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Expanded Late Wisconsinan ice cap and ice sheet margins in the western Queen Elizabeth Islands, Arctic Canada
... Breakup of modern ice shelves around the Antarctic Peninsula, in response to warming of the atmosphere (Scambos et al., 2004, Pritchard andVaughan, 2007;Banwell et al., 2013) and ocean , Paolo et al., 2015Wouters et al., 2015), has heightened long-standing concerns about the possible instability of parts of the West Antarctic Ice Sheet (Mercer, 1978;Payne et al., 2004;Oppenheimer, 1998;Skvarca et al., 1999;De Angelis and Skvarca, 2003;Joughin and Alley, 2011;Scambos, 2017;Gagliardini et al., 2010;Joughin et al., 2014b;Rignot et al., 2013Rignot et al., , 2014Dow et al., 2018;Smith et al., 2021). Similar concerns have been raised about accelerated ice discharge around Greenland based on satellite radar interferometry since 1996 (Rignot and Kanagaratnam, 2006) as well as from the loss of tidewater glaciers debouching from the Greenland Ice Sheet following the recent removal of buttressing ice shelves and floating glacier tongues (Nick et al., 2009;Pritchard et al., 2009;Joughin et al., 2014a;Mouginot et al., 2015). In both Antarctica and Greenland, the removal of floating margins has triggered accelerated ice flow and drawdown that has extended hundreds of kilometres inland towards interior ice divides (Scambos et al., 2004, Scambos, 2017Dupont and Alley, 2005;Glasser et al., 2011;Rignot et al., 2004;Fürst et al., 2016) increasing ice export to the ocean. ...
... J.H. England, R.D. Coulthard, M.F.A. Furze et al. Quaternary Science Reviews 286 (2022) 107524 Projections of future sea level rise based on ice sheet modelling remain contentious due to uncertainties surrounding the evolution of fast glacier flow in Antarctica and Greenland (Payne et al., 2004;Alley et al., 2005;Nick et al., 2009;Pritchard et al., 2009Pritchard et al., , 2011Shuman et al., 2011;Mouginot et al., 2015;Pollard et al., 2015;DeConto and Pollard, 2016;Robel et al., 2019). Contrasting models of West Antarctic ice retreat predict 21st century sea level rise ranging from 45 cm to >1 m (Edwards et al., 2015;DeConto and Pollard, 2016;Parizek et al., 2019;Serousi et al., 2020). ...
... Towards the end of the last glaciation, between~14.5 and 13.7 cal ka BP, the NW LIS underwent rapid regional retreat across 600 km, withdrawing eastward from the continental shelf of the Arctic Ocean to northern Victoria Island ( Fig. 1; England et al., 2009;Lakeman and England, 2013;Nixon and England, 2014;Vaughan, 2014). There, its land-based margin stabilized for~2000 years, until it readvanced abruptly, forming the VMSIS. ...
We document the unparalleled depositional and chronological record of an 80,000 km² ice shelf from the former Laurentide Ice Sheet whose advance and retreat spanned only ∼400 years (11.6–11.2 cal ka BP). Its catastrophic breakup (∼150 years; 11.3–11.2 cal ka BP) coincided with a rapidly warming atmosphere and ocean, paralleling reported conditions impacting the ongoing evolution of marine-based margins of Antarctica and Greenland. Our record highlights the instability of the NW Laurentide Ice Sheet whose marine-based ice streams and ice shelves retreated in channels of intermediate depth (400–600 m) that lack reverse slopes, suggesting that current ice dynamics models may underestimate the sensitivity of similar margins in Antarctica to ongoing global warming. Our reconstruction is based on > 700 km of coastal field surveys complemented by radiocarbon dates stratigraphically encompassing the full lifespan of the former Viscount Melville Sound Ice Shelf. Notably, this detailed record demonstrates that large ice shelves can be removed rapidly, cautioning against dismissing predictions of possible 21st century mean-sea-level rise >1 m derived from the ongoing retreat of marine-based ice sheets. We emphasize that during the last deglaciation of the western Canadian Arctic Archipelago, prominent ice streams transitioned into ice shelves whose depositional landforms are pervasive, demonstrating their pivotal role in the evisceration of the NW Laurentide Ice Sheet. The VMSIS provides an exceptionally detailed geological record whose tightly constrained chronology of growth, maintenance and breakup serves as an instructive example for those modern marine-based margins whose instability is being investigated to address estimates of possible future sea level rise.
... On nearby Melville Island, we show that ice persisted for longer than what was suggested by Dyke et al. (2003). Following Nixon and England (2014), we show remnant ice in the form of island-based ice caps, especially in western Melville Island, which has some high-elevation plateaux (Fig. 4). Our updated maps show a near-synchronous ice retreat from eastern M'Clure Strait and western Viscount Melville Sound at~11.5 ka (England et al., 2009). ...
... Our updated maps show a near-synchronous ice retreat from eastern M'Clure Strait and western Viscount Melville Sound at~11.5 ka (England et al., 2009). We make further refinements on Melville Island between 12 ka and 10 ka to follow extensive mapping, geomorphology and radiocarbon work (Nixon and England, 2014). We also retain a remnant ice lobe over northeastern Melville Island until 9 ka following the work of Hanson (2003). ...
... England et al. (2006) presented an updated interpretation of the deglaciation of the Innuitian Ice Sheet (see Fig. 3) suggesting that the pattern of ice retreat in this region may have been more rapid than what was suggested by Dyke et al. (2003). This new interpretation depicted many of the Central and Eastern Queen Elizabeth Islands as hosts to local ice dispersal centres at 18 ka (England et al., 2006(England et al., , 2009Nixon and England, 2014). We adopt these changes to the broad region of the Innuitian Ice Sheet, mostly consisting of minor adjustments to the ice margin and the most substantive change being an accelerated rate of ice retreat over marine regions at~9 ka (Fig. 5 and Figs. ...
The duration and intensity of the early Holocene freshening events has been explained by subglacial outburst(s) from Lake Agassiz, the collapse of a Laurentide Ice Sheet ‘saddle’ overlying Hudson Bay, and/or a combination of the two events. Our field evidence provides new radiocarbon ages and geomorphologic observations to assess the deglacial history of this important region, allowing for revision of the sequence of events. We show that the collapse of the Hudson Bay Ice Saddle in southwestern Hudson Bay occurred between 8.57 ±0.28 ka BP and 8.11 ±0.19 ka BP. This event was preceded by at least one subglacial-drainage event through numerous newly-mapped subglacial channels onshore of southwestern Hudson Bay. Lake Agassiz may have experienced multiple subglacial drainage events prior to the final HBIS collapse, accounting for the timing discrepancies in freshwater cooling observed in the North Atlantic. Importantly, this new work links the chronology of events on the southwest (land-based) side of the HBIS to the northeast (marine-based) side of the ice sheet. Additionally, this work provides a potential analog for the behaviour of other ice sheets whose beds lie well below sea level, such as the West Antarctic Ice Sheet, during periods of warming climate.
... On nearby Melville Island, we show that ice persisted for longer than what was suggested by Dyke et al. (2003). Following Nixon and England (2014), we show remnant ice in the form of island-based ice caps, especially in western Melville Island, which has some high-elevation plateaux (Fig. 4). Our updated maps show a near-synchronous ice retreat from eastern M'Clure Strait and western Viscount Melville Sound at~11.5 ka (England et al., 2009). ...
... Our updated maps show a near-synchronous ice retreat from eastern M'Clure Strait and western Viscount Melville Sound at~11.5 ka (England et al., 2009). We make further refinements on Melville Island between 12 ka and 10 ka to follow extensive mapping, geomorphology and radiocarbon work (Nixon and England, 2014). We also retain a remnant ice lobe over northeastern Melville Island until 9 ka following the work of Hanson (2003). ...
... England et al. (2006) presented an updated interpretation of the deglaciation of the Innuitian Ice Sheet (see Fig. 3) suggesting that the pattern of ice retreat in this region may have been more rapid than what was suggested by Dyke et al. (2003). This new interpretation depicted many of the Central and Eastern Queen Elizabeth Islands as hosts to local ice dispersal centres at 18 ka (England et al., 2006(England et al., , 2009Nixon and England, 2014). We adopt these changes to the broad region of the Innuitian Ice Sheet, mostly consisting of minor adjustments to the ice margin and the most substantive change being an accelerated rate of ice retreat over marine regions at~9 ka (Fig. 5 and Figs. ...
