Fast-flowing Ice Streams Research Articles

Skjoldryggen terminal moraine on the mid-Norwegian shelf

Terminal moraines are relatively large ridges of diamictic glacial debris produced at the outermost margins of past glaciers and ice sheets. Their identification on land is important in mapping the maximum extent of Quaternary ice sheets (e.g. Svendsen et al. 2004; Benn & Evans 2010). In marine environments where fast-flowing ice streams reach the shelf edge during full-glacial intervals, moraine ridges are not usually present and the seafloor is characterized by parallel-to-flow streamlined sediments (Ottesen & Dowdeswell 2009). Submarine terminal moraine ridges, by contrast, appear more typical of slower-flowing marine ice margins (Dahlgren et al. 2002; Dowdeswell & Elverhoi 2002; Dowdeswell et al. 2016). A very large ridge, about 150 km long and up to approximately 200 m high relative to the adjacent shelf, is located on the outermost part of the mid-Norwegian continental shelf edge between about 65° and 66° N (Fig. 1a). It is known as Skjoldryggen (‘skjold’ means ‘shield’ and ‘ryggen’ means ‘the ridge’ in Norwegian). Along most of its length, the ridge has a steeper offshore slope relative to …

Mega-scale glacial lineations, Peel Sound, Canadian Arctic Archipelago

The presence of streamlined glacial landforms in the submarine geological record from high-latitude fjords and channels provides evidence with which to identify the manner and direction of glacier flow and to infer subglacial processes. Numerous linear and curvilinear elongate features extending up to 15 km in length have been recognized in Peel Sound, Canadian Arctic Archipelago. These well-preserved sedimentary landforms reflect the former presence of a fast-flowing ice stream that flowed through Franklin Strait and Peel Sound. Multibeam swath-bathymetric imagery from an overdeepened region of Peel Sound near its junction with Franklin Strait, where the trend of the channel changes from NE to north (Fig. 1b, c), is shown in Figure 1a. Water depths here range from 390 to 460 m. Parallel ridge and groove bedforms form prominent seabed features on all the survey transects that …

Glacially eroded cross-shelf troughs surrounding Iceland from Olex data

Iceland is surrounded by a continental shelf of around 100 km in width, sloping gently towards a shelf break at 250–400 m below sea level. Incised into the shelf are numerous cross-shelf troughs positioned radially and mostly reaching the shelf-edge. The troughs are typically about 100 km long and 20 km wide. They vary in depth between 15 and 350 m and are interpreted to have formed by glacial erosion beneath fast-flowing ice streams or outlet glaciers of former Icelandic ice sheets that extended to the shelf-break (Ingolfsson & Norddahl 1994). Bathymetric images combining interpolated depth-soundings from Olex (multiple single-beam echo-sounder lines; http://www.olex.no/index_e.html) and IBCAO v. 3.0 (Jakobsson et al. 2012) databases show that the Icelandic continental shelf is incised by numerous linear to curvilinear depressions, positioned radially (Fig. 1a). These troughs are generally relatively short, narrow and straight in the eastern and southern areas of the continental shelf, large and long on the western shelf, and deep, long, sinuous and distributive on the northern shelf (Fig. 1). Their locations and geometries are shown in Figure 1a and their dimensions recorded in the table. Cross-profiles of the troughs vary from symmetrical and flat-bottomed to asymmetrical and U- or V-shaped (Fig. 1e). Prominent breaks …

Post-glacial sand drifts burying De Geer moraines on the continental shelf off North Norway

Glacial landforms are continuously modified by sedimentary processes after deglaciation. Examples of post-depositional modification occur on the continental shelf off Vesteralen, North Norway, an area that is thought to have been covered by a relatively slow-flowing or stagnant ice dome during the Last Glacial Maximum (LGM) (Ottesen et al. 2005). In this period, the Lofoten-Vesteralen archipelago would have been largely sheltered from fast-flowing ice streams of the Fennoscandian Ice Sheet draining NE and SE of the area. Multibeam bathymetry nevertheless reveals evidence of local glacial activity on the continental shelf offshore Vesteralen, including glacial lineations indicative of faster ice-flow in cross-shelf troughs (Elvenes 2009). After the ice retreated from the shelf, glacial and glacimarine deposits on the surrounding shallow banks were eroded and reworked by waves and currents. Later winnowing of shell material has led to deposition of bioclastic sand in sand fields (Boe et al. 2009). On the present-day seafloor, sandy sediment drifts are migrating over moraines and other glacial deposits, burying and obscuring the surficial signatures of glaciation. The narrow continental shelf extending about 20–40 km off the Vesteralen archipelago is characterized by many shallow banks (40–120 m deep) intersected by 100–500 m deep, steep-sided troughs. These troughs may be cross-shelf or coast-parallel, and several contain series of transverse, evenly spaced ridges …

