Arjen P. Stroeven

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.

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.

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.

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.

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.

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.

The Mount Feather Diamicton of the Sirius Group: an accumulation of indicators of Neogene Antarctic glacial and climatic history

Abstract A paucity of data from the Antarctic continent has resulted in conflicting interpretations of Neogene Antarctic glacial history. Much of the debate centres on interpretations of the glacigene Sirius Group strata that crop out as discrete deposits along the length of the Transantarctic Mountains and in particular on its age and the origin of the siliceous microfossils it encloses. Pliocene marine diatoms enclosed within Sirius Group strata are inferred to indicate a dynamic East Antarctic ice sheet that was much reduced, compared with today, in the early–middle Pliocene and then expanded again in the late Pliocene. However, the geomorphology of the Dry Valleys region is interpreted to represent a relatively long-lived (middle Miocene–recent) and stable polar climatic regime similar to that of today. The Mount Feather Diamicton infills a palaeovalley at ca. 2500 m on the NE flank of Mount Feather in the Dry Valleys region and has been included within the Sirius Group. We obtained four shallow cores (COMRAC 8, 9, 10 and 11) from beneath the permafrost boundary in the Mount Feather Diamicton in order to understand its origin and relationship with the surrounding landscape. Detailed studies of these cores (stratigraphy, sedimentology, palaeontology, micromorphology, petrography and fabric) have yielded new data that demonstrate a much more complex climatic and glacial history for the Mount Feather Diamicton than in previous interpretations. The data indicate that the Mount Feather Diamicton was deposited beneath a wet based glacier fed from a larger ice sheet behind the Transantarctic Mountains. It is, however, unlikely that this ice sheet overtopped Mount Feather (2985 m). A near-in situ non-marine diatom assemblage was recovered from 90 cm depth in COMRAC 10 and indicates a maximum depositional age of Late Miocene for the Mount Feather Diamicton. A subsequent glacial episode has distributed a boulder blanket across the surface of the diamicton. Other post-depositional processes include drying, infilling of surface layers with aeolian sediment, and the development of melt-water runnels. We interpret these combined data to indicate the persistence of more temperate climatic and glacial conditions in the vicinity of Mount Feather until at least the Late Miocene.

Age of Sirius Group on Mount Feather, McMurdo Dry Valleys, Antarctica, based on glaciological inferences from the overridden mountain range of Scandinavia

Abstract A striking difference in the geomorphology of the ice-sheet overridden lowlands and ranges in Scandinavia and the Wilkes Basin–Transantarctic Mountains region in Antarctica is the occurrence of basal till sequences (of the Sirius Group) on summits and interfluves in the Transantarctic Mountains and the lack of basal tills on corresponding surfaces in Scandinavia. From the pattern of glacially eroded surfaces and preserved preglacial surfaces in northern Sweden, we infer altitudinal and lateral patterns of glacial erosion and deposition governed by topography-dependent basal thermal regimes. Furthermore, summits and interfluves in northern Sweden are often remnants of preglacial surfaces. Consequently, these preglacial surface remnants were preserved underneath patches of cold-based ice. We conclude that the subglacial thermal zonation of ice sheets overriding peripheral mountain ranges is self-sustained by topography; i.e., cold-based ice relates to topographic highs and warm-based ice to topographic lows. We use these inferences to explore the glaciological implications for the Sirius Group sequence on Mount Feather, McMurdo Dry Valleys. A glaciologically plausible explanation for this deposit and similar Sirius Group sequences on dissected summit and interfluve surfaces in the Transantarctic Mountains is that deposition occurred before the bulk of the relief formed. The present relief in the Mount Feather vicinity is at least 11 Ma old. Consequently, the Sirius Group at Mount Feather is at least of middle Miocene age.

On marine microfossil transport and pathways in Antarctica during the late Neogene: Evidence from the Sirius Group at Mount Fleming

There are two extreme views of the evolution of the Pliocene Antarctic Ice Sheet. Dynamicists argue for ice-sheet reduction and reexpansion on the basis of Pliocene marine diatoms in a glacial deposit, the Sirius Group, that is widespread in the Transantarctic Mountains. Stabilists argue from other evidence that the Antarctic cryosphere remained essentially constant in area and volume; they propose marine diatom transport by eolian processes and emplacement into terrestrial glacial strata. Hence, the inferred source area and transport mechanism of marine diatoms are of critical importance. We tested the reduction hypothesis on an important outcrop of the Sirius Group at Mount Fleming, South Victoria Land. We observed very few, unidentifiable, marine diatom fragments in Sirius Group strata. In contrast, marine diatoms enclosed in a surface diamicton covering the Sirius Group were more abundant and identifiable. Our study further indicates that the Sirius Group at Mount Fleming was not deposited by the East Antarctic Ice Sheet but rather by alpine ice originating on the Transantarctic Mountains. On the basis of both data sets, we infer that marine diatoms postdate Sirius Group deposition at Mount Fleming and that transport was by wind, and we advance alternative scenarios for their source and transport pathways.

Towards a GIS assessment of numerical ice-sheet model performance using geomorphological data

A major difficulty in assimilating geomorphological information with ice-sheet models is the lack of a consistent methodology to systematically compare model output and field data. As an initial step in establishing a quantitative comparison methodology, automated proximity and conformity analysis (APCA) and automated flow direction analysis (AFDA) have been developed to assess the level of correspondence between modelled ice extent and ice-marginal features such as end moraines, as well as between modelled basal flow directions and palaeo-flow direction indicators, such as glacial lineations. To illustrate the potential of such an approach, an ensemble suite of 40 numerical simulations of the Fennoscandian ice sheet were compared to end moraines of the Last Glacial Maximum and the Younger Dryas and to glacial lineations in northern Sweden using APCA and AFDA. Model experiments evaluated in this manner were ranked according to level of correspondence. Such an approach holds considerable promise for optimizing the parameter space and coherence of ice-flow models by automated, quantitative assessment of multiple ensemble experiments against a database of geological or glaciological evidence.