Age and weathering status of granite tors in Arctic Finland (~ 68° N

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Geomorphology 94

Age and weathering status of granite tors in Arctic Finland (~68° N)

R.G. Darmody a,⁎, C.E. Thorn b, M. Seppälä c, S.W. Campbell d, Y.K. Li e, J. Harbor f

a Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana—Champaign, Urbana, IL 61801, USAb Department of Geography, University of Illinois at Urbana—Champaign, Urbana, IL 61801, USA

c Department of Geography, University of Helsinki, Helsinki, Finlandd Independent Scholar, Tontitown, AR 72770, USA

e Department of Geography, University of Missouri—Columbia, Columbia, MO 65211, USAf Department of Geography and Environmental Sciences, University of Colorado at Denver and Health Sciences Center, Denver, CO 80217, USA

Received 6 November 2006; accepted 12 April 2007Available online 6 May 2007

Abstract

Cold-based ice along the Fennoscandian glacial ice divide in northern Finland allowed preservation of old landscape features.Because there was little or no movement at the ice-land surface contact, erosion was at a minimum, and relict landscape featuressuch as tors can be found. We investigated two such granitic tors located at Pyhä-Nattanen (68°07.335′ N, 27°22.207′ E, 508 m asl)and Riestovaara (68°02.613′ N, 27°09.003′ E, 390 m asl) in subarctic Finland. At Pyhä-Nattanen, the sampling sequence includedbedrock material and grus taken from within horizontal cracks, which are so prevalent at the site as to make the tor resemble a stackof pancakes. At Riestovaara, where the outcrop is more subdued, in addition to bedrock and grus samples, soil samples were alsoextracted from a pit dug in an embryonic soil forming on the bedrock surface.

Based on cosmogenic nuclide dating, both tors greatly predate recent glaciation. The tor at Pyhä-Nattanen, which is a moreprominent landscape feature, had longer minimum 10Be and 26Al apparent exposure ages, 89.1±6.7 kyr and 69.6±6.4 kyr,respectively, than did the tor at Riestovaara, 55.8±4.5 kyr and 46.5±4.0 kyr, respectively. Both of them are significantly older than theregional last deglaciation age, indicating that both sites have survived multiple episodes of glaciation. Analyses of the 10Be and 26Almeasurements in accordance with marine oxygen isotope records (DSDP 607) indicate that the tors have undergone a minimumestimated total exposure-burial durations of 997 kyr (Pyhä-Nattanen) and 858 kyr (Riestovaara) and survived at least 14–16 episodesof glaciation. Weathering, as measured by porosity determined with a microprobe, was somewhat more advanced in the Pyhä-Nattanen granite samples than in the Riestovaara granite. However, with both granites, rock porosity did not change to a depth of 4 cmbelow the rock surface or vary by lichen cover/noncovered surfaces, indicating that weathering had progressed to a stage where recentlichen growth is overwhelmed by the long weathering history. Other measures of weathering, including total elemental analyses, didnot detect significant differences among at-a-site samples, perhaps because all samples are highly weathered and we did not have asample of unweathered granite for comparison. All samples, rock, grus, and soil were within the grus weathering range as indicated bychemical weathering indices. The soil forming adjacent to the tor at Riestovaara exhibited only slight development despite the greatapparent age of the landscape. The regolith in which the soil is formingmust therefore be much younger than the exposed bedrock andmost likely represents a post-deglaciation accumulation of grus spanning only the last 9 kyr.© 2007 Elsevier B.V. All rights reserved.

Keywords: Cosmogenic nuclides; Chemical weathering; Tors; Northern Finland; Glacial erosion; Relict landscapes; Grus

⁎ Corresponding author. Tel.: +1 217 333 9489; fax: +1 217 244 3219.E-mail address: [email protected] (R.G. Darmody).

0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2007.04.006

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11R.G. Darmody et al. / Geomorphology 94 (2008) 10–23

1. Introduction

Tors are the focus of an unusually rich geomorpho-logical literature. This is because they have arousedgreat curiosity in and of themselves, but also becausethey have been and continue to be landforms used toexplain the geomorphological history of the surroundingareas. The present paper falls unequivocally into thesecond category, and for the former we limit ourselvessimply to noting the paper by Twidale (2002) that hasthe benefit of placing tor formation in a broader context.Interest here focuses upon tors as sites of a bare bedrockchemical weathering regime as they are ridge-summitphenomena at least partially free of soil cover. Thisavenue of investigation may be enhanced by analysis ofthe adjacent derivative grus and embryonic soil foundon and around most tors or similar features (Marquetteet al., 2004). However, previous papers (e.g., André,2004; Phillips et al., 2006) have shown that the con-temporary weathering story for tors may be complicatedin previously glaciated environments by the larger- andlonger-scale landscape history.

In addition to determining the nature of weatheringprocesses, determining weathering rate, absolutely andrelatively, is important (e.g., Thorn et al., 2006a,b).Investigation of weathering rates on bedrock surfaceshas been greatly enhanced by the emergence ofcosmogenic nuclide techniques (Gosse and Phillips,2001), which provide the first real opportunity to date thesurface exposure of bedrock and thereby to determine

Fig. 1. Map of Finnish tor research sites

directly the expanse of time over which the measuredchemical weathering has taken place. For this reason, wecollected rock samples from the bare rock top of each torto determine 10Be, 26Al, and, 36Cl cosmogenic nuclidedates.With these, one can estimate the length of burial ofthe site beneath the Fennoscandian ice sheet. Conse-quently, the cosmogenic nuclide dates presented here notonly underpin the chemical analyses, but add a pair ofdated sites to the burgeoning literature on cold-based icesheets in Scandinavia (e.g., Kleman et al., 1997; Fabelet al., 2002; Stroeven et al., 2002a,b; Li et al., 2005;Stroeven et al., 2006; Harbor et al., 2006). The impact ofcold-based ice on the landscape varies from an almostentirely protective one to one that permits varyingdegrees of modest modification. Hall and Phillips (2006)have shown in the CairngormMountains of Scotland thatthe morphology of tors previously covered by cold-based ice may be combined with cosmogenic nuclidedating to construct a meaningful geomorphologicalhistory. Similar research has been conducted in theParkajoki region of northeastern Sweden (Hätterstrandand Stroeven, 2002; Stroeven et al., 2002a), which at∼67-40′N, 23-15′ E, lies slightly south and∼250 km tothe west of our research area.

