Description: The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component contributing to the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The ice stream geometry and surface velocities in the onset region of the NEGIS are not yet sufficiently well reproduced by ice sheet models. We present an assessment of the basal conditions of the onset region in a systematic analysis of airborne ultra-wideband radar data. Our data yield a new detailed model of ice-thickness distribution and basal topography in the upstream part of the ice stream. We observe a change from a smooth to a rougher bed where the ice stream widens from 10 to 60 km, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. The observation of off-nadir reflections that are symmetrical to the bed reflection in the radargrams suggests that these structures are elongated subglacial landforms, which in turn indicate potential streamlining of the bed. Together with basal water routing pathways, our observations hint to two different zones in this part of the NEGIS: an accelerating and smooth upstream region, which is collecting water, with reduced basal traction, and in the further downstream part, where the ice stream is slowing down and is widening, with a distribution of basal water towards the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology.

Description: The Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the Federal Institute for Geosciences and Natural Resources (BGR) collected around 150 hours of new gravity, magnetic and ice-penetrating radar data from east and south of Princess Elisabeth station in Dronning Maud Land between 2013 and 2015. Survey lines were spaced 10 km apart. The 2013/2014 and 2014/2015 used different gravimeters; a LaCoste and Romberg AirSea gravimeter (LCR) at constant barometric altitude and a Gravimetric Technologies GT2A gravimeter at constant ground separation. Both surveys used a Scintrex Cs-3 caesium vapour magnetometer mounted in a tail boom with compensation for the airframe calculated using a fuselage-mounted three-component fluxgate magnetometer. The GT2A gravity data reflect the effects of short-wavelength density contrasts between basement rocks and the ice sheet more reliably than the LCR data. Cross-over analysis suggests the repeatability of data collection with the GT2A lies at the sub-milliGal level. A broad subglacial channel that separates eastern Sør Rondane from the Yamato Belgica Mountains is evident in the gravity data. In the south of the survey region, the data reveal a dendritic pattern of subglacial valleys that converge towards the SW. Strong NS-trending magnetic anomalies coincide with the Yamato-Belgica Mountains. Further west, subtler ESE-trending anomalies confirm proposals that the SE Dronning Maud Land province continues into the region south of eastern Sør Rondane. An unexpected feature of both data sets is the apparent termination of the anomaly patterns associated with the province at a NNW-trending anomaly running south of Princess Elisabeth.

Description: We investigate the propagation of seismic waves in anisotropic ice. Two effects are important: (i) sudden changes in crystal orientation fabric (COF) lead to englacial reflections; (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded travel times. Velocities calculated from the polycrystal elasticity tensor derived for the anisotropic fabric from measured COF eigenvalues of the EDML ice core, Antarctica, show good agreement with the velocity trend determined from vertical seismic profiling. The agreement of the absolute velocity values, however, depends on the choice of the monocrystal elasticity tensor used for the calculation of the polycrystal properties. We make use of abrupt changes in COF as a common reflection mechanism for seismic and radar data below the firn–ice transition to determine COF-induced reflections in either data set by joint comparison with ice-core data. Our results highlight the possibility to complement regional radar surveys with local, surface-based seismic experiments to separate isochrones in radar data from other mechanisms. This is important for the reconnaissance of future ice-core drill sites, where accurate isochrone (i.e. non-COF) layer integrity allows for synchronization with other cores, as well as studies of ice dynamics considering non-homogeneous ice viscosity from preferred crystal orientations.

Description: The area around Syowa Station, the Japanese Antarctic wintering station in Lützow-Holm Bay, is widely considered to a junction of the continents of Africa, India, Madagascar, and Antarctica, according to a reconstruction model of Gondwana that considers the suture between East and West Gondwana. This area is therefore key investigating the formation of Gondwana. To reveal the tectonic evolution that contributed to Gondwana's formation in this area, joint Japanese-German airborne geophysical surveys were conducted around Syowa Station in January 2006 during the 47th Japanese Antarctic Research Expedition, from 67°S to 73°S latitude and from 35°E to 45°E longitude. Ice radar, magnetic, and gravity data were obtained from onshore areas. Several characteristic features that are possibly related to the tectonic evolution of Gondwana were inferred, primarily from magnetic anomalies, as well as from gravity anomalies and bedrock topography. The boundaries of the Lützow- Holm Complex, the Yamato-Belgica Complex, and the Western Rayner Complex are defined, but the inland extension of the boundary between the Lützow-Holm and the Yamato-Belgica Complexes is unknown south of 71°S. The main geological structural trends of the Lützow-Holm Complex derived from magnetic anomalies are NW-SE and are concordant with the geological results in the coastal region. However, nearly NE-SW-trending magnetic anomalies cut across the NW-SW magnetic anomaly trends, and NE-SW right lateral strike-slip faults were deduced from the magnetic and the gravity anomaly data of the Lützow-Holm Complex. The Lützow-Holm Complex was sub-divided into four blocks based on the estimated strike-slip faults. These strike-slip faults may have been generated during a younger stage of Pan-African orogeny, after the formation of NW-SE-striking geological structures. Cape Hinode, which is considered an allochthonous unit in the Lützow-Holm Complex according to its surface geology, may have originated from the Rayner Complex and been transported by right lateral strike-slip motions. These results provide new constraints on the tectonic evolution of Gondwana during the Pan-African orogeny.

Description: We investigate the propagation of seismic waves in anisotropic ice. Two effects are important: (i) sudden changes in crystal orientation fabric (COF) lead to englacial reflections; (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded traveltimes. Velocities calculated from the polycrystal elasticity tensor derived for the anisotropic fabric from measured COF eigenvalues of the EDML ice core, Antarctica, show good agreement with the velocity trend determined from a vertical seismic profiling. The agreement of the absolute velocity values, however, depends on the choice of the monocrystal elasticity tensor used for the calculation of the polycrystal properties. With this validation of seismic velocities we make use of abrupt changes in COF as common reflection mechanism for seismic and radar data below the firn–ice transition to investigate their occurrence by comparison with ice-core data. Our results highlight the possibility to complement regional radar surveys with local, surface-based seismic deployment to separate isochrones in radar data from other mechanisms. This is important for the reconnaissance of future ice-core drill sites, where accurate isochrone (i.e. non-COF) layer integrity allows for synchronization with other cores, as well as studies of ice dynamics considering non-homogeneous viscosity from preferred crystal orientations.

Description: Basal melt of ice shelves is a key factor governing discharge of ice from the Antarctic Ice Sheet as a result of its effects on buttressing. Here, we use radio echo sounding to determine the spatial variability of the basal melt rate of the southern Filchner Ice Shelf, Antarctica, along the inflow of Support Force Glacier. We find moderate melt rates with a maximum of 1.13 m/a about 50 km downstream of the grounding line. The variability of the melt rates over distances of a few kilometres is low (all but one 〈0.15 m/a at 2 km distance), indicating that measurements on coarse observational grids are able to yield a representative melt rate distribution. A comparison with remote-sensing-based melt rates revealed that, for the study area, large differences were due to inaccuracies in the estimation of vertical strain rates from remote sensing velocity fields. These inaccuracies can be overcome by using modern velocity fields.