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Chris R Stokes - One of the best experts on this subject based on the ideXlab platform.
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reconciling records of Ice streaming and Ice Margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide Ice sheet
Quaternary Science Reviews, 2018Co-Authors: Martin Margold, Chris R Stokes, Chris D ClarkAbstract:Abstract This paper reconstructs the deglaciation of the Laurentide Ice Sheet (LIS; including the Innuitian Ice Sheet) from the Last Glacial Maximum (LGM), with a particular focus on the spatial and temporal variations in Ice streaming and the associated changes in flow patterns and Ice divides. We build on a recent inventory of Laurentide Ice streams and use an existing Ice Margin chronology to produce the first detailed transient reconstruction of the Ice stream drainage network in the LIS, which we depict in a series of palaeogeographic maps. Results show that the drainage network at the LGM was similar to modern-day Antarctica. The majority of the Ice streams were marine terminating and topographically-controlled and many of these continued to function late into the deglaciation, until the Ice sheet lost its marine Margin. Ice streams with a terrestrial Ice Margin in the west and south were more transient and Ice flow directions changed with the build-up, peak-phase and collapse of the Cordilleran-Laurentide Ice saddle. The south-eastern marine Margin in Atlantic Canada started to retreat relatively early and some of the Ice streams in this region switched off at or shortly after the LGM. In contrast, the Ice streams draining towards the north-western and north-eastern marine Margins in the Beaufort Sea and in Baffin Bay appear to have remained stable throughout most of the Late Glacial, and some of them continued to function until after the Younger Dryas (YD). The YD influenced the dynamics of the deglaciation, but there remains uncertainty about the response of the Ice sheet in several sectors. We tentatively ascribe the switching-on of some major Ice streams during this period (e.g. M'Clintock Channel Ice Stream at the north-west Margin), but for other large Ice streams whose timing partially overlaps with the YD, the drivers are less clear and Ice-dynamical processes, rather than effects of climate and surface mass balance are viewed as more likely drivers. Retreat rates markedly increased after the YD and the Ice sheet became limited to the Canadian Shield. This hard-bed substrate brought a change in the character of Ice streaming, which became less frequent but generated much broader terrestrial Ice streams. The final collapse of the Ice sheet saw a series of small ephemeral Ice streams that resulted from the rapidly changing Ice sheet geometry in and around Hudson Bay. Our reconstruction indicates that the LIS underwent a transition from a topographically-controlled Ice drainage network at the LGM to an Ice drainage network characterised by less frequent, broad Ice streams during the later stages of deglaciation. These deglacial Ice streams are mostly interpreted as a reaction to localised Ice-dynamical forcing (flotation and calving of the Ice front in glacial lakes and transgressing sea; basal de-coupling due to large amount of meltwater reaching the bed, debuttressing due to rapid changes in Ice sheet geometry) rather than as conveyors of excess mass from the accumulation area of the Ice sheet. At an Ice sheet scale, the Ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating Ice sheet, the final elimination of which was mostly driven by surface melt.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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major changes in Ice stream dynamics during deglaciation of the north western Margin of the laurentide Ice sheet
Quaternary Science Reviews, 2009Co-Authors: Chris R Stokes, Chris D Clark, Robert D StorrarAbstract:Abstract Victoria Island lies at the north-western limit of the former North American (Laurentide) Ice Sheet in the Canadian Arctic Archipelago and displays numerous cross-cutting glacial lineations. Previous work suggests that several Ice streams operated in this region during the last (Wisconsinan) glaciation and played a major role in Ice sheet dynamics and the delivery of Icebergs into the Arctic Ocean. This paper produces the first detailed synthesis of their behaviour from the Last Glacial Maximum through to deglaciation (∼21–9.5 cal ka BP) based on new mapping and a previously published radiocarbon-constrained Ice sheet Margin chronology. Over 70 discrete Ice flow events (flow-sets) are ‘fitted’ to the Ice Margin configuration to allow identification of several Ice streams ranging in size from large and long-lived (thousands of years) to much smaller and short-lived (hundreds of years). The reconstruction depicts major Ice streams in M'Clure Strait and Amundsen Gulf which underwent relatively rapid retreat from the continental shelf edge at some time between ∼15.2 and 14.1 cal ka BP: a period which encompasses climatic warming and rapid sea level rise (meltwater pulse-1a). Following this, overall retreat was slower and the Ice streams exhibited asynchronous behaviour. The Amundsen Gulf Ice Stream continued to operate during Ice Margin retreat, whereas the M'Clure Strait Ice Stream ceased operating and was replaced by an Ice divide within ∼1000 years. This Ice divide was subsequently obliterated by another short-lived phase of Ice streaming in M'Clintock Channel ∼13 cal ka BP. The timing of this large Ice discharge event coincides with the onset of the Younger Dryas. Subsequently, a minor Ice divide developed once again in M'Clintock Channel, before final deglaciation of the island shortly after 9.5 cal ka BP. It is concluded that large Ice streams at the NW Margin of the Laurentide Ice Sheet, equivalent in size to the Hudson Strait Ice Stream, underwent major changes during deglaciation, resulting in punctuated delivery of Icebergs into the Arctic Ocean. Published radiocarbon dates constrain this punctuated delivery, as far as is possible within the limits imposed by their precision, and we note their coincidence with pulses of meltwater delivery inferred from numerical modelling and ocean sediment cores.
