Middle Weichselian Research Articles

Upper Pleistocene stratigraphy at the Medininkai site, eastern Lithuania: a continuous record of the Eemian‐Weichselian sequence

Eemian—Weichselian sequences, located outside the maximum limit of the Late Weichselian ice sheet, provide excellent opportunities for the discovery of continuous sedimentary records encompassing the whole Last Interglacial/Glacial cycle. Such a sequence is recorded in a borehole (117P) through the succession in a small kettlehole lake located at Medininkai, eastern Lithuania. The succession consists of peat, gyttja and silt deposited on top of a Saalian till. Pollen and plant macrofossil analysis, lithological analysis, U/Th dating and mineral magnetic measurements on the sediments have allowed 19 lithostratigraphic units and 16 local pollen assemblage zones (LPAZ) to be identified. The palaeocarpological record reveals a clear transition from the Saalian Glacial to the Weichselian stadial and interstadial phases. The mineral magnetic parameters suggest a good correlation between the concentration of magnetic minerals and stadial and interstadial periods. The Merkine (Eemian) Interglacial and two Early Weichselian Interstadials, Jonionys 1 (Brörup) and Jonionys 2 (Odderade), separated by cryomers, are identified. Intervals interpreted as analogous to the Middle Weichselian Denekamp and Hengelo interstadials are also recognized on the basis of pollen assemblages. The results show alternating periglacial and interstadial palaeoenvironments in Lithuania during the Early and Middle Weichselian and are of importance for Late Pleistocene palaeoenvironmental reconstruction of the Baltic area as a whole.

Upper Pleistocene stratigraphy at the Medininkai site, eastern Lithuania: a continuous record of the Eemian-Weichselian sequence

Eemian-Weichselian sequences, located outside the maximum limit of the Late Weichselian ice sheet, provide excellent opportunities for the discovery of continuous sedimentary records encompassing the whole Last Interglacial/Glacial cycle. Such a sequence is recorded in a borehole (117P) through the succession in a small kettlehole lake located at Medininkai, eastern Lithuania. The succession consists of peat, gyttja and silt deposited on top of a Saalian till. Pollen and plant macrofossil analysis, lithological analysis, U/Th dating and mineral magnetic measurements on the sediments have allowed 19 lithostratigraphic units and 16 local pollen assemblage zones (LPAZ) to be identified. The palaeocarpological record reveals a clear transition from the Saalian Glacial to the Weichselian stadial and interstadial phases. The mineral magnetic parameters suggest a good correlation between the concentration of magnetic minerals and stadial and interstadial periods. The Merkine (Eemian) Interglacial and two Early Weichselian Interstadials, Jonionys 1 (Brorup) and Jonionys 2 (Odderade), separated by cryomers, are identified. Intervals interpreted as analogous to the Middle Weichselian Denekamp and Hengelo interstadials are also recognized on the basis of pollen assemblages. The results show alternating periglacial and interstadial palaeoenvironments in Lithuania during the Early and Middle Weichselian and are of importance for Late Pleistocene palaeoenvironmental reconstruction of the Baltic area as a whole. (Less)

Non-glacial paleoenvironments and the extent of Weichselian ice sheets on Severnaya Zemlya, Russian High Arctic

The extent of the Barents-Kara Sea ice sheet (northern Europe and Russia) during the Last Glacial Maximum (LGM), in Marine Isotope Stage (MIS) 2 is controversial, especially along the southern and northeastern (Russian High Arctic) margins. We conducted a multi-disciplinary study of various organic and mineral fractions, obtaining chronologies with 14C and luminescence dating methods on a 10.5 m long core from Changeable Lake (4 km from the Vavilov Ice Cap) on Severnaya Zemlya. The numeric ages indicate that the last glaciation at this site occurred during or prior to MIS 5d-4 (Early Middle Weichselian). Deglaciation was followed by a marine transgression which affected the Changeable Lake basin. After the regression the basin dried up. In late Middle Weichselian time (ca 25–40 ka), reworked marine sediments were deposited in a saline water body. During the Late Weichselian (MIS 2), the basin was not affected by glaciation, and lacustrine sediments were formed which reflect cold and arid climate conditions. During the termination of the Pleistocene and into the Holocene, warmer and wetter climate conditions than before led to a higher sediment input. Thus, our chronology demonstrates that the northeastern margin of the LGM Barents-Kara Sea ice sheet did not reach the Changeable Lake basin. This result supports a modest model of the LGM ice sheet in northern Europe determined from numeric ice sheet modelling and geological investigations.

