evidence for floatation or near floatation in the mouth of kamb ice stream west antarctica prior to stagnation 1 ice penetrating radar profiles reveal distinctive internal structure within the flat ice terrains that bound the lower reaches of the now stagnant kamb ice stream west antarctica slightly warped but continuous upper layers overlie extensive deep line diffractors located at a uniform depth several hundreds of meters above the bed on the basis of their orientation and morphology we interpret the line diffractors to be the tips of basal crevasses at the boundary between these terrains and the inter ice stream ridges internal layers are strongly downwarped and the deepest layers are truncated at the bed results from simple kinematic ice flow modeling indicate that the observed layer pattern requires a cumulative basal melt of 120 350 m over a time period that is less than 300 years fields of basal crevasses are typically observed in ice shelves however isolated basal crevasses can also be found in grounded ice when the subglacial water pressure is close to overburden on the basis of the presence of these crevasses we argue that these terrains were once floating or near floating in the past we combine model results with estimates of recent ice thickness change to suggest that floatation was transitory and the region may have become fully grounded roughly 200 years prior to stagnation of kamb ice stream

twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

self similarity of the largest scale segmentation of the faults implications for earthquake behavior article i nfo article history earthquakes are sensitive to the along strike segmentation of the faults they break especially in their initiation propagation and arrest we examine that segmentation and search whether it shows any specific properties we focus on the largest scale fault segmentation which controls the largest earthquakes it is well established that major segments within faults markedly shape their surface cumulative slip length profiles segments appear as large slip bumps separated by narrow pronounced slip troughs inter segments we use that property to examine the distribution location number length of the major segments in 927 active normal faults in afar east africa of various lengths 0 3 65 km cumulative slips 1 1300 m slip rates 0 5 5 mm yr and ages 10 4 10 6 yr this is the largest fault population ever analyzed to identify the major bumps in the slip profiles and determine their number location and length we analyze the profiles using both the classical fourier transform and a space frequency representation of the profiles the s transform which is well adapted for characterizing local spectral properties our work reveals the fol lowing results irrespective of their length 70 of the slip profiles have a triangular envelope shape in conflict with the elastic crack concept irrespective of their length the majority of the faults at least 50 70 have a limited number of major segments between 2 and 5 and more commonly equal to 3 5 the largest scale segmentation of the faults is thus self similar and likely to be controlled by the fault mechanics the slip deficits at the major inter segment slip troughs tend to smooth as the faults accumulate more slip resulting in increased connection of the major segments the faults having accumulated more slip therefore generally appear as un segmented 10 30 our observations therefore show that whatever the fault on which they initiate large earthquakes face the same number of major segments to potentially break the number of segments that they eventually break seems to depend on the slip history structural maturity of the fault

rise of the ellsworth mountains and parts of the east antarctic coast observed with gps 1 using gps observations from 1996 to 2011 we constrain postglacial rebound in antarctica sites in the ellsworth mountains west antarctica are rising at 5 4 mm yr 95 confidence limits as in the postglacial rebound model of peltier but 10 mm yr slower than in the model of ivins and james therefore significant ice loss from the ellsworth mountains ended by 4 ka and current ice loss there is less than inferred from grace gravity observations in studies assuming the model of ivins and james three sites along the coast of east antarctica are rising at 3 to 4 2 mm yr in viscous response to holocene unloading of ice along the queen maud land coast and elsewhere kerguelen island and seven sites along the coast of east antarctic are part of a rigid antarctica plate o higgins northern antarctic peninsula is moving southeast at 2 3 0 6 mm yr relative to the antarctic plate

the rotational stability of an ice age earth summary predictions of glaciation induced changes in the earth s rotation vector exhibit sensitivities to earth structure that are unique within the suite of long wavelength observables associated with glacial isostatic adjustment henceforth gia and despite nearly a quarter of a century of research these sensitivities remain enigmatic previous predictions of present day true polar wander tpw speed driven by gia have indicated for example a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment phase or chemical of the density discontinuity at 670 km depth nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere however his results have introduced an intriguing paradox in particular predictions generated using a model with an extremely high viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness mitrovica as cited by nakada has suggested that the paradox originates from an inaccuracy in the traditional rotation theory e g wu peltier associated with the treatment of the background equilibrium rotating form upon which any load and rotation induced perturbations are superimposed we revisit these issues using a new treatment of the linearized euler equations governing load induced rotation perturbations on viscoelastic earth models we demonstrate that our revised theory in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity resolves the apparent paradox calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors indeed previously noted sensitivities of tpw speed predictions to the thickness and rheology elastic versus viscous of the lithosphere largely disappear in the application of the new theory significant errors are also incurred by neglecting the stabilizing influence of the earth s excess ellipticity finally we demonstrate that the contribution from rotational feedback on predictions of present day rates of change of the geoid sea surface and crustal velocities are overestimated by the traditional rotation theory and this has implications for analyses of ongoing satellite e g grace missions and geodetic gps surveys

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

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

the effect of directivity in a psha framework summary we propose a method to introduce a refined representation of the ground motion in the framework of the probabilistic seismic hazard analysis psha this study is especially oriented to the incorporation of a priori information about source parameters by focusing on the directivity effect and its influence on seismic hazard maps two strategies have been followed one considers the seismic source as an extended source and it is valid when the psha seismogenetic sources are represented as fault segments we show that the incorporation of variables related to the directivity effect can lead to variations up to 20 per cent of the hazard level in case of dip slip faults with uniform distribution of hypocentre location in terms of spectral acceleration response at 5 s exceeding probability of 10 per cent in 50 yr the second one concerns the more general problem of the seismogenetic areas where each point is a seismogenetic source having the same chance of enucleate a seismic event in our proposition the point source is associated to the rupture related parameters defined using a statistical description as an example we consider a source point of an area characterized by strike slip faulting style with the introduction of the directivity correction the modulation of the hazard map reaches values up to 100 per cent for strike slip unilateral faults the introduction of directivity does not increase uniformly the hazard level but acts more like a redistribution of the estimation that is consistent with the fault orientation a general increase appears only when no a priori information is available however nowadays good a priori knowledge exists on style of faulting dip and orientation of faults associated to the majority of the seismogenetic zones of the present seismic hazard maps the percentage of variation obtained is strongly dependent on the type of model chosen to represent analytically the directivity effect therefore it is our aim to emphasize more on the methodology following which all the information collected may be easily converted to obtain a more comprehensive and meaningful probabilistic seismic hazard formulation

