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1.
Jasper Knight 《地球表面变化过程与地形》2016,41(2):277-288
Interpretation of subglacial processes and environments can be usefully informed by the stratigraphy and structures of sediments preserved within drumlins, and the down‐ice variability of these sediments. Drumlins in Clew Bay, western Ireland, were formed by westward late Pleistocene ice flow onto the Atlantic shelf, but the depositional processes and environments of these sediments remain uncertain. This study describes and interprets the drumlin stratigraphy and sediment properties and structures from two drumlins on the south side of Clew Bay. Drumlin sediments are dominated by massive to stratified diamicton deposited subglacially by deformation of flows of varying rheology. Folds, shears and clastic dikes within the sediments suggest active subglacial water flow related to variations in ice flow and ice‐bed coupling. The distal ends of the drumlins are dominated by stratified diamicton and gravel units with sandy interbeds. These sediments reflect deposition in a leeside subglacial cavity formed coevally with bedform development. A model is proposed that can account for sediment stratigraphic patterns and drumlin formation. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
2.
Jeremy C. Ely Chris D. Clark Matteo Spagnolo Anna L.C. Hughes Chris R. Stokes 《地球表面变化过程与地形》2018,43(5):1073-1087
Drumlins are subglacial bedforms streamlined in the direction of ice flow. Common in deglaciated landscapes, they have been widely studied providing rich information on their internal geology, size, shape, and spacing. In contrast with bedform investigations elsewhere in geomorphology (aeolian and fluvial dunes and ripples for example) most drumlin studies derive observations from relict, and thus static features. This has made it difficult to gain information and insights about their evolution over time, which likely hampers our understanding of the process(es) of drumlin formation. Here we take a morphological approach, studying drumlin size and spacing metrics. Unlike previous studies which have focussed on databases derived from entire ice sheet beds, we adopt a space‐for‐time substitution approach using individual drumlin flow‐sets distributed in space as proxies for different development times/periods. Framed and assisted by insights from aeolian and fluvial geomorphology, we use our metric data to explore possible scenarios of drumlin growth, evolution and interaction. We study the metrics of the size and spacing of 36 222 drumlins, distributed amongst 71 flow‐sets, left behind by the former British‐Irish Ice Sheet, and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis. Through characterizing and analysing the shape of the probability distribution functions of size and spacing metrics for each flow‐set we argue that drumlins grow, and potentially migrate, as they evolve leading to pattern coarsening. Furthermore, our findings add support to the notion that no upper limit to drumlin size exists, and to the idea that perpetual coarsening could occur if given sufficient time. We propose that the framework of process and patterning commonly applied to non‐glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
3.
Spindle- and parabolic-shaped drumlins examined at fifty-five localities in northern Ireland possess stratification sequences on their lee-side flanks. These forms lack the distinctive steep stoss- and tapering lee-ends of classical drumlins and tend to occur in linear zones transverse to late Pleistocene ice-flow. In most cases (90 per cent) the stratified deposits infill embayments excavated in the lee-side of barkhanoid forms and, in the remainder (10 per cent), they are superimposed on the lee-side of whaleback forms. The stratification sequences developed as a result of sedimentation in interconnected subglacial water-filled cavities and are unlike remanie proglacial sediments moulded by ice into drumlin form. Stratigraphic evidence indicates that the lee-side sequences developed during drumlin streamlining, which supports the view that subglacial hydraulic processes played an important role in drumlin formation. 相似文献
4.
Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley & Sons, Ltd. 相似文献
5.
Numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady‐state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice‐sheet‐thickness‐modulated, down‐ice‐decreasing hydraulic heads. In relation to non‐glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice–bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo‐ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice–bed interface. Owing to the abundance of low‐permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
6.
