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Ice Margin (ice + margin)
Selected AbstractsMeltwater discharge through the subglacial bed and its land-forming consequences from numerical experiments in the Polish lowland during the last glaciationEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2009Jan A. Piotrowski Abstract 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. [source] Hydrologic and geomorphic effects of temporary ice-dammed lake formation during jökulhlaupsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2003Matthew J. Roberts Abstract Glacial outburst ,oods (jökulhlaups) occur frequently in glaciated environments, and the resultant ,ooding causes geomorphic change and, in some instances, damage to local infrastructure. During some jökulhlaups, ,oodwater is stored temporarily in ice-marginal locations. In July 1999, a linearly rising jökulhlaup burst from Sólheimajökull, Iceland. During this remarkable event, subglacial ,oodwater pooled transiently in two relict ice-dammed lake basins, before draining suddenly back into Sólheimajökull. The signi,cance of such rapid formation and attendant drainage of temporary ice-dammed lakes during jökulhlaups has not been addressed. Consequently, this paper: (i) assesses the hydrologic and geomorphic effects of temporary ice-dammed lake formation caused by lake-basin ,retro-,lling'; and (ii) discusses the impact and signi,cance of transient retro-,lling under jökulhlaup conditions. Pre- and post-,ood ,eldwork at Sólheimajökull enabled the impact and signi,cance of lake-basin retro-,lling to be assessed. Field evidence demonstrates that the July 1999 jökulhlaup had an unusually rapid rise to peak discharge, resulting in subglacial ,oodwater being purged to ice-marginal locations. The propensity for temporary retro-,lling was controlled by rapid expulsion of ,oodwater from Sólheimajökull, coincident with locations suitable for ,oodwater storage. Floodwater inundated both ice-marginal lake basins, permitting signi,cant volumes of sediment deposition. Coarse-grained deltas prograding from the ice margin and boulders perched on scoured bedrock provide geomorphic records of sudden retro-,lling. The depositional characteristics of lake-basin deposits at Sólheimajökull are similar to jökulhlaup sediments documented in proglacial settings elsewhere; however, their depositional setting and association with ice-marginal landforms is distinctive. Findings suggest that temporary ice-dammed lake formation and drainage has the capacity to alter the shape of the ,ood hydrograph, especially if drainage of a temporary lake is superimposed on the original jökulhlaup. Deposits associated with lake-basin retro-,lling have a long-term preservation potential that could help to identify temporary ice-dammed lake formation in modern and ancient glacial environments. Copyright © 2003 John Wiley & Sons, Ltd. [source] Validation of ECMWF (re)analysis surface climate data, 1979,1998, for Greenland and implications for mass balance modelling of the ice sheetINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2001Edward Hanna Abstract Climate (re)analysis products are potentially valuable tools, when properly verified, for helping to constrain the surface mass balance of the Greenland Ice Sheet (GIS). Monthly surface fields from European Centre for Medium-Range Weather Forecasts (ECMWF) operational- and re-analyses spanning 1979,1998 were validated using in situ data (surface air pressure and temperature, precipitation, cloud cover, short-/all-wave radiation, and wind speed/direction). These validation data are from coastal or near-coastal Danish Meteorological Institute (DMI) synoptic stations, inland Greenland Climate Network (GC-Net) and University of Wisconsin Automatic Weather Stations (AWSs), and two energy balance stations near the southern ice margin. The ECMWF analyses closely reproduce the seasonal patterns and interannual variations of much of the in situ data. Differences in the mean values of surface air pressure and temperature can mainly be ascribed to orography errors in the analyses' schemes, compared with the latest available accurate digital elevation model. Much of the GIS margin as modelled by ECMWF was too cold, on average by 4°C, and ECMWF precipitation averaged some 136% of the DMI station values. The misrepresentation of the (relatively) steep ice-sheet margin, which tends to be broadened and systematically over-elevated by several hundred metres, orographically reduced temperature and enhanced precipitation there in the ECMWF models. The cloud-cover comparison revealed not dissimilar annual mean cloud covers (ECMWF ,8%) but the ECMWF analyses had too little cloud and were too ,sunny' during the critical summer melt-season. ECMWF-modelled surface albedo in summer was ,11% lower than GC-Net values, which was mainly responsible for the disagreement of modelled surface short-wave radiation fluxes with observations. Model albedo and cloud errors need to be rectified if the analyses are