British Ice Sheet Research Articles

Form and Flow of the Last British Ice Sheet

Form and Flow of the Last British Ice Sheet

What happened to the last British Ice Sheet? Ice streaming, flow switching and grounding events during demise

What happened to the last British Ice Sheet? Ice streaming, flow switching and grounding events during demise

Complex interaction of subglacial hydrology and deforming bed processes and its implications for ice marginal stability: the example of the Middle Pleistocene British Ice Sheet from eastern England

Subglacial drainage is known to have a profound influence on glacier and ice sheet stability. Many scientists consider that such processes acted to cause abrupt climatic shifts in the North Atlantic region during Termination 1, and believe that meltwater-induced instabilities in the modern-day Greenland and Antarctic ice sheets could have similar climatic impacts in the future. Understanding the role played by porewater content and/or pressure, coupled with its temporal and spatial distribution within the subglacial bed, appear to be critical factors. These act to control the rheology and style of deformation within the subglacial deforming bed, and in-turn, the amount and distribution of ‘stick-slip’ at the ice-bed interface which facilitates ice motion. Within this paper, we present a model based upon sedimentological and structural evidence from Middle Pleistocene glacial deposits in Eastern England, which illustrate the symbiotic relationship between subglacial deforming bed processes, subglacial drainage within a pressurised channel system, and ice advance. The model shows the development of large-scale subglacial folds and thrusts within a subglacial melange that relate to temporal changes in porewater content and/or pressure within the subglacial bed. These acted to control the development and stability of subglacial drainage beneath the margins of the British Ice Sheet, ultimately leading to an increase in the efficiency of subglacial drainage and the subsequent collapse of the subglacial shear zone. This potentially slowed or even stopped ice advance leading to the part-collapse of the ice margin.

A comparison of Terminations I and II from a marine sediment core proximal to the former British Ice Sheet

A comparison of Terminations I and II from a marine sediment core proximal to the former British Ice Sheet

Predicting and investigating subglacial lakes of the former British and Fennoscandian Ice Sheets

Predicting and investigating subglacial lakes of the former British and Fennoscandian Ice Sheets

British Ice Sheet dynamics inferred from North Atlantic ice-rafted debris records spanning the last 175 000 years

British Ice Sheet dynamics inferred from North Atlantic ice-rafted debris records spanning the last 175 000 years

Development of a subglacial drainage system and its effect on glacitectonism within the polydeformed Middle Pleistocene (Anglian) glacigenic sequence of north Norfolk, Eastern England

Abstract The efficiency of subglacial drainage is known to have a profound influence on subglacial deformation and glacier dynamics with, in particular, high meltwater contents and/or pressures aiding glacier motion. The complex sequence of Middle Pleistocene tills and glacial outwash sediments exposed along the north Norfolk coast (Eastern England) were deposited in the ice-marginal zone of the British Ice Sheet and contain widespread evidence for subglacial deformation during repeated phases of ice advance and retreat. During a phase of easterly directed ice advance, the glacial and pre-glacial sequences were pervasively deformed leading to the development of a thick unit of glacitectonic melange. Although the role of pressurised meltwater has been recognised in facilitating deformation and melange formation, this paper provides evidence for the subsequent development of a channelised subglacial drainage system beneath this part of the British Ice Sheet filled by a complex assemblage of sands, gravels and mass flow deposits. The channels are relatively undeformed when compared to the host melange, forming elongate, lenticular to U-shaped, flat-topped bodies (up to 20–30 m thick) located within the upper part of this highly deformed unit. This relatively stable channelised system led to an increase in the efficiency of subglacial drainage from beneath the British Ice Sheet and the collapse of the subglacial shear zone, potentially slowing or even arresting the easterly directed advance of the ice sheet.

