the tidal spectrum underneath antarctic ice shelves a year long tidal record has been obtained from beneath the george vi ice shelf antarctica an unusual feature of the record is a significant response in tidal species 3 to 7 these harmonics are practically absent from records further north on the west coast of the antarctic peninsula but are present in all tidal height records from george vi sound a strong ter diurnal signal also exists in the tidal currents under the ice shelf nonlinearity also occurs in the tidal motion of the ronne and ekstrom ice shelves but has not been reported from the ross ice shelf the tidal dynamics of several antarctic ice shelves have therefore been modified by a region of strong nonlinear response to tidal forcing if nonlinear tides in the semidiurnal band are present on the ross ice shelf they could account for difficulties in modeling the area s weak semidiurnal tides an anelastic component in the deformation of the ice at the grounding line is tentatively proposed as the mechanism responsible the positioning of recording pressure sensors in pairs on the seafloor and at the ice shelf base will allow this hypothesis to be tested and also provide a value for the power dissipated by tidally induced flexure at the grounding line

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

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

ice shelf flexure and tidal forcing of bindschadler ice stream west antarctica abstract viscoelastic models of ice shelf flexure and ice stream velocity perturbations are combined into a single efficient flowline model to study tidal forcing of grounded ice the magnitude and timing of ice stream response to tidally driven changes in hydrostatic pressure and or basal drag are found to depend significantly on bed rheology with only a perfectly plastic bed allowing instantaneous velocity response at the grounding line the model can reasonably reproduce gps observations near the grounding zone of bindschadler ice stream formerly ice stream d on semidiurnal time scales however other forcings such as tidally driven ice shelf slope transverse to the flowline and flexurally driven till deformation must also be considered if diurnal motion is to be matched

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

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

dynamics of stick slip motion whillans ice stream antarctica article i nfo the stick slip motion and associated seismic emissions of whillans ice stream wis west antarctica are two of the many recent observations of unexpected ice sheet behavior that are challenging traditional models of rapid glacier motion here we find that the wis slip events repeatedly nucleate from a sticky spot located in the middle of the ice stream acting similar to an asperity in traditional models of earthquake physics this region shows less motion than surrounding areas during the inter slip periods thus concentrating stress and producing a pulse of seismic energy at the onset of slip the propagating rupture breaks through an additional asperity in the northern part of the ice stream producing another pulse of seismic energy 6 12 min after initiation both asperities are regions of higher hydraulic potential than surrounding regions suggesting they may have greater bed friction due to reduced water lubrication tidal pacing of the stress accumulation combined with fault healing controls the applied stress at failure with higher stress giving faster propagation of the rupture front and higher slip velocities these differences are reflected in the timing of the teleseismic arrivals our results highlight both the great sensitivity of large ice streams to small changes in external forcing and the importance of limited regions of the subglacial bed in controlling their motion as well as providing insights to the mechanics of repeating earthquakes

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

ice shelf elevation changes due to atmospheric pressure variations the inverse barometer effect ibe is the isostatic response of ocean surface height to changes in atmospheric pressure p air at a rate of about 1 cm hpa 1 the ibe is a significant contributor to variability of ice shelf surface elevation ice as we demonstrate with simultaneous global positioning system measurements of ice and local measurements of p air from the amery brunt and ross ice shelves antarctica we find that an ibe correction is justified for frequencies covering the weather band 0 03 0 5 cpd cycles per day the ibe correction reduces the standard deviation of the weather band signal of ice from 9 cm to 3 cm with this correction the largest remaining high frequency error signal in ice is the inaccuracy of the present generation of antarctic tide models estimated to be of order 10 cm for most of antarctica

