Subglacial Processes

ABSTRACTStable isotopes of water (δ18O and δ2H) were measured in the debris-laden ice underlying an Antarctic blue ice moraine, and in adjoining Law Glacier in the central Transantarctic Mountains. Air bubble content and morphology were assessed in shallow ice core samples. Stable isotope measurements plot either on the meteoric waterline or are enriched from it. The data cluster in two groups: the ice underlying the moraine has a δ2H:δ18O slope of 5.35 ± 0.92; ice from adjoining portions of Law Glacier has a slope of 6.69 ± 1.39. This enrichment pattern suggests the moraine's underlying blue ice entrained sediment through refreezing processes acting in an open system. Glaciological conditions favorable to warm-based sediment entrainment occur 30–50 km upstream. Basal melting and refreezing are further evidenced by abundant vapor figures formed from internal melting of the ice crystals. Both the moraine ice and Law Glacier are sufficiently depleted of heavy isotopes that their i...

The drumlins at Fláajökull, a non-surge-type glacier, Iceland provide the opportunity to investigate a modern drumlin-forming environment. Field observations indicate that the drumlins were formed sub-marginally along with an end moraine between the time period of 1966–1995. The fact that the drumlins did not extend beyond the end moraine, suggests that they were formed during this period. The Fláajökull drumlins consist of glaciofluvial outwash or till in their cores and have been streamlined with basal till. The Boulton hypothesis specifies that drumlin formation is driven by strain gradients in the glacier bed caused by hydrologic heterogeneity. This hypothesis was tested by measuring the anisotropy of magnetic susceptibility (AMS) of 457 till samples from two drumlins in the glacier forefield. In the first drumlin, two end member states of strain indicated that the basal till was sheared on the up-glacier side of the ridge (toward the overdeepened basin) and compressed on the down-glacier end with upward extrusion. These fabrics were likely formed in a recessional push-moraine on the drumlin surface. AMS fabrics in the other drumlin indicate that the tills were sheared in directions 10° and 12° northwest of the drumlin long axis; however, in one case the fabric was precisely sheared along the drumlin axis. Shear plane orientations conform to the drumlin morphology and were observed in both the drumlin core and the overlying basal till carapace in relation to the drumlin long axis. Shearing azimuths at the stoss end of the drumlin indicate some tendency for divergence suggesting higher basal resistance in the bed. New observations of hydrometer tests indicate that the Fláajökull tills are texturally similar to basal till from the Múlajökull drumlin field in central Iceland and the Horicon till, a late Wisconsin-age basal till from the Green Bay Lobe of the Laurentide Ice Sheet. In addition, ring-shear calibrations on the Horicon till provide a basis for estimating the state of strain for the tills at Fláajökull.

This thesis reexamines the glacial landscape of the central Avalon Penin sula , ewfoundland, Canada. Specifically, it focuses on an area of moraine ridges , previou s studies of which have generated conflicting interpretation of gene sis and deglacial significance. For instance, it has been interpreted as a series of recessional moraine s, and Although this thesis bears only my name , it is only with the help of many individuals was I able to complete it. Firstly , thanks goes to my field assistants, Christa

This thesis reexamines the glacial landscape of the central Avalon Penin sula , ewfoundland, Canada. Specifically, it focuses on an area of moraine ridges , previou s studies of which have generated conflicting interpretation of gene sis and deglacial significance. For instance, it has been interpreted as a series of recessional moraine s, and Although this thesis bears only my name , it is only with the help of many individuals was I able to complete it. Firstly , thanks goes to my field assistants, Christa

Growth of mid-latitude ice sheets during the glacial cycles of the Quaternary repeatedly reorganises the pattern of groundwater flow on a continent-wide scale. Relatively small scale non-glacial catchments are replaced by catchments which are integrated on the scale of continental ice sheets. Simulations are presented of the response to glaciation of a large part of the western European groundwater system during the last two (Saalian, Weichselian) glacial cycles. A two-dimensional model along an ice sheet flowline from western Sweden to The Netherlands illustrates the impact of glaciation on flow in the vertical plane, and a vertically integrated model illustrates its impact on areal patterns of flow. Hydraulics heads, hydraulic gradients and flow velocities are increased far above their modern values, and relatively shallow aquifers are completely flushed out during glacial periods. There are significant implications for groundwater chemistry and geological structures. Large seepage pressures generated near to ice sheet margins and major impacts on the distribution of effective pressures will produce structures such as hydrofractures, sediment dykes, sediment volcanoes, loading structures etc. The model can be readily applied to hydrocarbon reservoirs.

The mountainous area of west-central Sweden has a varied glacial morphology and three unique landform assemblages appear to mark areas of different basal thermal conditions during the decay of late Weichselian ice. A glacial geomorphological map was constructed through interpretation of aerial photographs at a scale 1: 60 000. The relative abundance of landforms interpreted to indicate a frozen bed, a thawed bed, and late-frozen to thawed-bed conditions were used for a first-order reconstruction of the basal temperature pattern of the decaying ice sheet. The features used to outline areas with different temperature regimes during deglaciation are glacial scouring, flutings, eskers, meltwater channels, boulder blankets, Rogen moraines, relict surfaces and thermoerosion features. In the northern part of the area,as well as along most of the water divide to the Atlantic Ocean, the ice was warm-based during deglaciation. Glacially scoured bedrock, fluting/ drumlinization, subglacial eskers and extensive glacial lake sediments characterize the area. In the high mountain area, lateral meltwater channels are short and curve downslope to form chutes or eskers. The basal ice conditions changed from frozen-to thawedbed in a late stage of the late Weichselian cycle in most of the southern part of the area, shown by the Rogen moraines on both sides of the ice divide. However, substantial areas in the eastern and southern low mountains had frozen-bed conditions during the complete late Weichselian. In the high mountains in the north, only the summits were frozen.

