Late Weichselian Glacial Maximum Research Articles

Resetting of sediments mobilised by the LGM ice-sheet in southern Norway

Former geological field investigations in the Rondane area, east-central southern Norway, have proposed that the maximum Fennoscandian ice-sheet coverage occurred during the Late Weichselian Glacial Maximum (LGM, ca. 20 ka) and that subsequent glaciofluvial sediments were first deposited in the early Holocene (after 10 ka). However, recent field investigations with ages from three internally consistent quartz optically stimulated luminescence (OSL) age series show an apparent deglaciation of northern Rondane in the period 20.0–13.8 ka. We examine here the possibility that these ages are too old because the sediment was not completely zeroed prior to deposition. Our investigations of incomplete bleaching use modern analogues, small aliquots, and single grains of quartz. First, the symmetric shape of small aliquot equivalent dose distributions suggests that the sediment was probably well bleached at deposition. This is supported by 5 modern analogue equivalent doses ( D e) of 0.6 Gy, ∼1.5% of the typical D e from the deglaciation sediments. Finally, from single grain studies on three samples, we conclude that there is no evidence for poor bleaching in these samples; thus the weighted mean gives the best estimates of D e, and these are completely consistent with both large and small aliquot estimates for these samples. These comparisons between large aliquots, modern analogues, small aliquots and single-grain analyses help to validate the OSL ages and confirm the complete resetting of these sediments prior to deposition.

Late Quaternary ice sheet history of northern Eurasia

The maximum limits of the Eurasian ice sheets during four glaciations have been reconstructed: (1) the Late Saalian (>140 ka), (2) the Early Weichselian (100–80 ka), (3) the Middle Weichselian (60–50 ka) and (4) the Late Weichselian (25–15 ka). The reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in Russia and Siberia, and with marine seismic- and sediment core data. The Barents-Kara Ice Sheet got progressively smaller during each glaciation, whereas the dimensions of the Scandinavian Ice Sheet increased. During the last Ice Age the Barents-Kara Ice Sheet attained its maximum size as early as 90–80,000 years ago when the ice front reached far onto the continent. A regrowth of the ice sheets occurred during the early Middle Weichselian, culminating about 60–50,000 years ago. During the Late Weichselian the Barents-Kara Ice Sheet did not reach the mainland east of the Kanin Peninsula, with the exception of the NW fringe of Taimyr. A numerical ice-sheet model, forced by global sea level and solar changes, was run through the full Weichselian glacial cycle. The modeling results are roughly compatible with the geological record of ice growth, but the model underpredicts the glaciations in the Eurasian Arctic during the Early and Middle Weichselian. One reason for this is that the climate in the Eurasian Arctic was not as dry then as during the Late Weichselian glacial maximum.

Arctic Ocean deep-sea record of northern Eurasian ice sheet history

The sediment composition of deep-sea cores from the central Arctic Ocean, the Fram Strait, and the Yermak Plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200,000 years. Layers with high amounts of coarse, terrigenous ice-rafted debris (IRD) and often high contents of smectite were deposited during extensive glaciations in northern Eurasia, when ice sheets reached the northern continental margins of the Barents and Kara seas and discharged icebergs into the Arctic Ocean. Intercalated layers with relatively low IRD and smectite contents, but abundant planktic foraminifers in the coarse fraction were deposited during periods of Atlantic Water inflow to the Arctic Ocean and seasonally open waters (leads) in a sea ice cover with only few icebergs in the Arctic Ocean. High IRD contents in the sediments reflect the presence of ice sheets on the Kara and Barents seas shelves and the hinterland during the entire oxygen isotope stage 6 (ca 190–130 ka), in substage 5b (ca 90–80 ka), at the stage boundary 5/4 (around 75 ka), and in late stage 4/early stage 3 (ca 65–50 ka). These results are in excellent correlation with those from recent field work in northern Scandinavia, European Russia, Siberia, and on the shelves. Relatively low amounts of IRD in central Arctic Ocean sediments from the Late Weichselian glacial maximum (ca 24–18 ka) correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time. Oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the Arctic Ocean. The synchroneity of freshwater events and IRD input suggests a common source. Strongest events were associated with deglaciations of the Barents and Kara seas after the ice sheets had blocked the outflow of large rivers for several millennia. The outflow of freshwater from large ice-dammed lakes occurred at ca 130, 80–75, and 52 ka. Freshwater events in the central Arctic Ocean during the last deglaciation (ca 18 ka) were relatively small compared to the previous events. This indicates that during most of the Late Weichselian glacial maximum a river outflow from northern Siberia to the Arctic Ocean was possible. Atlantic Water inflow to the Arctic Ocean and seasonally open waters in the ice (leads) occurred during the interglacials of oxygen isotope stage 1 and substage 5e, during several interstadials (stage 3, substages 5a and 5c), and to a lesser degree within stadials and glacials (stages 2, 4, and 6). With the exception of the interglacials, these periods were times of strong ice growth on the continents as revealed by terrestrial data. The coincidence suggests that open waters in the Arctic Ocean and the Nordic Seas were an important moisture source (in addition to more southerly sources) which fostered the growth of ice sheets on northern Eurasia.

