rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

full depth englacial vertical ice sheet velocities measured using phase sensitive radar we describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling using a ground based phase sensitive radio echo sounder pres during seven antarctic field seasons we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on berkner island roosevelt island fletcher promontory and adelaide island recorded changes in reflector positions yield full depth profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides we interpret these variations by comparing them to the results of a full stokes simulation of ice divide flow qualitatively validating the model and demonstrating that we are directly detecting an ice dynamical phenomenon called the raymond effect using pres englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice we discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pres measured vertical velocities into age modeling

abandoned penguin rookeries as holocene paleoclimatic indicators in antarctica penguins are sensitive indicators of the antarctic climate and of the environmental parameters that limit their presence and distribution paleoenvironmental data obtained from the study of abandoned penguin rookeries pygoscelis adeliae along the victoria land coast in antarctica indicate 14c date of 11 13 ka for the oldest abandoned rookery and supply new information about the timing of glacier retreat in southern victoria land after the last glacial maximum the continuous presence of the adelie penguins is documented from 7 ka according to our data the limiting factors that control the presence of penguins along the coast of victoria land changed during the holocene whereas several colonies were occupied for very long periods other sites were used for more or less extended periods and then abandoned the greatest diffusion of rookeries occurred between 3 and 4 ka a period of particularly favorable environmental conditions that has never been repeated it was followed by a sudden decrease in the number of penguin rookeries shortly after 3 ka this event has been attributed to an increase of the sea ice extension and may have been correlated to a worldwide phase of climate change near the subboreal subatlantic boundary a minor phase of penguin reoccupation occurred locally in the eighth to fourteenth centuries a d because the presence and number of penguins reflect the state of health of the antarctic marine ecosystem it is important to evaluate the variations in their distribution in the past in the absence of human induced changes

problems and possible solutions concerning radiocarbon dating of surface marine sediments ross sea antarctica abstract radiocarbon accelerator mass spectrometric ams dates on the acid insoluble fraction from 38 core tops from the western ross sea antarctica are used to address these questions 1 what are the apparent ages of sediments at or close to the present sediment water interface 2 is there a statistically significant pattern to the spatial distribution of core top ages and 3 is there a correction factor that can be applied to these age determinations to obtain the best possible holocene downcore chronologies ages of core top sediments range from 2000 to 21 000 14c yr b p some old core top dates are from piston cores and probably represent the loss of sediment during the coring process but some core top samples 6000 14c yr b p may represent little or no holocene deposition four possible sources of variability in dates 6000 14c yr b p n 28 are associated with 1 different sample preparation methods 2 different sediment recovery systems 3 different geographic regions and 4 within sample lateral age variability statistical analysis on an a posteriori design indicates that geographic area is the major cause of variability there is a difference in mean surface sediment age of nearly 2000 yr between sites in the western ross sea and sites east of ross bank in south central ross sea the systematic variability in surface age between areas may be attributed to a variable sediment accumulation rates sar surface age is inversely related to sar b differences in the percentage of reworked dead carbon between each area and or c differences in the co2 exchange between the ocean and the atmosphere

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

recumbent folding of divide arches in response to unsteady ice divide migration arches in stratigraphic layers directly under a flow divide raymond bumps are predicted by models of steady ice sheet flow and have been observed in several ice domes here we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position then again becomes stationary leaving the arched layers in a flank position as they are then carried downstream with the flow these abandoned arches can develop into recumbent folds these folds can occur over a wide range of divide migration speeds the shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location if the divide offset is abrupt pre cores or material lines comprising core relative isochrones can be used to quickly identify portions of an abandoned raymond bump that would be overturned at any future ice core site downstream if as appears to be the case in greenland the divide is never stable long enough to produce a mature arch folds of this type would not occur the most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams

the crossover stress anisotropy and the ice flow law at siple dome west antarctica we used observations and modeling of siple dome west antarctica a ridge ice divide to infer the importance of linear deformation mechanisms in ice sheet flow we determined the crossover stress a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms by combining measurements of ice properties with in situ deformation rate measurements and a finite element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal orientation fabric we found that a crossover stress of 0 18 bar produces the best match between predicted and observed deformation rates for siple dome this means that including a linear term in the flow law is necessary but generally the flow is still dominated by the nonlinear glen n 3 term the pattern of flow near the divide at siple dome is also strongly affected by crystal fabric measurements of sonic velocity which is a proxy for vertically oriented crystal fabric suggest that a bed parallel shear band exists several hundred meters above the bed within the ice age ice

holocene raised beaches at terra nova bay victoria land antarctica abstract more than 40 14c dates for raised beaches at terra nova bay ranging from the present to 7505 230 yr b p supply minimum or maximum limiting ages that bracket a relative sea level curve for this part of victoria land most samples consist of guano and penguin remains collected from abandoned nesting sites on beach deposits up to four 14c dates in stratigraphic sequence have been obtained in two soil profiles other samples consist of marine shells collected both within and on the surface of raised beaches all 14c ages for organisms that lived or fed in circumantarctic waters were corrected for a large reservoir effect the calibrated ages delimit a first relative sea level curve for victoria land rates of uplift ranged from about 10 mm yr following deglaciation to about 2 mm yr in the last 3 millennia widespread presence of adelie penguin nesting sites suggests that terra nova bay was deglaciated before 7065 250 yr b p 7059 6439 cal yr b p when environmental conditions in the coastal area were similar to the present ones