... On Ellesmere Island and neighboring Axel Heiberg Island, more than 20 radiocarbon ages from shells and organic material, largely in shoreline areas, have been dated to between 37.5 ka and 32.5 ka (England, 1990;England, 1996;England et al., 2000;Ó Cofaigh et al., 2000). To the west on Melville and Banks islands, 13 radiocarbon ages on shell fragments (largely contained in tills) suggest local ice-free conditions between 37.5 ka and 32.5 ka (England et al., 2009;Nixon and England, 2014). To depict our best estimate of Innuitian ice at 35 ka, we show the present-day ice extent for the region (Fig. 12A). ...
... On Ellesmere Island and neighboring Axel Heiberg Island, radiocarbon ages from shells and organic material, largely in shoreline areas, have been dated to between 32.5 ka and 27.5 ka (England, 1978;England, 1990;England, 1996;England et al., 2000;Ó Cofaigh et al., 2000). To the west on Melville and Banks islands, radiocarbon ages on shell fragments (largely contained in tills) suggest local ice-free conditions between 32.5 ka and 27.5 ka (England et al., 2009;Nixon and England, 2014) prior to being overrun by ice. Following these data, our estimate of Innuitian ice assumes present-day ice extent over the Queen Elizabeth Islands, which we also consider as the minimum ice extent. ...
The Laurentide Ice Sheet was the largest global ice mass to grow and decay during the last glacial cycle (~115 ka to ~10 ka). Despite its importance for driving major changes in global mean sea level, long-term landscape evolution, and atmospheric circulation patterns, the history of the Laurentide (and neighbouring Innuitian) Ice Sheet is poorly constrained owing to sporadic preservation of stratigraphic records prior to the Last Glacial Maximum (LGM; ~25 ka) and a case-study approach to the dating of available evidence. Here, we synthesize available geochronological data from the glaciated region, together with published stratigraphic and geomorphological data, as well as numerical modelling output, to derive 19 hypothesised reconstructions of the Laurentide and Innuitian ice sheets from 115 ka to 25 ka at 5-kyr intervals, with uncertainties quantified to include best, minimum, and maximum ice extent estimates at each time-step. Our work suggests that, between 115 ka and 25 ka, some areas of North America experienced multiple cycles of rapid ice sheet growth and decay, while others remained largely ice-free, and others were continuously glaciated. Key findings include: (i) the growth and recession of the Laurentide Ice Sheet from 115 ka through 80 ka; (ii) significant build-up of ice to almost LGM extent at ~60 ka; (iii) a potentially dramatic reduction in North American ice at ~45 ka; (iv) a rapid expansion of the Labrador Dome at ~38 ka; and (v) gradual growth toward the LGM starting at ~35 ka. Some reconstructions are only loosely constrained and are therefore speculative (especially prior to 45 ka). Nevertheless, this work represents our most up-to-date understanding of the build-up of the Laurentide and Innuitian ice sheets during the last glacial cycle to the LGM based on the available evidence. We consider these ice configurations as a series of testable hypotheses for future work to address and refine. These results are important for use across a range of disciplines including ice sheet modelling, palaeoclimatology and archaeology and are available digitally.
... A suite of new radiocarbon dates and glacial geomorphological mapping, however, has clearly indicated that the LIS inundated Banks Island during the LLGM (England et al., 2009;Lakeman et al., 2012Lakeman et al., , 2013. Similar methods and new dates have also extended the ice margin over the entirety of Melville Island and onto Eglington Island (Nixon et al., 2014) and it is likely that the ice also overran Prince Patrick Island and extended onto the continental shelf along the entire IIS margin (see Stokes et al., 2016). At the same time, a large body of work has used cosmogenic dating of high-elevation erratics close to fjord mouths (e.g. on Baffin Island) to demonstrate that a relatively thick LIS must have terminated on the continental shelf during the LLGM (e.g. ...
The last deglaciation of the Laurentide Ice Sheet (LIS) was associated with major reorganisations in the ocean-climate system and its retreat also represents a valuable analogue for understanding the rates and mechanisms of ice sheet collapse. This paper reviews the characteristics of the LIS at its Last Glacial Maximum (LGM) and its subsequent deglaciation, with particular emphasis on the pattern and timing of ice margin recession and the driving mechanisms of retreat. The LIS initiated over the eastern Canadian Arctic ~116-110 ka (MIS 5d), but its growth towards the LGM was highly non-linear and punctuated by several episodes of expansion (~65 ka: MIS 4) and retreat (~50-40 ka: MIS 3). It attained its maximum position around 26-25 ka (MIS 2) and existed for several thousand years as an extensive ice sheet with major domes over Keewatin, Foxe Basin and northern Quebec/Labrador. It extended to the edge of the continental shelf at its marine margins and likely stored a sea-level equivalent of around 50 m and with a maximum ice surface ~3,000 m above present sea-level. Retreat from its maximum was triggered by an increase in boreal summer insolation, but areal shrinkage was initially slow and the net surface mass balance was positive, indicating that ice streams likely played an important role in reducing the ice sheet volume, if not its extent, via calving at marine margins. Between ~16 and ~13 ka, the ice sheet margin retreated more rapidly, particularly in the south and west, whereas the north and east underwent only minimal recession. The overall rate of retreat decreased during the Younger Dryas (YD), when several localised readvances occurred. Following the YD, the ice sheet retreated two to five times faster than previously, and this was primarily driven by enhanced surface melting while ice streams reduced in effectiveness. Final deglaciation of the Keewatin and Foxe Domes, left a remnant Labrador Dome that disappeared ~6.7 ka.
... Fig. 7). The thick red line shows the updated LGM ice margin (following recent work [48][49][50][51][52][53][54] ). Underlying topography from GTOPO30 digital elevation data 47 . ...
The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent or potentially underway in West Antarctica, but others predicting a more limited response. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean-climate forcing. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but-at the ice-sheet scale-their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet.
... LIS extent is shown for the Last Glacial Maximum (LGM) and at 10.2 cal ka BP, from Dyke et al. (2003). Note that the LIS has recently been shown to extend to the continental shelf at the LGM in many regions (e.g., Briner et al., 2006;Shaw et al., 2006;Kleman et al., 2010;England, 2012, 2013;Jakobsson et al., 2014;Nixon and England, 2014). The locations of Figs. ...
This paper presents a comprehensive review and synthesis of ice streams in the Laurentide Ice Sheet (LIS) based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed. The inventory includes 117 ice streams, which have been identified based on a variety of evidence including their bedform imprint, large-scale geomorphology/topography, till properties, and ice rafted debris in ocean sediment records. Despite uncertainty in identifying ice streams in hard bedrock areas, it is unlikely that any major ice streams have been missed. During the Last Glacial Maximum, Laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets. Large ice streams had extensive onset zones and were fed by multiple tributaries and, where ice drained through regions of high relief, the spacing of ice streams shows a degree of spatial self-organisation which has hitherto not been recognised. Topography exerted a primary control on the location of ice streams, but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock, and where networks of ice streams switched direction repeatedly and probably over short time scales. As the ice sheet retreated onto its low relief interior, several ice streams show no correspondence with topography or underlying geology, perhaps facilitated by localised build-up of pressurised subglacial meltwater. They differed from most other ice stream tracks in having much lower length-to-width ratios and have no modern analogues. There have been very few attempts to date the initiation and cessation of ice streams, but it is clear that ice streams switched on and off during deglaciation, rather than maintaining the same trajectory as the ice margin retreated. We provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30% of the margin was drained by ice streams at the LGM (similar to that for present day Antarctic ice sheets), but this decreases to 15% and 12% at 12 cal ka BP and 10 cal ka BP, respectively. The extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate, or internal ice dynamical feedbacks unrelated to climate (or both) is largely unknown and represents a key area for future work to address.
Reconstructions of former ice sheets and glaciers provide important palaeoglaciological information about their behaviour in response to climate changes. Glacial trimlines record both the margin positions and palaeo ice thickness, enabling the production of empirically constrained 3-Dimensional reconstructions. However, the literature review into the characteristics, interpretation, and use of glacial trimlines here presented shows that these features have been under-utilised and are poorly described in the existing literature, with a confusing terminology currently in use. A new classification scheme and terminology for trimline identification and interpretation is developed to better facilitate further research into these common features of glacierised and formerly glaciated landscapes.
