Daniel Steinhage

Balance velocities and measured properties of the Antarctic ice sheet from a new compilation of gridded data for modelling

Abstract This paper presents a new compilation of gridded datasets for three-dimensional modelling of the Antarctic ice sheet. These are for surface elevation, ice thickness, bedrock elevation and accumulation rate as interpolated on a 281 × 281 mesh with 20 km spacing, and encompass all the ice sheet and surrounding continental shelf. Data sources include the Bamber digital-elevation model from ERS-1 radar-altimeter data, a redigitization of available ice-thickness data, the Giovinetto accumulation data, recent ice-thickness data from British and German expeditions as well as accumulation data from German and Norwegian expeditions. In particular, new data were incorporated for the Filchner-Ronne Ice Shelf and for Dronning Maud Land, Antarctica, arising from the EPICA pre-site survey. Special attention was devoted to matching the various data sources carefully, both among themselves and across the grounding line and below the ice shelves, to enable ice-sheet expansion and retreat in dynamic situations. As an application, the balance flow is calculated over the entire ice sheet using a two-dimensional finite-difference scheme and compared with a previous assessment. This brought to light the existence of ice-streaming features extending well inland. A detailed zoom over Dronning Maud Land exhibits the general flow characteristics of interest for locating a future deep-drilling site. As a by-product, an updated value of 26.4 × 106km3 was obtained for the total volume of the ice sheet and ice shelves, or equivalent to 61.1 m of global sea-level rise after removal of the ice sheet and subsequent oceanic invasion and isostatic rebound. The total accumulation over the grounded ice sheet, including the Antarctic Peninsula, is 1924 Gta−1, or between 5 and 20% higher than earlier estimates. Including all the ice shelves, the value is 2344 Gt a−1.

New maps of the ice thickness and subglacial topography in Dronning Maud Land, Antarctica, determined by means of airborne radio-echo sounding

Since the austral summer 1994/95 the Alfred Wegener Institute (AWI) hascarried out airborne radio echo sounding measurements (RES) in Antarctica withits newly designed RES system. Since 1995/96 an ongoing presite survey foran ice coring drill site in Dronning Maud Land (DML) has been carried out.This survey is part of the EPICA programme (European Project for Ice Coring inAntarctica). It covers an area of 948000 km2 with more than 49500 km ofairborne RES obtained from more than 200 hours of flight operation flown from1994-1997. Within this paper first results of the airborne RES survey will begraphically summarized as newly derived maps of the ice thickness and thesubglacial topography as well as a 3D view of surface and subglacial bed andoutcrop topography, revealing a total icevolume of 1.48 million km3.

Improved method to determine radio-echo sounding reflector depths from ice-core profiles of permittivity and conductivity

We present a technique that modifies and extends down-hole target methods to provide absolute measures of uncertainty in radar-reflector depth of origin. We use ice-core profiles to model wave propagation and reflection, and then cross-correlate the model results with radio-echo sounding (RES) data to identify the depth of reflector events. Stacked traces recorded with RES near the EPICA drill site in Dronning Maud Land, Antarctica, provide reference radargrams, and dielectric properties along the deep ice core form the input data to a forward model of wave propagation that produces synthetic radargrams. Cross-correlations between synthetic and RES radargrams identify differences in propagation wave speed. They are attributed to uncertainties in pure-ice permittivity and are used for calibration. Removing conductivity peaks results in the disappearance of related synthetic reflections and enables the unambiguous relation of electric signatures to RES features. We find that (i) density measurements with }-attenuation or dielectric profiling are too noisy below the firn-ice transition to allow clear identification of reflections, (ii) single conductivity peaks less than 0.5 m wide cause the majority of prominent reflections beyond a travel time of about 10 μs (∼900 m depth) and (iii) some closely spaced conductivity peaks within a range of 1-2m cannot be resolved within the RES or synthetic data. Our results provide a depth accuracy to allow synchronization of age-depth profiles of ice cores by RES, modeling of isochronous internal structures, and determination of wave speed and of pure-ice properties. The technique successfully operates with dielectric profiling and electrical conductivity measurements, suggesting that it can be applied at other ice cores and drill sites.

Past and present accumulation rate reconstruction along the Dome Fuji-Kohnen radio-echo sounding profile, Dronning Maud Land, East Antarctica

Abstract We used internal ice layers from a radio-echo sounding profile between the Kohnen and Dome Fuji deep drilling sites to infer the spatio-temporal pattern of accumulation rate in this sector of Dronning Maud Land, East Antarctica. Continuous internal reflection horizons can be traced to about half of the ice thickness and have a maximum age of approximately 72.7 ka BP. To infer palaeo-accumulation rates from the dated layers, we derived the thinning functions from a flow calculation with a high-resolution higher-order model of Dronning Maud Land embedded into a three-dimensional thermomechanical model of the Antarctic ice sheet. The method takes into account complex ice-flow dynamics and advection effects that cannot be dealt with using traditional local approaches. We selected seven time intervals over which we determine the average accumulation rate and average surface temperature at the place and time of origin of the layer particles. Our results show lower accumulation rates along eastern parts of the profile for the late Holocene (0–5 ka BP) than are shown by existing maps, which had no surface control points. During the last glacial period we find a substantially lower accumulation rate than predicted by the usual approach linking palaeo-accumulation rates to the condensation temperature above the surface inversion layer. These findings were used to fine-tune the relation between accumulation rate and temperature.

Converging flow and anisotropy cause large-scale folding in Greenland's ice sheet

The increasing catalogue of high-quality ice-penetrating radar data provides a unique insight in the internal layering architecture of the Greenland ice sheet. The stratigraphy, an indicator of past deformation, highlights irregularities in ice flow and reveals large perturbations without obvious links to bedrock shape. In this work, to establish a new conceptual model for the formation process, we analysed the radar data at the onset of the Petermann Glacier, North Greenland, and created a three-dimensional model of several distinct stratigraphic layers. We demonstrate that the dominant structures are cylindrical folds sub-parallel to the ice flow. By numerical modelling, we show that these folds can be formed by lateral compression of mechanically anisotropic ice, while a general viscosity contrast between layers would not lead to folding for the same boundary conditions. We conclude that the folds primarily form by converging flow as the mechanically anisotropic ice is channelled towards the glacier.

Surface topography and ice flow in the vicinity of the EDML deep-drilling site, Antarctica

Interpretation of ice-core records requires accurate knowledge of the past and present nsurface topography and stress–strain fields. The European Project for Ice Coring in Antarctica (EPICA) ndrilling site (75.00258°S, 0.06848°E; 2891.7 m) in Dronning Maud Land, Antarctica, is located in the nimmediate vicinity of a transient and forking ice divide. A digital elevation model is determined from the ncombination of kinematic GPS measurements with the GLAS12 datasets from the ICESat. Based on a nnetwork of stakes, surveyed with static GPS, the velocity field around the drilling site is calculated. The nannual mean velocity magnitude of 12 survey points amounts to 0.74ma–1. Flow directions mainly vary naccording to their distance from the ice divide. Surface strain rates are determined from a pentagonshaped nstake network with one center point close to the drilling site. The strain field is characterized by nalong-flow compression, lateral dilatation and vertical layer thinning.

Characteristics and small-scale variability of GPR signals and their relation to snow accumulation in Greenland's percolation zone

We investigate snowpack properties at a site in west-central Greenland with ground- penetrating radar (GPR), supplemented by stratigraphic records from snow pits and shallow firn cores. GPR data were collected at a validation test site for CryoSat (T05 on the ExpGlaciologiques Internationales au Groenland (EGIG) line) over a 100 m × 100 m grid and along 1 km sections at fre- quencies of 500 and 800 MHz. Several internal reflection horizons (IRHs) down to a depth of 10 m were tracked. IRHs are usually related to ice-layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season. Warm conditions in the following melt season can change these characteristics by percolating meltwater. In cold conditions, smaller melt volumes at the surface can lead to faint IRHs. The absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data. Our GPR-derived depth of the 2003 summer surface of 1.48 m (measured in 2004) is confirmed by snow-pit observations. The distribution of IRH depths on a 1 km scale reveals a gradient of increasing accumu- lation to the northeast of about 5 cm w.e. km −1 . We find that point measurements of accumulation in this area are representative only over several hundred metres, with uncertainties of about 15% of the spatial mean.

Internal structure of the ice sheet between Kohnen station and Dome Fuji, Antarctica, revealed by airborne radio-echo sounding

Abstract This study aims to demonstrate that deep ice cores can be synchronized using internal horizons in the ice between the drill sites revealed by airborne radio-echo sounding (RES) over a distance of >1000km, despite significant variations in glaciological parameters, such as accumulation rate between the sites. In 2002/03 a profile between the Kohnen station and Dome Fuji deep ice-core drill sites, Antarctica, was completed using airborne RES. The survey reveals several continuous internal horizons in the RES section over a length of 1217 km. The layers allow direct comparison of the deep ice cores drilled at the two stations. In particular, the counterpart of a visible layer observed in the Kohnen station (EDML) ice core at 1054 m depth has been identified in the Dome Fuji ice core at 575 m depth using internal RES horizons. Thus the two ice cores can be synchronized, i.e. the ice at 1560 m depth (at the bottom of the 2003 EDML drilling) is ∼49ka old according to the Dome Fuji age/depth scale, using the traced internal layers presented in this study.

Small-scale spatio-temporal characteristics of accumulation rates in western Dronning Maud Land, Antarctica

Spatio-temporal variations of the recently determined accumulation rate are investigated using ground-penetrating radar (GPR) measurements and firn-core studies. The study area is located on Ritscherflya in western Dronning Maud Land, Antarctica, at an elevation range 1400-1560 m. Accumu- lation rates are derived from internal reflection horizons (IRHs), tracked with GPR, which are connected to a dated firn core. GPR-derived internal layer depths show small relief along a 22 km profile on an ice flowline. Average accumulation rates are about 190 kg m -2 a -1 (1980-2005) with spatial variability (1σ) of 5% along the GPR profile. The interannual variability obtained from four dated firn cores is one order of magnitude higher, showing 1σ standard deviations around 30%. Mean temporal variations of GPR- derived accumulation rates are of the same magnitude or even higher than spatial variations. Temporal differences between 1980-90 and 1990-2005, obtained from two dated IRHs along the GPR profile, indicate temporally non-stationary processes, linked to spatial variations. Comparison with similarly obtained accumulation data from another coastal area in central Dronning Maud Land confirms this observation. Our results contribute to understanding spatio-temporal variations of the accumulation processes, necessary for the validation of satellite data (e.g. altimetry studies and gravity missions such as Gravity Recovery and Climate Experiment (GRACE)).

Regional and temporal variation of accumulation around NorthGRIP derived from ground-penetrating radar

Abstract During the summer of 2003, a ground-penetrating radar survey around the North Greenland Icecore Project (NorthGRIP) deep ice-core drilling site (75˚06’N, 42˚20’W; 2957ma.s.l.) was carried out using a shielded 250 MHz radar system. The drill site is located on an ice divide, roughly 300 km north-northwest of the summit of the Greenland ice sheet. More than 430 km of profiles were measured, covering a 10 km by 10 km area, with a grid centered on the drilling location, and eight profiles extending beyond this grid. Seven internal horizons within the upper 120 m of the ice sheet were continuously tracked, containing the last 400 years of accumulation history. Based on the age-depth and density-depth distribution of the deep core, the internal layers have been dated and the regional and temporal distribution of accumulation rate in the vicinity of NorthGRIP has been derived. The distribution of accumulation shows a relatively smoothly increasing trend from east to west from 145 kgm–2a–1 to 200 kg m–2 a -1 over a distance of 50 km across the ice divide. The general trend is overlain by small-scale variations on the order of 2.5 kgm–2a-1 km- 1 , i.e. around 1.5% of the accumulation mean. The temporal variations of the seven periods defined by the seven tracked isochrones are on the order of ± 4% of the mean of the last 400 years, i.e. at NorthGRIP ± 7 kg m–2 a-1. If the regional accumulation pattern has been stable for the last several thousand years during the Holocene, and ice flow has been comparable to today, advective effects along the particle trajectory upstream of NorthGRIP do not have a significant effect on the interpretation of climatically induced changes in accumulation rates derived from the deep ice core over the last 10 kyr.