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1.
While contributing <1 m equivalent eustatic sea‐level rise the British Isles ice sheet produced glacio‐isostatic rebound in northern Britain of similar magnitude to eustatic sea‐level change, or global meltwater influx, over the last 18 000 years. The resulting spatially variable relative sea‐level changes combine with observations from far‐field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment, local ice‐sheet history and global meltwater influx. After a review of the attributes of relative sea‐level observations significant for constraining large‐scale models of the isostatic adjustment process we summarise long records of relative sea‐level change from the British Isles and far‐field locations. We give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the British Isles and far‐field for a range of Earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the British Isles. For the first time we show a good degree of fit between relative sea‐level observations and predictions that are based upon global Earth and ice model parameters, independently derived from analysis of far‐field data, with a terrain‐corrected model of the British Isles ice sheet that includes extensive glaciation of the North Sea and western continental shelf, that does not assume isostatic equilibrium at the Last Glacial Maximum and keeps to trimline constraints of ice surface elevation. We do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local‐scale detail of the ice model in order to provide a fit for all sites, but argue that the next stage should be to incorporate an ice‐sheet model that is based on quantitative, glaciological model simulations. We hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology, glaciology, sea‐level change, Earth rheology and quantitative modelling. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

2.
During the last main phase of glaciation (26–13 ka) an ice‐cap developed in southwest Ireland and ice, from a dispersal centre in the vicinity of Kenmare, flowed north through the Gap of Dunloe in the Macgillycuddy's Reeks. On surrounding hillsides a weathering limit separates ice‐moulded bedrock, on low ground, from frost‐weathered terrain above. Assessment of bedrock dilation joint characteristics, Schmidt hammer R‐value data and clay‐sized mineral contents of basal soil samples, demonstrate significant contrasts in the degree of weathering above and below this limit. The weathering limit declines in altitude along former ice flow‐lines and is confluent with morainic deposits on the eastern side of the Gap. This supports the assertion that the high‐level weathering limit is a periglacial trimline that marks the former maximum upper limit of the body of ice which occupied the Gap of Dunloe during the Last Glacial Maximum (LGM). Reconstruction of the former ice‐surface profile from periglacial trimline limits on the eastern side of the Gap yields a mean estimate for basal shear stress of 106.5 kPa. This value suggests that the ice mass which occupied the Gap of Dunloe at the LGM was warm based and flowed on a bedrock substrate. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

3.
X-ray diffraction analyses of soils above and below a periglacial trimline developed across the basalts of the Trotternish Escarpment (Isle of Skye, Scotland) demonstrate that gibbsite is restricted to soils above the trimline. This suggests that the gibbsite is a relict of pre-Late Devensian weathering, and that the trimline did not develop after the last ice sheet achieved its maximum thickness. The sharpness of the boundary between frost-weathered regolith and gibbsitic soils upslope and ice-scoured bedrock associated with gibbsite-free soils downslope suggests that the trimline represents the altitude of the last ice sheet at its maximum thickness rather than a former boundary between passive cold-based ice and erosive warm-based ice. These findings illustrate how identification of high-level periglacial trimlines and associated contrasts in clay mineralogy provide a means for constraining reconstructions of the form of the last ice sheets.  相似文献   

4.
Degree of rock surface weathering was measured on sites in Oldedalen and Brigsdalen, where dates of deglaciation have been estimated. and on an altitudinal transect on the slopes of Skåla. representing one of the highest supra-marine reliefs in western Norway. The Schmidt hammer is useful only for distinguishing sites deglaciated during the Little Ice Age from those deglaciated during the Lateglacial and early Holocene. Degree of roughness of granitic augen gneiss bedrock surfaces was quantified from profiles measured in situ using a micro-roughness-meter and profile gauge. There is a significant increase in surface roughness above a clear trimline at c. 1350 m a.s.I. but no significant increase above a higher trimline previously proposed as the vertical limit of the last ice sheet in this area (c. 1560 m a.s.I.). The roughness of boulder surfaces on the summit blockfield does not direr significantly from the roughness of bedrock surfaces downslope as far as the lower trimline. These unexpected results suggest that bedrock surfaces between the two trimlines were not glacially abraded during the Late Weichselian, so that the upper trimline is unlikely to represent the vertical limit of ice during either the Late Weichselian or a subsequent readvance. Preliminary results of 10Be dating of surface quartz samples from above the lower trimline support the proposal that the site was not abraded during the last glaciation. The results can be interpreted in two ways: (1) The upper trimline represents the vertical limit of a pre-Late Weichselian advance. During the Late Weichselian the mountains were completely covered but surfaces down to the lower trimline were protected by cold-based ice. (2) The lower trimline marks the vertical limit of the Late Weichselian ice and the upper limit an older and more extensive glaciation.  相似文献   

