holocene stratigraphy and depositional history of the narragansett bay system rhode island u s a an extensive seismic reflection survey has been used to gain further knowledge of holocene stratigraphy and depositional history in the narragansett bay system nbs the early holocene stream dissected surface beneath the nbs is interpreted as having been flooded by the holocene sea in a manner suggested by oldale o hara s 1980 sea level rise curve the sea initially is believed to have penetrated the pre nbs east passage trunk valley about 9000 yr bp and subsequently spread landward via the trunk valley and its branches the holocene sediments display stratigraphic relationships that differ spatially at passage mouths the basal unconformity is inferred to be covered over with some 3 m of paralic and 5 m of marine sands and silts separated by a transgressive unconformity in contrast the interior sequences reveal a a valley section up to more than 15 m thick in which the regressional unconformity is overlain by probable lower fluvial and or estuarine sand silt facies that commonly grades upward to an estuarine silt clay facies and b an interfluve section in which a basal transgressive unconformity is blanketed by an estuarine nearshore sand silt facies that locally may change upward to a silt clay facies primarily holocene silt clay accretion produced by sedimentation processes associated with net non tidal estuarine circulation infilled the evolving nbs depositional bodies lenticular in shape and comprised of 12 m or more of sediment developed in lowlands near gould island 9000 yr bp in upper narragansett bay 7500 yr bp around hope island 7500 yr bp and in mt hope bay 6250 yr bp with an average minimal sedimentation rate of 1 6 2 2 mm yr 1 silt clay deposition commonly gas bearing has buried the basal relief in most of the nbs upper and middle portions except for middle east passage a comparison of nbs sedimentation with that of chesapeake delaware and hudson estuaries shows that the estuaries to the south have accumulated more sediment over a slightly longer period yet with the exception of the higher rate in the hudson estuary the sedimentation rates appear to be similar

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

stratigraphy sedimentology and engineering aspects of holocene organo calcareous silts suva harbour fiji abstract a deposit of post glacial organo calcareous silt 25 40 m thick blankets the floor of suva harbour and its marginal inlets including draunibota bay on the southeast coast of viti levu fiji this deposit is notable for its involvement in a large number of foundation failures in the area the organo calcareous silt is the latest sedimentary infill over a late pleistocene erosional surface which is dissected characteristically down to around 50 m below current mean sea level at the harbour margins and to at least 90 m at the seaward end of the study area the gross distribution of pleistocene and holocene sedimentary units is strongly influenced by pliocene graben formation in the mio pliocene bedrock the silt deposit forms part of a transgressive and highstand systems tract related to the latest glacio eustatic sea level rise and includes a series of lobate delta deposits currently prograding at the mouths of streams entering the harbour the silt was deposited in a lagoonal estuarine environment of restricted circulation in a marginal back barrier reef situation and received a variety of marine carbonate and terrigenous sediment contributions as well as organic substances from both autochthonous and allochthonous sources undisturbed sampling indicates the silt to be homogeneous non layered and thoroughly bioturbated but a distinct layering with a spacing in the order of 1 5 m probably related to periodic influxes of terrigenous material is obvious on continuous seismic profiles acoustic turbidity due to biogenic gas in thick accumulations over palaeo channels may well represent zones of high pore fluid pressure and indicate lower effective soil strength there is also evidence that the passage of the overpressured gas front results in hydraulic fracturing and consequent degassing of the silt the silt deposit displays a slight upward coarsening in response to shoaling conditions accompanied by an upward increase in both organic carbon and calcium carbonate proportions and a decreasing terrigenous contribution the increase in organic carbon is closely mirrored by increased diagenetic development of bacterially deposited agglomerates of iron sulfide crystals in the pore spaces the resulting pore space microstructure appears to exert a direct control on the in situ void ratio and hence directly affects the consolidation and strength characteristics of the sediment radiocarbon dates from the silt deposit indicate a consistent rate of deposition in the draunibota palaeo channel of 4 3 m 1000 yr since the marine transgressive front passed the site some 9600 yr b p the dates extend the reliable local sea level data for southeast viti levu back some further 4000 years and support an earlier contention that sea level reached its current position here at around 4000 4500 yr b p about 2000 years later than on the coast of southeast australia water depth at the cement works bridge site reached a maximum of 20 m some 5000 yr b p and subsequently has gradually shoaled to its present position just below mean sea level

correction to sub catchment melt and long term stability of ice stream d west antarctica by byron r parizek richard b alley sridhar anandakrishnan and howard conway

