Earth Surf. Dynam., 2, 383401, 2014
www.earth-surf-dynam.net/2/383/2014/
doi:10.5194/esurf-2-383-2014
Author(s) 2014. CC Attribution 3.0 License.
B. W. Goodfellow1,2, A. P. Stroeven1, D. Fabel3, O. Fredin4,5, M.-H. Derron5,6, R. Bintanja7, andM. W. Caffee8
1Department of Physical Geography and Quaternary Geology, and Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden
2Department of Geology, Lund University, 22362 Lund, Sweden
3Department of Geographical and Earth Sciences, East Quadrangle, University Avenue, University of Glasgow, Glasgow G12 8QQ, UK
4Department of Geography, Norwegian University of Science and Technology (NTNU),7491, Trondheim, Norway
5Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
6Institute of Geomatics and Risk Analysis, University of Lausanne, 1015 Lausanne, Switzerland
7Royal Netherlands Meteorological Institute, Wilhelminalaan 10, 3732 GK De Bilt, the Netherlands
8Department of Physics, Purdue University, West Lafayette, Indiana, USA
Correspondence to: B. W. Goodfellow ([email protected])
Received: 22 January 2014 Published in Earth Surf. Dynam. Discuss.: 10 February 2014 Revised: 9 June 2014 Accepted: 24 June 2014 Published: 21 July 2014
Abstract. Autochthonous blockeld mantles may indicate alpine surfaces that have not been glacially eroded. These surfaces may therefore serve as markers against which to determine Quaternary erosion volumes in adjacent glacially eroded sectors. To explore these potential utilities, chemical weathering features, erosion rates, and regolith residence durations of mountain blockelds are investigated in the northern Swedish Scandes. This is done, rstly, by assessing the intensity of regolith chemical weathering along altitudinal transects descending from three blockeld-mantled summits. Clay / silt ratios, secondary mineral assemblages, and imaging of chemical etching of primary mineral grains in ne matrix are each used for this purpose. Secondly, erosion rates and regolith residence durations of two of the summits are inferred from concentrations of in situ-produced cosmogenic 10Be and 26Al in quartz at the blockeld surfaces. An interpretative model is adopted that includes temporal variations in nuclide production rates through surface burial by glacial ice and glacial isostasy-induced elevation changes of the blockeld surfaces. Together, our data indicate that these blockelds are not derived from remnants of intensely weathered Neogene weathering proles, as is commonly considered. Evidence for this interpretation includes minor chemical weathering in each of the three examined blockelds, despite consistent variability according to slope position. In addition, average erosion rates of 16.2 and 6.7 mm ka1,
calculated for the two blockeld-mantled summits, are low but of sufcient magnitude to remove present block-eld mantles, of up to a few metres in thickness, within a late Quaternary time frame. Hence, blockeld mantles appear to be replenished by regolith formation through, primarily physical, weathering processes that have operated during the Quaternary. The persistence of autochthonous blockelds over multiple glacialinterglacial cycles conrms their importance as key markers of surfaces that were not glacially eroded through, at least, the late Quaternary. However, presently blockeld-mantled surfaces may potentially be subjected to large spatial variations in erosion rates, and their Neogene regolith mantles may have been comprehensively eroded during the late Pliocene and early Pleistocene. Their role as markers by which to estimate glacial erosion volumes in surrounding landscape elements therefore remains uncertain.
Published by Copernicus Publications on behalf of the European Geosciences Union.
Arcticalpine blockelds in the northern Swedish Scandes: late Quaternary not Neogene
384 B. W. Goodfellow et al.: Late Quaternary not Neogene
1 Introduction
Autochthonous blockelds are diamicts comprised of clay-to boulder-sized regolith formed through in situ bedrock weathering (Potter and Moss, 1968; Nesje et al., 1988; Ballantyne, 1998; Boelhouwers, 2004). They are classically a feature of periglaciated landscapes, where they frequently mantle mountain summits and plateaus assumed to have undergone up to tens of metres of (non-glacial) erosion during the Quaternary (Dahl, 1966; Ives, 1966; Sugden, 1968, 1974; Nesje et al., 1988; Rea et al., 1996; Ballantyne, 1998;Small et al., 1999; Goodfellow et al., 2009; Rea, 2013). According to this interpretation, blockelds indicate surfaces that persisted as nunataks or were inundated by non-erosive cold-based ice during glacial periods. Blockeld-mantled surfaces may provide useful markers for quantifying Quaternary glacial erosion volumes in surrounding landscapes (Nesje and Whillans, 1994; Glasser and Hall, 1997; Kleman and Stroeven, 1997; Staiger et al., 2005; Goodfellow, 2007;Jansson et al., 2011). However, recent studies of landscape evolution and Quaternary sediment budgets along the Norwegian margin (Nielsen et al., 2009; Steer et al., 2012) imply that, rather than providing these markers, autochthonous blockeld-mantled surfaces have also undergone surface lowering of some hundreds of metres through the action of a Quaternary glacial and periglacial buzz saw. The origins, ages, and erosion rates of blockelds remain enigmatic, so their utility for indicating non-glacially eroded surfaces and for estimating Quaternary erosion volumes is contentious.The weathering characteristics, erosion rates, and residence durations of present autochthonous blockeld regoliths in the periglaciated northern Scandinavian Mountains (Scan-des) are investigated in this study.
Autochthonous blockelds in periglaciated landscapes are frequently hypothesized to be remnants of Neogene weathering proles (Caine, 1968; Ives, 1974; Clapperton, 1975;Nesje et al., 1988; Rea et al., 1996; Boelhouwers et al., 2002; Andr, 2003; Marquette et al., 2004; Sumner and Meiklejohn, 2004; Fjellanger et al., 2006; Paasche et al., 2006; Andr et al., 2008; Strmse and Paasche, 2011). In this model, block production is initiated through chemical weathering of bedrock during the Neogene by a warmer-than-present climate. Regolith stripping occurred during the colder Quaternary, subaerially exposing rock made more porous by chemical weathering. Enhanced access by water permitted efcient frost shattering of this rock, which was periglacially reworked to produce blockeld mantles that armour these surfaces, making them resistant to further modication (Boelhouwers, 2004).
If chemical weathering depends upon a warm climate, certain characteristics of blockelds are incompatible with a Quaternary origin. These characteristics include the presence of saprolite (Caine, 1968) and/or secondary minerals, espe-
cially kaolinite and gibbsite (Rea et al., 1996; Fjellanger et al., 2006; Andr et al., 2008; Strmse and Paasche, 2011), and clay abundances exceeding about 10 % of the ne matrix (clay, silt, sand) by volume (Rea et al., 1996; Strmse and Paasche, 2011). An additional circumstantial argument is that there are apparently no actively forming blockelds (Boelhouwers, 2004), with the exception of those developing on highly frost susceptible limestone in the Canadian Arctic (Dredge, 1992). Also, blockeld-mantled surface remnants do not appear to have been glacially eroded (Sugden, 1968, 1974; Kleman and Stroeven, 1997; Fabel et al., 2002;Marquette et al., 2004; Stroeven et al., 2006; Goodfellow, 2007). Taken together, the evidence seemingly indicates that blockelds may pre-date the last glacialinterglacial cycle.Field observations in conjunction with geochemical features may indicate regolith residence durations extending back into the Neogene (Rea et al., 1996; Whalley et al., 2004; Andr et al., 2008; Strmse and Paasche, 2011).
The Neogene-origin model is not universally accepted, and some researchers conclude that blockelds in periglaciated landscapes are entirely Quaternary features (Dahl, 1966; Dredge, 1992; Ballantyne, 1998, 2010;Ballantyne et al., 1998; Goodfellow et al., 2009; Goodfellow, 2012). In this model, blockelds are produced through synergistic physical (e.g. through frost-cracking) and chemical weathering processes (Whalley et al., 2004) that operate independently of preconditioning by Neogene processes.
Key evidence supporting the Quaternary-origin model includes slow formation of clay-sized regolith and secondary minerals through chemical weathering. This is indicated, rstly, by a low ratio of clay to silt across a sample batch (clay< 0.5 [notdef] silt), compared with higher ratios
(clay> 0.5 [notdef] silt) in regoliths located in non-periglaciated
settings (Goodfellow, 2012). Secondly, low abundances of secondary minerals are mixed in with abundant primary minerals (Goodfellow, 2012). The secondary mineral assemblages may span a range of leaching intensities including low (minerals with interstratied primary and secondary layers), moderate (2 : 1 layer minerals such as vermiculite), high (1 : 1 minerals such as kaolinite), and extreme (Al- and Fe-oxides such as gibbsite and haematite). This may reect the effect of hydrological heterogeneities on weathering intensity in blockelds and the varying susceptibility of different primary minerals to chemical weathering. These mineral assemblages differ from those occurring in subtropical and tropical regoliths, which are generally simpler and dominated by high volumes (i.e. > 30 % of the regolith) of kaolinite and Al- and Fe-oxides (Meunier et al., 2007; White et al., 1998;Goodfellow, 2012).
In the Quaternary-origin model, it is further argued that blocks form by frost cracking within the regolith, near the base of the permafrost active layer where liquid water accumulates and seasonally refreezes (Dahl, 1966; Anderson,
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 385 1998; Small et al., 1999; Hales and Roering, 2007; Goodfellow et al., 2009; Ballantyne, 2010). This mechanism mighttherefore explain the apparent absence of frost cracking ofclasts comprising blockeld surfaces while further highlighting a possible key role of Quaternary, rather than Neogene,weathering processes in blockeld formation.
A critical problem with ascertaining blockeld ages and origins is that it has, until recently, been impossible to measure blockeld erosion rates or regolith residence durations. However, measurements of terrestrial cosmogenic nuclide (TCN) concentrations now offer some insight into these issues. Erosion rates of 1.112.0 mm ka1 have been inferred for subaerially exposed bedrock within alpine block-elds (Small et al., 1997; Bierman et al., 1999; Staiger et al., 2005; Phillips et al., 2006). These rates may be lower than in the surrounding blockelds, because exposed bedrock sheds, rather than retains, water (Small et al., 1999;Cockburn and Summereld, 2004; Phillips et al., 2006).However, regolith erosion rates in summit blockelds may remain low because of armouring of gently sloping surfaces by cobbles and boulders (Granger et al., 2001; Boelhouwers, 2004). For example, erosion rates of 13.414.0 mm ka1 have been measured in plateau blockelds in the Wind River
Range, Wyoming, which have not been inundated by glacial ice (Small et al., 1999). Where non-erosive cold-based ice has buried blockelds during glacial periods (Sugden and Watts, 1977; Kleman and Stroeven, 1997; Bierman et al., 1999; Httestrand and Stroeven, 2002; Briner et al., 2003;Marquette et al., 2004), time-averaged erosion rates are further lowered. Subaerial exposure and burial durations of blockeld regoliths might then extend back in time to the early Quaternary or late Neogene. By combining measurements of TCN concentrations in bedrock or regolith with an inferred history of surface burial by ice sheets from benthic 18O records (Fabel et al., 2002; Stroeven et al., 2002; Li et al., 2008), it is possible to estimate minimum time spans over which present blockeld regoliths have mantled surfaces (i.e. minimum regolith residence durations).