Streamlined hard beds formed by palaeo-ice streams: A review

Fast-flowing ice streams occur within modern ice sheets and also operated in Pleistocene ice sheets. The reconstruction of palaeo-ice streams normally relies on the mapping of mega-scale glacial lineations (MSGLs) and drumlins composed of soft sediment, mainly till. Analysis of new satellite imagery and digital terrain models, demonstrates the presence of large fields of kilometre-scale glacial lineations comprising rock drumlins, megagrooves and megaridges. In this paper we describe and analyse a number of such ‘hard-bed’ landform systems from the former Laurentide and British–Irish ice sheets, occurring in a variety of palaeo-ice stream settings. These are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams. Bedrock properties such as hardness, fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms. Elongate streamlined forms on hard crystalline rock, as on the Canadian Shield, only form under special circumstances; in contrast, sedimentary strata are highly susceptible to form streamlined hard beds, specifically if bedrock strike is parallel to ice flow. Large-scale elongate rock bedforms are erosional in origin, formed by preferentially focused abrasion or by lateral plucking, depending on bedrock type. Many palaeo-ice stream footprints previously mapped in the Laurentide Ice Sheet on the basis of soft-bed bedforms are shown to be significantly larger, extending up-ice across sedimentary strata and onto Precambrian crystalline rocks. Hard-bed streamlined forms further show that ice streaming does not necessitate a deformable bed, but can equally occur on smooth hard beds.

Late Cenozoic evolution of high-gradient trough mouth fans and canyons on the glaciated continental margin offshore Troms, northern Norway—Paleoclimatic implications and sediment yield

Trough mouth fans contain information about the evolution of high-latitude continental margins, including rates of glacial sedimentation and hinterland erosion. Here, the late Cenozoic evolution of high-gradient trough mouth fans and canyons on the Troms margin, northern Norway, is reconstructed. Paleocanyons were active prior to the Quaternary glaciations. Glaciomarine and glaciofluvial conditions prevailed between ca. 2.7 Ma and ca. 1.5 Ma, and ice sheets possibly reached the paleo–shelf break at least once. The minimum average sedimentation rate of this period was 0.22 m/k.y. From ca. 1.5 Ma to ca. 0.7 Ma, the glaciations intensified, and fast-flowing ice streams reaching the shelf break were established in the cross-shelf troughs. Subglacial deformation till was deposited at the outer shelf and later reworked by debris flows and turbidity currents. The Fennoscandian Ice Sheet started to route much of its ice mass to the north and south of the study area, and so the Troms margin possibly was a low-ice-flow sector from this time, with a minimum average sedimentation rate of 0.15 m/k.y. During the last ca. 0.7 m.y., ice streams continued to traverse the troughs, while sluggish-flowing ice prevailed on the banks. A minimum average sedimentation rate of 0.14 m/k.y. is estimated for this period. The minimum total erosion and erosion rate for the Quaternary are 50–140 m and 0.02–0.05 m/k.y., respectively. Compared with previous studies from other areas, this implies up to one order of magnitude variation in average glacial erosion rates along the western sector of the Fennoscandian–Barents Sea ice sheets. This is interpreted to be due to the size and bedrock composition of the catchment areas and the timing of ice growth and ice-sheet dynamics. In addition, the steep preglacial continental slope promoted high sediment flow velocity for the glacigenic sediments, causing much of the debris flows to transform into turbidity currents, which efficiently transported sediments across the slope and thereby maintained its steep gradient.

A boundary layer model for ice stream margins

The majority of Antarctic ice is discharged via long and narrow fast-flowing ice streams. At ice stream margins, the rapid transition from the vertical shearing flow in the ice ridges surrounding the stream to a rapidly sliding plug flow in the stream itself leads to high stress concentrations and a velocity field whose form is non-trivial to determine. In this paper, we develop a boundary layer theory for this narrow region separating a lubrication-type ice ridge flow and a membrane-type ice stream flow. This allows us to derive jump conditions for the outer models describing ridge and stream self-consistently. Much of our focus is, however, on determining the velocity and shear heating fields in the margin itself. Ice stream margins have been observed to change position over time, with potentially significant implications for ice stream discharge. Our boundary layer model allows us to extend previous work that has determined rates of margin migration from a balance between shear heating in the margin and the cooling effect of margin migration into the colder ice of the surrounding ice ridge. Solving for the transverse velocity field in the margin allows us to include the effect of advection due to lateral inflow of ice from the ridge on margin migration, and we demonstrate that this reduces the rate of margin migration, as previously speculated.

Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate

Abstract. We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rate anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions and ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise.

Automated drumlin shape and volume estimation using high resolution LiDAR imagery (Curvature Based Relief Separation): A test from the Wadena Drumlin Field, Minnesota

Resolving the origin(s) of drumlins and related megaridges in areas of megascale glacial lineations (MSGL) left by paleo-ice sheets is critical to understanding how ancient ice sheets interacted with their sediment beds. MSGL is now linked with fast-flowing ice streams but there is a broad range of erosional and depositional models. Further progress is reliant on constraining fluxes of subglacial sediment at the ice sheet base which in turn is dependent on morphological data such as landform shape and elongation and most importantly landform volume. Past practice in determining shape has employed a broad range of geomorphological methods from strictly visualisation techniques to more complex semi-automated and automated drumlin extraction methods. This paper reviews and builds on currently available visualisation, semi-automated and automated extraction methods and presents a new, Curvature Based Relief Separation (CBRS) technique; for drumlin mapping. This uses curvature analysis to generate a base level from which topography can be normalized and drumlin volume can be derived. This methodology is tested using a high resolution (3m) LiDAR elevation dataset from the Wadena Drumlin Field, Minnesota, USA, which was constructed by the Wadena Lobe of the Laurentide Ice Sheet ca. 20,000years ago and which as a whole contains ~2000 drumlins across an area of ~7500km2. This analysis demonstrates that CBRS provides an objective and robust procedure for automated drumlin extraction. There is strong agreement with manually selected landforms but the method is also capable of resolving features that were not detectable manually thereby considerably expanding the known population of streamlined landforms. CBRS provides an effective automatic method for visualisation of large areas of the streamlined beds of former ice sheets and for modelling sediment fluxes below ice sheets.

Assessment of heat sources on the control of fast flow of Vestfonna ice cap, Svalbard

Abstract. Understanding the response of fast flowing ice streams or outlet glaciers to changing climate is crucial in order to make reliable projections of sea level change over the coming decades. Motion of fast outlet glaciers occurs largely through basal motion governed by physical processes at the glacier bed, which are not yet fully understood. Various subglacial mechanisms have been suggested for fast flow but common to most of the suggested processes is the requirement of presence of liquid water, and thus temperate conditions. We use a combination of modelling, field, and remote observations in order to study links between different heat sources, the thermal regime and basal sliding in fast flowing areas on Vestfonna ice cap. A special emphasis lies on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate recently. We use the ice flow model Elmer/Ice including a Weertman type sliding law and a Robin inverse method to infer basal friction parameters from observed surface velocities. Firn heating, i.e. latent heat release through percolation of melt water, is included in our model; its parameterisation is calibrated with the temperature record of a deep borehole. We found that strain heating is negligible, whereas friction heating is identified as one possible trigger for the onset of fast flow. Firn heating is a significant heat source in the central thick and slow flowing area of the ice cap and the essential driver behind the ongoing fast flow in all outlets. Our findings suggest a possible scenario of the onset and maintenance of fast flow on the Vestfonna ice cap based on thermal processes and emphasise the role of latent heat released through refreezing of percolating melt water for fast flow. However, these processes cannot yet be captured in a temporally evolving sliding law. In order to simulate correctly fast flowing outlet glaciers, ice flow models not only need to account fully for all heat sources, but also need to incorporate a sliding law that is not solely based on the basal temperature, but also on hydrology and/or sediment physics.

Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

This paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the Antarctic Peninsula Ice Sheet (APIS) from the Last Glacial Maximum (LGM) through deglaciation to the present day. These data are used to reconstruct grounding-line retreat in 5 ka time-steps from 25 ka BP to present. Glacial landforms and subglacial tills on the eastern and western Antarctic Peninsula (AP) shelf indicate that the APIS was grounded to the outer shelf/shelf edge at the LGM and contained a series of fast-flowing ice streams that drained along cross-shelf bathymetric troughs. The ice sheet was grounded at the shelf edge until ∼20 cal ka BP. Chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains, as well as cosmogenic nuclide dates on erratics and ice moulded bedrock. Retreat in the east was underway by about 18 cal ka BP. The earliest dates on recession in the west are from Bransfield Basin where recession was underway by 17.5 cal ka BP. Ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid-shelf. The timing of initial retreat decreased progressively southwards along the western AP shelf; the large ice stream in Marguerite Trough may have remained grounded at the shelf edge until about 14 cal ka BP, although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka BP. Between 15 and 10 cal ka BP the APIS underwent significant recession along the western AP margin, although retreat between individual troughs was asynchronous. Ice in Marguerite Trough may have still been grounded on the mid-shelf at 10 cal ka BP. In the Larsen-A region the transition from grounded to floating ice was established by 10.7–10.6 cal ka BP. The APIS had retreated towards its present configuration in the western AP by the mid-Holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier, by the start of the Holocene. Mid to late-Holocene retreat was diachronous with stillstands, re-advances and changes in ice-shelf configuration being recorded in most places. Subglacial topography exerted a major control on grounding-line retreat with grounding-zone wedges, and thus by inference slow-downs or stillstands in the retreat of the grounding line, occurring in some cases on reverse bed slopes.

Deformation-induced melting in the margins of the West Antarctic ice streams

Flow of glacial ice in the West Antarctic Ice Sheet localizes in narrow bands of fast-flowing ice streams bordered by ridges of nearly stagnant ice, but our understanding of the physical processes that generate this morphology is incomplete. Here we study the thermal and mechanical properties of ice-stream margins, where flow transitions from rapid to stagnant over a few kilometers. Our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation-induced melting. We propose a 2-D model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction. We limit temperature to the melting point to estimate melt rates based on latent heat. Using rheology parameters as constrained by laboratory data and observations, we conclude that a zone of temperate ice is likely to form in active shear margins.

Glacially-megalineated limestone terrain of Anticosti Island, Gulf of St. Lawrence, Canada; onset zone of the Laurentian Channel Ice Stream

Anticosti is a large elongate island (240 km long, 60 km wide) in eastern Canada within the northern part of a deep water trough (Gulf of St. Lawrence) that terminates at the Atlantic continental shelf edge. The island's Pleistocene glaciological significance is that its long axis lay transverse to ice from the Quebec and Labrador sectors of the Laurentide Ice Sheet moving south from the relatively high-standing Canadian Shield. Recent glaciological reconstructions place a fast-flowing ice stream along the axis of the Gulf of St. Lawrence but supporting geologic evidence in terms of recognizing its hard-bedded onset zone and downstream streamlined soft bed is limited. Anticosti Island consists of gently southward-dipping limestone plains composed of Ordovician and Silurian limestones (Vaureal, Becscie and Jupiter formations) with north-facing escarpments transverse to regional ice flow. Glacial deposits are largely absent and limestone plains in the higher central plateau of the island retain a relict apparently ‘preglacial’ drainage system consisting of deeply-incised dendritic bedrock valleys. In contrast, the bedrock geomorphology of the lower lying western and eastern limestone plains of the island is strikingly different having been extensively modified by glacial erosion. Escarpments are glacially megalineated with a distinct ‘zig-zag’ planform reflecting northward-projecting bullet-shaped ‘noses’ (identified as rock drumlins) up to 2 km wide at their base and 4 km in length with rare megagrooved upper surfaces. Drumlins are separated by southward-closing, funnel-shaped ‘through valleys’ where former dendritic valleys have been extensively altered by the streaming of basal ice through gaps in the escarpments. Glacially-megalineated bedrock terrain such as on the western and eastern flanks of Anticosti Island is elsewhere associated with the hard-bedded onset zones of fast flowing ice streams and provides important ground truth for the postulated Laurentian Channel Ice Stream (LCIS) within the Gulf of St. Lawrence sector of the Laurentide Ice Sheet.

Sustained high basal motion of the Greenland ice sheet revealed by borehole deformation

AbstractIce deformation and basal motion characterize the dynamical behavior of the Greenland ice sheet (GrIS). We evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the GrIS. We find a sustained high amount of basal motion contribution to surface velocity of 44–73% in winter, and up to 90% in summer. Measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice-flow model as a consequence of stress transfer from slippery to sticky areas. This effect necessitates the use of high-order ice-flow models, not only in regions of fast-flowing ice streams but in all temperate-based areas of the GrIS. The agreement between modeled and measured deformation rates confirms that the recommended values of the temperature-dependent flow rate factor A are a good choice for ice-sheet models.

Late Weichselian local ice dome configuration and chronology in Northwestern Svalbard: early thinning, late retreat

The chronology and configuration of the Svalbard Barents Sea Ice Sheet (SBSIS) during the Late Weichselian (LW) are based on few and geographically scattered data. Thus, the timing and configuration of the SBSIS has been a subject of extensive debate. We present provenance data of erratic boulders and cosmogenic 10Be ages of bedrock and boulders from Northwest Spitsbergen (NWS), Svalbard to determine the thickness, configuration and chronology during the LW. We sampled bedrock and boulders of mountain summits and summit slopes, along with erratic boulders from coastal locations around NWS. We suggest that a local ice dome over central NWS during LW drained radially in all directions. Provenance data from erratic boulders from northern coastal lowland Reinsdyrflya suggest northeastward ice flow through Liefdefjorden. 10Be ages of high-elevation erratic boulders in central NWS (687–836 m above sea level) ranging from 18.3 ± 1.3 ka to 21.7 ± 1.4 ka, indicate that the centre of a local ice dome was at least 300 m thicker than at present. 10Be ages of all high-elevation erratics (>400 m above sea level, central and coastal locations) indicate the onset of ice dome thinning at 25–20 ka. 10Be ages from erratic boulders on Reinsdyrflya ranging from 11.1 ± 0.8 ka to 21.4 ± 1.7 ka, indicate an ice cover over the entire Reinsdyrflya during LW and a complete deglaciation prior to the Holocene, but apparently later than the thinning in the mountains. Lack of moraine deposits, but the preservation of beach terraces, suggest that the ice covering this peninsula possibly was cold-based and that Reinsdyrflya was part of an inter ice-stream area covered by slow-flowing ice, as opposed to the adjacent fjord, which possibly was filled by a fast-flowing ice stream. Despite the early thinning of the ice sheet (25–20 ka) we find a later timing of deglaciation of the fjords and the distal lowlands. Several bedrock samples (10Be) from vertical transects in the central mountains of NWS pre-date the LW, and suggest either ice free or pervasive cold-based ice conditions. Our reconstruction is aligned with the previously suggested hypothesis that a complex multi-dome ice-sheet-configuration occupied Svalbard and the Barents Sea during LW, with numerous drainage basins feeding fast ice streams, separated by slow flowing, possibly cold-based, inter ice-stream areas.

The marine-based NW Fennoscandian ice sheet: glacial and deglacial dynamics as reconstructed from submarine landforms

The configuration of the marine-based NW Fennoscandian ice sheet during the Last Glacial Maximum (LGM) and deglaciation is reconstructed using detailed swath bathymetry and high-resolution seismic data. The investigated area covers about 10,000 km2 of the continental shelf off Troms, northern Norway. Large scale morphology is characterized by cross-shelf troughs, coast-parallel troughs and banks. Based on mega-scale glacial lineations (MSGL), lateral and shear zone moraines and grounding zone systems, the extent and dynamics of the ice sheet during the LGM are deduced. MSGL indicate fast-flowing ice streams in the cross-shelf troughs, while the glacial morphology on the banks indicates more sluggish ice here. The marine-based part of the Fennoscandian ice sheet was sourced from ice domes in the east via fjord and valley systems inshore. Using a balance flux approach, we estimate palaeo-ice stream velocities during the LGM to be approximately 350 m/year. Three deglaciation events have been reconstructed: i) During the Torsken-1 event the ice sheet halted or readvanced to form groundings zone wedges (GZW) and the Torsken moraine, ii) Several still-stands or readvances characterized the ice behaviour on the shallower banks during the Torsken-2 event, iii) During the Flesen event, prominent end moraines in the inner parts of the troughs and banks were deposited. The locations of the end moraines and GZW in the troughs indicate that the retreat of marine-based ice streams in areas of reverse bed slope was episodic, probably mainly due to the variation in widths of the cross-shelf troughs.

Surface undulations of Antarctic ice streams tightly controlled by bedrock topography

Abstract. Full Stokes flow-line models predict that fast-flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent of the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity. We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness.

Antarctica's lost landscape

The evolution of Earth's largest hidden landscape beneath the East Antarctic Ice Sheet is poorly understood. Analyses of offshore sediments confirm that the ice incised deep troughs that host fast-flowing ice streams today, while older landscape features have been preserved.

Inversion for basal friction coefficients with a two-dimensional flow line model using Tikhonov regularization

We present results of basal friction coefficient inversion. The inversion was performed by a 2D flow line model for one of the four fast flowing ice streams on the southern side of the Academy of Sciences Ice Cap in the Komsomolets Island, Severnaya Zemlya archipelago. The input data for the performance of both the forward and the inverse problems included synthetic aperture radar interferometry ice surface velocities, ice surface elevations and ice thicknesses obtained by airborne measurements (all were taken from Dowdeswell <em>et al.</em>, 2002). Numerical experiments with: i) different sea level shifts; and ii) randomly perturbed friction coefficient have been carried out in the forward problem. The impact of sea level changes on vertical distribution of horizontal velocity and on shear stress distribution near the ice front has been investigated in experiments with different sea level shifts. The experiments with randomly perturbed friction coefficient have revealed that the modeled surface velocity is weakly sensitive to the perturbations and, therefore, the inverse problem should be considered ill posed. To mitigate ill posedness of the inverse problem, Tikhonov’s regularization was applied. The regularization parameter was determined from the relation of the discrepancy between observed and modeled velocities to the regularization parameter. The inversion was performed for both linear and non-linear sliding laws. The inverted spatial distributions of the basal friction coefficient are similar for both sliding laws. The similarity between these inverted distributions suggests that the changes in the friction coefficient are accompanied by appropriate water content variations at the glacier base.

The seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets

Grounding-zone wedges form where halts of decades to centuries during ice-sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast-flowing ice streams. Thirty grounding-zone wedges were identified on two-dimensional seismic-reflection profiles across the continental shelves of NE and NW Greenland. Grounding-zone wedges close to the seafloor are probably of late Weichselian age; others are on paleoshelves, buried beneath thick prograding glacial sediments. Several Greenland-shelf grounding-zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water. Grounding-zone wedges are asymmetric in the ice-flow direction with steeper ice-distal sides. Typical grounding-zone wedges are approximately 5–20 km long and 50–100 m thick, with a lateral width of several tens of kilometers. Forty grounding-zone wedges from the Greenland, Norwegian, and Antarctica margins show considerable variability about these values. Grounding-zone wedge dimensions are controlled by sediment flux, the duration of halts in ice retreat, subice cavity shape, and ice-stream width. Low-angle ice-shelf cavity roofs immediately beyond the grounding-zone probably restrict vertical accommodation space, preventing formation of high-amplitude ridges. Grounding-zone wedges are mainly transparent or chaotic on seismic profiles, probably resulting from delivery of diamictic debris. Offlapping reflectors represent progradation into an ice-roofed cavity. Reflector truncations of grounding-zone wedge bases indicate erosion during initiation. Channels are present within some grounding-zone wedges; meltwater flow is under high pressure, and V-shaped incisions suggest high-energy flow. Channels are not ubiquitous within Greenland grounding-zone wedges and may represent nonsteady meltwater release, perhaps through lake drainage. Where ice-sheet mass loss is dominated by meltwater runoff, grounding-zone wedges probably contain more sorted sediment than those from Greenland. Simple models of grounding-zone wedge architecture are presented. Grounding-zone wedges indicate episodic rather than catastrophic ice-sheet retreat and are a mechanism for ice-shelf stabilization because wedge growth counteracts collapse induced by ice-sheet thinning and sea-level rise.