Signature and timing of the Kattegat Ice Stream: onset of the Last Glacial Maximum sequence at the southwestern margin of the Scandinavian Ice Sheet

At the end of the Middle Weichselian (30–25 ka BP) a glacier advance from southern Norway, termed the Kattegat Ice Stream, covered northern Denmark, the Kattegat Sea floor and the Swedish West Coast during onset of the Last Glacial Maximum (LGM) at the southwest margin of the Scandinavian Ice Sheet. The lithostratigraphic unit deposited by the ice stream is the till of the Kattegat Formation (Kattegat till). Because morphological features have been erased by later glacial events, stratigraphic control and timing are decisive. The former ice stream is identified by the dispersal of Oslo indicator erratics from southern Norway and by glaciodynamic structures combined with glaciotectonic deformation of subtill sediments. Ice movement was generally from northerly directions and the flow pattern is fan‐shaped in marginal areas. To the east, the Kattegat Ice Stream was flanked by passive glaciers in southern Sweden and its distribution was probably governed by the presence of low permeability and highly deformable marine and lacustrine deposits. When glaciers from southern Norway blocked the Norwegian Channel, former marine basins in the Skagerrak and Kattegat experienced glaciolacustrine conditions around 31–29 ka BP. The Kattegat Ice Stream became active some time between 29 ka BP and 26 ka BP, when glaciers from the Oslo region penetrated deep into the shallow depression occupied by the Kattegat Ice Lake. Deglaciation and an interlude with periglacial and glaciolacustrine sedimentation lasted until c. 24–22 ka BP and were succeeded by the Main Glacier Advance from central Sweden reaching the limit of Late Weichselian glaciations in Denmark around 22–20 ka BP, the peak of the LGM. This was followed by deglaciation and marine inundation in the Kattegat and Skagerrak around 17 ka BP.

Signature and timing of the Kattegat Ice Stream: onset of the Last Glacial Maximum sequence at the southwestern margin of the Scandinavian Ice Sheet

At the end of the Middle Weichselian (30–25 ka BP) a glacier advance from southern Norway, termed the Kattegat Ice Stream, covered northern Denmark, the Kattegat Sea floor and the Swedish West Coast during onset of the Last Glacial Maximum (LGM) at the southwest margin of the Scandinavian Ice Sheet. The lithostratigraphic unit deposited by the ice stream is the till of the Kattegat Formation (Kattegat till). Because morphological features have been erased by later glacial events, stratigraphic control and timing are decisive. The former ice stream is identified by the dispersal of Oslo indicator erratics from southern Norway and by glaciodynamic structures combined with glaciotectonic deformation of subtill sediments. Ice movement was generally from northerly directions and the flow pattern is fan-shaped in marginal areas. To the east, the Kattegat Ice Stream was flanked by passive glaciers in southern Sweden and its distribution was probably governed by the presence of low permeability and highly deformable marine and lacustrine deposits. When glaciers from southern Norway blocked the Norwegian Channel, former marine basins in the Skagerrak and Kattegat experienced glaciolacustrine conditions around 31–29 ka BP. The Kattegat Ice Stream became active some time between 29 ka BP and 26 ka BP, when glaciers from the Oslo region penetrated deep into the shallow depression occupied by the Kattegat Ice Lake. Deglaciation and an interlude with periglacial and glaciolacustrine sedimentation lasted until c. 24–22 ka BP and were succeeded by the Main Glacier Advance from central Sweden reaching the limit of Late Weichselian glaciations in Denmark around 22–20 ka BP, the peak of the LGM. This was followed by deglaciation and marine inundation in the Kattegat and Skagerrak around 17 ka BP.

