Esker Systems Research Articles

Geographic object‐based image analysis (GEOBIA) of the distribution and characteristics of aeolian sand dunes in Arctic Sweden

AbstractCurrent climate change in the Arctic is unprecedented in the instrumental record, with profound consequences for the environment and landscape. In Arctic Sweden, aeolian sand dunes have been impacted by climatic changes since their initial formation after the retreat of the last glacial ice sheet. Dune type, location and orientation can therefore be used to explore past wind patterns and landscape destabilisation in this sensitive area. However, knowledge of the full spatial extent and characteristics of these dunes is limited by their inaccessibility and dense vegetation cover. Geographic object‐based image analysis (GEOBIA) permits the semi‐automatic creation of reproducible parameter‐based objects and can be an appropriate means to systematically and spatially map these dunes remotely. Here, a digital elevation model (DEM) and its derivatives, such as slope and curvature, were segmented in a GEOBIA context, enabling the identification and mapping of aeolian sand dunes in Arctic Sweden. Analysis of the GEOBIA‐derived and expert‐accepted polygons affirms the prevalence of parabolic dune type and reveals the coexistence of simple dunes with large coalesced systems. Furthermore, mapped dune orientations and relationships to other geomorphological features were used to explore past wind directions and to identify sediment sources as well as the reasons for sand availability. The results indicate that most dune systems in Arctic Sweden were initially supplied by glaciofluvial and fluvial disturbances of sandy esker systems. Topographic control of wind direction is the dominant influence on dune orientation. Further, our approach shows that analysing the GEOBIA‐derived dune objects in their geomorphological context paves the way for successfully investigating aeolian sand dune location, type and orientation in Arctic Sweden, thereby facilitating the understanding of post‐glacial landscape (in)stability and evolution in the area.

Distribution, characteristics and formation of esker enlargements

Eskers are primarily ridges of glaciofluvial sediment deposited in subglacial, englacial and supraglacial conduits. They are typically straight to sinuous features, however, their planform morphology can be highly diverse. Esker enlargements are spatially confined ridge sections that are significantly wider than the trunk ridge (typically 250–400 m) and that reconverge downflow. The enlargements include complex ridge networks or coherent sediment bodies. We mapped >1400 esker enlargements across Fennoscandia and Keewatin, Canada, to investigate their distribution and morphological characteristics. Esker enlargements are less abundant below the marine limit, and tend to become more abundant in areas of faster ice retreat. They form local clusters along particular ridges, and can occasionally be traced laterally between adjacent esker systems. Based on morphological observations, we link their formation to roof collapses in subglacial conduits. The distribution of esker enlargements indicates that subglacial conduit collapse became an increasingly significant process during the final stages of deglaciation of both the Scandinavian and Laurentide ice sheets, and may have exerted a positive feedback on ice sheet retreat at land-terminating ice margins.

Evidence for glacial geological controls on the hydrology of Maine (USA) peatlands

Abstract Freshwater pools commonly form eccentric crescent patterns in peatlands, an important atmospheric methane (CH4) source, and show an apparent spatial association with eskers in some deglaciated regions. However, the role of underlying permeable glacial deposits such as eskers in regulating hydrogeology, and perhaps even carbon cycling, in peatlands is rarely considered. In this study, ground-penetrating radar imaging and direct coring confirmed that clustered pools coincide with buried esker crests in contact with peat soil in Caribou Bog and Kanokolus Bog in Maine (USA). Hydraulic head and geochemical data combined with lidar indicate vertical water flow from shallow peat toward the permeable esker crests, suggesting enhanced downward transport of labile organic carbon that presumably accelerates rates of methanogenesis in deep peat. Eskers might therefore serve as proxies for enhanced CH4 production in deep peat, as supported by differences in dissolved CH4 profiles depending on proximity to pools. Geographic data compiled from multiple sources suggest that many peatlands with eccentric pools appear to be located proximal to esker systems in Maine and Fennoscandia. These geological factors may be important, previously unrecognized controls on water and the carbon cycle in peatlands.

