Johan Kleman

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.

Preservation of landforms under ice sheets and ice caps

Abstract This article addresses the question of whether or not distinct glacial and non-glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration. A review of recent work documents the existence of pre-last stadial landforms and landscapes in areas covered by the Fennoscandian and Laurentide ice sheets. A substantial number of independent works indicate that landforms such as eskers, drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia. Full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice-sheet base was continuously frozen to its bed. Larger “robust” landforms, such as large drumlins, appear to have been preserved even under wet-based conditions. In glaciated areas, patches preserved under dry (cold)-based conditions provide important windows towards the past, showing landscapes that were destroyed in surrounding areas affected by wet-based and eroding ice. Some consequences for the research fields of non-glacial geomorphology, archaeology and botany include the possibility of subglacial museums and refugia. A time/ space model describes geomorphological access to information from older events in glaciated areas.

Landscape preservation under Fennoscandian ice sheets determined from in situ produced 10Be and 26Al

Some areas within ice sheet boundaries retain pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were preserved under ice. Differentiating between these alternatives has significant implications for paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic 10Be and 26Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces, provide consistent last deglaciation exposure ages (∼8–13 kyr), confirming ice sheet overriding as opposed to ice free conditions. However, these ages contrast with exposure ages of 34–61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were preserved with little erosion through multiple glacial cycles. Based on the difference in radioactive decay between 26Al and 10Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845−418+461 kyr. These results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models, and in landscape development models. Subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as potential long-term subglacial biologic refugia.

Preglacial surface remnants and Quaternary glacial regimes in northwestern Sweden

Abstract We present a detailed map of the distribution of preglacial surface remnants in the Kebnekaise region of northwestern Sweden. In this mountain area we discern four important large-scale geomorphological units, each representing a specific set of erosional agents and formative conditions. These are: (i) intact preglacial surface remnants, characterized by gentle slopes, round summits, wide shallow valleys, and an absence of rock basins; (ii) preglacial surface remnants showing signs of minor glacial erosion and deposition; (iii) glacially scoured surfaces, including glacial troughs; (iv) deep fluvial valleys cut into the preglacial surface. The pattern of glacial erosion is explained as the result of three specific modes of glaciation known to have existed during the last 120,000 years, and inferred to have repeatedly prevailed during the last 2.75 million years: cirque glaciation, mountain ice sheets, and Fennoscandian ice sheets. A deep-ocean oxygen-isotope record of foraminifera from the North Atlantic (DSDP 607) was used to infer the temporal extent of these modes of glaciation during the last 2.75 million years. We interpret the preglacial landscape preservation and the pattern of glacial erosion in terms of the configuration, the basal thermal regime, and the duration of such glaciation events. The average subglacial thermal regime of both ice sheet types was frozen on the uplands and melting in the main valleys, where outlet glaciers and ice-streams formed. The pre-glacial landscape is best preserved at intermediate elevations, low enough not to have been covered by cirque glaciers, and apparently high enough not to have experienced melted-bed conditions and subglacial erosion during ice sheet overriding events. In a narrow high-relief zone along the elevation axis, interglacial fluvial erosion was morphologically important. The absence of glacial erosion on uplands in this zone allowed fluvial erosion to commence on the same locations during each ice-free interval. In contrast, no persistent fluvial valley pattern could develop in zones subjected to repeated glacial scouring and hence, derangement of fluvial patterns.

RECONSTRUCTION OF PALAEO‐ICE SHEETS: THE USE OF GEOMORPHOLOGICAL DATA

The article discusses the nature of the glacial inversion problem, which is defined as the extraction of time-slice ice-sheet flow patterns from the patchy and partly overprinted landform record present in former ice-sheet areas. A coherent inversion model for derivation of flow patterns and interior ice-sheet configuration from geomorphological data is presented. Glacial landscapes are classified according to the three criteria of internal age gradients, presence or absence of meltwater traces aligned to flow traces, and basal condition (frozen bed/thawed bed) inferred from morphology. The inversion model uses landscapes classified accordingly, spatially delineated into fans, as input data. Relative chronologies at fan intersections are used to sort fans in a relative-age stack that can be linked to stratigraphic (dating) information.

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.

The Palimpsest Glacial Landscape in Northwestern Sweden: Late Weichselian deglaciation landforms and traces of older west-centered ice sheets

ABSTRACTThe glacial landscape in northwestern Sweden is a palimpsest of landforms created by several ice sheets. Using crosscutting relationships and other morphological criteria, the glacial and glaciofluvial landforms formed during the Late Weichselian recession were discriminated and mapped separate from landforms from older glacial events. The geographical pattern of the last deglaciation could thus be reconstructed on the basis of “young” landforms only. The Late Weichselian ice sheet was over large areas continuously cold based. Only in limited zones were thawed-bed conditions reached during the deglaciation. The older land-forms all reflect ice flow and meltwater drainage from smaller ice sheets related to the mountain chain. A marginal position, characterized by lateral moraines, of such an ice sheet was discovered along the eastern rim of the mountain chain. This marginal zone is interpreted to represent a temporary halt in the recession of an older west-centered ice sheet, possibly during isotop...

Glacial land forms indicative of a partly frozen bed

In parts of the core area of the Fennoscandian ice sheet relict periglacial surfaces occur. The boundary between periglacial and glacial landscapes is often sharp and erosional, with fluting truncating patterned ground. The periglacial surfaces are older than the last ice sheet and are interpreted to represent patches of continuous frozen-bed conditions. A specific land-form assemblage occurs at the edges of such patches. On the basis of three type localities along the eastern rim of the Scandinavian mountains, four thermal boundary land forms, characteristic of the frozen-patch environment, are defined. Stoss-side moraines and transverse till scarps, not previously described, are interpreted to have formed in detachment zones where soil frozen to the glacier overlies thawed soil. The detachment zones are located where subglacial warming raises the phase-change surface (water/ice) until it intersects the soil layer upand down-glacier from residual frozen-bed patches. The up-glacier ends of frozen-bed patches are located on topographic highs, but downglacier the location of lateral sliding boundaries is occasionally independent of topography. The identification of relict surfaces and thermal boundary forms can improve paleo-ice-sheet models by providing estimates of the extent of frozen-bed conditions.

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.