Dating the extinction of European mammoths: new evidence from Estonia

Five mammoth molars, four pieces of tusk and one bone fragment found in different parts of Estonia were radiocarbon dated using the AMS method. Six of the analysed samples gave an infinite date. These remains originate most likely from an ice-free period during the Early Weichselian or early Middle Weichselian. The molar from Mooste, southeast Estonia was dated at 31,000 BP, and seems thus to belong to the same faunal complex as several other previously dated mammoth specimens recovered in Finland and other Nordic countries. Two molars from Puurmani, central Estonia gave an age of ca 10,000 BP, suggesting a late relict mammoth population living in northeast Europe during the Late Pleistocene/Early Holocene transition.

The coversands and timing of Late Quaternary earthquake events along the Peel Boundary Fault in the Netherlands

AbstractThe coversands along the Peel Boundary Fault in the Netherlands were investigated by a luminescence dating approach combining Infrared Optically Stimulated Luminescence (IRSL) and Thermoluminescence (TL) methods. At the Neer trench, ten samples were collected and investigated in order to set up an independent chronological framework for the deposition history of the fluvio-aeolian and aeolian sediments and hence the timing of Late Weichselian and Holocene earthquake events. Five sedimentary units could be distinguished by this chronological approach. The oldest fluvio-aeolian unit yielded a mean deposition age of 35.9±0.4 ka and is designated to correlate with the Middle Weichselian. An IRSL age estimate of 20.1 ±2.9 ka was determined for the sediment that most likely represents the Older Coversands I, and a mean luminescence age of 15.1±1.2 ka for deposits just below the Beuningen gravel bed. The aeolian sediment from above the Beuningen horizon yielded an IRSL age estimate of 9.4±1.0 ka. The youngest deposits from the colluvial wedge yielded <6.9±0.7 ka BP, and so an earthquake event was likely to occur during the Middle or Late Holocene, as evidenced by the luminescence age estimates.

Late Pleistocene and Holocene Vegetation and Climate on the Taymyr Lowland, Northern Siberia

Pollen records from perennially frozen sequences provide vegetation and climate reconstruction for the last 48,000 14C years in the central part of Taymyr Peninsula. Open larch forest with Alnus fruticosa and Betula nana grew during the Kargin (Middle Weichselian) Interstade, ca. 48,000–25,000 14C yr B.P. The climate was generally warmer and wetter than today. Open steppe-like communities with Artemisia, Poaceae, Asteraceae, and herb tundralike communities with dwarf Betula and Salix dominated during the Sartan (Late Weichselian) Stade, ca. 24,000–10,300 14C yr B.P. The statistical information method used for climate reconstruction shows that the coldest climate was ca. 20,000–17,000 14C yr B.P. A warming (Allerød Interstade?) with mean July temperature ca. 1.5°C warmer than today occurred ca. 12,000 14C yr B.P. The following cooling with temperatures about 3°–4°C cooler than present and precipitation about 100 mm lower corresponds well with the Younger Dryas Stade. Tundra–steppe vegetation changed to Betula nana–Alnus fruticosa shrub tundra ca. 10,000 14C yr B.P. Larch appeared in the area ca. 9400 14C yr B.P. and disappeared after 2900 14C yr B.P. Cooling events ca. 10,500, 9600, and 8200 14C yr B.P. characterized the first half of the Holocene. A significant warming occurred ca. 8500 14C yr B.P., but the Holocene temperature maximum was at about 6000–4500 14C yr B.P. The vegetation cover approximated modern conditions ca. 2800 14C yr B.P. Late Holocene warming events occurred at ca. 3500, 2000, and 1000 14C yr B.P. A cooling (Little Ice Age?) took place between 500 and 200 14C yr ago.