to be used effectively to drive energy balance models of Greenland snowmelt. ECMWF wind speed averaged 66% (62%) of the DMI station (AWS) values. The validation results provide useful insights into how one can best improve the ECMWF Greenland climate data for use in glaciological and climatological studies. Copyright © 2001 Royal Meteorological Society [source] Seafloor glacial features reveal the extent and decay of the last British Ice Sheet, east of Scotland,JOURNAL OF QUATERNARY SCIENCE, Issue 2 2009Alastair G. C. Graham Abstract Three-dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1,2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast-flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd. [source] Marginal formation of De Geer moraines and their implications to the dynamics of grounding-line recession,JOURNAL OF QUATERNARY SCIENCE, Issue 2 2005Mattias Lindén Abstract De Geer moraine ridges occur in abundance in the coastal zone of northern Sweden, preferentially in areas with proglacial water depths in excess of 150 m at deglaciation. From detailed sedimentological and structural investigations in machine-dug trenches across De Geer ridges it is concluded that the moraines formed due to subglacial sediment advection to the ice margin during temporary halts in grounding-line retreat, forming gradually thickening sediment wedges. The proximal part of the moraines were built up in submarginal position as stacked sequences of deforming bed diamictons, intercalated with glaciofluvial canal-infill sediments, whereas the distal parts were built up from the grounding line by prograding sediment gravity-flow deposits, distally interfingering with glaciolacustrine sediments. The rapid grounding-line retreat (ca. 400,m,yr,1) was driven by rapid calving, in turn enhanced by fast iceflow and marginal thinning of ice due to deforming bed conditions. The spatial distribution of the moraine ridges indicates stepwise retreat of the grounding line. It is suggested that this is due to slab and flake calving of the ice cliff above the waterline, forming a gradually widening subaqueous ice ledge which eventually breaks off to a new grounding line, followed by regained sediment delivery and ridge build-up. Copyright © 2005 John Wiley & Sons, Ltd. [source] Deformation styles as a key for interpreting glacial depositional environmentsJOURNAL OF QUATERNARY SCIENCE, Issue 6 2003Danny McCarroll Abstract Lithostratigraphical and lithofacies approaches used to interpret glacial sediments often ignore deformation structures that can provide the key to environment of formation. We propose a classification of deformation styles based on the geometry of structures rather than inferred environment of formation. Five styles are recognised: pure shear (P), simple shear (S), compressional (C), vertical (V) and undeformed (U). These dictate the first letter of the codes; the remaining letters conveying the evidence. This information can be used to reconstruct palaeostress fields and to infer physical properties of sediments when they deformed. Individual structures are not diagnostic of particular environments but the suite of structures, their relative scale, stratigraphical relationships, and orientation relative to palaeoslopes and to palaeoice-flow directions can be used to infer the environment in which they formed. This scheme is applied at five sites in west Wales. The typical succession is interpreted as subglacial sediments overlain by meltout tills, flow tills and sediment flows. Paraglacial redistribution of glacial sediments is widespread. Large-scale compressional deformation is restricted to sites where glaciers readvanced. Large-scale vertical deformation occurs where water was locally ponded near the ice margin. There is no evidence for glaciomarine conditions. Copyright © 2003 John Wiley & Sons, Ltd. [source] Moraine development at a small High-Arctic valley glacier: Rieperbreen, SvalbardJOURNAL OF QUATERNARY SCIENCE, Issue 6 2001Astrid Lyså Abstract Ice-cored lateral and frontal moraine complexes, formed at the margin of the small, land-based Rieperbreen glacier, central Svalbard, have been investigated through field observations and interpretations of aerial photographs (1936, 1961 and 1990). The main focus has been on the stratigraphical and dynamic development of these moraines as well as the disintegration processes. The glacier has been wasting down since the ,Little Ice Age' (LIA) maximum, and between 1936 and 1990 the glacier surface was lowered by 50,60 m and the front retreated by approximately 900 m. As the glacier wasted, three moraine ridges developed at the front, mainly as melting out of sediments from debris-rich foliation and debris-bands formed when the glacier was polythermal, probably during the LIA maximum. The disintegration of the moraines is dominated by wastage of buried ice, sediment gravity-flows, meltwater activity and some frost weathering. A transverse glacier profile with a northward sloping surface has developed owing