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

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

local insolation changes enhance antarctic interglacials insights from an 800 000 year ice sheet simulation with transient climate forcing abstract the antarctic ice sheet storing 27 million cubic kilometres of ice has the potential to contribute greatly to future sea level rise yet its past evolution and sensitivity to long term climatic drivers remain poorly understood and constrained in particular a long standing debate questions whether antarctic climate and ice volume respond mostly to changes in global sea level and atmospheric greenhouse gas concentrations or to local insolation changes so far long term antarctic simulations have used proxy based parameterizations of climatic drivers presuming that external forcings are synchronous and spatially uniform here for the first time we use a transient three dimensional climate simulation over the last eight glacial cycles to drive an antarctic ice sheet model we show that the evolution of the antarctic ice sheet was mostly driven by co2 and sea level forcing with a period of about 100 000 yr synchronizing both hemispheres however on precessional time scales local insolation forcing drives additional mass loss during periods of high sea level and co2 enhancing the antarctic interglacial and putting northern and southern ice sheet variability temporarily out of phase in our simulations partial collapses of the west antarctic ice sheet during warm interglacials are only simulated with unrealistically large southern ocean subsurface warming exceeding 4 c overall our results further elucidate the complex interplay of global and local forcings in driving late quaternary antarctic ice sheet evolution and the unique and overlooked role of precession therein

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

fault interactions and large complex earthquakes in the los angeles area faults in complex tectonic environments interact in various ways including triggered rupture of one fault by another that may increase seismic hazard in the surrounding region we model static and dynamic fault interactions between the strike slip and thrust fault systems in southern california we find that rupture of the sierra madre cucamonga thrust fault system is unlikely to trigger rupture of the san andreas or san jacinto strike slip faults however a large northern san jacinto fault earthquake could trigger a cascading rupture of the sierra madre cucamonga system potentially causing a moment magnitude 7 5 to 7 8 earthquake on the edge of the los angeles metropolitan region

paleo constraints on sea level rise palsea ice sheet and sea level responses to past climate warming abstract here we summarize the motivation and issues surrounding the responses of ice sheets and sea level to past climate warming as part of the paleo constraints on sea level rise palsea working group papers in this special issue of quaternary science reviews focus on the timescale of glaciations during the late pliocene the magnitude of ice sheet fluctuations and volume leading up to and during the last glacial maximum the timing and persistence of ice sheet impacts on deglacial and future relative sea level change and relative sea level change during peak interglacial climate a more dynamic cryosphere is noted under both late pliocene and last glacial cycle climate conditions while relative sea level changes during the last deglaciation appear to correspond closely with individual ice sheet deglaciation lastly relative sea level change during peak interglacial conditions may have fluctuated by as much as a meter although the sources of such variability greenland antarctica or elsewhere remain elusive

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

cosmogenic nuclide exposure ages from the pensacola mountains adjacent to the foundation ice stream antarctica we describe glacial geological observations and cosmogenic nuclide exposure ages from the schmidt williams and thomas hills in the pensacola mountains of antarctica adjacent to the foundation ice stream fis our aim is to learn about changes in the thickness and grounding line position of the antarctic ice sheet in the weddell sea embayment between the last glacial maximum lgm and the present glacial geological observations from all three regions indicate that currently ice free areas were covered by ice during one or more past ice sheet expansions and that this ice was typically frozen to its bed and thus non erosive permitting the accumulation of multiple generations of glacial drift cosmogenic nuclide exposure age data from glacially transported erratics are consistent with this interpretation in that we observe both i samples with holocene exposure ages that display a systematic age elevation relationship recording lgm to present deglaciation and ii samples with older and highly scattered apparent exposure ages that were deposited in previous glacial interglacial cycles and have experienced multiple periods of surface exposure and ice cover holocene exposure ages at the thomas and williams hills upstream of the present grounding line of the fis show that the fis was at least 500 m thicker prior to 11 ka and that 500 m of thinning took place between 11 and 4 ka however exposure age data from the schmidt hills downstream of the present grounding line of the fis show no evidence for lgm thickening of the fis and in fact provide some evidence that the fis could have been no more than 200 m thicker than present at the lgm if all these observations are correct they imply that the lgm and early holocene ice surface slope in the vicinity of the present grounding line was steeper than present which is inconsistent with glaciological model predictions of possible lgm ice sheet configurations specifically scenarios in which the lgm grounding line of the fis advanced to the outer continental shelf appear inconsistent with exposure age data from the schmidt hills whereas scenarios in which the fis grounding line did not advance at the lgm appear inconsistent with exposure age data from the williams and thomas hills

ice flow reorganization in west antarctica 2 5 kyr ago dated using radar derived englacial flow velocities we date a recent ice flow reorganization of an ice divide in the weddell sea sector west antarctica using a novel combination of inverse methods and ice penetrating radars we invert for two dimensional ice flow within an ice divide from data collected with a phase sensitive ice penetrating radar while accounting for the effect of firn on radar propagation and ice flow by comparing isochronal layers simulated using radar derived flow velocities with internal layers observed with an impulse radar we show that the divide s internal structure is not in a steady state but underwent a disturbance potentially implying a regional ice flow reorganization 2 5 1 8 2 9 kyr b p our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary these findings increase our knowledge of the glacial history of a region that lacks dated constraints on late holocene ice sheet retreat and provides a key target for models that reconstruct and predict ice sheet behavior

