GIS dataset: geomorphological record of terrestrial-terminating ice streams, southern sector of the Baltic Ice Stream Complex, last Scandinavian Ice Sheet, Poland

Here we present a comprehensive dataset of glacial geomorphological features covering an area of 65 000 km2 in central west Poland, located along the southern sector of the last Scandinavian Ice Sheet, within the limits of the Baltic Ice Stream Complex. The GIS dataset is based on mapping from a 0.4 m high-resolution digital elevation model derived from airborne light detection and ranging data. Ten landform types have been mapped: mega-scale glacial lineations, drumlins, marginal features (moraine chains, abrupt margins, edges of ice-contact fans), ribbed moraines, tunnel valleys, eskers, geometrical ridge networks, and hill–hole pairs. The map comprises 5461 individual landforms or landform parts, which are available as vector layers in GeoPackage format at https://doi.org/10.5281/zenodo.4570570 (Szuman et al., 2021a). These features constitute a valuable data source for reconstructing and modelling the last Scandinavian Ice Sheet extent and dynamics from the Middle Weichselian Scandinavian Ice Sheet advance, 50–30 ka, through the Last Glacial Maximum, 25–21 ka, and Young Baltic advances, 18–15 ka. The presented data are particularly useful for modellers, geomorphologists, and glaciologists.

The importance of topography in the occurrence of glacial valleys in Belarus

The relief of the ice-sheet bed predetermines the location of the valleys both in vast regions and in local, relatively limited areas. The influence of the relief in a large region on the formation of valleys occurs in lowland, more dissected northern and western regions of Belarus. Here, the bedrock relief plays a key role in appearance the hollow-like Vidzovsky, Polotsk and Surazh glacial erosion depressions and in isolation of the elevations separating them. The distribution of these largest forms of glacial erosion is associated with the regional picture of the glacial ice flow in topographic depressions, increased erosion of the ice bed, which developed under the glacial streams and lobes of the Scandinavian ice sheet. The degree and character of the relief dissection, the size, surface slope and orientation of the depressions, river paleo-valleys, elevations, and escarps had a leading role in the location of valleys in local, relatively limited areas. Their significance for the formation of valleys was to determine the local picture of the glacial flow and the areas where stresses were concentrated at the bed and glacial erosion intensified. The formation of tunnel valleys was possible in depressions and in the dissected relief of the glacial bed, which contributed to the accumulation of meltwater, the formation of subglacial lakes, as well as the concentration of meltwater in the subglacial channels.

GIS dataset: geomorphological record of terrestrial-terminating ice streams, southern sector of Baltic Ice Stream Complex, last Scandinavian Ice Sheet, Poland

Here we present a comprehensive dataset of glacial geomorphological features covering an area of 65 000 km2 in central west Poland, located along the southern sector of the last Scandinavian Ice Sheet, within the limits of the Baltic Ice Stream Complex. The GIS dataset is based on mapping from a 0.4 m high-resolution Digital Elevation Model derived from airborne Light Detection and Ranging data. Ten landform types have been mapped: Mega-Scale Glacial Lineations, drumlins, marginal features (moraine chains, abrupt margins, edges of ice-contact fans), ribbed moraines, tunnel valleys, eskers, geometrical ridge networks and hill-hole pairs. The map comprises 5461 individual landforms or landform parts, which are available as vector layers in GeoPackage format at http://doi.org/10.5281/zenodo.4570570 (Szuman et al., 2021a). These features constitute a valuable data source for reconstructing and modelling the last Scandinavian Ice Sheet extent and dynamics from the Middle Weichselian Scandinavian Ice Sheet advance, 50–30 ka BP, through the Last Glacial Maximum, 25–21 ka BP and Young Baltic Advances, 18–15 ka BP. The presented data are particularly useful for modellers, geomorphologists and glaciologists.

Tephrochronological evidence of a later Younger Dryas ice-sheet maximum in central Norway

<p>The Scandinavian Ice Sheet responded time-transgressively to the Younger Dryas (Greenland Stadial 1) cold event with large regional variations. Around Trondheimsfjorden in central Norway, the Tautra Moraines and the Hoklingen Moraines have long been assumed to have formed by glacial readvances during this event, as they have been dated to c. 12.7 and 11.6 cal. ka BP respectively (Olsen et al., 2015), mainly based on radiocarbon dating of often marine fossils. The Tautra Moraines, being the outer ridges of the two, should thus represent the maximum ice-sheet extent in this region during the Younger Dryas.</p><p>This ice-front position established a pro-glacial lake west of present-day Leksvik village on the Fosen peninsula (Selnes, 1982), which covered the Lomtjønnin lakes and Lomtjønnmyran fens, and drained through a spillway via Lake Rørtjønna. Some 20 km inland (northeast) from this location, inside the Tautra Moraines, the location of the Damåsmyran bog was covered by the ice sheet at that time.</p><p>By examining sediments from these sites for occurrences of volcanic ashes (visible and cryptotephra), combined with radiocarbon dating, we find that the ice front remained at the Tautra Moraines until the late Younger Dryas, contrary to the previous chronology (and overriding the suggested formation age of the Hoklingen Moraines). These findings comply with several recent reconstructions of the deglaciation at other sites in western (Lohne et al., 2012; Mangerud et al., 2016) and southern Norway (Romundset et al., 2019) and are a strong example of the usefulness of tephrochronology in the reconstruction of past ice-sheet dynamics.</p><p> </p><p><strong>References</strong></p><p>Lohne, Ø.S., Mangerud, J. & Svendsen, J.I. (2012) Timing of the Younger Dryas glacial maximum in Western Norway. <em>Journal of Quaternary Science</em>, vol. 27, pp. 81–88.</p><p>Mangerud, J., Aarseth, I., et al. (2016) A major re-growth of the Scandinavian Ice Sheet in western Norway during Allerød–Younger Dryas. <em>Quaternary Science Reviews</em>, vol. 132, pp. 175–205.</p><p>Olsen, L., Høgaas, F. & Sveian, H. (2015) Age of the Younger Dryas ice-marginal substages in Mid-Norway—Tautra and Hoklingen, based on a compilation of 14C-dates. <em>Norges geologiske undersøkelse Bulletin</em>, vol. 454, pp. 1–13.</p><p>Romundset, A., Lakeman, T.R. & Høgaas, F. (2019) Coastal lake records add constraints to the age and magnitude of the Younger Dryas ice-front oscillation along the Skagerrak coastline in southern Norway. <em>Journal of Quaternary Science</em>, vol. 34, pp. 112–124.</p><p>Selnes, H. (1982) Paleo-økologiske undersøkelser omkring israndavsetninger på Fosenhalvøya, Midt-Norge. Thesis at the Department of Botany, University of Trondheim.</p>

Ice sheet unzipping in the mountain-piedmont region of west-central Sweden: complex late-deglaciation of the Scandinavian ice sheet

<p>Regional-scale glacial geomorphological maps provide important empirical data for reconstructions of former ice sheets, which may serve as analogues for the behaviour of modern ice sheets under climate warming. In particular, the extensive LiDAR-derived record of former ice sheet beds, provides an outstanding archive from which to infer former ice sheet behaviour. The stacking together and analysis of, tens of thousands of individual landforms, based on their spatial coherency, provides a powerful tool to reconstruct ice flow dynamics, temporally evolving ice divide positions and the “unzipping” of ice sheets into separate masses during deglaciation. In this study, we develop a glacial geomorphological dataset focussing on the mountain-piedmont region of Jämtland in west-central Sweden. We focus on this region because it is where the last (Weichselian) ice sheet is believed to have unzipped into separate domes and was inundated by vast ice dammed lakes. Jämtland also records a complex temporal evolution of subglacial processes and was formerly mapped without the benefit of a LiDAR-based elevation model. The dataset was created by mapping in GIS and covers an area of 50 000 km<sup>2</sup> and almost 88 000 landforms, including glacial lineations, crag-and-tails, ice marginal moraines, lateral meltwater channels, eskers, and glacial lake shorelines. We use this unique dataset–in terms of spatial density and resolution–and quantitatively analyse cross-cutting relationships to establish a relative ice flow chronology. Our key findings include 1) a previously unmapped landform system, formed by the Early-to-Middle Weichselian westward expansion of a mountain centred ice sheet, and 2) a complex early Holocene deglaciation sequence with ice sheet unzipping occurring in southern and east-central Jämtland. The ice sheet split into a larger sheet retreating northward and a smaller ice sheet remaining southeast of the mountain piedmont. Our results provide new insights into the late deglaciation of the Scandinavian Ice Sheet.</p>

Ice-dammed lakes of Scandinavia - a key to the pattern and chronology of the final decay of the Scandinavian Ice Sheet

<p>Here we present the use of ice-dammed lake-related landforms and sediments for reconstructing the final phases of decay of the Scandinavian Ice Sheet.</p><p>In the late stages of the deglaciation, extensive glacial lakes were dammed between the easterly retreating Scandinavian Ice Sheet and the water divide within the mountains to the west. Using high-resolution airborne LiDAR-data, shorelines and other landforms relating to these ice-dammed lakes have now been discovered over larger areas and in greater numbers than previously known, opening a treasure trove of palaeoglaciological information of vast potential for reconstructing the final decay phase of the Scandinavian Ice Sheet.</p><p>The geomorphological imprint of the ice-dammed lakes is of particular importance in northern Scandinavia, as geological evidence pertaining unequivocally to the final ice sheet decay is sparse. Its interpretation is complicated since the ice sheet is thought to have mainly been cold-based during final decay, inhibiting sliding at the ice-bed interface and limiting the construction (or destruction) of landforms indicative of the changing shape and flow of the ice sheet. Furthermore, dated sediment sequences marking the onset of ice-free conditions are woefully few in northern Scandinavia. Likewise, available cosmogenic nuclide exposure dates provide high age uncertainty and inadequate geographical cover, leaving the timing and location of final ice sheet decay still elusive.</p><p>Using examples from northern and central Scandinavia, we show that ice-dammed lakes are an intricate part of the deglacial dynamics and show how mapping and dating them offer a solution to these problems. Even with a frozen ice-bed interface, surface melting and meltwater drainage creates landforms unequivocally associated with ice sheet decay: drainage channels, dammed lake shorelines, and deltas. Meltwater drainage routes and ice-dammed lakes are therefore powerful tools for reconstructing a disintegrating ice sheet; a ponded lake reveals the location of its requisite ice-dam, and drainage pathways reveal ice-free conditions. A dated sequence of ice-dammed lake sediments can therefore constrain both ice and lake coverage at that time for a much larger area than the dated site itself. Furthermore, the extent of different ice-dammed lake stages and their requisite ice-damming positions enables the pattern of ice margin change to be traced, and the relative age of ice-marginal positions determined using cross-cutting relations. The shorelines’ present-day tilts are also used to inform patterns and magnitudes of postglacial isostatic uplift, information otherwise lacking from the continental interior but of particular importance for modelling former ice sheet volumes and understanding the crustal response to ice sheet loading. Reconstructing the extents and timing of ice-dammed lakes and the study of related landforms and deposits can therefore greatly improve our understanding of the final decay of the Scandinavian Ice Sheet and provide potential analogues for the predicted future behaviours of modern ice sheets.</p>