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

late holocene ice flow reconfiguration in the weddell sea sector of west antarctica abstract here we report late holocene ice sheet and grounding line changes to the weddell sea sector of west antarctica internal radio echo layering within the bungenstock ice rise which comprises very slow flowing ice separating the fast flowing institute and moller ice streams reveals ice deformed by former enhanced flow overlain by un deformed ice the ice rise surface is traversed by surface lineations explicable as diffuse ice flow generated stripes which thus capture the direction of flow immediately prior to the creation of the ice rise the arrangement of internal layers can be explained by adjustment to the flow path of the institute ice stream during either a phase of ice sheet retreat not longer than 4000 years ago or by wholesale expansion of the grounding line from an already retreated situation not sooner than 400 years ago some combination of these events involving uplift of the ice rise bed during ice stream retreat and reorganisation is also possible whichever the case the implication is that the ice sheet upstream of the bungenstock ice rise which currently grounds over a 1 5 km deep basin has been and therefore may be susceptible to significant change

four years of automated measurements of seismic moments at papeete using the mantle magnitude mm 1987 1991 abstract we report on a 4 year experiment of routine automatic determination of seismic moments using the mantle magnitude m m we have developed a system performing automatic detection and location of distant earthquakes on the basis of three component broadband records at a single seismic station this system which has been operational since 1987 at papeete tahiti computes m m from the spectral amplitude of the mantle rayleigh waves based on a dataset of 474 earthquakes we show that m m provides a excellent estimate of the quantity log 10 m 0 20 where m 0 is the seismic moment in dyn cm as published subsequently by the harvard and usgs groups the average residual is 0 07 units of magnitude and the standard deviation 0 22 units this method which necessitates minimal hardware has powerful applications in the field of tsunami warning

low post glacial rebound rates in the weddell sea due to late holocene ice sheet readvance many ice sheet reconstructions assume monotonic holocene retreat for the west antarctic ice sheet but an increasing number of glaciological observations infer that some portions of the ice sheet may be readvancing following retreat behind the present day margin a readvance in the weddell sea region can reconcile two outstanding problems i the present day widespread occurrence of seemingly stable ice streams grounded on beds that deepen inland and ii the inability of models of glacial isostatic adjustment to match present day uplift rates by combining a suite of ice loading histories that include a readvance with a model of glacial isostatic adjustment we report substantial improvements to predictions of present day uplift rates including reconciling one problematic observation of land sinking we suggest retreat behind present grounding lines occurred when the bed was lower and isostatic recovery has since led to shallowing ice sheet re grounding and readvance the paradoxical existence of grounding lines in apparently unstable configurations on reverse bed slopes may be resolved by invoking the process of unstable advance in accordance with our load modelling

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

ice temperature and high geothermal flux at siple dome west antarctica from borehole measurements a verlical temperature profile through the west antarctic ice sheet wais at the summit of siple dome reveals an elevated geothermal flux this could be the root cause for the existence of a dynamic ice stream system in the wais siple dome is still frozen on its bed but adjacent ice streams have bed temperatures at the pressure melting point of ice although present day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice indirect ellbcts like a rapid disintegration of the floating ice shelves or additional melt water input at the surface could give rise to speed up of the ice streams with an ensuing rise in sea level ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration within a relatively short period of time of at least part of the wais

convergence and downwelling at a river plume front abstract the small scale structure of the circulation and hydrography at the frontal boundary of the connecticut river plume in long island sound has been resolved using a novel combination of instruments a towed acoustic doppler current profiler adcp and a rigid array of current meters and conductivity temperature sensors observations were made during the latter half of the eastward ebb tide when the river plume was well established and the front was moving to the west at approximately 0 3 m s 1 two across front transects revealed a horizontal convergence rate in the across front velocity components at 0 6 m of 0 05 0 1 s 1 this was associated with a salt induced horizontal density gradient of 10 2 kg m4 observations obtained during a period in which the towed adcp was caught in the zone of maximum surface convergence showed significant downwelling with a near surface maximum of 0 2 m s 1 vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0 6