Late Quaternary ice sheet history of northern Eurasia

The maximum limits of the Eurasian ice sheets during four glaciations have been reconstructed: (1) the Late Saalian (>140 ka), (2) the Early Weichselian (100–80 ka), (3) the Middle Weichselian (60–50 ka) and (4) the Late Weichselian (25–15 ka). The reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in Russia and Siberia, and with marine seismic- and sediment core data. The Barents-Kara Ice Sheet got progressively smaller during each glaciation, whereas the dimensions of the Scandinavian Ice Sheet increased. During the last Ice Age the Barents-Kara Ice Sheet attained its maximum size as early as 90–80,000 years ago when the ice front reached far onto the continent. A regrowth of the ice sheets occurred during the early Middle Weichselian, culminating about 60–50,000 years ago. During the Late Weichselian the Barents-Kara Ice Sheet did not reach the mainland east of the Kanin Peninsula, with the exception of the NW fringe of Taimyr. A numerical ice-sheet model, forced by global sea level and solar changes, was run through the full Weichselian glacial cycle. The modeling results are roughly compatible with the geological record of ice growth, but the model underpredicts the glaciations in the Eurasian Arctic during the Early and Middle Weichselian. One reason for this is that the climate in the Eurasian Arctic was not as dry then as during the Late Weichselian glacial maximum.

Arctic Ocean deep-sea record of northern Eurasian ice sheet history

The sediment composition of deep-sea cores from the central Arctic Ocean, the Fram Strait, and the Yermak Plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200,000 years. Layers with high amounts of coarse, terrigenous ice-rafted debris (IRD) and often high contents of smectite were deposited during extensive glaciations in northern Eurasia, when ice sheets reached the northern continental margins of the Barents and Kara seas and discharged icebergs into the Arctic Ocean. Intercalated layers with relatively low IRD and smectite contents, but abundant planktic foraminifers in the coarse fraction were deposited during periods of Atlantic Water inflow to the Arctic Ocean and seasonally open waters (leads) in a sea ice cover with only few icebergs in the Arctic Ocean. High IRD contents in the sediments reflect the presence of ice sheets on the Kara and Barents seas shelves and the hinterland during the entire oxygen isotope stage 6 (ca 190–130 ka), in substage 5b (ca 90–80 ka), at the stage boundary 5/4 (around 75 ka), and in late stage 4/early stage 3 (ca 65–50 ka). These results are in excellent correlation with those from recent field work in northern Scandinavia, European Russia, Siberia, and on the shelves. Relatively low amounts of IRD in central Arctic Ocean sediments from the Late Weichselian glacial maximum (ca 24–18 ka) correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time. Oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the Arctic Ocean. The synchroneity of freshwater events and IRD input suggests a common source. Strongest events were associated with deglaciations of the Barents and Kara seas after the ice sheets had blocked the outflow of large rivers for several millennia. The outflow of freshwater from large ice-dammed lakes occurred at ca 130, 80–75, and 52 ka. Freshwater events in the central Arctic Ocean during the last deglaciation (ca 18 ka) were relatively small compared to the previous events. This indicates that during most of the Late Weichselian glacial maximum a river outflow from northern Siberia to the Arctic Ocean was possible. Atlantic Water inflow to the Arctic Ocean and seasonally open waters in the ice (leads) occurred during the interglacials of oxygen isotope stage 1 and substage 5e, during several interstadials (stage 3, substages 5a and 5c), and to a lesser degree within stadials and glacials (stages 2, 4, and 6). With the exception of the interglacials, these periods were times of strong ice growth on the continents as revealed by terrestrial data. The coincidence suggests that open waters in the Arctic Ocean and the Nordic Seas were an important moisture source (in addition to more southerly sources) which fostered the growth of ice sheets on northern Eurasia.