Fifty-six new radiocarbon dates from driftwood (mainly Larix, Picea and Populus spp.) collected from the modern and raised shorelines of Melville and Eglinton islands (western Canadian High Arctic) are presented and compared to other driftwood collections from the Canadian Arctic Archipelago (CAA) and Greenland. By documenting the species (provenance) and spatio-temporal distribution of driftwood at various sites across the Arctic, regional characterizations of former sea-ice conditions and changes in Arctic Ocean circulation patterns may be deduced. The earliest postglacial invasion of the Canadian Arctic Archipelago by driftwood is recorded on central Melville Island at c. 11 cal. ka BP, suggesting that the modern circulation pattern of Arctic Ocean surface water southeast through the archipelago was established >1000 years earlier than previously proposed. Throughout most of the Holocene until c. 1.0 cal. ka BP, the rate of driftwood delivery to the western Arctic islands was low (~1 recorded stranding event per 200 years) and intermittent, with the longest break in the record occurring between c. 3.0 and 5.0 cal. ka BP. This 2000-year hiatus is attributed to a period of colder temperatures causing severe sea-ice conditions and effectively making the coasts of the western Arctic islands inaccessible. After c. 1.0 cal. ka BP, driftwood incursion increased to maximum Holocene levels (~1 recorded stranding event every 20 years). Driftwood identified to the genus level as Larix that was delivered at this time suggests that the Trans Polar Drift current was regularly in its most southwestern position, related to a dominantly positive Arctic Oscillation mode. The Little Ice Age appears to have had little impact on driftwood entry to the western Canadian Arctic Archipelago, indeed the general abundance in the latest Holocene may record infrequent landfast sea ice.
Amundsen Gulf and adjoining Dolphin and Union Strait and Coronation Gulf form the southwestern end of the Northwest Passage adjacent to the Beaufort Sea. Extensive high resolution multibeam sonar imagery and sub-bottom profiles of the seabed have been acquired, primarily in Amundsen Gulf, by ArcticNet and the Ocean Mapping Group at the University of New Brunswick. These data reveal a variety of seabed landforms including mega-scale glacial ridge and groove lineations, drumlins, moraines, iceberg scours, bedrock outcrops, and discontinuous sediment deposits of variable thickness. The lineations are widespread, especially in southeastern Amundsen Gulf. They resemble modern and paleo bedforms reported from Antarctica, Svalbard, Greenland and other Canadian Arctic channels, where they have been ascribed to ice streams.
The glacial sole marks on the seabed in Amundsen Gulf and regional data from the adjacent mainland and islands outline the configuration of a glacial ice stream from the Laurentide Ice Sheet that occupied Amundsen Gulf and adjoining waterways during the Late Wisconsinan. Part of the northwestward flowing ice stream was deflected around the Colville Mountains on Victoria Island and rejoined the main ice stream in Amundsen Gulf by way of Prince Albert Sound. The grounded Amundsen Gulf ice stream extended northwestward to the outer slope in the Beaufort Sea where it was buttressed by Arctic Shelf Ice. Maximum ice stream extent is inferred to have been coincident with the Late Glacial Maximum. Multi-sequence ice-contact sediments and stratigraphic relations with glaciomarine sediments indicate that several ice advances and retreats occurred in the northwestern part of the gulf. Final retreat from the maximum position began prior to 13,000 cal yr BP and terrestrial dates indicate that the retreating ice front had reached Dolphin and Union Strait by about 12.5 cal ka BP.
The large physiographic elements of Prince of Wales Island consist of several stepped planation surfaces incised by broad meandering fluvial channels that predate formation of the archipelago. Erosion surfaces likely correlated with Sverdrup Basin Mesozoic clastic fills. Most of the island is covered by thick drift, largely till. A few subtill nonglacial deposits are likely Sangamonian. Wisconsin Glaciation left a single till sheet with three cross-cutting landscape assemblages, each recording a phase and direction of flow. The island occupies part of the tail of a zone of dispersion of shield debris, >700km from source, perhaps resulting from phases 1 and 2. During phase 3, debris was dispersed >120km eastward from the island, most strongly in plumes representing ice streams. Postglacial sediments are mainly raised beaches with minor deltaic and alluvial sediment. -from Authors
Optimized regional climate simulations are conducted using the Polar MM5, a version of the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), with a 60-km horizontal resolution domain over North America during the Last Glacial Maximum (LGM, 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). The objective is to describe the LGM annual cycle at high spatial resolution with an emphasis on the winter atmospheric circulation. Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. Polar MM5 produces a substantially different atmospheric response to the LGM boundary conditions than CCM3 and other recent GCM simulations. In particular, from November to April the upper-level flow is split around a blocking anticyclone over the LIS, with a northern branch over the Canadian Arctic and a southern branch impacting southern North America. The split flow pattern is most pronounced in January and transitions into a single, consolidated jet stream that migrates northward over the LIS during summer. Sensitivity experiments indicate that the winter split flow in Polar MM5 is primarily due to mechanical forcing by LIS, although model physics and resolution also contribute to the simulated flow configuration. Polar MM5 LGM results are generally consistent with proxy climate estimates in the western United States, Alaska, and the Canadian Arctic and may help resolve some long-standing discrepancies between proxy data and previous simulations of the LGM climate.
The aim of the paper is to analyse landscapes of glacial erosion associated with the Laurentide ice sheet at its maximum and to relate them lo the three main variables affecting glacial erosion, namely former basal thermal regime of the ice sheet, the topography of the bed, and the geology of the bed. The key to the analysis is the comparison of the distribution of landscape types with the simulated pattern of the basal thermal regime of the former ice sheet.
Landscapes of area scouring are found to be associated with zones of basal melting and occur beneath much of the former ice-sheet centre and in those places where the topography favoured converging ice flow. The landscape type may also have formed beneath cold-based ice when it was carrying debris inherited from an up-stream zone of regelation. Areas with little or no sign of glacial erosion occur primarily in the north in the Queen Elizabeth Islands but they also occur on uplands associated with diverging ice flow; they coincide with areas calculated to have been covered by cold-based ice devoid of debris. Landscapes of selective linear erosion are common on uplands near the eastern periphery of the ice sheet. In these situations, pre-existing valleys channelled ice flow and created a situation where there was warm-based ice over the valleys and cold-based protective ice over the intervening plateaux. Variations in the permeability of the bedrock base have modified the landscape pattern, mainly in those areas where there was a change from one basal thermal regime to another. In general, permeable rocks tend to have experienced less erosion than impermeable rocks.
Using lake-basin density as an indication of the intensity of glacial erosion, a zone of maximum erosion is identified and this forms a ring between the centre of the former ice sheet and its periphery. This ring coincides with a zone where melt water from the ice-sheet centre is calculated to have frozen on to the bottom of the ice sheet. This regelation incorporated basal debris into the ice, forming a basal layer 20-50 m thick and afforded an efficient means of debris evacuation.
A conceptual model is developed and hangs round the following postulates: (1) Landscapes of glacial erosion are related primarily to the basal thermal regime of the ice sheet.
(2) Landscapes of glacial erosion are equilibrium forms related to maximum glacial conditions. This implies that at some stage in the Pleistocene the Laurentide ice sheet was in a stable maximum condition for a long period of time.
(3) Mechanisms allowing evacuation of debris rather than those of abrasion or fracture may be the most important in influencing the amount of erosion achieved by an ice sheet.
(4) Cold-based ice may accomplish erosion if it contains debris.
The debris-covered ice-margins of three largely cold-based glaciers in central Spitsbergen were investigated to reconstruct their formation and degradation. Clast shapes indicate dominant englacial and supraglacial transport with a smaller subglacial component. Emplacement of material is inferred to have been through meltout along flowlines due to the relatively uniform and continuous debris cover along the glacier margins; no evidence of thrusting has been found. Degradation of all three belts is rapid and involves debris flows at unstable places—e.g., the margins of meltwater channels. Resultant exposure of underlying ice initiates or accelerates melting, thereby leading to further debris flows. Hence, once degradation starts, a self-reinforcing cycle that removes material from the glacier commences. Landform preservation potential on millennial time scales in a high-arctic, continuous permafrost environment is thus limited. This work has implications for the interpretation of Pleistocene landform associations that use modern analogues from Svalbard.
For more than four decades, the reporting of 14C dates on marine molluscs from Arctic Canada has been notable for the lack of consistently applied marine reservoir corrections. We propose that the common approach of reporting Canadian Arctic marine 14C dates using presumed time-invariant reservoir corrections be abandoned in favour of calibration of 14C dates, using the current standard protocol. This approach best facilitates inter- and intra-regional correlation, and correlation with other geochronometers. In order to enable the consistent calibration of marine 14C dates from Arctic Canada, we analysed a 14C database of 108 marine mollusc samples collected live between 1894 and 1956, and determined regional reservoir offset values (ΔR) for eight oceanographically distinct regions. The following new ΔR values should be used for 14C calibration: NW Canadian Arctic Archipelago, 335 ± 85 yrs; Foxe Basin, 310 ± 90 yrs; NE Baffin Island, 220 ± 20 yrs; SE Baffin Island, 150 ± 60 yrs; Hudson Strait, 65 ± 60 yrs; Ungava Bay, 145 ± 95 yrs; Hudson Bay, 110 ± 65 yrs; and James Bay, 365 ± 115 yrs.