to the higher insolation along the south-facing ice margin. This asymmetric geometry also strongly affects the supraglacial drainage pattern. Lateral moraines have formed along both sides of the glacier, although the insolation aspect of the glacier has resulted in the development of a moraine 60 m high along its northern margin. Copyright © 2001 John Wiley & Sons, Ltd. [source] Deglaciation of the Irish Sea Basin: a critique of the glaciomarine hypothesisJOURNAL OF QUATERNARY SCIENCE, Issue 5 2001Danny McCarroll Abstract The glaciomarine model for deglaciation of the Irish Sea basin suggests that the weight of ice at the last glacial maximum was sufficient to raise relative sea-levels far above their present height, destabilising the ice margin and causing rapid deglaciation. Glacigenic deposits throughout the basin have been interpreted as glaciomarine. The six main lines of evidence on which the hypothesis rests (sedimentology, deformation structures, delta deposits, marine fauna, amino-acid ratios and radiocarbon dates) are reviewed critically. The sedimentological interpretation of many sections has been challenged and it is argued that subglacial sediments are common rather than rare and that there is widespread evidence of glaciotectonism. Density-driven deformation associated with waterlain sediments is rare and occurs where water was ponded locally. Sand and gravel deposits interpreted as Gilbert-type deltas are similarly the result of local ponding or occur where glaciers from different source areas uncoupled. They do not record past sea-levels and the ad hoc theory of ,piano-key tectonics' is not required to explain the irregular pattern of altitudes. The cold-water foraminifers interpreted as in situ are regarded as reworked from Irish Sea sediments that accumulated during much of the late Quaternary, when the basin was cold and shallow with reduced salinities. Amino-acid age estimates used in support of the glaciomarine model are regarded as unreliable. Radiocarbon dates from distinctive foraminiferal assemblages in northeast Ireland show that glaciomarine sediments do occur above present sea-level, but they are restricted to low altitudes in the north of the basin and record a rise rather than a fall in sea-level. It is suggested here that the oldest dates, around 17 000 yr BP, record the first Late Devensian (Weichselian) marine inundation above present sea-level. This accords with the pattern but not the detail of recent models of sea-level change. Copyright © 2001 John Wiley & Sons, Ltd. [source] Phylogeography of Douglas-fir based on mitochondrial and chloroplast DNA sequences: testing hypotheses from the fossil recordMOLECULAR ECOLOGY, Issue 9 2010PAUL F. GUGGER Abstract The integration of fossil and molecular data can provide a synthetic understanding of the ecological and evolutionary history of an organism. We analysed range-wide maternally inherited mitochondrial DNA and paternally inherited chloroplast DNA sequence data with coalescent simulations and traditional population genetic methods to test hypotheses of population divergence generated from the fossil record of Douglas-fir (Pseudotsuga menziesii), an ecologically and economically important western North American conifer. Specifically, we tested (i) the hypothesis that the Pliocene orogeny of the Cascades and Sierra Nevada caused the divergence of coastal and Rocky Mountain Douglas-fir varieties; and (ii) the hypothesis that multiple glacial refugia existed on the coast and in the Rocky Mountains. We found that Douglas-fir varieties diverged about 2.11 Ma (4.37 Ma,755 ka), which could be consistent with a Pliocene divergence. Rocky Mountain Douglas-fir probably resided in three or more glacial refugia. More variable molecular markers would be required to detect the two coastal refugia suggested in the fossil record. Comparison of mitochondrial DNA and chloroplast DNA variation revealed that gene flow via pollen linked populations isolated from seed exchange. Postglacial colonization of Canada from coastal and Rocky Mountain refugia near the ice margin at the Last Glacial Maximum produced a wide hybrid zone among varieties that formed almost exclusively by pollen exchange and chloroplast DNA introgression, not seed exchange. Postglacial migration rates were 50,165 m/year, insufficient to track projected 21st century warming in some regions. Although fossil and genetic data largely agree, each provides unique insights. [source] Cold-climate origin of the enclosed depressions and wetlands (,spungs') of the Pine Barrens, southern New Jersey, USAPERMAFROST AND PERIGLACIAL PROCESSES, Issue 4 2001Hugh M. French Abstract The ,frost-thaw' or thermokarst-lake-basin hypothesis, first invoked by P. E. Wolfe in 1953 to explain the enclosed depressions and shallow basins (,spungs') of southern New Jersey, is re-examined. The most probable explanation is that they formed in late Wisconsinan times as deflation hollows, or ,blowouts', when strong katabatic winds flowed southwards from the