Switching flow patterns within the last ice sheet in northern Scotland

Synopsis A confluence zone existed within the northern sector of the last British Ice Sheet (BIS) where ice flowing from the Northern Highlands met ice streaming out of the Moray Firth and across the plain of Caithness. At Wester Clett, a complex 35m thick sequence of glacigenic sediments provides important evidence of dynamic interactions between these ice lobes that are consistent with recent computer simulations of the behaviour the last BIS. Sedimentological evidence suggests that early advance of Northern Highlands ice, probably between 33.5 and 30.5 ka, was followed by a first incursion of Moray Firth ice. Ice flowing out of Strath Halladale then deposited sands and gravels along its margin. Later, around the time of the Last Glacial Maximum, a powerful flow of Moray Firth ice diverted Northern Highlands ice to flow to the NW across the present coastline, depositing tills of mixed provenance. Cosmogenic exposure dates of c . 18 ka for summits in southern Caithness indicate that the ice sheet thinned subsequently, but flow to the NW continued. The youngest till and its associated moraine systems record a final movement of Northern Highlands ice. Subsequent ice retreat was accompanied by uncoupling of ice lobes at the Caithness–Sutherland border soon after 16 ka.

Reconstructing flow paths of the Middle Pleistocene British Ice Sheet in central-eastern England: the application of regional soil geochemical data

High-density regional geochemical data for surface soils in central England and East Anglia reveal that much of their geochemical character is inherited from the tills that they are developed upon. Multivariate statistical analysis highlighted three significant element associations of Al–Fe–Ga–K–La–Mg–Rb, Ca–Sr and K–Fe accounting for almost 93% of the geochemical variability of soils derived from tills. Provenancing the geochemical signatures of the latter elements enabled the construction of ice flow paths associated with two different Middle Pleistocene ‘chalky’ till sheets. A lower till sheet relating to ‘Pennine’ ice flowing from west to east across the region, and an upper till sheet deposited by North Sea ice moving into northern East Anglia, and to the west of the Fen Basin, before fanning-out across central England. Overall, geochemical signatures of different till units are largely derived from local bedrock sources, with dilution and a new geochemical signature acquired as the ice flows over and incorporates new bedrock lithologies. The results show that high resolution soil geochemical data provides a further proxy with which the flow paths of former ice sheets can be delineated.

Drainage pathways beneath ice sheets and their implications for ice sheet form and flow: the example of the British Ice Sheet during the Last Glacial Maximum

AbstractThe drainage system beneath ice sheets is poorly understood, even though it is a major determinant of their basal boundary condition, with implications for their form, flow and stability, and particularly of the ice streams that are crucial dynamic components. Understanding can be enhanced by evidence from the beds of former ice sheets. Glaciological principles are applied to geological evidence in the Vale of York that demonstrate occupancy by a major ice stream of the British ice sheet, with deduced gravitational shear stresses in the range 4–24 kPa, compared with a typical glacier shear stress of about 80–100 kPa, and horizontal velocities in the terminal zone in the range of 400–950 ma1. It is shown that the ice stream was in an almost buoyant state, with effective pressures in the range 50–100 kPa for a range of ice pressures between 2000 and 5000 kPa. Similar deductions are made for other ice streams on the southern flank of the British ice sheet. The esker system in the area of the ice stream represents locations of subglacial drainage tunnels that are suggested to be the key drainage elements in ice sheets where excess surface water penetrates to the bed. A theory of channel‐groundwater coupling which self‐organises in response to the rate of water recharge and the transmissivity of the subglacial bed is applied to the sub‐ice stream esker system. When matched to local geological evidence, the theory predicts the distribution of basal water pressures, the rate of recharge of surface water to the glacier bed, local shear stresses, the areal pattern of sedimentary processes including bed deformation, and their seasonal fluctuations. Predictions of the annual recharge rate of 950 mm are consistent with the modern values of modern mean annual rainfall in the Ouse (Vale of York) catchment of 906mm. The calculated mean annual discharge of the four major subglacial rivers in 55 m3s−1, compared with a mean flow of the modern Ouse of 49 m3s−1. A transient analysis shows the response time of water pressures in the underlying aquifer to changes in surface recharge to be of the order 10–100 days, suggesting the possibility of instabilities at the ice‐bed interface. It is not possible, however, to determine whether the Vale of York ice stream was characterized by stable or instable (surging) flow. Copyright © 2010 John Wiley & Sons, Ltd.

