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

antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

uplift and tilting of the shackleton range in east antarctica driven by glacial erosion and normal faulting unravelling the long term evolution of the subglacial landscape of antarctica is vital for understanding past ice sheet dynamics and stability particularly in marine based sectors of the ice sheet here we model the evolution of the bedrock topography beneath the recovery catchment a sector of the east antarctic ice sheet characterized by fast flowing ice streams that occupy overdeepened subglacial troughs we use 3d flexural models to quantify the effect of erosional unloading and mechanical unloading associated with motion on border faults in driving isostatic bedrock uplift of the shackleton range and theron mountains which are flanked by the recovery slessor and bailey ice streams inverse spectral free air admittance and forward modeling of topography and gravity anomaly data allow us to constrain the effective elastic thickness of the lithosphere te in the shackleton range region to 20 km our models indicate that glacial erosion and the associated isostatic rebound has driven 40 50 of total peak uplift in the shackleton range and theron mountains a further 40 50 can be attributed to motion on normal fault systems of inferred jurassic and cretaceous age our results indicate that the flexural effects of glacial erosion play a key role in mountain uplift along the east antarctic margin augmenting previous findings in the transantarctic mountains the results suggest that at 34 ma the mountains were lower and the bounding valley floors were close to sea level which implies that the early ice sheet in this region may have been relatively stable

continued deceleration of whillans ice stream west antarctica 1 earlier observations indicated that whillans ice stream slowed from 1973 to 1997 we collected new gps observations of the ice stream s speed in 2003 and 2004 these data show that the ice stream is continuing to decelerate at rates of about 0 6 yr2 with faster rates near the grounding line our data also indicate that the deceleration extends over the full width of the ice plain extrapolation of the deceleration trend suggests the ice stream could stagnate sometime between the middle of the 21st and 22nd centuries

erosion driven uplift in the gamburtsev subglacial mountains of east antarctica the relative roles of climate and tectonics in mountain building have been widely debated central to this debate is the process of flexural uplift in response to valley incision here we quantify this process in the gamburtsev subglacial mountains a paradoxical tectonic feature in cratonic east antarctica previous studies indicate that rifting and strike slip tectonics may have provided a key trigger for the initial uplift of the gamburtsevs but the contribution of more recent valley incision remains to be quantified inverse spectral free air admittance and bouguer coherence methods indicate that unusually for continents the coherence between free air gravity anomalies and bedrock topography is high 0 5 and that the elastic thickness of the lithosphere is anomalously low 15 km in contrast to previously reported values of up to 70 km the isostatic effects of two different styles of erosion are quantified dendritic fluvial incision overprinted by alpine style glacial erosion in the gamburtsevs and outlet glacier type selective linear erosion in the lambert rift part of the east antarctic rift system 3d flexural models indicate that valley incision has contributed ca 500 m of peak uplift in the gamburtsevs and up to 1 2 km in the lambert rift which is consistent with the present day elevation of oligocene miocene glaciomarine sediments overall we find that 17 25 of gamburtsev peak uplift can be explained by erosional unloading these relatively low values are typical of temperate mountain ranges suggesting that most of the valley incision in the gamburtsevs occurred prior to widespread glaciation at 34 ma the pre incision topography of the gamburtsevs lies at 2 2 5 km above sea level confirming that they were a key inception point for the development of the east antarctic ice sheet tectonic and or dynamic processes were therefore responsible for ca 80 of the elevation of the modern gamburtsev subglacial mountains