continental ice margin across the sparsely vegetated, tundra terrain of the Pine Barrens. Wedge structures and cryoturbation phenomena suggest the existence of either permafrost or deep seasonal frost, and imply mean annual air temperatures of between ,0.5 °C and ,6 °C. When the groundwater table rose in late-glacial times, the hollows became ponds or wetlands. These were utilized as early as 12,000 years ago by palaeoindian and early archaic cultures as hunting camp sites. Today, many of these wetlands are drying up as the regional water table falls in response to increased water usage from agriculture and urbanization. Copyright © 2001 John Wiley & Sons, Ltd. RÉSUMÉ L'hypothèse gel-dégel ou de bassins thermokarstiques, invoquée pour la première fois par P.E. Wolfe en 1953 pour expliquer les dépressions fermées et les bassins peu profonds (spungs) du sud du New Jersey, est réexaminée. L'explication la plus probable est que ces dépressions se sont formées à la fin du Wisconsin comme des creux de déflation au moment où des vents catabatiques violents soufflaient vers le sud sur la marge de la calotte glaciaire et sur la végétation éparse de la toundra qui couvrait les Pine Barrens. Des structures en coin et des cryoturbations suggèrent l'existence, soit d'un pergélisol, soit d'un gel profond saisonnier, et impliquent des températures annuelles de l'air comprises entre , 0.5 et , 6°C. Quand la nappe aquifère s'est élevée à la fin de la glaciation, les dépressions sont devenues des mares ou des zones humides. Elles furent utilisées comme sites de camps de chasse, il y a 12000 ans par des Paléoindiens et des cultures archaïques primitives. Aujourd'hui, beaucoup de ces terres humides disparaissent à la suite de l'abaissement régional de la nappe aquifère résultant d'un accroissement de l'utilisation d'eau par l'agriculture et l'urbanisation. Copyright © 2001 John Wiley & Sons, Ltd. [source] Stratigraphy, sedimentology, age and palaeoenvironment of marine varved clay in the Middle Swedish end-moraine zoneBOREAS, Issue 2 2010MARK D. JOHNSON Johnson, M. D. & Ståhl, Y. 2009: Stratigraphy, sedimentology, age and palaeoenvironment of marine varved clay in the Middle Swedish end-moraine zone. Boreas, 10.1111/j.1502-3885.2009.00124.x. ISSN 0300-9483 Deglaciation of the Middle Swedish end-moraine zone and age of the sediment in and between the moraines have been discussed for about a hundred years. The goal of this project was to determine the stratigraphy and age of the sediment in and between the moraines. Inter-moraine flats are underlain by clay, 10,25 m thick, overlying thin sand and gravel or till on bedrock. The clay is overlain by a few metres of sand and gravel. Much of the clay beneath the flats consists of rhythmites that grade from grey to red and are 2,74 cm thick. Our interpretation of these rhythmites as being varves is supported by grain size and mineralogical and elemental variations. Foraminifera and ostracods show that the clay was deposited in an arctic marine environment, while radiocarbon dating of the microfossils indicates that the clay was deposited 12 150 cal. 14C years ago, during the Younger Dryas chronozone (YD). Most of the optical stimulated luminescence dates on the clay are much older, containing quartz sand that was insufficiently bleached. The stratigraphy indicates that the moraines are composed of YD clay pushed into ridge forms during ice-front oscillations. It is not possible to determine how far north the Scandinavian Ice Sheet retreated prior to the YD advance. We neither support nor reject the suggestion that the ice margin retreated to the northern edge of Mt. Billingen during the Allerød, causing the Baltic Ice Lake to drain. [source] 14C-dated fluctuations of the western flank of the Scandinavian Ice Sheet 45,25 kyr BP compared with Bølling,Younger Dryas fluctuations and Dansgaard,Oeschger events in GreenlandBOREAS, Issue 2 2010JAN MANGERUD Mangerud, J., Gulliksen, S. & Larsen, E. 2009: 14C-dated fluctuations of the western flank of the Scandinavian Ice Sheet 45,25 kyr BP compared with Bølling,Younger Dryas fluctuations and Dansgaard,Oeschger events in Greenland. Boreas, 10.1111/j.1502-3885.2009.00127.x. ISSN 0300-9483. We present 32 accelerator mass spectrometry (AMS) 14C dates obtained on well-preserved bones from caves in western Norway. The resulting ages of 34,28 14C kyr BP demonstrate that the coast was ice-free during the so-called Ålesund Interstadial. New AMS 14C dates on shells aged 41,38 14C kyr BP are evidence of an earlier (Austnes) ice-free period. The Ålesund Interstadial correlates with Greenland interstadials 8,7 and the Austnes Interstadial with Greenland interstadials 12,11. Between and after the two interstadials, the ice margin reached onto the continental shelf west of Norway. These events can be closely correlated with the Greenland ice core stratigraphy, partly based on identification of the Laschamp and Mono Lake palaeomagnetic excursions. We found that the pattern of the NGRIP ,18O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1,GS 1 (Bølling,Younger Dryas) were strikingly similar, which leads us to suggest that the underlying causes of these climate shifts could have been the same. We therefore discuss some aspects of glacial fluctuations during the Bølling,Younger Dryas in order to elucidate processes during Dansgaard,Oeschger events. [source] Origin of Pleistocene outwash plains in various topographic settings, southern PolandBOREAS, Issue 2 2009TOMASZ SALAMON The style of Pleistocene outwash sedimentation in the foreland of the central European Mountains (the Carpathians and Sudetes) was controlled to a large extent by the topography. The deposits of three outwash plains formed in various morphological situations in front of the Upper Odra Lobe during the Odranian glaciation (older Saalian) are described here to show the conditions of their development and to reveal the relation between outwash plain sedimentology and proglacial topography. One outwash plain was formed between the mountain front and the ice-sheet margin, which advanced into the zone of fore-mountain alluvial fans. This outwash, deposited parallel to the ice margin, was under the considerable influence of extraglacial rivers flowing from the mountains. The second outwash was deposited in a small valley dipping away from the ice sheet and successively buried by glaciofluvial sediments. It evolved from a narrow valley sandur to an unconfined outwash plain. The third one was formed in a relatively broad, dammed valley dipping towards the ice sheet, where proglacial lake base level controlled the pattern of outwash channels as well as the character of the sedimentation. The studied outwash plains have different sedimentary successions. Their sedimentary profiles differ from each other even in the neighbouring valleys, indicating that distinct depositional conditions existed at the same time in closely spaced areas. It is suggested that the glaciomarginal deposition was controlled mostly by the orientation of the valleys and the inter-valley areas relative to the ice-sheet front. Size and morphology of valleys and interfluves were also important. Depending on their orientation, the outwash plains were fed by meltwaters in various ways; the dip of their surfaces was markedly different and the dynamics of the proglacial river systems were diverse. [source] Response of the Rhine,Meuse fluvial system to Saalian ice-sheet dynamicsBOREAS, Issue 3 2008FREEK S. BUSSCHERS A new reconstruction of the interaction between the Saalian Drente glaciation ice margin and the Rhine,Meuse fluvial system is presented based on a sedimentary analysis of continuous core material, archived data and a section in an ice-pushed ridge. Optically Stimulated Luminescence (OSL) was applied to obtain independent age control on these sediments and to establish a first absolute chronology for palaeogeographical events prior to and during the glaciation. We identified several Rhine and Meuse river courses that were active before the Drente glaciation (MIS 11-7). The Drente glaciation ice advance into The Netherlands (OSL-dated to fall within MIS 6) led to major re-arrangement of this drainage network. The invading ice sheet overrode existing fluvial morphology and forced the Rhine,Meuse system into a proglacial position. During deglaciation, the Rhine shifted into a basin in the formerly glaciated area, while the Meuse remained south of the former ice limit, a configuration that persisted throughout most of the Eemian and Weichselian periods. An enigmatic high position of proglacial fluvial units and their subsequent dissection during deglaciation by the Meuse may partially be explained by glacio-isostatic rebound of the area, but primarily reflects a phase of high base level related to a temporary proglacial lake in the southern North Sea area, with lake levels approximating modern sea levels. Our reconstruction indicates that full ,opening' of the Dover Strait and lowering of the Southern Bight, enabling interglacial marine exchange between the English Channel and the North Sea, is to be attributed to events during the end of MIS 6. [source] Late Quaternary development of the southern sector of the Greenland Ice Sheet, with particular reference to the Qassimiut lobeBOREAS, Issue 4 2004ANKER WEIDICK The evolution of the southern Greenland Ice Sheet is interpreted from a synthesis of geological data and palaeoclimatic information provided by the ice-sheet cores. At the Last Glacial Maximum the ice margin would have been at the shelf break and the ice sheet was fringed by shelf ice. Virtually all of the present ice-free land was glaciated. The initial ice retreat was controlled by eustatic sea level rise and was mainly by calving. When temperatures increased, melt ablation led to further ice-margin retreat and areas at the outer coast and mountain tops were deglaciated. Retreat was interrupted by a readvance during the Neria stade that may correlate with the Younger Dryas cooling. The abrupt temperature