Ice Rumples Research Articles

Brief communication: Rapid acceleration of the Brunt Ice Shelf after calving of iceberg A-81

Abstract. The Brunt Ice Shelf, Antarctica, accelerated rapidly from a velocity of 900 to 1500 m a−1 during 6 months, following calving of a 1500 km2 iceberg on 22 January 2023. The immediate response to calving was observed as a change to the rate of acceleration and not to velocity directly. Acceleration increased by a factor of 10, with a second, smaller calving at the end of June 2023, leading to further tripling of acceleration. The acceleration was caused by the reduction of buttressing at the McDonald Ice Rumples, leading to high localised strain rates, which reduce the strength of the remaining ice shelf.

Topography reconstruction and evolution analysis of outlet glacier using data from unmanned aerial vehicles in Antarctica

Reconstruction of glacial topography is important for assessing the ice dynamics of glaciers in the past and understanding how they may respond to climate change in the future. As an emerging strategy, unmanned aerial vehicles (UAVs) have been successfully used in glaciology applications to reconstruct surface topography and monitor the short-term dynamics of glaciers. However, none of these studies have focused on the ice dynamics of outlet glaciers in Antarctica. In this study, based on a combination of UAVs and a base station, we investigated Dalk Glacier, a typical marine-terminating glacier in East Antarctica, during two Chinese National Antarctic Research Expeditions from 2019 to 2020. By applying structure-from-motion and multi-view-stereo photogrammetry, high-resolution orthomosaics and digital elevation models of the glacial topography were reconstructed with centimeter-level accuracy via in situ validation, thus marking the first application of UAV observations in the monitoring of Antarctic outlet glaciers. Topographic evolution of the glacier was quantitatively analyzed from various aspects including ice velocity, surface elevation, crevasses, and ice front calving. A maximum ice velocity of ∼ 310 m a-1 at its terminus was observed, which was ∼ 90 m a-1 greater than that in satellite-based studies. We analyzed in detail the spatially heterogeneous changes in the ice velocity and surface elevation of the glacier under the influence of an ice rumple at its calving terminus. Combined with satellite images and existing datasets, we hypothesize that the large ice rumples at the glacier terminus could deform the glacier and potentially damage its structural integrity, thereby limiting the growth of its terminus.

Hysteretic evolution of ice rises and ice rumples in response to variations in sea level

Abstract. Ice rises and ice rumples are locally grounded features found in coastal Antarctica and are surrounded by otherwise freely floating ice shelves. An ice rise has an independent flow regime, whereas the flow regime of an ice rumple conforms to that of the ice shelf and merely slows the flow of ice. In both cases, local highs in the bathymetry are in contact with the ice shelf from below, thereby regulating the large-scale ice flow, with implications for the upstream continental grounding line position. This buttressing effect, paired with the suitability of ice rises as a climate archive, necessitates a better understanding of the transition between ice rise and ice rumple, their evolution in response to a change in sea level, and their dynamic interaction with the surrounding ice shelf. We investigate this behaviour using a three-dimensional full Stokes ice flow model with idealised ice rises and ice rumples. The simulations span end-member basal friction scenarios of almost stagnant and fully sliding ice at the ice–bed interface. We analyse the coupling with the surrounding ice shelf by comparing the deviations between the non-local full Stokes surface velocities and the local shallow ice approximation (SIA). Deviations are generally high at the ice divides and small on the lee sides. On the stoss side, where ice rise and ice shelf have opposing flow directions, deviations can be significant. Differences are negligible in the absence of basal sliding where the corresponding steady-state ice rise is larger and develops a fully independent flow regime that is well described by SIA. When sea level is increased, and a transition from ice rise to ice rumple is approached, the divide migration is more abrupt the higher the basal friction. In each scenario, the transition occurs after the stoss-side grounding line has moved over the bed high and is positioned on a retrograde slope. We identify a hysteretic response of ice rises and ice rumples to changes in sea level, with grounded area being larger in a sea-level-increase scenario than in a sea-level-decrease scenario. This hysteresis shows not only irreversibility following an equal increase and subsequent decrease in sea level but also that the perturbation history is important when the ice rise or ice rumple geometry is not known. The initial grounded area needs to be carefully considered, as this will determine the formation of either an ice rise or an ice rumple, thereby causing different buttressing effects.

