David M. Rippin

Water-Content of Glacier-Ice: Limitations on Estimates from Velocity Analysis of Surface Ground-Penetrating Radar Surveys

The rheology of ice is strongly controlled by its liquid water content. Since water content and its distribution also exert a strong control on radar propagation velocity and attenuation, this provides a potential remote technique for assessing ice-water content. A suite of surface ground-penetrating radar (GPR) surveys have been undertaken on two glaciers, Tsanfleuron Glacier in the European Alps and Bakaninbreen, Svalbard in order to determine their water content variation with depth. Common-offset surface radar profiling shows both glaciers have a two-layered structure, with a shallow layer characterised by low returned radar power and a deeper layer characterised by strong scattering. The thickness of these layers varies rapidly across the glaciers. In order to provide a robust interpretation of the properties of the layers within these glaciers we present a quantitative semblance analysis of two common midpoint surveys, making estimates of layer thickness and water content. This analysis includes a M...

East Antarctic ice stream tributary underlain by major sedimentary basin

Marine and rift sediments exert a fundamental control on ice stream flow in the West Antarctic Ice Sheet, and hence on its mass balance and stability. In contrast, most ice streams in the much larger East Antarctic Ice Sheet are thought to be relatively stable features resting on till, perhaps underlain by crystalline rock. Any geological controls on East Antarctic Ice Sheet enhanced flow remain largely unknown. We present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the East Antarctic Ice Sheet is underlain by subglacial sediments ∼3 km thick and that these are influencing the flow regime of the overlying ice. We show that subglacial sediments are important in modulating ice dynamics, not just for the West Antarctic Ice Sheet, but also for its much larger neighbor, and suggest that the sedimentary basin identified here may contain information on the Neogene glacial history of this part of the East Antarctic Ice Sheet.

The basal roughness of Pine Island Glacier, West Antarctica

We assess basal roughness beneath Pine Island Glacier (PIG), West Antarctica, based on a recent airborne radio-echo sounding dataset. We identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of PIG. The central portion and two of its tributaries are particularly smooth, but the majority of the tributaries feeding the main trunk are rougher. We interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the West Antarctic ice sheet in the Pliocene or Pleistocene, and, conversely, a lack of marine sedimentation where the bed is rough. Importantly, we also identify a patchy distribution of marine sediments, and thus a bed over which the controls on flow vary. While there is a notable correspondence between ice velocity and bed roughness, we do not assume a direct causal relationship, but find that an indirect one is likely. Where low basal roughness results in low basal resistance to flow, a lower driving stress is required to produce the flux required to achieve mass balance. This, in turn, means that the surface in that area will be lower than surrounding areas with a rougher bed, and this will tend to draw flow into the area with low bed roughness. Since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially, there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin.

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.

Basal topography and ice flow in the Bailey/Slessor region of East Antarctica

An airborne radio-echo sounding campaign carried out in the upper reaches of Bailey Ice Stream and Slessor Glacier, in Coats Land, East Antarctica, has revealed that tributaries of enhanced flow lie within well-defined basal troughs and are separated from each other by bed highs. These new data indicate significant differences in ice thickness compared with those estimated in the Bedmap database. A numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas. Most flow in the enhanced flow tributaries of Slessor Glacier may also be explained by ice deformation alone. However, although ice deformation is also significant in the Bailey Ice Stream tributary, a large amount of basal motion is also required to fully explain flow velocities here. It is proposed that the trough in which Bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times. Along with water produced by geothermal heating, frictional heating, and through a reduction in the pressure melting point, basal motion may therefore be facilitated by the presence of a deformable, saturated till layer at the bed.

Spatial and temporal variations in surface velocity and basal drag across the tongue of the polythermal glacier midre Lovénbreen, Svalbard

We present results of a detailed investigation of surface motion across the tongue of a polythermal glacier, midre Lovenbreen, Svalbard, during the 1999 summer. Surface velocities in the warm-based upper tongue increased during periods of enhanced surface melting and rainfall events, and force-balance analysis indicates that these velocity variations were locally forced, probably by fluctuations in subglacial water pressure. Surface speed-ups were also observed on the cold-based lower tongue (which acted as a sticky spot, through which there was minimal subglacial drainage for most of the summer), but these were largely non-locally forced by longitudinal coupling to the faster-moving ice up-glacier. On one occasion, however, a large, rapid input of surface water to the glacier reduced the basal drag beneath the cold-based lower tongue, presumably due to hydraulic jacking. This resulted in locally forced enhanced surface velocities across the entire tongue, accompanied by a breaching of the lower tongue and an outburst of subglacially stored water.

Basal conditions beneath enhanced-flow tributaries of Slessor Glacier, East Antarctica

Radio-echo sounding data are used to investigate bed roughness beneath the three enhanced-flow tributaries of Slessor Glacier, East Antarctica. Slow-moving inter-tributary areas are found to have rough beds, while the bed of the northernmost tributary is relatively smooth. A reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary, and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times, so marine sediments may have accumulated here. Together, these factors are further support for the dominance of basal motion in this tributary, reported elsewhere. Conversely, although the other two Slessor tributaries may have water routed beneath them, they would not have been below sea level before the growth of the ice sheet, so cannot be underlain by marine sediments. They are also found to be rough, and, within the range of uncertainties, it is likely that basal motion does not play a major role in the flow of these tributaries. Perhaps the most interesting area, however, is a deep trough where flow rates are currently low but the bed is as smooth as the northern Slessor trough. It is proposed that, although ice deformation currently dominates in this trough, basal motion may have occurred in the past, when the ice was thicker.

The role of ice thickness and bed properties on the dynamics of the enhanced-flow tributaries of Bailey Ice Stream and Slessor Glacier, East Antarctica

Abstract Airborne radio-echo sounding investigations in the upper reaches of Bailey Ice Stream and Slessor Glacier, Coats Land, East Antarctica, have shown that enhanced-flow tributaries are associated with well-defined areas of relatively thicker ice, and are separated from each other by areas of relatively thinner ice. A numerical modelling study has revealed that while internal ice deformation might account for all the observed flow in inter-tributary areas and the majority in the Slessor tributaries, a significant proportion of the flow of Bailey tributary is attributable to basal motion. Further, investigations of depth-corrected basal reflection power indicate that the bed underlying both Bailey and Slessor enhanced-flow tributaries is significantly smoother than in the slower-moving inter-tributary areas. It is thus proposed that enhanced motion within Bailey tributary (and also perhaps Slessor) may be facilitated by a reduction in basal roughness, caused by the accumulation of water and/or sediments within subglacial valleys, or by the erosion and smoothing of bed obstacles.

The glacial geomorphology of the Antarctic ice sheet bed

Abstract In 1976, David Sugden and Brian John developed a classification for Antarctic landscapes of glacial erosion based upon exposed and eroded coastal topography, providing insight into the past glacial dynamics of the Antarctic ice sheets. We extend this classification to cover the continental interior of Antarctica by analysing the hypsometry of the subglacial landscape using a recently released dataset of bed topography (BEDMAP2). We used the existing classification as a basis for first developing a low-resolution description of landscape evolution under the ice sheet before building a more detailed classification of patterns of glacial erosion. Our key finding is that a more widespread distribution of ancient, preserved alpine landscapes may survive beneath the Antarctic ice sheets than has been previously recognized. Furthermore, the findings suggest that landscapes of selective erosion exist further inland than might be expected, and may reflect the presence of thinner, less extensive ice in the past. Much of the selective nature of erosion may be controlled by pre-glacial topography, and especially by the large-scale tectonic structure and fluvial valley network. The hypotheses of landscape evolution presented here can be tested by future surveys of the Antarctic ice sheet bed.

Ice‐flow structure and ice dynamic changes in the Weddell Sea sector of West Antarctica from radar‐imaged internal layering

Recent studies have aroused concerns over the potential for ice draining the Weddell Sea sector of West Antarctica to figure more prominently in sea-level contributions should buttressing from the Filchner-Ronne Ice Shelf diminish. To improve understanding of how ice-stream dynamics there evolved through the Holocene, we interrogate Radio-Echo Sounding (RES) data from across the catchments of Institute and Moller Ice Streams (IIS and MIS), focusing especially on the use of internal layering to investigate ice-flow change. As an important component of this work, we investigate the influence that the orientation of the RES acquisition-track with respect to ice flow exerts on internal layering, and find that this influence is minimal unless a RES flight track parallels ice flow. We also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice-flow regime. Our findings suggest that ice in northern IIS, draining the Ellsworth Subglacial Highlands, has retained its present ice-flow configuration throughout the Holocene. This contrasts with less topographically-constrained ice in southern IIS and much of MIS, whose internal layering evinces spatial changes to the configuration of ice flow over the past ~10,000 years. Our findings confirm Siegert et al.’s (2013) inference that fast flow was diverted from Bungenstock Ice Rise during the Late Holocene, and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the IIS and MIS catchments as the West Antarctic Ice Sheet has thinned since the Last Glacial Maximum.