Late Quaternary palaeoenvironment and chronology in the Trænadjupet Slide area offshore Norway

The northern mid-Norwegian continental slope was studied based on high-resolution side-scan sonar data, multibeam bathymetry, high-resolution and multichannel seismics together with gravity cores. Sedimentary provinces identified include a partly buried slide on the eastern, inner Vøring Plateau, an area dominated by glacigenic debris flows south-west of the Trænadjupet Slide, the Trænadjupet Slide, and an area of glacimarine sedimentation and a slide scar north-east of the Trænadjupet Slide. The Trænadjupet Slide affected an area of about 14 100 km 2 and mobilised about 900 km 3 of sediments. Little is known about the areal extent and volume of the older events. The glacigenic debris flows and glacimarine sediments were deposited while the Fennoscandian ice sheet was at the shelf break during the late Weichselian glacial maximum (prior to 13.2 14C kyr BP). Hemipelagic and/or contouritic sedimentation prevailed during the Holocene period. Two large slide scars were probably formed sometime prior to or during the late Weichselian glacial maximum (inner Vøring Plateau and north-east of the Trænadjupet Slide) and another during the mid-Holocene interglacial period immediately prior to 4000 14C kyr BP (the Trænadjupet Slide). The two older scars may represent one event or two separate events. Deposition of poorly permeable glacigenic sediments over high-water-content fine-grained hemipelagic and/or contourites may have prevented water escape and increased failure potential. Thus continental slope areas of episodically high sediment input of glacigenic sediments are prone to failure as illustrated by this study, which has identified at least two large slope failures. Failures have occurred both during glacial maxima, periods of climate deterioration and low global eustatic sea level, and during interglacials as today with improved climatic conditions and a high global eustatic sea level.

An isostatic test of the hypothesis of ice-free mountain areas during the last glaciation

There has long been a discussion on whether parts of the highest mountains of Norway were totally ice-covered, or if ice-free areas existed during the Late Weichselian glacial maximum. In this study the hypothesis of a very thin ice sheet was tested by modelling the isostatic response, using an Earth model with layered mantle viscosity overlain by an elastic lithosphere. The theoretical pattern of the present rate of uplift and the tilting history for the western coast of Norway based on a thin ice model show significant deviations from the observations, which seems to rule out the thin ice model as a viable option. However, this is based on the assumption that the present Fennoscandian uplift is caused by glacial isostasy alone. If there really existed large ice-free areas in the highest mountains of Norway in the last glacial maximum, the present Fennoscandian uplift must be ascribed to another mechanism in addition to glacial isostasy.

Maximum extent of the Eurasian ice sheets in the Barents and Kara Sea region during the Weichselian

Based on field investigations in northern Russia and interpretation of offshore seismic data, we have made a preliminary reconstruction of the maximum ice‐sheet extent in the Barents and Kara Sea region during the Early/Middle Weichselian and the Late Weichselian. Our investigations indicate that the Barents and Kara ice sheets attained their maximum Weichselian positions in northern Russia prior to 50 000 yr BP, whereas the northeastern flank of the Scandinavian Ice Sheet advanced to a maximum position shortly after 17000 calendar years ago. During the Late Weichselian (25 000–10000 yr BP), much of the Russian Arctic remained ice‐free. According to our reconstruction, the extent of the ice sheets in the Barents and Kara Sea region during the Late Weichselian glacial maximum was less than half that of the maximum model which, up to now, has been widely used as a boundary condition for testing and refining General Circulation Models (GCMs). Preliminary numerical‐modelling experiments predict Late Weichselian ice sheets which are larger than the ice extent implied for the Kara Sea region from dated geological evidence, suggesting very low precipitation.

Terrestrial biotas and environmental changes during the late Middle Weichelian in north Jutland, Denmark

Studies of glaciotectonized sub-till lacustrine deposits in Lønstrup Klint and Lodbjerg Klint, northern Jylland, Denmark, show the presence of plant and animal remains from the Middle Weichselian "mammoth steppe". Radiocarbon dates on picked-out plant fragments yield ages around 30 ka BP, and a macrofossil plant assemblage in the lake sediments indicates that uplands were covered by an open, tree-less vegetation with herbs and scattered shrubs. Mean July temperatures were probably at least 6°C lower than at present. The datings suggest a change from marine to glacio-lacustrine environments in Vendsyssel slightly before 30 ka BP. Glacial thrusting and subsequent deposition of the youngest till in the region was connected with the Late Weichselian glacial maximum, the Jylland stade, and glaciation commenced some time after 30 ka BP.

The retreat of the Barents Sea Ice Sheet on the western Svalbard margin

The deglaciation of the continental shelf to the west of Spitsbergen and the main fjord, Isfjorden. is discussed based on sub‐bottom seismic records and scdirncnt cores. The sea lloor on the shelf to the west of Isfjorden is underlain by less than 2 m of glaciomarine sediments over a firm diamicton interpreted as till. In central Isfjordcn up to 10 m of deglaciation sediments were recorded, whereas in cores from the innermost tributary, Billefjorden, less than a meter of ice proximal sediments was recognized between the till and the ‘normal’ Holocene marine sediments. We conclude that the Barents Sea Ice Sheet terminated along the shelf break during the Late Weichselian glacial maximum. Radiocarbon dates from thc glaciomarine sediments above the till indicate a stepwise deglaciation. Apparently the ice front rctrcatcd from the outermost shelf around 14. 8 ka A dramatic increase in the flux of line‐grained glaciomarine sediments around 13 ka is assumed to reflect increased melting and/or current activity due to a climatic warming. This second stage of deglaciation was intcrruptcd by a glacial readvance culminating on the mid‐shelf area shortly after 12.4 ka. The glacial readvance, which is correlated with a simultaneous readvance of the Fennoscundian ice sheet along the western coast of Norway, is attributed to the so‐called ‘Older Dryas’ cooling event in the North Atlantic region. Following this glacial readvance the outer part of Isljorden became rapidly deglaciated around 12.3 ka. During the Younger Dryas the inner fjord branches were occupied by large outlet glaciers and possibly the ice liont terminated far out in the main fjord. The remnants of the Harcnts Sea Ice Shcet melted quickly away as a response to the Holocene warming around 10 ka.

Deglaciation history and palaeoceanography of the western Spitsbergen margin since the last glacial maximum

Abstract Two high resolution cores from the Spitsbergen margin were studied to increase the understanding of the palaeoceanography and deglaciation of the Spitsbergen continental margin. Planktonic and benthic foraminiferal δ 18 O records along with foraminiferal abundance counts and dropstone analysis were used to interpret the palaeoceanography of the Spitsbergen margin and to link the marine record with deglaciation of the Barents Sea Ice Sheet. High foraminiferal and dropstone abundances during the Late Weichselian glacial maximum show that this period was characterized by seasonally ice-free conditions. The initial deglaciation recognized along the Spitsbergen margin is dated at 14.1 ka BP and represents the break up of the Barents Sea Ice Sheet, though some input from the Spitsbergen ice dome is likely. This initial deglaciation was produced by glacio-isostatic relative sea-level rise and/or by increased solar insolation after the minimum of the Late Weichselian. The first unequivocal deglaciation of Spitsbergen ice occurred at 13 ka, whereas the oceanographic regime was still dominated by polar waters. The beginning of the Holocene was heralded by a second deglaciation phase of the Spitsbergen ice mass. The first influx of North Atlantic waters causing rapid iceberg calving and melting was recorded at this time. Dropstone input virtually ended at approximately 9 ka BP when most glaciers had retreated beyond their present day positions. A short relatively cool episode produced by the retreat of the North Atlantic waters during the early Holocene was recorded along the Spitsbergen margin.

Late Pleistocene stratigraphy, glaciation chronology and Middle Weichselian environmental history from Klintholm, Møn, Denmark

A new and fundamental outline of the Late Pleistocene geological history in SE Denmark is obtained from examination of exposed cliff sections along the Baltic coast of M0n. Multi­diciplinary studies including lithostratigraphic classification, analyses of glaciotectonic struc- tures, biostratigraphic studies, amino acid chronology and radiocarbon and luminescence dating indicate the following: A shallow boreo-lusitanian sea that characterized the Eemian interglaciation (130-115 ka BP) was followed by periglacial conditions during the Early Weichselian (l 15-75 ka BP). In the early part of the Middle Weichselian (75--c. 40 ka BP) Baltic glaciers invaded the region twice, intenupted by an ice free and periglacial interval around 50 ka BP. The first ice stream left a reddish coloured till dominated by exotic rock-types of eastern Baltic provenance. From about 40-35 ka BP to slightly before 20 ka BP ameliorated conditions with periglacial terrestrial and lacustrine environments are recorded. A lake basin in the western part of the Baltic was surrounded by a low releif mammoth-steppe with an almost treeless vegetation of grasses, sedges, heather and dwarf shrubs. Increasing amounts of diamicton dominated by Palaeozoic shale and limestone towards the top of the muddy and slightly organic lake sediments suggest ice-rafting in the basin by the end of the late Middle Weichselian. Deposits suffered strong glaciotectonic deformation during the Late Weichselian glacial maximum (25-15 ka BP).