Mechanics and dynamics of pinning points on the Shirase Coast, West Antarctica

Abstract. Ice rises and rumples, sites of localised ice-shelf grounding, modify ice-shelf flow by generating lateral and basal shear stresses, upstream compression, and downstream tension. Studies of pinning points typically quantify this role indirectly, through related metrics such as a buttressing number. Here, we quantify the dynamic effects of pinning points directly, by comparing model-simulated stress states in the Ross Ice Shelf (RIS) with and without a specific set of pinning points located downstream of the MacAyeal and Bindschadler ice streams (MacIS and BIS, respectively). Because ice properties are only known indirectly, the experiment is repeated with different realisations of the ice softness. While longitudinal stretching, and thus ice velocity, is smaller with the pinning points, flow resistance generated by other grounded features is also smaller. Conversely, flow resistance generated by other grounded features increases when the pinning points are absent, providing a non-local control on the net effect of the pinning points on ice-shelf flow. We find that an ice stream located directly upstream of the pinning points, MacIS, is less responsive to their removal than the obliquely oriented BIS. This response is due to zones of locally higher basal drag acting on MacIS, which may itself be a consequence of the coupled ice-shelf and ice-stream response to the pinning points. We also find that inversion of present-day flow and thickness for basal friction and ice softness, without feature-specific, a posteriori adjustment, leads to the incorrect representation of ice rumple morphology and an incorrect boundary condition at the ice base. Viewed from the perspective of change detection, we find that, following pinning point removal, the ice shelf undergoes an adjustment to a new steady state that involves an initial increase in ice speeds across the eastern ice shelf, followed by decaying flow speeds, as mass flux reduces thickness gradients in some areas and increases thickness gradients in others. Increases in ice-stream flow speeds persist with no further adjustment, even without sustained grounding-line retreat. Where pinning point effects are important, model tuning that respects their morphology is necessary to represent the system as a whole and inform interpretations of observed change.

Measuring the location and width of the Antarctic grounding zone using CryoSat-2

Abstract. We present the results of mapping the limit of the tidal flexure (point F) and hydrostatic equilibrium (point H) of the grounding zone of Antarctic ice shelves from CryoSat-2 standard and swath elevation data. Overall we were able to map 31 % of the grounding zone of the Antarctic floating ice shelves and outlet glaciers. We obtain near-complete coverage of the Filchner–Ronne Ice Shelf. Here we manage to map areas of Support Force Glacier and the Doake Ice Rumples, which have previously only been mapped using break-in-slope methods. Over the Ross Ice Shelf, Dronning Maud Land and the Antarctic Peninsula, we obtained partial coverage, and we could not map a continuous grounding zone for the Amery Ice Shelf and the Amundsen Sea sector. Tidal amplitude and distance south (i.e. across-track spacing) are controlling factors in the quality of the coverage and performance of the approach. The location of the point F agrees well with previous observations that used differential satellite radar interferometry (DInSAR) and ICESat-1, with an average landward bias of 0.1 and 0.6 km and standard deviation of 1.1 and 1.5 km for DInSAR and ICESat measurements, respectively. We also compared the results directly with DInSAR interferograms from the Sentinel-1 satellites, acquired over the Evans Ice Stream and the Carlson Inlet (Ronne Ice Shelf), and found good agreement with the mapped points F and H. We also present the results of the spatial distribution of the grounding zone width (the distance between points F and H) and used a simple elastic beam model, along with ice thickness calculations, to calculate an effective Young modulus of ice of E=1.4±0.9 GPa.

Dynamics of Dalk Glacier in East Antarctica Derived from Multisource Satellite Observations Since 2000

Monitoring variability in outlet glaciers can improve the understanding of feedbacks associated with calving, ocean thermal forcing, and climate change. In this study, we present a remote-sensing investigation of Dalk Glacier in East Antarctica to analyze its dynamic changes. Terminus positions and surface ice velocities were estimated from Landsat and Sentinel-1 data, and the high-precision Worldview digital elevation model (DEM) was generated to determine the location of the potential ice rumple. We detected the cyclic behavior of glacier terminus changes and similar periodic increases in surface velocity since 2000. The terminus retreated in 2006, 2009, 2010, and 2016 and advanced in other years. The surface velocity of Dalk Glacier has a 5-year cycle with interannual speed-ups in 2007, 2012, and 2017. Our observations show the relationship between velocity changes and terminus variations, as well as the driving role of the ice rumple. Ice velocity often increases after calving events and continuous retreats. The loss of buttressing provided by an ice rumple may be a primary factor for increases in ice velocity. Given the restriction of the ice rumple, the surface velocity remains relatively stable when the glacier advances. The calving events may be linked to the unstable terminus caused by the ice rumple.