is the east antarctic ice sheet stable the greenland and east and west antarctic ice sheets are assessed as being the source of ice that produced an eemian sea level 6 m higher than present sea level the most probable source is total collapse of the west antarctic ice sheet accompanied by partial collapse of the adjacent sector of the east antarctic ice sheet in direct contact with the west antarctic ice sheet this conclusion is reached by applying a simple formula relating the floating fraction of ice along flowlines to ice height above the bed increasing the floating fraction lowered ice elevations enough to contribute up to 4 7 m to global sea level adding 3 3 m resulting from total collapse of the west antarctic ice sheet accounts for the higher eemian sea level partial gravitational collapse that produced the present ice drainage system of amery ice shelf contributes 2 3 m to global sea level these results cast doubt on the presumed stability of the east antarctic ice sheet but destabilizing mechanisms remain largely unknown possibilities include glacial surges and marine instabilities at the respective head and foot of ice streams

reconstructing recent landslide activity in relation to rainfall in the llobregat river basin eastern pyrenees spain abstract a chronology of recent landslides in the upper basin of the llobregat river eastern pyrenees has been reconstructed from technical reports field reconnaissance and dendrogeomorphological analysis the precipitation conditions responsible for triggering and reactivating landslides have been deduced by analysis of rainfall records from two rain gauges located in the area two different rainfall patterns relate to landslide occurrence i without antecedent rainfall high intensity and short duration rains trigger mostly debris flows and shallow slides developed in colluvium and weathered rocks a rainfall threshold of around 190 mm in 24 h initiates failures whereas more than 300 mm in 24 48 h are needed to cause widespread shallow landsliding ii with antecedent rain moderate intensity precipitation of at least 40 mm in 24 h reactivates mudslides and both rotational and translational slides affecting clayey and silty clayey formations in this case several weeks and 200 mm of precipitation are needed to cause landslide reactivation dendrochronological analysis shows that landslide activity in the eastern pyrenees was relatively low between 1926 and 1959 but has since increased significantly with reactivation now occurring about every 3 years

the ellsworth subglacial highlands inception and retreat of the west antarctic ice sheet antarctic subglacial highlands are where the antarctic ice sheets first developed and the pinning points where retreat phases of the marine based sectors of the ice sheet are impeded due to low ice velocities and limited present day change in the ice sheet interior west antarctic subglacial highlands have been overlooked for detailed study these regions have considerable potential however for establishing the locations from which the west antarctic ice sheet originated and grew and its likely response to warming climates here we characterize the subglacial morphology of the ellsworth subglacial highlands west antarctica from ground based and aerogeophysical radio echo sounding res surveys and the moderate resolution imaging spectroradiometer modis mosaic of antarctica we document well preserved classic landforms associated with restricted dynamic marine proximal alpine glaciation with hanging tributary valleys feeding a significant overdeepened trough the ellsworth trough cut by valley tidewater glaciers fjord mouth threshold bars down ice of two overdeepenings define both the northwest and southeast termini of paleo outlet glaciers which cut and occupied the ellsworth trough satellite imagery reveals numerous other glaciated valleys terminating at the edge of deep former marine basins e g bentley subglacial trench throughout the ellsworth subglacial highlands these geomorphic data can be used to reconstruct the glaciology of the ice masses that formed the proto west antarctic ice sheet the landscape predates the present ice sheet and was formed by a small dynamic ice field s similar to those of the present day antarctic peninsula at times when the marine sections of the west antarctic ice sheet were absent the ellsworth subglacial highlands represent a major seeding center of the paleo west antarctic ice sheet and its margins represent the pinning point at which future retreat of the marine based west antarctic ice sheet would be arrested

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

effective porosity and seepage velocity in column tests on compacted clay a series of column and tank tests were conducted using bromide to estimate seepage velocity through compacted clays bromide concentrations in effluent and sectioned core pore water were measured the seepage velocity estimated from the concentration depth profile data obtained from sectioned core pore water samples was approximately 50 greater than that estimated from the breakthrough curve data the discrepancy is thought to be caused by the soil sample compression during coring and sectioning the effective porosities estimated from the column tank breakthrough curves ranged from 89 to 104 of the total porosities determined from the weight volume phase relationship the hydrodynamic dispersion coefficient had a linear relationship with the seepage velocity when the mass transport parameters of contaminants through a soil liner are to be estimated using column tests it is recommended that tracer tests be conducted for the correct estimation of the seepage velocity