Radiocarbon Dating Organic Detritus: Implications for Studying Ice Sheet Dynamics

We present here a description of the 14C dating method used at the Institute of Geology, Estonian Academy of Science. We discuss results of geochronological studies of several stratigraphic sections, from which we estimate the age of the Late Weichselian (Late Valdaian) glacial maximum. 14C and paleobotanical data indicate that biodetrital materials comprise organic debris from various sources and suggest only a maximum age of investigated strata (16,000 BP).

Weichselian stratigraphy and palaeoenvironments at Bellsund, western Svalbard

A coastal cliff facing the ocean at the west coast of Spitsbergen has been studied, and seven formations of Weichselian and Holocene age have been identified. A reconstruction of the palaeoenvironment and glacial history shows that most of the sediments cover isotope stage 5. From the base of the section, the formation 1 and 2 tills show a regional glaciation that reached the continental shelf shortly after the Eemian. Formation 3 consists of glacimarine to marine sediments dated to 105,000–90,000 BP. Amino acid diagenesis indicates that they were deposited during a c. 10,000‐year period of continuous isostatic depression, which indicates contemporaneous glacial loading in the Barents Sea. Foraminifera and molluscs show influx of Atlantic water masses along the west coast of Svalbard at the same time. Local glaciers advanced during the latter part of this period, probably due to the penetration of moist air masses, and deposited formation 4. A widespread weathering horizon shows that the glacial retreat was succeeded by subaerial conditions during the Middle Weichselian. Formation 5 is a till deposited during the Late Weichselian glacial maximum in this area. The glaciation was dominated by ice streams from a dome over southern Spitsbergen, and the last deglaciation of the outer coast is dated to 13,000 BP. A correlation of the events with other areas on Svalbard is discussed, and at least two periods of glaciation in the Barents Sea during the Weichselian are suggested.

Late Weichselian glacial maximum on Andøya, North Norway

Morphological, seismic and lithostratigraphic investigations of a moraine deposit at Bleik (the Bleik moraine), northern Andøya, show short‐distance transported till overlying long‐distance transported predominantly glaciofluvial ice‐marginal deposits. Consolidated glaciomarine sediments from a core at present sea‐level, c. 400 m distally to the moraine complex, contain 31 species of foraminifera, among which Cassidulina reniforme, Islandiella helenae and Trifarina fluens dominate, and fragments of the molluscs Mya truncata and Astarte sp. and the echinoid Strongylocentrotus sp. Amino acid analyses of the foraminifera Cibicides lobatulus and the mollusc Mya truncata indicate ages between 22,000 and 16,000 BP. Radiocarbon dating of fragments of Mya truncata from the upper part of the core gave an age of 17,940 ± 245 BP, while a dating of unidentified shell fragments from the lower part gave an infinite age (>40,000 BP). The sediment was probably disturbed by icebergs beyond the end moraine zone, and the radiocarbon and amino acid dating of Mya truncata therefore represent a maximum age for this process. This new evidence indicates two phases with a higher relative sea‐level than at present at Bleik, c. 18,000 and >40,000 BP. The Bleik moraine probably represents the early Late Weichselian glacial maximum (c. 22,000 BP), while the underlying deglaciation deposit and associated beach‐ridge (Bruvollen) is of pre‐Late Weichselian age. Moraine ridges 3–4 km to the south of Bleik probably indicate advances of local glaciers between 22,000 and 18,000 BP.