Holocene Formation of Henry Ice Rise, West Antarctica, Inferred From Ice‐Penetrating Radar

Ice rises are regions of grounded ice embedded within floating ice shelves. The deformation of ice past them increases the back stress generated by the ice shelf, slowing the flow of the ice sheet. We present ground‐based ice‐penetrating radar data from Henry Ice Rise in the Ronne Ice Shelf, West Antarctica, that indicates regrounding during the Holocene. Relic crevasses and melt synclines are observed upstream of the present‐day grounding line. We conclude that these features formed during a previous flow configuration, from which the grounding line has since advanced to its current position. In agreement with previous work, our observations can be explained if initial grounding of the ice shelf occurred on a bathymetric high, forming an ice rumple that migrated upstream and temporarily ungrounded over the topographic high. The grounding line then advanced, preserving relic basal crevasses in the newly grounded ice. Using a simple ice‐flow model, we simulate the burial of these crevasses. While accounting for uncertainty in accumulation, firn density, radar‐derived depth, ice‐thickening history, initial crevasse height, and glacial isostatic adjustment, we estimate a burial time of 6 ± 2 kyr before present for the oldest relic crevasses, indicating that the ice rise formed at approximately this time. This potentially increased the buttressing generated by the Ronne Ice Shelf, causing thickening and advance of the ice sheet. By dating the formation and providing details of ice‐rise formation, these new results can provide useful constraints on both large‐scale ice‐sheet models and models of ice‐rumple and ice‐rise formation.

Progressive Degradation of an Ice Rumple in the Thwaites Ice Shelf, Antarctica, as Observed from High-Resolution Digital Elevation Models

Ice rumples are locally-grounded features of flowing ice shelves, elevated tens of meters above the surrounding surface. These features may significantly impact the dynamics of ice-shelf grounding lines, which are strongly related to shelf stability. In this study, we used TanDEM-X data to construct high-resolution DEMs of the Thwaites ice shelf in West Antarctica from 2011 to 2013. We also generated surface deformation maps which allowed us to detect and monitor the elevation changes of an ice rumple that appeared sometime between the observations of a grounding line of the Thwaites glacier using Double-Differential Interferometric SAR (DDInSAR) in 1996 and 2011. The observed degradation of the ice rumple during 2011–2013 may be related to a loss of contact with the underlying bathymetry caused by the thinning of the ice shelf. We subsequently used a viscoelastic deformation model with a finite spherical pressure source to reproduce the surface expression of the ice rumple. Global optimization allowed us to fit the model to the observed deformation map, producing reasonable estimates of the ice thickness at the center of the pressure source. Our conclusion is that combining the use of multiple high-resolution DEMs and the simple viscoelastic deformation model is feasible for observing and understanding the transient nature of small ice rumples, with implications for monitoring ice shelf stability.

Recent rift formation and impact on the structural integrity of the Brunt Ice Shelf, East Antarctica

Abstract. We report on the recent reactivation of a large rift in the Brunt Ice Shelf, East Antarctica, in December 2012 and the formation of a 50 km long new rift in October 2016. Observations from a suite of ground-based and remote sensing instruments between January 2000 and July 2017 were used to track progress of both rifts in unprecedented detail. Results reveal a steady accelerating trend in their width, in combination with alternating episodes of fast ( > 600 m day−1) and slow propagation of the rift tip, controlled by the heterogeneous structure of the ice shelf. A numerical ice flow model and a simple propagation algorithm based on the stress distribution in the ice shelf were successfully used to hindcast the observed trajectories and to simulate future rift progression under different assumptions. Results show a high likelihood of ice loss at the McDonald Ice Rumples, the only pinning point of the ice shelf. The nascent iceberg calving and associated reduction in pinning of the Brunt Ice Shelf may provide a uniquely monitored natural experiment of ice shelf variability and provoke a deeper understanding of similar processes elsewhere in Antarctica.

Five decades of strong temporal variability in the flow of Brunt Ice Shelf, Antarctica

ABSTRACTData showing velocity changes on the Brunt Ice Shelf (BIS), Antarctica, over the last 55 years are presented and analysed. During this period no large-scale calving events took place and the ice shelf gradually grew in size. Ice flow velocities, however, fluctuated greatly, increasing twofold between 1970 and 2000, then decreasing again to previous levels by 2012 after which velocities started to increase yet again. In the observational period, velocity changes in the order of 10% a−1 have commonly been observed, and currently velocities are increasing at this rate. By modelling the ice flow numerically, we explore potential causes for the observed changes in velocity. We find that a loss of mechanical contact between the BIS and the McDonald Ice Rumples following a local calving event in 1971 would explain both the increase and the subsequent decrease in ice velocities. Other explanations involving enlargement of observed rift structures are discounted as the effects on ice flow are found to be too small and the spatial pattern of velocity change inconsistent with data. The most recent phase of acceleration remains unexplained but may potentially be related to a recent re-activation of a known rift structure within the BIS.

