Geomorphological signature and flow dynamics of The Minch palaeo‐ice stream,northwest Scotland

Large‐scale streamlined glacial landforms are identified in 11 areas of northwest Scotland, from the Isle of Skye in the south to the Butt of Lewis in the north. These ice‐directional features occur in bedrock and superficial deposits, generally below 350 m above sea level, and where best developed have elongation ratios of >20:1. Sidescan sonar and multibeam echo‐sounding data from The Minch show elongate streamlined ridges and grooves on the seabed, with elongation ratios of up to 70:1. These bedforms are interpreted as mega‐scale glacial lineations. All the features identified formed beneath The Minch palaeo‐ice stream which was ca. 200 km long, up to 50 km wide and drained ca. 15 000 km² of the northwest sector of the last British‐Irish Ice Sheet (Late Devensian Glaciation). Nine ice‐stream tributaries and palaeo‐onset zones are also identified, on the basis of geomorphological evidence. The spatial distribution and pattern of streamlined bedforms around The Minch has enabled the catchment, flow paths and basal shear stresses of the palaeo‐ice stream and its tributaries to be tentatively reconstructed. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

The evolution of the terrestrial-terminating Irish Sea glacier during the last glaciation

Here we reconstruct the last advance to maximum limits and retreat of the Irish Sea Glacier (ISG), the only land-terminating ice lobe of the western British Irish Ice Sheet. A series of reverse bedrock slopes rendered proglacial lakes endemic, forming time-transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, optically stimulated luminescence (OSL) bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples. This chronology constrains what is globally an early build-up of ice during late Marine Isotope Stage 3 and Greenland Stadial (GS) 5, with ice margins reaching south Lancashire by 30 ± 1.2 ka, followed by a 120-km advance at 28.3 ± 1.4 ka reaching its 26.5 ± 1.1 ka maximum extent during GS-3. Early retreat during GS-3 reflects piracy of ice sources shared with the Irish-Sea Ice Stream (ISIS), starving the ISG. With ISG retreat, an opportunistic readvance of Welsh ice during GS-2 rode over the ISG moraines occupying the space vacated, with ice margins oscillating within a substantial glacial over-deepening. Our geomorphological chronosequence shows a glacial system forced by climate but mediated by piracy of ice sources shared with the ISIS, changing flow regimes and fronting environments.     

Eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank (Celtic Sea)

The architecture of a tidal sand bank in the south-eastern Celtic Sea was examined using very high-resolution seismic surveys. The bank comprises four depositional units. The lowest unit 1 is characterized by gently dipping (1–8°) strata that strike parallel to the length of the bank. Unit 1 is erosionally overlain by unit 2, which forms the bulk of the bank. This unit consists of stacked sets of downcurrent-dipping (7–12°) master bedding formed by climbing, sinuous-crested tidal dunes that are up to 20 m high. These deposits are locally incised by an anastomosed channel network (unit 3) that may represent a buried swatchway system. The upper part of the bank comprises wave-related deposits that are mainly preserved on the bank flanks (unit 4). The outer bank surface is erosional. The bank is believed to have formed during the last post-glacial sea-level rise. The facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy, mainly characterized by the thickening of dune cross-bed sets in unit 2. The majority of bank growth is inferred to have occurred in water depths of the order of 60 m. This evolution was controlled by relative sea-level rise, which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep, cross-cutting channels of unit 3. The transition to wave-dominated sedimentation in unit 4 is related to the decay of resonance with continued sea-level rise.     

New age constraints for the maximum extent of the last British–Irish Ice Sheet (NW sector)

This paper presents the first terrestrial age constraints from the outer continental shelf for the maximum extent of the NW sector of the last British–Irish Ice Sheet. Cosmogenic ¹⁰Be ages from eight glacially transported boulders on the island of North Rona show that the Late Devensian (Late Weichselian) British–Irish Ice Sheet overrode the island at its maximal stage and retreated c. 25 ka BP. These new dates, supported by other geological evidence, indicate that the north‐western part of the ice sheet was most extensive between 27 and 25 ka BP, reaching the outer continental shelf during the global eustatic sea‐level minimum at the Last Glacial Maximum. Copyright © 2012 British Geological Survey/Natural Environment Research Council copyright 2012. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