5.
Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km2 in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses.  相似文献   

6.
Analysis of soil samples from above and below trimlines representing the upper limit of glacial erosion at the Last Glacial Maximum demonstrates that soils with prolonged weathering histories above such trimlines yield significantly different mineral magnetic signatures from soils below trimlines. The nature of the contrast is conditioned by lithology. Basalt soils above the trimline yield significantly higher values of concentration‐dependent magnetic parameters (χ, χarm, IRM3T, soft IRM and hard IRM) than those below the trimline, due probably to transformation of non‐magnetic iron‐bearing minerals into magnetic forms. Conversely, for sandstone soils most magnetic parameters yield significantly lower values for above‐trimline samples, probably reflecting loss of ferrimagnetic minerals by dissolution and oxidation to aniferrimagnetic forms. These significant contrasts represent a new approach to validating high‐level weathering limits as periglacial trimlines cut at the Last Glacial Maximum. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

7.
Geomorphological evidence indicates that Donegal was formerly occupied by an ice dome that extended offshore to the west, northwest and north and was confluent with adjacent ice masses to the east and south. Erosive warm‐based ice over‐rode almost all the highest mountains, implying an ice‐divide altitude greater than 700 m. Only six peripheral summits escaped glacial modification, implying either that they remained above the ice surface as nunataks or supported a thin cover of protective cold‐based ice. Gibbsite, a pre‐last glacial weathering product, is preferentially represented on summits that escaped glacial modification. Cosmogenic 10Be exposure ages of 18.6 ± 1.4 to 15.9 ± 1.0 k yr for coastal sites confirm that Donegal ice extended offshore at the last glacial maximum. Reconstruction of the form of the Donegal ice dome suggests a former minimum ice thickness of ～500 m close to the present coastline in the west and northwest, and ～400 m near the coast of the Inishowen Peninsula in the north, with the ice extending at least 20 km across the adjacent shelf to the west and northwest. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

