Slow, patchy landscape evolution in northern Sweden despite repeated ice-sheet glaciation
Published:January 01, 2006
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A.P. Stroeven, J. Harbor, D. Fabel, J. Kleman, C. Hättestrand, D. Elmore, D. Fink, O. Fredin, 2006. "Slow, patchy landscape evolution in northern Sweden despite repeated ice-sheet glaciation", Tectonics, Climate, and Landscape Evolution, Sean D. Willett, Niels Hovius, Mark T. Brandon, Donald M. Fisher
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The conventional assumption that erosion by ice sheets is pervasive and effective in landscape evolution is tested in northern Sweden using geomorphic mapping and cosmogenic nuclide analyses of formerly glaciated surfaces. The following evidence indicates that recent glaciations in this region have produced only slow and patchy landscape evolution: (1) Geomorphic mapping shows that at least 20% of the repeatedly glaciated study region in the northern Swedish mountains has landforms that are relict, i.e., clearly nonglacial in origin. (2) The contrast between cosmogenic apparent exposure ages from relict landforms in the northern Swedish mountains and from overlying glacial erratics and juxtaposed glacially eroded bedrock surfaces, which are consistent with last deglaciation, implies that the relict landforms have been preserved through multiple glacial cycles. (3) Apparent 10Be and 26Al exposure ages for tor summit bedrock surfaces in the northern Swedish lowlands reveal that these relict landforms have survived at least eleven exposure and ten burial events with little or no erosion over the past ∼1 m.y. (4) The northern Swedish lowland and mountains are primarily covered by glacial landforms. However geomorphic mapping suggests that even these landforms may have undergone limited erosion during the last glacial cycle. Cosmogenic 10Be and 36Cl data from what appear to be heavily scoured areas in one glacial corridor indicate erosion of only ∼2 ± 0.4 m of bedrock during the last glaciation. These results suggest that in some areas the overall modification produced by ice sheets may be more restricted than previously thought, or it has occurred preferentially during earlier Quaternary glacial periods.
In the late nineteenth century, Ramsay (1864, 1876), Judd (1876), Bonney (1871, 1874), and Irving (1883), among others, had a protracted debate about whether landscape modification by ice was purely superficial, or if it was in fact largely responsible for the great depth and unique form of glaciated valleys. Although the significance of glacial erosion was still doubted by some influential geologists in the early twentieth century (e.g., Bonney, 1910; Garwood, 1910), since the 1880's it has generally been accepted that glaciers and ice sheets are capable of significant erosion (Davies, 1969). This notion that ice sheets are pervasive agents of erosion has been fostered by:
The visual impression of huge amounts of erosion in glaciated mountain landscapes; i.e., steep-sided glacial troughs, overdeepened basins, which, particularly in coastal settings, are impressive because they have been inundated by the sea (fjords);
the ubiquity of lakes and lake basins in formerly glaciated regions, and the virtual absence of these in regions that have never been glaciated;
the large volumes and ubiquitous occurrence of glacial and glacifluvial deposits; and
measurements of subglacial erosion by mountain glaciers (the primary source of field knowledge).
Hence, it is the overwhelming presence of a diverse array of evidence of the actions of ice that has led to the popular notion of pervasive ice-sheet modification of underlying landscapes. In addition, there is abundant offshore evidence for the erosion, transport, and deposition of large amounts of debris by glaciers. Typically, these sediments occur in packages called fans, a word that links to the idea that these deposits relate to the location of the grounding line of former ice streams (e.g., Solheim et al., 1996).
Notwithstanding the indisputable evidence for local large volumes of glacial erosion (carving of fjords, U-shaped valleys, and piedmont lakes) and deposition (offshore fans), the pervasiveness of ice-sheet erosion has continued to be questioned (Glasser and Hall, 1997; Hall and Sugden, 1987; Kleman and Stroeven, 1997; Lidmar-Bergström, 1997; Sugden, 1968, 1974, 1976). Alternative models include the possibility that individual ice sheets may have a patchy impact on the landscape, but that the combined impact of consecutive ice sheets affect most of the landscape at some point in time. Several types of observations support this alternative model. First, the large-scale geomorphology of many mountainous regions in formerly glaciated areas indicates large spatial differences in glacial erosion (Sugden 1968, 1974). For example, the typical geomorphology of these mountains is a patchwork of glaciated terrain and relict upland surfaces (Ahlmann, 1919; Gjessing, 1967; Kleman and Stroeven, 1997; Peulvast, 1988; Reusch, 1901; Sugden and Watts, 1977; Wråk, 1908). Glacial terrain is recognized through the presence of U-shaped valleys, cirques, horns and arrêtes, hanging valleys, mountain asymmetry, and the ubiquitous presence of lakes (e.g., Sugden and John, 1976). Relict (or remnant) upland surfaces, on the other hand, are recognized through the presence of winding V-shaped valleys, mountain symmetry, tors, weathering mantles, and an absence of (water-filled) rock basins (Kleman and Stroeven, 1997; Stroeven et al., 2002b; Sugden and Watts, 1977). This patchwork pattern of glacial erosion and preservation could either result from restricted ice extents (glaciers limited to valleys) or, where we know that large ice sheets covered the mountains, such as in northern Sweden, from complex patterns of ice-sheet basal thermal regimes.