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evolution of late glacial Ice Marginal lakes on the northwestern canadian shield and their influence on the location of the dubawnt lake palaeo Ice stream
Palaeogeography Palaeoclimatology Palaeoecology, 2004Co-Authors: Chris R Stokes, Chris D ClarkAbstract:Abstract During deglaciation of the North American Laurentide Ice Sheet large proglacial lakes developed in positions where proglacial drainage was impeded by the Ice Margin. For some of these lakes, it is known that subsequent drainage had an abrupt and widespread impact on North Atlantic Ocean circulation and climate, but less is known about the impact that the lakes exerted on Ice sheet dynamics. This paper reports palaeogeographic reconstructions of the evolution of proglacial lakes during deglaciation across the northwestern Canadian Shield, covering an area in excess of 1,000,000 km2 as the Ice sheet retreated some 600 km. The interactions between proglacial lakes and Ice sheet flow are explored, with a particular emphasis on whether the disposition of lakes may have influenced the location of the Dubawnt Lake Ice stream. This Ice stream falls outside the existing paradigm for Ice streams in the Laurentide Ice Sheet because it did not operate over fined-grained till or lie in a topographic trough. Ice Margin positions and a digital elevation model are utilised to predict the geometry and depth of proglacial lakes impounded at the Margin at 30-km increments during deglaciation. Palaeogeographic reconstructions match well with previous independent estimates of lake coverage inferred from field evidence, and results suggest that the development of a deep lake in the Thelon drainage basin may have been influential in initiating the Ice stream by inducing calving, drawing down Ice and triggering fast Ice flow. This is the only location alongside this sector of the Ice sheet where large (>3000 km2), deep lakes (∼120 m) are impounded for a significant length of time and exactly matches the location of the Ice stream. It is speculated that the commencement of calving at the Ice sheet Margin may have taken the system beyond a threshold and was sufficient to trigger rapid motion but that once initiated, calving processes and losses were insignificant to the functioning of the Ice stream. It is thus concluded that proglacial lakes are likely to have been an important control on Ice sheet dynamics during deglaciation of the Laurentide Ice Sheet.
Jason P Briner - One of the best experts on this subject based on the ideXlab platform.
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younger dryas Ice Margin retreat in greenland new evidence from southwestern greenland
Climate of The Past, 2021Co-Authors: Svend Funder, Nicolaj K Larsen, Laura B. Levy, Jesper V Olsen, Jason P Briner, Anita H L Sorensen, Anders A Bjork, Anders Schomacker, Kurt H KjaerAbstract:Abstract. Cosmogenic 10Be dates from bedrock knobs on six outlying tiny islands along a stretch of 300 km of the Southwest Greenland coast, indicate that the Greenland Ice Sheet (GrIS) Margin here was retreating on the inner shelf close to the coast during the Younger Dryas (YD) cold period. A survey of recently published 10Be and 14C-dated records show that this unexpected behaviour of the Ice-Margin has been seen also in other parts of Greenland, but with very large variations in extent and speed of retreat even between neighbouring areas. In contrast to this, landforms appearing in high resolution bathymetry surveys on the shelf, have recently been suggested to indicate YD readvance or long-lasting Ice-Margin still stand on mid shelf, far from the coast. However, these features have been dated primarily by correlation with cold periods in the Ice core temperature records, and therefore cannot inform about the Ice-Margin/climate relation. Ice-Margin retreat during a YD cooling has been explained by advection of warm subsurface water melting the Ice-Margin, and by increased seasonality of the climate with the temperature drop mainly in winter, with high impact on sea Ice extent and duration, but little effect on glacier mass balance. This study therefore adds to the complexity of the climate/Ice-Margin relation, where local factors may for some time overrule or mute overall temperature change. It also points to the urgent need for climate-independent dating of the rich treasure trove of information coming from the shelf in these years.
