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We present a high-resolution airborne radar data set (EGRIP-NOR-2018) for the onset region of the Northeast Greenland Ice Stream (NEGIS). The radar data were acquired in May 2018 with the Alfred Wegener Institute's multichannel ultra-wideband (UWB) radar mounted on the Polar 6 aircraft. Radar profiles cover an area of ∼24 000 km 2 and extend over the well-defined shear margins of the NEGIS. The survey area is centered at the location of the drill site of the East Greenland Ice-Core Project (EastGRIP), and several radar lines intersect at this location. The survey layout was designed to (i) map the stratigraphic signature of the shear margins with radar profiles aligned perpendicular to ice flow, (ii) trace the radar stratigraphy along several flow lines, and (iii) provide spatial coverage of ice thickness and basal properties. While we are able to resolve radar reflections in the deep stratigraphy, we cannot fully resolve the steeply inclined reflections at the tightly folded shear margins in the lower part of the ice column. The NEGIS is causing the most significant discrepancies between numerically modeled and observed ice surface velocities. Given the high likelihood of future climate and ocean warming, this extensive data set of new high-resolution radar data in combination with the EastGRIP ice core will be a key contribution to understand the past and future dynamics of the NEGIS. The EGRIP-NOR-2018 radar data products can be obtained from the PANGAEA data publisher (https://doi.pangaea.de/10.1594/PANGAEA.928569 ;). [ABSTRACT FROM AUTHOR]

We present a high-resolution airborne radar data set (EGRIP-NOR-2018) for the onset region of the Northeast Greenland Ice Stream (NEGIS). The radar data were acquired in May 2018 with Alfred Wegener Institute's multichannel ultra- wideband (UWB) radar mounted on the Polar6 aircraft. Radar profiles cover an area of ~24000 km2 and extend over the well- defined shear margins of the NEGIS. The survey area is centred at the location of the drill site of the East Greenland Ice-Core Project (EastGRIP) and several radar lines intersect at this location. The survey layout was designed to: (i) map the stratigraphic signature of the shear margins with radar profiles aligned perpendicular to ice flow, (ii) trace the radar stratigraphy along several flow lines and (iii) provide spatial coverage of ice thickness and basal properties. While we are able to resolve radar reflections in the deep stratigraphy, we can not fully resolve the steeply inclined reflections at the tightly folded shear margins in the lower part of the ice column. The NEGIS is causing the most significant discrepancies between numerically modelled and observed ice surface velocities. Given the high likelihood of future climate and ocean warming, this extensive data set of new high-resolution radar data in combination with the EastGRIP ice core will be a key contribution to understand the past and future dynamics of the NEGIS. The EGRIP-NOR-2018 radar data products can be obtained at the PANGAEA Data Publisher (https://doi.pangaea.de/10.1594/PANGAEA.928569; Franke et al. 2021a). [ABSTRACT FROM AUTHOR]

The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for 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 geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km2, centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting. [ABSTRACT FROM AUTHOR]

Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450-1700m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700-2030m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030-2385m depth) is an indicator of shear dominance. Region 5 (below approx. 2385m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared with results from a numerical ice sheet model (PISM, isotropic) for comparison of strain rate trends predicted from the large-scale geometry of the ice sheet and borehole logging data. This comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet. This article is part of the themed issue 'Microdynamics of ice'. [ABSTRACT FROM AUTHOR]

Antarctica is surrounded by a variety of large, medium and small sized ice shelves, glacier tongues and coastal areas without offshore floating ice masses. We used the mosaic of the Radarsat-1 Antarctica Mapping Project (RAMP) Antarctic Mapping Mission 1 (AMM) to classify the coastline of Antarctica in terms of surface structure patterns close to the calving front. With the aid of an automated edge detection method, complemented by manual control, the surface structures of all ice shelves and glacier tongues around Antarctica were mapped. We found dense and less dense patterns of surface structures unevenly distributed over the ice shelves and ice tongues. Dense surface patterns are frequent on fast flowing ice masses (ice streams), whereas most ice shelves show a dense surface pattern only close to the grounding line. Flow line analyses on ten ice shelves reveal that the time of residence of the ice along a flow path and-associated with it-the healing of surface crevasses can explain the different surface structure distribution close to the grounding line and the calving front on many ice shelves. Based on the surface structures relative to the calving front within a 15 km-wide seaward strip, the ice shelf fronts can be separated into three classes. The resulting map of the classified calving fronts around Antarctica and their description provide a detailed picture of crevasse formation and the observed dominant iceberg shapes. [ABSTRACT FROM AUTHOR]