The Deposition History of the Coversands along the Bree Fault Escarpment, NE Belgium

AbstractThe coversands along the Bree fault escarpment, NE Belgium, were investigated by a combined dating approach including infrared optically stimulated luminescence (IRSL), thermoluminescence (TL) and radiocarbon methods. Four trenches were excavated cutting the fault scarp near the village of Bree in northeast Belgium. Altogether 17 luminescence samples and seven radiocarbon samples were investigated in order to set up a more reliable and precise chronological frame for the local coversand stratigraphy and the timing of Late Quaternary earthquake events. The chronological results indicate at least five accumulation periods for the coversand units in the area of interest. The oldest coversands were deposited during Saalian or Early Weichselian followed by coversands deposited during the Early and/or Middle Weichselian. At least three coversand units can be distinguished for the time span of the Late Weichselian to Holocene. Significant TL age underestimation of more than 20% compared to IRSL was found for samples from Trench 4. Most of the IRSL age estimates are in agreement with radiocarbon dates from the same section.

Paleoseismological analysis of the Rurrand fault near Jülich, Roer Valley graben, Germany: Coseismic or aseismic faulting history?

AbstractA first trench has been excavated for paleoseismological analysis in the German part of the Roer Valley graben, which has experienced several historical earthquakes with a maximum intensity up to VIII on the MSK-scale.The trench has exposed the Rurrand fault as a complex fault zone with at least five separate, SW-dipping, normal fault strands displacing an early Pleistocene terrace of the Rhine river by more than 7 m. The major part of the observed deformation was produced during or after deposition of an overlying unit of stratified loess of middle Weichselian to probably Saalian age. The faulting history is shown to be episodic, with different fault strands active at different times. Growth faulting that would be indicative of continuous, aseismic fault motion has not been observed. Our stratigraphic control is not sufficient to constrain the timing and to provide evidence of the coseismic nature for each observed fault displacement. However, two units of structureless, gravelly loess are interpreted as the result of extensive solifluction triggered by two large surface-rupturing events. This is suggested by the position of these units, which is controlled by the main faults, and by their remarkably young age (< 400 cal. BC), indicated by radiocarbon and OSL datings and by the presence of historic brick fragments. At least two faults show moderate activity that is even younger. Our interpretation is not in agreement with earlier hypotheses that ongoing vertical movements of circa 1 mm/a in the German part of the Lower Rhine graben are the result of aseismic fault creep, but is in line with the results of similar investigations on the southwestern border fault of the Roer Valley Graben in Belgium, which demonstrates the need for further paleoseismological research in this region. The Rurrand fault is presently experiencing aseismic slip on its superficial portion, induced by extensive groundwater lowering for mining purposes. This ongoing deformation seems to be expressed in the trench as diffuse bundles of anastomosing cracks extending up to, and in some cases even into the plough zone, rather than as sharp fault planes which are typical of older, tectonic fault movements.

The chronology of a large ice-dammed lake and the Barents–Kara Ice Sheet advances, Northern Russia

Beach and shoreface sediments deposited in the more than 800-km long ice-dammed Lake Komi in northern European Russia have been investigated and dated. The lake flooded the lowland areas between the Barents–Kara Ice Sheet in the north and the continental drainage divide in the south. Shoreline facies have been dated by 18 optical stimulated luminescence (OSL) dates, most of which are closely grouped in the range 80–100 ka, with a mean of 88±3 ka. This implies that that the Barents–Kara Ice Sheet had its Late Pleistocene maximum extension during the Early Weichselian, probably in the cold interval (Rederstall) between the Brørup and Odderade interstadials of western Europe, correlated with marine isotope stage 5b. This is in strong contrast to the Scandinavian and North American ice sheets, which had their maxima in isotope stage 2, about 20 ka. Field and air photo interpretations suggest that Lake Komi was dammed by the ice advance, which formed the Harbei–Harmon–Sopkay Moraines. These has earlier been correlated with the Markhida moraine across the Pechora River Valley and its western extension. However, OSL dates on fluvial sediments below the Markhida moraine have yielded ages as young as 60 ka. This suggests that the Russian mainland was inundated by two major ice sheet advances from the Barents–Kara seas after the last interglacial: one during the Early Weichselian (about 90 ka) that dammed Lake Komi and one during the Middle Weichselian (about 60 ka). Normal fluvial drainage prevailed during the Late Weichselian, when the ice front was located offshore.