topographic steering of enhanced ice flow at the bottleneck between east and west antarctica hypothesized drawdown of the east antarctic ice sheet eais through the bottleneck zone between east and west antarctica would have significant impacts for a large proportion of the antarctic ice sheet earth observation satellite orbits and a sparseness of radio echo sounding res data have restricted investigations of basal boundary controls on ice flow in this region until now new airborne res surveys reveal complex topography of high relief beneath the southernmost weddell ross ice divide with three subglacial troughs connecting interior antarctica to the foundation and patuxent ice streams and siple coast ice streams these troughs route enhanced ice flow through the interior of antarctica but limit potential drawdown of the eais through the bottleneck zone in a thinning or retreating scenario these topographically controlled corridors of enhanced flow could however drive ice divide migration and increase mass discharge from interior west antarctica to the southern ocean

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

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

tree ring correlation between prehistoric landslides and abrupt tectonic events in seattle washington radiocarbon ages of submerged trees on landslide deposits in lake washington seattle indicate that the most recent slides in three separate areas may have occurred simultaneously about 1000 years ago tree ring crossdating shows that seven bark bearing trees from one of these recent slides and a tree 23 kilometers to the northwest in a probable tsunami deposit on the shore of puget sound died in the same season of the same year the close coincidence among the most recent lake landslides a probable tsunami abrupt subsidence and other possible seismic events gives evidence for a strong prehistoric earthquake in the seattle region

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

organized flow from the south pole to the filchner ronne ice shelf an assessment of balance velocities in interior east antarctica using radio echo sounding data 1 ice flow through central antarctica has the potential to transmit accumulation changes from deep interior east antarctica rapidly to the shelf but it is poorly constrained owing to a dearth of ice velocity observations we use parameters derived from airborne radio echo sounding res data to examine the onset areal extent and englacial conditions of an organized flow network tributaries feeding an ice stream draining from the south pole to the filchner ronne ice shelf we classified res flight tracks covering the region according to whether englacial stratigraphy was disrupted i e internal layers diverged significantly from the surface and bed echoes or undisrupted i e internal layers closely parallel surface and basal topography and we calculated subglacial roughness along basal reflectors where satellite measured surface ice flow speeds are available covering 39 of the study region regions of fast and tributary flow correspond with res flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow this suggests that disrupted internal layering and smooth subglacial topography identified from res profiles can be treated as indicators of past or present enhanced flow tributaries where neither satellite nor ground based ice flow measurements are available we therefore use these res derived parameters to assess the balance flux modeled steady state flow regime between the south pole and filchner ronne ice shelf the res analysis confirms that an organized flow network drains a wide region around the south pole into the filchner ronne ice shelf however the spatial extent of this network as delineated by the res data diverges from that predicted by currently available balance flux models

subglacial geology and geomorphology of the pensacola pole basin east antarctica the east antarctic ice sheet eais is underlain by a series of low lying subglacial sedimentary basins the extent geology and basal topography of these sedimentary basins are important boundary conditions governing the dynamics of the overlying ice sheet this is particularly pertinent for basins close to the grounding line wherein the eais is grounded below sea level and therefore potentially vulnerable to rapid retreat here we analyze newly acquired airborne geophysical data over the pensacola pole basin ppb a previously unexplored sector of the eais using a combination of gravity magnetic and ice penetrating radar data we present the first detailed subglacial sedimentary basin model for the ppb radar data reveal that the ppb is defined by a topographic depression situated 500 m below sea level gravity and magnetic depth to source modeling indicate that the southern part of the basin is underlain by a sedimentary succession 2 3 km thick this is interpreted as an equivalent of the beacon supergroup and associated ferrar dolerites that are exposed along the margin of east antarctica however we find that similar rocks appear to be largely absent from the northern part of the basin close to the present day grounding line in addition the eastern margin of the basin is characterized by a major geological boundary and a system of overdeepened subglacial troughs we suggest that these characteristics of the basin may reflect the behavior of past ice sheets and or exert an influence on the present day dynamics of the overlying eais

