Irish Ice Sheet Research Articles

Behavioural tendencies of the last British–Irish Ice Sheet revealed by data–model comparison

ABSTRACTIntegrating ice‐sheet models with empirical data pertaining to palaeo‐ice sheets promotes advances in the models used in sea‐level predictions and can improve our understanding of past ice‐sheet behaviour. The large number of empirical constraints on the last British–Irish Ice Sheet make it ideal for model–data comparison experiments. Here, we present an ensemble of 600 model simulations, which are compared with data on former ice‐flow extent, flow geometry and deglaciation timing. Simulations which poorly recreate data were ruled out, allowing us to examine the remaining physically realistic simulations which capture the ice sheets' behavioural tendencies. Our results led to a novel reconstruction of behaviour in the data‐poor region of the North Sea, insights into the ice stream, potential ice‐shelf and readvance dynamics, and the potential locations of peripheral ice caps. We also propose that the asynchronous behaviour of the British–Irish Ice Sheet is a consequence of the geography of the British Isles and the merging and splitting of different bodies of ice through saddle merger and collapse. Furthermore, persistent model–data mismatches highlight the need for model development, especially regarding the physics of ice–ocean interactions. Thus, this work highlights the power of integrating models and data, a long‐held aim of palaeoglaciology.

The timing and magnitude of the British–Irish Ice Sheet between Marine Isotope Stages 5d and 2: implications for glacio‐isostatic adjustment, high relative sea levels and ‘giant erratic’ emplacement

ABSTRACTThe extent, chronology and dynamics of the pre‐Marine Isotope Stage (MIS) 2 last British–Irish Ice Sheet (BIIS) are not well known. Although the BRITICE‐CHRONO Project has detailed the maximum extent and retreat phases of the last BIIS for the period after 30 ka and into the Last Glacial Maximum (LGM), the Project identified several pre‐existing datasets and generated new data that implied glaciation pre‐dating the LGM but which post‐dated the Last Interglacial (Eemian; MIS5e); these data are reviewed here. There are no dated till units but are other indicators clearly indicative of glaciation: deep‐sea ice‐rafted detritus flux into the adjacent NE Atlantic, cosmogenic rock‐exposure age dating from glaciated surfaces in Wales and the island of Lundy (Bristol Channel), and optically stimulated luminescence (OSL) ages of proximal glacifluvial sequences on the Isle of Lewis (Outer Hebrides) and in the Cheshire Basin. Taken together these indicate BIIS inception during MIS5d, growth into MIS4 and evidence for dynamic retreat–advance phases during MIS3. OSL evidence for high relative sea level indicated by raised beaches in southern Ireland during MIS4 and 3 at a time of lowered glacio‐eustatic sea level indicates substantial glacial isostatic loading, explained by the early growth of the BIIS during the last cold stage. High relative sea level during MIS4 and 3 coincident with adjacent calving ice sheet margins provides an explanation for the rafted giant erratics found around the shores of southern Britain and Ireland.

Palaeoenvironmental reconstruction of Loch Duart (NW Scotland, UK) since the Last Glacial Maximum: implications from a multiproxy approach

ABSTRACTA sediment core from the salt marsh fringing Loch Duart, NW Scotland, UK, containing Lateglacial to Holocene sediments, was analysed using a multi‐element geochemical approach to elucidate the relative sea level (RSL) and palaeoenvironmental changes associated with the deglaciation of the British and Irish Ice Sheet. Elemental and isotopic measurements of rhenium, osmium, carbon and nitrogen, X‐ray fluorescence scanning, radiocarbon dating, and foraminiferal analysis produced a suite of data that complements the existing biostratigraphic framework. This suite of bio‐, litho‐ and chemostratigraphic analyses permits discussion of RSL changes that reflect the interplay between post‐glacial eustatic rise and glacio‐isostatic adjustment. The osmium‐isotope (187Os/188Os) data, coupled with a new age–depth model, depict an RSL fall between 16.8 and 14.1 ka cal bp at an average rate of 2 mm a−1. Falling RSL culminates in basin isolation and is followed by subsequent marine inundation from 11.6 ka cal bp. This RSL record preserves the local interaction between glacial isostatic adjustment and glacio‐eustatic sea‐level change, a relationship that is reflected in the sediment's 187Os/188Os signature. This is the first known application of the osmium isotope system in an isolation basin that is shown to be a viable proxy for RSL change, a technique that could be applied to glacially influenced isolation basins globally.