Subglacial bedforms of the last British Ice Sheet

Please click here to download the map associated with this article. Subglacial bedforms are a characteristic feature of formerly glaciated landscapes. Formed parallel (e.g. drumlins) or transverse (ribbed moraine) to ice flow their preserved distribution is a valuable record of ice sheet configuration. To date these landforms have been underused in Britain such that we have only a simple static view of the flow patterns of the last British Ice Sheet, with glimpses of complexity recorded in just a few locations. Arguably, this is because of the lack of a suitable, ice-sheet scale, map of subglacial bedform distribution. We present the first consistent and countrywide map of subglacial bedforms for Britain produced from systematic mapping. Individual landforms were mapped from relief-shaded renditions of high-resolution elevation data and Landsat TM imagery. The outline or crest-line of each bedform was manually captured by onscreen digitisation directly into a Geographic Information System (GIS) database. Over 39,000 landforms were identified revealing new lineation patterns, multiple instances of cross-cutting/superimposition, and adding additional detail to the known distribution established by local field surveys. A near complete representation of the British subglacial bedform population, the map is a major step towards a detailed model of British Ice Sheet configuration by providing a suitable basis for reconstructing the flow evolution at the ice sheet scale, and a geomorphological framework for the interpretation of sedimentological and stratigraphical data. The map is presented at a scale of 1:525,000.

Cosmogenic isotope (36Cl) surface exposure dating of the Norber erratics, Yorkshire Dales: Further constraints on the timing of the LGM deglaciation in Britain

Cosmogenic isotope (36Cl) surface exposure dating of four of the erratic boulders at Norber in the Yorkshire Dales National Park, northwest England, yielded mean ages of ∼22.2±2.0ka BP and ∼18.0±1.6ka BP for their emplacement. These two mean values derive from different 36Cl production rates used for exposure age calculation. The ages are uncorrected for temporal variations in production rates and may underestimate the true ages by 5–7%. The former age, although implying early deglaciation for this area of the British ice sheet, is not incompatible with minimum deglaciation ages from other contexts and locations in northwest England. However, the latter age is more consistent with the same minimum deglaciation ages and geochronological evidence for ice-free conditions in parts of the northern sector of the Irish Sea. Within uncertainties, the younger of the mean ages from Norber may indicate that boulder emplacement was associated with North Atlantic Heinrich event 1. The limited spatial (downvalley) extent of the Norber boulders implies that at the time of their deposition the ice margin was coincident with the distal margin of the erratic train. Loss of ice cover at Norber was followed by persistent stadial conditions until the abrupt opening of the Lateglacial Interstadial when large carnivorous mammals colonised the area. The 36Cl ages are between ∼3.0ka and ∼13.0ka older than previous estimates based on rates of limestone dissolution derived from the heights of pedestals beneath the erratics.

British Ice Sheet dynamics inferred from North Atlantic ice‐rafted debris records spanning the last 175 000 years

AbstractSediment core MD04‐2822 from the Rockall Trough, northeast Atlantic, provides lithological evidence for the expansion of the British Ice Sheet (BIS) onto the Hebridean margin during both the penultimate (Marine Isotope Stage (MIS) 6/Wolstonian/Saalian) and last glaciation (Devensian/Weichselian). A composite chronology is constructed using: (i) the benthic foraminiferal oxygen isotope record, matched to an orbitally tuned stacked record of global ice volume (SPECMAP); (ii) the synchronisation of the planktonic foraminiferal (Neogloboquadrina pachyderma (sinistral)) and ITRAX XRF Ca records to the Greenland ice core (NGRIP) back to ca. 123 ka; and (iii) the synchronisation of the same N. pachyderma (sinistral)) and ITRAX XRF Ca records to the atmospheric methane record of the Antarctic ice core (EPICA Dome C) beyond 123 ka. The first offshore evidence for the expansion of the last BIS within MIS 4 is presented, suggesting extensive glaciation of northwestern Britain during the Devensian/Weichselian as early as 70 ka. The proximity of this long sediment core to the glaciated margin of northwestern Scotland makes it ideal to capture the ice‐rafted debris (IRD) signal of the BIS for the last 175 ka, via a continuous flux record of IRD. British Geological Survey © NERC 2009. All rights reserved.

Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model of ice sheet growth, retreat and dynamics

The ice sheet that once covered Ireland has a long history of investigation. Much prior work focussed on localised evidence-based reconstructions and ice-marginal dynamics and chronologies, with less attention paid to an ice sheet wide view of the first order properties of the ice sheet: centres of mass, ice divide structure, ice flow geometry and behaviour and changes thereof. In this paper we focus on the latter aspect and use our new, countrywide glacial geomorphological mapping of the Irish landscape (>39 000 landforms), and our analysis of the palaeo-glaciological significance of observed landform assemblages (article Part 1), to build an ice sheet reconstruction yielding these fundamental ice sheet properties. We present a seven stage model of ice sheet evolution, from initiation to demise, in the form of palaeo-geographic maps. An early incursion of ice from Scotland likely coalesced with local ice caps and spread in a south-westerly direction 200 km across Ireland. A semi-independent Irish Ice Sheet was then established during ice sheet growth, with a branching ice divide structure whose main axis migrated up to 140 km from the west coast towards the east. Ice stream systems converging on Donegal Bay in the west and funnelling through the North Channel and Irish Sea Basin in the east emerge as major flow components of the maximum stages of glaciation. Ice cover is reconstructed as extending to the continental shelf break. The Irish Ice Sheet became autonomous (i.e. separate from the British Ice Sheet) during deglaciation and fragmented into multiple ice masses, each decaying towards the west. Final sites of demise were likely over the mountains of Donegal, Leitrim and Connemara. Patterns of growth and decay of the ice sheet are shown to be radically different: asynchronous and asymmetric in both spatial and temporal domains. We implicate collapse of the ice stream system in the North Channel – Irish Sea Basin in driving such asymmetry, since rapid collapse would sever the ties between the British and Irish Ice Sheets and drive flow configuration changes in response. Enhanced calving and flow acceleration in response to rising relative sea level is speculated to have undermined the integrity of the ice stream system, precipitating its collapse and driving the reconstructed pattern of ice sheet evolution.

Lateglacial ice-cap dynamics in NW Scotland: evidence from the fjords of the Summer Isles region

The seaboard of western Scotland is a classic fjord landscape formed by glaciation over at least the last 0.5 Ma. We examine the glacial geology preserved in the fjords (or sea lochs) of the Summer Isles region of NW Scotland using high-resolution seismic data, multibeam swath bathymetry, seabed sediment cores, digital terrain models, aerial photographs, and field investigations. Detailed analyses include seismic facies and lithofacies interpretations; sedimentological and palaeoenvironmental analyses; and radiocarbon dating of selected microfauna. Our results indicate that the Pleistocene sediments of the Summer Isles region, on- and offshore, can be subdivided into several lithostratigraphic formations on the basis of seismic character, geomorphology and sedimentology. These are: subglacial tills; ice-distal and glacimarine facies; ice-proximal and ice-contact facies; moraine assemblages; and Holocene basin fill. The submarine landscape is also notable for its large-scale mass-movement events – the result of glaciodynamic, paraglacial or seismotectonic processes. Radiocarbon dating of marine shells indicate that deglaciation of this part of NW Scotland was ongoing between 14 and 13 ka BP – during the Lateglacial Interstadial (Greenland Interstadial 1) – consistent with cosmogenic surface-exposure ages from previous studies. A sequence of numerous seafloor moraine ridges charts oscillatory retreat of the last ice sheet from a buoyant calving margin in The Minch to a firmly grounded margin amongst the Summer Isles in the early part of Lateglacial Interstadial (GI-1) (pre-14 ka BP). Subsequent, punctuated, frontal retreat of the ice mass occurred in the following ∼1000 years, during which time ice-cap outlet glaciers became topographically confined and restricted to the fjords. A late-stage readvance of glaciers into the inner fjords occurred soon after 13 ka BP, which calls into question the accepted limits of ice extent during the Younger Dryas Stadial (Greenland Stadial 1). We examine the wider implications of our chronostratigraphic model, discussing the implications for British Ice Sheet deglaciation, Lateglacial climate change, and the style and rates of fjord sedimentation.

