Ice Margin Retreat Research Articles

Timing of the deglaciation in southern Patagonia: Testing the applicability of K-Feldspar IRSL

The timing of the ice margin retreat of the Late Glacial Patagonian Ice Sheet (PIS) in southern Patagonia has been the object of discussion for many years. In order to resolve questions about the complex response of the PIS to past climate change, any geological interpretation and data modelling need evaluation against an absolute chronology. The aim of this project is to investigate the applicability of OSL dating to sediments from southern Patagonia; in particular, we examine the dating potential of K-feldspar IRSL signals. Samples were collected from landforms interpreted as being deposited during deglaciation of the PIS, with an expected age range of 17 and 22 ka, and from recently deposited sediment. We measure small aliquots and single grain distributions using an IR50 SAR protocol with IRSL stimulation at 50 °C following a preheat at 250 °C (held for 60 s). Uncertainties are assigned to our individual dose estimates based on the over-dispersion (OD) observed in laboratory gamma dose recovery experiments (22% for small aliquots and 18% for single grains). Then the possible effects of incomplete bleaching and differential fading are examined. For our natural samples we observe environmental ODs between 30 and 130% and mean residual doses between ∼30 and 80 Gy. Minimum age models are used to identify the part of the dose population that is most likely to have been well-bleached and results from these models are compared. The models give ages that are consistent with each other; this may imply that they successfully identified the fully-bleached grains in the distributions, although there are some discrepancies between our small aliquot and single grain data. We observe large fading rates (on average 7.9 ± 0.6%/decade for large aliquots) but nevertheless a comparison of our fading corrected ages with the expected age range shows that 2 out of 3 ages are consistent with geological interpretation and an established radiocarbon and cosmogenic nuclide chronology. We conclude that these investigations suggest that fading corrections can be based on laboratory average small aliquot/single grain fading rates. The third age is supported by an alternative geological interpretation, and the two ages consistent with the existing chronology imply that in the Strait of the Magellan the hills of the Brunswick peninsula (>70 m.a.s.l) were deglaciated at around 22 ka.

Tunnel valleys: Concepts and controversies — A review

Although erosion of valleys by subglacial meltwater was first proposed more than 100years ago, the processes that produced these valleys under past ice sheets have remained a topic of vigorous debate. The most commonly hypothesized mechanisms have included gradual formation by sediment deformation into the incipient valleys under steady state conditions and subsequent transport by meltwater to the ice margin; time-transgressive formation along a retreating ice margin by drainage of surface meltwater to the bed or release of impounded meltwater behind a marginal permafrost wedge, probably catastrophically; and rapid erosion during widespread, catastrophic basal sheetflood events.Recent application of new and enhanced imaging techniques has led to the identification of many new tunnel valleys both on land and offshore throughout the regions glaciated during former ice ages. In addition, new advances in understanding of subglacial hydrology, both from modelling and field instrumentation have helped to constrain the drainage of meltwater from past ice sheet beds. This work elucidates the role of subglacial groundwater flow systems in evacuating meltwater from the bed, but also shows that groundwater flow alone was not enough to maintain ice bed meltwater pore pressures below the level of ice flotation. Both modelling and field observations indicate that subglacial conduits oriented parallel to glacial flow lines reorganize groundwater flow systems into basins containing lateral flow toward conduits cut upward into the ice or downward into the bed. Because of the inability of groundwater flow to account for all the meltwater produced by basal melting, other drainage mechanisms involving both non-channelized and channelized meltwater flow have been proposed. The notion of catastrophic drainage of impounded basal meltwater is still favored by many, and the discovery of hundreds of subglacial lakes beneath the Antarctic Ice Sheet and the likelihood of similar lakes beneath Pleistocene ice sheets provides a source of water for those advocating catastrophic formation for tunnel valleys. Proponents of catastrophic drainage consider the valleys to be channels reflecting the width and perhaps depths of flows that formed them.Tunnel valleys produced by the Scandinavian Ice Sheet have been described and mapped on land as well as on the sea bed. Valleys and networks of valleys, some of which are totally buried, have been linked to 3 or more glaciations. Tunnel valley networks from the last (Weichselian) glaciation in Denmark can be related to specific ice margins developed during ice retreat. In North America, most research has focused on tunnel valleys associated with ice margins formed by the last (Wisconsin) glaciation along the southern margin of the Laurentide Ice Sheet.