application of tandem accelerator mass spectrometer dating to late pleistocene holocene sediments of the east antarctic continental shelf abstract glacial recession from the antarctic continental shelf is recorded by glacial marine diamictons sands and overlying siliceous oozes in order to clarify the chronology for this sequence use was made of the university of arizona tandem accelerator mass spectrometer tams for 14 c dating small samples of benthic and planktonic foraminifera were selectively removed from diamictons graded sands and surface sediments which were recovered from the wilkes land continental shelf and slope east antarctica organic carbon was also utilized as a source for tams dating of the siliceous oozes and muds uncorrected ages varied from 14 260 140 to 3230 200 yr b p carbon fixed by phytoplankton and foraminifera is strongly influenced by old glacial derived co 2 thus reservoir corrections of up to 5500 yr are needed for the 14 c dates iceberg turbation reworks foraminifera so that dates from resulting deposits diamictons are interpreted as maximum ages the consistency of corrected ages from the shelf along with the sedimentologic interpretation suggests a rather recent recession perhaps mid holocene for this portion of the east antarctic ice sheet further application of the tams method should help clarify other problems concerning the late quaternary glacial history of antarctica

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

grounding line migration from 1992 to 2011 on petermann glacier north west greenland we use satellite radar interferometry to investigate changes in the location of the petermann glacier grounding line between 1992 and 2011 the grounding line location was identified in 17 quadruple difference interferograms produced from european remote sensing ers 1 2 data the most extensive time series assembled at any ice stream to date there is close agreement 20 6 cm between vertical displacement of the floating ice shelf and relative tide amplitudes simulated by the arctic ocean dynamics based tide model 5 aodtm 5 arctic tide model over the 19 a period the groundling line position varied by 470 m on average with a maximum range of 7 0 km observed on the north east margin of the ice stream although the mean range 2 8 km and variability 320 m of the grounding line position is considerably lower if the unusually variable north east sector is not considered our observations demonstrate that large isolated movements cannot be precluded thus sparse temporal records should be analysed with care the grounding line migration observed on petermann glacier is not significantly correlated with time r 2 0 22 despite reported ice shelf thinning and episodes of large iceberg calving which suggests that unlike other ice streams on the south west margin of the greenland ice sheet petermann glacier is dynamically stable

short term and long term tremor migration patterns of the cascadia 2004 tremor and slow slip episode using small aperture seismic arrays 1 tectonic tremor has been recorded at many subduction zones including the nankai cascadia mexican and alaskan subduction zones this study the first to use small aperture seismic arrays to track tremor deployed three small aperture seismic arrays along the cascadia subduction zone during a tremor and slow slip episode in july 2004 the tremor was active during virtually all up to 99 minutes of the analyzed tremor episode using 5 min sample windows individual wave phases were tracked across the arrays and used to derive slowness vectors these were compared with slowness vectors computed from a standard layered earth model to derive tremor locations locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume the migration between volumes was not smooth and the movement of the sources within the volume followed no specific pattern overall migration speeds along the strike of the subduction zone were between 5 and 15 km d smaller scale migration speeds between volumes reached speeds up to 2 km min uncertainties in the best locations were 5 km in epicenter and 10 km in depth for this data set and processing methodology tremor does not locate predominately on the primary subduction interface our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed at least in part of low frequency earthquakes broadly distributed in location

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

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

the presence of polynyas in the weddell sea during the last glacial period with implications for the reconstruction of sea ice limits and ice sheet history cores from a contourite drift on the continental slope in the nw weddell sea antarctica contain up to four sandy foraminifer rich layers wa1 4 with dispersed ice rafted debris ird interbedded with silty clayey sands and sandy muds accelerator mass spectrometry ams 14c dating on neogloboquadrina pachyderma from the foraminifer rich layers wa2 and wa3 yielded ages spanning the last glacial maximum lgm and marine isotope stage mis 2 20 319 28 543 cal yr bp and the middle part of mis 3 41 349 43 242 cal yr bp respectively we attribute the high abundances of planktonic and benthic foraminifera to deposition in a seasonally open marine environment given that the nw weddell sea shelf was covered by grounded ice at the lgm and probably during mis 2 and mis 3 and that diatom based reconstructions place the northern limit of winter and summer sea ice coverage to the north of the core sites during the last glacial period our results document the presence of a seasonal or perennial polynya in the nw weddell sea the polynya may have been formed by katabatic winds blowing off the antarctic peninsula ice sheet apis when it was grounded at the continental shelf break although polynya formation through upwelling of deep water cannot be ruled out in addition we argue that previously published data from the southern southeastern and southwestern weddell sea as well as the ross sea may indicate the widespread occurrence of polynyas along the antarctic continental margin during the last glacial period the widespread occurrence of polynyas could help explain how i aabw formation through brine rejection continued throughout the lgm and ii antarctic shelf benthos survived glacial periods the presence of polynyas through glacial periods also has implications for sea ice reconstructions from ice core records possibly biasing the signal towards a reduced sea ice cover finally we propose that the fringes of the apis which probably influence the presence or absence of polynyas in the nw weddell sea are sensitive to changes in antarctic temperature that may be related to a reduction in oceanic heat transport from the north atlantic