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holocene evolution of the western greenland Ice sheet assessing geophysical Ice sheet models with geological reconstructions of Ice Margin change
Quaternary Science Reviews, 2015Co-Authors: Nicolas E Young, Jason P BrinerAbstract:Abstract Geophysical Ice-sheet models are used to predict future Ice-sheet dimensions and, in turn, these projections help estimate the magnitude of eustatic sea-level rise. Before models can confidently predict Ice-sheet behavior, they must be validated by being able to duplicate the geological record of Ice-sheet change. Here, we review geological records of Greenland Ice Sheet (GrIS) change, with emphasis on the warmer-than-present middle Holocene, and compare these records to published studies that numerically simulate GrIS behavior through the Holocene. Geological records are concentrated in West and Southwest Greenland, which are also the regions where the GrIS Margin likely experienced the greatest distance of inland retreat during the middle Holocene. Several records spanning from Melville Bugt to Jakobshavn Isfjord in western Greenland indicate the GrIS achieved its minimum extent between ∼5 and 3 ka, and farther south in the Kangerlussuaq region, new data presented here indicate the Ice Margin reached its minimum extent between ∼4.2 and 1.8 ka. In the Narsarsuaq region in southern Greenland, the GrIS likely achieved its minimum configuration between ∼7 and 4 ka. We highlight key similarities and discrepancies between these reconstructions and model results, and finally, we suggest that despite some degree of inland retreat, the West and Southwest GrIS Margin remained relatively stable and close to its current position through the Holocene thermal maximum.
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age of the fjord stade moraines in the disko bugt region western greenland and the 9 3 and 8 2 ka cooling events
Quaternary Science Reviews, 2013Co-Authors: Nicolas E Young, Jason P Briner, Dylan H Rood, Robert C Finkel, Lee B Corbett, Paul R BiermanAbstract:Abstract Retreat of the western Greenland Ice Sheet during the early Holocene was interrupted by deposition of the Fjord Stade moraine system. The Fjord Stade moraine system spans several hundred kilometers of western Greenland's Ice-free fringe and represents an important period in the western Greenland Ice Sheet's deglaciation history, but the origin and timing of moraine deposition remain uncertain. Here, we combine new and previously published 10 Be and 14 C ages from Disko Bugt, western Greenland to constrain the timing of Fjord Stade moraine deposition at two locations ∼60 km apart. At Jakobshavn Isfjord, the northern of two study sites, we show that Jakobshavn Isbrae advanced to deposit moraines ca 9.2 and 8.2–8.0 ka. In southeastern Disko Bugt, the Ice sheet deposited moraines ca 9.4–9.0 and 8.5–8.1 ka. Our Ice-Margin chronology indicates that the Greenland Ice Sheet in two distant regions responded in unison to early Holocene abrupt cooling 9.3 and 8.2 ka, as recorded in central Greenland Ice cores. Although the timing of Fjord Stade moraine deposition was synchronous in Jakobshavn Isfjord and southeastern Disko Bugt, within uncertainties, we suggest that Jakobshavn Isbrae advanced while the southeastern Disko Bugt Ice Margin experienced stillstands during the 9.3 and 8.2 ka events based on regional geomorphology and the distribution of 10 Be ages at each location. The contrasting style of Ice-Margin response was likely regulated by site-specific Ice-flow characteristics. Jakobshavn Isbrae's high Ice flux results in an amplified Ice-Margin response to a climate perturbation, both warming and cooling, whereas the comparatively low-flux sector of the Ice sheet in southeastern Disko Bugt experiences a more subdued response to climate perturbations. Our chronology indicates that the western Greenland Ice Sheet advanced and retreated in concert with early Holocene temperature variations, and the 9.3 and 8.2 ka events, although brief, were of sufficient duration to elicit a significant response of the western Greenland Ice Sheet.