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

sedimentary framework of the southern maine inner continental shelf influence of glaciation and sea level change abstract although the tidally influenced shoreline of maine is longer than that of virtually any other state almost no research on its geology has been published in order to go some way towards remedying this 1500 km of high resolution seismic reflection data and 800 km of sidescan sonar imagery have been collected on the basis of these data and observations made during ten submersible dives more than 800 bottom samples were collected and evaluated for texture and composition the understanding of the sedimentary framework of the southern maine shelf and the processes that maintain it are summarized and future research directions to evaluate the strategic mineral potential are indicated in the past 14 000 years the maine shelf has experienced a deglaciation and two marine transgressions separated by a regression the deglaciation was accompanied by the first transgression and deposited till interbedded with up to 40 m of glaciomarine sediment the presumpscot formation across the shelf the first transgression culminated about 12 500 yrs b p and its landward limit is marked by large glaciomarine deltas 50 100 km landward of the present day coast sea level fell until about 9500 yrs b p when shorelines were cut at about the 65 m depth and some large lowstand deltas were deposited sea level has risen since then and in the general absence of modern river sediment input marine processes have reworked the older sediment five shelf environments have been defined in terms of their surficial sediment and stratigraphy nearshore ramps are sandy regions extending to about 30 m deep offshore of sandy beaches these may be reworked lowstand deltas and possess the thickest bodies of sand in the region nearshore basins are mud filled troughs seaward of coastal areas lacking significant river input slumping glaciomarine deposits provide most of the holocene mud that floors these basins rocky zones are extensive areas of exposed rock most common in the 30 50 m depth range these are areas of high carbonate productivity and provide shell fragments to adjacent areas the outer basins are muddy regions that begin at the 65 m shoreline and extend beyond the 100 m isobath to the deep gulf of maine finally shelf valleys are former stream courses that connect nearshore ramps and basins to the outer basins these bedrock framed valleys are filled with sand and mud and are apparently still active conduits for the offshore movement of sediment the mineralogy of the shelf sediment is complex and it is not yet clear whether glacial homogenization has obliterated distinctions between river drainage basins and the adjacent shelf studies are continuing into northern maine and analysis of the heavy mineralogy of the shelf sediment utilizing a 2 m humphrey spiral to process large grab samples and cores has begun

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

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

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

climate and ocean variability during the middle and late holocene recorded in laminated sediments from alfonso basin gulf of california mexico a laminated sequence core bap96 cp 24 38 12 n 110 33 24 w 390 m depth from the alfonso basin in bay of la paz southern gulf of california contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability the records provide a regional scenario of the middle and late holocene suggesting two major climatic regimes and several millennial scale events conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr bp promoting the intensification of evaporation processes and the prevalence of the gulf of california water in the basin these conditions correlate with strong droughts in the middle holocene of north america and with minimal incursion of tropical waters into the gulf of california proxies indicate a warm scenario and the dominance of the equatorial surface water in the alfonso basin from 2400 to 700 cal yr bp suggesting the intensification of enso cycles a climatic signal between 1038 and 963 cal yr bp may be correlated with global signal of the medieval warm period several cooling events are recognized at 5730 3360 2700 1280 and 820 cal yr bp and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the basin

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

sub catchment melt and long term stability of ice stream d west antarctica 1 the apparent long term persistence and short term variability of the siple coast ice streams of west antarctica are tied to regional thermal conditions and local basal lubrication numerical simulations indicate that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of fast moving modern and ice age ice streams despite their tendency to freeze to the bed and would allow additional thinning and grounding line retreat

seismic and geodetic evidence for grounding line control of whillans ice stream stick slip events the tidally modulated stick slip events of whillans ice stream in west antarctica produce seismic energy from three locations near the grounding line using ice velocity records obtained by combining time series from colocated broadband seismometers and gps receivers installed on the ice stream during the 2010 2011 and 2011 2012 austral summers along with far field seismic recordings of elastic waves we locate regions of high rupture velocity and stress drop these regions which are analogous to asperities in traditional seismic fault studies are areas of elevated friction at the base of the ice stream slip events consistently initiate at one of two locations near the center of the ice stream where events associated with the ross sea high tide originate or a grounding line spot where events associated with the ross sea low tide initiate as well as occasional high tide events following a skipped low tide event the grounding line site but not the central site produces rayleigh waves observable up to 1000 km away through fast expansion of the slip area grounding line initiation events also show strong directivity in the downstream direction indicating initial rupture propagation at 1 5 km s compared to an average of 0 150 km s for the entire slip event following slip initiation additional seismic energy is produced from two sources located near the grounding line first at the downstream end of subglacial lake engelhardt and second toward the farthest downstream extent of the ice stream this evidence suggests that the stronger higher friction material along the grounding line controls motion throughout the stick slip region