Late Quaternary ice sheet, lake and sea history of southwest Scandinavia – a synthesis

Based on a large number of new boreholes in northern Denmark, and on the existing data, a revised event‐stratigraphy is presented for southwestern Scandinavia. Five significant Late Saalian to Late Weichselian glacial events, each separated by periods of interglacial or interstadial marine or glaciolacustrine conditions, are identified in northern Denmark. The first glacial event is attributed to the Late Saalianc. 160–140 kyr BP, when the Warthe Ice Sheet advanced from easterly and southeasterly directions through the Baltic depression into Germany and Denmark. This Baltic ice extended as far as northern Denmark, where it probably merged with the Norwegian Channel Ice Stream (NCIS) and contributed to a large discharge of icebergs into the Norwegian Sea. Following the break up, marine conditions were established that persisted from the Late Saalian until the end of the Early Weichselian. The next glaciation occurredc. 65–60 kyr BP, when the Sundsøre ice advanced from the north into Denmark and the North Sea, where the Scandinavian and British Ice Sheets merged. During the subsequent deglaciation, large ice‐dammed lakes formed before the ice disintegrated in the Norwegian Channel, and marine conditions were re‐established. The following Ristinge advance from the Baltic, initiatedc. 55 kyr BP, also reached northern Denmark, where it probably merged with the NCIS. The deglaciation,c. 50 kyr BP, was followed by a long period of marine arctic conditions. Around 30 kyr BP, the Scandinavian Ice Sheet expanded from the north into the Norwegian Channel, where it dammed the Kattegat ice lake. Shortly after,c. 29 kyr BP, the Kattegat advance began, and once again the Scandinavian and British Ice Sheets merged in the North Sea. The subsequent retreat to the Norwegian Channel led to the formation of Ribjerg ice lake, which persisted from 27 to 23 kyr BP. The expansion of the last ice sheet startedc. 23 kyr BP, when the main advance occurred from north–northeasterly directions into Denmark. An ice‐dammed lake was formed during deglaciation, while the NCIS was still active. During a re‐advance and subsequent retreatc. 19 kyr BP, a number of tunnel‐valley systems were formed in association with ice‐marginal positions. The NCIS finally began to break up in the Norwegian Sea 18.8 kyr BP, and the YoungerYoldiaSea inundated northern Denmark around 18 kyr BP. The extensive amount of new and existing data applied to this synthesis has provided a better understanding of the timing and dynamics of the Scandinavian Ice Sheet (SIS) during the lastc. 160 kyr. Furthermore, our model contributes to the understanding of the timing of the occasional release of large quantities of meltwater from the southwestern part of the SIS that are likely to enter the North Atlantic and possibly affect the thermohaline circulation.

A 1.2 Ma record of glaciation and fluvial discharge from the West European Atlantic margin

The correlation of continental sedimentary records with the marine isotope stratigraphy is a challenge of central importance in Quaternary stratigraphy, particularly in Western Europe where long records of glaciation on land areas are particularly rare. Here we demonstrate for the first time the interrelationship of events during the last 1.2 Ma in an ocean-sediment core from the Bay of Biscay (NE Atlantic), SW of the Channel. The identification of discharge variations from tributary river systems to the ‘Fleuve Manche’ palaeoriver during glacio-eustatic sea-level lowstands demonstrates the correlation of the marine sediment stratigraphy to the expansion and recession of the European ice-sheets. The amplitude and chronology of European ice-sheet oscillations since the late Early Pleistocene is discussed and our results demonstrate that the first coalescence of the Fennoscandian and British ice-sheets in the North Sea basin ca 450 ka ago caused a profound change in lowstand European drainage alignment. This change caused a rerouting of Fennoscandian and eastern British ice-sheets-derived meltwaters from northwards into the Nordic Seas to southwards into the eastern North Atlantic thereafter. Besides allowing a thorough synchronisation of the European ice-sheet palaeogeography with the well-dated records of palaeoceanographical changes, our results improve the stratigraphy of the English Channel palaeovalleys and will provide important constraints on paleoclimatic scenarios considering the impact that such rerouting and meltwater surges might have on the stability of the oceanic conveyor belt.