Second, from the cross-cutting patterns of sets of glacial lineations (glacial morphology formed in a flow-parallel direction), it was concluded that glaciers have only very gently remolded pre-existing morphological features during subsequent glacial stages (Kleman et al., 1997). In these situations, neither erosion nor deposition was pervasive enough to remove the pre-existing glacial morphology. One reason for this apparent inability to change the landscape surface could be that glacial erosion is a very slow process. This explanation is presumably at odds with abundant observations on contemporary glaciers regarding the relative efficacy of glacial evacuation of loose debris and even erosion into competent bedrock. Another reason could be that the conditions appropriate for landscape modification by ice sheets only occurred during short periods of time (a transient feature). Because this explanation appears more likely, there now is a growing awareness that ice sheets can in fact protect the landscape for extensive periods of time (André, 2004; Clarhäll and Kleman, 1999; Dyke, 1993; Hättestrand and Stroeven, 2002; Kleman, 1992, 1994; Kleman and Borgström, 1990; Kleman and Hättestrand, 1999).
Third, the existence of interstadial and interglacial sediments in formerly glaciated regions signifies the relative inability of subsequent ice sheets to erode these sediments (Lagerbäck and Robertsson, 1988; Mangerud et al., 1979). In comparison to the preservation of “undated” glacial morphology, the presence of interstadial or interglacial sediments helps define the length of the time during which these sediments were subglacially preserved. Similarly significant, but harder to date, are occurrences of periglacial features or glacial deposits underneath more recent tills.
Finally, an abundance of direct observations and remotely sensed data indicate that contemporary ice sheets, ice caps, and polar glaciers are able to preserve older geomorphological features, sediments, or vegetation. For example, Arctic ice caps have retreated across older geomorphological features such as raised shorelines (e.g., Jonsson, 1983), the East Antarctic Ice Sheet retreated across Pliocene marine sediments during the Holocene (Fabel et al., 1997; Pickard, 1986), and the melting Greenland Ice Sheet and Arctic ice cap margins have exposed vegetation in growth position (Bergsma et al., 1984; Falconer, 1966; Goldthwait, 1960; Holmgren et al., 1984). The implication of all these observations is that although the ice masses once covered these locations, they were unable to erode their substrate.
Even though it is now becoming apparent that large areas underneath former ice sheets were either preserved or only slightly modified during the last glacial cycle, we have much less evidence for the behavior of earlier ice sheets. Based on the non-glacial nature of some mountain upland surfaces (Kleman and Stroeven, 1997; Sugden, 1968, 1974; Sugden and Watts, 1977) and extensive lowland surfaces (Lidmar-Bergström, 1997; Rudberg, 1954; Wråk, 1908), however, it can be argued that these have remained unaffected by glacial erosion for the entire duration of the Cenozoic ice ages (Hättestrand and Stroeven, 2002; Kleman and Stroeven, 1997). Moreover, relict glacial depositional morphology may be of a variety of ages, and glacial erosional features may be relict features. These questions are pertinent to the issue of whether the long-term evolution of landscapes in some areas of repeated glaciation is slow and patchy, and are addressed in this paper by means of cosmogenic nuclide studies performed in northern Sweden.