Equifinality and preservation potential of complex eskers

Eskers are useful for reconstructing meltwater drainage systems of glaciers and ice sheets. However, our process understanding of eskers suffers from a disconnect between sporadic detailed morpho‐sedimentary investigations of abundant large‐scale ancient esker systems, and a small number of modern analogues where esker formation has been observed. This paper presents the results of detailed field and high‐resolution remote sensing studies into two esker systems that have recently emerged at Hørbyebreen, Svalbard, and one at Breiðamerkurjökull, Iceland. Despite the different glaciological settings (polythermal valley glacier vs. active temperate piedmont lobe), in all cases a distinctive planform morphology has developed, where ridges are orientated in two dominant directions corresponding to the direction of ice flow and the shape of the ice margin. These two orientations in combination form a cross‐cutting and locally rectilinear pattern. One set of ridges at Hørbyebreen is a hybrid of eskers and geometric ridges formed during a surge and/or jökulhlaup event. The other sets of ridges are eskers formed time‐transgressively at a retreating ice margin. The similar morphology of esker complexes formed in different ways on both glacier forelands implies equifinality, meaning that care should be taken when interpreting Quaternary esker patterns. The eskers at Hørbyebreen contain substantial ice‐cores with a high ice:sediment ratio, suggesting that they would be unlikely to survive after ice melt. The Breiðamerkurjökull eskers emerged from terrain characterized by buried ice that has melted out. Our observations lead us to conclude that eskers may reflect a wide range of processes at dynamic ice margins, including significant paraglacial adjustments. This work, as well as previous studies, confirms that constraints on esker morphology include: topographic setting (e.g. confined valley or broad plain); sediment and meltwater availability (including surges and jökulhlaups); position of formation (supraglacial, englacial or subglacial); and ice‐marginal dynamics such as channel abandonment, the formation of outwash heads or the burial and/or exhumation of dead ice.

The geomorphology and sedimentology of eskers in north-central Ireland

Eskers are common glacial landforms in Ireland and form regionally-extensive networks that reflect stages of ice retreat of this sector of the late Devensian (Weichselian) British–Irish ice sheet. However, the sedimentary structures and composition of these eskers are poorly known, despite being able to offer valuable insight into subglacial hydrological processes during deglaciation. This study presents detailed morphological and sedimentary evidence from six esker systems in north-central Ireland, where adjacent and interacting ice domes were characterized by changes in ice-bed thermal and hydrological properties as well as significant ice dome centre migration. Morphologically, esker ridges in this area vary in length, size and continuity, with some esker systems ascending over hills (thus crossing watersheds) whereas other systems are confined to basin settings. Esker systems commonly appear to feed terminal outwash and glaciolacustrine deltas, but several systems do not show a clear terminus. The internal sediments within eskers vary from concentrically-bedded sands and gravels formed within enclosed tunnels, to interbedded sands, gravels and diamictons, occasionally with water-induced soft-sediment deformations, that are likely formed in interlobate settings. In contrast with some previous glacier models that assume that esker systems are isochronous and were formed in solely subglacial settings, the esker systems described here are interpreted as being of different ages, formed in variously subglacial to ice-marginal settings, and thus cannot be used as a single snapshot of the disposition of ice domes during stages of late Devensian ice retreat.

Esker systems in the Gulf of Bothnia

Eskers, straight-to-sinuous ridges of waterlain sand and gravel, and their erosional meltwater channel counterparts are geomorphological imprints of the palaeo-glacial hydrological system. Esker networks hundreds of metres to hundreds of kilometres long are widespread features of Quaternary glaciations and are used to reconstruct regional and continental patterns of deglaciation. However, they have been less commonly reported from sedimentary marine environments. Esker systems from the southern Gulf of Bothnia are imaged with unprecedented clarity in new multibeam datasets which reveal a highly connected subglacial hydrological network and its relationship to an underlying drumlin field. Sinuous, elongate sediment ridges thread through a drumlin field which rises from about 80 to 50 m water depth in the southern Gulf of Bothnia (Fig. 1a–i). Individual ridge segments range from 4 km long and are c. 50–200 m wide and c. 2–20 m high. They connect into much longer esker chains over 15 km long, broken only by small gaps. The longer esker paths form a broadly dendritic network, with up to fourth-order ‘tributaries’, converging southward in the direction of ice-flow. Within tributary sections, localized braiding and anabranching occur; occasionally isolated distributaries do not reconnect to the main …

