Glaciology of the British Isles Ice Sheet during the last glacial cycle: form, flow, streams and lobes

The last ice sheet over the British Isles, together with other mid-latitude Pleistocene ice sheets, and in contrast to the modern ice sheets of Greenland and Antarctica, had a relatively low profile, low summit elevation and extensive, elongated lobes at its margin. A thermo-mechanically coupled numerical ice sheet model, driven by a proxy climate, has been used to explore the properties that would have permitted these characteristics to develop. The approach, the key to quantitative palaeoglaciology, is to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle. Simulations show how a British ice sheet may have been confluent with a Scandinavian ice sheet during some parts of its history and how unforced periodic and asynchronous oscillations could occur in different parts of its margins. Marginal lobes are a reflection of streaming within the ice sheet. Such streams can be ephemeral, dynamic streams located because of ice sheet properties, or fixed streams whose location is determined by bed properties. The simulations that best satisfy constraints of extent, elevation and relative sea levels are those with major fixed streams that strongly draw down the ice sheet surface. In these, the core upland areas of the ice sheet were cold based at the Last Glacial Maximum, basal streaming velocities were between 500 and 1000 ma⁻¹ compared with surface velocities of 10–50 ma⁻¹ in inter-stream zones, shear stresses were as low as 15–25 kPa in streams compared with 70–110 kPa in upland areas and 60–84% of the ice flux was delivered to the margin via streams.     

Wastage features of the inland ice sheet in central South Norway

Detailed field mapping of different lateral phenomena, striae, texture and till fabric forms the basis of a reconstruction of five deglaciation phases in the east Jotunheimen-Gudbrandsdalen area, a land scape with moderate relief in the vicinity of the ice divide. During wastage, the inland ice sheet separated into several ice lobes situated in valleys enclosed by large ice-free uplands. The slope of the ice surface varied with local changes in the ice movement pattern and with the breaking of ice dams, which caused reversal of drainage from ice-dammed lakes. Non-climatic, large margianl moraines are thought to have fomed as a result of locally increased steepness of the ice surface. By tracing the deglaciation phases through two different valley systems which converge in the lake Mjøsa area, deglaciation events in the ice divide zone are correlated with marginal deposits at the front of the ice lobes.     

Thermo-mechanical facies representative of fast and slow flowing ice sheets: the Weichselian ice sheet,a central west Poland case study

This study deals with an issue of thermo-mechanical facies, reflecting specific thermal and mechanical properties of the subglacial environment. The main objective of this study was to develop a model of glacitectionic deformation and its sedimentary record beneath fast and slow flowing ice sheets, based on investigations conducted in Wielkopolska (west central Poland). Sedimentary structures, mainly at the contact between subglacial tills and glacifluvial sediments, were recognized to delineate typical facies associations in a Weichselian glacigenic succession. Each association was interpreted as a record of the different depositional environments related to different subglacial conditions. Those investigations suggest the substratum was composed of frozen and dry, and wet and mobile spots, and four thermo-mechanical facies were distinguished: A – is representative of slower ice flow, dry and cold subglacial conditions, where driving stresses and normal effective pressure were high; B – is also related to slow ice flow and occurrence of cold subglacial permafrost, but with little amount of unfrozen water (however, higher than in facies A), with similar physical characteristics of the ice sheet as facies A; thermo-mechanical facies C and D represent wet and warm ice sole, with low normal effective pressure and driving stresses, thus lowering sediments’ shear strength and enabling high ice-flow velocities. We suggest that these facies have specific and non-random location, thereby revealing the relationship between subglacial thermo-mechanical conditions and ice sheet dynamics. Slow moving, cold-based ice occurred along ice sheet margins and inter-stream areas, whereas fast-moving, warm-based, well-lubricated ice, was typical of the axial parts of ice streams.     