Age limits on the Late Quaternary evolution of the upper Loire River

The Loire River is one of the largest rivers in Europe, draining a basin area of southern France which exceeds 100,000 km 2. Despite its significance, the Loire remains poorly studied and has a very limited amount of chronological control on the deposited sedimentary record of fluvial activity. These factors, coupled with the availability of a set of 10 radiocarbon dates on younger (c., <5 ka) Loire terraces, have encouraged us to commence an intensive programme of optical dating on terraces from the upper Loire. We have obtained relatively good agreement between single-aliquot regeneration (SAR)-based age estimates and AMS dates, and have extended the chronology of the terraces back into the middle Weichselian. Our data indicate that the key shift in fluvial style from a braided gravel-bedded system through to a sand and silt dominated system occurred at around the time of the Pleistocene–Holocene transition.

Recurrent interaction between the Norwegian Channel Ice Stream and terrestrial‐based ice across southwest Norway

The occurrence of till beds alternating with glaciomarine sediment spanning oxygen isotope stages 6 to 2, combined with morphological evidence, shows that the southwestern fringe of Norway was inundated by an ice stream flowing through the Norwegian Channel on at least four occasions, the last time being during the Late Weichselian maximum. All marine units are deglacial successions composed of muds with dropstones and diamictic intrabeds and a foraminiferal fauna characteristic of extreme glaciomarine environments. Land‐based ice, flowing at right angles to the flow direction of the ice stream, fed into the ice stream along an escarpment formed by erosion of the ice stream. Each time the ice stream wasted back, land‐based ice advanced into the area formerly occupied by the ice stream. During the last deglaciation of the ice stream (c. 15 ka BP), the advance of the land‐based ice occurred immediately upon ice stream retreat. As a result, the sea was prevented from inundating the upland areas, allowing most of the glacioisostatic readjustment to occur before the land‐based ice melted back at about 13 ka BP. This explains the low Late Weichselian sea levels in the area (10–20 m) compared with those of the Middle Weichselian and older sea‐level high stands (∼200 m). Regional tectonic movements cannot explain the location of the observed marine successions. The highest sea level recorded (&gt;200 m) is represented by glaciomarine sediments from the Sandnes interstadial (30–34 ka BP). Older interstadial marine sediments are found at somewhat lower levels, possibly as a result of subsequent glacial erosion in these deposits. Ice streams developed in the Norwegian Channel during three Weichselian time intervals. This seems to correspond to glacial episodes both to the south in Denmark and to the north on the coast of Norway, although correlations are somewhat hampered by insufficient dating control.

