ALBERT

Description: Radio-echo sounding of the Antarctic and Greenlandic ice sheets often reveals a layer in the lowest hundreds of meters above bedrock more or less free of radio echoes, known as the echo-free zone (EFZ). The cause of this feature is unclear, so far lacking direct evidence for its origin. We compare echoes around the EPICA drill site in Dronning Maud Land, Antarctica, with the dielectric properties, crystal orientation fabrics and optical stratigraphy of the EPICA-DML ice core. We find that echoes disappear in the depth range where the dielectric contrast is blurred, and where the coherency of the layers in the ice core is lost due to disturbances caused by the ice flow. At the drill site, the EFZ onset at ~2100 m marks a boundary, below which the ice core may have experienced flow induced disturbances on various scales. The onset may indicate changing rheology which needs to be accounted for in the modeling of ice sheet dynamics.

Description: Radio-echo sounding of the Antarctic and Greenlandic ice sheets often reveals a layer in the lowest hundreds of meters above bedrock more or less free of radio echoes, known as the echo-free zone (EFZ). The cause of this feature is unclear, so far lacking direct evidence for its origin. We compare echoes around the EPICA drill site in Dronning Maud Land, Antarctica, with the microstructural and dielectrical properties of the EPICA-DML ice core. We find that echoes disappear in the depth range, where the coherency of the layers is lost due to disturbances caused by the ice flow. At the drill site, the EFZ onset at ~2100 m marks a boundary, below which the ice core may have experienced flow induced disturbances on various scales. The dating of the climate record becomes increasingly difficult below 1900 m, until correlation with the Dome C record is lost below 2417 m depth. The onset also indicates changing rheology which needs to be accounted for in the modeling of ice sheet dynamics.

Description: The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle- to increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2020–2030 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile and synthetic RES data. Our observations allow to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics. As the conductivity profile of the EPICA shows no distinctive peak at this depths, we exclude changes in conductivity as the reflector origin. This is supported by application of numerical forward modelling of electromagnetic wave propagation, based on the conductivity profile, which is able to reproduce nearby reflections, but fails to reproduce this one. Because of background noise, the permittivity profile based on dielectric does not show prominent signals at these depths. We therefore interpret the observed reflector to originate from this change in crystal fabric.

Description: The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle to an increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2025–2045 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile, synthetic RES data, and a RES profile with different electromagnetic polarisation azimuths. The reflector's magnitude shows maximum values for polarisation parallel to the nearby ice divide and disappears for polarisation perpendicular to it, identifying the orientation of the girdle to lie in the vertical plane parallel to the ice divide. Observations allow us to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics and modeling.

Description: We present the thermal distribution in the confluence area of Gorner- and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1–5 and 40 MHz. The higher-frequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold–temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

Description: East Antarctica is the keystone to Gondwana and is fundamental to the understanding of continental breakup and the distribution of continents since the Jurassic, with further implications for the formation of today’s oceans and ice sheets and evolution of climate. Analysis of multiple geophysical datasets in East Antarctica, including radio-echo sounding, potential fields and seismic datasets have revealed the distribution of sedimentary basins within East Antarctica. Differences in the morphology and orientation of sedimentary basins define lithospheric domains separated by basement-dominated regions. The basement highs are defined by multiscale linear features evident in gravity, bed topography and seismic tomography models. These boundaries, we suggest, indicate the margins of former continental blocks that were assembled in the Precambrian to form East Antarctica. Rheological contrasts at block margins controlled later deformation during Phanerozoic extension. First, the formation of variably-oriented sedimentary basins in the Devonian to Triassic is consistent with reactivation of prior architecture and the distribution of major basin-dominated regions is indicative of differences in lithospheric rheology and composition - warmer and compositionally more fertile lithosphere is prone to subsidence. Second, the basement highs are aligned with key features of Gondwana breakup in the Jurassic to Eocene including the Africa-Madagascar-Sri Lanka triple junction, the Kerguelen Plateau, the George V fracture zone of Australian-Antarctic basin and the Macquarie Ridge. We suggest that the differential lithospheric structure of East Antarctica including mantle, crust and basins led to the localisation of these features and fundamentally controlled the geometry of Gondwana breakup.

Description: We present a probabilistic approach to the inversion of surface wave dispersion data from glacial environments. This is intended to (i) assess non-linearity and non-uniqueness, and (ii) properly quantify resolution and trade-offs. For this, we use seismic data from Distributed Acoustic Sensing (DAS) experiments deployed on the Vatnajökull ice sheet located on Grímsvötn volcano in Iceland, and the Northeast Greenland Ice Stream (NEGIS). Our method is based on a regularisation-free Bayesian inference approach, implemented using a Hamiltonian Monte Carlo (HMC) algorithm. Exploiting derivative information for efficient sampling of high-dimensional model spaces, HMC approximates the posterior probability densities of all model parameters. Applied specifically to multi-mode surface wave dispersion measurements, HMC yields probabilistic models of 1-D anisotropic stratified media parameterised in terms of the P-wave velocities Vpv and Vph, the S-wave velocities Vsv and Vsh, the anisotropy parameter η, and density ρ. The benefits of this approach, not only from a glaciological perspective, include regularisation-free estimates of firn and ice properties, models that are not a priori biased by the exclusion of all parameters except S-wave speed, and some level of direct access to the vertical density profile.

Description: Reliable knowledge of ice discharge dynamics for the Greenland ice sheet via its ice streams is essential if we are to understand its stability under future climate scenarios as well as their dynamics in the past. Currently, active ice streams in Greenland have been well mapped using remote-sensing data while past ice-stream paths in what are now deglaciated regions can be reconstructed from the landforms they left behind. However, little is known about possible former and now defunct ice streams in areas still covered by ice. Here, we use radio-echo sounding data to decipher the regional ice-flow history of the northeastern Greenland ice sheet on the basis of its internal stratigraphy. Based on the same data set we also establish age-depth distributions in the ice stream and determine the physical properties of the ice, notably the distribution of anisotropy. By creating a three-dimensional reconstruction of time-equivalent horizons, we map folds deep below the surface that we then attribute to the deformation caused by now-extinct ice streams. We propose that locally this ancient ice-flow regime was much more focused and reached much farther inland than today’s and was deactivated when the main drainage system was reconfigured and relocated southwards. The insight that major ice streams in Greenland might start, shift or abruptly disappear will affect future approaches to understanding and modelling the response of Earth’s ice sheets to global warming. EGRIP radar consortium: Daniela Jansen, Steven Franke, Tobias Binder, Paul D. Bons, Dorthe Dahl-Jensen, Reinhard Drews, Graeme Eagles, Olaf Eisen, Reza Ershadi, Tamara Annina Gerber, Prasad Gogineni, Aslak Grinsted, Veit Helm, Angelika Humbert, Christine Hvidberg, David Lilien, Heinrich Miller, Charles O'Neill, John D. Paden, Nicholas Rathmann, Daniel Steinhage, Nicolas Stoll, Kyra Streng, Fernando Valero-Delgado, Ilka Weikusat, Julien Westhoff, Tun Jan Young, Ole Zeising