Periglacial phenomena in the wooded areas of Northern Sweden - relicts from the Tärendö Interstadial

Frost-shattered bedrock and ventifacts interpreted to be abraded by drifting snow or ice particles occur frequently in the wooded areas of northernmost Sweden. Ice-wedge casts and periglacial involutions are encountered more sporadically. The phenomena cannot be explained by the present or the Holocene climate and demonstrably pre-date the last deglaciation. The relation to glacial and non-glacial stratigraphy, and to ice flow during the latest glacial stade, strongly suggests that the periglacial activity dates from the local Tärendö Interstadial, tentatively correlated with Odderade. The presence of the periglacial features implies not only that different morphological formations survived the erosive impact of the last continental ice sheet but also that in wide areas the very ground surface, including ventifacts in primary position, is the same today as during the interstadial. A similar situation appears to prevail in northernmost Dalecarlia and parts of Härjedalen and Jämtland in central Sweden.     

Constraints on the Late Weichselian ice sheet over the Barents Sea from observations of raised shorelines

Direct evidence for Late Weichselian grounded glacier ice over extensive areas of the Barents Sea is based largely on indirect observations, including elevations of old shorelines on Svalbard and arguments of isostatic rebound. Such isostatic models are discussed here for two cases representing maximum and minimum ice-sheet reconstructions. In the former model the ice extends over the Kara Sea, whereas in the latter the ice is limited to the Barents Sea and island archipelagos. Comparisons of predictions with observations from a number of areas, including Spitsbergen, Nordaustlandet, Edgeøya, Kong Karls Land, Franz Josef Land, Novaya Zemlya and Finnmark, support arguments for the existence of a large ice sheet over the region at the time of the last glacial maximum. This ice sheet is likely to have had the following characteristics, conclusions that are independent of assumptions made about the Earth's rheological parameters. (i) The maximum thickness of this ice was about 1500–2000 m with the centre of the load occurring to the south and east of Kong Karls Land. (ii) The ice sheet extended out to the western edge of the continental shelf and its maximum thickness over western Spitsbergen was about 800 m. (iii) To the north of Svalberg and Frans Josef Land the ice sheet extended out to the northern shelf edge. (iv) Retreat of the grounded ice across the southern Barents Sea occurred relatively early such that this region was largely ice free by about 15,000 BP. (v) By 12,000 BP the grounded ice had retreated to the northern archipelagos and was largely gone by 10,000 BP. (vi) The ice sheet may have extended to the Kara Sea but ice thicknesses were only a fraction of those proposed in those reconstructions where the maximum ice thickness is centered on Novaya Zemlya. Models for the palaeobathymetry for the Barents Sea at the time of the last glacial maximum indicate that large parts of the Barents Sea were either very shallow or above sea level, providing the opportunity for ice growth on the emerged plateaux, as well as on the islands, but only towards the end of the period of Fennoscandian ice sheet build-up.     

Dynamics of the Late Weichselian ice sheet on Svalbard inferred from high-resolution sea-floor morphology

High-resolution bathymetric mapping of the fjords and continental shelf around the Svalbard archipelago shows an extensive pattern of large- and medium-scale submarine landforms formed by differences in ice-flow regimes. Mega-scale glacial lineations, lateral moraines, transverse ridges and glaciotectonic features are superimposed on the large-scale fjord, shelf and cross-shelf trough morphology of the margin. From these landforms we have inferred the flow and dynamics of the last ice sheet on Svalbard. Major fjords and their adjacent cross-shelf troughs have been identified as the main routes for ice streams draining the ice sheet. On the west coast of Svalbard major pathways existed along Bellsund, Isfjorden and Kongsfjorden. Along the northern Svalbard margin most of the ice drained through the Woodfjorden cross-shelf trough and Wijdefjorden-Hinlopen strait. Extensive areas with trough-parallel glacial lineations in the cross-shelf troughs suggest fast ice flow by palaeo-ice streams. Lateral ice-stream moraines, several tens of kilometres in length, have been mapped along the margins of some of the cross-shelf troughs, identifying the border zone between fast ice flow and stagnant or slow-flowing ice on intervening banks. Several general implications can be drawn from the interpretation of the glacier-derived submarine landforms around Svalbard. Firstly, the Late Weichselian ice sheet was partitioned into fast-flowing ice streams separated by slower moving ice. Secondly, our submarine morphological evidence supports earlier sedimentological, stratigraphical and chronological studies in implying that a large ice sheet reached the shelf edge around almost all of western and northern Svalbard in the Late Weichselian. The idea of a relatively restricted ice sheet over Svalbard, with ice-free conditions in some areas of the west coast at the Last Glacial Maximum, is therefore unlikely to be correct. Thirdly, the ice sheet appears to have retreated more rapidly from the cross-shelf troughs and outer fjords, although sometimes this occurred in a punctuated pattern indicated by grounding-zone wedges, and more slowly from the intervening shallower banks. In addition, a grounding zone for the ice sheet has been mapped at the shelf edge 10-20 km off the northwest coast of Svalbard, suggesting that ice did not reach the adjacent Yermak Plateau during the Late Weichselian.     

