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1.
This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.  相似文献   

2.
Understanding the pace and drivers of marine-based ice-sheet retreat relies upon the integration of numerical ice-sheet models with observations from contemporary polar ice sheets and well-constrained palaeo-glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine-geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice-marginal landforms, to reconstruct the timing and rate of ice-sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine- to a terrestrially-based ice-sheet margin. Seafloor bathymetric data in the form of moraines and grounding-zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid-shelf where it formed a large grounding-zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice-free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice-sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio-isostatically induced, high relative sea level. Net rates of ice-sheet retreat across the shelf were slow (62–19 m a−1) and reduced (8 m a−1) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine-based sectors of ice sheets.  相似文献   

3.
《Quaternary Science Reviews》2005,24(14-15):1673-1690
Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred ⩾18.3 kyr 14C BP along the northwestern Irish coast, in agreement with a deglacial age of ∼22 36Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m ∼16.7 14C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL ∼15.0 14C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr 14C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 14C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level.  相似文献   

4.
During decline of the last British–Irish Ice Sheet (BIIS) down‐wasting of ice meant that local sources played a larger role in regulating ice flow dynamics and driving the sediment and landform record. At the Last Glacial Maximum, glaciers in north‐western England interacted with an Irish Sea Ice Stream (ISIS) occupying the eastern Irish Sea basin (ISB) and advanced as a unified ice‐mass. During a retreat constrained to 21–17.3 ka, the sediment landform assemblages lain down reflect the progressive unzipping of the ice masses, oscillations of the ice margin during retreat, and then rapid wastage and disintegration. Evacuation of ice from the Ribble valley and Lancashire occurred first while the ISIS occupied the ISB to the west, creating ice‐dammed lakes. Deglaciation, complete after 18.6–17.3 ka, was rapid (50–25 m a?1), but slower than rates identified for the western ISIS (550–100 m a?1). The slower pace is interpreted as reflecting the lack of a calving margin and the decline of a terrestrial, grounded glacier. Ice marginal oscillations during retreat were probably forced by ice‐sheet dynamics rather than climatic variation. These data demonstrate that large grounded glaciers can display complex uncoupling and realignment during deglaciation, with asynchronous behaviour between adjacent ice lobes generating complex landform records.
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5.
The Upper Garonne Basin included the largest glacial system in the Pyrenees during the last glacial cycle. Within the long-term glacial retreat during Termination-1 (T-1), glacier fluctuations left geomorphic evidence in the area. However, the chronology of T-1 glacial oscillations on the northern slopes of the Central Pyrenees is still poorly constrained. Here, we introduce new geomorphological observations and a 12-sample dataset of 10Be cosmic-ray exposure ages from the Ruda Valley. This U-shaped valley, surrounded by peaks exceeding 2800 m a.s.l., includes a sequence of moraines and polished surfaces that enabled a reconstruction of the chronology of the last deglaciation. Following the maximum ice extent, warmer conditions prevailing at ~15–14 ka, during the Bølling–Allerød (B–A) Interstadial, favoured glacial retreat in the Ruda Valley. Within the B–A, glaciers experienced two phases of advance/stillstand with moraine formation at 13.5 and 13.0 ka. During the early Younger Dryas (YD), glacial retreat exposed the highest surfaces of the Saboredo Cirque (~2300–2350 m) at 12.7 ka. Small glaciers persisted only inside the highest cirques (~2470 m), such as in Sendrosa Cirque, with moraines stabilising at 12.6 ka. The results of this work present the most complete chronology for Pyrenean glacial oscillations from the B–A to the YD.  相似文献   

