Radio-echo Sounding Research Articles

Gravity Survey on the Mizuho Plateau, East Antarctica: SEAL Seismic Exploration in 2001-2002

A gravity survey on Mizuho Plateau was carried out in austral summer of 2001-2002 by the 43rd Japanese Antarctic Research Expedition (JARE-43), as one of the geophysical measurements of deep seismic exploration. Gravity measurements were conducted at 151 points in approximately 1 km interval along the 150 km profile. Considering the gravity effect from the ice-sheet, Free-air and Bouguer anomalies were calculated by using precise GPS locations. Furrowed and basin-like negative Free-air anomalies were observed in middle and northern parts of the profile. Bouguer anomalies were calculated by two bedrock elevation data obtained from seismic refraction and radio-echo sounding. High resolution model for bedrock elevation by radio-echo sounding was applied in determining the crustal thickness.

Subglacial hydrological connectivity within the Byrd Glacier catchment, East Antarctica

AbstractIce, Cloud and land Elevation Satellite (ICESat) repeat-track laser altimetry has identified 17 sites within the Byrd Glacier catchment, East Antarctica, where rapid ice-surface height changes have occurred, which have been interpreted as evidence for ‘active’ subglacial lakes. Here we present evidence from a new radio-echo sounding (RES) survey at 11 of these locations to understand the bed conditions associated with the proposed hydrological activity. At none of the sites examined did we find evidence in support of substantial pooled basal water. In the majority of cases, along-track RES bed reflection amplitudes either side of the locations of surface height change are indistinguishable from those within the features. These results indicate that, in most cases, hypothesized ‘active’ lakes are not discrete radar targets and are therefore much smaller than the areas of surface height change. In addition, we have identified three new relatively large subglacial lakes upstream of the region where most ‘active’ subglacial lakes are found, in an area where the hydraulic gradient is significantly lower. Our results suggest that substantial and long-lasting basal water storage in the Byrd Glacier catchment occurs only under low hydraulic gradients, while coast-proximal sites of hydraulic activity likely involve small or temporary accumulations of basal water.

Constraining past accumulation in the central Pine Island Glacier basin, West Antarctica, using radio-echo sounding

AbstractThe potential for future dynamical instability of Pine Island Glacier, West Antarctica, has been addressed in a number of studies, but information on its past remains limited. In this study we use airborne radio-echo sounding (RES) data acquired over Pine Island Glacier to investigate past variations in accumulation pattern. In the dataset a distinctive pattern of layers was identified in the central part of the glacier basin. We use these layers as chronological identifiers in order to construct elevation maps of the internal stratigraphy. The observed internal layer stratigraphy is then compared to calculated stratigraphy from a three-dimensional ice-flow model that has been forced with different accumulation scenarios. The model results indicate that the accumulation pattern is likely to have changed at least twice since the deposition of the deepest identified layer. Additional RES data linked to the Byrd ice core provide an approximate timescale. This timescale suggests that the layers were deposited at the beginning of or during the Holocene period. Thus the widespread changes occurring in the coastal extent of the West Antarctic ice sheet at the end of the last glacial period could have been accompanied by changes in accumulation pattern.

Active lakes of Recovery Ice Stream, East Antarctica: a bedrock-controlled subglacial hydrological system

AbstractA connected system of active subglacial lakes was revealed beneath Recovery Ice Stream, East Antarctica, by ICESat laser altimetry. Here we combine repeat-track analysis of ICESat (2003–09), Operation IceBridge laser altimetry and radio-echo sounding (2011 and 2012), and MODIS image differencing (2009–2011) to learn more about the lake activity history, the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system. We extend the lake activity time series to 2012 for the three lower lakes and capture two major lake drainages. One lake underwent a large deflation between 2009 and 2011 while another lake, which had been continuously filling between 2003 and 2010, started to drain after 2011. Most of the active lakes are located in a ~ 1000 km long bedrock trough under the main trunk of Recovery Ice Stream, whose base is ~ 1500– 2000 m below present-day sea level. The hydrologic system beneath Recovery Ice Stream is controlled by this unusually pronounced bedrock topography, in contrast to most Antarctic systems studied to date, which are controlled by the ice surface topography. Hydrologic connections among the lakes appear to be direct and responsive, and we reproduce the lake activity using a simple subglacial water model. We discuss potential causes of non-steady hydrologic behavior in major Antarctic catchments.

Topography, ice thickness and ice volume of the glacier Pedersenbreen in Svalbard, using GPR and GPS

Pedersenbreen is a small valley glacier (ca. 6 km2 in 2009), ending on land, located in north-western Spitsbergen, Svalbard. Ground-based radio-echo sounding in April 2009, using a 100-MHz commercial radar and a self-made low-frequency radar, revealed a polythermal structure. The radar was coupled with a global positioning system device to geo-reference the traces. Each radar profile was manually edited to pick the reflection arrival time from the interface between ice and bedrock. Travel times were converted to ice thickness using a velocity of 0.165 m/ns, which was estimated from common mid-point measurement. Then the surface topography, bedrock topography, ice thickness contours and ice volume were derived using interpolation methods. Because it was difficult to distinguish the reflection wave from the background with the 100-MHz radar in some of the thickest areas of Pedersenbreen, we used a 5-MHz radar of our own design to fill in this gap. The maximum thickness of Pedersenbreen reaches 183±9 m, and the ice volume is 0.393±0.047 km3 in 2009. Comparing these data with the surface topographical data available for 1936 indicates a mass loss of nearly 12% during the past 73 years.Keywords: GPR; GPS; glacier topography; ice volume; ice thickness; Pedersenbreen(Published: 20 February 2014)Citation: Polar Research 2014, 33, 18533, http://dx.doi.org/10.3402/polar.v33.18533

Interannual changes of the floating ice shelf of Petermann Gletscher, North Greenland, from 2000 to 2012

AbstractPetermann Gletscher, northwest Greenland, drains 4% of the Greenland ice sheet into Nares Strait. Its floating ice shelf retreated from 81 to 48 km in length during two large calving events in 2010 and 2012. We document changes in the three-dimensional ice-shelf structure from 2000 to 2012, using repeated tracks of airborne laser altimetry and ice radio-echo sounding, ICESat laser altimetry and MODIS visible imagery. The recent ice-shelf velocity, measured by tracking surface features between flights in 2010 and 2011, is ~1.25 km a−1, ~15–30% faster than estimates made before 2010. The steady- state along-flow ice divergence represents 6.3 Gta−1mass loss through basal melting (~5Gta−1) and surface melting and sublimation (~1.0Gta−1). Airborne laser altimeter data reveal thinning, both along a thin central channel and on the thicker ambient ice shelf. From 2007 to 2010 the ice shelf thinned by ~5 m a−1, which represents a non-steady mass loss of ~4.1 Gta−1. We suggest that thinning in the basal channels structurally weakened the ice shelf and may have played a role in the recent calving events.

Greenland subglacial lakes detected by radar

AbstractSubglacial lakes are an established and important component of the basal hydrological system of the Antarctic ice sheets, but none have been reported from Greenland. Here we present airborne radio echo sounder (RES) measurements that provide the first clear evidence for the existence of subglacial lakes in Greenland. Two lakes, with areas ~8 and ~10 km2, are found in the northwest sector of the ice sheet, ~40 km from the ice margin, and below 757 and 809 m of ice, respectively. The setting of the Greenland lakes differs from those of Antarctic subglacial lakes, being beneath relatively thin and cold ice, pointing to a fundamental difference in their nature and genesis. Possibilities that the lakes consist of either ancient saline water in a closed system or are part of a fresh, modern open hydrological system are discussed, with the latter interpretation considered more likely.

The Ellsworth Subglacial Highlands: Inception and retreat of the West Antarctic Ice Sheet

Antarctic subglacial highlands are where the Antarctic ice sheets first developed and the “pinning points” where retreat phases of the marine-based sectors of the ice sheet are impeded. Due to low ice velocities and limited present-day change in the ice-sheet interior, West Antarctic subglacial highlands have been overlooked for detailed study. These regions have considerable potential, however, for establishing the locations from which the West Antarctic Ice Sheet originated and grew, and its likely response to warming climates. Here, we characterize the subglacial morphology of the Ellsworth Subglacial Highlands, West Antarctica, from ground-based and aerogeophysical radio-echo sounding (RES) surveys and the Moderate-Resolution Imaging Spectroradiometer (MODIS) Mosaic of Antarctica. We document well-preserved classic landforms associated with restricted, dynamic, marine-proximal alpine glaciation, with hanging tributary valleys feeding a significant overdeepened trough (the Ellsworth Trough) cut by valley (tidewater) glaciers. Fjord-mouth threshold bars down-ice of two overdeepenings define both the northwest and southeast termini of paleo-outlet glaciers, which cut and occupied the Ellsworth Trough. Satellite imagery reveals numerous other glaciated valleys, terminating at the edge of deep former marine basins (e.g., Bentley Subglacial Trench), throughout the Ellsworth Subglacial Highlands. These geomorphic data can be used to reconstruct the glaciology of the ice masses that formed the proto–West Antarctic Ice Sheet. The landscape predates the present ice sheet and was formed by a small dynamic ice field(s), similar to those of the present-day Antarctic Peninsula, at times when the marine sections of the West Antarctic Ice Sheet were absent. The Ellsworth Subglacial Highlands represent a major seeding center of the paleo–West Antarctic Ice Sheet, and its margins represent the pinning point at which future retreat of the marine-based West Antarctic Ice Sheet would be arrested.

Snowdrift modelling for the Vestfonna ice cap, north-eastern Svalbard

Abstract. The redistribution of snow by drifting and blowing snow frequently leads to an inhomogeneous snow mass distribution on larger ice caps. Together with the thermodynamic impact of drifting snow sublimation on the lower atmospheric boundary layer, these processes affect the glacier surface mass balance. This study provides a first quantification of snowdrift and sublimation of blowing and drifting snow on the Vestfonna ice cap (Svalbard) by using the specifically designed snow2blow snowdrift model. The model is forced by atmospheric fields from the Polar Weather Research and Forecasting model and resolves processes on a spatial resolution of 250 m. The model is applied to the Vestfonna ice cap for the accumulation period 2008/2009. Comparison with radio-echo soundings and snow-pit measurements show that important local-scale processes are resolved by the model and the overall snow accumulation pattern is reproduced. The findings indicate that there is a significant redistribution of snow mass from the interior of the ice cap to the surrounding areas and ice slopes. Drifting snow sublimation of suspended snow is found to be stronger during spring. It is concluded that the redistribution process is strong enough to have a significant impact on glacier mass balance.

Evidence for a water system transition beneath Thwaites Glacier, West Antarctica

Thwaites Glacier is one of the largest, most rapidly changing glaciers on Earth, and its landward-sloping bed reaches the interior of the marine West Antarctic Ice Sheet, which impounds enough ice to yield meters of sea-level rise. Marine ice sheets with landward-sloping beds have a potentially unstable configuration in which acceleration can initiate or modulate grounding-line retreat and ice loss. Subglacial water has been observed and theorized to accelerate the flow of overlying ice dependent on whether it is hydrologically distributed or concentrated. However, the subglacial water systems of Thwaites Glacier and their control on ice flow have not been characterized by geophysical analysis. The only practical means of observing these water systems is airborne ice-penetrating radar, but existing radar analysis approaches cannot discriminate between their dynamically critical states. We use the angular distribution of energy in radar bed echoes to characterize both the extent and hydrologic state of subglacial water systems across Thwaites Glacier. We validate this approach with radar imaging, showing that substantial water volumes are ponding in a system of distributed canals upstream of a bedrock ridge that is breached and bordered by a system of concentrated channels. The transition between these systems occurs with increasing surface slope, melt-water flux, and basal shear stress. This indicates a feedback between the subglacial water system and overlying ice dynamics, which raises the possibility that subglacial water could trigger or facilitate a grounding-line retreat in Thwaites Glacier capable of spreading into the interior of the West Antarctic Ice Sheet.

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

AbstractThis 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.

The ice thickness distribution of Flask Glacier, Antarctic Peninsula, determined by combining radio-echo soundings, surface velocity data and flow modelling

AbstractAn interpolated bedrock topography is presented for Flask Glacier, one of the tributaries of the remnant part of the Larsen B ice shelf, Antarctic Peninsula. The ice thickness distribution is derived by combining direct but sparse measurements from airborne radio-echo soundings with indirect estimates obtained from ice-flow modelling. The ice-flow model is applied to a series of transverse profiles, and a first estimate of the bedrock is iteratively adjusted until agreement between modelled and measured surface velocities is achieved. The adjusted bedrock is then used to reinterpret the radio-echo soundings, and the recovered information used to further improve the estimate of the bedrock itself. The ice flux along the glacier center line provides an additional and independent constraint on the ice thickness. The resulting bedrock topography reveals a glacier bed situated mainly below sea level with sections having retrograde slope. The total ice volume of 120 ±15 km3 for the considered area of 215 km2 corresponds to an average ice thickness of 560 ± 70 m.

Gravity Survey along the Mizuho Traverse Routes, East Antarctica: SEAL Seismic Exploration in 1999-2000

A gravity survey was conducted along the Mizuho traverse routes in austral summer of 1999-2000 by the 41st Japanese Antarctic Research Expedition (JARE-41). The main purpose of the survey was to obtain detail gravity anomaly along the traverse routes, where deep seismic exploration was simultaneously conducted in order to obtain a fine crustal structure of the Mizuho Plateau. By using SCINTREX (CG-3M) gravity meter, the survey was carried out at 160 sites with about 1 km interval in a distance of 190 km from S16 to Z20 on the traverse routes. Free-air and Bouguer anomalies were calculated using precise locations by GPS measurements, by taking into account an effect of the thick icesheet. The furrowed negative Free-air anomalies are identified around H192, where middle points of the whole traverse routes. Two bedrock elevation models derived both by seismic refraction analyses and radio-echo sounding might correspond to the bottom and the top of a mixture layer composed from ice-moraine rocks over the bedrock surface. Thickness of the crust estimated by Bouguer anomalies is 0.5 - 1 km larger toward inland at the terminal point of Z20 along the profile.

The climate memory of an Arctic polythermal glacier

Knowledge of glacier equilibrium-line altitude (ELA) changes and trends in time is essential for future predictions of glacier volumes. We present a novel method for determining trends in ELA change at McCall Glacier, Alaska, USA, over the last 50 years, based on mapping of the cold–temperate transition surface (CTS), marking the limit between cold and temperate ice of a polythermal glacier. Latent heat release from percolating meltwater and precipitation keeps the ice column temperate in the accumulation area. A change from accumulation to ablation zone reduces this heat release, leading locally to glacier ice cooling. By mapping the CTS along the whole glacier length using radio-echo sounding and employing a thermodynamic model, the timing of the cooling was determined, from which past ELAs were constructed. These are in accord with mass-balance measurements carried out on McCall Glacier since the 1950s. We show that with a warming climate, McCall Glacier tends to cool in a counter-intuitive way.