British–Irish Ice Sheet and polar front history of the Goban Spur, offshore southwest Ireland over the last 250 000 years

Deep Sea Drilling Program (DSDP) Site 548 was cored in 1984 at a water depth of 1256 m on the Goban Spur, offshore southwest Ireland. Coring retrieved a ~100‐m‐thick Pleistocene contourite sequence. This study uses planktonic foraminiferal assemblage and benthic foraminiferal oxygen isotope analyses to establish an age model for the upper 40 m of this core. This site's multidisciplinary analyses of planktonic foraminiferal assemblages, lithic grains, facies and calcium carbonate concentration reveal a 250 000‐year record of the North Atlantic polar front variability and British–Irish Ice Sheet (BIIS) history. The sequence is characterized by alternations of ice rafted debris (IRD) laden pelagic mud facies with calcium carbonate‐rich silty sand contourite facies that track glacial/interglacial cycles. The polar front migrated southward across the area several times during glacial maxima and stadial periods, while warmer Mediterranean Outflow Water (MOW) flowed northward across the region during interglacial and interstadial periods depositing contourites. Lithic analyses reveal a complex history of IRD deposition associated with iceberg calving from the Laurentide Ice Sheet and northwest European ice sheets, mainly the BIIS. Comparison between the Goban Spur (DSDP Site 548) and the Celtic Margin (MD03‐2692) and central North Atlantic Integrated Ocean Drilling Program (IODP) Site U1308 suggests differences between the ‘non‐Laurentide Ice Sheet’ Heinrich Events (HE) 6 and 3 at the Goban Spur, with IRD from the BIIS being prominent during HE 6 and IRD from other European ice sheets north of the BIIS likely being more dominant during HE 3. The nature of lithics in IRD‐rich horizons during Terminations 3, 3A, 2 and 1 suggests significant iceberg calving episodes preceding BIIS retreat during the onset of interstadial intervals.

Reconstruction of the palaeo‐sea level of Britain and Ireland arising from empirical constraints of ice extent: implications for regional sea level forecasts and North American ice sheet volume

ABSTRACTReconstructions of palaeo‐sea level are vital for predicting future sea level change and constraining palaeo‐ice sheet reconstructions, as well as being useful for a wide array of applications across Quaternary Science. Previous reconstructions of the palaeo‐sea level of Britain and Ireland relied on a circular tuning of glacio‐isostatic models: input ice sheet thicknesses and extents were iteratively altered to fit relative sea level data. Here we break that circularity by utilizing new data from the BRITICE‐CHRONO project, which constrains the position of the British–Irish ice sheet margin through time, and we compare derived glacio‐isostatic modelling to the rich relative sea level record. We test a combination of plausible ice thickness scenarios which account for the uncertainty of ice margin position over the North Sea, demonstrating the region where regional sea level data could distinguish between different glaciation scenarios. Our optimal reconstruction is then combined with several global‐scale reconstructions. As the signal of the British–Irish Ice Sheet is constrained, we demonstrate how the relative sea level record of Britain and Ireland can be used to test reconstructions of far‐field ice sheets (e.g. Antarctica, Eurasia and the Laurentide). The derived palaeo‐topography data are likely to be useful for multiple disciplines. Finally, our improved method of sea level reconstruction impacts predictions of contemporary vertical land motion.

New metrics reveal the evolutionary continuum behind the morphological diversity of subglacial bedforms

Understanding the formation of subglacial bedforms is primordial to constrain ice-meltwater-bed interactions and the dynamics of past and present ice sheets. However, the difficulty to observe active subglacial bedforms below present-day ice sheets implies that their formation and evolution are essentially deduced from the inversion of morphological and sedimentological data from palaeo-subglacial bedforms. In this study, the morphological characteristics of subglacial bedforms are explored with a new approach based on the combination of three dimensionless morphometric indices i.e., circularity index, sinuosity index and elongation component ratio. We measure the spatial distribution of these indices on an unpublished database composed of ~13,500 digitized bedforms taken from two selected portions of the Irish Ice Sheet and Laurentide Ice Sheet beds considering that (1) all subglacial bedforms in each region were formed under a single ice flow configuration and (2) the bedforms may have developed either orthogonal, parallel, or oblique to ice displacement directions. Our results reveal a morphometric and spatial continuum along which ribbed bedforms of various lengths and shapes – ranging from circular to elongated flow-transverse forms – are incipient bedforms that can evolve into transitional sinuous ribbed bedforms, then into predominantly flow-parallel sinuous bedforms that progressively realign to form streamlined bedforms. Assuming the glaciological contexts of selected study areas, this continuum may provide a new geomorphic criterion to constrain spatial variations in subglacial conditions (e.g. ice flow velocity, bedrock characteristics, meltwater pressure).

Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE‐CHRONO reconstruction

The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling.

Using novel methods to track British and Irish Ice Sheet dynamics since the Late Pleistocene, along the west Porcupine Bank, NE Atlantic

Extensive research has been undertaken to elucidate the glacial history of the British Irish Ice Sheet (BIIS) in the NE Atlantic. BRITICE-CHRONO has compiled terrestrial and marine based evidence, to provide an empirical reconstruction of ice sheet expansion and retreat during the Late Pleistocene. Across the Irish margin, particular focus has been given to seafloor sediments which contain ice-rafted debris (IRD). However, there are few publications on IRD from areas proximal to the maximum extent of the BIIS, which would offer further insights on the behaviour of the ice sheet during (de)glacial events. Previous exploratory surveys of the west Porcupine Bank (wPB) visually identified IRD on the seafloor and these present a new study site to investigate the extent of the BIIS and the course of its icebergs. Moreover, there are uncertainties about the effects of icebergs on the marine life and cold-water corals occupying the nearby Porcupine Bank Canyon. Assessing a sediment core containing an IRD analogue for the wPB would thus, have a dual purpose. In the past however, coring missions of the wPB using traditional coring methods (i.e. piston and gravity cores) were unsuccessful. Here, we utilized a novel ROV-mounted vibrocoring procedure to capture a 0.75 m IRD-bearing sediment core from the wPB. Then further novel analytical methods (computed tomography-based IRD-detection) were used to quantify IRD every 0.02 cm to provide the highest resolution record of BIIS related IRD to date. From this, several fluxes of IRD deposition onto the wPB between 31.6 and 9 ka BP were revealed and corroborated by other published records from across the NE Atlantic. It was shown that the wPB IRD fluxes occur simultaneously with other parts of the margin. The IRD signal also shows that iceberg calving occurred on the wPB during the Younger Dryas. Grain-size analysis of the core allowed for a reconstruction and interpretation of the palaeoenvironmental conditions during these IRD flux events and shows that BIIS-derived glaciers had a major impact on hydrodynamic conditions in the wPB. Subsequently, intensive scouring led to a major hiatus in the core during 27.3–17.2 ka BP. These results are a useful addition to BIIS literature on this part of the shelf. Furthermore, it shows that bottom currents were influenced by (de)glacial events, an important finding when considering the presence of nearby current-dependant benthos.

A 10Be-dated record of glacial retreat in Connemara, Ireland, following the Last Glacial Maximum and implications for regional climate

Late Pleistocene stadials are traditionally viewed as periods of severe, year-round cooling centred on the North Atlantic, where the abrupt onset and termination of stadial conditions is widely, though not unequivocally, thought to reflect the impact on Atlantic Meridional Circulation of meltwater fluxes from fringing ice masses. Widespread subaerial melting of land-based ice, however, implies a state of glacier-climate disequilibrium that, according to some conceptual models, had to persist for the duration of each stadial event rather than immediately prior to it. Yet, to date, the spatial extent of any such melting remains unclear, as do the magnitude and duration of any cognate atmospheric warming. To help address this ambiguity, we reconstructed the timing and nature of deglaciation of the Connemara ice centre, Ireland, which, being located immediately downwind of the North Atlantic, affords an ideal location for assessing terrestrial expressions of stadial and non-stadial conditions. We report 15 new cosmogenic beryllium-10 ages of glacial erratic boulders that reveal the timing and magnitude of fluctuations in this sector of the former Irish ice sheet during Heinrich Stadial 1 (HS1). Coupled with geomorphic mapping, these results indicate rapid, widespread deglaciation of the Connemara ice centre at approximately 17 ka, under full stadial conditions, and that the ice margin retreated back to regional source areas rather than eastward towards the centre of the island. Recognising the potential divergence from the traditional view of cold North Atlantic stadials, we propose that the rate and nature of deglaciation in Connemara during HS1 likely reflects enhanced subaerial melting during the summer ablation season. • Fifteen new 10 Be dates constrain the deglaciation of an ice centre in Connemara, Ireland to Heinrich Stadial 1. • Our chronology contrasts with the current paradigm and suggests summer melting occurred despite anomalous winter cooling. • The Connemara deglacial record supports a broader pattern of enhanced seasonality in the North Atlantic during stadials.