rise at the Younger Dryas,Holocene transition led to a fast retreat of the ice margin, and after ,9 ka BP the ice sheet was smaller than at present. Expansion of the ice cover began in the Late Holocene, with a maximum generally during the Little Ice Age. The greatest changes in ice cover occurred in lowland areas, i.e. in the region of the Qassimiut lobe. The date of the historical maximum advance shows considerable spatial variability and varies between AD 1600 and the present. Local anomalous readvances are seen at possibly 7,8 ka and at c. 2 ka BP. A marked relative sea level rise is seen in the Late Holocene; this is believed to reflect a direct glacio-isostatic response to increasing ice load. [source] Are long subglacial bedforms indicative of fast ice flow?BOREAS, Issue 3 2002CHRIS R. STOKES It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bed-forms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios 10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main ,trunk' and then diverging towards the inferred ice margin. The ,bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets. [source] Late Pleistocene glacial and lake history of northwestern RussiaBOREAS, Issue 3 2006EILIV LARSEN Five regionally significant Weichselian glacial events, each separated by terrestrial and marine interstadial conditions, are described from northwestern Russia. The first glacial event took place in the Early Weichselian. An ice sheet centred in the Kara Sea area dammed up a large lake in the Pechora lowland. Water was discharged across a threshold on the Timan Ridge and via an ice-free corridor between the Scandinavian Ice Sheet and the Kara Sea Ice Sheet to the west and north into the Barents Sea. The next glaciation occurred around 75,70 kyr BP after an interstadial episode that lasted c. 15 kyr. A local ice cap developed over the Timan Ridge at the transition to the Middle Weichselian. Shortly after deglaciation of the Timan ice cap, an ice sheet centred in the Barents Sea reached the area. The configuration of this ice sheet suggests that it was confluent with the Scandinavian Ice Sheet. Consequently, around 70,65 kyr BP a huge ice-dammed lake formed in the White Sea basin (the ,White Sea Lake'), only now the outlet across the Timan Ridge discharged water eastward into the Pechora area. The Barents Sea Ice Sheet likely suffered marine down-draw that led to its rapid collapse. The White Sea Lake drained into the Barents Sea, and marine inundation and interstadial conditions followed between 65 and 55 kyr BP. The glaciation that followed was centred in the Kara Sea area around 55,45 kyr BP. Northward directed fluvial runoff in the Arkhangelsk region indicates that the Kara Sea Ice Sheet was independent of the Scandinavian Ice Sheet and that the Barents Sea remained ice free. This glaciation was succeeded by a c. 20-kyr-long ice-free and periglacial period before the Scandinavian Ice Sheet invaded from the west, and joined with the Barents Sea Ice Sheet in the northernmost areas of northwestern Russia. The study area seems to be the only region that was invaded by all three ice sheets during the Weichselian. A general increase in ice-sheet size and the westwards migrating ice-sheet dominance with time was reversed in Middle Weichselian time to an easterly dominated ice-sheet configuration. This sequence of events resulted in a complex lake history with spillways being re-used and ice-dammed lakes appearing at different places along the ice margins at different times. [source] Modeling the deglaciation of the Green Bay Lobe of the southern Laurentide Ice SheetBOREAS, Issue 1 2004CORNELIA WINGUTH We use a time-dependent two-dimensional ice-flow model to explore the development of the Green Bay Lobe, an outlet glacier of the southern Laurentide Ice Sheet, leading up to the time of maximum ice extent and during subsequent deglaciation (c. 30 to 8 cal. ka BP). We focus on conditions at the ice-bed interface in order to evaluate their possible impact on glacial landscape evolution. Air temperatures for model input have been reconstructed using the GRIP ,8O record calibrated to speleothem records from Missouri that cover the time periods of c. 65 to 30 cal. ka BP and 13.25 to 12.4 cal. ka BP. Using that input, the known ice extents during maximum glaciation and early deglaciation can be reproduced reasonably well. The model fails, however, to reproduce short-term ice margin retreat and readvance events during later stages of deglaciation. Model results indicate that the area exposed after the retreat of the Green Bay Lobe was characterized by permafrost until at least 14 cal. ka BP. The extensive drumlin zones that formed behind the ice margins of the outermost Johnstown phase and the later Green Lake phase are associated with modeled ice margins that were stable for at least 1000 years, high basal shear stresses (c. 100 kPa) and permafrost depths of 80,200 m. During deglaciation, basal meltwater and sliding became more important. [source] | |||||||||||||