8.
High‐resolution swath bathymetry and TOPAS sub‐bottom profiler acoustic data from the inner and middle continental shelf of north‐east Greenland record the presence of streamlined mega‐scale glacial lineations and other subglacial landforms that are formed in the surface of a continuous soft sediment layer. The best‐developed lineations are found in Westwind Trough, a bathymetric trough connecting Nioghalvfjerdsfjorden Gletscher and Zachariae Isstrøm to the continental shelf edge. The geomorphological and stratigraphical data indicate that the Greenland Ice Sheet covered the inner‐middle shelf in north‐east Greenland during the most recent ice advance of the Late Weichselian glaciation. Earlier sedimentological and chronological studies indicated that the last major delivery of glacigenic sediment to the shelf and Fram Strait was prior to the Holocene during Marine Isotope Stage 2, supporting our assertion that the subglacial landforms and ice sheet expansion in north‐east Greenland occurred during the Late Weichselian. Glacimarine sediment gravity flow deposits found on the north‐east Greenland continental slope imply that the ice sheet extended beyond the middle continental shelf, and supplied subglacial sediment direct to the shelf edge with subsequent remobilisation downslope. These marine geophysical data indicate that the flow of the Late Weichselian Greenland Ice Sheet through Westwind Trough was in the form of a fast‐flowing palaeo‐ice stream, and that it provides the first direct geomorphological evidence for the former presence of ice streams on the Greenland continental shelf. The presence of streamlined subglacially derived landforms and till layers on the shallow AWI Bank and Northwind Shoal indicates that ice sheet flow was not only channelled through the cross‐shelf bathymetric troughs but also occurred across the shallow intra‐trough regions of north‐east Greenland. Collectively these data record for the first time that ice streams were an important glacio‐dynamic feature that drained interior basins of the Late Weichselian Greenland Ice Sheet across the adjacent continental margin, and that the ice sheet was far more extensive in north‐east Greenland during the Last Glacial Maximum than the previous terrestrial–glacial reconstructions showed. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
This paper investigates the processes governing bedrock bedform evolution in ice sheet and ice stream areas in central West Greenland, and explores the evidence for a cross‐shelf ice stream at the Last Glacial Maximum (LGM). To the east of Sisimiut the formation of streamlined bedforms with high elongation ratios and high bedform density has been controlled by geological structure and topography in slow‐flowing ice sheet areas. At the coast, the effects of regional flow convergence, caused by coastal fjord orientation, routed ice into the Sisimiut/Itilleq area where it formed an ice stream onset zone. This funnelled ice into an offshore trough (Holsteinsborg Dyb), resulting in a southwesterly regional ice flow direction and the formation of a topographically routed ice stream (Holsteinsborg Isbrae). To the south of this, striae and bedform evidence show that local valley glaciers initially flowed east to west across the coast, but were later redirected by the Itilleq Fjord ice which turned southwestward due to diffluent flow and deflection by Holsteinsborg Isbrae. Roches moutonnées in this area have low elongation ratios and high bedform density, but do not provide unequivocal support for ice streaming, as they are a product of both bedrock structure and changes in ice flow direction, rather than enhanced flow velocities. Cosmogenic surface exposure ages limit maximum ice sheet surface elevation to ca. 755–810 m above sea level in this region. Such ice thickness enabled Holsteinsborg Isbrae to reach the mid/outer continental shelf during the LGM, and to contribute to the formation of a trough mouth fan and the Outer Hellefisk moraines. Initial deglaciation across this region was driven by rising sea level and increasing air temperatures prior to the Bølling Interstadial at ca. 14.5 cal. ka BP. Between 12 and 10 cal. ka BP both increased air and ocean temperatures post the Younger Dryas, and peak sea‐level rise up to the marine limit, caused accelerated thinning and marginal retreat through calving, although dating evidence suggests ice streams remained along the inner shelf/coast boundary until at least ca. 10 cal. ka BP, their longevity maintained by increased ice thickness and ice discharge. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Ascertaining the location of palaeo‐ice streams is crucial in order to produce accurate reconstructions of palaeo‐ice sheets and examine interactions with the ocean–climate system. This paper reports evidence for a major ice stream in Amundsen Gulf, Canadian Arctic Archipelago. Mapping from satellite imagery (Landsat ETM+) and digital elevation models, including bathymetric data, is used to reconstruct flow‐patterns on southwestern Victoria Island and the adjacent mainland (Nunavut and Northwest Territories). Several flow‐sets indicative of ice streaming are found feeding into the marine trough and cross‐cutting relationships between these flow‐sets (and utilising previously published radiocarbon dates) reveal several phases of ice stream activity centred in Amundsen Gulf and Dolphin and Union Strait. A large erosional footprint on the continental shelf indicates that the ice stream (ca. 1000 km long and ca. 150 km wide) filled Amundsen Gulf, probably at the Last Glacial Maximum. Subsequent to this, the ice stream reorganised as the margin retreated back along the marine trough, eventually splitting into two separate low‐gradient lobes in Prince Albert Sound and Dolphin and Union Strait. The location of this major ice stream holds important implications for ice sheet–ocean interactions and specifically, the development of Arctic Ocean ice shelves and the delivery of icebergs into the western Arctic Ocean during the late Pleistocene. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

11.
Models of glacio‐hydroisostatic sea‐level change have been published for the British Isles that are broadly consistent with the observational evidence, as well as with glaciological constraints. It has been argued, however, that the models fail to represent sea‐level change along the Irish Sea margins and in southern Ireland for the post‐deglaciation period. The argument rests on the interpretation of the depositional environment of the elevated ‘Irish Sea Drift’ on both sides of the Irish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the British Isles are consistent with the former interpretation in that sea‐levels on either side of the Irish Sea, south of about the Isle of Man, are not predicted to have risen above present sea‐level at any time since the deglaciation of the Irish Sea. This implies that ice over both the Irish Sea and Ireland was relatively thin (ca. 600–700 m over Ireland). If the glaciomarine interpretation of the elevated Irish Sea Drift is correct, then the maximum ice thickness over central and southern Ireland would have to reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea‐level change to be consistent with the Holocene evidence, this thick ice sheet could not have extended to the eastern side of the Irish Sea. Nor could it have been very thick at its northern and western limits. If such an ice model is extreme and incompatible with glaciological observations then the alternative is to accept the interpretation of the Irish Sea Drift as terrestrial in origin. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

