Ice-bed Interface Research Articles

Ice tectonic deformation during the rapid in situ drainage of a supraglacial lake on the Greenland Ice Sheet

Abstract. We present detailed records of lake discharge, ice motion and passive seismicity capturing the behaviour and processes preceding, during and following the rapid drainage of a 4 km2 supraglacial lake through 1.1-km-thick ice on the western margin of the Greenland Ice Sheet. Peak discharge of 3300 m3 s−1 coincident with maximal rates of vertical uplift indicates that surface water accessed the ice–bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four global positioning system (GPS) receivers located around the lake record the opening and closure of the fractures through which the lake drained. We hypothesise that the majority of discharge occurred through a 3-km-long fracture with a peak width averaged across its wetted length of 0.4 m. We argue that the fracture's kilometre-scale length allowed rapid discharge to be achieved by combining reasonable water velocities with sub-metre fracture widths. These observations add to the currently limited knowledge of in situ supraglacial lake drainage events, which rapidly deliver large volumes of water to the ice–bed interface.

The instability theory of drumlin formation and its explanation of their varied composition and internal structure

Despite their importance in understanding glaciological processes and constraining large-scale flow patterns in palaeo-glaciology, there is little consensus as to how drumlins are formed. Attempts to solve the ‘drumlin problem’ often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles. This is a key strength of the instability theory, which has been described in a suite of physically-based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface. Encouragingly, model predictions of bedform height and length are consistent with observations and, furthermore, the theory has been applied to a range of subglacial bedforms and not just drumlins. However, it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency. This paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins. We summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition (e.g. bedrock, till, glaciofluvial sediments, and combinations thereof). Contrary to a view which suggests that the presence of some sedimentary sequences (e.g. horizontally stratified cores) is inconsistent with the theory, we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre-date drumlin formation, the duration and variability of ice flow, and the balance between erosion and deposition (till continuity) at the ice–bed interface. We conclude that the instability theory is compatible with (and potentially strengthened by) what is known about drumlin composition and, as such, offers the most complete and promising solution to the drumlin problem to date.

Influence of subglacial conditions on ice stream dynamics: Seismic and potential field data from Pine Island Glacier, West Antarctica.

[1] We interpret seismic reflection and airborne potential field data acquired on Pine Island Glacier, West Antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics. Immediately beneath the glacier is a mixture of soft, deforming sediments and harder, non-deforming sediments. Beneath this, a sedimentary basin lies under part of the main glacier, with another under one of its slower-moving tributaries. A tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south, which also include a thick, rift-related magmatic intrusion. The boundary correlates with changes in the basal roughness, ice flow speed, and basal drag. Smoother bed, faster flow, and lower basal drag characterize the thicker sedimentary sequences, to the north, but there is no corresponding lateral change in the acoustic properties of the bed. Changes in the sub-bed (i.e., deeper than the ice-bed interface) lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier. Subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary, leading to increased basal drag and reduced ice flow in the future. We conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present-day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future.

Measurements of In-Flight Cross-Track Antenna Patterns of Radar Depth Sounder/Imager

Antenna arrays with low sidelobes in the cross-track direction are needed for sounding and imaging ice-sheets margins including outlet glaciers. Weak radar signals from the ice-bed interface are often masked by off-vertical surface clutter from extremely rough crevassed surfaces in ice-sheet margins. Synthetic aperture radar (SAR) processing can be used to synthesize a large array for reducing clutter in the along-track direction. Low-side-lobe transmit- and receive-antenna patterns must be generated from a limited size array in the cross-track direction. Airborne antenna pattern measurements are critical to verifying pattern characteristics in the presence of a non-ideal ground plane and neighboring aircraft structures, as well as in-flight operational dynamics. In this paper, we describe a set of airborne measurements performed to determine and optimize antenna patterns for the very high frequency (VHF) array used to sound and image polar ice sheets. We measured antenna patterns by flying over a relatively smooth ice surface at an altitude of about 2700 m. The pattern data were obtained by processing the surface echoes with aircraft rolled from left to right over more than five cycles. We also simulated antenna patterns using a three-dimensional computer model of the entire airborne platform and compared with experimental results. The discrepancies between the measured and simulated results are less than 2.7 dB for 85% of the data samples. The measured pattern data will be used to optimize our array processing algorithms.