The development of large erosive subglacial forms in unconsolidated sediments is generally attributed to the eroding power of subglacial meltwater flowing under high pressure conditions. Most explanations, however, differ in the source of meltwater and the speed at which it erodes the subglacial bed. Based on the geometry of deep tunnel valleys and glacial basins in northwestern Europe, a reconstruction of subglacial hydrological conditions during the development of subglacial depressions is made. It is demonstrated that the flow of subglacial meltwater in subglacial channels under high glaciostatic pressures is only capable of eroding large volumes of sediment as long as there is imminent glaciohydrological instability. For the thick aquifers in northwestern Europe, this instability is achieved when large quantities of supraglacial meltwater are available. Furthermore, a theoretical definition is given for maximum depression depth to be reached by subglacial erosion. It is shown that this maximum depth is strongly related to average air temperatures during deglaciation and that glacier bed lowering is to be expected during any final phase of glaciations. The theoretical framework presented enables a tentative comparison between large-scale glacial morphology of different glaciations in northwestern Europe. 相似文献
7.
Martin J. Siegert Martyn Tranter J. Cynan Ellis‐Evans John C. Priscu W. Berry Lyons 《水文研究》2003,17(4):795-814
Our understanding of Lake Vostok, the huge subglacial lake beneath the East Antarctic Ice Sheet, has improved recently through the identification of key physical and chemical interactions between the ice sheet and the lake. The north of the lake, where the overlying ice sheet is thickest, is characterized by subglacial melting, whereas freezing of lake water occurs in the south, resulting in ～210 m of ice accretion to the underside of the ice sheet. The accreted ice contains lower concentrations of the impurities normally found in glacier ice, suggesting a net transfer of material from meltwater into the lake. The small numbers of microbes found so far within the accreted ice have DNA profiles similar to those of contemporary surface microbes. Microbiologists expect, however, that Lake Vostok, and other subglacial lakes, will harbour unique species, particularly within the deeper waters and associated sediments. The extreme environments of subglacial lakes are characterized by high pressures, low temperatures, permanent darkness, limited nutrient availability, and oxygen concentrations that are derived from the ice that provides the meltwater. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
8.
Andrew Finlayson Emrys Phillips Ívar Örn Benediktsson Lucas K. Zoet Neal R. Iverson Jez Everest 《地球表面变化过程与地形》2019,44(1):367-380
The interaction between drumlins and overriding glacier ice is not well studied, largely due to the difficulty of identifying and accessing suitable active subglacial environments. The surge-type glacier Múlajökull, in central Iceland, overlies a known field of actively forming drumlins and therefore provides a rare opportunity to investigate the englacial structures that have developed in association with ice flow over the subglacial drumlins. In this study detailed ground-penetrating radar surveys are combined with field observations to identify clear sets of up-glacier and down-glacier dipping fractures at Múlajökull's margin. These are interpreted as conjugate shear planes or P- and R-type Reidel shears that developed and filled with saturated sediment derived from the glacier bed, during a previous surge. The fracture sets exhibit focused spatial distributions that are influenced by the subglacial topography. In particular, down-glacier dipping fractures are strongly focused over drumlin stoss slopes. These fractures, although well developed at depth, were mostly unable to transmit basal water and sediment up to the glacier surface during the surge cycle. In contrast, up-glacier dipping fractures formed over drumlin lee sides and in more gently sloping swales, and more frequently connected to the glacier surface, providing a pathway for the evacuation of basal water and water-saturated sediment. The study suggests that the subglacial drumlins under Múlajökull's margin have influenced the nature and distribution of englacial fractures, which could potentially contribute to spatial variations in basal water pressure during a surge. BGS © UKRI 2018 相似文献
9.
10.
Glacial curvilineations found along the southern sector of the Laurentide Ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes 
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下载免费PDF全文 First discovered in Poland, glacial curvilineations (GCLs) are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150 m wavelength, found within kilometres‐wide valleys interpreted as being produced by meltwater flowing subglacially. Their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent Polish examples. From these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods. Here we report, for the first time, GCLs found along the southern sector of the Laurentide Ice Sheet in three northern states of the USA. Using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations. We find aspects of their context and morphology difficult to explain using the existing hypothesis. We instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys, or that occur near subglacial lake shorelines. Further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements, which may have occurred by rotational or translational slope failure or by creep deformation. Our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma‐frozen ground. Under such circumstances, sediment blocks thawed by the water may then easily glide over a frozen décollement at low slope angles; analogous to subaerial active‐layer glides in permafrost environments. Permafrost spring sapping may have provided lines of weakness for slope failure. If the requirement for permafrost is found to hold, then GCLs may become an important indicator of the palaeo‐distribution of permafrost. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
11.