A lithostratigraphy of Weichselian glacial and interstadial deposits in Denmark

A lithostratigraphic model of Weichselian tills and inter-till deposits in Denmark is erected. Deposits comprise 1) Till, other diamicts and meltwater sediments deposited during glaciated stadials and 2) Interstadial and stadial fluvial, lacustrine and marine sediments deposited under climatic ameliorations and ice free conditions. Stratigraphic successions are correlated using lithological and palaeoecological evidence and they are linked to the marine sequences in northern Denmark named the Skærumhede series. Three stadials with one or more glaciation events succeeding Eemian interglacial conditions have been recognized. The oldest, Ristinge stadial, is most likely from the early Middle Weichselian situated between the Odderade and Moershoofd interstadials. It comprises The Ristinge, Ringshøj and Lovns tills of Baltic origin is found in the eastern and central parts of Denmark. The post-Hengelo, Klintholm stadial from the late Middle Weichselian, is represented by the Baltic Klintholm till found in the easternmost part of the country and possibly the Esrum diamicton in the buried Esrum valley belong to this stadial. The post-Sandnes interstadial, Late Weichselian Jylland stadial comprise the Kattegat Till of Norwegian provenance is found in northern Denmark. It is followed by the Mid Danish, Grenå, Fårup, Store Klinthøj, Himmerland tills of middle Swedish origin, and deposited by the ice-stream which reached the Main Stationary Line from northeasterly directions. The East Jylland and North Sjælland tills of respectively Baltic and Swedish provenance overlie the former tills in eastern Denmark. These are succeeded by the Bælthav Till of Baltic origin which is confined to the Danish islands. Late glacial deposits from the Bølling-Allerød oscillation comprises the upper boundary for Weichselian deposits. The compilation of the stratigraphic model serves the purpose of combining and bringing forward regional and local stratigraphic studies from the past decades and to set up a frame-work for dating the age and duration individual glaciation events and interstadials.

New radiocarbon dates from Finnish mammoths indicating large ice-free areas in Fennoscandia during the Middle Weichselian

New radiocarbon dates from Finnish subfossil mammoth material (Mammuthus sp.), transported by glacial ice, range in age from ca. 32000 to ca. 22500 yr BP. These results suggest that there was a larger ice-free area in Fennoscandia during the Middle Weichselian than previously assumed. In addition, two dates are also presented for bones found in clay with a different transport history. Copyright © 1999 John Wiley & Sons, Ltd.

New remains of Pleistocene Ovibos moschatus from Germany and its geographic and stratigraphic occurrence in Europe

All remains of Pleistocene muskoxen from central and western Europe are attributable to a single species, Ovibos moschatus. Its occurrence was restricted mainly to the area north of the Pyrenees and Alps, covering lowlands and mountain areas up to 1600 m. In the Middle Pleistocene, Ovibos occurred in a cold phase, well before the Elsterian, and again during the Saalian and Weichselian. In the Late Pleistocene, Ovibos co-occurred with other faunal elements indicative of severe continental climatic conditions during the early and late Weichselian. During the middle Weichselian, the genus seems to have retreated. A list of European localities that have yielded Ovibos moschatus, inclusive of geographical references, is provided.

Late quaternary sedimentation and glacial history of the western Svalbard continental margin

Glacigenic sediments recovered in shallow cores from the western Svalbard continental slope, are subdivided into five facies associations based on grain-size, sedimentary structure, mineralogy, petrography and geochemistry. Two diamicton facies are recognised, one of which is interpreted as hemipelagic mud with variable amounts of ice-rafted debris (IRD), and the other as a product of mass-movement. The diamictons are associated with melting of icebergs during glacial melt events and debris flow deposition on the submarine fans during peak glaciation, respectively. Laminated-to-layered mud and turbidites seem to be closely related to eustatic fall in sea level and erosion of banks located on the continental shelf, resulting in accumulation of fine-grained organic-rich deposits and thin silt- and sand layers on the continental slope. Middle Weichselian was characterised by several phases of extensive iceberg production and input of IRD. The first phase (60−55 ka) is correlated with a glacier advance on Svalbard, but the following phases (54≌44 ka) can not be correlated with glacier advances on land. This demonstrates that there may have been more glaciations than recorded on Svalbard, and that ice sheets at high northern latitudes fluctuated more frequently than previously assumed. The composition of the IRD deposited after 54 ka, suggests that the ice sheets during these advances were located in the eastern Svalbard-Barents Sea area. The Late Weichselian growth of the Svalbard-Barents Sea Ice Sheet occurred probably in two steps; Nucleation of a continental-based ice sheet between 27 and 22.5 ka, followed by a rapid advance to the shelf edge when the ice margin reached the soft, clay-rich sediments on the continental shelf. Eustatic lowering of the sea level during the initial phase led to bank exposure and erosion, bringing large quantities of fine-grained, organic-rich sediments to the deep-sea. During peak glaciation, glaciers provided a localised supply of sediments to the shelf troughs and the upper slope, which were redistributed by debris flows. The marine-based Svalbard-Barents Sea Ice Sheet started to retreat around 14.5 ka through massive iceberg discharge. The retreat was interrupted by a short-lived advance, accompanied by erosion of the shelf banks and redistribution of organic-rich sediments to the continental slope. The final ice recession started around 12 ka and ended close to 9 ka, when the fjords of Svalbard were essentially ice-free. Large amounts of meltwater released during the removal of the Svalbard-Barents Sea Ice Sheet, cooled down the surface waters of the Norwegian-Greenland Sea and formed a widespread sea-ice cover between 12 and 8 ka. Input of terrigenous material was greatly reduced during the early part of Holocene. The present interglacial is characterised by erosion of the shelf and upper slope by bottom currents, leaving a prominent lag deposit.