The primary method used in our effort to estimate amounts, rates, and patterns of glacial erosion is the interpretation of cosmogenic radionuclide concentrations in samples from rock surfaces. The cosmogenic radionuclides 10Be (half-life = 1.51 × 106 yr), 26Al (half-life = 7.1 × 105 yr), and 36Cl (half-life = 3.01 × 105 yr) are produced in rocks near the ground surface by reactions with secondary and tertiary cosmic ray neutrons and muons (Lal and Peters, 1967). These radionuclides are commonly used in studies of landscape evolution because they only form within a few meters of Earth's surface in quartz (10Be,26Al) and feldspar (36Cl), which are ubiquitous minerals in crustal rocks and sediments (cf. Bierman, 1994; Cerling and Craig, 1994; Fabel and Harbor, 1999; Gosse and Phillips, 2001; Nishiizumi et al., 1993). The in situ production provides a means of determining the amount of time the mineral has been at or near the ground surface (apparent exposure age; Lal, 1991). Because nuclide production decreases with depth, removal of two or more meters of irradiated rock during a glacial event will create a zero age surface (Fabel et al., 1997). In this context, areas known to have been ice covered should have exposure ages equivalent to deglaciation if they were significantly eroded by ice, and older exposure ages if they suffered limited erosion or were completely protected.
Depending on the level of sophistication of the data set for the different subregions studied, we employed one or more of the following three interpretations. First, single radionuclide evidence was used to acquire minimum limiting ages and maximum limiting erosion rates for these sites. This represents the simplest interpretation of the data, as it assumes continuous exposure (i.e., no burial by ice). Second, when both 26Al and 10Be were measured on the same samples, we used the 26Al/10Be ratio and the individual radionuclide concentrations to calculate minimum limiting total exposure and shielding (burial) durations, closely following the Bierman et al. (1999) approach. Finally, we calculated the minimum total history required to produce the exposure and shielding duration for samples with multiple isotopes, using the Deep Sea Drilling Project (DSDP) site 607 marine benthic foraminifera oxygen isotope record (Lazarus et al., 1995; Raymo et al., 1989; Ruddiman et al., 1989) as a proxy for the approximate duration of periods of ice-sheet cover versus ice-free conditions (Fabel et al., 2002).
The sampling strategy reflects the questions posed in the introduction in that we collected (1) samples of relict upland bedrock surfaces (upland tors), (2) samples of bedrock surfaces created by significant glacial erosion (“glacial surfaces”: on uplands and in the valleys), and (3) samples of bedrock or boulders from deglaciation features (i.e., meltwater channels, rock slope failures) and of erratics (ice-transported boulders). Field sampling procedures and laboratory procedures followed standards set by others, and have been detailed elsewhere (e.g., Fabel et al., 2002; Stroeven et al., 2002a, 2002c).
The cosmogenic radionuclide results given in Table 1 refer to the three geomorphological settings of interest: (1) individual mountain blocks, (2) individual glacial valleys, and (3) the northern Swedish lowland (Fig. 1). The first region is represented by eight bedrock samples for which cosmogenic radionuclides in a simple approach would indicate that they have been preserved through at least the last glacial cycle, because their apparent exposure ages are older than local deglaciation ages (32.7 ± 2.0 to 72.6 ± 4.4 ka). Local deglaciation ages of the relict bedrock surfaces are represented by four dates on erratic boulders (7.4 ± 0.5 to 12.7 ± 0.8 ka) and two dates from glacial surfaces (cut transverse to ice flow), formed when the ice sheet plucked bedrock to considerable (>2 m) depth (9.7 ± 0.7 and 10.8 ± 0.7 ka; cf. Fabel et al., 2002).
In the deep glacial valleys, bare bedrock outcrops are a common feature (Fig. 2). Because these valleys appear to have been deepened locally by hundreds of meters of glacial erosion, the expectation is that bedrock samples reflect deglaciation ages. Two samples derived from the bottom (10.3 ± 1.7 and 14.3 ± 0.9 ka), slopes (9.5 ± 1.5 and 17.3 ± 2.8 ka), and rock bastions within these valleys (13.9 ± 0.9 and 47.6 ± 2.9 ka) show a broad range of ages. They can be compared to local deglaciation ages derived from erratics and the upland glacial surfaces mentioned before and the ages of two boulders in a rock slope failure (9.1 ± 0.6 and 13.1 ± 2.1 ka). The rock fall was triggered by, and deposited onto, a decaying ice surface (cf. Rapp, 1960).