Jane K. Hart Kirk Martinez Philip J. Basford Alexander I. Clayton Graeme M. Bragg Tyler Ward David S. Young 《地球表面变化过程与地形》2019,44(9):1769-1782
We investigate the spatial and temporal englacial and subglacial processes associated with a temperate glacier resting on a deformable bed using the unique Glacsweb wireless in situ probes (embedded in the ice and the till) combined with other techniques [including ground penetrating radar (GPR) and borehole analysis]. During the melt season (spring, summer and autumn), high surface melt leads to high water pressures in the englacial and subglacial environment. Winter is characterized by no surface melting on most days (‘base’) apart from a series of positive degree days. Once winter begins, a diurnal water pressure cycle is established in the ice and at the ice/sediment interface, with direct meltwater inputs from the positive degree days and a secondary slower englacial pathway with a five day lag. This direct surface melt also drives water pressure changes in the till. Till deformation occurred throughout the year, with the winter rate approximately 60% that of the melt season. We were able to show the bed comprised patches of till with different strengths, and were able to estimate their size, relative percentage and temporal stability. We show that the melt season is characterized by a high pressure distributed system, and winter by a low pressure channelized system. We contrast this with studies from Greenland (overlying rigid bedrock), where the opposite was found. We argue our results are typical of soft bedded glaciers with low englacial water content, and suggest this type of glacier can rapidly respond to surface-driven melt. Based on theoretical and field results we suggest that the subglacial hydrology comprises a melt season distributed system dominated by wide anastomosing broad flat channels and thin water sheets, which may become more channelized in winter, and more responsive to changes in meltwater inputs. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
12.
One of the key issues associated with the hypothesis of catastrophic subglacial drainage of the Livingstone Lake event is whether flows of such large magnitudes are physically feasible. To explore this issue, a one‐dimensional hydraulic network flow model was developed to investigate the range of peak discharges and associated flow parameters that may have been carried by a tunnel channel network in south‐east Alberta, Canada. This tunnel channel network has been interpreted elsewhere to carry large discharges associated with subglacial meltwater flows because of the convex longitudinal profiles of individual channels. This computational modelling effort draws upon established and verified engineering principles and methods in its application to the hydraulics of this problem. Consequently, it represents a unique and independent approach to investigating the subglacial meltwater hypothesis. Based on the modelling results, it was determined that energy losses resulting from friction limit the maximum peak discharge that can be transported through the tunnel channel network to 107 m3 s−1, which is in reasonable agreement with previous estimates of flood discharges for proposed megafloods. Results show that flow through channels with convex longitudinal profiles occurs when hydraulic head exceeds 910 m (Lost River) and 950 m (Sage Creek) , respectively. These are considerably below the maximum head capable of driving flow through the system of 1360 m, beyond which ice is decoupled from the bed across the pre‐glacial drainage divide. Therefore, it is concluded that these model results support the hypothesis of catastrophic subglacial drainage during the Livingstone Lake event. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
13.
The orientation of several landforms, e.g. drumlins, flutes, crag-and-tails, and mega-scale glacial lineations, records the direction of the overlying ice flow that created them. Populations of such features are used routinely to infer former ice-flow patterns, which serve as the building blocks of reconstructions of palaeo ice-sheet evolution. Currently, the conceptualisation of flow patterns from these flow-direction records is done manually and qualitatively, so the extractable glaciological information is limited. We describe a kriging method (with Matlab code implementation) that calculates continuous fields of ice-flow direction, convergence, and curvature from the flow-direction records, and which yields quantitative results with uncertainty estimates. We test the method by application to the subglacial bedforms of the Tweed Valley Basin, UK. The results quantify the convergent flow pattern of the Tweed Palaeo-Ice Stream in detail and pinpoint its former lateral shear margins and where ice flowed around basal bumps. Ice-flow parameters retrieved by this method can enrich ice-sheet reconstructions and investigations of subglacial till processes and bedform genesis. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
14.