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varve and radiocarbon dating support the rapid advance of jakobshavn isbrae during the little Ice age
Quaternary Science Reviews, 2011Co-Authors: Jason P Briner, Nicolas E Young, Elizabeth K Thomas, H A Stewart, S Losee, S TruexAbstract:Abstract Large outlet glaciers draining the Greenland Ice Sheet significantly influence overall Ice sheet mass balance. Considerable short term (years to decades) retreat and fluctuations in velocity of Jakobshavn Isbrae, western Greenland, illustrate the complex nature by which large outlet glaciers respond to climate change, making predictions of future Ice sheet change challenging. To provide a longer-term view (centuries), we investigate the geological record of Jakobshavn Isbrae change. We use continuous sediment records from lakes that were influenced by the recent advance of Jakobshavn Isbrae, which took place during the Little Ice Age. In particular, we explore the use of annually laminated lake sediments (varves) to precisely constrain the advance of the Ice Margin as it approached its late Holocene maximum extent. We find that the Ice Margin advanced recently, at least after ∼1650 to ∼1700 AD, and more likely ∼1800 AD. We suggest that during this period Jakobshavn Isbrae advanced at a rate that was similar to its historically documented average retreat since ∼1850 AD. Our results indicate that Jakobshavn Isbrae, and presumably other large marine calving glaciers, have the ability to advance quickly in response to climate forcing.
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using proglacial threshold lakes to constrain fluctuations of the jakobshavn isbrae Ice Margin western greenland during the holocene
Quaternary Science Reviews, 2010Co-Authors: Jason P Briner, Nicolas E Young, H A Stewart, William Philipps, S LoseeAbstract:Abstract The future response of the Greenland Ice Sheet (GIS) and its potential contribution to sea level rise are uncertain. Rapid changes of Greenland’s outlet glaciers over the past decade have made it difficult to extrapolate Ice sheet change into the future. This significant short-term variability highlights the need for longer-term, geologic (e.g., Holocene) records of Ice Margin fluctuations. However, a major challenge with reconstructing the GIS during the Holocene stems from it having been smaller than it is at present, thus traditional glacial geologic approaches are not suitable. We use radiocarbon-dated sediment sequences from seven proglacial-threshold lakes spanning ∼50 km of the western GIS Margin near Jakobshavn Isbrae to constrain the timing of early Holocene deglaciation, the duration that this sector of the western GIS was smaller than its present configuration, and the timing of its advance during Neoglaciation. Our reconstructions suggest deglaciation ∼7300 cal yr BP, minimum Ice extent ∼6000–5000 cal yr BP and smaller-than-present Ice configuration until at least ∼2300 cal yr BP for the Ice Margin south of Jakobshavn Isbrae, and until ∼400 cal yr BP for the Ice Margin north of Jakobshavn Isbrae. One relatively large proglacial lake that became briefly Ice-free during the middle Holocene lies in a catchment that likely extends 10s of km inland beneath the GIS, suggesting significant middle Holocene retreat of this portion of the GIS. The overall pattern of Ice sheet change is inconsistent with existing Ice sheet model reconstructions for this region, but is consistent with numerous paleoclimate proxy and relative sea level data. These continuous lacustrine records corroborate, but provide closer age control than, existing non-continuous records of radiocarbon-dated reworked bivalves from historical moraines in the region. Reconstructing Ice Margin change from proglacial-threshold lakes is one of few approaches with the potential to constrain smaller-than-present Ice sheet extent.
Henrik Hojmark Thomsen - One of the best experts on this subject based on the ideXlab platform.
