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1.
The deglacial history of the central sector of the last British–Irish Ice Sheet is poorly constrained, particularly along major ice‐stream flow paths. The Tyne Gap Palaeo‐Ice Stream (TGIS) was a major fast‐flow conduit of the British–Irish Ice Sheet during the last glaciation. We reconstruct the pattern and constrain the timing of retreat of this ice stream using cosmogenic radionuclide (10Be) dating of exposed bedrock surfaces, radiocarbon dating of lake cores and geomorphological mapping of deglacial features. Four of the five 10Be samples produced minimum ages between 17.8 and 16.5 ka. These were supplemented by a basal radiocarbon date of 15.7 ± 0.1 cal ka BP, in a core recovered from Talkin Tarn in the Brampton Kame Belt. Our new geochronology indicates progressive retreat of the TGIS from 18.7 to 17.1 ka, and becoming ice free before 16.4–15.7 ka. Initial retreat and decoupling of the TGIS from the North Sea Lobe is recorded by a prominent moraine 10–15 km inland of the present‐day coast. This constrains the damming of Glacial Lake Wear to a period before ∼18.7–17.1 ka in the area deglaciated by the contraction of the TGIS. We suggest that retreat of the TGIS was part of a regional collapse of ice‐dispersal centres between 18 and 16 ka.
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2.
The glacial geomorphology of the Waterville Plateau (ca. 55 km2) provides information on the dynamics of the Okanogan Lobe, southern sector of the Cordilleran Ice Sheet in north‐central Washington. The Okanogan Lobe had a profound influence on the landscape. It diverted meltwater and floodwater along the ice front contributing to the Channeled Scabland features during the late Wisconsin (Fraser Glaciation). The glacial imprint may record surge behaviour of the former Okanogan Lobe based on a comparison with other glacial landsystems. Conditions that may have promoted instability include regional topographic constraints, ice marginal lakes and dynamics of the subglacial hydrological system, which probably included a subglacial reservoir. The ice‐surface morphology and estimated driving stresses (17–26 kPa) implied from ice thickness and surface slope reconstructed in the terminal area also suggest fast basal flow characteristics. This work identifies the location of a fast flowing ice corridor and this probably affected the stability and mass balance of the south‐central portion of the Cordilleran Ice Sheet. Evidence for fast ice flow is lacking in the main Okanogan River Valley, probably because it was destroyed during deglaciation by various glacial and fluvial processes. The only signature of fast ice flow left is the imprint on the Waterville Plateau. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

3.
The Northern Prince Gustav Ice Stream located in Prince Gustav Channel, drained the northeastern portion of the Antarctic Peninsula Ice Sheet during the last glacial maximum. Here we present a chronology of its retreat based on in situ produced cosmogenic 10Be from erratic boulders at Cape Lachman, northern James Ross Island. Schmidt hammer testing was adopted to assess the weathering state of erratic boulders in order to better interpret excess cosmogenic 10Be from cumulative periods of pre-exposure or earlier release from the glacier. The weighted mean exposure age of five boulders based on Schmidt hammer data is 12.9 ± 1.2 ka representing the beginning of the deglaciation of lower-lying areas (< 60 m a.s.l.) of the northern James Ross Island, when Northern Prince Gustav Ice Stream split from the remaining James Ross Island ice cover. This age represents the minimum age of the transition from grounded ice stream to floating ice shelf in the middle continental shelf areas of the northern Prince Gustav Channel. The remaining ice cover located at higher elevations of northern James Ross Island retreated during the early Holocene due to gradual decay of terrestrial ice and increase of equilibrium line altitude. Schmidt hammer R-values are inversely correlated with 10Be exposure ages and could be used as a proxy for exposure history of individual granite boulders in this region and favour the hypothesis of earlier release of boulders with excessive 10Be concentrations from glacier directly at this site. These data provide evidences for an earlier deglaciation of northern James Ross Island when compared with other recently presented cosmogenic nuclide based deglaciation chronologies, but this timing coincides with rapid increase of atmospheric temperature in this marginal part of Antarctica.  相似文献   