The Early and Middle Weichselian interstadials in the central area of the Scandinavian glaciations

In the central parts of the Scandinavian glaciations, in Norway, Sweden and Finland, there are a number of Early Weichselian sites with fresh-water sediments, which, by comparison with sites further south, can be correlated with either the Brorup or Odderade interstadials. Few of these sites can with any certainty be correlated with just one of the two. In northern Jutland and at thecoast of south-western Norway there are Early Weichselian marine sediments, some of those in Norway having been interpreted as interstadial. Middle Weichselian fresh-water sediments have only been found in Denmark and southern Sweden, whereas marine sediments occur in northern Jutland and at the coast in Norway, in the same areas as the Early Weichselian marine sediments. Most of the Middle Weichselian sites must be referred to as representing an interstadial complex, as a close correlation with the five interstadials identified in northwest Germany and The Netherlands is not possible. A close correlation has been made between the fluctuations in the deep-sea oxygen-isotope curve and the Weichselian history in Scandinavia, the justification for which is discussed.

The Margin of the Last Barents-Kara Ice Sheet at Markhida, Northern Russia

The Markhida Moraine is a broad zone of ice marginal landforms and deposits running E-W across the Pechora lowland in northern Russia. It was formed along the southern margin of an ice sheet advancing onto the coast from the Kara Sea. Previous researchers (Grosswald, 1980, 1983, 1994a: Grosswald et al., 1974) concluded that the moraine was formed during a readvance of the Barents-Kara ice sheet after 9000 14C yr B.P. This conclusion was based on the occurrence of deformed lacustrine sediments dated 9000 to 10,000 14C yr B.P. between two presumed basal tills in the type section at Markhida. A reinvestigation of the section by the authors revealed only one basal till overlain by lacustrine sediments and diamictons, yielding radiocarbon dates between 9000 and 10,000 14C yr B.P. Luminescence dating from the sand underlying the till gave Middle Weichselian ages. Deformation structures and fabric in the sediments overlying the till solely reflect gravity-driven processes, not glaciotectonic deformation as claimed by previous investigators. We conclude that most of these sediments were finally deposited during very slow (retarded) melting of a large body of buried glacier ice and the underlying permafrost. Thus, the Markhida Moraine, as an ice marginal feature, was formed prior to 10,000 14C yr B.P., probably during the Late Weichselian, but possibly as early as the Middle Weichselian.