Long continuous dendritic eskers offshore of Southampton Island, northern Hudson Bay

Eskers are long and sinuous ridges of glacifluvial sand and gravel formed subglacially or englacially in meltwater tunnels (e.g. Brennand 2000; Storrar et al. 2014). Eskers have been mapped in many formerly glaciated regions of Canada (e.g. Prest et al. 1968; Lajeunesse 2008; Storrar et al. 2014). They reveal pathways of meltwater drainage underneath the Laurentide Ice Sheet. The original geomorphology of eskers observed on land in air photos and satellite images is generally never perfectly preserved due to erosion by postglacial sub-aerial processes or to partial flooding in lakes and ponds. For this reason, eskers commonly emerge to form discontinuous ridges. Multibeam-bathymetric data collected in South Bay, Southampton Island (northern Hudson Bay; Fig. 1a–d) reveal remarkably well-preserved esker systems. The pristine state of preservation of these eskers is attributed to: (1) their occurrence in a subaqueous environment, where glacial landforms are generally better preserved than in a terrestrial setting (Ottesen & Dowdeswell 2006); and (2) the weak seafloor currents and low sedimentation rates that have prevailed in South Bay since deglaciation (Josenhans et al. 1988). Fig. 1. Long dendritic eskers in South Bay, Southampton Island, northern Hudson Bay. ( a ) Location …

Aerial LiDAR analysis in geomorphological mapping and geochronological determination of surficial deposits in the Sodankylä region, northern Finland

Geomorphological mapping based on an aerial light detection and ranging (LiDAR)-derived digital elevation model, field observations, ground penetrating radar measurements and test pit surveys was carried out in a glaciogenic environment in Sodankylä, central Finnish Lapland. The mapping area covered about 370 km2, with the LiDAR data having a pixel size of 2 m × 2 m and vertical resolution 0.3 m. The geomorphology of the area consists of large till-covered hills, ground moraine plains, glaciofluvial sand and gravel deposits composed of esker systems and related delta and outwash formations of the Weichselian cold stages, followed by pro-glacial glaciolacustric and post-glacial lacustric and fluvial sand/silt deposits. Furthermore, large areas in topographical depressions are covered by Holocene mires. The benefits of LiDAR data compared to traditional aerial-photo-based interpretation were in more detailed identification of surface deposits and more precise edging of the morphologies. This reflected more accurately the true ground surface in areas of dense vegetation. As an example, based on the LiDAR mapping it was possible to distinguish several till-covered delta and sandur deposits which based on OSL dating date back to the Early Weichselian stadial (74–89 ka). The results of the mapping project have been collated and published in a Quaternary geological map of the Sodankylä region.

Controls on the location, morphology and evolution of complex esker systems at decadal timescales, Breiðamerkurjökull, southeast Iceland

AbstractThis paper uses detailed mapping of eskers to address three questions which are important for reconstructing meltwater behaviour beneath contemporary and ancient ice masses: ‘What controls the morphology of simple and complex esker systems?’, ‘How do esker systems evolve through time?’ and ‘Are esker patterns compatible with groundwater controlled hydraulic spacing of esker tunnels?’. Esker crestlines and widths are mapped on the Breiðamerkurjökull foreland for eight time slices between 1945 and 2007, from high resolution (~50 cm) aerial photography, permitting their long‐term morphological evolution to be analysed in a high level of detail. We find that complex eskers develop where meltwater and sediment is abundant, such that sediment clogs channels, forming distributary eskers. Isolated eskers are simpler and smaller and reflect less abundant meltwater and sediment, which is unable to clog channels. Eskers may take several decades to emerge from outwash deposits containing buried ice and can increase or decrease in size when ice surrounding and underlying them melts out. It has been suggested that groundwater–channel coupling dictates the spacing between eskers at Breiðamerkurjökull. Our results do not dispute this, but suggest that the routing of sediment and meltwater through medial moraines is an additional important control on esker location and spacing. These results may be used to better understand the processes surrounding esker formation in a variety of geographical settings, enabling a more detailed understanding of the operation of meltwater drainage systems in sub‐marginal zones beneath glaciers and ice sheets. Copyright © 2015 John Wiley & Sons, Ltd.