response of the connecticut river estuary to late holocene sea level rise abstract the morphology of the modern connecticut river estuary has been inherited from an earlier tidal river the holocene evolution of the estuary has been dictated by the relative submergence of the coastline and the sediment trapping efficiency of the estuary and tidal river system the mid holocene tidal river system extended to the modern mouth of the estuary and subaerial floodplains stretched across the modern submerged shoals of the estuary submergence in excess of the long term sedimentation rate converted these floodplains into open backwater coves beginning about 4000 yr bp in the southern estuary and about 2700 yr bp in the upper estuary and it also initiated estuarine circulation in the tidal river approximately 1700 yr bp an abrupt decrease in the submergence rate caused freshwater marshes to prograde across the coves and across the remaining floodplain surfaces at 1000 yr bp salt marsh peat at the mouth of the estuary indicates estuarine circulation similar to that of t he modern estuary the marshes in the northern estuary have supported freshwater vegetation since their inception thus the continuum of modern environments present along the length of the estuary models the evolution of a single reach of the estuary through time continued submergence will increase the tidal prism and intensify estuarine circulation this effect will be regulated by the large excess sediment supply available to fill the volume of the estuary created by relative sea level rise

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

quaternary sequence stratigraphy of the brisbane river delta moreton bay australia abstract the quaternary geological history of the brisbane river delta front and prodelta was interpreted using seismic stratigraphic and sequence stratigraphic principles during the last major glacial sea level episode the floor of the bay was exposed and eroded by stream channels in the subsequent postglacial marine transgression the palaeochannels of the brisbane and pine rivers were backfilled with mud sea level continued rising until about 6500 years ago and a highstand has persisted since then during the quarternary sediment deposition in moreton bay was controlled by sea level fluctuations channel migration and palaeotopography there were at least four depositional episodes prior to the last glacial period plus the presenet postglacial episode periods of low sea level occurred between highstands and the sea bed exposed and eroded eight major seismic reflectors were recognised and six seismic sequences defined the seismic sequences represent five highstand systems tracts and one transgressive systems tract the relationships between eustacy and system tract type were applied to glacio eustatic sea level curves to determine relative ages of sequences in moreton bay the application of sequence stratigraphy to high frequency sea level fluctuations was demonstrated the stratigraphic system established within the context of sequence stratigraphy will provide a reference for future work within the region

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

numerical investigations of the slow down of whillans ice stream west antarctica is it shutting down like ice stream c we investigate whether the recent slow down of whillans ice stream wis west antarctica may lead to its complete stoppage in the near future using a numerical model basal resistance to ice stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing we implement a basal drainage system which acts to hinder ice stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing the ice module is a standard flowline model with parameterized key out of plane effects raymond 1996 the most important result of our modeling effort is that we never obtained a slow down that would not be followed by a complete stoppage within 5100years wis slow down canbe avoided in the model if large basal water input rates are assumed to satisfy basal freezing in com parison thetestedperturbationsinwiswidthandmassbalancehada relativelysmalleffect onthetendencyof the simulated ice streamto slowdown these resultsunderscorethe need for morequantitativeconstraints ontheefficiencyof sub ice streamwaterdrainage we con jecture that the present day slow down of wis will evolve to shut down in the next few decades unless anadditionof basalwaterprevents freeze on drivenbed strengthening

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

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

reconstructed topography of southern new england prior to isostatic rebound with implications of total isostatic depression and relative sea level abstract topographic models of the late quaternary landscape prior to isostatic rebound aid interpretation of glacial lake water levels and geomorphic features as well as ice thickness mantle viscosity and lithospheric strength the well established limit of glacial marine inundation in central and northern new england provides a test of total isostatic depression relative sea levels reflecting differing magnitudes of isostatic rebound show that published values of isostatic depression in southern new england would inundate the landscape in southern new england with marine water south of the limit of late pleistocene marine inundation this suggests that isostatic depression at the terminal margin of the laurentide ice sheet lis was 50 m a first order estimate of ice thickness based on total isostatic depression and the observed uplift profile in southern new england supports the thin ice models in new england ice thickness ranged from 100 m at the southeastern margin of the lis to 1000 m at a point 300 km north of the terminal margin this regional model places constraints on continental ice sheet and geophysical models that should be considered in the future

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

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

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

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

recent elevation changes on the ice streams and ridges of the ross embayment from icesat crossovers 1 we analyze differences in icesat elevation estimates at orbital crossover locations to determine short term rates of elevation change for small regions within the ross embayment of the west antarctic ice sheet a linear regression of crossover elevation difference against time difference gives an estimate of the mean elevation change rate during the icesat mission to date we observe prevalent elevation change in the south with uplift in the upstream end of kamb ice stream at 0 24 0 30 m a 1 and thinning in the parts of whillans ice stream mercer ice stream and the adjacent conway and engelhardt ice ridges at 0 05 0 18 m a 1 these rates of elevation change are too large to be explained by the 0 02 0 03 m a 1 formal regression error by seasonal height variations or by accumulation or densification rate variability suggesting that they reflect real variations stemming from ice dynamics of the region