A Last Glacial Maximum pollen record from Bodmin Moor showing a possible cryptic northern refugium in southwest England

AbstractA late Devensian palynological record is presented from Dozmary Pool (Bodmin Moor, southwest England), beyond the southern limit of the Last Glacial Maximum (LGM) British Ice Sheet. The pollen assemblages indicate predominantly herbaceous tundra–steppe communities but also include elevated levels (typically 10–20%) of conifer tree pollen (Picea, Pinus, Abies) and lower but persistent percentages of broadleaf tree pollen during the LGM. This record is seemingly at odds with the orthodox view of an entirely treeless tundra–steppe environment for this region and elimination of tree species from the British Isles during glacial maxima. Long‐distance pollen transport seems an unlikely explanation for the tree pollen considering distance to the nearest known refugia, except possibly for Pinus. Reworking of the tree pollen, often invoked in these circumstances, remains a possible alternative, especially given the abundance of these trees in the region during early Devensian interstadials. However, this explanation has been challenged by studies reporting plant macrofossil and faunal evidence for survival of temperate biota during glacial maxima and from climate modelling work that suggests some trees could have survived the glacial extremes in areas well beyond the recorded glacial refugia. Assuming reworking was not a major factor, the Dozmary Pool pollen record is consistent with the ‘cryptic northern refugia hypothesis’ that invokes survival of trees in small, scattered populations under locally favourable conditions during glacial maxima. Copyright © 2009 John Wiley & Sons, Ltd.

The paleo–ice stream in Vestfjorden, north Norway, over the last 35 k.y.: Glacial erosion and sediment yield

The basal erosion rate and sediment yield of the Vestfjorden paleo–ice stream, a marine-based part of the northwestern Fennoscandian Ice Sheet, were elucidated during a complete advance-retreat cycle using high-resolution seismic data and multi-beam bathymetry. In most of the area, there is a semitransparent, continuous upper seismic unit showing a southwestward (down-fjord) progradational pattern with megascale glacial lineations on top, interpreted to represent a subglacial deformable till. The till formed a continuous bed beneath most of the Vestfjorden paleo–ice stream, except for the upstream parts, where ice streaming is inferred to have occurred above bedrock. The till thickness is up to an order of magnitude larger than those reported from modern ice streams due to a more efficient transfer of material from the source area to the ice streambed. The till is draped by glaciomarine and marine sediments. The average sedimentation rate in the paleo–ice stream drainage route during the period from ca. 35 to 11 ka was ~2.6 m/k.y. This equals an average sediment discharge of 35 × 106 t/yr and an average sediment yield of 6.4 × 103 t km−2 yr−1. From this, we estimate an average erosion depth of ~40 m or rate of ~1.7 mm/yr in the sediment source area, the encircling fjord, and valley landscape. These values are in agreement with the results reported from similar settings in southern Norway, but they are nearly twice the figures reported for arctic ice-stream erosion. They are also significantly higher than the rates estimated from the British Ice Sheet. In conclusion, glacial erosion beneath paleo–ice streams seems to have been more efficient in subarctic regions.

Late Wisconsinan glacial landsystems on Atlantic Canadian shelves: New evidence from multibeam and single‐beam sonar data

Multibeam sonar surveys in the past decade, augmented by single‐beam data from the OLEX charting system, reveal landsystems on Atlantic Canadian shelves that are diagnostic of Late Wisconsinan ice‐sheet dynamics. Four landsystems are described. (1) The Bay of Fundy landsystem comprises two contrasting sets of bedforms, and is interpreted as evidence of topographically controlled fast‐flowing ice adjacent to slower‐moving ice. (2) The German Bank landsystem off southwest Nova Scotia is comprised of glacially fluted terrain overprinted by De Geer moraines and arcuate recessional moraines. We infer that a flow of grounded glacial ice out of the Bay of Fundy was followed by steady retreat, punctuated by at least one major re‐advance. (3) The Placentia Bay landsystem consists of a convergent field of streamlined landforms with superimposed De Geer moraines, overprinted in one area by flutings. We infer that this landsystem was formed in the onset zone of fast‐flowing ice, and that overprinting was due to a re‐advance of ice from offshore. (4) The south coast of Newfoundland landsystem, which includes arcuate, fjord‐mouth moraines and a coast‐parallel, fluted moraine, indicates strong topographic control on a retreating marine ice margin as it reached a fjord coastline. These submarine glacial landsystems are not inconsistent with a conceptual model showing Late Wisconsinan ice advance to shelf edges, rapid calving retreat along deepwater channels and slower retreat of ice margins grounded in shallow water. The re‐advances documented two of the study areas have parallels in the Last British Ice Sheet, confirming that the reorganization of marine‐based ice sheets, caused by calving in embayments, led to internally forced re‐advances.