Late Noachian and early Hesperian ridge systems in the south circumpolar Dorsa Argentea Formation, Mars: Evidence for two stages of melting of an extensive late Noachian ice sheet

The Dorsa Argentea Formation (DAF), extending from 270°–100° E and 70°–90° S, is a huge circumpolar deposit surrounding and underlying the Late Amazonian South Polar Layered Deposits (SPLD) of Mars. Currently mapped as Early-Late Hesperian in age, the Dorsa Argentea Formation has been interpreted as volatile-rich, possibly representing the remnants of an ancient polar ice cap. Uncertain are its age (due to the possibility of poor crater retention in ice-related deposits), its mode of origin, the origin of the distinctive sinuous ridges and cavi that characterize the unit, and its significance in the climate history of Mars. In order to assess the age of activity associated with the DAF, we examined the ridge populations within the Dorsa Argentea Formation, mapping and characterizing seven different ridge systems (composed of nearly 4,000 ridges covering a total area of ~300,000km2, with a cumulative length of ridges of ~51,000km) and performing crater counts on them using the method of buffered crater counting to determine crater retention ages of the ridge populations. We examined the major characteristics of the ridge systems and found that the majority of them were consistent with an origin as eskers, sediment-filled subglacial drainage channels. Ridge morphologies reflect both distributed and channelized esker systems, and evidence is also seen that some ridges form looping moraine-like termini distal to some distributed systems. The ridge populations fall into two age groups: ridge systems between 270° and 0° E date to the Early Hesperian, but to the east, the Promethei Planum and the Chasmata ridge systems date to the Late Noachian. Thus, these ages, and esker and moraine-like morphologies, support the interpretation that the DAF is a remnant ice sheet deposit, and that the esker systems represent evidence of significant melting and drainage of meltwater from portions of this ice sheet, thus indicating at least some regions and/or periods of wet-based glaciation. The Late Noachian and Early Hesperian ages of the ridge systems closely correspond to the ages of valley network/open basin lake systems, representing runoff, drainage and storage of liquid water in non-polar regions of the surface of Mars. Potential causes of such wet-based conditions in the DAF include: 1) top-down melting due to atmospheric warming, 2) enhanced snow and ice accumulation and raising of the melting isotherm to the base of the ice sheet, or 3) basal melting associated with intrusive volcanism (volcano-ice interactions). The early phase of melting is closely correlated in time with valley network formation and thus may be due to global atmospheric warming, while the later phase of melting may be linked to Early Hesperian global volcanism and specific volcano-ice interactions (table mountains) in the DAF. Crater ages indicate that these wet-based conditions ceased by the Late Hesperian, and that further retreat of the DAF to its present configuration occurred largely through sublimation, not melting, thus preserving the extensive ridge systems. MARSIS radar data suggest that significant areas of layered, potentially ice-rich parts of the Dorsa Argentea Formation remain today.

Morphometry and pattern of a large sample (>20,000) of Canadian eskers and implications for subglacial drainage beneath ice sheets

Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an ice margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing ice sheet dynamics. Comparison of esker patterns with an existing ice margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that ice surface was an important control on their location and that the conduits were, in places, close to ice overburden pressure.