We observed the large tabular iceberg A-38B in the time period from the calving event at the Ronne Ice Shelf in 1998 until its grounding near South Georgia in 2004 by means of various space-borne sensors. The initial ice thickness was determined by radar altimetry, the iceberg shape from radar imaging and optical instruments. The freeboard change at different stages of the drift was derived from ICESat Laser altimeter profiles. The analysis of the satellite data confirms that the decay of A-38B is governed mainly by basal erosion. We then used a numerical model for simulating iceberg melting and fitted the melt rate by varying the turbulent exchange parameters for temperature (γ T) and salt (γ S) to match the results obtained by ICESat data. Our results show that the iceberg passed through three melting regimes characterized by iceberg drift velocity: (1) In the Weddell Sea melting conditions are similar to the situation under an ice shelf with strong tidal currents which corresponds to a γ T of 1.0 ⁎ 10−4 m s−1. (2) In the Scotia Sea, where the iceberg drifts unhindered with the ocean current and is surrounded by its own melt water, γ T, is 0.4 ⁎ 10−4 m s−1. (3) At the grounding position friction velocity is again high due to tidal currents and γ T is 1.8 ⁎ 10−4 m s−1. γ S is set to 0.00505 γ T in all cases. The analysis shows that ICESat GLAS data together with satellite imagery can provide better estimates of turbulent exchange parameters, which is a step forward in improving the knowledge of fresh water input from melting icebergs into the Southern Ocean. [Copyright &y& Elsevier]

Abstract: Electrical properties of sediments encompass information about other linked physical properties, such as porosity and thermal conductivity. In situ measurements of electrical resistivity offer a cost- and time effective method for obtaining a comprehensive overview of near surface sediment physical properties. They also provide a quality check for in situ measurements of related sediment properties or for data obtained from sediment core analysis. We present results from the deployment of a sediment penetrating in situ probe which logs electrical resistivity of marine or limnic sediments up to 4 mbsf. Additionally, acceleration and pressure are recorded during sediment penetration. As the integration of acceleration delivers penetration depth with respect to time, a resistivity–depth profile can be derived. The measurements presented were performed during cruise SO-149 in the Cascadia Basin and at the eastern flank of the Juan de Fuca Ridge. The in situ probe is attached to a Lister-type heat flow instrument; therefore simultaneous measurements of thermal and electrical resistivities are possible. Strongly influenced by porosity, both data sets show good correlation. Continuous recordings of electrical resistivity can be used to validate and supplement the thermal data, which is only measured in depth intervals of 30 cm. Comparison with other independent measurements such as porosity measured on sediment cores also confirm the correct operation of the probe. [Copyright &y& Elsevier]