New geomorphic evidence for a multi‐stage proglacial lake associated with the former British–Irish Ice Sheet in the Vale of Pickering, Yorkshire, UK

ABSTRACTFormer proglacial lakes provide insight into the temporal and spatial evolution of past ice sheet dynamics and the glaciohydrological systems associated with them. Glacial Lake Pickering was a proglacial lake in North Yorkshire impounded by the British–Irish Ice Sheet. However, geomorphic evidence for its existence and extent is weak and contested. This paper presents, for the first time, systematic mapping of geomorphic features from high‐resolution (50 cm to 5 m) digital terrain model data and undertakes palaeogeographical reconstructions to establish Palaeolake Pickering's extent and levels. Results show only weak evidence for a 70‐m lake. This and the fact that the present landscape configuration is unable to impound such a large lake suggests that it either did not exist or is much older and that considerable landscape erosion has since taken place. There is good geomorphological evidence to support lake levels between 45 and 20 m. The new palaeogeographical reconstructions show these lake levels were initially controlled by the balance between the North Sea and the Vale of York Lobes of the British–Irish Ice Sheet with lower and later levels impounded by the Filey moraine. A multi‐stage Palaeolake Pickering with various extents and levels possibly spanning more than one glacial period has implications for other British–Irish Ice Sheet palaeolake extents reconstructed only from undifferentiated lacustrine sediments and their maximal elevation. The existence of an extensive multi‐phase proglacial Palaeolake Pickering along with other similar ones in Eastern England would have impacted mass loss and frontal dynamics via subaqueous melting and iceberg calving, potentially playing a role in controlling the dynamic behaviour of the North Sea Lobe.

The development and impact of an ice‐contact proglacial lake during the Last Glacial Termination, Palaeolake Riada, central Ireland

ABSTRACTThe retreat of the last Irish Ice Sheet across the Irish midlands resulted in the formation of an ice‐contact lake, Palaeolake Riada, along its eastern margin during recession. However, the full extent and characteristics of the lake and its impact on ice dynamics are unknown, as glacigenic landforms and deposits relating to the lake have not been mapped in sufficient detail to allow reconstruction of ice‐marginal behaviour. This paper presents new mapping of subglacial, ice‐marginal and glaciolacustrine landforms and deposits in the Irish midlands. Glacial landforms and landform associations across this wide area have been identified using aerial photographs and high‐resolution digital elevation models derived from LiDAR and radar data. Reconstructions of successive ice‐margin retreat positions are combined with glacio‐isostatically adjusted digital elevation models to identify the probable position of lake outflows and reconstruct the development of the proglacial Palaeolake Riada through time. These reconstructions indicate that at its largest, Palaeolake Riada had an ice‐contact margin of 113 km, and covered an area of over 2300 km2. Initially, ice recession was concentrated around meltwater discharge outlets, but these became less important as the lake expanded and thermo‐mechanical erosion occurred along the entire subaqueous margin.

Glacitectonic evidence of ice sheet interaction and retreat across the western part of Dogger Bank (North Sea) during the Last Glaciation