Topographically‐independent ice flow in northwestern British Columbia: implications for Cordilleran Ice Sheet reconstruction

Studies in southern British Columbia have shown that Cordilleran Ice Sheet flow was controlled by topograph, even in full glacial time. New ice‐flow evidence from the Nass River region, northern British Columbia, however, indicates that ice was thicker there and that the continental ice‐sheet phase of glaciation was reached. Inspection of high elevation sites has revealed a suite of ice‐flow indicators (mainly striae) undetected by earlier work. These suggest that at the Last Glacial Maximum (Fraser Glaciation), ice flowed southwestward across the Nass River region from an ice divide that probably was located in the Skeena Mountain area. Comparisons with adjacent work allow this divide to be mapped over a wide area. The results suggest that maximum ice thicknesses in the northern part of the Cordilleran Ice Sheet were larger than reported previously. The location of storm tracks in full glacial time may have played an important role in the production of an ice sheet that was thicker in northern British Columbia than it was in the southern half of the province. During deglaciation, ice thinned and gradually became confined to fiords and valleys, resulting in numerous and variable ice‐flow directions at that time. Topographic control was thus exerted on ice flow only after the glacial maximum was reached, despite the significant amount of relief in this region. Copyright © 2002 John Wiley & Sons, Ltd.     

Sea‐level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling

Models of glacio‐hydroisostatic sea‐level change have been published for the British Isles that are broadly consistent with the observational evidence, as well as with glaciological constraints. It has been argued, however, that the models fail to represent sea‐level change along the Irish Sea margins and in southern Ireland for the post‐deglaciation period. The argument rests on the interpretation of the depositional environment of the elevated ‘Irish Sea Drift’ on both sides of the Irish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the British Isles are consistent with the former interpretation in that sea‐levels on either side of the Irish Sea, south of about the Isle of Man, are not predicted to have risen above present sea‐level at any time since the deglaciation of the Irish Sea. This implies that ice over both the Irish Sea and Ireland was relatively thin (ca. 600–700 m over Ireland). If the glaciomarine interpretation of the elevated Irish Sea Drift is correct, then the maximum ice thickness over central and southern Ireland would have to reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea‐level change to be consistent with the Holocene evidence, this thick ice sheet could not have extended to the eastern side of the Irish Sea. Nor could it have been very thick at its northern and western limits. If such an ice model is extreme and incompatible with glaciological observations then the alternative is to accept the interpretation of the Irish Sea Drift as terrestrial in origin. Copyright © 2001 John Wiley & Sons, Ltd.     

Sedimentary evidence for deforming bed conditions associated with a grounded Irish Sea glacier,southern Ireland

Along the south coast of Ireland, a shelly diamict facies, the Irish Sea Till, has been variously ascribed to subglacial deposition by a grounded Irish Sea glacier or to glacimarine sedimentation by suspension settling and iceberg rafting. Observations are presented here from five sites along the south coast to directly address this question. At these sites, sedimentary evidence is preserved for the onshore advance of a grounded Irish Sea glacier, which glacitectonically disturbed and eroded pre‐existing sediments and redeposited them as deformation till. Recession of this Irish Sea glacier resulted in the damming of ice‐marginal lakes in embayments along the south coast, into which glacilacustrine sedimentation then took place. These lake sediments were subsequently glacitectonised and reworked by overriding glacier ice of inland origin, which deposited deformation till on top of the succession. There is no evidence for deposition of the Irish Sea diamicts by glacimarine sedimentation at these sites. The widespread development of subglacial deforming bed conditions reflected the abundance of fine‐grained marine and lacustrine sediments available for subglacial erosion and reworking. Stratigraphical and chronological data suggest that the advance of a grounded Irish Sea glacier along the south coast occurred during the last glaciation, and this is regionally consistent with marine geological data from the Celtic Sea. These observations demonstrate extension of glacier ice far beyond its traditional limits in the Celtic Sea and on‐land in southern Ireland during the last glaciation, and remove the stratigraphical basis for chronological differentiation of surficial glacial drifts, and thus the Munsterian Glaciation, in southern Ireland. Copyright © 2001 John Wiley & Sons, Ltd.     