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

tide induced lateral movement of brunt ice shelf antarctica 1 accurate position data show that brunt ice shelf has an irregular three dimensional motion driven by the ocean tide a 30 day gps record from halley station shows not only the expected vertical tidal displacement but also a significant variation in horizontal velocity the maximum forward velocity occurs about four hours before the maximum tidal height a phase relationship that suggests ocean currents may be responsible however values of ice water friction coefficients needed to make this explanation plausible appear to be too high to be physically reasonable alternative explanations invoke interactions between the ice shelf and a grounded area at the ice front our findings suggest that significant errors might result if these motions are ignored when interpreting double difference satellite sar interferograms where the common assumption is that the horizontal velocity of an ice shelf has no tidal dependence

rapid erosion drumlin formation and changing hydrology beneath an antarctic ice stream what happens beneath a glacier affects the way it flows and the landforms left behind when it retreats direct observations from beneath glaciers are however rare and the subglacial environment remains poorly understood we present new repeat observations from west antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record we interpret erosion at a rate of 1 m a 1 beneath a fast flowing ice stream followed by cessation of erosion and the formation of a drumlin from mobilized sediment we also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed all these changes occurred on time scales of a few years or less this variability suggests that an ice stream can reorganize its bed rapidly and that present models of ice dynamics may not simulate all the relevant subglacial processes

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

geodetic imaging of time dependent three component surface deformation application to tidal timescale ice flow of rutford ice stream west antarctica we present a method for inferring time dependent three component surface deformation fields given a set of geodetic images of displacements collected from multiple viewing geometries displacements are parameterized in time with a dictionary of displacement functions the algorithm extends an earlier single component i e single line of sight framework for time series analysis to three spatial dimensions using combinations of multitemporal multigeometry interferometic synthetic aperture radar insar and or pixel offset po maps we demonstrate this method with a set of 101 pairs of azimuth and range po maps generated for a portion of the rutford ice stream west antarctica derived from data collected by the cosmo skymed satellite constellation we compare our results with previously published insar mean velocity fields and selected gps time series and show that our resulting three component surface displacements resolve both secular motion and tidal variability

ocean tides and heinrich events climate varied enormously over the most recent ice age1 for example large pulses of ice rafted debris2 originating mainly from the labrador sea3 were deposited into the north atlantic at roughly 7 000 year intervals with global climatic implications3 here we show that ocean tides within the labrador sea were exceptionally large over the period spanning these huge abrupt ice movements which are known as heinrich events we propose that tides played a catalytic role in liberating iceberg armadas during that time

radar interferometry detection of hinge line migration on rutford ice stream and carlson inlet antarctica satellite synthetic aperture radar sar interferometry is employed to map the hinge line or limit of tidal flexing of rutford ice stream and carlson inlet antarctica and detect its migration between 1992 and 1996 the hinge line is mapped using a model fit from an elastic beam theory

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

tidal modulation of ice shelf flow a viscous model of the ross ice shelf three stations near the calving front of the ross ice shelf antarctica recorded gps data through a full spring neap tidal cycle in november 2005 the data revealed a diurnal horizontal motion that varied both along and transverse to the long term average velocity direction similar to tidal signals observed in other ice shelves and ice streams based on its periodicity it was hypothesized that the signal represents a flow response of the ross ice shelf to the diurnal tides of the ross sea to assess the influence of the tide on the ice shelf motion two hypotheses were developed the first addressed the direct response of the ice shelf to tidal forcing such as forces due to sea surface slopes or forces due to sub ice shelf currents the second involved the indirect response of ice shelf flow to the tidal signals observed in the ice streams that source the ice shelf a finite element model based on viscous creep flow was developed to test these hypotheses but succeeded only in falsifying both hypotheses i e showing that direct tidal effects produce too small a response and indirect tidal effects produce a response that is not smooth in time this nullification suggests that a combination of viscous and elastic deformation is required to explain the observations

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

tidal observations on floating ice using a single gps receiver 1 we measure ocean tides as experienced by sea ice or ice shelves using a single gps receiver such a method allows for tidal measurements to be made with the same precision regardless of its distance from a gps base station since such a base station is not required single epoch observations are shown to be accurate to about 50 mm when compared with concurrent bottom pressure gauge bpg observations the differences between the gps and bpg observations are largely free from power at tidal frequencies furthermore harmonic tidal analyses show that these gps observations allow the correct determination of diurnal and semi diurnal tidal constituents meaning that observations of this type can be assimilated into numerical tide models