Testing techniques to quantify drumlin height and volume: synthetic DEMs as a diagnostic tool 下载免费PDF全文
下载免费PDF全文 Glacial bedform height (H) and volume (V) likely preserve important information about the behaviour of former ice sheets. However, large systematic errors exist in the measurement of H and V. Three semi‐automated methods to isolate drumlins from other components of the landscape (e.g. trees, hills) as portrayed by NEXTMap have recently been devised; however, it is unclear which is most accurate. This paper undertakes the first quantitative comparison of such readily implementable methods, illustrating the use of statistically representative ‘synthetic landscapes’ as a diagnostic tool. From this analysis, guidelines for quantifying the 3D attributes of drumlins are proposed. Specifically, to avoid obtaining incorrect estimates caused by substantial systematic biases, interpreters should currently take three steps: declutter the digital elevation model for estimating H but not V; remove height data within the drumlin; then interpolate across the resultant hole to estimate a basal surface using Delaunay triangulation. Results are demonstrated through analysis of drumlins in an area in western central Scotland. The guidance arguably represents the best current advice for subglacial bedforms in general, highlighting the need for more studies into the quality of mapped data using synthetic landscapes. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
15.
Richard Clark 《地球表面变化过程与地形》1998,23(12):1135-1139
Field examination and review of previous studies show that glacial landforms are widespread in southeast Northumberland and across the local altitude range. Ice-contact deposits occur on the main valley floors. Numerous subglacial channels descend to or close to present valley floors, and many of the lesser valleys have carried meltwater and glaciofluvial sediment. In major part, the land surface retains its end-glacial form and age. Evidence for a landscape largely end-glacial in form and age rather than one produced since loss of ice cover by laterally diverse erosion indicates that rates of denudation over the last 15 ka have been less than those obtained from height differences between actual surface and a reconstructed notional end-glacial surface. Copyright © 1998 John Wiley & Sons, Ltd. 相似文献
16.
Jens-Ove Nslund 《地球表面变化过程与地形》1997,22(5):441-455
On the high altitude polar plateau of Amundsenisen, western Dronning Maud Land, East Antarctica, a subglacial valley, with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta, was studied by radio echo sounding. In addition, a small, probably glacial, valley was mapped within the same subglacial massif. Basal ice temperatures were calculated using field data on precipitation, air temperature and ice sheet thickness. Discoveries of old landforms which have been preserved more or less intact beneath the former Fennoscandian and Laurentide ice sheets have received increasing attention during the last decade. The aim of this study is to investigate whether preservation of landforms occurs under the East Antarctic Ice Sheet, and to discuss under that climatological and glaciological circumstances preservation may take place. The results show that the ice sheet covering the investigated localities is frozen to bed, and therefore has an insignificant erosional capability. The observations suggest that a large-scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold-based ice sheet marginal zone for a long time period. The process of glacial preservation, recognized for bedrock features and tentatively observed for sediment accumulations, should act on similar large-scale landforms under any cold-based ice sheet, present or past. On the basis of existing studies of the age and stability of the East Antarctic Ice Sheet, a Middle Pliocene age is suggested for the preserved landforms. The presence of the presumed sediment-filled valley further indicates that no prolonged periods of basal melting have occurred at the Amundsenisen study area during the ice sheet history, which includes the Quaternary glaciation periods. Finally, calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation. © 1997 by John Wiley & Sons, Ltd. 相似文献
17.