The mountainous area of west-central Sweden has a varied glacial morphology and three unique landform assemblages appear to mark areas of different basal thermal conditions during the decay of late Weichselian ice. A glacial geomorphological map was constructed through interpretation of aerial photographs at a scale 1:60 000. The relative abundance of landforms interpreted to indicate a frozen bed, a thawed bed, and late-frozen to thawed-bed conditions were used for a first-order reconstruction of the basal temperature pattern of the decaying ice sheet. The features used to outline areas with different temperature regimes during deglaciation are glacial scouring, flutings, eskers, meltwater channels, boulder blankets, Rogen moraines, relict surfaces and thermoerosion features.In the northern part of the area, as well as along most of the water divide to the Atlantic Ocean, the ice was warm-based during deglaciation. Glacially scoured bedrock, fluting/ drumlinization, subglacial eske...

Subaqueous debrites on a fan were recognized at Rzucewo (N Poland). Analysis of debrite fabric enabled debris-flow palaeodirections to be traced on the fan. The long axis azimuth and dip direction of lithic clasts (gravels) as well as soft-sediment clasts (SSC) were measured in the debrites. The results obtained indicate a palaeotransport direction to NNW, similar to the palaeocurrents interpreted from the asymmetrical ripples. However, detailed measurements of both gravels and SSC orientation show variability of palaeoflow directions between NW and NE sectors. In the proximal part of the fan, gravels indicate variable dip directions and dip angles, and a mostly scattered distribution of a-axis orientation. In the medial and distal parts of the fan, clasts are better ordered and usually their a-axes are oriented upslope or transverse to the palaeoslope inclination direction. Probably during the decelerating and halting of debris flows, the compression processes induced clast rotation and a change of inclination. In the distal part of the fan, debrites reveal a distinct variability of palaeoflow directions caused by flattened topography in which debris-flow lobes split and spread freely in different directions. Finally, we conclude that the individual sets of clast fabric usually indicate only local debris-flow directions.

Sliding of glacial ice over its base is typically the dominant flow mechanism of glaciers and ice sheets. The term "sliding" is often used in a loose sense to include the deformation of "soft" till beds and the differential motion between basal ice and underlying bedrock. The amount of sliding controls fast (Clarke, 1987) and slow (Maier et al., 2019) ice flow regimes. Sliding variations are powerful enough to halt, or even reverse, ice stream flow (Conway et al., 2002) or rupture ice tongues, leading to massive ice avalanches (Faillettaz et al., 2015; Kääb et al., 2018). Nevertheless, sliding mechanisms remain elusive, leading to uncertain predictions of ice sheet motion and stability, and ultimately sea level rise (

ABSTRACTWe evaluate the variability in basal friction for Kronebreen, Svalbard, a fast-flowing tidewater glacier. We invert 3 years (2013–15) of surface velocities at high temporal resolution (generally 11 days), to estimate the changing basal properties of the glacier. Our results suggest that sliding behaviour of Kronebreen within a year is primarily influenced by changes in water input patterns during the meltwater season and basal friction is highly variable from a year to another. At present, models usually employ parameterisations to encompass the complex physics of glacier sliding by mathematically simulate their net effect. For such ice masses with strong seasonal variations of surface melt, the spatio-temporal patterns of basal friction imply that it is neither possible nor appropriate to use a parameterisation for bed friction that is fixed in space and/or time, at least in a timescale of a few years. Basal sliding may not only be governed by local processes such as basal ...

Over large areas of the western interior plains of North America, hummocky moraine (HM) formed at the margins of Laurentide Ice Sheet (LIS) lobes that flowed upslope against topographic highs. Current depositional models argue that HM was deposited supraglacially from stagnant debris-rich ice ('disintegration moraine'). Across southern Alberta, Canada, map and outcrop data show that HM is composed of fine-grained till as much as 25 m thick containing rafts of soft, glaciotectonized bedrock and sediment. Chaotic, non-oriented HM commonly passes downslope into weakly-oriented hummocks ('washboard moraine') that are transitional to drumlins in topographic lows; the same subsurface stratigraphy and till facies is present throughout. These landforms, and others such as doughnut-like 'rim ridges', flat-topped 'moraine plateaux' and linear disintegration ridges, are identified as belonging to subglacially-deposited soft-bed terrain. This terrain is the record of ice lobes moving over deformation till derived from weakly-lithified, bentonite-rich shale. Drumlins record continued active ice flow in topographic lows during deglaciation whereas HM was produced below the outer stagnant margins of ice lobes by gravitational loading ('pressing') of remnant dead ice blocks into wet, plastic till. Intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show 'hybrid' drumlins whose streamlined form has been altered by subglacial pressing ('humdrums') below dead ice. The presence of hummocky moraine over a very large area of interior North America provides additional support for glaciological models of a soft-bedded Laurentide Ice Sheet.