The primary objective of this research was aninvestigation of the weathering status of bare bedrockand associated grus in the subarctic environment ofnorthern Finland. This represents geographical expan-sion of our earlier investigations of present-daychemical weathering in Norway and Sweden involving

, Pyhä-Nattanen and Riestovaara.

Fig. 2. (A) View NE from Riestovaara to Pyhä-Nattanen. Elevation ofPyhä-Nattanen, 508 m asl, the intervening valley is at 250 m asl. (B)Tor at Pyhä-Nattanen summit. (C) Collecting grus sample F8 frominterstices of tor at Pyhä-Nattanen.

Fig. 3. (A) Summit of Riestovaara, soil pit location indicated by arrow.(B) Soil pit at Riestovaara, tape marked in decimeters.

12 R.G. Darmody et al. / Geomorphology 94 (2008) 10–23

soils and bedrock (e.g., Darmody et al., 2000, 2002;Campbell et al., 2002; Dixon et al., 2002a,b; Darmodyet al., 2005a,b). The overall findings of the Norwegianand Swedish work have been to establish, contrary toolder beliefs, that biogeochemical processes are activein cold environments and comparable in nature to thosefound in mid-latitude, temperate contexts. However, themore subdued, and frequently mantled, landscape ofnorthern Finland represents a greater challenge whenseeking bedrock exposures, one where tors are appeal-ing landforms for sampling.

2. Research area and sites

The research area (Fig. 1) is located about 170 kmnorth of the Arctic Circle in northern Finland, althoughthe environment is commonly considered subarctic. Thespecific research sites are hills in an otherwise low-reliefarea. Pyhä-Nattanen is at 68°07.335′ N, 27°22.207′ E,with a summit elevation of 508 m asl. Riestovaara is at68°02.613′ N, 27°09.003′ E, about 13 km SE of Pyhä-Nattanen and has a summit elevation of 390 m asl. Theintervening lowland is about 250m asl. Twomain factorsdominate the landscape in the study region: fells that areresidual hills and ridges formed of Precambrian granites,and relatively flat lowlands surrounding the fells andcovered by large mires. Elevations in the region rangefrom 240 to 543 m. Pyhä-Nattanen and Riestovaara areamong the highest fells in the region, which includeTerävä Nattanen at 543m and Seinätunturi at 520m a.s.l.Relative relief varies from 200 to 300 m. Fell slopes arecovered by large, flat blocks and felsenmeer.

The lowlands are characterized by layers of preglacialin situ weathered granites and granulite tens of meters inthickness originating probably from Palaeozoic andTertiary times when subtropical conditions prevailed


(Hyyppä, 1987; Islam et al., 2002). A thickness of asmuch as 100 m of saprolite and weathered bedrock hasbeen found in the region (Virkkala, 1955).

The Weichselian glacial maximum in this region tookplace some 20,000 yr ago and the area was deglaciatedabout 9500 to 9200 B.P. as the ice margin retreated fromeast to west (Johansson, 1995; Fredin, 2002). About100 kmnorth of our study area, Kaitanen (1989) estimatedthat the ice thickness during the glacial maximum barelyexceeded 1200 m and that the ice cover was generallyb600 m in thickness during the other glacial phases.Kaitanen also suggested that the ice was cold-based,which would essentially prevent basal erosion. Our studyarea is located close to the ice divide of the last glaciation;therefore, very stable glacial conditions dominated thearea and the ensuing till mantle, where present, is veryshallow. Such a situation favors survival of the bedrockphenomena such as tors, and perhaps that of the preglacialweathering matrix in some locations.

The view of summit tors in previously glaciatedregions has changed from unglaciated indicators of theglacial limit, often located on nunataks (e.g., Ives, 1966),to that of landforms that frequently survived glaciation bycold-based ice (e.g., André, 2004; Phillips et al., 2006).Both of these latter papers provide independentlydeveloped, but remarkably similar, models of tors thathave survived cold-based ice glaciation. The models donot simply present tors as wholly unmodified byglaciation, but as features that may have experiencedvarying degrees of modification. This view has importantimplications for evaluation of cosmogenic nuclideexposure dating. Furthermore, “The imbalance between

Table 1Interpretation of 10Be and 26Al Finnish data compared to published data from

Location Elevation(m asl)

10Beconcentration(105 atoms g−1) a

26Alconcentration(105 atoms g−1) a

26Al/10B

FinlandPyhä-Nattanen 508 4.44±0.20 20.94±1.34 4.70±0.Riestovaara 390 2.80±0.15 14.14±0.80 5.04±0.

Northern SwedenNaakakar-hakka 360 3.84±0.17 20.50±0.98 5.34±0.Lamuvaara 300 3.08±0.14 15.78±1.14 5.12±0.Kiuhtislompolo 350 1.82±0.11 – –

a Samples were collected at latitude N67°N and measured nuclide concentb Minimum exposure age was calculated assuming zero erosion at samplec Minimum burial time was calculated from 26Al/10Be ratio assuming that th

buried rapidly by ice.d Total history is based on the DSDP 607 marine oxygen isotope proxy rec

the sample was first exposed to arrive at the present-day 10Be and 26Al consamples were derived using a cutoff value of 3.7‰ to deter mine ice-free andAll ages are minimum estimates assuming zero erosion during the glacial an

rapid regolith formation and removal and slower bedrockerosion creates tors” (Phillips et al., 2006, p. 241). Animportant inference for this study is that the bedrockchemical weathering and the grus/soil stories may notreflect identical histories. The work described in Phillipset al. (2006) is expanded upon in Hall and Phillips (2006)where a nuanced description of the impact of cold-basedice upon tor form in the CairngormMountains of Scotlandis presented and combined with a set of associatedcosmogenic nuclide exposure dates.