Investigating the last deglaciation of the Scandinavian Ice Sheet in southwest Sweden with 10Be exposure dating

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

A review and analysis of varve thickness records from glacial Lake Ojibway (Ontario and Quebec, Canada)

Varve thickness records from glacial Lake Ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the Laurentide Ice Sheet during the early Holocene. The demise of Lake Ojibway occurred when an ice dam of the shrinking Laurentide Ice Sheet failed, perhaps releasing enough freshwater to impede Atlantic meridional overturning circulation and precipitate the 8200 cal BP climatic cooling event. Previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long. Between varves numbered 1 to 1500, average rates of ice margin retreat increased from 140 m/y to as high as 400 m/y. Beginning prior to varve year 1800, and lasting to around year 2000, ice readvanced along a front at least 300 km wide during an episode called the Cochrane. The total distance of the advance was at least 75 km, and averaged around 200 m/y. During the advance, a series of ∼50 anomalously thick varves were deposited, the cause of which is uncertain. A widespread unconformity occurred during the end of the Cochrane advance, which was followed at around varve year 2060 by the return of varves, which thicken abruptly. Unlike previous varves, these varves include ice-rafted pellets of Lake Ojibway sediment. These varves may have resulted from rapid melting and calving, rather than a second advance, and may relate to basin re-filling following a lowstand. There are no direct dates on the Lake Ojibway varves, but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater (including Lake Agassiz overflow), diverted from routes to the Lake Superior basin, into the Lake Ojibway basin. Dates from the Lake Superior basin suggest that the diversion of this water occurred at around 9040 cal BP. Applying this date to varve 1528 constrains the age of the Lake Ojibway varves numbered 1 to 2100 to between 10,570 and 8470 (±200) cal BP.

Paired bedrock and boulder 10Be concentrations resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland

We measured in situ cosmogenic 10Be in 16 bedrock and 14 boulder samples collected along a 40-km transect outside of and normal to the modern ice margin near Sikuijuitsoq Fjord in central-west Greenland (69°N). We use these data to understand better the efficiency of glacial erosion and to infer the timing, pattern, and rate of ice loss after the last glaciation. In general, the ages of paired bedrock and boulder samples are in close agreement ( r 2 = 0.72). Eleven of the fourteen paired bedrock and boulder samples are indistinguishable at 1 σ; this concordance indicates that subglacial erosion rates are sufficient to remove most or all 10Be accumulated during previous periods of exposure, and that few, if any, nuclides are inherited from pre-Holocene interglaciations. The new data agree well with previously-published landscape chronologies from this area, and suggest that two chronologically-distinct land surfaces exist: one outside the Fjord Stade moraine complex (∼10.3 ± 0.4 ka; n = 7) and another inside (∼8.0 ± 0.7 ka; n = 21). Six 10Be ages from directly outside the historic (Little Ice Age) moraine show that the ice margin first reached its present-day position ∼7.6 ± 0.4 ka. Early Holocene ice margin retreat rates after the deposition of the Fjord Stade moraine complex were ∼100–110 m yr −1. Sikuijuitsoq Fjord is a tributary to the much larger Jakobshavn Isfjord and the deglaciation chronologies of these two fjords are similar. This synchronicity suggests that the ice stream in Jakobshavn Isfjord set the timing and pace of early Holocene deglaciation of the surrounding ice margin.