The manuscript "Subglacial drainage characterization from eight years of continuous borehole data on a small glacier in the Yukon Territory, Canada" by Rada and Schoof, describes and interprets an eight year dataset of observations from dozens of boreholes on a small mountain glacier in the Yukon. The dataset is extremely rich and shows complexity at diurnal and seasonal temporal scales at both local and regional spatial scales. The boreholes show support for the standard model of subglacial hydrology of an inefficient distributed system punctuated by an efficient channelizing system of limited spatial extent. However, the data also provide strong evidence for discon-

The subglacial drainage system is one of the main controls on basal sliding, but remains only partially understood. Here we use an 8-year dataset of borehole observations on a small, alpine polythermal valley glacier in the Yukon Territory to assess qualitatively how well the established understanding of drainage physics explains the observed temporal evolution and spatial configuration of the drainage system. We find that the standard picture of a channelizing drainage system that evolves towards higher effective pressure explains many features of the dataset. However, our dataset underlines the importance of hydraulic isolation of parts of the bed. We observe how disconnected portions of the bed systematically grow towards the end of the summer season, causing the drainage system to fragment into progressively more distinct subsystems. We conclude with an adaptation of existing drainage models that aims to capture the ability of parts of the bed to become hydraulically disconnected due to basal cavities of finite size becoming disconnected from each other as they shrink.

The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations.

The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations.

The study provides a morphologic classification of eskers formed on a soft bed in Poland during the Saalian and Elsterian Glaciations. The classification is based on morphometry, including esker fragmentation, length, sinuosity, ridge elongation and presence of tributary ridges. Five esker types and a total of ten subtypes were distinguished. The most common esker types to the south of the LGM in Poland include 1acontinuous, short, straight esker ridges, 1bcontinuous, short, sinuous esker ridges, and 4csegmented, long, sinuous eskers. The eskers were formed in subglacial N-, R-, N-R and open channels. A synchronous model of esker formation was dominant, but some time-transgressively formed eskers consisting of beads also occur. The analysed eskers have common features with eskers on a hard bed and some of them originated in a similar way, but many eskers exhibit dissimilarity due to different conditions of their formation. The study shows that current esker morphogenetic classifications need to be extended to also include the character of eskers on soft beds.

Interpreting past glacial dynamics from the glacial record requires that the depositional environments of glacial sediments and landforms be understood. In the case of interlobate deposits, models that incorporate various components of pro, supra and subglacial deposition have been developed and tested in the northern Kettle Moraine (nKM), Wisconsin; a large interlobate deposit that formed between the Green Bay and Lake Michigan lobes of the Laurentide Ice Sheet during the last deglaciation. In this paper, we interpret a new genesis for the nKM using sediment analysis and distribution along with landform distribution. In Sheboygan County, the nKM consists of two steep-sided, high-relief, hummocky ridges separated by a low elevation and lowrelief central axis. Gravel in the bounding hummocky ridges is well-sorted and well-rounded. Some bedding is collapsed. Large, isolated moulin kames are restricted to the axis area and composed of relatively poorly sorted, more angular gravel and diamicton. The distribution of these different sediments and landforms are explained by the accumulation of supraglacial debris that insulated the ice below the axis of the nKM, while the melting of cleaner ice on either side formed channels on the ice surface. As deglaciation proceeded, a substantial thickness of well-rounded, stream-deposited sand and gravel accumulated on ice in the bounding channels. Eventual collapse of this sediment formed the two hummocky ridges. Poorly sorted debris along the axis fell and slid into moulins and larger collapse areas in the ice. Thus, differential debris insulation and ice ablation controlled the mainly supraglacial deposition of this part of the nKM.

We present new glacier mass-balance field data from Glaciar Bahía del Diablo, Vega Island, northeastern Antarctic Peninsula. The results provided here represent glacier mass-balance data over a 10 year period (2001–11) obtained by the glaciological and geodetic methods relying on field measurements. Glacier surface digital elevation models (DEMs) were obtained in 2001 and 2011 from a kinematic GPS field survey with high horizontal and vertical accuracies. In situ mass-balance data were collected from yearly stake measurements. The results attained by the two methods agree, which may be considered a measure of their accuracy. A cumulative mass change of –1.90 ± 0.31 m w.e. over the 10 year period was obtained from the annual mass-balance field surveys. The total mass change derived from DEM differencing was –2.16 ± 0.23 m w.e.

Antarctic mass balance and contribution to sea level rise are dominated by the flow of ice through narrow conduits called ice streams. These regions of relatively fast flow drain over 90% of the ice sheet and generate significant amounts of frictional heat at the ice stream margins where there is a transition to slow flow in the ridge. This heat can generate temperate ice and a sharp transition in flow speed between the stream and the ridge. Within zones of temperate ice, meltwater is produced and drains to the bed. Here we model the downstream development of a temperate zone along an ice stream shear margin and the flow of meltwater through temperate ice into a subglacial hydrologic system. The hydrology sets the basal effective pressure, defined as the difference between ice overburden and water pressure. Using the southern shear margin of Bindschadler Ice Stream as a case study, our model results indicate an abrupt transition from a distributed to channelized hydrologic system wi...

