ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

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

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

first airborne gravity results over the thwaites glacier catchment west antarctica recent satellite observations of thwaites glacier in the amundsen sea embayment west antarctica have shown that the glacier is changing rapidly the causes of its dynamic behavior are uncertain but are of concern because this glacier has the most negative mass balance of all antarctic glaciers to better understand thwaites glacier s subglacial setting we conducted a multi instrumented aerogeophysical survey of its catchment and present here the first gravity results we employed a new gravimeter and it performed well despite extreme conditions and an unusual survey design the unleveled free air gravity anomalies have a 2 3 mgal rms error and a 9 km spatial resolution despite slightly higher than standard noise levels the free air anomalies correlate well with radar derived subglacial topography the new airborne gravity data assist in interpreting radar identified bedrock features and are an ideal basis for future studies of subglacial geology and its control on the dynamics of thwaites glacier

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

new 3d bathymetry and sediment distribution in lake vostok implication for pre glacial origin and numerical modeling of the internal processes within the lake abstract a new distribution of water and unconsolidated sediments in subglacial lake vostok east antarctica was developed via inversion of airborne gravity data constrained by 60 seismic soundings a model was developed for host rock with a density of 2550 kg m 3 that was inferred from prior 2d modeling our 3d bathymetry model of lake vostok corresponds better with seismic data rms of 125 m than two previous models based on the same gravity dataset the good match in both water and sediment thicknesses between the gravity model and seismic measurements confirms two major facts about lake vostok 1 the lake is hosted by sedimentary rocks and 2 the bottom of the lake is covered with a layer of unconsolidated sediments that does not exceed 300 m in the southern basin and thickens almost to 400 m in the northern basin our new bathymetry model suggests much shallower water thicknesses up to twice the previous estimates in the middle and northern parts of the lake while the water layer is thicker in the southern basin numerical modeling of the internal processes in the lake reveals the relevance of our new bathymetry model to the basal mass balance a significant decrease in transport is observed in the shallower northern basin as well as a decrease of 33 in the turbulent kinetic energy however only minor differences were observed in the distribution of the calculated freezing and melting zones compared to previous models estimates for the sedimentation rates for six possible mechanisms were made possible sedimentation mechanisms are 1 fluvial and periglacial i e those that are active prior to the establishment of a large subglacial lake 2 deposition due to overlying ice sheet including melting out of the ice as well as bulldozering by the overriding ice and 3 suspended sediments from subglacial water flow including those deposited by periodical subglacial outbursts the estimates for these mechanisms show that unconsolidated sediments of the observed thickness are most consistent with a lake that existed before glaciation

bathymetry and geological structures beneath the ross ice shelf at the mouth of whillans ice stream west antarctica modeled from ground based gravity measurements 1 new gravity data reveal important geologic controls on the location and behavior of the grounding zone of whillans ice stream wis west antarctica grounding zones of ice sheets and contiguous ice shelves are important for understanding ice sheet dynamics as key processes that influence the grounded ice and its discharge into the ocean occur in these regions here we model the bathymetry and shallow geological structures beneath the ross ice shelf in an embayment of the wis grounding zone using gravity data collected on the ground in conjunction with seismic and radar data we find that the region of shallow ocean water 50 m is extensive oceanographic models suggest that grounding zones exhibiting thin ocean cavities with gently sloping ice ocean interfaces are likely to be tidally well mixed leading to slower basal melting than would occur in a thicker stratified water column beneath the ocean water column we model a fault and a sedimentary basin in a half graben filled with two layers of sediments the total thickness of the sediment layer is 900 to 1250 m in the half graben and 600 to 800 m on the upthrown block and the basement depth is no more than 2000 m we observe that the upper softer sediment is thinnest near the modern grounding line and may possibly pinch out near our grid and that the modeled fault is roughly parallel to part the grounding we therefore hypothesize that the wis grounding line stabilized in its current location in part due to the subglacial geology

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

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

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

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

late quaternary evolution of reedy glacier antarctica abstract fresh deposits above the margins of reedy glacier show that maximum ice levels during the last glaciation were several hundred meters above present near the glacier mouth and converged to less than 60 m above the present day surface at the head of the glacier exposure ages of samples from five sites along its margin show that reedy glacier and its tributaries thickened asynchronously between 17 and 7 kyr bp at the quartz hills located midway along the glacier maximum ice levels were reached during the period 17 14 kyr bp farther up glacier the ice surface reached its maximum elevation more recently 14 7 10 2 kyr bp at the caloplaca hills 9 1 7 7 kyr bp at mims spur and around 7 kyr bp at hatcher bluffs we attribute this time transgressive behavior to two different processes at the glacier mouth growth of grounded ice and subsequent deglaciation in the ross sea embayment caused a wave of thickening and then thinning to propagate up glacier during the lateglacial and holocene increased snow accumulation on the east antarctic ice sheet caused transient thickening at the head of the glacier an important result of this work is that moraines deposited along reedy glacier during the last ice age cannot be correlated to reconstruct a single glacial maximum longitudinal profile the profile steepened during deglaciation of the ross sea thinning at the quartz hills after 13 kyr bp while thickening upstream near its confluence with mercer ice stream rapid thinning beginning prior to 7 8 kyr bp reduced the level of reedy glacier to close to its present level thinning over the past 1000 years has lowered the glacier by less than 20 m

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

etruscan archaeometallurgy record in sediments from the northern tyrrhenian sea the island of elba has been known since antiquity as the major source of metals especially iron for the etruscan civilisation unfortunately the archaeometallurgic record of the island has been destroyed during our century so the present knowledge of this history is very poor and only historical sources constrain the timing we present in this article indirect evidence of this activity buried in the sea deep that will shed new light on the archaeometallurgic history of the western mediterranean magnetic properties of marine sediments collected in the corsica channel imply enrichment in ferrimagnetic minerals and trace metals related to anthropic impact accelerator mass spectrometry 14c ages suggest that the contamination started about 4000 years ago we attribute it to emissions of fly ash from the etruscan smelting technologies for metal production these results show that archaeometallurgic activity started on elba island in the ii millennium bc much earlier than previously assumed

subglacial lake whillans seismic observations of a shallow active reservoir beneath a west antarctic ice stream abstract active subglacial lakes concentrate the distribution of water beneath ice sheets in both space and time seismic and surface observations from subglacial lake whillans slw west antarctica reveal that this active lake forms a persistent albeit fluctuating reservoir beneath whillans ice stream imaging and phase observations using active source seismic data show that slw is a perpetually shallow feature when surveyed near its low stand a water column was resolvable by seismic techniques along only 5 km of the 45 km profiled with a maximum depth of less than 8 m satellite altimetry shows that the high stand adds no more than 3 4 m to this this water column presents a suitable drill site at s 84 240 w 153 694 elsewhere the majority of the bed appears wet with soft sediment or water thicknesses of less than the imaging resolution of our data of approximately 2 m the surface expression of the active lake previously revealed by icesat elevation data and image differencing generally corresponds to the seismic estimate of soft sediment or water with notable exceptions occurring at the upstream and downstream ends of the lake these exceptions indicate that slw s water column is very shallow or absent in places at low stands or has disconnected or transiently active and inactive portions

basal ice sequences in antarctic ice stream exposure of past hydrologic conditions and a principal mode of sediment transfer 1 the brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in kamb ice stream the upper sequence 7 3 m comprises clear ice and layers with dispersed and stratified debris the lower sequence 8 2 m consists of accretion ice with alternating layers of stratified and solid debris the two sequences have volumetric debris contents of about 5 and 20 respectively we infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the siple coast plain where basal freezing currently dominates the basal thermal regime the basal ice layer contains the equivalent of 2 1 0 4 m of frozen sediment the high volume of sediment is a result of debris rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on off cycles fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the ross sea our results show that sediment wedges near grounding lines may form by melt out of basal ice debris this is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override coulomb plastic substrates

diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica 1 recent gps observations from a spatially extensive network across ronne ice shelf show significant daily ice flow variations at all sites the almost synchronous horizontal displacements occur at diurnal and semidiurnal tidal periods during spring tides displacements velocities and strains near the ice front have superimposed oscillations that are 300 of their mean values and occur over a six hour period resulting in regular ice shelf flow reversals close to ice stream grounding lines however the horizontal diurnal and semidiurnal signals decay and almost vanish from our analysis we conclude that ice shelves respond primarily elastically to tidal tilting thus accounting for the observed diurnal and semidiurnal flow variations and their amplification toward the ice shelf front our findings suggest that detailed modeling of these data could provide improved ice shelf and ice stream models for correctly simulating ice shelf flow and predicting future ice sheet evolution citation makinson k m a king k w nicholls and g hilmar gudmundsson 2012 diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica geophys res lett 39 l10501 doi 10 1029 2012gl051636

effect of sedimentation on ice sheet grounding line stability sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding line retreat in response to a rise in relative sea level of at least several meters recent antarctic changes thus cannot be attributed to sea level rise strengthening earlier interpretations that warming has driven ice sheet mass loss large sea level rise such as the 100 meter rise at the end of the last ice age may overwhelm the stabilizing feedback from sedimentation but smaller sea level changes are unlikely to have synchronized the behavior of ice sheets in the past

a model of tidally dominated ocean processes near ice shelf grounding lines 1 glaciological processes at grounding lines which divide floating ice shelves from grounded ice sheets may strongly influence the dynamics and evolution of inland ice therefore understanding the oceanic forcing on ice shelves in this region is of importance to predictions of cryospheric change and sea level rise as the ocean cavity shallows toward the grounding line tidal mixing becomes proportionately more important until a tidal front forms beyond which the water properties are vertically homogenized the extent of this mixed zone is relevant to several questions because a fully mixed region behaves differently to the stratified ocean offshore in this study a highly simplified one dimensional model is used to examine the size properties and sensitivities of the mixed zone the model suggests that most grounding line mixed zones are small implying that the usual models representing a stratified ocean are generally valid if tidal mixing is also taken into account modeled mixed zones can be significant in near freezing regions with vigorous tides and a shallowly sloping cavity but even these areas are smaller than previously proposed it therefore seems that upwelling of warm water rather than mixing in tidal zones generally maintains ice shelf basal melting near grounding lines where mixed zones are present the model suggests that they insulate the grounding line from offshore ocean waters the model illustrates the origin of ice shelf water plumes and confirms that unlike elsewhere in the cavity melting in the mixed zone increases linearly in response to ocean warming

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

j f larsen c a neal j r schaefer a m kaufman and zhong lu the 2008 phreatomagmatic eruption of okmok volcano aleutian islands alaska chronology deposits and landform changes this book provides a detailed description of the spectacu lar 5 week long 2008 eruption at okmok volcano vei 4 located on umnak island in the central aleutian volcanic arc black 1975 miller and smith 1987 burgisser 2005 it is avery good example of successful joint effort within a geolog ical survey during an eruption response not only providingone of the most complete documentations of historicalphreatomagmatic eruptions but also illustrating the complex ity of such events and their strong impacts on local sedimen tation rates landscape variations vent dynamics and thehydrogeological setting phreatomagmatic and phreatic explosions are not rare inthe life span of any volcano but they are commonlyoverlookedbecauseoftheirsmall sizeandrapiderosioncom paredtoothervolcanicproducts sincetheyarequitecommonin the current volcanic activity worldwide difficulties ariseduringeruptionresponsesbecauseofthelackofreportsavail able for comparison as well as the scarcity and discontinuityof related deposits in the geological record therefore thedescriptions presented in this book provide excellent refer ences for a better understanding of these short lived but lo cally hazardous events okmok s report of investigations 2015 2iscomplemen tary to other on going publications which are referred tothroughout the text it is aimed largely at the professionalgeologist but because of its wealth of superb illustrations incolor itcouldbeappreciatedbyvolcanologyamateurs andbyother scientists interested in extremely rapid landscape evolu tion due to volcanism furthermore the free access to thebook in pdf format and the supplementary dataset providean excellent opportunity to gain a wide audience and serve asan important reference source for future comparative studies chapter 1 gives an introduction to the 2008 eruption pointing out the onset of dramatic changes in the local geo morphology caused by the opening explosions the multiplic ity of syn eruptivevents and the lackof clear seismic precur sors the latter of which make phreatomagmatic events

discovery of till deposition at the grounding line of whillans ice stream we report on the discovery of a grounding line sedimentary wedge till delta deposited by whillans ice stream west antarctica our observation is that grounding line deposition serves to thicken the ice and stabilize the position of the grounding line the ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium thus the grounding line will tend to remain in the same location despite changes in sea level until sea level rises enough to overcome the excess thickness that is due to the wedge further our observation demonstrates the occurrence of rapid subglacial erosion sediment transport by distributed subglacial till deformation and grounding line sedimentation which have important implications for ice dynamics numerical modeling of ice flow and interpretation of the sedimentation record