In this study we test whether blockelds in the northern Swedish Scandes are remnants of intensely weathered Neo-gene regoliths or are formed solely by Quaternary weathering processes. We do this, rstly, by investigating the intensity of chemical weathering through grain size, X-ray diffraction, and scanning electron microscopy (SEM) analyses of block-eld ne matrix along three hillslope transects. Secondly, we examine regolith residence durations of two summit block-elds through the combination of apparent surface exposure durations, measured through TCN analyses, with burial durations, determined through an ice sheet model driven by benthic 18O records. Incorporating an elastic lithosphere, relaxed asthenosphere (ELRA) bedrock model, the ice sheet model is also used to study the effects of bedrock isostatic response to glacial loading and unloading on nuclide production rates (which vary with elevation above sea level) and subsequent regolith residence durations. Because uncertain-
Figure 1. Map of the study areas in the northern Swedish Scandes. The map location and sample sites along three hillslope transects are shown in the adjoining panels. Autochthonous blockelds mantling low-gradient convex summits appear to be eroded by diffusive processes, such as regolith creep, and erosion of autochthonous block-elds on steep slopes appears dominated by soliuction. Colluvial boulder drapes provide evidence of shallow landsliding and form allochthonous blockelds on the steepest regolith-mantled slopes. These comprise slope segments 1, 2, and 3, respectively, and are the focus of regolith sampling in this study. Allochthonous blockelds also form in till sheets (< 1 m thick) deposited on some glacially
eroded summits (Goodfellow et al., 2008) and on some high-altitude non-glacially eroded surface remnants (Kleman and Stroeven, 1997; Fabel et al., 2002; Goodfellow et al., 2008). Pits along each transect are numbered according to those given in Tables 1 and 2 and S1 in the Supplement. Grey areas of the maps are cliff faces, talus slopes, or surfaces modied by glacial erosion or deposition (covered in > 1 m thick tills on the transect maps). In the top right panel,
mixed colluvium and till drape the landscape below Alddasorru and Duopteohkka in the grey area west of these summits and comprises segment 4 for regolith sampling in this study. 10Be bedrock exposure ages from sites close to those used in this study are reproduced from Fabel et al. (2002) and Stroeven et al. (2006).
ties associated with calculations of regolith residence durations are large, we conne our enquiry to an order of magnitude question: are regolith residence durations likely conned to the late Quaternary (< 1 Ma) or do they extend to the early Quaternary/late Neogene? A Neogene origin would imply low Quaternary-averaged surface erosion rates and a utility of autochthonous blockeld-mantled surfaces as markers from which to estimate glacial erosion depths in surrounding landscapes. In contrast, the implications of a Quaternary origin are more ambiguous. These could not exclude tens of metres, to perhaps more than one hundred metres, of surface lowering during the Plio-Pleistocene transition of currently blockeld-mantled surfaces, resulting in a lowered utility of these surfaces as markers from which to estimate glacial erosion depths in surrounding landscape sectors.
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
386 B. W. Goodfellow et al.: Late Quaternary not Neogene2 Study area
Blockelds were examined along hillslope transects descending from three summits in the northern Swedish Scandes (Fig. 1): Alddasorru (68 25[prime] N, 19 24[prime] E; 1538 m above sea level (a.s.l.)), Duopteohkka (68 24[prime] N, 19 22[prime] E;
1336 m a.s.l.), and Tarfalatjrro (67 55[prime] N, 18 39[prime] E; 1626 m a.s.l.). The transects intersect slope segments shaped by contrasting assemblages of surface processes. Diffusive processes, such as regolith creep, have apparently shaped gently convex summits and soliuction has dominated on higher gradient downslope segments. On the steepest, lowermost slopes imbricated blocks and boulder sheets indicate that shallow landsliding and boulder tumbling have been active in addition to soliuction. Further deposition of transported material has occurred at the concave bases of these slopes where scattered boulders are embedded in, and rest upon, a ne-matrix-rich regolith. The abundance of the ne matrix here and numerous boulders, some of non-local lithology, resting on the ground surface also indicate an additional contribution of glacial till. The Tarfalatjrro tran-sect terminates 73 m below the summit in an autochthonous blockeld-mantled saddle and intersects only the diffusion-dominated segment. In contrast, the Alddasorru transect terminates on the steep mass-wasting segment 278 m below the summit, whereas the Duopteohkka transect intersects each of these segments and the soliuction-dominated segment and terminates 270 m below the summit on the mixed colluvium and till segment (Fig. 1).
Each blockeld is developed on amphibolite. Litho-logical variations were not observed along either of the Alddasorru or Duopteohkka transects except for some granitic glacial erratics. In contrast, plagioclase-porphyritic and highly schistic amphibolites were observed along the Tarfalatjrro transect, which terminates at its lower end on metapsammite. Glacial erratics also occur occasionally on Tarfalatjrro but were not observed along the prole.
The blockelds along each transect form areally continuous mantles, and bedrock outcrops are generally absent.An exception occurs on the narrow, ridge-like summit of Duopteohkka, where bedrock is frequently exposed. The blockeld surfaces are dominated by cobbles and boulders (> 90 % area) with patches of ne matrix visible only in the centres of interspersed periglacially sorted circles (mean diameters of 1.52.0 m) on Alddasorru, Tarfalatjrro, and on the upper anks of Duopteohkka. The Duopteohkka summit blockeld is not periglacially sorted, and ne matrix is absent from the surface. Ventifacted boulders and loess deposits are absent from each transect.
The study area is located where the Arctic maritime climate of Norway converges with the continental climate of northern Sweden. The mean annual air temperature (MAAT) on Tarfalatjrro during 19461995, as inferred from records of nearby Tarfala Research Station at 1130 m a.s.l. (Fig. 1), was approximately 6 C and mean annual precipitation
(MAP) was about 500 mm (Grudd and Schneider, 1996). More recent data from a permafrost monitoring borehole on Tarfalatjrro indicate a MAAT at 2 m above the ground of 4.3 C and mean annual ground temperatures of 2.8
and 3.0 C, at 0.2 and 2.5 m depth respectively, over
20032005 (Isaksen et al., 2007). The closest meteorological station to Alddasorru and Duopteohkka is located at 380 m a.s.l. at the Abisko Scientic Research Station. (Fig. 1). It has recorded a MAAT of 0.9 C and MAP
of about 320 mm (Eriksson, 1982), which is a warmer and drier climate than occurs on Tarfalatjrro. Based on this MAAT we infer that MAATs are also below zero on Alddasorru and Duopteohkka. Permafrost is present on each of the three summits, with the monitoring borehole on Tarfalatjrro indicating a distinct warming trend and a present active layer thickness of 1.41.6 m (Isaksen et al., 2001, 2007). Snow covers Tarfalatjrro from about October to May, although strong winds limit the maximum snow depth to about 0.3 m (Isaksen et al., 2001). Similar temperature, snow, and permafrost conditions are expected and assumed for Alddasorru and Duopteohkka. Vegetation along each transect is restricted to lichens, mosses, and occasional grasses, except for the base of the Duopteohkka transect, which is well grassed. Stable lichen-covered surface clasts indicate that, although they have occurred in the past, large-scale periglacial sorting and geliuction processes appear to be now largely inactive. However, upfreezing of pebbles and creep and geliuction processes over a few tens of centimetres remain active.
The northern Swedish Scandes have been repeatedly glaciated during the Quaternary, with cirque glaciation inferred to have been dominant before 2.0 million years ago (Ma), mountain ice sheets dominant between 2.0 and 0.7 Ma, and Fennoscandian ice sheets developing over the last 0.7 Ma (Kleman and Stroeven, 1997; Kleman et al., 2008). Current glaciation in the region is conned to small icecaps and small cirque and valley glaciers. During glacial periods, relatively high-altitude surfaces such as Tarfalatjrro, Alddasorru, and Duopteohkka were either exposed as nunataks or covered by cold-based ice sheets (Stroeven et al., 2006). The occasional erratics on Tarfalatjrro and abundant granitic erratics on Alddasorru and the anks of Duopteohkka conrm former ice sheet coverage as late as 12 ka (Fabel et al., 2002; Stroeven et al., 2006). However, the presence of autochthonous blockelds and the absence of till sheets and glacial erosion features, such as striated bedrock out-crops, indicate extremely minor glacial modication of Tarfalatjrro and Alddasorru. Clear evidence of glacial processes is also absent from Duopteohkka. We therefore consider the presently thin autochthonous blockeld and out-cropping bedrock to be attributable to slope transport processes operating across this narrow summit, although some glacial entrainment of blocks cannot be entirely discounted.
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 3873 Methods3.1 Field techniques
To determine the composition of blockeld regoliths, a total of 26 pits were hand excavated along the three hills-lope transects during August 20042006: 7 pits on Alddasorru, 10 pits on Duopteohkka, and 9 pits on Tarfalatjrro (Fig. 1). Blockeld sections were examined in 15 of these pits, which were excavated across sorted circles, from clast-dominated rings to ne matrix-rich circle centres, or into clast-rich soliuction lobes. The pit excavated into the summit of Duopteohkka was an exception because the thin regolith (< 0.5 m) and frequent bedrock exposures prevented periglacial sorting. The pits were excavated either until large amphibolite slabs prohibited sampling of deeper sections, the water table was intersected, or bedrock was reached. Fine matrix samples were taken from 16 blockeld pits for grain size, XRD, and SEM analyses. For replication purposes, at least three ne matrix samples were taken from each of the four surface process segments: (1) low-gradient diffusive,(2) soliuction slope, (3) steep mass-wasting, and (4) concave depositional, if and where they occur on the three tran-sects. Segment 3 was only sampled on the Alddasorru tran-sect, and only one sample was analysed from segment 2 on the Alddasorru transect. Because we previously found that only minor variations occur in ne matrix granulometry and secondary mineralogy with depth beneath the ground surface (Goodfellow et al., 2009), only one sample was analysed from each pit. An exception occurred for the Alddasorru summit pit, from which surface, 0.16 m, and 0.60 m depth samples were processed. For comparative purposes, ne matrix samples were taken for grain size, XRD, and SEM analyses from tills covering Ruohtahakorru (Fig. 1; 68 09[prime] N, 19 20[prime] E; 13421346 m a.s.l.; three samples from0.5, 0.9, and 1.2 m depth) and Nulpotjkka (Fig. 1; 67 48[prime] N,
18 01[prime] E; 1405 m a.s.l.; one sample from 0.9 m depth).
Two quartz clasts were collected from the summit surfaces of Duopteohkka and Tarfalatjrro for measurements of in situ-produced 10Be and 26Al concentrations. Sampling was undertaken on summits to eliminate the possibility of these clasts having been transported and buried by slope processes, which would complicate estimates of regolith residence durations from measurements of 10Be and 26Al concentrations.This constraint, coupled with the scarcity of summit vein quartz, limited our sampling for TCN analyses to these two sites. Zero vertical mixing was assumed for the vein quartz clast (4 cm thick) sampled from the Duopteohkka summit (Fig. 2a, b). This is because this long clast (0.19 m) resided on the surface of a thin regolith (0.3 m depth), and periglacially sorted circles were absent from this site. Because of the absence of glacial erratics from Duopteohkka and the presence of quartz veins in these blockelds, we considered the sampled clast to be locally derived. On Tarfalatjrro, the clast (34 cm thick) was taken from a shattered
Figure 2. Sample sites for surface quartz clasts on the summits of (a, b) Duopteohkka and Tarfalatjrro (c, d). The quartz sampled on Duopteohkka (arrow in b) was an isolated mass attached to an amphibolite block whereas the quartz clast sampled on Tarfalatjrro (arrow in d) was part of a frost-shattered quartz vein that extended
8 m across the blockeld surface (arrows in c).
quartz vein ( 8 m in length) that extended 8 m across the
blockeld surface (Fig. 2c, d). Zero vertical mixing of the sampled clast through the regolith prole was again assumed because the quartz vein was clearly expressed at the block-eld surface, periglacially sorted circles did not intersect the quartz vein, and the ne matrix required for periglacial sorting (Ballantyne and Harris, 1994, pp. 8596) was sparse, resulting in limited vertical and lateral sorting of clasts.While both samples were taken from autochthonous amphibolite blockelds, the narrow, high curvature summit of Duopteohkka, which also displays frequent bedrock out-crops, contrasts with the broad, comparatively low curvature, comprehensively regolith-mantled summit of Tarfalatjrro. A higher erosion rate and shorter regolith residence duration was therefore expected for Duopteohkka.