Typologies of genetic tills in the Sierra de Gredos (Spanish Central System)

Despite the fact that tills are crucial to the reconstruction of former glaciations, they are the most difficult deposits to interpret and classify. The only glacial diamictons that can be classified as true tills are those from subglacial environments because diamictons produced supraglacially are mass-flow deposits. In order to reconstruct the genesis of tills, it is critical to understand the processes taking place at glacier beds. The processes of deformation, melt-out, flowage, sliding, lodgement, meltwater reworking, and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members on a continuum of till types. The end members range from glaciotectonically folded and faulted stratified material to texturally homogeneous diamicton. Because subglacial processes vary both spatially and temporally, tills or complex till sequences contain superimposed signatures of transportation and deposition at the ice–bed interface. Till classification must reflect the range of products encompassed by the subglacial till production continuum and therefore, we can presently unequivocally identify: (i) glaciotectonite (rock or sediment that has been deformed by subglacial shearing/deformation but retains some of the structural characteristics of the parent material); (ii) subglacial traction till (sediment deposited by a glacier sole either sliding over and/or deforming its bed, the sediment having been released directly from the ice by pressure melting and/or liberated from the substrate and then disaggregated and completely or largely homogenized by shearing); and support the theoretical case for (iii) melt-out till (sediment released by the melting of stagnant or slowly moving debris-rich glacier ice and directly deposited without subsequent transport or deformation).

Complex interaction of subglacial hydrology and deforming bed processes and its implications for ice marginal stability: the example of the Middle Pleistocene British Ice Sheet from eastern England

Subglacial drainage is known to have a profound influence on glacier and ice sheet stability. Many scientists consider that such processes acted to cause abrupt climatic shifts in the North Atlantic region during Termination 1, and believe that meltwater-induced instabilities in the modern-day Greenland and Antarctic ice sheets could have similar climatic impacts in the future. Understanding the role played by porewater content and/or pressure, coupled with its temporal and spatial distribution within the subglacial bed, appear to be critical factors. These act to control the rheology and style of deformation within the subglacial deforming bed, and in-turn, the amount and distribution of ‘stick-slip’ at the ice-bed interface which facilitates ice motion. Within this paper, we present a model based upon sedimentological and structural evidence from Middle Pleistocene glacial deposits in Eastern England, which illustrate the symbiotic relationship between subglacial deforming bed processes, subglacial drainage within a pressurised channel system, and ice advance. The model shows the development of large-scale subglacial folds and thrusts within a subglacial melange that relate to temporal changes in porewater content and/or pressure within the subglacial bed. These acted to control the development and stability of subglacial drainage beneath the margins of the British Ice Sheet, ultimately leading to an increase in the efficiency of subglacial drainage and the subsequent collapse of the subglacial shear zone. This potentially slowed or even stopped ice advance leading to the part-collapse of the ice margin.

Investigating the effects of glacial shearing of sediment on luminescence

Previously reported low luminescence of basal glacial sediment has raised the possibility that processes operating at the ice–bedrock interface have the potential to reset (or ‘bleach’) natural luminescence signals (Swift et al., 2011). This finding indicates that certain types of glacial sediment (for example, sub-glacial diamicts) might be amenable to dating using luminescence-based techniques. Using a purpose-built ring-shear apparatus situated in a light-controlled environment, we have investigated the potential for mineral grains to be reset when subjected to conditions similar to those experienced by sediment that has undergone transport at the ice–bedrock interface. Reported here are the preliminary results of an initial experiment that used medium quartz sand with a naturally-acquired palaeodose of ∼4.3 Gy that had been obtained from a relict dune system. Incremental sampling during the shearing experiment and measurements were made to track changes in the luminescence properties of the sand as strain/shearing increased. The results indicate that increased strain/shearing resulted in an increase in the number of zero-dose grains and evolution of the De distribution from unimodal to multimodal. In light of the very much longer shearing distances that sub-glacial sediment would endure in nature, these results would appear to suggest that geomechanical processes at the ice–bed interface of glaciers and ice sheets may be a viable mechanism for resetting sediment.

Seasonal variations in Greenland Ice Sheet motion: Inland extent and behaviour at higher elevations

We present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west Greenland Ice sheet margin. In situ measurements of air temperature and surface ablation, and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes. We find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect, with sites nearest the ice sheet margin experiencing greater annual variations in ice motion (15–18%) than those above 1000 m elevation (3–8%). Patterns in the timing and rate of meltwater delivery to the ice–bed interface provide key controls on the magnitude of hydrologically-forced velocity variations at each site. In the lower ablation zone, the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system. At sites in the upper ablation zone, a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur. Our data suggest that land-terminating sections of the Greenland Ice Sheet will experience increased dynamic mass loss in a warmer climate, as the behaviour that we observe in the lower ablation zone propagates further inland. Findings from this study provide a conceptual framework to understand the impact of hydrologically-forced velocity variations on the future mass balance of land-terminating sections of the Greenland Ice Sheet.