The highest tors in our study region are 7 to 9 m inheight and 40 to 50 m in diameter. They are the exposedsurfaces of Nattanen-type granite batholiths (Haapalaet al., 1987; Front et al., 1989). We have not investigatedany aspect of their morphology systematically, althoughthe tor at Pyhä-Nattanen appears to bear a strongresemblance to tors of the Parkajoki region in nearbySweden (Hättestrand and Stroeven, 2002). Comprehen-sive impressions of the tor complex at Pyhä-Nattanenappear in Fig. 2, while an overview of the much moresubdued summit at Riestovaara, as well as a view of thesoil pit dug there, appears in Fig. 3. Mean annual airtemperature in the research area is about −1 °C, withFebruary the coldest month at −14 °C, and July thewarmest at +14 °C. Mean annual precipitation is∼450 mm, with 250 mm of that as snow; and meansnow depth inMarch is∼70 cm (Atlas of Finland, 1987).Actual snow depth on the fell summits and on the tors isunknown because much of it presumably blows off.However, hoar ice accumulations are common (Seppälä,2004), which may affect the amount of cosmic radiationtors receive. Vegetation at the research sites is a sparse

northern Sweden

e 10Be minimumexposure age(kyr) b

26Al minimumexposure age(kyr) b

Minimumburial time(kyr) c

Minimumtotal history(kyr) d

37 89.1±6.7 69.6±6.4 502 99739 55.8±4.5 46.5±4.0 371 858

35 79±17 70±15 216 48544 64±14 54±12 301 765

37±8 – – –

rations were normalized to sea level using Stone (2000).surface.e sample originally experienced a long period of exposure and then was

ord (Lazarus et al., 1995) using an iterative process to determine whencentrations (Finnish samples). The histories for the northern Swedencovered periods (Kleman and Stroeven, 1997; Stroeven et al., 2002a,b).d inter-glacial periods.

Table 2Interpretations and 36Cl data for samples from Pyhä-Nattanen and Riestovaara

Location Elevation(m asl)

36Cl/Clratio(10−15) a

Cl(ppm)

CaO(wt.%)

K2O(wt.%)

36Cl exposure age (kyr) b

Zero erosion 1.1 mm/kyr of erosion 1.6 mm/kyr of erosion

Pyhä-Nattanen 508 662±38 97.1 0.88 4.80 52.1±3.0 48.4±2.8 47.5±2.7Riestovaara 390 547±35 70.7 0.61 5.20 33.1±2.1 31.9±2.0 31.5±2.0a Ratio corrected using a 35Cl/Cl procedural blank value of 43±15×10−15.b 1σ AMS measurement uncertainty is reported in this table. Two surface erosion rates are used to show the sensitivity of the 36Cl ages to possible

surface erosion rates (Bierman et al., 1999; Stroeven et al., 2002a,b).


pine forest at Riestovaara and the lower elevations ofPyhä-Nattanen, with tundra at the summit of the latter.

3. Materials and methods

3.1. Field methods

At Pyhä-Nattanen, bedrock samples were taken byhand with a hammer and chisel; and grus was retrievedby hand from interstices between horizontal bedrockcracks, which are so prevalent that the tor resembles astack of pancakes (Fig. 2B, C). At Riestovaara, wherethe outcrop is more subdued, in addition to bedrock andgrus samples, soil samples were also collected from a pitdug in an embryonic soil forming on the bedrock surfacein one portion of the outcrop. Because there wasessentially no soil at Pyhä-Nattanen, we sampled a 15-cm-thick peat section from one of the small patches ofthin peat that blankets depressions in the rock outcrop(Luoto and Seppälä, 2000). Specific grus samplescollected at Pyhä-Nattanen included F2, taken from asmall joint in the tor; F6, taken from beneath a looserock slab on the tor top; and F8, from a large horizontaljoint in the side of the tor (Fig. 2C). Grus samples fromRiestovaara included F1 from a wide joint and F5 from athin joint in the outcrop.

3.2. Cosmogenic nuclide dating

Rock samples taken with a hammer and chisel fromsummit flat bedrock surfaces provided material forcosmogenic nuclide exposure age dating. Standard

Table 3Mean porosity (%) of granite along micro-transect from surface into the roc

Surface Location T

0–50 50–100 10

Lichen-free Pyhä-Nattanen 11.4 9.9 9.Riestovaara 2.0 1.4 1.

Lichen-covered Pyhä-Nattanen 10.4 9.6 8.Riestovaara 1.5 1.3 1.

sample information collected in the field includedshielding, sample thickness, longitude, latitude, eleva-tion, and lithology. PRIME Lab provided samplepreparation and accelerator mass spectrometry (AMS)measurements of 36Cl, 10Be, and 26Al. Approximately40 g of pure quartz was separated for each sample for10Be and 26Al analyses following the procedures ofKohl and Nishiizumi (1992). The quartz was thendissolved in HF and HNO3 and spiked with ∼0.7 mg9Be carrier. Total Al concentrations in quartz weremeasured by atomic absorption spectrophotometry inaliquots of solutions used to prepare targets for AMSanalysis. The 10Be/9Be and 26Al/27Al ratios weremeasured by AMS and corrected by full chemistryprocedural blanks. Exposure ages were calculated using10Be and 26Al production rates of 5.1±0.3 atoms g−1

yr−1 and 31.1±1.9 atoms g−1 yr−1 at sea level and highlatitude (N60°), respectively, and scaled to site specificaltitude and latitude using Stone's (2000) model. Thecorrection for sample thickness was calculated using anattenuation coefficient of 150 g cm−2 and density of2.75 g cm−3. The 26Al/10Be ratios were used todetermine minimum periods of surface burial by iceusing standard methods described in Fabel et al. (2002)and Stroeven et al. (2002a,b).