west antarctic ice stream discharge variability mechanism controls and pattern of grounding line retreat west antarctic ice streams show pronounced flow variability in their downstream reaches with changes stranding formerly fast flowing ice and redirecting discharge a simple model in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach can explain this behavior downstream thinning steepens the temperature gradient near the bed increasing upward heat flow and the tendency toward basal freezing the basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning that is the fastest flowing ice the most successful rapid outflows are regions where basal water from elsewhere is likely to be consumed freezing here leads to episodic slow downs and redirections of flow the history of which appears in satellite imagery as ice rises distorted streaklines and margin jumps created when discharge migrates to areas with more favorable basal conditions one compelling consequence of this process is that it makes catastrophic collapse less likely if discharge currents are forced to slow when they become too fast thin then there may be an upper bound on the retreat rate and discharge flux of the west antarctic ice sheet wais ice stream system under the present climate

the temperature regime of fimbulisen antarctica numerical simulations of the temperature regime of the ice shelf fimbulisen antarctica are presented a vertical temperature profile s1 of fimbulisen has been measured at the extension of jutulstraumen in which the temperature decreases with depth the three dimensional steady state temperature field was computed by a finite element technique horizontal flow velocities and surface accumulation rates were derived from observations the basal melt rate distribution arose from an assumption of balance in the mass continuity equation the computed basal melt rate distribution a b indicates that the highest basal melt rates up to 15 m a 1 occur at the inflow gate of jutulstraumen and low basal melt rates 0 6 ma 1 occur in the slower moving parts where the ice shelf overhangs the continental shelf a b 1 2 m a 1 the resulting temperature field indicates that fimbulisen consists of a cold middle part built up by the extension of jutulstraumen and warmer ice masses in slow moving areas to the west and east furthermore model runs were set up in which the atmospheric temperatures increased in 1 k steps the results suggest that the warming effectively increases the temperatures throughout the ice column in the slower moving parts therefore enhancing shear at the margins of the extension of jutulstraumen

characteristics of the sticky spot of kamb ice stream west antarctica amplitude analysis of reflection seismic data reveals the presence of highly variable bed conditions under the main sticky spot and adjacent regions of the kamb ice stream kis formerly ice stream c the sticky spot which is a zone of bed that imparts high basal resistance to ice flow is situated on a local topographic high composed of consolidated sediments or sedimentary rock any meltwater draining from upglacier along the base of the ice is routed around the sticky spot the ice over the sticky spot includes in at least some places a seismically detectable basal layer containing a low concentration of debris which locally thickens to 40 m over a topographic low in the bed the ice contact basal material ranges from dilated and highly porous to more compacted and stiff and perhaps locally frozen the softer material is preferentially in topographic lows but there is not a one to one correspondence between basal character and basal topography we speculate that the 40 m thick frozen on debris layer formed by glaciohydraulic supercooling of lake drainage events along a basal channel during the former active phase of the ice stream we also speculate that loss of lubricating water perhaps from piracy upstream contributed to the slowdown of the ice stream with drag from the sticky spot playing an important role and with the basal heterogeneity greatly increasing after the slowdown of the ice stream

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

drilling to the beds of the greenland and antarctic ice sheets a review we give a review of drilling to the beds of the greenland and antarctic ice sheets with a history of just a century we use a chronological rather than a geographical ordering to emphasize the way drilling has developed the review is divided into two parts core drilling and hole only drilling for access to the beds or the deep interior of the ice sheets this is an overview not a comprehensive report on each project while we have endeavoured to include all projects that fit our selection criteria we have not provided all the technical details

extensive retreat and re advance of the west antarctic ice sheet during the holocene to predict the future contributions of the antarctic ice sheets to sea level rise numerical models use reconstructions of past ice sheet retreat after the last glacial maximum to tune model parameters 1 reconstructions of the west antarctic ice sheet have assumed that it retreated progressively throughout the holocene epoch the past 11 500 years or so 2 4 here we show however that over this period the grounding line of the west antarctic ice sheet which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf retreated several hundred kilometres inland of today s grounding line before isostatic rebound caused it to re advance to its present position our evidence includes first radiocarbon dating of sediment cores recovered from beneath the ice streams of the ross sea sector indicating widespread holocene marine exposure and second ice penetrating radar observations of englacial structure in the weddell sea sector indicating ice shelf grounding we explore the implications of these findings with an ice sheet model modelled re advance of the grounding line in the holocene requires ice shelf grounding caused by isostatic rebound our findings overturn the assumption of progressive retreat of the grounding line during the holocene in west antarctica and corroborate previous suggestions of ice sheet re advance 5 rebound driven stabilizing processes were apparently able to halt and reverse climate initiated ice loss whether these processes can reverse present day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity parameters that are difficult to measure and to incorporate into ice sheet models