1] Interferometric synthetic-aperture radar data collected by ERS-1/2 and Radarsat-1 satellites show that Antarctic Peninsula glaciers sped up significantly following the collapse of Larsen B ice shelf in 2002. Hektoria, Green and Evans glaciers accelerated eightfold between 2000 and 2003 and decelerated moderately in 2003. Jorum and Crane glaciers accelerated twofold in early 2003 and threefold by the end of 2003. In contrast, Flask and Leppard glaciers, further south, did not accelerate as they are still buttressed by an ice shelf. The mass loss associated with the flow acceleration exceeds 27 km 3 per year, and ice is thinning at rates of tens of meters per year. We attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf. The magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change.
At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock. The diamictons are estimated to form during the last glacial maximum (LGM) and an earlier glacial event, possibly between OIS 4 to 5d. Both erosional events were presumably caused by the grounding of ice shelves originating from the Laurentide ice sheet. Broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor. Postglacial sedimentation evolved from iceberg-dominated environments to those controlled by sea-ice rafting and marine processes in the Holocene. A prominent minimum in planktonic foraminiferal δ18O is identified in deglacial sediments at an estimated age near 13,000 cal yr BP. This δ18O minimum, also reported elsewhere in the Amerasia Basin, is probably related to a major Laurentide meltwater pulse at the Younger Dryas onset. The Bering Strait opening is also marked in the composition of late deglacial Chukchi sediments.
Rapidly-flowing sectors of an ice sheet (ice streams) can play an important role in abrupt climate change through the delivery of icebergs and meltwater and the subsequent disruption of ocean thermohaline circulation (e.g., the North Atlantic's Heinrich events). Recently, several cores have been raised from the Arctic Ocean which document the existence of massive ice export events during the Late Pleistocene and whose provenance has been linked to source regions in the Canadian Arctic Archipelago. In this paper, satellite imagery is used to map glacial geomorphology in the vicinity of Victoria Island, Banks Island and Prince of Wales Island (Canadian Arctic) in order to reconstruct ice flow patterns in the highly complex glacial landscape. A total of 88 discrete flow-sets are mapped and of these, 13 exhibit the characteristic geomorphology of palaeo-ice streams (i.e., parallel patterns of large, highly elongated mega-scale glacial lineations forming a convergent flow pattern with abrupt lateral margins). Previous studies by other workers and cross-cutting relationships indicate that the majority of these ice streams are relatively young and operated during or immediately prior to deglaciation. Our new mapping, however, documents a large (> 700 km long; 110 km wide) and relatively old ice stream imprint centred in M'Clintock Channel and converging into Viscount Melville Sound. A trough mouth fan located on the continental shelf suggests that it extended along M'Clure Strait and was grounded at the shelf edge. The location of the M'Clure Strait Ice Stream exactly matches the source area of 4 (possibly 5) major ice export events recorded in core PS1230 raised from Fram Strait, the major ice exit for the Arctic Ocean. These ice export events occur at ∼12.9, ∼15.6, ∼22 and 29.8 ka (14C yr BP) and we argue that they record vigorous episodes of activity of the M'Clure Strait Ice Stream. The timing of these events is remarkably similar to the North Atlantic's Heinrich events and we take this as evidence that the M'Clure Strait Ice Stream was also activated around the same time. This may hold important implications for the cause of the North Atlantic's Heinrich events and hints at the possibility of a pan-ice sheet response.
During the past year, 1969-1970, both the 2-L (Dyck and Fyles, 1962) and 5-L (Dyck et al. , 1965) counters were routinely operated. A 1-L counter was finally constructed with acceptable characteristics (see description below) and was operated in July in place of the 5-L counter. The 2-L counter was operated exclusively at 2 atm. The 5-L counter was operated at 1 atm, except for October and November, 1969, when it was operated at 4 atm. The 1-L counter was operated at 1 atm.
All C 14 measurements in this date list were made with the 2 L counter described in our first date list (GSC I). Ages were calculated on a C 14 half life of 5570 ± 30 yr and 0.95 of the activity of the NBS oxalic-acid standard, and are quoted in years before 1950.
Prince Patrick and Eglinton islands have a polar desert climate and a landscape of coastal plains and dissected plateaux with limited vegetation cover. Use of a properly damped surveyor's compass is possible, however, magnetic declination changes markedly over short distances and large temporal variations are present. Bedrock of the report area is divisible into four major successions. These include: 1) 14 to 18 km of Proterozoic(?) and/or older bedrock above the Mohorovicí Discontinuity; 2) 10 to 14 km of thermally overmature but variably tectonized ("Franklinian") strata that range from Vendian(?) at the base through Upper Devonian at the top; 3) less than 1 km grading to more than 7 km of thermally mature and immature, relatively undeformed Carboniferous through Lower Cretaceous strata of the Sverdrup Basin, including up to 2 km of Middle Jurassic through Upper Cretaceous strata preserved in four peripheral basins and numerous small grabens; and 4) 70 m to more than 600 m of unconsolidated Pliocene sand, gravel, and peat, and related seismically defined Neogene strata of the Arctic Continental Terrace Wedge. The Franklinian succession is further subdivided into siliciclastic rocks of the Devonian clastic wedge (up to 6000 m thick), subsurface Lower Devonian and older strata of the Prince Patrick Platform, and correlative seismically defined deep-water strata of Canrobert Trough. A thrust-fold belt imaged seismically in the northeast is continuous with folds known at the surface on northwestern Melville Island, and folded Devonian strata are everywhere separated from Carboniferous and younger rocks by a profound angular unconformity. Other lower Paleozoic folds extend under southwestern Prince Patrick Island. A Carboniferous rift system located under the Sverdrup Basin margin has developed on the eroded roots of the Paleozoic fold belt. The rift formed in the Early Carboniferous (Serpukhovian), expanded to the southwest during the later Carboniferous, and was partly inverted during the Early Permian. Mid-Permian through early Middle Jurassic was a time of passive subsidence and progressive basin expansion toward the southwest. During Sverdrup Basin subsidence, four intracratonic basins, separated by Devonian "basement" highs, developed to the southwest between Middle Jurassic and Late Cretaceous time. An array of northerly trending horsts and grabens also developed during this time, part of a rift system that provides a geological record of the early development of the Arctic Ocean basin. Potential exists for far-travelled hydrocarbons within the Permo-Carboniferous and Jurassic-Cretaceous rift systems and in stratigraphic traps on the margins of the Mesozoic basins. Subbituminous coal seams to 1.5 m occur in Lower Cretaceous strata, and deposits of manganese carbonate are widespread in Campanian sandstone of Eglinton Island.
The stratigraphy of Lower Ordovician to Upper Devonian rocks on Melville Island is divisible into a lower carbonate-dominated portion and an upper clastic portion. The Lower Ordovician to lower Middle Devonian carbonate-dominated succession consists of three successive shale basin/carbonate platform sequences, each with a distinct distribution pattern for the two megafacies. Middle to Upper Devonian clastic strata abruptly but conformably overlie carbonate Sequence C, and were deposited in a foreland basin, in front of the southwestward advancing Ellesmerian orogenic belt. Three tectonostratigraphic sequences make up this clastic wedge. A profound angular unconformity, indicating Ellesmerian uplift and erosion, separates deposits of the Franklinian Mobile Belt from succeeding deposits of the Sverdrup Basin. -from Author
The sedimentary rocks of Melville Island can be grouped into three sedimentary successions that were controlled by regional tectonics. The groups are: 1) Infill of the Franklinian Geosyncline: the rocks are upper Precambrian to Upper Devonian carbonates succeeded by evaporites and black shales. These rocks are overlain by Devonian clastic rocks. 2) Sverdrup Basin rocks ranging from Pennsylvanian to early Tertiary. Carboniferous rocks are mainly sandstone, conglomerate and minor limestone. Bitumen-bearing Triassic rocks are found on northwestern Melville Island. The youngest rocks of the Sverdrup succession are the Maastrichtian and Paleocene strata of the Eureka Sound Group. 3) Beaufort Formation strata, consisting of gravel and sand containing wood, discontinuously overlie older rocks. They are assigned to the Upper Tertiary. -from Authors
Banks Island is a polar desert where continental ice sheets spreading from a dispersal centre to the SE reached their maximum extent on at least three occasions. In each case the glacial episode was preceded and followed by a marine episode. The three successive glacial episodes are the Banks, Thomsen, and Laurentide. The last two were separated by the Cape Collinson interglaciation. Lobes of ice of Laurentide age impinged on different parts of the Island, and morainic systems related to these lobes are described, and in some cases divided into two stades.-after Author
Late Wisconsinan age glacial landforms and deposits indicate that an ice shelf of at least 60,000 km2 flowed northwestward into Viscount Melville Sound, probably from the M'Clintock Dome of the Laurentide Ice Sheet. The ice shelf overlapped coastal areas and laid Winter Harbour Till up to 125 m above present sea level on the southern coast of Melville Island, to 135 m on Byam Martin Island, to possibly 90 m on the northeast tip of Banks Island, and to 150 m on the north coast of Victoria Island. The contemporary sea level was 50 to 100 m higher than present (it now rises eastward). A maximum age of 10,340 ± 150 yr B.P. for the till, and thus the ice-shelf advance, is provided by shells in marine sediments which underlie it, whereas a minimum age of 9880 ± 150 yr B.P. is provided by overlying shells that postdate the ice advance. The major advance of shelf ice into Viscount Melville Sound may be the result of the rapid disintegration of the M'Clintock Dome while the climate ameliorated in the western Arctic.