Supposed area-wasting of the Weichselian ice sheet in Denmark

The deglaciation at the end of the Weichselian in the Danish area has previously been considered to occur as a frontal wastage. Since the glacier ice was assumed to be debris-free, the wasting should be characterized by outwash plains and successions of end-moraines. The almost complete lack of sandur plains in the eastern part of the area and indications from recent investigations of widespread occurrence of flow till justify a re-evaluation of the mentioned deglaciation model.
Two morphological features have a general occurrence: the plains and the 'tunnel' valleys. The plains appear stepwise in the landscapes, and are frequently limited by steep slopes. Topmost is a subcircular kame-like hill. Sedimentologically, the plains mainly consist of melt water deposits, and the scattered occurrences of till are interpreted as flow till. The plains continue from the open landscape into the 'tunnel' valleys where they appear as terraces.
These features are considered to have been formed during the deglaciation. The almost horizontal surface of the ice sheet over large areas caused a sensitivity to changes in the climate. The wasting of the ice may therefore be expected to affect large areas almost simultaneously. On the assumption that the ice contained debris, an increasing amount of clastic matter was released on the ice surface. This material was concentrated in the depressions. If such a depression perforated the ice, the content of sediments settled on the substratum and a plain was established. During continued wasting the thickness of the ice decreased and the depressions were enlarged. They assumed the character of sandur plains. As still larger areas of these supraglacial sandurs rested on the basement the successive lower situated plains were formed. The latest ice was preserved where the 'tunnel' valleys are situated to-day.     

Geomorphic traces of a Weichselian ice stream in the Wielkopolska Lowland, western Poland

Geomorphological analysis of a digital elevation model reveals an extensive zone with uniformly oriented elongated landforms in the middle and eastern Wielkopolska Lowland, directly to the north of the maximum extent of the Weichselian Ice Sheet. Individual linear landforms are up to 10 km long, a few hundred metres wide, and with only a few metres of relief. The belts of linear landforms visible on the surfaces of the uplands are disrupted by subglacial channels and younger river valleys. The character and distribution of both landform types, in relation to the outlines of marginal zones of the Weichselian ice lobes, indicate that their origin was subglacial. The elongated landforms are interpreted as mega-scale glacial lineations (MSGLs) characteristic of palaeo-ice stream zones. The MSGLs occur in a zone 70 km long and 80 km wide and are distinctly divergent towards the maximum extent of the ice sheet. This arrangement demonstrates that they are the record of the terminal zone of the ice stream, whose full size was likely in the order of a few hundred kilometres in length.     

Late Weichselian/Devensian ice sheets in the North Sea and adjacent land areas.

A considerable discussion concerning the extent of the last Scandinavian and Scottish ice sheets has continued for several years. In contrast to earlier models based on an ice sheet extending to the edge of the continental shelf, recent proposals favor a limited geographical and vertical extent and imply that the Scandinavian and British ice sheets did not coalesce in the North Sea. These models indicate an ice-free, open embayment in the northern North Sea and areas of dry land in the southern North Sea region during the Late Weichselian/Devensian glacial maximum. Late Weichselian ice-sheet profiles from the North Sea to the adjacent land areas of southern Norway have been tentatively reconstructed. Low-gradient profiles in the present shelf areas are explained by unconsolidated, deformable sediments on the continental shelf inducing subglacial water pressure and low basal shear stress beneath marginal parts of the Scandinavian ice sheet. Combined with higher basal shear stress conditions in the present mainland areas, this explains the slightly concave and convex shape of the reconstructed ice-sheet profiles in the present coastal and inland areas of western Norway, respectively.     