4.
《Quaternary Science Reviews》1999,18(8-9):993-1019
Outcrops bearing stained, striated facets indicative of north–northeastward moving ice, truncated by unstained, striated facets indicative of various younger flows occur in the Caniapiscau area of north–central Quebec. This is the first report of differential staining of striated facets in the region. We propose that the staining occurred in an ice-free interval of probable interglacial age. This early ice flow probably occurred during ice retreat toward the Quebec highlands. Ice flow and glacial transport data from the southern Hudson Bay and James Bay basins indicate that the next major regional ice flow was toward the northwest and resulted from the expansion of an Early Wisconsinan glacier in the Quebec highlands. The northern part of this flow was diverted northwestward through Hudson Bay, and the southern part southwestward across James Bay, following a progressive counterclockwise rotation of flow. A zone of intersection (ZI) of two major glacier bedform systems, often referred to as the horseshoe-shaped Labrador Ice Divide, represents the head of a large northward convergent ice-flow system that extended to Ungava Bay and beyond. The Ungava flow propagated southward and captured the head of the opposing flow from an outflow centre located east of Caniaspiscau reservoir. We propose that this capture event correlates with the Gold Cove Advance in Ungava Bay and on Baffin Island at about 9900 14C yr BP. It is the largest advance of Quebec-Labrador ice yet proposed for the region. This correlation is based on the relative ice-flow chronology, accommodation of glacial lakes Naskaupi and McLean in the deglaciation sequence, the constraints placed on Last Glacial Maximum (LGM) ice configuration by the postglacial uplift pattern and events in the deep-sea record. Therefore, the Ungava ice-flow pattern is not a relict pre-Wisconsinan glacial landscape as recently proposed.  相似文献   

5.
Glacial varves can give significant insights into recession and melting rates of decaying ice sheets. Moreover, varve chronologies can provide an independent means of comparison to other annually resolved climatic archives, which ultimately help to assess the timing and response of an ice sheet to changes across rapid climate transitions. Here we report a composite 1257‐year‐long varve chronology from southeastern Sweden spanning the regional late Allerød–late Younger Dryas pollen zone. The chronology was correlated to the Greenland Ice‐Core Chronology 2005 using the time‐synchronous Vedde Ash volcanic marker, which can be found in both successions. For the first time, this enables secure placement of the Lateglacial Swedish varve chronology in absolute time. Geochemical analysis from new varve successions indicate a marked change in sedimentation regime accompanied by an interruption of ice‐rafted debris deposition synchronous with the onset of Greenland Stadial 1 (GS‐1; 12 846 years before AD 1950). With the support of a simple ice‐flow/calving model, we suggest that slowdown of sediment transfer can be explained by ice‐sheet margin stabilization/advance in response to a significant drop of the Baltic Ice Lake level. A reassessment of chronological evidence from central‐western and southern Sweden further supports the hypothesis of synchronicity between the first (penultimate) catastrophic drainage of the Baltic Ice Lake and the start of GS‐1 in Greenland ice‐cores. Our results may therefore provide the first chronologically robust evidence linking continental meltwater forcing to rapid atmosphere–ocean circulation changes in the North Atlantic.  相似文献   

6.
This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice‐stream system that drained ～3% of the entire British?Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice‐stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid‐shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft‐bed (sediment) and hard‐bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast‐flowing ice‐sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice‐bed coupling in a zone of flow convergence associated with ice‐stream onset; and a downstream bedform evolution (short drumlins to km‐scale glacial lineations) suggesting an ice‐flow velocity transition associated with a bed substrate and roughness change in the ice‐stream trunk. Chronology is still lacking for the timing of ice‐stream demise; however, the seismic stratigraphy, absence of moraines or grounding‐line features, and presence of well‐preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long‐lived, ice‐sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice‐discharge values for the Minch ice stream (12–20 Gt a?1) are comparable to high mass‐flux ice streams today, underlining it as an excellent palaeo‐analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.  相似文献   

7.
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   

8.
This paper presents the results of a detailed study of a complex hydrofracture system and host diamictons exposed within a longitudinal section through an elongate drumlin located to the west of Cemlyn Bay, Anglesey, NW Wales. This complex, laterally extensive sand, silt and clay filled hydrofracture system was active over a prolonged period and is thought to have developed beneath the Late Devensian (Weichselian) Irish Sea Ice Stream as it overrode this part of NW Anglesey. The sediment-fill to the hydrofracture system is deformed with kinematic indicators (folds, thrusts, augen) recording a SW-directed sense of shear, consistent with the regional ice flow direction across this part of the island. The lack of any geomorphological evidence for active retreat of the Irish Sea ice across Anglesey has led to the conclusion that hydrofracturing at the Cemlyn Bay site occurred within the bed of the Irish Sea Ice Stream whilst this relatively faster flowing corridor of ice was actively overriding the island. Shear imposed by the overriding ice led to the development of a subglacial shear zone which facilitated the propagation of the hydrofracture system with the laterally extensive feeder sills occurring parallel to Y-type Riedel shears. Although a subglacial setting beneath the active Irish Sea Ice Stream can be argued for the Cemlyn Bay hydrofracture system, its relationship to the formation of the ‘host’ drumlin remains uncertain. However, evidence presented here suggests that hydrofracturing may have occurred during the later stages or post landform development in response to the migration of overpressurised meltwater within the bed of the Irish Sea ice; possibly accompanying the local thinning and shutdown of the Irish Sea Ice Stream on Anglesey.  相似文献   