6.
We present an 8000‐year history spanning 650 km of ice margin retreat for the largest marine‐terminating ice stream draining the former British–Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0–25.3 ka, accelerating into the Celtic Sea to reach maximum limits 25.3–24.5 ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea‐level rise, mega‐tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (～550 m a?1) that slowed (～100 m a?1) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (～200 m a?1) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer‐term (>1000 a) patterns and rates of ice‐marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present‐day ice streams. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
Based on high‐resolution TOPAS acoustic data, bathymetric data sets and sediment cores from the Norwegian Channel, the last retreat of the Norwegian Channel Ice Stream has been investigated. Mapping of ice‐marginal features such as grounding‐zone wedges and terminal moraines off western Norway suggest that the retreat of the grounding line in this part of the channel was interrupted by frequent stillstands, whereas the channel south of the threshold at Jæren does not have crossing ice‐marginal landforms. Three main seismic units have been identified, and, based on their seismic characteristics, in addition to study of sediment cores, these units are interpreted as till (U1), glacial marine sediment (U2) and Holocene hemipelagic sediment (U3). Based on new and published radiocarbon dates of the lower part of U2, combined with dates from the adjacent areas, it is concluded that the grounding line started to retreat from the shelf edge at about 19 ka and that the inner part of Skagerrak was ice free at 17.6 ka. This gives an average retreat rate of 450 m a−1, which is generally higher than mean retreat rates estimated for other palaeo‐ice streams (15–310 m a−1).  相似文献   

8.
The offshore and coastal geomorphology of southwest Greenland records evidence for the advance and decay of the Greenland Ice Sheet during the Last Glacial Maximum. Regional ice flow patterns in the vicinity of Sisimiut show an enlarged ice sheet that extended southwestwards on to the shelf, with an ice stream centred over Holsteinsborg dyb. High level periglacial terrain composed of blockfield and tors is dated to between 101 and 142 ka using 26Al and 10Be cosmogenic exposure ages. These limit the maximum surface elevation of the Last Glacial Maximum ice sheet in this part of southwest Greenland to ca 750–810 m asl, and demonstrate that terrain above this level has been ice free since MIS 6. Last Glacial Maximum ice thickness on the coast of ca 700 m implies that the ice sheet reached the mid to outer continental shelf edge to form the Outer Hellefisk moraines. Exposure dates record ice surface thinning from 21.0 to 9.8 ka, with downwasting rates varying from 0.06 to 0.12 m yr−1. This reflects strong surface ablation associated with increased air temperatures running up to the Bølling Interstadial (GIS1e) at ca 14 ka, and later marine calving under high sea levels. The relatively late retreat of the Itilleq ice stream inland of the present coastline is similar to the pattern observed at Jakobshavn Isbræ, located 250 km north in Disko Bugt, which also retreated from the continental shelf after ca 10 ka. We hypothesise that the ice streams of West Greenland persisted on the inner shelf until the early Holocene because of their considerable ice thickness and greater ice discharge compared with the adjacent ice sheet.  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.
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11.
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   

12.
Uummannaq Fjord, West Greenland, held the Uummannaq Ice Stream system that drained an estimated ~6% of the Greenland Ice Sheet (GrIS) during the Last Glacial Maximum. Published ages for the final deglaciation in Uummannaq Fjord vary from as early as c. 9.8 ka to as late as c. 5.3 ka. Assessing this variability requires additional chronological controls to improve the deglaciation history of central West Greenland. Here, we combine 14C dating of lake sediment cores with cosmogenic 10Be exposure dating at sites adjacent to the present GrIS margin in the central‐inland sector of the Uummannaq Fjord system. We find that ice retreated to or within the present GrIS margin at 10.8±0.2 ka (n = 6). Although this ‘final deglaciation’ to or within the present GrIS margin across the Uummannaq Fjord system varies from c. 10.8 to 5.3 ka, all chronologies indicate collapse from the continental shelf to the inner fjords at c. 11.0 ka, which occurred at a net retreat rate of 300–1100 m a−1. The Uummannaq Fjord system deglaciated c. 1000 years earlier than the major fjord system to the south, Disko Bugt. However, similarly rapid retreat rates of the two palaeo‐ice stream systems suggest that their collapse may have been aided by high calving rates. The asynchronous deglaciation of the GrIS throughout the Uummannaq Fjord system probably relates to the influence of varying fjord geometry on marine glacier behaviour.  相似文献   