The Late Weichselian glacial maximum on western Spitsbergen inferred from offshore sediment cores

The Late Weichselian glacial history of the continental shelf off western Spitsbergen is discussed, based on acoustic sub-bottom records and sediment cores. The outer part of Isfjorden and the inner shelf to the west of this fjord are characterized by a thin veneer (10–20 m) of glacigenic sediments and absence of ice-marginal features. Towards the outer shelf the sediment thickness increases significantly, and exceeds 500 m at the shelf edge. Possible moraine complexes were identified in this outer part. Sediment cores from the inner shelf sampled a firm diamicton, interpreted as till, beneath soft glaciomarine sediments. Radiocarbon dates on shells from the clay resting directly on the till, suggest an age of around 12,500 yrs B.P. for the base of the marine sequence. We argue that grounded ice covered the sites shortly before. In contrast to suggestions that the fjords and coast were partly ice free during the Late Weischselian, we conclude that the ice must have reached out onto the continental shelf.

Paleoclimatic inferences from glacial fluctuations on Svalbard during the last 20 000 years

The climate history of western Spitsbergen, Svalbard is deduced from variations of glaciers during the last 20 000 years. A major depression of the regional equilibrium line altitude (ELA) occurred during the Late Weichselian glacial maximum (18000–13000y ago) when low summer temperatures may have caused year-round snow accumulation on the ground. This rapid expansion of the glaciers also indicates nearby moisture sources, suggesting partly open conditions in the Norwegian Sea during the summers. A rapid glacial retreat around 13 000–12 500 y BP was caused by a sudden warming. During the Younger Dryas the ELA along the extreme western coast of Spitsbergen was not significantly lower than at present. In contrast to Fennoscandia, the British Isles and the Alps, there is no evidence for readvance of local glaciers during Younger Dryas on western Spitsbergen. This difference is attributed to a much dryer climate on Spitsbergen and probably only slight changes in sea surface temperatures. In addition, summer melting in this high arctic area is more sensitive to orbitally increased insolation. Around 10 000 y BP another rapid warming occurred and during early and mid Holocene the summer temperatures were significantly higher than at present. A temperature decline during the late Holocene caused regrowth of the glaciers which reached their maximum Holocene position during the last century.

The Kapp Ekholm section, Billefjorden, Spitsbergen: a discussion

A radiocarbon date of 46,300 was obtained on shells in living position above the till exposed near Kapp Ekholm that G. S. Boulton (1979) dated at younger than 11,000 years B.P., his Billefjorden Stage, and which he assumed represented the Late Weichselian glacial maximum. According to a correlation of earlier published descriptions, J. A. Lavrushin (1969) had previously obtained similar ‘old’ dates, while L. S. Troitsky et al. (1979) obtained TL‐dates of 47,000 and 53,000 above this till. These dates require a reassessment of the timing of the Billefjorden Stage.

˝Stagnant-Ice˝ Topography and its Relation to Drumlin Genesis, with Reference to South-Central Ulster (Abstract Only)

Previous work on drumlin genesis considers the problem at the ice-rock interface and ignores the environmental significance of inter-drumlin sediments and superimposed landform associations. Present work by the author examines two drumlin fields in south-central Ulster, Northern Ireland, and proposes a mechanism of drumlin formation which is time-dependent and results from changes of thermal regime through time.Drumlin exposures in south-central Ulster reveal a particular facies association in drumlin tills showing considerable lithological variation; striae, roches moutonnées, and basal lodgement till beneath drumlins indicate an event early in the last glaciation characterized by a temperate thermal regime. This association is overlain by a basal melt-out till, characterized by striated clasts and containing numerous pockets of stratified sediment. The melt-out till is related to changes in provenance of erratics in the till, associated with a shift in the centre of ice accumulation during the course of glaciation.The stratigraphy suggests that a considerable amount of drift was formed prior to drumlin formation and that a change of thermal state may have characterized the shift in the ice accumulation zone. This change is also suggested by evidence of high-level melt-water transport in the Sperrin Mountains, indicating a sub-polar or sub-temperate thermal regime existing in the early part of the late Weichselian deglaciation.A model based on thermal convection theory of polar ice sheets (Hughes 1976) is proposed in an attempt to relate the juxtaposition of drumlins and glaciofluvial assemblages in southcentral Ulster. In an ice accumulation zone, as characterized this area during the late Weichselian glacial maximum, advection is generally absent and convection is the dominant flow in the ice sheet. In this environment motive ice may develop in the basal zone, resulting from vertical buoyancy stresses acting downward on a column in the ice sheet. This flow environment may coexist within an ice sheet which is "stagnant" and perhaps subject to supraglacial insolation melting, depending on the overall climatic regime. If convection is dominant, it may create areas of basal compression and tension, acting to establish co-existing zones of "hot" and "cold" ice; hot-ice zones are subject to basal erosion, leading to migration of subglacial sediments to points of englacial storage above the density inversion layer. Cold-ice zones act to protect underlying sediments and result in isolation of drift nuclei, protection being afforded by the yield strength of frozen drift being greater than either debris-laden or clean ice.If the model is tenable, it seems likely that drift nucleation in drumlin fields may have been accomplished before the drumlin formation or streamlining event. Vertical migration of subglacial materials also explains the similarity of sediment sources of glacigenic and glaciofluvial assemblages which occur in juxtaposition, and, without reduction in drumlin density. The sequence of events in south-central Ulster may be summarized as follows: (1)Build-up of ice, associated with sub-drumlin erosion forms and basal lodgement till. Change of thermal regime from temperate to sub-temperate.(2)Shift in the ice-accumulation zone from highland to lowland, indicated by erratic dispersals of glaciogenic sediments. Change of thermal regime from sub-polar to polar at the late Weichselian glacial maximum.(3)High-level insolation melting with concomitant basal freezing. Initiation of polygonal array in basal zone. Change of thermal regime from polar to sub-polar or sub-temperate.(4)Down-wastage of ice and nunatak formation. Formation of high-level melt-water phenomena. Deposition of basal melt-out till. Further development of basal polygonal array. Till nuclei established and englacial sediment isolated. Change of thermal regime from sub-polar to sub-temperate.(5)Change of thermal state to temperate. Uncoupling of ice from bedrock. Streamlining of till nuclei and reworking of englacial sediment by melt water.(6)Final disintegration of ice.