a baseline antarctic gia correction for space gravimetry abstract within the past decade newly collected gps data and geochronological constraints have resulted in refinement of glacial isostatic adjustment gia models for antarctica these are critical to estimating and understanding ice mass changes at present day a correction needs to be made when using space gravity for ice mass balance assessments due to the fact that any vertical movements of the solid earth masquerade as changes in ice mass and these must be carefully removed the main upshot of the new antarctic gia models is a downward revision of negative ice mass trends deduced from the gravity recovery and climate experiment grace resulting from a reduced gia correction this revision places grace inferred trend in mass balance within the one sigma uncertainty of mass balance deduced by altimetry because uncertainties in holocene ice history and the low viscosity rheology beneath the west antarctic ice sheet wais continue to vex further improvement in predictions of present day gia gravity rate more emphasis has been given to regional scale gia models here we use a bayesian method to explore the gravimetric gia trend over antarctica both with and without the impact of a late pleistocene antarctic ice loads along with the contribution of oceanic loads we call this model without loads associated with antarctica a baseline for regional gia models to build upon we consider variations of the radial mantle viscosity profile and the volume of continental scale ice sheets during the last glacial cycle the modeled baseline gia is mainly controlled by the lower mantle viscosity and continental levering caused by ocean loading we find that the predicted baseline gia correction weakly depends on the ice history this correction averages to 28 4 16 5 41 9 95 confidence gt yr of which 23 6 5 3 are contributed by east antarctica in contrast with late pleistocene antarctic proximal ice included the total modeled mass trend due to gia is 73 7 30 1 114 7 gt yr a baseline gia correction of 28 4 gt yr is of order 50 of the mean net mass trend measured during the period 1992 2017 the statistical analysis provides tools for properly synthesizing any regional antarctic gia model with a self consistent far field component this may prove important for accounting for both global and regional 3 d variations in mantle viscosity

balancing the last glacial maximum lgm sea level budget abstract estimates of post last glacial maximum lgm sea level rise are not balanced by the estimated amount of ice melted since the lgm we quantify this missing ice by reviewing the possible contributions from each of the major ice sheets this missing ice amounts to 18 1 9 6 m of global sea level rise ocean expansion accounts for 2 4 0 3 m of this discrepancy while groundwater could contribute a maximum of another 1 4 m to this offset after accounting for these two potential contributors to the sea level budget the shortfall of 15 6 9 6m suggests that either a large reservoir of water e g a missing lgm ice sheet has yet to be discovered or current estimates of one or more of the known lgm ice sheets are too small included within this latter possibility are potential inadequacies of current models of glacial isostatic adjustment

abrupt mid holocene ice loss in the western weddell sea embayment of antarctica abstract the glacial history of the westernmost weddell sea sector of antarctica since the last glacial maximum is virtually unknown and yet it has been identified as critical for improving reliability of glacio isostatic adjustment models that are required to correct satellite derived estimates of ice sheet mass balance better knowledge of the glacial history of this region is also important for validating ice sheet models that are used to predict future contribution of the antarctic ice sheet to sea level rise here we present a new holocene deglacial chronology from a site on the lassiter coast of the antarctic peninsula which is situated in the western weddell sea sector samples from 12 erratic cobbles and 18 bedrock surfaces from a series of presently exposed ridges were analysed for cosmogenic 10be exposure dating and a smaller suite of 7 bedrock samples for in situ 14c dating the resulting 10be ages are predominantly in the range 80 690 ka whereas bedrock yielded much younger in situ 14c ages in the range 6 0 7 5 ka for samples collected from 138 385 m above the modern ice surface from these we infer that the ice sheet experienced a period of abrupt thinning over a short time interval no more than 2700 years in the mid holocene resulting in lowering of its surface by at least 250 m any late holocene change in ice sheet thickness such as re advance postulated by several modelling studies must lie below the present ice sheet surface the substantial difference in exposure ages derived from 10be and 14c dating for the same samples additionally implies ubiquitous 10be inheritance acquired during ice free periods prior to the last deglaciation an interpretation that is consistent with our glacial geomorphological field observations for former cold based ice cover the results of this study provide evidence for an episode of abrupt ice sheet surface lowering in the mid holocene similar in rate timing and magnitude to at least two other locations in antarctica

an assessment of forward and inverse gia solutions for antarctica in this work we assess the most recent estimates of glacial isostatic adjustment gia for antarctica including those from both forward and inverse methods the assessment is based on a comparison of the estimated uplift rates with a set of elastic corrected gps vertical velocities these have been observed from an extensive gps network and computed using data over the period 2009 2014 we find systematic underestimations of the observed uplift rates in both inverse and forward methods over specific regions of antarctica characterized by low mantle viscosities and thin lithosphere such as the northern antarctic peninsula and the amundsen sea embayment where its recent ice discharge history is likely to be playing a role in current gia uplift estimates for regions where many gia models have traditionally placed their uplift maxima such as the margins of filchner ronne and ross ice shelves are found to be overestimated gia estimates show large variability over the interior of east antarctica which results in increased uncertainties on the ice sheet mass balance derived from gravimetry methods

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

lower satellite gravimetry estimates of antarctic sea level contribution a new model of glacial isostatic adjustment used in conjunction with grace satellite data suggests that ice loss from antarctica is contributing 0 19 millimetres per year to global mean sea level which is substantially less than previous grace based estimates estimates for recent mass loss from antarctica vary widely making it difficult to calculate any subsequent contributions to sea level rise one prominent approach uses the grace gravity recovery and climate experiment satellites to detect changes in gravity but the final estimates are dependent on the modelled response of the continent to changes in ice loading over time and to changes in lithospheric elastic rebound matt king and colleagues apply a new model of this glacial isostatic adjustment to the grace data and estimate antarctic mass loss between 2002 and 2010 to be about 69 gigatonnes per year much lower than previous estimates and equivalent to an annual sea level rise of about 0 2 millimetres recent estimates of antarctica s present day rate of ice mass contribution to changes in sea level range from 31 gigatonnes a year gt yr 1 ref 1 to 246 gt yr 1 ref 2 a range that cannot be reconciled within formal errors3 time varying rates of mass loss2 4 5 6 contribute to this but substantial technique specific systematic errors also exist3 in particular estimates of secular ice mass change derived from gravity recovery and climate experiment grace satellite data are dominated by significant uncertainty in the accuracy of models of mass change due to glacial isostatic adjustment7 8 gia here we adopt a new model of gia developed from geological constraints which produces gia rates systematically lower than those of previous models and an improved fit to independent uplift data9 after applying the model to 99 months from august 2002 to december 2010 of grace data we estimate a continent wide ice mass change of 69 18 gt yr 1 0 19 0 05 mm yr 1 sea level equivalent this is about a third to a half of the most recently published grace estimates2 5 which cover a similar time period but are based on older gia models plausible gia model uncertainties and errors relating to removing longitudinal grace artefacts destriping confine our estimate to the range 126 gt yr 1 to 29 gt yr 1 0 08 0 35 mm yr 1 sea level equivalent we resolve 26 independent drainage basins and find that antarctic mass loss and its acceleration is concentrated in basins along the amundsen sea coast outside this region we find that west antarctica is nearly in balance and that east antarctica is gaining substantial mass