Coastal valleys in the west part of Mid-Wales, such as the Mawddach, Dysynni, Tal-y-llyn and Dyfi, acted as corridors for ice which drained the Welsh Ice Cap during the Devensian. Analyses of detailed digital elevation models, and interpretation of satellite images and aerial photographs, show the existence of large variations in the amount of glacial modification between these valleys. Although all the valleys are glacially over-deepened along late Caledonian fault lines, only the Dyfi basin exhibits a dendritic pattern, with Vshaped cross-profiles and valley spurs typical of valleys formed by fluvial processes. Connectivity analysis of the Dyfi basin shows that it exhibits an almost completely dendritic pattern with connectivity and values of 0.74 and 1.01, respectively, with little glacial modification of the preglacial fluvial valley pattern in the form of glacial valley breaching. Several examples of glacial meltwater incision into a well-developed pre-existing river valley system, causing river capture across watersheds, have been identified in the Dyfi basin. The degree of preservation of the preglacial fluvial valley system within the Dyfi basin indicates limited modification by glacial processes, despite the area being subjected to glacier activity during the Late Devensian at least. It is possible that major parts of the basin were covered by cold-based or slow-moving ice, close to, or under, a migrating ice-divide, with the major ice drainage occurring along the weaker zone of the Pennal Fault along which teh Dyfi valley is located, causing minor adjustments to the surrounding interfluves and uplands. It is proposed here that the general river valley morphology of the Dyfi basin is of a preLate Devensian age.

Subglacial hydrology plays an important role in the ice sheet dynamics as it determines the sliding velocity of ice sheets and also drives freshwater into the ocean. Modeling subglacial water has been a challenge for decades, and only recently new approaches have been developed such as representing subglacial channels and thin water sheets by separate layers of variable permeability. We extend this concept by modeling a confined and unconfined aquifer system (CUAS) in a single layer. The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We also performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms governing the opening and closure of channels. Furthermore, we applied the model to the North East Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model in the Ice Sheet System Model (ISSM) shows considerable improvements of modeled velocities in the coastal region. 1 Introduction Subglacial water has been identified as a key component in glacial processes, it is fundamental in driving large ice flow variations over short time periods. Recent studies show considerable progress in modeling these subglacial networks and coupling them to ice models. Water pressure strongly influences basal sliding and can therefore be considered a fundamental control on ice velocity and ice-sheet dynamics. Generally, two fundamentally different types of drainage are identified: discrete channel / conduit systems and distributed water sheets or thin films. Distributed flow mechanisms are, for example, linked cavities (Lliboutry, 1968), flows through sediment/till (Hubbard et al., 1995), or thin water sheets (Weertman, 1957); those are considered to be an inefficient and slow system to transport water. Channels (Rothlisberger, 1969; Shreve, 1972; Nye, 1976) are seen as discrete single features or arborescent networks. It is assumed that these channelized or efficient drainage systems able to drain large amounts of water 1

Predicting how climate change will affect glacier and ice sheet flow speeds remains a large hurdle toward accurate sea level rise forecasting. Increases in surface melt rates are known to accelerate glacier flow in summer, whereas in winter, flow speeds are believed to be relatively invariant. Here we show that wintertime flow speeds on nearly all major glaciers throughout Alaska are not only variable but are inversely related to melt from preceding summers. For each additional meter of summertime melt, we observe an 11% decrease in wintertime velocity on glaciers of all sizes, geometries, climates, and bed types. This dynamic occurs because interannual differences in summertime melt affect how much water is retained in the subglacial system during winter. The ubiquity of the dynamic indicates it occurs globally on glaciers and ice sheets not frozen to their beds and thus constitutes a new mechanism affecting sea level rise projections.

Sea-level rise constitutes a significant risk for over 600 million people in the Low-Elevation Coastal Zone. Considerable uncertainty exists over the magnitude of possible future sea-level rise, because of poorly understood processes governing the stability of ice sheets (continental sized glaciers). One such uncertainty is how meltwater interacts with ice under a warming climate. Understanding of this process is limited by the inaccessibility of the subglacial zone, which lies beneath 100s or even 1000s of metres of ice. One approach to address this uncertainty is to investigate areas where ice sheets have retreated, i.e., where their beds are easily accessible. Eskers are landforms that record the location and dimensions of former subglacial meltwater channels, and are common in glaciated regions. Recent years have seen a dramatic increase in the availability of high-resolution Digital Elevation Models (DEMs) of glaciated regions, providing the opportunity to make detailed measurements of eskers from remotely sensed data. Manual mapping of these features at the required level of detail is not feasible over the large areas occupied by palaeo-ice sheets (e.g. most of Canada). We propose an automated method for detecting eskers in hillshaded digital elevation models, based on Convolutional Neural Networks (CNN). The automated method maps esker locations to facilitate detailed morphometric study of their form. Multiple CNN models are trained and tested via a specially-designed algorithm with built-in mechanism for selecting an optimal model. Training and testing imagery data were obtained from a test area in Canada, consisting of 1041 esker positive JPEG files and 37000 esker negative JPEG files. The CNN model performance on previously unseen images with and without eskers yields high sensitivity and specificity respectively and we use the model outputs to elicit esker features from the images. Discussions focus on how timely identifying esker locations enhance our understanding of why, how, and how fast the sea level rise might happen. We also highlight the importance of gaining such knowledge in a timely manner within the context of the United Nations Sustainable Development Goals (SDGs)-particularly SDG #13 and others relating to poverty and food security.