The mode of degradation of Pleistocene permafrost in West Siberia

Continuous permafrost 300 to 600 m thick is mapped in West Siberia beyond the Arctic Circle. To the south, it splits into two uneven layers to form a swallow-tail structure in cross-section. The surficial layer, which is about 50 m thick, occurs discontinuously between 66 ° and 64°N (in isolated patches up to the 60th parallel) and is thought to be a product of the Late Holocene cooling. The second, much thicker layer, separated from the surficial permafrost by a 100-200m thick talik, extends to the 60th parallel and must be a Pleistocene relict. Its fossil nature appears in near-zero temperatures and non-gradient curves of the vertical temperature profile (Yershov, 1989). The relict layer is commonly thought to be of the Late Weichselian age when solid permafrost reached the 51st parallel, i.e. shifted by 1500-1600 km south from its present limit (Baulin et al., 1984). Only Zemtsov (1976) related it to the Middle Pleistocene maximum glaciation. The study of fossil glacial ice contained in the thick discontinuous permafrost near the Arctic Circle has shown that relict permafrost must have survived since Early Weicheslian time (Astakhov and Isayeva, 1988). There are different ideas about the process which changed the ubiquitous solid permafrost of the Pleistocene into the present discontinuous swallowtail structure south of the Arctic Circle. The usual reasoning based on temperature profiles, freeze-andthaw structures in sediments and computer simulations does not allow unambiguous conclusions, particularly if geological data are disregarded. Balobayev et al. (1983) have suggested that the abrupt southern boundary of the modern continuous permafrost reflects a shoreline of a Late Weichselian proglacial lake that fringed the ice sheet along the Arctic Circle. Sedimentological research was discovered no traces of large proglacial lakes (Astakhov, 1989, 1992). Most investigators believe that the deep talik over the relict permafrost developed during the Holocene climatic optimum. The popular view, ascribing great mobility to the West Siberian permafrost, suggests that south of the Arctic Circle a permafrost layer approximately 500--600 m thick could form and vanish during the Late Pleistocene-Holocene (Baulin et al., 1981). This apparently contradicts geological data about the conservative nature of Siberian permafrost. The above mentioned lower Weichselian fossil ice and fresh-looking (not decayed) frozen wood of infinite radiocarbon age has survived within the permafrost near the surface (Astakhov and Isayeva, 1988). These facts show that: (i) no deep climatic fluctuations affected the permafrost since the Early Weichselian except the Holocene warming; and (ii) the Holocene thawing of the Arctic permafrost tens of metres deep (Baulin et al., 1981) must have been local. The apparent discrepancy between the data on great inertia of the Pleistocene permafrost in the Arctic and deep taliks registered south of the Arctic Circle must be attributed to heterogeneity of the degradation process in the different geographical zones (Astakhov, 1990). The divergence of geological processes in the Arctic from central West Siberia can be judged from the comparison of the sedimentary records. Facies related to the stable Weichselian permafrost show an arid and very continental climate from the Kara Sea to the Kazakh steppes. They are thick loess sheets (changing laterally into sand dunes), which are frozen with syngenetic ice veins in the Arctic, and thawed with ice wedge casts in the centre and south of West Siberia. The only zonal difference appears in the volume of water-lain facies within this principally aeolian formation. In the Arctic, aqueous sediments are mostly comprised of glaciofluvial sands and glaciolacustrine varves underlying the loess and dunes. Within the uppermost aeolian, mostly sandy formation, waterlain sediments are scarce, especially during 22 to 15 ka BP (Astakhov, 1991). Weichselian sediments in thermokarst sink-holes have not been described, even along the major river valleys. The situation is somewhat different south of the 64th parallel where numerous lenses of sandy, limnic rhythmites with dates from infinite to 22 ka BP are encompassed by loess-like silts with palaeosols and ice-wedge casts. They are especially abundant along the transverse Ob valley and the crest of the Siberian Hills. Such rhythmites, up to 15 m thick ('Kolpashevo sand' by Arkhipov et al., 1980), have nothing to do with large ice-damned lakes, because they:.(i) occur at all hypsometric levels from 40 to 120 m; (ii) contain seams of loess, palaeosols, cryoturbated horizons and ice-wedge casts; (iii) make oval and doughnut-shaped hillocks of thermokarst inversion topography; and (iv) are never traced laterally for more than a few hundred metres (Astakhov, 1989, 1992). These features indicate that in central West Siberia, thermokarst was active in the Middle Weichselian, probably along