Clas Hättestrand

Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model

The evolution of ice-sheet configuration and flow pattern in Fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model, i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets. The model uses mapped flow traces and deglacial melt-water landforms, as well as relative chronologies derived from cross-cutting striae and till lineations, as input data. Flow-trace systems were classified into four types: (i) time-transgressive wet-bed deglacial fans, (ii) time-transgressive frozen-bed deglacial fans, (iii) surge fans, and (iv) synchronous non-deglacial (event) fans. Using relative chronologies and aggregation of fans into glaciologically plausible patterns, a series of ice-sheet Configurations at different time slices was erected. A chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume. Geological evidence exists for several discrete ice-sheet configurations centred over the Scandinavian mountain range during the early Weichselian. The build-up of the main Weichselian Fennoscandian ice sheet started at approximately 70 Ka, and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern Norway. This configuration had a flow pattern which is poorly reproduced by current numerical models of the Fennoscandian ice sheet. At the Last Glacial Maximum the main ice divide was located overthe Gulf of Bothnia. A major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the Scandinavian mountain chain. Widespread areas of preserved pre-late-Weichselian landscapes indicate that the ice sheet had a frozen-bed core area, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

Frozen-bed Fennoscandian and Laurentide ice sheets during the Last Glacial Maximum

The areal extents of the Laurentide and Fennoscandian ice sheets during the Last Glacial Maximum (about 20,000 years ago) are well known, but thickness estimates range widely, from high-domed to thin, with large implications for our reconstruction of the climate system regarding, for example, Northern Hemisphere atmospheric circulation and global sea levels. This uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials, a knowledge of which would better constrain reconstructions of ice-sheet thickness. Here we show that, in the absence of direct data, the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen- and thawed-bed conditions. We argue that ribbed moraines were formed by brittle fracture of subglacial sediments, induced by the excessive stress at the boundary between frozen- and thawed-bed conditions resulting from the across-boundary difference in basal ice velocity. Maps of glacial landforms from aerial photographs of Canada and Scandinavia reveal a concentration of ribbed moraines around the ice-sheet retreat centres of Quebec, Keewatin, Newfoundland and west-central Fennoscandia. Together with the evidence from relict landscapes that mark glacial areas with frozen-bed conditions, the distribution of ribbed moraines on both continents suggest that a large area of the Laurentide and Fennoscandian ice sheets was frozen-based—and therefore high-domed and stable—during the Last Glacial Maximum.

A relict landscape in the centre of Fennoscandian glaciation: cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles

The presence of well-developed tors, boulder fields, and weathering mantles in the Parkajoki area of northeastern Sweden, near the centre of Fennoscandian glaciation, has been used to suggest that these landscapes were preserved during all glacial cycles since ice-sheet initiation in the late Cainozoic. This implies that all successive large-scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation. Cosmogenic 10Be and 26Al data from three tors and a meltwater channel in the Parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles. Apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at ∼11 ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion. Single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles. This estimate is based on (i) the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the DSDP 607 marine benthic foraminifer oxygen isotope record, in conjunction with (ii) a record of Fennoscandian ice sheet flow traces (and hence, ice sheet extent), and (iii) noting that cosmogenic nuclides are accumulated only during ice free periods. In addition, mean cosmogenic 26Al/10Be concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1.6 m Ma−1. Thus, the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles.

A relict landscape in the centre of Fennoscandian glaciation: Geomorphological evidence of minimal Quaternary glacial erosion