High-resolution 2D seismic data from the western side of Dogger Bank (North Sea) has revealed that the glacigenic sediments of the Dogger Bank Formation record a complex history of sedimentation and penecontemporaneous, large-scale, ice-marginal to proglacial glacitectonism. The resulting complex assemblage of glacial landforms and sediments record the interplay between two separate ice masses revealing that Late Devensian ice sheet dynamics across Dogger Bank were far more complex than previously thought, involving the North Sea lobe of the British and Irish Ice Sheet, advancing from the west, interacting with the Dogger Bank lobe which expanded from the north. The active northward retreat of the Dogger Bank lobe resulted in the development of a complex assemblage of arcuate thrust-block moraines (≤15 km wide, >30 km long) composed of highly folded and thrust sediments, separated by sedimentary basins and meltwater channels filled by outwash. The impact of the North Sea lobe was restricted to the western margin of Dogger Bank and led to deep-seated (100–150 m thick) glacitectonism in response to ice-push from the west. During the earlier expansion of the North Sea lobe, this thrust and fold complex initially occupied a frontal marginal position changing to a more lateral ice-marginal position as the ice sheet continued to expand to the south. The complex structural relationships between the two glacitectonic complexes indicates that these ice masses interacted along the western side of Dogger Bank, with the inundation of this area by ice probably occurring during the last glaciation when the ice sheets attained their maximum extents.

Transit boosting of trans-Channel katabatic winds by depressions associated with the west European storms tracks: impact on Pleistocene loess deposits

Onshore and offshore sedimentological, geochemical, geomorphological, palaeontological and geochronological studies of loess deposits located under and around the English Channel revealed that they were transported by katabatic winds generated by the British–Irish Ice Sheet. Katabatic winds, which are low-altitude wind flows, were able to jump over the low southern British hills but were stopped by the higher Brittany and Normandy hills. This regional topography is interrupted by a north–south corridor linking the northern and southern shores of Brittany where loess propagated down to the mouth of the Loire River. This long transit shows that the total distance travelled by the katabatic wind was around 750 km, which represents an unusual distance for the propagation of this wind under continental conditions. Strong similarities with Antarctica and Greenland, where well documented cases of katabatic winds are known, show that the transit of the trans-Channel katabatic winds were strongly enhanced by the seasonal drift of storms propagating in an eastward direction along the axis of the English Channel. This increasing strength of the north–south katabatic flux was probably at the origin of the transport of loess particles down to the mouth of the Loire River.

Multiple drivers of Late Holocene paraglacial sediment reworking in Ireland

On the east coast of Ireland, glacigenic sediments and landforms reflect both phases of late Devensian ice retreat, and postglacial slope readjustment under a paraglacial process regime. These different phases of landscape development are commonly difficult to identify in the absence of geomorphological, sedimentary and dating evidence for the style, magnitude and timing of postglacial landscape change. Here, sediments and radiocarbon dates on raised coastal sediments adjacent to a prominent retreat moraine of the late Devensian Irish ice sheet provide evidence for one such extended phase of paraglacial sediment reworking, in the 8th and 10th centuries CE. The timing of this phase of sediment reworking provides evidence for climatic deterioration during the European late Dark Ages/early Medieval Warm Period, and is consistent with evidence across northwest Europe for enhanced landscape geomorphic change at this time. From this evidence it is inferred that landscape morphological change dominantly reflects climatic rather than anthropogenic forcing, although the latter cannot be wholly discounted. Further, this evidence highlights the sensitivity to Holocene climate forcing of coastal paraglacial systems that contain abundant glacigenic sediment supply.

Impact of katabatic winds on the environment of Neanderthals during the Middle Palaeolithic in westernmost Europe

ABSTRACTCompilation of the offshore and onshore altitudinal limits of the loess deposits of western France and southern England shows that they were deposited by low‐level wind fields. These relate to (i) the deflation of silt‐rich sediment extracted from the outwash plains of the not far distant British–Irish Ice Sheet and from the palaeo‐rivers of the Channel, and (ii) the existence of north and north‐western palaeo‐winds deduced from particle size analysis and heavy mineral distribution, and suggest (iii) that loess particles were transported by strong katabatic winds blowing from the northern ice‐covered regions towards Brittany and Normandy. Comparison between the main orientation of Neanderthal shelters and the direction of the katabatic winds shows that they were perpendicular to each other. The dominant orientation of the shelters was apparently ruled by these winds. A small‐scale study concentrating on the penultimate glaciation shows that in contrast to Brittany and Normandy where loess deposits accumulated on north‐facing cliffs, in England the same particles were deposited on the leeside of the hills. The existence of deflation zones, violently swept by Marine Isotope Stage 6 katabatic winds south of the British–Irish ice sheet, was probably at the origin of the restricted number of Neanderthals at that time in England. © 2021 John Wiley & Sons, Ltd.