rapid early holocene deglaciation in the ross sea antarctica deglaciation of the ross sea following the last ice age provides an important opportunity to examine the stability of marine ice sheets and their susceptibility to changing environmental conditions insufficient chronology for ross sea deglaciation has helped sustain i the theory that this region contributed significantly to meltwater pulse 1a mwp 1a and ii the idea that ross sea grounding line retreat occurred in a swinging gate pattern hinged north of roosevelt island we present deglaciation records from southern transantarctic mountain glaciers which delivered ice to the central ross sea abrupt thinning of these glaciers 9 8 kyr bp coincided with deglaciation of the scott coast 800 km to the north and ended with the ross sea grounding line near shackleton glacier this deglaciation removed grounded ice from most of the central and western ross sea in less than 2 kyr the ross sea sector neither contributed nor responded significantly to mwp 1a

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

mechanical analysis of pinning points in the ross ice shelf antarctica ice shelves regulate the rate of ice sheet discharge along much of the antarctic coastline pinning points sites of localised grounding within floating ice can in turn regulate the flow and thickness of an ice shelf while the net resistive effect of ice shelves has been quantified in a systematic way few extant pinning points have been examined in detail here complete force budgets are calculated and examined for ice rises and rumples in the ross ice shelf west antarctica the diverse features have different effects on ice shelf mechanics that do not depend simply on their size but may we conclude depend on the properties of seafloor materials

high variability of climate and surface mass balance induced by antarctic ice rises ice rises play key roles in buttressing the neighbouring ice shelves and potentially provide palaeoclimate proxies from ice cores drilled near their divides little is known however about their influence on local climate and surface mass balance smb here we combine 12 years 2001 12 of regional atmospheric climate model racmo2 output at high horizontal resolution 5 5 km with recent observations from weather stations ground penetrating radar and firn cores in coastal dronning maud land east antarctica to describe climate and smb variations around ice rises we demonstrate strong spatial variability of climate and smb in the vicinity of ice rises in contrast to flat ice shelves where they are relatively homogeneous despite their higher elevation ice rises are characterized by higher winter temperatures compared with the flat ice shelf ice rises strongly influence smb patterns mainly through orographic uplift of moist air on the upwind slopes besides precipitation drifting snow contributes significantly to the ice rise smb the findings reported here may aid in selecting a representative location for ice coring on ice rises and allow better constraint of local ice rise as well as regional ice shelf mass balance

radar reflections reveal a wet bed beneath stagnant ice stream c and a frozen bed beneath ridge bc west antarctica digital airborne radar data were collected during the 1987 88 antarctic field season in nine gridded blocks covering the downstream portions of ice stream b 6 km spacing and ice stream c 11 km spacing together with a portion of ridge bc between them an automated processing procedure was used for picking onset times of the reflected radar pulses converting travel times to distances interpolating missing data converting pressure transducer readings correcting navigational drift performing crossover analysis and zeroing remanent crossover errors interpolation between flight lines was carried out using the minimum curvature method maps of ice thickness estimated accuracy 20 m and basal reflection strength estimated accuracy 1 db were produced the ice thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features the reflection strength map shows pronounced contrasts between the ice streams and ridge bc and between the two ice streams themselves we interpret the reflection strengths to mean that the bed of ice stream c as well as that of ice stream b is unfrozen that the bed of ridge bc is frozen and that the boundary between the frozen bed of ridge bc and the unfrozen bed of ice stream c lies precisely below the former shear margin of the ice stream

outburst flooding and the initiation of ice stream surges in response to climatic cooling a hypothesis abstract outburst flooding from subglacial reservoirs and associated surging of ice streams are caused by processes following climatic cooling that produce ice shelf grounding on proglacial sills trapping subglacial water according to an hypothesis presented here glaciers often advance into proglacial water bodies cooling may allow ice shelf formation and advance causing grounding on a proglacial sill trapping subglacial water ice shelf freeze on to the sill and development of a local reversal in ice air surface slope over the sill then are likely allowing ice thickening and water overpressurization if basal thawing then occurs as is likely an outburst flood and a surge may be triggered these processes may have been involved in heinrich events generation of antarctic subglacial lakes and meltwater scouring of some regions

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

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

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream