topographically mediated ice stream subglacial drainage networks satellite laser altimetry reveals short timescale changes in antarctic ice sheet surface elevation that are suggested to be driven by subglacial water transport and storage here details of the interaction between the dynamics of ice stream flow subglacial water system and bed elevation relief are examined in the context of idealized heterogeneous bed geometries using a two way coupled model of ice and subglacial water flow we show that basal topography controls the temporal and spatial variability of the sub ice stream hydraulic system the orientation and characteristic dimensions of the topographic undulations determine the morphology connected subglacial ponds or channel like subglacial water features and timescales of the sub ice stream drainage system the short term several years to decades variability of the simulated coupled ice stream subglacial water system suggests that the short term surface variations detected in remote sensing observations may be indicative of a rapidly evolving subglacial water system our simulations also show that interaction between ice flow and the highly dynamic subglacial water system has a strong effect on effective stress in the ice large effective stress magnitudes arise over areas where the basal traction is characterized by strong spatial gradients that is transitions from high to low basal traction or vise versa these transitions migrate on multiyear timescales and thus cause large effective stress variability on the same temporal scales

on the relationship between surface and basal properties on glaciers ice sheets and ice streams the influence of basal disturbances on the steady state surface topography and on surface velocities of glaciers and ice sheets is investigated numerically this is done for finite amplitude basal perturbations in both bed topography and basal slipperiness using a nonlinear ice rheology and a nonlinear sliding law the effects of varying the exponent n in glen s flow law on transfer characteristics are mainly quantitative and do not affect qualitative aspects of the transfer amplitudes and phase shifts such as the number of maxima and inflection points when plotted as functions of wavelength in particular the well known maximum in bed to surface transfer amplitude for a newtonian medium at sufficiently high slip ratios ratio between mean sliding velocity and mean ice deformational velocity also forms for n 1 transfer amplitudes generally become smaller with increasing n for wavelengths less than about 3 times the mean ice thickness h for larger wavelengths the situation is reversed and transfer amplitudes increase with n for active ice streams characterized by high basal slipperiness low surface slopes 0 5 and fairly constant for all wavelengths longer than about 3 5h with increasing n and decreasing surface slope the lower limit of wavelengths over which horizontal stress gradients can be ignored increases markedly perturbation solutions for newtonian medium are found to give an accurate description of the transfer characteristics for bedrock amplitudes up to 50 ice thickness and for fractional amplitudes of slipperiness perturbations up to 0 5

ice sheet change detection by satellite image differencing abstract differencing of digital satellite image pairs highlights subtle changes in near identical scenes of earth surfaces using the mathematical relationships relevant to photoclinometry we examine the effectiveness of this method for the study of localized ice sheet surface topography changes using numerical experiments we then test these results by differencing images of several regions in west antarctica including some where changes have previously been identified in altimeter profiles the technique works well with coregistered images having low noise high radiometric sensitivity and near identical solar illumination geometry clouds and frosts detract from resolving surface features the etm sensor on landsat 7 ali sensor on eo 1 and modis sensor on the aqua and terra satellite platforms all have potential for detecting localized topographic changes such as shifting dunes surface inflation and deflation features associated with sub glacial lake fill drain events or grounding line changes availability and frequency of modis images favor this sensor for wide application and using it we demonstrate both qualitative identification of changes in topography and quantitative mapping of slope and elevation changes

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

a method of repeat photoclinometry for detecting kilometer scale ice sheet surface evolution small scale glaciological processes can drive large scale ice sheet behavior but remain underreported due to a paucity of surface elevation measurements in remote polar regions satellite images provide a relatively long record of spatially dense surface observations which allow us to investigate changes in ice sheet topography on the spatial scales of 1 10 km inferring surface topography from satellite images is an established technique but in previous efforts strict requirements for illumination conditions and image quality have led to a great quantity of discarded data relaxing quality requirements and fitting linear trends to the time series of image derived surface topography allow inclusion of more total signal and enable blending data from multiple platforms as a proof of concept we combine 121 moderate resolution imaging spectroradiometer images to develop a 250 m resolution map of surface elevation change at totten glacier antarctica achieving a 1 sigma uncertainty of 0 22 textrm m textrm a 1 for a 15 year period our method of repeat photoclinometry agrees with repeat laser altimetry while revealing clear patterns of surface elevation change associated with ice advection channelized ice shelf basal melt subglacial lake activity and possible grounding line migration

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

accelerator mass spectrometry is the future bigger or smaller abstract since its inception in the late 1970s accelerator mass spectrometry has become a powerful tool for measurement of trace amounts of natural radionuclides in this paper we review recent advancements in ams and discuss future directions of this powerful technique we highlight some recent developments including the introduction of smaller accelerators novel detection systems and the development of new analytical capabilities we believe that the future prospects for ams measurements are practically unlimited and that ams has a vital role to play in the exploration of space where radionuclide measurements are a key factor in the understanding of processes on other planets

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

episodic ice velocity fluctuations triggered by a subglacial flood in west antarctica height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes these signals have been mapped throughout antarctica on timescales of months to years but only broad connections have been made between active lakes and ice dynamics we present the first high frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event collected over 5 years 2010 2015 using global positioning system data on whillans and mercer ice streams west antarctica we observed three episodic ice velocity changes over 2 years where flow speed increased by up to 4 as well as an 11 month disruption of the tidally modulated stick slip cycle that dominates regional ice motion our observations reveal that basal conditions of an antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales which can bias observations used to infer long term ice sheet changes

the effects of transverse bed topography variations in ice flow models 1 a suite of ice flow models is subject to a performance test designed to investigate treatment of flow over variable basal topography using transfer functions developed by gudmundsson 2003 and numerical models of various complexity this study demonstrates that a widely used flowband model has strong limitations associated with its underlying assumptions and thus should be applied only in specific geometrical settings its performance can be significantly improved by using the laterally averaged basal topography instead of centerline topography in applications where spatial variability of flow fields is important hybrid ice flow models can be a viable alternative to flowband models in addition analysis of horizontal distributions of the various ice flow characteristics e g surface elevation velocity and horizontal stress components shows that field observations that focus on assessing these parameters only along the centerline of ice flow could be misleading these results also suggest that such spatial variability needs to be taken into account when designing field surveys of ice flow

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

the triggering of sub glacial lake drainage during the rapid glacier drawdown crane glacier antarctic peninsula ice surface altimetry from icesat 1 and nasa aircraft altimeter overflights spanning 2002 09 indicate that a region of lower crane glacier antarctic peninsula shows an unusual temporal pattern of elevation loss a period of very rapid drawdown 91 m a 1 between september 2004 and september 2005 bounded by periods of large but more moderate rates 23 m a 1 until september 2004 12 m a 1 after september 2005 the region of increased drawdown is 4 5 km 2 2 km based on satellite aster and spot 5 stereo image digital elevation model dem differencing spanning the event in a later differential dem the anomalous drawdown feature is not seen bathymetry in crane glacier fjord reveals a series of flat lying formerly subglacial deeps interpreted as lake sediment basins we conclude that the elevation change feature resulted from drainage of a small deep subglacial lake we infer that the drainage event was induced by hydraulic forcing of subglacial water past a downstream obstruction however only a fraction of crane glacier s increase in flow speed that occurred near the time of lake drainage derived from image feature tracking appears to be directly attributable to the event instead retreat of the ice front off a subglacial ridge 6 km downstream of the lake is likely the dominant cause of renewed fast flow and more negative mass balance in the subsequent 4 years

a mass flux perspective of the tidewater glacier cycle i explore the tidewater glacier cycle with a 1 d depth and width integrated flow model that includes a mass flux calving parameterization the parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity the model demonstrates variable sensitivity to climate from an advanced stable configuration a small warming of the climate triggers a rapid retreat that causes large scale drawdown and is enhanced by positive glacier dynamic feedbacks eventually the terminus retreats out of deep water and the terminus velocity decreases resulting in reduced drawdown and the potential for restabilization terminus readvance can be initiated by cooling the climate terminus advance into deep water is difficult to sustain however due to negative feedbacks between glacier dynamics and surface mass balance despite uncertainty in the precise form of the parameterization the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability it also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models

dynamics of active subglacial lakes in recovery ice stream recovery ice stream has a substantial number of active subglacial lakes that are observed with satellite altimetry to grow and drain over multiple years these lakes store and release water that could be important for controlling the velocity of the ice stream we apply a subglacial hydrology model to analyze lake growth and drainage characteristics together with the simultaneous development of the ice stream hydrological network our outputs produce a good match between modeled lake location and those identified using satellite altimetry for many of the lakes the modeled subglacial system demonstrates development of pressure waves that initiate at the ice stream neck and transit to within 100 km of the terminus these waves alter the hydraulic potential of the ice stream and encourage growth and drainage of the subglacial lakes lake drainage can cause large r channels to develop between basal overdeepenings that persist for multiple years the pressure waves along with lake growth and drainage rates do not identically repeat over multiple years due to basal network development this suggests that the subglacial hydrology of recovery ice stream is influenced by regional drainage development on the scale of hundreds of kilometers rather than local conditions over tens of kilometers