A new coupled ice sheet/climate model: description and sensitivity to model physics under Eemian, Last Glacial Maximum, late Holocene and modern climate conditions

Abstract. The need to better understand long-term climate/ice sheet feedback loops is motivating efforts to couple ice sheet models into Earth System models which are capable of long-timescale simulations. In this paper we describe a coupled model that consists of the University of Victoria Earth System Climate Model (UVic ESCM) and the Pennsylvania State University Ice model (PSUI). The climate model generates a surface mass balance (SMB) field via a sub-gridded surface energy/moisture balance model that resolves narrow ice sheet ablation zones. The ice model returns revised elevation, surface albedo and ice area fields, plus coastal fluxes of heat and moisture. An arbitrary number of ice sheets can be simulated, each on their own high-resolution grid and each capable of synchronous or asynchronous coupling with the overlying climate model. The model is designed to conserve global heat and moisture. In the process of improving model performance we developed a procedure to account for modelled surface air temperature (SAT) biases within the energy/moisture balance surface model and improved the UVic ESCM snow surface scheme through addition of variable albedos and refreezing over the ice sheet. A number of simulations for late Holocene, Last Glacial Maximum (LGM), and Eemian climate boundary conditions were carried out to explore the sensitivity of the coupled model and identify model configurations that best represented these climate states. The modelled SAT bias was found to play a significant role in long-term ice sheet evolution, as was the effect of refreezing meltwater and surface albedo. The bias-corrected model was able to reasonably capture important aspects of the Antarctic and Greenland ice sheets, including modern SMB and ice distribution. The simulated northern Greenland ice sheet was found to be prone to ice margin retreat at radiative forcings corresponding closely to those of the Eemian or the present-day.

Geochronology and paleoclimatic implications of the last deglaciation of the Mauna Kea Ice Cap, Hawaii

We present new 3He surface exposure ages on moraines and bedrock near the summit of Mauna Kea, Hawaii, which refine the age of the Mauna Kea Ice Cap during the Local Last Glacial Maximum (LLGM) and identify a subsequent fluctuation of the ice margin. The 3He ages, when combined with those reported previously, indicate that the local ice-cap margin began to retreat from its LLGM extent at 20.5 ± 2.5 ka, in agreement with the age of deglaciation determined from LLGM moraines elsewhere in the tropics. The ice-cap margin receded to a position at least 3 km upslope for ∼ 4.5–5.0 kyr before readvancing nearly to its LLGM extent. The timing of this readvance at ∼ 15.4 ka corresponds to a large reduction of the Atlantic meridional overturning circulation (AMOC) following Heinrich Event 1. Subsequent ice-margin retreat began at 14.6 ± 1.9 ka, corresponding to a rapid resumption of the AMOC and onset of the Bølling warm interval, with the ice cap melting rapidly to complete deglaciation. Additional 3He ages obtained from a flood deposit date the catastrophic outburst of a moraine-dammed lake roughly coeval with the Younger Dryas cold interval, suggesting a more active hydrological cycle on Mauna Kea at this time. A coupled mass balance and ice dynamics model is used to constrain the climate required to generate ice caps of LLGM and readvance sizes. The depression of the LLGM equilibrium line altitude requires atmospheric cooling of 4.5 ± 1 °C, whereas the mass balance modeling indicates an accompanying increase in precipitation of as much as three times that of present. We hypothesize (1) that the LLGM temperature depression was associated with global cooling, (2) that the temperature depression that contributed to the readvance occurred in response to an atmospheric teleconnection to the North Atlantic, and (3) that the precipitation enhancement associated with both events occurred in response to a southward shift in the position of the inter-tropical convergence zone (ITCZ). Such a shift in the ITCZ would have allowed midlatitude cyclones to reach Mauna Kea more frequently which would have increased precipitation at high elevations and caused additional cooling.

Ice‐free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records

Helmens, K. F. & Engels, S. 2010: Ice‐free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records. Boreas, 10.1111/j.1502‐3885.2010.00142.x. ISSN 0300‐9483.The traditional notion that Fennoscandia was glaciated throughout Marine Isotope Stages (MIS) 4–2, from c. 70 kyr BP to the deglaciation 15–10 kyr BP ago, has been challenged during the last decade. Recent studies have shown that climate and environmental settings during MIS 3 were more dynamic than previously assumed, and lacustrine sediment bodies indicate open‐water conditions for several sites in eastern Fennoscandia. In this study, three sediment sequences from western, eastern and northeast Finland are compared in detail with respect to their chronology, vegetation reconstruction and climatic inferences. OSL‐dating places the sediments in early MIS 3. Pollen evidence suggests the presence of isolated birch trees and open birch forest close to the retreating ice margin, in contrast to vegetation reconstructions from central Europe, which indicate tree‐less vegetation. Furthermore, reconstructions of climate using transfer functions have yielded surprising results, indicating present‐day summer temperatures in northeast Finland. The combined results suggest ice‐free and warm conditions in major parts of eastern Fennoscandia in early MIS 3, possibly during Greenland Interstadial (GIS) 14 around 53 kyr BP ago.