12.
Blockfields, weathering boundaries and marginal moraines have been mapped along a longitudinal transect from northern Andøya to Skånland in northern Norway. The degree of rock-surface weathering above and below glacial trimlines, clay-mineral assemblages and surface exposure dating based on in situ cosmogenic 10Be have been used to reconstruct the vertical dimensions and timing of the Last Glacial Maximum (LGM) of the Scandinavian Ice Sheet in this region. The cosmogenic exposure dates suggest that the lower blockfield boundary/trimline along the Andøya-Skånland transect represents the upper limit of the Late Weichselian ice sheet, with an average surface gradient of c . 9.5 m/km. The surface exposure dates from Andøya pre-date the LGM, suggesting that the LGM ice sheet did not reach mountain plateaux at northwest Andøya. The results thus support evidence from lake sediment records that the northern tip of Andøya was not covered by the Scandinavian Ice Sheet during the LGM.  相似文献   

13.
The influence of glacier hydrology on the time-dependent morphology and flow behaviour of the late Weichselian Scandinavian ice sheet is explored using a simple one-dimensional ice sheet model. The model is driven by orbitally induced radiation variations, ice-albedo feedback and eustatic sea-level change. The influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet, where a marginal zone of rapid sliding, thin ice and low surface slopes develops. Such a zone is absent when hydrology is omitted from the model, and its formation results in earlier and more rapid deglaciation than occurs in the no-hydrology model. The final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr BP. Channelised subglacial drainage develops only episodically, and is associated with relatively low meltwater discharges and high hydraulic gradients. The predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting. Lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surface profile.  相似文献   

14.
On North Harris and southeast Lewis a weathering limit separates glacially-moulded bedrock on low ground from frost-shattered bedrock and blockfields on high plateaux. Analysis of the depths of horizontal stress-release joints demonstrates significant contrasts in bedrock weathering above and below this boundary, and the survival of gibbsite only in soils above the weathering limit indicates that it represents the upper limit of Late Devensian glacial erosion. The weathering limit declines regularly in altitude on either side of the former ice shed, and is therefore interpreted as a periglacial trimline defining the upper limit of a locally-nourished ice mass at its maximum extent, rather than a former thermal boundary between protective cold-based and erosive warm-based ice. Calculated basal shear stress values are consistent with this interpretation. The configuration of the trimline indicates that at the last glacial maximum the area supported an ice cap that achieved a maximum altitude of ca. 700 m above present sea level and declined in altitude to the west-northwest and east-southeast at an average gradient of ca. 20 m km?1. Extrapolation of the dimensions of this ice cap suggests that it terminated ca. 7–10 km west of the present coast of Harris, and was confluent with mainland ice a short distance east of the present coastline.  相似文献   