Thinning of the ice sheet by melting: a preliminary model for calculating deglaciation rates over central Sweden about 10,000 B.P.

Thinning of the ice sheet took place by two different processes - by the melting of ice over the whole or part of the surface and by the outflow of ice from the central parts of the ice body. A simulation model should therefore include both types of processes. A first draft of a model for the melting is presented, together with some notes about other parts of a complete, complex model for the thinning of the ice.     

Palaeoglaciation of Bayan Har Shan,northeastern Tibetan Plateau: glacial geology indicates maximum extents limited to ice cap and ice field scales

Key locations within an extensive area of the northeastern Tibetan Plateau, centred on Bayan Har Shan, have been mapped to distinguish glacial from non‐glacial deposits. Prior work suggests palaeo‐glaciers ranging from valley glaciers and local ice caps in the highest mountains to a regional or even plateau‐scale ice sheet. New field data show that glacial deposits are abundant in high mountain areas in association with large‐scale glacial landforms. In addition, glacial deposits are present in several locations outside areas with distinct glacial erosional landforms, indicating that the most extensive palaeo‐glaciers had little geomorphological impact on the landscape towards their margins. The glacial geological record does indicate extensive maximum glaciation, with local ice caps covering entire elevated mountain areas. However, absence of glacial traces in intervening lower‐lying plateau areas suggests that local ice caps did not merge to form a regional ice sheet on the northeastern Tibetan Plateau around Bayan Har Shan. No evidence exists for past ice sheet glaciation. Copyright © 2009 John Wiley & Sons, Ltd.     

Reconstructing the last Irish Ice Sheet 1: changing flow geometries and ice flow dynamics deciphered from the glacial landform record

The glacial geomorphological record provides an effective means to reconstruct former ice sheets at ice sheet scale. In this paper we document our approach and methods for synthesising and interpreting a glacial landform record for its palaeo-ice flow information, applied to landforms of Ireland. New, countrywide glacial geomorphological maps of Ireland comprising >39,000 glacial landforms are interpreted for the spatial, glaciodynamic and relative chronological information they reveal. Seventy one ‘flowsets’ comprising glacial lineations, and 19 ribbed moraine flowsets are identified based on the spatial properties of these landforms, yielding information on palaeo-ice flow geometry. Flowset cross-cutting is prevalent and reveals a highly complex flow geometry; major ice divide migrations are interpreted with commensurate changes in the flow configuration of the ice sheet. Landform superimposition is the key to deciphering the chronology of such changes, and documenting superimposition relationships yields a relative ‘age-stack’ of all Irish flowsets. We use and develop existing templates for interpreting the glaciodynamic context of each flowset – its palaeo-glaciology. Landform patterns consistent with interior ice sheet flow, ice stream flow, and with time-transgressive bedform generation behind a retreating margin, under a thinning ice sheet, and under migrating palaeo-flowlines are each identified. Fast ice flow is found to have evacuated ice from central and northern Ireland into Donegal Bay, and across County Clare towards the south-west. Ice-marginal landform assemblages form a coherent system across southern Ireland marking stages of ice sheet retreat. Time-transgressive, ‘smudged’ landform imprints are particularly abundant; in several ice sheet sectors ice flow geometry was rapidly varying at timescales close to the timescale of bedform generation. The methods and approach we document herein could be useful for interpreting other ice sheet histories. The flowsets and their palaeo-glaciological significance that we derive for Ireland provide a regional framework and context for interpreting results from local scale fieldwork, provide major flow events for testing numerical ice sheet models, and underpin a data-driven reconstruction of the Irish Ice Sheet that we present in an accompanying paper – Part 2.     