major ice sheet change in the weddell sea sector of west antarctica over the last 5 000 years until recently little was known about the weddell sea sector of the west antarctic ice sheet in the last 10 years a variety of expeditions and numerical modelling experiments have improved knowledge of its glaciology glacial geology and tectonic setting two of the sector s largest ice streams rest on a steep reverse sloping bed yet despite being vulnerable to change satellite observations show contemporary stability there is clear evidence for major ice sheet reconfiguration in the last few thousand years however knowing precisely how the ice sheet has changed in the past and when would allow us to better understand whether it is now at risk two competing hypotheses have been established for this glacial history in one the ice sheet retreated and thinned progressively from its last glacial maximum position retreat stopped at or very near the present position in the late holocene alternatively in the late holocene the ice sheet retreated significantly upstream of the present grounding line and then advanced to the present location due to glacial isostatic adjustment and ice shelf and ice rise buttressing both hypotheses point to data and theory in their support yet neither has been unequivocally tested or falsified here we review geophysical evidence to determine how each hypothesis has been formed where there are inconsistencies in the respective glacial histories how they may be tested or reconciled and what new evidence is required to reach a common model for the late holocene ice sheet history of the weddell sea sector of west antarctica

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

the sedimentary legacy of a palaeo ice stream on the shelf of the southern bellingshausen sea clues to west antarctic glacial history during the late quaternary a major trough belgica trough eroded by a palaeo ice stream crosses the continental shelf of the southern bellingshausen sea west antarctica and is associated with a trough mouth fan belgica tmf on the adjacent continental slope previous marine geophysical and geological studies investigated the bathymetry and geomorphology of belgica trough and belgica tmf erosional and depositional processes associated with bedform formation and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments here we present multi proxy data from sediment cores recovered from the shelf and uppermost slope in the southern bellingshausen sea and reconstruct the ice sheet history since the last glacial maximum lgm in this poorly studied area of west antarctica we combined new data physical properties sedimentary structures geochemical and grain size data with published data shear strength clay mineral assemblages to refine a previous facies classification for the sediments the multi proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings 1 subglacial 2 proximal grounding line 3 distal sub ice shelf sub sea ice and 4 seasonal open marine in the seasonal open marine fades we found evidence for episodic current induced winnowing of near seabed sediments on the middle to outer shelf and at the uppermost slope during the late holocene in addition we obtained data on excess pb 210 activity at three core sites and 44 ams c 14 dates from the acid insoluble fraction of organic matter aio and calcareous micro fossils respectively at 12 sites these chronological data enabled us to reconstruct for the first time the timing of the last advance and retreat of the west antarctic ice sheet wais and the antarctic peninsula ice sheet apis in the southern bellingshausen sea we used the down core variability in sediment provenance inferred from clay mineral changes to identify the most reliable aio c 14 ages for ice sheet retreat the palaeo ice stream advanced through belgica trough after similar to 36 0 corrected c 14 ka before present b p it retreated from the outer shelf at similar to 25 5 ka b p the middle shelf at similar to 19 8 ka b p the inner shelf in eltanin bay at similar to 12 3 ka b p and the inner shelf in ronne entrance at similar to 6 3 ka b p the retreat of the wais and apis occurred slowly and stepwise and may still be in progress this dynamical ice sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass balance changes in the study area the glacial history of the southern bellingshausen sea is unique when compared to other regions in west antarctica but some open questions regarding its chronology need to be addressed by future work c 2010 elsevier ltd all rights reserved

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

ice age storm trajectories inferred from radar stratigraphy at taylor dome antarctica the taylor dome ice core east antarctica provides a climate record that extends through the previous interglacial we use ice dynamics modeling and geochemical records to infer the local accumulation rate history a pronounced accumulation reduction occurred during the last glacial maximum lgm the spatial gradient of accumulation across the surface of taylor dome indicates the modern local prevailing storm direction radar internal layering shows that this gradient was reversed during the lgm suggesting a reorganization of synoptic weather systems occurred possibly linked to the advance and retreat of the west antarctic ice sheet

rapid sea level rise from a west antarctic ice sheet collapse a short term perspective will worldwide sea level soon rise rapidly because of a shrinkage of the west antarctic ice sheet wais here i give a personal perspective of that probability the crucial question is not whether large changes in ice mass can occur but how likely it is that a large rapid change say a several fold increase in the 20th century rate of about 2 mm a 1 will occur in the next century or two from awest antarctic cause twenty years ago weertman proposed that a marine ice sheet is inherently unstable but weertman s analysis was based on a simple model of a marine ice sheet that did not include fast flowing wet based ice streams which are now known to dominate the grounded ice sheet modern analyses do not definitively determine just how ice streams affect the stability of the wais but it can at least be said that there is no compelling theoretical reason to expect a rapid rise in sea level from the wais triggered by ice shelf thinning of the three main ice drainage systems in the wais the one that flows into pine island bay might be a particularly likely site for accelerated flow since there is no ice shelf to restrain the inflowing ice streams yet measurements show that this system is not significantly out of mass balance if the ross embayment system which has undergone several sudden glacial reorganizations in the last thousand years were unstable one might expect a history of large changes in the total outflow of ice into the ross ice shelf yet the total outflow in the ross embayment has remained relatively unchanged despite the large internal perturbations a fact that points to a stable not an unstable system study of the third major drainage from the wais into the ronne ice shelf also suggests that there is no gross discordance between the present velocity vectors and flow tracers in the ice shelf although the evidence is limited in the light of the evidence for recent stability it is difficult to see how climate warming whether anthropogenic or natural could trigger a collapse of the wais in the next century or two thus i believe that a rapid rise in sea level in the next century or two from a west antarctic cause could only occur if a natural not induced collapse of the wais were imminent based on a concept of pseudo random collapse once per major glacial cycle i estimate the chances of that to be on the order of one in a thousand