Deglaciation of a marine-based ice sheet: Late Weichselian palaeo-ice dynamics and retreat in the southern Barents Sea reconstructed from onshore and offshore glacial geomorphology

Marine and terrestrial datasets (swath bathymetry, 2D and 3D seismic and remote sensing data) are used to map the megascale glacial geomorphology of the southern Barents Sea seafloor, northern Fennoscandia and the Kola Peninsula, giving new insights into the dynamics and behaviour of the Barents Sea Ice Sheet during retreat. The glacial geomorphology indicates clear spatial and temporal variations in ice dynamics, with evidence for both active ice streaming and frozen-bed conditions at the maximum and during deglaciation. A five-stage reconstruction of the Late Weichselian deglaciation is proposed. At the Late Weichselian maximum, ice covered the whole of the Barents Sea shelf and the configuration and dynamics of the ice sheet were controlled by the Bjørnøyrenna (Bear Island Trough) Ice Stream. This was fed by source areas in the central and north-eastern Barents Sea and in the Fennoscandian mainland. Deglaciation was initiated along the western continental margin. Available dating results show that this occurred after 19 cal ka BP. The Bjørnøyrenna Ice Stream retreated rapidly (ice margin retreat rates of ∼275 m a −1), but was periodically grounded, as evidenced by a well-developed series of grounding zone wedges. This was followed by an advance of two short-lived ice streams from the eastern sector of the ice sheet into the south-western Barents Sea. These ice streams advanced and retreated very rapidly with no evidence for ice margin standstills. Correlation to dated cores suggests an age of around 16 000 cal ka BP for this readvance. The period of operation of these two ice streams was coincident with maximum ice extent along the eastern margin in Russia. The western Barents Sea ice margin then retreated onshore, with deglaciation continuing at a reduced rate. The initiation of deglaciation coincided with rising global eustatic sea level. Acceleration of calving, due to grounding line floatation rapidly opened up a major calving bay in Bjørnøyrenna, encouraging rapid loss of the marine-based ice. The retreat rate stabilised only once the ice sheet retreated onshore, and calving ceased. We note that the observed pattern of rapid retreat, with high variability in ice streaming for the marine-based Barents Sea Ice Sheet, is similar to that of the marine-based sectors of the Late Weichselian British, Laurentide and West Antarctic Ice Sheets.

Depositional processes and the inferred history of ice-margin retreat associated with the deglaciation of the Cordilleran Ice Sheet: The sedimentary record from Flathead Lake, northwest Montana, USA