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

timing of iron oxide cu au u hydrothermal activity and nd isotope constraints on metal sources in the gawler craton south australia the eastern gawler craton hosts australia s premier uranium bearing iron oxide copper gold iocg belt the 500 km long olympic cu au u province in addition to the olympic dam cu u au and prominent hill cu au deposits numerous barren and weakly mineralized iocg prospects are present in the province new geochronological data for hydrothermal minerals combined with constraints from host rock ages demonstrate that alteration and associated iocg mineralization formed between 1570 and 1600 ma in three districts of the province this iocg hydrothermal activity temporally overlaps with magmatism of the hiltaba suite and gawler range volcanics titanites in paragenetically early magnetite bearing alteration in the olympic dam and prominent hill districts yield u pb ion probe ages of 1576 5 and 1567 10 ma respectively molybdenite in veins crosscutting magnetite biotite and albitic alteration in the moonta wallaroo district have re os ages of 1574 6 and 1599 6 ma respectively these represent minimum ages of the iocg alteration assemblages in this district a muscovite 40ar 39ar age of 1575 11 ma provides a minimum age of paragenetically later hematitic alteration in the olympic dam district neodymium isotope compositions were determined for 44 whole rock samples from barren and weakly mineralized cu au prospects and host rocks in the olympic dam and prominent hill districts the new geochronological framework enables comparison of the nd isotope data across two iocg districts at the time of formation of the olympic dam deposit ca 1590 ma magnetite rich weakly cu mineralized alteration from five prospects yields a relatively narrow range end 1590 values of 5 8 to 4 1 both hematite and magnetite rich alteration yield generally similar end 1590 values that match values from fresh and weakly altered paleoproterozoic metasedimentary and metagranitic rocks 6 6 to 3 5 as well as from most felsic hiltaba suite intrusions and gawler range volcanics in the eastern gawler craton ca 6 to 4 these data are consistent with crustal sources for ree and by implication for associated copper in the barren and weakly mineralized prospects mineralization and alteration in these minor iocg systems can be geochemically discriminated from the giant olympic dam deposit where greater inputs of mantle derived ree and other ore components are evident 2007 society of economic geologists inc

ice stream subglacial access for ice sheet history and fast ice flow the beamish project on rutford ice stream west antarctica and initial results on basal conditions

tidally driven ice speed variation at helheim glacier greenland observed with terrestrial radar interferometry we used a terrestrial radar interferometer tri at helheim glacier greenland in august 2013 to study the effects of tidal forcing on the terminal zone of this tidewater glacier during our study period the glacier velocity was up to 25 m d 1 our measurements show that the glacier moves out of phase with the semi diurnal tides and the densely packed melange in the fjord here detrended glacier displacement lags behind the forecasted tidal height by 8 hours the transition in phase lag between the glacier and the melange happens within a narrow 500 m zone in the fjord in front of the ice cliff the tri data also suggest that the impact of tidal forcing decreases rapidly up glacier of the terminus a flowline model suggests this pattern of velocity perturbation is consistent with weak ice flowing over a weakly nonlinear bed

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

rifting processes and ice flow modulation observed on mertz glacier east antarctica we investigated the evolution of two major rifts cutting across mertz glacier tongue east antarctica using a combination of satellite images and 60 day sets of gps data from two stations deployed either side of the western rift in 2007 the eastern rift began to open in the early 1990s and the western rift initiated in 2002 in conjunction with the collision of a large iceberg with the tongue velocity time series derived from the 2007 gps data exhibited strong variations at tidal periods modulated by sea surface height and sea surface slope and reproduced here with a conceptually simple model we found that opening of the western rift in 2002 leads to a dramatic change in behavior of the tongue as the large range in velocity 700 2400 m a 1 observed in 2000 was largely reduced 1075 1225 m a 1 in 2007 opening of the western rift decoupled the glacier from the transverse loading on the tongue driven by east west tidal circulation this loading previously induced time varying lateral drag which caused the large velocity range our results suggest changes in the mechanical behavior of an ice tongue impact the dynamics of the outlet glacier system and should be considered in longer term mass balance evaluations