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

distortion of isochronous layers in ice revealed by ground penetrating radar in addition to measuring ice sheet thickness ground penetrating radar can be used to delineate reflections in ice sheets1 2 3 these reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow here we present ground penetrating radar data from fletcher promontory antarctica which show arches and troughs in isochronous ice layers to a depth of 100 m we demonstrate that the origin of these features can be determined by their growth with depth and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope one arch appears to be the result of a local anomaly in vertical strain rate its proximity to the ice divide width and growth with depth indicate that this arch is one of a class of features postulated by raymond4 but not previously shown to exist in the field such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice flow

predictions of antarctic crustal motions driven by present day ice sheet evolution and by isostatic memory of the last glacial maximum detectable crustal motion and secular rate of change of solid surface gravity may be produced by the earth s response to present day and past ice mass changes in antarctica scenarios of present day ice mass balance previously utilized to explore the global geodetic signatures of the antarctic ice sheet produce elastic crustal responses that are typically bounded by uplift rates 5 mm yr horizontal motion 1 mm yr and solid surface gravity change rates 1 gal yr in a restricted locality one scenario produces uplift rates slightly in excess of 10 mm yr and correspondingly enhanced horizontal and gravity rates in contrast the viscoelastic response to ice mass changes occurring since last glacial maximum lgm exceeds 5 mm yr uplift over substantial portions of west antarctica for a wide range of plausible choices of timing and magnitude of deglaciation and mantle viscosity similarly viscoelastic gravity rate predictions exceed 1 gal yr decrease over large regions confirming suggestions that a global positioning system gps and absolute gravity based program of crustal monitoring in antarctica could potentially detect postglacial rebound a published revision to the climap model of the antarctic ice sheet at lgm herein called the d91 model features a substantially altered west antarctic ice sheet reconstruction this revision predicts a spatial pattern of present day crustal motion and surface gravity change that diverges strikingly from climap based models peak d91 crustal rates assuming deglaciation begins at 12 kyr and ends at 5 kyr are around 16 mm yr uplift 2 mm yr horizontal and 2 5 gal yr gravity tabulated crustal response predictions for selected antarctic bedrock sites indicate critical localities in the interior of west antarctica where expected responses are large and d91 predictions differ from climap based models by a factor of 2 or more observations of the postglacial rebound signal in antarctica might help constrain antarctic mass balance and contribution to sea level rise over the past 20 000 years

global positioning system measurements from eastern tibet and their implications for india eurasia intercontinental deformation we present and interpret global positioning system gps measurements of crustal motions for the period 1991 1998 for a network encompassing the eastern part of the tibetan plateau and its foreland relative to a eurasian frame defined by minimizing the velocities of 16 gps stations in europe central asia and siberia stations within all parts of the plateau foreland in south china move 6 10 mm yr east southeast indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland north of the plateau foreland stations move northeastward at 10 mm yr indicating that the northern boundary of the deformation zone lies north of the plateau with this realization of a eurasian frame the velocity of the gps station at bangalore in southern india implies that the northward motion of india is 5 12 mm yr slower than that predicted from the nuvel 1a plate reconstruction viewed relative to the south china block stations of the northeast plateau bounded on the north by the qilian shan and the altyn tagh fault move nne to ne with velocities ranging from 19 mm yr within the plateau to 5 11 mm yr in its foreland the altyn tagh fault shows left lateral slip of 10 mm yr at 95 e and shortening across the fault of 5 mm yr stations south and west of the xianshuihe xiaojiang fault system define a crustal fragment rotating clockwise at 10 mm yr relative to the south china block around the eastern himalayan syntaxis the gps measurements indicate no significant shortening 3 mm yr within the longmen shan of the central eastern plateau and its adjacent foreland although the longmen shan rise over 6 km in 100 km horizontal distance geological studies indicate that the deformational field was established diachronously in late miocene to pliocene time was characterized by no east west shortening of tibetan crust and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes

flow at ice divide triple junctions 2 three dimensional views of isochrone architecture from ice penetrating radar surveys 1 ground based surveys of ice divide triple junctions in two antarctic ice rises the fletcher promontory and berkner island have been carried out using low frequency ice penetrating radars these surveys have focused on understanding the internal layer architecture at and around the triple junctions with a particular emphasis on understanding how the raymond effect operates a variety of features have been observed including raymond cupolas under the junctions bump stacks of varying amplitude with distance from the divide double bumps crooked stacks implying divide motion and the presence of ridges containing weak and strong bump stacks it was generally not possible to elucidate the three dimensional structure by correlation of pick crossovers so instead a least squares cross correlation technique was used which computes nominal age surfaces using radial basis function interpolation double bumps are absent from the central cupola of fletcher promontory permitting us to infer that the ice here has thinned by around 500 m in the past 5000 years estimates of the age scale suggest that there will be recoverable eemian ice a technique for inferring the velocity profile at divides using layer traces and assumed velocities in the flanks is presented