The cosmogenic nuclide exposure history method is undergoing major developments in analytical, theoretical, and applied areas. The capability to routinely measure low concentrations of stable and radioactive cosmogenic nuclides has led to new methods for addressing long-standing geologic questions and has provided insights into rates and styles of surficial processes. The different physical and chemical properties of the six most widely used nuclides: 3He, 10Be, 14C, 21Ne, 26Al, and 36Cl, make it possible to apply the surface exposure dating methods on rock surfaces of virtually any lithology at any latitude and altitude, for exposures ranging from 102 to 107 years. The terrestrial in situ cosmogenic nuclide method is beginning to revolutionize the manner in which we study landscape evolution. Single or multiple nuclides can be measured in a single rock surface to obtain erosion rates on boulder and bedrock surfaces, fluvial incision rates, denudation rates of individual landforms or entire drainage basins, burial histories of rock surfaces and sediment, scarp retreat, fault slip rates, paleoseismology, and paleoaltimetry. Ages of climatic variations recorded by moraine and alluvium sediments are being directly determined. Advances in our understanding of how cosmic radiation interacts with the geomagnetic field and atmosphere will improve numerical simulations of cosmic-ray interactions over any exposure duration and complement additional empirical measurements of nuclide production rates. The total uncertainty in the exposure ages is continually improving. This article presents the theory necessary for interpreting cosmogenic nuclide data, reviews estimates of parameters, describes strategies and practical considerations in field applications, and assesses sources of error in interpreting cosmogenic nuclide measurements.
So far, the most complete and accurate sea-level record that encompassed the period between the Last Glacial Maximum and the present day is based on cores drilled offshore the Barbados coral reef [1,2]. This record suggests a non-monotonous sea-level rise punctuated by dramatic accelerations, the so-called Melt Water Pulse events, that correspond to massive inputs of continental ice. The most extreme of these events, the so-called MWP1-A, initially identified in the coral-based sea level record from the Barbados island, suggests a sea-level rise of ~20 meters between 14.1 and 13.6 ka [3,4]. However, this event remains enigmatic and controversial. Barbados Island is relatively close to the former North-American ice sheet and the island itself belongs to an accretionary prism overlying an active subduction zone. The possibility remains that the apparent sea-level record may be flawed by tectonic or isostatic complications. Several records are consistent with its occurrence [5,6], but no broad agreement emerges about its timing and amplitude. Because of this lack of consensus, the temporal relationship between the MWP1-A and the abrupt, millennial-timescale, climatic events that punctuated the last deglaciation is a subject of controversial debates [7,8]. Furthermore, the ice source responsible for such a step in sea-level rise is still elusive [9,10]. Consequently, it remains a key issue to fully confirm the existence and amplitude of the MWP-1A by a precise coral reef record in a far-field site located in a oceanic basin distant from Barbados. The recent IODP Expedition 310 "Tahiti Sea Level" offers a unique opportunity to extend the existing Tahiti sea-level curve that documents the deglacial sea level rise for the last 13.8 ka [5]. Located at a considerable distance from the major former ice sheets and characterized by slow and regular subsidence rates, the Tahiti coral reefs provide an ideal setting to constrain MWP events that are thought to have punctuated the last deglaciation. The offshore coring operations carried out during Expedition 310 recovered more than 400 m of post-glacial reef material, ranging from 122 to 40 m below modern sea level [11]. Post-glacial coral material was selected using strict mineralogical and isotopic screening criteria in order to preclude any post-mortem diagenetic alteration of the coral skeleton. More than 60 U-Th ages were obtained on various types of corals characterizing shallow to deeper environments that extend the previous Tahiti record to 16 ka and allow to document the sea-level rise during the key period of the MWP-1A. Our results confirm the occurrence of an acceleration of the sea-level rise during that period. However, the timing and duration of this event differ significantly from observations from Barbados [3,4]. These new results indicate that the MWP-1A occurred at about 14.6 ka BP, synchronously with the Bølling onset. This allows us to revisit the relationship between the MWP-1A and the climate history of the last deglaciation. Their implications in terms of the potential sources of the ice that generated the MWP-1A will be also discussed. [1] Fairbanks, 1989, Nature 342, 637. [2] Bard et al., 1990, Nature 346, 456. [3] Fairbanks et al., 2005, Quaternary Science Reviews 24, 1781. [4] Peltier and Fairbanks, 2006, Quaternary Science Reviews 25, 3322. [5] Bard et al., 1996, Nature 382, 241. [6] Hanebuth et al., 2000, Science 288, 1033-1035. [7] Weaver et al., 2003, Science 299, 1709-1713. [8] Stanford et al., 2006, Paleoceanography 21. [9] Clark et al., 1996, Paleoceanography 11, 563-577. [10] Clark et al.; 2002. Science 295, 2438-2441. [11] Camoin et al., 2007, Proc. IODP, 310
Past deglacial ice sheet reconstructions have generally relied upon discipline-specific constraints with no attention given to the determination of objective confidence intervals. Reconstructions based on geophysical inversion of relative sea level (RSL) data have the advantage of large sets of proxy data but lack ice-mechanical constraints. Conversely, reconstructions based on dynamical ice sheet models are glaciologically self-consistent, but depend on poorly constrained climate forcings and sub-glacial processes.
During the last glacial maximum of east-central Ellesmere Island, trunk glaciers inundated the landscape, entering the Smith Sound Ice Stream. Accelerator mass spectrometry (AMS) dates on individual shell fragments in till indicate that the ice advanced after 19 ka BP. The geomorphic and sedimentary signatures left by the trunk glaciers indicate that the glaciers were polythermal. The configuration and chronology of this ice is relevant to the reconstruction of ice core records from northwestern Greenland, the history of iceberg rafting of clastic sediments to northern Baffin Bay, the reopening of the seaway between the Arctic Ocean and Baffin Bay, and the regional variability of arctic paleoenvironments. Deglaciation began with the separation of Ellesmere Island and Greenland ice at fiord mouths ∼8-8.5 ka BP. Ice reached fiord heads between 6.5 and 4.4 ka BP. Trunk glacier retreat from the fiords of east-central Ellesmere Island occurred up to 3000 years later than in west coast fiords. This later retreat was favoured by (1) impoundment by the Smith Sound Ice Stream in Kane Basin until ∼8.5 ka BP, which moderated the impact of high summer melt recorded in nearby ice cores between ∼11.5 and 8.5 ka BP; (2) the shallow bathymetry and narrowness (<2 km) of the east coast fiords, which lowered calving rates following separation of Innuitian and Greenland ice; and (3) the likelihood of higher precipitation along east Ellesmere Island. Glaciers throughout the field area readvanced during the late Holocene. The greater advance of coastal glaciers is attributed to their proximity to the North Water polynya in Baffin Bay.
A sediment core from Lake BC01 (75°10.945′N, 111°55.181′W, 225 m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae and Salix, typical of a High Arctic plant community, whereas the Arctic herb Oxyria underwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000 cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000 cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000 cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000 cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500 cal yr BP but then decreased substantially by 3500 cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.
For the past half-century, reconstructions of North American ice cover during the Last Glacial Maximum have shown ice-free land distal to the Laurentide Ice Sheet, primarily on Melville and Banks islands in the western Canadian Arctic Archipelago. Both islands reputedly preserve at the surface multiple Laurentide till sheets, together with associated marine and lacustrine deposits, recording as many as three pre-Late Wisconsinan glaciations. The northwest corner of Banks Island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation (Banks Glaciation) considered to be of Matuyama age (>780 ka BP). Inside the limit of Banks Glaciation, younger till sheets are ascribed to the Thomsen Glaciation (pre-Sangamonian) and the Amundsen Glaciation (Early Wisconsinan Stade). The view that the western Canadian Arctic Archipelago remained largely ice-free during the Late Wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on Banks and Melville islands, both dated to ∼22 ka BP. These dates imply that these islands constitute the northeast extremity of Beringia.