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

rigorous gps data processing strategies for glaciological applications global positioning system gps data are now routinely used for many glaciological applications in some common cases systematic errors are unmodelled at the data processing stage although they are often presumed insignificant in this paper i investigate these assumptions for three different scenarios 1 measurements on a moving glacier 2 measurements on a floating ice shelf and 3 precise height determination over large elevation ranges such as for aircraft positioning in lidar laser altimeter missions in each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the gps processing software under certain conditions if these coordinate biases go undetected short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques more rigorous processing strategies are discussed that allow for bias free parameter estimation

rapid recent warming on rutford ice stream west antarctica from borehole thermometry 1 the antarctic peninsula has warmed faster than the global average rate of warming during the last century due to limited availability of long term meteorological records the geographical extent of this rapid warming is poorly defined we collected borehole temperature measurements in the upper 300 m of rutford ice stream west antarctica and employed an inverse modeling scheme with a heat diffusion advection equation to determine the recent surface temperature history of the borehole position our results reveal recent warming of 0 17 0 07 c decade 1 since 1930 this result suggests that at least in an attenuated form the rapid warming observed over the antarctic peninsula extends as far south as rutford ice stream this result agrees with other recent results that show a warming trend across much of the west antarctic ice sheet

acceleration of pine island and thwaites glaciers west antarctica recent satellite investigations revealed that in the 1990s the grounding line of pine island and thwaites glaciers west antarctica retreated several km the ice surface on the interior of the basins lowered 10 cm a 1 and pine island glacier thinned 1 6 m a 1 these observations however were not sufficient to determine the cause of the changes here we present satellite radar interferometry data that show the thinning and retreat of pine island glacier are caused by an acceleration of ice flow of about 18 2 in 8 years thwaites glacier maintained a nearly constant flow regime at its center but widened along the sides and increased its 30 15 mass deficit by another 4 in 4 years the combined mass loss from both glaciers if correct contributes an estimated 0 08 0 03 mm a 1 global sea level rise in 2000

ice fabric in an antarctic ice stream interpreted from seismic anisotropy here we present new measurements of an anisotropic ice fabric in a fast moving 377 ma 1 ice stream in west antarctica we use 6000 measurements of shear wave splitting observed in microseismic signals from the bed of rutford ice stream to show that in contrast to large scale ice flow models which assume that ice is isotropic the ice in rutford ice stream is dominated by a previously unobserved type of partial girdle fabric this fabric has a strong directional contrast in mechanical properties shearing 9 1 times more easily along the ice flow direction than across flow this observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers suggesting it is essential to consider anisotropy in data driven models to correctly predict ice loss and future flow in these regions we show how passive microseismic monitoring can be effectively used to provide these data

synthetic aperture radar interferometry over rutford ice stream and carlson inlet antarctica we describe the calibration and interpretation of interferograms generated from ers i synthetic aperture radar single look complex sar slc images of the rutford ice stream area ground surveys provide over 100 tie points with which to optimise the interferometric baselines that separate nominally repeated satellite orbits covariant tie point errors are dealt with by constructing an error covariance matrix for the expected values of the unwrapped interferometric phases at the tie points with baseline parameters that minimise the weighted residual variance rms tie point residuals of less than i cm in slant range are obtained these are attributed to a combination of interferometric phase noise movement survey errors and inadequate slope information the image set used is inadequate for isolating the influences of topography and movement so the glaciological conclusions to be drawn are limited nevertheless the interferograms confirm that the whole of the upper 50 k m of carlson inlet flows at a speed less than a tenth of that of the neighbouring rutford ice stream also confirmed are the entry of faster moving ice into the lower reaches of carlson inlet and the position of part of the carlson inlet grounding line in general the distribution of the residuals suggests no significant differences in ice movement between 1978 and 1992 an exception is the neighbourhood of the shear margin between rutford ice stream and carlson inlet where inconsistencies between ground surveys over the periods 1984 86 and 1994 96 and interferograms from 1994 a n d 1996 suggest fluctuations in velocity of up to 10 m year 1