9.
Lake Vättern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice‐sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vättern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high‐resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vättern, are used here together with a unique 74‐m sediment record recently acquired by drill coring, and with onshore LiDAR‐based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying >120‐m deglacial sequence. Three distinct retreat–re‐advance episodes occurred in southern Lake Vättern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re‐advanced from north of Visingsö to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag‐and‐tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north‐central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a?1. The pronounced topography of the Vättern basin and its deep proglacial‐dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.  相似文献   

10.
The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.  相似文献   

11.
Trimlines separating glacially abraded lower slopes from blockfield‐covered summits on Irish mountains have traditionally been interpreted as representing the upper limit of the last ice sheet during the Last Glacial Maximum (LGM). Cosmogenic 10Be exposure ages obtained for samples from glacially deposited perched boulders resting on blockfield debris on the summit area of Slievenamon (721 m a.s.l.) in southern Ireland demonstrate emplacement by the last Irish Ice Sheet (IIS), implying preservation of the blockfield under cold‐based ice during the LGM, and supporting the view that trimlines throughout the British Isles represent former englacial thermal regime boundaries between a lower zone of warm‐based sliding ice and an upper zone of cold‐based ice. The youngest exposure age (22.6±1.1 or 21.0±0.9 ka, depending on the 10Be production rate employed) is statistically indistinguishable from the mean age (23.4±1.2 or 21.8±0.9 ka) obtained for two samples from ice‐abraded bedrock at high ground on Blackstairs Mountain, 51 km to the east, and with published cosmogenic 36Cl ages. Collectively, these ages imply (i) early (24–21 ka) thinning of the last IIS and emergence of high ground in SE Ireland; (ii) relatively brief (1–3 ka) glacial occupation of southernmost Ireland during the LGM; (iii) decoupling of the Irish Sea Ice Stream and ice from the Irish midlands within a similar time frame; and (iv) that the southern fringe of Ireland was deglaciated before western and northern Ireland.  相似文献   

12.
Only a few chronological constraints on Lateglacial and Early Holocene glacier variability in the westernmost Alps have hitherto been obtained. In this paper, moraines of two palaeoglaciers in the southern Écrins massif were mapped. The chronology of the stabilization of selected moraines was established through the use of 10Be cosmic ray exposure (CRE) dating. The equilibrium line altitude (ELA) during moraine deposition was reconstructed assuming an accumulation area ratio (AAR) of 0.67. Ten pre‐Little Ice Age (LIA) ice‐marginal positions of the Rougnoux palaeoglacier were identified and seven of these have been dated. The 10Be CRE age of a boulder on the lowermost sampled moraine indicates that the landform may have been first formed during a period of stable glaciers at around 16.2±1.7 ka (kiloyears before AD 2017) or that the sampled boulder experienced pre‐exposure to secondary cosmic radiation. The moraine was re‐occupied or, alternatively, shaped somewhat before 12.2±0.6 ka when the ELA was lowered by 230 m relative to the LIA ELA. At least six periods of stable ice margins occurred thereafter when the ELA was 220–160 m lower than during the LIA. The innermost dated moraine stabilized at or before 10.9±0.7 ka. Three 10Be CRE ages from a moraine of the Prelles palaeoglacier indicate a period of stationary ice margins at or before 10.9±0.6 ka when the ELA was lowered by 160 m with respect to the end of the LIA. The presented 10Be CRE ages are in good agreement with those of moraines that have been attributed to the Egesen stadial. Assuming unchanged precipitation, summer temperature in the southern Écrins massif at ~12 ka must have been at least 2 °C lower relative to the LIA.  相似文献   