Abstract A series of piston cores from Flathead Lake, Montana, USA and a new radiocarbon date from the sedimentary record provide the basis for describing sedimentary processes related to deglaciation of the Flathead Lobe of the Cordilleran Ice Sheet and for interpreting the retreat history of the lobe. The oldest part of Flathead Lake sediment core records is Late Pleistocene in age and consists of cm-scale rhythmites of silt and clay, interpreted here as annual varves. Each varve contains a light-colored coarser-grained portion, inferred to represent deposition during peak annual runoff, and a darker-colored finer-grained portion interpreted to represent sediment accumulation during seasonal low-flow conditions. The coarser-grained portions, especially in the stratigraphically older sections of each core, contain sedimentary structures that reflect traction transport. Based on these sedimentary structures, their facies characteristics, and their spatial distribution within the lake, we interpret the thicker, light-colored portion of each varve to be deposited by hyperpycnal flows caused by seasonal melt events rather than more classic turbidity currents. Immediately overlying Late Pleistocene rhythmites in all Flathead Lake cores is a unique, significantly coarser-grained dm-scale silt bed with a median grain size up to 50 µm. This silt bed has a sharp, locally erosional base and fines upward but does not contain any other sedimentary structures. In contrast to the rhythmites, we interpret this silt bed to represent a single, short-lived catastrophic sedimentation event generated by a large glacial outburst flood. Overlying this distinct bed are several other cm-scale beds of mainly silt that exhibit a basal upward-coarsening followed by an upward-fining median grain size profile. We interpret these beds and their grain size trends as reflecting the rising and falling hydrograph limbs of outburst floods derived from more distal sources located in the upstream parts of the upper Flathead watershed. The sediment record from Flathead Lake, together with results from geologic and geomorphologic 1:24,000 scale mapping around the lake margins, provide a series of constraints regarding the paleogeographic evolution of the area during deglaciation. Overall upward-thinning and upward-fining of the varved portion of the sediment core records reflects northward retreat of the southern Flathead Lobe ice margin starting at latest 14,475 ± 150 cal yr BP, the depositional age of the oldest varve in any of our cores. The depositional age of silt beds overlying the varved records is constrained as between 14,150 ± 150 cal yr BP and 13,180 ± 120 cal yr BP. Within the available chronostratigraphic constraints, the outburst floods interpreted to have delivered this silt to the Flathead Lake basin also downcut a bedrock nick point below the Flathead Lake outlet, oriented a series of large boulders downstream, and deposited a series of large flood bars on the lower Flathead River floodplain.

Calibrating a glaciological model of the Greenland ice sheet from the Last Glacial Maximum to present-day using field observations of relative sea level and ice extent

We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.

Early MIS 3 glacial lake evolution, ice-marginal retreat pattern and climate at Sokli (northeastern Fennoscandia)

Lake development at Sokli, northern Finland, is traced through the analysis of diatoms and other siliceous micro-fossils in a 2-meter thick minerogenic, laminated clay-silt deposit dated to the early part of Marine Isotope Stage (MIS) 3. Fluctuating water levels and changes in lake extent depicted by the siliceous micro-fossil record, together with lithology, suggest that an important part of the sediment sequence was deposited in a glacial lake. The proxy-based glacial lake evolution is tested using a Digital Elevation Model and geomorphologic evidence including eskers dated to the early MIS 3 Tulppio Interstadial at Sokli. Despite the apparent ice-dammed nature of the lake, the sediment is relatively rich in fossils and there are limited signs of re-deposition of older fossil material. The siliceous micro-fossil record together with data from other proxies previously analysed in the same sediment samples provides a coherent picture of past environmental changes around the Sokli site. This is most probably due to the sheltered position of the coring-site in a lake embayment. Quantitative climate reconstructions based on the diatom record show mean July air temperatures as high as present-day values at Sokli, and the temperature ranges indicated by the diatom record are in agreement with temperature reconstructions based on chironomids. The position of Sokli in the northeastern portion of the central area of the Scandinavian glaciations and the northern retreat pattern implies that an important part of eastern Fennoscandia was deglaciated during the early MIS 3 warming event.

Geomorphological evidence towards a de‐glacial control on volcanism

AbstractA number of theoretical, conceptual and numerical models exist for de‐glacial controls on volcanism, but geological evidence is scarce. We describe and explain a regional topographic and geomorphic expression of sub‐glacial volcanism, namely that at Kverkfjöll, Iceland. This area comprises a series of parallel sub‐glacially‐erupted volcanic edifices, which together give excellent 3D geological exposure. These ridges are orientated north‐south or along the most probable line of LGM ice margin retreat. We combine topographic and geomorphic observations to explain the changing style of volcanism in space, and attribute this to the recession and downwasting of the LGM ice sheet. Specifically, we observe that there is no spatial pattern to pillow lava edifice heights or volumes, indicating that fissure eruptions beneath the LGM ice sheet were of similar dynamics in space. However, hyaloclastite and hyalotuff deposits are restricted to proximal and high elevation positions. Furthermore, lithofacies are split by erosional contacts and hyalotuffs are faulted and intruded. These observations together suggest that as the overlying ice sheet became thinner volcanic activity became more explosive. Volcanic activity also appears to have ‘retreated’ towards the Kverkfjöll central volcano. Glacial outburst floods or ‘jökulhlaups’ were prevalent during de‐glaciation, partly because sub‐glacial meltwater could not be impounded due to the high gradient bedrock topography. For northern Iceland, our proposed sequence of landscape development suggests two major glacial advances during the Holocene, at least one of which at Kverkfjöll probably coincided with volcanic activity and a jökulhlaup. Future work should look to establish an absolute dating control and/or chronology. Copyright © 2009 John Wiley & Sons, Ltd.