controls on last glacial maximum ice extent in the weddell sea embayment antarctica the weddell sea sector of the antarctic ice sheet is hypothesized to have made a significant contribution to sea level rise since the last glacial maximum using a numerical flowline model we investigate the controls on grounding line motion across the eastern weddell sea and compare our results with field data relating to past ice extent specifically we investigate the influence of changes in ice temperature accumulation sea level ice shelf basal melt and ice shelf buttressing on the dynamics of the foundation ice stream we find that ice shelf basal melt plays an important role in controlling grounding line advance while a reduction in ice shelf buttressing is found to be necessary for grounding line retreat there are two stable positions for the grounding line under glacial conditions at the northern margin of berkner island and at the continental shelf break global mean sea level contributions associated with these two scenarios are 50 mm and 130 mm respectively comparing model results with field evidence from the pensacola mountains and the shackleton range we find it unlikely that ice was grounded at the continental shelf break for a prolonged period during the last glacial cycle however we cannot rule out a brief advance to this position or a scenario in which the grounding line retreated behind present during deglaciation and has since re advanced better constraints on past ice sheet and ice shelf geometry ocean temperature and ocean circulation are needed to reconstruct more robustly past behavior of the foundation ice stream

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

evolution of a coupled marine ice sheet sea level model we investigate the stability of marine ice sheets by coupling a gravitationally self consistent sea level model valid for a self gravitating viscoelastically deforming earth to a 1 d marine ice sheet shelf model the evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat we find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat in simulations with shallow reversed bed slopes and or small external forcing the drop in sea level can be sufficient to halt the retreat the rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry and a viscous component due to past ice and ocean load changes when the ice sheet model is forced from steady state on short timescales 500 years viscous effects may be ignored and grounding line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux on longer timescales an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

glacial isostatic adjustment on a rotating earth summary we extend and complete previous work to compute the influence of perturbations to the rotation vector on a suite of observables associated with glacial isostatic adjustment gia we emphasize observables relevant to present and future geodetic missions for example present day 3 d crustal motions relative sea level change and geoid or absolute sea level variations our calculations adopt spherically symmetric self gravitating maxwell viscoelastic earth models while incorporating realistic mass ice plus ocean load and rotation variations the predicted rotation induced signals are dominated by the influence of true polar wander tpw the spatial geometry of the tpw induced relative sea level geoid and radial velocity fields is primarily that of a degree two order one surface spherical harmonic the spatial variation of the horizontal velocity vectors is given by the gradient of this harmonic the peak radial and horizontal velocities are of the order of 0 5 mm yr 1 however we show that this value is sensitive to the adopted profile of mantle viscosity we also demonstrate that an accurate prediction of tpw induced sea level and 3 d crustal deformation rates requires that a realistic number of glacial cycles be incorporated into the ice load history we conclude that geodetic observations of the gia process should be analysed using a gia theory valid for a rotating planet finally we also consider variations in rotation driven by simple present day polar melting scenarios and predict the influence of these variations on a suite of geophysical observables we find that the rotational feedback associated with greenland melting is capable of significantly perturbing both relative and absolute sea level variations

variation of large elastodynamic earthquakes on complex fault systems 1 one of the biggest assumptions and a source of some of the biggest uncertainties in earthquake hazard estimation is the role of fault segmentation in controlling large earthquake ruptures here we apply a new model which produces sequences of elastodynamic earthquake events on complex segmented fault systems and use these simulations to quantify the variation of large events we find a number of important systematic effects of segment geometry on the slip variation and the repeat time variation of large events including an increase in variation at the ends of segments and a decrease in variation for the longest segments

measurements of time variable gravity show mass loss in antarctica using measurements of time variable gravity from the gravity recovery and climate experiment satellites we determined mass variations of the antarctic ice sheet during 2002 2005 we found that the mass of the ice sheet decreased significantly at a rate of 152 80 cubic kilometers of ice per year which is equivalent to 0 4 0 2 millimeters of global sea level rise per year most of this mass loss came from the west antarctic ice sheet

continuous gps measurements of postglacial adjustment in fennoscandia 1 geodetic results 1 data collected under the auspices of the bifrost gps project yield a geographically dense suite of estimates of present day three dimensional 3 d crustal deformation rates in fennoscandia johansson et al 2002 a preliminary forward analysis of these estimates milne et al 2001 has indicated that models of ongoing glacial isostatic adjustment gia in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3 d velocity field in this study we revisit our previous gia analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the bifrost rate estimates to establish insight into the physics of the gia response in the region we begin by decomposing a forward prediction into the three contributions associated with the ice ocean and rotational forcings from this analysis we demonstrate that recent advances in postglacial sea level theory in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin involve peak signals that are larger than the observational uncertainties in the bifrost network the forward analysis is completed by presenting predictions for a pair of fennoscandian ice histories and an extensive suite of viscoelastic earth models the former indicates that the bifrost data set provides a powerful discriminant of such histories the latter yields bounds on the assumed constant upper and lower mantle viscosity nu um nu lm specifically we derive a 95 confidence interval of 5 x 10 20 less than or equal to nu um less than or equal to 10 21 pa s and 5 x 10 21 less than or equal to nu lm less than or equal to 5 x 10 22 pa s with some preference for elastic lithospheric thickness in excess of 100 km the main goal of the bayesian inverse analysis is to estimate the radial resolving power of the bifrost gps data as a function of depth in the mantle assuming a reasonably accurate ice history we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle we conclude that the bifrost data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer however these data provide little constraint on viscosity in the bottom half of the mantle finally elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity

constraining models of postglacial rebound using space geodesy a detailed assessment of model ice 5g vm2 and its relatives summary using global positioning system very long baseline interferometry satellite laser ranging and doppler orbitography and radiopositioning integrated by satellite observations including the canadian base network and fennoscandian bifrost array we constrain in models of postglacial rebound the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth we test model ice 5g vm2 t90 rot which well fits many hundred holocene relative sea level histories in north america europe and worldwide ice 5g is the deglaciation history having more ice in western canada than ice 4g vm2 is the mantle viscosity profile having a mean upper mantle viscosity of 0 5 1021 pa s and a mean uppermost lower mantle viscosity of 1 6 1021 pa s t90 is an elastic lithosphere thickness of 90 km and rot designates that the model includes rotational feedback earth s response to the wander of the north pole of earth s spin axis towards canada at a speed of 1 myr 1 the vertical observations in north america show that relative to ice 5g the laurentide ice sheet at last glacial maximum lgm at 26 ka was 1 much thinner in southern manitoba 2 thinner near yellowknife northwest territories 3 thicker in eastern and southern quebec and 4 thicker along the northern british columbia alberta border or that ice was unloaded from these areas later thicker or earlier thinner than in ice 5g the data indicate that the western laurentide ice sheet was intermediate in mass between ice 5g and ice 4g the vertical observations and grace gravity data together suggest that the western laurentide ice sheet was nearly as massive as that in ice 5g but distributed more broadly across northwestern canada vm2 poorly fits the horizontal observations in north america predicting places along the margins of the laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr 1 in ice 4g and 3 mm yr 1 in ice 5g in disagreement with the observation that the interior of the north american plate is deforming more slowly than 1 mm yr 1 substituting vm5a t60 for vm2 t90 that is introducing into the lithosphere at its base a layer with a high viscosity of 10 1021 pa s greatly improves the fit of the horizontal observations in north america ice 4g vm5a t60 rot predicts most of the north american plate to be moving horizontally more slowly than 1 mm yr 1 in agreement with the data ice 5g vm5a t60 rot well fits both the vertical and horizontal observations in europe the space geodetic data cannot distinguish between models with and without rotational feedback in the vertical because the velocity of earth centre is uncertain and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors

a deglacial model for antarctica geological constraints and glaciological modelling as a basis for a new model of antarctic glacial isostatic adjustment abstract we present a new reconstruction of the antarctic ice sheets between 20 ka bp and the present day our reconstruction is derived using a numerical model to generate a physically consistent ice surface across the whole of the continent we define the extent of the ice sheet at five time slices 20 15 10 5 and 0 ka bp assuming an equilibrium state for the 20 ka bp time slice and a transient state for the deglacial time slices the evolution of the ice sheet within the numerical model is driven by variations in temperature accumulation rate and relative sea level in order to reconstruct the concave profile of the ice sheet in marine grounded regions such as the ross and weddell seas we force our model to develop channels of faster flow by defining greater basal sliding along the trajectory of former ice streams we find a strong dependence upon the basal sliding parameters and also the position of the grounding line we use an extensive data base of geological and glaciological data to constrain our ice sheet reconstruction grounding line extent is prescribed from marine geological data and we test ice sheet thickness against onshore geological data at 62 sites of the five time slices considered our 20 ka bp reconstruction is the best constrained by data and has an rms misfit of 147 6 m when compared to observations of ice thickness change between 20 ka bp and the present day across all time slices there are large regions of the ice sheet which are poorly constrained especially after 20 ka bp we estimate the spatial distribution of uncertainty in our ice sheet reconstruction and note that the solutions are least reliable in regions of complex topography we predict that the antarctic ice sheets contributed 9 1 5 m of eustatic sea level to the global ocean between 20 ka bp and the present and our reconstruction with minimum misfit contributes 8 m eustatic sea level during this period these values which we argue are an upper bound are lower than many previous estimates the reconstructed pattern of ice unloading can serve as a new input for glacial isostatic models

the dynamics of strike slip step overs with linking dip slip faults fault step overs with linking dip slip faults are common features on long strike slip fault systems worldwide it has been noted by various researchers that under some circumstances earthquakes can jump across fault step overs to cascade into large events while under other circumstances rupture is arrested at step overs there is also evidence that fault step overs may be preferential locations for earthquake nucleation the present work uses the 3d finite element method to model the dynamics of strike slip fault systems with step overs and linking dip slip faults i find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step over leading to a larger event additionally dilational step overs with linking normal faults are more prone to through going rupture than compressional step overs with linking thrust faults this difference is due to the sign of the normal stress increment on the dip slip fault caused by slip on the strike slip segments slip on the strike slip segments causes a negative unclamping normal stress increment on the linking normal fault in a dilational step over whereas the opposite effect occurs on the linking thrust fault in a compressional step over even in cases for which both dilational and compressional step overs can experience through going rupture dilational step overs typically experience higher slip particularly on the linking normal fault in the compressional case rupture nucleation on the linking thrust fault may increase the likelihood of through going rupture compared to nucleation on one of the strike slip segments near the intersections between the fault segments the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field the results help to emphasize the importance of two way interactions between nearby fault segments during the earthquake rupture process the results also may have implications for the probability of large earthquakes along geometrically complex strike slip fault systems and may help explain why step overs sometimes act as barriers and other times as nucleation locations for large earthquakes

reconstruction of changes in the weddell sea sector of the antarctic ice sheet since the last glacial maximum the weddell sea sector is one of the main formation sites for antarctic bottom water and an outlet for about one fifth of antarctica s continental ice volume over the last few decades studies on glacial geological records in this sector have provided conflicting reconstructions of changes in ice sheet extent and ice sheet thickness since the last glacial maximum lgm at ca 23 19 calibrated kiloyears before present cal ka bp terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the weddell sea ice sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding line advance at the lgm other marine geological and geophysical studies concluded that subglacial bedforms mapped on the weddell sea continental shelf subglacial deposits and sediments over compacted by overriding ice recovered in cores and the few available radiocarbon ages from marine sediments are consistent with major ice sheet advance at the lgm reflecting the geological interpretations different ice sheet models have reconstructed conflicting lgm ice sheet configurations for the weddell sea sector consequently the estimated contributions of ice sheet build up in the weddell sea sector to the lgm sea level low stand of 130 m vary considerably in this paper we summarise and review the geological records of past ice sheet margins and past ice sheet elevations in the weddell sea sector we compile marine and terrestrial chronological data constraining former ice sheet size thereby highlighting different levels of certainty and present two alternative scenarios of the lgm ice sheet configuration including time slice reconstructions for post lgm grounding line retreat moreover we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research the aim of our study is to provide two alternative interpretations of glacial geological datasets on antarctic ice sheet history for the weddell sea sector which can be utilised to test and improve numerical ice sheet models