modeling 5 years of subglacial lake activity in the macayeal ice stream antarctica catchment through assimilation of icesat laser altimetry subglacial lakes beneath antarctica s fast moving ice streams are known to undergo 1 km 3 volume changes on annual timescales focusing on the macayeal ice stream macis lake system we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from ice cloud and land elevation satellite icesat laser altimetry we construct a steady state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the icesat derived filling or draining rates the modeled volume change rates of five large subglacial lakes in the downstream portion of macis are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered however the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream overall we show that for this fast flowing ice stream most subglacial lakes receive 90 of their water from distant distributed sources throughout the catchment and we confirm that water is transported from regions of net basal melt to regions of net basal freezing our study provides a geophysically based means of validating subglacial water models in antarctica and is a potential way to parameterize subglacial lake discharge events in large scale ice sheet models where adequate data are available

glaciological twins basally controlled subglacial and supraglacial lakes subglacial lakes beneath ice streams of antarctica and supraglacial lakes observed on the flanks of the greenland ice sheet may seem to be unrelated the former derive their water from energy dissipation associated with basal friction the latter from atmospherically driven surface melting however using numerical models of ice and water flow it is shown here that they share a common relationship to basal conditions that implies that surface lakes or depressions that could host lakes under warmer atmospheric conditions and basal lakes might exist in tandem

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

seismic stratigraphy and subsidence evolution of the northwest sumatra fore arc basin abstract new seismic reflection data collected with the indonesian rv baruna jaya iii under a french indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern sumatra within the sumatra fore arc domain the direction of subduction of the indian oceanic crust below the sunda arc is normal in front of java and oblique along sumatra as a result parts of the sumatra margin are moving northwards along two large strike slip faults parallel to the subduction trench the sumatra fault in sumatra and the mentawai fault at sea which separates the accretionary prism from the fore arc basin we describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore arc domain after widespread uplift and erosion during the paleogene the fore arc basin becomes subsident during the miocene as evidenced by shelf deposits organized in two transgressive regressive sequences 2a 2b produced by tectonics and eustatism this is interpreted as the result of a limited e w extension linked to the incipient play of the sumatra and mentawai faults in pliocene quaternary times the fore arc basin is segmented into several sub basins aceh simeulue and nias basins by compressional zones or by strike slip faults the subsidence rate increased strongly producing sequence 3 with two or four subunits for the pliocene and sequence 4 for the quaternary during this period local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore arc basin the sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone

causes of sudden short term changes in ice stream surface elevation 1 recent satellite borne observations of antarctica s ice streams show sudden spatially confined surface elevation changes that are interpreted as caused by subglacial water movement using a numerical model of idealized ice stream flow coupled to various simple treatments of subglacial bed conditions we demonstrate that ice stream flow dynamics significantly modulates the surface elevation expression of processes taking place at the ice stream bed this modulation means that observed surface elevation changes do not directly translate to basal elevation changes e g inflation or deflation of subglacial water pockets of equal magnitude and shape thus subglacial water volume change is not directly proportional to the area integral of surface elevation changes model results show that ambiguities in interpretation of surface elevation changes can be overcome with additional measurements such as of surface velocity change and through development of methodology designed to understand transfer of basal change to surface change

thirteen years of subglacial lake activity in antarctica from multi mission satellite altimetry the ability to detect the surface expression of moving water beneath the antarctic ice sheet by satellite has revealed a dynamic basal environment with implications for regional ice dynamics grounding line stability and fluxes of freshwater and nutrients to the southern ocean knowledge of subglacial activity on timescales important for near term prediction of ice sheet fluctuations decadal to century is limited by the short observational record of nasa s ice cloud and land elevation satellite icesat laser altimetry mission used to generate the last continent wide survey 2003 08 here we use synthetic aperture radar interferometric mode data from esa s cryosat 2 radar altimetry mission 2010 present which samples 45 of the icesat derived subglacial lakes to extend their time series to the end of 2016 the extended time series show that there have been surface height changes at 20 of the 45 lakes since 2008 indicating that some of these features are persistent and potentially cyclic while other features show negligible changes suggesting these may be transient or nonhydrological features continued monitoring of active lakes for both height and velocity changes as well as developing methods for identifying additional lakes is critical to quantifying the full distribution of active subglacial lakes in antarctica