A major ice drainage pathway of the last British–Irish Ice Sheet: the Tyne Gap,northern England

The Tyne Gap is a wide pass, situated between the Scottish Southern Uplands and the English Pennines that connects western and eastern England. It was a major ice flow drainage pathway of the last British–Irish Ice Sheet. This study presents new glacial geomorphological and sedimentological data from the Tyne Gap region that has allowed detailed reconstructions of palaeo‐ice flow dynamics during the Late Devensian (Marine Isotope Stage 2). Mapped lineations reveal a complex palimpsest pattern which shows that ice flow was subject to multiple switches in direction. These are summarised into three major ice flow phases. Stage I was characterised by convergent Lake District and Scottish ice that flowed east through the Tyne Gap, as a topographically controlled ice stream. This ice stream was identified from glacial geomorphological evidence in the form of convergent bedforms, streamlined subglacial bedforms and evidence for deformable bed conditions; stage II involved northerly migration of the Solway Firth ice divide back into the Southern Uplands, causing the easterly flow of ice to be weakened, and resulting in southeasterly flow of ice down the North Tyne Valley; and stage III was characterised by strong drawdown of ice into the Irish Sea Ice Basin, thus starving the Tyne Gap of ice and causing progressive ice sheet retreat westwards back across the watershed, prior to ice stagnation. Copyright © 2009 John Wiley & Sons, Ltd.     

Quaternary palaeoceanographic changes in the northern South China Sea (ODP Site 1146): radiolarian evidence

The abundance and accumulation rates of siliceous microfossils in the northern South China Sea, including radiolarians, diatoms and sponge spicules, increased during most glacial intervals within the past 1100 kyr. Similar trends are observed in the index of thermocline surface radiolarians (TSR), diatom accumulation rates (DAR), charcoal accumulation rates (CAR) and the abundance of radiolarian species Cycladophora davisiana davisiana. Decreasing sea‐surface temperature accompanied by increased seasonality since 900 ka is indicated by a decline in the tropical radiolarian assemblage, including Tetrapyle octacantha and Octopyle stenozona, and by an increase in the subtropical assemblage, including Pterocorys zancleus, Peromelissa phalacra and Ommatartuts tetrathalamus tetrathalamus. Rapid increases at about 800 to 700 ka of siliceous microfossils, charcoal, subsurface and intermediate radiolarians, as well as the TSR index and the DAR, imply a fundamental shift in climate and a shoaling thermocline. Although these fundamental changes in the silicious fauna and flora of the South China Sea take place within the context of a developing 100‐kyr cycle, they do not change in step with changing sea‐level as indicated by marine δ¹⁸O. This is most clearly illustrated by the step‐like increase in silica accumulation (radiolaria, diatoms and sponge spicules) at 680 ka. Rather, these fundamental changes probably reflect intensified surface productivity associated with enhanced East Asian winter‐monsoon circulation. Copyright © 2003 John Wiley & Sons, Ltd.     

Subglacial bedform geomorphology of the Irish Ice Sheet reveals major configuration changes during growth and decay

The belated realisation that ribbed (Rogen) moraines form such an integral part of Irish geomorphology, and the piecemeal approach to previous drumlin mapping, is probably responsible for the highly contrasting views of palaeoflow patterns of the Irish Ice Sheet. Using a high resolution (25 m) digital elevation model we present morphological maps of a large part (100 × 100 km) of the so‐called ‘Drumlin Belt’ of north central Ireland. The landforms comprise mostly ribbed moraine much larger than found elsewhere (up to 16 km in length), which in places are superimposed on each other. Contrary to most prior assessments we find the bedform record to contain numerous and overlapping episodes of bed formation (ribbed moraine, drumlins and crag‐and‐tails) that provide a palimpsest record of changing flow geometries. These demonstrate an ice sheet with a centre of mass and flow geometry that changed during growth and decay. Using distinctive flow patterns and relative age relationships between them we reconstruct ice sheet evolution into four phases during a single glacial cycle. In phase 1 (early in the glacial cycle), Scottish and local ice coalesced to form a northeast‐centred Irish Ice Sheet. As it grew its centre of mass migrated southwards, culminating in a major N–S divide positioned down the east of Ireland (phase 2, ca. Last Glacial Maximum). During retreat, the centre of mass migrated at least 120 km northwards and became established in northwest Ireland and at this point a dramatic bedforming event produced one of the world's largest and most contiguous ribbed moraine fields (phase 3). Final deglaciation is thought to be by fragmentation into many topographically controlled minor ice‐caps (phase 4). Rather than any dramatic or unexpected behaviour, the reconstructed phases indicate a relatively predictable pattern of ice sheet growth and decay with changes in centres of mass, and does not require major readvances or ice‐stream events. Copyright © 2001 John Wiley & Sons, Ltd.     