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

an ice sheet remembers are we witnessing the early stages of the collapse of the west antarctic ice sheet in his perspective ackert explains that to answer this question we must study the behavior of the ice sheet not just in recent decades but over thousands of years he highlights the report by stone et al who document slow but steady thinning of the ice sheet over the last 9300 years punctuated by more rapid thinning as the ice margin approaches the record is consistent with earlier suggestions that the ice sheet is still responding to changes in climate and sea level that occurred thousands of years ago making it difficult to predict its response to present climate warming

early deglaciation of the british irish ice sheet on the atlantic shelf northwest of ireland driven by glacioisostatic depression and high relative sea level understanding the triggers and pace of marine based ice sheet decay is critical for constraining the future mass loss and dynamic behaviour of marine based sectors of the large polar ice sheets in greenland and antarctica numerical models which seek to predict this behaviour need to be calibrated against data from both contemporary and palaeo ice sheets and the latter requires accurate reconstruction of former ice sheet extent dynamics and timing marine geophysics sediment cores benthic foraminiferal assemblages and radiocarbon dating are used to reconstruct the extent of the last british irish ice sheet biis and the timing and style of its retreat on the atlantic shelf northwest of ireland shelf edge moraines and subglacial till recovered in cores from the outer continental shelf are dated to younger than 26 3 ka cal bp and indicate an extensive ice sheet at the last glacial maximum lgm that was grounded to the shelf edge nested arcuate moraines record the subsequent episodic retreat of the ice sheet across the shelf lithofacies and associated foraminiferal assemblages demonstrate that this retreat occurred in a glacimarine environment as a grounded tidewater margin and that high relative sea level and cold waters prevailed during retreat radiocarbon dates indicate that the timing of initial ice sheet retreat from the shelf edge occurred in the interval between 26 3 and 24 8 ka cal bp during the period of minimum global eustatic sea level and that the ice sheet had retreated to the mid shelf by 24 8 ka cal bp the donegal bay moraine a large moraine at the mouth of donegal bay records a major stillstand and readvance of the ice sheet during deglaciation between 20 2 and 17 9 ka cal bp estimated retreat rates of 5 5 35 m a 1 across the shelf demonstrate that retreat was slow it is noteworthy that retreat was initiated in the absence of ocean warming and when eustatic sea level was at a minimum the sea level rise that initiated deglaciation from the shelf edge therefore is inferred to have been a product of local glacio isostatic crustal depression rather than external forcing this demonstrates that marine based sectors of ice sheets can trigger their own demise internally through glacio isostatic adjustment and it provides an explanation for the early retreat of the biis on the atlantic shelf during the global lgm glgm

sea ice control of plio pleistocene tropical pacific climate evolution marine proxies of sea surface temperature sst indicate that the tropical pacific thermal gradient intensified through the plio pleistocene and peaked during pleistocene glaciations the cause of this variability which has been linked to the initiation of the walker circulation is uncertain here we hypothesize that plio pleistocene tropical climate variability was coupled to high latitude southern hemisphere climate change specifically sea ice extent we use a coupled ocean atmosphere general circulation model to investigate the influences of sea ice extent and atmospheric co2 on the tropical pacific thermal structure in the model co2 radiative forcing in the absence of any sea ice feedbacks has little influence on the tropical sst gradient a 180 ppm reduction in co2 causes the sst gradient to decrease by 0 4 c in comparison an expansion of southern hemisphere high latitude sea ice reduces tropical ssts and enhances the sst gradient in the tropical pacific by as much as 3 5 c tropical cooling is primarily due to the advection and upwelling of waters into the eastern equatorial pacific that were cooled by sensible heat loss at the sea ice margin in the southern pacific an energy balance analysis indicates that the ocean heat flux into the eastern equatorial pacific decreases by 44 this mechanism provides an intimate coupling between the tropical pacific and the high latitude southern hemisphere through the thermocline circulation

deglaciation of fennoscandia abstract to provide a new reconstruction of the deglaciation of the fennoscandian ice sheet in the form of calendar year time slices which are particularly useful for ice sheet modelling we have compiled and synthesized published geomorphological data for eskers ice marginal formations lineations marginal meltwater channels striae ice dammed lakes and geochronological data from radiocarbon varve optically stimulated luminescence and cosmogenic nuclide dating this is summarized as a deglaciation map of the fennoscandian ice sheet with isochrons marking every 1000 years between 22 and 13 cal kyr bp and every hundred years between 11 6 and final ice decay after 9 7 cal kyr bp deglaciation patterns vary across the fennoscandian ice sheet domain reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins for example the ice sheet margin in the high precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice marginal deposits in many fjords and coastal valleys retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern sweden and rapid retreat of the calving ice margin in the baltic basin our reconstruction is consistent with much of the published research however the synthesis of a large amount of existing and new data support refined reconstructions in some areas for example the lgm extent of the ice sheet in northwestern russia was located far east and it occurred at a later time than the rest of the ice sheet at around 17 15 cal kyr bp we also propose a slightly different chronology of moraine formation over southern sweden based on improved correlations of moraine segments using new lidar data and tying the timing of moraine formation to greenland ice core cold stages retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet with the lowest rates on the high elevation and maritime norwegian margin retreat rates compared to the climatic information provided by the greenland ice core record show a general correspondence between retreat rate and climatic forcing although a close match between retreat rate and climate is unlikely because of other controls such as topography and marine versus terrestrial margins overall the time slice reconstructions of fennoscandian ice sheet deglaciation from 22 to 9 7 cal kyr bp provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the fennoscandian ice sheet and are an important resource for testing and refining ice sheet models