glacial isostatic adjustment of scandinavia and northwestern europe and the radial viscosity structure of the earth s mantle summary during the last glacial maximum lgm large ice sheets covered scandinavia the barents sea and the northern british isles subsequent to the lgm the ice sheets disappeared and the solid earth readjusted towards a new isostatic equilibrium the glacial isostatic adjustment process is documented in numerous observations for example palaeoshorelines covering the last deglaciation phase and ongoing crustal deformations monitored by gps stations for example the bifrost project in this study we use palaeoshoreline data from scandinavia the barents sea and northwestern europe nw as well as radial crustal velocities from the bifrost campaign to infer the radial viscosity structure of the earth s mantle underneath scandinavia and nw europe a global inverse procedure based on the neighbourhood algorithm allows us to explore the hypothesis of a low viscosity zone in the upper mantle which has been proposed in the literature our results indicate a low viscosity zone underneath the barents sea with viscosities between 1019 and 1020 pa s in a depth interval of 160 200 km no such low viscosity zone is found underneath scandinavia and no clear indication for such a zone underneath nw europe the thickness of the rheological lithosphere increases from 60 to 70 km underneath nw europe and the barents sea towards values exceeding 120 km underneath scandinavia

dynamic transfer of rupture across differently oriented segments in a complex 3 d fault system 1 we simulate spontaneous dynamic propagation of rupture across two adjacent fault segments subject to a triaxial compressive stress regime these segments have different orientations and hence different focal mechanisms vertical strike slip and dipping thrust numerical simulations using a biem boundary integral equation method have revealed that under a typical triaxial homogeneous compressive stress regime where the magnitude of the intermediate principal stress lies halfway between those of the maximum and minimum ones ruptures of different focal mechanisms are not likely to occur simultaneously in a single rupture event propagation of rupture from a vertical strike slip fault segment to a pure dip slip normal reverse fault segment is possible only when the stress field is close to uniaxial compression or when the intermediate stress magnitude is close either to the minimum or the maximum one these findings are useful for evaluating possible earthquake scenarios along fault systems with complex 3d geometries

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

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

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

antarctic contribution to sea level rise observed by grace with improved gia correction antarctic volume changes during the past 21 thousand years are smaller than previously thought and here we construct an ice sheet history that drives a forward model prediction of the glacial isostatic adjustment gia gravity signal the new model in turn should give predictions that are constrained with recent uplift data the impact of the gia signal on a gravity recovery and climate experiment grace antarctic mass balance estimate depends on the specific grace analysis method used for the method described in this paper the gia contribution to the apparent surface mass change is re evaluated to be 55 13 gt yr by considering a revised ice history model and a parameter search for vertical motion predictions that best fit the gps observations at 18 high quality stations although the gia model spans a range of possible earth rheological structure values the data are not yet sufficient for solving for a preferred value of upper and lower mantle viscosity nor for a preferred lithospheric thickness grace monthly solutions from the center for space research release 04 csr rl04 release time series from january 2003 to the beginning of january 2012 uncorrected for gia yield an ice mass rate of 2 9 29 gt yr the new gia correction increases the solved for ice mass imbalance of antarctica to 57 34 gt yr the revised gia correction is smaller than past grace estimates by about 50 to 90 gt yr the new upper bound to the sea level rise from the antarctic ice sheet averaged over the time span 2003 0 2012 0 is about 0 16 0 09 mm yr

postglacial sea level change on a rotating earth summary we present a complete derivation of the equation governing long term sea level variations on a spherically symmetric self gravitating maxwell viscoelastic planet this new sea level equation extends earlier work by incorporating in a gravitationally self consistent manner both a time dependent ocean continent geometry and the in uence of contemporaneous perturbations to the rotation vector of the planet we also outline an e cient pseudo spectral numerical methodology for the solution of this equation and present a variety of predictions based on a suite of earth models of relative sea level rsl variations due to glacial isostatic adjustment gia these results show that the contribution to the predicted rsl signal from gia induced perturbations to the rotation vector can reach 7 8 m over the postglacial period in geographic regions where the rotationally induced signal is a maximum this result is sensitive to variations in the adopted lower mantle viscosity and is relatively insensitive to variations in the adopted lithospheric thickness we also show that the rotationally induced component of rsl change is su cient to in uence previous estimates of late holocene melting events and ongoing sea level change due to giawhich were based on a rsl theory for a non rotating earth in particular estimates of antarctic melting over the last 5 kyr based on the amplitude of sea level highstands from the australian region may require an adjustment downwards of the order of 0 5 m of equivalent sea level rise furthermore present day rates of sea level change are perturbed by as much as 0 2 mm yr 1 by the rotational component of sea level change and this has implications for gia corrections of the global tide gauge record over the period from the last glacial maximum to the present we predict a distinctly non monotonic variation in the rotation induced component of rsl this is in agreement with our previous preliminary study milne mitrovica 1996 but contrasts signi cantly with predictions presented by han wahr 1989 and bills james 1996 we demonstrate that the disagreement arises as a consequence of approximations adopted in the latter studies we furthermore refute an assertion by bills james 1996 that previously published constraints on mantle viscosity and ice sheet histories which did not incorporate a rotation induced rsl component are largely invalidated by this omission