The northern Swedish lowland region is characterized by low-relief surfaces that slope toward the Baltic depression. One of these low relief surfaces is characterized by a sudden absence of glacial lineations and the presence of numerous tors (Hättestrand and Stroeven, 2002), four of which were dated (37.0 ± 2.3 ka to 76.6 ± 4.7 ka). They have radionuclide inventories in excess of the local deglaciation inventory, represented by a bedrock sample from a meltwater channel (11.0 ± 0.8 ka; Stroeven et al., 2002c). More typically, however, the northern Swedish lowland region is characterized by extensive sets of (partly cross-cutting) glacial lineations and an associated system of eskers. Drumlins are the most impressive glacial lineations, in terms of amounts of material eroded and deposited. A pilot study including one apparent exposure date concluded that drumlins are complex features that have formed over multiple glacial cycles (41.8 ka; cf. Hättestrand et al., 2004).
The cosmogenic radionuclide data gathered in this study strongly support the concept that some ice sheets have a minimal influence on the long-term landscape evolution of formerly glaciated regions. The results from apparent exposure ages are consistent with expectations, based on the patterns of erosion as deduced from geomorphological evidence (Hättestrand and Stroeven, 2002; Stroeven et al., 2002b). The evidence is all counter to the concept that recent ice sheets are pervasive agents of erosion. This view can only be maintained as a possible hypothesis for localities in northern Sweden that have not yet been studied using cosmogenic nuclide approaches.
Relict Uplands (∼900–2100 m above sea level)
The evidence from the mountains clearly shows that relict surfaces, as defined by geomorphology (Figs. 2A, 2B, and 2E), are also “relict” in terms of bedrock samples having inherited cosmogenic radionuclide inventories. For the first time, it has been possible to advance beyond the semiqualitative statements that geomorphology can offer, and determine the evolution history (minimum exposure age and burial period) for these surfaces. Fabel et al. (2002) integrated the evidence from Mount Tjuolmma, the best locality in the mountains, and arrived at a conservative youngest age for the relict bedrock surface of 845−418 +461 ka. This result is quantitatively consistent with recent results from mountain summits in other formerly glaciated regions (Briner et al., 2003; Marsella et al., 2000; Small et al., 1997).
Hence, from a long-term landscape evolution point of view, these relict surfaces have been insignificantly lowered during the Cenozoic ice ages. Processes that would act to lower these surfaces are frost weathering and periglacial slope processes, such as solifluction. Of these options, periglacial processes dominate as indicated by (1) the absence of linear features of fluvial origin, (2) the ubiquitous presence of gentle convex-concave slope profiles, (3) the presence of sporadic permafrost, (4) the ubiquitous presence of active and relict periglacial phenomena across these uplands, and (5) the presence of tors (our sample localities). In conjunction with the sporadic presence of permafrost (Lundqvist, 1962; Sollid et al., 2000), transport of solutes in (melt)water through the active layer probably promoted widespread but low rates of long-term landscape lowering (Stroeven et al., 2002b).
Glacial Valleys (∼500–900 m above sea level)
The two samples from Riksgränsen, although sampled from roche moutonnée surfaces at the bottom of a prominent glacial valley that forms the Torneträsk depression (Fig. 2C), (which is the most likely location for severe glacial scouring), can test for the concept of selective linear erosion (Sugden, 1968). Because the glacial valleys occur adjacent to relict surfaces at intermediate and high elevations (Kleman and Stroeven, 1997), the interpretation is that there have been large spatial differences in rates of glacial erosion. The preglacial fluvial drainage pattern and relief of the landscape have dictated these spatial differences in erosion. When ice first started to grow on these mountains, it preferentially deepened existing valleys and depressions. Because pressure-melting conditions at the base of an ice sheet are required for both basal sliding and extensive glacial erosion to occur, it is likely that these conditions were first met where the ice sheet was thickest, such as in pre-existing valleys. The early glaciers and ice sheets thus enhanced the existing relief, which created a positive-feedback effect on subglacial temperature and pressure-melting patterns (Mazo, 1991; Oerlemans, 1984). Pre-existing depressions and valleys aligned to the ice-flow direction were preferentially eroded, and other areas were subject to much less or no erosion, a trademark of the process of selective linear erosion. A comparison of our data from Riksgränsen with older bedrock ages indicates a magnitude of ∼2 ± 0.4 m of valley-bottom erosion of bedrock during the last glacial cycle (Stroeven et al., 2002a). This rock loss depth yields an erosion rate for the glacial corridor that is at least an order of magnitude higher than maximum erosion rates over relict surfaces, which typically are 1–2 m/m.y., a result that is clearly consistent with the concept of selective linear erosion.
Although two features formed by the process of selective linear erosion are typical ingredients in this landscape, i.e., sharp glacial surface–preglacial surface boundaries and hanging valleys, which are formed by glacial valley widening and deepening, respectively, and although the pattern of erosion determined by cosmogenic nuclide dating is consistent with the concept of selective linear erosion, the magnitude of erosion of the trunk valley is surprisingly lower than expected. Considering that the larger glacial valleys have not been deepened by more than ∼400 m (Stroeven et al., 2002b) (although some valleys appear to have experienced as much as almost 900 m of erosion [Kleman and Stroeven, 1997]), 2 m of erosion per glacial cycle (e.g., Riksgränsen) is insufficient to explain the presence of these depressions. These results indicate that the overall modification produced by ice sheets along glacial corridors may either be more restricted than previously thought, or it has occurred preferentially during earlier Quaternary glacial periods. The potentially old age of significant deepening of the glacial valleys is corroborated by the presence of inherited nuclides in a clearly glacially scoured bedrock surface on the Bálddavárri rock bastion in the Kaitum depression (47.6 ± 2.9 ka; Table 1).
Northern Swedish Lowland (∼0–500 m above sea level)
The northern Swedish lowland is characterized by a stepped “plains with residual hills” morphology generally at 300–400 and 400–550 m above sea level (e.g., Fredén, 1994, p. 50–51), referred to as the Muddus Plains (Wråk, 1908). The Muddus Plains are regarded as the youngest preglacial fluvial surfaces of the region, the youngest of which, then, was the last base level for the evolution of the preglacial fluvial drainage system in the mountain range. If this interpretation is correct, the implication is that the northern Swedish lowland has not been significantly deepened (Lidmar-Bergström, 1997), save for an array of piedmont lakes that fringe the mountain range and probably relate to the inception stages of successive Fennoscandian ice sheets (Fredin, 2002; Kleman, 1992). The area with tors investigated in this study (Figs. 1 and 2D) is part of the lower Muddus surface, and the results qualitatively support the notion that these surfaces have not been degraded significantly. In fact, when Stroeven et al. (2002c) considered the mean value of the two tors yielding both 26Al and 10Be results, they concluded that the tors have survived for at least 605 k.y., and experienced a maximum erosion rate of 1.6 m/m.y.
These results are also in concert with the cosmogenic radio-nuclide results from the relict surfaces in the mountains. If the determined rate of erosion proves to be typical for the northern Swedish lowland as a whole, and the apparent exposure age of the Teletöisentunturi drumlin is provisionally consistent with this assumption, then the general subglacial condition of Fennoscan-dian ice sheets has been one of preservation or deposition. Because lineation systems overwhelmingly relate to the deglaciation of the last ice sheet, the inference is that successive Fennoscandian ice sheets were largely cold-based during inception and growth stages (east of the mountain range). Destruction of the cold-based core occurred during deglaciation and predominantly along fastflowing corridors (e.g., Kleman and Hättestrand, 1999).
Geomorphological and cosmogenic evidence indicates that erosion over relict surfaces, which comprises ∼20% of the area in the northern Swedish mountains and most of the northern Swedish lowland, has been negligible. These relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice-sheet and landscape development models, and as potential refuges for biota that can survive long periods of frozen conditions. The geomorphological and cosmogenic nuclide evidence from the northern Swedish mountains also provides strong support for a model of patchy and slow landscape evolution as a result of repeated ice-sheet glaciation in this region. Although this may not hold for all ice sheets, it is a model that should be tested for other areas of repeated ice-sheet glaciation, as it has potentially significant implications for ice-sheet reconstructions, landscape development models, and interpretations of sedimentary sequences in ocean basins.
The authors are indebted to Andrew Meigs for a very helpful review, and to Colin E. Thorn and Robert G. Darmody of the University of Illinois, who supplied the 36Cl results reported in this paper through National Science Foundation grant BCS-9818667. Swedish Natural Sciences Research Council grants G-AA/GU 12034-300 and G-AA/GU 12034-301 to Stroeven, and National Science Foundation grants OPP-9818162 and OPP-0138486 to Harbor financially supported this work.
Tectonics, Climate, and Landscape Evolution
- alkaline earth metals
- cosmogenic elements
- exposure age
- glacial erosion
- glacial geology
- glaciated terrains
- ice sheets
- landform evolution
- radioactive isotopes
- relict materials
- Scandinavian ice sheet
- upper Quaternary
- Western Europe
- northern Sweden