orbitally forced sedimentary rhythms in the stratigraphic record is there room for tidal forcing the imprint of orbital cycles which result from the varying eccentricity of the earth s orbit and changes in the orientation of its axis have been recognised throughout the phanerozoic rock record variations in insolation and their effect on climate are generally considered to be the sole transfer mechanism between the orbital variables and cyclic sedimentary successions common oceanographic principles however show that the ocean tide also responds to variations in the orbital parameters the ocean tide has not yet been considered to be a valid additional transfer mechanism for the orbital variations in geological studies of milankovitch cycles in sedimentary successions the insolation paradigm offers satisfying explanations and the role of long term variations of the ocean tide has not yet been appreciated variations in the ocean tide related to changing eccentricity at present 0 0165 theoretical maximum 0 0728 affect a variety of oceanographic and sedimentary processes in addition to the widely accepted paradigm of orbitally forced insolation changes the tidal transfer of orbital signals may explain certain less well understood aspects of orbitally induced cycles in the stratigraphic record related to ocean mixing organic productivity and tidal processes in shallow seas and deep water variations of the ocean tide in relation to the 18 6 year lunar nodal cycle which has no insolation counterpart by which they may be obscured indeed show that these relatively small variations can produce significant effects in sedimentary environments that are sensitive to variations in the strength of the ocean tide in analogy with the 18 6 year lunar nodal cycle orbital variations of the tide on milankovitch time scales are likely to have affected sedimentary systems in the past

parametric modelling of the geometrical ice ocean interaction in the ekstroemisen grounding zone based on short time series summary due to the impact of ocean tide the ekstroemisen ekstroem ice shelf antarctica still shows vertical displacements at its assumed grounding line these can reach amplitudes of 5 cm the low tide amplitudes are smaller than the corresponding high tide amplitudes this indicates that the ice body touches the bedrock during the low tides an elastic feedback of the ice body can be observed as little as 1 km behind the assumed grounding line the maximum vertical displacements can still reach 1 cm these results are based on data collected in the grounding zone of the ekstroemisen during the austral summer of 1997 this region has been studied by using a combined sensor field that makes continuous records using global positioning system gps gravimeter and tiltmeter instruments to derive the elastic response of the ice body to the ocean tides the resulting time series which has to be analysed is biased by outliers incomplete data or complex trend functions it is nearly impossible to analyse the very short time periods without assuming hypotheses it turns out that standard algorithms like correlation functions various fourier transforms or functional regressions are not suitable to solve the parametrization of time series in the region of the grounding zone where several kinematic and gravimetric processes overlap after a thorough analysis of these time series with an average length of 8 d we managed to homogenize the results of the hybrid and non simultaneous observations we were able to create a set of robust parameters to describe the ice shelf motion in the grounding zone based on non linear programming

bedform topography and basal conditions beneath a fast flowing west antarctic ice stream a grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath similar to 140 km 2 of rutford ice stream west antarctica the subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water saturated sediments the two troughs are filled with deforming sediment whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream the deforming bed is very flat along flow but undulates across flow sliding areas show rougher bed topography cross stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction these bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation in places they form finger like mounds of material which extend into the sliding region further downstream mean bedform height is 22 m mean width is 267 m and many of them extend for at least 1 2 km along flow we interpret most of these bedforms as drumlins and one as a mega scale glacial lineation the juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas variable both spatially and temporally any theory of subglacial sediment rheology must also be able to account for our conclusion that at any given time pervasive deformation extends at least a few metres into the bed and can persist over a considerable area many km 2 bedform geometry and basal conditions concur with interpretations of former ice streams with evidence for increasing elongation ratio with distance downstream however those studies also identified bedrock cropping out at the ice bed interface for which there is no evidence on rutford ice stream c 2008 elsevier ltd all rights reserved