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

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

the ross sea s contribution to eustatic sea level during meltwater pulse 1a abstract radiocarbon dates obtained from ross sea sediments combined with constraints on maximum ice extent based on sedimentological data and ice sheet modeling results indicate that the ross embayment antarctica is unlikely to have been a substantial contributor to meltwater pulse mwp 1a at 12 5 14c ka the ice sheet in the ross embayment during the last glacial maximum is estimated to have contained ice volume equivalent to 3 6 m eustatic sea level rise and does not show evidence of rapid retreat grounded ice advance occurred during initial late pleistocene eustatic rise and ice sheet retreat began coeval with or subsequent to mwp 1a chronological and sedimentological data from the ross embayment indicate that deglaciation in this region has occurred throughout the holocene and the maximum possible contribution of eustatic rise from the ross embayment is

geometry and dynamics of an east antarctic ice sheet outlet glacier under past and present climates 1 the behavior of east antarctic ice sheet eais outlet glaciers both at present and during the recent geological past is of considerable interest both from the point of view of understanding the mechanics of contemporary glacier dynamics and also with regard to epoch scale ice sheet stability during plio pleistocene climate transitions here we use a glacier flowline model that incorporates the effects of longitudinal stresses to numerically simulate ferrar glacier first under present day environmental conditions and subsequently under both colder and warmer climate regimes representing the last glacial maximum lgm and mid pliocene peak warmth respectively using airborne radar profiles insar derived surface velocities ice core and geological data for empirical constraint we present a diagnostic simulation that uses an iterative method to closely reproduce observed dynamics our model suggests that the glacier is largely cold based under present conditions flows predominantly by way of internal deformation and cascades over bedrock ridges due to the combined action of changes in cross sectional valley geometry local steepening of the glacier surface and the non local effects of longitudinal coupling time dependent evolutionary simulation of a lower profile glacier under a colder drier lgm climate predicts flow velocities lower than present with minimal bedrock erosion or basal till flux conversely the warmer than present mid pliocene climate produces a more dynamic glacier that is warm based and sliding along much of its bed we propose that eais outlet glaciers such as the ferrar respond dynamically along their length in response to changing environmental forcings with most significant changes taking place in their lower reaches adjustment to perturbations in upper catchments is more muted

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

sea level changes since the last glacial maximum an appraisal of the ipcc fourth assessment report the ipcc fourth assessment report ar4 stated that global sea level rose by about 120 m between the last glacial maximum lgm and 3 2 ka and did not rise again until the late 19th century a review of relevant literature demonstrates that the rise may have been larger by 10 15 m that the timing of stabilisation is poorly constrained within the last 7 ka bp and that the onset of modern rates of sea level rise is not synchronous and uniform across the globe the lgm lowstand as defined in the ar4 is based on the classic barbados sea level history but the sea level curve from this island may not be an accurate eustatic approximation if glacial isostatic adjustment gia processes were significant as suggested by various authors the barbados sea level curve revealed the existence of meltwater pulse mwp 1a at around 14 000 cal a bp questions about the source and causal mechanism of mwp 1a are still being explored by field scientists and gia modellers these problems have direct relevance for contemporary measurements of ice mass changes because space geodetic measurements rely on information provided by gia models to separate land motion from ice mass changes global patterns of holocene sea level change are controlled by regional sea level variability not only through isostatic and tectonic processes but also through steric effects longer wavelength gravitational changes produced by changing ice ocean mass flux and hemispheric scale perturbations in the earth s rotation eustasy is therefore merely a concept not a measurable quantity constraining the late holocene ice melt contribution to global sea level rise and determining regional sea level rise patterns in the 18th and 19th centuries are important future tasks for sea level scientists these will establish the baseline contributions to 20th and 21st century sea level rise and will help to determine the causes of modern rates of sea level rise constraining historical sea level accelerations and short term ice mass flux will aid models of ice sheet dynamics and help to reduce current uncertainties in future sea level predictions the ar4 shows that regional sea level variability is clearly present in sea level change patterns for the past 50 a and an important challenge for the next ipcc assessment is to incorporate this variability into regional future sea level rise scenarios copyright 2009 john wiley sons ltd

late quaternary ice surface fluctuations of hatherton glacier transantarctic mountains abstract former longitudinal profiles of hatherton glacier an outlet through the transantarctic mountains constrain nearby polar plateau elevations and ice shelf grounding in the southwestern ross embayment four gravel drift sheets of late quaternary age beside hatherton glacier are from youngest to oldest hatherton britannia i britannia ii and danum the hatherton drift limit is uniformly 20 to 70 m above the present ice surface the britannia ii drift limit is within 100 m of the present surface of uppermost hatherton glacier but is 450 m above middle hatherton glacier extrapolation of this profile downglacier indicates a surface elevation 1100 m above the present ross ice shelf the britannia i drift limit is parallel to but 50 100 m below britannia ii drift the danum drift limit is parallel to but 50 100 m above the britannia ii profile from correlation with drifts near mcmurdo sound and from local 14 c dates we assign an early holocene age to hatherton drift a late wisconsin age to britannia drifts and an age of marine isotope stage 6 to danum drift by our age model the upper reaches of hatherton glacier and presumably the adjacent polar plateau have not exceeded their current elevations by more than 100 150 m during the last two complete global glacial interglacial cycles whereas the middle and lower reaches of hatherton glacier have thickened considerably during the last two global glaciations late wisconsin and marine isotope stage 6 the effect of ice shelf grounding probably was the major control of these changes of hatherton glacier holocene ice surface lowering probably represents the last pulse of grounding line recession in the southwestern ross embayment