15.
The glacial isostatic adjustment (GIA) of the British Isles is complex due to the interplay between local and non‐local signals. A number of recent studies have modelled the GIA response of the British Isles using relative sea‐level data. This study extends these previous analyses by using output from a numerical glaciological model as input to a GIA model. This is a necessary step towards more realistic GIA models, and although there have been similar studies for the major late Pleistocene ice sheets, this is the first study to do so for the British Isles. We test three reconstructions, classed as ‘minimal’, ‘median’ and ‘maximal’ in terms of their volume at maximum extent, and find it is possible to obtain good data–model fits. The minimal reconstruction is clearly preferred by the sea‐level data. The ice reconstructions tested were not constrained by geomorphological information of past ice extent (lateral and vertical). As a consequence, the reconstructions extend further than much of this information suggests, particularly in terms of ice thickness. It is notable, however, that the reconstructions produce good fits to many sea‐level data from central, mountainous regions (e.g. Scottish highlands), which lends support to the suggestion that trimlines, often used as an constraint on the palaeo ice surface location, are in fact features formed at the transition from warm‐ to cold‐based ice and so mark a minimum constraint on the ice surface altitude. Based on data–model misfits, suggestions are made for improving the ice model reconstructions. However, in many locations, the cause of the misfit could be due to non‐local, predominantly Fennoscandian ice and so interpretation is not straightforward. As a result, we suggest that future analyses of this type consider models and observations for both Fennoscandia and the British Isles. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
The Tyne Gap is a wide pass, situated between the Scottish Southern Uplands and the English Pennines that connects western and eastern England. It was a major ice flow drainage pathway of the last British–Irish Ice Sheet. This study presents new glacial geomorphological and sedimentological data from the Tyne Gap region that has allowed detailed reconstructions of palaeo‐ice flow dynamics during the Late Devensian (Marine Isotope Stage 2). Mapped lineations reveal a complex palimpsest pattern which shows that ice flow was subject to multiple switches in direction. These are summarised into three major ice flow phases. Stage I was characterised by convergent Lake District and Scottish ice that flowed east through the Tyne Gap, as a topographically controlled ice stream. This ice stream was identified from glacial geomorphological evidence in the form of convergent bedforms, streamlined subglacial bedforms and evidence for deformable bed conditions; stage II involved northerly migration of the Solway Firth ice divide back into the Southern Uplands, causing the easterly flow of ice to be weakened, and resulting in southeasterly flow of ice down the North Tyne Valley; and stage III was characterised by strong drawdown of ice into the Irish Sea Ice Basin, thus starving the Tyne Gap of ice and causing progressive ice sheet retreat westwards back across the watershed, prior to ice stagnation. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

17.
In the UK, a combination of outcrop mapping, satellite digital elevation models, high‐resolution marine geophysical data and a range of dating techniques have constrained the maximum limit and overall retreat behaviour of the British and Irish Ice Sheet (BIIS). The changing styles of deglaciation have been most extensively studied in the west and north‐western sectors of the BIIS, primarily using offshore geophysical surveys. The surviving record in the southern, terrestrial sector is fragmentary, permitting only large‐scale (tens of kilometres) and longer timescale (c. 1 ka) reconstructions of ice‐margin movement, with limited information on deglacial processes. Here we present a high‐resolution study of the retreat behaviour for a section of the southern ice‐margin from Windermere in the Lake District, using high‐resolution two‐dimensional multi‐channel seismic data, processed using prestack depth migration. By combining the seismic stratigraphy with landform morphologies, extant cores and seismic velocity measurements, we are able to distinguish between: over‐consolidated till; recessional moraines; De Geer moraines; flowed till/ice‐front fan; supra‐/en‐glacial melt‐out till; and subsequent glaciolacustrine/lacustrine sedimentation. The results reveal a complex and active valley glacier withdrawal from Windermere that changed character between basins and produced two small, localized areas of ice‐stagnation and downwasting. This study indicates that similar active ice‐margin retreats probably took place in other valleys of the Lake District during the Late Devensian deglaciation rather than the previously held view of rapid ice‐stagnation and downwasting. When combined with the regional terrestrial record, this supports a model of early ice loss in terrestrial England compared with other parts of the UK. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
Matthias Kuhle   《Tectonophysics》2007,445(1-2):116
Since 1973 new data were obtained on the maximum extent of glaciation in High Asia. Evidence for an ice sheet covering Tibet during the Last Glacial Period means a radical rethinking about glaciation in the Northern Hemisphere. The ice sheet's subtropical latitude, vast size (2.4 million km2) and high elevation (6000 m asl) are supposed to have resulted in a substantial, albedo-induced cooling of the Earth's atmosphere and the disruption of summer monsoon circulation. Moraines were found to reach down to 460 m asl on the southern flank of the Himalayas and to 2300 m asl on the northern slope of the Tibetan Plateau, in the Qilian Shan region. On the northern slopes of the Karakoram, Aghil and Kuen-Lun mountains, moraines occur as far down as 1900 m asl. In southern Tibet radiographic analyses of erratics suggest a former ice thickness of at least 1200 m. Glacial polish and roches moutonnées in the Himalayas and Karakoram suggest former glaciers as thick as 1200–2700 m. On the basis of this evidence, a 1100–1600 m lower equilibrium line (ELA) has been reconstructed, resulting in an ice sheet of 2.4 million km2, covering almost all of Tibet. Radiometric ages, obtained by different methods, classify this glaciation as isotope stage 3–2 in age (Würmian = last glacial period). With the help of 13 climate measuring stations, radiation- and radiation balance measurements have been carried out between 3800 and 6650 m asl in Tibet. They indicate that the subtropical global radiation reaches its highest energies on the High Plateau, thus making Tibet today's most important heating surface of the atmosphere. At glacial times 70% of those energies were reflected into space by the snow and firn of the 2.4 million km2 extended glacier area covering the upland. As a result, 32% of the entire global cooling during the ice ages, determined by the albedo, were brought about by this area — now the most significant cooling surface. The uplift of Tibet to a high altitude about 2.75 Ma ago, coincides with the commencement of the Quaternary Ice Ages. When the Plateau was lifted above the snowline (= ELA) and glaciated, this cooling effect gave rise to the global depression of the snowline and to the first Ice Age. The interglacial periods are explained by the glacial-isostatic lowering of Tibet by 650 m, having the effect that the initial Tibet ice – which had evoked the build-up of the much more extended lowland ices – could completely melt away in a period of positive radiation anomalies. The next ice age begins, when – because of the glacial-isostatic reverse uplift – the surface of the Plateau has again reached the snowline. This explains, why the orbital variations (Milankovic-theory) could only have a modifying effect on the Quaternary climate dynamic, but were not primarily time-giving: as long as Tibet does not glaciate automatically by rising above the snowline, the depression in temperature is not sufficient for initiating a worldwide ice age; if Tibet is glaciated, but not yet lowered isostatically, a warming-up by 4 °C might be able to cause an important loss in surface but no deglaciation, so that its cooling effect remains in a maximum intensity. Only a glaciation of the Plateau lowered by isostasy, can be removed through a sufficiently strong warming phase, so that interglacial climate conditions are prevailing until a renewed uplift of Tibet sets in up to the altitude of glaciation.An average ice thickness for all of Tibet of approximately 1000 m would imply that 2.2 million km3 of water were stored in the Tibetan ice sheet. This would correspond to a lowering in sea level of about 5.4 m.  相似文献   