This chapter reviews that the Late Wisconsinan North American ice sheet complex consisted of three major ice sheets: (1) the Laurentide Ice Sheet, which was centred on the Canadian Shield, but also expanded across the Interior Plains to the west and south, (2) the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia, and (3) the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 7°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres rather than from the Canadian Shield. All of the peripheral ice sheets were confluent at the Last Glacial Maximum (LGM) with the Laurentide Ice Sheet, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector, named for areas of inception mid probable areas of outflow at LGM and located respectively east, west and north of Hudson Bay. The chapter presents revised maps of North American deglaciation at 500-year and finer resolution. These maps represent an updating of a series prepared nearly two decades ago for the INQUA 1987 Congress.
Changes in late Neoglaical climate resulted in extensive modification of Arctic terrestrial ice cover. A substantial reduction in terrestrial ice cover in the Queen Elizabeth Islands (QEI) following the `Little Ice Age' (LIA) (~AD 1250—1900), is indicated by widespread, light-toned patches of poorly vegetated terrain, extending back to the modern ice mass. These patches display abrupt outer margins (trimlines), which record the former position and maximum extent of perennial snow/ice and, in many cases, mark the former equilibrium-line altitude (ELA). Trimlines surrounding terrain formerly covered by LIA perennial snow/ice were mapped using multispectral classification approaches applied to high-resolution satellite imagery. ELAs were reconstructed from trimlines associated with former perennial snow/ice produced by long-term snowline lowering. Between the end of the LIA and 1960, the area of terrestrial ice in the QEI decreased by 37% (62 387 km2). Most of this reduction (94%) occurred in the eastern QEI where the majority of the ice exists today; however, a 100% reduction in ice cover occurred on many of the western islands by 1960, an effect largely controlled by the subtle topography of these islands. The reconstructed LIA ELA trend surface was used with the 1960 mapped ELAs to calculate spatial variations in the change in height (Δh) of the ELA trend surface throughout the QEI during the first half of the twentieth century. ELA Δh between the LIA and 1960 reveal a high degree of local variability in the mountainous regions, ranging from 0 to >600 m; however a strong regional-scale pattern of change is shown over the QEI as a whole.
Uplands of the Canadian Arctic Islands supported Late Wisconsinan ice caps that developed two landscape zones reflecting basal thermal conditions regulated by long-sustained ice flow patterns. Central cold-based zones protected older glacial and preglacial landscapes while peripheral warm-based zones scoured and otherwise altered their beds.
Some geomorphic effects are independent of ice cap scale, others vary with scale. For ice caps of 30 km radius or more, scour-zone width remains proportionally constant to flowline length under similar flow conditions. But intensity of scouring, ice moulding of drift and rock eminences, size and abundance of subglacial meltwater features, and development of end moraines increase with ice cap size.
Ice caps became entirely cold based early in retreat as the boundary between warm and cold ice shifted outward, probably because ice thinned and flow slackened. The frozen margins deflected meltwater, thus maximizing formation of lateral meltwater channels throughout retreat. The landform record of cold-based glaciers in this region is easily interpreted. Hence, regional ice sheet models invoking or based on the premise that cold-based ice leaves no geomorphic record seem untenable.
Despite the importance of rapidly-flowing ice streams to ice sheet mass balance, their incorporation into numerical ice sheet models is a major scientific challenge. This introduces large uncertainties in model output and inhibits a more complete understanding of the role of ice streams in overall ice sheet stability. Recent computational advances have enabled more realistic representations of ice streaming but few studies have attempted to compare model output against known locations of ice streams. This paper compares predictions of ice streaming derived from a large ensemble analysis of a Glacial Systems Model of the Laurentide Ice Sheet against independent geological evidence compiled from previously published studies. Although the precise dating of paleo-ice stream locations is problematic, our analysis includes comparisons at six different time-steps (18 to 10 cal ka BP) during deglaciation. Results indicate that the model is successful in predicting all of the major marine-terminating ice streams but there is mixed success in simulating terrestrial ice streams in the right place and at the right time, which is vital in guiding future model development. The model also reveals that whilst some ice streams persist throughout deglaciation the focus of mass loss associated with ice streaming switches through time with dynamic changes in ice stream catchments and tributaries. This implies that major changes in ice stream activity are to be expected in a deglaciating ice sheet, with important implications for contemporary ice sheet dynamics.
Eleven paleogeographic maps and a summary ice retreat map outline the history of advance, retreat, and readvances of the Laurentide Ice Sheet along with associated changes in proglacial drainage and relative sea level oscillations for Late Wisconsinan and Holocene times. The text outlines pertinent chronological control and discusses the paleoglaciology of the ice sheet, with attention to location and migration of ice divides, their attendant domes and saddles, and to ice streams, ice shelves, and mechanisms of deglaciation. -from Authors
Dark brown pumice has been discovered recently on raised beaches of Ellesmere and Devon Islands, and in archeological sites on Baffin Island. It is similar in appearance and chemical composition to pumice associated with raised marine features throughout northern Europe, especially along the coasts of Norway and Spitsbergen. The source area for the pumice is uncertain, but Iceland is a good possibility.Radiocarbon dates on driftwood and whale bones imbedded in beaches at the "pumice level", as well as at higher and lower elevations, indicate that the pumice arrived approximately 5000 years ago.The pumice serves as a time-line and provides a means of correlating widely-separated marine features. Because these features now occur at different elevations, the amount and direction of tilt can be calculated. Also, former ice centers can be delineated, as the areas which have undergone the greatest uplift are those where the ice cover was once thickest. In Arctic Canada the "pumice level" rises westward along Jones Sound—from 16.5 m a.s.l. at the mouth of South Cape Fiord, Ellesmere Island, to 24.0 m at the eastern tip of Colin Archer Peninsula, Devon Island, ca. 130 km away. It also rises northwestward toward the head of South Cape Fiord.The Jones Sound information, plus radiocarbon dates from elsewhere in the Queen Elizabeth Islands indicating the approximate position of the shoreline at the same time, shows that there is a region in the eastern and central part of the archipelago where >25 m of uplift has occurred during the last 5000 years. This region, including considerable areas that are now sea, is believed to have been covered by a major ice sheet during the last glaciation.
The Beaufort Formation, in its type area on Prince Patrick Island, is a single lithostratigraphic unit, a few tens of metres thick, consisting of unlithified sandy deposits of braided rivers. Organic beds in the sand have yielded more than 200 species of plants and insects and probably originated during the Pliocene, when the area supported coniferous forest. This Beaufort unit forms the thin eastern edge of a northwest-thickening wedge of sand and gravel beneath the western part of the island. These largely unexposed beds, up to several hundred metres thick, include the Beaufort unit and perhaps other older or younger deposits. On the islands northeast and southwest of Prince Patrick Island (Meighen Island to Banks Island), the name Beaufort Formation has been applied to similar deposits of late Rrtiary age. Most recorded Beaufort beds on these islands are stratigraphically and paleontologically equivalent to the "type" Beaufort, but a few sites that have been called Beaufort (such as Duck Hawk Bluffs and the lower unit at Ballast Brook, on Banks Island) differ stratigraphically and paleontologically from the "type" Beaufort. This paper recommends that these deposits (probably middle Miocene) and others like them be assigned new stratigraphic names and not be included in the Beaufort Formation as now defined. Informal names Mary Sachs gravel (Duck Hawk Bluffs) and Ballast Brook beds are proposed as an initial step. Formal use of the name Beaufort Formation should be restricted to the western Arctic Islands.