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

numerical simulation of spontaneous rupture processes on twonon coplanar faults the effect of geometry on fault interaction summary analyses of earthquake sources have revealed that the earthquake rupture process is complex and that the rupture does not occur on a single plane earthquake faults are often composed of several subfaults and rupture propagation tends to decelerate or terminate at places where the fault strike changes these observations imply that fault geometry including fault steps and fault strike change plays an important role in earthquake rupture complexity in this paper we calculate the spontaneous rupture processes of two non coplanar faults in 2 d in plane problems attempting to clarify the eiect of fault geometry we consider two simple modelsmodels in which two faults are either parallel or perpendicular to each other we calculate spontaneous rupture propagation on the faults bya nite diierence method andwe then compare the results in our simulations rupture initially grows on the main fault and stress perturbation from the main rupture then triggers rupture on the secondary fault propagation of the main fault rupture controls a spatio temporal pattern ofstress diierence in the uniform elastic medium which determines the rupture process of the secondary fault the rupture propagation and termination of the secondary fault are signi cantly diierent between the two models the diierence is obvious when rupture of the main fault is arrested and the secondary fault is located near the arrested end of the main fault when the secondary fault is parallel to the main fault rupture can propagate ahead on the secondary fault however when the secondary fault is perpendicular to the main fault rupture is either not triggered on the secondary fault or soon terminates if triggered this variation of the rupture process implies that fault interaction depending on geometry can explain the termination and change of rupture at places where the fault strike varies this shows the importance of the fault geometry in studying spontaneous dynamic rupture processes

airborne radar evidence for tributary flow switching in institute ice stream west antarctica implications for ice sheet configuration and dynamics despite the importance of ice streaming to the evaluation of west antarctic ice sheet wais stability we know little about mid to long term dynamic changes within the institute ice stream iis catchment here we use airborne radio echo sounding to investigate the subglacial topography internal stratigraphy and holocene flow regime of the upper iis catchment near the ellsworth mountains internal layer buckling within three discrete topographically confined tributaries through ellsworth independence and horseshoe valley troughs provides evidence for former enhanced ice sheet flow we suggest that enhanced ice flow through independence and ellsworth troughs during the mid to late holocene was the source of ice streaming over the region now occupied by the slow flowing bungenstock ice rise although buckled layers also exist within the slow flowing ice of horseshoe valley trough a thicker sequence of surface conformable layers in the upper ice column suggests slowdown more than 4000 years ago so we do not attribute enhanced flow switch off here to the late holocene ice flow reorganization intensely buckled englacial layers within horseshoe valley and independence troughs cannot be accounted for under present day flow speeds the dynamic nature of ice flow in iis and its tributaries suggests that recent ice stream switching and mass changes in the siple coast and amundsen sea sectors are not unique to these sectors and that they may have been regular during the holocene and may characterize the decline of the wais

reconstructing the last glacial maximum ice sheet in the weddell sea embayment antarctica using numerical modelling constrained by field evidence abstract the weddell sea embayment wse sector of the antarctic ice sheet has been suggested as a potential source for a period of rapid sea level rise meltwater pulse 1a a 20 m rise in 500 years previous modelling attempts have predicted an extensive grounding line advance in the wse to the continental shelf break leading to a large equivalent sea level contribution for the sector a range of recent field evidence suggests that the ice sheet elevation change in the wse at the last glacial maximum lgm is less than previously thought this paper describes and discusses an ice flow modelling derived reconstruction of the lgm ice sheet in the wse constrained by the recent field evidence the ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break a range of modelled ice sheet surfaces are instead produced with different grounding line locations derived from a novel grounding line advance scheme the ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea level estimates between approximately 1 4 and 3 m for this sector this paper describes the modelling procedure in detail considers the assumptions and limitations associated with the modelling approach and how the uncertainty may impact on the eustatic sea level equivalent results for the wse

abrupt holocene ice sheet thinning along the southern soya coast lutzow holm bay east antarctica revealed by glacial geomorphology and surface exposure dating abstract geological reconstruction of the retreat history of the east antarctic ice sheet eais since the last glacial maximum lgm is essential for understanding the response of the ice sheet to global climatic change and the mechanisms of retreat including a possible abrupt melting event such information is key for constraining climatic and ice sheet models that are used to predict future antarctic ice sheet ais melting however data required to make a detailed reconstruction of the history of the eais involving changes in its thickness and lateral extent since the lgm remain sparse here we present a new detailed ice sheet history for the southern soya coast lutzow holm bay east antarctica based on geomorphological observations and surface exposure ages our results demonstrate that the ice sheet completely covered the highest peak of skarvsnes 400 m a s l prior to 9 ka and retreated eastward by at least 10 km during the early to mid holocene ca 9 to 5 ka the timing of the abrupt ice sheet thinning and retreat is consistent with the intrusion of modified circumpolar deep water mcdw into deep submarine valleys in lutzow holm bay as inferred from fossil foraminifera records of marine sediment cores thus we propose that the mechanism of the abrupt thinning and retreat of the eais along the southern soya coast was marine ice sheet instability caused by mcdw intrusion into deep submarine valleys such abrupt ice sheet thinning and retreat with similar magnitude and timing have also been reported from enderby land east antarctica our findings suggest that abrupt thinning and retreat as a consequence of marine ice sheet instability and intrusion of mcdw during the early to mid holocene may have led to rapid ice surface lowering of hundreds of meters in east antarctica

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

comparison of deterministic and stochastic earthquake simulators for fault interactions in the lower rhine embayment germany time dependent probabilistic seismic hazard assessment requires a stochastic description of earthquake occurrences while short term seismicity models are well constrained by observations the recurrences of characteristic on fault earthquakes are only derived from theoretical considerations uncertain palaeo events or proxy data despite the involved uncertainties and complexity simple statistical models for a quasi period recurrence of on fault events are implemented in seismic hazard assessments to test the applicability of statistical models such as the brownian relaxation oscillator or the stress release model we perform a systematic comparison with deterministic simulations based on rate and state dependent friction high resolution representations of fault systems and quasi dynamic rupture propagation for the specific fault network of the lower rhine embayment germany we run both stochastic and deterministic model simulations based on the same fault geometries and stress interactions our results indicate that the stochastic simulators are able to reproduce the first order characteristics of the major earthquakes on isolated faults as well as for coupled faults with moderate stress interactions however we find that all tested statistical models fail to reproduce the characteristics of strongly coupled faults because multisegment rupturing resulting from a spatiotemporally correlated stress field is underestimated in the stochastic simulators our results suggest that stochastic models have to be extended by multirupture probability distributions to provide more reliable results