19.
The offshore sector around Shetland remains one of the least well-studied parts of the former British–Irish Ice Sheet with several long-standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice-Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice-sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi-proxy suite of new isotopic age assessments, including 32 cosmogenic-nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector-wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP , this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km2, accompanied by several ice-margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re-growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km2 to ca. 15 000 km2 between 19 and 18 ka BP , stabilizing at ca. 2000 km2 by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP , and may have involved one or more subsidiary ice centres on now-submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine-influenced Shetland Ice Cap.  相似文献   

20.
The Holocene sea-level high stand or “marine limit” in Wilkes Land, East Antarctica, reached 30 m above present sea level at a few dispersed sites. The most detailed marine limit data have been recorded for the Windmill Islands and Budd Coast at the margin of the Law Dome ice cap, a dome of the East Antarctic Ice Sheet (EAIS). Relative sea-level lowering of 30 m and the associated emergence of the Windmill Islands have occurred since 6900 14C (corr.) yr B.P. Numerical modeling of the Earth's rheology is used to determine the glacio-isostatic component of the observed relative sea-level lowering. Glaciological evidence suggests that most of EAIS thickening occurred around its margin, with expansion onto the continental shelf. Consequently, a regional ice history for the last glacial maximum (LGM) was applied in the glacio-isostatic modeling to test whether the observed relative sea-level lowering was primarily produced by regional ice-sheet changes. The results of the modeling indicate that the postglacial (13,000 to 8000 14C yr B.P) removal of an ice load of between 770 and 1000 m from around the margin of the Law Dome and adjacent EAIS have produced the observed relative sea-level lowering. Such an additional ice load would have been associated with a 40- to 65-km expansion of the Law Dome to near the continental shelf break, together with a few hundred meters of ice thickening on the adjoining coastal slope of the EAIS up to 2000 m elevation. Whereas the observed changes in relative sea level are shown to be strongly influenced by regional ice sheet changes, the glacio-isostatic response at the Windmill Islands results from a combination of regional and, to a lesser extent, Antarctic-wide effects. The correspondence between the Holocene relative sea-level lowering interpreted at the margin of the Law Dome and the lowering interpreted along the remainder of the Wilkes Land and Oates Land coasts (105°–160° E) suggests that a similar ice load of up to 1000 m existed along the EAIS margin between Wilkes Land and Oates Land.  相似文献   

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