New relative sea-level curves for the southern Scott Coast,Antarctica: evidence for Holocene deglaciation of the western Ross Sea

Here we present new relative sea-level (RSL) curves developed from Holocene-aged raised beaches along the southern Scott Coast of the western Ross Sea, Antarctica. Fifty-four dates of marine shells, seal skin and elephant seal remains incorporated within raised beaches during storms afford a chronology for these curves. All of the curves show the same pattern and timing of RSL change within a small range of error. The best-dated curve suggests that final unloading of grounded Ross Sea ice from the southern Scott Coast and McMurdo Sound region occurred shortly before 6500 ¹⁴C yr BP. This age is consistent with glacial geological evidence that places deglaciation between 5730 and 8340 ¹⁴C yr BP. Our data strongly suggest that grounding-line retreat of the Ross Sea ice sheet southward through the McMurdo Sound region occurred in mid- and late Holocene time. If this is correct, then rising sea level could not have driven ice recession to the present-day grounding line on the Siple Coast, because global deglacial sea-level rise was essentially accomplished by mid-Holocene time. Copyright © 1999 John Wiley & Sons, Ltd.     

Morphological and sedimentary responses to ice mass interaction during the last deglaciation

During decline of the last British–Irish Ice Sheet (BIIS) down‐wasting of ice meant that local sources played a larger role in regulating ice flow dynamics and driving the sediment and landform record. At the Last Glacial Maximum, glaciers in north‐western England interacted with an Irish Sea Ice Stream (ISIS) occupying the eastern Irish Sea basin (ISB) and advanced as a unified ice‐mass. During a retreat constrained to 21–17.3 ka, the sediment landform assemblages lain down reflect the progressive unzipping of the ice masses, oscillations of the ice margin during retreat, and then rapid wastage and disintegration. Evacuation of ice from the Ribble valley and Lancashire occurred first while the ISIS occupied the ISB to the west, creating ice‐dammed lakes. Deglaciation, complete after 18.6–17.3 ka, was rapid (50–25 m a^?1), but slower than rates identified for the western ISIS (550–100 m a^?1). The slower pace is interpreted as reflecting the lack of a calving margin and the decline of a terrestrial, grounded glacier. Ice marginal oscillations during retreat were probably forced by ice‐sheet dynamics rather than climatic variation. These data demonstrate that large grounded glaciers can display complex uncoupling and realignment during deglaciation, with asynchronous behaviour between adjacent ice lobes generating complex landform records.

     

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

Compilation 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.     

The last British Ice Sheet: A review of the evidence utilised in the compilation of the Glacial Map of Britain

This paper reviews the evidence presently available (as at December 2003) for the compilation of the Glacial Map of Britain (see [Clark C.D., Evans D.J.A., Khatwa A., Bradwell T., Jordan C.J., Marsh S.H., Mitchell W.A., Bateman, M.D. , 2004. Map and GIS database of glacial landforms and features related to the last British Ice Sheet. Boreas 33, 359–375] and http://www.shef.ac.uk/geography/staff/clark_chris/britice.html) in an effort to stimulate further research on the last British Ice Sheet and promote a reconstruction of ice sheet behaviour based on glacial geology and geomorphology. The wide range of evidence that has been scrutinized for inclusion on the glacial map is assessed with respect to the variability of its quality and quantity and the existing controversies in ice sheet reconstructions. Landforms interpreted as being of unequivocal ice-marginal origin (moraines, ice-contact glacifluvial landforms and lateral meltwater channels) and till sheet margins are used in conjunction with available chronological control to locate former glacier and ice-sheet margins throughout the last glacial cycle. Subglacial landforms (drumlins, flutings and eskers) have been used to demarcate former flow patterns within the ice sheet. The compilation of evidence in a regional map is crucial to any future reconstructions of palaeo-ice sheet dynamics and will provide a clearer understanding of ice sheet configuration, ice divide migration and ice thickness and coverage for the British Ice Sheet as it evolved through the last glacial cycle.     