Evidence for rapid ice flow and proglacial lake evolution around the central Strait of Magellan region,southernmost Patagonia

This paper presents a detailed palaeoglaciological reconstruction of ice sheet dynamics in the Seno Skyring, Seno Otway and Strait of Magellan region of the former Patagonian Ice Sheet, with a particular focus on previously hypothesised zones of rapid ice flow and the evolution of proglacial lakes. Geomorphological mapping from a combination of satellite imagery and oblique and vertical aerial photographs reveals a variety of glacial landforms that are grouped into several discrete flow‐sets and associated ice margin positions. The most distinct features are represented by flow‐sets of highly elongate streamlined glacial lineations on both sides of the Strait of Magellan. Based on the shape and dimensions of the flow‐sets and their abrupt lateral margins, a transverse and longitudinal variation in glacial lineation length and elongation ratio, and the reported presence of a potentially deformable bed and thrust moraines, the flow‐sets are interpreted as zones of rapid ice flow within the Otway and Magellan lobes. We hypothesise that this provides evidence for contemporaneous surge‐like advances within the lobes, which may explain the asymmetry in the lobate margin positions on either side of the strait. The mechanisms that initiated rapid flow are unclear, but are likely to have been influenced by internal factors such as a change in thermal/hydrological conditions at the bed. The topography of the region suggests ice‐dammed lakes would have formed as the ice lobes retreated. The westernmost of the former lakes, Lake Skyring, is delimited by a series of palaeo‐shorelines surrounding the present‐day lake Laguna Blanca and we reconstruct lake evolution based on manipulation of a digital elevation model. The size and orientation of meltwater channels and a large outwash plain indicate that Lake Skyring drained eastwards towards the Strait of Magellan, probably quite rapidly. We conclude that the potential for quasi‐independent surge‐like behaviour within adjacent lobes raises the possibility that, during climate‐driven ice expansion, some advances in this region may have been partly controlled by secondary internal feedback mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.     

The last ice sheet in Snowdonia

In Snowdonia there is a marked contrast between weathered summits, often with well‐developed blockfields or tors, and lower ice‐moulded terrain. The boundary is interpreted as a trimline marking the upper surface of the last ice sheet. This interpretation is supported by the presence of gibbsite, an end‐product of prolonged weathering, at the base of soils above but not below the trimline. The reconstructed ice surface reaches about 850 m above present sea‐level along an ice divide running NE–SW through the massif. There is no evidence to support the popular view that ice centred further south extended over Snowdonia, and breaching to form the major glacial troughs can be explained by the action of local ice. The field evidence presented here demonstrates that most models of the southern British and Irish Sea ice sheets are significantly flawed, the earliest being far too thick and the most recent far too thin. Copyright © 2000 John Wiley & Sons, Ltd.     

Pollen analysis of the 1973 ice core from Devon Island glacier,Canada

Meltwater from a 299-m-long ice core was filtered and analyzed for fossil pollen and spores. Pollen concentration was higher in the late Holocene and interglacial intervals (ca. 7 liter^?1) than in the early Holocene and Wisconsinan (ca. 1–2 liter^?1) ones. The late Holocene and interglacial assemblages were dominated by Alnus (alder), whereas the early Holocene and Wisconsinan ones were dominated by Betula (birch) and Artemisia (sage). During the Holocene and probably the last interglaciation, most of the pollen and spores were blown a minimum of 1000 km from low arctic shrub tundra and adjacent subarctic Picea (spruce) forest; these areas were dominated by the arctic air mass during the summer pollinating season. During the Wisconsinan-early Holocene, glacier ice and arctic air were more widespread and pollen sources were more distant; thus, at this time relatively little pollen was incorporated into the ice.The Devon ice-core data suggest that there should have been pollen in the continental ice sheet of Wisconsin time. When the ice sheet retreated this pollen would be carried by meltwater and redeposited with silt and clay together with contemporary pollen, producing an ecologically anomalous assemblage.     