Isolated, Late Miocene volcanogenic sequences in northern Alexander Island, Antarctic Peninsula, form an unusual, cogenetic association of volcaniclastic, sandy-gravelly lithofacies (including tillites) and volcanic (lava/hyalocalstite) lithofacies. Using simple lithofacies analysis and theoretical considerations of hydrodynamic effects of subglacial eruptions, valley-confined volcanic activity beneth thin, wet-based ice is suggested. The Alexander Island successions are complete enough to be regarded as model sequences for this uncommonly recorded type of eruptive/depositional activity. The sedimentary lithofacies represent resedimented tuffs and meltout or flow tills, which were probably deposited in subglacial ice tunnels eroded or enlarged by volcanically heated meltwater. The volcanic lithofacies formed by the interaction of hot magma with the ice tunnel walls (generating abundant meltwater) and water-saturatedsediments, resulting in the formation of heterogeneous masses of lava and hyaloclastite. There is no obvious sequence organisation in the sedimentary sections. This is probably due to a complex interplay of eruption-related and environmental hydrodynamic factors affecting the relative proportions of water and entrained sediment. 相似文献
18.
Bulk runoff and meteorological data suggest the occurrence of two meltwater outburst events at Finsterwalderbreen, Svalbard, during the 1995 and 1999 melt seasons. Increased bulk meltwater concentrations of Cl? during the outbursts indicate the release of snowmelt from storage. Bulk meltwater hydrochemical data and suspended sediment concentrations suggest that this snowmelt accessed a chemical weathering environment characterized by high rock:water ratios and long rock–water contact times. This is consistent with a subglacial origin. The trigger for both the 1995 and 1999 outbursts is believed to be high rates of surface meltwater production and the oversupply of meltwater to areas of the glacier bed that were at the pressure melting point, but which were unconnected to the main subglacial drainage network. An increase in subglacial water pressure to above the overburden pressure lead to the forcing of a hydrological connection between the expanding subglacial reservoir and the ice‐marginal channelized system. The purging of ice blocks from the glacier during the outbursts may indicate the breach of an ice dam during connection. Although subglacial meltwater issued continually from the glacier terminus via a subglacial upwelling during both melt seasons, field observations showed outburst meltwaters were released solely via an ice‐marginal channel. It is possible that outburst events are a seasonal phenomenon at this glacier and reflect the periodic drainage of meltwaters from the same subglacial reservoir from year to year. However, the location of this reservoir is uncertain. A 100 m high bedrock ridge traverses the glacier 6·5 km from its terminus. The overdeepened area up‐glacier from this is the most probable site for subglacial meltwater accumulation. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
19.
We present field observations from Bláhnúkur, a small volume (<0.1 km3) subglacial rhyolite edifice at the Torfajökull central volcano, south-central Iceland. Bláhnúkur was probably emplaced during the last glacial period (ca. 115–11 ka). The characteristics of the deposits suggest that they were formed by an effusive eruption in an exclusively subglacial environment, beneath a glacier >400 m thick. Lithofacies associations attest to complex patterns of volcano-ice interaction. Erosive channels at the base of the subglacial sequence are filled by both eruption-derived material and subglacial till, which show evidence for deposition by flowing meltwater. This suggests that meltwater was able to drain away from the vent area during the eruption. Much of the subglacial volcanic deposits consist of conical-to-irregularly shaped lava lobes typically 5–10 m long, set in poorly sorted breccias with an ash-grade matrix. A gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface, driven by magma-water interaction as the lava flowed over poorly consolidated, waterlogged debris. Sets of columnar joints on the upper surfaces of lobes are interpreted as ice-contact features. The morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2–5 m high. Avalanche deposits mantling the flanks of Bláhnúkur appear to have been generated by the collapse of lava lobes and surrounding breccia. A variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes. Collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias. Much larger lava flows were emplaced in the latter stages of the eruption. Columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100–200 m in length. There is little evidence for magma-water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater. As rhyolite magma rose to the base of the glacier, the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated. We present evidence that the cavity system evolved during the eruption, reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate. 相似文献