ice stream flow switching during deglaciation of the southwestern barents sea ice streams dominate the discharge of continental ice sheets recent observations and reconstructions have revealed that large scale reorganizations in their flow trajectory flow switching can occur over relatively short time scales however the underlying causes of such behavior and the extent to which they are predictable are poorly known this paper documents a major episode of ice stream flow switching during the late weichselian deglaciation of the southwestern barents sea and explores various hypotheses for its causation regional bathymetric data show that two ice streams that had similar adjoining topographically constrained source areas had very different trajectories and dynamics on the outer shelf at the late weichselian maximum the hakjerringdjupet ice stream flowed westward along the cross shelf trough of hakjerringdjupet while the soroya trough ice stream flowed northward into ingoydjupet forming a tributary of the bjornoyrenna ice stream initial retreat of the hakjerringdjupet ice stream was rapid but with episodic periods of grounding as it retreated onto the higher rougher topography of the inner shelf we infer a reduction in ice velocity and a dramatic decrease in the pace of retreat as recorded by nested sequences of recessional moraines following and probably in response to this we suggest that there was a short lived surge readvance of an adjacent lobe onto fugloybanken in contrast the adjacent soroya trough ice stream remained active throughout deglaciation before retreating rapidly with no stillstands or readvances we argue that the different retreat histories of the ice streams were determined by variations in bed topography bathymetry which modulated the grounding line response to sea level variation such a mechanism is likely to be an important control on the long term behavior of marine based ice streams and outlet glaciers in antarctica and greenland and suggests that gathering data on their subglacial topography should be a priority

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

chronology of late wisconsin ice retreat from the western ross sea antarctica lithologic data from marine sedimentary cores and accelerator mass spectrometer ams radiocarbon dates indicate that grounded ice did not advance to the western ross sea continental shelf edge during the last glacial maximum lgm a chronology of the timing of ice retreat was provided by 26 ams dates obtained from 12 cores dates ranging from 20 to 29 ka suggest that the outer continental shelf beyond ca lat 74 s was not covered by grounded glacial ice prior to and during the early stages of the lgm 14c dates just above transitions from subglacial diamictons to marine muds indicate that the area around the drygalski ice tongue was deglaciated by at least 11 5 ka the ross ice shelf reached its present day position near ross island by about 7 ka a hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf the hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

ice elevation near the west antarctic ice sheet divide during the last glaciation 1 interior ice elevations of the west antarctic ice sheet wais during the last glaciation which can serve as benchmarks for ice sheet models are largely unconstrained here we report past ice elevation data from the ohio range located near the wais divide and the onset region of the mercer ice stream cosmogenic exposure ages of glacial erratics that record a wais highstand 125 m above the present surface date to 11 5 ka the deglacial chronology prohibits an interior wais contribution to meltwater pulse 1a our observational data of ice elevation changes compare well with predictions of a thermomechanical icesheet model that incorporates very low basal shear stress downstream of the present day grounding line we conclude that ice streams in the ross sea embayment had thin lowslope profiles during the last glaciation and interior wais ice elevations during this period were several hundred meters lower than previous reconstructions citation ackert r p jr s mukhopadhyay b r parizek and h w borns 2007 ice elevation near the west antarctic ice sheet divide during the last glaciation geophys res lett 34 l21506 doi 10 1029 2007gl031412

holocene ice recession at polygon spur reedy glacier antarctica the holocene history of outlet glaciers affords information on the behavior and mechanisms controlling the extent of the east antarctic ice sheet here we present both new radiocarbon and recalcul

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

lithospheric thickness antarctic deglaciation history and ocean basin discretization effects in a global model of postglacial sea level change a summary of some sources of nonuniqueness the global model of postglacial relative sea level variations that has been developed over the past decade is employed to investigate the constraints that it may be invoked to place on the timing of the deglaciation of west antarctica the analyses presented here confirm the suggestion of p wu and w r peltier 1983 geophysical journal of the royal astronomical society74 377 450 that the model of this event employed inj a clark and c s lingle 1979quaternary research11 279 298 may be simply modified to rectify the misfits between theory and observation that are otherwise obtained at southern hemisphere sites a large number of southern hemisphere relative sea level data are shown to require that the retreat of antarctic ice substantially lagged the retreat of northern hemisphere ice if the deglaciation of antarctica was abrupt the time of onset of antarctic deglaciation is thereby shown to coincide with the time of most rapid northern hemisphere deglaciation sensitivity tests are performed which demonstrate that this result is relatively insensitive to the discretization employed to represent the ocean basins the only exception to this general rule obtains at some coastal sites at which a trade off is revealed between the delay of west antarctic melting and the thickness of the lithosphere required to reconcile the observed local variations of relative sea level at such sites which are all in the far field of the ice sheets some attention must be paid to the accuracy of the local finite element representation of the oceans and to the details of the antarctic deglaciation history

antarctic ice sheet elevation impacts on water isotope records during the last interglacial knowledge of how the antarctic ice sheet ais has varied in response to past climates can inform the prediction of future ais behaviour water stable isotope records from antarctic ice cores tradi

distinguishing subglacial till and glacial marine diamictons in the western ross sea antarctica implications for a last glacial maximum grounding line analyses of lithology stratigraphy and tephra from marine sediment cores collected from the western ross sea during cruises eltanin 32 and 52 and deep freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge subglacial till occurs as the lowest unit in most cores landward south of approximately 74 s while seaward of approximately 74 s the lowest diamicton units are glacial marine diamictons glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore distinct tephra layers stratification higher diatom and foraminifera abundances higher sand content and radiocarbon dates in chronological order downcore sand sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events one likely to have been derived from the mount melbourne volcano and the other from the pleiades volcano radiocarbon dates from sediment above and below the tephra layer in one of these cores df87 32 show that deposition indicative of open water conditions occurred between 22 and 26 ka in the western ross sea

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

exposure ages from mountain dipsticks in mac robertson land east antarctica indicate little change in ice sheet thickness since the last glacial maximum past changes in east antarctic ice sheet eais volume are poorly known and diffi cult to measure yet are critical for predicting the response of the ice sheet to modern climate change in particular it is important to identify the sources of sea level rise since the last glacial maximum lgm and ascertain the present day stability of the world s largest ice sheet we present altitudinal transects of 10 be and 26 al exposure ages across the framnes mountains in mac robertson land that allow the magnitude and timing of eais retreat to be quantifi ed our data show that the coastal eais thinned by at most 350 m in this region during the past 13 k y this reduction in ice sheet volume occurred over a 5 k y period and the present icesheet profi le was attained ca 7 ka in contrast to the west antarctic ice sheet which continues to retreat today combined with regional offshore and terrestrial geologic evidence our data suggest that the reduction in eais volume since the lgm was smaller than that indicated by contemporary ice sheet models and added little meltwater to the global oceans stability of the ice margin since the middle holocene provides support for the hypothesis that eais volume changes are controlled by growth and decay of northern hemisphere ice sheets and associated global sea level changes

late wisconsin and early holocene glacial history inner ross embayment antarctica abstract lateral drift sheets of outlet glaciers that pass through the transantarctic mountains constrain past changes of the huge ross ice drainage system of the antarctic ice sheet drift stratigraphy suggests correlation of reedy iii reedy glacier beardmore beardmore glacier britannia hatherton darwin glaciers ross sea mcmurdo sound and younger terra nova bay drifts radiocarbon dates place the outer limits of ross sea drift in late wisconsin time at 24 000 13 000 yr b p outlet glacier profiles from these drifts constrain late wisconsin ice sheet surface elevations within these constraints we give two extreme late wisconsin reconstructions of the ross ice drainage system both show little elevation change of the polar plateau coincident with extensive ice shelf grounding along the inner ross embayment however in the central ross embayment one reconstruction shows floating shelf ice whereas the other shows a grounded ice sheet massive late wisconsin holocene recession of grounded ice from the western ross embayment which was underway at 13 040 yr b p and completed by 6600 6020 yr b p was accompanied by little change in plateau ice levels inland of the transantarctic mountains sea level and basal melting probably controlled the extent of grounded ice in the ross embayment the interplay between the precipitation low late wisconsin and high holocene values and the influence of grounding on outlet glaciers late wisconsin thickening and late wisconsin holocene thinning with effects dying out inland probably controlled minor elevation changes of the polar plateau

holocene sea level database for the atlantic coast of the united states we have constructed a database of holocene relative sea level rsl observations for the atlantic coast of the united states the database contains 492 index points which locate the position of rsl in time and space and 344 limiting dates which constrain the minimum or maximum limit of former sea level the majority of the index points in the database are from 6 ka bp to present with only 7 older than 6 ka bp spatially index points are distributed between maine and south carolina but there is an absence of data from georgia and the atlantic coast of florida the database is sub divided into 16 regions based on the distance from the former laurentide ice sheet and are classified depending upon their susceptibility to compaction the index points and limiting data demonstrate that rsl did not exceed present 0 m during the holocene except potentially in regions 1 and 2 eastern maine and southern maine rates of rsl change were highest during the early holocene and have decreased over time due to the diminishing response of the earth s mantle to glacial isostatic adjustment and reduction of ice equivalent meltwater input along the atlantic coast of the united states the linear rate of rsl rise prior to 4 ka bp ranged from 0 5 to 4 5 mm a 1 compared to 0 6 1 8 mm a 1 from 4 ka bp to ad 1900 the database suggests minimal 0 3 mm yr changes in these rates of rsl rise during the late holocene deglaciation of the laurentide ice sheet caused the spatial variability captured by the database the maximum rate of late holocene and ongoing rsl rise occurred in mid atlantic regions new jersey and inner delaware because of collapse of the peripheral forebulge

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

ice rise stratigraphy reveals changes in surface mass balance over the last millennia in dronning maud land we use ice flow modelling to simulate the englacial stratigraphy of blaskimen island an ice rise in dronning maud land and elucidate the evolution of this data sparse region we apply a thermo mechanically coupled elmer ice model to a profile along flowlines and through the ice rise summit where surface mass balance smb flow velocity and ice stratigraphy were recently measured we conclude that i the ice rise is presently thickening at a rate of 0 5 0 6 m ice equivalent per year m ieq a 1 which is twice an earlier estimate using the field data and the input output method ii present thickening started 20 40 years in the past before which the ice rise was in a steady state iii smb contrast between the upwind and downwind slopes was stronger than the present value by 23 or 0 15 m ieq a 1 prior to 1100 years ago since then this contrast has been decreasing overall we surmise that these smb changes are likely a result of synoptic scale atmospheric changes rather than local atmospheric changes controlled by local ice topography our technique effectively assimilates geophysical data avoiding the complexity of ice flow beneath the ice divide thus it could be applied to other ice rises to elucidate the recent glacial retreat

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

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

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