deglacial history of the west antarctic ice sheet in the weddell sea embayment constraints on past ice volume change the retreat history of the west antarctic ice sheet wais since the last glacial maximum is important for understanding the process of rapid deglaciation constraining models that seek to predict the future trajectory of the ice sheet and for estimating rates of sea level change here we report new glacial geologic data from the southwestern weddell sea embayment that demonstrate that this part of the wais was thinner than previously suggested and that there was progressive thinning of the ice sheet by 230 480 m since ca 15 ka we use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the weddell sea at the last glacial maximum the volume of this ice would have added between 1 4 and 2 0 m to postglacial sea level rise and would not have been sufficient to contribute significantly to meltwater pulse 1a a rapid rise in sea level 14 200 yr ago

a 3 d coupled ice sheet sea level model applied to antarctica through the last 40 ky abstract we present results from a three dimensional ice sheet shelf model of antarctica coupled to a gravitationally self consistent global sea level model that incorporates maxwell viscoelastic deformation of the solid earth the coupled model captures complex post glacial changes in sea level associated with the gravitational deformational and rotational effects of the evolving surface mass ice plus ocean load over the global ocean including at the grounding lines of marine based ice the simulations are initiated at 40 ka and we focus on ice distributions and sea levels from the last glacial maximum to present our results extend and confirm the key conclusions of earlier work using a simplified 1 d ice sheet model by demonstrating that the sea level coupling has a significant stabilizing influence on marine ice sheet grounding lines the feedback of sea level changes into the ice sheet model acts to slow down the retreat and advance of the grounding line relative to simulations in which the full coupling is not incorporated differences in ice thickness between these simulations can reach 1 km close to the grounding line finally we perform preliminary comparisons of our results to relative sea level rsl histories and gps derived present day uplift rates at sites near the margins of antarctica we find that the coupling improves fits to uplift rates in several regions and that the rsl predictions of the coupled model yield a fit to the observations that is comparable to recent uncoupled simulations in which the underlying earth model was varied to obtain a best fit to the rsl histories

late holocene relative sea levels near palmer station northern antarctic peninsula strongly controlled by late holocene ice mass changes abstract many studies of holocene relative sea level rsl changes across antarctica assume that their reconstructions record uplift from glacial isostatic adjustment caused by the demise of the last glacial maximum lgm ice sheets however recent analysis of gps observations suggests that mantle viscosity beneath the antarctic peninsula is weaker than previously thought which would imply that solid earth motion is not controlled by post lgm ice sheet retreat but instead by late holocene ice mass changes if this hypothesis is correct one might expect to find holocene rsl records that do not reflect a monotonic decrease in the rate of rsl fall but show variations in the rate of rsl change through the holocene we present a new record of late holocene rsl change from torgersen island near palmer station in the western antarctic peninsula that shows an increase in the rate of relative sea level fall from 3 0 1 2 mm yr to 5 1 1 8 mm yr during the late holocene independent studies of the glacial history of the region provide evidence of ice sheet changes over similar time scales that may be driving this change when our rsl records are corrected for sea surface height changes associated with glacial isostatic adjustment gia the rate of post 0 79 ka land uplift at torgersen island 5 3 1 8 mm yr is much higher than the rate of uplift recorded at a nearby gps site at palmer station prior to the larsen b breakup in 2002 ad 1998 2002 ad

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

probabilities for jumping fault segment stepovers 1 seismic hazard analysis relies heavily on the segmentation of faults the ability of ruptures to break multiple segments has a big impact on estimated hazard current practice for estimating multiple segment breakage relies on panels of experts voting on their opinions for each case here we explore the probability of elastodynamic ruptures jumping segment stepovers in numerical simulations of segmented fault systems we find a simple functional form for the probability of jumping a segment stepover as a function of stepover distance an exponential falloff with distance we suggest this simple parameterization of jumping probabilities combined with sparse observational data to fix the lengthscale parameter as a new approach to estimating multisegment earthquake hazard

deglacial history of the pensacola mountains antarctica from glacial geomorphology and cosmogenic nuclide surface exposure dating the retreat history of the antarctic ice sheet is important for understanding rapid deglaciation as well as to constrain numerical ice sheet models and ice loading models required for glacial isostatic adjustment modelling there is particular debate about the extent of grounded ice in the weddell sea embayment at the last glacial maximum and its subsequent deglacial history here we provide a new dataset of geomorphological observations and cosmogenic nuclide surface exposure ages of erratic samples that constrain the deglacial history of the pensacola mountains adjacent to the present day foundation ice stream and academy glacier in the southern weddell sea embayment we show there is evidence of at least two glaciations the first of which was relatively old and warm based and a more recent cold based glaciation during the most recent glaciation ice thickened by at least 450 m in the williams hills and at least 380 m on mt bragg progressive thinning from these sites was well underway by 10 ka bp and ice reached present levels by 2 5 ka bp and is broadly similar to the relatively modest thinning histories in the southern ellsworth mountains the thinning history is consistent with but does not mandate a late holocene retreat of the grounding line to a smaller than present configuration as has been recently hypothesized based on ice sheet and glacial isostatic modelling the data also show that clasts with complex exposure histories are pervasive and that clast recycling is highly site dependent these new data provide constraints on a reconstruction of the retreat history of the formerly expanded foundation ice stream derived using a numerical flowband model

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

thickening of the ice stream catchments feeding the filchner ronne ice shelf antarctica 1 earlier ice sheet mass balance estimates disagree with satellite altimeter measured thickening of the catchment area that feeds the filchner ronne ice shelf antarctica in an effort to reconcile these differences we generated a new flux gate mass balance estimate using improved elevation and velocity data our results show a 39 26 gton yr thickening whereas earlier flux gate results indicated minor thinning our results are consistent with altimeter derived estimates of elevation change this suggests that altimeter observed thickening on the east antarctic plateau may be the result of some combination of a 20th century accumulation increase and a continuing response to the 50 accumulation increase that began in the early holocene