Deglaciation ages and meltwater routing in the Fort McMurray region, northeastern Alberta and northwestern Saskatchewan, Canada

A field-based reconstruction of the deglacial paleogeography in the Fort McMurray area permits: 1) constraining the timing of meltwater routing to the Arctic from the present Hudson Bay drainage basin; and 2) minimum-age estimates for ice-margin positions that can be used to constrain ice-sheet modeling results. A downslope recession of the Laurentide Ice Sheet resulted in a series of proglacial lakes forming between the ice margin and higher land to the southwest. The paleogeography of these lakes is poorly constrained in part from the masking effect of boreal forest vegetation and map-scale issues. However, recent space-shuttle based DEMs increase the number and spatial extent of moraines identified within the study area resulting in a coherent pattern of ice margin retreat focused on the Athabasca River valley. An intensive lake-coring program resulted in a minimum ten-fold increase in the radiocarbon database used to limit moraine ages. Results indicate that deglaciation in this region was younger than previously reported, and it is likely that the meltwater could not drain northward to the Arctic Ocean from any source southeast of the Fort McMurray area until approximately 9850–9660 14C BP.

Structural controls on englacial esker sedimentation: Skeiðarárjökull, Iceland

AbstractWe have used ground-penetrating radar (GPR) to observe englacial structural control upon the development of an esker formed during a high-magnitude outburst flood (jökulhlaup). The surge-type Skeiðarárjökull, an outlet glacier of the Vatnajökull ice cap, Iceland, is a frequent source of jökulhlaups. The rising-stage waters of the November 1996 jökulhlaup travelled through a dense network of interconnected fractures that perforated the margin of the glacier. Subsequent discharge focused upon a small number of conduit outlets. Recent ice-marginal retreat has exposed a large englacial esker associated with one of these outlets. We investigated structural controls on esker genesis in April 2006, by collecting >2.5km of GPR profiles on the glacier surface up-glacier of where the esker ridge has been exposed by meltout. In lines closest to the exposed esker ridge, we interpret areas of englacial horizons up to ~30m wide and ~10–15m high as an up-glacier continuation of the esker sediments. High-amplitude, dipping horizons define the base of esker materials across many lines. Similar dipping surfaces deeper in the profiles suggest that: (1) the dipping surfaces beneath the esker are englacial tephera bands; (2) floodwaters were initially discharged along structurally controlled englacial surfaces (tephra bands); (3) the rapid increase in discharge resulted in hydrofracturing; (4) establishment of preferential flow paths resulted in conduit development along the tephra bands due to localized excavation of surrounding glacier ice; and (5) sedimentation took place within the new accommodation space to form the englacial structure melting out to produce the esker.

Major changes in ice stream dynamics during deglaciation of the north-western margin of the Laurentide Ice Sheet

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.

Seafloor glacial features reveal the extent and decay of the last British Ice Sheet, east of Scotland

AbstractThree‐dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1–2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast‐flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd.