Sediments beneath modern ice sheets exert a key control on their flow, but are largely inaccessible except through geophysics or boreholes. In contrast, palaeo-ice sheet beds are accessible, and typically characterised by numerous bedforms. However, the interaction between bedforms and ice flow is poorly constrained and it is not clear how bedform sizes might reflect ice flow conditions. To better understand this link we present a first exploration of a variety of statistical models to explain the size distribution of some common subglacial bedforms (i.e., drumlins, ribbed moraine, MSGL). By considering a range of models, constructed to reflect key aspects of the physical processes, it is possible to infer that the size distributions are most effectively explained when the dynamics of ice-water-sediment interaction associated with bedform growth is fundamentally random. A 'stochastic instability' (SI) model, which integrates random bedform growth and shrinking through time w...

Triangular hummocks of subglacial origin have been identified in Sweden and Finland due to the increased resolution provided by LiDAR imagery. Their triangular shape is distinctive and recognizable as clearly identifiable landforms. These forms have been previously mapped in some cases as dead-ice hummocks, but geomorphic relationships with eskers, flutes ribbed moraine and De Geer moraines show these to be subglacial. We refer to these new landforms as 'murtoos.' Morphometric measurements show murtoos to be 50 to 200 m long and 50 to 100 m wide. The orientation of their apices strongly correlates with local ice-flow orientation. They form preferentially on beds that slope down-ice. In many cases, they occur in patches in an ice-flow parallel path with eskers, defining corridors we believe to be of subglacial meltwater origin. Murtoos are composed primarily of heterogeneous diamicton with variable amounts of bedded sand and gravel. Murtoos are most common where glacier-melt rates were high during deglaciation (Bølling-Allerød and Holocene), and they are absent where extensive frozen-bed conditions were present. We suggest murtoos are a landform produced as a glacier-bed adjustment to increased delivery of supraglacial meltwater during deglaciation.

Triangular hummocks of subglacial origin have been identified in Sweden and Finland due to the increased resolution provided by LiDAR imagery. Their triangular shape is distinctive and recognizable as clearly identifiable landforms. These forms have been previously mapped in some cases as dead-ice hummocks, but geomorphic relationships with eskers, flutes ribbed moraine and De Geer moraines show these to be subglacial. We refer to these new landforms as 'murtoos.' Morphometric measurements show murtoos to be 50 to 200 m long and 50 to 100 m wide. The orientation of their apices strongly correlates with local ice-flow orientation. They form preferentially on beds that slope down-ice. In many cases, they occur in patches in an ice-flow parallel path with eskers, defining corridors we believe to be of subglacial meltwater origin. Murtoos are composed primarily of heterogeneous diamicton with variable amounts of bedded sand and gravel. Murtoos are most common where glacier-melt rates were high during deglaciation (Bølling-Allerød and Holocene), and they are absent where extensive frozen-bed conditions were present. We suggest murtoos are a landform produced as a glacier-bed adjustment to increased delivery of supraglacial meltwater during deglaciation.

The aim of this study is to present the first evidence of triangular-shaped till landforms and related erosional features indicative of subglacial drainage within the ice stream bed of the Scandinavian ice sheet in Finland. Previously unidentified grouped patterns of Quaternary deposits with triangular landforms can be recognized from LiDAR-based DEMs. The triangular landforms occur as segments within geomorphologically distinguishable routes that are associated with eskers. The morphological and sedimentological characteristics as well as the distribution of the triangular landforms are interpreted to involve the creep of saturated deforming till, flow and pressure fluctuations of subglacial meltwater associated with meltwater erosion. There are no existing models for the formation of this kind of large-scale drainage systems, but we claim that they represent an efficient drainage system for subglacial meltwater transfer under high pressure conditions. Our hypothesis is that the routed, large-scale subglacial drainage systems described herein form a continuum between channelized (eskers) and more widely spread small-scale distributed subglacial drainage. Moreover, the transition from the conduit dominated drainage to triangular-shaped subglacial landforms takes place about 50e60 km from the ice margin. We provide an important contribution towards a more realistic representation of ice sheet hydrological drainage systems that could be used to improve paleoglaciological models and to simulate likely responses of ice sheets to increased meltwater production.

Yang et al.: A new surface meltwater routing model flow occurs even in the bare-ice ablation zone. Together, both interfluves and open channels combine to modulate the timing and discharge of meltwater reaching IDC outlet moulins, prior to further modification by englacial and subglacial processes.

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.

Figure 6 shows the PC2 score distribution for a time transgressive (fluctuating flow) drumlin flow set in the northeast of the study area. Subglacial bedform morphology indicates general ice flow direction to the northwest. Bedrock in the flow set area is dominated by basalt with a rhyolite body situated in the southeast. Lowest scores (blue) are associated with the elements Rb, Th, Ce and La and can be seen extending from the rhyolite to the west and also approximately 8 km to the northwest , in alignment with the ice flow direction indicated by bedform morphology. Principal component analysis (PCA) has been applied to between 23 and 27 elements for a total of 5729 soil samples that occur on till in the study area, both at regional scale and in targeted areas based on drumlin flow sets (Fig.1b). PCA assigns each element a loading value which is used to calculate scores for each sample point. It follows that if a sample point is dominated by high loading elements it will have a high score, and if low loading elements are dominant it will return a low score. Sample point scores are therefore representative of groups of elements that can be linked to bedrock source areas. The scores are mapped to assess the degree and direction of subglacial transport. 2. METHOD 3. RESULTS SAMPLE SELECTION Samples from areas of till superficial geology DATA PREPARATION Organic soils removed, element selection, normalisation