Late Weichselian ice sheet of Northern Eurasia

A considerable portion of Northern Eurasia, and particularly its continental shelf, was glaciated by inland ice during late Weichsel time. This was first inferred from such evidence as glacial striae, submarine troughs, sea-bed diamictons, boulder trains on adjacent land, and patterns of glacioisostatic crustal movements. Subsequently, the inference was confirmed by data on the occurrence and geographic position of late Weichselian end moraines and proglacial lacustrine deposits. The south-facing outer moraines in the northeastern Russian Plain, northern West Siberia, and on Taimyr Peninsula are underlain by sediments containing wood and peat, the radiocarbon dating of which yielded ages of 22,000 to 45,000 yr B.P. The youngest late-glacial moraines are of Holocene age: the double Markhida moraine in the lower Pechora River basin, presumably associated with “degradational” surges of the Barents Ice Dome, is underlain by sediments with wood and peat dated at 9000 to 9900 yr B.P.: this suggests that deglaciation of the Arctic continental shelf of Eurasia was not completed until after 9000 yr B.P. The reconstructed ice-front lines lead to the conclusion that the late Weichselian ice sheet of Northern Eurasia (proposed name: the Eurasian Ice Sheet) extended without interruptions from southwestern Ireland to the northeastern end of Taimyr Peninsula, a distance of 6000 km: it covered an area of 8,370,000 km 2, half of which lay on the present-day continental shelves and a quarter on lowlands that were depressed isostatically below sea level. Hence, the ice sheet was predominantly marine-based. A contour map of the ice sheet based both on the dependence of the heights of ice domes upon their radii and on factual data concerning the impact of bedrock topography upon ice relief has been constructed. The major features of the ice sheet were the British, Scandinavian, Barents, and Kara Ice Domes that had altitudes of 1.9 to 3.3 km and were separated from one another by ice saddles about 1.5 km high. At the late Weichselian glacial maximum, all the main ice-dispersion centers were on continental shelves and coastal lowlands, whereas mountain centers, such as the Polar Urals and Byrranga Range, played only a local role. The portions of the ice sheet that were grounded on continental shelves some 700 to 900 m below sea level were inherently unstable and could exist only in conjunction with confined and pinned floating ice shelves that covered the Arctic Ocean and the Greenland and Norwegian Seas. The Eurasian Ice Sheet impounded the Severnaya Dvina, Mezen, Pechora, Ob, Irtysh, and Yneisei Rivers, and caused the formation of ice-dammed lakes on the northern Russian Plain and in West Siberia. Until about 13,500 yr B.P. the proglacial system of lakes and spillways had a radial pattern; it included large West Siberian lakes, the Caspian and Black Seas, and ended in the Mediterranian Sea. Later, the system became marginal and discharged proglacial water mainly into the Norwegian Sea.