A Late Pleistocene channelized subglacial meltwater system on the Atlantic continental shelf south of Ireland

The study of palaeo‐glacial landforms and sediments can give insights into the nature and dynamics of ice sheets. This is particularly the case with regards to the subglacial record, which is challenging to observe in contemporary glaciated settings and hence remains only partially understood. The subglacial hydrological system is an essential component of ice dynamics, where increased water pressure enhances ice motion and sediment deformation, thus reducing ice‐bed contact. Tunnel valleys are large, sinuous, steep‐sided incisions that, together with smaller scale meltwater channels, indicate subglacial meltwater discharge beneath large ice sheets. Through the use of high‐resolution marine geophysical data, a system of buried and exposed tunnel valleys, possible subglacial or proglacial meltwater channels and palaeo‐fluvial valleys have been identified across the shelf of the Celtic Sea between Ireland and Britain. The presence of steep‐sided and overdeepened tunnel valleys is indicative of a large channelized meltwater drainage system beneath the former Irish Sea Ice Stream, the most extensive ice stream to drain the last British–Irish Ice Sheet. After the rapid ice expansion across the Celtic Sea shelf around 28–26 ka, the tunnel valleys were carved into both bedrock and glacigenic sediments and are associated with rapid ice stream retreat northwards into the Irish Sea Basin between 25.6 and 24.3 ka. The presence of a major subglacial meltwater system on the relatively shallow shelf suggests that significant erosive meltwater discharge occurred during the last deglaciation and highlights the important contribution of meltwater to the retreat of the British–Irish Ice Sheet on the continental shelf.

Exploring controls of the early and stepped deglaciation on the western margin of the British Irish Ice Sheet

ABSTRACTNew optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE‐CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice‐streams (MSIS). Shelf‐edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf‐wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still‐stands. By ~23–22 ka the outer shelf (~25 000 km2) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a–1 vs. ~2–6 m a–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine‐terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice‐stream retreat from mid‐shelf to the coast.

Timing, pace and controls on ice sheet retreat: an introduction to the BRITICE‐CHRONO transect reconstructions of the British–Irish Ice Sheet

ABSTRACTMotivated to help improve the robustness of predictions of sea level rise, the BRITICE‐CHRONO project advanced knowledge of the former British–Irish Ice Sheet, from 31 to 15 ka, so that it can be used as a data‐rich environment to improve ice sheet modelling. The project comprised over 40 palaeoglaciologists, covering expertise in terrestrial and marine geology and geomorphology, geochronometric dating and the modelling of ice sheets and oceans. A systematic and directed campaign, organised across eight transects from the continental shelf edge to a short distance (10s of kilometres) onshore, was used to collect 914 samples which yielded 639 new ages, tripling the number of dated sites constraining the timing and rates of change of the collapsing ice sheet. This special issue synthesises these findings of ice advancing to the maximum extent and its subsequent retreat for each of the eight transects to produce definitive palaeogeographic reconstructions of ice margin positions across the marine to terrestrial transition. These results are used to understand the controls that drove or modulated ice sheet retreat. A further paper reports on how ice sheet modelling experiments and empirical data can be used in combination, and another probes the glaciological meaning of ice‐rafted debris.

Timing and pace of ice‐sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

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

Pattern, style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf

AbstractPredicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast‐flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice‐stream‐dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic‐nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector‐wide ice‐sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice‐sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice‐sheet sector undergoing large size variations in response to sub‐millennial‐scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine‐terminating ice‐sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice‐sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice‐sheet build‐up in NW Scotland, with the Minch ice‐stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding‐zone wedges and moraines on the mid‐ and inner continental shelf attest to significant stabilizations of the ice‐sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present‐day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea‐level c. 26 kabp, followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice‐shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp, as the grounding‐line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding‐line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp, ice‐sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine‐instability‐triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid‐Greenland Stadial 2, with an ice‐accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice‐sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp, before significant ice‐mass losses thereafter. At the glacial termination, c. 14.5 kabp, glaciers fed outwash sediment to now‐abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice‐sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice‐sheet glaciation dominated by ice‐streaming were interspersed with periods of much more restricted ice‐cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice‐sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice‐sheet health but also to the acceleration of ice‐sheet demise via marine ice‐sheet instabilities.