The evolution of the western rift area of the Fimbul Ice Shelf, Antarctica

Abstract. This paper studies the evolution of a zone in the Fimbul Ice Shelf that is characterised by large crevasses and rifts west of Jutulstraumen, an outlet glacier flowing into Fimbulisen. High-resolution radar imagery and radio echo sounding data were used to study the surface and internal structure of this rift area and to define zones of similar characteristics. The western rift area is dominated by two factors: a small ice rumple that leads to basal crevasses and disturbs the homogeneity of the ice, and a zone with fibre-like blocks. Downstream of the rumple we found down-welling of internal layers and local thinning, which we explain as a result of basal crevasses due to the basal drag at the ice rumple. North of Ahlmannryggen the ice loses its lateral constraint and forms individual blocks, which are deformed like fibres under shear, where the ice stream merges with slower moving ice masses of the western side. There, the ice loses its integrity, which initiates the western rift system. The velocity difference between the slow moving western part and the fast moving extension of Jutulstraumen produces shear stress that causes the rifts to form tails and expand them to the major rifts of up to 30 km length.

Glaciological characteristics of Institute Ice Stream using remote sensing

We assess the ice flow of Institute Ice Stream (IIS; 81.5°S, 75°W) and the adjacent Ronne Ice Shelf using satellite images and geophysical parameters from recent continent-wide compilations. Landsat image pairs from the 1980s and 1990s are used to determine ice velocity. Peak speed is 398 ± 10 m a−1. Several mappings using images spanning an eleven-year period indicate this speed and the pattern of ice flow throughout the mapped portion of the stream is constant to within ± 20 m a−1. Combining catchment extent (141 700 km2) with surface accumulation, mass input to IIS is 25.1 ± 2 Gt a−1. Mean ice thickness across the grounding line is 1177 m. Mass flux to the Ronne Ice Shelf, determined from these values and our velocity profile, is 22.7 ± 2 Gt a−1. Topographic mapping using photoclinometry, coupled with ice thickness and ice velocity, permits an assessment of driving force versus flow speed. This indicates wide variations in basal resistance. Despite evidence of present-day near-balance and constant speed in the ice stream trunk, a recent change in outflow is implied by folding of shelf streaklines near Korff Ice Rise. This may be a result of changing shelf thickness or erosion of Doake Ice Rumples.

Theory of shallow ice shelves

Ice shelves consist of two layers, an upper layer of meteoric ice nourished by the flow from the connected inland ice and precipitation, and a lower layer of marine ice that is built by the melting and freezing processes at the ice-ocean interface and the accretion of frazil ice from the underlying ocean. The governing thermomechanical equations in the two layers are formulated as are the boundary and transition conditions that apply at the free surface, the material interface between the meteoric and the marine ice and the ice-ocean interface. The equations comprise in the bulk mass balances for the ice and the salt water (in marine ice), momentum balance and energy balance equations, and at the boundaries kinematic equations as well as jump conditions of mass, momentum and energy. The side boundary conditions involve a prescription of the mass flow along the grounding line from the inland ice and a kinematic law describing the mass loss by calving along the floating ice-shelf front. An appropriate scaling, in which the shallowness of the ice shelves is used, gives rise to the development of a perturbation scheme for the solution of the three-dimensional equations. Its lowest-order approximation – the shallow-shelf approximation (SSA) – shows the ice flow to be predominantly horizontal with a velocity field independent of depth, but strongly depth-dependent temperature and stress distributions. This zeroth order shallow-shelf approximation excludes the treatment of ice rumples, ice rises and the vicinity of the grounding line, but higher-order equations may to within second-order accuracy in the perturbation parameter accommodate for these more complicated effects. The scaling introduced finally leads to a vertical integrated system of non-linear partial integro-differential equations describing the ice flow and evolution equation for temperature and the free surfaces.

Seabed topography under the southern and western Ronne Ice Shelf, derived from seismic surveys

Seismic reflection measurements of ice thickness and water-column thickness have been made over the southern and western Ronne Ice Shelf, from which, the seabed elevation has been determined. A 2300 km traverse, covering an area over which little bathymetric data previously existed and at a station interval of 15 km, resulted in 152 new measurements of the seabed elevation. The seismic velocity in the ice shelf was determined using shallow refraction surveys for the snow and firn in the upper 100 m of the ice shelf, and by applying a model assuming a simple temperature profile for the deeper ice. Surface elevations were calculated assuming the ice to be in hydrostatic equilibrium. The estimated error in the seabed elevations is ±15 m. With the addition of the new data there is now complete coverage of the seabed elevation and water-column thickness for Ronne Ice Shelf at a spatial resolution of between 10–100 km. The seabed in the area to the south of Korff and Henry ice rises and the Doake Ice Rumples is about 100 m shallower than had previously been speculated, which will affect the validity of previous assumptions of water circulation and ice-ocean interaction in this area.

Tidal flexure at ice shelf margins

Hinge zones accommodate the differential tidal movement between floating ice shelves, which move with the sea tide, and grounded ice sheets, which do not. Observations on Rutford Ice Stream and Ronne Ice Shelf, Antarctica, using the kinematic method of Global Positioning System, have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone. The form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique. The model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty. The same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from Doake Ice Rumples, Bach Ice Shelf, and Ekström Ice Shelf, Antarctica, and Jakobshavns Glacier, Greenland. I conclude that the elastic beam model with a single value of the elastic modulus (0.88±0.35 GPa) adequately describes almost all the data and so can usefully be applied elsewhere. Earlier studies of tidal dissipation in ice shelf hinge zones, based on a viscous or transient creep rheology, showed that perhaps 30% of global tidal energy dissipation could be occurring in ice shelf hinge zones. This study suggests, however, that an elastic rheology may be equally appropriate, in which case this estimation of the tidal energy dissipation is excessively high.

Satellite imagery, a base line for glacier variation study on James Ross Island, Antarctica

Sequential visible and radar satellite imagery is used to document the fluctuations of 39 outlet glaciers on James Ross Island (JRI; 64° S, 58° W), Antarctic Peninsula. The monitoring time interval of 17.4 years extends from late 1975 until early 1993. The study is based on synergistic use of different sensors for monitoring glacier variations. The digitally enhanced Landsat TM images also allowed definition of boundaries between grounded and floating ice within Röhss Bay and detection of ice rumples, resulting in an improved measurement of the areal extent of JRI. In addition to glacier boundaries, the analyzed ERS-1 SAR images provide information on morphological properties of snow and ice. The satellite data show a general glacier retreat since 1975, resulting in clear decrease of overall glacier area on JRI. Of 39 glaciers measured, 33 glaciers retreated in the period 1975–93, 15 of which showed significant retreat in comparison to glacier size. This is in accordance with the pronounced atmospheric-warming trend recorded during the last two decades on the Antarctic Peninsula. The analysis of glacier areas on JRI thus provides a base line for detection of future changes.

Satellite imagery, a base line for glacier variation study on James Ross Island, Antarctica

Sequential visible and radar satellite imagery is used to document the fluctuations of 39 outlet glaciers on James Ross Island (JRI; 64° S, 58° W), Antarctic Peninsula. The monitoring time interval of 17.4 years extends from late 1975 until early 1993. The study is based on synergistic use of different sensors for monitoring glacier variations. The digitally enhanced Landsat TM images also allowed definition of boundaries between grounded and floating ice within Röhss Bay and detection of ice rumples, resulting in an improved measurement of the areal extent of JRI. In addition to glacier boundaries, the analyzed ERS-1 SAR images provide information on morphological properties of snow and ice. The satellite data show a general glacier retreat since 1975, resulting in clear decrease of overall glacier area on JRI. Of 39 glaciers measured, 33 glaciers retreated in the period 1975–93, 15 of which showed significant retreat in comparison to glacier size. This is in accordance with the pronounced atmospheric-warming trend recorded during the last two decades on the Antarctic Peninsula. The analysis of glacier areas on JRI thus provides a base line for detection of future changes.

The central part of the Filchner-Ronne Ice Shelf, Antarctica: internal structures revealed by 40MHz monopulse RES

About 1200 km of surface-based radio-echo sounding (RES) profiles were measured during the German Antarctic field season 1989-90 on the Filchner Ronne Ice Shelf (FRIS), Antarctica, with a 40 MHz monopulse sounder developed in Münster. In the area investigated, downstream of Doake Ice Rumples and Henry Ice Rise, a basal layer of marine ice up to 400 m thick was found below the meteoric ice, which is less than 100 m thick in some places. High absorption losses and low bottom reflectivity of the basal layer prevented sounding of the total ice thickness, except for areas near the ice edge with strong melting. From common mid-point (CMP) measurements, the velocities of electromagnetic waves in the ice shelf were determined, to obtain precise depths from travel times for airborne and surface-based soundings. The continuous profiling yielded the total ice thickness in the northern part and a variety of internal structures, including lavers in the meteoric and the marine ice, remnants of crevasses, varying reflections of the meteoric-marine interface reflection (MMR). From these data, it is shown that the most-disturbed MMR signatures originate at grounding zones at Doake lee Rumples and at both sides of Henry Ice Rise. The status of the meteoric ice bottom is preserved by the formation of marine ice. Hence, structures remain widely unchanged over 400 km of flow. Internal features that can be correlated on parallel profiles perpendicular to the flow provide an independent means for the construction of flow lines. Even in areas with a smooth MMR, reflection amplitudes show variations by a factor of 2. Their spatial frequency is maximal on profiles prependicular to the now direction. A possible explanation for the amplitude variations is the varying salinity of the top of the marine ice.

The central part of the Filchner-Ronne Ice Shelf, Antarctica: internal structures revealed by 40MHz monopulse RES

About 1200 km of surface-based radio-echo sounding (RES) profiles were measured during the German Antarctic field season 1989-90 on the Filchner Ronne Ice Shelf (FRIS), Antarctica, with a 40 MHz monopulse sounder developed in Münster. In the area investigated, downstream of Doake Ice Rumples and Henry Ice Rise, a basal layer of marine ice up to 400 m thick was found below the meteoric ice, which is less than 100 m thick in some places. High absorption losses and low bottom reflectivity of the basal layer prevented sounding of the total ice thickness, except for areas near the ice edge with strong melting.From common mid-point (CMP) measurements, the velocities of electromagnetic waves in the ice shelf were determined, to obtain precise depths from travel times for airborne and surface-based soundings.The continuous profiling yielded the total ice thickness in the northern part and a variety of internal structures, including lavers in the meteoric and the marine ice, remnants of crevasses, varying reflections of the meteoric-marine interface reflection (MMR). From these data, it is shown that the most-disturbed MMR signatures originate at grounding zones at Doake lee Rumples and at both sides of Henry Ice Rise. The status of the meteoric ice bottom is preserved by the formation of marine ice. Hence, structures remain widely unchanged over 400 km of flow. Internal features that can be correlated on parallel profiles perpendicular to the flow provide an independent means for the construction of flow lines. Even in areas with a smooth MMR, reflection amplitudes show variations by a factor of 2. Their spatial frequency is maximal on profiles prependicular to the now direction. A possible explanation for the amplitude variations is the varying salinity of the top of the marine ice.

The central part of the Filchner-Ronne Ice Shelf, Antarctica: internal structures revealed by 40MHz monopulse RES

About 1200 km of surface-based radio-echo sounding (RES) profiles were measured during the German Antarctic field season 1989-90 on the Filchner Ronne Ice Shelf (FRIS), Antarctica, with a 40 MHz monopulse sounder developed in Münster. In the area investigated, downstream of Doake Ice Rumples and Henry Ice Rise, a basal layer of marine ice up to 400 m thick was found below the meteoric ice, which is less than 100 m thick in some places. High absorption losses and low bottom reflectivity of the basal layer prevented sounding of the total ice thickness, except for areas near the ice edge with strong melting. From common mid-point (CMP) measurements, the velocities of electromagnetic waves in the ice shelf were determined, to obtain precise depths from travel times for airborne and surface-based soundings. The continuous profiling yielded the total ice thickness in the northern part and a variety of internal structures, including lavers in the meteoric and the marine ice, remnants of crevasses, varying reflections of the meteoric-marine interface reflection (MMR). From these data, it is shown that the most-disturbed MMR signatures originate at grounding zones at Doake lee Rumples and at both sides of Henry Ice Rise. The status of the meteoric ice bottom is preserved by the formation of marine ice. Hence, structures remain widely unchanged over 400 km of flow. Internal features that can be correlated on parallel profiles perpendicular to the flow provide an independent means for the construction of flow lines. Even in areas with a smooth MMR, reflection amplitudes show variations by a factor of 2. Their spatial frequency is maximal on profiles prependicular to the now direction. A possible explanation for the amplitude variations is the varying salinity of the top of the marine ice.