REPLY TO THE DISCUSSION BY C.D. ROWE AND N.R. BACKEBERG ON THE PAPER "RECONSTRUCTION OF THE ORDOVICIAN PAKHUIS ICE SHEET, SOUTH AFRICA" BY H.J. BLIGNAULT AND J.N. THERON (SAJG,113, 335-360, 2010)

Rowe and Backeberg (this volume) compares the morphology of cm scale structures formed by fluidization with similar looking structures of the Fold Zone (tens of m scale) for which Blignault and Theron (2010) proposed a model of folding essentially by buckling. The Blignault and Theron model is an integration of structure, glaciological aspects and lithofacies associations as observed over the full outcrop area of ca 300 km. Some of the comments made by Rowe and Backeberg require clarification. They are of the opinion that folding by buckling results in “an essentially unlimited long axis”. However, the extent of fold axes is a function of the extent of the stress field causing the folding. Blignault and Theron (2010) noted that research published during the last ten years on the ice-bed interface, shows how ‘sticky spots” vary spatially and temporally as water pressure at the interface varies. The implication is …

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

The rheology and hydro-mechanical interactions at the ice–bed interface form an important component of the glacier system, influencing glacier dynamics and the formation of till. We demonstrate that the sand-rich till at Briksdalsbreen in Norway, undergoes deformation throughout the year. On the bulk rheology scale, till deformation exhibits elastic behaviour during the winter, when water pressures are low; and linear viscous behaviour after a critical yield stress of 35 kPa, when water pressures are high during the spring and summer. On the clast and matrix scale, low water pressures, correspond with high case stress variability and till temperatures. Meltwater driven, stick-slip, glacier velocity increases were transmitted through a relatively strong till grain network, causing brittle deformation. Intermediate water pressures, during late summer were linked to intermediate case stress variability and high till temperatures associated with the heat generated from stick-slip motion. High water pressures in the till were associated with low case stress variability and low, meltwater controlled, till temperatures, and occurred in the spring and autumn. Once the till was saturated, the ductile till absorbed any stick-slip velocity increases. We discuss, with examples, the different till forming processes associated with these changing conditions, demonstrating that the resultant till will represent a complex amalgamation of all of these processes.

RECONSTRUCTION OF THE ORDOVICIAN PAKHUIS ICE SHEET, SOUTH AFRICA

This reconstruction of the Late Ordovician, glacigenic Pakhuis Formation of the Table Mountain Group is based on a variety of diverse datasets. Field observations were made over the full 300 km extent of exposure in a north-south direction. The thin glacigenic package constitutes a minor interlude in the development of a subcontinental, Palaeozoic, siliciclastic basin. The Pakhuis Formation is of special interest because it includes features like the extensive penecontemporaneous folding, an erosional unconformity and the uniform lithological character of the sandy diamictites. A model is presented by which an unusual concurrence of circumstances produced the special features of the Pakhuis glacigenics. These are the ice invasion of a developing basin, an onset zone within basin and a deep trough harboring an ice-stream. Subglacial, grounding zone and iceberg facies associations with their lateral variations are developed. The temperate, glaciation of relative short duration, comprised one major and a second minor advance. During the first advance the grounded ice sheet with slow-moving sheet-flow reached ice-stream characteristics along at least part of a trough. The occurrence of an ice-stream within a slow-moving, low-profile ice sheet provides the glaciological environment for ice-stream draw-down, flanking infill of ice, resulting in compressional ice-flow from the sides. The fundamental requirement for buckling (i.e. the development of the Fold Zone) is layer parallel shortening achieved by the compressional ice-flow. Ice-bed interface features like deforming–bed structures are developed, as well as outsize flutes for which an alternative bulk strain model is proposed. The glacial retreat was rapid, interrupted only by the second, minor advance. A marine environment is attested by the postglacial fauna of a black shale.