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comparison between greenland Ice Margin an Ice core oxygen 18 records
Annals of Glaciology, 2002Co-Authors: Niels Reeh, Hans Oerter, Henrik Hojmark ThomsenAbstract:Old Ice for paleoenvironmental studies retrieved by deep core drilling in the central regions of the big Ice sheets can also be retrieved from the Ice-sheet Margins. The d18O-content of the surface Ice was studied at 15 different Greenland Ice-Margin locations. At some locations, two or more records were obtained along closely spaced parallel sampling profiles, showing good reproducibility of the records. We present Ice-Margin d18O- records reaching back into the Pleistocene. Many of the characteristic d18O-variations known from Greenland deep Ice-cores can be recognized, allowing an approximate time scale to be established along the Ice-Margin records. A flow line model is used to determine the location on the Ice sheet where the Margin-Ice was originally deposited as snow. The Pleistocene-Holocene d18O-change at the deposition sites is determined by comparing the d18O-values in the Ice-Margin record to the present _18O-values of the surface snow at the deposition sites. On the northern slope of the Greenland Ice sheet, the Pleistocene-Holocene d18O-change is c. 10 per mil in contrast to a change of 6-7 per mil at locations near the central Ice divide. This is in accordance with deep Ice-core results.We conclude that d18O-records measured on Ice from the Greenland Ice-sheet Margin provide useful information about past climate and dynamics of the Ice sheet, and thus are important (and cheap) supplements to deep Ice core records.
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comparison between greenland Ice Margin and Ice core oxygen 18 records
Annals of Glaciology, 2002Co-Authors: Niels Reeh, Hans Oerter, Henrik Hojmark ThomsenAbstract:Old Ice for paleoenvironmental studies retrieved by deep core drilling in the central regions of the big Ice sheets can also be retrieved from the Ice-sheet Margins. The d18O-content of the surface Ice was studied at 15 different Greenland Ice-Margin locations. At some locations, two or more records were obtained along closely spaced parallel sampling profiles, showing good reproducibility of the records. We present Ice-Margin d18O- records reaching back into the Pleistocene. Many of the characteristic d18O-variations known from Greenland deep Ice-cores can be recognized, allowing an approximate time scale to be established along the Ice-Margin records. A flow line model is used to determine the location on the Ice sheet where the Margin-Ice was originally deposited as snow. The Pleistocene-Holocene d18O-change at the deposition sites is determined by comparing the d18O-values in the Ice-Margin record to the present _18O-values of the surface snow at the deposition sites. On the northern slope of the Greenland Ice sheet, the Pleistocene-Holocene d18O-change is c. 10 per mil in contrast to a change of 6-7 per mil at locations near the central Ice divide. This is in accordance with deep Ice-core results.We conclude that d18O-records measured on Ice from the Greenland Ice-sheet Margin provide useful information about past climate and dynamics of the Ice sheet, and thus are important (and cheap) supplements to deep Ice core records.
David J A Evans - One of the best experts on this subject based on the ideXlab platform.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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polyphase deformation at an oscillating Ice Margin following the loch lomond readvance central scotland uk
Sedimentary Geology, 2002Co-Authors: Emrys Phillips, David J A Evans, Clive AutonAbstract:The sequence of glacitectonic disturbance of an Ice-contact delta during the initial stages of deglaciation following the Loch Lomond Readvance is examined. An Ice-Marginal, Gilbert-type deltaic sequence exposed in Drumbeg quarry, Drymen (Scotland) displays a polyphase deformation history that is punctuated by periods of erosion and deposition. Progradation of the delta into Ice-dammed Lake Blane led to a temporary stabilisation of the Loch Lomond glacier during recession from its Loch Lomond Readvance maximum position. This was followed by a NE-directed readvance into the Ice-contact slope of the delta, resulting in proglacial deformation (D1) and the formation of a thrust-block ridge. In the middle to outer parts of the delta complex, sedimentation continued uninterrupted. Subsequent retreat of the Ice was accompanied by erosion and deposition as the Ice-contact slope was re-established. A second phase of readvance resulted in subglacial, ductile shearing (D2) and deposition of a diamicton during NE-directed over-riding of the delta sediments by the Ice. The direction of Ice flow subsequently changed towards the SE. This was followed by a further retreat of the Ice and re-establishment of the fan-delta complex. The complexity of the glacitectonic sequence at Drumbeg records the impact of an oscillating Ice Margin responding to either relative deepening of the lake waters in contact with the receding snout, or climatic controls. It provides further evidence that many of the Scottish Highland glaciers were subject to active recession rather than stagnation at the end of the Loch Lomond Readvance.