For 36Cl, ∼30 g of whole rock sample was dissolved,and the whole rock wet chemical technique was used toextract Cl for AMS measurement (Zreda et al., 1991).Chemical element analysis was performed to determinethe production rates of the 36Cl for each sample. The 36Clages were calculated using the CHLOE spreadsheetprogram (Phillips and Plummer, 1996). We used the

k from tor research sites in Finland

ransect depth below rock surface (μm)

0–150 150–200 200–250 250–300 4000

9 9.4 10.0 9.7 10.28 1.3 1.7 1.4 1.37 8.5 9.9 10.1 10.25 2.3 1.2 1.4 1.3

Table 4Particle size distribution of the soil profile at Riestovaara and of grus from Pyhä-Nattanen and Riestovaara

Horizon Depth a Gravel Class Sand Silt Clay VCoS CoS MS FS VFS CoSi FSi

O 0–6 48 LCoS 85 13 2 37 20 14 11 4 8 5E 6–14 50 LCoS 85 12 3 46 16 9 9 5 7 5Bw 14–28 50 CoS 88 11 1 60 13 6 5 4 7 4BC 28–46 61 CoS 94 5 1 74 15 3 2 1 3 2C 42–57 59 CoS 90 8 2 48 21 11 6 3 6 2Pyhä-Nattanen Grus (F2) 41 CoS 98 2 0 61 25 7 3 1 2 0

Grus (F6) 39 CoS 88 10 2 53 16 9 7 3 6 4Grus (F8) 31 CoS 94 6 0 43 25 15 8 2 4 2

Riestovaara Grus (F1) 84 CoS 99 1 0 61 31 5 1 0 1 0Grus (F5) 27 LCoS 76 22 2 29 21 12 9 5 14 8

Class; LCoS, loamy coarse sand, CoS, coarse sand; VCoS, very coarse sand; CoS, coarse sand; MS, medium sand; FS, fine sand; VFS, very fine sand;CoSi, coarse silt; FSi, fine silt.a Depth in cm, particle size in %.


production rates for 36Cl reported by Swanson and Caffee(2001) because they were calibrated at relatively highlatitudes (∼48°). In their work, the total production ratesfor 36Cl are 91±5 atoms 36Cl (g Ca)−1 yr−1 due to Ca,228±18 atoms 36Cl (g K)−1 yr−1 due to K, and 762±28neutron (g air)−1 yr−1 for secondary neutrons.

Our calculated apparent exposure ages were based onthe assumption of zero erosion at the sample surface.Other factors such as sediment and snow cover were notconsidered in the calculation because of the lack ofappropriate data. Given the exposed nature of thesampling sites, we presume they are typically wind-blown and snow-free. For 10Be and 26Al, snow andsediment cover will reduce nuclide production of thesample, thus our calculated ages will be minimumestimates of the surface exposure. For 36Cl, the ages werecomplicated because of the contribution of neutron-

Table 5Mineralogy of b4 μm fraction of peat, grus, and soil from Finnish tor resea

Sample Quartz Mica Feldspar

Pyhä-NattanenPeat 0 a 100 0Grus (F2) 5 45 40Grus (F6) 10 55 25Grus (F8) 10 35 45

Riestovaara soil horizonsO 10 65 20E 10 35 40Bw 15 55 15BC 5 40 30C 10 30 30Grus (F1) 5 30 30Grus (F5) 10 15 40

a Mineral abundances estimated from XRD peak areas and rounded to the

capture reactions. Overall, we estimate that the impact ofsnow cover would be b5% of the 36Cl ages.

3.3. Rock porosity

Chemical weathering increases rock porosity undervirtually all circumstances (e.g., Dorn and Brady, 1995;Brady et al., 1999; Gordon, 2005), and we used back-scatter electron (BSE) images of thin sections to estimateporosity of granite collected from both sites following themethod of Dorn (1995). We also sampled lichen-coveredand lichen-free rock surfaces to test the effects of lichengrowth on granite porosity. Porosity was determined onreplicated 2500 cm2 rectangles obtained with a micro-probe. Percentage values are relative, and the unweath-ered rock does not have a percentage value of zero. Forthis reason, we created multiple depth transects. Each

rch sites

Chlorite Vermiculite Interstratified

%

0 0 05 5 05 5 00 10 0

0 5 00 10 50 15 00 25 00 30 05 30 00 35 0

nearest 5%.

Table 6Total C and N, and extractable elements of peat, grus, and soil from Finnish tor research sites

Sample C N pH B.S.%

S P Ca Mg K Na B Fe Mn Cu Zn Al

% mg kg−1

Pyhä-NattanenPeat 45.5 1.6 3.4 15 29 35 1022 159 82 38 0.50 62 2 0.7 8.9 237Grus (F2) 0.2 0.1 5.0 21 38 78 152 40 65 37 0.62 210 4 1.2 1.4 569Grus (F8) 1.3 0.1 4.6 28 68 201 499 36 40 32 0.68 302 2 1.6 1.4 1183

Soil horizons at RiestovaaraO 18.6 0.6 3.7 7 30 72 305 60 93 25 0.47 175 8 1.1 5.1 780E 0.9 0.1 4.6 9 21 16 81 20 29 26 0.45 123 b1 0.8 0.8 532Bw 2.1 0.1 4.9 7 54 18 96 26 36 22 0.49 102 b1 0.7 0.6 2293BC 0.6 0.1 5.3 22 49 10 213 27 33 27 0.62 149 1 0.9 0.8 2012C 0.2 0.1 5.3 42 51 13 163 27 28 25 0.62 140 b1 0.9 0.8 1837Grus (F5) 3.3 0.2 5.1 24 73 15 648 38 66 35 0.56 76 1 0.7 1.1 2077


transect had sample rectangles at 0–50, 50–100, 100–150, 150–200, 200–250, and 250–300 cm depth beneaththe rock surface. Results from the transects werecompared to matching rectangles sampled at a depth of∼4 cm within the rock interior that, for investigativepurposes, we assumed to represent “unweathered” orbaseline values.

Analysis of variance (ANOVA) of rank-order trans-formed data was used because raw percentage data arenot appropriate for parametric analysis (Conover andInman, 1981). Use of multiple-way ANOVA wasprecluded by uneven n values in the cells. Rather thanovercome this problem by randomly removing some

Table 7Major element oxide content (wt. %) of peat, granite rocks, grus, and soil sa

Sample SiO2 Al2O3 Fe2O3 MnO M

Pyhä-NattanenPeat 54 17 4.2 0.04 2.Grus (F2) 76 12 1.7 0.02 0.Grus (F8) 75 13 1.9 0.03 0.Granite 1 73 13 2.2 0.03 0.Granite 2 74 13 2.3 0.03 0.

RiestovaaraGrus (F1) 77 12 1.3 0.01 0.Grus (F5) 67 14 3.5 0.03 0.Granite 1 73 14 1.6 0.01 0.Granite 2 75 13 1.9 0.01 0.