Abstract The Parkajoki area in northeastern Sweden is situated near the central area of Fennoscandian glaciation. Despite its location, the area is dominated by landforms induced by subaerial weathering and erosion processes, such as well-developed tors and associated saprolites, boulder fields, and boulder depressions. The glacial geomorphology is dominated by lateral and proglacial meltwater channels. Subglacial imprints indicative of thawed-bed conditions and reshaping by glacier sliding (e.g., fluting, drumlins, striae) are lacking. Hence, most of the landscape still exhibits a preglacial appearance. Because of its location, near the central area of glaciation, we attribute preservation to frozen-bed conditions of overriding ice sheets. The widespread distribution of well-developed tors and boulder fields, and the degree of chemical weathering of the bedrock, indicate that the area has been protected from glacial erosion during all glacial cycles since ice sheet initiation in the late Cenozoic. Unlike most other areas with tors in glaciated regions, the Parkajoki area is uniquely situated in the lowlands at 150–400 m.a.s.l. Moreover, this relict landscape is surrounded by glacial landscapes (including drumlins, ribbed moraine, and eskers) of varying age and at similar elevation. Hence, topographical reasons for this area being persistently cold-based cannot be invoked. By inference, we conclude that strain heat release never managed to cancel the initial subglacial permafrost conditions. We attribute this to divergent ice flow towards the convex outline of the ice sheet margin during deglaciations and to the relative roughness of the area compared to its surroundings. The implication is that to explain preservation throughout the Quaternary, all large-scale glaciations must have had a similar evolution concerning ice sheet configuration and internal dynamics as the last glaciation for which good constraints on evolution and outline are available.

RIBBED MORAINE FORMATION

Ribbed (Rogen) moraines are conspicuous landforms found in interior parts of formerly glaciated areas. Two major theories for ribbed moraine formation have been suggested in recent years: (i) the shear and stack theory, which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow, followed by basal melt-out of transverse moraine ridges, and (ii) the fracturing theory, according to which ribbed moraines form by fracturing of frozen pre-existing till sheets, at the transition from cold- to warm-based conditions under deglaciating ice sheets. In this paper, we present new data on the distribution of ribbed moraines and their close association with areas of frozen-bed conditions under ice sheets. In addition, we show examples of ribbed moraine ridges that fit together like a jig-saw puzzle. These observations indicate that fracturing and extension of a pre-existing till sheet may be a predominant process in ribbed moraine formation. In summary, we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory, while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines. One implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen-bed to thawed-bed conditions under former ice sheets.

Ribbed moraines in Sweden — distribution pattern and palaeoglaciological implications

Abstract A detailed map of ribbed moraines in central and northern Sweden is presented. They are frequent features over much of the mapped area, and the distribution has well defined limits. The most abundant ribbed moraines are situated in close connection to areas that are inferred to have been cold-based during the retreat of the Late Weichselian Ice Sheet, while outer, fully warm-based areas, lack ribbed moraine. Observations have also been made on a more local scale, where cold-based patches grade into ribbed moraines via a zone of a broken-up till cover. A model of origin is presented, which includes contracting of a frozen core area of a retreating ice sheet. At the transition from non-sliding to sliding conditions there will be high stresses resulting in local extensional flow at the base of the ice sheet. A rising phase change surface (frozen-thawed material) would cause detachment and ‘boudinage-like’ breaking up of a pre-existing till cover, thawed at the bedrock contact, but still frozen to the ice sole. The orientation of ice flow during the formation of the ribbed moraines closely matches the ice flow directions of the deglaciating Late Weichselian Ice Sheet. Thus, ribbed moraines are considered to be deglaciation landforms, typical of shrinking cold-based zones of a retreating ice sheet.

Zooming in on frozen-bed patches : scale-dependent controls on Fennoscandian ice sheet basal thermal zonation

In this paper, we explore geomorphological evidence allowing a first-order reconstruction of the extent and pattern of frozen-bed conditions under the last Fennoscandian ice sheet. We mapped relict landscapes, i.e. glacial landforms and subaerially developed ground surfaces predating the last ice sheet and marking sustained frozen-bed conditions, at four different spatial scales. At the ice-sheet scale, relict landscapes are most abundant between the Last Glacial Maximum ice divide and the elevation axis of the Scandinavian mountain range. The location of frozen-bed zones was mainly a function of dispersal centre location (low surface temperatures and small strain heating) and small ice thickness over the eastern flank of the mountain range. At the mesoscale (260 × 360 km map area), the pattern of relict surfaces is governed by inward-cutting ice-stream erosion. Topographical control was weak, but relation to flow pattern was strong, with the major frozen-bed zone located where ice flow was strongly divergent. At the regional scale (40 × 65 km map area) in hilly terrain, topographical control was strong with relict surfaces only appearing above a plane dipping in the up-ice direction. At the local scale (12 × 14 km map area), control by topography was likewise strong, but the detailed boundary pattern was irregular, with specific landforms occurring both up- and down-ice of frozen patches.

Quantifying the erosional impact of the Fennoscandian ice sheet in the Tornetrask-Narvik corridor, northern Sweden, based on cosmogenic radionuclide data

Despite spectacular landform evidence of a dominant role for glacial action in shaping landscapes under former northern hemisphere ice sheets, there is little quantitative evidence for rates and patterns of erosion associated with specific glaciations. Here we use cosmogenic nuclide data to assess rates of subglacial erosion underneath the Fennoscandian ice sheet. By testing whether there are remnant nuclide concentrations in samples taken from sites that include both relict areas and features and landscapes typically associated with vigorous glacial erosion, we can constrain the level and pattern of modification that resulted from the last glaciation. Cosmogenic 10Be and 36Cl data from the Torneträsk region confirm the temporal and spatial variability of glacial erosion suggested by geomorphological mapping. At some sites, glacial erosion estimates in what appear to be heavily scoured areas indicate erosion of only c. 2 ± 0.4 m of bedrock, based on cosmogenic nuclide inheritance. This implies that the generation of severely scoured terrain in this study area required multiple glaciations. The overall modification produced by ice sheets along glacial corridors may be more restricted than previously thought, or may have occurred preferentially during earlier Quaternary glacial periods.

Evidence for a relict glacial landscape in Quebec-Labrador

Abstract Two major glacial landform systems occur in Labrador. These are the radial lineation and esker swarm reflecting Late Wisconsinan decay, and the Ungava Bay lineation and esker swarm reflecting convergent northward flow. In some sectors the two landform systems give conflicting evidence regarding deglaciation pattern. We have interpreted the ice sheet dynamics in Labrador from morphological data, using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns. In the George River area we have found that the Ungava Bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation. A similar overprint also exists along the intersection line in west-central Labrador. These relations show that the previously accepted relative ages of the two landform systems (Hughes, 1964; Boulton and Clark, 1990a,b; Klassen and Thompson, 1993) have to be reversed. We interpret the Ungava Bay lineation and esker swarm to represent a 0.25 × 106km2 pre-Late Wisconsinan relict landscape, formed during the deglaciation of an older ice sheet and later preserved in a dry-based central zone of the Labrador dome.

Glacial geomorphology of the Bayan Har sector of the NE Tibetan Plateau

Abstract Please click here to download the map associated with this article. We here present a detailed glacial geomorphological map covering 136,500 km2 of the Bayan Har sector of the northeastern Tibetan Plateau—an area previously suggested to have nourished the most extensive Quaternary glaciers of the Tibetan Plateau. The map, presented at a scale of 1:650,000, is based on remote sensing of a 90 m SRTM digital elevation model and 15/30 m Landsat ETM+ satellite imagery. Seven landform types have been mapped; glacial valleys, glacial troughs, glacial lineations, marginal moraines, marginal moraine remnants, meltwater channels and hummocky terrain. A large number of glacial landforms exist, concentrated around mountain blocks protruding above the surrounding plateau area, testifying to former glacial activity. In contrast, large plateau areas of lower altitude lack glacial landforms. The mapped glacial geomorphology indicates multiple former glacial advances primarily by valley and piedmont glaciers, but lends no support to the hypothesis of ice sheet scale glaciation in the area. The presented glacial geomorphological map demonstrates the usefulness of remote sensing techniques for mapping the glacial geomorphology of the Tibetan Plateau, and it will be used for reconstructing the paleoglaciology of the Bayan Har sector of the northeastern Tibetan Plateau.