Marginal formations of the last Kara and Barents ice sheets in northern European Russia

Glacial landforms in northern Russia, from the Timan Ridge in the west to the east of the Urals, have been mapped by aerial photographs and satellite images supported by field observations. An east-west trending belt of fresh hummock-and-lake glaciokarst landscapes has been traced to the north of 67°N. The southern boundary of these landscapes is called the Markhida Line, which is interpreted as a nearly synchronous limit of the last ice sheet that affected this region. The hummocky landscapes are subdivided into three types according to the stage of postglacial modification: Markhida, Harbei and Halmer. The Halmer landscape on the Uralian piedmont in the east is the freshest, whereas the westernmost Markhida landscape is more eroded. The west-east gradient in morphology is considered to be a result of the time-transgressive melting of stagnant glacier ice and of the underlying permafrost. The pattern of ice-pushed ridges and other directional features reflects a dominant ice flow direction from the Kara Sea shelf. Traces of ice movement from the central Barents Sea are only discernible in the Pechora River left bank area west of 50°E. In the Polar Urals the horseshoe-shaped end moraines at altitudes of up to 560 m a.s.l. reflect ice movement up-valley from the Kara Ice Sheet, indicating the absence of a contemporaneous ice dome in the mountains. The Markhida moraines, superimposed onto the Eemian strata, represent the maximum ice sheet extent in the western part of the Pechora Basin during the Weichselian. The Markhida Line truncates the huge arcs of the Laya-Adzva and Rogovaya ice-pushed ridges protruding to the south. The latter moraines therefore reflect an older ice advance, probably also of Weichselian age. Still farther south, fluvially dissected morainic plateaus without lakes are of pre-Eemian age, because they plunge northwards under marine Eemian sediments. Shorelines of the large ice-dammed Lake Komi, identified between 90 and 110 m a.s.l. in the areas south of the Markhida Line, are radiocarbon dated to be older than 45 ka. The shorelines, incised into the Laya-Adzva moraines, morphologically interfinger with the Markhida moraines, indicating that the last ice advance onto the Russian mainland reached the Markhida Line during the Middle or Early Weichselian, before 45 ka ago.     

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

This study investigates the marginal subglacial bedrock bedforms of Jakobshavns Isbrae, West Greenland, in order to examine the processes governing bedform evolution in ice stream and ice sheet areas, and to reconstruct the interplay between ice stream and ice sheet dynamics. Differences in bedform morphology (roche moutonnee or whaleback) are used to explore contrasts in basal conditions between fast and slow ice flow. Bedform density is higher in ice stream areas and whalebacks are common. We interpret that this is related to higher ice velocities and thicker ice which suppress bed separation. However, modification of whalebacks by plucking occurs during deglaciation due to ice thinning, flow deceleration, crevassing and fluctuations in basal water pressure. The bedform evidence points to widespread basal sliding during past advances of Jakobshavns Isbrae. This was encouraged by increased basal temperatures and melting at depth, as well as the steep marginal gradients of Jakobshavns Isfjord which allowed rapid downslope evacuation of meltwater leading to strong ice/bedrock coupling and scouring. In contrast to soft-bedded ice stream bedforms, the occurrence of fixed basal perturbations and higher bed roughness in rigid bed settings prevents the basal ice subsole from maintaining a stable form which, coupled with secondary plucking, counteracts the development of bedforms with high elongation ratios. Cross-cutting striae and double-plucked, rectilinear bedforms suggest that Jakobshavns Isbrae became partially unconfined during growth phases, causing localised diffluent flow and changes in ice sheet dynamics around Disko Bugt. It is likely that Disko Bugt harboured a convergent ice flow system during repeated glacial cycles, resulting in the formation of a large coalesced ice stream which reached the continental shelf edge.     