Triangular hummocks of subglacial origin have been identified in Sweden and Finland due to the increased resolution provided by LiDAR imagery. Their triangular shape is distinctive and recognizable as clearly identifiable landforms. These forms have been previously mapped in some cases as dead-ice hummocks, but geomorphic relationships with eskers, flutes ribbed moraine and De Geer moraines show these to be subglacial. We refer to these new landforms as 'murtoos.' Morphometric measurements show murtoos to be 50 to 200 m long and 50 to 100 m wide. The orientation of their apices strongly correlates with local ice-flow orientation. They form preferentially on beds that slope down-ice. In many cases, they occur in patches in an ice-flow parallel path with eskers, defining corridors we believe to be of subglacial meltwater origin. Murtoos are composed primarily of heterogeneous diamicton with variable amounts of bedded sand and gravel. Murtoos are most common where glacier-melt rates were high during deglaciation (Bølling-Allerød and Holocene), and they are absent where extensive frozen-bed conditions were present. We suggest murtoos are a landform produced as a glacier-bed adjustment to increased delivery of supraglacial meltwater during deglaciation.

We apply cross-correlation to Pléiades satellite images to generate accurate, high-resolution monthly surface velocity maps of Monte Tronador glaciers between March and June 2012. Measured surface displacements cover periods as short as 19 days, with a precision of ∼0.58 m (11 m a−1). These glaciers follow a radial flow pattern, with maximum surface speeds of ∼390 m a−1 associated with steep icefalls. The lower reaches of the debris-covered tongues of Verde and Casa Pangue glaciers are almost stagnant, whereas Ventisquero Negro, another debris-covered glacier, shows acceleration at the front due to calving into a proglacial lake. Low-elevation debris-covered glacier tongues show increasing velocities at the beginning of the accumulation season, whereas higher-elevation, clean-ice tongues reduce their speed during this period. This contrasting behavior is probably in response to an increase in water input to the subglacial system from winter rainfall events at low elevations and a de...

Retreat of Mendenhall Glacier nearJuneau, Alaska, U.S.A., has exposed a bedrock ridge spotted with “siltskins”, patchy coatings of calcite-cemented clay-to sandsized lithic grains. Coatings 0.5–20 mm thick occur in two distinct morphologies. Thin, striated siltskins coat mainly stoss faces. Thicker, corrugated siltskins on lee faces consist of parallel micro-ridges elongated downslope. Thin-section analysis shows that siltskins consist of a basal, calcite-rich layer overlain by microlaminated layers of calcite-cemented lithic grains. Scanning electron microscope (SEM) energy-dispersive spectrometer (EDS) analysis of laminae and surfaces shows laterally persistent Ca/Si differences. Isotopic values for δO18 and δO13 ranged from −19.52‰ to −12.74‰ and −6.18‰ to −3.44‰, respectively, consistent with deposition from subglacial waters of varying isotopic composition and with derivation of carbon from inorganic sources. Corrugated siltskins are complex depositional features modified by er...

The subglacial drainage system is one of the main controls on basal sliding, but remains only partially understood. Here we use an 8-year dataset of borehole observations on a small, alpine polythermal valley glacier in the Yukon Territory to assess qualitatively how well the established understanding of drainage physics explains the observed temporal evolution and spatial configuration of the drainage system. We find that the standard picture of a channelizing drainage system that evolves towards higher effective pressure explains many features of the dataset. However, our dataset underlines the importance of hydraulic isolation of parts of the bed. We observe how disconnected portions of the bed systematically grow towards the end of the summer season, causing the drainage system to fragment into progressively more distinct subsystems. We conclude with an adaptation of existing drainage models that aims to capture the ability of parts of the bed to become hydraulically disconnected due to basal cavities of finite size becoming disconnected from each other as they shrink.

The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations.

Consecutive phases of de-icing of ice-cored moraines and the formation of dead-ice moraine were monitored over a 4-year period at the terminus of the Ko È tlujo È kull glacier, Iceland. Particularly, the transition from partially ice-cored moraine with isolated dead-ice blocks to the ice-free landscape receives attention in this paper in order to link the ®nal melting processes to the architecture of the sedimentary end product. In the current humid sub-polar climate of south Iceland de-icing of partially ice-cored moraines results chie¯y from melting along the bottom surface of ice-cores with an annual average rate of 25 cm. The ®nal de-icing is associated with an interrelated group of re-sedimentation processes and surface features. Series of sinkholes evolve at the toe of dead-ice blocks, which initiate retrogressive rotational sliding or backslumping of the ice-cored slopes and the formation of distinct edges and fractures in the adjacent basins. Although backslumping is the dominant process in this phase of re-sedimentation, structures resulting from this process are rarely recognized in the ice-free landscape. As ice-cores gradually diminish the effect of the latest re-sedimentation events will overprint or destroy most existing sedimentary characteristics. Thus, in the ice-free landscape, structures mainly related to the formation of sinkholes and fractures remain imprinted on the sediment succession. Generally, no inversion of the topography occurs during the ®nal phase of de-icing. The overall topography recognized in the late phase of the fully ice-cored terrain is merely lowered and the amplitude of the relief reduced as de-icing progresses. The sediment architecture of the icefree landscape is characterized by heterogeneous and often slumped diamict sediments with variable thickness and lateral distribution; clast orientation is related to the direction of slopes, and boulders are found in isolated groups or in linear arrangements.

Recent studies of subglacially precipitated carbonate deposits and associated solutional furrows have provided interesting new insight on subglacial water films, as well as on chemical exchange at the glacier bed. Considerable information on the film thickness and its temporal and spatial variability has been gained by analyzing several properties of subglacial carbonate deposits including: (1) the morphology of surface features aligned parallel to ice flow, (2) the laminated structure, and (3) the size distribution of fine rock fragments presumably transported in the film prior to their incorporation in the deposits. Chemical analyses of water from pro-glacial streams, together with calculations of CaCO3solubility and mass balance, show that the channelized water is chemically distinct from the film water in which CaCO3precipitates, and that subglacial precipitation is not possible where there is a considerable water flux through the film in excess of that associated with regelatio...

Deglaciated bedrock surfaces in limestone areas often exhibit extensive patterning by solutional furrows and carbonate deposits that occur in close association with undulations in the bed topography. These features clearly result from subglacial dissolution and precipitation of calcite on the bed — induced, for instance, by melting and freezing in a regelation water film — but little is known about the observed morphology. In particular, it is intriguing that (i) the solutional furrows, whose formation requires explanation, are collectively organized into arcuate patterns, with characteristic spacing, and (ii) a fluted or “spiculed” surface texture is ubiquitous on the calcite deposits. Herein, we propose specific mechanisms for such patterning based on a theory where chemical processes in the water film are coupled to regelation physics. Solutional furrows reflect locally enhanced dissolution along stoss surfaces, where CO2-rich bubbles advected in the ice from up-glacier come into...

We develop theoretically a description of a possible subglacial drainage mechanism for glaciers and ice sheets moving over saturated, deformable till. The model is based on the plausible assumption that flow of water in a thin film at the ice-till interface is unstable to the formation of a channelized drainage system, and is restricted to the case in which meltwater cannot escape through the till to an underlying aquifer. In describing the physics of such channelized drainage, we have generalized and extended Röthlisberger’s model of channels cut into basal ice to include “canals” cut into the till, paying particular attention to the role of sediment properties and the mechanics of sediment transport. We show that sediment-floored Röthlisberger (R) channels can exist for high effective pressures, and wide, shallow, ice-roofed canals cut into the till for low effective pressures. Canals should form a distributed, non-arborescent system, unlike R channels. For steep slopes typical of...

Recent studies of subglacially precipitated carbonate deposits and associated solutional furrows have provided interesting new insight on subglacial water films, as well as on chemical exchange at the glacier bed. Considerable information on the film thickness and its temporal and spatial variability has been gained by analyzing several properties of subglacial carbonate deposits including: (1) the morphology of surface features aligned parallel to ice flow, (2) the laminated structure, and (3) the size distribution of fine rock fragments presumably transported in the film prior to their incorporation in the deposits. Chemical analyses of water from pro-glacial streams, together with calculations of CaCO3solubility and mass balance, show that the channelized water is chemically distinct from the film water in which CaCO3precipitates, and that subglacial precipitation is not possible where there is a considerable water flux through the film in excess of that associated with regelatio...

R ece nt studies of subglacially precipitated carbonate deposits and associated solutiona l furrows have prov ided inte rest ing new insight on subglacial water films, as well as on chemical exchange at the glacier bed. Considerable informa tion on the film thi ckn ess and its temporal and spatial variability h as been gai ned by analyzing several properties of subglacial carbonate deposits including: (I) the m o rphology of surface features aligned parallel to ice flow, (2) the laminated stru cture, and (3) the size distribution of fin e rock fragm ents presumably tra nsported in the film prior to their incorporation in the d epos its. Chemical analyses of water from pro-glacial streams, together with calculations of CaC0 3 solub ility and mass balance, show tha t the cha nnelized water is chemically distinct from the film water in which Ca CO] precipita tes, and that subglacial precipitat ion is not possible where there is a considerable water flux through the film in excess of that associated with regela tion slidi ng. The principal implication of these studies is th a t a temperate cirq ue glacier is cha racteristically separated fro m its bed by a thin wate r film, probably m icrom eters in thickness; however, the film appears to occasionally thicken, at least locally b y as much as a hundred fold in exceptional cases. Furthermore, the water flux and /or solute concent ration in the basal film undergoes periodic, probably seasonal, variations possibl y rela ted to varia tions in the amount of water reaching and flowing through the basal film. R EsuME. Film d'eall de regel sOlls-glaciaire. Des etudes rece ntes d e preci pitation sous glaciaire de calcite et d es sillons de dissoluti o n qui leur son t associes ont apporte d'interessants nou veaux a pen;:us sur les films liquides sous-gl aciaires, comm e sur les echanges chimiques a u ni veau du lit glaciaire. Des informations importantes sur I'epaisseur des fi lms, et le ur variabilite tem p o rell e et spatiale ont ete acquises par I' analyse d e plusieurs proprietes d es calci tes sous gla ciaires. Pa rmi el les, citons: (1) la morphologie des caracteres superficiels align es para ll i:lement it l'ecoulement de la glace, (2) la structure laminaire et (3) la distribution d es dimensions des fin s fr agments de roche que I' on presume e tre tra nsportes par le film avant leur incorporation dans les depots. L es a nal yses chimiques des eaux des e missaires proglaciaires, ainsi que les calculs de la solubilite et du bilan d e masse de CaCO, montrent que cette eau est chimiquement differente de I'eau du film dans laquelle CaC0 3 pn:cipite, et que la precipitation sou s glaciaire n'est pas p ossible la ou il y a un flux considera ble d'eau transportee par le fi lm en sus de celle liee avec gl issement p ar regel. La principale concl usion it tirer des e tudes sous-glaciaires est qu'un petit glacier de cirque est typiquement separe d e son lit par un mince film d' eau, d'epaisseur p eut-etre micrometrique; cepend an t le film apparaJt occasionnellem e nt epaissi au moins localement, jusqu ' il cent foi s da ns certa ins cas exceptionnels, peut-etre pour evacuer les debits variables d' eau d e percolation du glacier. De plus, le d ebit liquide e t /ou la teneur en matiere dissoute du film liquide du fond presentent des variations p e riodiques, peut-Nre saiso nnii:res, que I'on peut relier aux variatio ns d es quantites d'eau atteignant le film au fond par percolation il travers la glace. ZUSAMMENFASSUN G. Wasserjilme allS subglazialer R egelation. Neuere Studien an subglazial ausgefallten Calziten und damit verbundene U :isungsfurchen haben ne u e, interessante Einsichten uber subglaziale Wasserfilme und liber d e n chemischen Austausch a m Gletscherbett vermittelt. Wesentliche Informa tione n liber die Filmdicke und deren zeitli che und raumliche V erande rbarkeit wurden aus der Analyse ei niger Eigenschaft en subgla zialer Calzite gewonnen, darunter: (I) die Morphologi e von Oberflachenerscheinungen, die parallel zum Eisfluss gerichtet sind, (2) die Blatterstruktur, (3) die G rossenverteilung kleiner Felsfragmente, die vermutlich vor ihrer Ablagerung im Film transportiert wurden. Chemische Analysen des Wassers in Vorfeldstromen zeige n zusammen mit Be rechnungen der L6sbarkeit von CaCO, und der M asse nbilanz, dass das \"'asser che misch verschieden ist vom Filmwasser, in dem CaCO, gefallt wird, und dass subglaziale AusfUllung nicht stattfinden ka nn, we nn ein betrachtli che r Wasserfluss uber j e nen hinaus, der mit dem R egelationsgleiten verbunden ist, durch den Film vorhanden ist. Aus dem Studium der subglazialen Prozesse [olgt vor allem, dass ein kleiner Kargletsch er charakteristisch von seinem Bett durch einen dunnen Wasserfilm, vielleicht nur Mikrometer dick, getrennt ist; der Film sch eint sichjedoch gel egentlich, zumindest lokal, in Ausnahmefiill en bis zum Hundertfachen zu verstarken, vielleicht urn den Durchfluss unterschiedlich er Wassermengen zu erm6glichen. Des weiteren erfiihrt der Wasserfluss und/oder die Losungskonzentration im Film per iodisch e, vielleicht jahreszeitliche Schwa nkunge n, die moglicherweise mit den Schwankungen der Wassermenge, die d e n Film am Untergrund durch Sickerung durch das Eis erreicht, in Beziehung stehen.

Shuttle radar topography data are used to construct a digital elevation model for the Island of Newfoundland. The model provides a new means of visualizing the landscape of the Island, and allows many large-scale glacial features to be identified. Use of these images in interpreting surficial geology is demonstrated using examples from the Northern Peninsula, St. George's Bay, northeast Newfoundland and the Avalon Peninsula. The model is particularly useful for identifying large-scale oriented landforms such as flutings, drumlins and crag-and-tail features. Preliminary mapping of these features offers insights into the ice-flow patterns and glacial history of Newfoundland.

In the northern half of the Walker Lake map area, three glaciofluvial landforms  eskers, transverse ridges, and irregular mounds, were identified, interpreted, and related to the deglacial history. Two types of eskers are present. Type-A eskers cut across topography and formed early during deglaciation, representing englacial channels. The larger type-B eskers follow major valleys and formed throughout deglaciation, representing subglacial and/or supraglacial channels. Irregular mounds and transverse ridges formed in the later stages of deglaciation and represent accumulations of material along topographic lows on the surface of the ice sheet that correspond to the major valleys in the area. This integrated hypothesis explains the distribution of the eskers and the fact that the irregular mounds and transverse ridges occur in linear zones that are commonly associated with eskers. Current Research 2002-C16 1 D.J. Utting et al. Résumé : Dans la moitié nord de la région cartographique de Walker Lake, trois formes de terrain fluvioglaciaires (des eskers, des crêtes transversales et des monticules irréguliers) ont été relevées, interprétées et associées à l'évolution de la déglaciation. Deux types d'eskers sont distingués. Les eskers de type A sont discordants par rapport au relief; leur formation remonte au début de la déglaciation et ils constituent la trace de chenaux intraglaciaires. Les eskers de type B suivent les vallées principales; ils sont apparus tout au long de la période de déglaciation et ils représentent des chenaux sous-glaciaires ou supraglaciaires. Des crêtes transversales et des monticules irréguliers se sont formés vers la fin de la déglaciation par accumulation de matériaux dans des dépressions à la surface de la nappe glaciaire qui coincident avec les principales vallées de la région. Cette hypothèse intégrée explique la répartition des eskers et le fait que les crêtes transversales et les monticules irréguliers se présentent dans des zones linéaires qui sont associées généralement aux eskers.