the influence of subglacial hydrology on the flow of kamb ice stream west antarctica 1 ice streams on the siple coast west antarctica have a complex history of flow because their basal motion is governed by time varying basal conditions although the mechanical interaction between ice and till is well established very little is known about the potential effect of regionally scaled water transport in a basal water system which has only recently become apparent to investigate the combined effect of hydrological and mechanical processes we developed the hydrology ice and till model in which ice flow is coupled to a coulomb plastic till layer and a basal water system consisting of discrete conduits when the model is applied to kamb ice stream kis results confirm that it is capable of oscillating between fast and stagnant modes of flow we show that when subglacial conduits are disregarded or do not extend to the grounding line the oscillatory behavior of the ice stream is governed by the basal thermal regime when conduits extend to the grounding line the modelled ice stream oscillation period is increased peak speeds are reduced and oscillations may ultimately cease if the volume of water supplied is sufficiently high three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years although varying according to hydrological activity our results indicate that kis could reactivate this century given its hydrological setting and 170 years of stagnation

variability of basal melt beneath the pine island glacier ice shelf west antarctica observations from satellite and airborne platforms are combined with model calculations to infer the nature and efficiency of basal melting of the pine island glacier ice shelf west antarctica by ocean waters satellite imagery shows surface features that suggest ice shelf wide changes to the ocean s influence on the ice shelf as the grounding line retreated longitudinal profiles of ice surface and bottom elevations are analyzed to reveal a spatially dependent pattern of basal melt with an annual melt flux of 40 5 gt a 1 one profile captures a persistent set of surface waves that correlates with quasi annual variations of atmospheric forcing of amundsen sea circulation patterns establishing a direct connection between atmospheric variability and sub ice shelf melting ice surface troughs are hydrostatically compensated by ice bottom voids up to 150 m deep voids form dynamically at the grounding line triggered by enhanced melting when warmer than average water arrives subsequent enlargement of the voids is thermally inefficient 4 or less compared with an overall melting efficiency beneath the ice shelf of 22 residual warm water is believed to cause three persistent polynyas at the ice shelf front seen in landsat imagery landsat thermal imagery confirms the occurrence of warm water at the same locations

holocene strath terraces climate change and active tectonics the clearwater river basin olympic peninsula washington state the 400 km 2 clearwater river basin located on the pacific flank of the actively uplifting olympic mountains of western washington state contains a well preserved flight of holocene fluvial terraces we have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic fluvial active tectonic and climatic processes that operate at holocene spatial and temporal scales detailed field mapping reveals three prominent holocene straths and their overlying terrace deposits terrace ages fall into three broad ranges ca 9000 11 000 yr b p qt4 4000 8000 yr b p qt5 and 0 3000 yr b p qt6 terrace deposit stratigraphy sedimentology and age distributions allow us to consider two alternative models for their genesis the favored model states that the terrace ages are coincident with lateral incision of the clearwater channel emplacement of the terrace alluvium and the carving of the straths vertical incision of the clearwater channel was primarily relegated to the brief 1000 yr intervals when we have no record of terraces alternatively the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision in both models we envision a clearwater river channel at or near capacity with a temporally variable rate of both lateral and vertical incision small deviations from this at capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel we consider several causes for variable hillslope sediment flux in this tectonically active setting including holocene climate change and ground accelerations related to earthquakes holocene rates of vertical incision are reconstructed along nearly the entire clearwater valley from the wide distribution of dated terraces incision rates clearly increase upstream mimicking a pattern documented for pleistocene terraces in the same basin however the rates are 2 3 times those determined for the pleistocene terraces the faster holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift however we favor an alternative explanation in which the holocene rates represent a channel rapidly reacquiring its stable graded concavity following protracted periods of time in the pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation these results illustrate how even in tectonically active settings representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial interglacial cycle