Soil horizons at RiestovaaraO 61 8 1.4 0.01 0.E 75 11 2.4 0.01 0.Bw 67 13 3.5 0.02 0.BC 74 12 2.2 0.02 0.C 77 11 2.1 0.02 0.

values from cells, individual one-way ANOVAs wereused. In short, the value of large n values was placedabove the problems of using repetitive ANOVAs.Results reported below all invoke the 5% confidencelevel (p≤0.05). The lichen-free and lichen-coveredtransects from each site were not “paired” in any formalsense and, therefore, are not appropriate for a paired t testor similar analysis.

3.4. Rock and soil laboratory analyses

Soil samples collected by horizon from a hand-dugpit were oven-dried and passed through a 2-mm sieve

mples from Finnish tor research sites

gO CaO Na2O K2O TiO2 P2O5

08 8.98 3.3 4.8 0.47 3.0518 0.59 2.8 5.0 0.20 0.0424 0.75 3.2 4.5 0.28 0.0634 0.88 3.6 4.8 0.29 0.0735 0.78 3.4 5.0 0.29 0.07

15 0.59 2.8 4.9 0.14 0.0457 0.94 3.0 3.8 0.36 0.0822 0.70 3.8 4.9 0.20 0.0529 0.61 3.3 5.2 0.25 0.05

08 0.36 1.8 3.4 0.21 0.0727 0.65 2.6 4.3 0.41 0.0373 1.05 2.5 3.1 0.40 0.0636 0.72 2.6 3.9 0.22 0.0531 0.63 2.5 3.8 0.19 0.04


before analyses. Particle-size analysis was by sieving forsands and gravels and by hydrometer for silt and clay(Gee and Bauder, 1986). A 1:1 soil to distilled watersuspension was used to determine pH (McLean, 1982).Extractable elements were determined by inductivelycoupled plasma spectroscopy on 1:10 soil:Mehlich 3extracts (Mehlich, 1984) at a commercial lab (BrooksideLabs, New Knoxville, OH). Cation exchange capacity(CEC) was estimated by summation of Mehlich 3extracted cations. Base saturation (BS) was determinedusing exchangeable H estimated from SMP buffer pH atpH 7.8 (McLean, 1982). Organic carbon and nitrogencontent were determined with a Leco C/N analyzer.Because there were no free carbonates in the soil, total Cwas attributed solely to organic matter.

Total elemental analyses of rock, grus, and soilsamples were determined with lithium metaborate/tetra-borate fusion followed by ICP-MS analyses at acommercial lab (Activation Laboratories Ltd., Ancaster,Ontario). Replicates and standards were run along withthe samples to insure quality control of the analyses. Theresults of chemical analyses were used to calculate severalwidely used weathering indices (Darmody et al., 2005b).

Mineralogy of the b4-μm soil fraction was deter-mined by X-ray diffraction on a Scintag diffractometerwith Co radiation. Semiquantitative estimates of themineral concentrations were made from peak areasobtained from Mg- and K-saturated and heated samples(Klages and Hopper, 1982).

4. Results and discussion

4.1. Cosmogenic nuclide apparent exposure ages

Apparent exposure ages for Pyhä-Nattanen andRiestovaara (Table 1) are considerably older than themost recent deglaciation age for the region, about 9200to 9500 BP (Johansson, 1995; Fredin, 2002), butconsistent with other published apparent exposureages for tors in northern Scandinavia (Stroeven et al.,2002a,b). Ages that are older than the last deglaciationindicate that the sites survived one or more periods ofglaciation without significant surface erosion (b2 m).The concentrations of cosmogenic nuclides in samplesthus reflect the accumulation of nuclides during periodswithout ice cover, minus decay of the nuclides duringperiods of ice cover. Surface erosion can also reducenuclide concentrations. However, field evidence such asintact weathering features suggests that glacial erosionat these sites has been insignificant since developmentof the features. Because of the different half-lives of thenuclides, we expect each to yield a different apparent

exposure age; and the ratios of the nuclide concentra-tions can be used to infer the cumulative length of timethat each sample was buried by ice during multipleepisodes of glaciation (Bierman et al., 1999; Fabel et al.,2002; Stroeven et al., 2002a,b).

The 10Be and 26Al apparent exposure ages for thePyhä-Nattanen sample are 89.1±6.7 kyr and 69.6±6.4 kyr, respectively. The 26Al/10Be ratio of 4.70±0.37indicates aminimumburial time of 502 kyr. Similar resultswere interpreted for the Riestovaara sample. The apparentexposure ages for 10Be and 26Al are 55.8±4.5 kyr and46.5–±4.0 kyr, respectively. The 26Al/10Be ratio of 5.04±0.39 indicates a minimum burial time of 371 kyr. Thus,both sites have survived multiple episodes of glaciationwithout erosion sufficient to completely remove thepreglacial nuclide inventory (∼2 m). We used an iterativeprocess to simulate variations of 26Al and 10Be concen-trations and ratios, in accord with the ice occupationhistory (Kleman and Stroeven, 1997) inferred from theDSDP 607 marine oxygen isotope proxy record (Lazaruset al., 1995), to determine a best fit for the originalexposure age. This analysis indicated that the minimumtotal exposure-burial histories for Pyhä-Nattanen andRiestovaara samples are 997 kyr and 858 kyr, respectively,implying that the sample surfaces have survived at least 16and 14 episodes of glaciation, respectively.

The 36Cl apparent exposure ages for Pyhä-Nattanenand Riestovaara are consistent with the 10Be and 26Alapparent exposure ages (Table 2). Because of thedifferent decay rates of 10Be (half-life 1.5 Ma), 26Al(half-life 0.7 Ma), and 36Cl (half-life 0.3 Ma), if asample was repeatedly covered by ice, the pattern ofapparent exposure ages should be: 10Be oldest, 26Alintermediate, and 36Cl youngest. For Pyhä-Nattanen andRiestovaara samples, the zero erosion 36Cl apparentexposure ages were 52.1±3.0 kyr and 33.1±2.1 kyr,respectively, obviously younger than their corresponding10Be and 26Al ages. This is consistent with our inter-pretation of repeated ice cover during glacial–interglacialperiods for these sample sites.