13.
We measured in situ cosmogenic 10Be in 16 bedrock and 14 boulder samples collected along a 40-km transect outside of and normal to the modern ice margin near Sikuijuitsoq Fjord in central-west Greenland (69°N). We use these data to understand better the efficiency of glacial erosion and to infer the timing, pattern, and rate of ice loss after the last glaciation. In general, the ages of paired bedrock and boulder samples are in close agreement (r2 = 0.72). Eleven of the fourteen paired bedrock and boulder samples are indistinguishable at 1σ; this concordance indicates that subglacial erosion rates are sufficient to remove most or all 10Be accumulated during previous periods of exposure, and that few, if any, nuclides are inherited from pre-Holocene interglaciations. The new data agree well with previously-published landscape chronologies from this area, and suggest that two chronologically-distinct land surfaces exist: one outside the Fjord Stade moraine complex (～10.3 ± 0.4 ka; n = 7) and another inside (～8.0 ± 0.7 ka; n = 21). Six 10Be ages from directly outside the historic (Little Ice Age) moraine show that the ice margin first reached its present-day position ～7.6 ± 0.4 ka. Early Holocene ice margin retreat rates after the deposition of the Fjord Stade moraine complex were ～100–110 m yr?1. Sikuijuitsoq Fjord is a tributary to the much larger Jakobshavn Isfjord and the deglaciation chronologies of these two fjords are similar. This synchronicity suggests that the ice stream in Jakobshavn Isfjord set the timing and pace of early Holocene deglaciation of the surrounding ice margin.  相似文献   

14.
Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic 10Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the 10Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic 3He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes.  相似文献   

15.
The net effect of ice‐flow shifts resulting in the dilution or reworking of clasts on a single preserved till sheet is often unknown yet has major implications for palaeoglaciology and mineral exploration. Herein, we analyse variations in till clast lithologies from a single till sheet, within palimpsest‐type Glacial Terrain Zones in NE Manitoba, Canada, to better understand sediment–landform relationships in this area of high landform inheritance. This near‐ice‐divide area is known to consist of a highly fragmented subglacial landscape, resulting from spatio‐temporal variations in intensity of reworking and inheritance throughout multiple glacial events (subglacial bed mosaic). We show that a seemingly homogenous ‘Keewatin’ till sheet is composed of local (>15 km) and continental‐scale (～100‐km‐long carbonate train and 350–600 km long Dubawnt red erratic train) fan, irregular (amoeboid) or lobate palimpsest dispersal patterns. Local dispersal is more complex than the preserved local landform flowset(s) record, but appears consistent with the overall glacial history reconstructed from regional flowset and striation analyses. The resultant surface till is a spatial mosaic interpreted to reflect variable intensities in modification (overprinting) and preservation (inheritance) of a predominately pre‐existing till sheet. A multi‐faceted approach integrating till composition, regional landforms, ice‐flow indicators, and stratigraphic knowledge is used to map relative spatio‐temporal erosion/reworking intensity.  相似文献   

16.
Hormes, A., Akçar, N. & Kubik, P. W. 2011: Cosmogenic radionuclide dating indicates ice‐sheet configuration during MIS 2 on Nordaustlandet, Svalbard. Boreas, 10.1111/j.1502‐3885.2011.00215.x. ISSN 0300‐9483.0300‐9843 Glacial geological field surveys, aerial image interpretation and cosmogenic radionuclide (CRN) dating allowed us to reconstruct the ice‐sheet configuration on Nordaustlandet, the northernmost island of the European sector on the margin of the Arctic Ocean. The timing of deglaciation was investigated by determining the 26Al and 10Be ages of glacially scoured bedrock, weathered periglacial blockfields and glacial erratic boulders. Only 10Be ages were useful for our interpretations, because of unresolved analytical problems with 26Al. Fjords and lowlands on Nordaustlandet yielded Late Weichselian 10Be ages, indicating that actively erosive ice streams scoured the coastal fjord bathymetry during marine isotope stage (MIS) 2. In Murchisonfjorden, ground‐truthed air‐photograph interpretation and 10Be ages of boulders indicated a cold‐based glacier ice cover during MIS 2 on higher plateaus. 10Be ages and lithological studies of erratic boulders on higher and interior plateaus of Prins Oscars Land (>200–230 m a.s.l.) suggest that the Mid‐Weichselian glaciation (MIS 4) might have been more extensive than that during MIS 2.  相似文献   

17.
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   

18.
Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483. Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS 14C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 14C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 14C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 14C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 14C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.  相似文献   

19.
We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using 10Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that 10Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between 10Be and lichenometric ages reveal that 10Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

20.
Surging of the southwestern part of the Laurentide Ice Sheet   总被引：1，自引：0，他引：1  
The southwestern part of the Laurentidc Ice Sheet, in central North America, repeatedly surged during the last part of the Wisconsin Glaciation. Evidence includes the extreme lobation of the ice margin, the gentle slopes of lateral moraines and other marginal features, a radiocarbon chronology indicating extremely rapid marginal advance and retreat, and the abundance of supraglacial flow till. Rapid ice movement was caused by subglacial water and was probably limited to areas of slowly permeable substrate, which slowed the escape of the water.  相似文献   

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