatial pattern of accumulation across the ridges we find that the divide position is most steady at siple dome somewhat steady at ridge de and highly variable at ridge bc data from siple dome and ridge de show evidence for steady northward motion of the ice divide for the past few thousand years the layers beneath ridge bc suggest a 5 km northward shift of the divide position within the past several hundred years assuming the divide shifts are all due to changing elevation of the bounding ice streams we infer the relative elevation history for segments of ice streams b e the northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from e to b with most dramatic recent thinning 100 m in 103 years of ice stream b relative to ice stream c analysis of the internal layer pattern across the ridges indicates a south north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases the inferred accumulation distribution is nearly uniform on the northern flanks decreases sharply within a few ice thicknesses across the divides and then decreases gradually farther to the south the north south decrease is strongest for ridge de and weakest for ridge bc this spatial pattern and the reduction in gradient strength with distance from the amundsen sea is consistent with the hypothesis that storms from the amundsen sea carry moisture first south then west over west antarctica and deposit more snow on the windward side of ridges due to orographic lifting this pattern has been stable for at least the past several thousand years