Holocene shore displacement and deglaciation chronology in Norrbotten, Sweden

The coastal zone of Norrbotten, northern Sweden, was gradually inundated by the Ancylus Lake following the retreating ice margin and forming a highest coastline approximately 210 m above the present sea level. The succeeding shore displacement is reconstructed based on lithological investigations and radiocarbon datings of identified isolation sequences from 12 cored lake basins. The highest lake basins, along with two basins above the highest shoreline, suggest ice-free conditions already at 10 500cal. yr BP This is at least 500 years earlier than previously thought and implies rapid ice-sheet break-up in the Gulf of Bothnia. The shore displacement (RSL) curve represents a forced regression of successively decreasing rate through the Holocene, from 9m/100yr to 0.8 m/100yr. During the first 1000–1200 years, the isostatic uplift is exponentially declining, followed by a constant uplift rate from c. 9500 cal. yr BP to 5500–5000 cal. yr BP. The last 5000 years seem to be characterized by a low but constant rebound rate. The development of the Ancylus Lake stage of the Baltic may also be discerned in the Norrbotten RSL curve, suggesting that the chronology of the Ancylus Lake stages may have to be revised. The Littorina transgression is also reflected by the RSL curve shape. In addition, a series of early to mid-Holocene beach terraces were OSL-dated to allow for comparison with the 14C-dated shore displacement curve. Interpretations of these ages and their relation to former sea levels were clearly more problematic than the dating of the lake basin isolations.

Evolution of lakes in the Huron basin: Deglaciation to present

Water bodies, ancestral to the present lakes including Lake Huron, first appeared in the southern Great Lakes basin about 15,500 14C years (18,800 cal years) BP during the oscillatory northward retreat of the last (Laurentide) ice sheet from its maximum position south of the Great Lakes watershed. Glacial lakes, impounded by a retreating ice margin on their northern shores, were continuously present after 13,000 14C (15,340 cal) BP for 3000 14C (3900 cal) years. Drainage routings varied in time through the Erie and Michigan basins to the Mississippi River system, a probable source for colonizing aquatic organisms, then to the Ontario basin, and finally northeastward to the Ottawa River valley via the isostatically-depressed North Bay outlet by 10,000 14C (11,470 cal) BP. Water levels were generally low between 10,000 and 7500 14C (11,470 and 8300 cal) BP and may have risen several tens of metres for short periods due to overflow of meltwater from upstream subglacial reservoirs or from glacial lakes impounded by residual ice in the Hudson Bay watershed. About 8000 14C (8890 cal) BP glacial runoff bypassed the Great Lakes, and Huron basin waters descended into hydrologic closure under the influence of the early Holocene dry climate. With increasing precipitation and water supply about 7500 14C (8300 cal) BP the Huron water body again overflowed its North Bay outlet. Differential isostatic uplift (fastest to the north-northeast) raised this outlet and lake level relative to the rest of the basin. The lake finally overflowed southern outlets at Chicago and Port Huron-Sarnia by 5000 14C (5760 cal) BP (during the Nipissing highstand). Enhanced erosion of the latter outlet and continued differential uplift of the basin led to the present configuration of Lake Huron and Georgian Bay.

Quaternary Stratigraphy in Northwestern Maine: A Progress Report

A preliminary Quaternary stratigraphy for northwestern Maine can be assembled from interpretation of ice-flow indicators (dispersal and erosional), exposed sections, and drill-hole logs. Evidence from the ice-flow indicators delineates at least four regions each with different ice-flow histories. The distribution of these regions may result from an eastward invasion of Laurentide source ice during the early portion of the late Wisconsinan and subsequent development of a local ice dome during the closing portion of the Late Wisconsinan substage. Exposed sections contribute the following probable sequence of events to the stratigraphy: 1) deposition of alluvial fans, 2) deposition of a gray compact till beneath eastward flowing (Laurentide) ice, 3) deposition of a brown till beneath northward flowing local ice, 4) deglaciation by a southward retreating ice margin. These events appear continuous and have all been provisionally assigned a Late Wisconsinan age. Drill-hole logs confirm the sequence derived from the exposed sections and allow extension of the Quaternary stratigraphy. The drill-log data show three associated groups of sediments that may in turn result from at least three separate ice margin advances and recessions. The uppermost group of sediments is correlated with those found in exposed sections. The position of the drill-hole logs in an over-deepened basin suggest erosion by at least one even earlier glaciation.