Neogene glacigenic debris flows on James Ross Island, northern Antarctic Peninsula, and their implications for regional climate history

Detailed sedimentological and microtextural analyses of newly-discovered late Neogene diamictites and other coarse-grained facies, mostly sandwiched between hyaloclastite of the James Ross Island Volcanic Group and Cretaceous sandstone and mudstone, indicate deposition mainly by glacigenic debris flows. The deposits on James Ross Island (northern Antarctic Peninsula) constrain the depositional setting, ice–bed dynamics and regional palaeoclimate. The sequences on James Ross Island vary in age but are mainly late Miocene and Pliocene. Unlike Neogene sedimentary sequences elsewhere in Antarctica, those on James Ross Island are unusually well-dated by a combination of 40Ar/ 39Ar and 87Sr/ 86Sr analyses on fresh interbedded lavas and pristine bivalve molluscs, respectively. The Sr isotopic ages of the debris flows cluster around 4.74, 4.89, 5.44, 5.78, and 6.31 Ma and probably date relatively warm periods in the northern Antarctic Peninsula region, when the bivalves lived under ice-poor or seasonally ice-free conditions. The bivalves are often remarkably well-preserved, lack adhering lithified sediment and, in at least two locations, are large, mainly unfragmented and sometimes articulated, suggesting that they were alive immediately prior to their incorporation in subaqueous debris flows at the margins of an advancing glacier. These fossiliferous glacigenic debris flows signify episodes of ice expansion during relatively warm periods, or “interglacials”, of the late Miocene and Pliocene. The James Ross Island glacigenic sedimentary successions attain thicknesses of up to 150 m and extend over 4 km laterally. The high volume of glacigenic sediment delivery implicit in the James Ross Island successions indicates that a series of dynamic ice fronts crossed the region during the late Miocene and Pliocene epochs. Associated evidence, in the form of clast abrasion (including striations and faceting) and bedrock erosion, is indicative of basal sliding and subglacial sediment deformation active at the ice–bed interface and wet-based temperate or polythermal regimes, prior to remobilisation. The evidence further suggests two local ice caps on James Ross Island during the warm periods, as well as ice-overriding by the Antarctic Peninsula Ice Sheet from the west and northwest.

An assessment of fluting and “till esker” formation on the foreland of Sandfellsjökull, Iceland

Intensive sedimentological analysis of tills, parallel-sided flutings, and a conduit fill (till esker) on the recently deglaciated foreland of Sandfellsjökull (a temperate, actively receding piedmont lobe draining the eastern margin of the Mýrdalsjökull ice cap in southern Iceland) reveals that subglacial sediment was emplaced predominantly as a dilatant A horizon after an initial phase of glacitectonite production from local outwash. Clast macrofabric strengths weakened through time, presumably in response to increasingly wetter substrate conditions, until the emplacement of the fluted till when clast alignment was more tightly constrained. The fluting phase was initiated only after boulders were plucked from an adjacent bedrock cliff and lodged at the ice–till interface. This suggests that the parallel-sided flutings constrained shearing at the ice–bed interface in a similar fashion to a fault gouge, thereby strengthening clast alignments but allowing the till to mould around larger obstacles to produce herringbone fabric patterns. Strong clast macrofabric strengths in the till esker are not parallel to the ice flow direction, but instead align parallel with the local axis of the meandering ridge. This suggests that the till esker was likely produced by the squeezing of a dilatant till into an elongated cavity or R-channel after meltwater evacuation, followed by localized deformation, constrained by the channel walls, as the channel moved downvalley because of ice flow. The short period over which the deformation took place was enough to impart weaker fabrics in parts of the conduit that lie transverse to the ice flow direction and stronger fabrics where the conduit trends toward parallelism with the ice flow.

Dynamic distributed drainage implied by the flow evolution of the 1996–1998 Adventure Trench subglacial lake discharge

Abstract The transport of subglacial water beneath the East Antarctic Ice Sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities, and ultimately global climate. Periodic outbursts are a critical mechanism in this process. Recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice-bed interface of the Adventure Subglacial Trench between 1996 and 1998 (Wingham et al., 2006. Rapid discharge connects Antarctic subglacial lakes. Nature 440, 1033–1036). Using radar echo sounding data from the Adventure Subglacial Trench region in conjunction with the previously reported satellite observations, along with some basic modeling, we calculate a mass budget and infer a flow mechanism for the 1996–1998 event. The volume released from the source lake exceeded the volume received by the destination lakes by ~ 1.1 km 3 . This discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward. The downstream release of water from the destination lakes continued until at least 2003, several years after the 1998 cessation of surface subsidence at the source lake. By 2003 a total of 1.5 km 3 or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes. The temporal evolution of discharge from the outlet can be simulated with the classic ice-walled semicircular channel model, if and only if the retreat of the source lake shoreline is taken into account. Further downstream, the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable. Along these reaches mechanical lifting of the ice roof and/or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge. A distributed system is also consistent with the 3-month delay between water release at the source lake and water arrival at the destination lake. Observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system. Ultimately the presence of large subglacial lakes along the flow path of the 1996–1998 Adventure Subglacial Trench flow path delayed the arrival of water to points downstream by approximately 12 months.