4.2. Rock porosity

Measured porosity for Pyhä-Nattanen granite sam-ples ranged from 11.4 to 8.5%. Porosity range for theRiestovarra samples was less, 2.3 to 1.2% (Table 3). Inaddition, the apparent depth of weathering at both sitesexceeded the microprobe sampling depth as evidencedby the similarity in porosity values over the samplingdepth range for all samples. Our assumption that thesample at 4 cm would represent an unweathered controlwas incorrect because it did not exhibit the minimum

Table 8Minor and trace element concentrations (ppm) of peat, granite rocks, grus, and soil from Finnish tor research sites

Sample Sc Be V Cr Co Ni Cu Zn Ga Ge As Rb Sr Y

Pyhä-NattanenPeat 4 2 17 10 7.8 −20 a 131 1,260 18 1.3 5 159 281 25Grus (F2) 2 2 7 10 1.5 −20 −10 −30 17 1.1 3 177 106 9Grus (F8) 2 3 8 10 1.5 −20 −10 −30 20 1.4 3 172 112 10Granite 1 3 3 10 10 1.6 −20 −10 −30 22 1.5 3 182 127 18Granite 2 4 3 9 10 1.7 −20 −10 −30 21 1.4 3 193 123 13

RiestovaaraGrus (F1) 2 2 12 10 1.2 −20 −10 −30 19 1.4 3 206 93 4Grus (F5) 4 3 31 30 5.5 −20 −10 −30 21 1.4 3 154 104 11Granite 1 1 3 5 10 0.5 −20 −10 −30 20 1.4 3 195 92 8Granite 2 3 3 11 10 1.3 −20 −10 −30 20 1.5 3 219 87 9

Soil horizons at RiestovaaraO 1 2 11 10 0.5 −20 −10 −30 12 1.1 11 132 51 5E 2 2 27 23 2.5 −20 −10 −30 21 1.7 3 174 82 6Bw 6 2 47 54 5.4 −20 −10 −30 22 1.4 3 132 100 8BC 4 2 26 26 4.0 −20 17 −30 20 1.1 3 163 89 6C 2 2 17 22 2.9 −20 12 78 17 1.1 3 152 77 5

Zr Nb Mo Ag In Sn Sb Cs Ba La Ce Pr Nd Sm EuaNegative values indicate concentrationsbdetection limit.


porosity value. We were unable to reach unweatheredrock, which presumably is well below 4 cm deep,because we did not have a rock drill available when wecollected our granite samples. Because the granites atthe two sites differ somewhat, interpretation of theirporosity is constrained. However, the tor at Pyhä-Nattanen is better preserved and stands at greater reliefthan the Riestovarra tor. In addition, it is likely mucholder (Table 1), so greater porosity there might beexpected.

Lichen growth did not influence depth or amount ofweatheringwith these samples. No statistically significantdifferenceswere found among the total number of shallow(0–50 cm depth) porosity measurements beneath lichen-free surfaces (n=40), lichen-covered surfaces (n=40),and the “inner”measures at 4 cm depth (n=100) at Pyhä-Nattanen (Table 3). Identical analysis at Riestovaaradistinguished only between the lichen-free outer measure-ments and those at 4 cm, but not among the othercomparisons. This pattern strongly suggests that the rock-weathering profile has advanced beyond the 4 cm depth,with or without a lichen-covered surface. The failure todistinguish between lichen-free and lichen-coveredsurfaces is counterintuitive and not in accordance withour ownwork at Storbreen (Thorn et al., in review) or thatreported by others (e.g., Etienne, 2002). The most likelyexplanation is that the weathering exhibited at these sitesis the product of an infinitely longer amount of time than alichen's life span and has, consequently, reached an

advanced stage in which lichen-cover at any one spot hascome and gone on multiple occasions.

We successfully used techniques applied here todifferentiate rock porosity over time spans of 250 yr inthe Jotunheimen Mountains of Norway (Thorn et al., inreview). While differing rock types preclude meaningfulcomparisons, the Norwegian results provide confidence inour approach. The essentially unvarying porosities withdepth in the Finnish rocks may represent threshold valuesbeyond which disintegration and grus formation takesplace. Consequently, we suggest that at Pyhä-Nattanen andRiestovaara weathering has progressed to a point where alldepths considered have reached some sort of limit, one thatadmittedly differs between the two sites. As grus is presentin the vicinity of the tors at both sites, and as the tors areknown to be of great age, this raises the possibility thatfurther rock weathering simply results in surface spallingand disintegration as grus is produced. However, such asuggestion must be viewed with considerable skepticismuntil a sampling design specifically created to address theissue is executed. This would have to include a rock drillcapable of reaching unweathered material, if it exists onthese very old tors.

4.3. Soil and grus physical properties

The soil profile at Riestovaara was: O horizon, 0–6 cm; E horizon, 6–14 cm; Bw horizon, 14–28 cm; BChorizon, 28–46 cm; C horizon, 46–58 cm; R, and granite

Table 8 (continued)

Zr Nb Mo Ag In Sn Sb Cs Ba La Ce Pr Nd Sm Eu

Pyhä-Nattanen270 26 28 0.6 −0.2 13.7 0.6 0.6 1280 34 72 6.2 21 3.4 0.9173 9 −2 −0.5 −0.2 1.5 −0.5 −0.5 1219 14 47 2.6 8 1.5 0.7195 14 −2 −0.5 −0.2 9.2 −0.5 0.5 1113 14 80 2.8 10 1.8 0.7298 15 −2 −0.5 −0.2 1.7 −0.5 0.6 1208 23 137 5.0 17 3.2 0.9302 16 3 −0.5 −0.2 1.8 −0.5 0.6 1256 11 76 2.4 9 1.9 0.8

Riestovaara118 12 −2 −0.5 −0.2 1.4 −0.5 0.5 817 20 54 2.6 8 1.2 0.4241 17 −2 −0.5 −0.2 1.8 −0.5 0.9 622 46 83 7.0 23 3.5 0.7178 16 −2 −0.5 −0.2 1.0 −0.5 0.6 716 19 102 3.0 9 1.5 0.4191 18 −2 −0.5 −0.2 2.7 −0.5 0.7 782 22 131 4.1 13 2.2 0.5

Soil horizons at Riestovaara125 14 −2 −0.5 −0.2 40.7 −0.5 −0.5 507 7 15 1.0 3 0.6 0.2232 21 −2 −0.5 −0.2 3.2 −0.5 0.7 663 6 15 1.0 4 0.8 0.3170 16 −2 −0.5 −0.2 2.0 −0.5 0.9 587 14 35 2.4 9 1.7 0.5109 12 −2 −0.5 −0.2 2.0 0.8 0.8 654 13 31 2.2 8 1.3 0.489 11 −2 −0.5 −0.2 1.3 −0.5 0.7 625 14 30 2.3 7 1.2 0.4


bedrock at 58 cm. The site was on a nearly level benchnear the summit rock outcrop. The E horizon was verylight colored, and the Bw was reddish brown, indicatingthat podzolization was ongoing (Fig. 3B). The soil profilewas coarse textured, predominatelymade up of gravel andcoarse sand, with no more than a trace of clay (Table 4).This is similar to the texture of the grus we sampled in torcracks and joints and indicates that the soil is essentiallyan accumulation of gruswith a slight pedogenic overprint.In addition, all the soil and grus samples were within thetextural range defined for grus (Migoñ and Thomas,2002). This embryonic stage of pedogenesis is alsoevidenced in comparisons of the grus and soil mineralogyand chemistry, which are quite similar.

4.4. Mineralogy

Mineralogy of the b4 μm fraction of the grus and soilwas dominated by primary minerals, quartz, mica, andfeldspars (Table 5). The dominant secondary mineralfound in the samples was vermiculite. The abundance ofvermiculite increased with depth into the soil, while mica,its presumed precursor, decreases with depth. This is notwhat would be expected in a normal weathering profilewhere secondary minerals form at the expense of primaryminerals from the surface downward. However, recentaeolian inputs of unweathered materials can explain thisanomaly. The mineral fraction isolated from the peatcollected at the windswept summit of Pyhä-Nattanen was

essentially all mica (Table 5). The shallow bedrockdepression where the peat accumulated would have beeneasily cleaned of any loose material upon deglaciation,and moss above the peat makes an ideal aeolian sedimenttrap. This would explain the peat's mica content if themineralogy of the local windblownmaterial was primarilymica. In addition, mica's platy habit would make it alikely candidate for wind transport (Seppälä, 2004). Suchan interpretation is supported by the particle sizedistribution of the soil profile. Silt, the most readilywind-transported particle size, increases toward the soilsurface (Table 4). High vermiculite content is also foundin the grus within tor cracks, particularly at Riestovaara.Grus accumulates (at least in part) as mica in thecompetent rock weathers to vermiculite, although thereare probably also some aeolian inputs. This interpretationof the mineralogy findings is based on too few samples tobe definitive. Additional sampling would lend confidenceto the hypothesis.

4.5. Chemistry

Although the soil examined in the field exhibitedonly slight podzolic horizonation (Fig. 3B), we foundchemical evidence of incipient podzolization: loss oforganic C and extractable Al in the E horizon and acorresponding increase of organic C and Al in the Bwhorizon (Table 6). Overall, soil pH was low anddecreased toward the surface from a high of 5.3 to

Table 8 (continued )

Sample Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U

Pyhä-NattanenPeat 3.8 0.8 5.0 1.0 3.1 0.5 2.9 0.4 7.7 1.7 −1 a 0.9 87 −0.4 12.3 2.0Grus (F2) 1.2 0.2 1.5 0.3 1.0 0.2 1.0 0.1 4.6 0.7 −1 1.0 7 −0.4 6.7 1.5Grus (F8) 1.5 0.3 1.9 0.4 1.2 0.2 1.2 0.2 5.5 1.0 −1 1.3 14 −0.4 13.6 2.1Granite 1 2.7 0.5 3.1 0.6 2.1 0.3 1.9 0.3 8.4 1.0 −1 1.3 12 −0.4 18.0 2.4Granite 2 1.8 0.3 2.2 0.5 1.4 0.2 1.4 0.2 8.5 1.0 −1 1.3 13 −0.4 21.3 1.7

RiestovaaraGrus (F1) 0.8 0.1 0.7 0.1 0.5 0.1 0.5 0.1 3.5 0.6 0.4 1.3 12 −0.4 21.4 1.7Grus (F5) 2.5 0.4 2.1 0.4 1.3 0.2 1.3 0.2 6.8 1.1 −1 0.8 7 −0.4 33.2 2.3Granite 1 1.0 0.2 1.3 0.3 0.9 0.2 1.1 0.2 5.9 1.1 −1 1.1 10 −0.4 31.2 2.2Granite 2 1.3 0.2 1.5 0.3 1.0 0.2 1.3 0.2 6.1 1.3 −1 1.3 11 −0.4 40.9 2.4

Soil horizons at RiestovaaraO 0.5 0.1 0.7 0.1 0.5 0.1 0.6 0.1 4.1 0.9 −1 0.4 −5 −0.4 8.6 1.1E 0.7 0.1 0.9 0.2 0.7 0.1 0.8 0.1 6.3 1.4 −1 1.1 7 −0.4 11.6 1.5Bw 1.5 0.3 1.5 0.3 1.0 0.2 1.0 0.2 4.9 0.8 −1 0.6 −5 −0.4 14.2 1.5BC 1.1 0.2 1.0 0.2 0.6 0.1 0.7 0.1 3.1 0.6 −1 0.6 6 −0.4 10.5 1.2C 1.0 0.2 0.9 0.2 0.6 0.1 0.6 0.1 2.6 0.6 −1 0.8 7 −0.4 9.1 1.1


3.7. The dominant extractable cation in the soil was Ca,followed by Mg, K, and Na. Because of the leaching ofbase cations, the base saturation was low, 7% at thesurface, but increased to 42% at depth. Grus extractablechemistry was similar to the soil chemistry, againillustrating that the soil represents a minor pedogenicoverprinting of a grus deposit.

Table 9Weathering indices based on oxide ratios of soil, grus, and rock from tor res

Sample Si /Al Si / (Al+Fe+Ti) (Ca+Mg+Na) /Ti (S

Pyhä-NattanenGrus (F2) 6.3 5.4 18 6.Grus (F8) 5.9 5.0 15 6.Granite 1 5.4 4.6 16 5.Granite 2 5.6 4.7 16 5.

RiestovaaraGrus (F1) 6.4 5.7 26 6.Grus (F5) 4.9 3.8 12 5.Granite 1 5.4 4.7 23 5.Granite 2 5.7 4.9 16 5.

Soil horizons at RiestovaaraO 7.5 6.2 11 7.E 6.5 5.2 9 6.Bw 5.0 3.9 11 5.BC 6.0 5.0 17 6.C 6.8 5.6 18 6.a CWI = Chemical Weathering Index, based on oxide ratios, (Al+Fe+Ti+b CIA = Chemical Index of Alteration, (Al) / (Al+Ca+Na+K)×100 (Mig

Because of minimum replication, statistical analyses ofthe chemistry data are not possible. However, some generalcomments can be made. Total elemental analysis of rocksamples (Table 7) indicated that the two sites were similarto each other and to publishedvalues (Haapala et al., 1987).Major oxide abundance is in the order of Si, Al, K, Na, Fe,Ca, Mg, Ti, P, and Mn. The grus samples were not much

earch sites in Finland

i+Ca+Na) / (Al+Ti) (Na+K) / (Fe+Mn) CWI a CIAb

5 4.5 15 590 3.9 16 606 3.7 17 598 3.6 16 59

6 5.8 15 590 1.9 24 646 5.4 16 599 4.4 16 59

6 3.6 33 596 2.8 16 601 1.6 25 671 2.9 18 639 2.9 16 62

H2O) / (all chemical components)×100.oñ and Thomas, 2002).

Fig. 5. Weathering ratios, soil/rock at Riestovaara. Values b1 indicatethat soil is more weathered than the rock.


different chemically from the competent rock. The peatsample from Pyhä-Nattanen was lower in Si, but higher inAl, Fe, Mg, Ca, and P than the other samples. Minor andtrace elements were also similar in the rock and grussamples, both within sites and between sites (Table 8). Anexception is Ba, which was in greater concentrations atPyhä-Nattanen. As with the major elements, the peatsample at Pyhä-Nattanen was higher in many of the minorand trace elements (including Co, Cu, Zn, Sr, Y, Nb, Mo,and Pb), indicating that vegetation at the site concentratedthese elements. The ratio of Si to Al in the peat was 3:1,which supports the finding that the mineral fraction of thepeat is composed exclusively of mica (Table 5).

Common weathering ratios did not reveal anysignificant or consistent trends, all samples wereweathered (Table 9). Weathering was so extensive thatall samples were within the grus range as defined bychemical weathering ratios (Migoñ and Thomas, 2002).This indicates that the tors are as weathered as the grus,although the soil samples tended to have somewhat higherweathering ratios than the grus or rock. To help reveal anysubtle trends, the ratios of the weathering indices of thegranite and grus at the two sites were calculated. Acomparison of the weathering index ratios of the grus(GG) and granite (GR) again indicates that there is notmuch difference in their weathering at Pyhä-Nattanen(Fig. 4). In addition, the ratio of the weathering indices ofthe granites from the two sites indicates that the Pyhä-Nattanen granite is slightly more weathered than theRiestovaara granite, possibly because of its coarsertexture, more exposed position, or greater age. Thesefindings are independently verified by our mineralogy

Fig. 4. Weathering ratios of granite rock (GR) and grus (GG) fromFinnish tor sites. Values N1 indicate that grus is more weathered or thatthe rock at a site is more weathered.

results, which indicate the presence of more secondaryspecies in the grus at Riestovaara (Table 5).

The soil at Riestovaara was somewhat moreweathered than the granite, as indicated by the ratiosof their weathering ratios being b1 (Fig. 5). Anexception to this is the Si/Al ratio that indicated thatthe soil was less weathered than the rock because it hadless Al. The general message of the chemistry data, aswith the other parameters evaluated, is that the soilrepresents only a slightly modified grus accumulation.

5. Conclusions

Cosmogenic nuclide dating is an invaluable tool ingeomorphological work. It allows us to answer theformerly unanswerable question posed for old bedrocklandscape features: “How old is it?” Our interpretation ofthe tors we observed in the field as prominent yet fragilefeatures exhibiting advanced weathering as preglacialremnants is confirmed. Cosmogenic nuclide dating ofthese Finnish tors gives minimum 10Be and 26Al apparentexposure ages of 89.1±6.7 kyr and 69.6±6.4 kyr at Pyhä-Nattanen and of 55.8±4.5 kyr and 46.5±4.0 kyr atRiestovaara. Analyses of 10Be and 26Al measurements inaccordance with marine oxygen isotope records (DSDP607) indicate a minimum exposure-burial history of997 kyr and 858 kyr for these two sites, which indicatesthat they not only predate the most recent glaciation,but also have survived at least 14–16 previous glacialcycles.

This considerable weathering history is paramount inthe interpretation of our ancillary data. The rock porositydata indicate that the granite at both sites is highlyweathered, beyond the reach of our hand samples andwithout regard to lichen cover, which is consistent withadvanced weathering and long exposure. Although therewere inconsistencies in the results from the chemicalweathering ratios used, in part because of the small


number of samples, the indices generally indicated that allmaterials evaluated (rock, grus, and soil) were similarlyand extensively weathered. Podzolization is occurring inthe soil as evidenced by the field morphology and thedistribution of C and Al in the soil profile and by thedecrease in pH as compared to the parent rock, although itis in its very early stages. The distribution of silt and themineralogy of the b4 μm fraction of the soil and grusindicates that aeolian additions to the soil may bemaskingweathering at the soil surface where it should be expected.Despite the very old age of the landscape as evidenced bythe tor exposure age, the soils adjacent are only slightlydeveloped.What appears to have happened is that the torshavewithstood a probable ridge-crest scouring of surficialdebris (grus) and, consequently, the current soils aredeveloping in a grus deposit with some aeolian inputs thatpost-date the last glaciation, which is b9 kyr old.

Acknowledgements

Fieldwork for this research was supported by theUniversities of Illinois and Helsinki and analytical work bygrant BCS-0319515 from theNational Science Foundation.

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Age and weathering status of granite tors in Arctic Finland (~ 68° N

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