Geomorphological evidence from the Lleyn Peninsula constraining models of the magnitude and rate of isostatic rebound during deglaciation of the Irish Sea Basin

A stepped series of sand and gravel terraces on the Lleyn peninsula of North Wales is used to test the magnitude and rate of isostatic depression required by the recently proposed glacimarine model of deglaciation of the Irish Sea Basin. A relative sea-level fall of 70 m is required while the ice remained pinned at the north Lleyn coast. Even taking the maximum known rate of isostatic uplift, the margin would have to remain stationary for 1400 years. It seems more reasonable to interpret the Lleyn terraces, and similar features around the Irish Sea Basin, as glacifluvial and glacilacustrine.     

Late Devensian deglaciation of the Tyne Gap Palaeo‐Ice Stream,northern England

The deglacial history of the central sector of the last British–Irish Ice Sheet is poorly constrained, particularly along major ice‐stream flow paths. The Tyne Gap Palaeo‐Ice Stream (TGIS) was a major fast‐flow conduit of the British–Irish Ice Sheet during the last glaciation. We reconstruct the pattern and constrain the timing of retreat of this ice stream using cosmogenic radionuclide (¹⁰Be) dating of exposed bedrock surfaces, radiocarbon dating of lake cores and geomorphological mapping of deglacial features. Four of the five ¹⁰Be samples produced minimum ages between 17.8 and 16.5 ka. These were supplemented by a basal radiocarbon date of 15.7 ± 0.1 cal ka BP, in a core recovered from Talkin Tarn in the Brampton Kame Belt. Our new geochronology indicates progressive retreat of the TGIS from 18.7 to 17.1 ka, and becoming ice free before 16.4–15.7 ka. Initial retreat and decoupling of the TGIS from the North Sea Lobe is recorded by a prominent moraine 10–15 km inland of the present‐day coast. This constrains the damming of Glacial Lake Wear to a period before ∼18.7–17.1 ka in the area deglaciated by the contraction of the TGIS. We suggest that retreat of the TGIS was part of a regional collapse of ice‐dispersal centres between 18 and 16 ka.

     

New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly

The southernmost terrestrial extent of the Irish Sea Ice Stream (ISIS), which drained a large proportion of the last British–Irish Ice Sheet, impinged on to the Isles of Scilly during Marine Isotope Stage 2. However, the age of this ice limit has been contested and the interpretation that this occurred during the Last Glacial Maximum (LGM) remains controversial. This study reports new ages using optically stimulated luminescence (OSL) dating of outwash sediments at Battery, Tresco (25.5 ± 1.5 ka), and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on Scilly Rock (25.9 ± 1.6 ka), which confirm that the ISIS reached the Isles of Scilly during the LGM. The ages demonstrate this ice advance on to the northern Isles of Scilly occurred at ∼26 ka around the time of increased ice‐rafted debris in the adjacent marine record from the continental margin, which coincided with Heinrich Event 2 at ∼24 ka. OSL dating (19.6 ± 1.5 ka) of the post‐glacial Hell Bay Gravel at Battery suggests there was then an ∼5‐ka delay between primary deposition and aeolian reworking of the glacigenic sediment, during a time when the ISIS ice front was oscillating on and around the Llŷn Peninsula, ∼390 km to the north. Copyright © 2017 The Authors. Journal of Quaternary Science Published by John Wiley & Sons, Ltd.

     

渤海和黄海北部潮间带底质中的介形虫

间带不同性质的底质中 ,介形虫动物群的特征和生态分布各不相同。在岩石滩和沙滩中介形虫较丰富、多样 ,前者具有 Xestoleberishanaii,Cythere lutea lutea,Loxoconcha hattorii,Aurila cymba等 ,而后者有 Sinocytheridea impressa,Pistocythereis bradyformis,Bicornucytherebisanensis,Cushmanidea subjaponica等。泥质滩中介形虫通常很少 ,有 Loxoconcha binhaiensis和L.ocellata等。受高潮和中潮影响的河口区 ,通常介形虫丰度和多样性的分布较低 ,都是适应环境变化很强的属种 ,并具有海、陆相介形虫分布的混合区段。朝向海洋方向 ,非海相介形虫的百分含量呈逐渐降低变化 ,而海相介形虫正好相反。