Retreat pattern of the Cordilleran Ice Sheet in central British Columbia at the end of the last glaciation reconstructed from glacial meltwater landforms

The Cordilleran Ice Sheet (CIS) covered much of the mountainous northwestern part of North America at least several times during the Pleistocene. The pattern and timing of its growth and decay are, however, poorly understood. Here, we present a reconstruction of the pattern of ice‐sheet retreat in central British Columbia at the end of the last glaciation based on a palaeoglaciological interpretation of ice‐marginal meltwater channels, eskers and deltas mapped from satellite imagery and digital elevation models. A consistent spatial pattern of high‐elevation (1600–2400 m a.s.l.), ice‐marginal meltwater channels is evident across central British Columbia. These landforms indicate the presence of ice domes over the Skeena Mountains and the central Coast Mountains early during deglaciation. Ice sourced in the Coast Mountains remained dominant over the southern and east‐central parts of the Interior Plateau during deglaciation. Our reconstruction shows a successive westward retreat of the ice margin from the western foot of the Rocky Mountains, accompanied by the formation and rapid evolution of a glacial lake in the upper Fraser River basin. The final stage of deglaciation is characterized by the frontal retreat of ice lobes through the valleys of the Skeena and Omineca Mountains and by the formation of large esker systems in the most prominent topographic lows of the Interior Plateau. We conclude that the CIS underwent a large‐scale reconfiguration early during deglaciation and was subsequently diminished by thinning and complex frontal retreat towards the Coast Mountains.

Irish Ice Sheet sector dynamics as indicated by the Tynagh mineral deposit erratic assemblages

Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an ice margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing ice sheet dynamics. Comparison of esker patterns with an existing ice margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that ice surface was an important control on their location and that the conduits were, in places, close to ice overburden pressure.

Drainage beneath ice sheets: groundwater–channel coupling, and the origin of esker systems from former ice sheets

The nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic. An experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in Europe, which we regard as a fossil imprint of major longitudinal drainage channels. We conclude that the large-scale distribution and spacing of major eskers is consistent with the theory of groundwater control, in which esker spacing is partly controlled by the transmissivity of the bed. It is concluded that esker patterns reflect the large-scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet's dynamic regime. The theory is then used to deduce: basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat; patterns of palaeo-groundwater flow and head distribution; and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin. Major channel/esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet, although major dynamic events are demonstrably associated with major shifts in the hydraulic regime. Modelling suggests: that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations, with groundwater flow predominantly transverse to ice flow; that the circulation pattern has the potential to create large-scale anomalies in groundwater chemistry; and that the spacing of channels will change through the glacial cycle, influencing water pressures in stream tunnels, subglacial hydraulic gradients and effective pressure. If the latter is reduced sufficiently, it could trigger enhanced bed deformation, thus coupling drainage to ice sheet movement. It suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle.

Subglacial drainage by groundwater-channel coupling, and the origin of esker systems: Part 1—glaciological observations

Overall This work is presented in two parts. Part I presents observations on the coupling between subglacial channel flow and groundwater flow in determining subglacial hydraulic regime and creating eskers from an Icelandic glacier that is suggested as an analogue for many parts of Pleistocene ice sheets. Part II develops a theory of perennial subglacial stream flow and the origin of esker systems, and models the evolution of the subglacial stream system and associated groundwater flow in a glacier of the type described in Part I. It is suggested that groundwater flow may be the predominant mechanism whereby meltwater at the glacier bed finds its way to the major subglacial streams that discharge water to glacier margins. Part I Boreholes drilled through an Icelandic glacier into an underlying till and aquifer system have been used to measure variations in head in the vicinity of a perennial subglacial stream tunnel during late summer and early winter. They reveal a subglacial groundwater catchment that is drained by a subglacial stream along its axis. The stream tunnel is characterised by low water pressures, and acts as a drain for the groundwater catchment, so that groundwater flow is predominantly transverse to ice flow, towards the channel. These perennial streams flow both in summer and winter. Their portals have lain along the same axes for the 5 km of retreat that has occurred since the end of the Little Ice Age, 100 years ago, suggesting that the groundwater catchments have been relatively stable for at least this period. In the winter season, stream discharges are largely derived from basal melting, but during summer, water derived from the glacier surface finds its way, via fractures and moulins, to the glacier bed, where it dominates the meltwater flux. Additional subglacial streams are created in summer to help drain this greater flux from beneath the glacier, through poorly integrated and unstable networks. Summer streams cease to flow during winter and tend not to form in the same places in the following summer. Perennial streams are the stable component of the system and are the main sources of extensive esker systems. Strong flow of groundwater towards low-pressure areas along channels and the ice margin is a source of major upwelling that can produce sediment liquefaction and instability. A theory is developed to show how this could have a major effect on subglacial sedimentary processes.