To correct for topographic shielding the surface geometries of the sampled blockelds and surrounding summits were measured with a clinometer and compass. Sample locations were recorded with a handheld GPS and on a 1:50 000 topographic map. Three amphibolite clasts and three ne matrix samples extracted in a cylinder of known volume were collected for regolith clast and matrix density measurements.
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
388 B. W. Goodfellow et al.: Late Quaternary not Neogene3.2 Fine matrix analyses
To determine the chemical weathering characteristics of blockelds, grain size, SEM, and XRD analyses were completed. Grain sizes were determined on dried samples with a Coulter LS particle size analyser. SEM analyses of chemical etching were performed on surface bulk ne matrix samples and semi-quantitative analyses of grain chemistry and mineralogy were completed according to energy dispersive spectrometer techniques (Goldstein et al., 2003). Thin sections for mineralogical interpretation of parent material were prepared from two Alddasorru rock samples.
For XRD analysis of clay mineralogies, the < 2 m size fraction of each sample was separated by settling, Mg-saturated, and puried with a ceramic lter to produce oriented samples. An initial XRD scan was performed at 269
2 with a scan speed of 0.02 2 s1 and a step size of 0.04 2 . Second and third scans were performed following ethylglycol saturation and heating of the samples to 550 C, respectively. These scans were performed at 235 2 with a scan speed of 0.0067 2 s1 and a step size of 0.04 2 .
Diffraction peaks were analysed with peak search software and manually reviewed using Brindley and Brown (1980) and Moore and Reynolds (1997).
3.3 Cosmogenic radionuclide analyses and ice sheet
modelling
Concentrations of 10Be and 26Al in samples of vein quartz were measured to estimate erosion rates and residence durations of blockeld regoliths. Clean quartz separates were processed for cosmogenic nuclide analyses through methods adapted from Kohl and Nishiizumi (1992) and Child et al. (2000). Accelerator mass spectrometry (AMS) measurement of the Tarfalatjrro sample was completed at PRIME Lab, Purdue University, USA, and AMS measurement of the Duopteohkka sample was completed at the SUERC AMS Laboratory, East Kilbride, UK. Measured TCN concentrations were corrected by full chemistry procedural blanks and normalized using the NIST 10Be standard (SRM4325) with a 10Be / 9Be ratio of (2.79 [notdef] 0.03) [notdef] 1011 and using a
10Be half-life of 1.36 [notdef] 106 a (Nishiizumi et al., 2007) and
the PRIME Lab 26Al standard (Z92-0222) with a nominal
26Al / 27Al ratio of 4.11 [notdef] 1011 and using an 26Al half-life
of 7.05 [notdef] 105 a (Nishiizumi, 2004). Errors in nuclide concen
trations include the quadrature sum of analytical uncertainty calculated from AMS counting statistics and procedural errors.
Apparent exposure ages were calculated from 10Be and
26Al concentrations using the CRONUS-Earth exposure age calculator (version 2.2; Balco et al., 2008) assuming zero erosion and the LalStone time-independent 10Be production rate model (Lal, 1991; Stone, 2000). The time-independent LalStone scaling was used here because we also used it for modelling regolith residence durations for reasons described
below. We do though cite in our results the full age ranges given for all production rate models incorporated into the CRONUS-Earth exposure age calculator. Corrections were applied for topographic shielding (scaling factors > 0.9998) and for sample thickness using a clast density of 2.65 g cm3.
Apparent exposure ages are not corrected for snow shielding because of high uncertainties, and a correction for vegetation shielding is not required. Quoted exposure age uncertainties (1 external) include nuclide production rate uncertainties and the concentration errors described above. The apparent exposure ages for these sites are minimum durations to which the effects on nuclide production rates of surface burial by snow and glacial ice, and elevation changes attributable to glacial isostasy are subsequently added.
Regolith residence durations, incorporating periods of subaerial exposure and burial by glacial ice, of the Duopteohkka and Tarfalatjrro summit blockelds were calculated from measured 10Be concentrations accordingly (rearranged from Lal, 1991):
Ni
N = [1 ]exp[notdef]( + )t[notdef] + , (1)
where N is nuclide concentration, with the subscript i indicating one step back in time; the nuclide half-life; t the time step, and where
=
E
[Lambda1] , (2)
where E is the erosion rate (cm a1), the regolith density, and [Lambda1] the attenuation mean free path, and
=
P N( + )
, (3)
where P is nuclide production rate. During periods of surface burial by ice sheets, Eq. (1) is simplied to
Ni
N = exp[notdef] t[notdef]. (4)
Regolith residence durations are obtained when zero nuclide concentrations are reached.
The source code for the CRONUS-Earth exposure age calculator (version 2.2; Balco et al., 2008) was not used for calculating these regolith residence durations because of the complexities introduced by accounting for depth- and time-averaged 10Be production rates. Rather, a sea-level high-latitude (> 60 ) 10Be production rate of4.59 [notdef] 0.28 atoms g1 a1, from the Nishiizumi et al. (2007)
10Be half-life of 1.36 [notdef] 106 a, was used in these calculations.
Production rates were scaled to latitude and altitude using Stone (2000) and a sea surface temperature of 5 C. The errors in regolith residence durations attributable to our use of simplied 10Be production rates are minor compared with uncertainties attributable to surface burial by snow and ice and isostatic responses to ice sheet loading and unloading.
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 389 A constant blockeld density of 2.60 g cm3 was assumedbased on a blockeld containing 15 % ne matrix, with adensity of 2.10 g cm3 (mean of three samples, 1 = 0.09),
and 85 % amphibolite, with a density of 2.79 g cm3 (mean of three samples, 1 = 0.02). The effects of regolith disso
lution were ignored because regolith residence durations that include multiple periods of surface exposure and burial by ice sheets preclude erosion rate calculations directly from nuclide concentrations. Furthermore, chemical weathering in these blockelds is likely to be minor (Goodfellow et al., 2009). Corrections were applied for shielding and for sample thickness using an attenuation mean free path of 160 g cm2 and a quartzite density of 2.65 g cm3. A correction for snow burial was also incorporated assuming 0.3 m of snow (Isaksen et al., 2007), with a density of 0.3 g cm3, for a duration of 7 months per year. The resulting annual shielding by snow of only 5.25 g cm2 is assumed to be representative of all ice-free periods. Associated uncertainties are, however, unknown but, because they are high, a sensitivity analysis was performed by increasing the depth of burial from 0.30 to0.50 m and increasing duration of burial from 7 to 10 months a year in a calculation of regolith residence duration.
To incorporate periods of surface burial by ice sheets into calculations of regolith residence durations, and to explore the effects of glacial isostasy on these calculations, we used a 3-dimensional ice-dynamical model forced by the Lisiecki and Raymo (2005) stack of global benthic 18O records and an ELRA bedrock model. Full details of the ice-dynamical model can be found in Bintanja et al. (2002, 2005). An ELRA model offers the best glacial isostasy approximation among the group of simple models, with its primary weakness being that it incorporates only one time constant (Le Meur and Huybrechts, 1996). Whereas self-gravitating visco-elastic spherical Earth models are the most accurate, they are much more complex and require greater computational power and time (Le Meur and Huybrechts, 1996). The ice sheet model was run at 40 km resolution, with a 100-year time step over the last 1.07 Ma. Although spatial resolution is coarse, using a 20 km grid provides minimal change in bedrock topography and calculations on a 50 m digital elevation model of the northern Swedish mountains produced the same mean elevations for 40 [notdef] 40 km squares centred on
the relevant model grid points. Furthermore, the wavelength of glacial isostasy is much longer than the topographic wavelength (Le Meur and Huybrechts, 1996). Regional ice sheet thickness is subsequently more important than local ice sheet thicknesses for determining isostatic response, and a grid size of some tens of kilometres appears justied. A limitation of our models treatment of isostasy is that it lacks an erosion component. However, the consequence of this on isostasy over successive late Quaternary glacial cycles is likely to be less important in this landscape of apparent selective linear erosion than in other alpine locations where glacial erosion was more aerially extensive. For example, Staiger et al. (2005) estimate limited net regional glacial erosion and
a low glacial erosion efciency for the Torngat Mountains in Labrador, Canada, which consist of similar lithologies to the Scandes and which were also subjected to selective linear glacial erosion during the Quaternary. Our model may also underestimate surface burial durations because smaller ice masses may have formed on summits in between the periods of regional ice sheet coverage that are captured in our model. However, ice sheet models that have the required resolution to account for the formation of these small ice masses are not run over multiple glacial cycles because of the high computational burden. Furthermore, the two summits used in our study are perhaps unlikely locations for small ice masses to form, because they are subjected to strong winds, or to persist well after larger ice masses have retreated, because of their locations on the crests of high ridges. We therefore consider our model to offer a reasonable approximation (with estimated [notdef]20 % error margins) of ice sheet burial durations
and glacial isostasy.
4 Results
4.1 Blockeld structure
Blockeld vertical sections along the Alddasorru, Duopteohkka, and Tarfalatjrro altitudinal transects display a number of common features (Fig. 3). Surface exposures of cobbles and boulders comprise the outer rings of periglacially sorted circles on low-gradient surfaces and delineate soliuction lobes on slopes. In addition to lichen covers, subaerially exposed clast surfaces on sorted circles display rounding through granular disintegration, which further conrms presently limited regolith mixing on low-gradient surfaces. In contrast, freshly exposed ne matrix and loose clasts indicate that some soliuction lobes remain active. In each setting, surface cobbles and boulders are underlain by a layer of gravel and larger clasts to average depths of 0.91.0 m beneath the ground surface. In the centres of ne matrix-rich sorted circle centres, surface layers of cobbles and boulders are usually underlain by ne matrix, granules and gravel, in which cobbles and boulders are embedded, to depths up to 0.7 m. Excavation depths generally ranged between 0.6 and 1.3 m, and the bottom of each pit typically consisted of boulders embedded in a ne matrix that ranged from damp to water saturated (Table S1 in the Supplement). The Duopteohkka summit pit was excavated to bedrock, which was reached at only 0.30 m.None of the pits revealed soil horizons or saprolite.
Although only about 10 % of the ground surface consists of ne matrix, it appears to comprise about 1020 % of the subsurface regolith (Table S1 in the Supplement). Surface ne matrix is most abundant on the section of the plateau into which Alddasorru pit 4 was dug ( 50 % of the sur
face area) and in the saddle into which Tarfalatjrro pits 69 were dug ( 30 % of the surface area). Sub-surface ne ma
trix appears most limited on the summits of Alddasorru and
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
390 B. W. Goodfellow et al.: Late Quaternary not Neogene
Figure 4. Quantities of clay and silt in ne matrix samples collected from blockelds (blue crosses) and till (red circles). All samples contain clay / silt ratios less than 0.5, which indicates ne matrix production under conditions that have been, at least seasonally, periglacial (Goodfellow, 2012). Fine matrix samples falling in the uncertainty zone may have been exposed to periglacial conditions during their formation (Goodfellow, 2012). All data have been divided by 65 to t on 01 scales.
Tarfalatjrro (515 %), and in the soliuction lobes on the slope of Duopteohkka (pits 4 and 5, 510 %; Table S1 in the Supplement). Here, cobbles and boulders were embedded in gravel throughout most of the subsurface, with only small accumulations of ne matrix on boulder tops or in poorly dened sorted circle centres.
In summary, blockeld sections indicate present surface stability as well as regolith sorting and transport, particularly during former periods of colder climate where blockeldmantled surfaces also remained free of glacial ice cover.Chemical weathering rates have been insufcient to produce soil horizons or saprolite.
4.2 Fine matrix granulometry
All blockeld ne matrix samples are sandy loams (Table 1;
US Department of Agriculture, 1993, p. 138). However, minor variations occur in the distribution of sand (1 = 7.5 %),
silt (1 = 6.9 %), and clay (1 = 1.3 %) according to sam
pling depth and the subsurface distribution of boulders, between which granules, pebbles, and gravel accumulate. Till samples vary between loamy sand, sandy loam, and silt loam, and have lower mean clay quantities (1.8 %, 1 = 1.1 %)
than the blockeld ne matrix (4.2 %, 1 = 1.3 %). Al
though clay abundances are higher in blockelds than in till,
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
Figure 3. Representative vertical sections of autochthonous block-elds. The photograph shows an excavation across one of the sparsely distributed sorted circles developed in the summit block-eld of Tarfalatjrro, and the line drawing summarizes the general features of autochthonous blockelds in a vertical section cut across a periglacially sorted circle. The ruler in the photograph is 1 m in length. Angular cobbles and boulders are embedded in ne matrix (clay, silt, sand) in sorted circle centres. Granules and gravel accumulate between clasts distributed vertically through the section that have subhorizontally oriented long axes. Conversely, the outer ring of the sorted circle is comprised of gravel, cobbles, and boulders, whereas granules accumulate near the base of the section. Clast surfaces are sub-rounded where they are subaerially exposed. The pit bases generally intersect large rock slabs and are wet.
B. W. Goodfellow et al.: Late Quaternary not Neogene 391 Table 1. Particle size distribution and secondary minerals in ne matrix from Alddasorru, Duopteohkka, and Tarfalatjrro transects, and from Ruohtahakorru and Nulpotjkka summit tills.
Locationb ElevationTransect and pit Samplea (slope segment) (m a.s.l.) Depth (m) Clay (%) Silt (%) Sand (%) Clay / silt Clay mineralsc
Alddasorru 1 BG-05-11 Summit (1) 1538 Surface 4.9 42.2 52.9 0.12 C, I, A, P, Q Alddasorru 1 BG-05-04 Summit (1) 1538 0.16 3.4 29.3 67.3 0.12 C, I, A, P, Q Alddasorru 1 BG-05-16 Summit (1) 1538 0.60 2.7 23.6 73.7 0.11 C, I, A, P, Q Alddasorru 2 BG-05-26 Slope (1) 1500 0.60 4.5 36.2 59.4 0.12 C, I, A, G?, P, Q Alddasorru 5 BG-05-67 Slope base (3) 1260 0.40 5.8 46.5 47.7 0.12 C, V, I, A, G, P, Q Alddasorru 6 BG-05-68 Slope base (3) 1260 0.40 4.1 37.0 58.8 0.11 C, V, I, A, G, P, Q Alddasorru 7 BG-05-69 Slope base (3) 1260 0.40 3.2 29.2 67.6 0.11 C, V, I, A, G, P, Q Duopteohkka 5 BG-05-64 Slope (2) 1200 0.40 4.0 40.6 55.4 0.10 C, V, I, A, P, Q Duopteohkka 6 BG-05-65 Slope (2) 1200 0.40 4.1 40.3 55.6 0.10 C, V, I, A, G, P, Q Duopteohkka 7 BG-05-66 Slope (2) 1200 0.40 4.4 38.4 57.3 0.11 C, V, I, A, G?, P, Q Duopteohkka 8 BG-05-70 Colluvium/till (4) 1060 0.40 1.1 14.1 84.8 0.08 C, V, I, A, P, Q Duopteohkka 9 BG-05-71 Colluvium/till (4) 1060 0.40 3.6 34.1 62.3 0.11 C, V, I, A, G, P, Q Duopteohkka 10 BG-05-72 Colluvium/till (4) 1060 0.40 1.5 20.8 77.7 0.07 C, V, I, A, G?, P, Q Tarfalatjrro 1 BG-04-25 Summit (1) 1626 1.25 1.5 39.5 59.0 0.04 C, I, A, P, Q Tarfalatjrro 2 BG-06-22 Summit (1) 1626 Surface 5.6 39.3 55.2 0.14 C, I, A, P, Q Tarfalatjrro 3 BG-04-26 Summit/slope (1) 1623 0.80 2.1 45.5 52.4 0.05 C, I, A, P, Q Tarfalatjrro 6 BG-05-83 Saddle (1) 1553 0.50 4.7 41.3 54.0 0.11 C, V, I, A, G, P, Q Tarfalatjrro 7 BG-05-84 Saddle (1) 1553 0.20 5.3 49.9 44.8 0.11 C, V, I, A, G, P, Q Tarfalatjrro 8 BG-05-85 Saddle (1) 1553 0.20 5.4 49.8 44.8 0.11 C, V, I, A, G, P, Q Tarfalatjrro 9 BG-06-07 Saddle (1) 1553 0.70 5.4 46.1 48.5 0.12 C, V, I, A, G, P, Q Ruohtahakorru BG-04-22 Summit till 1342 0.50 2.2 52.8 45.0 0.04 C, V, I, A, P, Q Ruohtahakorru BG-04-23 Summit till 1346 0.90 0.7 10.3 89.0 0.07 C, V, I, A, P, Q Ruohtahakorru BG-04-24 Summit till 1343 1.20 2.6 37.7 59.7 0.07 C, V, I, A, P, Q Nulpotjkka BG-04-27 Summit till 1405 0.90 0.7 47.2 52.1 0.01 C, V, I, A, P, Q
a BG-06-22 and BG-06-07 are representative of eight samples from Tarfalatjrro pit 2 and seven samples from Tarfalatjrro pit 9, respectively (Goodfellow et al., 2009). b Slope segments are numbered as follows: (1) diffusion-dominated summit, (2) soliuction-dominated slope, (3) steep mass wasting, (4) concave depositional, where regolith comprises colluvium and till. Till is indicated by the presence of clasts of different lithologies and a high abundance of ne matrix. c C chlorite; V vermiculite; I illite; A amphibole; G gibbsite (? indicates uncertain); P plagioclase (dominant feldspar); Q quartz.
clay comprises only 1.5 to 5.8 % of the ne matrix volume, which remains at the low end of the range (130 %) previously reported for other blockelds (e.g. Caine, 1968; Rea et al., 1996; Dredge, 2000; Marquette et al., 2004; Paasche et al., 2006; Table A.1 in Goodfellow, 2012). Clay / silt ratios are all 0.14 (Table 1; Fig. 4). These indicate a low intensity
of chemical weathering that is typical for regolith formation under, at least seasonal, periglacial conditions (Goodfellow, 2012).
4.3 Fine matrix mineralogy
XRD analyses of the clay-sized fraction of the regolith indicate the presence of primary and secondary minerals in all samples (Table 1, Fig. 5). Primary minerals are abundant and include chlorite, amphibole, and feldspar. The presence of these minerals, along with epidote, was also conrmed using thin sections. In addition to these primary minerals, small quantities of poorly crystallized Al- and Fe-oxyhydroxides are identiable by XRD in the Alddasorru and Tarfalatjrro summit samples. In contrast, vermiculite, gibbsite, and larger quantities of poorly crystallized oxyhydroxides are also identiable in concave locations, such as at the base of Alddasorru and in the Tarfalatjrro saddle. Gibbsite generally occurs together with poorly crystallized Al- and Feoxyhydroxides and vermiculite. A sample from the upper
slope of Alddasorru (BG-05-26; Fig. 5b) forms a possible exception, where poorly crystallized oxyhydroxides and gibbsite appear to be the only secondary minerals present. Up to two samples from the soliuction-dominated slope segment on the Duopteohkka transect are also gibbsite-bearing.All till samples contain poorly crystallized oxyhydroxides and vermiculite, but up to two samples of mixed colluvium and till at the base of the Duopteohkka transect also contain gibbsite. We are unable to distinguish kaolinite according to the standard XRD techniques we used because of the ubiquitous presence of chlorite (Moore and Reynolds, 1997,p. 234). However, we believe that kaolinite may be present in our samples in small quantities. Quartz and muscovite are also commonly present. Because of the scarcity of quartz in the amphibolitic parent rock, these likely represent aeolian additions to blockelds and/or are till components.
Chemical weathering was observed on only two albite grains under SEM: one through surface etching and a second through more general disintegration (Fig. 6). It was otherwise absent, even on easily weathered minerals such as amphibolite and epidote, in addition to most albite grains. This observation of sparsely weathered silt- and sand-sized grains complements the mixed primary and secondary mineralogy of clay-sized grains. Together, they provide an overall impression of generally minor chemical weathering.
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
392 B. W. Goodfellow et al.: Late Quaternary not Neogene
Figure 5. Four representative X-ray diffractograms of the clay-sized fraction (< 2 m) of ne matrix samples from (a) Alddasorru, (b) Alddasorru, (c) Nulpotjkka, and (d) Tarfalatjrro. For each sample three diffractograms are shown. In the bottom diffractograms the samples are untreated, in the middle diffractograms the samples are ethylglycolated, and in the top diffractograms the samples are heated to 550 C.
These diffractograms illustrate the range of minerals present (labelled) in the Alddasorru, Duopteohkka, and Tarfalatjrro blockelds, and in the till samples. Poorly crystallized oxyhydroxides produce a rise in the diffractogram baseline, which disappears on heating, at d spacings between 5 and 3.5 (pink-lled circles). Vermiculization of chlorite and/or mica is shown by peaks in the 1014 (yellow) area that
collapse to 10 on heating. Gibbsite is shown by peaks at 4.9 (green), which also collapse on heating.
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 393
Figure 6. SEM images indicating only slight chemical weathering of ne matrix. (a) Albite, with a chemically etched surface; the only etched grain identied (BG-05-84, Tarfalatjrro pit 7). (b) Chemically unaltered amphibole, typical of all SEM images of amphibole (BG-05-26, Alddasorru pit 2). (c) Disintegrating albite, possibly through chemical processes (BG-05-11, Alddasorru pit 1).(d) Chemically unaltered epidote, typical of all SEM images of epi-dote (BG-05-04, Alddasorru pit 1).
In summary, XRD and SEM analyses indicate chemical weathering in blockeld and till samples, albeit in limited quantities. Samples from concave blockeld sites appear most chemically weathered, and summit blockeld ne matrix appears the least chemically weathered. Chemical weathering of till samples displays an intermediate intensity.
4.4 Regolith residence durations
Apparent 10Be surface exposure durations for two surface quartzite clasts on Duopteohkka and Tarfalatjrro are33.5 [notdef] 3.2 ka and 81.8 [notdef] 7.8 ka, respectively (Table 2). These
ages are based on the time-invariant spallogenic production rate model (Lal, 1991; Stone, 2000; Balco et al., 2008). For Duopteohkka, this provides a younger age than any of the time-varying 10Be spallogenic production rate models included on the CRONUS-Earth exposure age calculator (version 2.2; Balco et al., 2008). The full apparent surface exposure age range is 33.5 [notdef] 3.2 ka to 35.6 [notdef] 4.4 ka. For Tar
falatjrro, the apparent surface exposure age range from these production rate models is 80.4 [notdef] 8.5 ka to 86.2 [notdef] 10.8 ka.
The 26Al / 10Be ratio of 6.57 [notdef] 0.43 for the quartz sample
of the Duopteohkka summit (Table 2) indicates no apparent burial (within error) by the Fennoscandian Ice Sheet. However, even a full exposure nuclide ratio does not exclude a complex exposure history including short intermittent periods of surface burial beneath an ice sheet. In contrast, the lower ratio of 5.92 [notdef] 0.41 for the quartz sample of the Tar-
Figure 7. Modelled ice sheet surface elevation, ice thickness, and bedrock response to ice sheet loading and unloading for(a) Duopteohkka and (b) Tarfalatjrro over the last 1.07 Ma. These data were generated using a 3-dimensional ice-dynamical model forced by the Lisiecki and Raymo (2005) stack of global benthic 18O records and the ELRA bedrock model (Bintanja et al., 2002, 2005).
falatjrro surface requires some previous period of burial, and, by inference, indicates periods of burial by glacial ice.
Model results for ice sheet surface elevations, ice sheet thicknesses, and glacial isostasy over multiple glacial cycles indicate that the Duopteohkka and Tarfalatjrro summits have been repeatedly covered by ice sheets (Fig. 7). According to the model, thicker ice has formed over Duopteohkka (a maximum of 1595 m compared with 1150 m for Tarfalatjrro) and burial durations have been longer on this summit. These data are consistent with what might be expected for the lower elevation of Duopteohkka (1336 m a.s.l. versus 1626 m a.s.l. for Tarfalatjrro). However, they seemingly contrast with inferences from the 26Al / 10Be ratio for
Duopteohkka (Table 2) of either no glacial burial of this summit or surface burial during short periods relative to intermittent full-exposure durations. The thickness and duration of ice cover may therefore be overestimated in our model, and a comparison with data from other models supports this possibility. Firstly, ice sheet thicknesses produced by our model are either similar to those indicated for our study areas by other ice sheet models (Fjeldskaar et al., 2000; Milne et al., 2004; Peltier, 2004; Steffen and Kauffman, 2005) or exceed them (Peltier, 1994; Kauffman et al., 2000; Lambeck et al., 2006; Steffen et al., 2006; Charbit et al., 2007). Secondly, the magnitude of isostatic rebound following the last glaciation is 497 m for Duopteohkka and 457 m for Tarfalatjrro.These values exceed those indicated by the isostatic rebound
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
394 B. W. Goodfellow et al.: Late Quaternary not Neogene Table 2. Cosmogenic nuclide data, apparent exposure ages, and nuclide ratios.
Location Elevation Thicknessb Shielding Quartz Be carrier 10Be / 9Bec [10Be]c,d,e,f Al carrier 26Al / 27Alc [26Al]c,f,g,h 10Be apparent 26Al apparent 26Al / 10Bec
Samplea ( N/ E) (m a.s.l.) (cm) factor (g) (mg) ([notdef]10
13) (106 atoms g1) (mg) ([notdef]10
13) (106 atoms g1) agec,i,j (ka) agec,i,j (ka) ratioAge [notdef] 1 (int) [notdef]1 (ext) Age [notdef] 1 (int) [notdef]1 (ext)
Duo 1 68.42/19.37 1330 3 1.0000 41.8211 0.3094 11.40 [notdef] 0.30 0.51 [notdef] 0.02 0.7222 50.10 [notdef] 1.05 3.52 [notdef] 0.19 33.5 [notdef] 1.2 [notdef] 3.2 32.7 [notdef] 1.8 [notdef] 3.4 6.57 [notdef] 0.43
Tar 1 67.61/18.52 1626 4 0.9998 54.2814 0.2741 44.70 [notdef] 1.20 1.55 [notdef] 0.05 0.9028 170.00 [notdef] 6.00 9.59 [notdef] 0.59 81.8 [notdef] 2.8 [notdef] 7.8 72.6 [notdef] 4.6 [notdef] 8.0 5.92 [notdef] 0.41
for these simple exposure conditions are also lowest for this scenario. These simple exposure ages are 34170 ka
for Duopteohkka and 82375 ka for Tarfalatjrro for the
range of erosion rates up to where the ages become asymptotic (Table 2; Fig. 8).
When intermittent surface burial by glacial ice is introduced, maximum erosion rates decrease to 17.5 mm ka1
and 7.1 mm ka1 for Duopteohkka and Tarfalatjrro, re
spectively (burial, 0 isostasy lines in Fig. 8). Regolith residence durations also increase for a given erosion rate up to the erosion rate where the ages become asymptotic. These ages vary from 110 to 370 ka and 166 to 450 ka for
Duopteohkka and Tarfalatjrro, respectively.
Increasing by 10 % the duration of each period of surface burial by glacial ice has negligible impact on maximum erosion rates for either summit (10 % more burial, isostasy in Fig. 8). However, long burial periods are reached on Duopteohkka at lower surface erosion rates than otherwise occur, resulting in longer regolith residence durations at these erosion rates. For example, at an erosion rate of 8 mm ka1, the regolith residence duration on Duopteohkka increases from 123 to 199 ka. A similar effect is induced by increasing the duration of snow cover from 7 to 10 months a year and the depth of snow from 30 to 50 cm (burial, isostasy, more snow lines in Fig. 8). Increasing the duration and depth of snow cover decreases maximum erosion rates.These values are now 15.7 mm ka1 for Duopteohkka and 6.3 mm ka1 for Tarfalatjrro.
In summary, the summits of both Duopteohkka and Tarfalatjrro appear to have been repeatedly inundated by glacial ice over the past 1.07 Ma. The durations of burial and depths of glacial isostatic depression have had notable impacts on regolith residence durations for each summit. It remains likely, however, that the residence durations of regolith mantles on both summits are conned to the late Quaternary.Modelled maximum erosion rates are 16.2 mm ka1 and
6.7 mm ka1 for Duopteohkka and Tarfalatjrro, respectively.
5 Discussion
Minimal chemical weathering of blockelds in the northern Swedish mountains is indicated by the following ne-matrix characteristics: clay / silt ratios 0.14 in all samples (n =
16), the presence of mixed primary and secondary minerals
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
map in the National Atlas of Sweden (Fredn, 2002, p. 101) by 150250 m, which again indicates a possible overestimation of ice sheet thicknesses and durations of ice coverage by our model. The key consequences of this for our subsequent analysis of regolith residence durations are that the lengths of the ice-free periods during which cosmogenic nuclides accumulate are likely underestimated, whereas nuclide decay periods during ice sheet burial are likely overestimated. If nuclides have accumulated in surface regolith more quickly than provided for in our model and nuclide decay has been less, inferred maximum erosion rates will be underestimated and regolith residence durations, for a given erosion rate, will be overestimated in our analyses. We consider the regolith residence duration calculations to remain valid for our purposes, however, because we are interested in an order of magnitude question (i.e. whether or not regolith residence durations are conned to the late Quaternary) and to be conservative in our interpretations prefer to err on the side of overestimating regolith residence durations.
Modelled regolith residence durations for Duopteohkka and Tarfalatjrro are shown in Fig. 8. Steps in these regolith residence duration curves indicate periods of surface burial by glacial ice. The timing of these steps in each modelled scenario varies according to erosion rate and durations of surface burial by glacial ice. The primary model output, which considers the effects of both ice sheet burial and bedrock isostasy on cosmogenic nuclide accumulation (labelled burial and isostasy), indicates a maximum surface erosion rate of 16.2 mm ka1 for Duopteohkka and 6.7 mm ka1 for Tarfalatjrro. As maximum surface erosion rates are asymptotically approached, maximum surface ages become innite. However, the regolith residence durations of Duopteohkka and Tarfalatjrro become asymptotic above cut-off values of 380 and 490 ka before present,
respectively. This indicates that the late Quaternary has likely offered sufcient time for the present regolith mantles on both summits to gain their respective 10Be inventories.
Four additional regolith residence duration scenarios help dene the sensitivity of derived maximum erosion rates to durations of surface burial by snow and glacial ice and to the magnitude of glacial isostasy (Fig. 8). As expected, maximum erosion rates are highest in the absence of former glaciation (0 burial, 0 isostasy lines in Fig. 8). These rates are 18.2 mm ka1 for Duopteohkka and 7.3 mm ka1
for Tarfalatjrro. Accordingly, regolith residence durations
B. W. Goodfellow et al.: Late Quaternary not Neogene 395
in clay-sized regolith (n = 16), and a scarcity of chemically
etched grains in bulk ne matrix (Table 1; Figs. 25). In addition, soil horizons and saprolite are absent from all blockeld sections (Table S1 in the Supplement). These ndings support those from Goodfellow et al. (2009) and their model of blockeld formation, primarily through physical weathering processes. Conversely, the data do not support blockeld initiation through intense chemical weathering under a warm, non-periglacial climate.
Chemical weathering intensity, although generally low, varies predictably along hillslope transects. Convex summit areas are the least chemically weathered, as indicated by the absence of well-crystallized secondary minerals (Table 1).This is possibly because these areas are drier (Table S1 in the Supplement) or because ne matrix may not be resident on summits long enough, before being transported downs-lope, for secondary minerals to become well-crystallized.Concave locations, such as at the Alddasorru slope base and the Tarfalatjrro saddle, exhibit the highest chemical weathering intensity, as indicated by the formation of vermiculite and gibbsite. This may be attributable to longer residence durations within the blockelds of ne matrix that has been transported downslope, wetter conditions, and/or changes in bedrock mineralogy (Table S1 in the Supplement). The relative paucity of summit ne matrix (Table S1 in the Supplement), which also displays lowest chemical weathering intensity (Table 1; Fig. 5), might therefore indicate erosion through, for example, surface creep and subsurface water ow. Altitudinal differences along the transects are generally insufcient for weathering variations to be clearly related to temperature changes, particularly along the 73 m Tarfalatjrro transect (Fig. 1). However, a slightly milder temperature regime and an extensive grass cover may enhance chemical weathering of the colluviumtill mixture at the base of the Duopteohkka transect (270 m below the summit), which contains vermiculized minerals and gibbsite (Table 1).Secondary mineral assemblages are consistent with contemporary climatic conditions and hillslope position, rather than indicating palaeoregoliths.
The presence of gibbsite in some ne matrix samples does not indicate a Neogene deep-weathering origin of block-elds. Although it is an end product of chemical weathering and can be abundant in intensely weathered regoliths, formation of limited quantities of gibbsite is apparently also favoured by the generally low temperatures and spatially and temporally variable hydrologic conditions that occur in alpine regoliths. The spatial distribution of gibbsite in the blockelds of the northern Swedish Scandes indicates that its precipitation may be favoured by wetter regolith conditions found, for example, in concave locations (Table 1).Seasonally abundant liquid water may result in some leaching along spatially discrete ow paths in these blockeld regoliths (Meunier et al., 2007; Goodfellow, 2012). Together with low temperatures, which inhibit chemical reactions, this may maintain the low concentrations of H4SiO4 along these
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
Figure 8. Regolith residence durations for (a) Duopteohkka and(b) Tarfalatjrro plotted against surface erosion rates. These are modelled using 10Be concentrations in regolith surface quartz clasts and incorporate periods of burial by ice sheets and changes in 10Be production rates attributable to glacial isostasy. Seasonal burial of ground surfaces by 30 cm of snow for 7 months of the year is included. Five different scenarios were modelled: (i) ice sheet burial duration and glacial isostasy (marked burial and isostasy in the plots), which is our primary model output; (ii) isostasy is removed (burial, 0 isostasy); (iii) a simple surface exposure history, from which burial and glacial isostasy are excluded (0 burial, 0 isostasy); (iv) the rst scenario is replicated, but snow depth and snow cover duration are increased to 50 cm and 10 months of the year, respectively (burial, isostasy, more snow); (v) each burial period is extended by 10 % and exposure periods commensurately shortened (10 % more burial, isostasy).
396 B. W. Goodfellow et al.: Late Quaternary not Neogene
Figure 9. Thermodynamic stability relations between feldspars, secondary minerals, and weathering solutions (top panels) and the relationship between gibbsite solubility and the pH of regolith water (bottom panel). Relatively low Na+ Ca2+, and K+ abundances and high H+ abundances favour secondary mineral formation. Solute concentrations are usually most favourable to kaolinite precipitation, but gibbsite forms where regolith waters have very dilute silica concentrations, similar to rainwater (adapted from Stumm and Morgan, 1981, p. 547, and Nesbitt and Young, 1984, p. 1524, with permission of Wiley and Elsevier, respectively). Gibbsite is least soluble where pH is 56, and its solubility increases as pH is elevated or lowered from this value (adapted from Wesolowski and Palmer, 1994). It is expected that these key conditions of low silica concentration and slight acidity occur in Arcticalpine blockelds, which favours gibbsite precipitation in these regoliths.
ow paths that are favourable to the precipitation of gibbsite rather than kaolinite (Fig. 9; Nesbitt and Young, 1984). Gibb-site may persist in blockelds because of the general absence of macro-vegetation and associated organic acids. This permits porewaters to remain slightly acidic, under which conditions gibbsite is least soluble (Fig. 9; Reynolds, 1971; May et al., 1979; Gardner, 1992; Wesolowski and Palmer, 1994). Marquette et al. (2004) measured a mean pH of 5.7 in alpine blockeld porewaters, which coincides with minimum gibb-site solubility (Fig. 9). As expected, gibbsite is present in those blockelds (located in NE Canada). The low temperature constraint on weathering reactions may further ensure the persistence of gibbsite rather than it being converted to kaolinite through resilication (Watanabe et al., 2010). The
presence of gibbsite in alpine regoliths is well documented (Green and Eden, 1971; Reynolds, 1971; McKeague et al., 1983; Bain et al., 1994; Rea et al., 1996; Dahlgren et al., 1997; Ballantyne, 1998; Ballantyne et al., 1998; Burkins et al., 1999; Marquette et al., 2004; Paasche et al., 2006; Watanabe et al., 2010; Hopkinson and Ballantyne, 2014), and it should not be used to indicate that regoliths have necessarily originated under a warmer-than-present pre-Quaternary climate.
Late Quaternary surface erosion rates on the Duopteohkka and Tarfalatjrro summits are low. These values are 16.2 mm ka1 (with a range of 15.718.2 mm ka1 for the modelled scenarios) and 6.7 mm ka1 (with a range of 6.37.3 mm ka1 for the modelled scenarios)
for Duopteohkka and Tarfalatjrro, respectively (Fig. 8).
The higher erosion rate for Duopteohkka likely reects enhanced regolith transport across the narrow, steeply sided, summit ridgeline. This is perhaps indicated by a patchy regolith only a few tens of centimetres thick and by older bedrock apparent surface exposure durations from relict non-glacial surfaces on a nearby part of Alddasorru (1380 m a.s.l.) and Olmohkka (1355 m a.s.l.) of 42.1 [notdef] 2.5
and 58.2 [notdef] 3.5 ka (analytical errors only), respectively
(Fig. 1; Fabel et al., 2002; Stroeven et al., 2006). The lower erosion rate for the broad, low gradient summit of Tarfalatjrro likely approaches the minimum limit for blockeld-mantled surfaces in this landscape, and is based on an apparent exposure duration which is similar to one derived from bedrock on nearby Drfalcohkka (1790 m a.s.l.) of 72.6 [notdef] 4.4 ka (analytical error only; Fig. 1; Stroeven
et al., 2006). Blockelds therefore appear to represent end-stage landforms (Granger et al., 2001; Httestrand and Stroeven, 2002; Boelhouwers, 2004) that effectively armour low-gradient surfaces, making them resistant to erosion and limiting further modication of surface morphology and regolith composition and thickness.
While average erosion rates of blockeld-mantled summits are low, they are of sufcient magnitude to remove
12 m thick regolith proles within a late Quaternary time frame, even accounting for periods of surface protection during burial by cold-based glacial ice (Figs. 78). If our conclusion that blockelds represent end-stage landforms resistant to further modication is correct, then it is reasonable to also assume steady-state conditions, where the rate of regolith production through weathering processes equals the erosion rate. Together, erosion and regolith production rates of 618 mm ka1 and geochemical features including mixtures of primary and secondary minerals in clay-sized regolith, low clay / silt ratios, and sparsely weathered primary mineral grains are consistent with blockeld formation under, at least, seasonal periglacial conditions during the late Quaternary.
The contrast between low rates of regolith production and erosion in blockeld mantles and the apparent comprehensive removal of presumably more-intensely weathered
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 397 Neogene regoliths from the northern Swedish Scandes is intriguing. One possible explanation for this contrast is thaterosion rates may have greatly outpaced regolith productionrates during the onset of cold Quaternary climatic conditions. This is because protective vegetative covers may havebeen lost during this period, and the intensity of periglaciation may have greatly increased. In Canadian Arctic plateaulandscapes similar to the Scandinavian Mountains, thereis evidence of extensive removal of Neogene regolith during the early Quaternary (Refsnider and Miller, 2013). Different conditions, including perhaps less intense Quaternary periglaciation, have allowed the persistence of pre-Quaternary saprolite remnants in low-altitude locations inScandinavia (Lidmar-Bergstrm, 1997). While our data indicate residence durations of present regoliths, they do notaddress the question of how long blockelds have mantledportions of the Scandes. Furthermore, they provide only minimum estimates of the ages of surfaces on which the block-elds reside. However, because the Plio-Pleistocene transition may have been a period of marked disequilibrium insurface processes, blockelds perhaps formed during or after this period.
Our data indicate relative stability of blockeld-mantled summits during the late Quaternary. They are therefore incompatible with the operation on these summits of a periglacial or glacial buzz saw during this period. However, they discount neither a buzz saw effect on topo-graphic relief during earlier periods, as has been previously suggested (Pedersen and Egholm, 2013), nor more rapid late Quaternary erosion rates on higher-gradient slopes. Field observations of extensive soliuction on slopes coupled with a marked absence of remnant glacial erosion features across a range of spatial scales, such as striae, roches moutonnes, rock drumlins, whalebacks, and crag and tails, are compatible with lowering of relief on blockeld-mantled surfaces through periglacial, rather than glacial, processes. We also speculate that formation of autochthonous blockelds on glacially eroded bedrock surfaces would be greatly inhibited. This is because of low water retention against, and low inltration into, subaerially exposed, glacially polished, and generally convex rock surfaces (Andr, 2002; Ericson, 2004; Hall and Phillips, 2006), which are essential for regolith production through chemical processes and frost action (Walder and Hallet, 1985; Anderson, 1998; Whalley et al., 2004; Dixon and Thorn, 2005; Goodfellow et al., 2009).Glacially scoured bedrock surfaces might therefore be resistant to regolith formation over timescales of 105106 years (or longer), and the sum of ice-free periods during the Quaternary may have been insufcient for weathering and erosive processes to have completely removed evidence of any early Quaternary glacial erosion from the landscape. We do consider though that (early) Quaternary periglacial processes may have modied presently blockeld-mantled surfaces to a greater extent than can be easily recognized (Anderson, 2002; Goodfellow, 2007; Berthling and Etzelmuller, 2011).
The utility of blockeld-mantled surfaces as markers from which to estimate Quaternary glacial erosion volumes in surrounding landscape elements remains uncertain. This is because potentially large spatial variations in rates of non-glacial erosion on blockeld-mantled topography may have lowered local relief and altered the topography from its preglacial conguration (Anderson, 2002; Goodfellow, 2007). In addition, presently blockeld-mantled surfaces may have undergone vertical erosion exceeding some tens of metres, as well as topographic reconguration, during the Plio-Pleistocene transition to a colder climate. Conrmation that these slowly eroding landforms persist over multiple glacialinterglacial cycles does, however, further demonstrate the utility of these landforms as key markers of nunataks and/or cold-based ice coverage during (at least) late Quaternary glacial periods.
6 Conclusions
Blockelds on three mountains in the northern Swedish Scandes were examined, and none of them appear to be remnants of thick, intensely weathered Neogene weathering proles, which has been the prevailing opinion for these regoliths. Minor chemical weathering is indicated in each of the three examined blockelds, with predictable differences according to slope position. Average erosion rates of 16.2
and 6.7 mm ka1 are calculated for two blockeld-mantled
summits, from concentrations of in situ-produced cosmogenic 10Be in surface quartz clasts that were inferred not to have been vertically mixed through the regolith. Although low, these erosion rates are of sufciently high magnitude to remove present blockeld mantles, which appear to be commonly < 2 m thick, within a late Quaternary time frame.
This nding remains valid even when accounting for temporal variations in 10Be production rates attributable to glacial isostasy and burial of ground surfaces by snow and cold-based glacial ice. Blockeld mantles appear to be replenished by regolith formation through, primarily physical, weathering processes that operated during the Quaternary.
The persistence of autochthonous blockelds over multiple glacialinterglacial cycles conrms their importance as key markers of surfaces that were not glacially eroded through, at least, the late Quaternary. However, presently blockeld-mantled surfaces may undergo potentially large spatial variations in erosion rates, and their regolith mantles may have been comprehensively eroded during the late Pliocene and early Pleistocene. Their role as markers by which to estimate glacial erosion volumes in surrounding landscape elements therefore remains uncertain.
The Supplement related to this article is available online at http://dx.doi.org/10.5194/esurf-2-383-2014-supplement
Web End =doi:10.5194/esurf-2-383-2014-supplement .
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
Ballantyne, C. K., McCarroll, D., Nesje, A., Dahl, S. O., and Stone,J. O.: The last ice sheet in north-west Scotland: Reconstruction and implications, Quaternary Sci. Rev. 17, 11491184, 1998.Berthling, I. and Etzelmller, B.: The concept of cryo-conditioning in landscape evolution, Quaternary Res., 75, 378384, 2011.Bierman, P. R., Marsella, K. A., Patterson, C., Davis, P. T., and Caffee, M.: Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southern Bafn Island: a multiple nuclide approach, Geomorphology, 27, 2539, 1999.
Bintanja, R., van de Wal, R. S. W., and Oerlemans, J.: Global ice volume variations through the last glacial cycle simulated by a 3-D ice-dynamical model, Quaternary Int., 9596, 1123, 2002.Bintanja, R., van de Wal, R. S. W., and Oerlemans, J.: Modelled atmospheric temperatures and global sea levels over the past million years, Nature, 437, 125128, 2005.
Boelhouwers, J. C.: New perspectives on autochthonous blockeld development, Polar Geogr., 28, 133146, 2004.
Boelhouwers, J., Holness, S. Meiklejohn, I., and Sumner, P.: Observations on a blockstream in the vicinity of Sani Pass, Lesotho Highlands, southern Africa, Permafrost Perigl. Process., 13, 251257, 2002.
Brindley, G. W. and Brown, G.: Crystal Structures of Clay Minerals and their X-ray Identication, Mineralogical Society, London, 495 pp., 1980.
Briner, J. P., Miller, G. H., Davis, P. T., Bierman, P. R., and Caffee,M.: Last Glacial Maximum ice sheet dynamics in Arctic Canada inferred from young erratics perched on ancient tors, Quaternary Sci. Rev., 22, 437444, 2003.
Burkins, D. L., Blum, J. D., Brown, K., Reynolds, R. C., and Erel,Y.: Chemistry and mineralogy of a granitic, glacial soil chronosequence, Sierra Nevada Mountains, California, Chem. Geol., 162, 114, 1999.
Caine, N.: The Blockelds of Northeastern Tasmania. Publication
G/6, Department of Geography, Research School of Pacic Studies, The Australian National University, Canberra, 127 pp., 1968.Charbit, S., Ritz, C., Philippon, G., Peyaud, V., and Kageyama,M.: Numerical reconstructions of the Northern Hemisphere ice sheets through the last glacial-interglacial cycle, Clim. Past, 3, 1537, doi:http://dx.doi.org/10.5194/cp-3-15-2007
Web End =10.5194/cp-3-15-2007 http://dx.doi.org/10.5194/cp-3-15-2007
Web End = , 2007.
Child, D., Elliott, G., Mifsud, C., Smith, A. M., and Fink, D.: Sample processing for earth science studies at ANTARES. Nuclear Instruments and Methods in Physics Research Section B, Beam Interactions with Materials and Atoms, 172, 856860, 2000.Clapperton, C. M.: Further observations on the stone runs of the
Falkland Islands, Biuletyn Peryglacjalny, 24, 211217, 1975.
Cockburn, H. A. P. and Summereld, M. A.: Geomorphological applications of cosmogenic isotope analysis, Progr. Phys. Geogr., 28, 142, 2004.
Dahl, R.: Block elds, weathering pits and tor-like forms in the Narvik mountains, Nordland, Norway, Geograf. Annal., 48A, 5585, 1966.
Dahlgren, R. A., Boettinger, J. L., Huntington, G. L., and Amundson, R. G.: Soil development along an elevational transect in the western Sierra Nevada, California, Geoderma, 78, 207236, 1997.
Dixon, J. C. and Thorn, C. E.: Chemical weathering and landscape development in mid-latitude alpine environments, Geomorphology, 67, 127145, 2005.
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
398 B. W. Goodfellow et al.: Late Quaternary not Neogene Acknowledgements. We thank Ulf Jonsell, Ben Kirk, Damian
Waldron, Mark Wareing, and staff at the Tarfala Research Station and Abisko Scientic Research Station for eld support, AnnE. Karlsen and Wieslawa Koziel (Geological Survey of Norway, Trondheim) for completing the grain size and XRD analyses, Bjrn Willemoes-Wissing (Geological Survey of Norway, Trondheim) for assisting with SEM, Maria Miguens-Rodriguez (SUERC, East Kilbride) for guiding Goodfellow through sample preparation for AMS, and Stewart Freeman for running AMS on sample Duo 1.Henriette Linge, an anonymous reviewer, and journal editor David Egholm are thanked for insightful comments that improved the manuscript. John Gosse, three anonymous journal reviewers and an associate editor are also thanked for helpful comments on an earlier version. The Swedish Society for Anthropology and Geography Andrefonden, Axel Lagrelius Fond, Hans and Lillemor Ahlmanns Fond, C. F. Liljevalchs Fond, Carl Mannerfelts Fond, and Lars Hiertas minne provided funding to Goodfellow, and Swedish Research Council grants 621-2001-2331 and 621-2005-4972, and a PRIME Lab SEED grant provided funding to Stroeven. Figure 9 is adapted from p. 547 in Stumm, W., Morgan, J. J., 1981, Aquatic Chemistry, 2nd Edn., John Wiley and Sons, New York, and from Fig. 1 in Nesbitt, H. W., Young, G. M., 1984, Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations, Geochimica et Cosmochimica Acta 48, 15231534, with permission of Wiley and Elsevier, respectively.
Edited by: D. Lundbek Egholm
References
Anderson, R. S.: Near-surface thermal proles in alpine bedrock: implications for the frost-weathering of rock, Arct. Alpine Res., 30, 362372, 1998.
Anderson, R. S.: Modeling the tor-dotted crests, bedrock edges, and parabolic proles of high alpine surfaces of the Wind River Range, Wyoming, Geomorphol., 46, 3558, 2002.
Andr, M. F.: Rates of postglacial rock weathering on glacially scoured outcrops (Abisko-Riksgrnsen area, 68 N), Geograf.
Annal., 84A, 139150, 2002.
Andr, M. F.: Do periglacial landscapes evolve under periglacial conditions?, Geomorphology, 52, 1491164, 2003.
Andr, M. F., Hall, K., Bertran, P., and Arocena, J.: Stone runs in the Falkland Islands: Periglacial or tropical?, Geomorphol., 95, 524543, 2008.
Bain, D. C., Mellor, A., Wilson, M. J., and Duthie, D. M. L.: Chemical and mineralogical weathering rates and processes in an upland granitic till catchment in Scotland. Water, Air Soil Pollut., 73, 1127, 1994.
Balco, G., Stone, J. O., Lifton, N. A., and Dunai, T. J.: A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements, Quaternary Geochronol., 3, 174195, 2008.
Ballantyne, C. K.: Age and signicance of mountain-top detritus,Permafrost Perigl. Process., 9, 327345, 1998.
Ballantyne, C. K.: A general model of autochthonous blockeld evolution, Permafrost Perigl. Process. 21, 289300, 2010.Ballantyne, C. K. and Harris, C.: The Periglaciation of Great
Britain, Cambridge University Press, Cambridge, 1994.
B. W. Goodfellow et al.: Late Quaternary not Neogene 399 Dredge, L.: Breakup of limestone bedrock by frost shattering andchemical weathering, Eastern Canadian Arctic, Arctic AlpineRes., 24, 314323, 1992.
Dredge, L. A.: Age and origin of upland block elds on Melville Peninsula, eastern Canadian Arctic, Geograf. Annal., 82A, 443 454, 2000.
Ericson, K.: Geomorphological surfaces of different age and origin in granite landscapes: An evaluation of the Schmidt hammer test, Earth Surf. Process. Landform., 29, 495509, 2004.
Eriksson, B.: Data rrandeSveriges temperaturklimat (Data concerning the air temperature of Sweden). SMHI Reports, Meteorology and Climatology, RMK 39, 1982.
Fabel, D., Stroeven, A. P., Harbor, J., Kleman, J., Elmore, D., and Fink, D.: Landscape preservation under Fennoscandian ice sheets determined from in situ produced 10Be and 26Al, Earth Planet.Sci. Lett., 201, 397406, 2002.
Fjeldskaar, W., Lindholm, C., Dehls, J. F., and Fjeldskaar, I.: Post-glacial uplift, neotectonics and seismicity in Fennoscandia, Quaternary Sci. Rev., 19, 14131422, 2000.
Fjellanger, J., Srbel, L., Linge, H., Brook, E. J., Raisbeck, G. M., and Yiou, F.: Glacial survival of blockelds on the Varanger Peninsula, northern Norway, Geomorphology, 82, 255272, 2006.
Fredn, C. (Ed): National Atlas of Sweden: Land and soils, SverigesNationalatlas, Vllingby, 208 pp., 2002.
Gardner, L. R.: Long-term isovolumetric leaching of aluminum from rocks during weathering: implications for the genesis of saprolite, Catena, 19, 521537, 1992.
Glasser, N. and Hall, A.: Calculating Quaternary glacial erosion rates in northeast Scotland, Geomorphology, 20, 2948, 1997.Goldstein, J., Newbury, D. E., Joy, D. C., Lyman, C. E., Echlin, P.,
Lifshin, E., Sawyer, L. C., and Michael, J. R.: Scanning Electron Microscopy and X-ray Microanalysis, 3rd Edn., Kluwer Academic/Plenum Publishers, New York, 689 pp., 2003.Goodfellow, B. W.: Relict non-glacial surfaces in formerly glaciated landscapes, Earth-Sci. Rev., 80, 4773, 2007.
Goodfellow, B. W.: A granulometry and secondary mineral nger-print of chemical weathering in periglacial landscapes and its application to blockeld origins, Quaternary Sci. Rev., 57, 121 135, 2012.
Goodfellow, B. W., Stroeven, A. P., Httestrand, C., Kleman, J., and Jansson, K. N.: Deciphering a non-glacial/glacial landscape mosaic in the northern Swedish mountains, Geomorphology, 93, 213232, 2008.
Goodfellow, B. W., Fredin, O., Derron, M.-H., and Stroeven, A. P.: Weathering processes and Quaternary origin of an alpine block-eld in Arctic Sweden, Boreas, 38, 379398, 2009.
Granger, D. E., Riebe, C. S., Kirchner, J. W., and Finkel, R. C.: Modulation of erosion on steep granitic slopes by boulder armoring, as revealed by cosmogenic 26Al and 10Be, Earth Planet.Sci. Lett., 86, 269281, 2001.
Green, C. P. and Eden, M. J.: Gibbsite in the weathered Dartmoor granite, Geoderma, 6, 315317, 1971.
Grudd, H. and Schneider, T.: Air temperature at Tarfala ResearchStation 19461995, Geograf. Annal., 78A, 115119, 1996.
Hales, T. C. and Roering, J. J.: Climatic controls on frost cracking and implications for the evolution of bedrock landscapes, J.Geophys. Res., 112, F02033, doi:http://dx.doi.org/10.1029/2006JF000616
Web End =10.1029/2006JF000616 http://dx.doi.org/10.1029/2006JF000616
Web End = , 2007.
Hall, A. M. and Phillips, W. M.: Weathering pits as indicators of the relative age of granite surfaces in the Cairngorm Mountains, Scotland, Geograf. Annal., 88A, 135150, 2006.
Httestrand, C. and Stroeven, A. P.: A relict landscape in the centre of Fennoscandian glaciation; geomorphological evidence of minimal Quaternary glacial erosion, Geomorphology, 44, 127143, 2002.
Hopkinson, C. and Ballantyne, C. K.: Age and Origin of Blockelds on Scottish Mountains, Scottish Geogr. J., 130, 116141, 2014.
Isaksen, K., Holmlund, P., Sollid, J. L., and Harris, C.: Three deep alpine-permafrost boreholes in Svalbard and Scandinavia, Permafrost Perigl. Process., 12, 1325, 2001.
Isaksen, K., Sollid, J. L., Holmlund, P., and Harris, C.: Recent warming of mountain permafrost in Svalbard and Scandinavia, J. Geophys. Res., 112, F02S04, doi:http://dx.doi.org/10.1029/2006JF000522
Web End =10.1029/2006JF000522 http://dx.doi.org/10.1029/2006JF000522
Web End = , 2007.
Ives, J. D.: Block elds, associated weathering forms on mountain tops and the Nunatak hypothesis, Geograf. Annal., 4, 220223, 1966.
Ives, J. D.: Biological refugia and the nunatak hypothesis, in: Arctic and Alpine Environments, edited by: Ives, J. D. and Barry, R. G., Methuen, London, 605636, 1974.
Jansson, K. N., Stroeven, A. P., Alm, G., Dahlgren, K. I. T., Glasser,N. F., and Goodfellow, B. W.: Using a GIS ltering approach to replicate patterns of glacial erosion, Earth Surf. Process Landf., 36, 408418, 2011.
Kauffman, G., Wu, P., and Li, G.: Glacial isostatic adjustment in Fennoscandia for a laterally heterogenous earth, Geophys. J. Int., 143, 262273, 2000.
Kleman, J. and Stroeven, A. P.: Preglacial surface remnants and Quaternary glacial regimes in northwestern Sweden, Geomorphology, 19, 3554, 1997.
Kleman, J., Stroeven, A. P., and Lundqvist, J.: Patterns of Quaternary ice sheet erosion and deposition in Fennoscandia and a theoretical framework for explanation, Geomorphology, 97, 7390, 2008.
Kohl, C. P. and Nishiizumi, K.: Chemical isolation of quartz for measurement of in situ-produced cosmogenic nuclides, Geochim. Cosmochim. Acta, 56, 35863587, 1992.
Lal, D.: Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models, Earth Planet. Sci. Lett., 104, 424439, 1991.
Lambeck, K., Purcell, A., Funder, S., Kjr, K. H., Larsen, E., and Mller, P.: Constraints on the Late Saalian to early Middle Weichselian ice sheet of Eurasia from eld data and rebound modeling, Boreas, 35, 539575, 2006.
Le Meur, E. and Huybrechts, P.: A comparison of different ways of dealing with isostasy: examples from modelling the Antarctic ice sheet during the last glacial cycle, Ann. Glaciol., 23, 309317, 1996.
Li, Y., Fabel, D., Stroeven, A. P., and Harbor, J.: Unraveling complex exposure-burial histories of bedrock surfaces under ice sheets by integrating cosmogenic nuclide concentrations with climate proxy records, Geomorphology, 99, 139149, 2008. Lidmar-Bergstrm, K.: A long-term perspective on glacial erosion,
Earth Surf. Process. Landf., 22, 297306, 1997.
Lisiecki, L. E. and Raymo, M. E.: A Pliocene-Pleistocene stack of 57 globally distributed benthic 18O records, Paleooceanography, 20, PA1003, doi:http://dx.doi.org/10.1029/2004PA001071
Web End =10.1029/2004PA001071 http://dx.doi.org/10.1029/2004PA001071
Web End = , 2005.