The climate memory of an Arctic polythermal glacier

AbstractKnowledge of glacier equilibrium-line altitude (ELA) changes and trends in time is essential for future predictions of glacier volumes. We present a novel method for determining trends in ELA change at McCall Glacier, Alaska, USA, over the last 50 years, based on mapping of the cold–temperate transition surface (CTS), marking the limit between cold and temperate ice of a polythermal glacier. Latent heat release from percolating meltwater and precipitation keeps the ice column temperate in the accumulation area. A change from accumulation to ablation zone reduces this heat release, leading locally to glacier ice cooling. By mapping the CTS along the whole glacier length using radio-echo sounding and employing a thermodynamic model, the timing of the cooling was determined, from which past ELAs were constructed. These are in accord with mass-balance measurements carried out on McCall Glacier since the 1950s. We show that with a warming climate, McCall Glacier tends to cool in a counter-intuitive way.

A camera and multisensor automated station design for polar physical and biological systems monitoring: AMIGOS

AbstractThe Automated Meteorology–Ice/Indigenous species–Geophysics Observation System (AMIGOS) consists of a set of measurement instruments and camera(s) controlled by a single-board computer with a simplified Linux operating system and an Iridium satellite modem supporting two-way communication. Primary features of the system relevant to polar operations are low power requirements, daily data uploading, reprogramming, tolerance for low temperatures, and various approaches for automatic resets and recovery from low power or cold shutdown. Instruments include a compact weather station, single- or dual-frequency GPS, solar flux and reflectivity sensors, sonic snow gauges, simplified radio-echo sounder, and resistance thermometer string in the firn column. In the current state of development, there are two basic designs. One is intended for in situ observations of glacier conditions. The other supports a high-resolution camera for monitoring biological or geophysical systems from short distances (100 m to 20 km). The stations have been successfully used in several locations for operational support, monitoring rapid ice changes in response to climate change or iceberg drift, and monitoring penguin colony activity. As of August 2012, there are nine AMIGOS systems installed, all on the Antarctic continent or in the surrounding ocean.

Removing the ice cap of Öræfajökull central volcano, SE-Iceland: Mapping and interpretation of bedrock topography, ice volumes, subglacial troughs and implications for hazards assessments

The ice covered active Öræfajökull central volcano forms a mountain range extending south from the central Vatnajökull ice cap, SE-Iceland. The high elevation span (7–2110 m a.s.l.) and extreme precipitation of this coastal part of Vatnajökull results in large mass turnover and high dynamic activity. Here we present bedrock and surface digital elevation models (DEMs) of Öræfajökull ice cap and its many outlets. The bedrock DEM is derived from radio echo sounding profiles and point measurements carried out in 1991–2012, and the surface from airborne LiDAR surveys in 2010–2011. At the centre of Öræfajökull is a ~14 km² caldera containing 4.3 km³ of ice, reaching ice thickness of 540 m. Most of the caldera drains meltwater eastwards to Kvíá river while the rest drains mainly westwards to Virkisá river. The caldera floor of Öræfajökull is smooth and volcanic mounds and ridges appear almost absent. An exception is a small topographic mound, beneath ~400 m of ice, near the water divides between Kvíá and Virkisá. The bedrock topography also suggests a separate caldera collapse, ~6 km² and ~150 m deep, within the main caldera. The subglacial topography implies an older highly eroded caldera north of the Öræfajökull summit (Hvannadalshnúkur 2110 m a.s.l.), similar in size to the present main caldera. The outlets of Öræfajökull, currently in some places up to 550 m thick, have excavated troughs reaching as far as 220 m below current sea level. Based on estimates of the present sediment transport rate in the rivers draining Öræfajökull, and the volume of the troughs, it would take over ~4000 years to form these troughs. Hence, it is unlikely that they were all excavated during the Little Ice Age. Marginal lakes will continue to grow and new ones form in the troughs as the outlets retreat in the coming decades, assuming current climate conditions or climate warming. The distribution of ice volume and area with elevation is however quite different from one outlet to another, suggesting variable glacier response to changing climatic conditions. A persistent temperature rise of 0.5–1.0°C may cause the lowest outlets to disappear completely, while the outlets with accumulation areas high up at the Öræfajökull caldera will survive even the warmest predicted climate scenarios.

Predicting radar attenuation within the Antarctic ice sheet

To better understand the ability of ice-penetrating radar to diagnose the subglacial environment from bed-returned power, we model the englacial radar attenuation of Antarctic ice. First, we use a one-dimensional thermo-mechanical model to evaluate the sensitivity of the depth-averaged attenuation rates to ice temperature as a function of surface accumulation rate, geothermal flux, and ice thickness. We find that attenuation is most sensitive to variations in geothermal flux and accumulation rate when the bed temperature is close to the pressure-melting point. But even if geothermal flux and accumulation rate remain fixed, attenuation can easily vary with ice thickness. Such high sensitivities show that one should not assume a uniform attenuation rate in the radar data analysis. Then, using ensembles of modeled ice temperatures with different boundary conditions, we generate multiple attenuation predictions for the Antarctic ice sheet and evaluate the resulting uncertainties. The largest contributor to uncertainty in these predictions is the geothermal flux. This uncertainty is localized within the deeper half of the ice sheet. By combining these temperature ensembles with ice-core chemistry data, we show that the sea salt adds little to the attenuation, but the contribution from acids accounts for ∼29% (inland) to ∼53% (coast) of the total attenuation. We conclude that improving radar diagnosis of the subglacial environment using bed-returned power requires both (1) better data-interpretation algorithms that account for attenuation variations and (2) better constraints of geothermal flux and bulk chemistry.

Volume and area changes of the Milne Ice Shelf, Ellesmere Island, Nunavut, Canada, since 1950

Ground penetrating radar (GPR) surveys of the 205 km2Milne Ice Shelf conducted in 2008 and 2009 are compared with radio echo sounding (RES) data from 1981 to provide the first direct measurements of thinning for any northern Ellesmere Island ice shelf. Our results show an average thinning for the ice shelf as a whole of 8.1 ± 2.8 m, with a maximum of >30 m, over this 28‐year period. Direct‐line comparisons along a 7.5 km transect near the front of ice shelf indicate a mean thinning of 2.63 ± 2.47 m over the same period. Reductions in areal extent (29%, 82 ± 8.4 km2: 1950–2009) and volume (13%, 1.5 ± 0.73 km3 water equivalent (w.e.): 1981–2008/2009) indicate that the Milne Ice Shelf has been in a state of negative mass balance for at least the last 59 years. A comparison of mean annual specific mass balance measurements with the nearby Ward Hunt Ice Shelf (WHIS) suggests that basal melt is a key contributor to Milne Ice Shelf thinning. Glacier inflow to the ice shelf has also reduced markedly over the past 28 years. The transition of ice shelf ice to lake ice was the most important source of mass loss. A 28.5 km2 epishelf lake now exists on the landward side of the ice shelf. Given these recent changes, disintegration of the Milne Ice Shelf will almost certainly continue in the future.

Precise analysis of ICESat altimetry data and assessment of the hydrostatic equilibrium for subglacial Lake Vostok, East Antarctica

SUMMARY Based on the Ice, Cloud and Land Elevation Satellite (ICESat) laser altimetry data, the hydrostatic equilibrium (HE) condition for the subglacial Lake Vostok, East Antarctica, is evaluated. A digital elevation model (DEM) of the ice surface is derived by a regional crossover adjustment. The analysis of the DEM and its comparison with GPS derived ice-surface elevations and an ice-surface DEM based on radar altimetry data reveal an overall accuracy of better than ± 0.7 m for the lake area. The DEM is combined with an ice-thickness model and a regional geoid model to determine the deviation of the local ice-surface height from HE. For large parts of the lake, the ice sheet fulfils the HE. Our results reveal a strong positive deviation of about 10 m along the lake shoreline. In addition, positive deviations are found in the northern part of the lake which coincide with ice rumples detected by radio-echo sounding. In the southern part of the lake, we find a linear negative deviation (−4.0 m) which coincides with the convoy route from Vostok station to Mirny base. In addition to the DEM, relative biases for the ICESat laser operational periods are determined in the regional crossover adjustment.