36cl terrestrial cosmogenic nuclide dating suggests late pleistocene to early holocene mass movements on the south face of aconcagua mountain and in the las cuevas horcones valleys central andes argentina

new boundary conditions for the west antarctic ice sheet subglacial topography beneath pine island glacier 1 predictions about future changes in the amundsen sea sector of the west antarctic ice sheet wais have been hampered by poorly known subglacial topography extensive airborne survey has allowed us to derive improved subglacial topography for the pine island glacier basin the trunk of this glacier lies in a narrow 250 km long 500 m deep sub glacial trough suggesting a long lived and constrained ice stream two tributaries lie in similar troughs others lie in less defined shallower troughs the lower basin of the glacier is surrounded by bedrock which after deglaciation and isostatic rebound could rise above sea level this feature would impede ice sheet collapse initiated near the grounding line of this glacier and prevent its progress into the deepest portions of wais the inland slope of the bed beneath the trunk of the glacier however confirms potential instability of the lower basin containing sufficient ice to raise global sea by 24 cm

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

a tsunami about 1000 years ago in puget sound washington water surged from puget sound sometime between 1000 and 1100 years ago overrunning tidal marshes and mantling them with centimeters of sand one overrun site is 10 kilometers northwest of downtown seattle another is on whidbey island some 30 kilometers farther north neither site has been widely mantled with sand at any other time in the past 2000 years deposition of the sand coincided to the year or less with abrupt probably tectonic subsidence at the seattle site and with landsliding into nearby lake washington these findings show that a tsunami was generated in puget sound and they tend to confirm that a large shallow earthquake occurred in the seattle area about 1000 years ago

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

glaciological and climatic significance of hercules dome antarctica an optimal site for deep ice core drilling 1 we present glaciological and climatological characteristics of hercules dome antarctica 86 s 105 w which demonstrate its potential as a deep ice core site annual layering in d ratios from a 72 m ice core collected by the us itase 2002 traverse indicate accumulation rates of 0 16 0 20 m yr ice equivalent over the last 300 years age control from stratigraphy seen in the radio echo sounding data collected during the same traverse suggests a rate of 0 09 0 11 m yr averaged over the past 18 000 years ice stratigraphy also indicates that the ice divide position has been stable through at least this period comparison of satellite derived temperature anomalies with atmospheric reanalysis data show that the site is sensitive to the two dominant patterns of climate variability in the high latitude southern hemisphere climate proxy data from a deep ice core at hercules dome would be indicative of changes in pacific southern hemisphere climate variability and may provide new information on rapid climate change events in antarctica the sensitivity of the site and the combination of relatively high accumulation rates low temperatures mean annual 35 c to 40 c and simple ice flow suggest that hercules dome is an ideal site for a future deep ice core

spatial and temporal variability in snow accumulation at the west antarctic ice sheet divide over recent centuries mean annual accumulation rates of 0 200 0 204 and 0 221 mweq a 1 respectively small scale spatial variability ssv was determined using an analysis of variance of accumulation in the ice core array thereby quantifying the uncertainty in individual accumulation records results indicate that the spatial variability was 0 030 mweq a 1 o r approximately 15 of the average annual accumulation an accumulation record representative of the wais divide local area over recent centuries was developed using a principal component analysis to identify the coherent accumulation signal the wais divide local record exhibited 14 interannual variability 1 standard deviation of the mean with the ssv reduced to 0 017 mweq a 1 correlations of the wais divide local accumulation record with atmospheric indices e g antarctic oscillation exhibited periods when the records oscillate in and out of phase thus reconstructing local and global atmospheric indices from wais divide accumulation records over recent centuries may prove problematic

a dynamic early east antarctic ice sheet suggested by ice covered fjord landscapes the east antarctic ice sheet has played a fundamental part in modulating climate and sea level during the past 30 million years understanding its history is crucial to evaluating its future behaviour and response to global warming airborne ice penetrating radar studies now reveal a fjord like landscape beneath several kilometres of ice in the east antarctic aurora subglacial basin the data confirm and provide a new constraint on the magnitude and dynamics of the oscillations of the east antarctic ice sheet during the late cenozoic which had previously been supported only by marine cores the first cenozoic ice sheets initiated in antarctica from the gamburtsev subglacial mountains1 and other highlands as a result of rapid global cooling 34 million years ago2 in the subsequent 20 million years at a time of declining atmospheric carbon dioxide concentrations2 and an evolving antarctic circumpolar current2 sedimentary sequence interpretation3 and numerical modelling4 suggest that cyclical periods of ice sheet expansion to the continental margin followed by retreat to the subglacial highlands occurred up to thirty times these fluctuations were paced by orbital changes and were a major influence on global sea levels5 ice sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of antarctic topographic lowlands here we show that the basal topography of the aurora subglacial basin of east antarctica at present overlain by 2 4 5 km of ice is characterized by a series of well defined topographic channels within a mountain block landscape the identification of this fjord landscape based on new data from ice penetrating radar provides an improved understanding of the topography of the aurora subglacial basin and its surroundings and reveals a complex surface sculpted by a succession of ice sheet configurations substantially different from today s at different stages during its fluctuations the edge of the east antarctic ice sheet lay pinned along the margins of the aurora subglacial basin the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level although the timing of the channel incision remains uncertain our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions each of which would have over deepened existing topographic depressions reversing valley floor slopes

detection of large prehistoric earthquakes in the pacific northwest by microfossil analysis geologic and palynological evidence for rapid sea level change 3400 and 2000 carbon 14 years ago 3600 and 1900 calendar years ago has been found at sites up to 110 kilometers apart in southwestern british columbia submergence on southern vancouver island and slight emergence on the mainland during the older event are consistent with a great magnitude m 8 earthquake on the cascadia subduction zone the younger event is characterized by submergence throughout the region and may also record a plate boundary earthquake or a very large crustal or intraplate earthquake microfossil analysis can detect small amounts of coseismic uplift and subsidence that leave little or no lithostratigraphic signature

prehistoric rock avalanches in the olympic mountains washington rock avalanches blocked streams in the olympic mountains southwest of puget sound during the past few thousand years limiting radiocarbon ages indicated that three or four of six avalanches occurred from 1000 to 1300 years ago or shortly thereafter most of the dates were from the outer preserved rings of trees drowned behind avalanche dams these three or four avalanches may be coeval not only with one another but also with abrupt tectonic deformation in western washington no rock avalanches in the olympic mountains are known to have resulted from storms or earthquakes during the past century the avalanches strengthen the case that a large prehistoric earthquake occurred in the puget sound region

abrupt uplift within the past 1700 years at southern puget sound washington shorelines rose as much as 7 meters along southern puget sound and hood canal between 500 and 1700 years ago evidence for this uplift consists of elevated wave cut shore platforms near seattle and emerged peat covered tidal flats as much as 60 kilometers to the southwest the uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform the tidal flats also emerged abruptly they changed into freshwater swamps and meadows without first becoming tidal marshes where uplift was greatest it adjoined an inferred fault that crosses puget sound at seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago the uplift and probable fault slip show that the crust of the north america plate contains potential sources of damaging earthquakes in the puget sound region

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

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

inland extent of the weddell sea rift imaged by new aerogeophysical data the weddell sea rift was a major focus for jurassic extension and magmatism during the early stages of gondwana break up and underlies the weddell sea embayment which separates east antarctica from a collage of crustal blocks in west antarctica newly collected aerogeophysical data over the catchments of institute and moller ice streams reveal the inland extent of the weddell sea rift against the ellsworth whitmore block and a hitherto unknown major left lateral strike slip boundary between east and west antarctica aeromagnetic and gravity anomalies define the regional subglacial extent of proterozoic basement middle cambrian rift related volcanic rocks jurassic intrusions and sedimentary rocks of inferred post jurassic age 2d and 3d magnetic depth to source estimates were used to help constrain joint magnetic and gravity models for the region the models reveal that proterozoic crust similar to that exposed at haag nunataks extends southeast of the ellsworth mountains to the margin of the coastal basins thick granitic jurassic intrusions are modelled at the transition between the ellsworth whitmore block and the thinner crust of the weddell sea rift and within the pagano shear zone the crust beneath the inland extension of the weddell sea rift is modelled as being either 4 km thinner compared to the adjacent ellsworth whitmore block or as underlain by an up to 8 km thick mafic underplate

the structure of west antarctica from geophysical studies the subglacial topographical and geological configurations of west antarctica and the boundary zone between east and west antarctica are poorly known yet they are crucial to our understanding of the tectonic evolution of the pacific proto atlantic margin of gondwanaland preliminary results are presented of simultaneous airborne radio echo and magnetic sounding of 106 km2 of west antarctica including detailed maps of ice sheet and bedrock surfaces magnetics radio echo and other available geophysical data suggest interpretations of sub glacial geology

fast flow signature in the stagnated kamb ice stream west antarctica among the major ice streams that drain west antarctica kamb ice stream formerly called ice stream c is unique in that it stagnated 150 yr ago but its former fast flow conditions are virtually unknown here we presentsurface based radar profiles of the ice stream s undulating internal stratigraphy which records these conditions our analysis of the profiles indicates that pre stagnation flow velocities averaged over a period 740 yr exceeded 350 m yr 1 in the trunk of the ice stream this velocity constraint would be lower if the ice had been thickening higher if thinning but suggests mass loss from the ice stream catchment that is of sufficient magnitude to reverse the gain estimated for today s siple coast region analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present day velocities useful for evaluating how west antarctic ice drainage has evolved

a history of pacific northwest earthquakes recorded in holocene sediments from lake washington large earthquakes can trigger slumping of the steep walls of lake basins and landslides in the drainage area resulting in turbidite deposition in the lake and increased detrital flux from inlets holocene sediments in piston cores from lake washington contain a series of terrigenous layers that were episodically deposited in the lake sedimentological geochemical and paleomagnetic analyses on nine piston cores show that the detrital layers are temporally and areally correlatable indicating basinwide disruptions these layers are opaque on x radiographs are coincident with magnetic susceptibility peaks have abundant aluminosilicate minerals are relatively coarse grained and have low organic carbon and biogenic silica contents the thicknesses and geographic distributions of the layers suggest that they are not due to floods or delta destabilization side scan swath imagery and subbottom profiling show that large slumps subaqueous landslides and debris flows are common along the margins of the lake a detailed chronology established from 21 radiocarbon ages on five cores show that a prominent turbidite was deposited about 1000 1100 years ago this turbidite apparently was triggered by a large earthquake that probably occurred on the seattle fault other depositional events in the sediment record at 1500 1700 2400 2500 and 2800 3200 years ago coincide with periods of landsliding that have been previously inferred from the dating of drowned trees in the lake more than 30 depositional events have occurred in the last 12 000 years and 21 disturbances have occurred since the deposition of the mazama ash about 7 600 years ago if all the events are due to earthquakes the puget sound region has been subject to major shaking every 300 to 400 years the strong intensities needed to trigger subaqueous slides may not be generated by just local sources such as the seattle fault but could also be caused by great subduction earthquakes occurring along the coast