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
400 B. W. Goodfellow et al.: Late Quaternary not Neogene Marquette, G. C., Gray, J. T., Gosse, J. C., Courchesne, F., Stockli,
L., MacPherson, G., and Finkel, R.: Felsenmeer persistence under non-erosive ice in the Torngat and Kaumajet mountains, Quebec and Labrador, as determined by soil weathering and cosmogenic nuclide exposure dating, Can. J. Earth Sci., 41, 1938, 2004.
May, H. M., Helmke, P. A., and Jackson, M. L.: Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25 C, Geochim. Cosmochim. Acta, 43, 861
868, 1979.
McKeague, J. A., Grant, D. R., Kodama, H., Beke, G. J., and Wang,C.: Properties and genesis of a soil and the underlying gibbsite-bearing saprolite, Cape Breton Island, Canada, Can. J. Earth Sci., 20, 3748, 1983.
Meunier, A., Sardini, P., Robinet, J. C., and Prt, D.: The petrography of weathering processes: facts and outlooks, Clay Min., 42, 415435, 2007.
Milne, G. A., Mitrovica, J. X., Scherneck, H.-G., Davis, J. L., Johansson, J. M., Koivula, H., and Vermeer, M.:. Continuous GPS measurements of postglacial adjustment in Fennoscandia:2. Modelling results, J. Geophys. Res., 109, 118, 2004.
Moore, D. M. and Reynolds, R. C.: X-ray Diffraction and the Identication and Analysis of Clay Minerals, 2nd Edn., University Press, Oxford, 378 pp., 1997.
Nesbitt, H. W. and Young, G. M.: Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations, Geochimi. Cosmochim. Acta, 48, 15231534, 1984.
Nesje, A. and Whillans, I. M.: Erosion of Sognefjord, Norway, Geomorphology, 9, 3345, 1994.
Nesje, A., Dahl, S. O., Anda, E., and Rye, N.: Block elds in southern Norway: Signicance for the Late Weichselian ice sheet, Norsk Geologisk Tidsskrift, 68, 149169, 1988.
Nielsen, S. B., Gallagher, K., Leighton, C., Balling, N., Svenningsen, L., Jacobsen, B. H., Thomsen, E., Nielsen, O. B., Heilmann-Clausen, C., Egholm, D. L., Summereld, M. A., Clausen, O. R., Piotrowski, J. A., Thorsen, M. R., Huuse, M., Abrahamsen, N., King, C., and Holger Lykke-Andersen, H.: The evolution of western Scandinavian topography: a review of Neo-gene uplift versus the ICE (isostasy-climate-erosion) hypothesis,J. Geodynam., 47, 7295, 2009.
Nishiizumi, K.: Preparation of 26Al AMS standards, Nuclear Instrume. Methods Phys. Res. B, 223224, 388392, 2004.Nishiizumi, K., Imamura, M., Caffee, M. W., Southern, J. R.,
Finkel, R. C., and McAninch, J.: Absolute calibration of 10Be AMS standards, Nuclear Instrum. Methods Phys. Res. B, 258, 403413, 2007.
Paasche, ., Strmse, J. R., Dahl, S. O., and Linge, H.: Weathering characteristics of arctic islands in northern Norway, Geomorphology, 82, 430452, 2006.
Pedersen, V. K. and Egholm, D. L.: Glaciations in response to climate variations preconditioned by evolving topography, Nature, 493, 2062010, 2013.
Peltier, W. R.: Ice age paleotopography, Science, 265, 195201,1994.
Peltier, W. R.: Global glacial isostasy and the surface of the ice-age Earth: the ICE-5G (VM2) Model and GRACE, Ann. Rev. Earth Planet. Sci., 32, 111149, 2004.
Phillips, W. M., Hall, A. M., Mottram, R., Field, L. K., and Sugden, D.: Cosmogenic 10Be and 26Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion, Geomorphology, 73, 224245, 2006.
Potter, N. and Moss, J. H.: Origin of the Blue Rocks block eld and adjacent deposits, Berks County, Pennsylvania, Geol. Soc. Am. Bull., 79, 255262, 1968.
Rea, B. R.: Blockelds (Felsenmeer), in: The Encyclopedia of Quaternary Science, edited by: Elias, S. A., Vol. 3, Elsevier, Amsterdam, 523534, 2013.
Rea, B. R., Whalley, B., Rainey, M. M., and Gordon, J. E.: Blockelds, old or new? Evidence and implications from some plateaus in northern Norway, Geomorphology, 15, 109121, 1996.
Refsnider, K. A. and Miller, G. H.: Ice-sheet erosion and the stripping of Tertiary regolith from Bafn Island, eastern Canadian Arctic, Quaternary Sci. Rev., 67, 176189, 2013.
Reynolds, R. C.: Clay mineral formation in an alpine environment,Clays Clay Mineral., 19, 361374, 1971.
Small, E. E., Anderson, R. S., Repka, J. L., and Finkel, R.: Erosion rates of alpine bedrock summit surfaces deduced from in situ 10Be and 26Al, Earth Planet. Sci. Lett., 150, 413425, 1997. Small, E. E., Anderson, R. S., and Hancock, G. S.: Estimates of the rate of regolith production using 10Be and 26Al from an alpine hillslope, Geomorphology, 27, 131150, 1999.
Staiger, J. K. W., Gosse, J. C., Johnson, J. V., Fastook, J., Gray, J. T., Stockli, D. F., Stockli, L., and Finkel, R.: Quaternary relief generation by polythermal glacier ice, Earth Surf. Process. Landform., 30, 11451159, 2005.
Steer, P., Huismans, R. S., Valla, P. G., Gac, S., and Herman, F.:
Bimodal PlioQuaternary glacial erosion of fjords and low-relief surfaces in Scandinavia, Nat. Geosci., 5, 635639, 2012. Steffen, H. and Kauffman, G.: Glacial isostatic adjustment of Scandinavia and northwestern Europe and the radial viscosity structure of the Earths mantle, Geophys. J. In., 163, 801812, 2005. Steffen, H., Kauffman, G., and Wu, P.: Three-dimensional nite-element modeling of the glacial isostatic adjustment in Fennoscandia, Earth Planet. Sci. Lett., 250, 358375, 2006. Stone, J. O.: Air pressure and cosmogenic isotope production, J.
Geophys. Res., 105, 2375323759, 2000.
Stroeven, A. P., Fabel, D., Hattestrand, C., and Harbor, J.: A relict landscape in the centre of Fennoscandian glaciation; cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles, Geomorphology, 44, 145154, 2002.
Stroeven, A. P., Harbor, J., Fabel, D., Kleman, J., Httestrand, C. El-more, D., Fink, D., and Fredin, O.: Slow, patchy landscape evolution in northern Sweden despite repeated ice-sheet glaciation, Geol. Soc. Am. Special Paper, 398, 387396, 2006.
Strmse, J. R. and Paasche, .: Weathering patterns in high-latitude regolith, J. Geophys. Res., 116, F03021, doi:http://dx.doi.org/10.1029/2010JF001954
Web End =10.1029/2010JF001954 http://dx.doi.org/10.1029/2010JF001954
Web End = , 2011.
Stumm, W. and Morgan, J. J.: Aquatic Chemistry, 2nd Edn., JohnWiley and Sons, New York, 1981.
Sugden, D. E.: The selectivity of glacial erosion in the Cairngorm Mountains, Scotland, Trans. Institute of British Geographers, 45, 7992, 1968.
Earth Surf. Dynam., 2, 383401, 2014 www.earth-surf-dynam.net/2/383/2014/
B. W. Goodfellow et al.: Late Quaternary not Neogene 401 Sugden, D. E.: Landscapes of glacial erosion in Greenland andtheir relationship to ice, topographic and bedrock conditions, in:Progress in Geomorphology: Papers in honour of D.L. Linton,edited by: Brown, E. H. and Waters, R. S., Institute of BritishGeographers Special Publication, Vol. 7, 177195, 1974.
Sugden, D. E. and Watts, S. H.: Tors, felsenmeer, and glaciation innorthern Cumberland Peninsula, Bafn Island, Can. J. Earth Sci.,14, 28172823, 1977.
Sumner, P. D. and Meiklejohn, K. I.: On the development of autochthonous blockelds in the grey basalts of sub-Antarctic Marion Island, Polar Geogr., 28, 120132, 2004.
US Department of Agriculture: Soil Survey Manual, Agricultural Handbook No. 18. Government Printing Ofce, Washington D.C., 532 pp., 1993.
Walder, J. S. and Hallet, B.: A theoretical model of the fracture of rock during freezing, Geol. Soc. Am. Bull., 96, 336346, 1985.Watanabe, T., Funakawa, S., and Kosaki, T.: Distribution and formation conditions of gibbsite in the upland soils of humid Asia: Japan, Thailand and Indonesia, 19th World Congress of Soil Science, Soil Solutions for a Changing World, 16 August 2010, Brisbane, Australia, 1720, 2010.
Wesolowski, D. J. and Palmer, D. A.: Aluminum speciation and equilibria in aqueous solution: V. Gibbsite solubility at 50 C and pH 39 in 0.1 molal NaCl solutions (a general model for aluminum speciation; analytical methods), Geochim. Cosmochim. Acta, 58, 29472969, 1994.
Whalley, W. B., Rea, B. R., and Rainey, M.: Weathering, block-elds, and fracture systems and the implications for long-term landscape formation: some evidence from Lyngen and ksfordjkelen areas in north Norway, Polar Geogr., 28, 93119, 2004.
White, A. F., Blum, A. E., Schulz, M. S., Vivit, D. V., Stonestrom,D. A., Larsen, M., Murphy, S. F., and Eberl, D.: Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: I. Long-term versus short-term weathering uxes, Geochim. Cosmochim. Acta, 62, 209226, 1998.
www.earth-surf-dynam.net/2/383/2014/ Earth Surf. Dynam., 2, 383401, 2014
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
Copyright Copernicus GmbH 2014
Abstract
Autochthonous blockfield mantles may indicate alpine surfaces that have not been glacially eroded. These surfaces may therefore serve as markers against which to determine Quaternary erosion volumes in adjacent glacially eroded sectors. To explore these potential utilities, chemical weathering features, erosion rates, and regolith residence durations of mountain blockfields are investigated in the northern Swedish Scandes. This is done, firstly, by assessing the intensity of regolith chemical weathering along altitudinal transects descending from three blockfield-mantled summits. Clay / silt ratios, secondary mineral assemblages, and imaging of chemical etching of primary mineral grains in fine matrix are each used for this purpose. Secondly, erosion rates and regolith residence durations of two of the summits are inferred from concentrations of in situ-produced cosmogenic <sup>10</sup>Be and <sup>26</sup>Al in quartz at the blockfield surfaces. An interpretative model is adopted that includes temporal variations in nuclide production rates through surface burial by glacial ice and glacial isostasy-induced elevation changes of the blockfield surfaces. Together, our data indicate that these blockfields are not derived from remnants of intensely weathered Neogene weathering profiles, as is commonly considered. Evidence for this interpretation includes minor chemical weathering in each of the three examined blockfields, despite consistent variability according to slope position. In addition, average erosion rates of ~16.2 and ~6.7 mm ka<sup>-1</sup>, calculated for the two blockfield-mantled summits, are low but of sufficient magnitude to remove present blockfield mantles, of up to a few metres in thickness, within a late Quaternary time frame. Hence, blockfield mantles appear to be replenished by regolith formation through, primarily physical, weathering processes that have operated during the Quaternary. The persistence of autochthonous blockfields over multiple glacial-interglacial cycles confirms their importance as key markers of surfaces that were not glacially eroded through, at least, the late Quaternary. However, presently blockfield-mantled surfaces may potentially be subjected to large spatial variations in erosion rates, and their Neogene regolith mantles may have been comprehensively eroded during the late Pliocene and early Pleistocene. Their role as markers by which to estimate glacial erosion volumes in surrounding landscape elements therefore remains uncertain.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer