Subglacial Meltwater Flow Research Articles

The Quaternary sedimentary history and dynamics of the deep-water margin off the Hudson Strait ice stream

Sites of ice-stream discharge to the continental slope build trough-mouth fans from glacigenic debris flows (GDFs), subglacial meltwater flows, and suspended sediment. Thus, past ice configurations may be interpreted from the sedimentological record. This study links the records in cores, high-resolution seismic profiles, and multibeam swath bathymetry of Hatton Slope and Fan to the changing glaciological history in Hudson Strait, one of the largest ice streams of the late Pleistocene Laurentide Ice Sheet. 14C-AMS dates, X-ray fluorescence geochemical profiles, and oxygen isotopes in sixteen cores document sediment supply during the last glacial cycle, and earlier history is interpreted from seismic data. The Frobisher Bay slope sector has a long record of GDF deposits from severe glacial periods between MIS 6 and 22, with little impact of meltwater discharge. Mass-transport events and aggrading gravel-floored valleys dominate the northern Hatton slope sector. The central slope sector shows the greatest shelf break progradation by till deposition, with kilometre-spaced gullies heading at 700–730 mbsl from the last major grounding of the ice-stream on the upper slope during Heinrich event 3 (H3). The gullies pass downslope into ∼5 km wide valleys, which have been sites of sediment bypass since widespread meltwater flow in MIS 6 and show little aggradation since MIS 16. Intervening >50 m high ridges show alternating levee accretion and erosion by shallow mass-transport events. Downslope on Hatton Fan, GDFs are common and fill wide erosional channels that lead to the sandy braid plain of the central Labrador Sea. The channels of the southern sector and Saglek Slope have channeled turbidity currents down the NAMOC, and stratified contourite deposits accumulated on the fan. Over the MIS 1–3 period widely represented by cores, ice was grounded in Hatton Basin and on the upper slope to 700 m water depth only immediately prior to H3. Older and younger H events had a floating ice-shelf in Hatton Basin, and sedimentation was dominated by the retreat of that ice shelf to Resolution Island. Distal cores show no systematic difference between the H1–top H3 interval and the base H3–H5 interval. Given the supply of carbonate detritus of consistent composition back to at least H5, any MIS 3 deglaciation of Hudson Bay would require improbably high growth rates of ice sheets in the centre of the continent.

Upper-flow regime bedforms in a subglacial triangular-shaped landform (murtoo), Late Pleistocene, SW Finland: Implications for flow dynamics and sediment transport in (semi-)distributed subglacial meltwater drainage systems

We know less about subglacial meltwater flow properties in distributed inefficient and semi-efficient systems in comparison to those of ice marginal eskers and proglacial environments. While previous studies have indicated the overall common presence of upper-flow-regime (UFR) bedforms in glacigenic settings, facies expressions of subglacial meltwater flows remain poorly documented. Three ca. 3 m deep and up to 70 m long trenches excavated across a triangle-shaped subglacial landform called a murtoo in a Lateglacial to Holocene meltwater route in SW Finland provide a detailed window into sedimentary structures presumably formed ca. 40–50 km away from the coeval subaqueous margin of the Fennoscandian Ice Sheet (FIS). The aim of this paper is to document small-scale bedforms, which formed subglacially by meltwater flow and to characterize the proximal and central parts of the studied murtoo during its early evolutionary phase. We defined seven main facies types that characterize the depositional processes of the unit. Overall, the studied deposits reflect increasing meltwater delivery through time and are characterized by abrupt lateral changes in sedimentary structures and grain size. While the initial deposits are dominated by massive and horizontally laminated silt with sand lenses interpreted as lower-flow-regime deposits, the latter sediments are characterized by sinusoidal stratification, sigmoidal cross-stratification and scours with backsets or chaotic fill interpreted as deposits of antidunes, humpback dunes, chutes-and-pools and cyclic steps of the upper-flow regime. The upper-flow-regime bedforms developed on a 1 m high and 15 m long bed slope and are associated with the formation of a short-lived enlarged water-filled cavity or pond, where supercritical density flows allowed for the deposition of upper-flow regime bedforms. The final coarse-grained murtoo head-bar development, characterized by planar-cross stratified gravel and pebbly sand, indicates avalanche processes that were controlled by grain size.Our results confirm that the core of the murtoo is depositional and meltwater processes played a key role in its deposition. Despite the subglacial setting with a subaqueous ice-sheet margin, the meltwater flow was not permanently characterized by pipe-flow conditions. Overall, the findings contribute to the understanding of semi-distributed subglacial meltwater systems during the retreat of a continental ice sheet (FIS) in a rapidly warming climate.

Genesis of subglacial triangular‐shaped landforms (murtoos) formed by the Fennoscandian Ice Sheet

AbstractThe purpose of this paper is to describe the internal structure and composition of recently discovered subglacial landforms called “murtoos” in order to interpret their formative processes and depositional environment. So far, murtoos have only been reported from Finland and Sweden, but they probably exist in all areas covered by past ice sheets. Murtoos mainly occur in fields along subglacial meltwater routes or corridors in close relation to eskers and ribbed moraine tracts. Murtoos were excavated perpendicular to the long axes of the triangular murtoo heads in six locations. Murtoos were found to be composed of silt/clay‐poor, sandy and gravelly diamictons interbedded with sorted sediments, and are suggested to be produced by pulsed, highly sediment‐concentrated flows during weak glaciotectonic deformation, indicating effective pressure close to zero. Murtoos can be divided into three main depositional units: 1) the core, 2) the murtoo body and 3) the murtoo mantle. The initial deposition of murtoos took place in a network of low canals and conduits or cavities with fluctuating stream flow likely over 50 km from the ice margin. Murtoos reveal an increasing influence of subglacial meltwater flow in rapidly widening broad and low conduits with increasing sediment transport over short distances. The results of this work suggest that murtoos were formed time‐transgressively over yearly meltwater cycles in a semi‐distributed drainage system not recognised before, in which was high‐pressure porewater conditions with rapid mobilisation of subglacial saturated sediments, a critical factor in the development of semi‐efficient drainage. Murtoos are suggested as missing element between distributed and channelised drainage systems not included in current glaciohydrological models or even in the theoretical basis of glacial hydrology.

Offshore murtoos indicate warm-based Fennoscandian ice-sheet conditions during the Bølling warming in the northern Gulf of Riga, Baltic Sea

Glacial landforms provide invaluable information on the dynamics and extent of past ice sheets. In planform view, murtoos are triangular-shaped subglacial landforms that have recently been described from areas previously covered by the Fennoscandian Ice Sheet and whose formation has been linked to meltwater activity. We document the first occurrence of offshore murtoos and associated eskers in a high-resolution multibeam and sub-bottom profile dataset from the northern Gulf of Riga, Baltic Sea. In this study area, the murtoos appear to be longer (range 43–748 m), narrower (range 12–394 m) and less high (range 0.5–7.8 m), on average, compared to those previously reported on land, although this may be partly due to post-glacial sediments that smooth the present-day seafloor topography and may have buried smaller murtoos. The direction of murtoo longitudinal axes suggests a local NNE–SSW paleo-ice stream direction. In sub-bottom boomer profiles, the murtoos are characterized by moderate amplitude, sub-parallel discontinuous internal reflectors, indicating a varying sandy diamicton and sand composition, which is compatible with the previously interpreted mode of deposition by pulsed subglacial meltwater flow. Eskers that are found in close association with the murtoos are shorter (mean length 538 m) and more curved than their counterparts on nearby land areas, and they lack a general alignment. In boomer profiles, the eskers are characterized by low-amplitude, sub-parallel continuous reflectors, suggesting a better-sorted sandy composition. The curved and poorly aligned external form of the eskers suggests deposition under restricted subglacial drainage conditions. We conclude that murtoos and related landforms in the northern Gulf of Riga were formed during the Bølling warming at ca. 14.5–14.1 cal kyr BP, when large amounts of meltwater were delivered to the ice-sheet bed.

Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

AbstractDeciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real‐time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km2. A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5–1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.

Inventory of glacial curvilineations (GCLs) at the southern periphery of the last Scandinavian Ice Sheet

Glacial curvilineations (GCLs) are enigmatic landforms recently discovered in the southern peripheral area of the last Scandinavian Ice Sheet (SIS). They consist of multiple parallel, winding ridges several metres high and up to several kilometers long, systematically arranged in groups of common geomorphic appearance. GCLs create spectacular landscapes whose origin generates much controversy. Expanding from the first GCLs finding in central Poland, we now present an inventory of other GCLs occurrences along the SIS margin in Poland, Germany and Denmark based on analysis of high-resolution LiDAR digital elevation models. We document a total of 59 glacial curvilineation fields and focus on the geomophic characteristics of the 43 most conspicuous ones. Each of these fields consists of smaller units called GCL swarms, 137 in total. The fields differ strongly in areal coverage (between 1 and 358 km2), number of swarms (between 1 and 21) and number of individual landforms (between 2 and 255). All GCL fields occur in tunnel valleys terminating at the ice margin, typically at various topographical levels. Our findings, interpreted in the context of glacial geological processes, support the origin of GCLs as erosional remnants of antecedent landscapes carved by channelized subglacial meltwater flows.

Diversity of murtoos and murtoo‐related subglacial landforms in the Finnish area of the Fennoscandian Ice Sheet

Murtoos are recently discovered triangular‐shaped subglacial landforms that form under warm‐based ice and in association with significant subglacial meltwater flow. They appear in distinct fields and commonly occur in the area that was covered by the Fennoscandian Ice Sheet during glacial periods. Murtoos potentially represent a transition form from non‐channelized to channelized subglacial drainage networks. In the present study, we analyse and classify murtoos and murtoo‐related landforms in the Finnish area of the Fennoscandian Ice Sheet based on their characteristics and appearance in LiDAR‐based digital elevations models. Combined with morphometric analyses, the observations suggest that five types of murtoos and murtoo‐related landforms are common and widespread in Finland: (i) triangle‐type murtoos (TTMs), (ii) chevron‐type murtoos (CTMs), (iii) lobate‐type murtoos (LTMs), (iv) murtoo‐related ridges and escarpments (MREs), and (v) other murtoo‐related polymorphous landforms (PMRs) that look like small mounds and ridges. The morphometric characteristics of the different types are described here in detail, and it is shown that they are spatially and geomorphologically related. In addition, we provide examples of murtoos other than the TTMs to demonstrate that different murtoo types and murtoo‐related landforms are composed of similar sediments and architectural characteristics. The diversity of murtoo landforms and the transition between distinct murtoo types indicate rapid and complicated variations in the configuration of subglacial hydrology at different spatial and temporal scales. This study emphasizes the essential role of subglacial meltwater in the shaping of glacial landscapes and the redistribution of large volumes of sediments during the deglaciation of the Fennoscandian Ice Sheet.

Upper slope processes and seafloor ecosystems on the Sabrina continental slope, East Antarctica

This study applies detailed seafloor bathymetry data and seafloor images to understand upper slope features and how these influence the distribution of seafloor biota on the East Antarctic margin. The East Antarctic slope on the Sabrina margin has been shaped by diverse processes related to repeated glaciation. Differences in the morphology of gullies on the upper slope enable an understanding of the likely processes that have been active on this margin. Gully morphology varies according to changes in slope gradient, which may have driven variations in sedimentation. Areas of lower slope angles may have led to rapid sediment deposition during glacial expansion to the shelf edge, and subsequent sediment failure. Typically, gullies in these areas are U-shaped, initiate well below the shelf break, are relatively straight and long, and have low incision depths, consistent with formation due to mass wastage. Areas of higher slope angles likely experienced enhanced flow of erosive turbidity currents during glaciations associated with the release of sediment-laden basal meltwaters. Sediment-laden subglacial meltwater flows typically create gullies such as those we observe that initiate at, or near, the shelf break; are V-shaped in profile; and have high sinuosity, deep incision depths and a relatively short downslope extent. The short downslope extent reflects a reduced sediment load associated with increased seawater entrainment as the slope becomes more concave in profile. These differences in gully morphology have important habitat implications associated with differences in the composition and beta-diversity of the seafloor communities. This upper slope region also supports seafloor communities that are distinct from those on the adjacent shelf, highlighting the uniqueness of this environment for biodiversity. Conservation strategies therefore need to consider slope and shelf communities as distinct and equally important components of the Antarctic ecosystem.

An automated method for mapping geomorphological expressions of former subglacial meltwater pathways (hummock corridors) from high resolution digital elevation data

Elongated tracts of hummocks or ‘hummock corridors’, exposed on palaeo-ice sheet beds, are believed to represent former subglacial meltwater pathways. Here, we present a method, coded in MATLAB, for automatically detecting and mapping hummock corridors from high-resolution digital elevation models (DEMs). Initially the DEM is filtered to remove bed roughness outside the size range of hummocks. A Fast Fourier Transform is then performed to determine the dominant orientation of hummock corridors and remove misaligned features. Finally, image segmentation is used to isolate and extract the hummock corridors as a binary mask. The automated approach is tested visually and statistically against detailed manual mapping in three areas of Canada and northern Scandinavia. Results show that while the automated method does not perfectly reproduce the manual mapping, it successfully captures the general configuration, morphometry (length and width) and location of hummock corridors, despite variation in expression across and between sites. This technique is ideally suited to take advantage of newly available high-resolution digital elevation data (e.g. the ArcticDEM), whose enormous volume makes large-scale manual mapping prohibitively time consuming. Its application will enable efficient and comprehensive mapping of the spatial distribution of hummock corridors across palaeo-beds that is necessary for deriving insights into their formation and the organisation of subglacial meltwater flow beneath ice sheets.

Submarine geomorphology of northeast Baffin Bay and its implications for local paleo-ice sheet dynamics

High-resolution bathymetric data from close to the northwest Greenland coast in northern Melville Bay, northeast Baffin Bay, reveal a range of glacial and nonglacial landforms. The glacial landforms include crag-and-tails, mega-scale glacial lineations (MSGL), rock drumlins, and roche moutonnées that indicate paleo-ice sheet dynamics. The nonglacial landforms include steep ridges that are interpreted as volcanic dykes (Neoproterozoic Thule dyke swarms and Paleoproterozoic Melville Bugt dyke swarms). These dykes are glacially overprinted and may have channeled ice and subglacial meltwater flow through narrow subglacial cavities.Some of the glacial landforms eroded into bedrock indicate a southward paleo-ice stream orientation; while other glacial landforms, including the sedimentary depositional landforms, indicate a westward paleo-ice stream orientation. The glacial landforms were likely produced during at least two epochs and under changing thickness of the ice streams. The glacial landforms eroded into the bedrock are likely older than the sedimentary glacial landforms and were likely produced by a thin south/southeastward ice stream that allowed cavity formation. The westward-oriented glacial landforms were likely produced by a thick ice stream that was probably active during the Last Glacial Maximum (LGM). The different orientations of the glacial landforms indicate a reorganization of the ice streams that may have occurred during retreat and subsequent readvance. This reorganization likely coincided with a migration of the ice divide from south to north.

Bedrock morphology reveals drainage network in northeast Baffin Bay

A subglacial drainage network underneath the paleo-ice sheet off West Greenland is revealed by a new compilation of high-resolution bathymetry data from Melville Bay, northeast Baffin Bay. This drainage network is an indicator for ice streaming and subglacial meltwater flow toward the outer shelf. Repeated ice sheet advances and retreats across the crystalline basement together with subglacial meltwater drainage had their impact in eroding overdeepened troughs along ice stream pathways. These overdeepenings indicate the location of a former ice sheet margin. The troughs inherit characteristics of glacial and subglacial meltwater erosion. Most of the troughs follow tectonic weakness zones such as faults and fractures in the crystalline bedrock. Many of these tectonic features correspond with the orientations of major fault axes in the Baffin Bay region. The troughs extend from the present (sub) glacial fjord systems at the Greenland coast and parallel modern outlet-glacier pathways. The fast flowing paleo-ice streams were likely accelerated from the meltwater flow as indicated by glacial landforms within and along the troughs. The ice streams flowed along narrow tributary troughs and merged to form large paleo-ice streams bedded in the major cross-shelf troughs of Melville Bay. Apart from the troughs, a rough seabed topography characterises the bedrock, and we see a sharp geomorphic transition where ice flowed onto sedimentary rock and deposits.

Subglacial blowouts in western Canada: insights into extreme meltwater pressures and hydrofracturing

This study interprets deformations of indurated 10s of metres thick bedrock strata by subglacial meltwater pressures at maximum levels, resulting in types of hydrodynamic structures not previously recognized. The structures provide insights into the range of extreme pressures possible with the backup of meltwaters sufficient to deform multi‐metre thick indurated beds, unlike elsewhere. Subglacial meltwater flows into the subsurface below the 50–100 m thick bitumen platform aquiclude of the Cretaceous Athabasca Oil Sands deposit were driven by the hydraulic head of the 1.5‐km‐thick Laurentide Ice Sheet. These meltwaters over‐pressured the regional Devonian aquifer waters, but the low permeability of the aquifer below the site of the Muskeg River Mine was insufficient to accommodate the voluminous influxes of subglacial meltwater. The resulting meltwater flows backed‐up, resulting in elevating aquifer pressures to maximum levels along the margins of the underlying Devonian Keg River reef mound and within water‐saturated Cretaceous sand beds offset to the east. The meltwater pressure build to extreme levels hydrofractured strata at sites along the margins of the Keg River mound preconditioned by dissolution‐induced subsidence in underlying salt beds. Release of the confining pressure upon withdrawal of the Laurentide Ice Sheet resulted in 45‐m‐high open blowout structures that punctured the bitumen platform at sites above the northern margin of the Keg River mound. Other blowout chimney sites above the southern margin of the mound were plugged because of insufficient pressure build to clear vents of all ejecta. Concurrently, pressured meltwater flows along the eastern margin resulted in dykes that cross‐cut and buckled Devonian limestone beds. Some dykes extended into overlying water‐saturated unconsolidated Cretaceous sand beds, channelling pressured water and hydroplastic mudflows that dissipated the extreme meltwater flow pressures and prevented puncturing of the overlying bitumen platform. These unusual deformation structures in western Canada resulted from catastrophic failure of hydrofractured rock zones responding to extreme meltwater pressures, in contrast to unconsolidated sediment deformations resulting from only moderately elevated meltwater pressures commonly observed elsewhere.

Seismic character of possible buried grounding-zone wedges in the Late Ordovician glacial rocks of Algeria

A relatively brief ice age in the Late Ordovician, about 445 Ma ago, when North Africa was located near the South Pole led to the deposition of terrestrial and marine glacial sediments across northern Africa (e.g. Le Heron & Craig 2008). Several types of glacial landforms have been identified previously in these glacial rocks, including sedimentary features interpreted as ice-proximal fans and tunnel valleys produced by subglacial meltwater flow (e.g. Hirst 2012). These glacigenic rocks had been buried beneath about 2000 m of overlying sediment and have been deformed subsequently by multiple orogenic phases. A number of asymmetrical depocentres have been identified in three-dimensional (3D) reflection-seismic records from Illizi Basin, Algeria (Fig. 1). They are interpreted as grounding-zone wedges (GZWs), by analogy with similar features in Quaternary sediments on the Greenland margin (Dowdeswell & Fugelli 2012). Fig. 1. ( a , c , d ) Seismic profiles through the long axes of three Late Ordovician buried grounding-zone wedges (GZWs, blue lines mark upper surfaces) in Illizi Basin, Algeria. VE×25, ×10 and ×10, respectively. Time sections from ISS™ (Independent Simultaneous Sources). 3D survey shot in 2009. We acknowledge Sonatrach, BP's partner in Algeria, for its part in …

Geomorphic signature of Antarctic submarine gullies: Implications for continental slope processes

Five quantitatively distinct gully types are identified on the Antarctic continental margin from swath bathymetric data of over 1100 individual features. The gullies differ in terms of length, width, depth, width/depth ratio, cross-sectional shape, branching order, sinuosity and spatial density. Quantitative analysis suggests that Antarctic gully morphology varies with local slope character (i.e. slope geometry, gradient), regional factors (i.e. location of cross-shelf troughs, trough mouth fans, subglacial meltwater production rates, drainage basin size), sediment yield and ice-sheet history. In keeping with interpretations of previous researchers, most gullies are probably formed by hyperpycnal flows of sediment-laden subglacial meltwater released from beneath ice-sheets grounded at the continental shelf edge during glacial maxima. The limited down-slope extent of gullies on the western Antarctic Peninsula is explained by the steep gradient and slope geometry at the mouth of Marguerite Trough, which cause flows to accelerate and entrain seawater more quickly, resulting in a reduction of the negative buoyancy effect of the sediment load. Due to pressure gradients at the ice-sheet bed caused by variations in ice thickness inside and outside palaeo-ice stream troughs, subglacial meltwater flow was generally focussed towards trough margins. This has resulted in gullies with larger cross-sectional areas and higher sinuosities at the trough margins. A unique style of gullying is observed off one part of the western Antarctic Peninsula, corresponding to an area in which the ice-sheet grounding line is not thought to have reached the shelf edge during the Last Glacial Maximum. We interpret the features in this area as the cumulative result of slope processes that operated over a long period of time in the absence of hyperpycnal meltwater flows.

Paleo ice flow and subglacial meltwater dynamics in Pine Island Bay, West Antarctica

Abstract. Increasing evidence for an elaborate subglacial drainage network underneath modern Antarctic ice sheets suggests that basal meltwater has an important influence on ice stream flow. Swath bathymetry surveys from previously glaciated continental margins display morphological features indicative of subglacial meltwater flow in inner shelf areas of some paleo ice stream troughs. Over the last few years several expeditions to the eastern Amundsen Sea embayment (West Antarctica) have investigated the paleo ice streams that extended from the Pine Island and Thwaites glaciers. A compilation of high-resolution swath bathymetry data from inner Pine Island Bay reveals details of a rough seabed topography including several deep channels that connect a series of basins. This complex basin and channel network is indicative of meltwater flow beneath the paleo-Pine Island and Thwaites ice streams, along with substantial subglacial water inflow from the east. This meltwater could have enhanced ice flow over the rough bedrock topography. Meltwater features diminish with the onset of linear features north of the basins. Similar features have previously been observed in several other areas, including the Dotson-Getz Trough (western Amundsen Sea embayment) and Marguerite Bay (SW Antarctic Peninsula), suggesting that these features may be widespread around the Antarctic margin and that subglacial meltwater drainage played a major role in past ice-sheet dynamics.

Würmian deglaciation of western Lake Geneva (Switzerland) based on seismic stratigraphy

Western Lake Geneva (le Petit-Lac) was filled during the Quaternary over a major erosion surface truncating the cemented, folded and thrusted Tertiary sediments of the foreland Alpine basin. The carving of the lake occurred during Quaternary glaciations with ice originating from the Rhone valley catchment basin flowing in two branches oriented SW and NE over the Swiss Plateau. Lake Geneva is situated on the South-Western branch of this paleo ice-cap. For the first time, a dense grid of high-resolution seismic profiles (airgun 5-inch 3, airgun 1-inch 3 and echosounder) has imaged the whole Quaternary sequence, providing a paleoenvironmental interpretation and a detailed reconstruction of the Rhone glacier retreat stages during glacial events that led to the formation of western Lake Geneva. The Quaternary sequence filling up the bedrock valley is exceptionally thick with up to 220 m of deposits and consists of glacial, glacio-lacustrine and lacustrine sediments. Fourteen seismic units have been defined (units U1–U14). Unit U1 represents the remnants of glacial deposits older than the last glacial cycle, preserved in the deepest part of the lake and in secondary bedrock valleys. Unit U2 represents gravel and sands deposited by meltwater circulation at the bottom of the glacial valley. Unit U3 is a thick, stratified unit marking the beginning of the deglaciation, when the Rhone glacier became thinner and buoyant and allowed the formation of a subglacial lake. Younger glacial units (units U4, U5, U7, U9, U11) are acoustically chaotic sediments deposited subglacially under the water table (undermelt tills), while the glacier was thinning. These glacial units are bounded by synform erosion surfaces corresponding to readvances of the glacier. The transition from a glacial to a glacio-lacustrine environment started with the appearance of a marginal esker-fan system (unit U6). Esker formation was followed by a small advance–retreat cycle leading to the deposition of unit U7. Then, the ice front receded and stratified sediments were deposited in a glacio-lacustrine environment (units U8, U10 and U12). This retreat was punctuated by two readvances – Coppet (unit U9) and Nyon (unit U11) – producing large push moraines and proglacial debris flows. Finally, a lacustrine environment with a characteristic lake current pattern and mass movement deposits took place (units U13 and U14). Except for unit U1, the sedimentary sequence records the Würmian deglaciation in a fjord-like environment occupied by a tidewater glacier with a steep, calving ice front. The presence of an esker-fan system reveals the importance of subglacial meltwater flow in continental deglaciation. Push-moraines and erosion surfaces below the glacier indicate at least 5 readvances during the deglaciation thus revealing that oscillations of ice front are the key process in deglaciation of perialpine fjord-lakes. The dating of these continental glacier fluctuations would allow correlation with oceanic and ice records and help to understand the climatic mechanisms between oceans and continents.

“Glacial curvilineations”: New glacial landforms produced by longitudinal vortices in subglacial meltwater flows

The glacial landscape of the Dobrzyń Plateau in central Poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans. The term “glacial curvilineations” is proposed to describe these enigmatic bedforms that occur as fields of parallel, sinuous ridges separated by troughs. The regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis. Tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau. We propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion. Curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel-forming flows. Parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow. Tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices. Arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini: they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau. Understanding curvilineation genesis may shed light on glacier hydrologic processes. Tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater, such as drainage of subglacial and/or supraglacial lakes. The extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet-like flows as wide as a few kilometres.

Morphology of the upper continental slope in the Bellingshausen and Amundsen Seas – Implications for sedimentary processes at the shelf edge of West Antarctica

Swath bathymetric and sub-bottom profiler data reveal a variety of submarine landforms such as gullies, slide scars, subtle shelf edge-parallel ridges and elongated depressions, and small debris flows along the continental shelf break and upper slope of West Antarctica. Gullies cutting through debris flow deposits on the Belgica Trough Mouth Fan (TMF) suggest formation after full-glacial deposition on the continental slope. The gullies were most likely eroded by sediment-laden subglacial meltwater flows released from underneath the ice margin grounded at the shelf edge at the onset of deglaciation. Scarcity of subglacial meltwater flow features on the outer shelf suggests that the meltwater reached the shelf edge mainly either through the topmost layer of soft diamict or in the form of dispersed flow beneath the ice, although locally preserved erosional channels indicate that more focused and higher-energy flows also existed. Concentration of gullies on the upper continental slope in front of the marginal areas of the major cross-shelf troughs, as contrasted to their axial parts, is indicative of higher-energy gully-eroding processes there, possibly due to additional subglacial meltwater flow from beneath the slow moving ice lying over the higher banks of the troughs. The shallow and sinuous gully heads observed on the outermost shelf within the Pine Island West Trough may indicate postglacial modification by near-bed currents resulting either from the subglacial meltwater flow from underneath the ice margin positioned at some distance landward from the shelf edge, or from the currents formed by brine rejection during sea ice formation. On the continental slope outside major troughs, slide scars as well as shelf-edge parallel ridges and elongated depressions indicate an unstable and failure-prone uppermost slope, although failures were probably mainly associated with rapid sediment loading during glacial periods. Complex, cauliflower- and amphitheatre-shaped gully heads biting back into the shelf edge suggest upslope retrogressive, multi-stage small-scale sliding as a contributing factor to the formation of gullies in these areas. Small debris fans immediately downslope of the slide scars suggest that small-scale debris flows have been the main downslope sediment transfer processes in the areas of weak or absent subglacial meltwater flow.

Geophysical survey of the intra-caldera icefield of Mt Veniaminof, Alaska

AbstractMt Veniaminof is a large active stratovolcano located on the Alaska Peninsula (56.2° N, 159° W). We present results of the first geophysical survey of the icefield that fills much of the 10 km×8 km caldera that was most recently modified during the last major eruption roughly 3700 BP. The subglacial topography and ice volume are derived from an 8MHz radio-echo sounding survey conducted in July 2005. Prominent internal reflectors are assumed to be isochronal ash/acid deposits related to local eruptions. Accumulation rates and basal melt rates are calculated using a Nye one-dimensional steady-state accumulation model applied at a location that approximates an ice divide and calibrated by matching internal reflectors with published records of recent local volcanic eruptions. The model yields order of magnitude estimates of the accumulation rate of 4ma–1 water equivalent and 2 ma–1 of basal melt. The subsequent geothermal flux of ∽19Wm–2 is similar to active hydrothermal vents in volcanic lakes. We suggest that these values represent an upper limit for the geothermal flux within the ice-covered regions of the main caldera. We also analyze likely subglacial meltwater flow paths to examine the implications of recent eruption activity at an active intra-caldera cinder cone. Two lava-producing eruptions from the cinder cone in 1983–84 and 1993–94 melted roughly 0.17km3 of ice. The lack of significant deformation of the internal stratigraphy to the south and east of the melt hole suggests that any subglacial drainage in those directions was entirely within subglacial deposits. We suggest that the more likely drainage route was northwest into a large outlet glacier.

Structurally-controlled fluvioglacial erosion features near Schefferville, Québec

The coincidence as to location of a late-Wisconsin ice disintegration centre, with an earlier ice dispersal centre, some 30 miles north-west of Schefferville, Québec, resulted in large scale englacial meltwater flow from north-north-west to south-south-east across the Schefferville area. The direction of flow was controlled by the englacial hydraulic gradient, controlled in turn by the ice surface gradient. The Schefferville area is underlain by Proterozoic metasediments, exhibiting a series of parallel folds and thrust-faults aligned NW-SE ; this structure is reflected in the marked parallelism of the ridges and valleys. When the englacial meltwater streams were let down on to this substrate, structurally controlled alignment of the meltwater channels resulted. Generally, this has resulted in remarkably straight channels aligned along the strike or along faults. In the Houston Mountain area, however, steeply pitching anticlinos in slates oroduced a striking series of subglacial arcuate channels. At several points in the Schefferville area, subglacial meltwater flow down a hillside at right angles to the strike, and in the opposite direction to the dip, in zones of steeply dipping, well-bedded sediments, has produced the features referred to in literature as vallons, and earlier explained as being largely periglacial in origin. These are a special type of structurally controlled subglacial chute, in which there has been only slight periglacial modification of the original fluvioglacial form.