Meltwater discharge through the subglacial bed and its land‐forming consequences from numerical experiments in the Polish lowland during the last glaciation

AbstractNumerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady‐state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice‐sheet‐thickness‐modulated, down‐ice‐decreasing hydraulic heads. In relation to non‐glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice–bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo‐ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice–bed interface. Owing to the abundance of low‐permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd.

Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica

AbstractRadar profiles of bed echo intensity can survey conditions at the ice–bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depth-averaged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dBkm–1 at Siple Dome to 15 dBkm–1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.

Bedform topography and basal conditions beneath a fast-flowing West Antarctic ice stream

A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ∼140 km 2 of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km 2). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream.

A laboratory study of particle ploughing and pore-pressure feedback: a velocity-weakening mechanism for soft glacier beds

AbstractIf basal-water discharge and pressure are sufficiently high, a soft-bedded glacier will slip over its bed by ploughing, the process in which particles that span the ice–bed interface are dragged across the bed surface. Results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity (velocity weakening). During ploughing at various velocities (15–400 m a−1), till was compacted in front of idealized particles, causing pore pressures there that were orders of magnitude higher than the ambient value. This excess pore pressure locally weakened the till in shear, thereby decreasing ploughing resistance by a factor of 3.0–6.6 with a six-fold increase in ploughing velocity. Characteristic timescales of pore-pressure diffusion and compaction down-glacier from ploughing particles depend on till diffusivity, ploughing velocity and sizes of ploughing particles. These timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred. Existing ploughing models do not account for velocity weakening. A new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38% but requires that excess pore pressures be measured. Velocity weakening by this mechanism may affect fast glacier flow, sediment transport by bed deformation and basal seismicity.

Polar Radar for Ice Sheet Measurements (PRISM)

We provide an overview of a project to develop and demonstrate radars for imaging the ice–bed interface, mapping shallow and deep internal layers, and measuring ice thickness. We developed an ultra-wideband very high-frequency (VHF) radar (120 to 300 MHz) for simultaneously measuring ice thickness, mapping internal layers at depth, and imaging the ice–bed interface, and an ultra high-frequency (UHF) ultra-wideband radar (500 to 2000 MHz) for fine-resolution mapping of near-surface internal layers. We used these radars to collect data at Summit Camp, Greenland, during July 2005, and the West Antarctic Ice Sheet (WAIS) deep drill core site during January 2006. These results provide us with an unprecedented view of basal characteristics and internal layers. We also developed an outreach program, a communications system, and rovers to facilitate geophysical measurements in the future.

Late Ordovician glacial record of the Anti-Atlas, Morocco

Late Ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the Anti-Atlas mountains of southern Morocco. These deposits are of economic significance as potential oil-bearing sandstones in the Tindouf and Boudenib basins and thus are here re-examined as analogues to subsurface hydrocarbon reservoirs. Glaciogenic deposits of the Upper Second Bani Formation rest unconformably upon underlying shallow marine clastic deposits. The unconformity is characterised by a series of palaeovalleys, some 0.5–1.0 km wide, and up to 100 m deep, which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a Late Ordovician ice sheet. The valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations, namely 1) a tabular sandstone association (shallow marine/shoreface deposits), 2) a massive sandstone and conglomerate (ice contact debrites), 3) meandriform sandstone deposits (ice proximal sandur), 4) stratified diamictites (ice-rafted debris) and 5) sigmoidally bedded sandstones (intertidal sandstones). Deformation in these sediments is ubiquitous and includes soft-sediment striated pavements, metre-scale duplex systems, thrust and fold belts of deformation affecting some tens of metres of sediment, and pervasive lineations. These features are interpreted to record the complex nature of deformation processes operating beneath a Late Ordovician ice sheet including sliding at the ice-bed interface, folding and deformation within the sediment column, and a series of complex ramps, detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet.