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Promontory ice rises are locally grounded features adjacent to ice shelves that are still connected to the ice sheet. Ice rises are an archive for the atmospheric and ice dynamic history of the respective outflow regions where the presence, absence, or migration of Raymond arches in radar stratigraphy represents a memory of the ice-rise evolution. However, ice rises and their inferred dynamic history are not yet used to constrain large-scale ice flow model spin-ups because matching the arch amplitudes includes many unknown parameters, e.g., those pertaining to ice rheology. In particular, anisotropic ice flow models predict gradients in ice fabric anisotropy on either side of an ice divide. However, this has thus far not been validated with observations. The ground-based phase-sensitive Radio Echo Sounder (pRES) has previously been used to infer ice fabric types for various flow regimes using the co-polarized polarimetric coherence phase as a metric to extract information from the birefringent radar backscatter. Here, we apply this technique using quad-polarimetric radar data along a 5 km transect across a ridge near the triple junction of Hammarryggen Ice Rise at the Princess Ragnhild Coast. A comparison with ice core data collected at the dome shows that the magnitude of ice fabric anisotropy can reliably be reconstructed from the quad-polarimetric data. We use the combined dataset also to infer the spatial variation of ice fabric orientations in the vicinity of the triple junction. The observations are integrated with airborne radar profiles and strain rates based on the shallow ice approximation. We then discuss whether estimated anisotropy from radar polarimetry on ice rises, in general, can be another observational constraint to better ice rises as an archive of ice dynamics.

Ice shelves buttress ice flow from Antarctica’s interior and provide the interface for ice-ocean interactions. Here, we present a comprehensive dataset collected with autonomous phase-sensitive radio-echo sounders (ApRES) on the Ekström Ice Shelf in East Antarctica. The data include a one year time series of basal melt near the grounding zone and > 40 repeat and quad-polarimetric observations covering the entire ice shelf with the transition to grounded ice and multiple lateral shear zones. The inferred melt rates are put into context in terms of their seasonality and with respect to spatial patterns which were mapped in the internal ice stratigraphy. The polarimetric backscatter show signs of anisotropic ice fabric and spatial changes can be traced coherently from the grounding line to the ice-shelf front. We investigate those signatures in conjunction with the vertical strain rates inferred from ApRES and ice-flow modelling to learn more about ice-shelf rheology, particularly with respect to the stress exponent n.

The internal ice stratigraphy as imaged by radar is an integrated archive of the atmospheric- oceanographic, and ice-dynamic history that the ice sheet has experienced. It provides an observational constraint for ice flow modeling that has been used for instance to predict age-depth relationships at prospective ice-coring sites in Antarctica’s interior. The stratigraphy is typically more disturbed and more difficult to image in coastal regions due to faster ice flow. Yet, knowledge of ice stratigraphy across ice shelf grounding lines and further seawards is important to help constrain ocean-induced melting and associated stability.Here, we present preliminary results of synthesizing information from radar stratigraphic characteristics from airborne and ground-based radar surveys that have been collected for specific projects starting from the 1990s onwards focusing on ice marginal zones of Antarctica. The key data is based on airborne surveys from the German Alfred Wegener Institute’s polar aircrafts equipped with a 150 MHz radar. In the meantime this system has been replaced by an ultra-wide band 150-520 MHz radar. The older data will provide a baseline with extensive coverage that can be used for model calibration and change detection over time. We aim to provide metrics of the radio stratigraphy (e.g. shape and slope of internal reflection horizons) as well as classified prevalent stratigraphy types that can be used to calibrate machine learning approaches such as simulation based inference. The data obtained will be integrated in coordination efforts within the SCAR AntArchitecture Action Group.

The internal stratigraphy of alpine glaciers entails information about its past dynamics and accumulation rates. It further can be used for intercalibrating the age-depth scales of ice cores. The internal ice stratigraphy is often imaged using radar, but similar to polar ice sheets the deeper stratigraphy is often difficult to resolve with classical pulsed radar systems. For polar ice sheets, the introduction of phase coherent radars has illuminated this former echo-free zone (EFZ) and now patterns of folded, buckled and disrupted ice stratigraphy are clearly visible. Unfortunately, the new airborne and ground-based radar systems applied in polar regions are typically too heavy to be deployed in an alpine environment.Here, we transfer the lightweight autonomous phase-sensitive radio-echo sounder (ApRES) to an alpine glacier targeting its echo-free zone (Colle Gnifetti, Italy/Switzerland). The ApRES is a coherent frequency modulated continuous wave radar with an integration time of 1 s per trace which we deployed in combination with a GNSS used in real time kinematic (RTK) mode. The latter allows repositioning of the antennas with sub-wavelength accuracy (approximately 5 cm) required to exploit the coherent signal. Like this, the radio-stratigraphy of the former EFZ at this site could be imaged using a matched filtering SAR method. The resulting radargrams cover former ice core sites (e.g., Ice Memory and KCC) and can be used to harmonize conflicting age-depth scales. This dataset will be analysed further in conjunction with ice-fabric measurements from ice cores to reveal how the anisotropic ice rheology imprints on the flow field of glaciers.

Mapping the ice bed interface with radar is challenging in many alpine glaciers where the ice is temperate, and in-ice absorption is high. It is also difficult in selected regions of polar ice sheets such as near grounding zones and in ice streams where clutter and rough beds increase incoherent volume scattering. The lack of information for the ice geometry impedes our process understanding, e.g., basal sliding (requires knowledge about the basal roughness) and the routing of subglacial water flow (requires knowledge on basal smoothness). The lack of observations to constrain variations in ice thickness on the sub-kilometre scale is thus still a bottleneck to confidently predict ice dynamics and expected rates of sea-level rise.A recent development in radioglaciology, namely the application of phase-coherent polarimetric radar, provides an excellent opportunity to overcome these limitations. Radar polarimetry has made significant strides in the last few years to constrain internal ice structure and their impact on the deformation of ice sheets, including the reconstruction of ice micro-structure parameters previously obtained from ice cores. Here, we suggest that the ice-bed interface can be identified in characteristic patterns of the polarimetric coherence phase. This new metric provides information in areas where the backscattered power amplitude does not show any signatures of the ice-bed interface. We provide examples for this across a wide range of glaciological settings, including cold (Colle Gnifetti, Switzerland) and temperate (Hintereisferner, Austria) alpine glaciers, thin grounding zones (Ekström Ice Shelf, East Antarctica) and thick ice domes (Dome C, East Antarctica). If this holds, then the ice thickness mapping in challenging glaciological settings should preferably be done using a quad-polarimetric acquisition geometry. For ground-based surveys, this can be done using an autonomous ice rover, for which we provide a proof-of-concept study on the Ekström Ice Shelf in Antarctica.

Acquisition of quad-polarimetric radar data on ice sheets gives insights about the ice-fabric variability with depth and consequently can deliver essential constraints on the spatially variable ice rheology. Polarimetric measurements are collected manually in most ground-based surveys, discretely sampling a limited profile range. Measurements are time-intensive and often do not cover critical areas such as shear zones where field safety is a concern. Autonomous rovers can provide an alternative that optimizes for time, sampling resolution and safety. Here, we present an autonomous acquisition technique of quad-polarimetric radar data using a rover. This technique is based on a previous layout that has proven its capacity to navigate in various snow conditions but did not yet actively trigger the geophysical instruments attached. We upgraded the rover with a novel Robotic Operating System (ROS2) that interfaces simultaneously with a real-time positioning GPS and an automatic phase-sensitive radio-echo sounder (ApRES) with multiple transmitters multiple receivers. Like this, the rover can autonomously steer to pre-destined waypoints and then take static measurements at those locations also in areas where field safety might be compromised. We demonstrate this proof-of-concept on the Ekström Ice Shelf Antarctica, where we acquired densely spaced polarimetric radar data measurements. The rover’s operating system offers many opportunities for other measurement principles, e.g., densely spaced co-polarized data suitable for synthetic aperture radar (SAR) processing.

The densification of polar firn that is subjected to horizontal strain rates is studied. A model for the enhanced densification of the firn by strain softening is developed. Strain softening describes an acceleration of power-law creep in the presence of high horizontal strain rates, which was suggested to explain the occurrence of exceptionally thin firn in the shear margins of ice streams. With the model the effect of strain softening is compared to other strain-driven densification mechanisms, like pure shear and strain heating, and to potential variations of temperature and accumulation rate. Thereby, strain softening is identified to dominate firn densification at high strain rates. A recorded density profile along a cross-section of the North-East Greenland ice stream (NEGIS) is reproduced with the presented model with good agreement in the shear margins. There, the thinning of the firn correlates with the location and magnitude of the shear margin troughs, which indicates that their formation is caused by strain softening. In regions with low strain rates the model overestimates the densification rate. Because of a particularly strong sensitivity of the model to low strain rates and the presence of non-zero strain rates on large parts of the Greenland Ice Sheet (GrIS), it is suggested that empirically tuned densification models already implicitly consider moderate horizontal strain rates. Besides the temperature and the accumulation rate, the effective horizontal strain rate is therefore proposed as a third forcing parameter, that needs to be considered in the development of a physics-based firn densification model.

Ice shelves are widely known to slow the transfer of Antarctic grounded ice to the ocean, especially if their flow is decelerated by local pinning points. Their longevity is influenced by variations in ice dynamics, surface accumulation and oceanic conditions in the ice shelf cavity. This is reflected in the ice shelf structure, which can be characterized by the shape of internal radar reflection horizons.We aim to map the internal ice shelf stratigraphy of ice shelves, starting with the narrow belt of ice-shelves in the Dronning Maud Land area. The final goal will be to evaluate these as a spatiotemporal archive of ice provenance and ice dynamics. The bulk of the data presented here were collected with AWI’s airborne multi frequency ultra-wideband radar and we combine these new observations with airborne and ground-based radar surveys from previous years. We present a consistent set of internal radar isochrones on a catchment scale for the Roi Baudoin area including the Ragnhild ice streams, the grounding-zone, the iceshelf and multiple ice rises. Using pattern matching technique we can link isochrones across different ice rises in the area, and hence provide first observational constraints on how ice rises jointly react to changes in atmospheric and oceanographic forcings. We also find a number of interesting features including dynamically induced dips in shear zones, truncating layers at the ice-shelf base, and the development of a meteoric ice layer distinguishing advected from newly accumulated ice in the iceshelf. The time series provided by radar observations over the last 10 years also offers the potential to map temporal changes. We use ice-flow modelling to provide age constraints for some internal layers and delineate portions within the shelf as a function of their advection history, hence marking areas of differing rheologies within the shelf. Taken together, this case study on a catchment scale is a primer to unravel the information stored in the isochronal stratigraphy of coastal Antarctica and contributes to international efforts (e.g., SCAR AntArchitecture) of mapping stratigraphy on ice sheet scales.