Glacial erratics collected on Melville Island, western Canadian Arctic Archipelago, were analyzed to determine their mainland provenance, thereby constraining their long-distance transport by the Laurentide Ice Sheet. These erratics can be broadly subdivided into three main lithologies: granite (n = 15), quartzite (n = 7), and diabase-diorite (n = 3). The granite erratics are most distinctive from a provenance perspective and can be further subdivided into three geochemical groups based on their potassium content: (1) a high-K2O group (K2O> 4.0 wt.%), (2) an intermediate-K2O group (K2O between 2.0 and 4.0 wt.%), and (3) a low-K2O group (K2O< 2.0 wt.%). In situ thin section laser ablation inductively coupled plasma mass spectrometer U-Pb zircon ages obtained for eight granite erratics yielded both Archean (2575 Ma) and a range of Paleoproterozoic (2472-1778 Ma) crystallization ages. In addition, three overprint ages were identified at 1.90, 1.84, and ~1.0 Ga. The most compelling constraint for a northward regional ice flow originating on the mainland are two high-precision conventional U-Pb zircon ages of 1969.5± 1.0 and 2472.3± 0.5 Ma, indicating that these granite erratics must have been derived from the 2.0-1.9 Ga Taltson-Thelon Orogen and the nearby 2.5-2.4 Ga Queen Maud Block, respectively. These granite-dominated terranes are located 600km due south and southeast of the collection area on Melville Island. Although it is unknown whether the final deposition of these erratics on Melville Island involved transport by one or more glaciations, it is apparent that this ice flow cannot be accommodated by the proposed north-south axis of the M'Clintock Ice Divide, the primary topographic feature of the northwestern Laurentide Ice Sheet during the last glacial maximum. The transport of erratics reported here would have required a former ice divide oriented east-west over the mainland, close to that proposed for the Ancestral Keewatin Divide. An east-west ice divide in this region is consistent with previously reported ice-flow indicators that document northward flow from the mainland and recent thermomechanically coupled ice-sheet numerical modeling that indicates former maximum ice thickness on the mainland immediately south of Melville Island.Des blocs erratiques recueillis sur l'île Melville, dans l'archipel arctique canadien, ont été analysés afin de déterminer leur provenance des régions continentales, encadrant ainsi leur transport sur de longues distances par l'Inlandsis laurentidien. Ces blocs erratiques peuvent être subdivisés selon trois principales lithologies : granite (n = 15), quartzite (n= 7) et diabase-diorite (n = 3). Les erratiques de granite sont les plus distinctifs du point de vue de la provenance et ils peuvent être subdivisés de nouveau en trois groupes géochimiques basés sur leur contenu en potassium : (1) un groupe à teneur élevée en K2O (K2O> 4,0 % poids), (2) un groupe à teneur intermédiaire en K2O (K2O entre 4,0 et 2,0 % poids) et (3) un groupe à faible teneur en K2O (K2O< 2,0 % poids). Des âges U-Pb in situ déterminés sur des zircons en lame mince par analyse à couplage inductif et spectrométrie de masse avec ablation au laser (LA-ICP-MS) de huit blocs erratiques de granite ont donné des âges de cristallisation archéens (2575 Ma) et paléoprotérozoïques (2472-1778 Ma). De plus, trois âges de surimpression ont été identifiés à 1,90, 1,84, et ~1,0 Ga. La contrainte la plus convaincante pour un écoulement régional de la glace vers le nord à partir du continent a été la détermination de deux âges conventionnels U-Pb de grande précision sur des zircons, soit 1969,5± 1,0 et 2472,3± 0,5 Ma, indiquant que ces blocs erratiques de granite doivent provenir respectivement de l'orogène Taltson-Thelon (2,0-1,9 Ga) et du bloc Queen Maud (2,5-2,4 Ga) avoisinant. Ces terrains dominés par des granites sont situés à 600km franc sud et sud-est du secteur de collecte sur l'île Melville.
Modern terrestrial glaciers in the Canadian High Arctic range from polythermal to cold-based. Where polythermal glaciers override thick unconsolidated sediment, longitudinal compression and glaciotectonic thrusting produce thrust-block moraines. In contrast, the dominant geomorphic record of cold-based glaciers consists of lateral and proglacial meltwater channels. Geomorphic and sedimentary evidence indicates that late Quaternary fiord glaciers were also characterized by variations in basal thermal regime. Erratic dispersal trains and striated bedrock record the flow of warm-based ice during the Last Glacial Maximum. Emergent grounding-line fans and morainal banks, deposited during deglaciation, consist of heterogeneous glaciomarine deposits that record well-developed subglacial drainage and high sedimentation rates. However, in other fiords, subaqueous outwash and fine-grained glaciomarine deposits are absent and deglaciation is recorded by lateral meltwater channels graded to raised glaciomarine deltas, suggesting these glaciers were predominantly cold-based during retreat. Regionally, deglacial depocentres are located at pinning points within fiords and a prominent belt of glaciogenic landforms at fiord heads records stabilization of ice margins during early Holocene retreat, rather than the limit of late Quaternary glaciation. Collectively, these observations refute previous reconstructions which inferred a climatically controlled switch from cold- to warm-based thermal conditions in fiord glaciers during early Holocene deglaciation, and indicate that the dominant controls on thermal regime were glaciological.
Ice streams are critical regulatory mechanisms in contemporary ice sheets. It has been inferred that they also had a significant effect on the dynamics of former ice sheets. Subsequently, many people have invoked their widespread occurrence from a variety of formerly glaciated areas. Hypothesised locations, however, have often out-weighed meaningful evidence. This paper addresses the problem, using the characteristics of contemporary ice streams as a basis for their identification from former ice-sheet beds. A convergence of knowledge gained from contemporary ice-stream research, coupled with theories of glacial geomorphology, allows several geomorphological criteria to be identified as suggestive signatures of ice-stream activity. It is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams. The criteria are used to construct conceptual land-system models of the beds of former ice streams, and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based.
Mass-balance measurements began in the Canadian High Arctic in 1959. This paper considers the >40 years of measurements made since then, principally on two stagnant ice caps (on Meighen and Melville Islands), parts of two ice caps (the northeast section of Agassiz Ice Cap on northern Ellesmere Island and the northwest part of Devon Ice Cap on Devon Island) and two glaciers (White and Baby Glaciers, Axel Heiberg Island). The results show continuing negative balances. All the glaciers and ice caps except Meighen Ice Cap show weak but significant trends with time towards increasingly negative balances. Meighen Ice Cap may owe its lack of a trend to a cooling feedback from the increasingly open Arctic Ocean nearby (Johannessen and others, 1995). Feedback from this ocean has been shown to be the main cause of this ice cap's growth and persistence at such a low elevation of 40 years of ground-based measurements.
A complex of glacial landforms on northeastern Victoria Island records diverse flows within the waning late Wisconsinan Laurentide Ice Sheet over an area now divided by marine straits. Resolution of this ice flow pattern shows that dominant streamlined landforms were built by three radically different ice flows between 11,000 and 9000 BP. Subsequent to the glacial maximum, the marine-based ice front retreated at least 300 km to reach northeast Victoria Island by 10,400 BP. Disequilibration at the rapidly retreating margin induced minor surges on western Storkerson Peninsula (Flow 1). Next, a readvance into Hadley Bay transported 10,300 BP shells, while a major ice stream over eastern Storkerson Peninsula (Flow 2) remoulded till into a drumlin field several hundred kilometres long and at least 80 km wide until flow ceased prior to 9600 BP. The ice stream surged into Parry Channel, covering 20,000 km2 with the Viscount Melville Sound Ice Shelf. Finally, Flow 2 drumlins on the northwest shore of M'Clintock Channel were cross-cut c. 9300 BP by advance of the grounded margin of a buoyant glacier (Flow 3), possibly an analogue of Flow 2 displaced farther south.
Analyses of the global process of glacial isostatic adjustment and post-glacial relative sea-level change continue to deliver important insights into Earth system form and process. One successful model of the related phenomenology is based upon a spherically symmetric internal viscoelastic structure for the solid Earth, which has been denoted VM2, and a model of the most recent deglaciation event of the current ice-age, denoted ICE-4G. The primary purpose of this paper is to describe several new a posteriori tests that have recently been performed to further investigate the quality of this global ‘solution’ to the inverse problem for both mantle viscosity and deglaciation history that is posed by the observables associated with this large-scale geodynamic phenomenon. I focus especially upon the ‘misfits’ of observations to the theoretical predictions of this model, which I am currently using to further refine its properties, and upon predictions made using it of geophysical signals that should soon become visible in the context of the Gravity Recovery and Climate Experiment (GRACE) satellite mission. Among the required refinements to ICE-4G, one that is necessary to eliminate a recently revealed misfit to space geodetic constraints on the present-day rate of radial motion at the Yellowknife location well to the west of Hudson Bay, and a similar misfit to absolute gravity measurements to the southwest of the Bay, is the insertion of a ‘Keewatin Dome’ of thick ice centred over Yellowknife with a ridge of ice extending to the south east. In the geomorphological literature, the existence of such a Keewatin Dome previously has been hypothesised but chronological control was lacking on the surface features that suggested its former existence. An important additional constraint that requires the late glacial existence of this important feature consists of new inferences of the Last Glacial Maximum lowstand of the sea from sites in the far field of the main concentrations of land ice. Copyright © 2002 John Wiley & Sons, Ltd.