Subglacial drainage by groundwater–channel coupling, and the origin of esker systems: part II—theory and simulation of a modern system

The observations from Breidamerkurjökull reported in Part I are used as a basis for a theory of coupled channel/groundwater flow that suggests that the large scale geometry of the hydraulic system self-organises as a consequence of coupling between groundwater and channel flow. The pattern of channel/esker distribution is largely determined by winter conditions and the sedimentary characteristics of eskers are largely determined by summer conditions, when large discharges of surface meltwater penetrate to the bed in the terminal zone. A simulation model is developed to explore evolution of the channel/groundwater system during a period of glacier growth. It suggests that as the glacier grows, channel/esker frequencies will increase as will the proportion of the melt flux discharged by channel flow, that groundwater impacts can extend to depth, that low water-pressure bulbs will extend downwards beneath channels, that groundwater upwelling towards channels are strong enough to create widespread conditions for liquefaction and that pressure shocks can occur as the channel system reorganises itself.

Extensive Hesperian‐aged south polar ice sheet on Mars: Evidence for massive melting and retreat, and lateral flow and ponding of meltwater

Local and regional Mars Orbiter Laser Altimeter (MOLA) topographic data support the presence of an extensive Hesperian‐aged volatile‐rich south polar deposit (the Dorsa Argentea Formation, Hd, and related units) underlying the present Amazonian‐aged polar cap (Api, residual ice, and Apl, layered terrain) and covering a surface area that could be as large as 2.94×106km2(about 2% of the surface of Mars), over twice the area of the present Amazonian‐aged south polar deposits. The deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present‐day polar deposits. Several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows, including a large valley near the crater Schmidt and the Argyre basin. Narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of Hd. The new topographic data support the interpretation of these ridges as eskers, representing meltwater distribution networks at the base of the receding deposit. Extensive development of large pits and depressions (cavi) have previously been interpreted as eolian etching or basal melting of ice‐rich deposits. Analysis of MOLA topography supports the interpretation that they represent basal melting of ice‐rich deposits and shows that they have links to the esker systems. Volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by Apl, and that some of the liquid water formed ponds and lakes in the distal parts of Hd. The presence of pedestal craters is further evidence of the removal of extensive volatile‐rich deposits and contributes to the quantitative measure of the former deposit thickness. Where did the melt products go? Inspection of the margins of the Dorsa Argentea Formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the Argyre basin, some 3.5–4.0 km below their origin. These channels do not exhibit tributaries. Their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant, and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the Argyre basin over 1000 km away. Estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5.9×106km3, equivalent to a global layer of water ∼20 m deep if the deposit consisted of ∼50% volatiles. A portion of these volatiles migrated across the surface to pond in adjacent valleys and basins, and into the groundwater system. A significant portion of the volatiles remain in the deposit, representing a net removal from the atmosphere and from the active hydrologic system in early to middle Mars history, and forming an accessible record of aqueous conditions and possible biological environments dating from that time.

Morphology of subglacial conduit deposits: control by bedrock topography, discharge flow variation, or both? A cautionary case study: Axelsberg, Nynäshamn, south central Sweden

The results of a case study of full-pipe conduit discharge into a Pleistocene proglacial lake provide improved knowledge for recognizing the sedimentology and genetic morphology of isolated glaciofluvial deposits. Most published studies have investigated the elongated deposits of esker systems within a stagnant or retreating ice sheets. Lack of genetic description for flow through full-pipe conduits where there are localized cross-sections of conduit has restricted recognition of changes in the dynamics of the flow in such situations. Axelsberg gravel pit, located about 40 km south of Stockholm, Sweden, shows a succession of glaciolacustrine deposits, both in the lee and on top of a Precambrian gneiss body that served as a local hill in the Baltic Ice Lake beyond the sediment-carrying conduit during deposition of the sediments in a sediment-carrying conduit. The succession is subdivided into five lithofacies, two of which are coarse gravel. These two facies are spatially separated from one another by an even coarser-grained gravel facies consisting of blocks up to 3 m in diameter, which occurs mainly on top of the gneiss hill. The lower gravel facies is interpreted as a remnant of the deposits formed during a phase of strong discharge into the lake through a subglacial conduit. The block-rich gravel facies formed at the phase of maximum velocity of the turbulent current, the concentration of blocks in this facies is explained by extremely strong turbulent flow conditions perhaps during a sudden, local discharge through the subglacial conduit. The presence of the gneiss hill just in front of the conduit may have created the exceptionally high-velocity turbulent current capable of transporting meters-large boulders. The upper, somewhat less coarse-grained but still boulder-rich facies was formed when the turbulent conditions had become less extreme. Boulders up to 1 m occur in this facies. The depositional features indicate that the succession in the Axelsberg gravel pit was formed within the stagnant frontal zone of the ice sheet. Lack of similar deposits farther to the north along the deglaciation path suggests a local, short-term event. As there is no evidence of such a rapid climate signal in the later part of the Younger Dryas documented by clay-varve chronology, the formation of this deposit as a response of the ice sheet to that would not be regarded as a probable alternative explanation. This study suggests that clastic sedimentary sections are among the reliable records of short-lived internally controlled glacial events, not necessarily climatically driven ones, in the past.

Structure and rhythmic pattern of glaciofluvial deposits north of Lake Vänern, south‐central Sweden

The glaciofluvial deposits of the Vanern plain, Värmland, form esker systems and trains. Esker trains are defined as series of short eskers, kames and other glaciofluvial deposits. Trains spreading fanlike from the same area are combined into esker systems. Some individual deposits are described. The general pattern differs from the long, coherent eskers typical of eastern Sweden. A rhythmic pattern of depositional units in the esker trains is found to correspond to a 10 to 11 yr cycle, possibly representing the sunspot cycle. Zones of alternating faster and slower retreat of the ice margin are identified. Two lines of slower retreat, the Karlstad and Fryken lines, correspond to the Norwegian Ski line, dated at 10 000 BP (conventional radiocarbon years) and to a slightly younger line. The spacing of the esker trains is controlled by topography in the north, but by basal ice melting, which controlled their flow and spacing on the Ratter terrain in the south.

A qualitative view of sub-ice-sheet landscape evolution

Qualitative analysis of sub-ice-sheet landscapes is based on processes inferred from landform and sediment. Striation and plucking give information on the dynamic and thermody namic conditions at the ice bed. Lodgement and melt-out tills indicate that basal accretion and freezing conditions were followed by melting during till deposition, under sliding ice in the case of lodgement and stagnant ice in the case of melt-out. Surging is inferred from evidence of low ice- sheet profiles. Melt-out appears to have taken place over large areas in periods of stagnation following surges. Interbedded stratified sediment and melt-out till indicate decoupling of the ice from its bed during relatively minor subglacial drainage events. The bulk of glacial erosion, transport and deposition took place during ice-sheet advance. Over-ridden proglacial sediment, lodgement/deformation and melt-out tills, with subglacial sorted beds, were eroded by cataclysmic meltwater outburst floods that produced erosional and depositional drumlins, flutings, Rogen moraines and vast tracts of scoured bedrock. These floods marked a dramatic change in glacier regime over large areas of ice sheets. Low ice-sheet profiles caused widespread stagnation; glacial erosion and transport came to a halt. Extensive esker systems attest to a regional, integrated subglacial drainage network. Tunnel channels, glaciotectonic landforms and hummocky moraine are other elements of the subglacial landscape related directly or indirectly to meltwater storage and outburst floods. Models of landscape evolution are based on the conclusions derived from the analysis of sediment, form and process.