Understanding the material properties and physical conditions of basal ice is crucial for a comprehensive understanding of Antarctic ice‐sheet dynamics. Yet, direct data are sparse and difficult to acquire. Here, we employ ultra‐wideband radar to map high‐backscatter zones near the glacier bed within East Antarctica's Jutulstraumen drainage basin. Our backscatter analysis reveals that the basal ice in an area of ∼10,000 km2 is composed of along‐flow oriented sediment‐laden basal ice units connected to the basal substrate, extending up to several hundred meters thick. Three‐dimensional thermomechanical modeling supports that these units form via basal freeze‐on of subglacial water that originated from further upstream. Our findings suggest that basal freeze‐on, and the entrainment and transport of subglacial material play a significant role in an accurate representation of material, physical, and rheological properties of the Antarctic ice sheet's basal ice, ultimately enhancing the accuracy and reliability of ice‐sheet modeling. Plain Language Summary: We investigate the lowermost portion of the ice column of the Antarctic ice sheet, known as basal ice. This part holds crucial information about how the ice sheet behaves when it flows. Using the principle of echo‐location, we use radar technology to scan the ice in East Antarctica's Jutulstraumen drainage basin. We discover that a significant portion of the basal ice in this region is filled with sediment and can be several hundred meters thick. To better understand how this distinctive type of ice forms, we used a mathematical model that suggested that these ice units likely form when water underneath the ice is transported and refreezes at particular locations. Our findings highlight the importance of these freezing and mixing of materials from beneath the ice. Understanding where these ice units are located provides important information for ice‐flow models to ultimately better understand how the Antarctic ice sheet behaves in the future. Key Points: We have identified high‐scattering basal ice in Jutulstraumen Glacier's onset region using radar, reaching several hundred meters from the bedBackscatter analysis suggests that the basal ice units contain unstratified point scatters and cause little radio‐glaciological loss3D thermo‐mechanical modeling implies that freeze‐on of basal meltwater generated upstream likely initially formed the basal ice units [ABSTRACT FROM AUTHOR]

Ice shelves surrounding the Antarctic perimeter moderate ice discharge towards the ocean through buttressing. Ice-shelf evolution and integrity depend on the local surface accumulation, basal melting and on the spatially variable ice-shelf viscosity. These components of ice-shelf mass balance are often poorly constrained by observations and introduce uncertainties in ice-sheet projections. Isochronal radar stratigraphy is an observational archive for the atmospheric, oceanographic and ice-flow history of ice shelves. Here, we predict the stratigraphy of locally accumulated ice on ice shelves with a kinematic forward model for a given atmospheric and oceanographic scenario. This delineates the boundary between local meteoric ice (LMI) and continental meteoric ice (CMI). A large LMI to CMI ratio hereby marks ice shelves whose buttressing strength is more sensitive to changes in atmospheric precipitation patterns. A mismatch between the steady-state predictions of the kinematic forward model and observations from radar can highlight inconsistencies in the atmospheric and oceanographic input data or be an indicator for a transient ice-shelf history not accounted for in the model. We discuss pitfalls in numerical diffusion when calculating the age field and validate the kinematic model with the full Stokes ice-flow model Elmer/Ice. The Roi Baudouin Ice Shelf (East Antarctica) serves as a test case for this approach. There, we find a significant east–west gradient in the LMI / CMI ratio. The steady-state predictions concur with observations on larger spatial scales (>10 km), but deviations on smaller scales are significant, e.g., because local surface accumulation patterns near the grounding zone are underestimated in Antarctic-wide estimates. Future studies can use these mismatches to optimize the input data or to pinpoint transient signatures in the ice-shelf history using the ever growing archive of radar observations of internal ice stratigraphy. [ABSTRACT FROM AUTHOR]

During the past 20 years, multi-channel radar emerged as a key tool for deciphering an ice sheet's internal architecture. To assign ages to radar reflections and connect them over large areas in the ice sheet, the layer genesis has to be understood on a microphysical scale. Synthetic radar trace modelling based on the dielectric profile of ice cores allows for the assignation of observed physical properties' variations on the decimetre scale to radar reflectors extending from the coring site to a regional or even whole-ice-sheet scale. In this paper we rely on the available dielectric profiling data of the northern Greenland deep ice cores: NGRIP, NEEM and EGRIP. The three records are well suited for assigning an age model to the stratigraphic radar-mapped layers, and linking up the reflector properties to observations in the cores. Our modelling results show that the internal reflections are mainly due to conductivity changes. Furthermore, we deduce fabric characteristics at the EGRIP drill site from two-way-travel-time differences of along and across-flow polarized radarwave reflections of selected horizons (below 980 m). These indicate in deeper parts of the ice column an across-flow concentrated c-axis fabric. [ABSTRACT FROM AUTHOR]

Abstract: We use the time-average of a series of satellite synthetic aperture radar (SAR) images from 2005 to 2007 to identify grounded icebergs in the western Weddell Sea. Satellite laser altimetry along repeated ground tracks confirms that regions of high mean backscatter are associated with stationary, tabular icebergs. The altimeter data also provide iceberg freeboard, from which we infer maximum ice draft assuming the lightly-grounded limit and a two-layer density model consistent with a source at Filchner–Ronne Ice Shelf. The maximum iceberg draft agrees very well with the ocean depth at a point where the bathymetry is well constrained by ship soundings. However, for an extensive region near 71°S, south of Larsen C Ice Shelf, the maximum grounding depth for several icebergs reveals the ocean to be locally more than 200m shallower than in bathymetry products. This previously uncharted bank is expected to have a profound effect on the northward flow of the western Weddell Gyre and the cross-slope transport of offshore warm deep water towards the eastern Antarctic Peninsula ice shelves. Passive microwave data also show that sea-ice concentration is reduced in the vicinity of this group of grounded icebergs, indicating an iceberg influence on ocean/atmosphere heat and freshwater exchanges. [Copyright &y& Elsevier]

Migration of subglacial water underneath thick Antarctic ice is difficult to observe directly but is known to influence ice flow dynamics. Here, we analyze a 6‐year time series of displacement maps from differential Sentinel‐1 SAR interferometry (DInSAR) in the upstream region of Jutulstraumen Glacier. Our results reveal short‐term (between 12 days and 1 year) interconnected subsidence‐ and uplift events of the ice surface, which we interpret as a pressure response to the drainage and filling of subglacial lakes. This indicates an episodic cascade‐like water transport with longer quiescent phases in a dynamically stable glacial setting. Abrupt events appear in the DInSAR time series and are confirmed by ICESat‐2 altimetry. The events can be traced for a 1‐year period along a ∼175 km flow path. We are able to observe the migration of subglacial water with unprecedented spatial and temporal resolution, providing a new observational baseline to further develop subglacial hydrological models. Plain Language Summary: Subglacial lakes and the movement of subglacial water play an important role in the way how ice flows in the Antarctic Ice Sheet. The drainage and filling of subglacial lakes is reflected in subsidence and uplift at the ice surface, which can be monitored by satellite based elevation measurements. In this study, we detect these elevation changes of the ice surface at the onset of Dronning Maud Land's largest glacier (Jutulstraumen Glacier, Antarctica). We register a number of connected events which show us where and when subglacial water moves downstream. We find that the water flows similarly to a self‐tipping swimming pool bucket: water beneath the ice is localized and abruptly moves from one place to another after some time has passed. Using airborne radar‐sounding techniques, we find that the water flows along preferential flowpaths in the subglacial system. These are the first confirmed subglacial water movements in central Dronning Maud Land, an area where no subglacial water flow has been observed previously. The episodic nature of water flow is a new observation that will help us to understand how the subglacial water forms its own pluming system beneath these large glaciers. Key Points: Sentinel‐1 DInSAR estimates reveal a ∼175 km flow path of a cascade‐like chain of subglacial water propagation over a 1‐year periodHigh‐resolution ultra‐wideband radar data show no characteristic lake bed reflections and suggests water transport via a local trough systemWe find evidence of highly dynamic water transport in a static glacial stetting providing a benchmark dataset to improve modeling results [ABSTRACT FROM AUTHOR]

Ever since the first deep ice cores were drilled, it has been a challenge to determine their original, in-situ orientation. In general, the orientation of an ice core is lost as the drill is free to rotate during transport to the surface. For shallow ice cores, it is usually possible to match the adjacent core breaks, which preserves the orientation of the ice column. However, this method fails for deep ice cores, such as the EastGRIP ice core in Northeast Greenland. We provide a method to reconstruct ice core orientation using visual stratigraphy and borehole geometry. As the EastGRIP ice core is drilled through the Northeast Greenland Ice Stream, we use information about the directional structures to perform a full geographical re-orientation. We compared the core orientation with logging data from core break matching and the pattern of the stereographic projections of the crystals' c-axis orientations. Both comparisons agree very well with the proposed orientation method. The method works well for 441 out of 451 samples from a depth of 1375–2120 m in the EastGRIP ice core. It can also be applied to other ice cores, providing a better foundation for interpreting physical properties and understanding the flow of ice. [ABSTRACT FROM AUTHOR]

The Northeast Greenland Ice Stream (NEGIS) is the largest active ice stream on the Greenland Ice Sheet (GrIS) and a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice core is drilled in the upstream part of the NEGIS, termed the East Greenland Ice-core Project (EastGRIP). Upstream flow can introduce climatic bias into ice cores through the advection of ice deposited under different conditions further upstream. This is particularly true for EastGRIP due to its location inside an ice stream on the eastern flank of the GrIS. Understanding and ultimately correcting for such effects requires information on the atmospheric conditions at the time and location of snow deposition. We use a two-dimensional Dansgaard–Johnsen model to simulate ice flow along three approximated flow lines between the summit of the ice sheet (GRIP) and EastGRIP. Isochrones are traced in radio-echo-sounding images along these flow lines and dated with the GRIP and EastGRIP ice-core chronologies. The observed depth–age relationship constrains the Monte Carlo method which is used to determine unknown model parameters. We calculate backward-in-time particle trajectories to determine the source location of ice found in the EastGRIP ice core and present estimates of surface elevation and past accumulation rates at the deposition site. Our results indicate that increased snow accumulation with increasing upstream distance is predominantly responsible for the constant annual layer thicknesses observed in the upper part of the ice column at EastGRIP, and the inverted model parameters suggest that basal melting and sliding are important factors determining ice flow in the NEGIS. The results of this study form a basis for applying upstream corrections to a variety of ice-core measurements, and the inverted model parameters are useful constraints for more sophisticated modelling approaches in the future. [ABSTRACT FROM AUTHOR]

The effect of grain size on strain rate of ice in the upper 2207 m in the North Greenland Eemian Ice Drilling (NEEM) deep ice core was investigated using a rheological model based on the composite flow law of Goldsby and Kohlstedt (1997, 2001). The grain size was described by both a mean grain size and a grain size distribution, which allowed the strain rate to be calculated using two different model end-members: (i) the microscale constant stress model where each grain deforms by the same stress and (ii) the microscale constant strain rate model where each grain deforms by the same strain rate. The model results predict that grain-size-sensitive flow produces almost all of the deformation in the upper 2207 m of the NEEM ice core, while dislocation creep hardly contributes to deformation. The difference in calculated strain rate between the two model end-members is relatively small. The predicted strain rate in the fine-grained Glacial ice (that is, ice deposited during the last Glacial maximum at depths of 1419 to 2207 m) varies strongly within this depth range and, furthermore, is about 4–5 times higher than in the coarser-grained Holocene ice (0–1419 m). Two peaks in strain rate are predicted at about 1980 and 2100 m depth. The prediction that grain-size-sensitive creep is the fastest process is inconsistent with the microstructures in the Holocene age ice, indicating that the rate of dislocation creep is underestimated in the model. The occurrence of recrystallization processes in the polar ice that did not occur in the experiments may account for this discrepancy. The prediction of the composite flow law model is consistent with microstructures in the Glacial ice, suggesting that fine-grained layers in the Glacial ice may act as internal preferential sliding zones in the Greenland ice sheet. [ABSTRACT FROM AUTHOR]

The surface mass balance (SMB) of the Larsen C ice shelf (LCIS), Antarctica, is poorly constrained due to a dearth of in situ observations. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of SMB over the LCIS. Continuous time series of snow height (2.5-6 years) at five locations allow for multi-year estimates of seasonal and annual SMB over the LCIS. There is high interannual variability in SMB as well as spatial variability: in the north, SMB is 0.40±0.06 to 0.41±0.04mw.e. year-1, while farther south, SMB is up to 0.50±0.05mw.e. year-1. This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.34mw.e. north of 66° S, and 0.40mw.e. south of 68° S. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial variations in SMB: a particularly strong and coherent reflection horizon below 25-44m of waterequivalent ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed synchronously across the ice shelf. Combining snow height observations, ground and airborne radar, and SMB output from a regional climate model yields a gridded estimate of SMB over the LCIS. It confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west, modulated in the west by föhn-induced sublimation. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification rather than SMB. [ABSTRACT FROM AUTHOR]

Impurities control a variety of physical properties of polar ice. Their impact can be observed at all scales – from the microstructure (e.g., grain size and orientation) to the ice sheet flow behavior (e.g., borehole tilting and closure). Most impurities in ice form micrometer-sized inclusions. It has been suggested that these μ inclusions control the grain size of polycrystalline ice by pinning of grain boundaries (Zener pinning), which should be reflected in their distribution with respect to the grain boundary network. We used an optical microscope to generate high-resolution large-scale maps (3 μm pix-1, 8 × 2 cm2) of the distribution of micro-inclusions in four polar ice samples: two from Antarctica (EDML, MIS 5.5) and two from Greenland (NEEM, Holocene). The in situ positions of more than 5000 μ inclusions have been determined. A Raman microscope was used to confirm the extrinsic nature of a sample proportion of the mapped inclusions. A superposition of the 2-D grain boundary network and μ-inclusion distributions shows no significant correlations between grain boundaries and μ inclusions. In particular, no signs of grain boundaries harvesting μ inclusions could be found and no evidence of μ inclusions inhibiting grain boundary migration by slow-mode pinning could be detected. Consequences for our understanding of the impurity effect on ice microstructure and rheology are discussed. [ABSTRACT FROM AUTHOR]

The flow of glaciers and polar ice sheets is controlled by the highly anisotropic rheology of ice crystals that have hexagonal symmetry (ice lh). To improve our knowledge of ice sheet dynamics, it is necessary to understand how dynamic recrystallization (DRX) controls ice microstructures and rheology at different boundary conditions that range from pure shear flattening at the top to simple shear near the base of the sheets. We present a series of two-dimensional numerical simulations that couple ice deformation with DRX of various intensities, paying special attention to the effect of boundary conditions. The simulations show how similar orientations of c-axis maxima with respect to the finite deformation direction develop regardless of the amount of DRX and applied boundary conditions. In pure shear this direction is parallel to the maximum compressional stress, while it rotates towards the shear direction in simple shear. This leads to strain hardening and increased activity of non-basal slip systems in pure shear and to strain softening in simple shear. Therefore, it is expected that ice is effectively weaker in the lower parts of the ice sheets than in the upper parts. Strain-rate localization occurs in all simulations, especially in simple shear cases. Recrystallization suppresses localization, which necessitates the activation of hard, non-basal slip systems. This article is part of the themed issue 'Microdynamics of ice'. [ABSTRACT FROM AUTHOR]

We performed numerical simulations on the microdynamics of ice with air inclusions as a second phase. Our aim was to investigate the rheological effects of air inclusions and explain the onset of dynamic recrystallization in the permeable firn. The simulations employ a fullfield theory crystal plasticity code coupled to codes simulating dynamic recrystallization processes and predict timeresolved microstructure evolution in terms of lattice orientations, strain distribution, grain sizes and grain-boundary network. Results show heterogeneous deformation throughout the simulations and indicate the importance of strain localization controlled by air inclusions. This strain localization gives rise to locally increased energies that drive dynamic recrystallization and induce heterogeneous microstructures that are coherent with natural firn microstructures from EPICA Dronning Maud Land ice coring site in Antarctica. We conclude that although overall strains and stresses in firn are low, strain localization associated with locally increased strain energies can explain the occurrence of dynamic recrystallization. [ABSTRACT FROM AUTHOR]

Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries. [ABSTRACT FROM AUTHOR]

Explosive seismic reflection data from Halvfarryggen, a 910m thick local ice dome of the Antarctic ice sheet, show numerous laterally continuous reflections within the ice between 300 and 870m depth. We compare the quality of data obtained with explosive sources with that obtained using a vibroseis source for detecting englacial reflections with a snowstreamer, and investigate the origin of englacial reflections. We find vibroseis in combination with a snowstreamer is ten times more productive than explosive seismics. However, englacial reflections are more clearly visible with explosives, which have a broader bandwidth signature, than the vibroseis, which is band-limited at the high-frequency end to 100 Hz. Only the strongest and deepest englacial reflection is detected with vibroseis. We interpret the majority of englacial reflections to originate from changes in the crystal orientation fabric in closely spaced layers, less than the vibro-seismic tuning thickness of 13.5 m. Phase analysis of the lowermost englacial reflector, 40m above the bed, indicates a sharp increase in seismic wave speed. We interpret this reflector as a transition to a vertical single-maximum fabric. Our findings support current results from anisotropic ice-flow models, that crystal fabric is highly anisotropic at ice domes, both laterally and vertically. [ABSTRACT FROM AUTHOR]

The EastGRIP (East GReenland Ice core Project) Ice Core is drilled in a highly dynamic area, the North East Greenland Ice Stream (NEGIS), with a surface velocity of 55 m/yr, representing high flow rates in this area. All previous deep ice cores in Greenland were mainly drilled to find undisturbed ice for climate reconstruction. In contrast, the main purpose of this ice core is to increase our understanding of ice flow and linking it to deformation features shown in the physical properties. Some of these deformation features can be made visible using the line scanner device. It scans a polished ice core slab illuminated by an indirect light source (similar to dark field microscopy) and thus makes internal features (e.g. impurities, bubbles, hydrates and partly grain boundaries) visible, creating a 10x165 cm image of the core. Light traveling though the core is reflected and scattered at these features thus causing the camera to detect a bright section where the impurity content is high ("cloudy bands"), whereas ice with a low impurity concentration will not reflect light and contribute a dark layer in the image ("clear bands"). This is used to make layering, i.e. the stratigraphy, visible. Ice from the last glacial period has a well layered stratigraphy resulting from fairly regular annual dust storms in spring to summer. As deformation increases and deformation modes change towards the bottom of the core, these layers will show disturbances and folding. A strong relationship between the δ18O of the water isotopes in the ice core and the impurity concentration, derived from the visual intensity of different layers, can be observed. The correlation of these two makes way for a very precise correlation of the visual stratigraphy and their δ18O age, to analyze differences in deformation modes of ice from different climatic periods.Main deformation in the upper part of the ice sheet is pure shear (stretching along the horizontal and thinning in the vertical) and simple shear in the bottom parts. The gradual change from pure to simple shear is seen in the development of small scale disturbances in the layers, such as wavy patterns. The evolution of these features into z- and s-folds is expected in greater depth. Hereby the layer will deform into a z- or s-shape, overturning a section of the layer. Wavy features, such as the ones seen here, have not been observed in other cores and could be associated to the highly dynamic drill site in NEGIS. [ABSTRACT FROM AUTHOR]

Crystal Orientation Fabric (COF) of c-axes in ice cores reveals information about deformation within ice sheets. While this is a well established analysis technique for deep ice cores from ice divides, information about COF in ice streams is just now becoming available: the EastGRIP ice core is situated inside the largest ice stream in Greenland, the North East Greenland Ice Stream (NEGIS). With the ongoing analysis of samples from the EastGRIP ice core, COF is now available down to 1714 m, revealing an extremely more rapid evolution of COF anisotropy with depth compared to all other ice cores. This enables us to study the ability of polarimetric radar measurements to infer an overall pattern of COF from measurements conducted at the surface. Depending on whether the COF is isotropic or anisotropic, a radar signal is reflected differently in terms of angle dependence and polarization. We conducted these polarimetric measurements around the EastGRIP drill site and we compare them to COF data obtained from 778 thin sections, prepared and measured at EastGRIP drill site. We investigate the hypothesis that the same pattern of COF can be retrieved from the polarimetric measurements as is available from the ice core. If confirmed, this would provide an addition constraint on the (an)isotropy at locations where no ice core is available. This would potentially provide quasi spatial coverage and greatly improve our understanding of the evolution of anisotropy over from ice divides to outlet glaciers. [ABSTRACT FROM AUTHOR]