13.
Considerable uncertainty surrounds the timing of glacier advance and retreat during the Younger Dryas or Loch Lomond Stade (LLS) in the Scottish Highlands. Some studies favour ice advance until near the end of the stade (c. 11.7 ka), whereas others support the culmination of glacier advance in mid‐stade (c. 12.6–12.4 ka). Most published 10 Be exposure ages reported for boulders on moraines or deglacial sites post‐date the end of the LLS, and thus appear to favour the former view, but recalibration of 33 10 Be ages using a locally derived 10 Be production rate and assuming rock surface erosion rates of zero to 1 mm ka?1 produces exposure ages 130–980 years older than those originally reported. The recalibrated ages are filtered to exclude anomalous data, and then employed to generate aggregate probability density distributions for the timing of moraine deposition and deglaciation. The results suggest that the most probable age for the timing of the deposition of the sampled outermost moraines lies in the interval 12.4–12.1 ka or earlier. Deglacial ages obtained for sites inside Loch Lomond Stadial glacier limits imply that glaciers at some or all of the sampled sites were retreating prior to 12.1 ka. Use of aggregated data does not exclude the possibility of asynchronous glacier behaviour at different sites, but confirms that some glaciers reached their maximum limits and began to retreat several centuries before the rapid warming that terminated the LLS at 11.7–11.6 ka, consistent with the retrodictions of recent numerical modelling experiments and with geomorphological evidence for gradual oscillatory ice‐margin retreat under stadial conditions.  相似文献   

14.
The now acknowledged thinning of the Greenland Ice Sheet raises concerns about its potential contribution to future sea level rise. In order to appreciate the full extent of its contribution to sea level rise, reconstruction of the ice sheet's most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment. We measured 10Be concentrations in 12 samples collected along longitudinal and altitudinal transects from Sisimiut to within 10 km of the Isunguata Sermia Glacier ice margin on the western coast of Greenland. Along the longitudinal transect, we collected three perched boulders and two bedrocks. In addition, we sampled seven perched boulders along a vertical transect in a valley within 10 km of the Isunguata Sermia Glacier ice margin. Our pilot dataset constrains the height of the ice sheet during the Last Glacial Maximum (LGM) between 500 m and 840 m (including the 120 m relative sea level depression at the time of the LGM, 21 ka BP). From the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12.3 ± 1.5 10Be ka (n = 2) and 8.3 ± 1.2 10Be ka (n = 3) to be ～6 cm a?1 over this time period. Direct dating of the retreat of the western margin of the Greenland Ice Sheet has the potential to better constrain the retreat rate of the ice margin, the thickness of the former ice sheet as well as its response to climate change. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

15.
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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16.
The behaviour of ice sheets as they retreated from their Last Glacial Maximum (LGM) positions provides insights into Lateglacial and early Holocene ice‐sheet dynamics and climate change. The pattern of deglaciation of the Laurentide Ice Sheet (LIS) in arctic fiord landscapes can now be well dated using cosmogenic exposure dating. We use cosmogenic exposure and radiocarbon ages to constrain the deglaciation history of Clyde Inlet, a 120 km long fiord on northeastern Baffin Island. The LIS reached the continental shelf during the LGM, retreated from the coastal lowlands by 12.5 ± 0.7 ka (n = 3), and from the fiord mouth by 11.7 ± 2.2 ka (n = 4). Rapid retreat from the outer fiord occurred 10.3 ± 1.3 ka (n = 6), with the terminus reaching the inner fiord shortly after 9.4 ka (n = 2), where several moraine systems were deposited between ca. 9.4 and ca. 8.4 ka. These moraines represent fluctuations of the LIS during the warmest summers since the last interglaciation, and this suggests that the ice sheet was responding to increased snowfall. Before retreating from the head of Clyde Inlet, the LIS margin fluctuated at least twice between ca. 7.9 and ca. 8.5 ka, possibly in response to the 8.2 ka cold event. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
Recent estimates of the timing of the last glaciation in the southern and western Uinta Mountains of northeastern Utah suggest that the start of ice retreat and the climate-driven regression of pluvial Lake Bonneville both occurred at approximately 16 cal. ka. To further explore the possible climatic relationship of Uinta Mountain glaciers and the lake, and to add to the glacial chronology of the Rocky Mountains, we assembled a range-wide chronology of latest Pleistocene terminal moraines based on seventy-four cosmogenic 10Be surface-exposure ages from seven glacial valleys. New cosmogenic-exposure ages from moraines in three northern and eastern valleys of the Uinta Mountains indicate that glaciers in these parts of the range began retreating at 22–20 ka, whereas previously reported cosmogenic-exposure ages from four southern and western valleys indicate that ice retreat began there between 18 and 16.5 ka. This spatial asynchrony in the start of the last deglaciation was accompanied by a 400-m east-to-west decline in glacier equilibrium-line altitudes across the Uinta Mountains. When considered together, these two lines of evidence support the hypothesis that Lake Bonneville influenced the mass balance of glaciers in southern and western valleys of the range, but had a lesser impact on glaciers located farther east. Regional-scale variability in the timing of latest Pleistocene deglaciation in the Rocky Mountains may also reflect changing precipitation patterns, thereby highlighting the importance of precipitation controls on the mass balance of Pleistocene mountain glaciers.  相似文献   