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

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

ice flow structure and ice dynamic changes in the weddell sea sector of west antarctica from radar imaged internal layering recent studies have aroused concerns over the potential for ice draining the weddell sea sector of west antarctica to figure more prominently in sea level contributions should buttressing from the filchner ronne ice shelf diminish to improve understanding of how ice stream dynamics there evolved through the holocene we interrogate radio echo sounding res data from across the catchments of institute and moller ice streams iis and mis focusing especially on the use of internal layering to investigate ice flow change as an important component of this work we investigate the influence that the orientation of the res acquisition track with respect to ice flow exerts on internal layering and find that this influence is minimal unless a res flight track parallels ice flow we also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice flow regime our findings suggest that ice in northern iis draining the ellsworth subglacial highlands has retained its present ice flow configuration throughout the holocene this contrasts with less topographically constrained ice in southern iis and much of mis whose internal layering evinces spatial changes to the configuration of ice flow over the past 10 000 years our findings confirm siegert et al s 2013 inference that fast flow was diverted from bungenstock ice rise during the late holocene and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the iis and mis catchments as the west antarctic ice sheet has thinned since the last glacial maximum

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

seismic amplification within the seattle basin washington state insights from ships seismic tomography experiments recent observations indicate that the seattle sedimentary basin under lying seattle and other urban centers in the puget lowland washington amplifies long period 1 5 sec weak ground motions by factors of 10 or more we computed east trending p and s wave velocity models across the seattle basin from seismic hazard investigations of puget sound ships experiments to better characterize the seismic hazard the basin poses the 3d tomographic models which resolve features to a depth of 10 km for the first time define the p and s wave velocity structure of the eastern end of the basin the basin which contains sedimentary rocks of eocene to holocene is broadly symmetric in east west section and reaches a maximum thickness of 6 km along our profile beneath north seattle a comparison of our velocity model with coincident amplification curves for weak ground motions pro duced by the 1999 chi chi earthquake suggests that the distribution of quaternary deposits and reduced velocity gradients in the upper part of the basement east of seattle have significance in forecasting variations in seismic wave amplification across the basin specifically eastward increases in the amplification of 0 2 to 5 hz energy correlate with locally thicker unconsolidated deposits and a change from crescent formation basement to pre tertiary cascadia basement these models de fine the extent of the seattle basin the seattle fault and the geometry of the basement contact giving insight into the tectonic evolution of the seattle basin and its influence on ground shaking

two post glacial earthquakes on the saddle mountain west fault southeastern olympic peninsula washington abstract two earthquakes ruptured the western strand of the saddle mountain fault after the cordilleran ice sheet began its retreat from the puget sound lowland about 17 ka the fault system includes multiple northeast trending thrust faults on the southeastern olympic peninsula delineated on light detection and ranging topography by prominent east side up scarps that deform vashon till and extend over 6 7 km the cargill creek trench on the saddle mountain west fault exposed faulted drift and postglacial colluvium below a 1 2 m high scarp steeply dipping northeast striking faults form a 10 m wide zone beneath the scarp structural relations in the trench indicate that two earthquakes produced at least 1 m of vertical separation additional folding or faulting beyond the trench raised the scarp to a total height of 1 7 m aligned cobbles dipping approximately 40 to the north attest to earthquake related tilting a cobble firmly lodged in till and split by a secondary fault indicates that hanging wall deformation involved a dextral component of slip however slip on the master fault may have involved a left lateral component based on slip indicators at adjacent trench sites in the region a single charcoal age and estimates of the onset of ice retreat limit the time of the oldest earthquake to 17 7 7 ka the most recent event occurred after 1 7 ka and may be the same earthquake that dammed price lake about 1100 yr ago known from related studies empirical relations suggest that the most recent earthquake was about m 7 0 2 and produced an estimated total slip of 1 3 1 7 m the total postglacial slip rate ranges from 0 1 to 0 3 m k yr these results support the hypothesis that the saddle mountain fault is part of a larger fault system that accommodates transpressive strain as the puget lowland crust migrates around the eastern flank of the olympic mountains

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years