Two mammoth fossils (presumably woolly mammoth, Mammuthus primigenius) from northwestern Banks and southwestern Melville Islands, Northwest Territories, Canada, have been radiocarbon-dated to the Last Glacial Maximum (LGM), at 21 000 and 22 000 14CYBP, respectively. These fossils not only are the northernmost mammoth records for North America, but also indicate that the Mammoth Steppe and Beringia extended eastward at least to Ballast Brook, Banks Island (74.3° N, 123.1° W), and possibly to the Cape James Ross area of Melville Island (75.7° N, 114.4° W). The specimens, a tibia and a tusk, probably represent woolly mammoths that moved northeastward from the Mackenzie Delta region during the LGM, when worldwide sea level had dropped about 120 m, leaving large tracts of sea bottom exposed off the Beaufort Sea coast and the west coast of Banks Island (then largely clear of glacial ice). Evidently herb tundra rich enough to supply the mammoths' needs characterized the regional landscape at that time. It is proposed that the term "Beringia" be used in the broad sense where evidence exists for a land connection between Asia and North America, regardless of its cause(s) and its supposed westerly or easterly limits, and that "Beringia" be used in a standard way: followed by its geological age in parentheses. Also, the term "Bering Isthmus" seems preferable to the commonly used "Bering Land Bridge.".
The "Little Ice Age" was the most recent period during which glaciers extended globally, their fronts oscillating about advanced positions. It is frequently taken as having started in the sixteenth or seventeenth century and ending somewhere between 1850 and 1890, but Porter (1981) pointed out that the "Little Ice Age" may 'have begun at least three centuries earlier in the North Atlantic region than is generally inferred'. The glacial fluctuations of the last millennium have been traced in the greatest detail in the Swiss Alps, where the "Little Ice Age" is now seen as starting with advances in the thirteenth century, and reaching an initial culmination in the fourteenth century. In the discussion here, evidence from Canada, Greenland, Iceland, Spitsbergen and Scandinavia is compared with that from Switzerland. Such comparisons have been facilitated by improved methods of calibrating radiocarbon dates to calendar dates and by increasing availability of evidence revealed during the current retreat phase. It is concluded that the "Little Ice Age" was initiated before the early fourteenth century in regions surrounding the North Atlantic.
After ~ 11,000 years of glacio-isostatically induced forced regression, geomorphological evidence indicates that the coastline of eastern Melville Island, western Canadian Arctic Archipelago, is now being transgressed. Recently developed coastal features associated with this transgression include: drowned gullies and small estuaries, barriers and lagoons, barrier islands, erosional notches, backstepping beaches, and drowned tundra vegetation and vehicle tracks dating from the 1970s. We mainly attribute this relative sea-level rise to the eastward migration of a peripheral crustal forebulge. Furthermore, the reported transgression also includes a component from recent eustatic sea-level rise during the 20th century. Recent earthquakes recorded in the Gustav-Lougheed Arch Seismic Zone located in Byam Martin Channel, 70 km east of Melville Island, suggest that neotectonics could also be involved in local relative sea-level adjustments. Other factors associated with global warming, especially the formation of an earlier shore-ice lead coupled with increased storm activity might also be responsible for some of the coastal changes. Our study indicates that the current zero isobase, separating areas of net transgression from those of net regression, is now located off the east coast of the island. Our field observations support recent glacio-isostatic modelling that shows the island is presently undergoing a transgression.
Portrayal of North American ice cover during the Last Glacial Maximum is dominated by the Laurentide Ice Sheet, leaving little detail for the adjacent Innuitian Ice Sheet (IIS). Four decades of geological fieldwork across the Queen Elizabeth Islands now warrant specific treatment of the IIS, including its chronology, configuration, dynamics and retreat. This reconstruction is relevant to the sedimentary history of the Arctic Ocean and to high latitude climate forcing. The IIS was composed of both an alpine and lowland sector. The advance of the alpine sector occurred as recently as 19 14C ka BP. Geological evidence configures outflow from alpine and lowland divides that produced several palaeo-ice streams, one extending northwestward across the Canadian Arctic Archipelago to the polar continental shelf. Retreat of the IIS commenced along its southwest margin ∼11.6 14C ka BP. However, most of the ice sheet remained on the continental shelf during the Younger Dryas. By ∼10 14C ka BP, marine-based ice experienced widespread calving through the western and central archipelago in response to Holocene warming and ongoing eustatic sea level rise. The sea penetrated the eastern archipelago by 8.5 14C ka BP, gutting the alpine sector of the IIS. Regional isobases record the glacioisostatic signature of the ice sheet, and are congruent with the primary geological evidence. The delayed buildup of the IIS was out-of-phase with the growth of the Laurentide Ice Sheet that occasioned climatic and glacio-eustatic forcing in the Innuitian region. Recent modelling experiments reinforce the hypothesis that growth of the Laurentide Ice Sheet culminated in a split jet stream that temporarily favoured augmented precipitation and growth of the IIS.
A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics.
The Late Wisconsinan advance of the Laurentide Ice Sheet started from a Middle Wisconsinan interstadial minimum 27–30 14C ka BP when the ice margin approximately followed the boundary of the Canadian Shield. Ice extent in the Cordillera and in the High Arctic at that time was probably similar to present. Ice advanced to its Late Wisconsinan (stage 2) limit in the northwest, south, and northeast about 23–24 14C ka BP and in the southwest and far north about 20–21 14C ka BP. In comparison to some previous reconstructions of ice extent, our current reconstruction has substantially more Late Wisconsinan ice in the High Arctic, where an Innuitian Ice Sheet is generally acknowledged to have existed, in the Atlantic Provinces, where ice is now thought to have extended to the Continental Shelf edge in most places, and on eastern Baffin Island, where ice probably extended to the fiord mouths rather than to the fiord heads. Around most of the ice margin, the Late Wisconsinan maximum ice extent either exceeded the extent of earlier Wisconsinan advances or it was similar to the Early Wisconsinan advance. Ice marginal recession prior to 14 14C ka BP occurred mainly in deep water and along the southern terrestrial fringe. However, Heinrich event 1 probably drew down the entire central ice surface at 14.5 14C ka BP sufficiently to displace the Labrador Sector outflow centre 900 km eastward from the coast of Hudson Bay. The onset of substantial ice marginal recession occurred about 14 14C ka BP in the northwest, southwest, and south but not until about 10–11 14C ka BP in the northeast and in the High Arctic. Thus, the period of maximum ice extent in North America generally encompasses the interval from ∼24/21 to 14 14C ka BP, or considerably longer than the duration of the LGM defined as occurring during a period of low global sea level as well as during a time of relative climate stability ∼18 14C ka BP. The interval of advance of much of the Laurentide Ice Sheet to its maximum extent (between ∼27 14C ka BP and ∼24 14C ka BP) coincides with a suggested interval of rapid fall in global sea level to near LGM levels.
Late Holocene sea-level highstands of amplitude are endemic to equatorial ocean basins. These highstands imply an ongoing and moderate, sub-mm/yr, sea-level fall in the far field of the Late Pleistocene ice cover that has long been linked to the process of glacial isostatic adjustment (GIA; Clark et al., 1978). Mitrovica and Peltier (1991) coined the term ‘equatorial ocean syphoning’ to describe the GIA-induced sea-level fall and they provided the first physical explanation for the process. They argued that water migrated away from far-field equatorial ocean basins in order to fill space vacated by collapsing forebulges at the periphery of previously glaciated regions. We provide a complete physical explanation for the origin of equatorial ocean syphoning, and the associated development of sea-level highstands, using numerical solutions of the equation that governs meltwater redistribution on spherical, viscoelastic Earth models. In particular, we separate the total predicted sea-level change into contributions associated with ice and meltwater loading effects, and, by doing so, isolate a second mechanism that contributes significantly to the ocean syphoning process. Ocean loading at continental margins induces a ‘levering’ of continents and a subsidence of offshore regions that has also long been recognized within the GIA literature (Walcott, 1972). We show that the influx of water into the volume created by this subsidence produces a sea-level fall at locations distant from these margins—indeed over the major ocean basins—that is comparable in amplitude to the syphoning mechanism isolated by Mitrovica and Peltier (1991).
Past reconstructions of the deglaciation history of the North American (NA) ice-sheet complex have relied either on largely unconstrained and limited explorations of the phase space of solutions produced by glaciological models or upon geophysical inversions of relative sea-level (RSL) data which suffer from incomplete geographic coverage of the glaciated regions, load history amplitude/timing ambiguities, and a lack of a priori glaciological self-consistency. As a first step in the development of a much more highly constrained deglaciation history, we present a synthesis of these two previously disjoint methodologies based on a large ensemble of glacial cycle simulations using a three-dimensional thermo-mechanically coupled ice-sheet model. Twenty glacial system model parameters, chosen so as to best cover the true deglacial phase space, were varied across the ensemble. Furthermore, a new high-resolution digitized ice margin chronology was imposed on the model in order to significantly limit the uncertainties associated with deglacial climate forcing. The model is simultaneously constrained by a large set of high-quality RSL histories, a space geodetic observation of the present-day rate of vertical motion of the crust from Yellowknife and a traverse of absolute gravity measurements from the west coast of Hudson Bay southward into Iowa.