18.
The retreat of the Barents Sea Ice Sheet on the western Svalbard margin   总被引：1，自引：0，他引：1  
The deglaciation of the continental shelf to the west of Spitsbergen and the main fjord, Isfjorden. is discussed based on sub-bottom seismic records and scdirncnt cores. The sea lloor on the shelf to the west of Isfjorden is underlain by less than 2 m of glaciomarine sediments over a firm diamicton interpreted as till. In central Isfjordcn up to 10 m of deglaciation sediments were recorded, whereas in cores from the innermost tributary, Billefjorden, less than a meter of ice proximal sediments was recognized between the till and the 'normal' Holocene marine sediments. We conclude that the Barents Sea Ice Sheet terminated along the shelf break during the Late Weichselian glacial maximum. Radiocarbon dates from thc glaciomarine sediments above the till indicate a stepwise deglaciation. Apparently the ice front rctrcatcd from the outermost shelf around 14. 8 ka A dramatic increase in the flux of line-grained glaciomarine sediments around 13 ka is assumed to reflect increased melting and/or current activity due to a climatic warming. This second stage of deglaciation was intcrruptcd by a glacial readvance culminating on the mid-shelf area shortly after 12.4 ka. The glacial readvance, which is correlated with a simultaneous readvance of the Fennoscundian ice sheet along the western coast of Norway, is attributed to the so-called 'Older Dryas' cooling event in the North Atlantic region. Following this glacial readvance the outer part of Isljorden became rapidly deglaciated around 12.3 ka. During the Younger Dryas the inner fjord branches were occupied by large outlet glaciers and possibly the ice liont terminated far out in the main fjord. The remnants of the Harcnts Sea Ice Shcet melted quickly away as a response to the Holocene warming around 10 ka.  相似文献   

19.
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   

20.
In this study we have obtained 17 cosmogenic exposure ages from three well‐developed moraine systems – Halland Coastal Moraines (HCM), Göteborg Moraine (GM) and Levene Moraine (LM) – which were formed during the last deglaciation in southwest Sweden by the Scandinavian Ice Sheet (SIS). The inferred ages of the inner HCM, GM and LM are 16.7 ± 1.6, 16.1 ± 1.4 and 13.6 ± 1.4 ka, respectively, which is slightly older than previous estimates of the deglaciation based on the minimum limiting radiocarbon ages and pollen stratigraphy. During this short interval from 16.7 ± 1.6 to 13.6 ± 1.4 ka a large part (100–125 km) of the marine‐based sector of the SIS in southwest Sweden was deglaciated, giving an average ice margin retreat between 20 to 50 m a?1. The inception of the deglaciation pre‐dated the Bølling/Allerød warming, the rapid sea level rise at 14.6 cal. ka BP and the first inflow of warm Atlantic waters into Skagerrak. We suggest that ice retreat in southwest Sweden is mainly a dynamical response governed by the disintegration of the Norwegian Channel Ice Stream and not primarily driven by climatic changes. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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