Robert D Storrar - One of the best experts on this subject based on the ideXlab platform.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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morphometry and pattern of a large sample 20 000 of canadian eskers and implications for subglacial drainage beneath Ice sheets
Quaternary Science Reviews, 2014Co-Authors: Robert D Storrar, Chris R Stokes, David J A EvansAbstract:Ice sheet flow is strongly influenced by the nature and quantity of meltwater entering the subglacial system. Accessing and monitoring contemporary drainage systems beneath Ice sheets is notoriously difficult, but it is possible to utilise the exposed beds of palaeo-Ice sheets. In particular, eskers record deposition in glacial drainage channels and are widespread on the exposed beds of former Ice sheets. However, unlike some other common glacial landforms (e.g. drumlins) there have been relatively few attempts to investigate and quantify their characteristics at the Ice sheet scale. This paper presents data on the distribution, pattern, and morphometry of a large (>20,000) sample of eskers in Canada, formed under the Laurentide Ice Sheet, including quantification of their length, fragmentation, sinuosity, lateral spacing, number of tributaries, and downstream elevation changes. Results indicate that eskers are typically very long (hundreds of km) and often very straight (mean sinuosity approximates 1). We interpret these long esker systems to reflect time-transgressive formation in long, stable conduits under hydrostatic pressure. The longest eskers (in the Keewatin sector) are also the least fragmented, which we interpret to reflect formation at an Ice Margin experiencing stable and gradual retreat. In many locations, the lateral distance between neighbouring eskers is remarkably consistent and results indicate a preferred spacing of around 12 km, consistent with numerical models which predict esker spacing of 8–25 km. In other locations, typically over soft sediments, eskers are rarer and their patterns are more chaotic, reflecting fewer large R-channels and rapidly changing Ice sheet dynamics. Comparison of esker patterns with an existing Ice Margin chronology reveals that the meltwater drainage system evolved during deglaciation: eskers became more closely spaced with fewer tributaries as deglaciation progressed, which has been interpreted to reflect increased meltwater supply from surface melt. Eskers show no preference to trend up or down slopes, indicating that Ice surface was an important control on their location and that the conduits were, in places, close to Ice overburden pressure.
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major changes in Ice stream dynamics during deglaciation of the north western Margin of the laurentide Ice sheet
Quaternary Science Reviews, 2009Co-Authors: Chris R Stokes, Chris D Clark, Robert D StorrarAbstract:Abstract Victoria Island lies at the north-western limit of the former North American (Laurentide) Ice Sheet in the Canadian Arctic Archipelago and displays numerous cross-cutting glacial lineations. Previous work suggests that several Ice streams operated in this region during the last (Wisconsinan) glaciation and played a major role in Ice sheet dynamics and the delivery of Icebergs into the Arctic Ocean. This paper produces the first detailed synthesis of their behaviour from the Last Glacial Maximum through to deglaciation (∼21–9.5 cal ka BP) based on new mapping and a previously published radiocarbon-constrained Ice sheet Margin chronology. Over 70 discrete Ice flow events (flow-sets) are ‘fitted’ to the Ice Margin configuration to allow identification of several Ice streams ranging in size from large and long-lived (thousands of years) to much smaller and short-lived (hundreds of years). The reconstruction depicts major Ice streams in M'Clure Strait and Amundsen Gulf which underwent relatively rapid retreat from the continental shelf edge at some time between ∼15.2 and 14.1 cal ka BP: a period which encompasses climatic warming and rapid sea level rise (meltwater pulse-1a). Following this, overall retreat was slower and the Ice streams exhibited asynchronous behaviour. The Amundsen Gulf Ice Stream continued to operate during Ice Margin retreat, whereas the M'Clure Strait Ice Stream ceased operating and was replaced by an Ice divide within ∼1000 years. This Ice divide was subsequently obliterated by another short-lived phase of Ice streaming in M'Clintock Channel ∼13 cal ka BP. The timing of this large Ice discharge event coincides with the onset of the Younger Dryas. Subsequently, a minor Ice divide developed once again in M'Clintock Channel, before final deglaciation of the island shortly after 9.5 cal ka BP. It is concluded that large Ice streams at the NW Margin of the Laurentide Ice Sheet, equivalent in size to the Hudson Strait Ice Stream, underwent major changes during deglaciation, resulting in punctuated delivery of Icebergs into the Arctic Ocean. Published radiocarbon dates constrain this punctuated delivery, as far as is possible within the limits imposed by their precision, and we note their coincidence with pulses of meltwater delivery inferred from numerical modelling and ocean sediment cores.