The subglacial thermal organisation (STO) of ice sheets

This paper examines ice-sheet wide variations in subglacial thermal regime and ice dynamics using the landform record exposed on the beds of former mid-latitude ice sheets (the Laurentide, Cordilleran, Fennoscandian and British-Irish Ice Sheets). We compare the landform patterns beneath these former ice sheets to the flow organisation beneath parts of the contemporary Antarctic Ice Sheet inferred from RADARSAT-1 Antarctic Mapping Project (RAMP) data. The evidence preserved in the landform record and observed on contemporary ice masses can be grouped into four major ice-dynamical components that collectively define the subglacial thermal organisation (STO) of ice sheets. These ice-dynamical components are frozen-bed patches, ice streams, ice-stream tributaries and lateral shear zones. Frozen-bed patches appear at a wide range of spatial scales, spanning four orders of magnitude. In some areas, frozen-bed zones comprise large proportions of the bed (e.g. near the ice divide in continental areas), whilst in other areas they constitute isolated “islands” in areas dominated by thawed-bed conditions. Ice streams, narrow zones of fast flow in ice sheets that are otherwise dominated by slow sheet flow, are also common features of Quaternary ice sheets. Tributaries to ice streams flow at velocities intermediate between full ice-stream and sheet flow, and may divert ice drainage from one primary ice-stream corridor to an adjacent one. Sharp lateral boundaries between landforms indicate sliding and non-sliding conditions, respectively. These lateral boundaries represent important discontinuities in the glacial landscape and mark the location of shear zones between thawed-bed ice streams and intervening frozen-bed areas. We use the landform evidence in the area around Great Bear Lake, Canada to trace the evolution of an ice-stream web through time, demonstrating that frozen-bed patches are integral components of this complex system. We conclude that frozen-bed patches are important for the stability of ice sheets because they laterally constrain and isolate peripheral drainage basins and their ice streams.     

Periglacial trimlines,former nunataks and the altitude of the last ice sheet in Wester Ross,northwest Scotland

High-level weathering limits separating ice-scoured topography from frost-weathered detritus were identified on 28 mountains in Wester Ross at altitudes of 700–960 m, and a further 22 peaks support evidence of ice scouring to summit level. Weathering limits are defined most clearly on sandstone and gneiss, which have resisted frost shattering during the Late Devensian Lateglacial, but can also be distinguished on schists and quartzite. Schmidt hammer measurements and analyses of clay mineral assemblages indicate significantly more advanced rock and soil weathering above the weathering limits. The persistence of gibbsite above weathering limits indicates that they represent the upper limit of Late Devensian glacial erosion. The regular decline of weathering-limit altitudes along former flowlines eliminates the possibility that the weathering limits represent former thermal boundaries between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines that define the maximum surface altitude of the last ice sheet. Calculated basal shear stresses of 50–95 kPa are consistent with this interpretation. Reconstruction of ice-sheet configuration indicates that the former ice-shed lay above 900 m along the present watershed, and that the ice surface descended northwestwards, with broad depressions along major troughs and localised domes around independent centres of ice dispersal. Extrapolation of the ice surface gradient and altitude suggests that the ice sheet did not overrun the Outer Hebrides, but was confluent with the independent Outer Hebrides ice-cap in the North Minch basin. Erratics located up to 140 m above the reconstructed ice surface are inferred to have been emplaced by a pre-Late Devensian ice sheet (or ice sheets) of unknown age. © 1997 John Wiley & Sons, Ltd.     

中国极地冰盖数值模拟研究进展北大核心CSCD

冰盖数值模拟是一种基于多源观测数据,通过构建并求解冰流动力学方程组,理解冰流运动物理机制以及诊断和预估其演化过程的方法,目前已被广泛应用于冰盖变化研究。本文简要介绍了极地冰盖数值模拟方法,归纳综述了近十余年我国学者在极地冰盖数值模拟方面的研究进展,厘清我国在冰盖数值模拟领域遇到的瓶颈和关键问题。阐述了如何与我国的极地冰盖科考优势区域深度结合,协同多源强化观测和数值模拟,研发和改进冰盖模式,提高冰盖模拟能力,对定量估算极地冰盖的物质平衡及其对未来海平面上升的影响做出实质贡献。通过逐步发展冰盖模式的研究能力,有望将来在冰盖关键动力过程和机制的科学认识上有所突破。